Theoretical Study On How The Non-uniformity Of The Sunlight Spot Influences The Performance Of The Concentrating Photovoltaic System

The technology of concentrating photovoltaics is to apply Fresnel lens or other concentrators to bring concentrating sunlight to the surface of the solar cells.In order to reduce cost,this technology replaces expensive solar cells with cheap Fresnel lens.However,when the concentrating device gathers sunlight,the heat is also concentrated.High temperature will make the power performance of solar cells decrease fiercely.In addition,in a real concentrated photovoltaic system,the light spot on the surface of solar cells is non-uniform.The non-uniformity of sunlight intensity and cell temperature make the performance of concentrated cells unstable.Therefore,the non-uniformity of the sunlight spot has an important influence on the generating performance of the concentrated solar cells.Iin 2002,Franklin et al.concluded that the open circuit voltage and the generation efficiency of the non-uniform concentrated solar cell decreased with the increase of the non-uniformity of sunlight spot.The higher the non-uniformity,the drop is larger.At the same time,Mellor et al.also found that the open circuit voltage,the filling factor and efficiency of solar cells deceased a little under the non-uniform sunlight.But in 2016,Xing Y et al.came to the conclusion looked "paradoxical".The results showed that in non-uniform sunlight situation which the concentration ratio is 500X,the power efficiency didn't decrease with the increase of the non-uniformity of the sunlight spot,when the area ratio of the radiator to the solar cell is 500.The efficiency increased with the increase of the non-uniformity when the ratio is 2500.The conclusion above indicates that the heat transfer intensity of the cooling system has an effect on the results related in non-uniform sunlight.Thus,this paper will summarize the significant factors which influence the generation performance of the solar cells,from the perspective of theoretical analysis.Then,detailed discussion will be carried out to clarify how these factors affect the generating performance of solar cells.Firstly,this paper will establish an efficiency model of the silicon solar cell.At first,the model must determine the definition equation of the photoelectric current in the concentrating situation.It is known from the equation that the photoelectric current is almost exclusively related to the concentrated sunlight.Then,the consumption of the carrier concentration is quantitatively analyzed by the radiation recombination,SRH recombination,the auger recombination and the surface recombination.Hence,the equation of the consumption of the carrier concentration is obtained about the voltage of the external circuit.According to the photoelectric carrier concentration and the consumption of the carrier concentration,the carrier concentration contributed to the external circuit is available,which is current of the external circuit.Solving the equation of the voltage and the current,the functions of the open circuit voltage,the working voltage,the working current and efficiency will be acquired.Based on the analysis,it is concluded that these are the influencing factors of the generation performance for silicon solar cells:the concentration ratio,the cell temperature(the heat transfer intensity of the cooling system)and the non-uniformity of the sunlight spot.Secondly,a temperature field model of the silicon cell is established according to the two-dimensional cylindrical unsteady heat conductivity model.After solving the discrete system of equations,the alternating direction iteration method is applied.Then,the tridiagonal matrix algorithm is used in each direction.Last,the relevant formulas are integrated,and the temperature field of the silicon cell will be figured out by programming in Visual Studio.Afterwards,when the cell temperature field is known,calculations can be carried out by the programs in MATLAB,according to the formulas of the performance parameters calculated in the efficiency model.The further research will be conducted to analyze variation trends of performance parameters such as the voltage,current and efficiency for silicon solar cells,respectively in the conditions of different concentration ratios,different coefficients of heat transfer in the cooling system and the non-uniformity of sunlight spot.During the calculations,it is assumed that the irradiation intensity of the non-uniform sunlight spot is reduced proportionally by the concentric rings,and the ratio is named as non-uniform ratio.The solar irradiance in each ring is equal.The center circle has the largest irradiance,which forms a geometric sequence with the irradiances of outer rings.Calculating the performance parameters,the entire silicon cell is regarded as the parallel forms of multiple independent rings.Finally,it is concluded that the short circuit current and the working current are almost exclusively associated with concentration ratios(or equivalent average concentration ratios),not affected by the cell temperature(cooling system heat transfer coefficient)and the non-uniform degree of the sunlight spot.In a non-uniform concentrated photovoltaic system,the current of the solar cell rises linearly with the increase of the concentration ratio.When the heat transfer intensity of the cooling system is determined,the opening circuit voltage will gradually decrease with the increase of the concentration ratio,as well as the working voltage and efficiency of the silicon cell.When the equivalent average concentration ratios are stable,the opening circuit voltage,with the working voltage and efficiency,will have a slight increase,and then gradually decrease with the increase of the non-uniform degree of the sunlight spot.At different distributions of the sunlight spot,the non-uniform ratios differ slightly,when the voltage and efficiency reach the top.At the same concentration ratio and the cooling condition,the non-uniformity has different influences on the performance of solar cells,with different distributions of the sunlight spot.Other conditions remained,the larger the main circle of the sunlight spot,the larger the radius of the concentric ring,the non-uniform sunlight spot will have less influences on the performance of solar cells.