THERMAL¦ 85 to 90 percent biomass, though during milder winters, that number has reached 95 percent. “With the artic blasts this year, it’s gotten to be a little more challenging,” Woolsey says. “The interesting thing about greenhouses is that the coldest part of the year is when we tend to be emptiest in terms of product, because we’re gearing up for spring. Toward the end of winter and beginning of spring we’re jammed full—right now (mid-March) we’re at 100 percent.” The weather, and how full the greenhouse is with product, significantly influences the facility’s heat load, according to Woolsey. Temperature throughout the greenhouse varies widely, with the warmest areas—about 72 degrees Fahrenheit—being present where plugs or baby plants are located, and the coolest where the finished crops are located, with an overall average temperature of about 65 degrees. So how are different temperatures in different regions of the greenhouse accomplished? In the main blocks of the greenhouse are four Argus climate control computers that contain customized climate models for each zone. “We have four weather stations, and the computer looks at those, outdoor temperature, light, wind and other things, and comes up with a hot water target for each zone. “We’re able to hit our targets right on the money,” Woolsey says. Behind the heating magic are four 8-MW Vyncke boilers, each rated at 29.5 MMBtu per hour. The boilers replaced 12 natural gas boilers that are still in place and operable, but used only as supplemental or back-up heat. “We try to never use them, but sometimes we’re forced to,” Woolsey says. What makes Metrolina’s current heating system so ideally suited to its needs is, from Woolsey’s perspective, its current storage set up, which includes 3 million gallons of hot water storage via five tanks of various sizes. “It allows us to store about 415 MW of heat,” Woolsey says. “It’s ideal for us because on a summer day, greenhouses typically use zero heat. It could be 20 degrees outside, but we’re not going to use any heat if there’s sun—a greenhouse
is a giant solar collector. We use sunlight hours as recharging periods.” Interestingly, the 32 MW of capacity the four boilers supply is not nearly enough heat for the greenhouse, if supplied in real time. “So having anywhere from eight to 12 hours of daylight, we can run those boilers at full output, and just store the heat away inside of pipes and tanks,” Woolsey explains. “At night, we’re able to draw on the full 32 MW of boilers, plus storage—it’s like a giant battery. If we didn’t have storage, we’d never make it through the night.
We would have to have twice as many boilers.” In order to accommodate the biomass system when it was installed three years ago, a new building was constructed on the side of the property to house the boilers, ash conveyors and multicyclone air filters. A nearby storage house can hold several hundred trailer loads of fuel, which is delivered every weekday, multiple times per day, and sometimes on weekends. “At max, we burn up to 12 tractor tailor loads per day,” Woolsey says, adding that fuel is sourced
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615.471.9299 MAY 2014 | BIOMASS MAGAZINE 35