Optimal operation and design of solar-thermal energy storage systems

Abstract

The present thesis focuses on the optimal operation and design of solar-thermal energy storage systems. First, optimization of time-variable operation to maximize revenue through selling and purchasing electricity to/from the grid is presented for a thermal energy storage system. Time-variable electricity prices and electricity buy-back from the grid to re-charge the energy storage is considered. The concentrated solar power on demand (CSPonD) concept, in which a salt pond receives solar energy, stores thermal energy, and delivers thermal energy to the power cycle is considered. Electric heaters are added to the CSPonD concept, allowing for periods of electricity buy-back from the grid to re-charge the energy storage. System-level models are developed and optimization of the design and operation is performed with local solvers. Three main case studies are considered: the first case study investigates the optimization of time-variable operation without electrical heating under time-invariant electricity price; the second case study optimizes the operation under time-variant electricity price without electric heaters; the third case study optimizes the operation under time-variant electricity price allowing charging of the pond using the grid electricity. The first case study reflects the time-invariant tariff model, whereas the second and third case studies consider a time-of-use feed-in-tariff. Two hourly price profiles are considered in order to assess the influence of it on the optimal design and operation of the thermal energy storage. The first electricity profile consists of a price profile that fluctuates moderately, and the second price profile fluctuates highly, including negative prices. The results show significant increase in the revenue when adding electric heaters. Under the moderately fluctuating electricity price, the use of heaters increases the revenue significantly, compared to the same case with no electric heaters considered. Under the highly fluctuating electricity price, the use of heaters more than doubles the revenue, compared to the same case with no electric heaters considered. Also, the performance analysis of a regenerative thermal energy storage system with enhancement heat transfer structures is presented. In a regenerative thermal storage system, thermal energy is transferred from a hot heat transfer fluid to the storage unit core elements during charge, and from the core elements to the cold heat transfer fluid during discharge. Herein, concrete as the solid storage material, nitrate solar salt as the heat transfer fluid, and aluminum plates as the heat transfer structures is considered. The discharge process from uniform initial temperature is studied with different configurations (pure concrete and concrete enhanced by transfer structures), operation strategies (laminar versus turbulent flow regimes), and dimensions. Results show a significant decrease in the cost of the thermal energy storage system when heat transfer structures are added, as well as a better performance in terms of discharge efficiency and discharge time period. The amount of solar salt needed for this configuration is approximately one fourth that required for a nitrate two-tank system operating with the same temperature difference.