Combined cycle with low-quality heat integration and water injection into the compressed air

The water injection after the compressor into the combustion air of a gas turbine and its evaporation by low-quality heat was analysed for different types of low-temperature heat sources and water-vapour contents of the pressurised air. To integrate low-quality heat—for example solar heat—into a conventional combined cycle efficiently and economically, the cycle must be changed in such a way that a maximum of low-temperature heat can be exploited. By injecting water into the compressed air, its evaporation takes place at partial pressure and therefore low-quality heat can be used. The new cycle uses only the stoichiometric air needed for the combustion. The air surplus for reducing the adiabatic temperature to the allowable gas turbine inlet temperature is substituted by water, which is injected into the compressed air and evaporated at partial pressure (i.e. at low temperature below 170 °C). After the heat recovery boiler the water-vapour content of the flue gas is recovered by partial condensation, cleaned and reused for injection. After presenting the features of the cycle, the paper concentrates on a preliminary design of the unit. Calculations were carried out for a variety of gas turbine inlet temperatures, pressure ratios and water-vapour contents. The results of the calculations are presented and the possibilities and difficulties of the low-quality heat integration will both be discussed.