Decarbonization of industrial sector through rock based high-temperature thermal energy storage: Experimental, numerical and techno-economic analysis

Heating accounts for approximately 50% of the final energy used globally, and it is one of the primary sources of CO₂ emissions contributing to global warming. Therefore, decarbonizing heating requires the utilization of renewable energy sources. Low-cost and adaptable integrated thermal energy storage unit systems are essential to take into consideration given the unpredictable characteristics of renewable energy sources and enable operational flexibility. Rock-based packed bed high temperature thermal energy storage could facilitate this much-needed change.

This thesis focuses on developing a strategy in order to understand and increase the performance of the novel hemispherical shaped vertical flow rock based thermal energy storage system and its techno-economic feasibility in integrating this storage with heat supply technologies in an industrial process. A detailed design and characterization of this novel high-temperature thermal storage and its components is described in this work. The experimental results for medium and high-temperature ranges logged from this pilot plant are used to have a better understanding of the packed bed thermal energy storage under different boundary conditions. A comprehensive numerical model adopted for the pilot storage is validated with the experimental results and used to assess the influence of different parameters on the storage efficiency and thermocline of the packed bed which were not experimentally possible due to financial constraints; this would provide a groundwork for designing the following industrial scale high temperature thermal storage. An upscaled version of this storage is developed to study the behavior of the storage unit under real conditions when integrated with a solar thermal collector field to supply a constant heat demand over a long period of time. In the end, an optimization tool will be used to perform economic optimization and evaluation on the system level for two different geographical locations (Spain and Denmark) with different weather and energy market conditions to provide optimal size and minimize operational costs for a given industrial heat demand. This optimization tool calculates the levelized heat cost for three different high-temperature thermal storage integration schemes with solar thermal collectors, electric heaters, and a high-temperature heat pump. The levelized cost of heat of these schemes is compared with the benchmark of a natural gas-fired boiler.

This research shows that rock-based thermal storage can be suitable for integrating with other heat supply technologies in an industrial application. The experimental round trip efficiency reaches almost 70% in the initial tests; this, however, can be increased by changing the parameters and is expected to improve under consecutive cycling. The charge efficiency recorded in the numerical study ranges between 77-94% depending on the boundary conditions. The heat loss is reduced for upscaled storage from 20% to 4% of the total heat storage over three days. Economically the addition of high-temperature thermal energy storage reduces the levelized cost of heat for all the schemes by decreasing both the operational cost and capital cost by providing flexibility in terms of electricity price and load shifting.