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New Ways to Harness More of the Sun's Energy Through Solar Concentrators
Solar concentrators hold great promise for delivering plentiful alternative energy and contributing to the world's energy needs in the future, since direct sunlight is by far the most abundant source of energy available on Earth. But while the basic technology behind solar panels has been around for decades and while increasing in popularity governmental financial incentives are still important in fueling the increasing number of installations around the world.
If you've ever focused a beam of light from a magnifying glass onto a piece of paper you notice two things right away: (1) The diameter of the light beam hitting the paper is much smaller than the diameter of the magnifying glass; and (2) the concentrated light is a lot warmer than the surrounding air … in fact at the right distance you can start a fire with a magnifying glass. In this example, you could say that the magnifying glass concentrated the sunlight. Therefore, a solar concentrator does exactly what its name implies: It takes the sunlight that strikes a wide area and bunches it together. But it doesn't just concentrate the sunlight; it also directs that sunlight to a very specific, smaller location. Unlike a solar tracker, which moves the solar panel so that the sun is striking at the best angle, a solar concentrator is stationary.
One of the barriers to the widespread use of solar power in the US is its cost, While costs to produce electricity from the sun have been falling steadily (while energy from fossil fuels has been increasing) it is still more expensive. If solar power generation is going to increase in the future, large-scale production will have to become cheaper—perhaps by either lowering the cost of solar cells or increasing the amount of power they generate. Solar Concentrators are one of the major avenues of research that many hope will lead to lower costs, and there are several approaches that are currently being followed.
The basic strategy of solar concentrators is to gather the light using inexpensive means and direct the light to very efficient solar cells that then produce electricity. The greater the efficiency of a solar cell, the more energy it can produce when in the sun. Commercially available flat-plate solar panels are currently around 19 percent efficient, but in the last several years, scientists have been able to do significantly better by making "multijunction cells.” These stack several layers of ultra-thin materials on top of each other. Each separate layer absorbs a particular range of colors (or wavelengths) of light, and together they convert a broader spectrum of the sun's energy into electricity. According to Sarah Kurtz and John Geisz of the , the latest, most efficient cells in the laboratory can achieve efficiencies greater than 40 percent (click here to read this news article). In their Energy Express paper, they predict that this cutting edge will continue to move, and the best solar cells may reach 45 or 50 percent efficiency with further work. While multi-junction solar cells can achieve higher efficiencies, their thin layers may rely on rare and expensive elements like indium and gallium in their design, and the cells themselves often must be constructed under exacting laboratory conditions. These factors make the best solar cells very expensive to produce. Consequently, such cells are best used in applications that can justify these costs – such as space applications, military installations (where the small size is attractive), and in situations where the sunlight can be concentrated and directed to the multi-junction cell. To read their article, "Multijunction solar cells for conversion of concentrated sunlight to electricity" by Sarah Kurtz and John Geisz click here.
One way to concentrating the sunlight that falls on the solar cells is to use Fresnel lenses. At the Ioffe Physical Technical Institute in St. Petersburg, Russia they are combining efficient solar cells with Fresnel lenses—optical elements similar to the sort of shaped glass that allows lighthouse torches or theatrical spotlights to focus light into a strong beam. They also use sun-tracking devices to automatically orient solar panels with the direction of the sun as it moves across the sky. Generally, the problem with this type of technology is that by intensifying the light that hits the solar cells it raises the heat of the solar cell causing it to lose efficiency, and thus necessitates the use of cooling systems to reduce the heat loss.
Holographic tuning makes it possible to produce a low-cost, heat-resistant, and efficient solar panel using a combination of holographic strips to concentrate the light above and below the solar panel (making bi-facial solar panels practical). For full details about this very promising technology and the company that is building PV panels using these strips, read our article about Prism Solar.
These concentrators consist of glass or plastic sheets coated by a thin film of dye molecules. The dye absorbs specific colors of sunlight and then re-emits light at longer wavelengths that gets trapped within the glass and converted into electricity by small solar cells attached to the edges of the concentrator. These concentrators are inexpensive and are a potential solution for lowering the cost of solar electricity, Baldo and colleagues say. In their paper, they are reporting a significant new improvement they achieved by suspending the dye molecules in a liquid crystal and controlling their orientation they report the highest light trapping efficiency by a luminescent solar concentrator to date. For more information click here.
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