With Thermionic Generation, the Steam Age Is Looking Sad

The vast majority of electricity in the world is funneled through one general technology: the electrical generator. Some source of mechanical energy is converted into electrical current via the movement of a magnet along an electrical conductor, or the other way around. Spin a magnet around the inside of a coil of copper wire and the effect will be the rotating magnetic field creating a sort of rotating electronic pressure within the coil, which is called voltage. That’s the end product, and the modern technologized, industrialized world is more or less based on it.

It’s a neat scheme because there are a lot of different ways you can produce a mechanical force for the generator: water moving through turbines in a hydroelectric plant, heat creating steam that turns turbines in a nuclear power plant or coal plant, just the regular old wind turning a turbine, or an internal combustion engine’s rotating driveshaft delivering mechanical energy to an alternator via a rubber belt. The basic idea has been around for almost 200 years, since Hungarian inventor Ányos Jedlik built his first “electromotor” in 1828.

In most generators there’s another, preliminary conversion of force: thermal to mechanical. For example, heat creates steam creates an increase of air pressure that spins the blades of a turbine. Gasoline combustion creates a extreme burst of heat and pressure that pushes a piston. What if it were possible to eliminate a step? What if we could just take heat, any old heat, and covert it directly into electricity with nothing in the middle?

Such a technology would not only be more efficient than a steam turbine, it could be used for very weak sources of heat, like the wasted heat generated by solar power or the excess heat created by a car engine. This is the project of Jochen Mannhart of the Max Planck Institute for Solid State Research and colleagues at Stanford University and the University of Augsburg: creating a so-called thermionic generator that achieves efficiency high enough to be useful. Their work is described in the free, open access Journal of Renewable and Sustainable Energy.

It works like this. Take two metallic plates and separate them with an extremely small vacuum space. One of the plates is hot (heated by one of the usual heat sources) while the other is cold. What happens is that the hot plate will kick out a bunch of electrons (you’re adding energy to it), which will then condense on the surface of the cold plate. The result is a difference in charges between the plates and, thus, usable electrical current. Neat.

Well, sort of. So far, thermionic generation is drooping down at about 10 percent efficiency. It’s been used so far to power TOPAZ Soviet spy satellites, but not much else. The technology’s advantage then was that it allowed for very lightweight nuclear generation, but the tradeoff with the TOPAZ reactions was a 50 percent drop in power output from a typical nuclear power plant. The problem is that a cloud of negative charge forms within the vacuum, preventing the hot plate from kicking out more electrons and effectively putting the brakes on the system’s output.

The Mannhart team overcame this by starting with an electrical field in between the plates that can be manipulated ("shaped") in such a way as to accelerate the electrons away from the hot plate and then decelerate them again when they arrive at the cold plate. (Note that this journey is only a couple of millionths of a meter long.) The result was 40 percent efficiency. Not bad. A typical steam turbine nuclear power plant sits at about 30 percent efficiency, with coal closer to 40 percent.

“Although the technical development of such thermoelectronic generators will require further substantial efforts,” the IRESR paper reads, “we conclude that a highly efficient transformation of heat to electric power may well be achieved.” Mannhart told Physics Today that the technology could be ready for the market in four to 20 years. It's not a bad thought: more efficiency means less reactors. That'd be a good thing.

@everydayelk