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Ground heat exchangers (GHEs), which are also called geothermal heat exchangers, have emerged as a promising and globally accepted way of exploiting shallow geothermal energy, for example ground-coupled heat pumps, ground heat storage. A GHE is essentially a pipe (e.g., U-, W-, or helical-shaped) in a vertical borehole or a foundation pile of a building, in which a circulating heat-carrying fluid absorbs (or discharges) heat from (or to) the ground.〔Li M, Lai ACK. Review of analytical models for heat transfer by vertical ground heat exchangers (GHEs): A perspective of time and space scales, Applied Energy 20015; 151: 178-191.〕〔Hellstrom G. Ground heat storage – thermal analysis of duct storage systems I. Theory. Lund: University of Lund; 1991.〕 GHEs can have various configurations. This article discusses two kinds of closed loop GHEs, i.e., borehole and foundation pile GHEs. The borehole type is the most common. It consists of one or two U-shaped pipes that are inserted into a vertical borehole and connected to a heat pump or a heating system to form a closed loop. A U-shaped channel usually comprises two small-diameter high-density polyethylene (HDPE) tubes thermally fused to form a U-shaped bend at the bottom.〔ASHRAE. ASHRAE handbook: HVAC applications. Atlanta: ASHRAE, Inc; 2011.〕 The space between the wall of the borehole and the U-shaped tubes is usually grouted completely with grouting material or, in some cases, partially filled with groundwater.〔Kavanaugh SK, Rafferty K. Ground-source heat pumps: Design of geothermal systems for commercial and institutional buildings. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.; 1997.〕 The depth of the hole (generally from 30 m to 200 m) depends strongly on local geological conditions and available drilling equipment. In a foundation pile GHE (or energy pile), the heat transfer tubes are inside the steel frame of a foundation pile. There are various possible shapes. Foundation piles are usually much shallower than boreholes and have a greater radius. Since energy piles generally require less land area, this technology is evoking increasing interest in the ground-source heat pumps community. == Analysis of heat transfer by GHEs == A huge challenge in predicting the thermal response of a GHE is the diversity of the time and space scales involved.Four space scales and eight time scales are involved in the heat transfer of GHEs. The first space scale having practical importance is the diameter of the borehole (~ 0.1 m) and the associated time is on the order of 1 hr, during which the effect of the heat capacity of the backfilling material is significant. The second important space dimension is the half distance between two adjacent boreholes, which is on the order of several meters. The corresponding time is on the order of a month, during which the thermal interaction between adjacent boreholes is important. The largest space scale can be tens of meters or more, such as the half length of a borehole and the horizontal scale of a GHE cluster. The time scale involved is as long as the lifetime of a GHE (decades).〔Li M, Li P, Chan V, Lai ACK. Full-scale temperature response function (G-function) for heat transfer by borehole ground heat exchangers (GHEs) from sub-hour to decades. Appl Energy 2014; 136: 197-205.〕 The short-term hourly temperature response of the ground is vital for analyzing the energy of ground-source heat pump systems and for their optimum control and operation. By contrast, the long-term response determines the overall feasibility of a system from the standpoint of life cycle. Addressing the complete spectrum of time scales require vast computational resources. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ground heat exchanger」の詳細全文を読む スポンサード リンク
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