Techno-economic evaluation of nitrate salt storage concepts at 620°C

Kurzfassung

The production of dispatchable electricity from volatile renewable energy such as wind and PV requires energy storage. Retrofitting existing fossil fired power plants with thermal energy storage, and ultimately transferring them to heat storage plants, is a promising and low-cost option for implementing large amounts of grid scale energy storage [1]. State of the art molten salt thermal energy storage is used up to a maximum temperature of 565 °C, while modern supercritical steam power plants operate at temperatures above 600°C for optimum efficiency. Previous work has shown that the thermal stability of molten nitrate salt can be improved by providing specially composed gas phases [2,3]. Therefore, the design of the gas system is decisive for the new generation of nitrate storage systems up to 620°C. This study presents a techno-economical comparison of conceptual designs including gas systems for large-scale molten salt thermal energy storage with operation temperatures of 620 °C. The interaction between the technological demand for high temperatures, requirements for material stability and the economic perspective are evaluated. The proposed storage concepts with nitrate salts at elevated temperatures all require a gas handling system to prevent salt degradation. In a first step, different gas handling systems are optimally designed based on net present value method. Examples are closed systems with active tank pressure control and half-open system with purge gas control. Previous work indicates that the formation of corrosive impurities (e.g. oxide ion) may be significantly reduced by a controlled gas atmosphere, but structural materials for 620 °C have not yet been identified. In order to evaluate the potential of the approach, we use a simplified assumption that similar structural material as for the 565 °C can be utilized. In a second step, the different storage technologies (two-tank and single tank) are assessed in combination with a supercritical power cycle. The cost of dispatchable power is used as a measure to compare the proposed storage concepts at 620°C with state of the art molten salt two tank storage at 565 °C. The techno-economic analysis of the gas handling system reveals an optimum for temperature and pressure in the case of a closed system with active tank pressure control. Furthermore, it is shown, that the cost of dispatchable power generation from a supercritical power plant with 620°C nitrate salt storage can be lower compared to a subcritical power plant with state of the art 565°C nitrate salt storage, depending on the cost of the charging power. Installing a gas handing system for nitrate salt thermal energy storage can increase the maximum temperature from 565°C to 620°C, ultimately increasing the amount of energy that can be stored per unit volume. Moreover, the full potential of the high efficiency of a supercritical steam power cycle can be exploited by implementing 620°C nitrate salt storage. The additional investment for the 620 °C storage system is compared to the benefits of a higher power block efficiency and potentially lower storage costs using the cost of dispatchable power generation.