Geothermal add-ons for heat pumps on the market today collect heat from the air or the ground. This new device uses a fluid that flows through a roof-mounted solar module to collect heat from the sun while an integrated photovoltaic cell generates electricity from the sun's visible light.
A standard rooftop silicon solar cell will miss about 75% of the energy provided by the sun at any given time because it can't collect light very far into the IR region (although a "black silicon" coating can extend this range somewhat). The design of the new solar-thermal device takes advantage of this longer-wave IR through an integrated array of clear tubes 5mm in diameter. They lie flat, and an oil blended with a dye flows through them. The visible sunlight shines into the clear tubes and the oil inside and is converted to electricity by a spray-on polymer PV layer on the back of the tubes. This process also heats the oil, which can then flow into a heat pump, for example, to transfer the heat into a home.
Unlike flat solar cells, the curve of the tubes inside the new device allows for the collection of both visible light and IR heat from nearly sunrise to sunset. Because of the general structure and the ability to capture light at oblique angles, the solar-thermal device can be building-integrated to mimic roofing tiles used today.
Tests of the solar-thermal device have shown 30% efficiency in converting solar energy to power (heat plus electricity). By comparison, a standard solar cell with a polymer absorber has shown no greater than 8% conversion efficiency, claim researchers.
"It's a systems approach to making your home ultra-efficient because the device collects both solar energy and heat," said David Carroll, director of the Center for Nanotechnology and Molecular Materials at Wake Forest University. "Our solar-thermal device takes better advantage of the broad range of power delivered from the sun each day." The research appears in the March issue of Solar Energy Materials and Solar Cells.
The research team will build the first square-meter-size solar-thermal cell this summer.