Minimum exergy destruction from endoreversible and finite-time thermodynamics machines and their concomitant indirect energy

A functional model of least exergy production (MLED) merges concepts of internal machine irreversibility, reservoir-to-machine thermal resistance, and reservoir-to-reservoir heat leaks with that of indirect energy used in the manufacture, operation and decommissioning of the engine. Thereupon an analytical solution yields the internal temperatures for the minimum destruction of exergy per unit work. In the absence of heat leaks or internal machine irreversibility, the corresponding cycle efficiency tends to the Carnot efficiency with zero indirect energy, and tends to the maximum power efficiency with large indirect energy. A similar approach is applied to a heat pump to yield an optimum coefficient of performance. It is proposed that with adequate databases of cycle irreversibility factors and indirect energy the MLED could be employed as part of a rapid, tentative first step in shortlisting the candidate technologies for localised power and heat supply. In a particular worked example (1) a proposal to replace centrally generated electricity with a local heat engine, fuelled with landfill gas, is rapidly shown to be worthy of a more detailed, structural analysis (2) for both the local and centralised heat engines optimum cycle efficiencies lie between the Carnot efficiency and the maximum power efficiency. •The approach accounts for exergy destroyed indirectly in the manufacture, operation and decommissioning of machines.•For heat engines, the optimum cycle efficiency lies in a range limited by the Novikov and Carnot efficiencies.•For heat pumps, an optimum coefficient of performance is achieved.•A worked example demonstrates rapid screening of alternative methods of power production.