Development of a high temperature and high power PCM storage for standby operation

Abstract

Thermal energy storages integrated in industrial processes allow for higher degrees of energy flexibility and reduction of fossil fuel use. The combination of latent heat thermal energy storage systems with water/steam processes can lead to optimized thermal gradients and therefore good system efficiencies. Storage units and systems have been proven at pilot scale. The integration in industrial processes remains a challenge, due to the size of the systems as well as the hurdles in design, permitting and build. This thesis encompasses the parametrization, design, build, integration and initial operation of a megawatt-scale latent heat thermal energy storage unit, producing superheated steam for an operating cogeneration plant and industrial process customers. The storage unit can produce superheated steam at more than 300 °C and 25 bar at a mass flow rate of 8 t/h for at least 15 minutes. The data of the dispatchable production of superheated steam for more than 20 minutes show the thermal power and capacity at 5.5 MW and 1.9 MWh. In order to develop this storage system and show the feasibility of the novel aspects of producing superheated steam in once-through operation, providing megawatt-scale thermal power and capacity, and integrating it into an operating system, various steps are involved. These steps are a combination of upscaling the thermal power and capacity, designing for a feasible build and for given system requirements, and system integration development. The upscaling in thermal power results in the development of a very dense fin structure and tight tube-spacing. The upscaling in capacity requires the development of a design model with capabilities for analysis of thermal losses and possible non-ideal flow through the headers, as well as more banal aspects such as transportability and weight considerations as well as physical filling capability with the pelleted salt during commissioning, and accessibility for permitting bodies. System integration in an operating system considers charging and discharging with the available components and maximization of benefits to the plant. This integration, requiring not just a design and optimization of the storage technology itself but of the whole system, is a novel point of view for the development of latent heat storage systems. It is not critical that the storage itself provide all of the parameters, but that the system integration makes this feasible.