Thermal Energy Storage In Geothermal Heat Pump Systems

A ground heat exchanger (GHX), as used in geothermal heat pump (GHP) systems, is inherently a thermal energy storage medium owing to the large thermal time constant of the ground but is conventionally not viewed as such by practitioners. Recently, sensible and latent thermal energy storage has received considerable attention in many applications, and with buildings, as with any energy storage device, the purpose is to reduce the size of primary energy components and equipment required to meet peak loads. Thermal energy storage has only been sparsely studied in GHP applications, and thus this dissertation focuses on taking advantage of some short-term and long-term opportunities for thermal energy storage in vertical GHXs with the objective of reducing the required size of the GHX, and therefore the capital cost. Regarding long-term (i.e., years) thermal energy storage in GHXs, past research has focused on so-called hybrid designs aimed at removing year-to-year thermal energy buildup or depletion in the ground. Little to no attention has been given to the effects of borehole field pattern on the thermal performance of vertical borehole heat exchangers. Existing design and simulation tools only account for rectangularly-shaped borefields, and this dissertation has extended recent works to account for hourly simulation borehole field patterns of arbitrary shape. Results show that significant reduction in size of the GHX can be realized in large, rectangular borefields if some portion of the boreholes in the center of the GHX are eliminated. Regarding the short-term (i.e., hours to days) thermal energy storage in GHXs, most design and simulation tools use the concept of a steady-state borehole thermal resistance, and thus do not account for hourly response of the GHX fluid and borehole grout. This dissertation has extended recent works to include these short-term effects. Further, the use and feasibility of phase-change material (PCM) storage tanks in GHP systems was also examined, and system simulation results show significant potential for GHX size reduction with inclusion of a PCM storage tank, but the system performance is quite sensitive to the PCM melt temperature due to significant hysteretic nature of the salt hydrate. Use of actual PCM heating and cooling curves are important considerations to address this design sensitivity. Moreover, it was determined that there is no unique optimum unless other factors are considered such as installation cost and physical constraints. Many combinations of GHX size and PCM mass are capable of achieving the design goal with similar annual electric energy consumption. Thus, a significant outcome of this dissertation has been the development of a freely-distributable design software tool that allows practitioners to simulate and cost optimize the design of a GHP system with PCM tanks and other thermal energy sources and sinks