Absorption-based heat storage for electrification of dynamic Power-to-chemicals processes

Heat integration is a well-established approach to increase the energy efficiency in chemical processes. However, conventional steady-state approaches fail in dynamically operated Power-to-chemicals processes where production rate needs to follow the renewable energy generation profiles. Herein, we present a novel strategy to store and release heat by exploiting the high reaction enthalpy of absorption of ammonia (or water) in cheaply available metal halides, where the heat storage temperature can be tuned between 300-500°C. Modelling of the system provides guidance for the tailored design of the heat storage system by balancing heat loses, heat transfer and heat storage rates, with a maximum theoretical efficiency of 81%. In addition, this heat integration strategy can store heat closer to the temperature at which will be subsequently utilized, maximizing the heat quality. The novel heat storage approach is demonstrated for the dynamic operation of a (exothermic) Haber-Bosch reactor for green ammonia synthesis. In comparison to simply storing sensible heat (e.g. ceramic material), the absorption-based heat storage system can store up to 12X more heat and maximizes the quality of the heat by minimizing the temperature swing. Additionally, it keeps the HB reactor hot for up to 10X longer than the reactor alone during idle operation, facilitating the fast re-starts with no energy requirements while increasing the production due to in-situ cooling. The dynamic reactor operation enabled by this new absorption-based heat storage approach highlights the importance of re-designing of Power-to-X processes with novel technologies to enhance their economic feasibility.