Technical Performance Comparison of Horizontal and Vertical Ground-Source Heat Pump Systems

Abstract

The configurations of ground heat exchangers (GHEs) play a significant role in the efficiency and sustainability of ground-source heat pump (GSHP) systems. However, there is a knowledge gap in understanding the performance differences between the horizontal and vertical GSHP systems in the same project under various heating and cooling demands. In this study, a technical performance comparison between GSHP systems coupled with horizontal ground loops and vertical boreholes under three scenarios of heating-to-cooling ratios (6 : 1, 2.4 : 1, and 1 : 1) was conducted. The simulations were based on a coupled thermal–hydraulic model for unsaturated soils that takes into account realistic ground surface boundary, GHE boundary, and the dynamics of heat pump efficiency. The GHEs were designed based on an experimental site located on the campus of a UK university. Results showed significant differences in the development of fluid temperatures and coefficient of performance (COP) of heat pumps between the horizontal and vertical GSHP systems due to the differences in the soil profiles and temperature boundaries. Both the fluid temperatures and heat pump COPs in the horizontal GSHP system reached a steady annual cycle after 2 years regardless of the heating-to-cooling ratios. For the vertical system, a general downward trend in the fluid temperatures and the COP of the heat pump in the heating mode can be found when a heating-to-cooling ratio was 6 : 1 or 2.4 : 1, while an overall upward trend in the fluid temperatures and the COP of the heat pump in the heating mode can be noted in the case of 1 : 1 heating-to-cooling ratio. Additionally, the heat pump operating in the cooling mode was off most of the time when a heating-to-cooling ratio was 6 : 1 or 2.4 : 1, while a declining trend in the COP of the heat pump in the cooling mode was exhibited in the case of a heating-to-cooling ratio of 1 : 1. The technical comparison reveals that the heating-to-cooling ratios would significantly affect the efficiency and sustainability of both GSHP systems.