Tonya Maxwell

tmaxwell@citizen-times.com

ASHEVILLE - Solar energy generated from rooftops could offset the need for the existing Lake Julian coal plant, according to data compiled by Google, though that thought experiment comes with caveats, experts say.

The number of Asheville rooftops that could serve as a source of solar energy – Google puts that number at more than 20,000 based on satellite imagery and other data – could generate 442 megawatts of power, according to data released by Google last month as part of Project Sunroof.

The Duke Energy coal plant in Asheville has a capacity of about 376 megawatts, while the planned natural gas plant slated to replace it in 2020 will generate up to 560 megawatts.

The Google data, a national effort called Project Sunroof, provides both a bird’s eye view of the citywide data and also allows homeowners to check their own addresses, where the tool estimates the hours of usable sunlight per year, square feet available for solar panels and expected cost savings over 20 years.

It estimates that 66 percent of Asheville roofs could be viable for solar panels.

“If their methodology is good, it’s incredible to see this kind of capacity in a city like Asheville,” said Julie Mayfield, co-director of area environmental group MountainTrue.

While the installation of solar panels on every viable roof isn’t pragmatic, the information suggests that 630 roofs have a solar capacity of 50 kilowatts to one megawatt.

One megawatt of solar energy can power 164 homes, according to the Solar Energy Industries Association.

“There are some big roofs out there that could get you a big bang for the buck and that are owned probably by governments or institutions that might have more capacity to install solar rather than [homeowners,]” Mayfield said. “It also shows we have a lot of flat roofs that are easier to work on and could provide a lot of capacity.”

In a paper released in December, one directed to policymakers, the National Renewable Energy Laboratory found that rooftop solar panels could generate up to 40 percent of energy the United States consumes.

It was a number that was surprising, said lead author Pieter Gagnon, an engineering analyst of solar policy and technoeconomics with the agency.

“It was significantly larger than previous numbers that had been floating around,” he said. “The last time there was a good effort on this was in 2008. There’s more buildings now, there’s been technological advancements – you can generate more power from the same sized roof – and with better data we were able to make a more accurate characterization and it turns out our earlier estimates were somewhat conservative. There’s actually more than we thought.”

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But he added that installing solar panels on every viable roof isn’t a solution to energy problems.

“The best way to solve a problem is to have a diverse portfolio,” Gagnon said. “Every forecast that I’ve seen for the grid to grow into the future still has a fair portion of traditional generation sources. They help to support the integration of solar and renewable sources because they’re more flexible. You can choose when to turn them on. It’s important to have that kind of support for rooftop [photovoltaics]. And there’s a lot of excitement about energy storage in a variety of ways and more intelligent buildings that can moderate their energy usage.”

The numbers provided by Project Sunroof for Asheville are a pleasant surprise, particularly given how difficult it has been historically to estimate rooftops – and the directions they face – in an area, said Bora Karayaka, an assistant professor in the department of engineering and technology at Western Carolina University.

“The biggest problem with renewable energy is real estate,” Karayaka said. “There are solar farms across the state, but people are renting their farmland, which means you aren’t going to be able to plant any agricultural products. Solar rooftops are great from the perspective that it’s free real estate. It’s there, no one is using it. This is an excellent potential. The rooftop is there. The real estate is there. So you aren’t going to impact the environment from that perspective.”

On the Project Sunroof site, Google recommends that homeowners contact a solar installer for more site specific information, a position echoed by Randy Wheeless, a spokesman for Duke Energy.

With an investment in solar, homeowners should do their homework, including choosing a sound installer, determining if panels will violate homeowners association regulations and examining small trees that one day might grow to shade out panels.

“That gas plant will run about 90 percent of the time, where a solar facility at best will run 22, 23 percent of the time,” Wheeless added. “So even though you’re generating a lot of electricity in the daytime, you’re obviously not generating at night. You’re not going to have that 24/7 coverage you would with a natural gas facility.”

Mayfield, who is also an Asheville city council member, said she forwarded the Project Sunroof information to the city’s Energy Innovation Task Force, which includes a Duke Energy representative, so that panel can examine the data.

The information is promising, she said, but isn’t a solution to Asheville’s power demands.

“All of the solar capacity would probably do little to address our peak demand, because our peak demand is in the winter mornings, frequently before the sun comes up,” she said. “As great as solar is, it is not the answer to all of our energy needs here in Western North Carolina because of when our peak occurs.”