How we can turn wasted heat into electricity

From watches that run on body heat to generators in factory smokestacks, thermoelectric materials have countless applications.

Muhammet ToprakThermoelectric materials can convert heat to electricity, meaning they have incredible potential to turn the waste heat all around us into a green source of energy. Prof. Muhammet Toprak of KTH - Royal Institute of Technology in Sweden develops these materials, and we speak with him about what we can expect from them in the future. To learn more about his research, follow his project on ResearchGate.

ResearchGate: What are thermoelectric materials?

Muhammet Toprak: Thermoelectric materials are solid-state materials that are capable of converting heat to electricity and vice versa. In some cases, they’re used in cooling devices that rely on electrical currents to remove heat. In others, they do the opposite, converting temperature gradients, or differences, to an electrical voltage. Thermoelectric devices are particularly favorable because they are solid-state devices, eliminating the need for maintenance of moving parts or a fluid to transfer the heat.

RG: How widespread is energy production from these materials?

Toprak: Thermoelectric materials have not yet reached widespread use and are most commonly used in portable cooling devices for cars. However, there are also many attempts to use thermoelectric devices for harvesting waste heat. For example, there are wristwatches that run on thermoelectric power from body heat, and several plans for designing wearable thermoelectric energy harvesters. Of course, the human body produces relatively low-grade heat. High-grade heat offers other opportunities, and thermoelectric generators have been developed to harvest energy from wood stoves and the exhaust tubes of trucks and automobiles.

RG: What are some other potential sources of waste heat?

Toprak: Fossil fuel-based energy sources still make up more than 80 percent of the total energy supply and are inefficient, meaning they produce a lot of waste heat. In a coal-fired power plant, at least 56 percent of the energy is wasted. Until the renewable energy sector grows enough to handle all our energy needs, we need to develop the technologies to harvest this wasted heat. Thermoelectric materials provide an opportunity for this, as they’re well suited for use in power plants, refractories, metallurgy, and other processes resulting in waste heat. Thermoelectric generators are also coupled with solar collectors, where solar thermal energy is converted to electrical power.

Thermoelectric power from waste heat in cars
Heat from a car's brakes or exhaust can generate power to run its radio, air conditioning, and electric windows. Credit: Timothy Vogel

About two thirds of the energy from gasoline used in cars and trucks is also wasted heat. One of the most important applications for thermoelectric generators is reducing the carbon footprint of transport by using the waste heat from the exhaust pipe to generate power. There are even attempts to use the heat from brakes for power generation. This power then can be used to charge the vehicle’s battery, or run electric windows, air conditioning, etc. to improve fuel economy.

In one project, we are planning to integrate the thermoelectric generators into factory chimneys where very hot steam is released into the atmosphere. This will be an interesting large-scale project to demonstrate the impact that’s possible with this technology.

RG: How do thermoelectric generators work?

Toprak: Typically, there are n-type and p-type thermoelectric elements, which are connected electrically in series and thermally in parallel. Thus, when placed under a thermal gradient, the charge carriers will generate a net current, producing electrical voltage. The energy conversion is direct, which means that scalability of the devices with good performance is not a big concern.

RG: What are the biggest challenges standing in the way of more widespread use of thermoelectric materials to produce energy?

Toprak: Currently, some thermoelectric materials are well proven for generating power with low-grade heat. At higher temperatures, where the grade of the heat is increased, the energy that can be harvested also increases. However, materials intended for use with intermediate and high temperatures have stability issues. Furthermore, electrical contact materials react with other materials, which changes their composition and degrades the device’s performance.

The crucial challenge is to make good electrical and thermal contacts. The large temperature gradient across the device puts mechanical stress on the contact points. Because of this, high-grade heat is usually cooled down before it can be harvested, meaning the highly energetic waste heat is still not used at its full potential.

RG: Are there any drawbacks to using thermoelectric materials?

Toprak: Some promising thermoelectric materials contain toxic or scarce elements, which prevent their production and use at a large scale. Alternative materials that are more abundant and benign are currently being developed to make this technology more appealing.

RG: What do you foresee for the future of thermoelectric energy harvesting?

Toprak: Although thermoelectric material development is on the right track, putting these materials into devices where the whole device is going to be cycled between very high temperatures and ambient temperature has not yet yielded long-lasting performance due to degradation. However, if we can solve the material related issues, I am confident that thermoelectric harvesting will be broadly used.

In terms of fabrication and processing, we can’t limit ourselves to conventional approaches. The way the material is fabricated and processed has an immense effect on its final performance. This means that even slight chemical modifications of the materials require extensive testing to identify the best processing conditions. This is time-demanding research, and it would not be appropriate to expect miracles in a short time. But the future of the technology is promising.

Featured image courtesy of Dylan Pankow, Christopher A. S. Harvey, Zaereth, and Jarek Tuszyński.