Energy and entropy analysis of closed adiabatic expansion based trilateral cycles

• Published in: Energy conversion and management 2016-07, Vol.119, p.49-59
• Main Authors: Garcia, Ramon Ferreiro, Carril, Jose Carbia, Gomez, Javier Romero, Gomez, Manuel Romero
• Format: Article
• Language: English
• Subjects: Adiabatic expansion; Adiabatic flow; Carnot factor; Entropy; Entropy analysis; Exergy; Heats (energies); Operating temperature; Rejection; Second law efficiency; Thermal efficiency; Thermoelectricity; Trilateral cycle
• ISSN: 0196-8904; 1879-2227
• DOI: 10.1016/j.enconman.2016.04.031

•The adiabatic expansion based TC surpass Carnot factor at low temperatures.•The fact of surpassing Carnot factor doesn’t violate the 2nd law.•An entropy analysis is applied to verify the fulfilment of the second law.•Correction of the exergy transfer associated with heat transferred to a cycle. A vast amount of heat energy is available at low cost within the range of medium and low temperatures. Existing thermal cycles cannot make efficient use of such available low grade heat because they are mainly based on conventional organic Rankine cycles which are limited by Carnot constraints. However, recent developments related to the performance of thermal cycles composed of closed processes have led to the exceeding of the Carnot factor. Consequently, once the viability of closed process based thermal cycles that surpass the Carnot factor operating at low and medium temperatures is globally accepted, research work will aim at looking into the consequences that lead from surpassing the Carnot factor while fulfilling the 2nd law, its impact on the 2nd law efficiency definition as well as the impact on the exergy transfer from thermal power sources to any heat consumer, including thermal cycles. The methodology used to meet the proposed objectives involves the analysis of energy and entropy on trilateral closed process based thermal cycles. Thus, such energy and entropy analysis is carried out upon non-condensing mode trilateral thermal cycles (TCs) characterised by the conversion of low grade heat into mechanical work undergoing closed adiabatic path functions: isochoric heat absorption, adiabatic heat to mechanical work conversion and isobaric heat rejection. Firstly, cycle energy analysis is performed to determine the range of some relevant cycle parameters, such as the operating temperatures and their associated pressures, entropies, internal energies and specific volumes. In this way, the ranges of temperatures within which the Carnot factor is exceeded are determined, where carbon dioxide, nitrogen, helium and hydrogen are considered as real working fluids, followed by an entropic analysis in order to verify 2nd law fulfilment. The results of the analysis show that within a range of relatively low operating temperatures, high thermal efficiency is achieved, reaching 44.9% for helium when the Carnot factor is 33.3% under a ratio of temperatures of 450/300K. With respect to entropy analysis, it is verified that the results of the latter demonstrate compliance