Could a Thermoelectric Wood Stove Pay for Itself?

By Ken Adler, AGH Senior Technical Advisor

Payback calculations are common in the residential solar photovoltaic industry where homeowners want to know how long it will take for them to recoup their initial investment. If you purchase panels outright, payback periods depend on a variety of factors including a utility’s price for electricity, tax incentives, and amount of daily sunlight hours. A range of 5 to 8 years is possible however, it can be as wide as 3 to 15 years.[1]

Answering the payback question for thermoelectric wood stoves is one of the objectives for the 2018 Wood Stove Design Challenge. In the meantime, there are several ways to begin answering this question with information already available. It is also useful to look at how use of a thermoelectric wood stove in combination with another energy-saving system, i.e., solar, could prove beneficial to the homeowner and thus both industries as well. For example, in northern states and Canada, a thermoelectric wood stove could reduce the number of residential panels needed and thereby save the homeowner thousands of dollars in panel costs.

Early Thoughts on Payback

The retail price of a thermoelectric module is around $57.50 for a 22-watt module, or $2.61 per watt.[2] One critical point to make here is that the power output of our 22-watt module assumes an optimal hot-side temperature of 300 C (572 F) and cool-side temperature of 30 C (86 F). This ideal temperature differential is very difficult to achieve in real world conditions, so the real-world cost per watt for thermoelectric modules will be higher. However, cost should decrease and efficiency improve with widespread adoption of thermoelectric modules, similar to what happened in the solar industry. For example, DOE estimated that the installed cost of a solar panel declined from $7.06 per watt in 2009 to $2.93 in 2016, a reduction of 60 percent.[3] If we go back to 1977, the cost of a solar panel was $77 per watt. It is not unreasonable to expect a decline for the cost of thermoelectric modules as economies of scale reduce production costs.

Of course, when a thermoelectric module is placed into a wood stove there are other associated costs. The primary cost by far is the heat exchange system. As I’ve discussed in a previous post, to generate at least 100 watts of power, it’s likely that a water-cooled heat exchange will be needed. The current retail price for a 100-watt water cooled thermoelectric generator, which includes eight thermoelectric modules, is $599, or $5.99 per watt. One question the competition will attempt to answer is how much this heat exchange will cost when it is integrated into the design of the wood stove.

Secondary cost considerations include the price of the wood stove, its installation, and fuel costs. The price for a larger size 50,000 BTU wood stove can range from $900 to over $4000, and the average consumer spends about $2,500. Since a thermoelectric wood stove would be providing both heat and electricity, it is difficult to separate out how much of the cost of the stove is for each function. The more crucial point for now is that many larger size stoves, which can generate up to 50,000 BTUs and meet the 2020 EPA NSPS standard, are available for as little as $1,300. While this does not include the cost of installation, it does suggest that the wood stove portion of the costs should not be a major obstacle.

The cost of installing a thermoelectric wood stove into a home should not necessarily be that much greater than the cost of installing a traditional wood stove. One additional cost will be attaching the power outputs from the thermoelectric wood stove to an inverter. However, if we assume that early adopters will already have or are planning to get a solar PV system (more on this below) the cost of the inverter would not be a major obstacle.

Finally, one can assume that the fuel cost for a thermoelectric wood stove is essentially zero because the wood stove is already being used to heat the home. A thermoelectric module will convert only 3 to 6 percent of the heat from a woodstove into electricity, while the remaining 94 to 97 percent passes through the module and is released as heat into the home. In other words, the module is only using a very small percentage of the heat generated by the stove to produce electricity.

Value in Combining Technologies

While more in-depth analysis is needed, it’s possible that a thermoelectric wood stove could help reduce the size and cost of solar PV systems in northern climates that have limited sunlight/solar radiation in winter. For example, a typical 5000 watt solar PV system in Vermont produces 6,280 kWh of electricity per year, while the same system produces 7,913 kWh in Los Angeles.[4] Most of this difference is due to the low winter time output in Vermont between October and February: For example, the Vermont system produces 239 kWh in December, as compared to the Los Angeles system’s 473 kWh. If the Vermont resident wanted to generate the same amount of power as in Los Angeles, they would need to increase the size of their solar PV system from 5000 watts to approximately 6300 watts. At the current cost of approximately $3.36[5] per watt installed for residential solar, this could cost the Vermont resident an additional $4,368 for additional solar panels.

Alternatively, instead of purchasing extra solar panels, the Vermont resident could invest in a thermoelectric wood stove to boost their winter time power output. As we mentioned in our previous blog, a wood stove with a 150 to 200-watt thermoelectric generator operating 16 - 20 hours per day could generate 93 to 124kWh of electricity per month, which would be a good boost to the Vermont output of 239 kWh in December. And, at 0.16 $/kWh for electricity in Vermont, the thermoelectric wood stove could save the homeowner an additional $15 to $20 per month.

While a real payback calculation for a thermoelectric wood stove will need to wait until prototypes go through more testing and we get results from the 2018 Wood Stove Design Challenge, the available information suggests thermoelectric wood stoves could help reduce the cost of residential solar installations, and potentially save homeowners thousands of dollars.


[1] http://solar-power-now.com/the-typical-solar-panel-payback-period/
[2] See our Resources page for a list of thermoelectric retailers.
[3] NREL. U.S. Solar Photovoltaic System Cost Benchmark. September 2016. In 1977, solar panels cost $77 per watt.
[4] NREL PVWatts Calculator
[5] EnergySage. Solar Marketplace Intel Report. April 2017.

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Hybrid Residential Solar and Thermoelectric Power Generation

by Ken Adler, Senior Technical Advisor at the Alliance for Green Heat

Some of you may be wondering about thermoelectric wood stoves and why we decided to include them in the 2018 Wood Stove DesignChallenge, which will be held in November 2018 on the Washington Mall.  Our goal of this competition is to support development and commercialization of a revolutionary thermoelectric wood stove that produces electricity equal to 50 percent or more of the winter time output of a residential solar photovoltaic system. By combining a thermoelectric wood stove and a residential solar PV system and home battery, like the TESLA Powerwall, we can support residential and grid-based distributive power goals, and incentivize greater investment in solar power. 

Specifically, thermoelectric wood stoves can help solve the problem of low winter time solar PV output in northern climates, where useful solar radiation is limited to 2 - 4 hours per day.

While a thermoelectric wood stove may sound revolutionary, the technology behind the stove has been used since the 1980s in oil and gas field operations, where methane gas provides a low-cost source of heat to power the thermoelectric generator. Wood stoves, like waste methane gas, can provide a free source of heat for the thermoelectric generator.

Alphabet Energy Thermoelectric Generator

Thermoelectric generators are like solar panels, however, instead of turning light into electricity they turn heat into electricity. To generate electricity, one side of a thermoelectric module is heated by the wood stove while the other side is cooled with either an air or water-cooled heat sink. For applications above 100-watts, water-cooled heat sinks are the most common approach because of their ability to extract greater amounts of heat from the thermoelectric module.

60-Watt Water Cooled Thermoelectric Generator

In northern climates like New England, Canada and northern Europe, low winter time solar radiation increases the cost and reduces the efficiency of solar PV systems, and the cost-effectiveness of battery storage systems like the Tesla Powerwall.  According to NREL, solar radiation in northern areas like Vermont peaks at 6.0kWh/m2 in June and declines to 1.7kWh/m2 in December. This means that an average 4,000-watt residential solar system will go from producing 571kWh in June to 191kWh in December--a 66% reduction is solar power output.  This project will demonstrate how a thermoelectric wood stove can cost-effectively supplement a solar PV system.

Building on our experience from 3 previous Design Challenges, we will work with wood stove manufacturers, universities and others to build and test 100 to 200-watt thermoelectric wood stoves that could effectively increase by 50% the winter time output of a 4,000-watt residential solar PV system.   

Thermoelectric generators are currently sold as accessories for wood stoves; however, these accessories are limited in size and efficiency. By integrating a thermoelectric generator into a wood stove we can achieve far greater power output, efficiency, and lower cost. For example, a wood stove with a 150 to 200-watt thermoelectric generator operating 20 hours per day could generate 93 to 124kWh of electricity per month, which compares favorably with the December solar PV output of 191kWh in Vermont.

Russian Thermoelectric Wood Stove 

(not certified for sale in the U.S.)

There are several reasons why now is the time to consider thermoelectric wood stoves. First, the price of the thermoelectric modules, which are a component of the TEG, has dropped substantially because they are now being mass produced in China.[1]  Second, the EPA’s recent wood stove NSPS regulation is helping to make new wood stoves cleaner and more efficient and, coupled with cordwood testing and automated features, a new generation of cleaner stoves could also generate electricity. Third, thermoelectric wood stoves can produce electricity up to 24 hours per day eliminating load management concerns common with solar and wind power. Lastly, the stoves are powered by local wood supplies, making their fuel low carbon and locally sourced.

The 2018 competition on the Mall will demonstrate the role thermoelectric wood stoves can play in promoting solar power, energy storage systems and biomass energy, while also reducing energy costs, supporting climate change goals, and increasing distributive power.   

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[1] The cost of a thermoelectric module has fallen below $2 per watt (uninstalled), compared with $3.50 per watt for solar panels (installed).

A reader responds to the wood vs. coal debate

In our October newsletter, we reprinted a column by Cory Hatch from the Jackson Hole News & Guide.  We found it to be a thoughtful piece by a person grappling with energy choices and trying to fairly weigh their costs and benefits.  We got a lot of positive feedback about the article but one reader, a former professor of thermodynamics thought it was important enough to clarify something the author said.

The column made many good points, and concluded:

"After you factor in the energy and associated emissions it takes to extract, refine and transport fossil fuels, wood starts to look pretty good again. If you harvest trees close to home, firewood is fairly efficient and renewable, even if storing that carbon again takes some time. Unfortunately, woodstoves have local impacts, too. Chimney smoke contains particulates, nitrogen oxides and other gasses that can degrade air quality and cause health problems, especially for people with respiratory or cardiovascular disease."

The author also noted: "according to the Sierra Club, burning coal for electricity is only about 35 percent efficient, whereas a modern wood stove is about 75 percent."

Professor Gael Ulrich took issue with that:

" It is true that wood is superior in that it is renewable and not a fossil fuel, and it does not contain sulfur or other elements that can be problems in coal stack emissions.  But his statement, attributed to the Sierra Club, implying that wood combustion is twice as efficient as coal combustion is incorrect.  The distinction is subtle and not appreciated by someone without a background in thermodynamics, but I will try to make the reason clear.  

Converting fuel energy to heat and then to electricity can never be done with 100% efficiency.  Even the most modern efficient power plants seldom succeed 40%.  Historically, that principle was elucidated by Carnot, and the theoretical maximum possible is known as the Carnot efficiency.  In short, electricity or "stored work” is a higher form of energy than heat.  Converting “heat" to “work” always represents an energy loss manifest as “waste heat.” 

Coal power plants probably don’t do much better that 35% as suggested, but wood-fired electricgenerators are even less efficient (for reasons that would require more explanation).  Converting the energy content of coal to heat can, on the other hand, be done with high efficiency, matching or exceeding that of biomass combustors.  

Thus, the statement as worded in Hatch’s column is misleading.  His arguments regarding global warming, etc. are ok.  

I thought you might be interested in the error in case someone else has not already called it to your attention."

Periodically we publish substantive comments such as this one on our blog. Scores of shorter comments are left on our Facebook site.  If you have a longer substantive comment, we can't promise to publish it but please don't hesitate to send to info@forgreenheat.org.

Pellet Boiler Also Makes Enough Electricity to Power Home

Austrian company OkoFEN has brought to market the first residential electricity producing pellet boiler, the Pellematic Smart_e. The boiler uses a pellet-condensing module from an earlier pellet boiler design, the Pellematic Smart and outfitted it with with a Microgen Stirling Engine. The engine uses a heated/cooled helium hydraulic system. The result of these two technologies is a unique boiler that is capable of outputting 14 kW (47,770 Btu/h) of thermal energy and 1 kW of electricity.

The Pellematic Smart_e can save households’ money on both heat and electricity bills. Switching from oil, propane, or electricity to pellets usually results in substantial savings. The heat is used for space heating of the living area and domestic hot water generation. Plus with the added Stirling Engine, the electricity you produce in your own house can offset part of a home’s monthly electricity bill. A full load of 24 hours can produce 24 kWh of electricity at 1kW.

At 24 kWh per day, the Pellematic Smart_e could generate enough electricity for most or all of a home’s average daily electricity needs. However, in order to create this electricity, the boiler must run at full capacity, which can create a substantial amount of excess heat. The surplus heat may be stored while the electricity is still produced but even with the heat storage capabilities, some of the heat may be lost, and the efficiency of the boiler falls. For the Pellematic Smart_e to meet the electricity needs of a house, it would have to extend far beyond the heating needs of that house.

“The issue is the large 14:1 ratio between thermal and electricity on this unit, and the (likely) narrow output curve,” explains Norbert Senf, one of the judges for the Wood Stove Design Challenge, “In a thermal electricity generating station, you get about a 2.5:1 ratio.”

Although it is perhaps not the most efficient when it comes to producing electricity, OkoFEN’s innovative boiler demonstrates the potential of this hybrid technology. The success of this product is likely to lead to further industry advancements to manufacture and distribute pellet boilers capable of producing even greater amounts of electricity.

For more information check out OkoFEN’s website here

Also, their FAQ page here

Do Electric Rates Impact Wood Heat?

Electric heat is clean in the home,
but not when its made and line
losses are significant.

When the price of oil goes up, it goes up for everyone.  But electricity rates vary widely by state with residents of some states paying double what they pay they other states.  New York and Connecticut saddled with the highest electricity rates in the country, which may have contributed to the rapid rise of wood heating in those states.  Conversely, in the south where electricity rates are the cheapest, the use of wood heat decreased in most states between 2000 and 2010. 

Electric heating has surged in recent years – though not as fast as wood heat – in part because of more efficient heat pumps.  Residential geothermal heat is slowing gaining traction but requires a lot of electricity to pump the heat from the earth through the home, leading some experts to regard it simply as very efficient electric heat, not as a renewable. (Electricity for geothermal heat costs an average of $10.43 per million Btus compared to $12.63 per million Btus if you purchase cordwood.)

The economic disadvantages of fuel oil and propane as heating fuels are often discussed, but electricity is a more complex story.  More than a third of American homes use electricity as the primary source of heat (US Census) and another 24% use it as a secondary heat source (EIA).  Granted many of them are in the south, where the heating load is smaller but more northern states can have surprisingly high rates of electric heating.  For example, 15% of homes in Connecticut, where electricity is second in price only to Hawaii, primarily use this form of heating (US Census).  

An electric boiler costs an estimated $35.05 per million Btu, according to EIA, and an electric space heater, a common appliance used for secondary heating, costs an estimated $34.32 per million Btu.  An EPA certified wood stove running at 72% efficiency, in contrast, is estimated to cost only $12.63 per million Btu.       

These EIA calculations assume that electricity costs only 11 cents per kWh.  However, half of the U.S. averages more than that amount.  In northeastern states such as New York and Connecticut where electricity costs over 17 cents, the cost of using an electric heater can be as high as $56.42 per million Btu.  This may make consumers think twice before buying an electrical space heater instead of a wood stove as a secondary space heater. 

Map of average U.S. electricity prices by the Alliance for Green Heat. Click to enlarge.

Consumer rates in Alaska, California, Maryland, New Jersey and Washington DC are on the high side, ranging from 14 - 17 cents per kWh (an average of $41 - $50 per Btu for electric heating).  The majority of homeowners in the south, west and midwest pay 9 - 13 cents per kWh for their electricity, which works out to about $26 - $38 per Btu.  That is a much cheaper rate than some states, but still twice the cost of heating with wood.

Pacific northwestern states with a strong hydro-electric presence and Appalachian states with an abundance of coal typically average electricity costs below 9 cents per kWh.  Idaho has the cheapest average electricity rate of any state at 7.99 cents per kWh, thanks to low demand and a large number of dams on the Snake River.    

To compare heating fuel costs in your home, download the EIA’s comparison calculator here: www.eia.gov/neic/experts/heatcalc.xls  

Residential Heating Fuels Show Diverse Growth Patterns

Wood was the fastest growing heating fuel nationally between 2000 and 2010, and in 25 states. But in some regions, electricity, natural gas, propane and even oil are experiencing rapid growth. Wood grew the fastest (+34.6%), followed by electricity (+26.8%) and natural gas (+4.9%), and both propane (-16%) and oil (-21.9%) experienced significant declines. But regional differences abound.

In decline just about everywhere else, the South was the only region to have seen substantial gains in residential oil use. Texas (84.6%) had the greatest increase in oil use of any state by far, with Arkansas (36.75) and Oklahoma (35.7%) rounding out the top three. In both Texas and Oklahoma, oil grew the fastest of any fuel source. In Arkansas, it finished second to electricity (48.2%).

The Northeastern United States experienced some of the biggest shifts in natural gas and propane use over the past ten years. Maine (44.8%), New Hampshire (39.4%) and Connecticut (27.1%) currently lead the U.S. in residential propane growth, and Vermont also ranks second among the states where natural gas is rising the fastest. In each of these New England states, however, wood still remains the fastest growing source of residential heating fuel.

Other than Vermont, gas heating rose the most in Nevada (51%) and Idaho (41.1%). Propane use saw large increases in Pennsylvania (22.2%) and Washington (21.1%), in addition to the aforementioned New England states.

Unlike wood and propane, the large increase of electricity in the U.S. is not confined to any particular geographic region. The states with the three biggest increases were Georgia (54.7%), Iowa (49.5%) and South Dakota (49.2%).

Wood heat use grew fastest in the Northeast and Great Lake States, and fell in most of the south. If history is any lesson, the South may rise again, and heating demographics will continue to provide a fascinating and often surprising growth trends.

Changes in rank

In addition to regional growth rates, state ranking of primary heating fuel use is another lens through which we can understand the growth of wood heat. In 2000, wood was among top the top four heating fuels in 26 states, exceeding at least propane or oil. In 2010, wood was among the top four fuels in 33 states.

In 2000, wood was the third most common heating fuel in two states (Oregon and Idaho), exceeding propane and oil. In 2010, it was the third most common fuel in five states (Idaho, Oregon, Vermont, Washington and West Virginia).

Finally, in 2000 and 2010, wood was the second most common heating fuel in one state (Maine) after heating oil.

Wood is not the most common heating fuel in any state, and it is unlikely that it ever will be – or ever should be.

AGH Board Members Detail Home’s Carbon Footprint

To understand carbon impacts of residential heat, it helps to get get personal. Three Alliance for Green Heat (AGH) board members and one AGH supporter - Jon Stimling, John Ackerly, Josh Elmore and Peter Caldwell - calculated the carbon footprint of their homes in order to better understand home heating carbon impacts. Each household employs at least one form of renewable energy. The four individuals emitted between .92 and 3.8 metric tons of carbon per year to heat their homes, which is well below the national average and even further below their respective state averages. 

Postscript: Ten years later, one Board member is nearly fossil fuel free in his house and cars.  And it was easier than many people may expect.

The lower-than-average emissions are due in part to the fact that each household uses wood or pellet stoves to provide some portion of their heating needs. The calculations were reached using the carbon calculator website; www.carbonfootprint.com, a well regarded program that assigns .18 metric tons of carbon for a ton of wood pellets or a cord of wood. The average American home produces about 12 tons of carbon a year, according to most authorities and 40 to 60% of that on average is used for heating.

The EPA estimates approximately 4 metric tons of carbon dioxide (CO2) per person per year (about 17% of total U.S. emissions) are emitted from people's homes. The individuals featured here all have household footprints of 3 to 7 metric tons, but if calculated using the EPA per capita average, these homes would emit 8 to 20 tons metric tons. The home with the lowest carbon impact from heat has a pellet furnace that can meet 100% of the home’s heating needs. One Board member has solar panels, which covers 100% of his electric load and another buys 100% wind power. Some variance in CO2 output can be attributed to how electricity is generated in each household’s particular state.

House #1: 
A 1,900 square foot home occupied by four people in Maryland has a total average output of 6.95 metric tons of CO2 a year. After purchasing 100% wind power, the footprint drops to 2.85. Natural gas, used as a backup heating fuel, is the largest contributor, totaling 2.49 metric tons of CO2 a year. The combustion of 2 cords of wood in an EPA certified LOPI Patriot circa 1995 accounts for the other .37 metric tons. The wood is obtained from the urban wood waste stream through local tree services working in the neighborhood.

Footprint	Metric tons of C02
Total Household	2.85
Total W/ Electricity	6.95
Total Heating Only	2.23
Natural Gas	2.49
Wood Cords	.37
Size of House	1900 Sq/ft
Number of Occupants	4

House #2:

A 1,800 square foot home occupied by five people in Colorado. This household emits 3.08 metric tons of CO2 a year on average. The largest contributor to this impact is due to the reliance on propane as a back up heating fuel. It is 2.90 metric tons of the total emissions. The remaining .18 metric tons of CO2 is attributed to the single cord of wood burned per year in an EPA certified 2002 Jotul. The wood is either self harvested or purchased from people hired to thin local forest for fire suppression. 100 percent of electricity is from solar panels installed near the house.

Footprint	Metric tons of C02
Total Household	3.08
Total Heating Only	2.36
Propane	2.90
Wood Cords	.18
Size of House	1800 Sq/ft
Number of Occupants	5

House #3:

A 2,650 square foot home in upstate New York occupied by two people. The household’s total CO2 footprint is 7.59 metric tons a year. This result is a combination of the home’s electricity consumption (9,733 kwh/yr), which contributes 3.79 metric tons of CO2 to the total, and heating oil (back-up fuel) which is responsible for 2.52 metric tons of CO2 a year. Finally, the combustion of 7 cords of wood a year in a Vermont Castings stove results in 1.29 metric tons of CO2 being released. The wood is self harvested and split by hand.

Footprint	Metric tons of C02
Total Household	7.59
Total Heating Only	3.81
Heating Oil	2.52
Wood Cords	1.29
Size of House	2650 Sq/ft
Number of Occupants	2

House #4:

A four person 2,500 square foot household in New Hampshire emits 7.02 metric tons of CO2 a year. The largest contributor to this total was electricity consumption (15,000 kwh/yr on average), which resulted in 4.65 metric tons of CO2 being released. Propane for cooking and hot water contributed 1.45 metric tons to the total. With the remaining .92 metric tons released through the combustion of 5 tons of wood pellets in a Harman PF100 furnace each year.

Footprint	Metric tons of C02
Total Household	7.02
Total Heating Only	.92
Propane	1.45
Wood Pellets	.92
Size of House	2500 Sq/ft
Number of Occupants	4

These four households demonstrate the wide array of energy saving measures and methods to offset CO2. There is no blanket, cure-all strategy to reduce fossil fuel consumption, however this group shows how relatively small measures like running a wood stove can help to decrease heavy CO2 emissions. Everyone in the thermal biomass or energy efficiency sector should consider doing their household carbon footprint as well as getting a home energy audit to get a more tangible understanding of these issues. Understanding your carbon footprint in comparison with other Americans is also useful. Finally, understanding the potential of strategies to offset CO2 from fossil fuels, like using a modern wood or pellet stove in your home, is the building block for appreciating how we as a society can intelligently leverage this technology.