Thermo Electric Generators

Being able to generate electricity by simple means anywhere where heat is available is a fascinating thought. The enormous potential of this technology is underlined simply by the fact that by harvesting additional electricity from waste heat it can significantly increase the efficiency of known technologies.

How it works

TEGs consist of two materials placed in contact with each other via bridges. Electricity can then be generated if there is a temperature difference between the two materials and the heat begins to move from one side to the other. (learn more about the thermoelectric effect)

TEGs have no moving parts and thus can be considered as mostly maintainance free.

Two general problems exist in such devices: high output resistance and adverse thermal characteristics. In order to get a significant output voltage a very high Seebeck coefficient is needed (high V/°C). A common approach is to place many thermo-elements in series, causing the effective output resistance of a generator to be very high (>10kOhm). Thus power is only efficiently transferred to loads with high resistance; power is lost across the output resistance otherwise. A generator with very high output impedance is effectively a temperature sensor, not a generator. Secondly, because low thermal conductivity is required for a good TEG, this can severely dampen the heat dissipation of such a device. For example it is not generally considered wise to place a TEG on an essential IC chip that requires cooling. Because of the low thermal conductivity of a TEG device, that IC is cooled at a slower rate.

TEGs' typical efficiencies are around 5-10%. A lot of research has been done to make this technology efficient. Together with the Fraunhofer Institut SAM-Thermoelectric technologies have developed a way to generate electricity at 30% efficiancy (see product description PDF). 

Uses of TEGs

TEGs in improved stoves

TEGs are an interesting way to improve ICS performance. For example Aprovecho and Biolite have attached a TEG driven Fan to a rocket stove. The Side Feed Fan Stove has shown significant results by reducing Fuel use by 42%, PM emission by 94%, and CO emissions by 91% compared to three-stone fire.

Small TEGs could also be used to improve the working conditions at the cooking place for example by additional lighting with LEDs. (see page 19 of the presentation linked above, to see how much light you could get out of a single candle)

TEGs in solar systems, engines and other big appliances

A license has been purchased by the O-Flexx GmbH from the above mentioned SAM-Thermoelectric technologies company. They are now developing very promising ways to improve different technologies by converting waste heat into electricity. The SAM TEGs are beeing used to increase the efficiancy of solar systems, automotive engines, transport and more.. (see their products and solutions). They also receive funding from the german government for their improvement of solar panels. These big appliances with the SAM Technology will be commercially launched by end of 2010.

TEGs as cooling devices

As Jean-Charles Peltier showed in 1834 the effect of thermoelectrics is versatile. Connecting a TE module to a low voltage will actively move heat from one side to the other. (Peltier effect)