Hybrid PV-Thermal Panel
The most efficient energy-gathering device for a homeowner is a roof that can both heat water and produce electricity.
The arithmetic is pretty simple: In bright midday sun, roughly 1,000 watts of solar radiation falls on every square meter of the Earth’s surface.
A high-quality silicon photovoltaic (PV) module can produce about 140 watts from that energy, and a thin-film module, laminated to a standing-seam metal roof, can produce about 65 watts per square meter.
It’s nice to have that power, but it’s obviously only a small fraction of the energy available. The rest is usually lost as heat. A metal roof typically cooks up to about 120°F at noon (50°C), or even 160°F (70°C) in midsummer.
That heat can be harvested by running hydronic tubes behind the roof panels. Water, or an anti-freeze solution, runs through the tubes and is heated to roughly the temperature of the roof panels. That heat can be used for space heating, industrial processes or domestic hot water.
Dawn Solar Systems Inc. in New Hampshire has installed nearly 200 building-integrated metal-roof water-heating systems, many of them faced with UniSolar PV laminate. Bill Poleatewich, president of Dawn Solar, notes that the heat harvested by these certified collectors can be equivalent to about 250 watts per square meter, depending on the thermal load.
At peak sun, a hybrid industrial PV-thermal system therefore may generate up to 4.8 times as much total power as the thin-film module alone. Since solar thermal energy harvests are limited by the size of the thermal load, a residential system may produce as little as 60 watts per square meter of usable heat energy, still nearly double the total solar electric harvest.
This is the most efficient use of space, especially on what Poleatewich calls “constrained rooftops.” Where space is at a premium — on the tops of tall buildings, for instance — a system designer can use the hybrid collector to maximize the total solar power harvested while boosting the efficiency of thermal systems like ground-source heat pumps.
The hybrid roof can reduce heating and cooling loads. It also carries heat away from the heat-sensitive PV modules, thus helping to maintain their efficiency. Empirical data collected by Dawn Solar suggests that the hydronic cooling effect may boost electrical production by up to 10 percent on a very hot day.
– SETH MASIA
Seth Masia is deputy editor of Solar Today.