Technical Feasibility Assessment of a Recuperator in Reversible Trans-critical CO2 Heat Pump/Rankine Cycle: Considering Scroll Machine Operation Boundaries

Carnot batteries are getting more attention rapidly, as a potential energy storage technology coupled with variable renewable energy. Trans-critical heat pumps (THP)/Rankine cycles (TRC) using a natural fluid CO2 perform well with a sensible hot storage of a large temperature spread, and their capital cost can be further reduced in a reversible configuration. This study proposes a reversible THP/TRC based on a reversible scroll machine with hot storage but without cold storage for a small-scale family farm, using the ambient air as the secondary fluid of the evaporator/condenser in THP/TRC, respectively. The study assesses the feasibility of a recuperator in the reversible THP/TRC under optimal and limited operation ranges, taking into account the maximum pressure and temperature of the scroll machine. In optimal operation, the recuperator improves the power-to-power efficiency (P2P) up to 28 % (recuperator efficiency is 0.75) and reduces the optimal high pressure of both THP/TRC. In the limited operation, P2P is improved by a perfect recuperator up to 57 %, and this value decreases with recuperator efficiency from 1 to 0.25. The extension of the highest scroll pressure contributes to a higher possible hot storage temperature and less affected conditions for basic THP/TRC without a recuperator. The affected conditions in the recuperated THP/TRC are reduced by the higher temperature that the scroll machine can withstand. This work introduces operation boundaries of a reversible scroll machine as constraints in a reversible CO2 Carnot battery, supplying some reference for its practical system design and operation conditions.