The following info is from our research poster created as part of my class that built and tested the heat exchanger:
A new heat exchanger was built to extract heat from hot compost piles. The system uses commonly available and recycled material to cheaply extract heat from compost piles for greenhouse heating and freshwater prawn production. First, a shallow solar pond was built inside a greenhouse to collect solar heat energy. Next, water from the pond was pumped into recycled 55 gallon drums forming the wall between the greenhouse and a hot compost pile. Heat transferred from the hot compost pile into the water flowing through the drums then returned to the pond. The pond acted as a heat storage battery for nighttime greenhouse heating. The drums also filtered the water using a swirl filtration and solids settling technique to improve water quality for freshwater prawns growing in the pond.
Heat extraction from compost piles is nothing new. Records from at least the 1940’s show decomposing straw was used to provide heat and carbon dioxide to improve plant growth inside greenhouses. During the 1970’s a French inventor named Jean Pain pioneered heat extraction techniques from compost piles using water. Pain built 20 foot wide and 10 foot tall round wood chip compost piles with coils of polyethylene pipe wrapped inside. Passing water through the pipes at 1.1 gallons per minute heated the water to 140°F. The massive piles of wood chips generated heat for eighteen months without turning the piles or blowers to stimulate aeration (Pain 1980).
During the 1980’s the New Alchemy Institute on Cape Cod placed hot compost piles inside a greenhouse separated by an insulated chamber. Air was blown through the compost piles and into the greenhouse through a soil filter to provide heat and carbon dioxide for plant growth. The compost was accessed through removable panels on the outside of the greenhouse and new compost was added weekly to generate continuous heat. The compost heating system kept the greenhouse 23° to 35°F warmer than outside nighttime minimums (Fulford 1986).
Commercial compost heat extraction systems are currently available through Agrilab Technologies. Compost is placed on top of insulated slabs and a fan draws hot vapor from the compost pile into a heat exchanger. The heat exchanger transfers the compost heat into water and the hot water is used in radiant floor heating or anywhere domestic or commercial hot water is needed. The Agrilab system does not require pipes inside the compost pile making the piles easy to build and turn with a tractor. An Agrilab system built to compost cow manure produces up to 120,000 BTUs per hour for radiant floor heating (www.agrilab.com).
Recently, several new techniques to extract compost heat were developed and tested at the Clemson University Student Organic Farm. Similar to the Agrilab system, the compost pile is placed on top of an insulated slab. Instead of using fans to extract hot vapor, water flowing through pipes inside the slab extract heat from the compost pile. Additional techniques embed pipes inside compost piles. The pipes are strategically placed to intercept hot vapor rising from natural ventilation processes inside the pile. The current project aims to build a multi-functional heat extraction system at low cost. The new heat exchanger forms the wall of the greenhouse supporting the wall from the increased weight of the adjacent compost pile. The system also filters water using centrifugal and gravitational forces for improving aquaculture inside heat storage ponds within the greenhouse.
Material and Methods
A series of 55 gallon drums were placed along the wall of a greenhouse. The drums were connected using 2 inch schedule 40 PVC pipe 40 inches above the base of the drum. Rubber Uniseals prevented the pipes from leaking at all drum penetration points. At the entry point to each barrel a 90 degree elbow was inserted onto the pipe end forcing the incoming water into a swirling motion. The water exited the barrel through a standpipe at the center of the barrel. The swirling action pushes solids out and away from the standpipe in the center and gravity pushes solids down to the bottom of the barrel. A drain on each barrel connected to a valve then a central drainpipe removes settled solids.
Water was pumped through the barrels using Little Giant 4M-MDQX-SO inline pump designed for pond filtration. Hobo data logger attached to the inflow and outflow of the system monitored heat gain of the flowing water at 15 minute intervals.
Over a one week period an approximate 35 cubic yard compost pile was built adjacent and over the top of the barrels. The compost was made using 2 parts wood chips and 1 part food waste recycled from the Clemson University campus. The ingredients were mixed using a P125 ABI manure spreader powered by a New Holland TC40 tractor.
In our 15 drum system, under a compost pile with an average temperature of 159⁰F, the temperature difference between the water going into the barrel system and the water coming out at a flowrate of 1.32 gallons per minute extracted about 8,712 BTUs/hour after stabilization. However if the system is treated like a battery and allowed to “charge” during the day enabling more significant heat build up before being run at night the same flowrate of 1.32 gallons per minute can extract an average of around 15,000 BTUs/hour.
While environmental impact is an important part of sustainability it’s also necessary for a project to be cost effective. At a flowrate of 1.32 gallons per minute the system extracts 8,712 BTUs/hour or 209,088 BTUs/day. With propane at $2.95/gallon the savings are about $6.65/day, and $1,198 over a 6 month period. So in a case where a grower already has an inline pump for aquaculture, which is likely, this heating system pays for itself in under 4 months and a grower who needed to also purchase a pump could expect full return on investment in under 5 months.
There is also some optimization that can be done, we found that if you use the system like a solar panel and allow the system to “charge” during the day by turning it off and only running the circulation at night you can double the BTUs extracted and average around 15,000 BTUs/hr.
1. Forms the wall of the greenhouse and supports the greenhouse from the weight of the compost.
2. Transfers heat from the compost into the adjacent greenhouse.
3. Water heated in the exchanger transfers heat to greenhouse ponds to heat greenhouses
4. Separates solids from the water to improve water quality for prawn production
5. Solids fertilize adjacent farm field
6. Solids collected in basin fertilize compost pile
7. Water drained from system irrigates farm field
Allison Acosta, Meredith McSwain, Carly Basinger, Ellie Lane, Charles Murray, Aaron Stiebohr, Charles Weinheimer, Michael Bartley, Mitchell Madsen
It is about 3' x 5' and a few feet tall, just a frame with a plastic cover. I plan to put it in my sunniest spot (have a mostly shady very small yard) and dig a trench in the back and fill
with compostables, and make it a hill against the back wall. I figure it will give me the most sunshine for the area. I consider this a way to extend the growing area and help protect
my veggies from my new neighborhood bunny(s).I live in the northeast and spring is slower to warm these past years, then it gets hot quickly and then its fall again. I am trying new strategies
throughout the yard. My newest idea is Margory Wildcraft squash pit video idea with a temporary topper to trap heat.
Shawn Jadrnicek wrote:...Similar to the Agrilab system, the compost pile is placed on top of an insulated slab. Instead of using fans to extract hot vapor, water flowing through pipes inside the slab extract heat from the compost pile. Additional techniques embed pipes inside compost piles. The pipes are strategically placed to intercept hot vapor rising from natural ventilation processes inside the pile...
I assume you mean a concrete slab? With pipes embedded? I am trying to picture this correctly. And what material pipes were used? I recall you saying once that you thought you could improve the system by using different pipes in contact with the compost, but don't remember the specifics... I know from DIY solar water research that PEX is a lousy conductor, but is also cheap, and a long length of PEX piping coiled up can potentially make up in surface area what it lacks in conductivity. If that was used in your system, I'd be very eager to hear more about your results.
And did you ALSO have pipes buried in the compost pile, as well as in the slab beneath it? Not sure I got that from your description.
Shawn Jadrnicek wrote:I go into all the details in the book but the concrete slab has about 900 feet of 3/4 inch pex pipe inside and gets around 15,000 btu's/hr when the compost is in the 150's. We've also run 1" pex through the middle of a windrow style pile and get around 4-6,000 btu's/hr from a single run down a 35 foot long 40 cubic yard pile and around 8,000 btu's/hr from two runs. Depending on flow rate of water and amount of pipes you can extract too much heat and cause compost temps to drop. Heat transfer is also dependent on moisture levels in compost pile.
Very interesting. Thanks for the info! I'm sure you present a more detailed analysis in the book, but based on the numbers you just quoted, it sounds like the 1" PEX mid-pile is the more efficient heat extractor. Although it probably presents more of a difficulty in terms of turning or moving the pile around without damaging the pipe. And I'm sure that if you monitor temperatures, you can adjust the flow rate to optimize the system for your needs.