Concentrated photovoltaics systems are designed to give electrical power at a lower cost than traditional photovoltaics. In order to achieve this, excellent performance from the multi-junction solar cells optimized for concentrators must be achieved, while minimizing the cost of optics, temperature control and other system balances.
Variance in incidence angles of light at the solar cells surface is a particular attribute of the lens-based concentrated photovoltaics. In addition, there has been a concern of inhomogeneity of the light distribution on the cell. These are some of the causes for low performance owing to an increase in series resistance and current mismatch between junctions. For this reason, most concentrators have been designed with an aim of minimizing the inhomogeneity by homogenizing secondary optical element.
Secondary Optical system reduces spatial as well as spectral inhomogeneity through several internal reflections of the incident light. A secondary optical system also adds secondary concentrations to a concentrated photovoltaic system. Unfortunately, as the performance is enhanced and the irradiance homogenized the average incident angle on the surface of the cell increases further. Therefore, concentrated photovoltaic cells in conjunction with a secondary optical system will experience a loss in performance by the angle of incidence variance caused by the optics.
Leon Bunthof and co-workers at Radboud University in The Netherlands studied in detail the effect of oblique illumination of concentrated photovoltaic solar cells performance. Previous studies on this angular dependence have been pegged to the optical coupling difference between junctions as a function of incident angle as well as temperature. In the current study, they focused on the total electrical output of concentrated photovoltaic solar cells as a function of the incident angle. Their work is published in Solar Energy.
The authors used solar cells equipped with an Anti-Reflection Coating for use with secondary optical system as well as front contact metal tabs, similar to solar cells typically applied in concentrator systems. The cell featured a silver front grid contact with fingers with inclined sides.
For uncoated cells, the transmittance of incident photons to the semiconductor element was dependent on the angle of incidence. Therefore, a cell equipped with anti-reflection coating were expected to have a different transmission curve, but these cells also indicated increased reflections at oblique angle of incidence.
The researchers observed that the solar cells performed considerable worse as the illumination became more oblique. For an angle of incidence of 83°, they recorded a drop in performance of up to 58%. This drop in the performance of the cell was referenced to the optical attributes of the anti-reflection coating considering that the computed angle of incidence dependent transmission through the coating correlated well with the observed angle of incidence dependent cell performance.
The second loss mechanism was found and referenced to the front contact grid by propagating the angle of incidence orthogonal to the grid fingers. For this case, an increasing illumination fraction interacted with the sides of the grid metal for increasing angle of incidence. Therefore, shape and orientation of the grid fingers became a critical source of cell performance loss for the oblique illumination. Additional 18% loss in current generation could then be attributed to front grid.
In order to determine the sensibility of application of a secondary optic element, the optical performance of several model secondary optics was evaluated by ray tracing simulations. The concentrating elements showed a clear increase in optical performance, that leads to a gain in electrical solar cell parameters, in these cases exceeding the loss caused by the elevated illumination angle.
The authors concluded from their study that grid orientation and design with respect to the optical system should be considered and optimized carefully using methods as described in the article, when designing concentrated photovoltaic systems.
In recent years the field of building-integrated photovoltaics has seen a tremendous growth. Concentrator systems especially, offer great benefits when incorporated in a building, as they allow the possibility for multi-functionality in the form of e.g. the direct use of heat, or the regulation of daylight entering the building interior. However the building incorporation puts design constraints (e.g. size, weight) on the concentrating solar systems, which therefore often feature much more complex optical systems than applied in conventional concentrators. This leads to stronger inhomogeneity in cell illumination profile, and a further elevated cell illumination angle. In designing such systems especially, it is of great importance to consider the function of each system component carefully, using experimental measurements, or ray tracing.
L.A.A. Bunthof, J. Bos-Coenraad, W.H.M. Corbeek, E. Vlieg, J.J. Schermer. The illumination angle dependency of CPV solar cell electrical performance. Solar Energy, volume 144 (2017), pages 166–174.Go To Solar Energy