Theory, Method And System Of Solar Energy Utilization With Photovoltaic-Photothermal-Thermochemical Complementation

Since the 21st century,the energy environment problems faced by mankind have become increasingly severe.It is urgently needed for clean energy to gradually replace the existing large-scale use of fossil energy.Solar energy has attracted people's extensive attention and research because of its huge amount and wide distribution.However,after many years of development,solar energy utilization technology still faces problems such as low efficiency,high cost,difficulty in energy storage,and unstable energy supply,leading to serious restrictions on its development.The slow development of solar energy utilization technology can be partly attributed to the incompleteness of the thermodynamic theory of solar energy utilization,which leads to the lack of theoretical guidance for the development of solar energy utilization technology and delays the intersection between the invention of the new method and the different utilization methods of solar energy.Supported by key project of National Natural Science Foundation of China,National Key Research and Development project and other scientific research project,this paper explores the thermodynamic theory of solar energy utilization with photovoltaic-photothermal-thermochemical complementation and proposes solar photovoltaic-photothermal-thermochemical hybrid methods and systems,during which the energy conversion mechanisms and performance enhancement methods in these systems are analyzed.The main contents and conclusions of this paper are as follows:(1)Based on the basic theory of thermodynamics,solar reversible conversion models under the conditions of solar surface and earth's surface are bulit,respectively.During the process of model construction,the causes of irreversible solar energy conversion and its influencing factors are analyzed.The three main factors affecting the irreversibility of solar energy conversion are the concentration ratio,the incident/exit radiation intensity difference of the blackbody,and the degree of spectral matching.The ultimate efficiencies of solar energy conversion under non-reversible conditions and under reversible conditions are given.The ideal efficiencies of solar photovoltaic conversion and solar photothermal conversion are analyzed,and complementary mechanisms of photovoltaic-photothermal complementation and photovoltaic-photothermal-thermochemical complementation are explored.(2)Thermodynamic and kinetic studies on solar photovoltaic-photothermal-methanol thermochemical complementary utilization methods and systems are performed.The influences of key design and operating parameters on the system efficiency,such as solar radiation intensity,reaction temperature and reaction pressure,have been analyzed.Results show that the net solar-to-electric efficiency can reach 43%(optimized to 52%),which is increased by 23%and 30%,respectively,compared with the efficiency of solar methanol decomposition system(35%)and solar photovoltaic system(33%).The efficiency enhancement is achieved by cascade utilization of solar energy and energy level upgrading of low-temperature solar thermal energy by high energy-level chemical energy.Then,the 24-h energy-supply characteristics of the system are analyzed.The results show that,by storing solar energy as chemical energy in syngas,the hybrid system has the function of energy storage and can provide continuous,on-demand energy supply around the clock.(3)Based on the solar spectral splitting method,a complementary method and system for solar spectral splitting photovoltaic-photothermal-methane reforming was proposed.Full-spectrum optimized utilization are enabled by converting the short-wavelength spectrum into electricity and low-temperature thermal energy through the photovoltaic cells and converting the long-wavelength spectrum into high-temperature thermal energy by the photothermal chemical reactor for the methane reforming reaction.The exergy efficiency during solar photovoltaic and solar thermal conversion reaches 52%.On the basis of spectrum optimized utilization,the system further achieves the conversion of lower-energy-level solar thermal energy to higher-energy-level chemical energy through the coupling of methane reforming.Finally,the net solar-to-electric efficiency of the system reaches 39%.In addition,compared with electricity supplied from national grid and heat supplied from industrial boiler,generating the same amount of electricity and thermal energy,the system can save 63%of fossil energy and reduce 76%of carbon dioxide,which results from the complementarity of solar energy and fossil energy.(4)An experimental study is carried out on the solar photovoltaic-photothermal-methanol thermochemistry hybrid method and system.Different hybrid reactors are designed and manufactured.Field coupling analysis model and a reactor performance test platform of the reactors are both established.Through the combination of simulation and experiment,the optimized structure of the reactor is obtained.Based on the optimized design of the reactor,a 500 W prototype of a photovoltaic-photothermal-methanol-thermochemical hybrid principle is developed.Under the outdoor actual operating conditions,the photovoltaic efficiency and photothermal-chemical conversion efficiency are tested,and solar-to-electric efficiency(33%)of the complementary prototype is finally obtained.