National Geographic News

Tanya Basu

National Geographic

Published November 14, 2013

In a chilly room at the University of Maryland in College Park, a small group of students is working to make wood stoves cool.

They gather around a black steel box, on top of which is a funnel, shepherding smoke away. They tensely watch the thermometer. The room is still cold, but it normally takes about 20 minutes for the stove to begin warming up.

Eventually someone sighs. Hunched shoulders relax.

"It's working," says Taylor Myers, captain of Mulciber, one of 14 teams from around the world that will be competing as finalists in the first-ever Wood Stove Decathlon, which begins Thursday and continues through the weekend on the National Mall in Washington, D.C.

The competitors are applying new materials, configurations, and high-tech control systems to an old problem: How do you burn wood with less smoke? (See related, “Quiz: What You Don't Know About Wood Energy.”)

It's an important question with implications for both health and energy efficiency at a time when high heating costs have more homeowners in cold climates turning to wood stoves in the winter. (See related, “High Fuel Costs Spark Increased Use of Wood for Home Heating.”)

Decathlon sponsors, including the New York State Energy Research and Development Authority (NYSERDA), the U.S. Forest Service, and a broad range of wood stove industry groups, are seeking to spur innovative ideas for remaking the wood stove for the modern age: high-efficiency, low emissions, and affordable.

Beyond the Franklin Stove

Wood stoves have a small, devoted following today in the industrialized world, but not long ago they were a staple in homes far beyond the cozy mountain cabin. Wood was the dominant source of fuel from the founding of the American colonies until 1885, when it was surpassed by coal. (See related “Quiz: What You Don’t Know About Home Heating.”)

Benjamin Franklin's stove, the mid-room cast iron furnace, was the efficiency breakthrough of its day, generating heat in all directions and providing warmth even after the fire was extinguished. But another Philadelphia inventor, David Rittenhouse, is credited with making a crucial safety improvement to the Franklin stove—a chimney to vent smoke.

Some 240 years later, organizers of the Wood Stove Decathlon call the event "the first high-profile technology competition for wood stoves since Ben Franklin's time."

Although the high-efficiency wood stoves on the market today, especially in Europe, are a far cry from the Franklin and Rittenhouse stoves, many consumers aren't aware of the options or are turned off by the cost of high-end appliances. The U.S. Environmental Protection Agency has a certification program to encourage consumers to switch to modern stoves that reduce smoke and dust, and cut heating expenses. But the Wood Stove Decathlon, modeled after the U.S. Department of Energy's popular Solar Decathlon, seeks to take innovation a step further.

The competition is seeking to spur ideas for a new generation of wood stoves that are greener, more aesthetically pleasing, easier to use,  safer, and more affordable.

"A lot of what wood stoves deal with is a PR problem," said John Ackerley, president for the Alliance for Green Heat, the event organizer.  "People associate them with what they saw when they were growing up."

Where There's Smoke

The biggest problem with wood stoves is the plume of smoke that rises from burning wood. Smoke "always has been a problem and always will be a problem," said John Gulland, a hearth products consultant in Killaloe, Ontario. Smoke is a pollutant and irritant, and it can be dangerous, particularly for children and older people. (Take the quiz: What You Don't Know About Wood Energy.)

It's difficult to come up with one system that will reduce smoke by burning wood consistently and efficiently because there are so many types of wood.

"You've got wood stoves in Alaska that burn spruce, in Washington state they burn Douglas fir, and in New England, they burn maple and oak," Gulland said. "There's no way you can have a highly refined combustion system that will burn all those things."

Wood quality is a problem as well: depending on precipitation, wood from one region may hold more moisture than the same type of wood from a different location.

"No microprocessor can control for [moisture]," Gulland said. "So the problem is that we have a nonstandardized fuel that varies widely."

Wood pellet stoves, widely used in Europe and gaining popularity in the United States, have proven to be an efficient solution to the problem of variability.

Pellets are "just sawdust that's been compressed," said the Alliance for Green Heat's Ackerley. "They're already very dry and very consistent. They can automatically be loaded into a firebox. You can control combustion in a firebox. It's more transportable."

But some consumers shun pellet stoves because they're looking for the warm look of logs burning.

Gulland, who has been in the industry for about 35 years, says trade-offs between form and function are common.

"Every manufacturer of a wood stove has to compromise . . . design to fueling practice," he said. "I've seen really great design come and leave the market because [it] could not cope with” the limitations of working with a variable fuel like wood.

Not Dampers, But a Controller

Students in the Fire Protection Engineering Department at the University of Maryland decided to take on the challenge of creating a wood stove that doesn’t sacrifice form for function. Naming their team Mulciber, after the Roman god of fire, the students focused on a key fire ingredient that is far less variable than the wood: the air. The team's innovation is a piece of equipment--a controller that continually adjusts the flow of air needed for combustion.

"In the typical wood stove, air flow is driven by combustion," said team captain Myers. "So as the fire heats up, it heats up the air inside the box and it draws air in from the outside room into the fire. That means your air flow is entirely dependent on how hot your fire is.” Wood stoves typically include dampers to close off the air flow slightly, but dampers are a one-size-fits-all effort that is not always effective.

Instead, the Maryland students designed a controller that will calibrate the air flow based on temperature measurements it takes inside the fire box. "Based on those measurements and [an] algorithm we program, [the controller] will control the air-flow rate," team member Mark McKinnon said.

Fans will adjust to control the air-flow rate—meaning the efficiency of combustion process will be markedly improved. The fans will react to the controller and vice versa, a "feedback loop," McKinnon said.

The team also incorporated catalysts and particulate scrubbers into its design to make the appliance burn cleaner. But the air-flow system is what makes Mulciber's wood stove cutting edge and unique.

"If you optimize the wood-burning process with sensors, you're doing so much more than you ever could with just tending to your stove with whatever wood you have lying around," said teammate Anita Alexander. "I think that's one of the biggest problems with wood stove technology right now. [wood stove owners] don't know the oxygen levels or what's going on with the fire-burning process."

Simplicity is key to the design of the stove.

"There's not a whole lot of moving parts," Myers said. "It's basically a big steel box that we blow air into. But even in that, we found the complexity it takes to manufacture it is intimidating."

Researching and building the stove proved to be pricey and time-consuming for the only student team to reach the finals of the Wood Stove Decathlon. Competing against professional engineers and commercial wood stove companies, the Mulciber team had no corporate ties or sponsorship, but was able to  secure some funding from the university. The students spent the better part of a year producing a prototype  unlike most in the market. The final product was produced on campus by the campus machine shop. "We started our work from scratch," explained Alexander. "We don't have any other influences but our own ideas."

In the end, the students relished the challenges.

"I think, just by being in the fire protection department, burning yourself a little bit is part of the job," McKinnon said with a chuckle.

Downdraft and Rocket Heaters

The University of Maryland team's air-flow controller is just one of the wood stove heating solutions that will be on display on what is expected to be a fittingly cold weekend in the U.S. capital. Some teams have sought more efficient burning by using turbulence or downdraft. There will be convection heat exchangers and "rocket" heaters—insulated vertical chimneys. Don't expect to see ugly, bulky boxes. The finalist stoves are sleek and chic, using everything from recycled steel oil barrels to soapstone, a powdery rock long praised for its heating capabilities.

The competition has real-world relevance. So many American households are returning to wood stoves that the U.S. Energy Information Administration (EIA), the government's main energy forecasters, last year began including an analysis of firewood and pellets in the annual Winter Fuels Outlook. EIA calculates that 2.5 million U.S. households used wood as their primary heating source in 2012, forecasting the number would grow 3 percent within a year, faster growth than for any other heating fuel. (See related, “No Freeze on Winter Energy Prices, Despite Natural Gas Boom.”)

Still, wood stoves are not typically seen as a modern renewable energy appliance, said Ackerley.

"Only one in ten homes in America have wood stoves," Ackerley continued. "Twenty-, thirty-, forty-year-olds don't associate the new [wood stoves] with being far more efficient than the old ones."

Typically, installation of a wood stove is treated as an energy efficiency improvement, subject to far more modest tax breaks than those available for solar energy, for example.

Ackerley hopes that the Wood Stove Decathlon contest judges, many of whom are state regulators engaged in energy and environmental issues, might be inspired  to push for greater recognition of the positive role cleaner wood stoves can play.

"Maybe states can develop their own ... incentives for them, do something stricter than Washington mandates," Ackerley said. "There's not an effective lobby, no big multinationals involved."

Ackerley and others said Europe, with higher energy costs and plenty of dense forests, has been far ahead of the United States in developing high-tech solutions for wood stoves. (Teams from Austria, Finland, and Denmark are among the finalists in the Wood Stove Decathlon.) (See related “Pictures: Hungary Literally Burns Money For Fuel.”)

In the United States, the allure of wood stoves competes with what Gulland refers to as "the cult of convenience." Unless a stove owner opts for a pellet stove, burning wood will require plenty of collecting, drying, and stacking. "Wood heating is the absolute opposite of convenient," Gulland said.

But members of Maryland's Mulciber team see a future for high-tech, high-efficiency wood stoves.

"I don't know why” wood stoves lost popularity in the United States, said Stanislav Stoliarov, the assistant professor mentoring the team. “Perhaps we thought we were going to walk away from this technology at some point. But that's a mistake. We're not walking away from it. This technology is going to stay with us."

Stoliarov thinks of firewood as "the perfect renewable fuel."

"You can 'regrow' it," he said. "We largely overlook the ideas that make it more efficient and clean."

At the Wood Stove Decathlon, the teams are hoping those ideas catch fire.

Follow Tanya Basu on Twitter.

Russell Davis
Russell Davis

It appears that wood furnace (stove, heater, boiler) efficiency is a inverse multiplier effect on pollution. (EPA measures are typically delusional at best)

Efficiency is a combination of burn efficiency and heat extraction efficiency.

The highest heat extraction efficiency is achieved by thermal cascades beginning with the highest temperature feasible dry steam injection water heating and ending with the condenser-like conditions of sub floor heating by exhaust gases (like the Romans did some 1500 years ago). 100% efficiency can be approached when including the heat of exhaust water condensation in cold weather.

Burn efficiency approaching 100% requires a rocket stove like insulated burn stack and a modest excess of oxygen in the exhaust to minimize the wasted heat and pollution in CO. Too much excess oxygen in the exhaust means that the volume of heated exhaust air is too high which reduces exhaust temperature and thus reduces heat transfer efficiency. Measured exhaust gas temperature is an excellent proxy measure of clean burn - the higher the better.

If the measure of wood furnace (stove, heater, boiler) excellence is not orthogonal from all points of view then the measure is incompetent except in deception.

Michael Fornetti
Michael Fornetti

I agree that residential wood heat is problematic on small units without complex controls and back end cleanup such as electrostatic precipators and baghouses.  In Europe district combined heat and power plants generate steam in bubbling bed or circulating fluid bed boilers, run the steam through turbines, extract steam at low pressure and heat water for district heat.  These units are very clean and efficient and are used where population densities are high and near wood resources.

In the US, we have the wood resources but population densities are too low to make the infrastructure economically feasible, so we have very little wood based combined heat and power.

I live in Norther Michigan an area where wood is going to waste, rotting in the forest so continued research on residential heaters and boilers is is great interest to me and I commend the students doing this important work!   

Vic Steblin
Vic Steblin

Many wood burners claim that heating a home with wood is clean, green, safe and cheap. I disagree with all these claims.

First, the clean claim. Scientific studies show that wood smoke has thousands of dangerous types of particles. This should be as obvious as comparing smoke to natural gas exhaust. Even the best EPA approved system with all the modern technological advances cannot match the liquid or gas fuels. The best “clean” is to wear layers of clothes in a colder house.

Second, the green claim. Green usually means sustainable, which works if the resource does not get overwhelmed. Sustainable is fine for rural areas or where trees vastly outnumber people. With more than 7 billion people on Earth, burning trees is clearly not a sustainable practice. Recent studies also show that the black carbon particles from smoke substantially add to the warming effect when the polar ice absorbs more solar heat. True environmentalists should NOT be burning wood.

Third, the safe claim. Natural practices like forest fires are not always safe. The claim that wood smoke is safe is disproved by epidemiological studies and the simple observation that people avoid campfire smoke.

Finally, the cheap claim. A top quality, modern wood stove with all the latest pollution reduction equipment is not cheap. The supporting equipment like shed, truck, saw, splitter, moisture meter and safety barriers are not needed with clean systems. And when the health effects of lots of wood smoke in an area are included in taxpayer costs, wood burning is off the chart. Fire department callouts for creosote fires and insurance costs of wood caused house fires swamp cleaner systems. Burning wood just transfers true costs to taxpayers.

In summary, wood burning is NOT clean, green, safe or cheap. Considerate people butt with this outdated practice in crowded areas. Officials should ban this silly practice when natural gas is available.

Vic Steblin
Vic Steblin

May the wood stove and all solid fuel burning go the way of the steam locomotive, outhouse, icebox and shooting in city limits.

José Manuel Quintáns Pazos
José Manuel Quintáns Pazos

It is a renewable fuel only if you grow up the same amount of wood than you burned up.

It is a carbon neutral fuel only if you can sink the same amount of carbon (forests, oceans, sequestration) than you release to the atmosphere by burning that wood.

So when we see a wider picture we acknowledge that the main factor is our attitude, but of course they demonstrate a good attitude with their efforts on research towards a more efficient technology.

Sooman Han
Sooman Han

It is new to me that a large portion of people in Europe and USA is still utilizing wood stoves, which disappeared a long time ago in South Korea. Innovation in wood stoves might be a solution to energy efficiency in some countries, but not in Korea and Japan. We cannot have  woods for stoves at all because of lack of natural resources like timber all over the country. We even have no place to install it in at home! might better find another way to dealing with this kind of problem.  

topi robi
topi robi

OMG is it news in 2013 "Instead, the Maryland students designed a controller that will calibrate the air flow based on temperature measurements" all wood gasifier stove use this technology, moreover secondary air + combustion control by o2 sensor, U'll get better results ;) if the goal is better combustion and not only temperature control.

topi robi
topi robi

OMG is it news in 2013 "Instead, the Maryland students designed a controller that will calibrate the air flow based on temperature measurements" all wood gasifier stove use this technology, moreover secondary air + combustion control by o2 sensor, U'll get better results ;) if the goal is better combustion and not only temperature control.

Salim Shaban
Salim Shaban

Drought conditions in Kenya have made a significant impact on the ability to grow food locally.The African Christians Organization Network has pioneered the use of cook stoves that uses the water hyacinth- invasive plant growing on the shores of Lake Victoria- as  fuel instead of wood,thus preserving the forests that help maintain water supplies and a more stable climate.ln addition, the biochar generated  by stoves can be recycled as a soil  nutrient, boosting the fertility of household gardens and can be used as well as  burning fuel.ACON collects the liquid extracted from water hyacinth to add this a long with biochar into chicken litter compost to produce organic fertilizer which more than 14,000 local farmers are using on their farms.

ACON uses a financial model that provides for employment for youth and women, dignity,improved health, energy, and food security for rural people in Western Kenya through the production, sales of stoves, fuel briquettes, organic fertilizer and eventually carbon credits.

Robert Lawton
Robert Lawton

I'd be much more interested in a wood stove design that optimized for electricity production with home heating an obvious byproduct. 

Tom Mengel
Tom Mengel

I like the concept of a local carbon loop system that would create less free atmospheric carbon and would need no complex and inefficient transport systems that just create more free carbon as a usage tax.  If wood pellets are just "sawdust" that has been formulated for consistency to burn completely and clean in a calibrated stove/heater then why not also use switch grasses and corn stalks and other local "carbon wastes" in their formulation?  We could have recycle centers act as the focus and provide something to sell back to the community to help them break even economically, clear out some of the stuff that fills our landfills now, and locally grow the difference (I'm thinking maybe hemp once it's lost it's stigma and come back into widespread cultivation).  Plus the wood pellets would have little impact on the local environment due to having storage/spillage issues (like oil and gas) and could be very low tech and efficient to control costs and make a common system available to third world areas where smoky and dangerous fuels are now used.  We could probably use existing acculturate base silos and farm transport equipment to handle the stuff.   This could keep the heating fuel cycle clean, short, local, and cheap.  Sounds win-win to me.

hiroki ito
hiroki ito

In our country, we cannot have a wood stove because of our narrow space of  houses. But we realize the total human warming machine is wood stove, putting aside the efficacy and affordability. I do want to have the wood stove in my house. I really expect the wood stove to have more popularity and more availability even in an urban area.

Justin Case
Justin Case

Carbon neutral fuel. If we can turn those long chain hydrocarbons into a little bit of carbon monoxide and carbon dioxide and water through burning the wood and then burning the smoke, we will have replaced a lot of fossil fuels.

I love this idea, and I am not surprised that those Finns are leading the pack. People already know generally that the Swedes are burning garbage to generate so efficiently and so cleanly that they are running out of garbage to burn. They are importing it from other countries. Woodlands are not garbage, but the point is that combustion itself, not necessarily the feedstock, can be optimized to the point that we can use it without choking ourselves in the process.

Salim Shaban
Salim Shaban

African Christians Organization Network  (ACON) is producing a clean, very efficient biochar producing cooking stove “The Moto Stove.”  Biochar can be used as organic fertilizer or clean burning household fuel, which improves health conditions for women and children by reducing indoor air pollution and the work and cost of gathering fuelwood. ACON makes biochar from an invasive plant, the Water Hyacinth, removed from Africa’s Great Lake Victoria. ACON uses a financial model that provides employment, dignity, improved health, energy, and food security for rural people in Kenya.

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