permaculture

[Toby Hemenway <hemenway@jeffnet.org>]

On Tuesday, June 17, 2003, at 04:58 PM, Marimike6@cs.com wrote:

> Wood's a good fuel for some very finite purposes. It is, however, polluting (very smoky, with carcinogenic components and carbon monoxide, for instance), inefficient (we would very soon strip all the earth's forests bare just trying to heat our houses) , heavy to haul and very expensive per therm. But it's a fine luxury fuel, for occasionally sitting by a roaring fire in the fireplace.

Wood definitely has a place in the mix of useful fuels. It's only polluting when burned inefficiently (oil would be even dirtier if burned in a fireplace; wood has only 1 percent the sulfur content of fuel oil). And it's decent on heat content: 9000 btu/lb, while fuel oil is 19,000 btu/lb (minus, of course, all the extraction costs, which can be much less for wood: me and my saw versus wells, refineries, and tankers). A well built, medium-size house needs about 10,000 btu/hr on a cold winter day, so a high-efficiency wood stove only needs a few logs a day. Wood is a marvelous way to store solar energy: non-toxic, local, and all those other functions too. My stove-building friend Dean Still says, "The Creator must love us very much to give us wood." So how do you get the most from wood? Mike is right; fireplaces aren't suitable for heating, and most wood stoves are only a little better. Masonry stoves are pretty good, using about half the wood of a metal stove. But new stove designs are even better. My friends at Aprovecho Research Center have been developing wood stoves that are about the most efficient on the planet, and they put out less pollution than any oil furnace.  ( http://www.efn.org/~apro/  with many articles on stoves.) The key is to separate the combustion from the place where heat is exchanged. 

In fireplaces and conventional wood stoves, the fire radiates its heat right where combustion takes place. That means the fire is constantly being cooled, and low-temperature combustion is very inefficient and very dirty. Aprovecho stoves pre-heat the intake air, and the firebox is insulated, so combustion is very, very hot, and the fuel is just about totally consumed. Then the super-hot flue gases pass into a heat exchanger that very efficiently extracts nearly all the heat; the chimney gas is not only perfectly clear, you can hold your hand over it because nearly all the heat has been extracted. No smoke, no creosote. A handful of sticks will heat a house for several hours. That also means very small wood can be used: branches instead of logs, so not only are you using about 20 percent as much wood as a conventional stove, you can use dead branches instead of whole trees.

A well managed woodlot will yield 1-2 cords of wood per acre per year sustainably, (a coppiced woodlot yields far more) which is more than enough to heat a big, well-insulated house with an Aprovecho stove. Passive solar will reduce that considerably. So I think in countries with a moderate amount of forest (the US has more than 200 million acres) fuel wood could be a major source of heat: renewable, low-polluting, and locally supplied. No wonder Holmgren likes it.

Toby
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For a look at my book on ecological gardening,
Gaia's Garden: A Guide to Home-Scale Permaculture, visit
http://www.chelseagreen.com/Garden/GaiasGarden.htm