Biomass · Assessing feedstock
Sun and wind show up on their own. Biomass has to be produced, collected, moved and stored, and the amount a property genuinely produces in a year is almost always smaller than the amount it appears to produce when you walk it.
This page works out that number for the three streams a property can carry, and converts each one into kilowatt-hours so you can set it against your own load list.
What you are counting
A property produces biomass energy in three forms, and each one suits a different set of equipment. Counting them together produces an encouraging total and a system that does not work, because no single machine can eat all three.
Measure them separately. Most properties turn out to have one real stream, a second that is marginal, and a third that is not there at all.
Measured in cords per year that the woodlot can give up without shrinking. Feeds gasification and steam, and heats the house either way.
Measured in what is collected daily at one point, not in what the animals produce. Feeds a digester, and nothing else here.
Measured in hours of burning per year. Feeds thermoelectric modules and Stirling engines, and costs you almost no extra work.
Stream one
The number that matters is annual increment: the volume the stand puts on each year, which is the volume you can remove each year and still have the same woodlot in twenty years. Standing timber is a stock and can be spent once. Growth is the income.
Extension and state forestry guidance across the northern hardwood range converges on roughly half a cord per acre per year under selective harvest, with a well-stocked and actively managed stand reaching about one cord. Softwood stands run higher. Use the conservative figure until you have several seasons of your own records.
From acres to kilowatt-hours
That last condition is the one that ends most wood-to-electricity plans. A household heating with wood in a cold climate burns somewhere between two and five cords a winter. If the woodlot yields five cords and the house needs four, the generation project has one cord to work with, and one cord is around 900 kilowatt-hours, which is roughly a month of ordinary household consumption.
Burning wood for heat returns most of its energy as heat. Burning it for electricity throws away the large majority and hands back a small fraction as power. When both uses compete for the same woodlot, heat almost always wins the argument on arithmetic alone, which is why the systems that do both at once, covered in the combined heat and power guide are the ones worth studying.
Stream two
This is the shortest assessment on the page, because the Environmental Protection Agency has already published the screen. Farms are considered preliminary candidates for anaerobic digestion at roughly 500 head of cattle, or 2,000 hogs on a liquid or slurry manure system[2]. Most digesters actually operating in the United States sit above 1,000 cows or 5,000 hogs.
Smaller projects exist and a few work well, but they clear a condition that has nothing to do with headcount: the manure has to be collected frequently, as a liquid or slurry, at a single location. Animals on pasture fail that test at any herd size, because the manure is never gathered. A digester cannot be fed from a field.
If you keep a few animals, this stream is not available to you, and the honest thing is to say so on the first screen rather than after a winter of reading.
What a cow is actually worth
Measured across six manure-only farm digesters over twelve months, daily biogas production averaged about 79 cubic feet per lactating cow equivalent[3]. At roughly 60 percent methane that is near 47,000 Btu of fuel per cow per day. Through an engine at realistic efficiency it is around 3 kilowatt-hours gross.
Gross is the operative word. A digester holds its contents near body temperature to keep the bacteria working, and in a cold climate it spends a large share of its own gas doing that. The smaller the vessel, the worse that ratio gets, because a small tank has more surface area for the volume it holds. This is the structural reason the screening threshold sits where it does, and it does not yield to enthusiasm.
The full treatment of what a digester is, what the gas does to an engine if it is not cleaned, and what comes out the other end is the subject of its own guide in this hub, which is being written now. It is worth reading even if your herd is nowhere near the threshold, because the same chemistry governs a manure lagoon you may already have.
Stream three
If a stove is already burning six months of the year, the fuel is bought, the labour is spent and the heat is being produced whether or not you take anything electrical from it. Everything you recover is additional, and the assessment is simply how many hours a year the stove is hot.
The catch is scale. Commercial thermoelectric modules convert somewhere around five to six percent of the heat passing through them into electricity[4], so a stove-top installation is measured in watts. A few modules handle lighting and phone charging. They do not handle a well pump.
Hours of burning across a full year, not across the coldest month. A stove running twelve hours a day for five months is about 1,800 hours, and that figure sets everything.
Output arrives only when the stove is hot, which in most climates means it arrives in the season when solar is worst. That complementarity is the real argument for this stream.
Worked example
A common shape of property, run through all three screens in the order above. The numbers are illustrative and chosen to demonstrate the method, not to describe your place.
Twelve acres, of which about nine are workable hardwood the tractor can reach. At half a cord per acre that is roughly 4.5 cords a year sustainably.
The house burns three and a half cords keeping itself warm. One cord is left. Through a gasifier that is somewhere near 900 kilowatt-hours a year, for a machine that wants feeding through every hour it runs. Verdict: the wood is real, and it is already spoken for.
Two cows, a few pigs, poultry. Everything is on pasture or deep bedding, and nothing is collected daily as a slurry at one point.
Against a screening threshold of 500 head, and against a collection requirement this property does not meet at all, the stream is absent rather than small. Verdict: not available. Compost it and move on.
Three and a half cords burned across roughly 1,800 hours in a season, in a stove that is already there and already paid for.
A modest thermoelectric installation running through those hours contributes a small, steady trickle at exactly the time of year the panels are producing least. Verdict: the only stream on this property worth equipment.
That is a realistic outcome and not a disappointing one. The property has been screened in an afternoon, one pathway has been identified as worth pursuing, and two have been ruled out before any money moved. Set the resulting figure against your own load list rather than against what the equipment is rated to produce.
Common mistakes
A forty-acre wood contains a great deal of timber and yields perhaps twenty cords a year. Sizing a system against the stock rather than the increment produces a machine that works beautifully for six years and then has no woodlot.
Steep ground, wet ground, and anything across a stream without a crossing are not part of the yield. Extraction distance decides this more than acreage does, and a few hundred feet of bad ground removes a stand from the calculation entirely.
Fuel burned for heat is not available to a gasifier, and the house has first claim in a cold climate. Any plan that quietly assigns the whole woodlot to generation has assumed the household stops heating.
A digester is fed daily from a single collection point. Manure distributed across pasture by the animals themselves is a soil input and not a feedstock, whatever the headcount suggests.
Moisture is energy that leaves as steam. Gasification is stricter about this than a stove is, and wood that heats a room acceptably at 30 percent moisture will not run a gasifier well at all.
A year's cords are not a year's steady output. Every figure on this page assumes somebody is present to cut, dry, load and tend, and that person's available hours are a harder constraint than the acreage.
Next
Most readers finish this page with one viable stream, and for the majority it is the heat already coming off a stove. That is the pathway with the smallest equipment cost, the least added labour, and the least that can go wrong, so it is the one to understand before any of the larger systems become worth pricing.
The guides that follow are being written in that order. Until they exist, the hub sets out what each one will cover and how the risky three are handled.
The eight guides in this hub, what each pathway demands, and why three of them are covered without build instructions.
Back to the hubThe six property-scale generation areas, and how they compare against each other for a given site.
See the sectionSources
Conversion figures on this page are screening arithmetic for deciding whether to look further, and they carry assumed efficiencies at every stage. Measure your own yield across several seasons, and confirm collection, storage and permitting requirements with your state and county authority before committing money. No figure here is taken from a company selling the equipment it describes.