Investigation of the performance of a combined Brayton/Brayton cycle with humidification

A humidified combined gas power cycle has been theoretically investigated in this paper. The topping cycle is a conventional air Brayton cycle, whereas the bottoming cycle has an air humidifier installed between the compressor and the recovery heat exchanger. Using mass and energy balance equations, the performance of the combined power plant has been calculated and its sensitivity to various operation parameters has been discussed. The energy efficiency and specific power generation varied linearly when the air mass flowrate in the bottoming cycle increased. The performance of the plant initially improved, reached a maximum, and then degraded when the topping cycle pressure ratio was increased. The same qualitative behavior was observed upon increasing the bottoming cycle pressure ratio while maintaining the topping cycle pressure ratio constant. The power output increased in a quasi linear fashion and the energy efficiency increased in a logarithmic fashion when the combustion chamber temperature was increased. In all these cases, humidification of the air after the bottoming cycle compressor proved to be a good means of improving the performance of the power plant, especially when the combustion temperature is low or the bottoming cycle pressure ratio is high.