Development of a Full-Scale Soil-Borehole Thermal Energy Storage System

Abstract

This study involves an evaluation of the design and construction process for a soil-borehole thermal energy storage (SBTES) system installed in a sandy-silt deposit. A series of simplified numerical simulations were performed to understand the role of different variables on the heat storage in the SBTES system. The results indicate that soils with lower thermal conductivity have less lateral heat loss, and that arrays with smaller borehole spacings permit more concentrated storage of heat at higher temperatures. INTRODUCTION Soil-borehole thermal energy storage (SBTES) systems are an approach to provide efficient renewable resource-based thermal energy to heat buildings (Gabrielsson et al. 2000; Sibbit et al. 2007; Zhang et al. 2012; McCartney et al. 2013). They function in a similar way to conventional ground-source heat pump (GSHP) systems, where a fluid is circulated within a closed-loop pipe network installed in vertical boreholes to shed or absorb heat from the surrounding subsurface. Different from conventional GSHP systems, SBTES systems are configured to store thermal energy collected from solar thermal panels during the summer, and discharge the heat to buildings during the winter. The boreholes are typically spaced much closer together in an SBTES system than in a conventional system. The temperature of the ground within the borehole array increases from its ambient temperature (approximately 10-20 °C) to 60-90 °C during heat injection. SBTES systems are a convenient alternative to other energy storage systems as they are relatively inexpensive, involve storage of renewable energy (solar thermal energy), and are space efficient as they are underground. Most SBTES systems involve direct circulation of fluid through the closed-loop boreholes during heat injection and extraction, without the use of a heat pump. Although soil-borehole thermal energy storage (SBTES) systems have been shown to be an effective tool for storing thermal energy collected from renewable sources such as solar panels (Sibbit et al. 2012), their transient response during heat injection is not well understood. The lack of understanding of their response prevents evaluation of strategies to minimize the lateral loss of heat from the borehole array, improving the efficiency of heat storage (i.e., difference between heat injected and heat extracted), and improving the rates of heat injection and extraction. Sizing of the borehole field is critical as an undersized borehole field may not provide the required heat capacity, while too large of a field will result in higher costs and a lower rate of heat transfer. To better understand 1608 IFCEE 2015 © ASCE 2015