Abstract
The Reverse electrodialysis heat engine (REDHE) combines a reverse electrodialysis stack for power generation with a thermal regeneration unit to restore the concentration difference of the salt solutions. Current approaches for converting low-temperature waste heat to electricity with REDHE have not yielded conversion efficiencies and profits that would allow for the industrialization of the technology. This review explores the concept of Heat-to-Hydrogen with REDHEs and maps crucial developments toward industrialization. We discuss current advances in membrane development that are vital for the breakthrough of the RED Heat Engine. In addition, the choice of salt is a crucial factor that has not received enough attention in the field. Based on ion properties relevant for both the transport through IEMs and the feasibility for regeneration, we pinpoint the most promising salts for use in REDHE, which we find to be KNO3, LiNO3, LiBr and LiCl. To further validate these results and compare the system performance with different salts, there is a demand for a comprehensive thermodynamic model of the REDHE that considers all its units. Guided by such a model, experimental studies can be designed to utilize the most favorable process conditions (e.g., salt solutions).
Highlights
A significant and rapidly growing share of renewable energy is produced intermittently, causing a mismatch between supply and demand
The present work gives a comprehensive overview of the reverse electrodialysis heat engine (REDHE) concept, where a reverse electrodialysis stack as a power unit is coupled in a closed loop with a thermal regeneration unit to restore the salinity gradient
High-performance polymers featuring a low area resistance while maintaining sufficient permselectivity are a main target in membrane development
Summary
A significant and rapidly growing share of renewable energy is produced intermittently, causing a mismatch between supply and demand. It is reported that by recovering hydrogen gas from the RED system, the produced energy can be 1.5 times higher (118 Wh m−3), compared to directly withdrawing electricity Both electricity and hydrogen production have large markets; there are limited technologies available to date for direct renewable hydrogen production. This review aims to: (1) give an overview of the state-of-the-art for the REDHE technology, its promises, and limitations, especially with regards to hydrogen production from waste heat; (2) compare different approaches for thermal solution regeneration; (3) highlight the most crucial membrane properties and trends in IEM development; and (4) summarize properties and suitability of different salts for the use in REDHEs, concerning both the regeneration unit and the power unit. We discuss differences in ionic transport across the IEM among salts
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