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Harvesting energy with flexible thermoelectrics

12 May 2013

A large proportion of the energy we produce disappears unused into thin air via waste heat. Tiny thermoelectric generators can tap some of this waste energy, using nothing more than a temperature gradient to produce electricity. Les Hunt reports.

Thermoelectric generators (TEGs) offer an attractive means of utilising heat that is otherwise lost to the atmosphere. However, producing these devices has been both difficult and expensive, as well as being compromised by the lack of suitable materials.

Fortunately, that may be about to change, thanks to recent work undertaken by the Fraunhofer Institute for Material and Beam Technology (IWS) in Dresden. At the Hannover trade fair last month IWS researchers presented a new manufacturing process via which these generators can be cost-effectively produced in the form of large-area flexible components using non-toxic synthetic materials.

Power station cooling towers are a fertile area for this method of energy harvesting. The researchers have concentrated their efforts on these 150m high concrete giants, to take advantage of the very large temperature differences between the hot rising water vapour within, and the cooler concrete skin of these towers. Aljoscha Roch of the IWS takes up the story:

“Thermoelectric generators currently have an efficiency of around eight percent. That sounds very small. But if we succeed in producing a TEG cost-effectively, on a large scale and from flexible materials, we can install them extensively on the insides of the concave cooling tower wall. In this way, through the enormous amount of energy produced in these power plants – around 1,500 litres of water are evaporated every minute – we could generate large quantities of electricity.”

Together with his colleagues at the IWS, Dr Roch has now moved a step closer to realising this goal. He and his colleagues have succeeded in 3D printing a TEG. The miniature generators are produced cost-effectively, on large surfaces in a manageable way, but more significantly, the materials used in their construction are environmentally friendly. Dr Roch again:

“TEGs are today largely produced by hand from components that contain toxic materials – lead, for example. We are now using modern 3D printing technology and harmless polymers that are electrically conductive.” During the 3D printing process, a thermoelectrically active polymer paste is used to produce the 20-30µm thick thermoelectric layers. “The generators have to be of a certain thickness in order to build up electrical voltage from temperature difference,” Dr Roch explains. “Currently available 3D printing processes could be very suitable for achieving the required depth.”

So, how is the electricity ‘harvested’ from these polymer generators which are only a few micrometers in size? In the cooling tower example, the effect of the hot water vapour in the tower makes the electrons in the generator migrate to its cooler side and an electrical potential difference results.

Even small temperature differences such as one degree are sufficient to produce this effect – a phenomenon known to physicists for almost 200 years. It has been the lack of efficient manufacturing methods and suitable materials that have so far hindered the wider use of thermoelectric energy harvesters. 

Hitherto, their uses have been confined to special applications, such as powering the electronics of orbiting satellites and automotive testing. In the latter example, when mounted on the exhaust pipe of the vehicle under test, TEGs can be used to supply current for the vehicle’s onboard electronics. To date, some 600W has been generated by these special TEGs; multiply that by the millions of vehicles currently in use on our roads and that equates to an energy saving of several thousand million kilowatt hours.

Cars and cooling towers are just two examples; waste heat is produced from a huge variety of industrial processes and human activities. Dr Roch believes that with TEGs installed on industrial production lines, in water treatment plants, throughout large data centres or on any type of system that expels a hot exhaust, a potentially large and hitherto untapped source of energy could easily be exploited.

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