A thin conductive material takes advantage of the temperature difference between its two sides to produce electricity. This is known as the Seebeck effect. 

Souce: Wikimedia Commons

The thermoelectric effect is the conversion of temperature variances to electric voltage and the other way around via a thermocouple. A thermoelectric mechanism creates a voltage when there is a separate temperature on each side of the device. When a voltage is applied to it, heat is transferred from one side to the other, producing a temperature difference. If you were to apply electricity to such a device, one side would get really cold and the other really hot. 

If a thermoelectric device is located on skin, it will produce power as long as the ambient air is at a temperature that is lower than the skin. A device that is one square centimeter in area can yield up to 30 microwatts. If these generators are located side by side, the amount of power being harvested is increased.

At MIT, researchers are working to improve the effectiveness of the circuitry that harnesses the small amounts of power generated by standard thermoelectric generators. Some batteries need to work a long time, such as medical devices that are implanted, such as biomedical monitoring or treatment procedures. This new technology being developed by MIT researchers could make battery replacement unnecessary. Such devices could be powered using the differences in temperature between the body and the surrounding air.

24-hour-a-day monitoring of medical conditions, such as heart rate, blood sugar or other biomedical data can be observed, through a simple gadget worn on a patient’s arm or leg and powered by the body’s temperature.

Rights granted: MDPI, Basel, Switzerland.

A thermoelectric device has the potential to produce energy. These devices are designed to take advantage of the differences of tens to hundreds of degrees. Newly designed devices have the ability to harness differences of just one or two degrees, producing small but usable amounts of electric power. The key to the new technology is a control circuit that elevates the energy output from the thermoelectric material and the storage system, such as a storage capacitor.

There's a potential for researchers to improve the efficiency of thermoelectric generators. As they now stand, a thermoelectric generator can only convert 0.4% of the heat energy into usable electrical power. The U.S. Department of Energy and the University of California-Berkeley are carrying out research to develop more efficient thermoelectric generators.

How do thermoelectric generators work? Why does a difference in temperature produce an electric current? An electric conductor has free charges that can move about. When an electric field is applied, these charges move and create an electric current. 

If you take a piece of metal and cause one end to be hot and the other end cold, the electrons on the hot side will have more energy and shift about more. These hotter electrons spread out, while on the cold end, the electrons have less energy. The amount of charge separation depends on the specific metal. If you use another metal with two ends at different temperatures this will have a different charge separation on the hot and cold ends because this metal is different than the first. Put together, these different metals will form a type of battery—a thermoelectric generator.

These thermoelectric generators are very inefficient. You will need huge temperature differences to get useful energy out of them. The good news is that these thermoelectric generators have no moving parts, which means they are small and quite reliable.

Harvesting waste heat from the human body has the potential to be an excellent portable power source, generating on average 58.2 W/m2 at resting metabolic rate. 

This device has a small profile, producing a new class of low-power wearable devices that do not require batteries. These devices can be directly integrated into clothing without the need to regularly charge or swap batteries. 

Wearable thermoelectric generators have an advantage over other power sources, such as mechanical energy devices, which require the user to be somewhat active, which is often not possible for elderly or bed-ridden patients. Photovoltaics don’t work when the user is in the dark, such as indoors or at night. Thermoelectric generators will create continuous power as long as there is a difference between the skin and surrounding temperature.

Photo: Kaist, South Korea

Thermoelectric Charger Uses Body Heat To Charge Smartphone

Humans generate heat as a side effect of metabolism, and to preserve core body temperature at approximately 98°F. This amounts to 58.2 W/m2 of the heat generated, although not all of it escapes through the skin—some is lost through exhalation and perspiration. Heat from the skin is transported to the lower temperature surroundings by convection and radiative transfer at rates of 1-10 mW/cm2 all through the body. The rate of transfer depends on the part of the body and arteries, which have the greatest heat transfer. Clothing also blocks heat transfer. Therefore, the average transfer over the entire body is approximately 5 mW/cm2. The best areas, which can be targeted is the radial artery in the wrist which has a heat flow of about 25 mW/cm2 at room temperature.

Converting skin heat to electricity is a challenge because these devices must be flexible, thin, and non- toxic. Thin thermoelectric generators have much lower conversion efficiency than those with large heat sinks. The good thing is that many useful wearable electronics have very low power consumption—in the microwatt range. A flexible wristband covered in thermoelectric generator modules was shown to be sufficient to capture accelerometer data from a user. Thin thermoelectric generators have been built into textiles such as shirts to harvest lower power while taking advantage of a larger surface area.

Photo: NASA

Thermoelectric generator for NASA's Mars Science Laboratory mission

Thermoelectric generators are being used for space instruments in the Mars Rover, Curiosity, and Voyager 2. They are harnessing long-lasting heat with Voyager operating for over 40 years using this type of power. Body sensors can be worn continuously if they can channel body heat to be energy independent. To achieve this, researchers need to develop new thermoelectric materials that are efficient at lower temperatures, non-toxic and inexpensive.

The content & opinions in this article are the author’s and do not necessarily represent the views of AltEnergyMag

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