Investigation On Energy Transmission Of Solar Receivers With Heat Transfer Fluid At Different Phases And Their Application

To improve solar-to-electric conversion efficiency of concentrated solar power systems and simplify the construction of the system,the concept of a central receiver with supercritical heat transport fluids has been proposed in this paper.As a result of the large variations in the thermal–physical properties of supercritical water in the pseudo-critical region,the feasibility of stable heat transfer model need to be further studied.Furthermore,the non-uniform heat flux distribution of the receiver may lead to unusual behaviors of heat transfer and flow.These problems complicate the mechanism of heat transfer of supercritical water in receiver with un-uniform heat flux distribution.Therefore,in order to improve the theory of energy conversion and the mechanism of irreversible losses of this receiver with supercritical fluid,the following work has been carried out:Firstly,the mechanism of energy conversion of solar receiver has been discussed deeply.The effort of various heat transfer fluids on the energy conversion process and the optimization principles of different supercritical fluids in diverse conditions have been investigated.According the stable heat transfer model using external characteristic parameters,the similarity method is adopted in this paper to study the operation performance of the receiver with supercritical fluid from theoretical perspective and experimental perspective.The performance of the receiver is evaluated with the parameters such as:the heat loss of receiver,the peak optical efficiency and the incident angle factor.The equations of thermal efficiency are developed based on the model and the measured data respectively.The objective of a vertical tube with concentrated incident solar heat flux on one side is proposed,the corresponding physical model and 3D mathematical model are given in this section.Secondly,a testing site of a small scale parabolic trough collectors is built to evaluate the practical characteristics and the influence of operation parameters such as:the heat loss of receiver,the peak optical efficiency and the incident angle factor.The standard deviation of the model in this paper is 1.4%while that of the method based on ASHRAE 93 standard is 2.6%,which proves the accuracy of the model.The PTC has different performance characteristics in different season,with high exergy efficiency and low energy efficiency in winter,low exergy efficiency and high energy efficiency in summer.Therefore,to utilize solar energy more efficiently,the small-scale parabolic trough collector is suggested to start tracing the sun after the value of DNI is more than 300 W/m² in the morning.Thirdly,the single heat exchange tube is the minimum valid unit,the flow and heat transfer characteristics of supercritical fluid in this tube play an important role in the solar-to-electric conversion efficiency of receiver.The temperature field and velocity field in the tube with concentrated incident solar heat flux on one side have been investigated.The effect of incident flux and mass flux on the heat transfer of supercritical water have also been studied.As for the tube,heat transfer takes places on the entire inner wall,though heat flux is small when a is more than 150°.A criterion characterizes the influence of natural convection on the turbulent vertical flows of supercritical fluids.Once the criterion is more than the limiting value of 10^-5,a mixed convection including forced and natural convection appears in the condition of uniform heat flux.For the tube with concentrated incident solar heat flux on one side,this limiting value is 10^-6,which means that heat transfer deterioration is ease to occur.To avoid deterioration in the heat transfer of supercritical water,the relation between the threshold incident solar heat flux and mass flux for the supercritical water has been provided.Furthermore,the Nusselt correlation which can be used to predict heat transfer coefficient of the vertical tube heated non-uniformly on one side by concentrated solar flux has been confirmed.Further,to better understand the specifics of heat transfer and pressure drop under non-uniform heat flux,it is necessary to perform a comparative study of the heat transfer and flow of supercritical water in a vertical tube heated by a uniform heat flux and a non-uniform heat flux.The results show that heat transfer deterioration occurs with pressure drop.Entropy generation mainly depends on the temperature gradient along the vertical tube's radial direction which is due to the thickness of the thermal boundary layer and the intensity of secondary flow.To balance the minimum irreversible losses and hydraulic resistance in the tube in a central receiver,the relationship between incident flux and mass flux is determined under the principle of minimum irreversible losses.Given the incident flux,the value of mass flux should be more the 4 times of the value of incident flux.Finally,for the solar powers with Direct Steam Generation?DSG?systems,although they have high solar-to-electric conversion efficiency,there is no effective approach to store the thermal energy.Utilizing the residual energy from the hybrid CSP-biogas plant to heat the anaerobic digester not only recovers the residual heat,but also helps to improve the energy conversion efficiency of the CSP-biogas hybrid power plant and to solve the problem of heat storage in CSP.To improve the controllability and efficiency of the hybrid power plant,a biogas production system with anaerobic treatment of liquid cattle manure under mesophilic conditions is designed.Due to the uncertainty of solar radiation intensity,the hybrid solar-biogas power plant could employee different operation models to ensure optimal performance of the system.Hybridizing solar energy with biogas energy not only improves the power stability every day but also increases power dispatchability.Compared to the CSP,the monthly power generation of the hybrid solar-biogas power system is more stable.