Scholarly article on topic 'Field Test of an Advanced Solar Thermal and Heat Pump System with Solar Roof Tile Collectors and Geothermal Heat Source'

Field Test of an Advanced Solar Thermal and Heat Pump System with Solar Roof Tile Collectors and Geothermal Heat Source Academic research paper on "Earth and related environmental sciences"

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{"Combined solar thermal and heat pump system" / "performance assessment" / "field test" / "in-situ monitoring"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Anja Loose, Harald Drück

Abstract The technological combination of solar thermal systems with heat pumps continues to be a highly topical subject in the market of sustainable domestic hot water and space heating systems. Nonetheless, objective performance test methods are not yet common standard. In this context field tests with six different combined solar and heat pump systems installed in single- and smaller multi-family houses in Germany have been performed under real operating conditions by the ITW/TZS. In this paper a novel heating system comprising solar roof tile collectors and so-called geothermal energy baskets is presented in detail and in-situ monitoring results are shown and discussed.

Academic research paper on topic "Field Test of an Advanced Solar Thermal and Heat Pump System with Solar Roof Tile Collectors and Geothermal Heat Source"

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Energy Procedia 48 (2014) 904 - 913

SHC 2013, International Conference on Solar Heating and Cooling for Buildings and Industry

September 23-25, 2013, Freiburg, Germany

Field test of an advanced solar thermal and heat pump system with solar roof tile collectors and geothermal heat source

Anja Loosea* and Harald Drucka

aInstitute for Thermodynamics and Thermal Engineering (ITW) Research and Testing Centre for Thermal Solar Systems (TZS) University of Stuttgart, Pfaffenwaldring 6, 70550 Stuttgart, Germany

Abstract

The technological combination of solar thermal systems with heat pumps continues to be a highly topical subject in the market of sustainable domestic hot water and space heating systems. Nonetheless, objective performance test methods are not yet common standard. In this context field tests with six different combined solar and heat pump systems installed in single- and smaller multi-family houses in Germany have been performed under real operating conditions by the ITW/TZS. In this paper a novel heating system comprising solar roof tile collectors and so-called geothermal energy baskets is presented in detail and in-situ monitoring results are shown and discussed.

© 2014 TheAuthors. Published by ElsevierLtd.

Selectionandpeerreview bythescientificconference committeeofSHC 2013under responsibilityofPSE AG Keywords: Combined solar thermal and heat pump system; performance assessment; field test; in-situ monitoring

1. Introduction

During the past years, a variety of combined solar thermal and heat pump (SHP) systems with different conceptual designs have appeared on the European market, claiming that higher seasonal performance factors of the overall systems can be achieved than with traditional, separated heating systems. The main background for this

* Corresponding author. Tel.: +49-(0)711-68563940; fax: +49-(0)711-68563503. E-mail address: loose@itw.uni-stuttgart.de

1876-6102 © 2014 The Authors. Published by Elsevier Ltd.

Selection and peer review by the scientific conference committee of SHC 2013 under responsibility of PSE AG doi:10.1016/j.egypro.2014.02.104

development is the expected increase of efficiency for both, the solar thermal system and the heat pump due to synergetic effects resulting from the mutual interaction of these sub-systems [1]. Recently, more than 100 market available SHC systems have been identified [2]. However, uniform and objective criteria for the evaluation of the combined solar and heat pump systems' thermal performances are not available up to now. Because of this corresponding test and assessment procedures are needed in order to be able to determine the energetic performance and the environmental impact of combined solar thermal and heat pump systems in an objective manner. Motivated by these facts the national research project WPSol (performance testing and ecological assessment of combined solar thermal and heat pump systems, project duration 2010 - 2013) has been initiated in order to develop performance test methods for such combined systems. Key activities within this project are among others field tests, i.e. in-situ monitoring of combined solar thermal and heat pump systems installed in real buildings.

2. Field tests of combined solar thermal and heat pump systems

Broad field tests of separate system technologies (such as solar thermal systems and heat pump systems) have been performed already for heat pump systems only [3] and for solar thermal combi systems without heat pumps [4], but not yet for the specific combination of solar thermal and heat pump systems. Although some of these combined systems have been monitored in single cases [5-7], a systematic study related to the in-situ performance for this system category is still missing. Therefore, field tests based on in-situ monitoring have been performed in international cooperation within the common IEA SHC Task 44 / HPP Annex 38 of the Solar Heating and Cooling Programme and the Heat Pump Programme of the International Energy Agency [8] in order to determine the thermal performance of combined solar thermal and heat pump systems under real operating conditions.

The main aim of the in-situ monitoring is on the one hand the detection of installation errors, optimization of the operation behavior of the entire systems and controlling functions for different operation modes as well as the dimensioning of the collector field and storage capacity, etc. On the other hand, measured data are necessary for the validation of numerical simulation models of combined solar thermal and heat pump systems. The combined solar thermal and heat pump systems which are monitored within the project WPSol represent a broad spectrum of different system concepts. Six heating systems installed in Germany have been equipped with measuring equipment and been monitored until August 2013. In this paper, one of the most innovative systems under investigation comprising a brine to water heat pump in combination with solar roof tile collectors and geothermal energy baskets is described in more detail and the results determined during the in-situ monitoring are presented and discussed.

3. Combined SHP system with solar roof tile collectors and geothermal heat source

3.1. System description

The system described in this paper consists of so-called geothermal energy baskets as heat source for a brine to water heat pump. Alternatively, "invisible" solar roof tile collectors (coated Aluminum absorbers in the shape of roof tiles) can be used as direct heat source for the heat pump, as well. These solar roof tiles act both as solar thermal collectors and as solar absorbers, which are also able to collect heat from the ambient air, respectively. Direct charging of a space heating buffer store and a domestic hot water buffer store by the solar roof tiles is also possible, as well as solar regeneration of the earth surrounding the energy baskets. A simplified hydraulic scheme is depicted in Fig. 1 and pictures of the geothermal energy baskets and the solar roof tile collectors are shown in Fig. 2.

The control of this bivalent system always checks which is the most effective energy source at a time and uses this source to provide the necessary temperature for domestic hot water, space heating and regeneration of the earth. If there is heat demand for domestic hot water or space heating the heat source with the highest temperature is chosen to charge the heat stores. If the brine temperature from the roof is high enough, the store is heated directly without using the heat pump. If there is no heat demand, the system chooses between solar thermal regeneration of the ground or bringing the temperature in the two buffer stores to a higher level, depending on the respective temperatures.

Fig. 1. Simplified hydraulic scheme of the monitored heating system

The control of the system also distinguishes between summer, winter and transition times. While during summer direct solar thermal domestic hot water preparation is favored, during winter solar thermal heat is offered to the heat pump directly as heat source, if possible and the temperature is high enough, and during autumn and spring the regeneration of the ground is prioritized. A configuration with combi store is available as well. Passive cooling is optional.

Fig. 2. (a) Geothermal energy baskets, (b) solar roof tiles [Pictures: A. Drexler, A. Loose]

3.2. Technical data and building description

The following technical specifications apply for the heating system monitored and for the corresponding building:

• Brine to water heat pump; COP 4.7 at B0/W35 (EN 14511), refrigerant R410a, nominal capacity 12 kW, scroll compressor

• Solar roof tile collectors oriented to south, 35 m2 collector area

• Geothermal energy baskets, 4 x 1.5 kW nominal abstraction capacity

• 400 l domestic hot water buffer store, 400 l space heating buffer store

• Multi-family house, 3 flats, 5 persons

• Year of construction: 1960, refurbishment in 2011

• Location: Füssen, Bavaria, Germany

• Heated living area: 280 m2, radiator heating system with low flow temperatures (radiators are oversized after retrofit of the building)

• Annual space heating demand: 18.890 kWh/a (67.5 kWh/m2a) (measured)

• Design supply and return temperature space heating: 29/27°C (measured)

• Annual DHW demand: 1.345 kWh/a (4.8 kWh/m2a) (measured)

• Design tapping temperature: 45°C.

3.3. Monitoring procedure

The system has been monitored from September 2012 until August 2013. Data were collected once per minute with an Ennovatis Smartbox as data logger and transferred once per day via a GSM mobile connection to the ITW. Pt 1000 temperature sensors were used for measurement of the ambient temperature and room temperature (boiler room), a solar irradiation sensor (Si cell) was also installed on the roof in the solar collector plane.

Heat meters (ultrasonic flow meters with two Pt 500 sleeve sensors each) were used for monitoring the heat flow in the heating loop, the two buffer store charging loops, the DHW loop and DHW circulation. Heat meters based on turbine type volume flow meters and Pt 500 temperature sensors for flow and return temperatures were used to measure the heat flow in the brine loops (i.e. solar loop, geothermal source and regeneration, primary and secondary loop of the heat pump). Electricity meters for heat pump, electric heating element as backup, control, primary circulation pump, store charging pump, heating loop, fresh water station and DHW circulation pump were installed. Fig. 3 shows the monthly energy balances for the period from September 2012 until August 2013 in Kilowatt-hours.

| 3500

5; 3000

2500 2000 1500 1000 500

V o~ ^ O" y V V-T V

Fig. 3. Monthly energy balances of the monitored combined solar and heat pump system for the period of one year

The central small bar of each column depicts the useful energy (space heating, domestic hot water, circulation and regeneration) including heat losses and measuring faults. At the outer part of each column, the energy sources are displayed. They consist of solar gains from the roof tile collectors, geothermal gains from the energy baskets and electrical energy consumed by the heat pump. An electric back-up was not needed during the monitored period.

3.4. Definition of new performance figures and monitoring results

During the course of IEA SHC Task 44 / HPP Annex 38 new performance figures for combined solar thermal and heat pump systems have been defined in international agreement [9]. These are used in the style of seasonal performance factors of monovalent heat pumps and are defined as produced heat quantity divided by the electricity consumption for several system boundaries. The most important performance factor for combined solar thermal and heat pump systems according to Task 44 / Annex 38 is the seasonal performance factor SPFshp for the overall system (SHP = solar heat pump):

_ f(QsH+QpHw)*dt t SHP Sl.iPel.i*dt ■

In contrast to the performance factor SPFshp+ the electricity consumption for space heating loop pump and domestic hot water circulation pump are not considered in this value. Attention should be paid to the fact that these thermal performance factors are not directly comparable to the seasonal performance factors which are commonly used to describe the performance of heat pumps without combination with solar thermal systems. The definition for combined solar and heat pump systems utilizes the mere useful energy output, while for field tests with monovalent heat pumps found in the literature generally the total amount of heat produced by the heat pump - i.e. including heat losses of the store - is used. Thus, the latter values are higher than the new SPFshp by definition. Seasonal performance factors can also be calculated for single components and sub-systems, as e.g. for the solar thermal collector field, for the heat pump or for the heat pump with solar assistance.

= i^ot = f(QHP>at und SPFsoiHp ifeo^p)^ ^ ;

The performance factor SPFSolHP serves for the direct comparison with the performance of the heat pump SPFHP and thus is an indicator for the added value given by the solar thermal sub-system.

Figure 4: Performance factors SPFhp, SPFshp and SHPsoihp of the system

* with Qsh - space heating, Qdhw - domestic hot water consumption, Pel,i - electricity consumption of all consumers (heat pump, circulation pumps, controller and electric heating element if applicable)

* SC - solar collector, Qsol - total solar gain, Qhp - heat produced by the heat pump, Pel,sol - electricity consumption of solar circulation pump and solar control, Pel,HP,i - electricity consumption of compressor, control and primary loop circulation pump of the heat pump, Pel,ctr - electricity consumption of the controller

The thermal performance factors for the system described here are shown on monthly basis in Fig. 4 and reached values of SPFHP = 3.77, SPFsys = 3.46 and SPFSolHP = 5.43 for the monitored period from September 2012 until August 2013. The performance of the heat pump in this system is therefore comparable to a conventional brine to water heat pump with borehole heat exchangers.

Figure 5: (a) Share of heat sources, (b) share of solar gains by application for the period Sept. 2012 until Aug. 2013

In Fig. 5a the contributions of the different heat sources of the system (solar thermal and geothermal) and the electricity consumed by the heat pump are depicted. It can be seen, that about one third of this contribution is solar thermal, comprising direct charge of buffer stores (15 %), direct use for the heat pump as heat source (47 %) and regeneration of the earth (38 %) as shown in Fig. 5b. The share of solar energy used directly as heat source for the heat pump amounts to 22 % of all heat sources, geothermal 56 % and electricity 22 %.

The seasonal differences made by the system's control of operation modes can be seen clearly from monitoring results. Fig. 6 depicts the monthly share of solar thermal energy gains split up into their different usage categories. While during summer direct use for domestic hot water preparation is favoured, during the heating period solar gains are provided to the heat pump's evaporator and in autumn and spring regeneration of the earth is preferred.

Solar to heat store ■ Solar to HP ■ Solar regeneration

Figure 6: Seasonal control of the solar gains for the period Sept. 2012 until Aug. 2013

From the solar thermal point of view, the fractional energy savings (fsav) can also be calculated as an additional performance factor, which is defined as the auxiliary heating demand of a solar combi system (Qaux) in relation to the energy demand of a reference system (Qref) operated with natural gas. Since in the case of heat pumps as backup of a solar thermal system the fossil energy used is electricity instead of gas, one can define a new figure fsav,PE which also considers the primary energy needed for the auxiliary heating demand:

sav,PE

, with Qaux — (Pel,HP + Pel.heating element) * fpE,electricity and

Qref = (.Qsh + Qdhw + Qlosses,store)

: fpE,,

Over the monitored period of one year the system reaches mean values of fsav = 82.4 % and fsav,PE = 58.4 %. These values indicate the share of environmental or rather renewable energy (solar thermal, geothermal and heat from the air) which is used during operation of the heating system on basis of final energy and primary energy, respectively, as compared to the fossil fuel operated reference system. Further solar thermal performance factors are the collector efficiency (ncol = solar gains/solar irradiation onto collector plane) and the specific collector gains per square meter and year. For the system under investigation these amount to ncol = 26 % for the period measured and specific gains were 292 kWh/m2a. It has to be taken into account that solar absorbers are in use which are less effective than e.g. flat plate collectors, yet also capable to use heat from the surrounding air in times with no or low solar radiation, which is included in the collector gains. According to IEA Task 44/Annex 38 also for the collector field a seasonal performance factor SPFSC can be calculated as already defined above in Eq. 2. The SPFSC and r|col are shown on monthly basis in Fig. 7.

Figure 7: (a) Performance factors SPFSC and monthly solar thermal yield in total, (b) collector efficiency

Figure 8: Electricity consumption of the monitored system for the period Sept. 2012 until Aug. 2013

The share of electricity demand for auxiliary devices such as circulation pumps and system control is a substantial value for the assessment of the performance of the overall system, as well, since it is expressed indirectly in the calculated value SPFSHP. Even if highly efficient sub-systems as e.g. a high performance heat pump are applied in the system, the performance of the overall system may be poor, if too much auxiliary electricity is needed.

§ As primary energy factors fPE values of 1.1 for natural gas and 2.6 for the electricity mix in Germany were used. Auxiliary electric energy for control and circulation pumps were not included in the calculation of Qaux according to EN 12976. Qiosses describes the heat losses of a conventional heat store for domestic hot water and is applied here with 644 kWh/a. tfconv - efficiency of gas boiler (factor 0.75).

The system under investigation showed rather good values with only 10 % share of auxiliary energy demand (Fig. 8). The use of the electric heating element which is integrated in the system was not necessary during the period monitored.

The heat losses of the two buffer stores amount for about 250 kWh/month in average. This corresponds to 13 % of the amount of heat charged to the buffer stores or 87 % storage efficiency. The monthly energy balances of the buffer stores are depicted in Fig. 9 with energy charged to the stores as positive and used energy as negative values.

Figure 9: Monthly energy balances of the two buffer stores for the period Sept. 2012 until Aug. 2013

The typical course of monthly space heating and domestic hot water demand of the system under investigation depending on the mean ambient temperature is shown in Fig. 10. In addition, the mean room temperature measured in the boiler room can be seen. Circulation has been stopped after February 2013 due to high heat losses and the surplus of electricity demand. The house was not inhabited during August 2013, i.e. there was no domestic hot water demand during this month.

Figure 10: Monthly space heating and domestic hot water demand in dependence of the ambient temperature

In addition to the thermal performance factors which have been newly defined by the experts participating in the IEA Task 44/ Annex 38 and presented above, also some figures of ecological relevance have been agreed on. Two of these are the Global Warming Potential (GWP) and the Primary Energy Ratio (PER):

GWp _ ÍXQFE*GWPj*dt rkgCQ2-Eq.-. ^ pER _ S£Pel,FE*CEDNRE,el*dt rkWhpE-. ** (4)

!(.QsH+QDHW>dt L kWhUE J !(.QsH+QDHW>dt ( )

Figure 11: (a) Global warming potential, (b) primary energy ratio of the system

Both values are shown in Fig. 11 on a monthly basis. By the GWP value the amount of greenhouse gas emissions as CO2-equivalents is expressed related to the amount of useful energy (for space heating and domestic hot water) per Kilowatt-hour. The value PER describes the amount of primary energy which has to be invested for earning one Kilowatt-hour of useful energy. For comparison, the values used for natural gas and for oil are given below in Eq. 5. Once the calculated values from measured data are smaller than these, the operation of the system is more ecologic than for a correspondent gas or oil fueled system.

GWP (gas): 0.307 kgCO2-Eq/kWh, GWP (oil): 0.318 kgCO2-Eq/kWh (5)

PER (gas): 1.194 kWhPE/kWhFE, PER (oil): 1.271 kWhPE/kWhFE.

4. Conclusion

In the context of the development of new performance test methods for combined solar thermal and heat pump systems the ITW/TZS has performed a field test with six of such systems with different conceptual designs. One of three systems comprising brine to water heat pumps has been presented in this paper including in-situ monitoring results for the period of one year. The heat pump using shallow geothermal and solar thermal heat as sources has been installed into a retrofitted multi-family house in Füssen (Germany) in summer 2012 and has been monitored from September 2012 until August 2013. The system provides domestic hot water (1.345 kWh/a) for four persons and a small child (which is a comparably very low value, yet measured as such) and space heating (18.890 kWh/a) for 280 m2 heated floor area. All heat flows and the electricity consumption of heat pump, control, circulation pumps and hydraulic transfer station have been monitored.

The measured seasonal performance factor of the heat pump as component (SPFHp = 3.77) corresponds to typical values for brine to water heat pumps in the field found in previous investigations, c. f. [3]. The overall system's

PE - primary energy, FE - final energy, UE - useful energy, NRE - non-renewable energy, GWP; (0.56 kg CO2/kWh) and CEDNRE,el, (3.13 kWhPE/kWhFE) are European average values for the CO2-equivalent (greenhouse gas emissions) and for the cumulative energy demand for the electricity mix, QSH - space heating, QDHW - domestic hot water consumption.

performance factor (SPFSHp = 3.46) can be seen as rather good for a system which has not been optimized yet, and which is situated in a retrofitted old building, though higher values have been predicted to be possibly reached by simulation studies.

The combined solar thermal and heat pump system presented here has got a big potential for even increased solar thermal contributions, especially during summer time, in combination with additional heat sinks as e. g. private swimming pools for single-family houses. Furthermore, the solar roof tile collectors are of special interest for applications in protected historical buildings, since they are available in different roof colors and thus indeed almost invisible.

It has to be underlined at this point that mere field test results from in-situ monitoring cannot be directly compared, since the results measured depend on a variety of boundary conditions such as location of the building, climatic conditions, building standard and space heating demand of the building and last but not least the consumer behavior. Only with simulations of the systems under reference conditions the results become definitely comparable. More results of field tests with combined solar thermal and heat pump systems performed by international research groups as participation to the IEA SHC Task 44/ HPP Annex 38, a comparison among these systems and also system simulations will be available in the corresponding handbook of the Task which will be published presumably in the beginning of 2014.

Acknowledgements

The project WPSol is partly funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit BMU) under grant number 0325967A. The authors gratefully thank for the support and carry the full responsibility of the content of this publication.

Thanks are also addressed to our industrial project partners Prometall-Fertigungstechnik GmbH, 87669 Rieden am Forggensee and Noventec GmbH, 87629 Füssen, Germany, for their contributions and support.

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