Investigations of thermal self-sufficient Residential Buildings with Solar Energy Systems

Investigations of thermal self-sufficient Residential Buildings with Solar Energy Systems P. Klanatsky 1, 2, F. Inschlag 1, 2, F. Hengel 1 and Ch. Heschl 1, 2 1 Center for Building Technology Forschung Burgenland, Eisenstadt, 7000, Austria 2 Department Energy-Environmental Management University of Applied Sciences, Eisenstadt, 7000, Austria SUMMARY The usage of solar thermal energy for space heating and domestic hot water production has been investigated for an energy-efficient semi-detached house in Austria. For this purpose, a high-resolution parameter study was carried out. To enhance the numerical efficiency of the dynamic system simulation approach reasonable model simplifications were derived and evaluated. Based on the proposed method more than 18.000 annual system simulations were computed and the influences to the solar coverage rate, auxiliary heater energy and the required thermal storage capacity were analysed. The results were presented in form of contour plots so that the ideal thermal storage capacity can be determined rapidly for different solar system configurations. INTRODUCTION Buildings are responsible for about 40% of total energy consumption and 36% of CO2 emissions in Europe. Therefore, regulatory frameworks such as the recast of the energy performance of buildings directive (EPBD, 2010) is a substantial measure to achieve the European Energy 2020 targets. The directive shall reduce the energy consumption of buildings and foster the nearly zero-energy building standards. In Central Europe the thermal energy demand of energy efficient buildings is responsible for about two-third of the total energy consumption. Consequently, the integration of thermal solar systems with well-balanced storage concepts to align the energy production and the energy demand are still key technologies to realize cost-effective solutions. Particularly the multitude of influencing factors such as installed collector type, user behavior, slope and azimuth angle etc. make a rapid optimizing of the storage capacity difficult. To get a better understanding about the effectively required storage capacity extensive overall system analysis are needed. Dongellini et al. (2015) developed a Simulink model (dynamic model) to simulate a solar heating system (composed by solar collectors coupled to a thermal storage) for DHW production. The authors shown the solar coverage factor (SCF) dependence of three different DHW profile loads. Antoniadis and Martinopoulos (2016) have been investigated the performance of a building integrated solar thermal system which utilizes seasonal thermal energy storage for domestic applications in order to optimize its sizing. For a typical building in Thessaloniki (Greece) the system was able to cover as much as 70% of the heating load requirement. Chotivisarut et al. (2009) have done simulations of a central solar heating system with seasonal storage using TRNSYS 17 to predict thermal performance and economic aspects. With a 50 m³ underground thermal storage tank and a 5 m² solar collector area, Chotivisarut et al. (2009) determined a solar fraction for this application of about 76%. Furthermore, Fuller and Aye (2007) and Pascual et al. (2014) have done investigations about seasonal thermal energy storage systems in existing buildings (small district heating network). In contrast to the authors who were mentioned before, a dynamic model (simulation software is TRNSYS 17) will be used to identify the fossil fuel substitution capabilities. For an energy-efficient semi-detached house in Austria, calculations were done to identify the required thermal storage capacity for space heating and domestic hot water production with only a solar thermal system (thermal self-sufficient). METHODS Because of the mismatch between the transient solar radiation and thermal energy demand (clouds, fog, day and night, summer and winter), a thermal storage tank is needed to make a semi-detached house in Austria self-sufficient. With the thermal storage tank, the generated solar thermal energy can be stored and used for space heating and domestic hot water supply (short-term or seasonal storage). Due to the bulk influencing boundary conditions, the structured derivation of an optimized system solution is still a challenge. Hence, the required thermal storage capacity in dependence of different kinds of solar thermal collectors (flat plate collector and an evacuated tube collector), variation of slope and azimuth angle and of the aperture area of the solar thermal collector and other factors was investigated. To evaluate the required thermal storage capacity for a thermal self-sufficient residential building following steps give a broad outline about the chosen approach: Two scenarios were modelled: In Scenario A system simulations with consideration of a thermal storage tank for space heating and domestic hot water production were carried out. In contrast to Scenario A the system simulations in Scenario B were realised with an ideal thermal storage tank. The simulation results of Scenario A were compared with the results of simulations of Scenario B. In detail the annual solar coverage and the required energy demand of the auxiliary heating systems for domestic hot water supply and space heating were compared. Implementation of simulations with an ideal thermal storage tank and calculation of the required thermal storage capacity for a thermal self-sufficient residential building in dependence of different kinds of solar thermal collectors (flat plate collector and an evacuated tube collector), variation of the slope, azimuth angle and the aperture area of the solar thermal collector. These investigations were done with transient simulations of the building model, the solar thermal system and the thermal storage system with the simulation software TRNSYS 17. ISBN: 978-0-646-98213-7 COBEE2018-Paper229 page 677

BOUNDARY CONDITIONS All simulations have been carried out for the same boundary conditions. These are the structure and type of the building (and also the orientation), the climate (location of the building), the domestic hot water profile load, the type of solar thermal collectors, the ventilation concept, the user behaviour and the lightning control. Building specification As mentioned before these studies were carried out for a semi-detached house in Austria (see Figure 1). The building has a ground and a first floor and no basement. The shading effect of nearby buildings were considered in this study. The insulation of the building, the glazing elements and the ventilation concept result in a mean space heating demand of approximately 22 kwh per unit area (gross area) and year. The fraction of the space heating demand for ventilation is about one third. In this building can be accommodated up to 4 persons (the attendance of the persons was defined). The setpoints for the room air temperatures (four zones were considered) were chosen by 21 C with a hysteresis of ±1 C. Floor heating systems (one in each zone) and a water/air heat exchanger supplies the rooms with heat to ensure the thermal comfort for the occupants in the winter months. Figure 1. Semi-detached house in Austria (yellow color) with glazing elements, roof and orientation. Climate data The chosen location of the described building is Graz in Austria (Central Europe; latitude: 46 59 N, longitude: 15 27 E, altitude: 342 m). Therefore, the typical meteorological year of Graz was used for specifying the climate data. Figure 2 shows the profile of the typical ambient temperature and (TRNSYS 2012) the typical global radiation on the horizontal. In the winter months (December, January, February) the ambient temperature is most of the time below 0 C and the global radiation on the horizontal surface is most of the time below 300 W/m². DHW profile load and DHW storage tank In this study, the DHW profile load (fraction of daily hot water usage in Figure 3) was considered. Furthermore, a storage tank with 500 liter, a height of 1.8 m and a diameter of 0.59 m was used to ensure the hot water supply in times without thermal output from the solar thermal collectors. With the fraction of daily hot water usage and the consumption of 60 liter hot water per person and day with a temperature of 45 C the dynamic discharge of the hot water storage tank could be calculated. Figure 3. Fraction of daily hot water usage. Solar thermal collector Two different solar thermal collectors have been considered, a flat plate collector (FPC) and an evacuated tube collector (ETC). The technical data of the chosen solar thermal collectors are given in Table 1. Figure 4 shows the collector thermal efficiency of the chosen collectors. The defined ambient temperature is 20 C in summer and -3 C in winter. In winter, the thermal efficiency of the flat plate collector with a mean fluid temperature of 60 C is much lower as the thermal efficiency of the chosen evacuated tube collector. Table 1. Solar thermal collectors technical data (flat plate collector FPC; evacuated tube collector ETC). Solar collector type FPC ETC 0 0.832 0.642 1 [W m -2 K -1 ] 3.920 0.885 2 [W m -2 K -2 ] 0.0126 0.0010 Figure 4. Collector thermal efficiency of the chosen collectors (winter: Tambient=-3 C, summer: Tambient=20 C). Figure 2. Typical ambient temperature profile and profile of the typical global radiation on the horizontal surface of the chosen location GRAZ in Austria. Thermal storage tank (used in Scenario A ) The investigation considered a thermal storage tank with a volume of 1000 liter (with a height of 2 m and a diameter of 0.80 m) respectively a volume of 1500 liter (with a height of 2.15 m and a diameter of 0.94 m). The thermal storage tank was used to store the generated solar thermal energy, which ISBN: 978-0-646-98213-7 COBEE2018-Paper229 page 678

can be used for space heating and domestic hot water production. Eight nodes were designed to discretise the cylindrical storage tank in height. The temperature of the node on the top was used to control the charging and/or discharging of the thermal storage tank. Ideal storage tank (used in Scenario B ) For the ideal consideration of the storage tank, a setpoint for the supply temperature of 80 C was used to control the mass flow for the charging process. Furthermore, the temperature difference between supply and return temperature was defined by a constant of 15 C. For comparative purposes, the heat capacity of the ideal storage tank was set to 55 kwh (vs. 1000 liter thermal storage tank) and 80 kwh (vs. 1500 liter thermal storage tank). For the discharging of the ideal storage tank only the energy demand was considered (without heat losses of the storage tank and temperature limitations). System description Figure 5 shows the hydraulic scheme which was considered in the dynamic model for Scenario A. The chosen configuration of the hydraulic system take into account the direct use of the solar thermal energy for space heating (for floor heating systems and for ventilation (water/air heat exchanger)) and domestic hot water production. In times of no heat loads, the thermal energy will be stored in a thermal storage tank (considered with heat losses, Scenario A ) until energy for space heating and/or domestic hot water supply will be required (e.g. at night). The hydraulic scheme of Scenario B considered instead of the thermal storage tank an ideal storage tank (considered without heat losses). Table 2. Used types in TRNSYS (TRNSYS (2012) and Thermal Energy System Specialists (2012)). type description 2 differential controller with hysteresis 4f stratified storage tank 6 auxiliary heaters 15 weather data reading and processing 23 PID controller 56 multizone building modeling 60d detailed fluid storage tank / vertical cylinder (DHW) 538 evacuated tube solar collector 539 glazed flat plate collector Settings of the solver in TRNYSYS 17 Following parameters were used to set the solver in TRNSYS 17: simulation time step: simulation time: evaluation time interval: solution method: 60 s 2 years 2 nd year tolerance integration: 0.001 tolerance convergence: 0.001 tolerance values: successive method; old solver relative before warning: 500 before error: 10000 before trace: 3000 diff. equation algorithm: 1, modified euler method In total, a parameter study with more than 18.000 system simulations with the simulation program TRNSYS 17 for a semi-detached house in Austria and a daily DHW consumption profile was carried out. RESULTS In the following subchapters, an extraction of the simulation results for an azimuth angle of 0 (orientation of the collector field to the south) will explained. The focus of the analysis of the simulation results was the annual solar coverage (Equation 1) and the required energy for auxiliary heaters. Solar coverage=1 8760h 0h Q auxiliarydt 8760h 0h Q demanddt (1) Figure 5. Hydraulic scheme for the space heating and domestic hot water production with solar thermal collectors. In Table 2 the used main TRNSYS components (TRNSYS, 2012) and TESS components (Thermal Energy System Specialists, 2012) are listed and described (so called types). Scenario A In Scenario A a thermal storage tank was used to store the generated solar thermal energy which can be used for space heating and domestic hot water production in times with no solar thermal energy. The simulation results of the system simulations with a 1000 liter thermal storage tank are shown in Figure 6. The left diagram on the top of Figure 6 shows the solar coverage of the semi-detached house with a flat plate collector (FPC) in ISBN: 978-0-646-98213-7 COBEE2018-Paper229 page 679

4th International Conference On Building Energy, Environment dependence of the aperture area and the slope of the collector field. In comparison, the right diagram on the top of Figure 6 shows the solar coverage by using evacuated tube collectors (ETC). With the usage of evacuated tube collectors, it is possible to increase the solar coverage. Additionally, the diagram on the bottom of Figure 6 shows the required energy for auxiliary heaters by using flat plate collectors (left diagram) and evacuated tube collectors (right diagram). ideal storage tank was set for comparative purposes to 55 kwh (vs. 1000 liter thermal storage tank) and 80 kwh (vs. 1500 liter thermal storage tank). The results of the system simulations with an ideal storage tank with 55 kwh are shown in Figure 8. The solar coverage of the semi-detached house with flat plate collectors (FPC) is shown in the left diagram on the top of Figure 8. In comparison, the right diagram on the top of Figure 8 shows the solar coverage by using evacuated tube collectors (ETC). As before, it is possible to increase the solar coverage with the usage of evacuated tube collectors. Supplementary, the diagrams on the bottom of Figure 8 show the required energy for auxiliary heaters by using flat plate collectors (left diagram) and evacuated tube collectors (right diagram). Figure 6. Solar coverage (top) and required energy for auxiliary heaters (bottom) for FPC (left) and ETC (right) with a 1000 liter thermal storage tank. The influence on the solar coverage and the required energy for auxiliary heaters using a thermal storage tank with 1500 liter instead of a 1000 liter tank shows Figure 7. With the usage of a thermal storage tank with higher volume of water, it is possible to increase the solar coverage respectively to reduce the required energy for auxiliary heaters (Figure 6 vs. Figure 7). By a small aperture area of the collector field, the influence of the bigger thermal storage tank is less than by a large aperture area. Figure 8. Solar coverage (top) and required energy for auxiliary heaters (bottom) for FPC (left) and ETC (right) with an ideal storage tank with 55 kwh. By using an ideal storage tank with 80 kwh the solar coverage and the required energy for auxiliary heaters are shown in Figure 9. Once again, with the usage of a storage tank with a higher thermal capacity, it is possible to increase the solar coverage respectively to reduce the required energy for auxiliary heaters (Figure 8 vs. Figure 9). This influence of the bigger storage tank is by a large aperture area of the collector field greater than by a small aperture area. Figure 7. Solar coverage (top) and required energy for auxiliary heaters (bottom) for FPC (left) and ETC (right) with a 1500 liter thermal storage tank Scenario B In Scenario B an ideal storage tank was used to store the generated solar thermal energy which can be used for space heating and domestic hot water production in times with no solar energy. As mentioned before the heat capacity of the ISBN: 978-0-646-98213-7 Figure 9. Solar coverage (top) and required energy for auxiliary heaters (bottom) for FPC (left) and ETC (right) with an ideal storage tank with 80 kwh. COBEE2018-Paper229 page 680

4th International Conference On Building Energy, Environment Comparison of the results of Scenario A and B In this subchapter, the simulation results of Scenario A were compared with the simulation results of Scenario B. In Figure 10, the results of the system simulations with FPC and a thermal storage tank are compared to the system with an ideal storage tank. The Figure shows the comparison of the solar coverage for the chosen hydraulic scheme with the smaller storage tanks (1000 liter vs. 55 kwh). By using a thermal storage tank the solar coverage is up to absolutely 10% higher because the required energy for the auxiliary heaters is up to approximately 700 kwh fewer (for a slope angle between 40 and 80 ; see Figure 10). The results of the system simulations with evacuated tube collectors and a thermal storage tank with an ideal storage tank are compared in Figure 12 and Figure 13. By using an evacuated tube collector both methods show a good accordance (see Figure 12 for the results with the small storage tanks (1000 liter vs. 55 kwh) and Figure 13 for the bigger storage tanks (1500 liter vs. 80 kwh)) to consider a storage tank. Figure 12. Comparison of the solar coverage (top) and required energy for auxiliary heaters (bottom) for ETC with a 1000 liter thermal storage tank (left) and ETC with an ideal storage tank with 55 kwh (right). Figure 10. Comparison of the solar coverage (top) and required energy for auxiliary heaters (bottom) for FPC with a 1000 liter thermal storage tank (left) and FPC with an ideal storage tank with 55 kwh (right). The effects on the solar coverage and the required energy for auxiliary heaters for the hydraulic scheme with bigger storage tanks (1500 liter vs. 80 kwh). are shown in Figure 11. Again, by using flat plate collectors (FPC) with a slope angle between 40 and 80 and an ideal storage tank the solar coverage will be underestimated (up to more than absolutely 10%) and the required energy for auxiliary heaters will be overestimated (up to more than 700 kwh). Figure 13. Comparison of the solar coverage (top) and required energy for auxiliary heaters (bottom) for ETC with a 1500 liter thermal storage tank (left) and ETC with an ideal storage tank with 80 kwh (right). Calculation of the required thermal storage capacity Figure 11. Comparison of the solar coverage (top) and required energy for auxiliary heaters (bottom) for FPC with a 1500 liter thermal storage tank (left) and FPC with an ideal storage tank with 80 kwh (right). ISBN: 978-0-646-98213-7 The left diagram in Figure 14 shows the required thermal storage capacity of an ideal storage tank for a self-sufficient semi-detached house with flat plate collectors in dependence of the aperture area and of the slope angle of the collector field. Areas with white background colour indicates, that the output of the solar collectors is less than the energy demand to charge the respective ideal storage tank. The right diagram in Figure 14 shows the required thermal storage capacity of an ideal storage tank for a self-sufficient semi-detached house with evacuated tube collectors. With the usage of evacuated tube collector it is possible to charge the ideal thermal storage tank (without heat losses) or use the generated solar thermal COBEE2018-Paper229 page 681

energy direct for space heating and domestic hot water production in the winter months. In this case, the required thermal capacity of the thermal storage is much lower. With the ideal consideration of the storage tank and fulfillment of the required thermal storage capacity, a solar coverage of 100% is possible for the taken boundary conditions. E.g., for a thermal self-sufficient residential building with 36 m² evacuated tube collectors (aperture area without shading) and a slope of 60 the required thermal storage capacity is appr. 500 kwh (chosen supply temperature for charging was 80 C). In the case of using the chosen flat plate collector, the collector thermal efficiency influences the accuracy of the dynamic approach with consideration of an ideal storage tank. The result is an overestimated required thermal storage capacity for a self-sufficient semi-detached house in Figure 14. The influence of the azimuth angle on the required thermal storage capacity shows Figure 15 (with an aperture area of 36 m²). CONCLUSIONS The usage of solar thermal energy for space heating and domestic hot water production has been investigated for a semi-detached house in Austria (Central Europe). In detail, the required energy for auxiliary heaters and also the solar coverage in dependence of different kinds of solar thermal collectors (flat plate collector and an evacuated tube collector), variation of slope, azimuth angle and the aperture area of the solar thermal collector and other factors was identified. Furthermore, a dynamic approach with an ideal consideration of the storage tank shows a good agreement with the results of a thermal storage tank by the utilization of evacuated tube collectors. Because of the lower thermal efficiency of the chosen flat plate collectors (related to the supply temperature), the comparison of the results of the thermal storage tank with the ideal storage tank shows a less accordance. Nevertheless, for the first step of the design process of the hydraulic scheme for a thermal self-sufficient semi-detached house in Austria, the ideal consideration of the storage tank allows an exactly enough determination of the required thermal storage capacity. ACKNOWLEDGEMENT Figure 14. Required thermal storage capacity dependency on the chosen solar thermal collector (azimuth angle = 0 ). Figure 15. Required thermal storage capacity dependency on the chosen solar thermal collector for 36 m² aperture area. DISCUSSION To evaluate the required thermal storage capacity for a thermal self-sufficient residential building a dynamic approach was shown. To that, the storage tank was considered under ideal assumptions (without heat losses, constant temperature difference between supply and return temperature in kind of charging). It could be identified that the collector thermal efficiency influences the accuracy of the dynamic approach (flat plate collector vs. evacuated tube collector). Furthermore, the influence of variation of the slope, azimuth angle and of the aperture area of the solar thermal collector and other factors was demonstrated. On the one hand, a large aperture area is necessary to reduce the thermal storage capacity for a thermal self-sufficient semi-detached energy efficient house in Austria. On the other hand, a solar coverage of appr. 80% should be possible with evacuated tube collectors with an aperture area more than 28 m² and a thermal storage tank with about 1500 liter. The disadvantage of this large aperture area (and a small thermal storage capacity) are long standstill times of the solar collector field in the summer months with a high fluid temperature in the collectors. Therefore, an external shading device to reduce the effective aperture area in the summer months should be considered (was not considered in these investigations). This project was funded by the "Klima- und Energiefonds" and was conducted within the research programme "Energieforschung (e!mission)". REFERENCES Antoniadis Ch. N. and Martinopoulos G. 2016. Optimization of a Building Integrated Solar Thermal System with Seasonal Storage, conference paper, conference: First International Conference on Building Integrated Renewable Energy Systems, At Dublin, Ireland, March 2016 Chotivisarut N., Kiatsiriroat T. and Aye L. 2009. Design of central solar heating with underground seasonal storage in Australia, Asian Journal on Energy and Environment, vol. 10, issue 1, pp. 28-34 Dongellinia M., Falcionia St. and Morinia G. L. 2015. Dynamic simulation of solar thermal collectors for domestic hot water production, ATI 2015-70 th Conference of the ATI Engineering Association, Energy Procedia, vol. 82, pp. 630 636 EPBD 2010/31/EU. 2010. Directive 2010/31/EU of the European Parliament and of the Council of 19 May 2010 on the Energy Performance of Buildings, Official Journal of the European Union, L153/13, Brussel Fuller R. and Aye L. 2007. Seasonal storage for solar thermal systems in Australia?, in Is Solar our only Nuclear option?, ANZSES Solar 07 Pascual C., Martinez A., Epelde M., Marx R. and Bauer D. 2014. Dynamic modeling of seasonal thermal energy storage systems in existing buildings, proceedings from the 55 th conference on simulation and modelling (SIMS 55), 21-22 October, Aalborg, Denmark Thermal Energy System Specialists. 2012. TESS COMPONENT LIBRARIES, Version 17.1.03 (2012-09- 07), Thermal Energy System Specialists, LLC, 22 North Carroll Street, Suite 370, Madison, WI 53703, USA TRNSYS. 2012. A transient system simulation program, TRNSYS Version v17.01.0016, Solar Energy Laboratory, University of Wisconsin, Madison ISBN: 978-0-646-98213-7 COBEE2018-Paper229 page 682