Fresnel lens and CPC Concentrators

Fresnel lens and CPC Concentrators

There are generally two types of concentrators: reflective and refracting. The main two are described below.

Refractive Concentrators – Fresnel lens

The Fresnel lens is transformed from a semi-circular lens. Figure 1 is a schematic diagram of a Fresnel lens concentrating light. The angle between the refraction surface of each tooth slot and the lens plane is determined by the refractive index of the lens material, the focal length of the lens, and the distance from each tooth to the center of the lens. The condensing multiple of this lens can theoretically reach 10,000 times, but in practice, it is usually dozens of times. Experiments have shown that at noon on a clear day, the temperature at the focal zone of the Fresnel lens can reach more than 20°C, which is 4 to 5 times higher.

Fresnel lens and CPC Concentrators
Figure 1 – Schematic diagram of Fresnel lens focusing

From the effect of temperature on the output characteristics of photovoltaic cells, it can be known that under high-intensity light radiation, if a high-power concentrator is used, the operating temperature of photovoltaic cells will rise sharply, which will reduce the conversion rate of photovoltaic cells and even damage the cells. Therefore, in order to improve the output energy of photovoltaic cells, a low-power concentrator with a concentration ratio of 1 to 10 should be used.

Reflective Concentrator – CPC Concentrator

The CPC concentrator belongs to the combined parabolic concentrator, which is a non-imaging low-focus concentrator. The reflection surface is made of aluminized film, which is relatively low in cost and simple in processing technology. It is designed according to the principle of marginal ray, which can collect the incident light within a given acceptance angle range to the receiver with an ideal concentration ratio. This concentrator not only provides a large acceptance angle, but also enables light within the acceptance angle to reach the photovoltaic cell after at most one reflection. It can receive direct solar radiation and partial scattered radiation, and can receive radiation around the sun that general concentrators cannot receive.

The CPC concentrator consists of two rotating paraboloids symmetrical to the central axis of the photovoltaic cell, and the vacuum tube encapsulating the photovoltaic cell is placed at the bottom. The bottom ends of the two parabolas are the foci of the opposite parabola, and the line connecting the focus of one parabola to the vertex must be parallel to the axis of the other parabola. As shown in Figure 2, the designed two-dimensional cross-sectional graph of the compound parabolic reflector consists of two parabolas and two involutes.

Fresnel lens and CPC Concentrators
Figure 2 – Cross-sectional 2D graphics of CPC reflector

When the incident angle is oblique |θi|<θa (maximum half acceptance angle), the solar light entering the opening needs only one reflection to focus on the focal point. When the incident angle |θi|>θa, the solar light entering the opening is reflected multiple times on the two parabolic mirrors and then leaves the opening and shoots into space. So as long as the incident angle satisfies -θa < θi < θ, all the light will converge on the solar panel after one or several reflections.

Taking the center of the vacuum tube as the origin to establish a rectangular coordinate system, the parametric coordinate equation of the CCP cross-section is obtained:



In the formula, L is the length of the tangent from the point of the reflector to the vacuum tube; Φ is the angle between the tangent and the negative direction of the Y-axis.

The ratio of the opening length of the CPC concentrator to the receiver area (the perimeter of the vacuum tube) is the focusing ratio C. For a CPC concentrator with gap g, its maximum focusing ratio can be expressed as:

Fresnel lens and CPC Concentrators