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.

Meet the teams: Backed by the EPA and DOE, top cookstove designers tackle the heatstove

This post is the sixth in a series introducing the 12 teams participating in the 2018 Wood Stove Design Challenge in November.

By John Ackerly, Ken Adler, and Shoshana Rybeck, Alliance for Green Heat

Cook stoves and international development 

Dean Still (left) with participants at an
Ethos conference near Seattle.

Dean Still leads the team at the Advanced Studies in Appropriate Technology Lab (ASAT) and has been involved with the international cook stove community for decades. He and his team have achieved many key breakthroughs in testing, stove design, instrumentation, and like most researchers and developers, they have had their share of dead ends, disappointments and flops.  

The New Yorker magazine featured Dean in a 2009 article about engineers who need to design stoves that cost no more than $10 so even the world’s poorest people can afford them. So, when we told Dean that cheap stoves by our standards were between $500 and $1,000, he laughed and said, “you would think that should be easy.”

ASAT won a $300,000 grant from the EPA to build a heat stove that makes electricity, thirty times more than the $10,000 that each team is getting from DOE through the Alliance for Green Heat. ASAT has another advantage that only a few teams have in the competition: they operate a testing lab so they can measure changes in emissions and efficiency with each iteration of the prototype.

ASAT's sister group, Aprovecho hosts
annual "stove camps" where innovators
work together to built and test designs

The challenge for ASAT is whether they translate their experience and skill making cookstoves into a successful heating stove that performs well in the lab and in the field. There is a tight timeline and ASAT is new to the way the EPA measures PM and efficiency for heat stoves—and the way that tests will be conducted at the Stove Challenge. Translating lab test protocols to real world emissions may be even tougher for cook stoves and oversight of cook stove labs is weak compared to EPA-approved heat stove labs. But the cook stove community is far more transparent, and test results and technologies are more freely shared in the common battle of a global enemy—daily exposure to wood smoke from traditional cooking practices.

ASAT’s strategy

Dean Still with Prince Charles, who
has been an advocate for the modern,
cleaner cookstoves

ASAT’s stove is unique from the start—it’s a rocket stove.  Rocket stoves are most common as a developing world cook stove where you feed small diameter fuel into an insulated chamber and let it burn the ends of the sticks. Rocket stoves designed for heating are usually called “rocket mass stoves” but the prototype coming to the competition in November 2018 will be a new, high tech, low mass hybrid of the Rocket stove.  

ASAT is using an expansive strategy and are integrating many innovative technologies, from fabric filters to laser smoke sensors that automatically adjust secondary air.  By keeping an open mind, the ASAT team is winnowing down technologies, but the danger is that they are doing this on a very short time-line and the prototype is still going through major changes with less than 6 months prior to the competition.

ASAT's lab in Cottage Grove, OR

ASAT is working to make a stove mostly for the Asian market that can benefit those who struggle to get heat and electricity independently with ultra-low emission rates. The stove will include an electrostatic filter for PM to capture escaping emissions. Dean says his stove will have “jets of pre-heated secondary air that create a zone of mixing like in the carburetor of a car where a combination of high temperatures, residence time and complete mixing result in close to complete combustion. The fan speed, the amount of smoke, CO, CO2, and temperature in stove will be shown on a LTD screen. A PM sensor in the chimney automatically adjusts the velocity of the secondary air jets.”

Thermoelectric technology 

With help from the EPA, ASAT is developing a stove that can be used to cook food, heat the home and create electricity mainly for rural households in Asia that have intermittent or no electricity. To do this, electronic designer and manufacturer Karl Walter and lab manager Sam Bentson have created a new type of TEG that fits into the evolving design. The team is currently working on the fifth iteration and are focusing on how to create reliable electricity in the most inexpensive way possible. 

Sam Bentson, ASAT Lab Manager

The team has been building their own TEG that is making electricity without the use of a water cooling system. Instead, their TEG uses a large radiator that is essentially an aluminum fin designed to move the most air at the lowest wattage possible. With the team solely using a radiator and not including a water cooled system, or even a fan, the model is far less expensive, which appeals to ASAT's dedication to making this as affordable as they can. The team hopes to develop a TEG for less than $100 that creates at least 15 watts of power, using a switch mode voltage regulator and usb connection. Karl Walter says, “Water cooling is great but there is also something to be valued in a simpler, robust approach.”

Creating an affordable cook stove with an attached TEG that does not have “moving parts” is no easy task. Thankfully, with the financial support of the DOE and EPA, ASAT has been able to fully delve into this project and work through the challenges they have faced. Karl says that for their team the largest challenges had to do with temperature differentials and power outputs. He says that finding a “realistic power output was difficult” as well as figuring out “what happens when you overheat the stove.” But, the team is on a two year contract for their EPA project and with a year remaining, they are optimistic about the future development of their product and plan to have a model prototype ready for market testing within the next 4 months.

ASAT follows the old adage to think globally and act locally. Their efforts to create stoves that benefit people around the world have had tremendous support from national and regional organizations and they hope to expose the powerful technology they’ve been working on to a larger audience at the challenge in November. 

Contact the team at:

Dean Still
managerstovetec@gmail.com

Sam Bentson
managerstovetec@gmail.com

Karl Walter
karl@waltech.com

Automated Wood Stoves Line Up to Prove their Value

Seven companies with automated wood stoves have been selected to be part of a collaborative workshop at Brookhaven National Lab this November. The goal of the workshop is to explore how automation can be both affordable and effective.

See photos, diagrams and short descriptions of each of the 7 stoves here.

Stove emissions are the result of three equally important factors: the stove, the operator and the fuel.  EPA testing and the NSPS focuses mainly on the stove, but excessive emissions also result from poor operation and unseasoned fuel. Automated stoves can tackle all three of these factors, resulting in emission improvements beyond what the EPA can currently assess.

The seven automated stoves are made by ClearStak, Kleiss Engineering, MF Fire, Flamekeeper, Wittus, Hwam and Aprovecho Research Lab. Some need an electric outlet, some make electricity, some use oxygen sensors and all are unique and innovative.

“This workshop is designed to enable participants to study affordable automation techniques and see which work best,” said John Ackerly, President of the Alliance for Green Heat. “If the EPA settles on a number at 2.5 grams or less in the NSPS, these technologies may also offer affordable and reliable solutions,” Ackerly added.

The Organizing Committee includes John Ackerly-AGH, Ellen Burkhard-NYSERDA, Tom Butcher-BNL, Prof. Phil Hopke - Clarkson University, Craig Issod-founder of Hearth.com, Mark Knaebe-Forest Service, Ben Myren-Myren Consulting, Rob Rizzo- Mass. Dept. of Energy, Norbert Senf - Masonry Heater Association, Dean Still-Aprovecho, Rod Tinnemore-WA Dept. of Ecology and Rebecca Trojanowski-BNL.

Because of space limitations, only about 50 people will be accepted to participate in the workshop.   Participants will be selected based on experience, not on a first come, first serve basis. The workshop is from November 4 – 7th and held at Brookhaven National Lab in Upton New York, about an hour and a half east of New City on Long Island.

The Workshop is the second chapter of the Wood Stove Design Challenge that kicked off last year on the National Mall with the Wood Stove Decathlon.  An updated version of one of the stoves, the MF Fire designed by University of Maryland graduate students, competed in the 2013 Wood Stove Decathlon.  Wittus and Hwam also competed in the Decathlon and are bring updated versions of their stove. ClearStak consulted with a number of teams at the Decathlon.

As of December 2016, a number of automated stoves have entered the market place, including one that competed in this challenge.