Energy performance evaluation of geothermal boreholes

Abstract

This paper presents the modeling of a geothermal heat pump system (GHP) with vertical borehole heat exchangers (BHE). It has been developed to study the feasibility of applying this system to cooling and heating buildings in Beirut using 3E evaluation criteria: energy, economy, and environment using TRNSYS simulation software. All model components are designed, two sizing methods are used for the pre-design of borehole heat exchangers, in which five cases are studied to select the proper BHE design, based on the evaluation of coefficient of performance, energy consumption, and the change in ground temperature. Simulation results achieve high efficiency of the modeled system; however, they point to a significant increase in the ground temperature. The feasibility analysis demonstrates that GHP system is economically and environmentally feasible compared to conventional air cooled heat pump system (AHP). In addition, the study of the effects of several design parameters of BHE on the performance of the GHP modeled system confirms that the length of BHE is the most critical design parameter, but the borehole separation spacing, the thermal conductivity of grout, and the borehole diameter also have an effect on the energy efficiency of the system. The findings obtained from this work assert on well-designed studies for any application of a GHP system in Beirut, taking into consideration effective solutions to reduce the increase in ground temperature.