Preliminary assessment of ground-source heat exchangers for cooling in Hawai‘i

Abstract

This study provides a preliminary assessment of ground source heat exchangers (GSHE) for cooling applications in the tropical environment of Hawai‘i. Recent demonstrations proved GSHE capacity for space cooling in tropical southeast Asia. We present multiple engineering scenarios in which GSHE could be feasible across Hawai‘i, highlighting regions of interest. Limestone and basalt provide the best host geology, feasible when air-ground temperature differences are >4 C and cooling is only required for Hawai‘i’s five summer months. This work investigates the effect of varying input parameters such as energy efficiency ratio, rock thermal conductivity, piping thermal resistance, temperature gradient, and fraction of time operating on the length of cooling loop required to successfully operate a GSHE. Maps of depth to water are provided, as well as calculations of Peclet numbers for basalt, limestone, and alluvium. Basalt’s Peclet number shows advection is a more dominant process than conduction in heat exchange; limestone has equivalent advective and conductive processes; alluvium is dominated by conduction. Thus, both basalt and limestone may have an increased capacity to efficiently act as heat sinks in Hawaiian environments as groundwater flow could additionally transfer heat through advection, not relying solely on the calculated capacity of conductive heat exchange.