A Study On Characteristic And Efficiency Optimization Of Organic Rankine Cycye Driven By Low-Enthalpy Heat

There are abundant low-enthalpy heat in China, such as solar, geothermal, biomass, industrial waste heat and so on. Recycling this part of the energy is help for energy conservation. For the temperature of low-enthalpy heat range in 70℃ to 150℃, the Organic Rankine Cycle technology is applied in this paper, thus achieving efficient utilization of it. To optimize the efficiency of the ORC, two main aspects are studied:optimizing the cycle design and applying the mixtures to the ORC with the purpose of reducing the loss of available energy. The main efforts are shown as following:1) Based on RKS equation of state, a calculation program has been elaborated for common refrigerants by using VB language, and error analysis of physical parameters for the four pure refrigerants (R245fa, R152a, R123 and R124) have been made; 2) Each part of the ORC system are modeled and analyzed, including the evaporator, the condenser, the expander, the working fluid pump and the indoor heat exchanger. And then a simulation program of the ORC system has been developed. And the net work output, cycle efficiency, exergy efficiency and thermal efficiency are set to be the cycle indicators to evaluate the performance of the ORC system; 3) Run and debugging the basic ORC system, CHP ORC system, and the ORC system equipped with IHE. And the performance of the ORC under different structures and parameters like the temperature of heat source and heat sink, the type of working fluid and the superheat are studied; 4) An advanced calculation program are elaborated for mixtures, and error analysis of physical parameters for the two mixtures (R152a/R245fa, R123/R124) have been made. Then a simulation program of the ORC system with mixtures has been developed. And the performance of the ORC systems between pure and mixtures are compared.The following conclusions can be obtained in this paper:(1) For the calculation program, apart from the liquid phase density, the relative errors of other thermophysical properties of the four pure refrigerants (R245fa, R152a, R123, R124) and two binary non-azeotropic mixtures (R152a/R245fa, R123/R124) are less than 3%, meet the accuracy requirements in this paper.(2) For the basic ORC with pure refrigerant, as the temperature of the heat source increases, the work output increases, while the cycle efficiency and exergy efficiency increase first and then decrease. And the optimum efficiencies for each cycle are in the range of 6.5% and 7.5% when the temperature differences between the heat source and heat sink are in the range of 55℃ to 75℃.(3) For the CHP ORC system with R245fa, the domestic hot water can be produced when the heat sink inlet temperature is higher than 40℃, and the corresponding exergy efficiency and overall thermal efficiency are 29% to 56% and 87% to 90% higher than those in the non-CHP ORC, respectively. Besides, the IHE has little effect on the improvement of work output and efficiencies for the CHP ORC.(4) For the ORC system with mixtures, the performance of the systems with mixtures of R152a/R245fa and R123/R124 are better than that of the systems with pure refrigerants of R152a, R245fa, R123 and R124. When the mole fractions of R152a/R245fa and R123/R124 are 0.6/0.4 and 0.7/0.3 respectively, the work output per unit mass of refrigerant are optimal, the maximum values are 24.346 kW and 19.243 kW respectively. And when the two mole fractions are 0.5/0.5 and 0.6/0.4 respectively, the cycle efficiency and exergy efficiency are optimal, the maximum cycle efficiency are 8.67% and 8.72% respectively, and the maximum exergy efficiency are 36% and 35.73%.The results of this paper have significance in improving the efficiency of using low-enthalpy heat. And the simulation programs of the ORC system with pure and mixtures refrigerants can provide theoretical basis and technical guidance for the experiment and practical application in the future.