Thermal Performance Analysis of High Temperature Borehole Thermal Energy Storage Design from an Insulation Perspective

Sammanfattning: Thermal energy storage is a growing concept within the sector of energy production. The concept is based on storage of excess thermal energy, for usage during high demand, as a mean of reducing the need for new production. By implementing thermal storage economic and environmental gains can be made as production can be kept more constant over time and less energy is wasted, furthermore the storage system can be used for cooling of thermal processes. This Master Thesis focuses on analysing the thermal performance of High Temperature Borehole Thermal Energy Storages from a design and insulation perspective. The aim of which was to highlight the parameters critical for finding an optimized design with regards to thermal performance and economic profitability-potential. The analysis was based on the implementation of TRNSYS for modelling and simulations for a reference HT-BTES, from which a general perspective was lifted. The reference project comes in the form of a feasibility study for a planned HT-BTES at Filborna in Sweden. The reference storage is to be made in sedimentary bedrock beneath an existing gypsum deposit, and make up the outset for analysis. The thermal performance, with regards to design and insulation, was analysed by performing a sensitivity study highlighting the relation between thermal performance and design and insulation parameters. Complementing this sensitivity study was an economic analysis. Secondarily environmental effects were evaluated. Limiting the analysis is the lack of modelling possibility for groundwater and highly conductive zones which may cause additional unpredicted thermal losses. Lastly the effects if implementing a heat pump into the system were analysed. The results from the sensitivity study showed clear indications of greater and minor dependencies between thermal performance and design/insulation parameters. Parameters such as “storage volume” showed tendencies for reduced thermal performance with increasing parameter value, whilst parameters such as “borehole depth” showed improved thermal performance with increasing parameter value. When considering the insulation aspect of HT-BTES the results were found quite intuitive. Increased insulation thickness, and decreased thermal conductivity of insulation, reduces top losses. It is from this evaluation of dependency between thermal conductivity of insulation and thermal performance that the insulation material perspective could be implemented. The material perspective accounts for effects on thermal properties from temperature and moisture content, highlighting the importance of moisture protection. When implementing an economic perspective into the thermal performance analysis regarding insulation design, the economic value was found in the extracted energy. This value needed to exceed the value the thermal energy had when it was stored, and cover for all cost aspects included in the system design. The main conclusion drawn from the project was that in order to derive modelling results close to reality, extensive work is needed for determining all parameters of the thermal storage model. The sensitivity study showed that uncertainties in parameters may yield significant impact on the final results.