Efficient Low-Grade Heat Harvesting Enabled by Tuning the Hydration Entropy in an Electrochemical System

Abstract

Harvesting of low-grade heat (<100 degrees C) is promising, but its application is hampered by a lack of efficient and low-cost systems. The thermally regenerative electrochemical cycle (TREC) is a potential alternative system with high energy-conversion efficiency. Here, the temperature coefficient (alpha), which is a key factor in a TREC, is studied by tuning the hydration entropy of the electrochemical reaction. The change of alpha in copper hexacyanoferrate (CuHCFe) with intercalation of different monovalent cations (Na+, K+, Rb+, and Cs+) and a larger alpha value of -1.004 mV K-1 being found in the Rb+ system are observed. With a view to practical application, a full cell is constructed for low-grade heat harvesting. The resultant eta(e) is 4.34% when TREC operates between 10 and 50 degrees C, which further reaches 6.21% when 50% heat recuperation is considered. This efficiency equals to 50% of the Carnot efficiency, which is thought to be the highest eta(e) reported for low-grade heat harvesting systems. This study provides a fundamental understanding of the mechanisms governing the TREC, and the demonstrated efficient system paves the way for low-grade heat harvesting.