Thermodynamic and economic evaluation of a novel heat supply design using low-pressure feedwater in a cogeneration plant

An innovative heat supply design for cogeneration systems has been proposed. Instead of the conventional heat source (extraction steam of the turbine), the low-pressure feedwater of the heat regeneration system is employed to provide district heat in the new scheme, through warming the supply-water of the primary heating network in a plate heat exchanger. A detailed thermodynamic analysis was conducted to evaluate the proposed design compared with the traditional one, based on a large-scale coal-fired power plant that needed to be retrofitted into cogeneration. The results show that the net power output of the cogeneration system is boosted by 4.68 MW owing to the novel concept. Furthermore, the total system efficiency and effective electric efficiency of the cogeneration system are promoted by 0.43 and 0.46 percentage points. This is because the exergy efficiency of the supply-water heating process is raised by 29.65 percentage points, and the exergy efficiency of the cogeneration system is improved by 0.42 percentage points. Finally, the new design was economically analyzed, and the results indicate that the net present value reaches 16543.2 thousand USD (8381.5 thousand USD more than that of the conventional design). Therefore, the proposed concept is feasible and profitable in engineering.