Groundwater survey and heat transport, modeling for optimum design of geothermal heat pump systems in Akita Plain

为了有效利用浅层矿床的地源热泵系统，对当地地下水流动系统的了解是非常重要的。本研究在日本秋田冲积平原进行，以澄清以下事项：(1)利用野外调查结果进行地下水流系统分析；(2)秋田平原全田地下水流模型的建立；(3)大型地源热泵系统长期运行性能的预测。在浅层钻孔中，测量地下水位，绘制地下水位图。在深井中，每隔21N测量一次地下水温度，以收集沿井的地下水流动剖面。在地下水流动模拟方面，利用三维地下水与热运移软件FEFLOW建立了秋田平原全田地下水模型。数值模拟结果表明，阿基特平原中部地下水平均流速为1.38×10~(-4)(m/d)，由东向西。现场测量结果与数值模拟结果吻合较好。单个地下热交换器(GHE)模型表明，在开采的含水层中存在Peclet数等于或大于0.1的地下水时，可以提高热采取率。利用多井模型，确定了每年至少两个月的蓄热运行的重要性。多井模型还表明，在不包括夏季储热操作的地源热电联产系统中，较大的地源热能间距是可取的。

For the efficient use of Ground Coupled Heat Pump (GCHP) systems on shallow deposits, the understanding of the local groundwater flow system is highly important. This research was conducted in Akita Alluvial Plain, Japan, to clarify the following items : (1) analysis of groundwater flow systems using field survey result, (2) development of field-wide groundwater flow model in Akita Plain, (3) prediction of long-term operation performance of a large-scale GCHP system.In field surveys, groundwater samplings were carried out in deep and shallow boreholes. In shallow boreholes, groundwater levels were measured to draw groundwater level maps. In deep boreholes, groundwater temperatures were measured every 21n to collectt groundwater flow profiles along wells. PH and EC were measured at well sites.For groundwater flow modeling, 3D groundwater and heat transport software "FEFLOW" was used to construct a field-wide groundwater model in Akita Plain. Numerical simulations showed that the average groundwater velocity in the central part of Akit.a Plain is 1. 38X 10-4 (m/d), from east to west. The field survey results showed good agreements with numerical simulation results. The model of a single Ground Heat Exchanger (GHE) showed that heat extraction rates are enhanced if groundwater flows with Peclet numbers equal or higher than 0.1 exist in the aquifer under exploitation. With the multi-well model, the importance of heat storage operation of att least two months per year was determined. The multi-well model also showed that larger spacing between GHEs is preferable in GCHP system that will not include heat storage operations during the summer months.