Half a croissant, on a plate, with a sign in front of it saying '50c'
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Rarefaction A/C

with cold sump
  [vote for,

This Idea is for a dual-loop A/C system that uses water in all its common phases as its primary coolant, and also as a "cold ballast" to enable shifting of external energy consumption to a cheaper and more thermodynamically efficient time period.


The COLD SIDE (distributing coolth for your climatic pleasure) is standard: a radiator (with a fan), utilizing a light glycol or propanol solution for transportation media. The (closed) loop goes in/out of the cold sump tank.

The COLD SUMP is a well-insulated tank that starts the night half filled with water and ends it three quarters filled with ice particles. A flap-valved pipe leads to the hot side, from which comes the control-valved water-return.

The RAREFACTOR, the beginning of the hot side,is a specialized vacuum pump: a piston in a cylinder, on a flywheel run by an electric motor. A line with an outwards flap-valve leads from this to the heat radiator.

Since we're using water for heat transport, the HOT SIDE (heat dissipation outside of the building) isn't limited to the boring, noisy fan/closed-radiator model: it can (and should) be a small fountain, a metal-heatsink-sculpture waterfall, a boiled-trout pond[1] or all of the above. A valved return line goes back into the cold-sump tank. For all intents and purposes this is same'ish a commercial cooling tower, but the temperature of the incoming water is markedly greater (and volumetrically smaller).


Storing coolth:
The rarefactor pulls water vapour out of the sump tank, reducing the pressure inside to a vacuum; the heat pond cools it, and it's fed back into the tank. Eventually the tank is filled with ice particles.

Releasing coolth:
These ice particles (which eventually becomes slush then water if all the cold's run out) cool the cooling side of the system when it's run during the day.[2]


The cold side is run by the building thermostat, and a timer (which is used if, for instance, nobody's going to be at home during the day).

The hot side is also run by a timer which turns it on at night.

The rarefactor is pretty neat: on the downstroke it pulls vapour out of the tank against external air-pressure, but (after the tank-side flap closes) the upstroke is powered by the pressure differential for almost the complete stroke.

The water return from the external pond runs through an adjustable pressure-valve, ensuring that the tank's internal pressure is at an optimum to efficiently accomodate the rarefactor operation, ie: there's a most-efficient peak vapour-pressure amount that depends partially on rarefactor cylinder-size, duty cycle, and mechanical losses.

Last but not least, some non-magical method of determining when the sump tank is full of ice hooked to the water return, and a simple float valve in the pond to add water lost to evaporation.


Example system: a daily cooling requirement of 25,000 BTU/hour x 12 hours

= 300K BTU
= 300MJ
= 3/4 tonne of water cooled from 20C water to 0C ice
= 1.5 cubic metres of dry snow/ice [3].

A tank of 2 cubic metres volume should easily accomodate the daily cooling needs with a bit of headroom.


Oh yes, during the winter, when you don't need the air conditioned, you can dump your mash into the tank, adjust the pressure valve, hang a bucket on the rarefactor outflow pipe and produce bio-ethanol.

The kitchen or bar fridge radiators could be placed into the sump tank for further energy savings (to also take advantage of the night-time energy savings and efficiency). And you could heat your pool, though you probably won't want to do that on days hot enough to be using the A/C.


[1] Latent Heat of Evaporation of water is the equivalent to an 80C increase in temperature, ie: the water dribbling out of the rarefactor will be 100C.

[2] Nothing's stopping the system from running both processes simultaneously, though you may have to wait a bit to get cold air out if you've used up all the cold, or it hasn't been run in awhile (same thing).

[3] Fresh-fallen flake snow is about a 6:1 density compared with water, but what we end up with would be particulate at about 2:1.

FlyingToaster, Apr 11 2011

CALMAC’s ICEBANK® thermal energy storage tanks http://www.calmac.c...roducts/icebank.asp
[Klaatu, Apr 23 2011]


       Okay, apart from wanting to know why nobody bothered voting, I've managed to tie my shoelaces together in a scientific fashion...   

       [post edit: untied for now : knotty bad science turned to good]
FlyingToaster, Apr 14 2011

       Well, with an opening line like that, the tempting thing for all of us to do would be to NEVER vote on this idea.   

       Okay, maybe not, but I'm more of a comments guy myself.
normzone, Apr 14 2011

       hmm, you have a point, but I don't get why we don't use plain old water instead of R-nnn, so I sortof expected somebody to pipe up with "Hah they do that all the time in Mozobrogovia" or "that was the original model 200 years ago" (which I somehow missed in my perusal of the subject).   

       (the "Baked Alaska" comment was gonna be mfe'd anyways to avoid future confusion of off-season perusers)   

       (and "boiled koi" changed to "boiled trout" to relieve chronosensitive bad taste)
FlyingToaster, Apr 14 2011

       Mmmm... koi...   

       Maybe I missed the part where you deal with the open-air evaporation problem. Also, instead of a (I assume) twin-valve cylinder-piston-flywheel mechanism, why not a double closed-circuit pump? Picture something like a blood pump crossed with a turbocharger; I just thought of it about three seconds ago, so I haven't had time to draw up a blueprint yet.
Alterother, Apr 15 2011

       Ah, I _did_ miss that bit. So this is attached to an external water source?
Alterother, Apr 15 2011

       The concept was originally modelled for a home's "central air" system, integrated with the furnace, using the same ductwork and circulation fans, etc.but it could work for large buildings. You could even make a portable room-to-room model and place a decorative fountain on the outside of a window or door.   

       I'm not sure where to put the tank: both indoor, outdoor and buried-outdoors have advantages. But the pond *has* to be outside. Oh wait, that's not what you were asking...   

       It doesn't rely on evaporative cooling from the pond to the open air (any that occurs is just incidental) rather heat-transfer in a fountain and/or heat-sink sculpture. The pond is there to give the water enough time to sink close to ambient temperature. Replacement water would just be either plumbed in, or in the cheaper cases something added to the outside faucet. Note that if you run out of water it won't self-destruct: the return valve from the pond back to the tank remains closed unless there's water in the pond; and the rarefactor won't be operated unless there's *some* vapour pressure in the tank because that would be a waste of effort.   

       Re: the pump... the nifty thing about the pump is that it operates elastically on pressure and phase change, as well as electrically.   

       The piston pulls rarefied water-vapour into the cylinder during the downstroke. The pressure-differential to the outside pushes the piston back for the upstroke, *and* the phase change from vapour to liquid will occur partways through the upstroke as well, which adds even more energy into the flywheel. You will in fact gain energy (okay, maybe ignore that last bit). When you stop the electric motor, the pump will run through a few more cycles from the flywheel.   

       The math for energy-return doesn't look too difficult:
(Swept-volume minus effluent-water volume) divided by swept-volume, minus friction.

       I'm not sure a turbine in any form can do that so easily....(?) granted there's much less friction in a single bearing assembly compared to a piston & cylinder. Looking forward to reading "Peristaltic Vacuum Pump" or whatever.
FlyingToaster, Apr 15 2011

       [ao] Mind you, my pump can't be a normal vacuum pump: it's built for rarefied water vapour in and hothothot water out. Pump energy-recovery depends on pressure differential and phase change during the upstroke. This means that there has to be a way of removing a relatively small amount of water from the cylinder each time: Any leftovers remaining on the cylinder or piston head re-evaporates into the cylinder during the next draw. Gnome with a squeegee perhaps.
FlyingToaster, Apr 15 2011

       //if you run out of water, it won't self-destruct//   

       I caught that bit, meant to complement you on it. Very clever.   

       Okay, now that I'm giving it more thought, I may actually sketch out and post this 'peristaltic vacuum pump,' as you coined it. I wasn't actually comparing it to a turbocharger in that it works with a turbine action, rather instead that the flow through one half of the pump powers the other half. I need to do two things before I proceed: 1) make sure it hasn't already been baked, as I've been boned for that in the past, and 2) make sure it's actually physically possible without violating various coefficients and principles and whatnot.
Alterother, Apr 15 2011

       Wait, the shift from liquid to vapor occurs _midway_ through the piston's upstroke? Is this accomplished through heat or pressure, and how do you generate enough of either when your lines are hardly pressurized and you're dissapating heat into the open air whilst simultaneously cooking up a mess of tasty koi?   

       I must have misunderstood something (else) in the process...
Alterother, Apr 15 2011

       Okay, now I get it. Nevermind.   

       You still need to come up with a failsafe for the piston itself; as you pointed out, the mechanism is friction-heavy, meaning that if the cool-side vapor were to fluctuate drastically, say due to a sudden change in ambient tempurature or a faulty day/night timer, ice particles could cause your piston to sieze. Maybe run the rarefactor line around the cylinder block a few times? That would also assist the phase shift _and_ mostly eliminate the need for squeegee gnomes, which I understand do not come cheap. They are, in fact, almost as expensive as helium....
Alterother, Apr 15 2011

       It might be an idea to prewarm the from-the-tank vapour before it hit the pump, though that would require further valving to avoid warming the tank (as is it's just one outwards-from-the-tank flap-valve). There *might* be a possibility of ice on that flap-valve (on the inside), though since it's connected to the warm side it would tend to just boil off. Ditto the return valve, but that's engineering.   

       Concerning the rarefactor, that would be outside so hardly a chance of sub-zero weather (or close to zero enough that the cold from the tank has an effect). Even if it were though, it wouldn't be any different from cranking over a car engine in the cold. The actual amount of water in each draw would be miniscule. I've got a calculation by [Wrongfellow] in my Didgeridoo Kettle post which results, applied to this, looks like I'd need a 200L cylinder to withdraw 1cc of water on one stroke. While doable (1/5 of a cubic metre isn't that big, really) I was hoping for something a bit smaller. I"ll eventually try to do the math myself to see. [edit: Did it: 206 litres of 0C steam = 1cc of water]   

       (hmm... something tells me I should reword "upstroke" and "downstroke")
FlyingToaster, Apr 15 2011

       Try "thrusting stroke" and "withdrawal".
spidermother, Apr 16 2011

       I'd still worry about that electric motor powering your flywheel in the event of a valve sticking or the cylinder icing up. I'm not as much of a math guy as, my grandfather (A lockheed radar engineer) or, for that matter, many of you guys, but what I _do_ have a lot of experience with is stuff that looks good on paper and goes all pear-shaped in the real world. Also, start-up might be a problem if the system isn't primed-- it'd take one hell of an electric motor to cycle a 200L cylinder on dry lines, unless the first few revolutions are very slow. We'd be talking somthing like a 6 or 12v engine starter, serious overkill once the device got up to running speed. Of course, now that I follow that tangent, you could wire the thing up like a dynamic motivator, draw a decent percentage (say 15-20%) of return off the flywheel once things got spinning.   

       Sorry, I tend to branch off a bit sometimes...
Alterother, Apr 17 2011

       //concerning the rarefactor, that would be outside//   

       A great idea, granted, but I don't know where you live. I live in Maine, specifically in the western mountains, where we have a funky climate that can be damn hot during the day and dip 20-25* (F) at night (from a high of 90-something to low of 65* and back in one 24-hour period) in the middle of summer; not a plummet into sub-zero, granted, but drastic and sudden enough to frost up the condenser, lines, and valves of a conventional A/C, or, say, an oxyfuel torch regulator, come morning.
Alterother, Apr 17 2011

       I'm a bit north in Toronto; we get a couple weeks of A/C weather during the summer at most but the weather doesn't dip down at night enough to worry about external freezing. There's no way ice is going to form within the pump during operation seeing as the input temperature of the water vapour is 0C and as soon as it repressurizes it climbs quickly and sharply. Ice which forms inside the tank on the valves is going to be the first to sublimate because the other side of the valve is above freezing, and outside the tank there's no low pressure zones so it will simply melt.   

       Trying to work my head around a 206L cylinder though, though bear in mind that figure was just for the purpose of getting 1cc of water per draw. So instead I'm thinking a linear free-piston pump: one long cylinder with an input and an output valve at each end, with a simple free-floating piston in the middle which is moved from one end to the other by a coil wrapped around the cylinder. As the water vapour in one side is contracting prior to being expelled as water, the other side is pulling in more water vapour from the tank. No flywheel required in any fashion... the very first draw will be a bit of a pain though because there's no contraction going on on the other side to help it along.   

       But there's still the squeegee problem. Any water that gets left in the cylinder will evaporate during the next draw and the end result is that proportionately less vapour gets drawn in from the tank. That 206L only draws 1cc of water out. One possible solution is to blow it out using hot air at the end of each stroke. I do get the feeling I'm missing something though.
FlyingToaster, Apr 17 2011

       [Alterother] You can get a º symbol, on a MS-DOS or Windows computer, by holding down <alt>, pressing <1><6><7> on the numeric keypad (not the numbers in the top row of the keyboard), then releasing <alt>. NumLock must be on.   

       //(and "boiled koi" changed to "boiled trout" to relieve chronosensitive bad taste)// Are you suggesting that I have to boil my koi for just the right time to get the best taste?   

       Could this idea be summerised as "vacuum assisted evaporative cooler"?   

       There's a thermodynamic inefficiency here, since you are generating coolth at about 0ºC, and dumping waste heat at about 100ºC. It's only possible to get good efficiency by keeping these temperatures close to the inside and outside temperatures respectively.
spidermother, Apr 17 2011

       //"vacuum assisted evaporative cooler"?//   

       The evaporation happens inside the cold sump tank (which is insulated)... you could stick evaporative stuff on the outside, but the effluent from the pump is at 100C so it's going to cool down quite a bit simply from exposure to the atmosphere (it could be simply dumped, about 130 litres to take the example system from 20C water to 0C ice, but then what would we do with the trout ?).   

       The 0C on the house side is fine, it's the perfect temperature for an air conditioner (more or less) and the 100C water is made by allowing 0C water vapour to pressurize to ambient, and it's immediately expelled from the system into the pond (where it eventually cools and is recycled back into the tank).   

       You could also dump it into the existing (or an extra) hot water heater to make things a bit less complicated on the A/C end. Then you'd just plumb in from the water mains to top up the cold sump tank.   

       Bear in mind that one of the purposes of the cold sump is the actual cold sump: use energy during the night to make coolth which is then used during the day: so you're going to get a loss from the thing just sitting around no matter what. Maybe another liquid with a freezing point at say 15C if you can find one, but water's pretty well the champion as far as latent heat is concerned.
FlyingToaster, Apr 17 2011

       Not sure about this, but perhaps if the hot side were rigid (able to contain a partial vacuum) and cooled to, say, 30ºC in the trout pond, then the water could condense at that temperature, at a lower pressure. This would reduce the work done by the pump, since it would be operating across a smaller pressure difference, which is another way of saying that the thermodynamic efficiency would be greater.   

       It's the usual sort of compromise, however. Decreasing the temperature gradient decreases the power consumption, but tends to require larger and more expensive heat exchangers.
spidermother, Apr 17 2011

       Ahhh, I think I see what you're talking about... you mean simply keep the outside pressure to the minimum required to condense the water vapour. That sounds relevant but complicated.   

       A couple annos ago I changed the pump from a single cylinder with flywheel (used to capture energy on the upswing) to a double headed linear free-piston setup which does away with the flywheel... does that make a difference ?
FlyingToaster, Apr 17 2011

       None of this makes sense to me. The water vapor (that you apparently imagine is hot) is only at room temperature, so isn't going to be cooled by your "heat pond." And your pump has no chance of working at all.
ldischler, Apr 17 2011

       Well, we're still working on the pump: the problem is that the massive volume of rarefied water vapour turns into a tiny amount of water, and if any water stays inthe cylinder it will reboil the next time the piston pulls out, making the next draw less effective.   

       But as far as "going from 0C water vapour (at .006atm or pascals or something) to 100C water (at atmospheric pressure)...   

       The water vapour coming into the pump is extremely rarefied and has boiled at 0degC. When it's been pulled into the pump and the piston's on the backswing, the pressure inside the cylinder increases, the vapour pressure increases and the water condenses. But it's still got 2260J/g energy, where does that go to ? 411 goes to increasing the temperature to 100C leaving 1849 to ensure that it's not only hot but actually still close to a roiling boil. Or at least that's the way I understand it.   

       There's still something with vapour pressure in there somewhere that not *all* the water is liquid, but I'm still poking at steam tables to see if they bite and haven't gotten around to working it out yet (even though it might, at least partially, solve the "squeegee problem").
FlyingToaster, Apr 17 2011

       If you're boiling water at a reduced pressure and temperature, when you reverse that, it will condense at the same pressure and temperature. The curves won't exactly match up, there will be hysteresis, but if you start with steam at 50C at a reduced pressure, for instance, when you repressurize, you'll go back in that neighborhood. If you run this very fast, I suppose the steam won't have time to condense. On the other hand, the water won't have time to boil, either. So maybe what you need is an asymmetry of the stroke. A slow pull, then a rapid compression.
ldischler, Apr 18 2011

       Einstein-Szilard Refridgerator, but they used more volatile stuff, still reticulated though. Good thought...
4whom, Apr 18 2011

       // if you start with steam at 50C at a reduced pressure, for instance, when you repressurize, you'll go back in that neighborhood// eh ? nah, this part I actually get.   

       When you put pressure on something it gets hotter right ? Likewise when you rarify something it gets colder. So if you have something that's already rarified and you let it return to normal pressure then it will be hotter than it was when it was rarified.   

       So, 0DegC steam at almost vacuum is brought to atmospheric pressure and it gets lots hotter: it gets to 100degC but it doesn't quite have enough oomph to go back into steam because it's at atmospheric pressure.
FlyingToaster, Apr 19 2011

       [spidermother] thanks for the tip, but I'm using an iPad I got for x-mas. Took me two weeks to figure out where the $@#%¥! hyphen is. Still working on the 'degree' symbol.   

       Toasty, love the free-floating piston idea. Seems not only easier for start-up, but more efficient in operation, since this device doesn't have to cycle very rapidly.
Alterother, Apr 23 2011

       [Ao] Thankyou; startup would be the interesting part since you'd be pushing against the atmosphere on the first stroke without any of the help from the other side of the piston that would be present for the rest of operation. Still more than a litle bit stuck on how to evacuate all the water from inside the cylinder though. I very roughly estimated that 300 "RPM" would bring the example-sized cold tank from room temperature water to ice in 8 hours.
FlyingToaster, Apr 23 2011

       Maybe delay your valve action to create slight negative pressure on the "out" stroke? I have no idea how you could arrange it to work like that, but it should vacuum that excess water right out.
Alterother, Apr 25 2011

       well... vapour pressure is 100x more at 100C than at 0C so I might be able to just spit it out like a watermelon seed... but probably not.
FlyingToaster, Apr 25 2011

       //well... vapour pressure is 100x more at 100C than at 0C// well, now i think we has your problem, right there.
4whom, Apr 25 2011


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