Within Our Reach?

With talk about "green" initiatives and alternative energy potential so prevalent in the current national conversation, solar energy is once again a bright topic.

Thanks to three Purdue faculty members in electrical and computer engineering, Purdue is playing a significant part in that discussion. Delivering high-efficiency solar energy to the masses may not be as far off as you would imagine. Professors Richard Schwartz, Jerry Woodall, and Jeffery Gray are hard at work on research and initiatives to turn solar power into useable energy, and through their national prominence are putting Purdue Engineering on the solar energy map.

Although profusely abundant, solar power remains a largely untapped commodity. Lowering the high cost of harnessing solar power and converting it into usable energy has kept engineers and scientists across the country busy for decades. Purdue researchers are focused on making solar energy affordable and efficient for mass production. Only by making it truly competitive with other "alternative" energies will solar be a viable option for everyday, widespread use.

The barriers standing in the way of solar energy's proliferation fall under three categories: capturing the sun's power, converting that raw power into energy we can use, and storing the energy during those times when sufficient sunlight isn't available. Then there's the matter of efficiency—right now, commercial silicon-based solar cells convert sunlight into electricity with only a 10-20 percent rate of efficiency. Add to that the reality of high manufacturing costs for the cells and current solar energy technology, and the cost is between three and six times greater than its fossil fuel counterparts.

Engineers face not only the challenge of increasing efficiency in the areas of capturing, converting, and storing the sun's power, but of making those advances while lowering manufacturing costs.

Seizing the elusive

Schwartz, professor of electrical and computer engineering, who retired in January 2009 after more than 40 years at Purdue, has made a career of work in silicon solar cells and photovoltaics. Progress in the field has been steady but slow over the years. "It's been a series of small steps over the past decades," he says.

Photovoltaic cells, which convert sunlight directly into electricity, currently are too costly to compete against cheaper forms of energy production, such as coal, natural gas, and nuclear fission reactors. Schwartz points out that the relatively new field of nanotechnology represents a potential new tool in solar energy development because it may offer ways of making superior cells at lower cost than is now possible. In addition to his faculty post, Schwartz served as co-director of Purdue's Birck Nanotechnology Center and also as dean of the Schools of Engineering from 1995 to 2002.

According to the American Solar Energy Society (ASES), a not-for-profit organization dedicated to increasing the use of solar energy, energy efficiency, and other sustainable technologies in the U.S., governmental investment in solar energy is finally gaining ground. As recently as October 2008, the U.S. House and Senate passed historic legislation that will massively increase the use of solar energy across the country. Renewable energy provisions in H.R.1424 include an eight-year extension of the 30 percent solar tax credit and removal of the monetary cap for residential solar electric installations.

For Jerry Woodall, the Barry M. and Patricia L. Epstein Distinguished Professor of Electrical and Computer Engineering, the concept of solar energy remains elusive in part because it is less tangible. "Energy is something you can put in a can—oil, natural gas. Those things are energy, and therefore already in a usable state. That's why they've always been our primary energy source," he says. "With solar, we have to convert power into energy, and do it in a way that's both efficient and economical."

"The price of a photovoltaic cell has to be low and the efficiency has to be relatively high. If the cell converts with a rate of at least 10 percent efficiency, you could give it away for free and nobody would take it. But when you get above that efficiency rate, and if the cost to manufacture it is still low, then you're talking about a usable device. And that's what engineers have been working on since the 1950s."

It's all in the numbers

In 2006, the Department of Energy (DOE) announced that with DOE funding, a concentrator solar cell produced by Boeing-Spectrolab had achieved a record-breaking 40 percent conversion efficiency rate. A milestone in solar energy research, the achievement is tempered by the three- to five-year lag from discovery to practical use in the field.

Getting to 50 percent efficiency, according to Woodall, can't be done with a single material like silicon. "The ways the multi-gap materials work is essentially you stack five materials, all with different band gaps, on top of each other. So the silicon still captures its share of the photons, but the ones it doesn't capture get picked up by the materials in front of and behind the silicon which have different band gaps.

When you do that, you increase efficiency," he says. "But, that is not enough. These high performance "tandem" cells are much more expensive than thin film low efficiency cells. Therefore, to make them economically viable two important R&D accomplishments have to be made: a low cost, high efficiency optical concentration technology and a low cost highly perfect crystalline template technology upon which the high performance cells can be fabricated. Much effort is currently being spent on these two issues."

Purdue's High Efficiency Solar Cell Research Group, housed in Purdue's Energy Center and of which Woodall is the director, takes parallel low-cost and high-efficiency approaches to solar cell design. Faculty research focuses on multi-junction, high-efficiency cells and thin film cells with reasonable efficiency but high affordability. Participating researchers have the fundamental knowledge and technical skills to enable a successful transition to economically and environmentally friendly solar power.

According to Jeffery Gray, associate professor of electrical and computer engineering, one important component in examining the efficiency of solar energy is modeling. Gray is one of the researchers leading that charge. "The reason we undertake detailed modeling," he explains, "is so we know exactly what photons and electrons are doing inside semiconductor materials—where they're moving to, and understanding the internal dynamics of the device so we can make predictions for the actual production of the cells."

Gray, whose research focuses on computer modeling of semiconductor devices, semiconductors physics, and photovoltaic devices, also serves as an advisor to the Purdue Solar Racing team. An initiative aimed at getting students interested and excited about solar energy potential, the team, under Gray's guidance, is committed to two philosophies: building a competitive solar vehicle and educating team members and the public of the possibilities of renewable energy.

Through research, student initiatives, and funding for key centers, Schwartz, Woodall, and Gray believe Purdue is helping the U.S. "catch up" with other countries on solar energy production. Schwartz points out that although the U.S. lags behind countries like Japan and Germany in the area of solar energy production, we lead the world in solar energy research. "Those countries had a number of external factors—many political—that drove down the cost of producing solar which in turn drove production up," he says. "But we're working with the same if not more advanced technologies here. It's just a matter of time before we can catch up on the production side of things."

The ASES points out that the U.S. boasted a 259-megawatt photovoltaic market in 2007, a market that is estimated to grow to 1,590 megawatts by 2010. This is impressive, especially when you consider the market was only 17 megawatts in 2000.

These numbers don't lie. There is no question that interest in solar energy conversion is growing. The increased interest in "green" economies and alternative answers, at once both trendy and smart, have only boosted the scientific community's ability to generate funds and explore making solar energy an efficient commodity. By all counts, Purdue ECE faculty are principal players in the race to solar energy efficiency.