Photo-thermal-tructural Properties Study Of Multi-heating Large-aperture Trough Concentrator System

The massive consumption of traditional fossil energy such as coal and oil,as well as environmental pollution,seriously restrict industrial development,people’s living standards and life health.Therefore,countries achieve the goal of"carbon neutralization"and the friendly development of industry and environment by developing renewable energy.As an indispensable part of renewable energy,parabolic trough solar photothermal power technology has been focused and vigorously developed.Using a larger-aperture parabolic trough concentrator（PTC）and increasing outlet temperature can reduce the investment cost,which has become the important research directions of the PTC system.In order to reduce the cost and promote the development of larger-aperture PTC system,this dissertation takes the concentrator with an aperture width of 8 m and a half angle of 80°as the research object.According to the theory of multi-heating,the absorber tubes（AT）with different shapes are developed at different temperatures section to make the outlet temperature of larger-aperture PTC system exceed 580℃and maximize the thermal efficiency.The main research contents and conclusions are as follows:Low temperature section（300-400℃）:According to the characteristics of relatively low temperature and small external radiation loss,a semi-circular AT with outer fins is constructed to increase the interception rate of solar rays and improve optical efficiency.At the same time,according to the deflection theory,a cross beam is added at the bottom of the semi-circular AT to form an integration between the beam and AT and reduce the deflection of AT.Through numerical simulation study,the optimal values are obtained when the radius is 50 mm and the fin width is 10mm,and the optical efficiency and thermal efficiency are 83.3%and 80.3%respectively.Among them,the optical efficiency contributed by the semi-circle part is 76.8%,and the fin part is only6.5%,so the semi-circle part plays a main role and the fin plays an auxiliary role.The height of the beam is 10 mm,the deflection of the AT is reduced by 4%（0.1 mm）,the average temperature is reduced by 1℃,the thermal efficiency is increased by 0.2%.The comprehensive enhanced heat transfer factorη_HTP=1.1,and the beam only increases the convective heat transfer area and does not form secondary flow.The sensitivity of thermal efficiency and surface temperature decreases when the flow rate exceeds 0.5 m/s（2.44 kg/s）.In addition,it is found that the local high temperature of the AT appears at the edge of the fin and is 50℃higher than the fluid temperature.Medium temperature section（400-500℃）:Comprehensive consideration of the interception factor and external radiation loss,the outer fins are removed to form a semi-circular AT.At the same time,a circular AT with flat reflector is also constructed.So the best choice in the medium temperature section is obtained through comparative study.The comparative study shows that:DNI=400-1000 W/m²,the average thermal efficiency of semi-circle AT with diameter of 100 mm is the highest,which is 2.5%higher than that of circle AT.At the same time,the uniformity of solar radiation flux on the AT’s surface is also the best and the uniformity is 38.8%.However,the deflection of the semi-circle AT with a diameter of 100 mm is the largest,which is 1.6 times（70mm）and 1.7 times（80 mm）,respectively.From the comprehensive comparison of thermal efficiency and deflection,the circular AT with a diameter of 80 mm is the most widely used.The semi-circular AT with a diameter of 100 mm has high efficiency and uniform of solar radiation flux,which is of the most research value.Therefore,the heat transfer enhancement of semi-circular AT is studied next.According to the field synergy theory,two extreme fins are added at the bottom of semi-circular AT,namely convex（short and thick）and ciliated（long and thin）,to enhance heat transfer.The results show that the thermal efficiency of convex finned tube is increased by 1.2%,and that of ciliated finned tube is better and increased by 1.6%.High temperature section（500-580℃）:According to the deflection theory,the deflection of the semi-circular AT in the high temperature section is large,so that it deviates from the original position and greatly reduces the optical efficiency.Therefore,the circular AT is selected,and the secondary reflector is constructed to improve efficiency.According to geometric optics,a flat secondary reflector is constructed,and a curved secondary mirror is also constructed by using adaptive algorithm.The two kinds of secondary mirrors are compared to choosing the best.The results show that the secondary mirror constructed by the adaptive algorithm makes the solar radiation flux on the AT’s surface more uniform,which is about twice that produced by the flat secondary mirror.However,the maximum half-angle of the primary concentrator is 50°,which can not meet the half angle requirements of the concentrator in this dissertation.Therefore,using flat secondary mirror in the circle AT,meanwhile the AT diameter of 70 mm is the best choice and the optical efficiency is 75%,which is 8.8%higher than without secondary mirror.At the same time,the thermal simulation results show that when DNI=1000 W/m²,the thermal efficiency is 60.5%.The strategy based on regulating flow is introduced to meet the requirements of outlet temperature580℃and maximum temperature lower than 600℃.DNI is 400 W/m²,600 W/m²,800 W/m²and 1000 W/m²,and the corresponding flow rate are greater than 2.21 kg/s,4.98 kg/s,8.29 kg/s and13.83 kg/s,respectively.Construction and research of multi-heating larger-aperture PTC system:According to the research results of each section,a multi-heating system is constructed to increase the fluid temperature from 300℃to 580℃.The results show that the optical efficiency is 79.1%,which is 4.5%higher than the traditional system with 80 mm circular AT.The total length of a single circuit is about 1400 m,and the mass flow is 6.1-19.9 kg/s when DNI=400-1000 W/m².The thermal efficiency is directly proportional to DNI,and the thermal efficiency is 64.3%（DNI=400 W/m²）,69.1%（DNI=600 W/m²）,71.4%（DNI=800 W/m²）and 72.8%（DNI=1000 W/m²）respectively.Meanwhile,the large-aperture PTC power station established in Dunhuang of Gansu Province（94.66 east longitude and 40.14 north latitude）has an annual average efficiency（η）of 24.50%,higher than the current of 20%.For the 50 MW PTC power with an investment of 1.8 billion RMB,if this system is adopted in PTC power station,the multi income is about 110 million in the service life.