Fundamental Theory And Experimental Study Of Reflective Concentrating Solar Energy Utilization

Efficient and cost-effective solar power generation is an important research issue nowadays.Concentrating photovoltaic system can greatly reduce the amount of the expensive solar cells and decrease the cost of the power generation.Using the spectral beam splitting technology,the specific spectral solar irradiance that is suitable for photovoltaic conversion will be transmitted onto the cell,so the heat toad of the cell can be reduced and the conversion efficiency will be increased. Meanwhile,the rest part of the solar irradiance will be recovered to increase the overall utilization efficiency of the solar energy.Therefore,this dissertation mainly focuses on the theoretical and experimental analysis of the concentrating system performance as well as the effect of the optical properties of the beam filter.First,basis on the theory of the spectrum radiation thermodynamics,the method of the solar energy utilization with beam splitting technology is analyzed. The two-stage reflective concentrating beam splitting system is proposed and two solutions of solar photovoltaic/thermal hybrid system are presented.Nondimensional analytical expressions of the geometric concentration ratio of the twostage parabolic tough reflective concentrating system are derived.Basis on the elementary principle of the optical thin film,the low-pass broad-cut filter employed in the two-stage concentrating splitting system has been designed using Needle optimization method.Using niobium pentaoxide and silicon dioxide as the high and low refractive index materials,respectively.The film coating is carried out with magnetron sputtering method,and the test results show that the reflectivity of the available spectrum for the silicon solar cell is above 95%.Based on the ray tracing algorithms,a detailed three-dimensional optical model is set up to evaluate the flux density distribution on the elements' surfaces,and the expression of the local flux density with effect of the spectral beam split is derived.The effects of the solar brightness distribution and optical errors(e.g.,alignment error and surface slope error) are also involved and the tracking error is also analyzed.The numerical calculation is carried out using the annual mean Buie CSR 6 sunshade, the flux density distributions on the beam filter,heat receiver and photovoltaic cell are obtained.Due to the shielding of the beam filter,the flux density on the central part of the splitting mirror surface is low while the rest part is uniform, and the flux density on the central part of the solar cell is higher than that on the sides.It is also shown that the distribution on the filter is slightly affected by its alignment error while it greatly affect the distribution on the photovoltaic receiver surface.The dual-axis sun tracking device is designed and manufactured.The rotation axis of the right ascension is arranged in north-south direction and runs at the speed of 15°per hour.Meanwhile the inner frame set on the two ends of the right ascension axis also rotates.The declination axis is set on the two ends of the inner frame in east-west direction.Using a hand-operated screw adjusting mechanism,the angle between the receive plane and the right ascension axis is controlled to be equivalent to the latitude every day.Tracking time is limited from 7 am to 5 pm,the concentrating area is 10 m~2,and the driver power is only 12 W.The tracking device is loaded the four-dish concentrator,semi-parabolic trough and Fresnel concentrator.The low-cost device is a promising system for that the control system is simple,the tracking precision is high.A semi-parabolic trough concentrating system is presented,which can realize high concentration ratio by avoiding the shielding of the receiver,and the antiwind performance is good.A detailed optical model is presented to evaluate the relations between the geometric concentration ratio,the relative aperture and the photovoltaic cell location,and the optimal inclination angles of the cell are given to obtain uniform flux density at different concentrator relative apertures.The spatial distribution of the flux density on the solar cell plane and the performances of the semi-parabolic trough concentrating photovoltaic system are experimentally studied.The spatial distribution of the flux density is investigated by the CCD method and calibrated by heat flow meter.The electrical performance of the photovoltaic system is experimentally studied.A beam filter coated with materials of high refractive index titanium dioxide and low refractive index silicon dioxide has been designed,which can transmit all the visible light and part of the near- infrared band and reflect the rest band irradiance.The influence of the spectral beam splitting technology on the cell temperature and the system efficiency is theoretically studied.The results show that spectral beam splitting technology on the concentrating photovoltaic system can reduce the cell temperature and increase concentration ratio together with photoelectric conversion efficiency.Two new reflective concentrating systems,the semi-parabolic-trough system and zigzag-plate system,which can obtain iso-intensity distribution using plate mirrors,are proposed.Considering the effect of the incident sun rays' nonparallelism, the design formulas and uniform flux density distribution for the semiparabolic -trough system are presented.As for the zigzag-plate system,the design formulas with the parallel rays are also derived.With the known position(including the tilt angle and the distance on the vertical direction) and the width of the receiver plane,the number and the width of each mirror as well as the tile angle and the position will be evaluated.Experimental device has been set up to test the flux density distribution of the concentrating system,and the photovoltaic performance experiment has also been carried out.The experimental results show that the flux density distribution of the zigzag-plate mirror concentrating system is more uniform than that of the semi-trough concentrating system using parabolic reflector,and the photovoltaic conversion efficiency of the zigzag-plate system is higher than that of the semi-parabolic system.The work was sponsored by the National Nature Science Foundation of China (No.50576092,50736005).