I just read an old article written by SF author Ben Bova about MHD power generators. MHD stands for magnetohydrodynamic: the basic idea is that you burn a fossil fuel at very high heat (approx 5000 degrees F) with traces of potassium; the heat turns the gas into a plasma and due to the ionized potassium becomes an electronic conductor; the plasma is pumped through a tube lined with magnets and the passage of the charged plasma past the magnets generates electricity. The supposed benefit is you create approximately 50% more usable power per amount of fuel burned than you would through a conventional steam turbine. In addition, the plasma is “cleaned” as it cools in order to recycle the potassium and this process removes other chemicals from the exhaust as a side-benefit, so the end product creates less pollution.
Now according to Bova, this technology is a great thing and we should all be using it. However due to political problems he says the project hasn’t been developed to practical levels and remains a laboratory experiment. (Bova, despite being a conservative, argues the project should be funded by the government rather than private industry.) Bova also argues that the United States, despite developing the original idea, will fall behind other countries like Japan, France, and the Soviet Union which are working on developing MHD generators for real world use.
As the above reference to the Soviet Union indicates, this article was not a recent one. Bova originally wrote it in 1982 and what I read was a 1990 update. But as far as I know, MHD power generation still remains a experimental operation at best.
My question then is whether MHD really is a good idea that could (and may still someday) work in the real world. Or was Bova glossing over problems that existed since the MHD was first invented back in the 1960’s? Or have there been problems that have been discovered since 1990 that are holding up development?
This smacks of that old kooky standby: “the oil companies are supressing this magical new technology”. Methinks there may be technical reasons MHD power generators aren’t in every municipality.
I don’t know anything about this particular technique, but the phrase “due to political problems… the project hasn’t been been developed” is a pretty reliable indicator that it’s bogus. If you really had a workable idea for a vastly more efficient use of fuel, your problem would be in dealing with all the investors who wanted a share.
Bova isn’t saying that there’s a conspiracy. What he basically said is that while MHD is a good idea it needs to be developed. That development will cost a good chunk of money. Corporate funds haven’t been enough to move it to full development and government funds are mostly going to areas like fusion and solar research.
Bova wrote a book about it Little Nemo, “THE FOURTH STATE OF MATTER, Plasma Dynamics and
Tomorrow’s Technology”. This site reviews the book and sings the praises of MHD generators.
This subject is one near and dear to me. My old next-door neighbor was an engineer for AVCO-EVERETT Corp., back in the 1870’s. This firm had a plasma guru (Dr, Adrain Kantrowitz) who had a project going to develop a MHD generator. Unfortunately, the process is very difficult to control, and the whole thing was cancelled.
According to the following press release it seems that our governmetn is working on MHDs.
They are aiming for a 10 megawatt generator. Considering this thing is meant to be flown around that seems pretty incredible. Anyway, as often happens with military development, the technology if useful usually finds its way into the marketplace eventually.
Yeah, thanks to military development, we now have things like: The Home Flamethrower Kit[sup]TM[/sup], Nuke 'Em[sup]TM[/sup] roach bombs, and Daisy Cutter Lawn Mowers[sup]TM[/sup]!
Actually the MSU/USAF project sounds like the kind of thing that Bova feels was holding back MHD development. His basic argument was that we needed to stop spending money building dozens of prototypes and put that money into building one real powerplant. He seems to feel that forty years if enough time to conduct experimental research and it’s now time to start delivering commercially available electricity.
True, a plasma can conduct electricity, but that just means it is the
medium in which electricity flows. How can plasma be the source of electricity?
In a highly charged plasma, the electrons are stripped from the nuclei? If you can somehow siphon off the negative electrons,
what are you going to do with all the positive nuclei?
They work, but there are some big practical problems.
The MHD generator produces power in the same way as any other type of generator - by moving a conductor in a magnetic field. This will work for any fluid conductor - liquid metal, salty water etc.
You put a magnetic field across the “stream” of moving plasma, e.g if your plasma stream is horizontal, you could put your field across it top-to-bottom. You then place two conductors on opposite sides of that stream, left and right, so that the plasma touches them and flows bewteen them. Hook these conductors to an external load (e.g. a light bulb) and a DC current flows left-to-right through the plasma stream at right-angles to the direction of flow and the direction of the magnetic field. Energy is extracted from the velocity of the plasma, slowing it down. (Electrons leave the plasma at one side and re-enter on the other - there is no problem with left-over positive nulcei.)
On its own, an MHD power generator isn’t very efficient, although it does allow an incredible amount of power to be generated from a very small unit, (Which is why the military like it. Good for running laser weapons, railguns etc.) The advantage for commercial power generation is that it can be used to increase the input temperature of your “heat engine”. The laws of thermodynamics tell us that to generate power from a heat source (e.g. a burning fuel), then the higher the temperature, the more efficient you can be. However, you need a system for extracting the power that can stand up to that high temperature.
The MHD generator extracts energy from the velocity of the plasma stream. First you need something that will convert the very high temperature of the plasma into velocity. That’s simple, it’s called a rocket nozzle and we know all about them. Then your MHD generator extracts some energy from the plasma jet, slowing it down, effectively reducing its temperature. Then you can feed that jet through a gas turbine, extracting some more energy and cooling it some more. Then you can use the hot gas turbine exhaust to boil water and run conventional steam turbines. Overall efficiency is good.
The practical problem of course is building a power plant that can stand up to a rocket exhaust running more or less continuously for the 30 years that is a typical power plant design life. With current technology, that’s a tall order. Combined-cycle power plants do exist where you omit the MHD generator but burn your fuel first in a gas turbine and then use the exhaust to boil water for steam turbines. The increase in input temperature allowed by this has increased fuel efficiency in the same way, but to a lesser degree.
I’d hate to see what 30 years of rocket-velocity gasses would do to an exhaust nozzle…! I’m no materials engineer, but I’m betting that even Stellite would erode away to nothing in pretty d*mn short order.
Well, I AM a materials engineer and you’re right on the money! Superalloys like Stellite are used for the turbines at the backs of jet engines, which have service lives of around 100 to 1000 hours depending on whether the engine is a military turbojet or a commercial airliner turbofan.
Since your rocket nozzle doesn’t have to actually lift anything, there are options for MHD generators not open to rocket engineers. You could shield the inside of the nozzle with a thin “blanket” of inert gas, or you could build the nozzles out of blocks of cheap refractory ceramic (in effect, high tech firebrick) and change them out as they erode. It may also be possible to use magnetic and electric fields to prevent the plasma from touching the nozzle walls at all.
The real sticking point is the current collecting electrodes, which have to be touching the hot plasma for the thing to work. They are going to erode something awful, and I can’t see any way round that.
I wonder if the collecting probes could be made of a sacrificial material, and simply be ‘fed’ into the plasma stream as as they erode…? Kinda like the electrodes on an arc-search light. As the electrodes erode, an automated system pushes the carbon electrode bars back together, maintaining the arc gap at a constant value, or at least constant until it runs out of material.
Of course, pure carbon would not be exactly the material of choice in an MHD system, I’d think.
