As the market for solar electric systems takes off, a growing number of technology companies are exploring hybrid systems, which reduce the use of coal and natural gas for energy generation.

One new technology theme that caught my attention in 2010 is a hybrid system that produces electricity while harvesting heat to create hot water or heat or cool a building.

The idea promises to use heat that would otherwise be lost into the air. The heat comes from crystalline silicon solar cells, which generally convert less than 20 percent of the sunlight that hits them into electricity.

PVT Solar started selling its rooftop systems to home builders in late 2009. Cogenra Solar installed its first pilot system on the grounds of a winery this past summer. I recently caught up with Peter Le Lievre, CEO of Chromasun, which plans to launch its first rooftop hybrid system in 2011.

Le Lievre co-founded Ausra, the concentrating solar thermal technology developer Areva bought in 2010. He said the idea of creating a hybrid system came while he was at Ausra. He started Chromasun in 2008 and set about creating a rooftop version of the solar thermal system. The company announced the close of a $3 million A round earlier this year.

Chromasun’s thermal system uses aluminum mirrors to concentrate the sun onto pipes that contain water or oil. The hot liquid goes to a heat exchanger to heat up water in a tank or to preheat a boiler, or it runs through a chiller to provide cooling. Using thermal energy reduces the reliance on natural gas or heating oil. Chromasun, based in San Jose, Calif., is initially targeting businesses and universities that might want the solar thermal system to run air conditioners.

Chromasun already has thermal-only systems in field trials. Santa Clara University in the Silicon Valley and Abu Dhabi Water and Electricity Authority are some of its customers.

To turn the thermal system into a hybrid Chromasun will mount a receiver lined with monocrystalline silicon solar cells, behind which a pipe captures and transfers heat to water coursing through it. The plan is to sell both the thermal-only systems and the hybrid, Le Lievre said.  

Overall, each concentrator runs 3.2-meters long by 1.2 meters wide by 0.3-meter high, and the aluminum mirrors will concentrate the sun 20 times, Le Lievre said. The system can heat water up to 212-degrees Fahrenheit (100-degrees Celsius). It will be mounted on single-axis tracker and encased in glass. A glass canopy protects the solar collectors and relievers from debris and bird nests that otherwise are hard to clean or remove, Le Lievre said.

“We see that hybrid systems are for people who already are looking at PV and say, ‘Why not harvest the heat as well?’ Because, you are going to get the heat for very little cost,” Le Lievre said. “Whereas with thermal, we think of commercial customers who don’t ordinarily put PV on their rooftops. They want to reduce natural gas consumption.”

Engineering a hybrid system involves figuring out a trade-off between electricity and heat generation. Operating a system at 150-degrees Fahrenheit (65.6-degrees Celsius) presents a good trade-off, Le Lievre said.

“The challenge with the hybrid is the operating temperature of the photovoltaic cells is limited to how hot you can run the system,” he added.

Solar cells lose their power output with prolonged exposure to high temperatures. Research shows that above 25-degrees Celsius (77-degrees Fahrenheit), crystalline silicon solar modules lose 0.5 percent of their output with every 1-degree Celsius increase. A hybrid system works at a higher temperature and produces hotter heat by slowing the flow of the fluid through the pipe. That means the heat lingers and lowers the solar cells’ performance. Conversely, the system can cool the cells by increasing the flow to whisk the heat away.

Figuring out how to harvest the heat without cooking the solar cells has stumped other technology developers in the past, according to Tim Merrigan, a senior program manager at the National Renewable Energy Laboratory.

Merrigan told me a few months back that the last time there was a wave of efforts to design hybrid systems was about 10-15 years ago, and many of those designs couldn’t remove the heat efficiently. A common approach back then was to put solar cells inside an enclosed, conventional solar thermal collector that used copper tubes to absorb the heat.

“By not having the right control over the fluid that was flowing through the system, it was increasing the temperatures of the PV cells. You weren’t getting the added benefit of the PV,” Merrigan said.

Developers of the new crop of hybrid systems claim they have found better ways to manage the trade-offs between electricity and heat production. In some cases, the developers use more sophisticated sensors and controls to do so.

Chromasun and its research partners in Australia recently won a $3.2 million grant to develop a next-generation hybrid system. The research will investigate methods to split light so that the solar cells will only receive a portion of the spectrum that they can use, said Le Lievre, who declined to provide details. The rest of the spectrum that typically turns into heat will be directed to the thermal receiver. This way, the cells will perform in a cooler environment. It also allows adjustments in the thermal part of the system to produce hotter heat.

The startup will contract with American and/or Chinese manufacturers to make components for its first-generation hybrid and thermal-only systems. The company already has a 16,000 square-foot assembly plant in San Jose that can put together 10 megawatts of thermal systems per year, Le Lievre said. The plan is to expand the manufacturing operation in late 2011 and locate assembly plants close to Chromasun’s customers.