Compost for Food and Heat

Basic Concept

At the end of Oct to beginning of Nov build a compost pile in the range of 3-4 tons, 300-400 ft3, from recently harvested corn stalk, sorghum stalks, legume stalk, grass clippings, wood chips, etc. If some of this needs to be stored it needs to be dry, covered from rain, and away from house since when dry it can easily be ignited, spontanious combustion is possible. Greenhouse is 4’ in the ground. Hot tub near the entrance rests on the ground. Aerobic compost septic 1m3, 3’x3’x3’ on the ground. Evapotranspiration wicking bed 8x8? has to be a little lower than the Aerobic compost Septic. The rest of the greenhouse is 4 ft of compost in the winter. Compost surrounding hot tub helps keep it warm. Insulation only needed on the top and bottom.

Jean Pain

Any thermal compost pile—Jean explained—can be sized according to the demand for heat that is anticipated. We’ll discuss, then, a heap built from about 16 tons of clippings. This happens to be equivalent to the amount of brush that can be removed from an average acre of timber during a year of normal stewardship . . . and such a pile also produces the right amount of humus to add to an acre of land that’s to be used for growing cereal grains.

After the twigs from such an area have been shredded, they’ll form a mound about 10 feet wide, 10 feet high, and 15 feet long. ref How to

Gaelan Brown

  • The Compost-Powered Water Heater (book)
  • Jean Pain method uses “Passive Airation” - requires high-carbon materials like yard waste or bark mulch soaked with water.
  • For negative-pressure airation - 1 ton of compost = 1000 Btu/h for 6-8 weeks = 1 million Btu total
  • For Jean Pain about half that? over 12 to 18 months.
  • PFRP- Process for Further Reduction of Pathogens
    • Above 132F for 5 or 7 days
    • Not part of PFRP, but then bring it down to about 120F to keep more finished compost
  • airflow 2 cfm - 20 cfm per ton

Conan Eaton

Conan Eaton - Auburn Star Farm Vermont

COMPOST WHEELIE BIN HOT WATER

Forced Air Composting System

Instructables

MEN 2nd Gen

…inner enclosure of chicken wire that was six feet wide by eight feet tall. The cage was then filled with a 3-to-1 wood-chip/manure mixture (as opposed to the 4-to-1 mix that made up our first heap) and wrapped with coils of 1” black semi-flexible plastic pipe.

In order to increase the heap’s ability to maintain and store a supply of hot water, we positioned a 30-gallon water heater tank immediately outside of the inner core, and—around both the central cylinder and the tank—added a second layer of compost three feet thick. Thus the entire cylinder-within-a-cylinder pile measures 8’ X 12’ … and weighs in at a hefty nine tons. ref

…registering an inner temperature of 140°F for two months and then leveling off again at a plateau of 130°F, which it sustained for another two months before dropping any further. ref

MEN 3rd Gen

5 feet high, 10 feet wide, and 12 feet long… weighs a solid six tons. ref

Grow More Vegetables - John Jeavins

location 1218: Locate compost pile under a deciduous tree (oak is best). 6 ft from trunk location 1236: minimum 3’x3’x3’, 1 cubic yard, 1,000 lbs. large 4’ high, 5’ wide, 10’ long location 1268 a thermophilic 113F to 149F cured compost has 1/3 to 1/2 the cured organic material of a mesophilic 50F to 113F compost.

So we are burning more carbon than necessary to get higher heat like 140F. Although if we can keep most of the pile below 113F, it could still work well. Legonella be a problem at those temps, though so keep the compost hot water loop separated from the hot water that goes to shower and faucets if temps are under 130F. And get temps of hot water tank over 130F with solar every few days.

location 968 40 beds per person or 8000ft2 for 2 people location 981 per 100ft2 bed 90-360lbs immature compost material to produce 15-60lb mature compost for application.

80beds x 180lbs = 14,400 lbs might be an average starting compost pile weight to feed 2 people and heat their house and hot water.

More Math

an acre… (9 tons of) fallen autumn leaves… 170,000,000 Btus or 50 MegaWatt-hours of energy ref

Scale for Average House

At the end of Oct to beginning of Nov build a compost pile in the range of 7-9 tons from recently harvested corn stalk, sorghum stalks, legume stalk, grass clippings, wood chips, etc. Construct a 12’ diameter cylinder 8’ tall with PEX pipe in the center similar to MEN 2nd Gen below. During the day a single solar-pv DC pump circulates the water from the compost pile to the domestic hot water tank then through the radiant heat under the floor. This works with the solar to provide hot water and space heat from Nov-Feb. Then in April the cured compost goes on a 8000ft3 garden that could provide all calories for 2 people, or even all the food if desired. It’s scary how close the food production and heat production numbers line up for 2 people living in one house.

Hot tub

One Peace Corps volunteer suggested that we could better utilize the heat by constructing a jacuzzi. Indeed, a simple pool could be easily constructed by excavating a depression in the middle of the compost bed and lining it with a heavy plastic or rubber sheet to contain the water. Then, by circulating the water in this pool back into the heat exchanger when it cooled below a desired level, a compost-powered hot tub could be built. ref ref video example

Decisions

Air vs Water heat transfer

Since water is needed for hot water and hot tub, it make sense to add one more water loop for pex tube in the slab. A 4th loop for a radiant heater in the bedroom could be added later. All run off of one pump.

Compost in Greenhouse vs Outside

In the greenhouse has the advantage of adding heat and CO2 to the greenhouse. It has the disadvantage of having to move material in and out and taking up a lot of space. Also it should not be left unattended for long periods of time due to fire risk.

Compost doesn’t need light, so it should go outside the greenhouse. Next to the greenhouse for air exchange is good. Perhaps in a little foam shed, eg hexayurt.

Passive vs Active Airation

Passive is picky about feedstocks. Lots of carbon material. Fine textured bark mulch woodchips soaked in lots of water. Put aeration pipes underneath to assist. Seems like a blower could be attached to these aeration pipes if it needs a little active boost from time to time. Needs to be a certain height? Active needs a fan, but it only needs to run once or twice a day for 15 minutes. It gives more flexability in feedstocks and pile size. It also can ramp up and down faster. 6-8 weeks vs 12-18 months. Let’s do active. Inside a little 1-4 yd3 insulated chamber.

Size

TBD - I’ll wait to read The Compost-Powered Water Heater