| Abstract |
In ground source heat pump (GSHP) systems, the enhancement of heat exchange rate per unit length of a ground heat exchanger (GHE) is an important factor with regards to the initial cost of the system. When the GHE is installed in a formation with fast groundwater flow, the heat exchange rates are generally larger than the case without groundwater flow due to the thermal advection effects of the groundwater flow. Alternatively, when the groundwater velocity is slow, the application of the GSHP system is more difficult due to the small thermal advection, which can results in high initial cost of installation. This research hence focuses on the enhancement of heat exchange rate of GHEs by water injection from land surface into the GHEs, which causes an artificial vertical groundwater flow in the GHEs. A series of thermal response tests (TRTs) was first carried out in an ungrouted GHE drilled in Fukuoka City, Japan. In the TRTs, water injection rates were varies as 0, 5, 10, 20 L/min, while keeping the heat load and the circulation rate constant. The results of TRTs showed that the water injection suppressed the rise of heat medium temperatures, which could improve the COP (Coefficient of Performance) of heat pumps. The rate of increase in heat medium temperatures strongly depended on the water injection rates, while excessive injection rates were found to be inefficient. During the TRT, the vertical temperature profiles in the GHE were monitored using an optical fiber thermometer, which showed that the water injection accelerated the recovery of ground temperatures after heating. A numerical model of the GHE was then constructed based on geological and groundwater information at the TRT location. The model was validated using the TRT results and was used in sensitivity studies to clarify the effect of injected water on the heat exchange performance. The sensitivity studies showed that a small-scale water injection of 1?5 L/min could improve the heat exchange rate by 20-52 %, while the rate of increase diminished when the injection rate exceeds 5 L/min. The sensitivity study also showed that the temperature of injected water has strong influence on the enhancement of heat exchange rates. |