Nancy Lamontagne
Researchers at the University of Arizona, led by Roger Angel, have undertaken the development of a solar energy module with one goal in mind: producing solar energy at a cost low enough to compete with fossil fuel without the need for government subsidies. So far, they are on track to accomplish this.
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Roger P. Angel |
Caption: Researchers completed an end-to-end test in which a reflector with 4 rectangular segments powers a receiver with 8 triple junction cells. The receiver’s ball lens glows with concentrated sunlight in this first test, which yielded 500W of DC power. Images courtesy of REhnu LLC.
“We took a clean sheet of paper and designed a system with the goal of achieving an installed capital cost of $1/watt,” Angel said. The resulting “energy telescope” concentrates sunlight in a way that is different from most other concentrated photovoltaic systems, which usually place a lens on every solar cell.
In the energy telescope, dish reflectors direct sunlight onto a ball lens. The light becomes highly concentrated at the focus near the center of the ball lens and emerges concentrated approximately 400 times. The light is further concentrated by optical funnels, resulting in light that is at 1200 times geometric concentration before it hits photovoltaic cells located just behind the funnels. These funnels are sized so that all the cells receive the same power and thus generate the same electrical current.
Sunlight from the reflector dish comes to a highly concentrated focus near the center of the ball lens and an array of optical funnels captures and further concentrates the light. Immediately behind the funnels would be the triple-junction photovoltaic cells (not shown). Images courtesy of REhnu LLC.
“Other groups have used dish systems, but they don’t use the same type of optics to control the light at its focus,” Angel said. These optics allow more energy to be made using half the area of photovoltaic cells.
In most systems the photovoltaic cells are the most expensive component, but this is not the case in the solar telescope. Thus high-tech, high efficiency cells can be used without increasing costs significantly. The solar telescope uses commercially available triple-junction solar cells, which have three junctions that each capture energy from different wavelengths of light. These solar cells have more than double the conversion efficiency of conventional (single-junction) cells.
Eight dish reflectors and their associated optics and photovoltaics as well as cooling and tracking components are all integrated into a spaceframe structure (pictured at right). Cost was at the forefront of this structure’s design as well. It was constructed in the lightest possible way using lightweight high strength low alloy steel in a mechanically efficient framework.
From the beginning, Angel has been focused on bringing the energy telescope technology from the laboratory to practice. To this end he founded REhnu, LLC, which has an exclusive license for the energy telescope solar technology developed at the University of Arizona.
So far the researchers have performed end-to-end testing using 8 adjacent optical funnels paired with eight 15-mm square solar cells, a subset of the 36 funnels and cells to be used in a complete receiver.
The 8 cells consistently produced over 500 watts of DC power. From the measured direct normal irradiance (DNI), they calculated they system’s end-to-end efficiency to be 25%. This scales up to an output close to 2.5 kW at 1000 W/m2 DNI solar flux for a full 36 cell receiver.
“Our goal is to have the first complete 20 kW energy telescope installed and operating by the end of the year,” Angel said. He recently received a $1 million grant that will be matched by REhnu, which will help him meet this goal and advance the technology.
