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Water-cooled lander
tanjent
post Aug 22 2007, 05:22 PM
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There is a recent posting on Emily's Planetary Society blog, which must be Doug's because she's not there herself, although her name is the only name on it. The subject is using water to cool a long-lived surface probe on Venus. It sounds far more practical than any of the other proposals for landing giant atomic-powered refrigerators, or developing a whole new family of high-temperature semiconductors, etc.

But I didn't understand the whispered criticism to the effect that the Ekonomov paper assumed that the water would absorb heat only from the one watt of power driving the instrument package itself. I simply can't believe that he went to the podium and presented his model without taking into account the fact that the surface of Venus is a pretty hot place, and that the proposed probe would be absorbing the ambient heat. This is an interesting proposal and I would like to understand both the original calculation of 50 days to bring the water to a boil, and the cited flaw in the calculation. I too find it hard to believe that it would take 50 days to bring water to a boil on the Venusian surface, but where exactly is the error, and what remains after we correct it?

Doug is busy of course, but I hope he will find the time to address this when he returns, if someone else hasn't done so by then.
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helvick
post Aug 22 2007, 06:32 PM
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QUOTE
..The concept is based around a 60-centimeter-wide pressure vessel, that's thermally isolated from a larger vessel using a vacuum. Inside the vessel are all the electronics you need, insulated from the otuside using 100 kilograms of water..

It appears that what is being suggested is a very small electronics package surrounded by the water within the 60cm internal vessel. A 60cm diameter sphere would hold ~110kg of water so you probably could get 100kg of water + a few kg of smarts into such a package.

The assumption is that you can effectively thermally isolate this internal block from the outer shell where you have the ultra high temperature power components and the sensors. I assume the objection came from someone who had some idea about how hard it is to build a thermal isolation system that keep the heat transfer across a 400deg+ temperature gradient into a 60cm core at <<1 watt. to be fair it does seem a bit fanciful but I'm no thermal expert.

Apart from that the numbers don't seem to add up (to me at any rate).

100kg of water. Specific heat capacity of 4.2 kJ.kg^-1.K^-1. Say we are heating that from an initial (arbitrary) temperature of 300K (25C) to 600K. That requires 4.2 * 100 * 300 kiloJoules. That's 126000 kiloJoules.
A Joule is a watt per second, or a kiloJoule is 0.2777 watt hours so 126000 is 35000 watt hours. Divide by 24 and you get 1458 days. Maybe he's talking about Venusian days?

The good news though is that the error is all in the right direction. This means that you could support an electronics package that produced 30 watts of power instead and still last 50 days before you had to start venting coolant. The other number they talked about then actually makes sense too. The latent heat of vapourisation of water is 2260kJ.kg^-1. Conveniently enough 30watts for one day is close to that at 2592kJ so you lose about 1.1kg of coolant per day once boiloff has started.

So maybe the system is viable for an electronics package that generates 1 watt of heat and a thermal isolation system that leaks 29 watts? That may still be impossible but it's certainly a lot easier to build than a perfect thermal isolation system.
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tty
post Aug 22 2007, 08:41 PM
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Why start at 300 K rather than 270 K? By shielding the package from the Sun during passage it should be quite possible to get the water to freeze. Note that melting a lump of ice takes about the same amount of heat as warming the resulting water from freezing point to 80 degrees centigrade. The only drawback is that the water container must be built so it is not damaged by the water freezing (ice is of course less dense than water). However you would probably have to do that in any case for safety.
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djellison
post Aug 22 2007, 09:03 PM
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Unfortunately - the guy's english wasn't very good, and his details were fairly thin on the ground. He talked about the power consumption of the electronics being only 1 watt to minimize the energy put into the water - and perhaps through langauge barrier rather than anything else - it seemd that he was infering that only 1W of energy would be heating the water. (and it was Earth days he was talking about - he wanted to reach a significant part of a Venus year - 100-200 Earth days)

Obviously - the cable from the batteries itself will be sinking more than that, ditto any other connectors to the 'outside' world in terms of instrumentation, comms, the vent for steam etc etc. I don't think the language barrier between the speaker and the audience helped when people were asking about the 1w etc. Think about what the rovers do when they're on 240 whrs - 10 watts average. Now think what a venus lander would do with 1 watt average.

There was another point made to me - a seismic instrument on 1 bps? Forget it. Seismic measurements from inside a boiling kettle? Crazy.

Like I said in the blog - most agreed that the principle would obviously work - but just not how he was describing it.

Doug
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RJG
post Aug 22 2007, 09:59 PM
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These calculations are based on the use of water. I remember using test loads for broadcast TV transmitters that employed the latent heat of evaporation of a Glauber's Salt (sodium sulphate) solution to get rid of the dissipated power. I believe that sodium sulphate has a significantly higher latent heat figure -but Wikipedia etc have let me down so I can't find the number right now.

If the specific heat + latent heat approach is a good one then obviously choosing the best compound will give maximum duration on the surface.

Rob
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tasp
post Aug 22 2007, 11:08 PM
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None of my steam traction engine information sheets goes up to 900F, but I suspect the pressure required to maintain the water as liquid will be large.

In fact, larger than the external atmospheric pressure, so the water will hit an equilibrium with venting into the high pressure atmosphere at a temperature less than the external temp. Might as well design the electronics to tolerater this temp/pressure, for maximum life.

Question for the chemistry majors here, is there a nice stable compound, that would not harm the electronics, and minimize that equlibrium temperature ??
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Greg Hullender
post Aug 23 2007, 06:16 AM
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Tasp: the idea was that the water is INSIDE a glorified thermos bottle, so it isn't exposed to 900F at all. Anyway, that's way over the critical temperature of water, so no amount of pressure could keep it liquid.

I'd say the first question to answer would be "given the best thermos we can make, with the atmosphere of Venus on the outside and holding 100 Kg of water initially at 4C and 1 atm on the inside, how will the water temperature vary as a function of time?" If the answer is, "it'll boil in hours," then there's nothing more to discuss. But if the answer is "it won't reach 100C for months, and even then it'll take two years for it to all boil away," then maybe there is something to it.

Anyone know enough about modern thermos technology to estimate this?

--Greg
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djellison
post Aug 23 2007, 12:03 PM
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The principle would be to have the pressure vessel at 90+atmospheres so the water would boil away at that critical point at 300 deg C (numbers are 'roughly')

Doug
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marsbug
post Aug 23 2007, 01:36 PM
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QUOTE (Greg Hullender @ Aug 23 2007, 07:16 AM) *
" how will the water temperature vary as a function of time?" If the answer is, "it'll boil in hours," then there's nothing more to discuss. But if the answer is "it won't reach 100C for months, and even then it'll take two years for it to all boil away," then maybe there is something to it.
--Greg

I usually try to avoid numbers but this intruiged me so heres a quick and dirty attempt:
Assume the thermal isolation system consists of a double walled shell with a good vacuum in between (say 10^-4 mbar) so that convection and conduction between the walls is close to zero, and that the shell is constructed of a low vapour pressure material such as tungsten, or is being actively pumped to keep it at that level. Assume also that connections between the walls are minimised (could fairly low tech heat tolerant machinery outside the shell be operated entirely by remote by an electronics package inside?) to ten square centimetres of cross sectional area, made of material with a low thermal conduction , say equal to aerogel at 0.03 w/m/k. As a random figure lets say the separation between walls is 10 cm, and the water starts at a temperature of 273 degrees Kelvin. The amount of heat transmitted through the connections would then be:


0.03= q/t * (L/(A*ΛT)
wher q/t is joules per secound, L is the length of the conducting connection, A is the total csa of the connections and ΛT is the temperature difference in kelvin.

So:
q/t= 0.03/ (0.1/(0.01*400)= 1.2 watts. Which looks ok.
BUT:
The major source of heat into the inner vessel will be radiation from the outer wall. Assuming that the outer wall reaches the same temperature as the venusian atmosphere fairly quickly, and that it can be approximated as a black body it will be radiating heat onto the inner wall at
W= σ*A*T^4
Where W is heat transfer in watts, σ is the Stefan-Boltzmann constant, A is the internal surface area of a sphere with an internal radius of 40 centimeters and T is the temperature of the outer wall in Kelvin (673 deg):
A= 4πr^2 = 4*π*0.4*0.4 = 2.01 meters square (ish)
W= σ*2.01*673^4= 23797.24 watts falling onto the inner shell? Or roughly 24 Kj per secound.
So water has specific heat capacity of 4.2 kJ.kg^-1.K^-1, so for 100 kg to reach 373 Kelvin from just above freezing requires 4.2kj*100*100= 42000 Kj. Assuming it heats linearly (which I know it doesn’t but can’t remember how to work it out properly) 42000/24 = 1750 seconds, or just under half an hour.
That doesn’t look very promising! Where did I go wrong anybody (bet its something very simple!)?. But it does assume one atmosphere of pressure, not ninety! Any chance I could look at how you worked it out for that figure doug?

Edit: Even if it works at ninety atmospheres I'd be impressed by a container that would hold a good vacuum for fifty days under those conditions without an active pump. I'd also be impressed by an active pump that would run for fifty days under venus surface conditions. Not saying it can't be done, just that I'd be impressed! biggrin.gif wink.gif


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djellison
post Aug 23 2007, 01:52 PM
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QUOTE (marsbug @ Aug 23 2007, 01:36 PM) *
Any chance I could look at how you worked it out for that figure doug?


I didn't work out anything - it's all the presenters work. I'll drag up the abstract PDF later smile.gif

Doug
Attached File(s)
Attached File  EPSC2007_A_00387.pdf ( 42.88K ) Number of downloads: 340
 
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AndyG
post Aug 23 2007, 02:22 PM
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Marsbug - you're maths is somewhat awry.

Think of the vessel as a Dewar/vacuum flask. It boils down (!) to the difference of the inner and outer shell temperatures ( = disturbingly high) and the emmissivity of the surface of the vessels, given that it's a radiation source of heat across the vacuum.

So your W= σ*A*T^4 wants to be W= e*σ*A*T^4

e can be as low as 0.02 for polished gold or silver. Plugging that in, the radiation input onto the inner sphere will be around 500W. Toastie!

This'll make for a vessel that'll last hours at most. Certainly not days and weeks.

Whilst looking at this (but you beat me to the post) I did spot some work on Si IC's conducted (!) at 600+C. That might be a better way to go (albeit more expensive).

Andy
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marsbug
post Aug 23 2007, 02:45 PM
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Thanks Doug and AndyG! This makes it seem a bit more plausable, and use of gettering answeres the objection of how to maintain a vacuum (shoulda thought of that!). Fifty days still seems like a long time to me, but a long term venus lander; what an idea! Going a bit off topic here but what science objectives could you fulfill with one?


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AndyG
post Aug 23 2007, 03:23 PM
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[Excessive quote removed, hopefully before Doug caught it!]

Other than seismological studies - and multiple landers would be needed for that - I think the dipping balloon has more going for it. With the right selection of heat-absorber, using the suggested technology, you could cool off above the altitude of your choice (Sodium freezes at 45km, Water freezes at the cloud tops around 58km) allowing for short-stay trips to the surface coupled with long duration study of Venus's meteorology and surface.

Andy
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helvick
post Aug 23 2007, 04:13 PM
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All the above makes sense but my understanding is that you can further reduce the heat flux by adding additional intermediate isolation layers between the inner and outer flask walls (see page 16 of this set of lecture notes for the detail). The net effect is an additional 1/(n+1) reduction in heat flux where n is the number of additonal layers so (in theory) you could reduce the flux rate down to ~50 watts with 9 additional inner walls. This approach is similar to that adopted by the JWST for its main shield.

The materials issues remain very tough though - the components all have to be very stable at up to 600K.
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algorimancer
post Aug 23 2007, 05:36 PM
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I wonder whether a silica aerogel might be a better solution as an insulator. Light-weight, great insulation, in contrast with the problem of maintaining a vacuum on the surface of Venus. Melting point of 1200 degrees celsius seems appropriate. Apparently the variant with carbon added to the mix makes an even better insulator.
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