Get More Energy from the Sun by Combining PV with Thermal

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Residential solar systems for electricity and heating water are most often installed separately, competing for limited roof space and the greatest amount of exposure to sunlight. Some newer hybrid models are overcoming this situation by combining photovoltaic (PV) and solar thermal technologies in an integrated system that uses the same surface area to capture untapped energy at prices comparable to stand-alone installations.

This PV-thermal, or simply PV-T, approach has been around for a couple of decades, but it has taken a backseat as the bulk of the solar industry continues to deliver the two technologies as separate devices. In response to growing interest in PV-T, the International Energy Agency began a research program in 2005 that eventually showed that when PV panels are mounted on top of air collectors, the total solar efficiency increased to more than 50 percent compared to the typical 15-18 percent for PV modules alone. That’s more useable energy from the same amount of solar radiation.

Basically, PV-T systems raise the solar panels up off the roof by a few inches, which allows sun-warmed air to circulate below the panels and be directed to intake vents through the roof. Just below the roof is a heat exchanger where thermal energy can heat water and enter the house’s ventilation system to warm interior spaces, or conversely, supply cooler outside air at night. An extra benefit is that the air circulating beneath the panels keeps their surface temperature lower, slightly increasing PV output.

Using air to capture thermal energy is like “turbo-charging your PV system,” according to Ryan Stepp of EchoFirst Inc., a Berkeley, Calif., company leading PV-T into a larger share of the residential market. Stepp explained the workings and outlined the benefits of his company’s Echo solar system during a workshop at the California Center for Sustainable Energy in San Diego, Calif.

How Does It Work?

The Echo solar system uses standard, out-of-the-box PV panels set on a specially designed mounting system that forms a shallow, sealed box closed on three sides and open at the lower end of the roof. The sun powers the panels and heats the air inside the box as it is drawn into roof vents leading to PVT’s patented Energy Transfer Module.

While the DC power is sent to an AC inverter for use in the house, the warm air is pulled by a fan into the module, about the size of a microwave oven, where it is filtered and enters a heat exchanger. Water is circulated into the exchanger from the home’s water heater where it is heated and sent back to the storage tank. On hot days, air from the exchanger is vented outside, and when it is cold, sent into the home’s ventilation ducting. In addition to wall-mounted controls, the entire system is Internet-enabled, which allows for monitoring and thermostat control from anywhere using a computer or any mobile device.

Meritage Homes, one of the nation’s top 10 homebuilders, as well as other leading green builders, are incorporating Echo solar systems as a standard feature in their eco-friendly communities. “Including Echo in our new homes helps make them approximately 70-80 percent more energy efficient than our competitors’ homes,” said C.R. Herro, Meritage’s vice president of environmental affairs.

What About the Cost?

Although the cost of PVT’s Echo solar system is higher than an equivalent-sized standard PV system, the additional savings resulting from water and space heating and system efficiency evens out the overall return on investment. Additionally, the system is able to satisfy a much larger percentage of the home’s total energy needs than basic solar electric or basic solar thermal systems.

Among the manufacturers of PV-T systems are SolarWall, Dawn Solar, Cogenra Solar, SolarRoofs.com and others.

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