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Solar Water Distillation Using Vacuum Pump

09/27/2016 1:18 PM

I am doing research about solar vacuum water distillation. The design model will consists of a glass cover, basin water equipped with reflecting mirror and insulation, which acts a solar cooker where heat will be directed, and controlled vacuum pump powered by solar panels to create vacuum inside the chamber. A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum to decrease the saturation temperature of water and to increase the efficiency and rate of water purification. Feed water in the basin is heated by solar energy, and the evaporated water is condensed by inner glass cover. The chamber is slightly in slanted position to collect condensed water stack in the glass cover and another tube which is the condenser is connected to the chamber where other evaporated water will pass through and condensed water will be collected by the same container tank where condensed water from glass cover is collected. Is this design will possibly work? Or can you give me some suggestions to improve my research? Thank you.

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#1

Re: Solar water distillation using vacuum pump

09/27/2016 1:42 PM

Well first of all the quantity you plan to produce is important, and the source water type is also important....These 2 parameters will determine the most cost effective method...

https://en.wikipedia.org/wiki/Solar_still

https://en.wikipedia.org/wiki/Concentrated_solar_still

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#2

Re: Solar Water Distillation Using Vacuum Pump

09/27/2016 2:15 PM

Isn't this deja vu all over again?

Low Pressure Water Distillation. Why is it Not Used More?

Why not just use the sun?

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#3

Re: Solar Water Distillation Using Vacuum Pump

09/27/2016 2:32 PM

The vacuum chamber should be a cyclone (also known as a flash chamber), this will spread the incoming water surface area out for evaporation, the vapors are removed from the top of the cyclone and run through the regenerated HX (to save energy) heating the incoming water prior to running through the condenser to be condensed siphoned/pumped off.

the water left in the cyclone will flood the bottom to be pulled away with a pump and rerun through the HX.

By rerunning it through the system, you save on energy of reheating the water. This you can do for a while until you reach a high strength waste (determined by you) and pumped out to be disposed of.

Ok, now you have a rough outline of the process. Now you can start balancing your systems, with a systems curve and thermal mass calculations.

I have used WASP in the past, it's very handy. But I will tell you, it still uses energy other then solar.

Your first calculation should be;

How many BTU/hr can you obtain/collect from solar?

This will drive your production and design of your still.

And again, Its the temperature that purifies the water, if your lowering the water's flash point by using vacuum, your losing the ability to sanitize.

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#4

Re: Solar Water Distillation Using Vacuum Pump

09/27/2016 2:39 PM

You are clueless!

Pulling a vacuum on water does not purify it!

Water vapor is the gaseous phase of water that gets sucked into the vacuum pump.

Stick with solar.

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#6
In reply to #4

Re: Solar Water Distillation Using Vacuum Pump

09/27/2016 6:20 PM

He's not just using the vacuum to purify it. It's just to get the boiling point lower os thusly less initial energy is needed to make the transition from liquid phase to vapor phase.

As far as I know the standard format for living organisms to be somewhat water based so bringing their insides to a boil regardless of pressure or temperature pretty much kills them.

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#7
In reply to #6

Re: Solar Water Distillation Using Vacuum Pump

09/27/2016 6:23 PM

Your guessing, aren't you.

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#10
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Re: Solar Water Distillation Using Vacuum Pump

09/27/2016 9:14 PM

Depends, What's the Op trying to remove?

Is he starting with pretreated municipal water, semi potable groundwater, river water before a flood, after a flood, Sea water, high organic level swamp water, raw sewage water, oil well production water, nuclear contaminated wastewater?

A large part of cleaning up water is knowing what it is you have to take out to begin with so that there no point designing the system to take out contaminates it will never encounter.

If you can't find the time to figure out what your dealing with assuming everything is in there is a fast way to end up expending way too much time and energy designing for nothing.

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#9
In reply to #6

Re: Solar Water Distillation Using Vacuum Pump

09/27/2016 6:51 PM

In the sort-of companion thread to this, it has been mentioned that the latent heat of evaporation increases slightly as the pressure is lowered. However, the sensible heat required to get the water from ambient to boiling decreases more. That makes the total heat be less, and also makes lower temperature heat sources be viable.

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#5

Re: Solar Water Distillation Using Vacuum Pump

09/27/2016 4:33 PM

There is about one way this might work:

Use charcoal to absorb the light, forget the mirror, and focus the sunlight using a large fresnel lens (you can buy these on Amazon, I think).

Next, since the water vapor will be essentially low pressure steam, it will help with killing pathogens, pyrogens, etc. You could expand this steam through a large turbine wheel (like the final stage of a steam turbine), and exhaust the turbine into a condenser unit, which will operate under some vacuum, up to about 700 mmHg. Even if air-cooled, you will have warmed water (send it through a heat exchanger if you can guarantee no leaks, no contamination) for preheating the source water, and you will be able to generate some power (a small amount).

You could use evaporation pond to get rid of the brine from your small "boiler".

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#8

Re: Solar Water Distillation Using Vacuum Pump

09/27/2016 6:26 PM

I started playing with this concept some years ago just as a hobby experiment relating to fine tuning my knowledge of heat pump systems.

I had planned to use a common R-22 based heat pump (old window air conditioner compressor) as the primary driver for the system and a pair of liquid to liquid heat exchangers for the evaporator and condenser units.

The intent was to use the heat pump to recycle the thermal energy plus use its input energy to keep the system up to working temperature as well along with a vacuum pump to bring the overall working temperatures down a fair amount.

The numbers seemed good and worth the time to play with but other things got in the way and I never rally got back to it.

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#11

Re: Solar Water Distillation Using Vacuum Pump

09/28/2016 6:26 AM

Do not be too disheartened by some of the locals. What you are describing is similar (though not the same) as systems used in some ships to generate purified water on board. (Though in those systems, the condensation happens outside the vacuum chamber.)

Look up "mightly ships" series and find the passenger liners for a demonstration.

Where your problem will lie is that for re-condensation inside the vacuum volume, you need to sacrifice a significant portion of your input energy. What you gain in lower "boiling" energy is sacrificed by how you describe your system.

You will also have some challenges with the three fluid interfaces to the chamber (Raw water in, distilled water out and concentrated raw water out) especially since the two outlets will need to suck against the internal vacuum.

Send us a report on prototype 1!!

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#14
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Re: Solar Water Distillation Using Vacuum Pump

09/28/2016 11:36 AM

On board ships, there are usually two or three options to pick from:

(1) Multiple effect flash distillation, heat is conserved by exchange with previous vessel.

(2) Vapor compression system with compressor (essentially pulls minimal vacuum on the water to reach lower boiling point), then compresses the vapor, and condenses it against the lower temperature in the boiling vessel.

(3) SWRO - seawater RO, 800+ psig applied pressure. mature technology.

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#12

Re: Solar Water Distillation Using Vacuum Pump

09/28/2016 11:30 AM

So really it's a sub-atmospheric solar still. If you want to improve your research, Google that, and get hold of a set of steam tables.

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#13

Re: Solar Water Distillation Using Vacuum Pump

09/28/2016 11:31 AM

From experience with marine steam condensers, drawing a vacuum will drop the boiling point of water down to the point that it boils and condenses directly to ice. On a few occasions, we froze a condenser solid and had to shut it down to thaw it out.

Typical operating pressures in the condenser were less than 1" mercury absolute pressure and we ran a multistage jet pump to draw the vacuum. What typically came out of the ejector were non-condensable gasses.

At that vacuum level, anything but a thick cylindrical shell will implode due to the vacuum.

That said, in normal bright sunlight with a black absorber and no light magnification, it is not unusual to be able to boil water at atmospheric pressure in an insulated solar hot water panel with double glass pane over the front. I had a neighbor install solar hot water and it was normal to see the relief valve at the top of the panel pop off and vent steam on an 80 degree summer day.

I wouldn't put the effort into a vacuum pump.

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#15
In reply to #13

Re: Solar Water Distillation Using Vacuum Pump

09/28/2016 7:36 PM

Indeed...

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#16
In reply to #15

Re: Solar Water Distillation Using Vacuum Pump

09/29/2016 10:57 AM

I like that idea. Really smart, since heat goes in where it is needed, not radiated, or convected away in the breeze.

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