The scientific community has taken on the challenge to develop innovative methods to exploit low-temperature (<100 °C) heat sources, having large potential to decrease the carbon footprint. In this review, we first summarise the novel proposed techniques, and then we propose a framework for comparing the performances reported in the literature based on two indices, which are currently critical for the technical and economic feasibility: energy efficiency and power density. Two techniques show the best performances; both use distillation to regenerate the working solution of a flow battery. In many papers, schemes based on additional heat exchangers are proposed with the aim of improving the efficiency. Such techniques are also discussed here: first, the power per unit surface of the heat exchangers is discussed as a performance index; then, a method for inherently ranking the techniques, independent of the use of additional heat exchangers, is provided. The analysis shows that the application of such schemes does not significantly change the ranking of the techniques, while having a detrimental effect on the complexity of the whole system. Finally, we discuss the performances of the various techniques based on general thermodynamic principles and methods; in particular, the meaning of the temperature versus entropy graph in this context is considered.
