Thermodynamic Investigation of a Novel Hybrid Carnot Battery with Thermal Storage at Two Temperature Levels

Energy storage is a critical step for the decarbonization of our society by exploiting renewable energies. Carnot batteries are promising technologies for storing excess electricity from renewables, but their efficiency is generally limited. The present work suggests a novel idea for enhancing the performance and flexibility of Carnot battery units. Specifically, a hybrid system that exploits both electricity and waste heat streams is suggested, incorporating two stages of latent thermal storage to increase flexibility. Moreover, the present system uses an innovative cascade high-temperature heat pump, with an absorption heat transformer in the low stage and a vapor-compression unit in the upper stage. The power production machine is a recuperative organic Rankine cycle. Practically, the absorption heat transformer exploits the waste heat stream by upgrading it and reducing the temperature lift that the vapor-compression heat pump produces, so there is increased electrical performance with suitable consumption of the waste heat stream. Also, the respective configuration without the absorption heat transformer is studied to make a suitable comparison. The results of this analysis show that the adoption of the AHT enhances the roundtrip efficiency from 26.30% up to 92.31% (relative enhancement) compared to the system without it. For the main scenario with a waste heat stream temperature at 100 °C, the roundtrip efficiency was found to be 72.86% and the exergetic efficiency as 35.02% for the system with the absorption heat transformer.