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Name: CJ
Status: student
Grade: 9-12
Country: South Africa
Date: Winter 2011-2012


Question:
Since water can boil at lower temperatures by lowering atmospheric pressure, will it be possible to build and sustain a steam engine at the lower boiling point of water?



Replies:
Yes, but such an engine's maximum horsepower output is reduced in proportion to the pressure, typically.

Jim Swenson


Andy - It is definitely "possible", but almost never practical enough to be worth doing.

The one real-life example I can think of is: the through-the-car-window antennas for cell-phones, GPS, etc. These have a cable from the transmitter box, carrying about 1-to-10 Watts of RF current, to a patch stuck to the inside of the window. At the same spot but on the outside face of the glass, is a similar patch which picks up the power radiated by the first patch, and a second length of cable running to a roof-top antenna. Notice that the wireless gap is only 0.5 cm, but the width of the patch is ~5 cm. Also, the freespace wavelength at that frequency is only 4-10 times the 5 cm size of the patch.

So I am sure it is possible to jump power across a 10-meter-wide street by having antenna-fences about 100-meters long and 30-meters high on both sides, and it also requires changing the 50 Hz to ( lightspeed / 1000_meters ) = 300 kHz. Which is fairly expensive to do for many kilowatts, but we are getting to where it might be affordable.

But then it would be a poor idea to have unprotected pedestrians walking in that section of that street. The electric fields would be pretty high. I think 300 kHz is a typical frequency for a Tesla coil . . .

It is conceivable to do a better-contained job by converting the kilowatts of power from 50 Hz to microwave frequencies, say 10 GHz, and beaming it from one parabolic dish antenna to another. Then the dangerous energy would only fill the space between the two 2-meter wide dishes, probably elevated well above everybody's head.. At the receiving end it would have to be converted back to 50Hz. So far, our best electronics can do all this with maybe 60% efficiency.

So it would waste half the power, get real hot, not last long, be extremely expensive, and leak very strong electromagnetic pollution.

Same story with lasers. Our best laser so far is maybe 40% efficient. And the solar-cells we would have to use to change the light back to electricity would loose some too. I think they might be 80% efficient if the laser wavelength matched the solar-cell bandgap exactly. Probably worse than that, in practice. The good thing about lasers is that the light-waves can be blocked by most solid objects. They do not sneak out and go everywhere like radio waves and electromagnetic pollution.

I think somebody will invent a 10-cm-diameter tungsten-carbide tunnel-drilling snake first, complete with eyes, ultrasonics, metal-detectors, etc.. Then maybe one could get that street crossed with a wire at reasonable cost.

Some engineering minds would really love to be able to beam power down from giant solar stations in space. But we are not yet starting a "space-power program", because we cannot do a good job of beaming power yet.

Jim Swenson


Yes. A steam engine could be operated at lower pressure, resulting in a lower temperature and energy content of the steam.

Hope this helps, Burr Zimmerman



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