Analysis Of Dual Souce Power Generation System Integrated With Solar Power Tower And Fired Coal

With restricted by the technology development and resource distribution,the phenomena that the electric power is mainly generated from the coal will not change in a long time.It is evaluated that even in 2050,more than 60 percent of total power is from coal-fired plant.However,since the environment problem is more and more serious and the bad weather like haze or coal rain appears frequently,the standards of pollution emission and saving index for different industries have been improved again and again,especially for the coal-fired system.It means the large-scale structure reforming and the up-to-date technology realization are definite necessary right now.Among all kinds of presented approaches,the complementation between coal and solar to reduce the coal consumption from the energy origin is one of the most promising options.In the regular,the solar aided power generation(SAPG)means to use the heat from parabolic trough(PT)to replace the extracted high quality steam whose temperature range is low and so is the solar power efficiency.Solar tower power technology(SPT)have the advantages of high concentration,high efficiency and high similarities,the potential in the combination between SPT and coal-fired plant is huge.In this paper,the dual source power unit with solar tower and coal-fired boiler are taken as the research target,the dual heat-source boiler model,the heliostat field model,the thermal receiver model,the thermocline storage model and the molten salt exchanger model were built and researched subsequently.The key questions around the integration scheme and the maximum of solar absorbency on the side of coal-fired plant and the improved optical algorithm of heliostat field and the optimization of heliostat distribution on the side of solar power tower were discussed deeply and carefully,by which finally the impacts of solar irradiation intensity and storage volume on the annual solar power efficiency and saved coal consumption on the whole system level were compared and discussed.Based on the data of one 660 MW coal-fired plant in the northwest China,a boiler model with two heat source,solar and fired coal,was encoded by the thermodynamic laws and the classic heat transfer equations.Considering the operation parameters of different heat exchangers in the boiler,two kinds of integration schemes can be presented,namely superheating scheme and subcooling scheme.The thermal analysis of dual source boiler simulations showed that just simply coupling the solar heat with the original coal-fired boiler can break the thermal balance at the end of flue gas duct and aggravate the overtemperature in the supheaters which can result in the safety issues.To solve these problems,a flue gas bypass could be added at the outlet of furnace.The results proved that the thermal load between the superheaters and the reheaters could be rebalanced and the overtemperature risk in the final superheaters and platen heaters could reduce,and the thermal efficiency could be improved as well.Take the solar collector data of PS10 as references,the solar power efficiencies of two schemes are respectively 6.1% and 2.44% higher than that of PS10 plant on the design rate.If selecting Solar Two plant,the thermal power generation efficiency of superheating scheme was 5.25% higher than that of Solar Two turbine system.On basis of the operation characteristic of 660 MW dual heat source boiler,the steam quality at the outlet of boiler could be selected as the limitation to discuss the maximum absorbency of solar heat in the integrated system.For the superheating scheme,the maximum solar absorbency is 119 MW while the maximum standard saved coal consumption rate and is 23.78g/kWh and the maximum solar contribution to power generation is 7.13%.For the subcooling scheme,the maximum solar absorbency is 90 MW while the maximum standard saved coal consumption rate and the maximum solar contribution to power generation are respectively 17.89g/kWh and 4.9%.Then the sensitivity analysis of influence factors for the absorbency showed both increasing the fouling factor of platen superheater and decreasing the fouling factor of final superheater can rise the maximum absorbency,but the effect of the former is more remarkable.Moreover,increasing the steam temperature at the outlet of platen superheater can raise the maximum absorbency of subcooling scheme.By discussing the merits and drawbacks of Monte-Carlo ray tracing method and flat mapping method,an improved code aiming to compute the shading and blocking factor was presented,which has higher prediction accuracy,higher calculation accuracy and higher calculation speed.The prediction accuracy of shading phenomenon was demonstrated between 90% and 98% which the accuracy of blocking factor was around 80% ~ 85%.What's more,the calculation speed of the new code can accelerate with the optimization of heliostat field proceeding.Based on the improved shading and blocking calculation code,the whole optical model of heliostat field was built and then testified by the data of PS10 concentrating field.On consideration of the search difficulty for the optimal heliostat distribution,a combined optimization algorithm was presented,by which different types of layout were compared and analyzed.The outcome showed that in the north field biomimetic spiral layout behaves better than the others,but for the circular field,radial staggered layout seemed in the lead while the spiral layout in the outset zone was the best.By encoding the logic flow through the sub-systems,such as the solar power tower collector,the packed-bed storage tank,the double-pipe or shell-pipe heat exchanger and dual heat source boiler,the annual simulation of the 660 MW dual source power unit in a typical meteorological year could be conducted.The outcome showed the annual solar power average efficiency and the amount of saved standard coal could be affected by the quality of DNI(Direct Normal Insolation)and the quantity of insolation respectively.If the value of DNI was high enough,a storage volume growth of 1×60MWht could improve the annual solar power efficiency of the integrated system about 0.4~0.8% when the solar multiple is 1.33.In the annual operation,the low mass flow rate of packed-bed might increase the thickness of thermocline layer,then degrade the discharge efficiency.Under different DNI input and storage volume,the annual solar power efficiency of superheating scheme was about 17.5%~18.8% while that of the subcooling scheme is approximately 14.0%~15.0%.And both the two schemes can save more than 9×103 tons coal in one year while the maximum could arrive at 1.4×104 tons.By comparisons of two integration schemes,it could be found that the lower the integration temperature,the higher the solar collector efficiency,but the temperature had no effect on the discharge efficiency of storage tank.