ADDISON COUNTY — Let’s face it; Vermont isn’t the sunniest of places. On average, there’s only a 51 percent chance it will be sunny here during daylight hours.
So as the state’s infant solar industry grows, how accurately can solar developers project how much energy an array will produce?
Vergennes city officials inked a deal with a solar developer in 2012 to lease city land for a solar project built by the Burlington firm Encore Redevelopment. In exchange, the city sees a credit on the power bill for its wastewater treatment plant based on the electric output of the array. The array went online on the last day of 2013 and just completed its first year of operation.
So how did it fare? Well, not as well as expected. The developer estimated the 150-kilowatt, net-metered array would produce 200,000 kilowatt hours of electricity annually. But in 2014 the array produced just 176,502 kWh, or 88 percent of the projection.
As a result, Vergennes saved less on its power bill than expected. The city did save $3,960.
Chad Farrell of Encore Redevelopment said the firm suspects some shading from trees may be to blame for the low output, and said that output projections are just that — estimates of what to expect.
“When we’re within 10 percent of projection, that’s pretty good,” Farrell said.
Farrell said his company will look to improve the output of the Vergennes array, and noted that over time, the fluctuations in output average out to meet expectations.
He said Encore, which has installed 7 megawatts of solar panels in Vermont, sought to site an array on the city’s wastewater treatment plant because the company seeks to use land that is unsuitable for other development.
“Anytime we can use otherwise unusable property, we always look to deploy on those types of sites, like landfills and roofs,” Farrell said.
PREDICTING OUTPUT
Overall, solar developers said that even in cloudy Vermont, solar isn’t a guessing game. Nathaniel Vandal of GreenPeaks Solar in Waitsfield said that using mathematical formulas, engineers can predict output with a high level of certainty.
He said typically, developers base their projections on the probability that an array’s output will exceed expectations.
“Many developers use the best estimate of ‘average’ production,” he said, explaining that estimate is one that will be exceeded in 50 percent of years, and not met in the other 50 percent.
This method, Vandal said, produces a low margin of error.
“Generally, a bad year and an exceptional year do not vary a tremendous amount,” he said. “Typically there is a 90 percent probability that the generation in a given year will meet or exceed the (average) estimate.”
That margin of error is created by an uncontrollable variable — the weather. Thankfully, there’s a handy collection of data, called the National Solar Radiation Database, which includes solar radiation data for the United States dating back to 1961 (it includes records for Vermont since 1991).
Vandal said the database gives developers the information they need to project how solar arrays will perform in a specific place for an extended period of time.
“A typical meteorological year data set provides designers and other users meteorological values to typify conditions at a specific location over a longer period of time, say 30 years,” Vandal said.
Vandal said GreenPeaks explains to investors, landowners and communities that solar array outputs vary from year to year, but those variations balance out to the calculated average.
“It is important to remember that these are 20- to 25-year investments,” Vandal said.
Duane Peterson of Waterbury solar development firm SunCommon said it was too early to report the output of its 150-kilowatt Community Solar Arrays, since they are only a few weeks old, but said there is a wealth of data that engineers can use to predict what areas are best to site arrays.
He said Addison County got high marks for the amount of sunlight it receives, called irradiance, lack of tree-shading and ample land with few steep hills or cliffs.
Peterson said if any of the arrays were to underperform, the company would pay investors the difference.
“The thousands of solar systems built across Vermont and throughout the nation have developed tremendous data that guides the design of every one of our arrays — so we are confident in our solar performance projections and are confident financially guaranteeing them,” Peterson said.
One uncertainty that developers cannot use historical data to predict, Vandal of GreenPeaks said, is climate change.
A White House climate change report released last year — the culmination of years of research by hundreds of scientists — predicted significant consequences for each region of the country as a result of a changing climate. For the Northeast, and mountainous Vermont, in particular, the report predicted a warmer, wetter climate with more cloud cover.
While climate change will likely determine the future of the solar industry in Vermont, Vandal said he’s comforted knowing that solar arrays replace electricity generated using carbon-heavy sources, like coal.
“The one thing that is certain is that each of these facilities is doing its part to combat it,” Vandal said.