Indoor Experimental Assessment of the Efficiency and Irradiance Spot of the Achromatic Doublet on Glass (ADG) Fresnel Lens for Concentrating Photovoltaics

The overall goal of this method is to assess the performance of the achromatic doublet on a glass Fresnel lens as new optics for concentrating photovoltaic systems. The method allows to determine both the transmission efficiency of the optics and its concentration ability by measuring the size of the spot cast by the lens. The evaluation of the optics is carried out by measuring how well it concentrates light on multi-junction solar cells.

These devices convert into electricity irradiance over a wide spectral bandwidth. In concentrated photovoltaics, the chromatic aberration reduces the maximum attainable concentration when using the refractive primary element. This limitation is avoided by using the achromatic doublet on glass Fresnel lens we have designed.

The design includes two different materials, a plastic and an elastomer, having different dispersion. That is, the refractive index variation is a function of the wavelength. The inexpensive manufacturing process includes the lamination of both materials on a glass substrate in order to obtain a parquet of lenses.

For every measurement, a silicone on glass Fresnel lens is used as benchmark. The solar simulator for concentrator solar cells, Helios 3030 from Solar Added Value, has been used to perform the measurements. This equipment is able to measure MJ solar cell under concentrated light of 1, 000 suns with controlled spectrum.

Place the top, middle, and bottom reference isotypes inside the solar simulator along with the solar cell to be measured. Place them as close together as possible in order to reduce errors due to non-uniform illumination at the measuring plane. Next, adjust the flash lamp height in order to reach the desired level of concentration.

Add the necessary filters to adjust the spectral distribution. Then, connect the isotypes and the cell to be measured to the data acquisition board of the solar simulator. Open the control software, and select an irradiance level, where both the top and middle isotypes indicate exactly the same irradiance level.

This is to confirm that the cell is measured under the target concentration level and spectrum. Then, run the simulator to start the IV test. For every point defined in the text file, the equipment polarizes the cell at the desired voltage, triggers the flash, and measures the current generated by the solar cell.

Repeat this process at different concentration levels to check that the photocurrent generated by the cell changes linearly with the concentration, confirming the reliability of the calibrated solar cell as light sensor for lens characterization. Mount the three-axes automated positioning platform inside the dark chamber of the solar simulator for concentrator photovoltaic systems. Then, mount the solar cell on the platform's moving holder in such a way that it is possible to control its position along X, Y, and Z axes, and connect it to the data acquisition board.

Next, clean and place the lens to be measured on the fixed support mounted on the automated positioning platform. Use the moving platform to center the solar cell with respect to the lens and to place it at the optimal focal distance. Then, use the spectroheliometer containing three isotype cells inside collimating tubes to assess the spectral conditions during measurement.

Close the simulator curtain to block all external light sources. Open the software controlling the solar simulator, and press the button Light pulse to trigger the xenon flash lamp. Next, determine the current generated by the solar cell as the measured value when top and middle isotypes indicate exactly the same irradiance level.

Write a text file with several lens-to-cell distances around the optimum value, and repeat the measurement for every position. Repeat all measurements, substituting the achromatic doublet on glass Fresnel lens with the silicone on glass Fresnel lens that will be used as a reference. On the same three-axes automated positioning platform that was previously used, mount the CCD camera.

Select the optimum focal distance. Adjust the holder to place the spot of light so that it is approximately centered on the CCD sensor. Then, add a short-pass filter to block light whose wavelength is longer than 650 nanometers.

In this manner, only the light that is converted into electricity by the top subcell within a multi-junction solar cell will be recorded. Trigger the xenon flash lamp, and synchronize the CCD camera to take a photograph of the light spot projected by the lens. Process the photograph to select an area that includes the spot, and calculate the centroid of the irradiance spot.

Calculate the diameter of the light spot projected by the lens. It is defined as the diameter of the circle containing 95%of the light that reaches the CCD camera sensor. Next, take one photograph for every position around the optimum focal distance that was previously defined.

Repeat the measurements with the short-pass filter to block light whose wavelength is shorter than 650 nanometers. In this case, only the light that is converted into electricity by the middle subcell within a multi-junction solar cell will be recorded. The previous measurements can be repeated by placing the lens under test inside a thermal chamber able to control its temperature.

The chamber wall needs to be transparent for all wavelengths of interest. The normalized values of the photocurrent generated by the solar cell, when illuminated by the achromatic doublet on glass or the silicone on glass Fresnel lens, are plotted as a function of the relative lens-to-cell distance. The achromatic doublet on glass lens show higher tolerance to displacement of the lens from its optimum position along the optical axis, thanks to its design.

The evolution of the spot diameter corresponding to the top and middle subcells within a multi-junction solar cell is plotted as a function of the lens-to-receiver distance for both lenses. The displaced curves in the silicone on glass sample is due to chromatic aberration. Since the refractive index for short wavelengths is higher, the focal point for blue light is closer to the lens.

Conversely, for the achromatic lens, the position of the minimum spot for blue light corresponds exactly with the minimum spot for red light, proving the lens achromatic behavior. The light spot enlargement, due to temperature variation for the silicone on glass lens, is larger than for the achromatic lens. In outdoor operating conditions with strong thermal excursion, using the achromatic lens would make the system performance more stable.

Once mastered, this technique allows the complete indoor characterization of optics for concentrated photovoltaic application, such as primary lenses or primary mirrors. The achromatic doublet on glass Fresnel lens developed at the Solar Energy Institute have been completely characterized using the proposed protocol. Both the optical efficiency and the spot size have been measured.

Using this method, we have been able to experimentally demonstrate the achromatic behavior of the ADG lens, its higher tolerance to a displacement with respect to the optimal focal distance, and the lower sensitivity to a temperature variation.