This is where thermoelectric generators or TEG’s come in. These are devices that can convert heat energy directly into electrical energy without any need for moving parts like turbines. Thermoelectric generators work by exploiting a temperature gradient between the two sides of the generator.
Think of it like this, If you take a piece of metal heat one end and simultaneously cool the other, the electrons surrounding the metal atoms at the hot end will have more energy than the equivalent electrons at the cooler end.
This means the hot electrons will be jiggling around faster than those at the cool end, so they’ll tend to move towards the cold end faster than the cold electrons will move towards the hot end. Eventually, the cold end will become more negatively charged and the hot end will become positively charged.
This phenomenon, where a temperature difference can create a voltage, is known as the thermoelectric effect, and it was first described as long ago as the 1820s by a German physicist called Thomas Johann Seebeck.
So it sounds simple, but one big problem is that the voltage produced is absolutely tiny. This can’t just be solved by connecting lots of pieces of metal together in series like you would to make a bigger battery, because the wires used to connect them, which are Also pieces of metal will produce a voltage in the wrong direction and opposed the voltages and the pieces of metal.
The way that scientists have solved this problem is to use a material that can conduct electricity using positively charged particles rather than electrons. Just like the electrons in the metal, the positively charged particles will move away from the hot side.
So now, if you link a chain of them together, the voltages will add up along the series allowing you to generate a useful amount of power.
Materials with this type of positive conducting property are called semi conductors. In fact, rather than heating pieces of metal to create the current, another type of semiconductor material which conducts using electrons and is much more efficient generator is used instead, sandwich between the two positive conducting semiconductors.
Another problem with thermoelectric devices is the types of materials that allow electrons to flow easily, so you can tap off the electricity also tend to be very good at conducting heat. So very quickly the temperature gradient driving the process is lost and the efficiency Falls.
To get around this, scientists are trying to find materials with a high electrical conductivity, but a low thermal conductivity.
One way to achieve this is to use certain metal alloys which can be used to form a lattice of different sized atoms that slow down the flow of heat while allowing electrons to move freely.
In the same way, certain arrangements of nanoparticles can trap heat and Slow it down. In general, the more joins or interfaces there are between the particles, the slower heat is able to move.
Right now, thermoelectric generator is produced using these technologies are only about 10 percent efficient at best, but in industries on the scale of a power station, even at this early stage, this could still contribute a huge fuel saving.
Another area that might soon benefit from the use of this technology is the auto industry. Just like a power plant over, 70 % of the energy locked up in the fuel you put in your car exits along the exhaust pipe as wasted heat.
As a result, a number of car manufacturers are now testing prototype TEG’s that can scavenge back some of this wasted heat from the exhaust and use it to run things like the air conditioning unit and the lights and to charge the batteries in hybrid vehicles.
Industry experts are predicting that we’ll see the first mainstream thermoelectric generator equipped cars as soon as 2017, but there’s one place where TEG’s are already proving their worth, space exploration, and particularly aboard probes going so far away that solar power isn’t an option.
Space provides an excellent working environment for TEG’s, because the average temperature in space is just three degrees above absolute zero, which takes care of the cold side of the generator very nicely.
Meanwhile, the hot side is provided by a radioactive source such as strontium 90, which produces heat as it decays. This type of radioisotope thermoelectric generator, was used in the Apollo missions and is currently powering the Cassini and Voyager spacecraft missions, millions of miles from home.