The sun’s energy can be increased when focused on a smaller area—just ask any kid who has burned holes in a sheet of paper with a magnifying glass. The same concept has been used for small camping stoves and enormous solar furnaces. And while solar panels that take advantage of concentrated sunlight to boost their power output already exist, scientists at the Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) in Freiburg, Germany, have taken the concept to a whole new level: They’ve created a micro-concentrated photovoltaic (micro-CPV) solar panel that has better efficiency but lower costs to manufacture.
Previous micro-CPV designs have faced multiple problems, including degradation over time and discoloration of the optical elements. Other designs often also required active thermal management systems to cool the components.
Innovative Micro-CPV Solar Panel Design
The team built a sub-module panel of about 200 square centimeters; while this comprised more than one micro-CPV cell in a 10-by-6 cell array, it was smaller than a standard commercial panel. Often, projects like this require sophisticated processes and expensive materials such as complex lenses and designs that required custom assembly procedures with precision alignment of the elements, but that’s not true in this case.
Instead, the researchers used glass substrates for their printed circuit boards. The photovoltaic modules can be assembled using a standard high-speed pick-and-place machine. And the concentrating lenses are made using silicon on glass technology.
The researchers estimate that their design uses less than one-thousandth the expensive semiconductor materials required by standard panels. They describe their work in a recent paper published in the IEEE Journal of Photovoltaics.
The system relies on planoconvex lenses to focus sunlight. The convex portion is made from silicone, which is a low-cost material with sufficient optical qualities. This is adhered to a flat glass surface, which has matching thermal expansion characteristics to maintain optical alignment throughout temperature changes.
The researchers first created a single cell unit using submillimeter-sized photovoltaic chips that measure 885 by 685 micrometers, with an active area of 585 by 585 µm. They then made a larger array module for testing. The matrix array used cells with chips that were 1,127 by 927 µm with an active area of 827 by 827 µm. The larger chip size meant that their placement was less critical than with the smaller chips, which makes assembly easier and more efficient. The chips use five-junction solar cells, which use multiple layers to capture the energy from different wavelengths of sunlight.
The prototype micro-CPV solar panel uses a matrix of cells to create an industry-standard 24-by-18 inch panel (61 by 45.7 centimeters) with an aperture area greater than 2,000 square centimeters.Fraunhofer ISE
In both cases, the chips were mounted on a glass substrate, which helped dissipate the heat from the panel. This was sufficient to control temperatures without the need for additional active cooling components, in spite of the concentrated sunlight. The outdoor tests were conducted with the panel mounted on a dual-axis tracking mechanism so that the lenses could reliably focus the direct sunlight.
“The combination of miniaturized components, additive manufacturing, parallelized processes, and self-alignment promises significant cost reduction for CPV, and will further benefit from learning curves in other major industries. At the same time, energy and resource consumption are cut down,” says Henning Helmers, the head of Fraunhofer ISE’s III-V Photovoltaics and Concentrator Technology department. This could lead to lower environmental impact across the entire life cycle, compared with other photovoltaic solutions.
The end result is a more efficient solar panel that can be made from low-cost materials with fewer semiconductor components using standard high-volume manufacturing processes already in use in the display industry. Not everyone is convinced about the economics of the design, however. Jenny Chase, solar analyst with BloombergNEF, points out that “semiconductor materials are inexpensive these days, and dual-axis tracking adds a lot of cost” to an installation.
The team tested the panel outdoors over the span of a year. The panel achieved 36 percent conversion efficiency under concentrator standard testing conditions (CSTC) that control for factors that can impact the results, including irradiance (the amount of sunlight reaching the panel), ambient temperature, and windspeed. The tests resulted in median values of 31.4 percent to 33.6 percent conversion efficiency in real-world conditions. According to various sources, the best commercial panels currently available typically have 19 percent to 24 percent conversion efficiency, so this new design extracts about 50 percent more power from the sun. After the year-long test, the team did not observe any significant degradation in the panel or its performance.
The micro-CPV development could lead to commercially available panels that cost less to build yet produce more energy, which would make solar power even more attractive as an alternative to other sources. The Fraunhofer ISE team is currently spinning out a company, Clearsun Energy, to bring this technology to market.