Performance Of Borehole Heat Exchanger Research Articles

Improved method and case study of thermal response test for borehole heat exchangers of ground source heat pump system

Thermal response test (TRT) is crucial for the determination of the ground thermal conductivity and the evaluation of the thermal performance of borehole heat exchangers (BHEs) of ground source heat pump (GSHP) system. This paper presented a novel constant heating-temperature method (CHTM) for TRT. Further, a type of improved TRT equipment was developed and the mathematical model to deal with test data was presented. Based on the measurement of the natural ground temperature distribution, an in situ TRT case was carried out. The experimental results showed that, compared with the conventional TRT with constant heating-flux method (CHFM), CHTM has an obvious advantage of reducing the time period reaching a steady heat-transfer state between the BHE and its surrounding soils. This improved TRT equipment can effectively operate under both heat-injection and heat-extraction modes, and its test data can accurately reflect the thermal properties of the soils as well as the thermal performance of the BHE under different operation conditions. Finally, the advantages and disadvantages between CHTM and CHFM were compared, which can provide a useful reference for the design of GSHP system.

Thermal performance of borehole heat exchanger under groundwater flow: A case study from Baoding

The performance of ground source heat pumps (GSHPs) depends strongly on the heat transfer between the soil and borehole heat exchangers (BHEs). In the present work, a thermal performance experiment of a BHE under groundwater flow was conducted in Baoding, China. Based on the measurement of the natural ground temperature profile, a simplified theoretical model was presented to estimate the characteristics of groundwater flow. The results showed that the presence of groundwater had an obvious influence on the temperature profile in the aquifer. Due to the strong groundwater advection, the thermal performance of the BHE was enhanced. The enhanced effect depends to a great extent on the distribution and thickness percentage of the ground layer with the greatest groundwater flow. In the present case, the heat injection and heat extraction of the BHE were enhanced on average by 9.8% and 12.9%, respectively, compared with the case without groundwater flow, when the total thickness of coarse sand and gravel layer as a percentage of the borehole depth was 10.6%. This enhanced effect is favorable for reducing the possible imbalance between heat injection and extraction from and to the ground, which is helpful for the long-term operation of GSHP systems.

Lecture Note : Geothermal Heat Pump System On the Shallow Subsurtace Thermal Regime and Groundwater Flow (3) Groundwater Flow Effect on the Performance of Borehole Heat Exchanger

Lecture Note : Geothermal Heat Pump System On the Shallow Subsurtace Thermal Regime and Groundwater Flow (3) Groundwater Flow Effect on the Performance of Borehole Heat Exchanger

Comparison of the thermal performance of double U-pipe borehole heat exchangers measured in situ

A borehole heat exchanger is a ground heat exchanger devised for the extraction or injection of thermal energy from/into the ground. The thermal performance of a borehole heat exchanger can be assessed with a response test. The response test method allows the in situ determination of the thermal conductivity of the ground in the vicinity of a borehole heat exchanger, as well as the effective thermal resistance of this latter. The response test method is briefly described before it is applied to several designs of double U-pipe borehole heat exchangers. The tests have shown the viability of the method. They reveal that the thermal resistance can be decreased by 30% when quartz sand is used instead of bentonite and when spacers are used to keep the plastic pipes in contact with the borehole wall. With a common heat extraction rate of 50 W/m of borehole length, the temperature gain in a heat pump evaporator is +2 K. Finally, a mobile device has been developed to offer the possibility of accomplishing a response test.