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Soldiers' Tools Go Solar

March 2007
By Henry S. Kenyon

New photovoltaic technology promises embedded power for handheld systems.

Warfighters soon may turn to the sun to recharge their battlefield electronics. The U.S government is developing highly efficient solar cells that will be built into batteries and tactical equipment such as night vision goggles, personal navigation devices and radios. The effort seeks to cut the number of spare batteries carried by soldiers to save weight and reduce logistics requirements.

The technical goal of the Defense Advanced Research Projects Agency’s (DARPA’s) Very High Efficiency Solar Cell (VHESC) program is to demonstrate at least 50 percent efficiency in a photovoltaic device. According to program manager Douglas Kirkpatrick, Arlington, Virginia, the work is progressing rapidly and has the potential to affect both the commercial and government markets.

The program began in 2005 as a proof of concept demonstration for a different approach to solar cell design made possible by new developments in optics. An important design requirement was to integrate photovoltaic cells directly into systems, avoiding the need to unfold a solar collector to recharge equipment. Kirkpatrick notes that reducing the demand for batteries is a pressing need because the logistics of supplying batteries to troops in the field is daunting. “They [batteries] really drive how much weight a soldier has to carry, which really drives the logistical pipeline,” he says.

Kirkpatrick considers the VHESC program a classic DARPA effort because it applies new technologies from various areas to meet a specific challenge. If it is successful, he believes, the technology can potentially impact many markets beyond the defense industry. “If you can make 50-percent efficient solar cells, then the majority of tactical battlefield electronic devices—and a very large marketspace of commercial electronic devices—can be very much like a calculator and effectively powered by an embedded, integrated solar cell,” he shares.

The $53 million program’s first phase ended in August 2006, passing its go-no-go criteria by a significant margin. Kirkpatrick explains that phase one was so successful that the program is being restructured to develop products from the technology more aggressively. Phase one demonstrated a lateral solar cell design proposed by the University of Delaware and a team consisting of university, business and government laboratories, including DuPont, BP Solar, Corning Incorporated, Emcore Corporation, Blue Square Energy, LightSpin, the Massachusetts Institute of Technology, the University of California at Santa Barbara, the University of Rochester and the National Renewable Energy Laboratory.

Traditional solar cells use a single layer of silicon. Conventional high-efficiency cells stack multiple layers in a vertical architecture to more effectively capture a broader range of sunlight. The vertical architecture cells are manufactured in a layering process similar to microchip construction. Each layer, or lattice, is designed to gather incoming photons from a specific range of spectrum, and each layer is grown on the layers beneath it. This structure requires that the atomic lattice structure of the material of each layer can grow in the layer below it, which limits the kinds of materials that can be used.

Lateral architectures consist of a variety of small photovoltaic cells arrayed next to each other. This architecture enables designers to use different light-sensitive materials to cover the solar spectrum. The structure allows materials such as silicon, gallium arsenide and indium phosphide lattices to be placed side by side in one solar cell without lattice matching issues. Lateral solar cells also incorporate an optical element that separates and focuses sunlight into spectral bands optimized for each of the elements.

Kirkpatrick explains that an inherent problem with conventional solar cells is their layered architecture. While the vertical lattice structure is optimized to capture energy from specific light frequencies in each of the layers, the design also causes traditional solar cells to lose up to 60 percent of their power because they can efficiently access only certain parts of the spectrum within the constraints of vertical layer by layer lattice matching. In a typical series-connected vertical architecture photovoltaic cell, each layer drives electrical current, but the device can drive only as much as the layer with the least current. Because of this inherent design issue, even conventional solar cells with theoretical efficiencies of 47 percent to 48 percent lose power, he notes.

Lateral architectures for photovoltaics are not a new idea. Kirkpatrick says that the concept originated in the 1970s, but the optics technology at the time was unable to increase spectral efficiency beyond 70 percent, which negated the design’s benefit. The VHESC program has achieved 93 percent to 94 percent spectrum-gathering efficiencies with the potential for rates greater than 97 percent. “We already have 94 percent [optically] efficient systems in the bag. That’s the difference,” he shares. “If you can make the optical system efficient, then that sets this [the power gains from lateral cell architecture] free. That’s the big deal.”

Dichroic coating is one of the key technologies used in developing the new solar cell’s spectrum-splitting optics. Commonly used inside light bulbs for track lighting, dichroic filters selectively pass a small range of colors while reflecting others. Kirkpatrick notes that the lighting industry invested heavily in developing these coatings in the 1980s, dramatically advancing the state of the art. “The guys in the 1970s didn’t have access to that technology,” he says.

One advantage of a lateral architecture is that the solar cell’s materials do not have to be monolithic, lattice-matched or series connected, either optically or electrically. This removes key constraints and efficiency losses inherent in the vertical architecture and allows cost-effective combinations of otherwise incompatible materials.

Although DARPA’s proof-of-concept lateral solar cell design is not at 50-percent power efficiency, it is more efficient than conventional designs. Kirkpatrick notes that a typical, state-of-the art, two-layer solar cell is 30-percent efficient. A 70-square-centimeter cell of this traditional design, small enough to be mounted on a mobile phone, can generate around 2.1 watts. By comparison, DARPA hopes to produce at least 50 milliwatts per square centimeter with a lateral architecture cell operating at a minimum efficiency of at least 40 percent. A 70-square-centimeter laterally designed cell would produce 3.5 watts.

Kirkpatrick cautions that highly efficient solar cells will not reduce the need to carry spare batteries, but they will greatly affect logistics and in-field sustainability by cutting demand. For example, a military single channel ground and airborne radio system (SINCGARS) radio uses a 210-watt-hour battery. This battery has an area of roughly 157 square centimeters that could house a solar cell. A 50-percent-efficiency solar cell mounted on the battery would produce about 7 watts. He notes that it takes three and a half days, at eight hours of solar exposure a day, to generate the energy in a SINCGARS battery. But with a solar cell charger embedded in each battery, a unit needs only four batteries for the radio. “That’s all you need—ever. You’ve got one battery in the radio, being drained. You put the other three batteries out to be recharged in situ. You integrate the solar cell directly with the battery pack, and instead of having to resupply or carry God knows how many of these [batteries], you carry four,” he says.

Besides proving the technology, DARPA also is examining cost-effective ways to produce lateral solar cells. Kirkpatrick observes that manufacturing high-efficiency solar cells becomes less expensive than manufacturing traditional photovoltaics because each cell’s individual components do not have to be constructed in layers.

The VHESC program seeks to create a six- or seven-band-gap photovoltaic system. A band gap is the energy difference between different parts of a semiconductor such as a photovoltaic cell. A semiconductor’s conductivity is dependent on and must exceed its band gap. DARPA researchers are developing a six-band-gap solar cell by putting three two-band-gap systems in parallel rather than stacking them. As the program moves forward, Kirkpatrick explains that it is shifting toward producing a platform design that can be easily manufactured at very high volumes.

 In addition, the project members are exploring biofabrication. This technology borrows concepts from biological processes to create structures and materials with the properties desired for photovoltaic cells. According to Kirkpatrick, while biofabrication allows the high-precision construction of three-dimensional structures from molecular components, the initial emphasis of the VHESC program is the potential to greatly reduce production costs of certain key components. Researchers proposed a variety of biofabrication methods during phase one, but Kirkpatrick expects that only a few approaches will be practical enough for inclusion in the program’s final iteration.

Kirkpatrick notes that biofabrication technology has made considerable progress, but he cautions that it is not ready for commercial development. “I don’t know whether this is going to happen. This is the highest risk component of the program by far, and it’s a relatively small investment in the program,” he says.

The VHESC program is different from other efforts to incorporate solar cells into uniforms and pieces of equipment. Kirkpatrick explains that the program began with the consideration that rechargeable batteries are present in small tactical devices. The goal is to make the solar cells part of an integrated battery charger. The project will test this concept on a warfighter-carried global positioning system unit called a Defense Advanced GPS Receiver (DAGR), which has a battery pack. The DARPA program will test integrating the photovoltaic cell onto the battery. A service member bearing the equipment will have one battery recharging while another one is in the device.

Although lateral solar cells have the potential to make some types of conventional solar cells obsolete, Kirkpatrick warns that their ultimate utility will come down to cost—the cost of production, installation, compatibility, design and architectural preferences. The DARPA program closely coordinates with civilian government initiatives such as the U.S. Energy Department’s solar cell program and the Solar America Initiative. Kirkpatrick relates that DARPA and these other government efforts provide each other with data to avoid replicating research. “They [other government agencies] pay close attention to this program. And they have said, ‘If this is a success, it’s going to change the way we do business,’” he says.

 

Web Resources
DARPA Very High Efficiency Solar Cell program: www.darpa.mil/sto/smallunitops/vhesc.html
Solar America Initiative: www1.eere.energy.gov/solar/solar_america/index.html