Thermo-chemical energy storage (TCES) has the potential to store very large amounts of energy within a small space and at low cost. This is achieved by converting thermal energy ('heat') to chemical energy via a reversible chemical reaction. For example, by heating a granular metallic oxide to the right temperature and at the right pressure, some of the oxygen is driven off (i.e., the substance is 'reduced') and heat is absorbed during the process. The resulting 'reduced oxide' is stable and can be stored for long periods without degradation. Heat can subsequently be recovered, when required, by passing air at elevated pressure over the reduced oxide. Some of the oxygen in the air is then absorbed but the remaining gas is heated by the reaction and (since it is also at high pressure) can be used to drive a gas turbine to generate electricity.

Other gas-solid reactions are also possible, including 'calcination' of limestone (i.e., heating it up to drive off carbon dioxide) and hydration (with steam) of e.g. calcium oxide. Each reaction has its own set of peculiarities which can be exploited to its advantage. For example, the carbon dioxide emerging from the calcination reaction can be compressed and liquefied. This in itself absorbs electrical energy (in order to drive the compressors) and constitutes an additional, surprisingly compact and stable form of energy storage, from which electricity can be recovered by using the high-pressure CO2 to drive a turbo-generator.

Compared with batteries, TCES has the potential to store energy at much lower cost per kilowatt-hour of storage capacity at grid scale, despite having a lower round trip efficiency. This is because TCES systems can be built based on unit operations and power plant technologies which scale up easily, compared to electrochemical systems. The efficiency for a thermo-chemical system is likely to be in the range 40 to 60%, however the 'conservation of energy', means that the remaining energy need not be wasted: it can be exploited for heating buildings, providing hot water or supplying heat for industrial processes. Furthermore, these systems offer the possibility to provide long duration storage without any safety hazards or pressurised storage facilities. How these technologies can contribute to various grid services, the scale needed and how best to locate them within the distribution network needs to be assessed. Many of the components have inertia, which will provide some frequency support, but the thermal response may limit service provision, particularly if waste heat is also being utilised.

In this grant we will develop and test new materials to enable more efficient and cost effective TCES processes. Issues investigated include the cycle stability of the materials, their capacities and rates of conversion. Lab scale testing will demonstrate key aspects of the cycles and provide information needed for design and modelling work to evaluate these processes. We will conduct modelling on the process flowsheet, with detailed component models to allow losses to be identified and the process and material combinations to be optimised. To understand the value of these technologies to society, we will conduct system level dynamic modelling to understand their ability to provide grid services under various scenarios, including those in which there is the provision of thermal energy for district/industrial heating applications. We will analyse and quantify the grid-scale integration potentialities of TCES technology by adopting a whole-system approach, thus its integration with electricity/heating/cooling/gas networks. This will allow us to unlock the opportunities offered by this novel multi-energy storage technology to enhance the flexibility of the energy grid as a whole, and thus enable a future energy system with a high penetration of renewables