Scholarly article on topic 'Polymeric Solar Collectors or Heat Pump? – Lessons Learned from Passive Houses in Oslo'

Polymeric Solar Collectors or Heat Pump? – Lessons Learned from Passive Houses in Oslo Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Michaela Meir, Espen Murtnes, Aylin Maria Dursun, John Rekstad

Abstract Energy monitoring has been performed for two passive houses in Oslo during 2012-2013. One house is heated by a solar heating system, the other with an air-to-water heat pump. The objective has been to investigate the need for additional energy supply in order to provide the required indoor comfort and prepare domestic hot water. If corrected for differences in domestic hot water consumption and indoor temperature the two houses require almost equal amounts of auxiliary energy. The solar energy gain would increase significantly if the solar collectors were placed more appropriate, with less shading due to neighboring buildings and vegetation. Both heating technologies could improve performance with minor system adaptations. It was shown that solar thermal heating can compete with heat pump techology even for locations as far north as Oslo, Norway.

Academic research paper on topic "Polymeric Solar Collectors or Heat Pump? – Lessons Learned from Passive Houses in Oslo"

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Energy Procedia 48 (2014) 914 - 923

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

September 23-25, 2013, Freiburg, Germany

Polymeric solar collectors or heat pump? - Lessons learned from

passive houses in Oslo

Michaela Meira,b, Espen Murtnesa, Aylin Maria Dursuna, John Rekstada,b

aUniversity of Oslo, Department of Physics, PO Box 1048, Blindern, 0316 Oslo, Norway bAventa AS, Trondheimsveien 436A, 0962 Oslo, Norway

Abstract

Energy monitoring has been performed for two passive houses in Oslo during 2012-2013. One house is heated by a solar heating system, the other with an air-to-water heat pump. The objective has been to investigate the need for additional energy supply in order to provide the required indoor comfort and prepare domestic hot water. If corrected for differences in domestic hot water consumption and indoor temperature the two houses require almost equal amounts of auxiliary energy. The solar energy gain would increase significantly if the solar collectors were placed more appropriate, with less shading due to neighboring buildings and vegetation. Both heating technologies could improve performance with minor system adaptations. It was shown that solar thermal heating can compete with heat pump techology even for locations as far north as Oslo, Norway.

© 2014TheAuthors. Publishedby ElsevierLtd.

SelectionandpeerreviewbythescientificconferencecommitteeofSHC2013underresponsibilityofPSEAG

Keywords: Polymeric solar collectors, energy monitoring, passive house, heat pump, lessons learned, collaboration building industry;

1. Introduction

Due to the fact that electricity is a dominant energy source in Norwegian buildings, the energy system is well suited for statistical studies. In the survey by Statistics Norway on the Energy consumption in households, 2009 18.5% of all households in Norway have installed a heat pump [1]. For detached dwellings this share was 33%. More recent values are presently not available, but the popularity of heat pumps has continued to increase. The study shows that the energy savings after the installation of a heat pump is rather marginal on an annual basis, as illustrated in Fig. 1. In contrast to heat pumps, the use of solar thermal energy is still very limited in Norway; the country is at the bottom level in Europe in terms of installed solar collector area with 4.5 m2 per 1000 inhabitants

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.2014.02.105

[2]. The installed collector area in the neighboring countries Sweden and Denmark are 49 m and 109 m per 1000 inhabitants respectively.

One reason could be the general acceptance that solar energy utilization might be less competitive due to high investment costs, the location far north and less solar radiation compared to other European countries. One aim of the present study is to investigate this assumed general acceptance by measuring the energy consumption in two almost identical buildings, one heated by a heat pump and one by a solar heating system.

It is well known from earlier studies that the energy consumption in homes strongly depends on the inhabitants and their individual user behaviour [3]. Hence, a large ensemble is necessary in order to gain statistical significance in a comparison of energy demands in buildings. In the present study this disadvantage is partly compensated by precise monitoring of a large number of parameters in real time, all related to the use of heat and the installations related to heat supply.

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Fig. 1. The graph shows the electric enery consumption before and after installation of a heat pump in 168 Norwegian households (positive number means that the electricity consumption has been reduced). The savings after installation are marginal (source: Statistics Norway [1]).

2. Description of studied houses

The present project consists of 17 detached passive houses situated at Mortensrud, a south suburb of Oslo, named Rudshagen passive house field (see Fig. 2) [4]. The houses were designed by SPOR Architects and built by OBOS Nye Hjem and Mesterhus AS. OBOS Nye Hjem is a daughter company of OBOS (Oslo Housing and Savings Society), the largest Nordic cooperative building association, which has built more than 100,000 homes, representing a quarter of Oslo's housing stock.

Fig. 2. Rudshagen passive house field in the south of Oslo with 17 detached houses (source: OBOS). [4]

In order to meet the passive house standard for detached houses, the heating systems were designed with air-to-water heat pumps. Towards the end of the planning period it was decided that one of the houses should have a solar

heating system instead. The solar collector system replaced the heat pump while most other parts of the heating system remained unchanged. The objective has been to measure the use of auxiliary energy supply for heating purposes in the solar heated house (House A) and compare to the neighboring house heated by a heat pump (House B). Both houses are shown in Fig. 3. The Figures 4 and 5 illustrate the ground level floor plan and the site map of House A and B. Table 1 lists the central components of the water-based heating system in House A and B. In both houses live young couples with small children.

Fig. 3. Rudshagen passive houses, left heated by a solar thermal system (A) and right by heat pump (B) (Source: T. Lauluten, Spor Architects)

2.1. House A: Facade-integrated, polymeric solar collectors

The heating system of House A consists of facade integrated, non-selective, polymeric collectors with an active are of 19.5 m2. House A and B are almost identical except that House A is mirror-inverted and relative to House B rotated 90° (Fig. 3). The solar heating system is a drain-back system and in order to guarantee a well-working drainback function the lower level of the collector field is placed higher than the top level of the buffer store. No windows were planned on the south-facing facade, hence the solar collectors were mounted on the upper part of the facade. The facade with main windows faces towards west. The solar heat as well as the auxiliary electric energy is delivered to the 800 l buffer store in the technical room, which is located outside of the thermal shell of House A (Fig. 4a, red).

The buffer store has an immersed, 100 l pressurized tank for the pre-heating of DHW. The final tapping temperature is provided by a 76 l boiler with electric heating. In contrast to House B, the heat buffer store contains system water, which circulates without intermediate heat exchangers in the solar loop, buffer store volume and floor system. Further details are given in Table 1 and by the black lines of the hydraulic scheme in Fig. 5.

House A House B Site map of surroundings

Fig. 4. Rudshagen passive houses: Floor plan of the 1st floor of solar heated House A and heat pump heated House B. The red square for House A indicates the technical room where the heat store is located. For House B the corresponding technical installations are inside the "well-insulated

passive house shell"; source: [4].

2.2. House B: Air-to-water heat pump

The heating system of House B and all other houses at Rudshagen, consists of an air-to-water heat pump, type HWS-601XW-E (Toshiba) [5]. As shown in the hydraulic scheme in Fig. 6 (black lines), the heat pump transfers the energy via a spiral heat exchanger to a 300 l heat store, which contains drinking water and a 3 kW auxiliary electric heater.

Table 1. Details of the heating system in House A (solar heated) and B (heat pump heated).

House A (solar heated) House B (heat pump heated)

Living area 116 m2 + 2.5 m2 116 m2

Window area, south 3.6 m2 11.9 m2

north 0 m2 4.2 m2

east 4.2 m2 0 m2

west 11.9 m2 3.6 m2

Primary heating system 19.5 m2 solar collectors, facade integrated non- Air-to-water heat pump HWS-601XW-E

selective, all polymeric collectors (AventaSolar) 50 Hz, nom. power 6 kW (0.6-6.0 kW)

Heat store 800 l heat buffer store; non-pressurized including 300 l heat store; pressurized; including

system water, DHW

- for DHW preheating by immersed tank - for DHW preheating

pressurized with volume of 100 l - serves floor heating via heat exchanger

- serves space heating; no intermediate heat - 3 kW auxiliary electric heater

exchangers to floor heating loop

- 3 kW auxiliary electric heater

- Active volume for solar thermal / auxiliary

(electric) heat supply differs (800 l / 540 l)

DHW boiler OSO Flexi benk RB 80; Volume: 76 l; Power: 1.95 kW; Thermostat pre-set: 75 °C

Heat emission system 1st + 2nd floor: floor heating in bathroom (10.9 m2) and corridor (14.4 m2), total 25.3 m2;

Additionally: 1st floor fan coil (water-to-air heat exchanger)

Fan coil Input power, fan motors 2 x 114 W

Heat recovery Villavent® VR400 DC & DCV/B / VR700 DVC & DCV

3. Energy monitoring

The monitoring project was financed by the Research Council of Norway through the project SILVER with project partners OBOS, Aventa AS and University of Oslo. The measurements were meant to go on from the beginning of February 2012 until the end of March 2013, but were later extended until the end of 2013. Three ALMEMO® 2890-9 dataloggers from Ahlborn [6] were set-up in both houses recording the parameters as shown in Figures 5 and 6.

Fig. 5. Hydraulic scheme of the solar heated house (A) in black color; instrumentation for energy monitoring in blue and green color.

Fig. 6. Hydraulic scheme of the heat pump heated house (B) in black color; instrumentation for energy monitoring in blue and green color.

During most of the monitoring phase the cycle time of all recorded parameters was 15 minutes. The cycle time was reduced to 5 minutes at the end of June 2013. For the electric energy consumption and the volume flow integrated values were read-out, and for the solar irradiance the average value over a sampling interval was written to file. For all other parameters instantaneous read-out data were recorded.

4. Analysis and method

In practice not all energy quantities can be measured in a complex system like homes populated with users who are living a normal life and are decoupled from such a comparative study. Hence methods were developed to project out the relevant energy quantities from a limited set of measured parameters. In the present case electric energy, which can be measured with high accuracy, is the only auxiliary energy source. The aim of this study is to determine

how much auxiliary energy is needed in order to provide the desired comfort in terms of room temperature and domestic hot water (DHW) heating.

Both the domestic hot water consumption and the indoor temperature level is determined by the users, and may differ significantly for the two houses. Hence, in order to make a reliable comparison these parameters needed to be measured with sufficient time resolution.

4.1. Domestic hot water preparation

In the present study it was not possible to record the instant tap temperature with high precision. As seen from Fig. 5 and Fig. 6 the energy used for preparing DHW is provided by different sources. In House A the DHW is preheated in the heat buffer store by a combination of solar- and electric energy. The final tap temperature is reached in a 76 l DHW boiler, which has a thermostat with fixed temperature. It is expected that occasionally an amount of water exceeding the boiler volume is tapped during a period of time, which is too short for the electric heating element in the boiler to maintain the thermostat set temperature.

The same is expected in House B. Here the DHW in the 300 l heat store is pre-heated by the heat pump or by the auxiliary energy source (electric energy). Also here the final tap temperature is reached in the 76 l DHW boiler.

Following method is used for determination of the mean tap temperature within a certain time, e.g. 24 hours. The total net energy consumption for DHW preparation QDHW is given by

QdHW = VP■ c (TDHW-Tcw) , (1)

where V is the withdrawn water volume, p is the density of water, c is the specific heat capacity of water, TDHW is the tap temperature (DHW) and Tcw the cold water temperature. The cold water temperature data were provided as weekly average values at nearby Bj0rndalen Pump Station by Oslo Water and Sewerage Department [7]. There are heat losses from the boiler, buffer store and pipes. These heat losses can be found from the DHW boiler's electric energy use, which is necessary to maintain the set temperature in periods without DHW withdrawal. During the DHW draw-off the heat loss is larger due to supply of heated water in the pipes connected to the boiler. Hence, the total energy use related to DHW preparation is QDHW (Eqn. 1) plus heat losses.

4.2. Total energy use for DHW preparation

The total energy used for DHW preparation corresponds to the supplied energy for DHW heating in the same period. The energy supply consists of two components: i) the energy content of the preheated water supplied from the buffer store to the DHW boiler, and ii) the electric energy £boiler delivered by the heating element in the DHW boiler. Total energy use for DHW preparation is

QdHW + Qloss = Eboiler + VP' C ([preheated - Tcw ) (2)

Here Tpreheated is the temperature of the preheated water supplied to the 76 l DHW boiler and Qloss the heat losses of the DHW boiler. The volume of domestic hot water V is the same as the withdrawn volume during the measured sequence. TDHW can be determined from Eqn. (1) and (2):

VP ■ c (DHW -Tpreheated)= Eboiler — Qloss (3)

The tapped water volume from the 76 l-boiler represents a certain heat capacity. This stored energy can be ignored by choosing time intervals starting from the early morning before any DHW is tapped, as the electric heater in the boiler brings the water temperature up to the set temperature during the night.

The tapping temperature has only been measured occasionally, in order to verify the set temperature of the thermostat in the boiler tank which is set to 75 °C. The DHW temperature varies with the amount of tapped water

due to the small volume of the boiler. The temperature of the preheated water supplied to the boiler Tpreheated is measured continuously, and the actual tapping temperature can thus be determined from Equation (2) where £boiler is the electric energy supply to the boiler, and all quantities refer to a certain time interval, e.g. 24 hours.

4.3. Solar gain

The solar gain (Psolar_gain) for House A can be determined in two ways. The standard method (heat flow method) is based on the volume flow dV/dt of the heat carrier in the solar collector circuit, and the temperature difference (Team ' Tforward) of the water in the return and forward pipes to the collectors

Psolar _ gain d^ P' C ' ((Treturn Tforward

Here p is the density and c the specific heat capacity of the heat carrier. The other method is a calorimetric study of the heat store. During a period of solar harvesting, the mean temperature of the heat buffer store increases. Knowing the heat capacity Cs of the store, the solar gain Qsolar is given by

Qsolar - Cs ' (T2 -Tl)

Ti is the average store temperature at the start of the measuring period, T2 is the average store temperature at the end of the period. The calorimetric method is expected to give smaller values for the solar gain than the heat flow method: When the solar collector system is operative, solar heat is supplied to the buffer store while energy related to space heating, DHW preheating, heat losses from the store and the connected pipes is withdrawn from the buffer store.

5. Performance and comparison

5.1. Electric energy consumption for DHW preparation and space heating

Figure 7 compares the monthly electric energy consumption (auxiliary energy) for domestic hot water preparation and space heating in the two houses. House A (solar heated house) reveals a significantly lower energy demand in the summer months, while the heat pump heated house has a smaller auxiliary energy demand during the winter months. This is expected considering the geographic location of Oslo, where the solar altitude above the horizon is 6.5° on December 21, noon. Due to shadow on the façade mounted collectors from surrounding buildings and vegetation the solar collector system was not in operation in the period from medio November to February.

HOUSE A * HOUSE B HOUSE A ■ HOUSE E

Fig. 7. Monthly electric energy consumption for domestic hot water (DHW) and space heating;

During one year the accumulated electric energy consumption for space heating and DHW preparation is approximately equal for both houses when only the data in Fig. 7 are considered. The dependency of the auxiliary energy use on the weather is seen in Table 2, listing two 12 months periods, (i) including the rainy summer months in 2012 and (ii) the summer with good solar radiation in 2013. Relative to the test reference year for Oslo where the total accumulated solar irradiation in July amounts 104 kWh/m2 [8], the measured irradiation in July 2012 was 71 kWh/m2 and in July 2013 was 91 kWh/m2.

Table 2. Electricity consumption related to space heating and domestic hot water preparation in the solar heated house (A) and in the heat pump heated house (B) during two 12 month periods: (i) with a "rainy summer" in 2012 and (ii) "good summer" in 2013 with regard to solar irradiance.

Period Electricity consumption Electricity consumption

Solar heated house (A) Heat pump heated house (B)

(i) 01.05.2012 - 30.04.2013 9 495 kWh 8 914 kWh

(ii) 01.10.2012 - 31.09.2013 9 147 kWh 9 521 kWh

5.2. Corrections

While the electric (auxiliary) energy consumption is directly measured, the total heat demand of House A and B is not straight forward to find from the recorded parameters. In the present work the difference in energy consumption for the two houses is determined instead.

The difference in DHW consumption has been measured in terms of volume and temperature. It was found that the DHW consumption in House B is larger than in House A and the average tapping temperatures differ. According to Subsection 4.2 the energy consumption for DHW preparation was calculated being 15.4 % higher in House B than in House A.

Since the two houses are almost identical (size and thermal insulation), the differences in the space heating demand are correlated to the differences in indoor temperature (assuming equal in-regulation of heat recovery and ventilation system). During the heating season the indoor temperatures in both houses were found to be approximately equal. House B has south facing windows in the second floor, in contrast to House A with windows toward west. It is observed that the indoor temperature in the second floor of House B is somewhat higher than in house A due to passive solar gains (but outside heating periods!).

The main difference of the space heating demand between House A and B relates to the technical room, a small annex of House A. All energy for space heating and preheating of DHW is supplied to installations in the technical room, which has a poor exterior insulation relative to the passive house shell (House A). In House B the corresponding installations are inside the "passive house shell". The heat loss coefficient, the outdoor- and indoor temperatures of the technical room of house A are known. From October to February the solar heating system is not active. Hence all energy supply in House A during this period is electric energy and the total heat loss from the technical room can be ascribed to electricity. We assume also that 50% of the heat loss from the technical room in the months March, April and September is ascribed to electricity. The total "electric energy losses" from the technical room during the measuring period (in Figure 8) is determined to1008 kWh.

Figure 8 shows the difference in the auxiliary energy consumption between the solar heated house (A) and the heat pump heated house (B) after corrections for the differences in DHW consumption and heat losses from the technical room. Here positive values imply that the consumption in House A is smaller than in House B, negative values that the consumption in House A is larger than in House B.

Fig. 8. Difference between House A and House B in electric energy consumption for heating (positive values imply that the consumption in House A is smaller than in House B, negative values that the consumption in House A is larger than in House B).

Fig. 9. Solar gain in 2013 determined with the calorimetric method and the heat flow method for House A.

The solar gain for House A was determined with the two methods in Eqn. 4 and 5: The results for 2013 are shown in Fig. 9. The annual solar gain is between 2 500 kWh and 3 000 kWh, depending on the ambient temperature and solar irradiation [8]. In the present study it was not aimed to determine the COP for the heat pump system.

6. Lessons learned

6.1. Experiences: Heating system / Performance

It is shown that the energy savings of the solar heating system gives at least as large energy savings as the system based on an air-to-water heat pump. This result is obtained in spite of the disadvantages in the present project where vegetation and neighboring buildings made the solar system passive during four winter months. With a solar collector mounted in a more favorable angle, and with a horizon free from obscuring objects and shadow on the collector field, the solar gain could be increased more.

It is evident that the energy consumption in both houses exceeds considerably the low expected values for passive houses. It should be emphasized, that in spite of this discrepancy the houses use less energy than conventional low energy houses. It is a challenge to identify the reasons for the discrepancy. A part of the explanation is related to the design of the heating systems. The energy used for DHW preparation is too high compared to the energy content of

the heated water. A small DHW boiler volume is compensated by a high set temperature of 75 °C and results in increased heat loss and lack of comfort. Further the temperature required by the space heating system, with a combination of floor heating (bath room and corridors) and a fanOcoil, is much higher than necessary. This significantly reduces the efficiencies of the heat pump and the solar heating system respectively. The space heating system could be improved by reducing the system's operational temperature. Appropriate changes in the system architecture would be beneficial both for the performance of the heat pump and the solar heating system.

6.2. Experiences: Collaboration with building industry

Concerning the building integrated solar collectors close collaboration with and acceptance by the building industry is considered to be a key for further increase of solar thermal installations. Presently the installation costs of solar thermal systems, which can be up to 50% of the total retailer price, represent a major part of the total end-user price [9]. Cost reductions can be achieved by designing solar components in collaboration with the building industry considering optimal substitution of conventional facade and roof cladding and installation procedures, which are adapted to the skills and workflow at the building site.

The design of the present collector concept aims to remove some of the main barriers for acceptance by the building industry. Low weight, modular structure and collector loop without hydraulic pressure gives the solar collector features similar to a building skin product similar to other roof- or façade covers and the collectors can consequently be mounted by any skilled construction worker saving time and costs. In the present case where the infrastructure at the building site was simultaneously used for the collector installation, the installation of the facade collectors were in the range of 10% of the total solar heating system costs.

The "lessons learned" from the solar heated house at Rudshagen are transferred to a follow-up project with 34 passive houses built in Oslo, all with polymeric collectors. The construction is started in autumn 2013.

Acknowledgements

The energy monitoring and the "lessons learned" in the collaboration with the building industry are sub-projects of the research project SILVER - Solar in Living Environments, supported by the Research Council of Norway's programme RENERGI (2011-2014). Financial support is kindly acknowledged.

References

[1] Statistics Norway. Energy consumption in households, 2009. Retrieved on 19.09.2013 from https://www.ssb.no/en/energi-og-industri/statistikker/husenergi/hvert-2-aar/2011-04-19. Published: 19 April 2011.

[2] Mauthner F, Weiss W. Solar Heat Worldwide - Markets and contribution to the energy supply 2011. Edition 2013. Retrieved on 10.09.2013 from http://www.iea-shc.org/solar-heat-worldwide.

[3] Peter M. On heat consumption in buildings - User influence and saving potentials through adapted behaviour and the use of solar energy. PhD-thesis, Faculty of Mathematics and Natural Sciences, University of Oslo, to be defended 2013/2014.

[4] Rudshagen passive house field, Website: http://www.rudshagen.no.

[5] http://www.toshiba-aircon.de/

[6] Ahlborn Mess- und Regelungstechnik GmbH, Germany; www.ahlborn.com

[7] Oslo Water and Sewerage Department (Oslo Vann- og Avl0psetaten), http://www.oslo.kommune.no/english/water_and_sewerage.

[8] Standard Norge. Tillegg M: Klimadata i Norsk Standard NS 3031:2007.

[9] Epp B. Challenge of the world market: "Europe is stuck with high installation costs", retrieved on 01.10.2013 from http://solarthermalworld.org.