Following the Sun

A field-deployed military unit faces a constant challenge to keep its collection of electronic devices running.

Communications gear, portable computers, cameras and sensors all hunger for energy. Diesel generator provide electricity to support tactical operations when power grids are unavailable. But that approach has drawbacks: heat, noise and the constant need for fuel. The sun, however, offers a source of power that exists wherever a unit might find itself. A growing range of photovoltaic (PV) technologies can convert that natural resource into electricity. A few solar-powered systems already have been fielded in Iraq, and the Army and Air Force continue to investigate the use of PV to support field operations.

The military, in fact, has emerged as an important solar patron and a stimulus for research and development. The defense sector has offered funding to help some solar companies “go up the learning curve,” said Rommel Noufi, group manager and principal scientist at the National Renewable Energy Laboratory. NREL is the nation’s main facility for renewable energy R&D.

Solar still has a way to go before it becomes commonplace in the field. Industry and government researchers continue to seek ways to boost the availability, reliability and efficiency of solar technology.

Areas of exploration include next-generation PV materials, such as organic polymers. Energy storage and management — dealing with solar power once it is generated — represents another important thrust.

Development is slow at times, and the ability to mass-manufacturer certain solar technologies can prove elusive. But as work continues, military buyers can expect to encounter cheaper, lighter and more flexible solar products.

Solar growth

Solar energy has been around for decades, but the rise in oil prices and carbon footprint concerns have rekindled interest in renewable energy. Market research company Frost and Sullivan estimates that 1.8 million PV units shipped in North America last year, with the market projected to climb at a 31.6 percent clip through 2014.

Silicon wafer technology, also referred to as monocrystalline silicon, accounts for most solar products shipping today and the vast bulk of the installed base. Although it is the oldest mass-manufactured solar technology, silicon remains popular because of its efficiency in converting sunlight into electricity, said Lucrecia Gomez, industry analyst for energy and power systems at Frost and Sullivan.

Silicon wafer solar cells are assembled on solar panels, which, in turn, can be grouped in solar arrays for bulk energy production. But silicon wafer-based panels can also be used in the field. Xacta, a business unit of integrator Telos, has deployed a monocrystalline solution for the Central Command Air Forces.

The solution, created last year under a $3.34 million task order, supports wireless local-area network nodes in remote locations in Southwest Asia.

Xacta was conducting site surveys for wireless LAN installations when it heard from its Air Force customer that some locations lacked power. Xacta responded with a solar solution, although the company hadn’t consciously planned to pursue alternative energy as a business endeavor.

“We’ve been thrust into the market,” said Tom Badders, business development director at Xacta Secure Networks Group.

In mission-critical deployments, the military gravitates toward solar technologies that have proven themselves, said Lynn Brenneman, director of engineering services at Xacta and the engineering lead responsible for the company’s solar projects. New solar technology, he said, “hasn’t hit the level of the stability and reliability history that the current technology has.”

However, newer technologies are making their way into military applications. Brenneman said Xacta uses monocrystalline silicon in its solutions but noted that panels featuring polycrystalline silicon are becoming quite common.

SmartSpark Energy Systems, for instance, built a version of its polycrystalline silicon-based solution for the Defense Department. SmartSpark offers ForeverPower, which combines a solar panel with the company’s power electronics and energy storage algorithms.

Brian Precious, SmartSpark’s director of marketing, said the company’s solar products are typically used in lower-power applications — operating a video camera or sensor, for example. Precious said the company is mulling the use of its solar technology for higher-power applications such as wireless broadband equipment.

Beyond silicon wafer and polycrystalline technology, amorphous silicon, cadmium telluride and copper indium gallium diselenide (CIGS) have also emerged as PV materials. Those materials are examples of thin-film PV cells. Cells of this type generally are less efficient than silicon wafer cells but can be manufactured at a lower cost.

Some thin-film materials, such as amorphous silicon and CIGS, can be manufactured on a flexible substrate. The crystalline technologies, in contrast, are produced on rigid panels. Flexible, thinfilm solar solutions are starting to see use in military circles.

On the Rangeresqe Web site, flexible solar panels based on CIGS are for sale alongside items such as rugged computers and desert boots. Rangeresqe tests soldier support systems and offers an online catalog of tactical warfighter gear, including Global Solar Energy’s Sunlinq line and the 30-watt P3-30 portable power pack.

Barron Worthington, owner of Rangeresqe, said the flexible solar panels can fold and fit in a soldier’s cargo pocket. “We have had a fair amount of interest,” he said. “It’s an area the military is going to want to continue to look at.”

The Army already uses versions of Global Solar’s products built to meet the service’s specifications.

Other technologies join thin film in the pursuit of flexibility. Konarka embeds solar cells in plastic and has worked with the Army to incorporate its technology into tents. Konarka’s tent prototype employs the company’s dye-sensitized solar-cell technology, based on organic polymer semiconductors, printed in liquid form on a plastic film.

Rick Hess, Konarka’s president and chief executive officer, said tents and Quonset huts offer considerable surface area that could yield solar energy.

“If we can laminate the plastic material to the standard tent material, that will allow the tent to generate power,” he said. “The concept is to try to reduce or eliminate diesel generators where possible. You don’t have to worry about the logistics of the fuel and the noise and the heat — which can be a problem when the military is in the field.”

Overcoming limitations

The light-to-electricity approach has made some progress in the military, but industry has yet to detect a storm of solar activity. Hess cited the military’s preference for careful evaluation as it gains experience with solar material and acquires an understanding of its capabilities.

Xacta’s Badders noted that defense customers only deploy solar where the technology is viewed as necessary, citing cost as one factor inhibiting greater prevalence. “It’s expensive,” he said. “It’s difficult to design.”

On the cost side, industry is working to boost manufacturing output, which will lower pricing. Noufi identified First Solar as having made such a breakthrough in cadmium telluride solar-cell production. First Solar manufactures PV modules designed for large-scale, grid-connected solar systems.

First Solar’s work has enabled the company to drop its modulemanufacturing cost below silicon, Noufi said. “They drove the cost way past silicon,” said Noufi, who is responsible for the cadmium telluride and CIGS technologies group at NREL.

CIGS companies have yet to achieve a similar manufacturing and cost breakthrough, Noufi said. But some CIGS companies “are making strong strides to increase their capacity.”

For both CIGS and cadmium telluride companies, improvements in solar-cell efficiency and manufacturing yield will reduce costs. On the design side, solar vendors must juggle several variables when deploying systems. SmartSpark, for one, addresses a number of design variables, including the load that needs to be powered and the location in which the system is to be installed.

“We developed an algorithm for” calculating what size solar panel is needed for a given climate and load, Precious said.

Other design considerations are energy storage and management.

An application with a high uptime requirement, for instance, calls for a solar solution with plenty of storage capacity. Otherwise, the system would fail to supply power during prolonged periods of low sunlight. Xacta’s solar solution deployed in Iraq requires a ton of batteries — 2,000 lbs. of batteries to power a wireless hot-spot unit, intrusion-detection sensor, backhaul radio, small switch and air conditioner.

Badders pointed to battery technology as a factor dampening large-scale use of solar technology.

“Batteries have already been an issue,” he said. “The battery is a technology that is very slow to develop.”

Battery life provides another sticking point, particularly in situations where it’s difficult to do maintenance. A lead acid battery may last one to three years, Precious said. But SmartSpark has tested ultracapacitors as an alternative energy storage system. Precious said ultracapacitors have the potential to operate at least 10 years but are more expensive than batteries.

SmartSpark’s energy storage algorithms aim to extend the life of ultracapacitors and batteries. The company’s electronics management approach maintains an energy storage device’s state of charge in a range that maximizes life. The lifespan of an energy storage system shrinks if it is repeatedly overcharged in sunlight or drained on days when the sun is obscured, Precious said.

Military buyers have much to ponder with solar power: the cost and performance of solar materials, weather conditions, availability expectations, and energy storage methods, among other elements. But that’s the price of harvesting the sun.