Initial Evaluation of Ground-source Heat Exchangers for Cooling Applications in Hawai‘i

We investigate the potential of ground source heat exchangers (GSHE) for HVAC, particularly in cooling applications, in the tropical environment of Hawai‘i. Recent studies in Southeast Asia showcase the capacity of GSHE for space cooling in tropical environments (Yasukawa and Uchida, 2018). In Hawai‘i, high efficiency HVAC is one of the most impactful energy savings initiatives based on lifetime energy demand and “Total Resource Benefit,” a metric that combines the cost of energy saved and the capacity avoided (Hawai‘i Energy 2018 Annual Report). For reference, 58% of the University of Hawai‘i’s energy usage is in HVAC. Our assessment presents multiple environmental and engineering scenarios in which GSHE could be feasible in Hawai‘i, highlighting regions of viability, in particular on the islands of Hawai‘i and O‘ahu. We utilize calculations from Glassley et al. (2012) and an array of literature values for the thermal properties of local Hawaiian rock types. We analyze varying input parameters such as energy efficiency ratio (EER), rock thermal conductivity, piping thermal resistance, temperature gradient, and fraction of time operating for the cooling loop. Our study shows that limestone and basalt units provide the best host geology in Hawai‘i, making GSHE feasible when air-ground temperate differences are >4 C for Hawai‘i’s five-month summer cooling period. To account for the effects of local groundwater flow, we interpolate depth to water maps as well as calculations of Peclet numbers for basalt, limestone, and alluvium. Peclet numbers for basalt and limestone imply advection may be a key process for heat transfer not captured in the original predictive equations. This, coupled with the high local water tables and groundwater flow rates around the state, suggest that basalt and limestone may have an increased capacity to act as heat sinks in Hawaiian environments with improved GSHE capacity beyond what is already established in this study.