All over the world heat fizzles out unused. But that needn’t be the case. Thermoelectric “energy harvesting” could generate any amount of electricity that is required. At last a startup has initiated a first commercial project.
At present, more than half the energy is lost as waste heat. Automobiles are just one prominent example. Only a third of the fuel consumed is used to move the vehicle. Stationary combustion machines, such as thermal power stations and industrial incineration plants are similarly wasteful.
This explains why scientists worldwide are working on projects to convert heat from combustion processes into electricity. The physics behind this have been known for almost 200 years. Back then, Johann Seebeck discovered that a voltage develops at a contact point of two different metals or semiconductors that are kept at different temperatures. The thermoelectric voltage depends on the combination of materials used and the temperature difference. 13 years later, physicist Jean Peltier discovered that this also worked the other way round: “Electric current causes a temperature difference.” Although this all sounds very simple, it continues to cause problems as regards the technical implementation.
Even though special applications have worked for many decades. For example, an atomic battery that converts the thermal energy of the spontaneous nuclear decay of a radionuclide into electrical energy has been powering the Voyager space probe for forty years. With no mechanical components it requires virtually no maintenance.
A more “terrestrial” application could see thermoelectric generators in future vehicles. This could reduce fuel consumption by up to fifteen percent. Electricity for the onboard electronics would then be recovered from exhaust heat loss. An initial system from BMW, which was presented to the public in 2008, generated maximum electric power of 200 watts. However, as yet there are no solutions ready for mass production.
A penny saved is a penny earned
But it can also be much less than that. Thermoelectric generators have an equally high potential in stand-alone power generation for devices with low energy requirements. Based on the energy harvesting principle, they generate energy from their surroundings.
For example, for wireless sensor networks in smart factories. For this purpose, scientists from the Institut für Integrierte Produktion Hannover (IPH) and the Institute for Physical Chemistry and Electrochemistry (PCI) at the University of Hannover have developed flexible thermoelectric generators from a calcium cobalt oxide paste that can be processed in a simple, inexpensive screen printing process that can also be scaled.
Printed thermoelectric generators from Otego, a startup based in Karlsruhe, Germany, are also aimed at the same market segment. No larger than a cube of sugar, the “oTEG” can be integrated easily into electronic products for supplying energy to wireless sensors for condition monitoring of machines and equipment in industry (condition-based maintenance) and also for building automation. Polymer materials make it flexible and impervious to impact and vibration. It can be used at temperatures from -55 to +85°C. Because of the output voltage in the single digit volt range, it can be used in standard electronic components.
With pencil and paper
Scientists at Helmholtz-Zentrum Berlin (HZB) have demonstrated that thermoelectric generators don’t necessarily require “complicated” materials or production processes. The inexpensive and environmentally friendly solution makes do with a normal pencil, photocopy paper, and a conductive synthetic varnish.
Specifically, the pencils (graphite) provide a voltage of about 0.875 millivolts with a temperature difference of 50 degrees Celsius. This result is similar to other, much more expensive nanocomposites that have been used in the past for flexible thermoelectric elements. Adding indium selenide would increase this value tenfold.
In the future, with these simple ingredients, thermoelectric components, which are extremely inexpensive, environmentally friendly, and non-toxic, could be printed on paper. Tiny, flexible components such as this could also be worn directly on the body and could use body heat to operate small devices or sensors.