Extended mapping and systematic optimisation of the Carnot battery trilemma for sub-critical cycles with thermal integration

Thermally integrated pumped thermal energy storage (TI-PTES) is a flexibility option to recover low-grade heat and provide overnight storage. Common criteria when designing such systems are the power-to-power efficiency (electricity recovery), the exergy efficiency (combined heat and electricity recovery) and the energy density (storage size). However, these are generally conflicting and multi-criteria optimisation is therefore required. Design guidelines have been proposed for some specific case studies but are still lacking for the remaining wide range of possible integrations. This work therefore presents a systematic multi-criteria analysis of a TI-PTES, consisting of a vapour compression heat pump, a sensible heat storage and an organic Rankine cycle, in an extended integration domain. Results show that the storage temperature levels are key variables, as they directly influence the conflict between the performance of the heat pump and the organic Rankine cycle. Also, the intensity of the conflict between the three criteria increases with the temperature difference between the source and the sink, mainly because of the power-to-power efficiency (the density and the exergy efficiency are much less conflicting with each other). Finally, the relevance of thermal integration in TI-PTES is questioned when it leads to a sharp deterioration in exergy efficiency and density.