By Ferreira, A.C.M.; Nunes, M.L.; Teixeira, S.F.C.F.; Leão, C.P&pe
A numerical optimisation method, previously applied to a large industrial cogeneration plant has been adapted for a small-‐scale unit delivering 125 kW of useful heat and 85 to 105 kW of electricity, suitable for medium-‐size buildings. The unit is based on a micro gas-‐turbine and includes an internal pre-‐heater (typical of these types of small-‐scale units) and a Water Heater. The mathematical model yields a non-‐linear objective function, subject to physical constraints of system operation. The electrical power production is one of the six decision variables that were selected for the optimisation algorithm where the objective function is defined as the maximization of annual worth of the CHP system. A sensitivity analysis was carried out and showed that the relationship between the electricity feed-‐in tariff and the natural gas price significantly affects the heat to power ratio and the annual worth of the system. Regarding the physical aspects of the major system components, the analysis disclosed that the most relevant and sensitive variables are the compressor pressure ratio, the internal pre-‐ heater effectiveness and the turbine inlet temperature.