Fundamental Research On Thermodynamics Of Radiation And Its Application

Solar radiation,as one radiation form,is different from normal thermal energy. The most distinct difference is that the frequency characteristic of radiation.The conversion of radiation such as photovoltaic conversion is frequency-dependent. Since the thermodynamic theory of radiation is still not quite perfect until now,for a long time most of the analysises of solar photovoltaic(PV) and thermal utilization are only based on thermal efficiency of the first law of thermodynamics,and the effective utilization of exergy of solar energy lacks fundamental theoretical guidance. Therefore,it may be an effective way to improve the efficiency,expercially the exergy efficiency of solar energy utilization and to reduce the cost of solar power generation,by reinforcing the foundational research of non-equilibrium thermodynamics of solar radiation.Firstly,the thermodynamic of spectral radiation is studied in this paper.A new parameter named spectral effective temperature T_λis proposed,which represents the thermodynamic quality of the photon.The relation between T_λand the wavelengthλ, is given asλT_λ= c₃=5.33016×10^-3 m·K.Using the proved equation of spectral effective temperature T_λand the expression of the photon energy hv,the entropy constant of the photon is given as s_λ=3.72680×10^-23 J/K.The effective blackbody temperature educed from s_λis proved to be equal to the blackbody radiation temperature,which confirms the validation of the concept of photon entropy constant.The exergy,entropy and enthalpy of the spectral blackbody radiation,the equilibrium cavity radiation,the radiation flux in open system are discussed by T_λand s_λ,as well as the entropy change in the process of the state transformation of photon gas.By analyzing the exergy of spectral radiation,the exergy efficiency of the spectral radiant energy available for photosynthesis,which is calculated by applying the cocept of spectral effective temperature,is proved to be higher than the Camot efficiency of heat source temperature being the sun's temperature.Thus the cause of the viewpoint in the literature that there is negative entropy production in photosynthesis can be explained.Secondly,taking nonequilibrium radiative heat transfer between two surfaces for example,the nonequilibrium thermodynamic of radiation is studied.The formulae of entropy flow,entropy generation,exergy flux and optimal temperature of absorbing surface for maximum exergy output are derived.Then,the nonequilibrium thermodynamic of diluted radiation whose intensity is weakened with the distance away from the radiation source is discussed.The concepts of radiation intensity coefficient and effective radiation temperature are introduced.Take solar thermal receiver and converter for example,the formulae of radiation-to-heat conversion efficiency,entropy generation,exergy output ratio and the optimal absorbing temperature of the receiver are established in terms of the effective radiation temperature.These formulae are the same in equation form with the corresponding formulae of constant-intensity radiative heat transfer.The influence of the variation of the radiation intensity is expressed by the effective radiation temperature in terms of the radiation intensity coefficient.The entropy generation factor is only related to the surface temperature of the radiation source and the absorbing temperature of the receiver.The influence of the solar concentration ratio n on the exergy output ratio of the solar receiver is also discussed.Thirdly,the method for calculating the irreversible loss of the exergy in radiant energy converters is presented.The formula for calculating the net entropy received by the receiver is also presented.For solar photovoltaic power generation,the expression of energy efficiency of the photon in the photovoltaic conversion is presented,the theoretical efficiency limit of photovoltaic conversion in the solar cell is derived,and the theoretical optimal band gap and cut-off wavelength of the solar cell are obtained.The driving force,the irreversible loss of available energy and the entropy increase in the photoelectric conversion are discussed and,the expression of available energy of the excited electron in the solar cell is presented.The results of the study of the nonequilibrium thermodynamic of diluted radiation whose intensity is weakened with the distance away from the radiation source indicate that,for solar energy utilization the incident radiosity of the solar receiver can be increased by solar concentration and then the exergy output ratio is increased.Therefore,experimental study of solar concentrating photovoltaic is developed.A Fresnel lens concentrator PV power system is designed and manufactured.Passive cooling is adapted by gluing the silicone cells onto an aluminum plate.The geometric concentration ratio is 23 and the real concentration ratio is 12.9.The outdoor test shows that,the maximum output efficiency is 7.60%. The maximum output power is 5.6 times compared with non-concentrating photovoltaic power gerneration by the same area solar cell.Based on the frequency-dependent theory of thermodynamic of radiation,the mechanism of utilization of spectrum splitting solar concentrating energy is discussed. The solar spectral availability function and the designed scheme of hybrid photovoltaic/photothermal(PV/T) power generation of two-stage reflective spectrum splitting solar concentrating energy are presented.The thermodynamic performance of the system is analyzed,and the generating investment and cost are compared. Theoretical analysis indicates that,for a PV/T spectrum splitting system at 30 suns concentration,the photovoltaic efficiency is about 0.095,the thermal power generation efficiency is 0.043,and the total power generation efficiency is 0.139. Compared with common PV systems,the electric power of unit area silicon cell increases to about 16 times,while the investment cost and the generating cost is significantly reduced.To analyze the characteristic of the scheme of PV/T power generation of spectrum splitting solar concentrating energy,the thermophysical models of five kinds of optical systems and radiant energy conversion devices are established,including fixed solar panels system,several times concentrating PV system,one-stage reflective parabolic concentrating system,reflective dish spectral beam splitting concentrating PV system and Fresnel lens concentrating PV system.The energy efficiencies and exergy efficiencies are analyzed.The formula for calculating the spectral solar radiation at Earth's surface is presented.The analysis indicates that area of solar cells can be significantly reduced by appropriately increasing the concentration ratio while the temperature of the solar cells is limited.Finally,the experiment of hybrid PV/T utilization of spectrum splitting parabolic reflective solar concentrating energy is presented and advices for improvement are proposed.To improve the scheme of the utilization of two-stage reflective spectrum splitting solar concentrating energy,several new schemes are introduced,and the related experiments and presented.As another choice of thermal using way of the hybrid PV/T utilization of spectrum splitting solar concentrating energy,the power generation performance of a solar-driven semiconductor thermoelectric generator is experimentally investigated.The relationship between the performance parameters and the solar radiation intensity are obtained,when the cold-side temperature is kept constant.The power generation performance of different area of semiconductor thermoelectric generator for different concentration ratio is compared.Advices to improve the power generation performance of semiconductor thermoelectric generator are presented and the feasibility of combination of thermoelectric generation and solar photovoltaic power generation is preliminary discussed.The work is financially supported by the National Natural Science Foundation of China(Grants No.50576092,50736005).