Research On Concentrated Photovoltaic Secondary Optical Element Based On Photonic Crystal Reflection Method

Fresnel-based concentrated photovoltaic(CPV)technology is one of the effective ways to achieve efficient and low-cost collection and utilization of solar energy.Secondary optical ele-ments(SOE)are often used to improve the acceptance angle,irradiance distribution uniformity and concentration ratio of CPV modules,thereby improving the performance of CPV module in practical applications.However,the introduction of SOE will also increase some optical surfaces,due to specular material absorption or Fresnel reflection will cause considerable light energy loss.This problem reduces the optical efficiency of the CPV module,which in turn increases the cost of power generation.Therefore,optimizing the additional surface introduced by the SOE be-comes an effective way to eliminate or minimize the absorption loss of the specular material and the Fresnel reflection loss.The existence of the omnidirectional photonic band gap(OPBG)in one-dimensional photonic crystals(1D PC)makes it possible to design perfect mirrors with omni-directional high reflection characteristics in specific wavelengths.Therefore,this paper proposes a reflective SOE based on a 1D photonic crystal heterostructure(PCH),and conducts related the-oretical and experimental research.The completed work and main conclusions include:Firstly,numerical analysis method is used to study the propagation characteristics of light in the dielectric layer,and then an one-dimensional photonic crystal heterostructure composed of 11 one-dimensional photonic crystals alternately cascaded is designed.The 1D PCH uses K9 glass as the substrate,and TiO2 and MgF2 as high and low refractive index materials.And each 1D PC has the same filling ratio and number of periods,and the lattice constant increases in geometric progression.Numerical simulation results show that the 1D PCH has an OPBG in the main spectral response band(380-1650nm)of solar cells,and the reflectivity is close to 100%.Then,a simulation model which can reflect real optical phenomena is established based on the key characteristics of the standard ground spectrum,the angle distribution of solar rays,the absorption coefficient of the material and the dependence of the refractive index on the wave-length.The performance of a CPV module equipped with the proposed SOE(a hollow truncated tetrahedral equilateral pyramid with photonic crystal heterostructure layer(HTPCH))is analyzed.At the same time,the performance of the CPV module without SOE is also analyzed to distin-guish the influence of the SOE on the performance of the CPV module.The simulation results show that the HTPCH SOE can not only significantly improve the acceptance angle and spot distribution uniformity of the CPV module,but also can overcome the Fresnel reflection loss and the absorption loss of the mirror material,thereby improving the efficiency of the CPV mod-ule.In addition,the performance of the CPV module equipped with a traditional reflective SOE(a hollow truncated tetrahedral equilateral pyramid with metallic film(HTPM))and a refractive SOE(a solid truncated tetrahedral equilateral pyramid lens(STP))is also analyzed to compare the optical performance of the HTPCH SOE with traditional reflective and refractive SOE.The simulation results show that compared with the HTPM and STP modules,the optical efficiency of the HTPCH module is increased by 13.47%and 5.71%,respectively.Finally,the trial production of the HTPCH SOE was completed,the corresponding indoor and outdoor experimental platforms were built,and preliminary experiments of various CPV modules were carried out based on the experimental platform.Based on the output,the short-circuit current,the maximum output power and the focus spot are analyzed.The results show that the HTPCH module presents the highest acceptance angle and irradiance distribution uniformity,and compared with the HTPM and STP modules,the short-circuit current of the HTPCH module is increased by 13.7%and 7.4%respectively,which is in good agreement with the simulation results.Outdoor testing also proved the rationality of the design.Compared with the other three types of CPV modules,the HTPCH module showed the highest system electrical efficiency.