| Abstract |
A large-scale geothermal heat pump (GHP) system, located in Hokkaido, Japan, consists of 78 vertical borehole heat exchangers (BHEs) of 85 meters deep. The GHP system has been operated since December 2008 for cooling and heating of the greenhouses in which Phalaenopsis orchid are cultivated. Since the heating load is much higher than the cooling load in Hokkaido, maintaining the operation performance of the system requires the consideration on the heat balance between heat extraction and disposal. Moreover it is also important to evaluate the influence of long-term heat exchange on ground temperatures because the influences are accumulated with time. Thus, in this research, a heat transport and groundwater flow numerical model (GHP system model), which has 78 vertical BHEs, simulates long-term heat pump operation to predict the future behavior of the heat exchange and to investigate the influence of GHP on the ground temperatures. The GHP system model requires the evaluation of underground physical properties as input such as ground thermal and hydraulic conductivities. In this study, these parameters are estimated using single borehole heat exchanger numerical model (Single BHE model). The estimated parameters are then used to construct the GHP system model, which is validated through a history matching. Then we performed the long-term simulations using the validated model with 4 cases of different balances of heat load per year, which consist of heating period of 9 months and cooling periods of 0-3 months. The heat extraction rate in the case of 1 month cooling decreases in the long run, while the heat extraction rate is well-maintained in the case of 2 month cooling period. As a consequence, even if amount of heat disposal and heat extraction with ground are not balanced, it is still possible to maintain the heat exchange rate for long time period. |