By Alex Hutchinson

1. Back-Contact Silicon PV Panels
Back-Contact Silicon PV Panels

When it comes to solar technology, no one is better equipped to separate the genuine potential from the hype than the Department of Energy, which spearheads the country’s solar research efforts. So it’s worth noting that the DOE’s choice for the brand-new 205-kilowatt solar installation on the roof of its Washington, D.C., headquarters, unveiled in September, was the unique high-efficiency solar panels built by Silicon Valley-based SunPower Corporation.

Energy Secretary Samuel Bodman called the choice “both practical and symbolic,” and he was right. It’s practical because, earlier this year, SunPower’s silicon photovoltaic cells demonstrated an efficiency of 23.4 percent—a record for large-scale, mass-produced cells. SunPower uses what’s called a “back-contact” design, which means that all the electrical contacts are on the back of a cell, leaving a larger area on the front of the cell exposed to the sun. Such designs have always been efficient, but it’s only in the last few years that manufacturing costs have become competitive.

It’s also symbolic, because SunPower provides a glimpse of what it takes to successfully bring a solar technology to market. Its back-panel PV design was developed at Stanford University in the early 1980s, and the company was founded in 1985. That’s a long time to get the product from the lab to the consumer, notes Larry Kazmerski, the director of the National Center for Photovoltaics at the National Renewable Energy Laboratory (NREL). “We have to shorten that timeline,” he says.

For SunPower, at least, the wait is over. With PV cells that the company boasts as 50 percent more efficient than conventional crystalline silicon cells, SunPower is moving full steam ahead, most recently with an agreement to build a 250-megawatt “solar ranch” in the California Valley. The project should begin delivering power in 2010 and will be—at least temporarily—the largest PV installation in the world.

2. Microinverters
Microinverters

People who want toinstall PV panels on their roofs soon run into a very basic electrical engineering problem: Solar panels produce DC (direct current) power, but the appliances we plug into the wall require AC (alternating current) power. There are plenty of systems that convert DC to AC, allowing solar power to feed into the grid or power home appliances directly, but they typically lose power in the process, add complexity and cost enough to discourage small-scale applications.

The solution is a new breed of small devices called “microinverters.” Connected directly to individual solar panels, microinverters enable each panel to output AC instead of DC power. They can also be combined with a wireless monitoring system that allows the performance of each panel to be monitored. “The combination of our inverter and monitoring system will promote large amounts of grid-connected solar power,” says Patrick Chapman of SmartSpark Energy, one of the companies rushing to release microinverter systems.

SmartSpark was one of 12 companies that received a total of $24 million from the Department of Energy in August to pursue “solar-energy grid integration”—a recognition that solar panels will need to integrate seamlessly into existing home electrical systems if they’re going to succeed. It’s been a neglected niche up to now, but there are signs that investors are taking note. Enphase Energy, which brought the first solar microinverter to market in June, received a $15 million injection of venture capital in September to help it cope with rapidly growing demand.

3. Concentrating PV Panels
Concentrating PV Panels

Most of the big solar power companies are pretty skittish about revealing precise numbers for the cost of their technology—primarily because it’s still too expensive when compared with alternatives like burning a pile of coal. So a startup called Sunrgi made quite a splash in April when it confidently predicted that its XCPV (Xtreme Concentrated Photovoltaics) solar panels (click for video) would soon be producing electricity at a cost of $0.05 per kilowatt-hour—easily cheap enough to be competitive with conventional sources of electricity. And they made it sound easy.

“In a little more than a year we were able to develop and successfully test XCPV,” said company co-founder Robert Block. “We expect the Sunrgi system to become available for both on- and off-grid power applications, worldwide, in 12 to 15 months.”

The key to Sunrgi’s technology: Using lenses to concentrate incoming sunlight by a factor of 1600, in the same way that a kid can fry an ant using a magnifying glass. This allows Sunrgi to use fewer of the expensive semiconductors that make up solar cells, saving money while getting the same amount of power out of the system. For now, though, the record stands at 40.8 percent, set in August by a prototype cell at the National Renewable Energy Laboratory that was illuminated with the concentrated light equivalent of 326 suns.

A host of other companies are also racing to make concentrating PV a commercial reality. Emcore, a New Mexico-based company whose solar cells have broken efficiency records in outer space, says that its “inverted metamorphic multijunction” solar cells will have up to 45-percent efficiency when used with concentrating lenses and mirrors. Solaria is another player, with a design that maximizes sunlight while minimizing silicon use.

4. CIGS Thin-Film Panels
CIGS Thin-Film Panels

A key factor holding back the growth of solar power is the price of silicon: A prolonged shortage has caused prices to soar to 10 times their previous levels—and the economic crisis isn’t exactly helping. Other semiconductors that can replace silicon in PV cells are also expensive, but because they can be used as “thin films,” far less of the material is required to build the cells.

The current market leader for thin-film technology is Arizona-based First Solar, which makes its PV cells from cadmium telluride and expects to ramp up its manufacturing capacity to over 1 gigawatt of solar modules in 2009. The technology isn’t perfect, though: The solar conversion efficiency is only about 10 percent, and cadmium is considered a hazardous material, which creates extra complications in the disposal of cad-tel panels.

The next big thing, NREL’s Kazmerski insists, is thin-film panels made of cadmium indium gallium selenide (CIGS). “Right now, CIGS is grabbing an incredible amount of attention,” he says. Earlier this year, researchers at NREL reached an important milestone when they built a CIGS cell with a record 19.9-percent efficiency—nearly double that reported by First Solar, and very similar to the efficiency of silicon panels.

“This is an important milestone,” NREL scientist Miguel Contreras says. “The thin-film people have always looked for matching silicon in performance, and we are reaching that goal.”

Not surprisingly, a swarm of young companies is racing to capitalize on the new CIGS technology, including San Jose-based Nanosolar, Santa Clara-based Miasole and Austin-based HelioVolt. “There are so many startups, it’s a bit scary,” Kazmerski says, “because not all of them will be around in a few years.” But if any of them succeed, the rewards will be immense.

5. Solar Thermal Storage
Solar Thermal Storage

Any discussion of solar power eventually has to face the obvious question: What happens when the sun goes down? Given the limited capacity and high cost of batteries, there hasn’t been an easy answer—until, perhaps, now. Construction is wrapping up on the Andasol 1 plant on the Guadix plateau in the Spanish province of Grenada. When it begins operations in late October, Andasol 1 will be the first commercial solar power plant to incorporate a thermal storage capacity, enabling it to run for about 7.5 hours when the sun is down or disappears behind a cloud.

“This [storage capacity] is the aspect that has captured the excitement of the utilities,” says Fred Morse, a former Department of Energy official who now works for Abengoa Solar. It represents a major shift in how we think about solar power, because instead of simply pumping electricity into the grid whenever the sun is shining, utilities can choose to save and deploy that power during peak periods, when electricity is at its most expensive.

The Andasol 1 plant, built by German company Solar Millennium AG, will be joined in 2009 by Andasol 2, and the two plants together will supply half a million people with power. The plants rely on “concentrating solar thermal” technology instead of the more familiar photovoltaic panels. Vast arrays of curved mirrors are used to focus sunlight on a liquid, heating it to over 750 F and boiling water to power a steam turbine. Storing power simply requires the equivalent of a giant thermos to store the heated liquid until steam is desired.

Abengoa plans to bring the technology to the U.S. with a storage-enabled 280-megawatt plant in Gila Bend, Arizona, that should be completed in 2011. And they’re looking for more opportunities, Morse says. “New Mexico has a request for bids out. So does Arizona and California. We’re pursuing every opportunity.”