Thermodynamic Optimisation of the Otto / Stirling Combined Cycle

Combined cycle systems are an established method for increasing primary energy efficiency of power generation systems. Some ongoing research is concerned with investigating the novel combined cycle system involving the Otto and Stirling thermodynamic cycles. The Otto cycle is to act as the topping cycle, with the Stirling cycle acting to recover heat from the exhaust for the purpose of additional power generation. The present work investigates the thermodynamic optimisation of the combined cycle system for the case of the engines operating under two imposed parametric constraints: 1) imposed heat addition to the Otto cycle, and 2) imposed maximum cycle temperature of the Otto cycle. These conditions are analogous to a specified fuel consumption of the engine and the metallurgical limit of the operating components respectively. The optimum work output for each scenario is analysed with respect to the particular physical constraints of the Stirling cycle heat exchangers – effectiveness, NTU, heat transfer coefficient and heat transfer area. Only interactions between the engine and the external source and sink are considered in this treatment. Regeneration within the cycle, as would typically be used within the practical engine, is considered as perfect. The existence of an optimum power output for the combined system is proven analytically. A numerical study is then presented to further investigate the performance for each of the parameters named above.