Performance Study Of Organic Rankine Cycle System And Working Fluid Selection At Low Condensation Temperature

The content of medium and low temperature heat energy is huge（geothermal energy,solar energy,industrial waste heat,etc.）,so it is of far-reaching significance for China’s energy structure transformation to effectively utilize these medium and low-grade heat energy.At present,organic Rankine cycle has become one of the most effective and widely used ways in the utilization of medium and low temperature heat energy.Based on the organic Rankine cycle power generation system,different from the research papers with heat source and evaporation temperature as the core variables,considering that the change of condensation temperature is an important factor affecting the performance of organic Rankine cycle power generation,especially the lower condensation temperature can significantly improve the system performance,this paper takes the perspective of condensation temperature as the starting point,the influence of working medium and working condition on system performance is systematically.Based on the thermodynamic relationship,a simplified thermodynamic analytical model of organic Rankine cycle power generation system is established,and compared with the previous research conclusions and results,the essential influencing factors behind the specific laws are analyzed.According to the calculation results of the theoretical numerical model,the error of the simplified formula applied to a variety of working fluids is verified,and a specific analysis is made,which further shows that the accuracy of the simplified theoretical analytical model is relatively high（the error less than 5%）.An organic Rankine cycle power generation model with variable condensation temperature is established by MATLAB.The effects of condensation temperature on the main components of the circulation system are studied.Taking the system exergy efficiency as the objective function,the relationship between the optimal evaporation temperature,the heat source temperature and condensation temperature is analyzed,and the optimal evaporation temperature function of many kinds of working fluids is obtained:T_opteva=K?T_{he at}（10）b?T_{co n}（10）c.In this paper,condensation temperature is taken as the core variable,and dozens of working fluid are compared and analyzed and their performance is optimized.When the heat source temperature is 140℃,considering the physical and chemical properties of the working fluid,the optimal circulating working fluid is R236ea under the conventional condensation temperature.And the net output power of butane is the largest under the ultra-low condensation temperature（-80℃and below）.However,if the problem of low condensation pressure is considered,propane will be the most promising working fluid at low condensation temperature.Taking critical temperature,eccentricity factor,condensation temperature and evaporation temperature as independent variables and cyclic net output work as dependent variables,the data are obtained by multivariable nonlinear fitting with global optimization algorithm,in which a=0.8394、b=0.005869、c=11.0089、d=10.5485,the determination coefficient reaches R²=0.982,which has high accuracy.The coupling systems of ORC-LAES-LNG is established,which reduces the lower limit of condensation temperature in organic Rankine cycle system,and widens the temperature range of heat source,improves the power generation efficiency of organic Rankine cycle system.Under the set parameters,the electric electric conversion rate of the coupling system is 81.8%,which has largely improved the system efficiency compared with the traditional LAES system.