Abstract
Finite-time thermodynamics with an ecological criterion is used to optimize the performance of an irreversible combined Carnot heat-engine. The irreversiblities come from two sources: (1) finite heat conductance for the three heat exchangers; (2) irreversibility inside the heat engine. The ecological function is defined as the power output minus the loss power or the product of the environmental temperature and the entropy rate. The ecological function is optimized with respect to two dimensionless cycle temperatures and thermal efficiency. The maximum ecological function and its corresponding power output and thermal efficiency are presented. It is shown that the ecological function is an important criterion for the design of an irreversible combined Carnot heat engine, considering not only the power output but also the thermal efficiency. When the three heat exchangers have equal heat transfer coefficients, the optimum heat-transfer area ratio for the middle heat exchanger is always equal to 1/3, independent of the irreversibility parameters for the top and bottom cycles and the heat reservoir temperature ratio. Furthermore, the optimum heat-transfer area ratio for the hotend heat exchanger decreases while that for the cold-end heat exchanger increases as the irreversibility parameters of the top and bottom cycles are increased.
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