Posted by: energymd | December 5, 2008

Heat free (nearly) with a compost furnace

The following is a post created as an adjunct to an educational science unit on alternative energy.   If you desire other specific information please leave a comment.  Thanks,  Marc.


Winters are exceptionally cold here in Wisconsin.  I got tired of trying to feed the insatiable appetite of the three oil burning furnaces that heat my home.  When oil prices started rising several years back, the cost of fuel became prohibitively expensive.  After surveying the available alternatives I decided to capture the excess bacterial heat from a compost pile to heat my home. 

First – the basics:  Mix a big pile of carbon and nitrogen organic materials (shredded brush or brown leaves and green grass for example) in the proper proportion (about 30:1), add water until it has the consistency of a damp sponge (about 45~55% moisture content), keep it aerated so the bacteria can breathe, put some garden hose into the pile and run it into the house where it will connect to a radiator (like the one in your car) then back to the pile.  A pump circulates water through the hose and radiator, and as the pile heats up the water, the water heats up the radiator and a fan moves air through the radiator to transfer the heat into the room!  Voila – nearly free heat!!!!! 

Some logistics:  I live and work on several acres of wooded land, so getting plant material isn’t a big deal.   I use heavy equipment to move the material around, so that helps a lot!  For those of you living on residential lots, using a scaled down version of this technology to preheat hot water is a very practical way to lower your monthly energy bill.          

Some specific data and rationale:  Why compost?  Because as best as I’m aware, it’s among the most practical and efficient processes (some say >97% eff – although I think that’s optimistic) for capturing, storing and recovering solar energy that is immediately available at a reasonable cost.  Published values vary, but typical are compost energetic values of 18-19 MJ/kg or 17,040-18,012 Btu/kg  on dry matter basis, 8~10,000 BTU/LB is frequently cited.  The compost I’m using weighs about 40lbs/ft2, and my design specification requires 750 BTU/ft2.  For the technically curious, compost heats to about 120~130 deg F through the action of mesophillic bacteria initially, at which point thermophillic bacteria take over and heat the pile to about 150~160 deg F for a while at which point the temps drop back to the 120~130 deg F range where the mesophillic bacteria take over again to finish the job of decomposition.  It can be a very complete combustion process, and an examination of my pile reveals white ash remaining as residual matter.  Once the thermophillic bacteria have taken over, you can’t keep your hand under the stream of water circulating through the pile or you will be burned – pretty cool huh?

Here are some pix to click – enjoy!