Modeling of Vertical Ground Loop Heat Exchangers with Variable Convective Resistance and Thermal Mass of the Fluid

The ability to predict the short-term behavior of ground loop heat exchangers (GLHE) is critical to the design and energy analysis of ground source heat pump (GSHP) systems. Thermal load profiles vary significantly from building to building – GLHE designs can be dominated by long-term heat build-up or short-term peak loads. In some extreme cases, where the GLHE design is dominated by short-term peak loads, temperatures in the GLHE can rise rapidly; say 5-10oC in one to two hours. For such short-term peak loads, the thermal mass of the fluid can significantly dampen the temperature response of the ground loop. The over prediction of the temperature rise (or fall) in turn can cause an over prediction of the required GLHE length. Furthermore, the temperature response can be damped by the fluid in the rest of the system, in addition to the fluid in the borehole. The temperature response also has a secondary impact on the predicted energy consumption of the system, as the COP of the heat pump varies with entering fluid temperature. Therefore, it is desirable to be able to model the short-term behavior accurately.