August 20, 2006 | General

Extracting Thermal Energy From Composting

BioCycle August 2006, Vol. 47, No. 8, p. 38
Commercial prototype hot water plant is powered by vapor discharged from the composting of animal manure and farm residuals.
Molly Farrell Tucker

A LIVESTOCK FARM in a remote Vermont town has the world’s first commercial-size composting facility that converts excess heat from the compost piles into energy for the farm. Diamond Hill Custom Heifers, located in Sheldon, Vermont, is using the heat transfer system of a Canadian company, Agrilab Technologies, Inc., to capture water vapor from compost to heat flooring in a calf barn and heat water to warm calves’ milk.
Diamond Hill’s owners, Terry and Joanne Magnan, raise approximately 2,000 milk calves and heifers for custom boarding and sale to the dairy industry. The Magnans had good reasons for starting a compost system. They were facing much higher energy costs and restrictions on manure spreading.
At a recent open house, Terry Magnan noted that higher prices for fuel and fertilizer and rising interest rates have increased the farm’s production costs 20 to 25 percent in the last two years. The farm is located about a dozen miles from Lake Champlain and is in the Missisquoi River watershed which flows into the Missisquoi Bay of Lake Champlain. All Vermont farmers must follow manure regulations under the Accepted Agricultural Practices (AAPs) law enforced by the Vermont Agency of Agriculture, Food and Markets. Some of these provisions include a winter spreading ban between December 15 and April 1 unless a permit/waiver is granted, and a manure spreading set back of 25 feet from streams and watercourses and 50 feet from wellheads.
Diamond Hill farm also meets the size threshold that requires a Medium Farm Operation (MFO) general permit by 2007. This includes plans for managing nutrients, animal wastes and livestock carcasses, and reducing soil erosion. Lastly, the state of Vermont has an EPA-approved plan to reduce phosphorus (P) in a Total Maximum Daily Load (TMDL) agreement by 2016. Vermont’s Governor James Douglas has accelerated achieving these targets by 2009. The plan calls for phosphorus reductions from nonpoint pollution sources including agriculture, municipal back roads, construction and logging activities, urban and municipal wastewater treatment facilities and septic systems.
Brian Jerose, a friend of Magnan’s and co-owner of WASTE NOT Resource Solutions, heard that Magnan was interested in composting the farm’s livestock manure and bedding. Jerose’s consulting firm specializes in compost site development and watershed protection issues, and has assisted several Vermont and New York farmers and butchers in developing composting systems.
“Terry had composted on a smaller basis around the farmstead, through stacking and turning bedded manures from dairy calves and heifers,” says Jerose. “He observed the heat and steam from these manure piles and connected the need for hot water/other heat on his farm. He knew that well-composted manure was also more easily spread on hay and corn fields and could become another source of revenue to the farm, diversifying the farm’s cash flow.”
“When this opportunity came up with Terry, I immediately thought of Agrilab’s system,” says Jerose. Agrilab Technologies, Inc. is a division of the Acrolab Group of Windsor, Ontario. Agrilab’s patent pending heat transfer technology extracts thermal energy from aerobic composting of farm manure. The company had built a pilot plant in Windsor, Ontario. “Diamond Hill Heifers gave us a great opportunity to showcase our technology on a much larger scale,” says Joseph Ouellette, president of Agrilab and Acrolab.
“We know that the composting process produces substantial quantities of water vapor at a temperature ranging from 55° to 65°C (131° to 149°F) which, if not captured, escapes to the atmosphere,” says Ouellette. “Our technology converts the energy in this water vapor into something more usable. Only surplus vapor is extracted, so the composting process doesn’t go dormant.”
The system Ouellette developed for Diamond Hill uses Isobar stainless steel super-thermal conductor heat pipes which condense heated water vapor from the compost piles and transfers the condensed vapor along the pipes, providing energy to heat water in an 800-gallon insulated bulk storage water tank. The heated water is then used to heat milk formula for the calves, and to provide radiant heat in the floor of the calf barn and in the mixing room to protect the plumbing from freezing. “The hot water dissolves the powdered calf formula better and makes it easier to ingest,” says Jerose. “The heated floor keeps the barn drier,” he adds, “and drying and good ventilation are important for calf respiratory health.”
Jerose helped Magnan apply for state and federal funding to install the Agrilab system. The Magnans contributed more than $120,000 in cash, labor and materials and received $247,000 in state and federal funds. The USDA Natural Resources Conservation Service (NRCS) awarded the Magnans a $197,000 Conservation Innovation Grant, one of 42 awarded in the U.S. in 2004. The Vermont Agency of Agriculture Food and Markets (VTAAFM) also provided $50,000 in Best Management Practice (BMP) funds. Agrilab Technologies contributed in-kind engineering, patent license use, and monitoring and operating equipment.
A number of agencies, businesses and individuals also contributed their services to the project including the University of Vermont (UVM) Extension Service; WASTE NOT Resource Solutions; City Soil and Greenhouse Company; Paul Stanley Crop Management Services; the Composting Association of Vermont; Paul Godin Mechanical; mechanical fabricator Aaron Robtoy and veterinarian Steve Wadsworth.
Developing the system for Diamond Hill presented several challenges. “The first challenge was to produce predictable and large quantities of water vapor without adversely affecting the composting process,” Ouellette explains. “The second was to capture the water vapor without compromising the composting process. The third challenge was converting the energy in the water vapor into more usable and storable forms of energy. The final challenge was ensuring that the system was economical and that the energy needed to capture and store the water vapor didn’t consume more energy than was being collected.” It took 18 months to design and build a composting barn, install the heat transfer system and get it operating. The composting barn was built during the summer and fall of 2005.
The composting barn has two bays, each approximately 52 feet wide and 60 feet long. A central, enclosed hallway separates the two bays. There is room for four windrows in each bay. Only one bay is currently being used. Construction of the composting floor in the second bay is planned for 2007.
The floor in the first bay was constructed in layers. “We wanted to isolate the composting pad from the ground to ensure that the water vapor generated by composting was not condensing on the concrete pad because of the heat sink properties of the ground substrate,” says Ouellette. Compacted sand was laid down first and covered with 2-inch-thick pink foam board insulation. In addition, 6 to 10 inches of reinforced concrete was poured over the foam board sheets.
An array of PVC pipes was then cast into the concrete to act as collectors of the water vapor. Forms attached to the upper surfaces of each pipe were removed after the concrete was poured, exposing the top of the pipes (2.5 to 3 inches in width). The exposed top of each pipe was cut away, leaving a slot along the top of its 60 foot length. This created a gutter that was covered with heavy-duty mesh screen along the length of the pipe. “The mesh screen allows tractors and loading equipment to move along the composting floor without damaging the pipes,” explains Jerose. A layer of wood chips was laid over the mesh to keep compost fines from falling through the grates.
Foil was wrapped around the base and sides of each pipe to raise the insulation value around the pipes to approximately R50 or R60. The remaining gaps in the floor were filled with roofing tar.
Four air blowers inside the center hallway of the composting barn draw ambient air from within the composting barn, and the air is pulled through the windrows from top to bottom. The air becomes heated water vapor as it is pulled through the windrows. Each blower pulls the vapor from four floor gutters under each 13-foot-wide windrow. Ductwork connects the 16 floor gutters to four manifolds where the blowers blow vapor through a 10-inch duct to a larger 50-foot-long corrugated PVC conduit. The conduit, which is covered with foil and bubble wrap insulation, runs down the side of the hallway. The conduit contains six stainless steel Isobars which run the length of the 50-foot conduit, and then enter the 800-gallon bulk water tank.
Vapor condenses on the Isobars and transfers energy to the water in the bulk tank. “As long as the water in the bulk tank is at a lower temperature than the hot water vapor, heat transfer will take place,” explains Ouellette.
The Isobars extend about six inches beyond the tank, in an insulated enclosure welded to the tank. A 24-inch discharge stack connects into the conduit. The discharge stack is vented up and outside of the hallway that houses the system. The exhaust vapor is controlled via a damper in the stack and is vented to the atmosphere or redirected to the compost windrows. “Manometers are being installed to measure air flow resistance, which will help in refining the damper and blower operation,” says Jerose. “In general, we don’t want to move the hot vapor over the Isobars too fast or less vapor will have a chance to condense. Damping the vent is balanced with forcing the blowers to operate at higher rpm, using more electricity.”
The bulk tank water circulates through a nearby hot water tank with a heat exchanger, similar to many residential/commercial hot water tanks. The outside “jacket” circulates the bulk tank water, which releases heat energy into an interior 50-gallon tank. This interior tank is refilled with 45°F well water, and depending on the rate of hot water use, can be heated by the “jacket” water above 130°F. This preheated water then moves to a final hot water tank where it is heated up to the targeted process water temperature of 150-155°F at the time of usage via an oil-burning water boiler. “The preheating reduces oil consumption in hot water heating in the same manner as a passive solar hot water preheater,” explains Jerose.
The heat transfer system is presently generating water temperatures of up to 130°F (55°C) in the tank, notes Ouellette. At peak times, the heat transfer system is producing more than 120,000 BTUs of energy per hour (2.9 million BTUs per day). “As the system is optimized, this is expected to increase to 150,000 to 200,000 BTUs per hour,” he adds.
A computer data acquisition system monitors 35 points in the compost windrows, ductwork, concrete floor, isobars, water tank, hallway and outside, and collects data in ASCI format. The data is downloaded on a 24 hour basis and converted into Excel and Quattro to assess system performance over a calendar year.
The farm has produced 10 windrows since January 2006, each containing 180 to 200 tons or 250 to 300 cubic yards of solid and liquid livestock manure and straw/sawdust/woodchip bedding. “The farm has both young weaned calves and middle-aged heifers, so there are diverse manure and bedding situations,” notes Jerose. The compost recipe is based on carbon, nitrogen, moisture content and the density of the materials. The materials are blended in a Kuhn Knight 5073 Vertical Maxx feed mixer that is also used to mix feed for the farm’s livestock. The mixer’s two augers chop up straw and hay in the bedding and homogenize the materials. No inoculant is added to the compost recipe. “Anytime you have manure in the mix, there is enough biological activity from the cow’s rumen to get the microbial decomposers going,” says Jerose.
The farm’s goal is to reduce the number of compost recipes to three. “One or two will use feedstocks solely from the farm, and the other recipe will include wood chips brought in to the farm,” adds Jerose. “The wood chips will be 10 to 20 percent of the mix by volume. Wood and bark chips provide more long-term carbon, which is the sugar/carbohydrate/energy the microbes need a lot of to do their work, along with making a more porous windrow structure.”
Currently there is a 25 to 1 ratio of carbon to nitrogen in the compost recipes. “If we can get the ratio higher,” points out Jerose, “there will be less odor and more nitrogen will be saved, but then we’ll have to bring in more carbon. Wood chips are excellent for porosity, air flow and heat production over time. But if it costs $500 to bring in a truckload of wood chips, will it produce enough additional energy to justify the cost?” Jerose notes that when just straw and sawdust bedding are used, the windrows compact sooner. “We had two piles that were 10 feet in height when first mixed, but settled down to five feet because of carbon dioxide going into the air or physical settling,” he says. Magnan used a front loader to flip the two five-foot piles together. “With more wood chips, we may be able to maintain taller piles without the piles compacting on themselves.”
Temperature probes are placed in different sections of the windrows. “Temperature is the best indicator of the activity of a pile. There can be a variation of 30°F so we measure at four points in a pile.” Temperatures have been running 130 to 160°F since January 2006. “With good irrigation and airflow, we get good temperatures even at the tops of the piles, as they dry quickly,” adds Jerose. The windrows have not been sampled for E. coli, salmonella or pathogens. “We track this through temperature monitoring. If we keep the temperatures at 131° for 15 days, it will reduce the pathogens.”
The windrows dry out through evaporation and air being pulled through piles. To reirrigate them, captured water that has leached out of the windrows is drained into a sump with a manhole cover, pumped up into an overhead sprinkler system and sprayed onto the windrows. “It’s a simple system of PVC pipes and a discharge hose.” After an average of 8-12 weeks of composting on the aerated floor, the windrows are moved to a nearby shed for further composting and curing, then sale or use. When the second composting bay is completed, Jerose predicts that a new windrow can be formed each week.
“The compost piles can freeze in the winter if we leave the blowers on too high, so we don’t leave them on all the time. The piles can maintain their heat if air isn’t continuously pulled through.” Variable speed controllers were installed in spring 2006 to control the RPMs of the air blowers and a computer set timer system controls the amount of time that they are on. “The blowers now alternate instead of being on constantly or on a manual cycle,” notes Jerose. “Alternating windrow aeration and having a good temperature monitoring system led to an improvement in performance.” The blowers are on for one and a half hours with the newest piles and for a half hour with the older piles. To further reduce windrow cooling during the winter months, the windrows will be covered and curtains will be installed on the barn walls.
Things will be done differently when the second half of the composting barn is constructed in 2007. “We’re going to continue to observe and monitor the system’s operation during fall-winter-spring 2006-2007,” says Jerose. “We will build the next wing based on what we’ve learned and the mistakes that we have made,” adds Ouellette.
“We’ll make sure that there is a tighter seal in the gutters in the floor of the barn,” continues Jerose. The PVC pipes curled in from the heat of the windrows and that left gaps in the floor, releasing moisture into the floor instead of into the pipes. Some of the leachate came into the hallway, with a strong ammonia odor. Sealing the pipes in the older bay and the new bay when it is built will solve this. “To keep the pipes from curling, we’ll either brace the pipes or pour the concrete differently by doing two pours with two layers of concrete,” he adds.
More insulation will be added to the concrete in the floor of the second bay. “We’ve learned to insulate as much as you can, and then double it,” says Ouellette. “The toughest place to insulate is the compost floor. It needs to sustain the weight of the compost piles, tractors and other equipment. And it needs to be isolated from the earth underneath, which is a great conductor of energy.” They are considering adding a subsurface layer of six-inch-diameter rock, as well as rock chips and dust to provide additional insulation and stability. “That will capture the energy we are losing,” adds Ouellette.
Ouellette says some condensation could be seen through the bubble wrap covering the 50-foot conduit in the hallway. “We’re planning to tear down the conduit this summer 2006 and check it for leaks and condensation problems and reseal it,” he notes. The performance of the heat transfer system will be measured by increasing demand on the system. “The demand will be increased,” explains Jerose, “by activating the radiant floor heating more frequently through the fall and winter.”
The computer data acquisition system used at Diamond Hill costs $5,000. The system currently has 35 sensors, but Ouellette believes that fewer sensors can be used in future systems. “The system could probably be monitored with one bimetal thermometer in each of the piles, one in the hallway and one in the bulk tank,” he predicts. Ouellette says the cost of purchasing a complete Agrilab heat transfer composting system depends on the purchaser’s energy expectations, feedstock availability, and the size of the compost operation.
Part of the Diamond Hill grant funds are being used to track the capital cost of building the facility; electricity, labor and equipment costs; how many gallons of fuel are being used to run the equipment, conserved manure nutrients and other factors, in order to track the payback of the system. “We feel that this system, with just the first wing operating, will generate enough energy to heat five typical Vermont homes annually,” says Ouellette.
Farmers can also sell by-products created by the heat transfer composting system to other businesses. Ouellette rattles off several businesses that could use these by-products: fish processing and meat packing plants, greenhouse operations, sawmills, greenhouses, and even car washes. “These businesses have a use for either heat energy or hot water or both, and would generate organic residuals suitable for composting or are located near a residual generator,” adds Jerose.
“Aqueous ammonia solutions can be captured and used for agricultural or industrial applications,” Ouellette notes. “Carbon dioxide, produced in large concentrations as a system by-product, is also a great plant growth accelerant. A dry ice plant would love to have a concentrated source of carbon dioxide. It would make manufacturing a lot cheaper instead of drawing latent carbon dioxide from the atmosphere. Hot water has great commercial value – it can be sold to a car wash.”
The Magnans use some of the compost on the farm, and began selling it in 2005 as a conservation mix for erosion control, straight compost, and a garden mix of 50 percent compost and 50 percent topsoil. The compost was approved for use in organic production by NOFA-VT (Northeast Organic Farming Association – Vermont chapter), in accordance with the USDA National Organic Program
Terry Magnan continues to explore new markets for Diamond Hill compost, including crop fields, gardens, landscaping, and erosion control. “Terry’s farm is located far away from population centers and there is already good bagged compost available,” notes Jerose, “so bulk sales are the primary effort.” Local gardeners and landscapers are the most frequent customers. Prices range from $25 to $40 per cubic yard. Some larger loads have been brought to construction sites and for use on recreation fields and adjacent slopes.

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