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posted by [personal profile] damerell at 05:52pm on 09/09/2014
(For those not familiar with the question, it is; given one pair of _Portal_ portals, that transmit matter between them instantaneously preserving velocity relative to the portal surface, what's the most useful thing we can do?)

My answer to the question is energy generation: drop a solid steel bar through the portals. If the portals are 2m^2 and we drop the bar at a very modest 10 m/s we get out 160MW.

It's trivial to extract this energy; electric railway locomotives have all the engineering we need. The Class 91 is an obsolete design now, but each bogie's 4 driving wheels can move nearly 250kW from one steel surface to another (an electric motor pretty well runs in reverse as a generator, the frictional considerations are the same, etc). We can also obviously drop the bar at the Class 91's c. 60 m/s top speed, generating just under a gigawatt from 16,000 steel wheels pressed against the bar. (Sanity check; each wheel then has just over 300 cm^2 of bar surface to itself). The bar is safely held by the infrastructure of wheels, electric generators, and the supporting framework for same; it can be sped up or slowed down by adjusting the load.

Obviously one could do much better with custom designed equipment, but this is clearly possible because it uses existing - 1980s - technology.

It's counterintuitive that the speed of the bar makes one get more free energy out, but it does make sense. Cyclists (who are also nerds) are familiar with this effect because, although power to overcome air resistance varies with the cube of speed, power input from gravity varies with speed. Another way of looking at it is that our figure for work done by gravity on the bar (which is where that 160MW comes from) should match our figure for potential energy created by teleporting stuff. 2m^2 (cross-section) * 10 m/s (speed of bar) * 8000 kg/m^3 (density of steel) * 100m (height teleported) * 10 J/kg/m (energy imparted to mass by being teleported up against gravity) is indeed 160MW. Now it's clear why a faster moving bar works better; more mass is being teleported every second.

I don't know what the air resistance losses are - although I hope they're low for a polished steel cylinder - but in principle we could run the mechanism in a low-pressure environment.
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damerell: NetHack. (normal)
posted by [personal profile] damerell at 06:52pm on 09/09/2014
I think there might be ethical concerns with that. :-/

Also, with electricity at 8p a kWh, if my free energy machine is so expensive to build and service that I can take only 10% of that as profit, I think I make ten [1] million quid every 2 months on the 60 m/s machine. (And I doubt 100 m is the upper limit on the length of the bar, it being a steel girder 2m^2 thick, supported on all sides).

One thing that concerns me about my plan is that, unlike the water-turbine option discussed elsewhere, if we lose our grip on the bar it will be impossible to catch it again. Eventually, presumably, even in a partially evacuated environment, surface friction will heat the bar to the point that molten steel is flung off it downwards at tremendous speeds. I suspect after that things would become very exciting (especially if an evacuated environment was punctured causing a sudden influx of air).

[1] Boo, order of magnitude error here originally.
Edited Date: 2014-09-09 06:53 pm (UTC)
hairyears: (Default)
posted by [personal profile] hairyears at 08:50pm on 10/09/2014
I recall that the rated power output of a Magnox reactor is 680 MW and they built them in pairs.

Nowadays, we would consider that a small power station. I think you're unambitious...

How much energy could you extract from a portal opening into the deeper levels of a gas giant? Or deep enough that the stream of expanding hydrogen wouldn't shatter the blades of a turbine - and we can keep the pressure gradient and the gas velocity down, with a really *long* sequence of expanding rotors and stators...

...And the 'dead' exhaust is hydrogen. Lots of hydrogen. Feed it, and compressed air, into the ignition chambers of another twenty gas-burning turbines.

Vent the exhaust to atmosphere: it's just water. Or, if the second-stage 'burner' turbines are fixing nitrogen into NOX in the ignition chambers, use the massive cooling effect of the first-stage expansion to liquefy the nitrogen out of air and run the burners on cold oxygen. Or liquid oxygen, if you can build a reasonable air-distillation plant around the first-stage turbine.

Oh, and you've got lots of electricity, nitrogen and hydrogen. Maybe you can fix some ammonia for fertiliser as a sideline.

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