April 2010

Prof. Humayun Mughal, CEO, Silicon CPV presented the following at the International Conference on Concentrating Photovoltaic Systems in Freiburg, Germany.

"Design of a 960x concentrating optics with good tracking error tolerance"

Figure 1

120x Receiver

A route to lower cost concentrators is to increase the level of concentration from the common 500x used in CPV systems. This higher level of concentration must be achieved with a good tracking error tolerance of around 1° so that tracking cost does not rise prohibitively.

A 120x system (Figure 1) has shown good tracking error tolerance and a new optical design has been developed which allows a similar design of primary lens coupled with a secondary to give geometric concentration ratio of 960 and allows only a 10° loss in energy at the focal point for 1° off axis alignment.

Design of primary lens

The 120x system was based on a 11 x 11 parquet with a focal length of 204mm. For use with a secondary collector the focal length is decreased to 160mm and a 31 x 31 tile has been designed. This lens (Figure 2) gives a uniform illumination at the entrance of the secondary. (Figure 3)

Figure 2

960x primary

Figure 3

Intensity at Secondary

Design of the secondary concentrator

The first design investigated was a hollow mirrored light pipe but this gave over 50% power loss for a 1° off axis alignment. A total internal reflecting prismatic structure was finally chosen. It was found that the necessary optical performance would be achieved with a high refractive index glass. The shape of the secondary is shown in figure 4.

Figure 4

Secondary with ray tracing

Off axis performance

The primary secondary combination has a geometric concentration ratio of 960. The off axis illumination distribution at the cell is shown in figure 5 showing less than 10% power loss at a 5 x 5mm cell area

Figure 5
Illumination on the cell at 1° off axis alignment

http://www.cpv-conference.org/cms