Integrating Compressed Air Energy Storage with Borehole Thermal Energy Storage: A Feasibility Study

There is an increasing number of renewable energy projects deployed to supply electrical energy, thermal energy, or both. The trend is mainly driven by the continuing growth in global energy demand and effort to deviate from the emission-intensive hydrocarbon society. Despite the relative advantages of renewables, compared to fossil fuels, their intermittent availability often imposes limitations on deployment, as it can cause a supply-demand imbalance issue. For overcoming the challenge, Energy Storage Systems (ESS) are integrated into renewable energy systems. This thesis focuses on integrating existing ESS technologies, Compressed Air Energy Storage (CAES) and Borehole Thermal Energy Storage (BTES). In this thesis, the integrated ESS system is referred to as an integrated CAES-BTES system. The integrated system stores excess electricity from renewable sources in CAES, and the heat of compression is stored in BTES. The thermal energy can be transferred to the expansion side of the CAES or outside of the system. The objective of this thesis is to conduct a first-order feasibility study on the design of the integrated CAES-BTES system. For the study, several existing configurations of CAES and BTES and their design methods outlined in the literature are reviewed first. A conceptual design of the integrated system is proposed, based on the literature review. From the literature review, key parameters that affect the overall performance of the ESS technologies are also identified. Then, a computer model of the integrated CAES-BTES system is produced with the Matlab®, which allows prompt thermodynamics simulation and sizing of the system, based on user-defined parameters, such as the energy demand and operational/physical constraints. A parametric study is conducted using the model to analyze the system's performance under different operating conditions. The integrated system's economic feasibility is determined relatively by comparing its Levelized Cost of Energy Storage (LCOS) with the other ESS technologies. A financial model is created using Microsoft Excel/VBA, which calculates the LCOS based on the technical and financial input parameters from the Matlab® model and existing literature. An integrated CAES-BTES with a power output of 1.5 MW that operates in a continuous daily cycle of 8 hours of discharge is considered in the parametric study. Based on the scenarios considered for the parametric study, the CAES system can achieve up to 60% and 40% round-trip efficiency for Adiabatic CAES (A-CAES) and Diabatic CAES (D-CAES), within the range of parameters. The results have demonstrated the more notable influence of the expander inlet temperature and the number of compression expansion stages on the system's efficiency. For the BTES side of the integrated system, the thermal properties of the storage medium and grout materials have shown significant impacts on the configurations. The LCOS is calculated for the integrated CAES-BTES system and compared to conventional D-CAES and A-CAES systems. Rated storage capacity of 2.5 GWh i considered for the compared systems. The LCOS analysis shows that when electricity cost is not accounted for, the integrated CAES-BTES has similar LCOS as the A-CAES system. For instance, their LCOS can be lower than D-CAES, when more than 1800 yearly full load hours are considered. The LCOS of the integrated system presents better cost-competitiveness than both conventional systems when electricity cost is higher than 0.03€/kWh. The results of the study provide the advantages of the integrated CAES-BTES not only over the CAES systems but potentially over other ESS technologies as well. This study concludes that integrated CAES-BTES can be a technically and economically feasible option if it is designed appropriately. It also demonstrates a potential design approach and determines critical parameters for the system's performance, which may be valuable for engineers and designers. However, as the thesis results are applicable to the first-order design case only, more detailed design methods may be considered for the specific design problem