Performance enhancement of a combined cycle using heat exchanger bypass control: A thermodynamic investigation

The purpose of the present study is to investigate the effect of connecting bypass valve between topping and bottoming cycles in a combined cycle system consisting of a regenerative topping gas turbine and bottoming air turbine cycles, the system explored using first and second laws of thermodynamics. The bypass valve placed between two heat exchangers and when this bypass valve opens the exhaust gasses of the turbine of topping cycle leave to the environment through both heat exchangers and when it is close the exhaust gasses of the turbine of topping cycle leave to the environment through the second heat exchanger only. The effects of the bypass hot gases on the work output, efficiency and the exergy destruction of the combined cycle plant are investigated parametrically and all pertinent parameters and function (compressor pressure ratio and turbine inlet temperature) are simulated and implemented to reports their effect on the overall performance. The result shows that gain in net work output is from 13.5% to 45% when the temperature at the inlet of turbine of topping cycle is 1400 K and 14%-19%, at turbine inlet temperature of 1000 K. And the gain in net efficiency of the cycle is from 15% to 31% when the temperature at the inlet of turbine of topping cycle is 1400 K and 4%-15%, at turbine inlet temperature of 1000 K.