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Material Breaks Record For Turning Heat Into Electricity
ananyo writes "A new material has broken the record for converting heat into electricity. The material had a conversion efficiency of about 15% — double that of one of the most well-known thermoelectrics: lead telluride (abstract). For decades, physicists have toyed with ways to convert heat into electricity directly. Materials known as thermoelectrics use temperature differences to drive electrons from one end to another. The displaced electrons create a voltage that can in turn be used to power other things, much like a battery. Such materials have found niche applications: the Curiosity rover trundling about on the surface of Mars, for example, uses thermoelectrics to turn heat from its plutonium power source into electricity. That doesn't mean that the material is ready to be used on the next Mars rover, however: NASA has been looking at similar materials for future space missions, but the agency is not yet convinced that they are ready for primetime."
In a heat-sink you want to carry heat away from an object. A termoelectric is by definition a poor heat-sink because it requires a temperature gradient to work. This gradient means that the material is a poor conductor of heat. If it were a good conductor then both sides would quickly reach the same temperature and it would stop working.
You seem to have a very poor grasp pf thermodynamics but to put it simple I will refer to the article itself.
"Building a better thermoelectric depends on finding materials that conduct electricity, but not heat"
There it is in plain language. Thermoelectrics are poor conductors of heat.
Also, nearly everything you say in your post is simply wrong. You do not convert heat into electricity. You use a heat gradient to cause an electric field. The electrons flow out one side and return to the other. You are not "piping" heat anywhere.
Thermo-electrics do not run on the heat gradient between themselves and the air. They run on the heat gradient between two sides of the material itself. I can have a block of ceramic at relatively uniform 1000C without it being 'maximally cool'.
Considering a thermoelectric device with a cold-side temperature of 350K and a hot-side temperature of 950K, respective waste-heat conversion efficiencies of ~16.5% and ~20% are predicted.
For a hot-side temperature of 950 K and a cold-side temperature of 350 K, the Carnot efficiency (i.e. the maximum possible efficiency of any device) is ~63%. So this is somewhere between 1/4 and 1/3 as efficient as it could possibly be. Large generators, such as combined cycle gas turbines are considerably more efficient, but these devices are small and silent. In other words: not bad.
Poor AC getting so mean comments.
Actually you ARE right, but only from a certain point of view. Firstly, you are right that thermoelectric materials take heat away, and thus cool down whatever they are attached to.
The critical point here is that merely cooling down is not enough for a heat sink. The heat sink has to be cooled down FAST. Faster than it's heat source is heating it. Thermoelectrics just can't turn heat into electricity fast enough to let a heat sink do it's job.
So it's not really a matter of thermoelectrics heating up heat sinks, they don't heat them up, they in fact cool them down, what is heating up the heat sink is the heat source (say a CPU or a power engine).
The problem is that no thermoelectric so far can transform heat intro electricity faster than a CPU turns electricity into heat.
But... the future refused to change.
Interesting idea, but a similar thing has been done for a century+ with the outgoing steam being used to preheat the incoming water. There's orders of magnitude of difference in energy gained between that and thermocouples at huge scales. However at small scales a steam plant is not possilbe while a thermocouple is.
Long time lurker, commenting because I know something about this one (doing my PhD in thermoelectrics).
First of all, you _can_ use thermoelectrics to cool things like CPUs or fridges, but don't expect to generate any energy from them when you do it because you need to be putting electricity into the system, essentially carrying the thermal energy with it. You will cool one end and heat the other end. If you've ever heard of a Peltier cooler then you know what I am talking about.
A good background can be found here: http://thermal.ferrotec.com/technology/thermal/thermoelectric-reference-guide/
Second, this is something people have been messing around with the nanostructure of tellurium alloys for ~20 years or so, with the sole purpose of reducing thermal conductivity. The figure of merit for thermoelectrics is ZT = thermopower^2 x electrical conductivity x temperature / thermal conductivity. You can't increase electrical conductivity without reducing thermopower and increasing thermal conductivity (as there is a lattice and an electrical contribution). Thermopower is more or less a function of the number of carriers (lower is better) and their effective mass, so this is difficult to increase without durastic changes in the crystal structure or killing electrical conductivity. This leaves thermal conductivity. If you increase disorder in the material you make it harder for thermal energy to travel through it, which as lead to lots of research on how you manage this without messing up your carrier conduction. These are known as PGEC (phonon glass electron crystal) materials.
Third, there are lots of applications of these (in heating/cooling and power conversion) if they can be made efficient and cheap. Anywhere you have a heat source pretty much. To use the classic car analogy, BMW, Ford, GE (amongst others) are looking at using a thermoelectric module to generate power for the car from the waste heat in the exhaust gases from the engine. This would increase the power of your engine by removing the alternator and also make the car lighter.
The problem is the efficient and cheap part. These kinds of thermoelectrics are based on tellurium, an element about as abundant in the earth's crust as platinum, but to my knowledge isn't specifically mined for. Most other elements involved are toxic heavy metals (Pb, Sb, Bi, etc.)... so these aren't exactly nice things to have around or to make.
This is where oxides come in. Made of lighter, more abundant, less toxic elements they are much cheaper to make (not just sourcing the materials, heath and safety too etc.), and are stable at much higher temperatures. As you know from Carnot, the higher the temperature a heat engine works at the more efficient it becomes; rather than 900 K (600C) you're looking at more like 1300 k (1000C) and upwards. Current high ZT oxides are things like NaxCoO2 and Ca3Co2O6, which have layered structures; one part is great at absorbing thermal energy (due to Na disorder for example) and the other is good at conducting electricity (like the CoO2 portion of NaxCoO2)
TL;DR
The way I see this paper: great proof of concept, PGECs are doing what they say on the tin and this will be great for low T applications. But for high power generation we need something more like the oxides which are cheaper, easier to produce, and work at higher temperatures.