Solar Barn - Fourth Quarter 2008
Renewable energy curing rides a “heat” wave
Many a grower has gone through long, hot days and warm, humid nights putting a crop through a cure. And as he has made his adjustments, checked his vents and his wet-bulb temperature, or shuffled from barn to barn looking at gauges and making subtle changes in humidity and temperature, he has likely paused, mopped his sweaty brow, looked at the long line of barns he has worked with, and caught the irony of his situation.
Here I am, he thinks to himself, working to get every barn to the perfect temperature to get a cure, while I stand in the heat that is not too far off being right to make a cure.
The grower knows that to get the cure he desires, he will have to use plenty of propane, natural gas or heating oil—costly and nonrenewable energy sources. And he does all this work while toiling in the free, renewable energy of the hot sun.
All this is not lost on Bob Pope.
Pope, together with his partner Richard Anderson, is working on harnessing the sun to lower growers’ curing costs. And while Pope cannot get a 100 percent solar cure, he has developed a curing barn that utilizes a hybrid energy source—solar energy—to reduce the barn’s consumption of fuel in the curing process.
Based out of Tarboro, N.C., in the heart of flue-cured tobacco country, Pope and Anderson teamed up to open Eastern Carolina Manufacturing Company on the site used by FarmTrac North America to build Long tobacco barns. The partners bought the site from FarmTrac in May 2008.
The company builds conventional Long barns, as it purchased the brand name from FarmTrac in the sale. The deal not only turns a profit but also provides a base of operations for Pope Manufacturing Company, Pope’s subsidiary operation that has developed a commercial version of a curing barn that partially uses solar energy.
Pope originally designed the barn when he was a student at North Carolina State University back in the late 1970s. He drew from the airflow principles taught to him by Dr. John W. Glover, one of the innovators of bulk tobacco curing. While a student doing independent studies with N.C. State’s Department of Biological and Agricultural Engineering, Pope focused on the savings derived from augmenting the primary energy source with solar energy.
After graduation, Pope started an investor group to develop a commercial version of the solar barn, which was constructed in 1977 and used in that curing season. Initial testing was very encouraging and six more barns were built in the next two years, but a dip in demand for new barns and relatively cheap fuel prices worked against the project. But the project verified Pope’s design was a fuel saver.
The idea was resurrected in March 2007, when he built another prototype of an updated version of the solar barn on Anderson’s farm.
Pope began with a 1997 Long model—an all-metal, 10-box barn that Anderson donated. He added eight inches to the bottom of the barn all around, boosting the plenum depth to 30 inches, which increased the barn’s ability to create static pressure below the tobacco and sped up the curing cycle.
Pope painted the barn with a flat black base, creating a solar absorber plate. He then added corrugated polycarbonate sheeting to the top and sides of the barn, positioned on special ribs a few inches from the absorber plate. The sheeting builds up heat in the space between the absorber plate and the sheeting, similar to parking a car in the hot sun.
“The polycarbonate skin lets the UV rays in like a car’s windshield,” said Pope. “The heat collects in the black metal like it would on black upholstery, and the heat builds up. The UV rays can’t go back out the polycarbonate, so heated air is trapped in the space between the black metal and the skin.”
The pre-heated air is drawn into the barn for curing. The black metal forms the roof and walls of the barn, so the back side of the metal conducts heat directly to the tobacco in the barn. Heat lost from the curing chamber collects in the heated air space outside the barn and returns to the barn.
Pope went back to a few of Glover’s original barn design innovations as well. He added a Venturi ring at the entrance to the fan, which creates an accelerated velocity of wind flow into the fan enclosure, dropping air pressure and easing restriction of the fan. He also added a turning vane to guide air under the bottom of the barn, which reduces air resistance. He installed a three-horsepower secondary fan to pull the hot air from the solar collector during yellowing. That fan boosts the 10-horsepower primary fan in subsequent stages of curing.
Another important improvement in the new barn was the installation of vortex generators—oval holes as part of the polycarbonate skin brackets that support the skin over the absorber plate. The vortex generators draw the heated air through them to create a spinning motion. That increases the volume of air coming from the absorber plate.
Pope added a set of misting heads near the bottom of the rear bulkhead to moisten the air moving through and bring the tobacco in order. The misting system also prevented overheating during the yellowing stage by adding evaporation to help cool the tobacco. Misting in the yellowing phase also aided yellowing and sped up the curing process.
Pope put his barn to the test on Anderson’s farm, curing alongside one of Anderson’s conventional 10-box barns. In three curing cycles, Pope’s barn cured at a rate of 32 percent lower fuel usage than the conventional barn, yielding 11.9 pounds of dried tobacco per gallon of propane compared to nine pounds per gallon of propane in conventional barns, with virtually the same cure quality. The barn also completed its cures a day earlier than the conventional barns.
Pope was quick to point out that he focused his entire attention on his new barn, which may have allowed him to be more efficient that he could have been in a conventional multi-barn operation.
“I was curing one barn,” he said, “and he was curing 40 or so.”
Pope is estimating that the solar barn used in regular curing cycles will save about 25 percent in a multi-barn operation. While that figure is a bit smaller than his research indicated, Pope feels that a grower will not be able to pay such individual attention to a barn in an operation as he did in curing his crop in the solar barn.
Still, the barn held to its initial promise. With the price of fuel soaring and a demand for fuel-saving products that involve renewable energy, Pope felt his time was right to market his barn.
Pope arranged for two demonstration sites during the 2008 curing season. One site is on the farm of Wallace and Ernest Roberts near Lawrenceville, Va. Philip Morris International has funded that site, which is being monitored by Dr. David Reed from Virginia Tech. Eastern Carolina Manufacturing Company has funded the other site, which is on the farm of James Corey in Robersonville, N.C. The initial results from both of those sites have been good, and the farmers involved say they are pleased with how their demonstration solar barns operate.
The project carries potential benefits well past the initial fuel savings. The reduced costs for curing will help a grower’s bottom line and strengthen U.S. flue-cured tobacco as an international commodity.
By using renewable energy, solar barns leave a much smaller carbon footprint in a world that constantly demands more earth-friendly measures be taken.