Vacuum Solar Still

[seandepagnier]

Since water evaporates at much lower temperatures with reduced pressure, it should be possible to build a solar still which operates at a much higher efficiency if it were in a vacuum.

For example, at 0.5 psi (instead of 14.7 psi) the water would boil at 80F (26C) which may well be the ambient temperature, and could even work at night? Please correct me if I am wrong here. It probably still needs a temperature differential to recondense. Are there some other limitations of evaporation I have overlooked?


I am thinking either a hand pump or electric pump would be suitable, and it would only need to be pumped once each time the still is refilled, not continuously. Maybe there is a way to refill without completely losing the vacuum.

What do you think?

[SvenG]

"Great minds" and all that ...

However, you would have to keep pumping as the evaporating water (low-temp steam) will fill the vacuum.



-Sven

[Sailmonkey]

you are missing one huge detail. The pump will need to run continously, as the water flashes to steam the pressure in the evaporator increases. Large ships don't use RO to make water, they use a flash evaporator. the engine suplies the heat, a feed pump provides the saltwater, a vacum pump provides the lower pressure and motive means to take the freshwater out of the evap chamber and move it into the condensing coil.
I think on a small scale RO trumps evap for any useable quantity other than basic liferaft survival.

[seandepagnier]

I realize now the pump would need to run continuously. Maybe with a pressure valve to set the pressure it runs at, it would be intermittent.

If it were electric, this shouldn't be too big of a deal. It could pump the water vapor into a second chamber which is much cooler and higher pressure and allows the water to condense more effectively.

I want to figure out a way to make more water from a small solar still so that it is not taking up too much deck space. It would be interesting to compare to RO in terms of energy consumption, but I dont think I can build a RO system as cheaply.

I am reading about using cheap 12 volt air compressors for tires as vacuum pumps, they can often be had for $5 or less... I already have one, but it is not rated for continous duty.

[tager]

This is an interesting idea. It just may be worth a try. The stills would not be inflatable, they would have to be acrylic or something equally rigid. This may end up becoming a storage issue.

[dr01allen]

it's a good idea and relatively simple.

you'll need a thermal collector in the sun to use as an evaporator (like a piece of copper pipe) and a condenser in a cool place (in the water, along side the hull?).

the system will need to be maintained under vacuum which can be done with a vacuum switch and pump. gases other than water vapor in the system will decrease effectiveness.

it will also need to allow new sea water in and purge old water out of the evaporator to minimize salt buildup. a bi-directional pump with a shut off valve attached to the evaporator should work.

water collected in the condenser will need to be pumped out. using the same type of pump on the evaporator and condenser will minimize spares. don't try to plumb the same pump for both purposes since you'll contaminate the fresh with salt water.

in operation:
-you'd charge the evaporator with salt water and pull a vacuum on the system. the system should be sized so that the system can run all day without recharging the evaporator.
-the thermal collector will heat the water and evaporate it, increasing the pressure in the system.
-the increased pressure will raise the dew point in the condenser and (as long as it is kept cooler than the dew point) water will condense, lowering the pressure in the system.
-the lower pressure will cause more water to evaporate from the collector, thus maintaining the cycle.

as water evaporates from the collector, the salt water gets saltier and the vapor pressure decreases. this requires a higher temperature or lower pressure to keep the water evaporating.

since the system is closed, the volume of liquid in the system is constant. when water has been transferred from the evaporator to the condenser, pumping it out will lower the pressure which helps keep water moving out of the saltier water.

refilling the evaporator will dilute the salts and increase the vapor pressure but also increases the system pressure by increasing the amount of liquid in the system and introducing dissolved gases.

a good design element would be to use a large volume of supply water (held under vacuum) feeding a conveniently sized evaporator. in the right configuration, the supply water will degas when the vacuum is pulled but the dissolved gases would collect in the supply tank and not impact the evaporator / condenser circuit.

the vacuum could then be maintained by pumping out the distilled water.

at night the system could be flushed and prepared for the next day.

I've used vacuum tube absorbers for solar heating and know from personal experience that they can produce super heated steam. using one for a boiler though can lead to massive corrosion problems. either use a material that can take concentrated hot salt water and steam or design it for easy replacement.

have phun.

da

[hellosailor]

The $5 tire pumps, and even the $25 tire pumps, are usually "disposable". Very much not a continuous duty device and I wouldn't expect them to pump a hard vacuum. Which is why pumps sold for working on AC systems are way more than $25, even for the cheapest ones.

Be interesting if you can work it out though.

[daddle]

Solar stills don't necessarily boil the water. They maintain enough of a temperature difference to evaporate on one side and condense on the other. Evaporation is not boiling.

I'm not sure a vacuum pump is much of a help.

As hootch makers know, boiling is not necessarily a good thing as too many nasty tasting volatile substances boil away first which might make for fishy tasting water.

A self contained solar still in the shape of a solar panel would be a fantastic product. Don't forget the designed-in rain gutter for catching the easy fresh water. A poor example here makes 1.5g/d.

[Pblais]

I want to figure out a way to make more water from a small solar still so that it is not taking up too much deck space. It would be interesting to compare to RO in terms of energy consumption, but I don't think I can build a RO system as cheaply.

Depends on the desire volume you require. One gallon a day vs 30 gallons starts to introduce different ideas. RO is predictable and runs in any weather. The ability to predict allows much larger volumes of water. I don't see the vacuum pump idea going any place helpful. It's like the idea of perpetual motion - be nice if there was some. It takes raw energy to spin straw into gold. The form it takes is optional.

[Minggat]

Originally Posted by Sailmonkey

I think on a small scale RO trumps evap for any useable quantity other than basic liferaft survival.

If this were true, I wouldn't have the work I do. I'd still have a job in water treatment. It would just involve more flash evap units.

I say this after having commissioned RO plants rated in MGD (Million Gallons Day). Although the largest one was a combination MSF (Multi Stage Flash) and RO. The plant was 200 MGD in Saudi Arabia. Still, smaller plants I've commissioned (87 MGD- RO only) are nothing to snort at.

It depend$ on what you have to work with.

When technology reaches the consumer, we often think it was developed for the consumer.

[MikeM7777]

In actuality the vacuum solar still will reach equilibrium at a relatively low temperature. The pressure given off by evaporation is exactly negated by the corresponding vacuum created by condensation. Mother Earth news has an article on how it is done at the following link although the scale of Mother's vacuum still is larger than what would be practical on most sailing yachts there may be scalable variations. Essentially the whole operation is kept under a vacuum until the process is finished. Additional needed water or mash can be drawn in by the vacuum. The collector is closed off from the rest of the system before emptying, etc.
If the link doesn't work just google it, there are many similar articles and patents.
Mother' s Home-Scale Vacuum Distillery
Ciao!
Mike