Comparative Heating Performances of Ground Source and Air Source Heat. Pump Systems for Residential Buildings in Shanghai

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1 Comparative Heating Performances of Ground Source and Air Source Heat Pump Systems for Residential Buildings in Shanghai Zhaohui Liu 1,2, Hongwei Tan 1,2,3* 1 School of Mechanical Engineering, Tongji University, Shanghai, China 2 Research Center of Green Building and New Energy, Tongji University, Shanghai, China 3 C UNEP-Tongji Institute of Environment for Sustainable Development, Tongji University, Shanghai, China * Corresponding hw_tan@tongji.edu.cn SUMMARY As the main heat sources/sinks of heat pump systems, the ambient air and ground may have different performance characteristics. In this study, the performance comparison of an Air Source Heat Pump (ASHP) and a vertical Ground Source Heat Pump (GSHP) for space heating is presented. Both systems were installed in a residential building in Shanghai, and the ASHP and GSHP were tested in a same room under the similar weather conditions. The performance of the ASHP system largely depends on the ambient conditions. Under the same outdoor air temperature conditions, the power consumptions of the ASHP on rainy days were higher than that on sunny days, and the COPs of the heat pump unit and the heat pump system on sunny days were higher than that on rainy days. During the heating season, the weighted average COPs of the ASHP unit and the heat pump system are 2.44 and 1.9, respectively. The seasonal average COPs of the GSHP unit and the heat pump system are 5.21 and 3.03, respectively. The experimental results indicate that the GSHP system was preferable to the ASHP system in terms of energy efficiency for space heating. PRACTICAL IMPLICATIONS The aim of the present work is to analyze the relative heating performance of ASHP and GSHP system in a residential building. The purpose is to help the designer and user make decision in selection of cold and heat sources for references. KEYWORDS Power consumption, Transmission power ratio, Outdoor air temperature, Coefficient of performance, Weather condition 1 INTRODUCTION Heat pumps are increasingly popular for heating and cooling in commercial and residential buildings due to their features of sustainability and superior energy efficiency. Among these, the air source heat pump (ASHP) and ground source heat pump (GSHP) systems are probably the most competitive technologies in the current market. Because of the different properties

2 between the ambient air and ground, the ASHP and GSHP system may have different performance characteristics. GSHP systems are promising due to their consistent performance year round (Nagano et al. 2006). In contrast to ground, ambient air is a superior source for heat pump in terms of availability, but the ambient air annual temperature fluctuation is detrimental to the performance of the ASHP system (Soltani et al. 2015), and The major problem of ASHP systems is that their heating capacity degrades as the outdoor air temperature drops below 0 ºC (Bertsch et al. 2008). However, the energy performance differences between the ASHP and GSHP system are not very clear. The performance comparison based on the laboratory data is not representative of site running performance, which take into account the typical transient behaviour and load patterns of real life systems (Urchueguía et al. 2008). Therefore, the performance comparison between the actual operation of the ASHP and GSHP systems during similar conditions is needed. Because the ground has relatively stable temperature, the coefficient of performance (COP) of GSHPs is usually higher than that of ASHPs (Rad et al. 2013; Wang et al. 2012). Petit et al. (1997) presented a techno-economic comparison of the ASHP system and horizontal GSHP system in South Africa. And Esen et al. (2007) reported a techno-economic comparison between a GSHP system and an ASHP system in Turkey. The aim of the present work is to analyze the relative heating performance of ASHP and GSHP system in a residential building. Both systems ASHP and GSHP were installed in parallel in the same building, and large amounts of monitoring data were obtained to analyze the heating performance and characteristics. 2 EXPERIMENTAL SETUP 2.1 House description The house demonstrates the environmental-friendly and sustainable building technologies, which is located at Tongji university, Shanghai, China. The Vacuum Insulation Panel (VIP) is employed in the source-side walls. The high-level insulation and source-side shading allow the cooling/heating load for HVAC system much less time than the ordinary house. The building area is about 58 m 2, and the Load peak value in heating mode was sized at 3 kw. 2.2 Description of the heat pump systems Figure 1. Schematic diagram of ASHP and GSHP system

3 The ground source heat pump and the air to water heat pump are installed for space heating/cooling. Figure 1 shows a schematic diagram for the heat pump systems. The both heat pumps are linked in parallel. However, only one of the heat pumps can work at the same time. The ASHP system is composed of four circuits: condenser fan, air circuit, the refrigerant circuit, water circuit and Air Handling Unit (AHU) circuit. The GSHP system consists of four circuits: the ground heat exchanger (GHE) circuit, the refrigerant circuit, water circuit and AHU circuit. The GSHP system consists of one vertical GHE of 100m depth. The specification and characteristics of the heat pump systems for the house are given in Table 1. Table 1. The specification of the ASHP and GSHP systems Equipment Technical information Heat pump Heat pump Ground heat exchanger(ghe) Manufacturer: DAIKIN; Model: NRZQA56AV2C; Heating capacity: 5.75KW; Power Input: 1.16KW Refrigerant: R410A COP: 4.96; Cooling capacity: 5.12KW; Power Input:1.57KW COP:3.26 Manufacturer: DAIKIN; Model: NRZQA56AV2C modified; Heating capacity: 5.75KW; Power Input: 1.16KW Refrigerant: R410A; COP: 4.96 Cooling capacity:5.12kw; Power Input:1.57KW COP:3.26 Type: vertical single U-tube; Pipe diameter:0.032m material: polyethylene; Pipe thickness:0.0035m borehole depth:100m 2.3 Test apparatus and measuring equipment The data acquisition system was designed to characterize the system performance. Several sensors were installed to monitor the most relevant parameters of the system. T-type thermocouples were installed to measure the temperature the indoor air temperature, the outdoor air temperature. Three wire PT100 temperature sensors were used to measure the inlet and outlet water temperature of source-side (GSHP) and the inlet and outlet water temperature of user-side (AHU), The source-side fluid flow rate (GSHP) and user-side (AHU) fluid flow rate were measured using an ultrasonic flowmeter. The power consumption of GSHP, ASHP, source-side circulating pump, refrigerant-water heat exchange unit (HD) and Air Handling Unit (AHU) were measured using power meters. Data from the temperature sensors are collected by a data acquisition module ICPCON I-7015P. 3 RESULTS AND DISCUSSION 3.1 Power consumption Figure.2 and Figure.3 respectively demonstrate hourly average power consumption for the ASHP system and the GSHP system in heating mode during a special day. The daily average power consumptions of the ASHP system and GSHP system are 2.25kW and 1.21kW, respectively. The power consumption of the GSHP system is more stable compared with the ASHP system. It is due to the performance of the ASHP system depend on the ambient conditions more than the GSHP system. The daily average power consumptions of the ASHP system is 86% higher than that of the GSHP system. For the ASHP system, the power

4 Figure 2. Hourly electrical energy consumption for the ASHP system in heating mode during a special day Figure 3. Hourly power consumption for the GSHP system in heating mode during a special day consumption of energy transport components (user-side circulating pump and AHU) accounts for about 19% of the total power consumption. For the GSHP system, the power consumption of energy transport components (the source-side circulating pump, the user-side circulating pump, AHU) accounts for about 43% of the total power consumption. The transport power ratio of the GSHP system is higher than that of the ASHP system, it can be attributed to two factors. First, the average power consumption (0.69 kw) of the GSHP unit is 38% of that of the ASHP system. Second, compared to the ASHP system, the source-side circulating pump for the GHE is employed in the GSHP system, and it leads to increasing of the power consumption of energy transport components. To a large extent, the performance of the ASHP system depends on the ambient conditions. Figure. 4 and Figure.5 display the power consumption results on sunny days and rainy days, respectively. From Figure.4 and Figure.5, it can be seen that the power consumption increased with the decreasing of the outdoor air temperature. Under the same outdoor air temperature condition, the power consumptions of the heat pump unit on rainy days were higher than that on sunny days. Take the same outdoor air temperature 4 ºC on both sunny days and rainy days Figure 4. The power consumption variations with outdoor air temperature for the ASHP system on sunny days (for heating) Figure 5. The power consumption variations with outdoor air temperature for the ASHP system on rainy days (for heating)

5 for instance, the power consumptions of the ASHP unit were 1.49 kw and 2.06 kw, respectively. And the power consumption on rainy days increased by 38% over that on sunny days. The power consumption of heat pump was higher when it was defrosting. Because the evaporator of heat pump frosted more easily on rainy days. The relative humidity is high on rainy days. From the instantaneous power consumption, it can be found that the heat pump began to defrost when the outdoor air temperature was lower than 6 ºC in rainy days. And in the sunny day, the heat pump began to defrost when the outdoor air temperature was lower than -5 ºC. In other words, the temperature range for defrosting condition on rainy days was wider than sunny days. Figure 6. The transmission power ratio variations with outdoor air temperature for the ASHP system on sunny days (for heating) Figure 7. The transmission power ratio variations with outdoor air temperature for the ASHP system on rainy days (for heating) Figure. 6 and Figure.7 indicate the transmission power ratio variations with outdoor air temperature for the ASHP system on sunny days and rainy days, respectively. At runtime, keeping the power consumption of energy transport components constant. The transmission power ratio increased with the increasing of the outdoor air temperature due to the power consumption of the heat pump unit decreased with the increasing of the outdoor air temperature. Under the same outdoor air temperature condition, the transmission power ratio of the ASHP system on sunny days was higher than that on rainy days. Take the same outdoor air temperature 4 ºC on both sunny days and rainy days for instance, the transmission power ratios of the ASHP system were 0.22 and 0.17, respectively. And the transmission power ratio of the ASHP system on sunny days increased by 29% over that on rainy days. 3.2 Coefficient of performance (COP) of the heat pump system It can be found that the changing trend of COP was consistent with the trend of outdoor air temperature. From several days experimental data, the COPs of the ASHP system on sunny days and rainy days were investigated with respect to the outdoor air temperature as shown in Figure.8 and Figure.9, respectively. From Figure.8 and Figure.9, it can be seen that the COPs of the ASHP and the ASHP system increased with the increasing of the outdoor air temperature. As the outdoor air temperature increased from -6 ºC to 6 ºC on sunny days, the COP of the ASHP unit itself increased linearly from 1.85 to 2.42 by 31%, and the COP of the ASHP system

6 Figure 8. Hourly average COP variations with hourly average outdoor air temperature for ASHP system on sunny days (for heating) Figure 9. Hourly average COP variations with hourly average outdoor air temperature for ASHP system on rainy days (for heating) increased linearly from 1.55 to 1.89, by 22%. In addition, as shown in Figure.9, as the outdoor air temperature increased from 1.5 ºC to 8.5 ºC on rainy days, the COP of the ASHP unit itself increased linearly from 1.33 to 1.82 by 37%, and the COP of the ASHP system increased linearly from 1.15 to 1.44, by 25%. Under the same outdoor air temperature conditions, the COPs of the heat pump unit and the heat pump system on sunny days were higher than that on rainy days. The power consumption of heat pump was higher when it was defrosting and the higher power consumption results in lower COP. Take the same ambient condition of outdoor air temperature 4 ºC on both sunny days and rainy days for instance, the COPs of the ASHP unit were 2.33 and 1.51, respectively. And the power consumption decreased by 35%. Figure 10. Hourly average COP variations with the difference between the indoor air temperature and the outlet temperature of GHE (for GSHP heating) Figure 11. Hourly average COP variations with the difference between the indoor air temperature and the outdoor air temperature (for ASHP heating)

7 Shown as Figure.10, the COPs of the GSHP system decreased with the increasing of the difference between the indoor air temperature and the outlet temperature of GHE. And the average COPs of the GSHP unit and GSHP system are 5.21 and 3.03, respectively. And Figure 11. displays that the COPs of the ASHP system decreased with the increase of the difference between the indoor air temperature and the outdoor air temperature. Compared Figure.11 with Figure.10, it can be found that the difference between the indoor air temperature and the outlet temperature of GHE was smaller than that between indoor air temperature and the outdoor air temperature, and the temperature difference had effects on the performance of heat pump. To compare the performance of ASHP system with GSHP system, assuming the ASHP system run under the outdoor air temperature 5.8 ºC which is the monthly average temperature of January, February and December. According to the relationship between COP and the outdoor air temperature shown as Figure 10. The COPs of ASHP unit itself and the ASHP system under the outdoor air temperature 5.8 ºC can be calculated to be 2.41 and 1.88, respectively. The COPs of GSHP unit and GSHP system increase 116 percent and 61 percent than that of ASHP unit and ASHP system, respectively. The significant improvements in COPs of the GSHP system over the ASHP system can be ascribed to several main factors. Firstly, the temperature difference between the evaporation and condensation of GSHP system is lower than ASHP system. And it is because of the average outdoor air temperature is lower than the ground temperature. Besides, the specific of air is significantly lower than that of water, which resulted in higher power consumption of the ASHP unit over GSHP unit. In addition, the frosting and defrosting conditions of the ASHP system lead to the higher power consumption. 3.3 Heating seasonal performance Figure 12. Bin distribution for the heating season (Nov.27th - Mar. 21th) in Shanghai, China The relationship between COP and the outdoor air temperature was employed to estimate the heating seasonal performance of ASHP system. The bin data for Shanghai city in China were utilized to calculate the hourly COPs during the heating season. The heating season starts on November 27th and ends on March 21th, assuming the outdoor air bin temperature range was

8 varied from -6 ºC to 15 ºC for space heating. The bin distribution of this range in 1 ºC increments shown as Figure 12. According the relationship between COP and the outdoor air temperature described in Fig 8, the weighted average COPs of the ASHP unit and the heat pump system are 2.44 and 1.9, respectively, during the heating season. The seasonal average COPs of the GSHP unit and the heat pump system are 5.21 and 3.03, respectively. During the heating season, the average COPs of GSHP unit and GSHP system increase 114 percent and 59 percent than that of ASHP unit and ASHP system, respectively. 4 CONCLUSIONS In this study, a performance comparison between the ASHP system and GSHP system for space heating was conducted in hot summer and cold winter zone. The performance of the ASHP system largely depends on the ambient conditions. Under the same outdoor air temperature conditions (4 ºC) the power consumptions of the ASHP on rainy days were higher (38%) than that on sunny days, and the COPs of the heat pump unit and the heat pump system on sunny days were higher (54%)than that on rainy days. For the ASHP system and the GSHP system, the power consumption of energy transport components accounts for about 19% and 43% of the total power consumption, respectively. The average COPs of GSHP unit and GSHP system were 114% and 59% higher than that of ASHP unit and ASHP system, respectively. In sum, for space heating, the GSHP system could perform better than the ASHP system in terms of energy efficiency. 5 REFERENCES Bertsch, S.S., Groll, E.A Two-stage air-source heat pump for residential heating and cooling applications in northern U.S. climates. International Journal of Refrigeration, 31(7), Esen, H., Inalli, M., Esen, M A techno-economic comparison of ground-coupled and aircoupled heat pump system for space cooling. Building and Environment, 42(5), Nagano, K., Katsura, T., Takeda, S Development of a design and performance prediction tool for the ground source heat pump system. Applied Thermal Engineering, 26(14), Petit, P., Meyer, J A techno-economic analytical comparison of the performance of airsource and horizontal-ground-source air-conditioners in South Africa. International Journal of Energy Research, 21(11), Rad, F.M., Fung, A.S., Leong, W.H Feasibility of combined solar thermal and ground source heat pump systems in cold climate, Canada. Energy and Buildings, 61, Soltani, R., Dincer, I., Rosen, M.A Comparative performance evaluation of cascaded air-source hydronic heat pumps. Energy Conversion and Management, 89, Urchueguía, J.F., Zacarés, M., Corberán, J.M., Montero, Á., Martos, J., Witte, H Comparison between the energy performance of a ground coupled water to water heat pump system and an air to water heat pump system for heating and cooling in typical conditions of the European Mediterranean coast. Energy Conversion and Management, 49(10), Wang, E., Fung, A.S., Qi, C., Leong, W.H Performance prediction of a hybrid solar ground-source heat pump system. Energy and Buildings, 47,