Thermally integrated innovative Carnot batteries to upgrade and dispatch low temperature sensible waste heat

This work focuses on innovative thermally integrated Carnot batteries exploiting low temperature sensible waste heat, available at temperatures between 70°C and 100°C, while adopting a sensible solid-based thermal energy storage system. The charging cycle is based on transcritical heat pumps operating with CO2-based mixtures, representing the most innovative aspect of the work, whereas the discharging cycle runs with conventional pure fluid ORC. The heat pumps adopting CO2 doped with a low fraction of hydrocarbons can achieve very high second law efficiencies (around 70%), even in a simplified cycle layout with a sensible heat source, storing heat in the low-cost storage operating between 50°C and 150°C. Similarly, an ORC with cyclopropane as working fluid is identified as the most promising solution for the discharging phase of the Carnot battery. Results show RTE of around 62% and 84% for the thermally integrated systems with waste heat sources at 70°C and 100°C, respectively, and an air-cooled ORC. Finally, calculations show that RTE close to 100% is possible with a water-cooled ORC. Both the computed values of RTEs and the high compactness and effectiveness of the innovative heat pumps highlight the necessity for additional research into thermally integrated Carnot batteries as a way to exploit and dispatch sensible waste heat.