Fundamental Theoretical And Experimental Study Of Thermophysics In Solar Energy Utilization

Because of the requirements for human social development and environmental protection, it is very urgent and important to pursue renewable and clean energy sources to replace limited and polluting fossil fuels. Solar energy is the most abundant and cleanest renewable energy source. How to convert solar energy into electricity efficiently and cheaply is one of the hotspots in the world. However, the classical radiation thermodynamics based on the model of photon gas in thermal equilibrium enclosed in a cavity is not suitable for the open and non-equilibrium solar radiation. In the other hand, the performances of photo-thermal, photovoltaic and hybrid Photovoltaic/Thermal utilizations are mainly analyzed by using the law of energy conservation, the second law of thermodynamics combined with spectral characteristics of solar energy has not been applied in the theoretical analysis work. Therefore, it is necessary to do a further study on the fundamental thermophysics for solar energy utilization, and the framework includes the characterization of spectral available energy, description of entropy, radiation thermodynamics system for the transfer and transform process of open and non-equilibrium solar radiation, thermal and quantum interaction between radiation energy and matters. On the basis of these theoretical researches, a scientific approach to using solar energy is proposed and the related experimental study is carried out.On the basis of the previous work of our group, the main research work of this thesis is as follows:Firstly, according to the fundamental theory of non-equilibrium state radiation thermodynamics, the non-equilibrium radiation, especially the energy state of the entrance of solar radiation receiver, is described and represented. The duality of light-matter interaction is distinguished, one is the macro energy characteristics in photo-thermal utilization and the other is quantum characteristics in photovoltaic utilization and actinism. According to the characteristics that solar energy is diluted in transfer process but keep the same spectrum, the decreased energy quality and entropy increase of the received solar energy in thermal utilization are calculated, and the quantum characteristics of spectral radiations on sun surface and the earth are validated the same in photovoltaic utilization and actinism. Secondly, the energy conversion mechanism and enhancement methods of the interaction between light and matters are studied. Starting from the modification of the interaction mechanism between light and matters, a photon wave model is proposed. The interaction between the photon wave driving force and atomic nucleus-atomic nucleus oscillator, and the one between the photon wave driving force and atomic nucleus-electronic oscillator are analyzed. The results are the same as those calculated by the classical wave theory, which illustrates the correctness of the photon wave model to a certain extent. Photon waves with different frequencies show either photo-thermal or quantum effects when they interact with molecules, phonons and atomic nucleus-electronic oscillators on the surfaces of matters. The radiation absorbed by molecules and phonons will be transformed into heat energy and the quantum effect concomitantly turns solar radiation into electricity and heat energy. The parameters which affect the absorptivity and the reflectivity of the semiconductor radiation receivers are analyzed. The results show that a semiconductor material with a small resonance transition frequency, a small energy band gap and a large damping coefficient can make a better usage of the available solar radiation.To increase the photoelectric conversion efficiency, the optical properties of a coating material for silicon solar cells are analyzed by using the photon wave model. Then the effects on the absorptivity of solar cells caused by the surface micro structures are studied, including the regular surface with different micro structure, regular and irregular surface micro structures and the different cone angles of the same regular surface micro structure. The results show that the absorptivity of the irregular micro structure is higher than the one of the regular micro structure. The Needle method is employed to optimize the spectral selective transmission coating for PV systems.To improve the total efficiency of solar radiation utilization, a two stage transmission and reflection concentrating PV/thermal system with beam splitting technique is proposed. A system composed by linear Fresnel lens, spectral selective transmission coating, solar thermal receiver and other components is proposed, and the optical analysis and efficiency of the PV/thermal system are made. The results show that the efficiency of the proposed PV/thermal system is higher than a traditional system undef the same conditions. The proposed system also takes advantage of simple structure, low cost, and easy manufacture. At last, to improve the low photoelectric conversion efficiency of solar cells due to the non-uniform light concentrated by traditional concentrators, a strip-focus Fresnel lens concentrator and a bending-plate reflection concentrator are proposed and designed. The Monte Carlo Ray Tracing Method is employed for the optical analysis of the two concentrators, in which the solar radiation cone angle is considered. And the effects of the structure parameters for the performance of the two concentrators are analyzed. The theoretical results show that the distributions of the concentrated light on the focal plane of the two concentrators are both with high uniformity. The experimental study of the bending-plate reflection concentration PV system is earned out by using a two-axis solar tracker. The flux distribution of the concentrated light of the bending-plate concentrator is measured by a CCD method, and the Ⅰ-Ⅴ characteristics of the silicon solar cell monomer and module are tested. The experimental results agree well with the theoretical results. The efficiencies of the silicon solar cell monomer and module in the bending-plate reflection concentrating PV (CPV) system are both higher than the ones in the parabolic trough CPV system. The cost of the bending-plate concentrator is much lower and easy manufacture, which make the bending-plate concentrator suitable for low and medium CPV system.