Collaborative optimization of regeneration unit and power generation unit in reverse electrodialysis heat engine for low-grade heat recovery

Abstract

Reverse electrodialysis heat engine (REDHE) emerges great potential to harvest the low-grade heat (LGH) produced during the industrial processes and renewable energy utilizations. In the closed-loop system, the power generation unit converts the salinity gradient power (SGP) to electric energy using the reverse electrodialysis (RED) cells, and the regeneration unit recovers the salinity gradient based on LGH. As two units are in series, the regeneration and power generation units should be collaboratively optimized to achieve a high performance of REDHE. Herein, a framework which can unitedly simulate the both units is built, which enables to divide the total efficiency of REDHE to the respective efficiencies of units. LiBr solution as the working fluid and the effects of initial salt concentration, concentration of the high concentration (HC) solution and temperature in RED cells are considered. Results show that the total efficiencies increase with the concentration of HC solution and the RED temperature due to the production of more chemical energy in the regeneration unit. However, a non-monotonic trend of the total efficiency with the initial concentration is discovered, which increases from 4.91% to 5.42% and then decreases to 4.69% when the initial concentration raises from 20 wt% to 25 wt% and then reaches 30 wt% at RED temperature of 25 ℃. Although enhancing the RED temperature to 50 ℃ leads the total efficiencies increase to 6.25%, 7.13% and 6.38%, respectively, the trend is unchanged. The respective efficiencies of units explain the phenomenon by that a higher salinity can decrease the efficiency of the power generation unit by reducing the ionic selectivity in the RED cells, but enhance the efficiency of the regeneration unit. Therefore, the highest total efficiency reached at initial concentration of 25 wt% is the trade-off between the efficiencies of two units. This study indicates that increasing the concentration of HC solution, the RED temperature, and carefully selecting a compromise initial concentration are helpful to achieve a high energy efficiency in a REDHE.