Heat Extraction From Cattle Manure Composting / Remediation

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Agrilab Technologies Inc. A unit of the Acrolab Group of Windsor, Ontario, Canada has developed and designed a system for the extraction thermal energy from the process of aerobic decomposition (composting) of farm waste. The system extracts energy from the hot water vapour generated during the composting process. The system then transfers that energy into an insulated bulk storage water tank for farm heating and process water applications. The system is essentially self powered with the exception of a small amount of electrical power needed to power four 120 VAC motor driven inline air blowers using 1/8 hp motors. In this particular application at Diamond Hill Custom Heifers, the waste takes the form of cattle manure and bedding materials.

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Diamond Hill Custom Heifers decided to compost a portion of the manure and bedding produced in its operation both for on-farm use and sale of the compost end product and to supplement the thermal energy requirements of the facility. The composting facility ,collaboratively designed by Terry Magnan, Joseph Ouellette, Brain Jerose and Bruce Fulford with input from Aaron Robtoy, Paul Godin and Dan Carswell, produces on a large scale, high grade compost approved for organic use. Further, the system extracts the thermal energy generated as a product of the decomposition (composting) process.

The facility consists of a two bay composting barn with an east and west composting floor separated by a central enclosed hallway/gallery. Each of the composting floors or bays is approximately 52 feet wide and 60 feet long. Each bay will permit active composting of between 700 and 800 tons of materials at one time. The four windrows in each bay routinely achieve and maintain temperature of 120 to 165 degrees F. for four to eight weeks after initial placement.

The floors are 6” to 10” poured reinforced concrete over compacted sand and closed cell expanded foam sheets typical of the type used in the installation of “in floor” electric heating systems. Below the concrete and above the insulation an array of PVC pipes (Fig 2) have been placed. They form four sections of four 8” pipes running under the floor generally equidistant from each other and oriented east to west for the full length of each bay or pad. Additionally, these pipes are insulated by being wrapped with a “Techfoil” material which raises the insulation value around the pipes to approximately R50 or R60.

These pipe arrays are manifolded together in groups of four contiguous pipes such that the manifolds, which are located in the hallway separating the two bays, provide one outlet per four pipes.

After the concrete floor has been poured to approximately 6' to 10' in thickness, forms attached to the upper surfaces of the pipes are removed causing a chord of the pipe approximately 2.5” to 3” wide, along the length of the pipe, to be exposed to view. That chord is then cut out leaving each pipe with a slot the full 70' length of the pipe 2” to 3” wide, 2” below the floor of the pad.

This allows for tractors and loading equipment to operate on the composting floor without crushing or damaging the gutter pipes.

The manifolded pipes, (Fig 5) which are in fact vapour collectors, have a 10' Fantech blower mounted vertically into 10' flexible duct. the flex ducts are then attached to a large 24' PVC corrugated conduct (Fig 6) in which an array of six Isobar superthermal conductors is situated. The vapour from the compost windrows is drawn through the gutters, through the ductwork and across the isolabs. The resulting trough, with the pipe below it and the opening centered along the length of the slot, is then covered with a heavy-duty mesh screen along the length of the trough.

These superthermal conductors known as Isobars reside within the 24” “condenser” conduit for 50 feet and then immediately enter an insulated 800 gallon capacity bulk tank extending through the tank and exiting out the other end. Note that shows the Isobars ready for insersion into the bulk tank. shows the Isobar array before the 24' conduit is fully installed.

The Isobars then exit the tank and extend roughly 6” beyond in an insulated enclosure welded to the tank surface. These Isobar extensions facilitate the charging valve train for the Isobars.

A 24” discharge stack is “T”d into the condenser conduit at or near the point where the conduit bulkheads against the bulk tank wall. The stack is vented up and outside of the hallway/gallery which houses the system. Exhaust vapour is controlled via a damper in the stack. Exhaust may be vented to the atmosphere. Plans are underway to duct this spent vapour back into the composting bay which will add water for irrigation and heat to the air drawn from above the composting pile. This will increase the effectiveness of the energy generation as well as assist in more efficient composting.

Four inline air blowers are attached to the outlet of the manifolds which in turn connect the four slotted floor 8” pipe arrays The warm vapour laden air is drawn through the slots cut in the tops from the floor embedded PVC pipes and is fed to the 24” condenser conduit containing the Isobar superthermal conductors.

The manifolds, connecting conduits, condenser conduit, and air blowers are all sealed to ensure that no air leaks are present and then insulated to R60+.

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Isobar Superthermal conductors are devices more technically defined as evacuated two phase heat exchangers. Isobars, ( Fig 7, 8 & 9) in this application are made of 3” stainless steel tube sections, sealed at each end and charged with a working fluid.

The characteristics of Isobars are such that any energy applied locally to any random portion of the Isobar is immediately and at exceptionally high speed transferred to all remaining portions of the surface of the Isobar. As an example if a propane torch flame was applied to one end of the Isobar over an area of 3 square inches, all the energy of that flame would immediately be distributed across the remainder of the Isobar at high speed to the point where you could touch the area where the flame had been applied seconds after the flame was removed.

Isobars are isothermal devices constantly achieving uniform temperatures on the surface as a result of random heat inputs. In this particular application, the hot water vapour condensing on the Isobars is immediately transferred at near sonic speeds to that portion of the Isobar that is in contact with the water in the bulk tank. As long as the water in the bulk tank is at a temperature below that of the hot compost water vapour, transfer will take place. Isobars are self powered. They do not require electrical power. They require no external energy source to activate them other than a difference in temperature from one location on its surface to another.

A mixture of cattle manure and bedding plus other constituents as necessary is blended to a specific recipe with the use of a feedmixer and loaded onto the floor to a height of 10 feet roughly and covering the complete composting bay or floor. The porosity of the blend is predictable. The natural composting process is enhanced by the use of the air blowers drawing ambient air from the barn through the composting piles from top to bottom where that air replaces water vapour at a temperature of from 90 to 150 F depending on the age and recipe of the materials.

The hot water vapour is drawn into the slotted 8” pipes embedded in the composting floor and taken to the condenser conduit through insulated interconnected ducts. The airflow through the system from the composting floor is controlled by the use of slide gate valves in the pipes as well as through speed controls governing the RPM of the air blowers. Because the duct and air blower system is highly insulated, once the system achieves steady state the vapour being drawn from the composting materials will not condense on the inner surfaces of the pipes, blowers and ducts which constitute the vapour path of the system, but remain in vapour state until it is in contact with the Isobar superthermal conductors.

Once in contact with the Isobar array, the hot water vapour generated by the composting reaction is drawn through the collector ducts and manifolds via the air blowers. There it condenses and yields not only its latent heat but also the energy associated with the temperature of the water condensate, which is at a higher temperature than the Isobars. As long as the water in the bulk tank is at a lower temperature than the hot water vapour, this heat transfer action will continue without the need for outside power other than that needed to drive the four air blowers. In the instance of a power outage, the Agrilab Heat Transfer System would continue to operate at a reduced level as a result of the natural draft caused by the negative pressure in the condenser duct created by the condensing vapour.

Isobar System Benefits: The Agrilab Isobar Heat Transfer System provides hot process water contained in an insulated bulk storage tank for use either as an adjunct to facility hot water heating systems or for direct hot process water.

The Agrilab Isobar Heat Transfer System can provide in some instances most if not all process water heating needs without the use of external power sources.

Quantifiable results: At present The Agrilab Isobar Heat Transfer System operating at Diamond Hill Custom Heifers is functioning in a timer-controlled alternating aeration cycle . The requirements of the system being to provide energy sufficient to raise the temperature of well water to a process level of 120 F. A double tank heat exchanger is connected to the 800-gallon bulk tank. This is an intermittent or batch activity that places little demand on the system. Tests applied to the system in this mode have indicated that a minimum transfer rate of 240,000 BTU/day has been achieved. If the system is under significant demand such that the temperature of the water in the insulated bulk tank is kept at a differential of 20 F with respect to the hot compost water vapour temperature, it is expected that the heat transfer rate will be in the range of 500,000+ BTU s/day. Monitoring the system for vapour temperatures and vapour flow rates will allow for better documentation of BTU production, loss and utilization. The use of heated water in the radiant floor heating system may significantly increase the value of the captured energy from the composting system. This is being tested presently and may periodically yield up to 3 million BTU/day or 120,000 BTU/hour.

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