Illustration: Diego Marmolejo.
If the 2,000 year old windmill is the predecessor of today’s wind turbines, the fireplace and the wood stove are the even older predecessors of today’s solar panels. Like solar panels, trees and other plants convert sunlight into a useful source of energy for humans. Throughout history, the burning of wood and other biomass provided households with thermal energy, which was used for cooking, heating, washing, and lighting.
Photosynthesis also underpinned all historical sources of mechanical power: it provided fuel for both human and animal power, as well as the building materials for water mills and windmills. Neither the old-fashioned windmill nor the old-fashioned wood stove produced electricity, but both can easily be adapted to do so. It suffices to connect an electric generator to a windmill, and to connect a thermoelectric generator to a wood stove.
Thermoelectric generators (or “TEGS”) are very similar to “photoelectric” generators – which we now call “photovoltaic” generators or solar PV cells. A photovoltaic generator converts light directly into electricity, and a thermoelectric generator converts heat directly into electricity. 1
A thermoelectric generator consists of a number of ingot-shaped semiconductor elements which are connected in series with metal strips and sandwiched between two electrically insulating but thermally conducting ceramic plates to form a very compact module. 2 They are commercially available from manufacturers such as Hi-Z, Tellurex, Thermalforce and Thermomanic.
A thermoelectric module. Image: Gerardtv (CC BY-SA 3.0)
A thermoelectric module. Image used with permission, Applied Thermoelectric Solutions LLC, How Thermoelectric Generators Work.
Stick a thermoelectric module to the surface of a wood stove, and it will produce electricity whenever the stove is used for cooking, space heating, or water heating. In the experiments and prototypes that are described in more detail below, the power output per module varies between 3 and 19 watts.
As with solar panels, modules can be connected together in parallel and series to obtain any voltage and power output that one needs – at least as long as there is stove surface left. As with solar panels, the electric current that is produced by the thermoelectric module(s) is regulated by a charge controller and stored into a battery, so that power is also available when the stove is not in use. A thermoelectric stove is usually combined with low voltage, direct current appliances, which avoids the conversion losses of using an inverter.
Thermoelectric stoves could be applied in many parts of the world. Most research is aimed at the global South, where close to 3,000 million people (40% of the global population) rely on burning biomass for cooking and domestic water heating. Some of these households also use the stove or fireplace for lighting (1,300 million people have no access to electricity) and for space heating during part of the year. However, there’s also research aimed at households in industrial societies, where biomass stoves and burners have increased in popularity, especially outside of cities.
Ever since the thermoelectric effect was first described by Thomas Seebeck in 1821, thermoelectric generators have been infamous for their low efficiency in converting heat into electricity. 3456 Today, the electrical efficiency of thermoelectric modules is only around 5-6%, roughly three times lower than that of the most commonly used solar PV panels. 4
However, in combination with a stove, the electrical efficiency of a thermoelectric module doesn’t matter that much. If a module is only 5% efficient in converting heat into electricity, the other 95% comes out as heat again. If the stove is used for space heating, this heat cannot be considered an energy loss, because it still contributes to its original purpose. Total system efficiency (heat + electricity) is close to 100% – no energy is lost. With appropriate stove design, the heat from electricity conversion can also be re-used for cooking or domestic water heating.
More Reliable than Solar Panels
Thermoelectric modules share many of the benefits of solar panels: they are modular, they require little maintenance, they don’t have moving parts, they operate silently, and they have a long life expectancy. 7 However, thermoelectric modules also offer interesting advantages compared to solar PV panels, provided that there’s a regularly used (non-electric) heat source in the household.
Although thermoelectric modules are roughly three times less efficient than solar PV panels, thermoelectric stoves provide a more reliable electricity supply because their power production is less dependent on the weather, the seasons, and the time of the day. In jargon, thermoelectric stoves have a higher “net capacity factor” than solar PV panels.
Even if a stove is only used for cooking and hot water production, these daily household activities still guarantee a reliable power output, no matter the climate. Furthermore, the power production of a thermoelectric stove matches very well with the power demand of householders: the times when the stove is used, are commonly also the times when most electricity is used. Solar panels, on the other hand, produce little or no electricity when household demand peaks.
Image: A Soviet thermo-electric generator based on a kerosene lamp, powering a radio, 1959. Source: The Museum of Retrotechnology.
Note that these advantages disappear when thermoelectric generators are powered by direct solar energy. Solar thermoelectric generators (or “STEGS”), in which thermoelectric modules are heated by concentrated sunlight, don’t compensate for the low efficiency of their modules due to higher reliability because they are just as dependent on the weather as solar PV panels are. 8910
Less Energy Storage
Because of its higher reliability, there’s no need to oversize the power generation and storage capacity of a thermoelectric system to compensate for nights, dark seasons or bad weather days, as is the case with a solar PV installation. Battery capacity only needs to be large enough to store electricity for use in between two firings of the stove, and there’s no need to add extra modules to compensate for periods of low power production.
Solar panels and thermoelectric stoves can also be combined, resulting in a reliable off-grid system with little need for energy storage. Such a hybrid system combines well with a stove that is only used for space heating. The thermoelectric modules produce most of the power in winter, while the solar panels take over in summer.
Cheaper to Install, Easier to Recycle
A second advantage is that thermoelectric modules are easier to install than solar panels. There’s no need to build a structure on the roof and an electric link to the outside world, because the whole power plant is indoors. This also prevents theft of the power source, a significant problem with solar panels in some regions.
All these factors make that power from a thermoelectric stove can be cheaper and more sustainable compared to power from solar PV panels. Less energy, materials and money are needed to manufacture batteries, modules, and support structures.
In terms of sustainability, there’s another advantage: unlike solar PV panels, thermoelectric modules are relatively easy to recycle. Although silicon solar cells themselves are perfectly recyclable, they are encapsulated in a plastic layer (usually “EVA” or ethylene/vinyl acetate polymer), which is critical to the long-term performance of the modules. 11 Removing this layer without destroying the silicon cells is technically possible, but so complex that it makes recycling unattractive from both a financial and energetic viewpoint. 1213 On the other hand, thermoelectric modules do not contain any plastic at all. 141516
Cooling the Modules
The electrical efficiency of a thermoelectric generator doesn’t only depend on the module itself. It’s also, in large part, influenced by the temperature difference between the cold and the hot side of the module. A thermoelectric module operating at half the temperature difference will only generate one quarter of the power. Consequently, improving the thermal management of a thermoelectric generator is a major focus in the design of thermoelectric stoves, as it allows to produce more power with less modules.
On the one hand, this involves locating the hottest spot(s) on a stove and fixing the modules there – provided that they can take the heat. Most stoves have surface temperatures from 100 to 300 degrees Celsius, while the hot side of bismuth telluride modules (the most affordable and efficient ones) withstands continuous temperatures of 150 to 350 degrees, depending on the model.
On the other hand, thermal management comes down to lowering the temperature of the cold side as much as possible, which can be done in four ways: air-cooled and water-cooled forced convection, which involves electric fans and pumps, and air-cooled and water-cooled natural convection, which involves the use of passive heat sinks that do not have a parasitic load on the system.
Active cooling usually has higher efficiency, even when the extra use of a fan or a pump is taken into account. However, passive systems are cheaper, operate silently, and are more reliable than active systems. In particular, the breakdown of a fan can be problematic, as it can lead to module failure due to overheating. 17
Thermoelectric Stoves with Heat Sinks
The first thermoelectric biomass stoves were built in the early 2000s, although the Soviets pioneered a similar concept in the 1950s with mostly electric radios powered by kerosene lamps. 6 In 2004, a team of Lebanese researchers retrofitted a typical cast-iron wood stove from local rural areas with a single 56 x 56 mm thermoelectric module they had made themselves. 18 The stove, which is used for cooking and baking as well as for space and water heating, is rather small (52 x 44 x 29 cm) and weighs 40 kg.
Image: The cast-iron stove used in the experiments. 18
The researchers screwed a 1 cm thick smooth aluminium plate to the hottest spot of the stove surface, fixed the module there, and attached a very large (180 x 136 x 125 mm) aluminium finned heat sink to its cold side. At a burning rate of 2.5 kg soft pine wood per hour, their experiments showed an average power output of 4.2 watts. Operating the wood stove for 10 hours per day (excluding the warm-up phase) thus supplies a rural Lebanese household with 42 watt-hours of electricity, enough to cover basic needs.
Image: TEG installation details and location on stove. 18
More modules and heat sinks can be added to increase power output, but of course the stove surface is limited, and as more modules are added they will be located in areas with a lower surface temperature, decreasing their efficiency. Another way to increase power production is to use an even larger heat sink, and/or a more expensive heat sink made from materials with higher thermal conductivity.
Thermoelectric Stoves with Fans
Most thermoelectric stoves that have been built to date use electric fans to cool the module, in combination with a much smaller heat sink. Although the fan can break and is a parasitic load on the system, it can simultaneously increase the efficiency of the stove by blowing hot air into the combustion chamber – slashing firewood consumption and air pollution roughly by half. Furthermore, fan-powered stoves avoid the building of a chimney and can rely on a horizontal exhaust pipe instead. 19 Consequently, self-powered, fan-cooled stoves make it possible to reduce firewood consumption and indoor air pollution in rural regions of the global South where people neither have access to electricity, nor the means to make a chimney through the roof.
A study of a forced-draft thermoelectric cookstove with one module showed a 4.5 watt power output, of which 1 watt is required to operate the fan. 20 The net power production (3.5 watts) is lower compared to that of the stove with only a heat sink (4.2 watts), but the fan-cooled stove uses only half as much firewood: it generates 3.5 watts net electricity at a burning rate of 1 kg of firewood per hour, while the passively cooled stove requires 2.5 kg of firewood to produce 4.2 watts.
Image: TEG-powered forced draft cooking stove. 20
An 80-days field test of a similar portable thermoelectric cookstove design in Malawi showed that the technology was highly valued by the users, with the stoves producing more electricity than was needed. Over the entire period, power production amounted to between 250 and 700 watt-hours of electricity, while electricity use was between 100 and 250 watt-hours. 21
Some fan-cooled thermoelectric cooking stoves are commercially available, often designed with backpackers in mind. Examples are the stoves from BioLite, Termomanic and Termefor, which advertise power outputs between 3 and 10 watts, depending on the design and the number of modules. 17
Thermoelectric Stoves with Water Tanks
The most efficient thermoelectric stoves are those in which the cold side of the module(s) is cooled by direct contact with a water reservoir. Water has lower thermal resistance than air, and thus cools more effectively. Furthermore, its temperature cannot surpass 100 degrees Celsius, which makes module failure due to overheating less likely.
Image: the principle of thermoelectric stove with passive water cooling. 17
When thermoelectric modules are water-cooled, the waste heat from their electricity conversion does not contribute to space heating, but to domestic water heating. Water-cooled thermoelectric stoves can be active (using a pump) or passive (no moving parts). 17
Most thermoelectric stoves with passive water cooling are small and only used for heating relatively small amounts of water. In fact, rather than the stove, it is most often a cooking pot that is equipped with thermoelectric modules. For example, the PowerPot is a commercially available backpacking type cooking pot with a thermoelectric module attached to the base, which can be directly placed on the top of a stove and advertises a power generation of 5-10 watts.
Image: multifunctional wood stove with passive water cooling. 22
A much larger and more versatile thermoelectric stove with passive water cooling was designed by French researchers, based on a large, multifunctional mud wood stove design from Morocco. 1922232425 They installed eight thermoelectric modules at the bottom of a built-in 30L water storage tank, which not only serves as the heat sink for the cold side of the generator, but also as the domestic hot water supply for the household. Furthermore, the stove is equipped with a self-powered electric fan and has a double combustion chamber to increase combustion efficiency.
Tests of a prototype generated 28 watts of power using two modules, while burning 1.5 kg of wood for cooking and/or heating. The fan used 15W, meaning that 13W of power remains for other uses. The stove also provided 60 litres of hot water per hour. Depending on the duration of two cooking sessions, between 35 and 55 watt-hour electricity was stored in a battery in a day. Note that here the researchers take into account the losses of the charge controller, the 6V battery, and the fan.
Thermoelectric Stoves with Pumps
Passive water cooling has a downside. As the temperature of the water in the tank increases, the difference between the cold and the hot side of the module will decrease, and so will the electrical efficiency. There either needs to be sufficient time between two firings of a stove to let the water cool down again, or the warm water should regularly be used and replaced by cold water. A pump makes this task more convenient.
Image: Prototype of a thermoelectric stove with water-cooled modules. 26
A 2015 prototype, in which a wood stove used for cooking and space and water heating was equipped with 21 thermoelectric modules cooled by a pumped water system, showed a power production from 25W (burning 1 kg of pine wood per hour) over 70W (4 kg wood/hour) to 166W (9 kg wood/hour). 26 The power output per module is as high as 7.9 watts, almost double the power output per module of the stove with natural air cooling. The pump uses 5W and the stove also has a fan to increase combustion efficiency, which consumes 1W. 2728
Thermoelectric Gas Boilers?
Thermoelectric generators with forced water cooling better fit the energy infrastructure in industrial societies, especially in households with central heating systems. More modules could be added, resulting in a power production that matches a relatively high energy lifestyle. However, there’s some caveats. First, central heating systems are only used for space and water heating, not for cooking, which makes their power production less reliable throughout the year. Second, only some central heating systems operate on biomass or wood pellet burners, while many more run on gas, oil or electricity.
Prototype of a thermoelectric wood-pellet burner. 29
Obviously, when the heat source is electric, it makes no sense to stick a thermoelectric module to it. A thermoelectric system is incompatible with the vision of a high-tech sustainable building where heating is done with an electric heat pump, cooking happens on an electric cooking stove, and hot water is produced by an electric boiler.
However, when the energy source is gas or oil, a thermoelectric boiler is as much of a low carbon solution as a grid-connected solar PV system on the roof. 30 A thermoelectric heating system doesn’t make a household independent of fossil fuels, but neither does a grid-connected solar PV installation. It relies on the (largely fossil fuel powered) power grid to solve energy shortages and excesses, and it usually counts on a fossil fuel powered central heating system for space and water heating.
Image: A 1 kW thermoelectric generator with forced-water cooling for low temperature geothermal resources. 31
A thermoelectric heating system that runs on fossil fuels also compares favourably to a large cogeneration power plant, which captures the waste heat of its electricity production and distributes it to individual households for space and water heating. In a thermoelectric heating system, heat and power are produced and consumed on-site. Unlike a central cogeneration power plant, there’s no need for an infrastructure to distribute heat and electricity. This saves resources and avoids energy losses during transportation, which amount to between 10 and 20% for heat distribution and between 3 and 10% (or much more in some regions) for power distribution.
A cogeneration power plant is more energy efficient (25-40%) in turning heat into electricity, meaning that in comparison a thermoelectric heating system supplies a larger share of heat and a smaller share of electricity. This is far from problematic, though, because even in Europe 80% of average household energy use goes to space and water heating.