Can passive heat collection be used to heat water to very high temperatures?

[russellsh2]

TL;DR Summary

Making a passive heat collector using the Evacuated Tube Solar Heat design

I ask this question making refference to the Evacuated Tube Solar Water Heaters that have alcohol in their core tube to transfer heat from the Evacuated Tube to the water. Is it possible to make a passive water heater that uses a step system of tubes filled with water in a gradually decreasing vacuum. In other words the first would be in a high vacuum lowering the boiling point drastically. The the bottom of the tube would collect heat and the vapor would rise to a small bulb that surrounds the bottom section of the next tube. The next tube would be at a slightly lower vacuum, making the boiling point slightly high than the first tube. It would collect heat from the first tube and turn to vapor and rise to its bulb that surrounds the next tube and so on till the last one deposites all the heat into the normal pressure water, thus using passive heat collection to heat water to very high temperature.

 

[BvU]

the vapor would rise to a small bulb that surrounds the bottom section of the next tube. The next tube would be at a slightly lower vacuum, making the boiling point slightly high than the first tube.
It would collect heat from the first tube and turn to vapor and rise to its bulb that surrounds the next tube

And there it would not transfer enough heat to the bulb that surrounds the next tube to bring that to the higher boiling point. so: no!

and so on till the last one deposites all the heat into the normal pressure water, thus using passive heat collection to heat water to very high temperature.

because that would go against the second law of thermodynamics.

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[russellsh2]

Im not trying to question your credentials. I just dont understand why it wouldnt transfer the energy. The Evacuated Tube actually does just what I described, but it uses alcohol instead of water. ()

 

[BvU]

Changing to a fluid with a lower boiling point (e.g. from water to alcohol) would work for heat transfer, but it doesn't bring you to a higher temperature.

My credentials may be doubtful, the second law is way above suspicion .

In the video the alcohol vapour is heated way above boiling point and condenses against the much lower temperature of the water. No violation of the second law occurs, because heat is transferred from higher to lower temperature.

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[russ_watters]

Nothing in that description talks about any temperature differences, which are required to drive heat transfer. If you have an idea about that, you haven't said it. But it looks to me like the entire system would establish equilibrium with the surrounding temperature and then nothing else would happen.

 

[russellsh2]

Thanks guys. I think I see. I think I overestimated the cooling ability of the phase change of the fluid. I know theres more but I dont know how to drscribe it. Let me ask you this then. Why even use alcohol in their design? Wouldnt there be a greater effect on the water if it was in contact with the 300deg grass?

 

[jack action]

In other words the first would be in a high vacuum lowering the boiling point drastically.

I'm not an expert on the system but from what I understood from your video, this is not what the vacuum does.

The vacuum serves as insulation between two layers of glass. No air - no material in fact - no heat conductivity possible towards the outside of the tube.

So the rays of the sun go through the glass and heat the inner tube filled with alcohol. The glass is insulated so no heat escapes, transforming the system into a very effective small greenhouse. This explains why the very high temperature inside while still cool to the touch.

 

[russellsh2]

You are correct. I was just toying with the idea of shifting the temperature that the phase change occurs by putting water in a vacuum. The greater the vacuum, the lower the temp. In other words boiling water at room temp.

 

[russ_watters]

Thanks guys. I think I see. I think I overestimated the cooling ability of the phase change of the fluid. I know theres more but I dont know how to drscribe it.

If you hook up a vacuum pump to a vessel filled mostly with water, the vacuum pump will evacuate the air and the water will start to boil. As it boils, the water will cool and it will reach an equilibrium temperature where the water conducts heat through the sides of the vessel to keep it boiling at the same rate (in terms of energy) that it is boiling away. This will always just absorb energy from the environment and remain cooler than the environment (until you run out of water). Propane tanks actually work this way (but at a higher than atmospheric pressure). If you turn off the vacuum pump (or stop the propane flow), it will simply warm back up to ambient temperature. There's no temperature gain above ambient.

Let me ask you this then. Why even use alcohol in their design? Wouldnt there be a greater effect on the water if it was in contact with the 300deg grass?

Well, in that case you are heating way above ambient via exposure to the sun, so it is much different from what you described. But the reason they use alcohol is probably because they don't want to boil the water that they are tying to heat.