Low-grade heat-based thermal cycles unconstrained by the Carnot factor doing work by cooling

The objective of the research is to convert heating and cooling energy into mechanical work capable of yielding high thermal efficiency while surpassing the Carnot factor (CF), even when using low grade heat sources. To achieve the proposed objectives, a closed process-based thermal cycle composed of two isochoric and two adiabatic path functions is studied, where one of the isochoric transformations is dedicated to heat absorption, and the other is dedicated to heat release. The analysis is carried out on the basis of double-acting cylinder-based engine types, operating according to a closed process-based thermal cycle with air, helium and hydrogen as the working fluids, characterised by performing mechanical work both in the expansion phase due to heat addition, and in the contraction phase due to heat extraction. The proposed thermal cycles yield acceptable performance, even at medium and low temperatures (that is, for an exceptionally low ratio of the high to the low temperatures of the heat source and heat sink, respectively). The main results derived from a case study operating between 300 and 340 (K) with air, helium and hydrogen, give an efficiency of 56.32 (%), 78.64 (%) and 56.54 (%) respectively, and while the specific work amounts 12.78 (kJ/kg), 71.24 (kJ/kg) and 182.90 (kJ/kg) respectively.