Abstract
Recently, the medium-depth geothermal heat pump systems (MD-GHPs) have been applied for space heating in China. Theoretically, the MD-GHPs use deep borehole heat exchangers (DBHEs) to extract heat from the medium-depth geothermal energy with the depth of 2~3 km, thus, improving the energy performance of whole systems obviously. This paper conducts field tests of nine conventional shallow-depth geothermal heat pump systems (SD-GHPs) and eight MD-GHPs to analyze the energy performance of heat pump systems, as well as heat transfer performance of ground heat exchangers. Then the comparative studies are carried out to analyze the difference between these two ground coupled heat pump systems. Field test results show that the outlet water temperature of DBHEs in MD-GHP can reach more than 30 °C with heat extraction of 195.2 kW~302.8 kW per DBHE with a depth of 2500 m, which are much higher than that of SD-GHPs. However, the heat pumps and water pumps in the ground side should be specially designed to fit the high-temperature heat source instead of following operation mode of SD-GHPs. Then with variable speed compressor which has high energy efficiency under a wide range of load rate and compressor ratio, and with the ground-side water pumps which efficiently operate under high water resistance and low flow rate, the COP of heat pumps and COPs of whole systems could reach 7.80 and 6.46 separately. Thus, the advantage of high-temperature heat source could be fully utilized to achieve great energy-saving effects.
Highlights
Space heating plays an important role and accounts for nearly 21% of energy consumption in buildings [1]
The total space heating area is 1.13 million m2, where 0.61 million m2 are equipped with shallow-depth geothermal heat pump systems (SD-GHPs), and 0.52 million m2 are equipped with medium-depth geothermal heat pump systems (MD-GHPs)
As for the heat transfer medium, water is applied in MD-GHPs, while glycol solution is commonly applied in SD-GHPs for anti-freezing
Summary
Space heating plays an important role and accounts for nearly 21% of energy consumption in buildings [1]. Among various kinds of heat pump systems, the ground-coupled heat pump systems (GCHPs), which extract heat from the geothermal energy, can perform efficiently and stably [4] It has been widely applied and studied worldwide [5]. Benefiting from high-temperature heat source, the medium-depth geothermal heat pump systems (MD-GHPs) could perform better than conventional GCHPs, which is named shallow-depth geothermal heat pump systems (SD-GHPs) in this paper. This method was first introduced and carried out in America and Europe, where the outlet water temperature of DBHEs could reach nearly 98 ◦ C in Hawaii [13] and 40 ◦ C in Switzerland [14]. Based on the field test results, the differences between these two GCHPs are figured out, so as to guide the system design, as well as optimize the control strategy of MD-GHPs
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