Friday, October 7, 2011

Biomass Energy??

Aesthetically, fireplaces are always nice. They even feel warm when you're up close. But I've always known they're terribly inefficient. After all, most of the heat goes straight up the chimney--around 80%. The real score could be even worse, because the rising hot air has to be replaced by cold air pulled in somewhere through the building envelope, which can make other rooms colder (per MythBusters test - but see discussion).

Last year while talking to a friend, he told me of seeing a fireplace insert that could easily heat the whole house. I started investigating and found that modern fireplace inserts are not just a little more efficient than open fireplaces--they can in fact be quite efficient and generate large amounts of heat.

In the current climate of energy-consciousness, along with the modern efficiencies of wood stoves and fireplace inserts comes a fancy newfangled term: biomass heating--which simply means the recovery of energy recently captured in biological material. It's what people have been doing for millennia, but now made chic. Biomass stoves can either burn traditional sticks of wood or wood pellets made of compressed sawdust. Pellet stoves can potentially be more efficient because the burn rate can be micromanaged: they have a hopper and conveyor belt that maintains the rate of new fuel added to the fire. The downside of pellet stoves is that the conveyor belt stops when the electricity goes off; also, you can't grow your own fuel.

The EPA (Environmental Protection Agency) serves a useful role when it comes to biomass stoves by providing standards and a certification of how clean the stoves burn. As opposed to regulation, this is information that permits you and me as consumers to compare complex and difficult-to-evaluate competing products. A fireplace insert is really just a wood stove that fits into the fireplace opening, so the list of certified "stoves" includes inserts as well.

So last spring we ordered two fireplace inserts--one for our home and one for the office. We went with a mid-range model from Napoleon (1402) that runs around $2,000 installed by Eric Harton (see his website). If you are out at the State Fair of Texas in the next few weeks, drop by his booth at the Embarcadero building booth #27. A nice guy who does good work.

Objections overcome
The smoke-filled air in Congo
obscures the world's largest volcano
--just 10 km away.
When I first started looking into biomass heating, I had no idea biomass could be a serious 21st century fuel. After all, I've seen mountain air painted gray by the smoke of cooking fires in Congo. It seemed so quaint--but especially dirty. So, there are several intellectual hurdles to overcome:
  1. "Doesn't biomass energy pollute the air with dirty smoke?"
    If you see and smell smoke from your neighbor's chimney, you can be sure they don't have an efficient wood-burning insert. Even older wood stoves or inserts produce 15 to 30 grams  per hour (gph) of particulate matter. An efficient stove produces a little smoke only in the first 15 minutes while it is warming up--then the visible smoke disappears. EPA certification requires less than 7.5 gph, with most new stoves scoring under 4. Our Napoleon model is rated at 3.5 gph.
  2. "Aren't we supposed to be planting trees? Cutting down trees can't be green."
    Except that it can be done sustainably. Replant trees and they will grow. Grow your own fuel. I planted many trees at my office (our old house) 10 to 15 years ago. Just from trimming them they provide a lot of wood. Look at how many tree trimmings go to the landfill each year. That's wasted biomass fuel. It's better to burn it instead. Some of our own trees need to be cut down soon because of overcrowding or because they're starting to shade our solar array. I plan to replant trees in the back of the yard just for fuel.
  3. "Burning wood is inefficient."
    It's true that an open fireplace radiates at best 25% of it's heat into the room and sends the rest up the chimney. A wood stove reverses the heat loss, giving off as much as 85% or more of it's heat into the house. It does this by slowly burning the wood at a high temperature of around 550°. The oven bricks heat up and slowly cook the wood. In addition, super-heated jets of air provide fresh oxygen to re-burn rising smoke before it actually leaves the oven. (There are also catalytic models that achieve slightly higher efficiencies. But these also add a couple hundred dollars of cost every few years to replace the catalyst, which is fragile and also subject to premature breakage.) In addition, electric blowers in most models circulate air around the stove and disperse the heat into the house. This is one point at which I think the stove makers could improve--why not use an external combustion engine (a Stirling engine) and just use the heat of the stove to power the blowers?
  4. "Doesn't burning wood release carbon into the atmosphere?"
    Tree's capture carbon and the sun's energy, and burning them releases the energy for use and the carbon back to the air to be recaptured by new trees. If dead trees rot instead, the same carbon is released back into the atmosphere. Either way, there is a closed carbon cycle. The issue raised with fossil fuels is the release of carbon back into the air that has been sequestered for millennia--potentially upsetting the current balance of components in the atmosphere.
  5. "Can you really heat a modern house with wood?"
    Look at the BTU/hour rating of a stove. Compare your furnace rating. A small gas furnace may put out 50,000 BTU/hour. Our Napoleon stove is rated at 11,400 to 70,000 BTU/hour--it depends greatly on the type of wood burned and how much you stoke the fire. Hard woods contain more heat content and will burn longer. Of course, a home's architecture affects how much of the home one heating "appliance" will warm. In general, biomass fuel will be considered a secondary and emergency source of heat by most users. But it could be quite a substantial source.
Payback period 'guestimate'
Currently I have no hard data to go on for our houses. I can only speculate. What if 50% of our heat this next winter could come from wood? For our home that would amount to $320 saved in our gas bills; for work that would be closer to $200. Subtract out the $100 each I spent to deliver 1/2 a cord (4' tall by 8' long stack) of wood at each location, and the savings is relatively small. The payback could be 10 to 20 years. However, I expect to be much more comfortable at the office because I will relax the extreme austerity measures I've implemented the last few years! And there is the ambiance to enjoy. I'll report back in the spring after we have actual data about savings at home and at the office.

The Hearth, Patio and Barbecue Association provides a calculator to give some sense of the relative potential cost savings of adding a new biomass stove. It's interesting and based on some 'typical' situation, but clearly there are more variables than just the cost of the new fuel vs. the cost of the old fuel. Especially critical is the question of how much the family actually makes use of the new 'appliance' (stove or insert).

After all, there's work involved in stoking the fire. But if you enjoy your fireplace anyway, it's a matter of enjoying more fruit from your labor. A good stove will burn the same amount of wood much longer because it burns the wood more slowly and thoroughly. Many stoves have a stated burn-time of 9 hours or so.

And what about splitting wood? Does that count as chores or exercise? Maybe both. For a city dweller, buying a new maul (the proper tool for splitting wood) and learning the technique for the first time brings a feeling of accomplishment. Even my younger son found it a challenge to see if he could split wood with one blow. It certainly gets one out-of-doors for a bit and builds a connection with God's creation--kind of like a campfire experience. You can't get that from a cold--or hot--furnace.

Tax Credit
Finally, biomass stoves are a beneficiary of recent green initiatives at the federal level. They are eligible for a 2011 Consumer Energy Efficiency tax credit of $300 if their thermal efficiency rating is at least 75%. While the emissions level of an insert is easy to identify on the listing of EPA-certified stoves, the thermal efficiency rating is harder to nail down. The EPA listing appears to show a set efficiency rating for all stoves by class: 63% for non-catalytic wood stoves, 72% for catalytic, 78% for pellet stoves. Manufacturers, however, often list efficiencies well above 80%. Apparently the tax credit efficiency rating must be certified by and obtained directly from the maker. Our Napoleon inserts are certified at 78.83% efficiency.


Peter followed Him at a distance, right into the high priest's courtyard. He was sitting with the temple police, warming himself by the fire. (Mark 14.54, cf. Luke 22.55-57)

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