Modeling And Dynamic Performance Simulation For Solar-driven Organic Rankine Cycle Power Generation System

There are plentiful low-grade energy sources such as solar energy,geothermal energy and waste heat energy in our country.The use of Organic Rankine Cycle to convert the low-grade thermal energy into mechanical work is of great significance to promote the energy conservation,emission reduction and environmental protection.However,the instability of the heat source and the presence of external disturbances seriously affect the safe and reliable operation of the system,resulting in decreased system efficiency.Therefore,it is necessary to optimize the control of the Organic Rankine Cycle power generation system.In view of the above problem,this paper studied the dynamic characteristics of the system under various disturbances and the performance of the off-design conditions.The research work carried out in this paper is as follows:1.Taking a solar energy Organic Rankine Cycle power generation test rig as the research object,the collector of the test rig is the parabolic trough collector,the heat exchangers are plate heat exchangers,and the expander is the scroll expander.The working fluid is R123,and the designed generating capacity is 8kW.Based on the moving boundary model,the dynamic heat transfer equations of the evaporator and the condenser were derived.The two-phase region used the average void theory to calculate the thermally physical parameters,flow and heat transfer property of the working fluid.The geometry of the scroll expander was analyzed,the leakage and heat transfer model of the working fluid in the expander was simplified,and the temperature and pressure of the working fluid in the expander which follows the rotational angle of the orbiting scroll were derived.According to the principle of the system,the coupling relationship of each component was determined,and the simulation model of the Organic Rankine Cycle system was established.2.The operating state of the system under the design conditions of cold and heat source(the heat source temperature 430 K,the heat conduction oil flow 2.0kg/s,the cold source temperature 303 K,the cooling water flow 1.5kg/s)is simulated,the dynamic responses of the system under the disturbance of heat transfer oil flow,cooling water flow,pump frequency and expander frequency are analyzed.3.With the heat conduction oil temperature between 426 K and 436 K,the cooling water temperature between 293 K and 303 K,and the pump frequency in the range of 38Hz-45 Hz,the performance of the system under off-design conditions are simulated.The main results of this paper are as follows:1.The dynamic simulation results of the system under the disturbance of heat conduction oil flow,cooling water flow,pump frequency and expander frequency are as follows: 1).The step of heat conduction oil flow is 0.2kg/s,and the self-balancing time of the system is 30 s.The evaporation pressure increases by 0.15 MPa,the superheat increases by 1.5°C,the output power increases by 0.03 kW,and the power generation efficiency decreases by 0.04%.2).The cooling water flow step is 0.2kg/s,the self-balancing time of the system is 25 s,the evaporation pressure reduces by 0.007 MPa,the superheat increases by 0.5°C,the output power increases by 0.07 kW,and the power generation efficiency increases by 0.03%.3).The frequency of pump step is 2Hz,the system working fluid flow increases by 0.017kg/s,the evaporation pressure increases by 0.15 MPa,the working fluid superheat decreases by 4.7°C,the expander output power increases by 0.25 kW,and the system power generation efficiency increases by 0.28%.4).When the frequency step of the expander is-2 Hz,the variation of the evaporation pressure of the system is approximate equivalent to the frequency step of pump,however,the response time is doubled,the superheat degree reduces by 1.5 °C.The effect of mass density shows a trend of decreasing first and then increasing.After stabilization,the output power phase reduces by 0.01 kW,and the system power generation efficiency increases by 0.1%.It can be obtained that the working state of the cycle fluid is less affected by the disturbance of the cooling water than the heat conduction oil;compared with the temperature disturbance of the heat conducting oil,the flow disturbance has little influence on the evaporation pressure and the superheat of the working fluid,and the system can also reach a new balance in a shorter time;the evaporation pressure and the superheat should be used as the controlled variable of the system;the change of the frequency of the expander can quickly respond to the change of the evaporation pressure,and the frequency of the pump can cause a large change to the superheat.2.Simulation results of off-design conditions show that during the process of changing the temperature of the heat conduction oil from 426 K to 436 K,the evaporation pressure and superheat increase significantly,and the output power of the expander also increases.The irreversibility increases in heat transfer process in the evaporator gradually reduced the power generation efficiency of the system from 8.67% to 8.52%.As the cooling water temperature changes from 293 K to 303 K,the mass flow rate of the working fluid in the system decreases with the increase of the cooling water temperature,the output power of the expander decreases,and the power generation efficiency of the system decreases continuously.Under the design conditions of cooling and heat source,the output power of the expander increases with the increase of the frequency of the pump,and the power generation efficiency of the system increases first and then decreases.When the pump frequency is 41 Hz,the efficiency reached a maximum of 8.64%.In this paper,the dynamic model of the Organic Rankine Cycle system is established.The influences of each disturbance on the operating state of the system and the performance of the system under off-design conditions are studied.It has certain significance for the anti-disturbance control and optimal adjustment of the system.