The Study Of Direct Steam Generation Solar Thermal Power System Based On Two-Stage Accumulators And Cascade Rankine Cycles

In direct steam generation(DSG)solar thermal power system,water is used as the working medium for solar collectors and heat storage simultaneously,resulting in simple system structure and attractive thermo-economic performance.Currently,commercial DSG solar thermal power plants usually use single-stage accumulators.The supplied saturated steam pressure and mass flow rate decrease during flashing heat discharge process.To avoid inefficient power generation,the final temperature drop of water in the accumulators is within 50℃,thereby leading to a limited storage capacity.To solve the crucial issue,this thesis proposed an innovative two-stage accumulators based on cascade Rankine cycles,including organic Rankine cycle(ORC)with good thermodynamic characteristics at low-medium temperature sources.The system involves unique two-step heat discharge.In the first step,heat is released via water vaporization in a high-temperature accumulator(HTA)to drive the top steam Rankine cycle,and the condensation heat of exhaust steam is used to drive the bottom ORC cycle.In the second step,the saturated water at a reduced temperature flows from the HTA to a low-temperature accumulator through a heat exchanger and the discharge heat is only used to drive the bottom ORC cycle.This thesis studied three types of two-stage accumulators-baed DSG systems using steam-organic Rankine cycles(SORC),cascade organic Rankine cycles(CORC)and evacuated flat-plate collector(EFPC),respectively.The specific research contents and innovations are as follows:(1)Relying on the second-tep heat discharge mode,the SORC cycle in the two-stage accumulators-based SORC-DSG system can avoid severe off-design operation,and the bottom ORC cycle can operate under rated condition for a long time.The results show that the temperature drop of water can be as high as 130～190℃during the second-step heat discharge process,resulting in an increase of the system’s heat storage capacity by 460%.The corresponding equivalent payback period is less than 5 years,indicating that the two-stage accumulators can improve the cost-effectiveness of DSG system.(2)A new indicator,namely equivalent heat-to-power efficiency(ηeq)is established to optimize the steam condensation temperature(T2)of the two-stage accumulators-based SORC-DSG system.The results show that ηeq is a better indicator that SORC thermal efficiency,the optimum steam condensation temperature(T2,opt)corresponding to the maximum equivalent heat-to-power efficiency(ηeq,max)is higher that that based on the maximum SORC thermal efficiency.The T2,opt andηeq,max of pentane are 139～190℃ and 20.93%～24.24%,respectively.(3)The adverse impact and erosion of steam water droplets on the steam turbine can be avoided by using CORC instead of SORC,and the techno-economic performance of the two-stage accumulators-based DSG system is further improved.The results show that when benzene,toluene or cyclohexane is used as the ORC working fluid,the maximum difference between ηeq,CORC and ηeq,SORC respectively corresponding to the two-stage accumulators-based CORC-DSG and SORC-DSG systems is about 1.95%,and ηeq,CORC,max is only 0.13%～0.98%lower than ηeq,SORC,max.(4)The two-stage accumulators-based EFPC-DSG system has low requirements for solar radiation resources,further expanding the applicable areas of the two-stage accumulators-based DSG solar thermal power system(such as the central and eastern regions of China).The total equivalent heat-to-power efficiency(ηsys,eq)of the system is above 6.0%when the global solar irradiation is higher than 600 W/m2.The monthly average output power of a two-stage accumulators-based EFPC-DSG system with 100 kW installed capacity in Beijing,Hefei and Guangzhou are 353.73～1122.31 kWh/day,75.15～871.47 kWh/day and 86.69～701.37 kWh/day,respectively.(5)The experimental results show that the EFPC collector has good performance at medium working temperature,and the collector’s thermal efficiency can reach 36.1%～50.6%at the temperature of 155～173℃.The condenser outlet pressure and the reduced coefficient of pressure ratio of expander are increased when the bottom ORC system contains non-condensable gases,and the output power is decreased by a maximum value of 114%,which means that the non-condensable gases in the bottom ORC system should be removed in time.