Scholarly article on topic 'Progress in building-integrated solar thermal systems'

Progress in building-integrated solar thermal systems Academic research paper on "Civil engineering"

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Abstract of research paper on Civil engineering, author of scientific article — Christoph Maurer, Christoph Cappel, Tilmann E. Kuhn

Abstract Solar building envelopes are attracting increasing interest. Building-integrated solar thermal (BIST) systems are one of the subgroups of solar building envelopes. This paper summarizes the most important contributions of recent years and extends them. First, BIST elements are defined and available BIST elements are presented. Then, the general functions which BIST systems can provide are presented and the conflict between the constant U and g values of simple planning software and the variable g and U values of BIST elements is discussed. Measurements to characterize BIST elements are presented as well as a design parameter space in which the current BIST elements are located and which can be used when developing innovative new components. Methods to evaluate and compare BIST technologies are presented. The substantial cost savings which were achieved in three building projects between 2002 and 2009 are discussed. Roles within the building process are presented, as well as the general methods and challenges for economic BIST calculations and one economic calculation as an example. Based on existing building processes, a vision for future BIST building process integration is presented. Simple BIST models, which need no programming, are provided with easy-to-use equations. The challenges of standards and regulations are outlined and future research topics are presented. This paper summarizes important recent contributions to BIST research as a basis for future progress in building-integrated solar thermal systems. Instead of aiming to cover all recent BIST developments, the focus is on BIST research findings which are relevant for cost reduction of BIST components and therefore necessary for the economic success of BIST technology. These are discussed, together with proposals for future research.

Academic research paper on topic "Progress in building-integrated solar thermal systems"

Solar Energy xxx (2017) xxx-xxx

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Solar Energy

journal homepage: www.elsevier.com/locate/solener

Progress in building-integrated solar thermal systems

Christoph Maurer *, Christoph Cappel, Tilmann E. Kuhn

Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, Germany ARTICLE INFO ABSTRACT

Article history:

Received 29 September 2016 Received in revised form 12 April 2017 Accepted 20 May 2017 Available online xxxx

Keywords:

Building-integrated solar thermal (BIST) Building-integrated solar system (BISS) Solar building envelope Facade

Solar architecture Solar thermal collector

Solar building envelopes are attracting increasing interest. Building-integrated solar thermal (BIST) systems are one of the subgroups of solar building envelopes. This paper summarizes the most important contributions of recent years and extends them. First, BIST elements are defined and available BIST elements are presented. Then, the general functions which BIST systems can provide are presented and the conflict between the constant U and g values of simple planning software and the variable g and U values of BIST elements is discussed. Measurements to characterize BIST elements are presented as well as a design parameter space in which the current BIST elements are located and which can be used when developing innovative new components. Methods to evaluate and compare BIST technologies are presented. The substantial cost savings which were achieved in three building projects between 2002 and 2009 are discussed. Roles within the building process are presented, as well as the general methods and challenges for economic BIST calculations and one economic calculation as an example. Based on existing building processes, a vision for future BIST building process integration is presented. Simple BIST models, which need no programming, are provided with easy-to-use equations. The challenges of standards and regulations are outlined and future research topics are presented. This paper summarizes important recent contributions to BIST research as a basis for future progress in building-integrated solar thermal systems. Instead of aiming to cover all recent BIST developments, the focus is on BIST research findings which are relevant for cost reduction of BIST components and therefore necessary for the economic success of BIST technology. These are discussed, together with proposals for future research. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CCBY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4XI/).

1. Introduction

Many papers on building-integrated solar thermal (BIST) systems have been written in the last few decades. This paper aims to present the most important contributions to BIST research, with a special focus on results obtained in the 21st century. It aims to provide an overview of previous progress and the current state of BIST systems to document the starting point for future progress of the BIST approach.

First, building-integrated solar thermal (BIST) systems will be defined in order to clarify the terminology. Then the need for building-integrated solar systems to achieve a cost-effective transformation to a renewable energy system will be outlined. The state of the art will be discussed as well as current challenges and available components.

1.1. Definitions

Most solar thermal collectors on buildings have been installed until now with rear ventilation, which means that there was an

* Corresponding author. E-mail address: christoph.maurer@ise.fraunhofer.de (C. Maurer).

air gap between the collector and the rest of the building envelope. This case is sketched in Fig. 1(a). This means that the solar thermal collector is surrounded, to a good approximation, by air of the ambient temperature. Therefore, the efficiency formula of (Cooper and Dunkle, 1980) uses only the ambient temperature and is widely used to calculate the solar thermal performance of such collector installations. When there is no rear ventilation, the case sketched in Fig. 1(b), then the collector performance is also influenced by the temperature of the building interior, which needs to be included in the formulas for accurate predictions. In the case of Fig. 1(b), the collector serves as insulation for the building envelope. Therefore, it can be called a ''multifunctional building envelope component" with possible benefits from this synergy. However, many building envelopes serve more than one function. Another important difference between the cases of Fig. 1(a) and (b) is that the energy flux between the collector and the interior of the building should be considered in the case of Fig. 1(b), but can typically be neglected in the case of Fig. 1(a). Energy simulation models of non-ventilated solar thermal installations should thus include the temperature of the interior and the energy flux to the interior.

It would be possible to define ''building-integrated" solar thermal as ''non-ventilated" solar thermal. However, building

http://dx.doi.org/10.1016/j.solener.2017.05.065 0038-092X/© 2017 The Authors. Published by Elsevier Ltd.

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Nomenclature

Acronym Explanation g ; g BAST > g BIST > go > go,BAST > go,BIST Collector efficiency

ai ,bist first-order collector efficiency coefficients in the BIST case, g = g0 - alx - a2x2G in general, in the BAST case, in

W/(m2 K) the BIST case, collector efficiency at x = 0, in the BAST case,

a2, a2 bastt,a2 bist second-order collector efficiency coefficients in in the BIST case,

general, in the BAST case, in the BIST case, W/(m2 K2) EN European standard

abists BIST area on the building envelope, m2 fbl fraction of back losses in the BAST case

BAST building-added solar thermal F'bast collector efficiency factor in the BAST case

BIPV building-integrated photovoltaics FMI functional mock-up interface

BIPVT building-integrated photovoltaic-thermal FMU functional mock-up unit

BISS building-integrated solar systems G solar irradiance, W/m2

BIST building-integrated solar thermal IEA International Energy Agency

Ca annual costs per square metre of the BIST installation for IFC Industry Foundation Classes

operation and maintenance, the electricity of the pump ISO International Organization for Standardization

and renting the space for the storage tank, €/m2 LCA life-cycle analysis

CdBISTS investment cost for the solar thermal function of the LCCwlcoh life cycle cost including the levelized cost of heat, €

building envelope including the necessary technical build- LCCwoLCoH life cycle cost without income for the solar thermal

ing plant, €/m2 heat, €

CidBisTimage added value of the building due to a better image per LCOH levelized cost of heat, €

square metre, €/m2 NPVwlcoh net present value including the levelized cost of heat, €

Cidbs investment cost for the technical building plant including qdBISTS non-renewable primary energy saved by the BIST system

the solar thermal function per square metre of BIST area, in one year for this specific application and per square

€/m2 metre of BIST area, kW h/m2

cidsenv investment cost per square metre for the additional solar Quse; Quse.BAST. Quse.bISt solar thermal collector gain in general, in the

thermal function of the building envelope, €/m2 BAST case and in the BIST case, W/m2

cirbs investment cost for the technical building plant of the ref- Qbists non-renewable primary energy demand in kW h per year

erence case per square metre, €/m2 for the BISTS case, kW h

cirenv investment cost per square metre of a conventional part of Qr non-renewable primary energy demand in kW h per year

the building envelope, €/m2 for the reference case, kW h

cisbs investment cost for the technical building plant for the Qsts heat which is supplied per year by the solar thermal

BISTS case per square metre, €/m2 system to the demand of the building, kW h

cisenv investment cost per square metre of a BISTS part of the .so shared object

building envelope, €/m2 r discount rate

cisubs subsidies per square metre collector, €/m2 Rfa thermal resistance between the absorber and the average

Crec costs per square metre for recycling the BISTS, €/m2 fluid temperature, m2 K/W

CsCO2 cost of saved carbon dioxide emissions, €/kg Ri, Ri,BAST, Ri,BiST thermal resistance between the absorber and the

csnrpe cost of non-renewable primary energy saved by the BIST temperature of the building interior in general, for the

system, €/(kW h) BAST case, for the BIST case, m2 K/W

CEN European Committee for Standardization SHC Solar Heating and Cooling Programme of the International

CO2 carbon dioxide Energy Agency

DTstag BAST temperature difference between the absorber and the T service life of the system in years

ambient air during stagnation in the BAST case, K Ta ,Tint. Tfav Temperature of the ambient air, of the building interior,

DIBt German Institute for Construction Technology (Deutsches of the fluid (average), K

Institut für Bautechnik) TCOwlcoh total cost of ownership including levelized cost of heat, €

DLL dynamic-link library TCOwoLCOH total cost of ownership without income for the solar

e average carbon dioxide emission per kW h of non- thermal heat, €

renewable primary energy which the reference case uses

instead of the BIST contributions, kg/(kW h)

envelopes can offer a large variety of functions, which will be discussed in Section 2.1 in detail. Two examples can illustrate the challenge of finding appropriate definitions. Rear-ventilated collectors can be installed without having a significant effect on the building interior. However, in general, they could be installed to act as acoustic insulation, too. Are they then part of the building envelope and do they need to be included in the definition of building-integrated collectors? Second, if an entire roof or facade is constructed of solar thermal collectors, matching the design of the building, e.g. with its windows, if this is perceived as ''the roof" or ''the facade" by people looking at the building, but there is rear ventilation, can these collectors really be named ''not building-integrated"? Because of the large number of functions of a building envelope, a sharp separation line between ''building-integrated" and ''not building-integrated" is arbitrary in general. For a detailed analysis of ''how building-integrated" an installation is, all the functions need to evaluated and compared to a typical reference

case, including aesthetics as a function of the building envelope. For this, the applicable functions would need to be collected, evaluated, e.g. by points between 0 and 10 and presented as a radar chart. As one example to illustrate this, Fig. 2 presents an installation of semi-transparent solar thermal facade collectors, which provide (at least) the functions of solar thermal performance, aesthetics and classic solar control requirements. Fig. 3 presents a radar chart for these functions. The evaluation of the classic solar control requirements was based on Kuhn (2017) and the evaluation of the aesthetics will be discussed together with Table 1 in Section 1.3.

Definitions already exist like the one for building-integrated photovoltaics (BIPV) from (EN 50583, 2016), which defines BIPV as PV modules which provide a function from the European Construction Product Regulation (European Parliament and European Council, 2011) for the function of the building. If a BIPV module is removed, it needs to be replaced by another building product.

Fig. 2. Photos of an installation of semi-transparent solar thermal facade collectors in Paris, France. © RobinSun.

aesthetics 10

classic solar-control solar thermal

requirements performance

Fig. 3. Radar chart of the functions of the BIST installation of Fig. 2.

Such a definition includes many PV installations because PV can be installed in a way that it provides at least some function like wind or noise protection or shading. On the other hand, such a definition does not differ between very well-integrated, multifunctional BIPV

Table 1

Example of an evaluation of the BIST installation of Fig. 2 to illustrate the methodology.

Field position and dimension 9

Visible materials 10

Surface texture 10

Surface colour 9

Module shape & size 8

Jointing 8

Total 9

that offer many functions of a high quality and BIPV that offers only one function of a low quality.

Building-integrated solar systems (BISS) and solar building envelopes are therefore defined here similarly to the definition of building-integrated solar thermal systems as building envelopes that use the solar irradiance to provide one function to the building and also provide at least one other function to the building. When a solar building envelope provides many functions to the building and a high quality of these functions, then it is very well integrated and very multifunctional. BIST elements form a subgroup of

BISS/solar building envelopes. Solar building envelopes are more general than BIST installations, since the function based on solar irradiance, which they require to meet the definition, can be a solar thermal function or any other solar function like solar control or PV.

The terms BIST collector, element and system are used in this publication with the following meanings: A BIST collector can be installed as part of the layer of a building envelope that faces the surroundings. A BIST element is a part of the entire building envelope which provides a solar thermal function and includes all layers of the building envelope from the exterior to the interior of the building. A BIST system (BISTS) is the combination of BIST elements) with technical building plant. This means that a BIST collector can be installed as a component of a BIST element, which in turn is usually a part of a BIST system.

On the one hand, the resulting graduated definition of''more" or ''less" ''building integration" or ''multifunctionality" is more complex than expected at the beginning of BIST research. The rating of the building integration also depends on the evaluators and further research is needed to develop methods for better comparison of the degree of building integration for different installations. On the other hand, this well reflects the large variety of buildings and possible synergy effects created by addressing several functions at the same time. ''In-roof" collectors for example are typically integrated into the plane of the roof but are still rear-ventilated.

Similarly, there a no strict boundaries between ''passive" solar systems and solar control and BIST systems. This paper focuses on BIST installations that can transport solar thermally generated heat to a storage unit so that it can be used at times with low irra-diance, because this typically allows a larger fraction of the solar irradiance to be used as heat.

In the same way, it is just a matter of definition whether building-integrated photovoltaic-thermal (BIPVT) systems are considered to be a part of BIST technology. This paper focuses on BIST elements in order to be more specific. However, publications on BIPVT systems are mentioned when they are important also for BIST considerations and many findings for BIST installations can be applied in a modified form to BIPVT systems.

1.2. Importance for the energy transformation

Studies of scenarios for predominantly renewable energy systems in 2050 such as (Henning and Palzer, 2014; Palzer and Henning, 2014) conclude that large capacities of wind, photovoltaic and solar thermal energy supply are needed and that the building stock simultaneously needs to reduce its energy demand to achieve cost-effective solutions for a 100% renewable electricity and heat supply in Germany in 2050. Depending on the cost assumptions, the quantities of renewables and energy savings vary. There are also scenarios without solar thermal systems. The annual total costs are likely to be lower than the costs for electricity and heat today. In (Henning and Palzer, 2015), the entire German energy system is modelled including the transformation costs. Depending on whether the prices for fossil fuels rise and whether prices for carbon dioxide emissions have to be paid, the cumulative costs of a 85% renewable energy system are lower or higher than maintaining the current German energy system. In summary, there are several scenarios with similar total costs and there is some uncertainty regarding the assumption of costs over several decades. However, the solar thermal energy supply needs to be extended considerably according to the analysed scenarios. Section 3.1 will present how building-integrated solar thermal collectors have already saved costs by the synergy of refurbishing the building and installing solar thermal collectors at the same time. It is therefore expected that building-integrated solar thermal collectors can contribute significantly to the necessary energy savings

reasons why the IEA SHC Task 56 on ''Building Integrated Solar Envelope Systems for HVAC and lighting" was started in the beginning of 2016 (IEA SHC Task 56).

1.3. State of research

Building-integrated solar thermal systems have been investigated for many decades (Morse, 1881; Fuschillo, 1975; Pierce, 1977). There have been many attempts to design solar building envelopes which use air as the heat transfer medium, use at least one glass pane and an absorbing layer and/or blinds and switch-able openings to transport the absorbed heat into the interior or the exterior, sometimes in combination with supplying or extracting air. The term Trombe wall for such building envelopes was coined because Felix Trombe (Trombe and Michel, 1974) worked in detail on a concept which was patented by Morse (1881). An overview of research on Trombe walls has been presented by Saadatian et al. (2012). Although many concepts looked promising, many demonstration buildings were built and many simulation models were developed (Lamnatou et al., 2015b, 2015c), few products are on the market and the building sector has not been penetrated by Trombe walls so far. One reason may be that switchable openings need to be designed and controlled carefully to avoid expensive maintenance.

Another challenge is that even in winter, there is often no heating demand at times with high irradiance due to the solar gains through the windows. Therefore, this review is focusing on active solar thermal building envelopes that typically use a pump or ventilator to transport solar thermal heat to a thermal storage unit so that it can contribute to the technical building services also at times with low irradiance. This section first discusses important general contributions to the topic chronologically, before presenting research on the large variety of BIST technologies.

In the 21st century, Bergmann and Weiß (2002) was the first and one of the most important contributions to the progress of BIST technology. Its analysis of several technical barriers for BIST technology is still pertinent (Cappel et al., 2014b).

For example, Bergmann and Weiß (2002) investigated 14 BIST installations and concluded that peak temperatures for the heat-transfer fluid of up to 195 °C can be reached during stagnation, heat-resistant materials need to be used and thermal expansion needs to be considered. Overheating of the interior was be prevented by 10 cm of mineral wool. In winter, up to 90% of the thermal losses through the building envelope can be prevented over the BIST areas during days with high irradiance. Regarding vapour transfer, solar thermal absorbers and casings typically represented a vapour barrier. In the case of installations without rear ventilation, the layers between the solar thermal collector and the interior should therefore be open to diffusion to allow vapour to leave the building skin through the interior surface during times when the collector is warmer than the interior of the building. Simulations and experiments were performed by Bergmann and Weiß (2002) and proved that there are no moisture problems in this case.

Regarding the hydraulics of the heat transfer fluid, the Tichel-mann interconnection is recommended. It can be used even for varying numbers of collectors in parallel. Without detailed simulations, the installer should be able to estimate good set points for the valves to achieve a flow distribution pattern, which extracts a similar amount of heat from each collector. Shading of course needs to be considered with the help of a sun path indicator or a simulation when a new installation is planned. It should also be remembered that the plants and buildings in the surroundings could change and that agreements are made, e.g. with neighbours to prevent difficulties in the future. In general, it can be stated that the yield of solar thermal collectors is much less sensitive to partial

and the supply of renewably generated heat. This is one of the shading than the electricity yield of PV systems.

A few years later, Matuska and Sourek (2006) analysed facade collectors experimentally and theoretically regarding the solar thermal efficiency and performance and the temperatures of the building interior. At the same time, the use of flat-plate collectors integrated into the facades of a high-rise residential building in Hong Kong was analysed (Chow et al., 2006).

Another publication which is still state of the art was presented by (Munari Probst and Roecker, 2007) with the topic of the aesthetic evaluation of BIST installations. Four aesthetic criteria are presented: The position and the dimension of the collector field (s) should be compatible with the architectural composition of the whole building. The surface texture and colour should interact positively with the other building skin materials, colours and textures. The shape and size of the solar thermal element should fit into the construction grid of the building and the other dimensions of the facade elements. Finally, the jointing between the solar thermal element and the surrounding building envelope should be designed such that the modular grid of the collector fields harmonizes with the entire building design.

This methodology was further developed by the IEA SHC Task 41 on ''Solar Energy and Architecture" which also presented valuable contributions (IEA SHC Task 41 Publications; Farkas and Horvat, 2012; IEA SHC Task 41, 2013; Munari Probst and Roecker, 2013; IEA SHC Task 41 Case Studies, 2014). To illustrate the methodology for the aesthetic evaluation, Table 1 presents one evaluation of the BIST installation of Fig. 2 as an example. The result of such an evaluation can be used in a radar chart as in Fig. 3. Of course, such an evaluation depends on the evaluator. If evaluation that is more objective is needed, the number of eval-uators could be increased to find a statistically significant result. The interactions between functional and aesthetic aspects in the planning of solar thermal building envelopes are also discussed by Krstic-Furundzic et al. (2017).

In this second decade of the twenty-first century, the simulation models for BIST elements were improved. There had been several detailed models of solar thermal collectors which could be applied for BIST elements (Oliva et al., 1991; Plantier et al., 2003; Hassan and Beliveau, 2007; Cadafalch, 2009; Molero Villar et al., 2009) and also detailed models of multi-skin facades. A connection to the closed-source building model in TRNSYS (Beckman et al., 1994) was developed by (Maurer and Kuhn, 2011) with the focus on short computing times and easy usability for planners. Another connection to the closed-source building model in TRNSYS was presented by (Hauer and Streicher, 2013). The currently most comprehensive overview of BIST simulation models was presented by (Lamnatou et al., 2015b, 2015c).

General reviews of BIST technologies have been published (Quesada et al., 2012a, 2012b; Buker and Riffat, 2015; Zhang et al., 2015). While Quesada et al. (2012a, 2012b) provides an extensive overview of solar facades, it does not provide much evaluation and comparison of the developed technologies.

In a separate subsection, D'Antoni and Saro (2012) provides a review of BIST elements with high thermal capacity such as those made with concrete.

Many examples of building-integrated photovoltaic-thermal (BIPVT) and BIST systems are presented by Buker and Riffat (2015), but this publication misses important contributions like (Bergmann and Weiß, 2002; Robin, 2002; Matuska and Sourek, 2006; Davidsson et al., 2012; Hauer and Streicher, 2013; Maurer et al., 2013). In Buker and Riffat (2015), much technical data from the reviewed papers is presented. Nevertheless, a comparison between the presented papers is difficult because the boundary conditions are not presented and the specific usage pattern, including the fluid inlet temperature, greatly affects the solar thermal performance. In Zhang et al. (2015), already the fundamental con-

on the irradiance and the collector operation mode, which can double the g value or solar heat gain coefficient of a BIST element (Maurer and Kuhn, 2012). The overview in Zhang et al. (2015) of standards for solar thermal components includes many countries, but some important standards for building products are missing. Eq. (5) of Zhang et al. (2015) seems to neglect multiple reflections and the collector efficiency factor F'. It would be helpful to know the original source of the equations. The overview presents only a part of the commercially available products presented by IEA SHC Task 41 (2013) and Cappel et al. (2014a), but some visual impressions of several buildings including BIST installations are presented which go beyond the scope of the comprehensive overview developed by the IEA SHC Task 41 (IEA SHC Task 41 Case Studies, 2014).

Different BIST systems are also presented by O'Hegarty et al. (2016). The proposed equation for the solar thermal performance is similar to the equation of Pflug et al. (2013). It is therefore likely that it can represent the solar thermal performance of BIST installations quite well but it remains unclear whether the same difficulties to calculate the energy flux to the building interior occur as in Pflug et al. (2013). O'Hegarty et al. (2016) presents a comprehensive schematic overview of possible BIST installations. The economic discussion does not yet include life cycle cost analysis.

Some of the functions, which are discussed in Section 2.1, are also mentioned by Kalogirou (2015). Only a few of the technologies, which were presented by the above-mentioned reviews, are also presented by Kalogirou (2015). However, two product technologies are presented which have not been mentioned by the other reviews.

Regarding the life-cycle analysis (LCA) of BIST systems, important contributions were presented by Lenz et al. (2012), Lamnatou et al. (2015a, 2016). They also contributed to the LCA overview of COST Action TU1205 BISTS (2015), which contains many photos of well-integrated BIST installations, too.

A review of building-integrated photovoltaic thermal (BIPVT) systems is presented by Yang and Athienitis (2016).

An overview of barriers preventing BIST technology from penetrating the market deeply is provided by Cappel et al. (2014b). One economic barrier is related to standards and incentives. If an innovative BIST product is not yet foreseen in the measurements specified by currently valid standards, it may be excluded from the necessary labels (e.g. solar keymark) for incentives. Another challenge is caused by the regulations themselves: As they differ from country to country and change from time to time, innovative companies need appreciable resources to check whether their products are still eligible for the incentives and whether they can be treated as an energy-saving measure according to the relevant local building code.

Regarding the barriers within the building process, many experts have predicted problems with the on-site interaction of different trades. However, people who have implemented BIST installations do not consider this to be a barrier, based on their experience. In the end, each tradesperson remains responsible for his work. However, it is true that the liabilities should be addressed clearly in the contracts, especially for large building projects. Section 3.2 will present some roles within BIST building processes, but many business models and forms of cooperation between professionals are expected to develop in the coming decades.

In the survey of Cappel et al. (2014b), customers were not aware of the option of facade-integrated solar thermal collectors and when they heard about them, they were typically afraid of additional costs and unaware of the cost savings which have already been achieved by BIST installations and which will be presented in detail in Section 3.1. Aesthetic issues occupied only the third

cept lacks the general dependence of the energy flux to the interior ranking for customers.

Amongst architects, Farkas and Horvat (2012) also showed that lack of knowledge is the greatest problem. BIST technology adds an additional function to the building envelope; it is therefore more complex than conventional building envelopes and the education system for architects needs to show them how they can handle this complexity easily in order to create attractive solar architecture. A recent survey on barriers to solar facades in general is presented by Prieto et al. (2017).

As the interest in BIST systems is increasing, many BIST technologies have been developed recently. Section 1.4 will present technologies which reached the stage of being commercially available. This section mentions research on BIST elements, which are not yet available on the market to the knowledge of the authors, but may be further developed to become products in the future.

Concepts with multiple building skins similar to the Trombe wall concepts continue to be investigated (Saelens et al., 2004; Manz and Frank, 2005; Saelens et al., 2008; Baldinelli, 2009). Several technologies were proposed to have stationary shading elements above or in front of windows which provide solar thermal energy, too (Abu-Zour et al., 2006; Palmero-Marrero and Oliveira, 2006, 2010; Li et al., 2016). There are many developments on BIPVT like (Buonomano et al., 2016; Athienitis et al., 2017) which exceed the scope of this paper.

Research on providing coloured BIST products with good solar thermal performance is also active (Tripanagnostopoulos et al., 2000; Kalogirou et al., 2005; Orel et al., 2007a; Orel et al., 2007b; Bonhote et al., 2009; Anderson et al., 2010; Mertin et al., 2014). Concentrating BIST elements were developed by Petrakis et al. (2009), Chemisana et al. (2013), and González-Pardo et al. (2014). BIST elements integrated into gutters were investigated by Motte et al. (2013) and Notton et al. (2014) and BIST elements as window shutters by Cristofari et al. (2017). A BIST collector which uses natural convection and liquid and air as heat transfer media in parallel was developed by Ji et al. (2011).

Plaster-integrated BIST components together with a facade-integrated heat pump were developed by Ruschenburg et al. (2011). Another coverless, low-cost BIST element was developed by Giovanardi et al. (2015). A low-cost BIPVT element was developed by Katsifaraki et al. (2014). A system for building-integrated solar elements was developed by Windholz et al. (2011), Gosztonyi et al. (2013), and Stark et al. (2014)). Experience with BIST systems was presented by Zhai et al. (2008) and Maurer et al. (2014).

Opaque absorbers between glass panes were developed by Heinzen et al. (2006) and Giovannetti et al. (2014). Stationary semitransparent solar thermal facade elements were developed by Robin (2002), Behling et al. (2013), and Maurer et al. (2013). A semitransparent stationary BIPVT element was developed by Zacharopoulos et al. (2017). Concentrating semitransparent BIST elements were developed by Sultana et al. (2012), Davidsson et al. (2010, 2012), and Ulavi et al. (2014). Solar thermal venetian blinds were investigated by Haeringer et al. (2017), Cruz Lopez (2011), Tripanagnostopoulos (2014), and Guardo et al. (2015). There is research to use liquids between glazing to reduce the cooling loads of a building and provide solar thermal energy at the same time (Chow et al., 2011; Gstöhl et al., 2011; Gil-Lopez and Gimenez-Molina, 2013; Lyu and Chow, 2015; Sierra and Hernández, 2017).

The use of heat pipes for BIST elements has been investigated by Albanese et al. (2012) and Rassamakin et al. (2013). Within 1EA SHC Task 39, BIST collectors made from polymers were developed (1EA SHC Task 39; Köhl et al., 2012) and more polymer BIST collectors are being investigated (Pugsley et al., 2017). A ceramic B1ST collector was developed by Yang et al. (2013). BIST elements with asphalt as the absorber were investigated by Bobes-Jesus et al. (2013) and Pugsley et al. (2017). For overheating protection, Hengstberger et al. (2016) analysed the usage of phase change

materials. Absorbers made from ultra-high-performance concrete were developed by Koch et al. (2012).

BIST collectors with air as the heat transfer medium were developed by Dowson et al. (2012), Gao et al. (2013), Kalogirou et al. (2017), and Norton et al. (2017). An overview of transpired BIST collectors is provided by Ashish et al.. Building-integrated vacuum tubes with air as the heat transfer medium were presented by Maurer et al. (2012b). The general method of Welz et al. (2014) for air collectors can also be applied to BIST collectors. A BIST collector with an integrated vacuum-insulated storage tank was developed by Smyth et al. (2017).

At Kapfenberg in Austria, a promising new approach was demonstrated: facade elements were prefabricated with maximum outer dimensions such that they can still be transported by a truck and which included BIST collectors and a part of the technical building plant (Sacherer et al., 2014). The prefabricated elements were then installed quickly on the existing building. One target is to save costs by prefabrication and by reducing the disturbance to the building occupants. BIST collectors with a trapezoidal form were developed by Visa et al. (2014) and Visa et al. (2017). BIST strip collectors are being developed which offer free choice of the material between the collector strips as well as the distance between the strips and the length of the strips (Morawietz et al., 2016).

1.4. Commercially available components

An overview of available products is not typically a scientific task in itself. However, if science aims to support the economic success of BIST systems with its methods, it is crucial to know which BIST technologies are already commercially available as products. Understanding the success of these elements helps to focus on the most important research topics. As there is a wide variety of BIST products, ways of structuring this variety are needed which are useful for the relevant stakeholders. This section is based on Cappel et al. (2014a) and presents BIST subcomponents first and then an overview of BIST products in a systematic way.

1.4.1. Subcomponents for BIST products

For glazed collectors, the texture of the cover glass pane can contribute to aesthetically successful BIST integration. An overview of some possible glass textures is provided by Fig. 4.

Another option is the sandblasting or etching of the glass surface to achieve direct-diffuse reflectance and transmittance instead of the direct-direct transmittance which allows visual contact through clear glass panes.

Screen-printing is another option, but this causes higher optical losses.

At EPFL, Mertin et al. (2014) developed a reflective filter which leads to a coloured visible reflection, but reduces the solar trans-mittance by less than 5%. Photos of such coated glass panes are presented in Fig. 5. When combined with a light-scattering layer, the structure of the absorber is not visible anymore. The materials are commercially available and can be manufactured with etched surfaces and patterns as in Fig. 6, if desired.

When transparent glass covers are used, the texture of the absorber influences the aesthetic integration. Some solar thermal collectors with a clearly visible absorber texture are presented by Fig. 7. The welding and the thickness of the absorber sheet influence the texture of the absorber.

Thicker absorber sheets reduce the bending of the absorber sheet between the tubes that carry the heat transfer medium.

A cost-effective solution is ultrasonic welding from the front of the absorber sheet to weld the sheet to the tube. Laser welding is also used often, but results in a lower thermal conductance between the absorber sheet and the tube. Both are presented in

Fig. 4. Glass with different textures over a solar cell (Munari Probst and Roecker, 2011).

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Fig. 7. Solar thermal collectors with strongly textured absorbers (Munari Probst and Roecker, 2011).

Fig. 8. Ultrasonic welding from the rear of the absorber sheet with an additional sheet surrounding the tube is presented in Fig. 9 next to an adhesively bonded sheet to connect the absorber sheet and the tube. In building integration, custom-made collectors are often needed and it is typically expensive to adjust welding machines for mass production to individual geometries. Therefore, it is helpful to know about the available absorber options. Another method is to manufacture long strips of absorber sheet and tube, to cut them to the right size and to weld header tubes to the small tubes.

1.4.2. BIST products

The following classification of BIST products first presents opaque BIST elements and the semi-transparent ones, because this difference is important for architects and the available areas for installation. The classification then differentiates according to the

thermal resistance between the absorber and the ambient air (unglazed, glazed, vacuum) because this has an aesthetic influence and a strong influence on the efficiency of the BIST system. Finally, the BIST elements are classified according to their heat transfer medium (air or liquid) because this also influences the interaction of the BIST system with the technical building services. This section includes information from (Cappel et al., 2014a; COST Action TU1205 BISTS, 2015; Zhang et al., 2015; IEA SHC Task 51, 2016). Even a comprehensive market overview can never be complete, but we consider it helpful to understand the opportunities for future BIST products better.

1.4.2.1. Opaque BIST products. In general, unglazed collectors do not reach the temperature levels of glazed collectors. Unglazed collectors with air as the heat transfer medium are therefore typically

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Fig. 8. Sheets and tubes connected by laser welding (left) and by ultrasonic welding (right) (Sapa, 2013).

Fig. 9. Sheets and tubes connected with an omega-shaped sheet by ultrasonic welding from the rear of the absorber sheet (left) and by a sheet bonded adhesively by pressure to the back of the absorber sheet (Cappel et al., 2014a).

Fig. 10. Schematic drawing of two corrugated sheets between which the heat transfer fluid flows (IEA SHC Task 41, 2013).

used for heating or pre-heating of supply air. The first concept for using the building envelope as an unglazed air collector was patented, marketed and installed in many buildings by Hollick (1982) and Solar Wall (2016), and also offers many colours and surface textures. The absorber can be transpired, may be used for night cooling and parts of the collector can be covered with PV or glazed to reach higher temperatures. Similar concepts are offered, e.g. by Enerconcept; Kingspan Ltd (2016) and Matrix Energy Inc. (2016) and by OM Solar Association, which have installed solar thermal roofs with a combination of unglazed and glazed areas in a large number of buildings.

Different technologies are used for unglazed collectors with a liquid heat transfer medium. Corrugated sheets, between which the fluid flows, are used by Energie Solaire SA (2016) and illustrated in Fig. 10. Polymer tubes were used by Englert (2016) and Rheinzink (2016) to activate their metal roofs. Another tube system is used by WAF-Fassadensysteme GmbH to extract heat from rear-ventilated panels. A special brass composition is used by ATMOVA (2016) for solar thermal tiles which resemble conventional tiles. Absorbers on top of each tile and with the same colour as the tiles are offered by NELSKAMP (2016). Fluid pipes below stone slats, bitumen or asphalt are offered, e.g. by Stobbe (2016).

Glazed air collectors have already been integrated into several buildings by Grammer Solar GmbH and are also offered by Environmental Solar Systems (2016). Glazed air collectors with light, diffusely reflecting glass covers are offered by EnerSearch. Facades which can also provide preheated supply air are offered by SCHANKULA Architekten Diplomingenieure (2016).

For glazed solar thermal collectors with liquid as the heat transfer medium, either mass-produced flat-plate collectors can be used which are designed for rear-ventilated installation but can also be integrated into the building envelope, or collectors which are especially designed for building integration can be used which can also be cost-effective as presented in Section 3.1. This paragraph limits itself to collectors which are especially constructed for building integration. Custom-made flat-plate BIST collectors are supplied for example by SolMetall GmbH, DOMA Solartechnik (2016), SIKO SOLAR GmbH (2016), ThuSolar GmbH (2016), and Winkler Solar (2016). There are even circular flat-plate collectors offered by Energetyka Solarna ENSOL (2016).

Many cases of successful architectural integration of its polymer collectors into the building envelope have been presented by Aventa Solar. Polymer collectors typically embody less grey energy than collectors made of glass and metal. Transparent curved glass tiles with an absorber below them are offered by SolTech Energy (2016) while, e.g. (IMERYS Toiture, 2016) offers flat solar thermal tiles. BIST collectors with a flat glass cover and small parabolic reflectors within the collector to reflect the solar radiation onto the absorber sheets are offered by Solarfocus. A collector including a storage tank for the ridge of the roof is offered by Inventum (2016). BIST elements with coloured diffuse reflection but small optical losses are offered, e.g. by SwissINSO.

An absorber which is completely integrated between two glass panes as an integrated glazing unit is offered by H+S Solar GmbH (2016). A BIST collector which is as thin as a glass pane at the edges is offered, e.g. by SOLTOP (2016) and S-Solar AB (2016) which can

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make the integration into a transom and mullion façade or overlapping collectors easier. One integration solution for its solar thermal collectors is offered by Wagner & Co Solartechnik GmbH which is tight against wind-driven rain and can be used in the roof as well as in the facade. A facade system into which solar thermal, PV and PVT elements can be mounted is, e.g. offered by C. Bosch GmbH (2016) and S-Solar AB (2016). Concrete facade elements which serve as thermal storage and solar thermal absorber are offered, e.g. by GAP3 solutions GmbH. Building envelopes with absorbers between glass panes are offered, e.g. by Heliopan Energie Fassaden and Heinrich Lamparter Stahlbau GmbH (2016).

Amongst BIST vacuum tube collectors, collectors are offered in which the absorber sheets within the vacuum tubes are tilted for good absorption of the solar radiation, e.g. by Schweizer-Energie (2016). Vacuum tube collectors which use air as the heat transfer medium and which can be manufactured according to the desired design are offered by Airwasol GmbH & Co. KG. Vacuum tubes within special tiles with a curved translucent cover were developed by REM. Vertical vacuum tubes including cylindrical storage tanks with absorbing surfaces are offered, e.g. by GLE Solar Energy.

1.4.2.2. Semi-transparent BIST products. Semi-transparent building-integrated solar thermal elements can be used, e.g. for solar control and visual contact between the interior and the exterior of the building. The first semi-transparent facade collector with a liquid heat-transfer medium was invented and installed in many buildings by Robin (2002). It is presented in Fig. 2 and is manufactured of absorber sheets hanging on horizontal pipes. By using reflective horizontal stripes on the interior glass pane, the solar shading is strong in summer and weak in winter, an effect that is known as seasonal shading. Facade systems like that mentioned above from Heliopan Energie Fassaden and Heinrich Lamparter Stahlbau GmbH (2016) can also be installed with transparent areas. Fixed perforated horizontal slats attached to a pipe which can be mounted between glass panes were developed by S-Solar AB (2016). A semi-transparent facade collector with horizontal vacuum tubes and concentrating perforated sheets was developed within a cooperation between RITTER XL SOLAR (2016) and WICONA (2016). A semi-transparent absorber with very small slats and intermediate glass panes was developed by Permasteelisa Group (2016). Many small openings in the absorber can allow good visual contact through the collector.

As presented, numerous building-integrated solar thermal elements are available and more are presently being invented and developed. Some products may not be currently available in small quantities, but could be produced again if the demand were large enough.

2. Functions and characteristics

Section 1 presented the definitions, the importance, the state of the art and the available components of building-integrated solar thermal elements, as well as the challenges that they face. In order to understand these challenges and opportunities better, this section presents the BIST functions and characteristics in detail before the economic aspects will be discussed in Section 3. First, the general functions that BIST elements can provide will be presented. Then the complexity of BIST building simulations is discussed regarding the solar heat gain coefficient or g value. Section 2.3 presents how BIST elements can be characterized in the laboratory. Section 2.4 presents a multidimensional design space, which includes the currently known BIST elements and is likely to include most future BIST elements, too. New BIST elements can be developed by combining the right parameters within this design space. In order to evaluate ideas for new BIST elements,

Section 2.5 presents evaluation criteria and discusses how they can be used for specific development questions.

2.1. Functions which BIST systems can provide

In general, a BIST element can provide the following functions when it is operating well in a system, which typically includes piping, a thermal storage tank, a pump and a membrane expansion vessel

2.1.1. Domestic hot water heating

This is the most common use of solar thermal systems. Instead of oil, gas, biomass or electricity, the sun is used to heat the water, e.g. to shower. Typically, the domestic hot water should provide a minimum temperature between 40 and 65 °C. If the BIST collectors cannot provide the minimum temperature, they can still be used to preheat the fresh water to reduce the demand for non-renewable energy sources.

2.1.2. Space heating

During the heating season, solar thermal collectors can reduce the demand for non-renewable energy sources to heat the rooms within a building. However, the temperature difference between the heat transfer medium within the collector and the ambient air strongly affects the efficiency of the BIST element. Therefore, BIST elements should be combined with low-temperature heat distribution systems such as floor heating and thermally activated building systems (TABS) which can operate with supply temperatures of 35 °C. If a radiator system is supplied with 60 °C, the same solar collectors provide less heat.

2.1.3. Other low-temperature heat demand

BIST systems that provide only low-temperature heat can still preheat domestic hot water or swimming pools or serve as a heat source for heat pumps.

2.1.4. Process heat

Within some buildings or their surroundings, there is a demand for heat with a temperature of more than 100 °C. For industrial processes as are needed, e.g. in a laundry or a hospital, specialized BIST systems could provide temperatures up to 400 °C. Up to now, there has been research on building-integrated concentrating solar thermal building envelopes. The available commercial products for solar thermal process heat are not yet focussing on building integration.

2.1.5. Dehumidification

Another function, which systems with BIST elements can provide, is dehumidification. Some old buildings need a supply of dry air in order to avoid moisture problems. In large buildings, air-handling units are typically installed, often including an energy recovery unit. BIST technology can be used to dehumidify the sorption unit. In this case, the compression chiller does not need to provide the energy for dehumidification and the compression chiller can also operate at the higher temperatures for space cooling instead of the low temperatures that would be needed for dehu-midification. The smaller temperature difference also increases the efficiency of the compression chiller. Finally, dehumidification that is needed, e.g. in agricultural processes can also be provided by BIST technology.

2.1.6. Space cooling

Besides electrically driven heat pumps, there are also thermally driven heat pumps, which can use solar thermal heat for cooling. Due to the decrease in the price of PV modules, photovoltaic and electrically driven heat pumps are sold more often and thermally

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driven heat pumps are recommended in cases where there is a large heat demand, e.g. for industrial processes.

If the supply air can be dried more than necessary by a dehu-midification process, it can be humidified again which can provide cooling to the supply air.

2.1.7. Storage

In principle, the renewably generated heat can be stored within the building envelope. However, conventional thermal storage tanks often have a cylindrical shape and building envelopes are typically large in two dimensions and thin in the third dimension. Therefore, a building-integrated thermal storage tank typically has a larger surface area. Since the losses from a tank depend on its surface area, a building-integrated thermal storage tank typically needs an increased thickness of the building envelope and/or more insulation material and/or insulation with lower conductivity to reach a similar thermal resistance between the interior and the exterior of the tank as conventional thermal storage tanks.

In general, thermal storage tanks are much cheaper per kW h of stored energy than electric storage units. The renewable heat of a BIST system can be used directly to heat a thermal storage tank, which can be dimensioned to store the heat for some hours, a few days or even between the cooling and the heating seasons.

Another option for ''building-integrated thermal storage" is to use the thermal mass of a building for heat storage (Maurer et al., 2013). When there is enough solar thermally generated heat, e.g. during the heating season, the temperature of the building can be increased above the minimum comfort temperature as long as the temperature within the building stays below the maximum comfort temperature. As the building envelope contributes to the thermal mass of the building, it can also be optimized to have a large thermal capacity.

2.1.8. Energy flux to the building interior

When solar thermal collectors are integrated into the building envelope without rear ventilation, they serve as insulation of the building envelope. When a BIST collector is not operated, e.g. because the storage tank is fully charged, the collector is typically warmer than the non-BIST building envelope, which decreases the heating demand and increases the cooling demand. With increasing insulation between the absorber and the building interior, this effect decreases. When a BIST collector is operated, the energy flux to the building interior depends on the collector operation. If the collector is operated at very low temperatures, the energy flux from the BIST area to the interior can be even smaller than the energy flux from the non-BIST area to the interior. If the collector is operated at very high temperatures, the energy flux to the interior is almost as large as in the case without collector operation. In general, the energy flux to the building interior also depends on the direction of the incident radiation, especially for angle-selective BIST products.

2.1.9. Solar control

As presented in Section 1.4.2.2, some BIST elements are semitransparent. Compared to transparent windows, the semitransparent BIST system provides solar control in order to reduce the cooling demand or overheating of the building (Kuhn, 2016).

2.1.10. Visual contact

Semi-transparent BIST elements can also provide visual contact between the interior and the exterior of the building. Since transparent areas are important in modern architecture, semitransparent BIST installations can offer visual contact and primary energy savings compared to an opaque wall, especially in the spandrel area (Maurer, 2012). At present, switchable semi-transparent

blinds which can either provide full visual contact and daylight or solar control and solar thermal performance (Fraunhofer Institute for Solar Energy Systems ISE, 2016).

2.1.11. Daylighting

If stationary semi-transparent BIST installations are used instead of opaque elements, they can reduce the demand for artificial lighting and the primary energy demand of a building (Maurer, 2012). If a transparent window is at a position that is not relevant for daylighting, like many spandrel areas, or if the transparent areas are much larger than necessary, some window areas can be replaced by stationary semi-transparent or opaque BIST components, without increasing the demand for artificial lighting.

2.1.12. Glare control

The glare control function can be divided into glare protection within the room and the prevention of outdoor glare. The prevention of discomfort glare in rooms due to bright objects in the vision area can be assessed with the daylight glare probability DGP metric (Wienold and Christoffersen, 2006). The influence of veiling glare (disturbing reflections on a computer screen) depends also on the quality and reflection properties of the screen. Both glare aspects (veiling reflections and bright objects in the vision area) can be reduced dramatically when semi-transparent BIST elements can block the direct radiation from the sun. For more details, see Kuhn (2016). It is important that new semi-transparent BIST elements are designed such that they do not create additional bright spots in the vision area, e.g. due to direct sunlight which is redirected into the room by glossy edges of an absorber or by similar effects.

2.1.13. Aesthetic function

The building envelope contributes greatly to the aesthetic perception of the building. BIST elements can contribute to high-quality architecture as well as to low-cost installations, which are still acceptable aesthetically.

2.1.14. Water tightness

BIST elements can provide areas, which are tight against rainwater and even wind-driven rain.

2.1.15. Air tightness

Some BIST installations do not allow air flow, which can contribute to low heating and cooling demands. Other BIST installations offer a controlled supply of solar heated air to contribute to the indoor air quality.

2.1.16. Technical building plant

In general, BIST elements can include piping and components of the technical building services like small air-handling units and heat pumps. This can reduce disturbance during installation to the people who live or work within the building because less work needs to be done inside the building. In addition, the BIST elements including technical building services can be prefabricated in a factory, which can save costs.

2.1.17. Other functions of building envelopes

BIST systems can offer many more functions, which some conventional building envelopes also offer. BIST products can and may contribute to mechanical resistance and stability, safety in case of fire, hygiene, health and the environment, safety in use, protection against noise, energy economy and the sustainable use of natural resources (European Parliament and European Council, 2011).

BIST elements are being developed, such as solar thermal venetian Good BIST elements should also be easy to maintain.

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2.2. Variable g and U value

Commercially successful solar thermal collectors have been modelled with efficiency curves (Cooper and Dunkle, 1980; ISO 9806, 2013) or with detailed models such as Oliva et al. (1991), Plantier et al. (2003), Hassan and Beliveau (2007), Cadafalch (2009), and Molero Villar et al. (2009). Planners who assist architects in calculating the energy demand of a building and designing the technical building plant usually work with values like the U value as the overall thermal transmittance of a building envelope element and the g value, which is also called the solar heat gain coefficient, solar factor or total solar energy transmittance. The U value is used to calculate the energy flux through a building envelope based on the thermal resistance and the g value to calculate the additional energy flux into the building caused by the solar radiation. For a simple opaque wall, this is a good approximation. However, the energy flux from the BIST element to the interior depends in general on the operation mode of the collector and on the irradiance, which means that the g value is not constant but variable depending on the operation and the irradiance (Maurer and Kuhn, 2012). A variable g value can account for a typical BIST element, which is operated only during periods of nonzero solar irradiance. Even more generally, a BIST element could also be operated at night, e.g. to release heat to the ambient. In this case, not even the U value of the BIST element at times without irradiance is a constant but depends on the temperature of the heat transfer medium in the BIST element. It is therefore obvious that new simulation models are needed, which model the energy flux of BIST elements correctly and rapidly including the energy coupling to the building. However, neither the easy-to-use software popular amongst planners nor the detailed software often used by scientists such as TRNSYS (Beckman et al., 1994), ESP-r (Clarke, 2001), EnergyPlus (Crawley et al., 2001), Modelica (Mattsson et al., 1998) and IDA ICE (Sahlin et al., 2004) has been able to model BIST elements.

A workaround to connect a multi-skin facade model with additional thermal zones to the building model of TRNSYS 14.2 was developed by Saelens (2002) and further extended in TRNSYS 15.3 (Saelens et al., 2008). It hands over the average surface temperatures and the transmitted solar radiation. A workaround to connect semi-transparent BIST elements to the closed-source multi-zone building model of TRNSYS 16 and 17 was developed by Maurer and Kuhn (2011) by modifying a window in order to supply the transmitted solar energy and the heat flux of the BIST model to the building interior. A third workaround was developed for TRNSYS 16 and 17 by Hauer and Streicher (2013) using a mass-less zone to connect opaque collectors to the wall of a TRNSYS building. Besides being accurate and fast, the coupling between BIST elements and building models should be easy to use for planners. For Maurer et al. (2013), the HVAC planners, IPB, used the model and the coupling of Maurer (2012) to perform detailed simulations of a high-rise building. A whole-year simulation in one-hour time intervals for one storey, including a second facade model for windows with venetian blinds, takes about 40 min on a 2.4 GHz dual-core processor, while the simulation of one room with BIST and window models takes 10 min.

In order to calculate the solar thermal performance and the heating and cooling demand of the building correctly, methods to integrate BIST models into all relevant simulation programs need to be developed, which reflect the fact that the g and the U values of BIST building envelopes are not constant.

2.3. BIST measurements

Measurement methods to characterize BIST elements in a labo-

(2017). First the optical measurements of different layers of BIST elements are presented, before the calorimetric measurements are discussed which provide the solar thermal performance and the energy flux to the building interior for defined boundary conditions.

2.3.1. Optical measurements

Optical measurements of components for BIST elements are especially interesting when a new BIST element is being developed or an existing one is to be optimized. As large incidence angles of the solar radiation often occur on building envelopes, the transmit-tance and reflectance should be measured as a function of incidence angle. If a BIST element includes spectrally selective layers, the optical measurements should also be spectrally resolved. Furthermore, if a BIST element includes several transparent layers, polarization-dependent measurements should be performed. For a double glazed cover, the absolute error due to neglecting the polarization at an incidence angle of 60° can be at least three percentage points in the transmittance. If the solar transmittance of the double glazing is small, the relative error can be large. For more than two glass panes, the error is even larger.

Measurement procedures for the transmittance and the reflectance of layers in general and for (near-)normal radiation are presented by EN 14500 (2008). The direct-direct transmittance and reflectance of thin samples can be measured for different angles of incidence with a small integrating sphere which can rotate around the sample as presented by Fig. 11 (Wilson, 2007). The direct-hemispherical transmittance can be measured by a test facility as presented by Fig. 12. The sample can rotate together with the large integrating sphere which measures the direct-hemispherical transmittance (Platzer, 1987). The direct-hemispherical reflectance can be measured by an irradiated rotat-able sample within an integrating sphere. Light-scattering and textured components may need suitable measurement facilities as described by Wilson et al. (2009). For materials with complex angle-dependent optical properties, photogoniometer measurements to determine the bidirectional scattering distribution function (BSDF) are recommended (Apian-Bennewitz, 2010). The hemispherical-hemispherical transmittance and reflectance can be calculated from the direct-hemispherical transmittances and reflectance at different incidence angles, e.g. with the formulas provided by EN 14500 (2008) for special cases which fulfil specified conditions of symmetry.

There are various methods to calculate the effect of multiple reflections in a stack of optical layers, e.g.

- approaches for layer systems based on spectral normal-hemispherical values like ISO 15099 (2003), ISO 9050 (2003), EN 13396 (2007), and EN 410 (2011),

- detailed layer systems with angle-dependent, spectrally resolved, polarization-dependent calculation for direct and for diffuse radiation based on Maheu et al. (1984) and Maheu and Gouesbet (1986),

- matrix methods like Klems (1993a, 1993b) and Ward et al. (2011),

- ray-tracing methods like Ward Larson and Shakespeare (1998).

When applied to opaque BIST elements, the result is typically the effective absorptance of each layer depending on the direction of incident radiation. In the case of semi-transparent BIST elements, the effective visible and solar transmittance of the BIST element depending on the direction of radiation from outside the building is typically needed and if possible also the reflectance of the BIST element from inside the building for accurate building

ratory were presented by Maurer et al. (2012a) and Maurer et al. performance simulations.

Fig. 11. Schematic drawing of a centrally mounted sample, which is irradiated from the left. A small integrating sphere can rotate around the sample to determine the direct-direct transmittance or reflectance for different incidence angles.

Fig. 12. Schematic drawing of a test facility for the direct-hemispherical transmit-tance. The sample (dotted) is mounted at the opening of an integrating sphere, which can be rotated around an axis passing through the sample for different incidence angles of the radiation from the left.

2.3.2. Calorimetric measurements

Typical measurements of the solar thermal performance according to ISO 9806 (2013) are only valid for rear-ventilated BIST installations because the same ambient temperature is assumed on all sides of the collector. If the energy flux from the BIST absorber to the building interior cannot be neglected, a simultaneous measurement of the solar thermal performance and of the energy flux to the building interior is necessary to characterize both quantities as a function of the temperature and mass flow of the heat transfer fluid and the temperatures of the ambient air and of the interior space. A methodology to measure the energy flux of a BIST element towards the building interior is presented by Kuhn (2014). A measurement example for a semi-transparent facade collector to

validate a BIST simulation model is presented by Maurer (2012). Calorimetric measurements are the most important measurements of BIST elements. Fig. 13 presents a cross-section through the indoor calorimeter for building envelope elements of the TestLab Solar Facades at Fraunhofer ISE. The ambient air temperature, the irradiance and the external heat transfer coefficient can be adjusted as well as the temperature of the artificial interior and the internal heat transfer coefficient. A black absorber has the temperature of the artificial interior and absorbs nearly all radiation that is transmitted through the sample. Heat flux sensors inside the calorimeter absorber measure the energy flux to the interior. At the same time, the fluid flow and the fluid inlet temperature through the BIST element are controlled by a thermostat. The solar thermal performance can be calculated, using the measured fluid output temperature as input data.

Additional temperature sensors can be applied, e.g. to investigate temperature distributions inside the BIST element. Additional edge insulation allows the measurement of centre-of-glazing values. The comparison of measurements with and without additional edge insulation is used to assess the quality of the edge design and allows predictions for BIST elements with different ratios of collector area and edge perimeter.

In addition to the indoor calorimeter, Fraunhofer ISE has also developed an outdoor test facility for real-size building envelopes (OFREE) which also provides the energy flux to the building interior as well as the solar thermal performance. It can measure BIST elements up to 3.77 m height and 1.5 t weight and track them at defined angles with respect to the sun. OFREE can also control the temperature of the artificial interior and the heat transfer coefficient. Although the ambient temperature cannot be adjusted, the direct solar radiation has a much smaller divergence and realistic spectral properties, so it is recommended for BIST elements with strong angular or spectral dependence.

BIST installations in buildings can and should be monitored to check their real-life behaviour. However, monitoring takes a lot of time and the boundary conditions can be controlled and measured less accurately than in the laboratory. Therefore, calorimetric

Fig. 13. Cross-section through the g value calorimeter. The sample (dotted area) is irradiated by solar simulator. The measurement chamber is air-conditioned and a thermostat controls the temperature of the black calorimeter absorber. Edge insulation (hatched area) can be applied to minimize edge effects.

measurements in a test laboratory are recommended to characterize BIST elements.

2.4. The design parameter space

When developing new BIST research topics, many different solutions are possible for each topic. Therefore, there are countless possible combinations of solutions to create a unique BIST element. BIST elements can then be evaluated according to numerous criteria.

From a more general point of view, the different design topics can be considered as ''dimensions" and the individual solutions as parameters within each dimension. This leads to a multidimensional design parameter space, in which a specific BIST element can be described by the parameter values in all dimensions of this design parameter space. It can be very helpful when designing new BIST elements to think about these design dimensions in order to invent better BIST elements. When these BIST elements are evaluated according to different criteria, also these criteria can be considered as dimensions of the evaluation space. For the development of better BIST elements, it can be helpful to think about these evaluation dimensions and about how some design parameters are linked to some evaluation results.

This methodology was presented by Maurer et al. (2015a). Of course, innovations tend to extend the future design parameter space, e.g. by creating new dimensions. The following sections present the design parameter space with a short explanation of the dimensions. A schematic drawing of this design parameter space is presented in Cappel et al. (2015a). The evaluation space is discussed in Section 2.5.

2.4.1. Physical parameters

2.4.1.1. Optical properties. Wavelength-dependent optical properties of materials can be used in combination with the spectral properties of the irradiance, e.g. to increase the solar absorptance or to create colour effects.

The angular dependence of optical properties can be used, e.g. to provide seasonal shading.

Materials can also change the polarization of the transmitted and reflected radiation, which may be used in new applications.

The albedo or ground reflectance can also be included into BIST designs. When the albedo of the surroundings is large, e.g. because

of snow, BIST elements with a large tilt angle (e.g. vertical collectors) can even have a higher solar thermal performance than a BIST element which is perpendicular to the direct solar radiation.

When shadows threaten to reduce the BIST performance significantly, a ray-tracing model should be used to calculate the solar thermal performance accurately.

The optical properties can also be switchable and/or can be designed to improve visual comfort, e.g. by preventing outdoor or indoor glare.

Lenses and mirrors can be used to concentrate the solar radiation.

Apart from the optical properties of single materials, materials can be combined to form a stack of optical layers, e.g. to reach a certain effective transmittance or absorptance.

2.4.1.2. Heat transfer properties. Heat transfer to the heat transfer medium, into the building and to the ambient is an important field for successful BIST designs. Heat transfer can be managed by influencing the conductive, the convective and/or radiative heat transfer and could be switchable.

2.4.1.3. Hydraulics. It is important that the heat transfer medium flows evenly through an absorber and the entire collector field in order to avoid unnecessary heat losses. The hydraulic system also needs to be stagnation-safe without developing excessive pressure when there is solar radiation but no mass flow. The processes during stagnation are discussed, e.g. by Streicher (2001).

2.4.1.4. Water vapour diffusion resistance. The water vapour diffusion resistance of BIST elements can be modified over a broad range. Collectors with glass covers and/or metal back-sheets do not permit the diffusion of water vapour from the inside of the building to the outside. Other constructions such as perforated metal sheets are permeable for water vapour. The diffusion resistance of a BIST element has to be adapted to the needs of the particular application in the building context in order to avoid vapour accumulation and condensation within the wall or roof. In the case of vapour-tight components, it should be considered that the temperature of BIST elements is often higher than the temperature of the room when the collector is irradiated by the sun, so that the

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water vapour can be transported to the inside of the building in order to avoid the accumulation of moisture in the wall.

2.4.1.5. Degradation. Degradation can affect all parts of the BIST element such as the absorber, the heat transfer medium, and the frame. Degradation can also affect all functions of the BIST element such as aesthetics and solar thermal performance. Leakage due to corrosion or thermal expansion needs to be prevented, for example.

2.4.1.6. PV option. For the BIST elements, there is always the option that PV cells are used as absorbers. This adds a large set of photovoltaic and electric parameters which are not presented in detail here, but can be found in the literature of building-integrated photovoltaic-thermal (BIPVT) elements.

2.4.2. Technical parameters

2.4.2.1. Technology. The design of a BIST element is influenced by the decision as to which functions should be addressed. For example, it needs to be decided whether the collector should typically be rear-ventilated or not, if semi-transparency is targeted, if flat-plate or vacuum collector technology should be used and whether the collector shall be stationary or movable.

2.4.2.2. Components. Regarding the components of a BIST element, first the glass will be discussed, then the absorber, the hydraulic connection of the BIST elements, the insulation and the back cover as well as the vapour diffusion barrier.

The glass panes can be smooth or structured, have anti-reflective and/or coloured properties, can be light-scattering and even switchable in their optical properties. Glass can be omitted completely in unglazed collectors.

The surface of the absorber can be, e.g. smooth or structured.

Regarding the geometry of the absorber, it is important to know whether the absorber is a flat plate, consists of strips or vacuum tubes and whether the outer dimensions are, e.g. limited to a rectangular shape or variable.

Absorbers can be made of metal, but also other materials like polymers, concrete, asphalt and plaster can be used.

The heat transfer medium can flow in pipes which are connected to the absorber using harp or meander configurations. It can also flow in channels, which are arranged inside the absorber with different geometrical configurations. Alternatively, the heat transfer medium flows around the absorber. The piping can also be made of many materials and heat transfer media such as air, water and water-glycol mixtures can be chosen. The following joining technologies have been used for the hydraulic system of the absorber: ultrasonic welding, laser welding, ultrasonic welding from the rear with an omega shaped sheet, adhesive bonding from the rear, roll bonding, blow-forming, and extrusion.

Absorbers can be coated, e.g. with conventional paint, solar paint optimized for high absorptance, sputtered selective coatings, low-selectivity paints (Orel et al., 2007a, 2007b) or be without a coating.

In general, the absorber could be movable.

Regarding the hydraulic connection of BIST elements, different sections of the collector field need to be connected in a way to ensure a homogeneous flow distribution. The position of the piping needs to be specified as well as the connection to technical building plant. There are different forms of venting the collectors, e.g. at the highest point or by alternating mass flows and a central air separator.

The insulation of a BIST element needs to be stable over the expected temperature range but can also be chosen from materials which embody only small quantities of grey energy.

Often a metal sheet is used to cover the back of a mass-produced flat-plate collector, but there are also BIST collectors that are delivered without a back cover or with a cover made of other materials such as wood.

Finally, BIST elements can be a vapour barrier or open for diffusion.

2.4.2.3. Mounting. BIST elements need to be mounted within the building system without creating thermal bridges or causing temperatures that exceed the allowable temperatures for the materials of the building envelope.

2.4.2.4. Type of the building envelope. BIST collectors can be designed for various building envelopes such as tiled roofs, curtain wall facades, transom and mullion facades, perforated facades and historic facades.

2.4.2.5. System. The characteristics of a BIST collector also influence the technical building plant that interacts with this collector.

BIST systems can be optimized for cases without a backup heat source or for cases with a backup such as a boiler fuelled with gas or oil. BIST systems can also be optimized, e.g. for the combination with a heat pump, a district heating system or a micro heat and power system.

The BIST system can also include a storage unit based on water, on phase changes or chemical reactions and can be local or shared within a district heating system.

The energy demand of the building and its surroundings is important for BIST collectors. Possible demands of the building, which BIST system can aim to meet, are presented in Section 2.1.

If PV is included in the BIST element, additional elements such as a suitable inverter are needed.

2.4.2.6. Building. BIST collectors can be specially designed for certain kind of buildings, e.g. for hotels, office buildings, farms or industrial buildings.

2.4.2.7. Ecodesign. It is important not only to design a BIST system with good annual values, but also to consider also the grey energy within a life cycle analysis.

2.4.3. Building process

2.4.3.1. Building phases. The design of a BIST collector needs to take all phases of the building process into account, such as planning, construction, operation and recycling. Furthermore, the BIST development needs to provide the necessary information to all stakeholders within the building information process at the right time.

2.4.3.2. Software. In the planning phase, calculation methods should be developed which need very few inputs and little time. For accurate calculation of larger BIST systems, detailed BIST models are needed.

2.4.3.3. Legal issues. If a new BIST system is to be financed by a contractor or a large credit institute, the contracts need to address all possible issues in a suitable way. If glare can be an issue for the surroundings, appropriate measures or agreements are recommended in order to avoid conflicts. When plants and buildings on other property could affect the BIST performance significantly, legally binding agreements are recommended and should be developed.

2.4.3.4. Business models. Conventional solar thermal collectors are typically sold in Europe by the manufacturer to a wholesaler, who then sells to an installer. When some BIST products aim to reduce costs by direct delivery to the construction site, this can

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influence the design, too. For maintenance, a suitable business model needs to be developed, too.

2.4.4. Dissemination

For innovative BIST elements, it is essential to ensure that all possible stakeholders know about the most important characteristics of the element. Therefore, a BIST element should be designed in a way that the most important topics can easily be understood.

2.4.5. Political issues

The BIST element can be designed in such that it complies with the financial support schemes, which are defined by political agendas.

2.4.6. Psycho-social issues

BIST elements need to be developed to be attractive to certain customer segments. For this, the development team should know about their decision-making processes and think about how different kinds of scepticism can be overcome.

2.5. Evaluation

As presented in Section 2.4, the design parameter space can be used to develop better BIST elements by thinking about better combinations of parameters. New BIST ideas then need to be evaluated according to different criteria. These criteria also can be considered as ''dimensions" of an ''evaluation space". It can be very helpful to think about the connection of certain parameter combinations to certain evaluation results. This section first presents general criteria, which can be used as dimensions of the evaluation, and then discusses criteria that are more specific.

The following criteria for new BIST research and development topics were presented by Maurer et al. (2015a):

• Functionality.

• Aesthetics.

• Ecological aspects.

• Economics.

• Availability/Feasibility.

For the evaluation of the aesthetics, the criteria of (Munari Probst and Roecker, 2013) are recommended:

• position of the collector field,

• dimension of the collector field(s),

• surface texture,

• colour,

• shape and size of the solar thermal element,

• jointing between the solar thermal element and the surrounding building envelope.

As mentioned in Section 1.3, the subjectivity of rating according to these criteria can be reduced by increasing the number of eval-uators and their experience combined with a representative choice of the evaluators.

On the one hand, the right criteria need to be chosen for each evaluation. If for example a header tube is being improved which is not visible from outside, the aesthetic criteria are not applicable. When a switchable thermal coupling is being developed for solar thermal venetian blinds, then the criteria of minimizing the opaque area fits better than the aesthetic criteria mentioned above. In this case, reliability is also more relevant than feasibility and the auxiliary energy demand and the thermal resistance are more applicable than ecological aspects in general. Furthermore, the start-up time of the switchable thermal coupling in order to allow

movement of the venetian blinds affects the price customers are willing to pay for the solar thermal venetian blinds.

In each step of product development, the criteria need to be adapted and an evaluation must be made in order to choose the most promising development paths. When a final BIST element is being evaluated, the buildings and the technical building plant in which it could be used need to be considered as well as how flexible it is to allow easy integration into different building envelopes. In general, a BIST element cannot be evaluated on its own. For example, if a BIST element provides a large thermal resistance between the interior and the exterior, this is typically good during the heating season. However, during summer nights it would be better to have a small thermal resistance in order to cool the building by air with the ambient temperature. Therefore no values or ranges of values can be defined which are generally good, but only with respect to specific cases. BIST elements can be characterized as a single BIST element, but can be evaluated only within a building system.

This raises the question as to how BIST elements can then be compared. In principle, one reference building could be imagined, which covers all possible cases and energy demands. However, this is not feasible because of the infinite number of possible cases. Furthermore, by excluding some possible building cases, innovative products, which have benefits mostly in these specific cases, would be ignored because their benefits cannot be presented. For each BIST building system, one or more appropriate reference building systems need to be simulated for comparison. A BIST product needs to offer the best cost-benefit ratio for a certain market segment to be successful. Of course, the market segments in which the BIST product is leading should be as large as possible.

Many BIST developments are presented with results that are difficult to compare to other BIST developments. Therefore, it is essential that the boundary conditions are clearly stated, that variations of the boundary conditions are discussed, that an evaluation of all important criteria is provided and that the market potential of this case is estimated. Only if these detailed results are available can companies compare and invest in the best BIST technology.

3. Economics

This section first presents an analysis of the investment costs of three completed BIST building projects. Then it describes three promising roles for actors in BIST building processes. The general challenges of the economic evaluation of BIST systems and finally a case study of the life-cycle cost of flat-plate collectors in the facade of a hotel are presented.

3.1. Savings already achieved by BIST installations

First, the costs of conventional flat-plate collectors with rear ventilation are discussed. Then three cases of BIST installations are presented and compared to the cost of the conventional collectors. This section is based on Cappel et al. (2015b).

3.1.1. Costs of conventional flat-plate collectors

It is difficult to obtain the real costs of conventional collectors including the costs of the collector subcomponents, the costs for the logistics and the discounts on the list prices. Detailed cost estimations were presented by Mangold (1996), Banse (2012), and Brandl (2014) updated them. Together with Mai (2010b, 2010a) the simplified cost overview of Table 2 was developed.

While the prime costs are necessary to run the company, including logistics and administration, a factor of 4 lies between the prime costs and the cost of the installed collectors. This is

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Table 2

Simplified overview of the costs of conventional flat-plate collectors.

[€/m2]

Cost of the absorber 30

Cost of manufacturing a collector 60 Prime costs 120

List price 240

Installed collector 480

Installed system 960

Fig. 14. Photo of the BIST installation in Augsburg © Lichtblau Architekten.

partly due to the wholesaler between the manufacturers and the installers.

3.1.2. Case 1: Refurbishment of a vicarage

The building is located in Augsburg, Germany, and was refurbished in 2009. It is illustrated in Fig. 14 including the BIST installation. The presented costs are without value-added tax in the year 2009. The conventional facade is made of fibre cement cladding and includes a composite thermal insulation system with a cost of 130 €/m2.

Table 3 presents a detailed overview of the costs of the BIST element. With 390 €/m2 it is 260 €/m2 more expensive than the conventional facade. However, this difference, which is the cost for

Table 3

Detailed overview of the BIST costs of case 1.

Costs in €/m2 without VAT Materials Labour Total

Wooden transom and mullion construction on 20 15 35

existing masonry

100 mm insulation made of resole foam 15 10 25

Watertight underlay 5 5 10

Totals for the carpentry facade work 40 30 70

Wooden box 120 mm deep with a back of 40 35 75

oriented strand board

Mineral wool insulation 50 mm deep 10 5 15

Solar thermal absorber made of aluminium 85 30 115

with selective coating

4 mm tempered glass with prism structure 25 10 35

Pressing frame with sealing 20 10 30

Delivery and installation on the wooden 10 40 50

transom and mullion construction

Total for the solar thermal components 190 130 320

Total 230 160 390

Fig. 15. Photo of the new building in Munich with BIST © Lichtblau Architekten.

the additional solar thermal function of the building envelope, is about 46% less than for a conventional collector installed with rear ventilation according to Table 2, and about 38% less if a cost for the installed conventional collectors of 420 €/m2 is assumed.

The list price of conventional rear-ventilated solar thermal collectors in Germany did not change much between 2002 and 2011 according to BSW - Bundesverband Solarwirtschaft e.V.. There are no current studies published about the market prices of solar thermal components in Germany, only singular data like Cappel et al. (2015b) which suggest that the prices today may still be in a similar range. Therefore, it is reasonable to compare the additional cost for the solar thermal function in the building envelope in 2009 and 2002 with the prices of Table 2. The two values for the conventional rear-ventilated collectors help to quantify the uncertainty of this comparison.

3.1.3. Case 2: New building in Munich

The building was newly constructed in Munich, Germany, and was finished in 2002. It is presented by Fig. 15. The presented costs are without value-added tax in the year 2002. The non-BIST facade is a wooden facade with vacuum insulation panels and a cost of 240 €/m2.Fig. 16

Fig. 16. Photo of the refurbished terraced house including BIST installation © Lichtblau Architekten.

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Table 4

Detailed overview of the BIST costs of case 2.

Costs in €/m2 without VAT Materials Labour Total

Rectangular frame, installed in two pieces on a 70 40 110

wooden wall

Vacuum insulation panel 30 mm 60 15 75

Thermal insulation layer made of glass fibre 10 5 15

matting 5 mm

Total for the carpentry facade work 140 60 200

Solar thermal absorber made of aluminium, 150 60 210

selective coating

Delivery and installation 30 40 70

Total for the solar thermal absorber only 180 100 280

4 mm tempered glass with prism structure 35 15 50

Pressing frame made of anodized aluminium 30 40 70

with sealing

Total for the cover glazing 65 55 120

Total 385 215 600

Table 5

Detailed overview of the BIST costs of case 3.

Costs in €/m2 without VAT Materials Labour Total

Plywood slats screwed to the masonry 30 20 50

Vacuum insulation panel 30 mm 60 15 75

Thermal insulation layer made of glass fibre 10 5 15

matting 5 mm

Total for the carpentry facade work 100 40 140

Solar thermal absorber made of aluminium, 150 60 210

selective coating

Delivery and installation 20 30 50

Total for the solar thermal absorber only 170 90 260

4 mm tempered glass with prism structure 35 15 50

Pressing frame made of galvanized steel with 20 20 40

sealing

Total for the cover glazing 55 35 90

Total 325 165 490

Table 4 presents a detailed overview of the costs of the BIST element of case 2. With 600 €/m2, it is 360 €/m2 more expensive than the non-BIST facade. However, the extra cost for this solar thermal function in the building envelope was 25% less than the cost of an additional conventional rear-ventilated collector with a price of 480 €/m2 and 14% less than a cheaper installed conventional collector with an assumed price of 420 €/m2.

3.1.4. Case 3: Refurbishment of a terraced house

The building is located in Munich, Germany, and was refurbished in 2002. It is illustrated in Fig. 15. The presented costs are without value-added tax in the year 2002. The refurbished facade without a solar-thermal function uses vacuum insulation panels and has a cost of 240 €/m2.

Table 5 presents a detailed overview of the costs of the BIST element of case 3. With 490 €/m2, it is 250 €/m2 more expensive than the refurbished facade without the solar-thermal function. However, this additional solar-thermal function of the building envelope was more than 40% less expensive than a conventional rear-ventilated solar thermal collector, even if a price of only 420 €/ m2 is assumed for the installed conventional collector.

3.1.5. Comparison of the three BIST installations with the BAST case

This section compares the investment costs of conventional

flat-plate collector installations and three BIST installations. All use spectrally selective coatings so it is assumed that they provide a similar collector performance. The BIST installations are also designed to have a maintenance cost that is as low as for conventional collectors. For the piping, storage tank and the rest

of the solar thermal systems, similar costs are expected for both technologies. Of course, the initial planning of a BIST installation takes more effort than reproducing such installations. In order to achieve high aesthetic quality with BAST collectors, additional planning efforts are needed, too. It is therefore assumed that the planning costs are comparable. Therefore, it is expected that the savings in investment costs contribute proportionally to savings of the total cost of ownership, which will be further discussed in Section 3.3.

Solar thermal experts tend to acknowledge a cost-saving potential to BIST technology. These research results demonstrate that substantial savings have already been achieved by using BIST elements, compared to conventional solar thermal collector installations. One reason for the savings can be the direct supply from the manufacturer to the installer without an intermediate wholesaler. According to Table 2, this can save much costs as long as the manufacturer can deliver the BIST collectors in an inexpensive way. Another reason are the synergetic effects of installing the BIST facade in a single effort instead of first finishing the building envelope and then modifying it again for the solar thermal installation. The scenarios for the energy system in 2050 of Henning and Palzer (2014) and Palzer and Henning (2014) show that solar thermal systems need to decrease their costs substantially as planned by International Energy Agency (1EA) (2010) in order to be an important part of the future energy system. BIST technology can already provide a significant part of these savings.

3.2. Actors in the building process

Within Cappel et al. (2015b), different roles were described for actors in BIST building processes. There are numerous possible roles for each actor and more will be developed. 1nstead of listing all possible roles, this section presents three of the most promising roles in BIST building processes.

3.2.1. Solar architect

When an architect has learned everything, which is necessary to realize B1ST buildings, he can organize the construction and refurbishment of buildings, interacting with the building owner, the collector manufacturer and planners, who can have their own business model adapted to this process and interactions. Compared to the conventional building processes, a solar architect needs to fulfil the tasks described in the next subsection, but needs support from the planner, manufacturer and installer to do so.

This paragraph names the necessary additional actions of a solar architect compared to the architect of a building without BIST technology. All tasks including the optional ones are presented in Cappel et al. (2015b). During the discussion of the boundary conditions, the architect needs to advise the building owner on the capabilities of and options for BIST systems. This includes different possible systems, estimations of the solar thermal performance, possible building code restrictions and subsidies. During the preliminary planning, a BIST system needs to be developed and the performance needs to be calculated as well as the investment costs. The architect also needs to clarify and explain to the building owner the relevant links regarding urban planning, design, functions, technologies, economics, ecology, building science, social and public aspects. As the next step, the architect develops the basic design where a specific BIST system needs to be chosen and more accurate calculations of the energy performance and costs are needed, such as the financial amortization. 1n addition, details of the construction need to be developed as well as the concept for the construction process, the subsidies and the maintenance. For the construction permit, contributions from experts for building physics, safety in case of fire, etc. may be needed. The solar architect also needs to provide his results to the other partners like

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the collector manufacturer so that the interfaces can be planned correctly. Negotiations may be needed to receive the building permit, especially when heritage sites are involved. The cost calculation needs to include the possible synergetic effects. During approval planning, the necessary documents have to be generated and discussed with the authorities. During execution planning, detailed construction drawings need to be developed which cover all necessary aspects. The results need to be distributed to the other stakeholders and the work needs to be distributed well amongst the different trades. The architect also needs to check the work of the collector manufacturer. During the preparation of the call for tenders, all tasks and amounts need to be listed. The cost calculation also needs to be updated, based on the detailed costs provided by the planner. During the call for tenders, the tenders need to be checked, evaluated and compared in detail. Often discussions with the bidders are necessary and the orders need to be specified so clearly that later difficulties are avoided. During building supervision, the BIST installation needs to be checked in accordance with the construction permit and general technical regulations. It is recommended that the planners check the object finally, too. The architect also needs to coordinate the BIST works and check the invoices after the installation and compare them with the original offers. The architect has to approve the construction works and provide documentation of faults. During site supervision, the architect needs to look for faults before the warranty period expires.

For technical and financial reasons, the architect should therefore buy the BIST components from a collector manufacturer instead of an installer and a wholesaler. The manufacturer of BIST elements need to deliver their products to the constructions sites at low costs. The key partners for the solar architect are the collector manufacturer and the planners and the key resource is the knowledge needed to construct cost-effective BIST buildings.

3.2.2. Flexible production of solar thermal absorbers

The large manufacturers of solar thermal collectors produce large numbers of absorbers and collectors of identical geometries in order to be competitive. However, the production facilities could be adjusted to produce custom-made absorbers for building integration similar to the absorbers provided for the cases 2 and 3 in Sections 3.1.3 and 3.1.4. Vaillant analysed these adjustment possibilities in Cappel et al. (2015b) for its flat-plate absorber production. The width of the absorber is limited by the width of the roll of absorber sheet metal. If wider absorbers are necessary, wider rolls as well as machines to process these rolls would be needed. Currently, the piping is delivered in a roll and welded to the absorber sheet in meanders before the header tubes are connected. Within the capabilities of the welding machine, the length of the absorber and meander could be decreased and increased up to a certain length, after which larger welding machines would be needed. The welding machine could be programmed to produce a large number of different absorber configurations. Plug-and-play fittings to connect absorbers are already used for conventional solar thermal collectors.

The proposed value consists of custom-made solar thermal absorbers for low prices. To be successful with such a business model, the cost structure needs to be optimized and this is affected for example by the costs of the logistics of offering many different geometrical configurations of absorbers and header tubes. Key partners for market introduction could be companies selling prefabricated houses or architects of large building projects like hotels with many BIST absorbers of the same geometry.

commissioning. According to the estimation of Table 2, a large amount of the cost of installed collectors is due to wholesalers and discounts because the installation of solar thermal collectors is very easy nowadays. Therefore, it seems to be a promising value proposition for general contractors to offer BIST buildings, but use inexpensive workers to install the BIST system except the connection to the technical building plant, and use a local installer only for the connection to the technical building plant, checking the installation, commissioning and maintenance. Such actors are already successful in the market for prefabricated houses and heat pumps in Germany. The general contractor of the proposed role needs BIST expertise, e.g. by employing an installer who advises the inexpensive workers regarding the installations and who may plan the BIST systems.

3.3. Economic BIST calculations

Quantifying the value of BIST technology is a complex task. First, the general method is presented, followed by a discussion of the challenges faced when applying the method. This section is based on (Maurer and Smyth, 2017a, 2017b).

3.3.1. General method

To calculate the costs of a BIST installation, the investment costs are estimated first. The investment cost per square metre for the additional solar thermal function of the building envelope cidsenv can be calculated as the difference between the investment cost per square metre of a BIST part of the building envelope cisenv and the investment cost per square metre of a conventional part of the building envelope cirenv as the reference:

Cidenv ~ cisenv — cirenv (1)

The first challenge is here to define the reference building envelope because in general, several references are possible and only one is needed for this first approximation. The same needs to be done with the technical building plant. The investment cost for the technical building plant including the solar thermal function per square metre of BIST area cidbs

Cidbs = (cisbs — cirbs)/ABIST (2)

is the difference between the investment cost for the technical building plant for the BISTS case cisbs and the investment cost for the technical building plant of the reference case cirbs divided by the area ABIST covered by the BIST elements.

Sometimes the value of a building increases greatly by the inclusion of BIST technology, e.g. gaining a better image, and this increase can exceed the financial benefits which are directly caused by the BIST system, for example lower expenses for fossil fuel. This difference cidBISTimage due to the image can be calculated by estimating the real estate price of the BIST building, subtracting the net present value of the BIST system cnpvBIST and the estimated real estate price of the same building but without BIST technology cirbuild, divided by the area of the BIST elements:

CidBISTimage = (ciBISTbuild — cnpvBIST — Qrbuild)/Abist (3)

If the difference cidBISTimage is estimated to be small, it can be set to zero for the following calculations.

The subsidies per square metre of collector cisubs also need to be clarified. The investment cost for the solar thermal function of the building envelope including the necessary technical building services cdBISTS is then calculated by:

CidBISTS = cidenv + cidbs — cisubs — cidBISTimage (4)

3.2.3. General contractor and local installer The next step is to calculate the levelized cost of heat (LCOH). For

Typically, an installer installs the solar thermal system this, the heat Q^ needs to be calculated which is supplied each year including the collectors, the piping, the storage tank and the by the solar thermal system to the demand of the building. The

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equation for the life cycle cost including the levelized cost of heat LCCwLCOH is needed and is also known as the total cost of ownership including LCOH TCOwLCOH, or the net present value NPVwLCOH times minus one. With the annual costs per square metre of the BISTS ca for operation and maintenance, the electricity for the pump and renting the space for the storage tank, the discount rate r, the calculated service life of the system in years T and the costs per square metre for recycling the BISTS crec, the equation for the LCOH is equal to

LCCwLCOH = TCOwLCOH = -NPVwLCOH

= (QdBBTS + (ca - LCOH * Qst )(1 + r)-"j + crec (1 + r)-TjABBTS

Eq. (5) is then set to zero, e.g. by using the solver within MS Excel to receive the right value for the LCOH.

Another interesting value for comparison is the cost of saved non-renewable primary energy csnrpe. For this, a coupled simulation of the building including the BIST elements and the technical building services is recommended. The result of such a simulation is typically the amount of non-renewable primary energy demand in kW h per year for the BISTS case QBISTS and the amount of non-renewable primary energy demand in in kW h per year for the reference case Qr. The non-renewable primary energy saved by the BIST system in one year for this specific application and per square metre of BISTS area qdBiSTS is then calculated by:

QdBISTS = (Qr " QBISTS)/ABISTS

with the area ABISTS of the building envelope covered by BISTS. QBISTS and qdBjSTS are not equal to Qsts but also include the influence of the BIST element on the energy demand of the building.

To calculate the cost of non-renewable primary energy saved by the BIST system csnrpe, the life cycle cost without income for the solar thermal heat LCCwoLCOH is needed, which is also known as the total cost of ownership without income for the solar thermal heat TCOwolcoh:

csnrpe —

CidBISTS + (En=1 Ca(1 + r) n) + Crec(1 + r)"

ELwO + r)"n

Together with an average carbon dioxide emission per kW h of non-renewable primary energy e, which the reference case uses instead of the BIST contributions, the cost of saved carbon dioxide emissions csCO2 can be calculated as:

CsCO2 :

Lsnrpe

3.3.2. Challenges in application

The following challenges apply not only for BISTS, but also for other renewable and non-renewable investments. However, the challenges are exceptionally large in new areas like innovative BIST systems, because many stakeholders do not have much experience with BIST technology yet and not much information, e.g. about detailed costs of past research projects is publicly available yet.

3.3.2.1. Challenge of influence. It is difficult to quantify the value of a BIST solution in general, because the performance of a BIST system depends on the location, the orientation, the building itself and the technical building plant. Specific cases can be analysed and optimized, but even if only one parameter is changed, the performance of the whole system can change appreciably. To address this issue, ''typical cases" shall be analysed in detail to identify parameter ranges, within which the parameters can be used as a good

approximation, and to identify parameters with a large influence on the performance.

3.3.2.2. Challenge of detail. At the time when important decisions are made during the building process, many details are not yet known. In addition, detailed calculations need valuable time. E.g., the detailed modelling and simulation of an innovative BIST envelope can require significant effort. A large public data base of the performance calculations and measurements of BIST installations together with examples of cost calculations could help new stakeholders for initial estimations of their BIST projects.

3.3.2.3. Challenge to quantify. The value of a building depends on data like the thermal resistance of the building envelope and the time since the last refurbishment of the building envelope. However, it also depends on subjective evaluations of the aesthetics of the building envelope and the image of the applied technologies, which are very difficult to quantify. Stakeholders can estimate these benefits and drawbacks based on their experience or on small surveys in which they can ask for an evaluation of several possible variants of a building envelope. In general, large statistical surveys on existing buildings with and without BIST installations would be very helpful.

3.3.2.4. Challenge of difference. While searching for the best reference for a BIST envelope, it should be noted that there is no perfect reference, which differs only in the solar thermal function from the BIST envelope. Even dummy elements may have the same visual appearance, but, e.g. their image for the customers is very different. Therefore, it will never be possible to quantify the value of the solar thermal function alone exactly. In fact, BIST envelopes are competing with many other possible building envelopes, from building envelopes with a focus on low investment costs through highly ecological building envelopes without a renewable energy supply to very expensive facades with a focus on the aesthetics of the building. On the one hand, simply the number of comparisons presents a challenge, which may be addressed by analysing only the most promising cases. On the other hand, the cases can be very different, e.g. regarding the technical building plant where the kind of difference can cause an additional challenge. If one system reduces the cooling load passively and another system provides a renewable heat supply for the heating demand using some electricity, then the cost of the carbon dioxide emissions is more difficult to compare than in cases where just the amount of heat differs. Such topics can be analysed in detail, but often need expertise to be treated correctly and accurately.

3.3.2.5. Challenge of future. No one knows exactly how the economics and technologies will develop. Therefore, many assumptions regarding the energy prices and discount rates are needed. For innovative products, the necessary maintenance and the service life can be estimated, but some uncertainty will always remain. To quantify the uncertainty, the following procedure based on the Monte-Carlo method is recommended. First the important output values and the most uncertain parameters are defined and the uncertainty of these parameters is estimated. Then a large number of random parameter sets is generated, based on the uncertainty of the individual parameters. Finally, the output values are calculated for each parameter set and are presented in histograms. By analysing these histograms, the risk of deviations within a certain range can be quantified.

3.4. Economic case study

An interesting case for a BIST installation is the refurbishment of a hotel building, which has a large facade, but little roof area,

C. Maurer et al. /Solar Energy xxx (2017) xxx-xxx

which is mostly needed for conventional technical building services such as cooling towers. This case study does not aim to optimize a system by varying many parameters but to quantify the cost of saved non-renewable primary energy csh in €/ (kW h) of a non-optimized system.

The following assumptions were made: The hotel has 100 beds and a daily demand of 100 litres of domestic hot water per bed (DIN Deutsches Institut für Normung e.V., 2009). The mains water temperature is assumed to be at 12 °C. The aesthetically integrated flat-plate collectors with rear ventilation are used, with efficiency coefficients of g0 = 0.789, a1 = 3.545 and a2 = 0.017 and an incidence angle modifier of 0.89 at an incidence angle of 50°. They cover an unshaded area of 57.5 m2 of the south facade. The heat transfer medium is a water-glycol solution with a thermal capacity of 3.48 kJ/(kg K), the collector circuit has a mass flow of 72 kg/h per m2 collector area. The load side extracts heat twice a day between 6 and 8 a.m. and 7 and 9 p.m. with the same mass flow. Frankfurt, Germany was chosen as the location and the weather data was generated with Meteonorm. The BIST collectors were connected via a heat exchanger to a stratified thermal storage tank with a volume of 5 m3 of water. It is modelled to display eight equally dimensioned temperature nodes. The incoming hot fluid from the collector enters the top node, while the inflow at mains water temperature occurs to the bottom node. Losses in the heat exchanger are not considered. The mass flow in the collector circuit is regulated by a controller, which operates with an upper cut-out temperature of 95 °C at the top of the storage tank and the temperature difference between the outlet of the collector and the bottom of the storage tank. If this temperature difference exceeds 5 K, the pump is switched on; if it falls below 3 K, it is switched off. The pump needs 60 kJ/h of electricity and heating of the fluid by the pump is neglected. The BIST collector only pre-heats of the mains water; it is further heated up to 60 °C by fossil fuels.

A whole-year energy simulation was performed within (Götz, 2015) and delivered a solar thermal performance of 891 kWh/ (m2 a) because the flat-plate collectors were operating at low temperatures for DHW preheating. For the additional cost for the BIST elements, 250 €/m2 were assumed as in Section 3.1.4 and the additional installation costs for the piping and the building system were estimated to be 28,500 €. For such an installation in Germany, 100 €/m2 subsidies can be received. The annual costs for maintenance and operation, electricity for the pump and renting the space for the storage tank were estimated to be 1363 €/a. A nominal discount rate of 2% was used, which corresponds, for example, to a real interest rate of 4% and an inflation rate of 2%. For the recycling, the cost or income was defined as zero. A calculated service life of 20 years was considered.

This simple BIST case study can easily be reproduced and modified. Table 6 presents results for some scenarios with and without the subsidies mentioned above, with nominal discount rates of 2% and 4% and with the assumed service life of 20 years, as well as the case when a service life of 30 years can be reached with the same investment costs. The resulting levelized costs of heat are

comparable to other heat sources. Compared to the calculations of the levelized cost of energy of Mauthner and Herkel (2016), the BIST system described above combines the advantages of a high demand for domestic hot water with the low investment costs of BIST technology.

When varying more parameters, the cost for the solar thermal function has an influence, of course, but the cost of the piping and storage tank account for two thirds of the investment cost and could be reduced, e.g. by using an existing tank instead of a separate tank for preheating.

In this case study, with the exception of the inflation, no additional increase of the cost of heat sources rhs was assumed but this could be done by changing Eq. (5) to

= CÍÍB¡STS + ¿(Ca(1 + r f - LCOH * Qst(1 + r + rhs)-n)

+0rec(1 + r) T)

This simple case study demonstrates that the life cycle cost analysis of a BIST system can be done without much effort, that it should be done when developing new products and that all values need to be provided so that variations can be calculated easily. If the energy flux from the absorber to the building interior needs to be considered, then the heating and cooling demand of the building need to be compared as well and included in the calculation as presented by Maurer (2012). For transparent and translucent solar thermal building envelopes, the difference in artificial lighting needs to be included, too. With simple or detailed BIST models like Maurer et al. (2013, 2015b), this is not much more effort than simulating a rear-ventilated BIST collector.

4. Integration into the building process

This section discusses current and future BIST building processes as well as new approaches to calculate the BIST performance without programming a detailed model. Finally, current standards and regulations are discussed.

4.1. Building processes and vision

Not all current building processes use 3D CAD drawings. In many cases, stakeholders of the planning and construction process use detailed models but exchange them, e.g. as 2D reports. Building information modelling (BIM) aims to improve this exchange and is a process which started in the 1970s (Eastman et al., 1974). The international non-profit organization, buildingSMART (buildingSMART, 2016), has published the Industry Foundation Classes (IFC) as an open format for exchanging information about building processes (ISO 16739:2013). The IFC are undergoing a continuous improvement process and more standards to improve the building process are being developed by buildingSMART, such as ISO 12006-2:2015 and ISO 12006-3:2007). However, a BIST building process can involve different kinds and quantities of

Table 6

Levelized cost of heat (LCOH) for BIST scenarios with and without subsidies, different discount rates and service lifes.

Service life (years)

[€/J]

[€/ (kW h)]

No subsidies

With subsidies

2% discount rate 4% discount rate

2% discount rate 4% discount rate

20 30 20 30

20 30 20 30

2.1E-08 1.8E-08 2.4E-08 2.0E-08

1.9E-08 1.6E-08 2.2E-08 1.9E-08

0.077 0.063 0.086 0.073

0.070 0.058 0.078 0.067

C. Maurer et al. /Solar Energy xxx (2017) xxx-xxx

information as well as many stakeholders. The research project SolConPro (Ed. Zublin et al., 2016) investigates what is needed for successful BISS building processes in addition to non-BISS building processes. An overview of additional tasks, which an architect of a BIST building needs to work on with other stakeholders, has been presented in Section 3.2.1. For example, the cover glass pane of a BIST collector can have a solar transmittance which depends only on the incidence angle, as is common for smooth float glass pane, or it can have complex angle-dependent properties, e.g. because of a structured surface. On the one hand, the next IFC version should provide a location within its format to allow this data to be exchanged within a building process. On the other hand, there are different formats and it may be difficult to extract the detailed information from a detailed model, to convert it into text format and to include it in a detailed BIST model. Therefore it is proposed that binary functions be exchanged in future versions of IFC together with a text file describing the name, the inputs, the parameters and the outputs of this function (Maurer et al., 2017). The binary functions can be used immediately with little risk of errors compared to the exchange of large data sets in text format. Binary functions are functions of source code which are compiled to machine code, e.g. into a dynamic-link library (DLL) or shared object (.so). With the descriptive text file, the binary functions can be addressed and used by other software. The descriptive text file and binary functions may comply with the strict definitions for functional mock-up units and interfaces (FMU, FMI) (fmi-standard.org, 2014) which makes the integration into software easier but requires more effort for creating the binary function. Binary functions can reduce the cost of the current building process for extracting and using detailed data. Furthermore, instead of two models, one identical model can be used, which is an advantage, especially when the model is complex and/or specific. However, there is another important advantage: 2D reports are exchanged nowadays also to keep business secrets confidential, e.g. the materials of a BIST element, which can be extracted from the source code of a detailed model but can hardly be extracted from a 2D report or a binary function. Including binary functions in the IFC could therefore lead to an exchange of higher quality. Stakeholders can save money and offer new services by using a function which they can apply together with all their knowledge to many cases and which they will not receive otherwise because of confidentiality issues.

The vision for future building processes including BIST technology is that the simple data is exchanged as text in IFC and the complex functions in machine code. The binary functions of the subcomponents of a BIST elements should be added to a binary function of the entire BIST element, which is then distributed, e.g. to the planner and the facility manager. A similar model exchange between the development of a BIST element and a

planner has been used for Maurer et al. (2013). Current investigations are addressing the question as to which simple data should be exchanged as text in IFC in addition to the binary functions in order to include all necessary information for BIST buildings in future versions of IFC.

4.2. Simple BIST models

One challenge of BIST elements is that detailed models need time and money to be developed and that neglecting the energy coupling between a BIST element and the building interior usually underestimates the solar thermal performance of the BIST element and the cooling demand and overestimates the heating demand of the building. Therefore simple BIST models were investigated in Maurer (2012) and Pflug et al. (2013) until good simple approaches were published in Maurer et al. (2015b). Fig. 17 presents an overview of the four approaches. Approach A and B are recommended for cases where the collectors which are integrated into the building envelope have been characterized with rear ventilation and the parameters g0, a1 and a2 according to Cooper and Dunkle (1980) and ISO 9806 (2013). They provide equations to adapt these three parameters to calculate the solar thermal performance and the energy flux to the building interior. Approach C is suitable for cases where monitoring data of a BIST element exists. Approach D was the simplest node model that provided acceptable results compared to a detailed physical model. It can be, e.g. fitted to BIST measurement results, which have been described in Section 2.3, or to stationary calculations. Approach D can also be adapted for BIPVT elements (Maurer et al., 2015c). As the equations of Approaches A and B of Maurer et al. (2015b) have not been optimized for easy use, the most important equations are presented here in a more user-friendly format.

When there is so much insulation that the heat exchanged between the absorber and the building interior can be neglected, then the collector efficiency of the BIST case at zero temperature difference between the heat transfer medium and the ambient g0 BIST is calculated by

g0 ,BAST

1 " fbl + fblFBAST

using the collector efficiency of the rear-ventilated (BAST = building-applied solar thermal) case at zero temperature difference between the heat transfer medium and the ambient air g0 BAST, the assumed fraction of back losses in the BAST case fbl and the collector efficiency factor of the BAST case F'BAST.

The second-order collector efficiency coefficient of the BIST case a2;BIST is assumed to equal the second-order collector efficiency coefficient of the BAST case a2 bast.

Fig. 17. Schematic drawings illustrating the different approaches for modelling building-integrated solar thermal collectors.

C. Maurer et al. /Solar Energy xxx (2017) xxx-xxx

a2 ,BIST — a2 ,BAST

The first-order collector efficiency coefficient of the BIST case a1 yBisr can be calculated by

ai bist —

a2,BAST * AT^tag,BAST ~ fblGg0,BAST + Ggo,BIST

stag,BAST

including the temperature difference between the absorber and the ambient during stagnation in the BAST case DTstagBAST at the standard irradiance G, which typically equals 1000 W/m2.

Approach B is recommended for cases where the heat flux between the absorber and the interior should not be neglected. First, the thermal resistance Ri>BAST between the absorber and the ambient at the rear of the BAST case and the thermal resistance Ri BIST between the absorber and the building interior in the BIST case need to be estimated as well as the thermal resistance between the fluid and the absorber Rfa. Then the solar thermal gain of the BIST case quse ,bist can be calculated by

quse,BIST = (quse,BAST Ri,BIST(Rfa + Ri,BAST) + Ri,BIST(Tfav - Ta)

+ Ri,BAST(Tint - Tfav)) /(Ri,BAST(Rfa + Ri,BlST)) (13)

with the average temperature of the heat transfer medium Tfav, of the ambient air Ta and of the building interior Tint and the solar thermal gain of the BAST case quseBAST calculated conventionally from g,bast, a1 ,bast and a2,bast.

For all approaches, the equations to calculate the temperature of the absorber and the energy flux between the absorber and the interior are provided by Maurer et al. (2015b).

Simple models are especially important in early planning stages, when little information is available. Adaptive simulation models, which can switch from a simple model with few inputs to a more accurate one with more inputs, could accompany planners from the early planning until the operation of the building.

permission can be very costly. Another solution is the ''approval for individual cases" (Zustimmung im Einzelfall). Building contractors must apply to the building supervisory authority of the federal state for every single building. The cost for an approval for individual cases can range from 400 to 26,000 € according to the web page (Schneider, 2015). It is expected that the requirements for further buildings with the same design of the solar thermal façade collector will not be as comprehensive as long as the building is constructed in the same federal state. One part of the approval is usually the fire safety assessment. In general, the fire safety requirements are stricter for taller buildings but also vary between different federal states.

Besides the approval, the regulations for subsidies are essential for innovative components. In the past, it has been difficult for products like BIST collectors with air as the heat transfer medium, because the old standard EN 12975 (CEN, 2006a, 2006b) did not provide a suitable measurement method. However, the solar keymark label based on EN 12975 was necessary for the subsidies. Without the subsidies, it was very difficult to compete with subsidized collectors. The current standard (ISO 9806, 2013) has defined a measurement method for collectors with air as the heat transfer medium.

Another challenge for companies is to keep track of the different regulations within different countries. Especially companies with innovative products need to be careful that their products are eligible for subsidies but keeping track of all regulations needs a lot of effort for small companies.

Because of the wide diversity in BIST technologies, numerous standards can be involved. A large number of standards which can be involved for building-integrated photovoltaics was collected by ConstructPV (in preparation). Many of these standards can also apply to some BIST technologies. For example, there are standards for the tightness of the building envelope regarding air and rain, for protection against noise and insects, for fire and impact safety and for materials like insulation and glass.

4.3. Standards and regulations

The following overview of standards and regulations for BIST technology is based on the report (Bueno et al., 2016). The requirements for the technical approval can vary strongly between different countries and even between regions due to different building regulations. One challenge is that (ISO 9806, 2013) specifies measurements with ambient air temperature also at the rear of the collector and does not provide parameters for a model which also provides the heat flux to the building interior, such as the Approach D of Section 4.2. Another challenge is that even for one BIST collector, there can be wide variation in the possible building integration regarding the thermal resistance and capacities between the absorber and the interior. Not all of these variants can be measured. Finally, a large variety of simulation models is needed to evaluate innovative BIST elements. Future standards should therefore aim to provide methods to calibrate BIST models so that they are accurate and reliable.

In general, approval from a local, regional or national building authority is needed to use BIST elements. In Germany, the DIBt (Deutsches Institut für Bautechnik) is responsible for the categorisation of building products and their approval. According to the list of critical products (Deutsches Institut für Bautechnik, 2015) published by DIBt, solar thermal collectors with a tilt angle of more than 75° or an area of more than 3 m2 need approval. In Germany, there are two different ways to achieve the approval. For ''general construction permission" (allgemeine bauaufsichtliche Zulassung), it is necessary to apply and to fulfil the requirements that are specified by the DIBt. Since these results in general permission, it is expected that many different tests including simulations and measurements will be required, which in turn means that the

5. Outlook

A roadmap for facade-integrated solar thermal elements was developed and is summarized for BIST installations here (Cappel et al., 2015a; Maurer, 2017). The references also describe the short-term and long-term visions, which were the basis for this R&D roadmap.

The most important research topics until 2020 concern effective dissemination and education for builders, architects and planners. They need to understand the benefits of BIST technology and know how to use them, just like other parts of the building. At the same time, research is needed to develop optimized methods for public incentives to contribute to a macroeconomically efficient energy transformation. Intelligent support for architects and planners needs to be developed, especially for the early planning stages. By analysing numerous real BIST projects in depth, the most economic BIST solutions can be recommended.

The second most important research topic is to develop further absorbers or collectors that have a long lifetime, an easy and failsafe installation and appealing aesthetics, while being manufactured industrially. The highest potential for reducing BIST prices is offered by developing new business models that omit the traditional three-stage distribution and use synergy among the labour branches on-site.

At the same time, specialized BIST solutions should be developed in order to penetrate wider markets from their respective niches. In general, holistic approaches are needed which focus on a certain BIST research topic but take the remaining research topics into account, too. A graphical summary of the most important research topics is presented by Fig. 18.

C. Maurer et al. /Solar Energy xxx (2017) xxx-xxx

Mass produced (partly) prefabricated ST façades New business models Cost 50 % of today

BIST first established in residential sector (SAH with 50 % First commercial solar facades for hospitals, hotels, etc. Special collectors demonstrated for niche markets More know-how among architects and installers, less complexity

BIST is one standard solution for refurbishment of residential and non-residential buildings SAH (high solar frac.) standard for ni buildings

Mass production for special collectors First heat networks with BIST demonstrated

SAH standard for building stock 20 -34 % of all buildings heat demai covered by (BI)ST BIST and heat networks integrated national or international energy s; BIST in standard tools for urban planners

Adapt products towards individual measures and mass production

Stagnation-proof coll. design Individual direct-flow absorbers, bionic structures Characterisation of air gaps between absorber and wall Evaluation of new decentralized hybrid systems (micro heat pumps, BIPVT) Heat pipes for collector applications, connection to hydraulics Adapt BIST design for easy certification as building product

Prefabrication

I New factories for prefabricated façade elements with BIST Integration of standard products in prefabricated houses Tool to digitalise geometry of facades, hydraulic optimisation

Hydraulics

Optimisation of flow behaviour for large collector arrays Investigation of boundary conditions that require pressure equalisation

single absorbers | Self-adjusting valves for matched-flow

Questions of stakeholders

Dissemination & translation of existing know-how e.g. via [4] Include BIST in education & training

Software & simulations

In depth analysis allocating BIST technology to dedicated application, taking building physics into account

Simulations & optimisation on district level

Derive rules of thumb, easy-to-use interfaces

New business models & boundary conditions

Optimized cooperation of the stakeholders and sales structure Recommendations for product placement, calculation of risk (insurance) Optimized public support for market-penetration

Fig. 18. Graphical summary of the most important research topics (Cappel et al., 2015a).

Regarding the other research topics mentioned in this paper, innovative BIST solutions will provide an even better cost-benefit ratio due to their multifunctionality. The simulation methods are going to improve in a way that manufacturers and planners will be able to provide accurate calculations at low costs also for innovative components. Furthermore, within the IEA SHC Task 56 it is planned to analyse whether the existing standards can be easily applied to innovative solar envelopes including BIST technology or whether modifications are needed.

6. Conclusions

First, building-integrated solar thermal elements were defined as having more functions for the building envelope than just the delivery of solar thermal heat. As more and more "multifunctional" building envelopes are developed, it becomes important to understand that there are no strict boundaries between building-integrated and building-added systems but that there is a continuous scale between low and high integration and multifunctionality.

Second, the state of the art of BIST technology was discussed and the available BIST products were presented. The authors are convinced that the development of new BIST technology can profit

significantly by studying the success and failure of existing commercial BIST products and the BIST developments of the past.

Third, the general functions which BIST systems can provide were presented and the conflict between oversimplification by some simple planning software and the real performance parameters (like the variable g and U values of BIST products) was discussed. Solar building envelopes that provide many functions are therefore more complex than simple building envelopes. However, it is mostly a matter of choosing the right methods to make multifunctional building envelopes easy to use in building projects.

Fourth, measurements to characterize BIST elements were presented. This is essential for decisions about which effects can be simulated and which test models should be measured to support the cost-effective development of new BIST technologies.

Fifth, a design parameter space was presented in which the current BIST elements are located and which can be used to develop innovative new parameters and/or parameter combinations. Thinking about the design parameters in a systematic way can generate new insight and systems that perform better than previous ones.

Sixth, methods were presented to evaluate BIST technologies during development and as a product in comparison to competing conventional technologies. This is crucial, because the budget that

is available for the development of a BIST technology needs to be spent as efficiently as possible in order to develop a very successful technology. Finding the right evaluation method at each step of the development process reduces the risk that major difficulties appear at a late stage of the development.

Seventh, the cost data of three BIST installations from 2002 until 2009 were analysed and substantially reduced investment costs compared to conventional rear-ventilated solar thermal collectors were identified. This documents that BIST technology does not have mere ''cost potential", but has already generated significant economic savings in the past.

Eighth, three roles within building processes with BIST involvement were presented to outline the scope of possible future business models involving BIST installations. On the one hand, every stakeholder in BIST-related building projects needs specific BIST knowledge. On the other hand, the stakeholders need a good distribution of roles and work to be successful.

Ninth, the general methods and challenges of economic BIST evaluations were presented as well as an example of an economic evaluation. This economic evaluation can be easily reproduced and adapted to other BIST cases.

Tenth, a vision for future BIST-related building processes is presented, based on the existing processes. The methods proposed in this section may further reduce the cost of buildings with integrated BIST systems.

Eleventh, simple simulation models for BIST components, which need no programming, are presented with easy-to-use equations. This is especially important for the decisive early planning stages.

Twelfth, the challenge of standards and regulations is outlined and future research topics are presented. Standards and regulations need to be improved so that they can be easily applied. The list of future research topics shows that many developments need to start now so that BIST technology can contribute significantly to the transition of the energy system in the coming decades.

With these twelve points, this paper aimed to provide an overview of the most important BIST research topics in recent years. Instead of mentioning all scientific BIST developments, the paper focused on the cost-relevant findings and the aspects that are necessary to make BIST technology more cost-effective and therefore commercially more successful in the future.

Acknowledgements

The authors thank Florian Lichtblau, Lotta Koch, Wendelin Sprenger, Steffen Franz, Helen Rose Wilson, Simon Frederik Haerin-ger, Jean-Marc Robin and John Hollick for valuable discussions. The authors also thank Inga Katharina Götz for performing the energy calculations of Section 3.4, Thomas Baumann from Ingenieurbüro Peter Berchtold for cost estimations for the same section and Helen Rose Wilson for proof-reading the manuscript.

This work was funded by the German Federal Ministry for Economic Affairs and Energy, based on a decision by the German Bundestag, and has received funding from the European Community's Horizon 2020 Programme under grant agreement no. 680441. The authors thank the European Union for support in COST Action TU1205 Building Integration of Solar Thermal Systems, too.

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