Performance Analysis And Heat Transfer Characteristics Research Of Mixtures As Working Fluids For The Low-medium Temperature Organic Rankine Cycle

Organic Rankine cycle(ORC) is one of the main efficient ways to utilize the low-medium temperature geothermal source, its performances are related to many respecters, such as conditions of heat source, structure of the system, choice of the heat exchanger, expander efficiency, working fluid of the cycle used, etc. Selecting appropriate working fluid is a key factor that affects system performance. Studies had found that zeotropic mixtures could effectively improve the efficiency of system,increase the electricity capacity in organic Rankine cycle. In order to optimize working fluids for low-medium temperature geothermally powered ORC, HFCs and HCs is mixed with each other, depending on flame retardancy of HFCs to overcome the strong flammability of HCs and also the mixtures of HFOs and HCs are researched which has lower cost than the pure HFOs. Based on the first and second laws of thermodynamics, the system model is established, and the characteristics of mixture and its influence on system performance are analyzed under different evaporation temperature and different composition proportion by using the simulation calculation in the article. Also the flow boiling heat transfer characteristics of the mixtures of R245 fa and R601 a in a horizontal single tube and shell side are investigated, the conclusions are as follows:Temperature glide makes the working fluid and heat source mach better, which can improve the system performance. But under the condition of a certain system,excessive glide maybe lead the working fluid evaporating un-completely, which will result in fluid hammer in the turbine and make a bad affection on the operation of the system.The maximum net power output of the mixtures appears at the proportion of the highest temperature glide, and it exist an optimal evaporating temperature. When the evaporating temperature exceeds a certain value, the net power output of some mixtures decrease. So based on the specific conditions to select appropriate blending ratio for mixtures, otherwise mixtures will not have advantages compared with the pure fluids.For the evaporating temperature above the optimal evaporating temperature, the net power output is inversely proportional to the thermal efficiency, so the thermal efficiency is not preferable to be only focused on the design of power plant.The performance of R245fa/R600a(0.9/0.1) is more preferable among the working fluids researched in the paper. Also in addition to the flammability, the performance of R245fa/R601a(0.4/0.6) is suitable.The loss in the evaporator is the main part of the energy loss of the whole system which decreased dramatically with the evaporation temperature. And the minimumenergy loss in the working medium pump can be neglected. Therefore, optimizing the structure of the heat exchanger can improve the heat-transfer level and reduce the irreversible loss of the evaporator, which is an effective way to enhance the efficiency of the system.Experimental studies have found that with the increase of the heat source temperature, the convection heat transfer coefficient of the mixtures showed a trend of decrease after the first growth along the length direction of the whole tube from inlet to outlet of the working fluid. With the increase of working fluid flow rate, the convection heat transfer coefficient increases, and the heat source temperature is higher, the greater the increase amplitude.Temperature glide has a certain influence on the heat transfer coefficient of the working fluid. The specific influence is associated with the characteristics of its pure components.