Abstract:

In this paper, exergy analysis is used to evaluate the performance of a micro-cogeneration unit using an alpha type Stirling engine. Exergetic efficiency is calculated for the whole system according to the second law of thermodynamics. Exergy analysis of each sub-system leads to the choice of the optimum physical parameters for minimum local exergy destruction ratios. Power generation, thermal efficiency and exergetic efficiency are used as successive objective functions. The best distribution of heat exchanger area is found by the optimization of each objective function. Energy, entropy and exergy balance are used for a control volume, assuming a steady-state operation of the engine, in order to deduce the overall irreversibility of the heat exchangers. Energetic, entropic and exergetic analyses are also presented for the whole system, taking into account the limiting physical constraints such as the maximum pressure and volume of the Stirling engine, imperfect heat exchangers, limited heat capacity of the source and the sink (temperature variation of each external fluid during heat transfer) and imperfect internal heat restitution in the regenerator. Hence, an endo- and exo-irreversible Stirling engine cycle is considered and optimized according to several performance criteria: energetic and exergetic. Functional diagrams are proposed in order to show internal and external irreversibilities (destroyed exergy flow and entropy flow generation) and to represent graphically entropy and exergy balance. The models developed in this work allow us to optimize the engine operation according to four criteria: maximum power, maximum thermal efficiency and maximum exergetic efficiency.