Experimental Study And Optimization Of Organic Rankine Cycle Power Generation System And Evaporator

Organic Rankine cycle(ORC)is one of the important technical approaches to transform low temperature heat source into high-grade electric energy.There are a lot of industrial waste heat and renewable low-grade energy such as solar energy and geothermal energy in China.Therefore,it is of great significance to carry out research on organic Rankine cycle power generation system for the efficient utilization of low-grade energy and improvement of energy supply in China.Although plenty of studies have been carried out by domestic and foreign scholars,there are many scientific and technical problems in the operation performance of variable load system and the optimization of evaporator from the system and component level,which need to be further studied.Therefore,the dissertation first carries out experimental study of organic Rankine cycle,then uses the second law of thermodynamics to simulate the system,and then carries out optimization of the system and the evaporator to enhance the heat transfer.Aiming at the thermodynamic performance of the system under the condition of variable load operation caused by power grid peaks and troughs,an organic Rankine cycle prototype with heat source power of 100 kW and maximum operating pressure of 1.0 MPa was established,and R245fa working fluid was selected for experimental study.Under the condition of constant heat source parameters,the thermodynamic performance of the system under different expander speed and load conditions was studied.It is found that the working fluid flow rate and evaporation temperature increase with the increase of expander speed and load power.The superheat of organic working fluid at the inlet of expander decreases.The expansion ratio of expander increases with the increase of load power and expander speed.The net efficiency of organic Rankine cycle system increases with the increase of load,when the load is the same,it increases with the increase of expander speed.In order to further improve the thermodynamic performance of the system,the simple system is studied by means of exergy analysis.For the low-grade heat source whose heat source temperature is between 100? and 150?,it is found that the system exergy efficiency increases with the increase of heat source temperature.When the heat source temperature is the same,the system exergy efficiency first increases and then decreases with the increase of evaporation temperature.By analyzing the exergy loss of heater,condenser,working fluid pump and expander,it is found that the exergy loss of heater is the largest under the same heat source and working fluid conditions.For example,when working fluid is R600a and heat source temperature is 140?,the exergy loss of heater accounts for 51%of the total exergy loss of the four parts.According to the principle of energy cascade utilization,a new configuration of two-stage organic Rankine cycle power generation system is proposed from the system point of view,so as to improve the thermodynamic performance of the system.Under the same parameters,the exergy efficiency of the two-stage power generation system is higher than that of the single-stage cycle system,and there is a maximum.For example,when the heat source temperature is 130? and the working fluid is R600a,the exergy efficiency of the two-stage cycle system is 10.1%higher than that of the single-stage system.The mass split ratio of the preheater in the double cycle system is related to the temperature of the working fluid and heat source.The range of the time split ratio is larger at low temperature and smaller at high temperature,and the change trend presents the shape of isosceles triangle.From the perspective of enhanced heat transfer of components,the mechanism of gas-liquid automatic phase separation for the enhanced heat transfer in the process of working fluid flow boiling in evaporator was proposed,and the heat transfer performance of parallel structure and phase separation structure was verified by experiments.It is found that in the saturated boiling region,due to the automatic phase separation of gas and liquid under the action of surface tension,the formed liquid flows on both sides of the heating zone/gas,which enhances the heat transfer ability in the flow boiling stage.For example,when the heat flux of the heat source is 120 kW/m2 and the flow rate of the working medium is 0.4 g/s,the local heat transfer coefficient(at 25 mm)of the split phase structure is increased by 20.4%and the average heat transfer coefficient is increased by 9.9%.In the nucleate boiling region,because the bubbles are small,they directly enter the next microchannel,and there is no gas liquid phase separation phenomenon,so the heat transfer coefficients of the two structures are basically the same.The structure of gas-liquid phase separation was optimized from two aspects of structure parameters and micro/nanostructured surface.The results show that the local heat transfer coefficient and the average heat transfer coefficient of the phase separation structure increase with the increase of the gas-liquid separation angle and the number of segments.For example,when the working fluid mass flow is 0.4 g/s and the heat source power is 120 kW/m2,the gas-liquid separation angle at 20° is higher than that at 10°.The local heat transfer coefficient at 25 mm increased by 39.9%,and the average heat transfer coefficient of heating zone increased by 28.4%.When the gas-liquid separation angle is 20°,the average heat transfer coefficient of 6-stage structure at 25 mm is 22.6%higher than that of 4-section structure,and the heat transfer enhancement factor is 1.6.For promoting heat transfer enhancement using micro/nanostructured surface,Ag-Ni composite micro/nanostructured surface was prepared by brush plating in the channel with 6-stage structure and 200 separation angle.It was found that the micro/nanostructured surface advanced the occurrence of saturated boiling of working medium,and enhanced the boiling heat transfer.For example,when the heating power is 120 kW/m2 and the mass flow rate of working fluid is 0.4 g/s,the local heat transfer coefficient of micro/nanostructured surface at 20 mm is 46.1 kW/m2·K,while that of 6-stage structure is 12.5 kW/m2·K,which indicate an increase by 3.6 times,and the average heat transfer coefficient of the micro/nanostructured surface is 16.1%higher than that of the 6-stage structure,and the heat transfer enhancement factor is 1.9.