New approach to thermal power plants operation regimes maximum power versus maximum efficiency

Abstract

This work deals with the optimization of the thermal power plants based on Carnot exoirreversible cycle. The external irreversibility is due to the heat transfer at finite temperature differences between the working fluid and the external heat sources. Two fundamental operation regimes will be defined: (i) the power regime and (ii) the economical–ecological regime. The optimization of a thermal power plant with defined heat exchanger thermal conductances provides a “maximum power operation regime”. At a given value of the total heat exchanger thermal conductance, imposed by the system size restriction, for different possible values of the total thermal conductance distribution, one results an infinite number of “maximum power regimes”. Each of these regimes leads to a specific pair of optimal values for the temperature differences but keeps the same value for the thermal efficiency ( η opt = 1 − T 0 / T ). Among the maximum power operation regimes, there is only one “maximum-maximorum power regime”. In the maximum power regimes the entropy generation is not minimal. The “maximum-maximorum power regime” is characterized by the highest entropy generation. Another method of thermal power plant optimization with fuel consumption, respectively, input heat flux imposed, shows the existence of an “economical–ecological operation regime”. This regime has a maximum thermal efficiency and a minimum entropy generation at corresponding maximum power production. The “economical–ecological” is obtained for a specific pair of optimal values for the temperature differences and demands the equipartition of the thermal conductances, corresponds to the optimal design condition. In the “economical–ecological regimes” characterized by low value of the temperature differences at the high temperature source, corresponding to supercritical thermodynamic conditions, the thermal power plant obtains a higher thermal efficiency than the optimal value of “power operating regimes”. Among the infinite number of economical–ecological regimes, corresponding to different imposed values of the input heat flux, there is one with the highest power output—“the maximum-maximorum power regime”.