Modelling Borehole Thermal Energy Storage of Renewable Resources as a Fluctuating Source for Charge

Abstract

Decarbonization of heating is essential to achieve net zero targets. As 80 % of heat in the UK is generated by combustion of natural gas, alternative renewable resources must be explored to meet this demand. Low enthalpy, shallow geothermal resources can contribute to space heating and cooling by utilizing ground sourced heat pumps, providing a low carbon energy supply. Borehole heat exchangers circulate heat transfer (working) fluid through a subsurface closed-loop system, to feed ground sourced heat pumps providing heating or cooling to adjacent buildings. Additionally, the temperate climate in the UK provides a promising setting for borehole thermal energy storage through coupling of seasonal sources of surplus heat or coolth (or even surplus low carbon electricity, e.g., wind or solar), with subsurface borehole heat exchangers. Yet, there has been limited uptake of borehole thermal energy storage systems in the UK, possibly due to a lack of familiarity or confidence in borehole thermal energy storage technology, high capital costs, lack of policy driving thermal energy storage or the widespread availability of gas boilers. This study investigates the potential use of shallow geothermal resources to supply heat to the King’s Buildings Campus of the University of Edinburgh, Scotland, via modeling of a borehole heat exchanger array to meet heating demand. Energy generated from surplus curtailed wind was modeled as a fluctuating source of charge generated by air source heat pumps. As part of this study, ‘whole systems’ modeling was undertaken with TRaNsient SYStem simulation tool (TRNSYS), but the subsurface component of this software is not capable of modelling groundwater flow. Therefore, this paper aims to 1) compare a numerical model developed with OpenGeoSys to the data generated from TRaNsient SYStem simulation tool for any discrepancies, and 2) evaluate the subsurface thermal response using OpenGeoSys, with and without regional groundwater flow imposed on the model. Results indicate that there is a high potential for the use of such systems which incorporate curtailed wind, but the demand profile of the building and supply of heat for charge will impact the subsurface thermal balancing of the system. Initial comparison of the two modelling environments showed they have a strong match, with modeled outlet temperatures for the first year typically within 2 ºC of each other. Furthermore, detailed subsurface evaluation with OpenGeoSys showed groundwater flow has a strong impact on such systems, negatively inhibiting thermal performance. Whether the storage of surplus electricity, with recovery for thermal energy, is attractive from an economic or exergy point of view remains an open question.