Thermodynamic analysis of multi-heating cycles working around the critical point

•Multi-heating cycles supply heat at low temperature along the regenerator.•Multi-heating increases exergy efficiency by reducing regenerator irreversibility.•Exergy efficiency of multi-heating cycle is up to 24% greater than in simple cycle.•Parametric analysis identifies the optimum conditions for multi-heating cycles. Multi-heating cycles reduce regenerator irreversibility, resulting in an increase of exergy efficiency. While a single heat source supplies the thermal power in simple regenerative cycles, in multi-heating cycles additional heat sources at lower temperatures supply part of this power along the regeneration. The greater the number of these additional heats, the higher the exergy efficiency is. An exergy analysis compares multi-heating and simple regenerative cycles working in the surroundings of the critical point. The main purpose is to identify the thermodynamic region in which there is a greater benefit of multi-heating, trying to abstract from the fluid as much as possible. The effect of compressor inlet temperature and limiting pressures is studied by means of parametric analysis. Cycles with the compressor working at pressures above the discontinuity line (set of coexistence line and Widom line) have small precooler irreversibilities and large regenerator irreversibilities. Multi-heating cycles take advantage of these conditions since they can reduce the regenerator irreversibilities and maintain the precooler irreversibilities small, leading to high exergy efficiencies. The results show that the exergy efficiency of multi-heating cycles may be up to 24% greater than in simple cycles.