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