h a l f b a k e r yOn the one hand, true. On the other hand, bollocks.
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The boiling temprature of water is 212. The ambient temprature on the moon, in the sun is 220 degrees. If a thin metalic container on the surface of the moon held water, it would heat the water to it's boiling point. The steam created could power a turbine. After powering the turbine the steam would
quickly cool into condensate, ready to be reheated back to steam. This would be a closed system, yeilding a potentially unlimited supply of energy, on the moon.
according to this the max moon temperature is 212 deg.
http://www.asi.org/...ce-temperature.html
would water boil at the same temperature on the moon? so many questions. [po, Oct 04 2004, last modified Oct 21 2004]
although this link says its warmer.
http://www.solarviews.com/eng/moon.htm
[po, Oct 04 2004, last modified Oct 21 2004]
Mercury Engine
http://www.halfbake...ea/Mercury_20Engine
A planet based energy capturing device. [st3f, Oct 04 2004]
Properties of Water and Steam
http://www.thermexc.../tables/vap_eau.htm
<Caution> Thermodynamics Information [suctionpad, Oct 04 2004, last modified Oct 21 2004]
[link]
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I guess either you'd have to dig into the ground to get a temperature gradient or have an engine that ran so slowly that it did 1 revolution every 29œ days. |
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With a little change in thinking:
When the sun is shining, heat the water so that it changes to vapour: use the pressure to generate power.
When the sun is not shining, the vapour will condense.
In a controlled system, the pressure can be anything you want it to be.
Isn't this just a heat pump with a long cycle time?
How do we intend to use the power? |
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Surely if you build a great big heat
collector panel to boil the water, the
shadow underneath would get blinkin'
cold very quickly and stay that way! |
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Po the boiling point of water varies with pressure lower pressure lower boiling point. This fact sticks in my head due to the British obsesion with Tea. |
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I know that little engine no 1 but I don't what the pressure would be on the moon! |
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Seems redundant with the idea linked by [st3f], you're just doing it on a different airless body. |
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The temprature in a shadow on the moon is a couple of hundred below zero so it's easy to cool the steam into condensate. As for the ambient preasure on the moon it's very low so the actually boiling point for the water would be well under our 212 as sea level. So, it would be easy to boil water into steam to drive a turbine and then to cool it off back into water. The difficult part would be to find the right amount of cooling to keep the water from freezing before it got back to the hot side, but that's what engeiners are for. |
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I don't think it would be so easy to cool down... there is no conduction or convection in a vacuum. |
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I expect it was cooled by radiating the heat (I was going to say "cooled by radiation").
The steam engine in question would need a larger cold side radiator than the hot side, since the temperature difference would be smaller than on the hot side. |
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Miasere, there is no set, fixed location for day-night on the moon. The moon does not have a dark side- it has a far side, a side always facing away from us, due to the fact the time it takes the orbit the earth is equal to the time it rotates on its axis. Sometimes the side facing us is lighted, sometimes not. |
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Also, you don't need 212 degrees to boil water if the pressure is lower. It will boil at lower temperatures, right? |
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However, what if the water was converted to steam in the light, then allowed to rise up into a darker area where it might cool, condense and sink back down into the sunlight. Perhaps with some clever piping you could create a "flow" of heated water rising, spinning a turbine for power without fully going to steam. |
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Don't see anything physically wrong with the system as described, although the actual energy available for capture is likely rather small. Simpler, and probably more efficient, would be a Stirling engine type of arrangement, although I wonder where your heat sink would be. |
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[Link] shows physical properties of water and steam across a range of pressures. Since the proposed system is enclosed, the absolute pressure could easily be controlled from high vacuum up to several hundreds of atmospheres (dependent on available materials). Unfortunately I was awake during my Thermodynamics lectures. |
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re [Zanzibar]'s idea of moving the engine around the moon: in Kim Stanley Robinson's book Blue Mars, there was a colony/city on Mercury which moved around the planet on giant tracks, keeping perpetually in the twilight zone. (Sounds spooky - perhaps that should be 'dawn zone') That was moved by the thermal expansion of the tracks as they moved from night into day, but if you're gonna use tracks it seems like it would be appropriate to use a steam engine. |
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I believe there's a region near each
pole which is in perpetual sunlight,
perhaps at the top of a peak. ISTR
some plans for a moonbase
involve these areas where solar
power is available 24/7 (or should
that be 29/365?) |
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Now, no convection in a vacuum I understand but no conduction? Heat cannot be conducted in a vacuum? If I radiantly heat one end of a steel rod in a vacuum the heat will never spread to the rest of the rod? |
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No convection or conduction in a vacuum, not at all, no-siree bob. Conduction within a metal bar that is in a vacuum, oh sure. |
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But a radiant heat source can transfer heat to an object not in contact with the heat source in a vacuum, no? |
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[bristolz] if you think big enough, then the whole planet Earth is in a vacuum... |
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Oh, I have. I was just looking for a little clarity. Hoping someone would make a distinction. Poking a little fun. You know, the usual. |
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Oh. Nevermind. I was just discerning between “through” and “in,” thinking it might be a significant distinction. Silly me. |
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Maybe a word on the actual mechanics would help clarify this. There would be two radiators that flip positions. The first radiator is on the light side. It gets hot enough to boil the water, increasing presure. The presure i.e. steam is relieved through a check valve, over a series of blades that convert the presure into mechanical energy. The mechanical energy could be used for anything being it powering a generator, a compressor, a pump... On the other side of the blades the steam is goes to the second radiator where it's quickly cooled. Once the top radiator has boiled off all of it's water, the two radiators rotate, putting the bottom one filled with water in contact with heat and the empty one in the shadow to cool of steam and collect water. This might work better in space than the moon, I wasn't thinking about the moons dark periods. |
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Theres a source of energy yet untapped , I know not where it is, but I doubt it
is on the moon ,but it might have something to do with it? |
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This is pretty clever, I think. Forward it on to Popular Science and see what they do with it. |
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If the energy of the system could be used to keep it in motion somehow, say you build a big rail around the moon or something, then your problems with being on the wrong side at the wrong time would be solved. |
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Or how's this: no recovered energy perhaps but a useful transport perhaps. Say you had a tube, encircling some minor circle around the moon. The tube would have gates every so often that could be raised or lowered, to act as check valves and keep the high-pressure and low-pressure sides of the system separated. A vehicle would ride the pressure differential around. |
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Do the rocks emerging into the dark side of the moon cool that quickly since the only modes of heat transfer available are radiant and conduction through the planet? |
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[BadBuddha] your last post makes it much clearer what you have in mind. Have some pastry. |
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Maybe use liquid Nitrogen? |
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Just to clarify, I don't believe there is an ambient temperature...just a solar temperature. Wouldn't ambient require an atmosphere? |
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For a modification of your idea, how about a spinning satellite with two chambers. The one facing the sun could boil the water, pressurising the sun facing side, then shoot into the shaded, freezing side. This would put it back to a liquid or gas. Then when the shaded side hit the sun, the process would repeat. |
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True! But, you couldn't attachethe moon to, say, a small satellite or space shuttle ;) |
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And, you're right. It probably would throw it out of orbit. Maybe counter weight powered by solar panels :) |
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The moon idea is much better...I was just trying to expand it. |
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I stopped reading the annos about half-way down, so please forgive me if this hasn't already been covered. |
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There are closed-loop steam generation plants located on earth, where the condensed steam is simply recycled back through the system. The lack of atmosphere plays no part in determining whether or not this would work on the moon. |
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All that's required is that there be a suitable source of heat and a suitable heat sink to re-cool the steam and reduce the workload of the compressor. Since the passage through the turbine extracts work from the steam, the steam cools a bit, but since work is also required to pump the liquid water back into the boiler under pressure, you don't get much out of a system without a cold sink. The system just heats up until it's in a fully gaseous state, whereby it becomes a zero-return device. Cooling the steam allows you to extract more energy than the pump requires, letting the system continue running. |
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On the moon, the lack of atmosphere is not a hindrance. The cold sink can be as simple as a large radiative plate located in shadow, with a view of space and insulation from direct or reflected solar radiation. |
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Even if a surface in direct sunlight only gets up to 212F, there is the possibility of using reflectors to concentrate the sunlight. |
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This could be a viable alternative to nuclear batteries for long-term lunar habitation. Bun. |
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I can't go along with this idea. While there are
engineering ways to get the sucker to work, there's no
need or economic benefit. Photovoltaics would give
power more cheaply than any complex contraption. Let's
not forget we have to get the stuff up there to begin
with. |
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