Review of osmotic heat engines for low-grade heat harvesting

Osmotic heat engines (OHEs) consisting of a separation unit for separating the salt solution into two solutions with different concentrations and a power generation unit for extracting electricity from the mixture of the two solutions generated in the separation unit has attracted increasing attention as an emerging “low-grade heat to power” technology. In this study, a comprehensive review of OHEs is presented for the first time. The common types and characteristics of the main component of osmotic heat engine system are overviewed. The architectures of the OHEs classified as reverse electrodialysis heat engines, pressure retarded osmosis heat engines are presented and reviewed. Some other state-of-art heat-to-work thermodynamic systems are also introduced briefly. Furthermore, the potential application of osmotic heat engines is discussed and a critical assessment of osmotic heat engines is conducted. Outlook and technological challenges outlining the research perspectives are also discussed in this review, and the results reveal that practical application of OHE considering the distinct characteristics of application scenarios should be further emphasized. Substantial improvement in membrane performance and cost would favor the emergence of OHE in a commercial scale.