Convection Heat Loss Characteristics Of The Heat-Pipe Receiver In Solar Dish/AMTEC Thermal Power System

Due to energy and environment problems, as well as increasing demand forelectricity, solar thermal power becomes one of the most promising and challengingtechnologies for its clean and renewable energy resource. Solar dish power system, asone of the three generic solar thermal power systems (trough, tower, and dish/enginesystems) is characterized by high efficiency, modularity, autonomous operation, and aninherent hybrid capability (the ability to operate on either solar energy or fossil fuel, orboth). On the one hand, the traditional solar dish thermal power systems such as thedish/Stirling system which directly converts the solar radiation into electricity are beingchallenged. On the other hand, solar receiver plays an important role in light–heatconversion for the dish/engine systems, and normally subjected to various modes ofheat loss. Among these, convection heat loss is a major contributor of the total energyloss of solar receiver. However, the convection heat loss mechanism of solar receiver isso complex that it is difficult to calculate. Therefore, the estimation of convection heatloss of solar receiver is a key input for the performance evaluation of solar dish powersystem; also, it is nowadays a priority research line in the field of solar energy.In this dissertation, on the basis of a thorough literature survey and a comparativestudy, firstly, a new conversion power unit—alkali metal thermal to electric converter(AMTEC) is proposed to cascade with the dish collector system, forming a new solarthermal power system, namely solar dish/AMTEC thermal power system. Meanwhile, anew configuration of heat-pipe receiver is introduced to realize isothermal light-heatconversion for the solar dish/AMTEC thermal power system. After that, taking the heat-pipe receiver as the object, three-dimensional numerical studies are performed to getinsight into the flow and heat transfer characteristics of air inside and in the vicinity ofthe receiver under no-wind and wind conditions. Based on the air thermophysicalproperty variations, the influence of relevant parameters, such as aperture position andsize, cavity aspect ratio, tilt angle, wall temperature and environmental wind on naturalor combined free-forced convection heat loss of the heat-pipe receiver is investigated.The combined convection heat loss of the heat-pipe receiver under wind condtion iscompared to the natural convection heat loss under no-wind condition. The impact ofwind on combined convection loss as well as the coupling characteristics of windinduced convection and the natural convection of air in the cavity are clearly presented, Accordingly, Nusselt number correlations that can estimate natural and combined free-forced convection heat loss with reasonable accuracy are proposed, and they arecompared with existing models. Then, based on the numerical results, the overallthermal-electric conversion efficiency of the solar dish/AMTEC thermal power systemis calculated; also, the relationship between the system efficiency and variousparameters is discussed in detail. Finally, an experiment using the electric heatingmethod is carried out to explore the effect of tilt angle, heat flux and aperture ratio etc.on the heat losses of a cylindrical cavity receiver. On the one hand, this dissertationenriches the theory of solar receiver convection heat loss; on the other hand, it providesscientific basis for the design and performance improvement of the solar dish thermalpower system. The main findings are as follows:①For the natural convection heat loss of heat-pipe receiver under no-windcondition, three-dimensional numerical results reveal that, due to the air thermophysicalproperty variation with temperature, the numerical results agree better with theexperimental measurements. Meanwhile, the natural convection heat loss and theconvection heat transfer coefficient increases with increasing wall temperature whileNusselt number decreases as cavity wall temperature rises. The impact of apertureposition on the natural convection heat loss of receiver is closely related to tilt angle,while the aperture size has similar effect for different tilt angles. The expansion ofconvection zone together with the augmentation of velocity magnitude is mainlyresponsible for the increment of natural convection heat loss with decreasing cavityaspect ratio. Moreover, a modified definition of aperture ratio is introduced, aiming toreflect the combined effect of the cavity aspect ratio and aperture size. In addition,experimental investigation finds that, when the input power is constant, the naturalconvection heat loss decreases significantly as the tilt angle increases while theconduction heat loss and the radiation heat loss increase slightly. Different from theresults for the constant wall temperature condition, the natural convection heat loosNusselt number increases slowly with the increase of natural convection heat flux;while it decreases linearly with increasing wall temperature. In addition, the influenceof tilt angle on the radiation heat loss for constant heat flux condition is greater than thatfor constant constant wall temperature condition.②For the combined free-forced convection heat loss of heat-pipe receiver underwind condition, three-dimensional numerical results show that, different from thediscipline that the natural convection heat loss decreases monotonically with increasing tilt angle under no-wind environment, the combined convection heat loss of receiver inwindy environment is affected by both the wind and the tilt angle, and is more complex.Under some certain wind conditions, the combined convection heat loss possibly bereduced below the natural convection value, and there exists critical wind speed thatminimizes the combined convection heat loss. Moreover, the combined convection heatloss of receiver at different inclinations becomes more and more indistinguishable aswind speed increases.③For the overall thermal-electric conversion performance of solar dish/AMTECthermal power system, the theoretical calculations indicate that, the variation of theoverall thermal-electric conversion efficiency of solar dish/AMTEC thermal powersystem with the operating temperature in wind condition is very similar to that of no-wind condition. The difference is that, the maximum overall efficiency of the solardish/AMTEC thermal power system reduces from20.7%in no-wind condition to19.0%in the wind condition. Furthermore, the overall efficiency of solar dish/AMTEC thermalpower system decreases with increasing wind speed, while the wind direction showsvery little effect. In view of its high conversion efficiency, the solar dish/AMTECthermal power system has the potential to become one of the most competitivetechnologies of solar energy utilization.