Modelling and optimization study on a high-temperature borehole thermal energy storage concept driven by power plant waste heat

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Insinööritieteiden korkeakoulu | Master's thesis
Sustainable Energy Conversion Processes
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Master's Programme in Advanced Energy Solutions (AAE)
High-temperature borehole thermal energy storage has potential to increase energy conservation and therefore, decrease the amount of waste heat. Among the existing seasonal thermal energy storage technologies, borehole thermal energy storage is one of the most price-competitive and mature technology. Borehole thermal energy storage enables power plant to store waste heat during summer for later use during high demand winter season with efficiency of 40–65 %. The effect of borehole spacing and length on the performance of borehole thermal energy storage has not been explored. This master’s thesis studies how borehole spacing and depth affects large-scale storage performance and investment cost. In order to study the effect, a numerical model, based on the method proposed by Al-Khoury and Bonnier (2006), is created using COMSOL Multiphysics software. In the method, fluid flow is modelled in a single U-tube pipe in a one-dimensional (1D) heat pipe element and heat transfer in bedrock is in the three-dimensional (3D) domain. In total, this study included 21 research cases. The created model was simulated with seven borehole spacings and three different ratios of storage width to height. Based on information found in the literature borehole spacing was varied at 0.5 meters interval from 2.0 m to 5.0 m. The storage width to depth –ratios used were 1.0, 0.67 and 0.5. The results of this master’s thesis indicate that the most economically optimal borehole spacing should be based on the determination of the optimal storage temperature drop and extraction of energy during discharging period within time t. Therefore, optimal spacing should be chosen based on the thermal properties of the storage medium, charging and discharging inlet temperatures of the fluid, required injected and extracted energy, and the length of the time period within which the power needs to be charged or discharged. Deeper boreholes do not seem to have a significant effect on storage performance or investment cost; however, this study did not consider increased heat losses due to suboptimal storage shape. Based on simulation results, the same capacity (4.46 GWh) may cost 1.5 million € more in suboptimal case than in the optimal one (=550 boreholes, borehole spacing 3.5 m and depth 172 m). The BTES solution modelled in this thesis has the potential to yield 2-5 MW thermal energy continuously and 10 MW thermal energy for one-hour peaks.
Vuorinen, Ville
Thesis advisor
Arola, Teppo
high-temperature borehole thermal energy storage, borehole heat exchangers, fluid flow, heat transfer, COMSOL Multiphysics, underground heat
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