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. 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. 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. 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.
Contents
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).
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.
Formalization of the heat transfer problem
The main questions that engineers may ask in the early stages of designing a GHE are (a) what the heat transfer rate of a GHE as a function of time is, given a particular temperature difference between the circulating fluid and the ground, and (b) what the temperature difference as a function of time is, given a required heat exchange rate. In the language of heat transfer, the two questions can probably be expressed as
where Tf is the average temperature of the circulating fluid, T0 is the effective, undisturbed temperature of the ground, ql is the heat transfer rate of the GHE per unit time per unit length (W/m), and R is the total thermal resistance (m.K/W).R(t) is often an unknown variable that needs to be determined by heat transfer analysis. Despite R(t) being a function of time, analytical models exclusively decompose it into a time-independent part and a time-dependent part to simplify the analysis.
Various models for the time-independent and time-dependent R can be found in the references.