In piezoelectricity, there exist (crystal) materials that can directly convert electricity into a change of pressure, or vibration, or micro-motion, and vice-versa: the same material can convert a change of pressure back into electricity. With the thermoelectric effect (known by its most well-known application, the peltier element), we have a method of directly converting electricity into heat transfer, usable for cooling and heating, which is reversible as well – a temperature gradient in the material can be converted back into electricity.
The interesting bit is that both these processes are not very efficient. The work seemingly magically – all you need is the appropriate material and electrical connections. No complicated machinery. No moving parts. The energy is converted directly, in-place.
So why aren’t we using these devices for all instances where electricity needs to be converted into pressure or motion (via the piezoelectric effect), or into heat or cold (via the thermoelectric effect)? We could throw out so many complicated machines and replace them with these extremely simple devices (provided that it’s possible to obtain the required materials in sufficient quantity).
Because the efficiency of these »magical« phenomena is too low. Too little of the energy put in is converted into the desired effect (or the effect can not be used very efficiently, as is the case with piezoelectric actors that only afford tiny amounts of motion), and the same goes for the reverse conversion. It’s strange, really, that nature seems to provide us with this kind of »magic«, but it keeps us from exploiting it fully, as if to say: Here’s a bit of a miracle, but you only get a little of it. I’m not going to give you this much power, are you sure you could handle it?
Just a thought.