Cogeneration Exergy Analysis

With the development of economy, the improvement of people's living standard and the growth of population, the energy consumption has increased. The shortage of energy prompted in awareness of energy conservation. Exergy analysis as a comprehensive, reasonable of thermodynamics analysis method is widely used in chemical, heating and cooling, etc. Combined heat and power can make full use of heat and achieve " make the best use of heat", improving energy efficiency greatly. The exergy analysis method is used to calculate whether there is more space for energy saving of cogeneration in this dissertation.Firstly, based on the mathematical model of exergy loss ,we calculated the exergy efficiency , exergy loss rate and the exergy loss coefficient of major equipment and systems of condensing steam power plant in rated condition. Compare with the result of exergy analysis and energy analysis, we found that exergy analysis is more comprehensive. By changing the temperature and pressure of main steam, we found that the exergy efficiency is decrease when temperature of main steam decrease, and when pressure of main steam decrease, the exergy efficiency also decrease.Then, by changing the amount of steam in regenerative system, and a portion of the steam is supplied to heat users, cogeneration reform was changed to a extraction heating unit. The exergy of supply water and exergy efficient of heat users are also added in this dissertation. We found that, extraction-type cogeneration can reach the aim of exergy-saving. Finally, based on the same parameters of main steam, by changing the exhaust pressure, comparison of condensing steam power plant and back-pressure cogeneration, we found that when the exhaust pressure is less than 0.1Mpa, both efficiency and exergy efficiency of condensing steam cycle power plant is increase when the exhaust pressure reduce; and when the exhaust pressure is greater than 0.1Mpa, the exergy efficiency of back pressure cogeneration reducewhen exhaust pressure is reduce.Based on the result of exergy loss of this three different power systems, the exergy loss of boiler and turbine is the most part of the system. Therefore we use white-box model of boiler and unit model of turbine to do a detail analysis. We found that the largest exergy loss of boiler systems generate is the irreversible process combustion process, followed by the heat with mass loss. Through reducing the amount of air leakage and provide an appropriate amount of air and reduce the exhaust temperature to reduce the loss of boiler exergy efficiency. In the turbine, exergy loss rate of condensing unit is the largest of turbine, followed by deaerator and the third is low pressure heater. By reducing leakage, enhanced insulation, reduce heat transmission, changed the calandria method, tested the water quality , these exergy loss of turbine could be reduce.