Proposal and study of a balanced hybrid Rankine–Brayton cycle for low-to-moderate temperature solar power plants

• Analysis of a hybrid Rankine–Brayton cycle with balanced recuperator.
• Configuration allows low irreversibility in all processes simultaneously.
• High efficiency for low temperature solar thermal power plants (below 500 °C).
• Fluid selection becomes important: Isobutane & R141b reach the highest efficiency.

A hybrid Rankine–Brayton cycle with balanced recuperator is proposed, as well as its operating conditions and potential working fluids, for low to moderate temperature solar applications (below 500 °C). The configuration allows the simultaneous decrease of the irreversibility at the heat source and sink and at the compression, mixing and regeneration processes. It is similar to the recompression and partial condensation CO2 cycles, but the selection of the fluid, which becomes a key aspect, introduces several advantages (improving performance and making the cycle simpler).The cycle working several fluids is optimised with a genetic algorithm and compared to other nine reference cases based on Rankine and Brayton cycles, conventional and advanced, also optimised. The proposed cycle, working with isobutane and R141b, reaches efficiency values higher than 39%, with compression ratios technologically feasible and suitable cooling temperatures. This efficiency is higher than those obtained with transcritical R125 Rankine cycles (11% higher), transcritical CO2 Rankine cycles (25%) and recompression CO2 cycles (34%), and it is comparable to that reached by complex regenerative steam Rankine cycles. Irreversibility is also comparable to regenerative Rankine cycles and lower than those of transcritical R125 (14% lower), transcritical CO2 (20%) and recompression CO2 cycles (37%).