Review of the best technologies for the exploitation of low-to-medium temperature heat sources: from ORC to sCO2 power cycles

Nowadays, the search for alternative sources for conversion of heat into power pushes towards the recovery of low grade heat streams and a wise use of renewable energies. Typically, Organic Rankine Cycle (ORC) power systems are used to exploit low-to-medium temperature sources. However, safety and environmental issues related to the use of organic fluids, along with thermodynamic limitations in the heat transfer process from the heat source, suggests looking beyond the standard ORC configurations. Thus, the scientific community has recently renewed the interest in alternative power cycles that can be classified into two main categories: thermodynamic cycles derived from modified layouts of the conventional ORC systems and supercritical/transcritical CO2 cycles. This study presents a critical review of these cycles operating with heat sources in the temperature range 90-500°C. A systematic categorization of the several architectures proposed in the literature is suggested. The new layouts search for the maximum power output by improving both cycle thermal efficiency and heat recovery effectiveness. The considered literature studies are subdivided according to the type of heat recovery power system and to its power size. Three classes of power size are identified: 0-50 kW, 100-900 kW and 1-13 MW. For each class, a thermodynamic comparison of the power systems is tempted coping with the variety of boundary conditions found in the literature. General indications about the best choice of the heat recovery power system are drawn according to the power size and the temperature of the heat source. Simple ORC systems appear the most competitive at small power ranges and for low temperature heat sources (below 200°C). Double pressure ORCs and transcritical cycles are a good alternative for higher sizes, due to the improved heat recovery effectiveness. Finally, transcritical CO2 systems outperform ORCs when the power output exceeds 1MW and for temperatures higher than 350°C.