Heat Pump Ground Coil Analysis With Thermal Interference

Abstract

When two horizontal ground coils are buried in the same trench, one on top of the other, the total energy exchange between coils and ground increases drastically over that of a single coil for the same trench length. However, because of the thermal interference between the two coils, the overall performance of the two-coil trench is very difficult to determine. Traditionally this type of problem is handled by the line source mirror-image approach. However, in applying the line source approach, the strength of the source is not known precisely and must be estimated. Most designers will estimate this value very conservatively in order to be on the safe side, and this will result in a longer coil than is actually needed. This paper provides a detailed mathematical model to describe the operation of the two-coil system that can calculate more realistically the performance of the ground coils, which in turn could make the ground coil heat pump system more economically competitive with other heating and cooling systems. The effect of thermal interference is clearly shown in the calculated soil temperature profile. The effect of fluid inlet positions, from upper or lower coils, is also studied. The comparison of measured and calculated daily energy absorbed from the ground indicated a maximum error of 27 percent, with the average error at less than 12 percent, calculated values are on the conservative side. This model is better than the line source approach for calculating the performance of two coils in the same trench. It can be used for design purposes, or used to check such coil designs by other methods.