Inaugural issue of OSA's Energy Express focuses on gaining energy with solar concentrators

New devices that concentrate the sun's rays, increase the efficiency of solar cells, and help to lower the cost of solar energy, are featured in the inaugural issue of Energy Express, a new special bi-monthly supplement to Optics Express, the open-access journal of the Optical Society (OSA). Edited by Bernard Kippelen of the Georgia Institute of Technology, Energy Express launches today and will focus on research that encompasses the science and engineering of light and its impact on sustainable energy development, the environment, and green technologies. Research featured in Energy Express will highlight the role optics plays in energy efficiency and sustainability, from making solar energy economical to providing energy from fusion, to studying new combustion processes that can ease carbon dioxide sequestration, to monitoring the nitrogen cycle, and to realizing significant energy savings using solid-state lighting, such as inorganic and organic light emitting diodes (LEDs and OLEDs). To underscore the importance of optics in energy, Energy Express will publish a series of Focus Issues dedicated to selected research areas. The first issue of Energy Express is a special Focus Issue dedicated to new developments in solar concentrators, a set of technologies that aim to increase the efficiency of the generation of solar energy. Roland Winston of the University of California Merced is the guest editor for this issue.

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, solar power still only contributes a tiny amount to the overall power consumed each year in the United States, for example. In 2008, it accounted for roughly 1 percent of U.S. renewable energy production, with all renewable sources together contributing only approximately 7 percent of the total amount of energy produced in the U.S. that year—the latest for which statistics are available from the U.S. Energy Information Administration.

One of the barriers to the widespread use of solar power in the U.S. is its cost. Currently, it is still significantly more expensive dollar-per-watt than power generated by burning coal, oil, or natural gas. 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.

Several articles featured in the inaugural issue of Energy Express focus on the latest emerging new technologies that aim to achieve this. These include new materials and designs for solar cells that can increase their efficiency; new approaches to concentrating the sun's rays to allow solar panels to make the most of the daylight; and applications of solar power that harness the sun's energy to remove the salt from seawater, generate chemical fuels, and power solid-state lighting.

Some highlights of the special Focus Issue on Solar Concentrators include:

LAYERED SOLAR CELL MATERIALS

One of the basic approaches to improving solar panels in the last several years has centered on increasing their efficiency—the amount of the sun's energy hitting the panels that they can convert into useable 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 U.S. National Renewable Energy Laboratory, the latest, most efficient cells in the laboratory can achieve efficiencies greater than 40 percent. 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.

The article, "Multijunction solar cells for conversion of concentrated sunlight to electricity" by Sarah Kurtz and John Geisz can be accessed at: http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-18-S1-A73.

CONCENTRATING LIGHT WITH LENSES AND SUN TRACKING

While multijunction 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.

One way to counter these costs is to increase the amount of energy the cells produce by concentrating the sunlight that falls on them. In one Energy Express paper, Valery Rumyantsev and colleagues at the Ioffe Physical Technical Institute in St. Petersburg, Russia describe such ways to maximize the amount of energy from solar cells by squeezing as much sunlight as they possibly can into them.

The Ioffe team is combining 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.

In another paper, Roland Winston and Weiya Zhang of the University of California Merced describe a non-tracking solar concentrator that does not rely upon precise sun-tracking devices, which may add to the cost and complexity of installing and maintaining solar panels, they observe, and which may not be practical on residential rooftops.

Winston and Zhang have developed a stationary solar concentrator that concentrates sunlight up to four times for eight hours a day, year-round. This has the potential to reduce the number of expensive solar cells in a solar panel by four, they say.

The article, "Solar concentrator modules with silicone-on-glass Fresnel lens panels and multijunction cells" by Valery D. Rumyantsev can be accessed at: http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-18-S1-A17.

The article, "Pushing concentration of stationary solar concentrators to the limit" by Roland Winston and Weiya Zhang can be accessed at: http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-18-S1-A64.

IMPROVED DYES FOR CONCENTRATING SOLAR ENERGY

Marc Baldo and colleagues at the Massachusetts Institute of Technology report the latest research on another way to eke out more power from solar panels. They use an inexpensive technology known as luminescent solar concentrators to increase the usable energy falling from the sun onto solar cells.

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. In their article, they report the highest light trapping efficiency by a luminescent solar concentrator to date.

The article, "Dye Alignment in Luminescent Solar Concentrators: I. Vertical Alignment for Improved Waveguide Coupling" by C.L Mulder, P.D. Reusswig, A. M. Velázquez, H. Kim, C. Rotschild, M.A. Baldo can be accessed at: http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-18-S1-A79.

SOLAR FUELS

In their Energy Express paper, Aldo Steinfeld of ETH Zurich in Switzerland and Alan Weimer from the University of Colorado at Boulder review how concentrated solar energy can be used to produce clean chemical fuels for the power and transportation sectors. Thermochemical processes that make use of concentrated solar radiation as an energy source to drive high-temperature endothermic reactions have the potential to achieve high solar-to-fuel energy conversion efficiencies.

They describe solar thermochemical cycles to split H₂O and CO₂ via metal oxide redox reactions. They also describe solar processes and reactors in which coal, for instance, can be converted into high-quality synthesis gas (mainly H₂ and CO), which can be further processed to Fischer-Tropsch liquid fuels with a solar-upgraded calorific value and, consequently, lower CO₂ emissions per kWh. Ultimately, these solar-driven processes provide means of storing intermittent solar energy in a transportable and dispatchable chemical form.

The article, "Thermochemical Production of Fuels with Concentrated Solar Energy" by Aldo Steinfeld and Alan W. Weimer can be accessed at: http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-18-S1-A100.

Source: Optical Society of America