Thermoelectric Wood Stoves, Solar Power, and the 2018 Wood Stove Design Challenge

By Ken Adler, Program Director for Thermoelectrics


Could the marriage of residential solar photovoltaic and thermoelectric wood stoves soon be a reality? The results of our 2018 Wood Stove Design Challenge suggests that it just might be, and a productive marriage at that! The Challenge was held on the National Mall in Washington, D.C., in a large tent, from November 8-12.

The U.S. Department of Energy (DOE) Bioenergy Energy Technology Office (BETO) helped fund the Challenge. In its press release announcing the Challenge, BETO referenced the Alliance for Green Heat’s stretch goal that integrating thermoelectric wood stoves with solar PV systems could increase a home’s wintertime power by 50 percent in northern climates, and provide a renewable energy alternative to oil and natural gas heating systems. This year’s winning thermoelectric stoves generated enough power to demonstrate that we are well on our way to meeting this goal.  

Short winter days, clouds, and snow are the Achilles heel for solar power in northern climates. However, solar power is critical to reducing CO2 pollution and climate change. In Vermont, a home may get only 2-3 hours of effective sunlight in December versus 6-7 hours in July. Here’s where thermoelectric wood stoves come in: Residents typically operate their wood stoves during the morning and evenings, when solar PV power is at its lowest.

Winning Stoves Generate Excitement

The Alliance’s Organizing Committee invited five teams from industry and academia to participate in the thermoelectric stove competition of the 2018 Challenge.  An independent panel of judges evaluated each stove and selected the first and second prize winners.  

During the 3-hour test period, the judges scored the stoves based on power output, particulate matter and carbon monoxide emissions, safety, degree of automation, and efficiency. (We’ll review the stoves’ emissions results in a later blog post.) After 30 minutes to warm up, electric power output was measured every ten minutes over the remaining 2½ hours. Table 1 provides a summary of the results.

Table 1. Summary of Thermoelectric Power Results

Stove Name	Maximum Power (Watts)	Total Energy in 2.5 hours (Watt-Hours)
E-Stove	268	276
Wiseway	139	235
Kd3	85	118
Englander	32	47
ASAT	14	28

Figure 1. E-Stove

First prize went to E-Stove by Wittus and HE Energy. The stove generated a maximum of 268 watts, an average of 161 watts, and a total of 276 watt-hours over 2.5 hours[1]. The E-stove, manufactured in Germany, is designed to be installed in a living room. It works like a hydronic boiler and generates enough hot water to heat a medium size or large home. For the test, the hot water was pumped through two radiator systems, which discharged the heat into the tent. The power control system can be connected directly to a battery, like a TESLA Powerwall, or supply a 220-volt output (110 volt for U.S. market). HE Energy expects the stove to be commercially available in fall 2019.

Figure 2. Wiseway Stove

Second prize winner, Wiseway by Vulcan Energy and Hi-Z Technology, generated a maximum of 139 watts, an average of 123 watts, and a total of 235 watt-hours over 2.5 hours. The average power output of the Wiseway wasn’t substantially less than the E-Stove, even though the maximum wattage was almost 50 percent less. The Wiseway combustion temperature was kept even by a gravity fed pellet system that continuously fed pellets into the combustion chamber.  In contrast, power output from the E-Stove dropped from a high of 268 watts to a low of 38 watts as the logs burned down.

The Wiseway uses a water-cooled thermoelectric generator (TEG) to produce electricity. The hot water from the TEG was pumped into a hot water tank and through a radiator to discharge heat. The TEG has a power control center that includes a USB port, and a 12-volt and 110-volt output. Vulcan Energy designed the Wiseway to demonstrate the company’s capabilities to custom design thermoelectric generators for pellet stoves. It has no immediate plans to commercialize this stove.

Figure 3. Kd3 Stove and Power Control Center

The Kd3 by Unforgettable Fire generated a maximum of 85 watts, an average of 49 watts, and a total of 118 watt-hours over 2.5 hours. The Kd3 is a downdraft stove designed to generate electricity, and heat water like a hydronic boiler for radiators and domestic hot water. It uses two thermoelectric generators by FireVolt to generate electricity for lighting a home or charging a battery. It includes a power control center to manage power output from additional power sources like solar or wind generation. Due to limitations of our DC load meter, the judges and Kd3 team faced several challenges to accurately measure the power output of the FireVolt thermoelectric generators. We conducted a third unofficial power test, which demonstrated the Kd3 could generate 127 watts of power. Unforgettable Fire expects to have the Kd5 commercially available in 2019.

Figure 4. Englander Stove

A team of engineering students from George Washington University retrofitted a thermoelectric generator to work with a stove donated by England’s Stove Works. It generated a maximum of 32 watts, an average of 22 watts, and a total of 47 watt-hours over 2.5 hours. The team used a TegMart thermoelectric generator, which has a power control center with a dedicated power output for a water pump, USB port, and 12-volt output for charging a battery. The event provided an excellent opportunity to introduce GW engineering students to wood stoves and thermoelectric principles.

Figure 5. Downdraft Rocket

The Downdraft Rocket by ASAT uses a low-cost thermoelectric module to generate a maximum of 14 watts, an average of 12 watts, and a total of 28 watt-hours over 2.5 hours. ASAT was not attempting to design a thermoelectric stove that maximized electric power output. Instead, their stove was designed for use in the developing world to cook food and generate enough electricity to power LED lights and/or charge a cell phone. The combustion chamber, catalytic converter, and chimney have low emissions to protect the health of users. Its passive air-cooled system requires less maintenance then a water-cooled system.

Comparing Solar and Thermoelectric the Output

When compared with daily wintertime solar power output, the E-Stove demonstrated that a thermoelectric stove could potentially generate an energy output equal to 50 percent of a 5kW residential solar PV system. The graph below shows the large variation in daily power output of a 5kW solar PV system during January in Burlington, Vermont.[2] For 15 days in January, the solar PV system generates less than 4,000 watt-hours of energy per day, due to limited daylight, clouds, and snow. If the E-Stove was operated for 20 hours per day, it could generate 2,208 watt-hours of energy, which equals 52 percent of the solar energy produced during these 15 days. However, for the entire month, the E-Stove would increase total energy output by only 38 percent.[3] 

Maine Energy System demonstrating their
Okofen pellet boiler with a Stirling engine.

Pellet boilers with Sterling engines could also supplement solar power. Maine Energy Systems demonstrated two such units at the Challenge, though the units weren’t eligible for a prize. The larger system for commercial buildings can produce 50-60kW of heat and 4-5 kW of electrical power. The smaller system, which will not be available in the United States until 2019, produces 9-16 kW of heat and 0.6-1.0 kW of electrical power. This type of system could solve the Achilles heel problem faced by solar power during the wintertime. The commercial system, which operated every day at the Challenge, was surprisingly quiet. The Sterling Engines will be expensive until larger scale production can reduce their price. 

Hurdles to Overcome

While thermoelectric stoves and Sterling Engines look promising, none of the thermoelectric stoves have been commercialized, they will be very expensive until they’re mass produced, and their installation and maintenance require an expertise in wood heat, plumbing, and electricity. These are daunting challenges, but then so is climate change. The 2018 Wood Stove Design Challenge is over, but the future of thermoelectric wood stoves is just beginning.

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[1] The power output of the E-Stove’s thermoelectric modules exceeded the 150-watt capacity of the Alliance’s DC load meter, so we used Team Wittus’ DC load meter.

[2] Based on DOE’s PVWatts Calculator.

[3] The Alliance for Green Heat and the Vermont Biomass Energy Resource Center has developed a proposal to provide a much more robust analysis of the potential role of thermoelectric stoves.

Thermoelectric Wood Stove, Solar Power, and a Floating Cabin!

Guest blog post, by Margy Lutz

Finally this winter, our thermoelectric wood stove generator is fully operational. Following our test runs, we placed the pump to recycle cold water down in the lake water under the cabin. In winter, it gets about 5 degrees C (41 F). That's plenty cold for a good differential between the 300 degrees C on the hot side.

Most system owners don't live in a float cabin four feet with a constant cold water source under the floor. The typical user has to use a recycled liquid (usually including a water/antifreeze mixture) that runs through a radiator for cooling.

In addition, a charge regulator/controller is used to protect the batteries and prevent overcharging. The model that came with our system has lights to let you know the status of the charging process.

Wayne likes to know more about the charge we are getting. He installed an ammeter and a volt meter. The switch in the middle controls the water pump down below the cabin. To maximize the charge to our cabin battery bank, we've installed a separate solar panel and two six volt batteries wired in a series to run the pump.

Living off the grid has its challenges, but having an alternative power sources has made our winters much brighter (pun intended). Do you generate power? What are some of the solutions that have worked for you? -- Margy

Postscript by Ken Adler, AGH Technical Advisor: 

Congratulations to Wayne and Margy on their thermoelectric wood stove. In a follow-up communication with Wayne, he reported that they are no longer using the system because the thermoelectric modules failed. Wayne doesn’t know why they failed, however, the most common reason for failure is overheating.  The modules can also fail if Bellville washers are not used to allow the module to expand and contract during heating and cooling. Wayne also reports,

Even when I was partially (marginally) operational, I produced less than 2 amps at 12V DC (23 watts) to recharge my cabin battery bank. This would have been enough to put a top-off charge on my cabin batteries (normally recharged via my solar system), particularly valuable in the winter when solar power is minimal and my wood stove is operating nearly 24-7. The primary reason for the low amperage was the need for a 1,8 amp 12V (21.6 watts) water pump to feed the cold side of the modules. In many ways, I reside in the perfect test location for this thermoelectric system, since 

I have a nearly infinite supply of very cold water 4 feet below my wood stove. I live in a floating cabin on Powell Lake BC, and the lake is extremely deep and very cold in all seasons. What an opportunity to serve as a source of cold water through the cooling system! The pump only needed to pump the cold water up 4 feet and then outflow back into the lake. Even with this tremendous advantage, I couldn’t get everything fully operational.

Does this make me a non-believer in thermoelectric from a wood stove? Absolutely not — I still believe this is an important future source of electrical power in my cabin, since even a top-off voltage during the solar-depraved Canadian winter would be worth the price. I’d be one of the first in line if a recreational property thermoelectric system was available, and I’d be quick to try again. Thus, I wish you all of the best with your preparation for the 2018 conference. I’ll be following the results closely.

In an earlier post, Wayne reports that he is using three 25 watt thermoelectric generators for a total rated power of 75 watts of output, however, he’s only getting 23 watts of power for his battery. Part of this is due to his pump, which is drawing almost 22 watts of power. If you are interested in building your own thermoelectric wood stove, there are a few improvements that you may want to consider. First, TEG suppliers (see our resources page) now sell more efficient lower wattage pumps. Second, consider starting with a thermoelectric generator rated for 100 to 200-watts. While this is more expensive, if you go with a smaller system much of your power will be consumed by the pumps and/or fans you need to cool the modules. Third, Bellville washers are critical for allowing the modules to expand and contract.

If you are interested in designing a thermoelectric wood stove for our 2018 Wood Stove Design Challenge, please visit our web site for more information. For more information on Wayne and Margy’s life on a floating cabin, please visit their blog at Powell River Books Blog.

For an overview of the potential of thermoelectric wood stoves, click here.

Fourth Wood Stove Competition to focus on automation and electricity generation

Today, the Alliance for Green Heat announced the fourth Wood Stove Design Challenge, returning to

Wittus, the winning team in the 2018
Wood Stove Design Challenge

the National Mall in Washington, DC in November 2018.

[For winners and results of the event, click here.]

The 2018 event will be free and open to the public and includes rigorous testing of the next generation of technology that can make wood stoves consistently cleaner, more efficient, easier to use and, like solar energy, a renewable source of electricity.

The fourth Wood Stove Design Challenge is modeled after the Department of Energy’s (DOE’s) Solar Decathlon, a competition between teams from universities worldwide to design more efficient and cheaper residential solar power.  Like the Solar Decathlon, the Wood Stove Challenge also attracts teams from around the world and focuses energy and resources on innovation and improved performance.  The stove competitions have been in partnership with the DOE Brookhaven National Lab, the New York State Energy Research and Development Administration (NYSERDA), the US Forest Service and others, the Osprey Foundation, among others. 

Participants will compete in two events:  One is to automate the wood stove with 21^st century technology like sensors and WIFI-enabled controls that improve combustion efficiency, reduce air pollution and improve ease of use.   The second competition will focus on thermoelectric wood stoves that generate electricity to power lights, cell phones, and WIFI-enabled controls. Thermoelectric generators are similar to solar PV systems except they turn heat instead of light into electricity.  When integrated with a residential solar PV system, a thermoelectric wood stove and battery power system, like the TESLA Powerwall, could effectively double the wintertime output of solar PV system in areas like northern United States, Canada and northern Europe.

Wood stoves are still used by 30 – 60% of homes in hundreds of rural and suburban counties around the country.  Yet, the technology revolution that has swept household appliances in the last 20 years has by-passed wood stove technology. 

Teams in the 2018 stove challenge will be competing for up to $50,000 in prizes.  The teams and exhibitors will also have a chance to showcase new technology on the National Mall just blocks away from the Department of Energy, the US Department of Agriculture and the Environmental Protection Agency. 

“This is a chance for students, back yard inventors, and wood stove manufacturers to re-invent this age-old technology for today’s environmentally conscious and time-conscious consumer,” said John Ackerly, founder of the Wood Stove Competition and President of the Alliance for Green Heat.  “An affordable, smart wood stove is achievable and could help millions of families reduce their reliance on gas and oil while significantly reducing  pollution,” Ackerly added.

“This is the first Wood Stove Challenge to promote wood stoves that generate electricity to power everything from a cell phone to an entire home.  Thermoelectric wood stoves, when integrated with solar PV systems and home batteries like the TESLA Powerwall, have the potential to make solar energy more affordable, reduce air pollution, and pave the way for a more sustainable energy future, “according to Ken Adler, Senior Technology Advisor at the Alliance for Green Heat and formerly with the U.S. EPA.

Previous Stove Design Challenges brought innovative stoves and a diverse array of stove and energy experts together on the National Mall in 2013, Brookhaven National Lab in 2014 and 2016.

Further details about participating and competing in this competition will be available late March, 2017. For more information about the  2018 competition, contact John Ackerly at jackery@forgreenheat.org. 

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The Alliance for Green Heat promotes modern wood and pellet heating systems as a low-carbon, sustainable and affordable energy solution. The Alliance works to advance cleaner and more efficient residential heating technology and hosts international stove design competitions to accelerate innovative stove technology.  Founded in Maryland in 2009, the Alliance is an independent non-profit organization and is tax-exempt under section 501c3 of the tax code.