5:15 PM - 6:30 PM
[AHW24-P03] Demonstration of Ground Source Heat Pump systems and variations of the subsurface temperatures in Saitama Prefecture
Keywords:ground source heat pump, subsurface temperature, groundwater flow
Geothermal energy systems are a type of renewable energy system. They are superior in energy conservation and are spreading in Japan and overseas. The ground-source heat-pump (GHP) system is widely used for room heating and cooling and hot water supply in large facilities and housings. It is important to quantitatively compare the operating efficiency, cost, and environmental effects of GHP and air source heat pumps (AHP) through experiments to promote the use of GHP in the future. The operating efficiency of both heat pumps varies with the weather conditions and heat loads at the installation sites and only the efficiency of GHP varies with the subsurface environmental conditions.
In 2018, the Local Government of Saitama Prefecture installed both GHP and AHP in five small-scale buildings in the prefecture. Among the five sites, four sites (Hanno City, Hanyu City, Miyashiro Town, and Kasukabe City) contain atmosphere-monitoring stations and the fifth (Kazo City) site is the Eco-Lodge of the Center for Environmental Science in Saitama. At the atmosphere-monitoring stations in four sites where the demonstration experiments were conducted, the continuous cooling and heating operation tests were conducted at the target room temperature (Cooling: 27°C, Heating: 25°C) of general housing to examine the operation efficiency of the GHP and AHP. Specifically, either of the HPs was operated alternately in summer and winter at 15-day intervals, and the SCOP was calculated by measuring the inlet/outlet temperatures, the circulating flow rate of the secondary circulating fluid, and power consumption. As a result, the average SCOP of the four sites in the cooling operation was 5.7 and 2.6 for the GHP and AHP, respectively, and 3.0 and 2.4 for GHP and AHP, respectively, in the heating operation. The efficiency of the GHP was higher than that of the AHP in all sites. The efficiency of the cooling operation was higher than that of the heating operation in the same site, and this tendency is consistent with the results of existing demonstration experiments.
In two sites (Kazo City and Miyashiro Town), observation wells (six wells at 2 m and one well at 5 m from the heat exchange well) were installed to investigate the temperature fluctuation in the vicinity of the heat exchange wells. In the eco-lodge, cooling and heating operations were conducted at the maximum load. The GHP was operated under a high load for 30 days to monitor the process of the rise and fall in subsurface temperature. The operation was halted for the next 60 days and the recovery process was monitored. This paper presents the results of the cooling operation. The temperature increased by 2°C at the observation well, 2 m away from the heat exchange well (West 2 m well). And large temperature rise was not observed at the well 5 m away (West 5 m well). Next, assuming that the heat propagates from the heat exchange well to the surroundings by heat diffusion, the temperature variation was calculated using a numerical simulator (FEFLOW) and employing the finite element method. The attached figure shows the areal temperature distribution 90 days after starting the monitoring at a depth of 40 m. It was confirmed that heat spreads concentrically from the heat exchange well to the circumference. The measured and calculated values at the observation well 2 m west were compared and they are consistent. This could be well explained by assuming that the heat propagates by thermal diffusion. In more detail, however, the measured temperatures at the west 2 m observation well and the east 2 m well increased at the same rate during the heating process, whereas the temperatures differed by 0.5°C in the recovery process (the east 2 m well was hotter than the west 2 m well). This suggests that variations in thermal properties and groundwater flow influence the observed temperatures. These factors shall be considered in our future studies.
In 2018, the Local Government of Saitama Prefecture installed both GHP and AHP in five small-scale buildings in the prefecture. Among the five sites, four sites (Hanno City, Hanyu City, Miyashiro Town, and Kasukabe City) contain atmosphere-monitoring stations and the fifth (Kazo City) site is the Eco-Lodge of the Center for Environmental Science in Saitama. At the atmosphere-monitoring stations in four sites where the demonstration experiments were conducted, the continuous cooling and heating operation tests were conducted at the target room temperature (Cooling: 27°C, Heating: 25°C) of general housing to examine the operation efficiency of the GHP and AHP. Specifically, either of the HPs was operated alternately in summer and winter at 15-day intervals, and the SCOP was calculated by measuring the inlet/outlet temperatures, the circulating flow rate of the secondary circulating fluid, and power consumption. As a result, the average SCOP of the four sites in the cooling operation was 5.7 and 2.6 for the GHP and AHP, respectively, and 3.0 and 2.4 for GHP and AHP, respectively, in the heating operation. The efficiency of the GHP was higher than that of the AHP in all sites. The efficiency of the cooling operation was higher than that of the heating operation in the same site, and this tendency is consistent with the results of existing demonstration experiments.
In two sites (Kazo City and Miyashiro Town), observation wells (six wells at 2 m and one well at 5 m from the heat exchange well) were installed to investigate the temperature fluctuation in the vicinity of the heat exchange wells. In the eco-lodge, cooling and heating operations were conducted at the maximum load. The GHP was operated under a high load for 30 days to monitor the process of the rise and fall in subsurface temperature. The operation was halted for the next 60 days and the recovery process was monitored. This paper presents the results of the cooling operation. The temperature increased by 2°C at the observation well, 2 m away from the heat exchange well (West 2 m well). And large temperature rise was not observed at the well 5 m away (West 5 m well). Next, assuming that the heat propagates from the heat exchange well to the surroundings by heat diffusion, the temperature variation was calculated using a numerical simulator (FEFLOW) and employing the finite element method. The attached figure shows the areal temperature distribution 90 days after starting the monitoring at a depth of 40 m. It was confirmed that heat spreads concentrically from the heat exchange well to the circumference. The measured and calculated values at the observation well 2 m west were compared and they are consistent. This could be well explained by assuming that the heat propagates by thermal diffusion. In more detail, however, the measured temperatures at the west 2 m observation well and the east 2 m well increased at the same rate during the heating process, whereas the temperatures differed by 0.5°C in the recovery process (the east 2 m well was hotter than the west 2 m well). This suggests that variations in thermal properties and groundwater flow influence the observed temperatures. These factors shall be considered in our future studies.