The use of combined heat and power (CHP) systems to produce both electric and thermal energies for medium-size buildings is on the increase, due to their high overall efficiency, high energy prices and political and social awareness. In this paper, an energy-economic study is presented. The main objective is to implement an analysis that will lead to the optimal design of a small cogeneration system, given the thermal power duration curve of a multi-family residential building. A methodology was developed to obtain this curve for a reference B-class building located in the North of Portugal. The CHP unit is based on a micro gas-turbine and includes an Internal Pre-Heater (IPH), typical of these types of small-scale units, and an external Water Heater (WH). A numerical optimization method was applied to solve the thermo-economic model. The mathematical model yields an objective function defined as the maximization of the annual worth of the cogeneration system. A purchase cost equation was used for each major plant component that takes into account size and performance variables. Seven decision variables were selected for the optimization algorithm, including performance of internal gas-turbine components and electrical and thermal powers. The results show that, the revenue from selling electricity to the grid and fuel costs have the greatest impact on the annual worth of the system. The optimal solution for the small CHP is sensitive to fuel price, electricity feed-in-tariff, capital cost and to the thermal load profile of the building. High European energy prices point towards future micro gas-turbines with better electrical efficiencies, achieved via a higher pressure-ratio compressor and turbine inlet temperature.

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