IEEE Spectrum – Todays armies marches on its batteries. Without them, soldiers can’t see in the dark, work their radios, or determine their positions. But even the best storage batteries—accounting for one-fifth of the load in a typical infantryman’s 45-kilogram pack—can’t last the week or so that field soldiers require. Better energy storage is taking too long, so far better solar power is also being pursued.
The most promising effort to create such superefficient photovoltaics (solar power) began in 2005, when Doug Kirkpatrick, a veteran of the optics industry, kick-started the Very High Efficiency Solar Cell (VHESC) program for the U.S. Defense Advanced Research Projects Agency (DARPA). He wanted a way to build modules from solar cells that would convert a full 50 percent of the solar energy they receive into direct current. That’s a jaw-dropping number when you consider that in 2005 the best laboratory devices were still shy of 40 percent efficiency and were improving by less than one percentage point per year.
Funneling Color: The DARPA design concentrates sunlight and splits it into “buckets” optimized to absorb at particular frequency ranges. The initial concept, not yet realized, was to split light into three ranges. High-energy photons would go to a single cell, while two or three cells stacked together would harness the power from mid-energy and low-energy cells, respectively. As of today, researchers have not been able to produce a good enough cell for the high-energy photons, so the prototype design has just two buckets—a “green” one for higher frequencies and a “red” one for lower ones
Kirkpatrick considered the needs of the end user—the soldier. He noted that each electrical gizmo offered a different-size area for placing solar cells. This would make it possible to put more cells on bigger gadgets and use them to charge up other gizmos, such as flashlights, which have very little real estate. But that was a risky strategy, because the failure of one piece of equipment would put the others out of commission, too. Therefore, Kirkpatrick decreed that each piece of gear would have its own solar power, limiting the solar module’s area to 10 square centimeters and setting cell efficiency at a minimum of 50 percent. The only other key specification for the module was power, which had to be at least 0.5 watts to recharge the batteries of the more common gadgets in an acceptable time.
Christiana Honsberg, now at Arizona State University, and Allen Barnett, now at the University of New South Wales, in Australia, argued that the best approach would be to concentrate sunlight, break it into its constituent colors, and project each color onto the kind of solar cell best suited to it. This meant placing the cells side by side instead of the conventional way, in a vertical stack.
Switching from a multijunction stack to a side-by-side array increases the maximum theoretical efficiency. That’s because in a stacked system each layer is normally connected in series and thus has to produce the same amount of current. Designers can adjust layer thicknesses and other parameters to try to meet this requirement, but that’s hard to do perfectly because the solar spectrum varies throughout the day—it’s redder in the morning and late afternoon, bluer at midday. So a triple-junction cell’s current is often limited to whatever the least productive layer can produce.
alculations show that without concentrated sunlight, efficiency should rise from 51.5 to 55.6 percent when you shift from three junctions to four. And moving to five or six junctions should raise efficiency to 58 percent and 59.6 percent. Real-world gains would of course be lower, but still, upping the concentration should assure good results. Barnett and Honsberg estimate that for a six-junction cell operating at 20 suns you get an efficiency of 54.3 percent, and at 100 suns, 55.6 percent. With much more concentration than that the cell might overheat, and it would be difficult to point the mirrors precisely.
Barnett and his colleagues initially developed tracking-free solar modules using concentrations of 20 suns. A mobile user could manually point the cells toward the sun.
Because of the difficulties with the high-energy bucket, the VHESC program has so far developed optical modules with merely low-energy cells, together with mid-energy variants that also capture some high-energy photons, although not as efficiently. In 2007, researchers measured the performance of cells made from indium gallium phosphide, gallium arsenide, silicon, indium gallium arsenide phosphide, and indium gallium arsenide, then calculated the contribution that each would deliver to a five-junction cell operating at 20 suns. They concluded that such a device could have a 42.9 percent efficiency, well ahead of the 40.7 percent record then held by a Spectrolab cell operating at a concentration of 240 suns.
Since then the program has defined its target differently, aiming not for a cell efficiency of 50 percent but rather for a module efficiency of 40 percent. This makes sense: Modules are always less efficient than cells, partly because imperfections prevent their optics from directing all the incident sunlight on the cell. And it’s the output power of the modules that matters. The 40 percent target is still plenty tough, given that the best commercial triple-junction photovoltaic modules have efficiencies of just over 30 percent when operating at concentrations of several hundred suns.