In recent years, cogeneration systems have gained increasing attention especially when dealing with distributed generation for residential buildings. One of the main problems with using cogenerative systems in residential building applications is that the demand for heat and electricity is not synchronized. For this reason, when the combined heat and power system operates during electricity peak hours (i.e. the rate of the electricity is higher), it could be profitable to store the heat in order to satisfy delayed demands. This paper presents a model for the calculation of the profitability of micro combined heat and power systems for residential building applications. The model takes into account hourly demands calculated by means of monthly and daily load profiles for heat and electricity. The system under consideration is composed of a CHP system, an auxiliary boiler and a heat-storage tank. The model is applied to a single-family dwelling in order to evaluate the effect of the size of the thermal energy storage unit on the energy and economic performance of four different prime movers (an internal combustion engine, a Stirling engine, a micro Rankine cycle and a thermophotovoltaic system). Thermal energy produced, electrical energy produced, self-consumed or exchanged with the grid, consumed natural gas, as well as differential cash flow with respect to separate generation and payback period are presented. The effect of the size of the thermal energy storage proves to be not linear with respect to the thermal power of the prime mover.
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