Fuel Cell Performance

> A question occured to me while reading this detailed analysis about home
> energy useage. "How much time / acreage was required to create
> (sunlight to photosynthsis) the fuel stocks depleted by an average house
> in a year?"

As a start and for a cold climate (Alaska):
http://www.gi.alaska.edu/ScienceForum/ASF3/302.html

"How much forest land does it take to keep a northerner's house
perpetually in firewood?

To answer this question we begin by defining the size and heating
requirement for an "average" interior Alaska house. Axel Carlson of the
University of Alaska's Cooperative Extension Service defined an average
house as one having about 1200 square feet of floor area and needing 176
million BTU's for heat each year. It would take about 1250 gallons of
fuel oil each year to heat such a house.

By photosynthesis, the plants growing on an acre of land can convert and
store about 200 million BTU's of solar energy each year. So, in
principle, it would seem that an acre could supply a house. In practice,
though we burn only tree trunks and perhaps the largest branches for
house heating. This plant material is produced at a far lower rate.

Forests considered to be just marginal for commercial wood production in
Alaska and Yukon will yield about a fifth of a cord of trunk wood per
acre each year. Since it takes about 12 cords of wood to supply the 176
million BTU's for our average house's annual heating, as much as 60
acres of marginally commercial forest land are needed to supply the
house year after year on a sustained basis.

Of course, the more productive forest lands bordering the Gulf of Alaska
will produce trunk wood at a far higher rate. Balsam poplar and black
cottonwood stands of southern Alaska are reported to produce well over a
cord of wood per acre annually. So perhaps a 10-acre stand of these
hardwoods could supply the heating requirements of a house indefinitely."

But, here is the "average" home energy use issue again. If I can point
to homes that use 1/10 the supplemental heat (in this case it would be
20 million BTUs because of better insulation), we can see that a "good"
house in Alaska perhaps would only need a one to six acres of hardwoods
-- if we ignored heat produced by occupants and local electrical useage.
One kWh of electricity is equivalent to 3412 Btu of heat. If the house
uses 24 kWh / day * 365 days per year = 8760 kWh * 3412 Btu = about 30
million Btu. So you can see this is in the ballpark. Note that obviously
heating needs may be larger in winter and there will be less sun for
passive solar, so I won't argue so much in this Alaska case that
everything could be generated by internal heat -- but you can see it is
close. If that wood is burned in an efficient cogeneration facility
producing heat plus electricity (33% efficiency converting to electric?)
you can see that about four or so acres of good hardwood land probably
be enough to generate all that houses power and heating needs
perpetually (or about twenty or so acres of marginal land).

Note that the US has about 3.5 million square miles of land area
http://www.infoplease.com/ipa/A0108355.html
and at about 1000 acres per square mile that's about 30 billion acres.
So, if there are about 100 million household in the US, and each need
around 20 acres of marginal forest land to produce power perpetually, it
would seem there is plenty of land to produce all the power needed for
energy efficient homes from trees using less than 1/10 of the available
US land according to these calculations. That sounds too good to be
true, so maybe there is an error in this calculation somewhere?

This was fun, but, long term given expected breakthroughs in PV cost and
pollution reduction, and all the effort involved in cutting wood and
getting rid of ash, I'd still recommend letting most of the wood rot in
place to provide habitat or voles and fungi, and using local PV instead.

--Paul Fernhout