Assessment and comparison of different arrangements of horizontal ground heat exchangers for high energy required applications

Abstract

Horizontal ground heat exchangers (GHEs) have a lower initial installation cost in comparison to vertical ones; however, they require more land area. In order to reduce the required land area of horizontal GHEs, their heat exchange rate per unit land area needs to be enhanced. A remedy to this problem is installing horizontal GHEs in an arrangement with maximum heat exchange rate per unit land area. However, such arrangements have not been introduced for different types of horizontal GHEs thus far. To this end, in this study, the thermal performance of various GHE pipe arrangements for three different types (linear, spiral, and slinky) of horizontal GHEs is evaluated. Comparing the obtained results, the arrangement that provides the maximum heat exchange rate per unit land area for each type is introduced. A 3D numerical model based on the finite-element method is developed to simulate the GHEs performance. In order to have a fair comparison, all studied arrangements are designed to have the same installation cost. The obtained results indicated that the linear GHE with quadruple-layer arrangement provides the most uniform heat distribution in the ground and, therefore, has the highest heat exchange rate per unit land area among all cases studied. This arrangement has about a 34% higher heat exchange rate per unit land area than the conventional single-layer arrangement. The staggered double-layer arrangement is the best arrangement for the slinky and spiral GHE types, which has about 22 and 7% higher heat exchange rate per unit land area than conventional single-layer arrangement, respectively. Furthermore, the thermal performance of the best arrangement for each type is evaluated for three different values of soil thermal conductivities.