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 |
 |
| Figure 2. Wiseway Stove |
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 |
 |
| Figure 4. Englander Stove |
 |
| Figure 5. Downdraft Rocket |
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.
[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.
