An enormous portion of low-grade heat (<100C) exists in the form of solar heat and waste heat from residential buildings and industries. However, converting low-grade heat using a conventional solid-state thermoelectric device-based system is challenging due to poor conversion efficiencies and low cost-effectiveness. Electrochemical heat-to-electricity conversion using a thermally regenerative electrochemical cycle (TREC) redox flow battery (RFB) can be considered a promising means for securing cost-effective low-grade heat storage with a sufficiently high-power rating due to the scalability of the redox flow batteries. Despite such benefits, the record conversion efficiency of the TREC RFB is less than 6%, which is far behind its theoretical maximum, which can be over 20% at a temperature gradient of 80C. This is attributable to the electrolyte design that does not consider its thermodynamic and electrochemical characteristics, which are the key aspects of TREC-based systems.

Herein, we propose developing an efficient TREC-based RFB system through a data- and modelling-driven screening of the redox chemicals and electrolyte design and developing a scalable demonstrator. Comprehensive analytic thermoelectrochemical model studies and empirical research will be carried out under dynamic operational parameters, such as temperature, flow rate, conductivities, resistance and etc., which have been poorly studied despite their high importance in TREC design. The long-term current-voltage characteristics obtained from both lab-scale and large area multi-stack flow cells will be used for further model development (empirical constants and other dependent variables for overpotential terms) using a feedback loop scheme (i.e., inverse-modelling process).

Critical key parameters of redox couples, including but not limited to thermogalvanic (Seebeck) coefficient, reaction entropy, solubility, conductivity, reaction rate and etc., will be collected and assessed by using data collection matrix followed by a series of comprehensive theoretical and experimental screening steps. Quantitatively, through the activities outlined above, this project is aimed to deliver a TREC RFB system with a record-breaking heat-to-chemical-to-electricity conversion efficiency (>10% at 60C temperature gradient at maximum power density) which is equivalent to 70% of the Carnot limit.

This project is a challenging and ambitious interdisciplinary engineering study, requiring a broad spectrum of collaborations. Theoretical modelling with the initial screening of redox couples and prototype system development will be conducted at Heriot-Watt University, while the University of Strathclyde will prepare and characterise the fundamental characteristics of candidate redox couples. Close partnership with various external experts will also be carried out for the success of the project. Aarhus Univ. (Prof. A. Bentien's group) will support external research stay opportunities for organic chemical treatment. KIST Europe (in Saarland Univ) also will provide access to state-of-the-art multi-stack battery testing facilities and research staff support for PDRA's research stay. AES Solar Ltd. shall participate as an industrial partner, providing technical support for heat-collector design. LIND Ltd. will also support the project with access to their commercial-scale testing facilities.

Outcomes from this work will be an important milestone in both energy storage and electrochemistry areas. We envisage technological advances for efficient and durable solar energy storage that promptly meet the needs of the times for the UK's zero-emission future. In addition, more importantly, the purpose of the project meets the needs of energy security considering recent international armed conflicts in Europe. Alongside the current UK's renewable energy roadmap policy for a carbon-neutral society, this project also fits well in the EPSRC's Energy Storage portfolio.