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
Termo Electric Generators (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 Improved Cookstoves (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.The improved combustion is achieved by returning waste heat into the system and improving the airstream inside the stove.
Another example is the Philips stove mentioned as example (F) in a stove study published on pciaonline.org in 2009. The study shows results of stove tests. The philips stove is also equipped with a TEG driven Fan and has shown great results in respect to particle and CO emission (see graphs on pages 16 and 19 of the study).
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 Side Feed Fan Stove presentation linked above, to see how much light you could get out of a single candle)
TEGs in Solar Systems, Engines and other 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 in Biogas appliances
TEGs are currently being tested as add-on for biogas cookers. Similar to the idea with woodfuel stoves the TEG would produce electricity to charge a mobile phone or provide light during the cooking process.
Combining Benefits
Rather then just developing with a focus on just one technology, developers are more looking into combining TEGs with other technologies such as solar water heaters and PV Panels to increase efficiency and to make these technologies economically more viable. At Massachusetts Institute of Technology (MIT) a flat panel hat been developed that combines solar water heating with electricity generation. This combination and a new light absorbing surface resulted in a panel that is claimed to be 7-8 times more efficient than previous solar-thermal electric devices.
www.sciencedaily.com/releases/2011/05/110501183821.htm
TEGs as Cooling Devices
As Jean-Charles Peltier showed in 1834 the effect of thermoelectrics is versatile. Connecting a termo electric (TE) module to a low voltage will actively move heat from one side to the other (Peltier effect).
Further Reading
- Micropower claims to have modules with 20-30% efficiency to provide power at 1-2$/Watt
- Calculations about the efficiency of a TEG in a car
- Announcement (and later) proceedings ECT 2011
- Homepage of ECT 2010
- Link collection on TEG producers, consultants, products....
