Optimization of combined heat and power cogeneration via modification of low-pressure regenerative system with absorption heat exchanger

The large exergy loss during heat transfer is a key problem for the combined heat and power cogeneration. Instead of the traditional heat source coming from high-grade extracted steam at the outlet of the intermediate-pressure turbine, a novel combined heat and power configuration is proposed for large heat demand by modifying the low-pressure regenerative system. By adding a new LP unit with sufficient extracted flow and heat exchange, a part of the heat source from high-grade extracted steam at the outlet of the intermediate-pressure turbine is replaced by low-grade extracted steam within the LP turbine, avoiding the off-design operation of the LP turbine in the traditional combined heat and power configuration. In addition, the new combined heat and power configuration is optimized by introducing an absorption heat exchanger. The absorption heat exchanger lowers the return temperature of the PHS, which further reduces the energy grade of the heat source. The thermodynamic results show that the electric efficiency of the novel combined heat and power configuration is increased by 4.24% in total at a given heat demand of 241.37 MW. Exergetic results reveal that the exergy loss of two heat exchange links is drastically reduced from 30.47 MW and 26.04 MW to 8.80 MW and 11.81 MW respectively. An economic analysis of the new CHP configuration indicates that the dynamic payback period counteracting the investment increment is only 2.15 years, and the net present value increment over a 25-year period can reach $55.818 million.