| On summarizing the current problems in solar cookers and according to the optical characteristics of Fresnel lens and principle of polar-axis tracking,a fixed-focus Fresnel lens solar concentrator is proposed as a new type of solar concentrator.It can combine with cavity receiver and fin-tube heat exchanger with latent heat storage to construct an indirect solar cooker,the fixed-focus Fresnel lens phase change solar cooker using polar-axis tracking.Firstly,the fixed focus concentrating characteristics of fixed-focus Fresnel lens solar concentrator are verified by experimental and numerical simulation.Then,the optical performance and thermal behavior of fixed-focus Fresnel lens solar concentrator using polar-axis tracking/cavity receiver system(fixed-focus Fresnel lens concentrating system)are investigated by numerical simulation and experimental methods,respectively.Finally,by applying the fixed-focus Fresnel lens concentrating system combined with fin-tube heat exchanger with latent heat storage to the solar cooker,the overall thermal performance of phase change solar cooker under different working modes are studied and analyzed experimentally.Focusing on the optical characteristics of fixed-focus Fresnel lens solar concentrator using polar-axis tracking,experiments and numerical simulation are carried out to study and verify the optical performance of it under different incident angles,different tracking error angles and different periodical adjustment errors.The results show that the solar concentrator can realize fixed-focus under different concentrated sunlight incident angles throughout the year.The maximum value of the relative optical efficiency loss of solar concentrator in the experiment for one day is 1.87%.The mean value and maximum value of the local concentration ratio of solar flux on the receiver are more than 86.64 and 1319.43,respectively.The allowable angle of tracking error should be less than 1°.When the tracking error angle is bigger than 1.5°,the relative optical efficiency loss increases sharply which is up to 17.42%.The allowable angle of periodical adjustment error should be less than 1°,too.Experimental results are in coincidence with optical simulation results and the peak value of simulation errors are less than 5%,which indicates that the simulation model is correct and the optical characteristics of solar concentrator can be verified by optical simulation method.Aiming at the problem of dead space of solar energy absorption and the non-uniform distribution of solar flux in traditional cavity receiver,the optical performance of fixed-focus Fresnel lens solar concentrator using polar-axis tracking/cavity receiver system is investigated by numerical simulation.The results show that the bottom reflective cone cannot improve the flux uniformity of cavity receiver completely but can increase the optical efficiency of system.Based on the bottom reflective cone angle 90°,cavity receiver surface absorptivity 0.85 and bottom reflective cone reflectivity 0.85,the average optical efficiencies using cavity receivers with bottom reflective cone of spherical,cylindrical,conical are 72.23%,68.37%and 76.40%respectively while that using their corresponding conventional cavity receivers are 68.49%,31.91%and 74.61%.The former significantly increased 3.74%,36.46%and 1.79%,respectively.The cavity receiver surface absorptivity and concentrated sunlight incidence angle have the most significant influence on the optical efficiency and flux uniformity,respectively.The conical cavity receiver with bottom reflective cone is most suitable for system.In view of the fact that the glass cover can reduce the heat loss of cavity receiver and increase the optical loss of solar system,the optical and thermal performance of fixed-focus Fresnel lens solar concentrator using polar-axis tracking/conical cavity receiver system is investigated through combination of numerical simulation and experimental method.The results show that concentrated sunlight incidence angle has a significant impact on the optical efficiency.The optical efficiency and time constant of system without glass cover are higher than that of system with glass cover.Comparing the thermal efficiencies of system with and without glass cover under different concentrated sunlight incidence angles,it can be seen that the thermal efficiency with glass cover for wide range of higher temperature differences is higher than that without glass cover.The F’η0(θ)and F’U1/C are dominant parameters in lower and higher temperature differences,respectively.In view of the low heat transfer efficiency of heat exchanger with latent heat storage and the mismatch of energy utilization time and space in the process of using solar cooker,the thermal behavior of the fixed-focus Fresnel lens phase change solar cooker using polar-axis tracking is also studied based on experimental method.The results show that the tilt angle of fin-tube heat exchanger with latent heat storage can not significantly improve its heat storage performance,but it can improve the temperature distribution of composite phase change paraffin to some extent,reduce the heat transfer temperature difference and improve the temperature distribution uniformity.When the phase change heat release is completed,the storage time of fin-tube heat exchanger with latent heat storage can reach 15.7 h,which meets family requirements for thermal time in the evening or the next morning.In the work model of direct heat collection,the oil temperature in oil tank can rise from 25℃ to 156℃in 54 min.In the work model of indirect heat storage,the heat storage efficiencies of solar cooker which from the beginning of heating to the occurrence of phase change heat storage,to the completion of phase change heat storage and to the end of the phase change heat storage experiment are 24.94%,29.43%and 26.25%,respectively.In the work model of thermal utilization of the fin-tube heat exchanger with latent heat storage,the circulating heat efficiencies of fin-tube heat exchanger with latent heat storage which from the beginning of heat releasing to the completion of phase change heat release and to the end of the phase change heat release experiment are 76.53%and 78.02%,respectively.The research methods and related research results of fixed-focus Fresnel lens concentrating system provide a theoretical guidance for the design and development of fixed focus concentrating system and an important reference for the application of multifunctional solar cooker integrated with building in the furture. |
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