Image credit: Bernd Sieker, used under Creative Commons license.
Dear Pablo: Does installing commercial rooftop solar PV (with the dark-colored PV cells) negate the effect of painting that same roof white to alleviate the "heat island" effect in cities?
When the sun's energy arrives at the Earth's surface it is either reflected or absorbed. When more energy is absorbed than normal, such as in a city with lots of dark asphalt and concrete, we get a "heat island" effect. We're exploring if solar panels contribute to this effect, and if so, whether or not this impact is offset by their benefits.
Basics of Absorbing the Sun's Energy
The average reflection coefficient (think of 1.00 as a perfect mirror, and 0.00 as a surface that absorbs all incoming energy), or albedo, of the earth is between 0.30 and 0.35. When humans move in and pave everything, that albedo decreases -- meaning that more solar radiation is absorbed. The albedo of fresh and worn asphalt is 0.04 and 0.12 respectively.
The average insolation (the term for the amount of the sun's energy reaching the earth) over all 24 hours of the day is 250 Watts per square meter, which is the amount of energy used by about 25 CFLs. Cutting the albedo in half by changing the reflectivity of the land effectively doubles the amount of energy absorbed. A square meter of asphalt might absorb an average 225 W/m2 per day, or 5.4 kilowatt-hours (kWh), worth of energy.
What Is A Cool Roof And How Can We Benefit From Lighter-Colored Roofing Materials?In building terminology, a cool roof is a roof covered in materials with a high solar reflectance and thermal emittance, or the ability to release heat quickly, rather than storing it and radiating it toward the inside of the building. While a cool roof does not need to consist of a mirror, they are often white, or lighter in color. One study revealed that, if every structure on earth were given a cool roof, the collective effect on radiative forcing, the measure of climate change impact, would be 0.01-0.19 W/m2 (By comparison, the net impact of human emissions on the earth is about 1.6 W/m2.)
How Much Heat Do Solar Panels Absorb?Photovoltaic panels range from blue to black but they are smooth and have an albedo around 0.3. But it is not the albedo itself that matters, it is the relative change in albedo from the status quo. Since most solar panels are roof-mounted, and most roofs are covered in dark tar-paper shingles, covering the roof with solar panels may actually represent a positive change in reflectivity. The solar panels would absorb 1.8 kWh per square meter per day, far less than the 5.4 kWh absorbed by asphalt. The same solar panel, assuming a 15% efficiency would also generate 0.9 kWh of electricity per square meter per day.
Although solar panels absorb heat much like a roof would, the fact that they are raised up off the roof significantly changes the amount of infrared radiation (heat) that makes it into the house. Think of it this way: the solar panel absorbs about 30% of the suns heat energy, re-emits half out toward the sky and half toward the roof, which absorbs about 30% of the heat emitted by the solar panel or only 5% of the sun's heat (30% of 50% of 30%). This concept is supported by a study by UC San Diego.
Do Solar Panels Contribute To The Heat Island Effect?Cities and their expansive hardscapes are certainly to blame for the heat island effect and, since the hardscapes and solar energy-absorbing roofs are already there, solar panels may actually represent a reduction in heat absorbtion. Add to that the fact that solar panels produce renewable energy that does not contribute to climate change like conventional sources like coal. Adding to that, the soot particles in the atmosphere from the burning of fossil fuels contribute a radiative forcing of 0.1 to 0.4 W/m2. Solar panels, on the other hand, are reducing the amount of climate change-causing emissions.
So if you're wondering about the heat island effect before installing rooftop solar, don't. According to the numbers, the solar panels will not only produce energy, but keep your home slightly cooler, which means you'll be using less energy in the first place, which will keep us all a little cooler.
Pablo Päster is a weekly columnist for TreeHugger.com and Principal Environmental Consultant at Hara Software. Send your questions to Pablo(at)TreeHugger.com or submit the via this form and connect to his RSS feed.
More TreeHugger Articles On Solar Panels and The Heat Island Effect:
Ask Pablo: Do Solar Panels Actually Contribute To Climate Change?
Heat Islands Becoming Heat Continents
New York City to Have More Frequent, Intense Heat Waves - Due to Climate Change & Urban Heat Island