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How Efficient is a Solar Barn?

Evaluating the efficiency of roof-mounted solar panels was the focus of a recent project at the West Central Research and Outreach Center (WCROC) in Morris, Minnesota. “We installed a solar photovoltaic system on the roof of the confinement swine finishing barn at WCROC in June 2015,” explains swine nutritionist Lee Johnston. “The solar system was sized with the idea that the electricity generated on the roof with a southern exposure to the sun could power the barn underneath.”

Measuring 35×150 feet, the barn houses 430 hogs for finishing.

The 27-kilowatt solar system was intended to provide the electric power consumed by the barn’s ventilation fans, natural gas heaters, lights, feed-auger motors, and a power washer. Natural gas supplies the heat.

Inverters convert the power generated by the solar panels from direct current (DC) to alternating current (AC).

Power generation is meeting the electrical draw. Over an 11-month period from October 2015 through August 2016, the solar system generated 25,500 kilowatt-hours of electricity. The operation of the finishing barn during that same time period consumed 19,400 kilowatt-hours of electrical energy.

mismatched power generation

Yet, on a monthly basis, there were mismatches in energy generation and consumption during that time. “In February through August 2016, production of electricity easily exceeded the power demands of the barn,” says Johnston. “However, during December 2015 and January 2016, power required to operate the barn far exceeded electricity produced by the solar system.

“The low production of electricity in December and January was caused primarily by a lot of cloudy days and snowdrifts forming on the panels that blocked the sun,” he says. “The Minnesota manufacturer of the panels stated that snow covering the panels would melt off in a couple of days, so there was no need to intervene. We tested that claim by not scraping the snow from the panels and learned that some intervention was needed to keep our panels clear in winter.”

The difficulty with drifting snow on the relatively flat-pitched roof caused the research team to choose ground-mounted solar systems for later installations on the farm.

Because the WCROC has no means of storing electrical energy, the excess energy produced by the roof-mounted panels in the months of greater sunlight was used to meet power demands in other parts of the farm. “To operate during times of low solar energy, the barn still relies on power from the commercial power grid,” says Johnston.

five-year payoff

The 2015 initial purchase price of the 27-kilowatt system was $86,000. State and federal rebates and incentives available at the time of installation could have reduced the final cost for producers.

With the local retail price of electricity at 10¢ a kilowatt-hour, the research team projected that the system could pay for itself in five years if a producer took advantage of all the incentives available.

When sizing a solar system for a building or a farm, Johnston recommends sizing the system as closely as possible to the power consumption of the farm or facility.

Sizing below the power demand could be more cost-effective than sizing above.

“Selling excess solar energy at retail price to a utility company is possible if the solar system is less than 40 kilowatts in size,” says Johnston. “Systems larger than 40 kilowatts can sell excess electricity to the utility company at a much lower rate called the avoided cost. It doesn’t make sense to install a big solar array (greater than 40 kilowatts) if the farm can’t use all the power generated. It will be difficult to pay for the extra investment when receiving the lower price for the extra energy generated.”

Net metering is the most common solar incentive that utilities provide. When solar panels produce excess power, net metering allows the energy to be sent to the grid. An equal amount of energy can then be withdrawn from the grid at no cost when the solar system is underproducing.

You can find out more about policies in your state at the Database of State Incentives for Renewables and Efficiency (dsireusa.org).

For information about the potential for solar energy in your area, visit the National Renewable Energy Laboratory at nrel.gov. 

LEARN MORE

Lee Johnston

320/589-1711, Ext. 2117

johnstlj@umn.edu

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