Thermoelectric Energy Harvesting
Contributed By Electronic Products
Temperature differences exist nearly everywhere in both natural and man-made environments. They are easily recognizable since they are often the source of personal discomfort. We dress warmly to reduce the body heat lost on a cold day, while heating and air conditioning improve comfort levels by setting indoor air temperatures to suit personal preferences. However, the often undesirable temperature gradient can be put to useful work with what is known as the thermoelectric effect.
The thermoelectric effect (TE) has been known for nearly two centuries. Thomas Johann Seebeck is credited with discovering that dissimilar metals placed across a temperature gradient could deflect a compass needle. The coefficient of a material or device to generate a voltage per unit of temperature is known as its Seebeck coefficient (stated in V/°C).
Thermoelectric devices are most often used as a heat pump for cooling and are commonly known as Peltier coolers. This stems from the discovery by Jean Charles Athanase Peltier in 1884 that passing a current through a junction between two types of metal would absorb heat at one side and generate heat at the other.
Portable solid-state cooling has proven extremely useful over the years, especially in devices such as infrared detectors. However, as Seebeck noticed, the device will work in reverse, and this is quite useful nowadays as a way to harvest heat energy and use it in micropower electronics.
A sandwich implementing an array of n- and p-type semiconductor legs (Figure 1) creates the typical TE module. The legs are pellets of either bismuth telluride or antimony telluride. The legs are strapped together to create a series electrical connection and a parallel thermal connection. The top and bottom of the module is most often aluminum oxide ceramic to provide electrical insulation and good thermal conductivity.
The Seebeck coefficient of a complete module depends on other factors, but for the raw bismuth telluride material, it can be up to −287 μV/°C for certain compositions. For a complete module with modest dimensions of around an inch square (single-stage modules are between 2 and 5 mm thick), it is possible to produce over 100 mV/°C.
A TE energy-harvesting system takes advantage of any temperature difference between its two surfaces. Temperature gradients are everywhere. We encounter a wide range of equipment operating at temperatures much higher than the ambient environment. Our own bodies are relatively warm considering core body temperature is 37°C. Skin temperatures are typically in the range of 32°C. For typical indoor air temperatures, a harvester attached to a person’s skin offers ∆T up to 10°C. Waste heat from a human body is insignificant compared to most of the machines we use every day. Anything warm to the touch, such as your laptop, will provide more harvesting potential than the 10° or so available from human skin at standard indoor air temperature. At the extreme end of readily available waste heat is internal combustion engine exhaust. The exhaust gases themselves run up to several hundred degrees, which is well above the melting temperature of the solder used to construct the thermoelectric modules, but this example illustrates how easily accessible very high temperature gradients are. It is also not too difficult to imagine a harvester with insulation designed to protect the TE module.
Whether a thermoelectric generator (TEG) is intended to harvest a temperature gradient of a few degrees or a hundred, the same set of design principles apply. Chief among these is the idea that the temperature gradient needs to be maintained. The charge is moved in a TE device by the flow of heat. Without a temperature difference, there is no flow of heat and no electrical output from the generator. Consider a TEG mounted on the surface of equipment with a surface temperature on the housing of 50°C. Initial output would be 5 V for a device with specifications similar to the example module mentioned earlier. However, the temperature of the entire TEG would continue to rise until it reached equilibrium with the equipment housing. Although a small gradient might continue, it is unlikely to be sufficient to produce a useful output power. A good heat sink design is critical to optimize the effectiveness of the TEG modules.
Available TE modules
Now we will look at some possibilities for creating a thermoelectric harvesting system. Very large TE modules are available up to 50 mm square and larger. One example is the Laird 926-1178-ND. The manufacturer specifications are geared toward use of the Laird module as a thermoelectric heat pump. This is typical of TE datasheets. However, a simple rule of thumb can be applied when choosing the best TE module for power generation. Best performance from a given size of module is usually provided by the device with the highest product of VMAX x IMAX.
For more discrete applications, experimentation, or basic proof-of-concept, much smaller TE modules are also available (Figure 2). Consider the 102-1679-ND from CUI Devices. This TE is also specified as a heat pump but will provide good harvesting performance considering that the rule of thumb value for this module is 17.8 VA.
There is a quicker solution to getting up and running: a thermal harvesting demonstration kit. The Enocean 1084-1012-ND evaluation kit (Figure 3) provides a complete thermal energy-harvesting system along with the STM 312C radio module for 315 MHz transmission. With the kit assembled, all the radio needs to send a message is for your thumb to press the TE module onto a cool surface. After a few seconds the TEG provides enough energy to power up the radio to send a message. Since the direction of heat flow determines the output polarity of the TEG, one simply needs to take care to press the correct side of the module.
Thermal energy harvesting is both intriguing and powerful. Heat sources abound and this usually wasted energy is easily scavenged by a thermoelectric generator. Thermoelectrics cover the complete range from small devices suitable for micropower devices for wireless sensor networks to large units meant as alternative energy sources or back-up power. For more information use the links provided to go to product pages on the Digi-Key website.
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