Tweaking of refractive surfaces to improve the irradiance uniformity in Fresnel lenses: Possibilities and limitations

In-depth study of the method for modifying the spatial and spectral irradiation distribution from a Fresnel lens concentrating sunlight on a multi-junction solar cell is proposed. An approach considered to constructing a Fresnel profile is based on the principle of independent control of the slope angles of the refracting faces of individual teeth (single lenses) to provide the required compromise between the level of decrease in local concentration in the center of the light spot and the allowable increase in its size to maintain an acceptable average sunlight concentration on solar cell. The limiting angle of rotation of the faces was established by the magnitude of the power capture coefficient (K_int) in a spot of a given size. Two options for the formation of the Fresnel profile are considered: with a constant and variable magnitude of the interception coefficient for sunlight directed by single lenses into focus. Additionally, monitoring was carried out for irradiance distributions within the spectral components of a light flux and photocurrent generation profiles for a top, a middle or a bottom subcell of a multi-junction solar cell. It is shown that with the redistribution of light energy from the center to the periphery of the focal spot, a decrease in the level of local sunlight concentration in its center by a factor of 1.5-2 exists. Simultaneously with the of light power “blurring” in the spot, the pronounced spectral redistribution of power was observed. As a result, an extremely undesirable current limitation mode was established on the significant part of the GaInP/Ga(In)As/Ge multi-junction solar cell from the bottom subcell. To eliminate this current limitation, the options for the defocused position of the solar cell relative to the design focal length of the FL are considered.