We present a new design of compound Fresnel-R concentrator which is composed of two lenses: a primary lens (Fresnel lens) that works by total internal reflection at outer sawteeth but refraction at inner sawteeth, and a ringed secondary lens that works by refraction. In contrast to previous Fresnel lens concentrators, this design increases the acceptance angle, improves the irradiance uniformity on the solar cell, and reduces the aspect ratio significantly. Meanwhile several sawteeth of the primary Fresnel lens can correspond to a same ring of secondary lens, which will efficiently lower the complexity of designing and manufacturing. Moreover, in order to reduce the influence of manufacturing tolerances and to increase the optical efficiency further, the central part of the bottom of the secondary lens which directly adhered to the solar cell is designed as a cone-shaped prism to collect the sunlight that does not reach the solar cell. Finally, we provide simulations and analyses of the design method an optical efficiency more than 80% and an aspect ratio smaller than 0.5 can be achieved.
The photovoltaic industry is growing rapidly today; however, it is still limited by the high cost of photovoltaic systems, especially the expensive semiconductor material. An effective way to reduce the cost is to cut down the amount of the semiconductor material by means of combination with concentrating optics. The Fresnel lens has been used as a concentrator in photovoltaic field for many years [
In addition to the excellent performance of the multijunction solar cell under high concentration sunlight [
With this background, in this paper a more realistic design method is proposed to design a high concentration compound Fresnel lens with a small aspect ratio, which combined the outer sawteeth that work by total internal reflection with the inner sawteeth that work by refraction. The critical radius where the sawteeth of Fresnel lens begins to perform TIR is derived by using simple trigonometry. Furthermore, the approach of simplifying design method that several sawteeth of the primary Fresnel lens can correspond to a same ring of secondary lens is also presented. Thus, only several aspherical rings are designed as secondary lens' front surface is done in the CPVs to obtain more uniform irradiance distribution. Finally, a cone-shaped prism to collect the sunlight that does not reach the solar cell is also added to consummate the CPVs, which will increase the optical efficiency further.
The CPV optics constitute a typical design problem that contains both the bundle coupling problem for obtaining maximum acceptance-concentration product and the prescribed irradiance to obtain uniform irradiance distribution on the solar cell area [
The design method includes two parts: design of primary Fresnel lens and design of secondary lens of CPV system. The former also includes two parts: design of the outer sawteeth of Fresnel lens that work by total internal reflection and design of the inner sawteeth that work by refraction. The front surface of secondary lens has several aspheric rings and is designed to optimize the spatial distribution of the light over the solar cell. The solar cell is adhered at the bottom of the secondary lens directly, making it simple to seal against moisture and prevent misalignment. Figure
Sketch map of compound Fresnel-R lens for CPV system.
First, we design the outer part of primary Fresnel concentrator that works by total internal reflection. Before that we need to determine the critical radius which cuts the Fresnel lens into two different parts: the outer part that works by TIR and the inner part that works by direct refraction, as shown in Figure
Determination of the critical radius. (a) The direct refractive sawtooth. (b) The TIR sawtooth. The sawteeth and the angle of incident ray are locally exaggerated, only for interpreting conveniently.
Figure
The change of incident angle
Achieving high reflectivity mirror is usually expensive and the degradation of the mirror quality could be faster than is desirable for a CPV system [
Design of the outer and inner sawteeth of primary Fresnel lens. The coordinate system is specified (
Secondly, we design the inner part of primary Fresnel concentrator that work by direct refraction. The design method is very similar to that of the outer sawtooth as described above. The only difference is that the rays (represented by red line) emitted from a plane wavefront
Note that in the calculation of these discrete points, the width of each sawtooth
The design method of secondary lens is depicted in cross-section in Figure
Design of secondary lens. (a) Specification of the type of secondary lens' front surface. (b) Calculation of the borders of secondary lens' front surface.
Next we need to calculate the borders of this ring of secondary lens' surface tracing the edge rays
Finally, in order to reduce the influence of manufacturing precision, a cone-shaped prism to collect the sunlight that does not reach the solar cell is added to consummate the CPVs, as shown with bold lines in Figure
Schematic drawing of the secondary lens whose
Considering the cost of the manufacture of a real lens, numerical simulation based on the most widely used Monte Carlo ray tracing optical software is an efficient way to validate the concentrator design. A simple high concentration circular Fresnel lens was designed and simulated to validate this novel design method. The specifications of the design parameters are diameter of Fresnel lens 51 mm, focus of Fresnel lens 21 mm, drag angle is 2°, diameter of secondary lens 8.6 mm, and diameter of solar cell 2 mm. The geometrical concentration is 625x and the aspect ratio of the entire optical system is smaller than 0.5. For this study we choose PMMA as the base Fresnel lens material for the optics. This is a very good option: high transmission factor, susceptible of molding manufacturing and with low cost. However, it is not suitable to be as the material of the secondary lens due to high temperature resulted from the high concentration, which will distort the plastic material. Thus the glass material of BK7 is adopted as the secondary lens material. The width of each groove is about 3 mm, which can be freely adjusted due to manufacturing condition.
As shown in Figure
Profile and 3D cutaway view of a compound Fresnel-R lens for CPV system.
This simulation was done by assuming that the refractive index of material PMMA and BK7 are 1.49 and 1.52 (
Relative transmission curve versus incident angle. The traditional Fresnel lens 1 and 2 represent the Fresnel concentrator without secondary lens and with secondary lens, respectively.
As depicted in Table
The main losses of this concentrator system, considering the normal incidence and divergence angle of solar.
The reason of losses | Losses | Transmission |
---|---|---|
Fresnel losses on the front surface of primary Fresnel lens | 96.06% | |
Fresnel losses on the sawteeth surface of primary Fresnel lens | 90.82% | |
Losses due to the drag angle of primary Fresnel lens' sawteeth | 86.32% | |
Fresnel losses on the surface of secondary lens | 82.77% | |
Total power efficiency on the surface of solar cell: 82.77% |
Spectral transmission power before and after the concentrator.
In 3D the irradiance distribution is relatively uniform in the radial direction but not in the azimuthal direction because of its rotational symmetry. The simulation results are shown in Figure
3D irradiance distribution on the cell when the sun ray is at normal incidence. (a) Traditional Fresnel lens. (b) Novel compound Fresnel lens.
A novel design of the compound Fresnel concentrator for CPV system was presented to increase the concentration factor, reduce the aspect ratio, and improve the optical performance on the solar cell. The simulation results have identified that using this method to design Fresnel concentrator, the aspect ratio (
The authors gratefully acknowledge the financial support from National Basic Research Program of China with Grant no. 2010CB227101 and the Innovation Program of Chinese Academy of Sciences.