Build Winning Desalination Plant with Sun Energy

[james2286]

hello people,

gday everybody! I am a first year engineering student and we just got an assignment where we have to make a desalination plant. The restrictions on the plant are as follows:
- plant must be within the confines of 1m by 1m by 0.5m
- plant must ONLY using the sun as a source of energy
- no chemicals may be added to the water

The final test will take place in a two hour session where the plant that is able to purify the most water with cost also taken into account, wins $1000.

I was wondering if anyone here can give me their thoughts / ideas for the best possible plant.

Any ideas no matter how stupid would be appreciated. Creativity and onnovation will be what makes this successful.

Thank you!

 

[Clausius2]

Apart of those desalination plants which work based on osmosis effects, once I heard that if you find yourself in an isolated island just on the middle of the ocean, the best method to purify water is to boil it. Thus, the salt will remain in the bottom, and you can destilate water causing vapor molecules to crash into a cool surface and so becoming pure liquid water again.

Ok, this is a rudimentary method, but if you are lost in such island, maybe you remind by words...

 

[Astronuc]

Well, osmosis would work, but the membrane maybe expensive.

Evaporation seems the most practical, as sun is source of energy. That could be achieved with a solar concentrator or evaporation pool.

And perhaps a condenser.

Since this is your assigment, you'll have to figure out the details.

As for materials, that's up to you the designer. But if I were to use metal, like stainless steel, I'd go with a 6-moly SS, like AL-6X, rather than a 3-moly (SS 316) for corrosion resistance in aerated seawater. AL-6X was designed for landing gear struts on naval aircraft.

 

[james2286]

what is the osmosis effect? Would this RELLY be a practical solution?

Here are some of things I've thought of to maximize the evaporation of the water. (assuming I am going to deslinate via distillation)
- Solar cells to heat water
- Base of container that has the water in it is painted black (to absorb heat)
- Maximize the surface area of the water that is exposed to the sun (maybe have multiple containers on top of each other at different levels so perhaps the sun is able to penetrate more surface area)
- Humidity - should I attempt to have the air above the sea water as humid or as dry as possible. From first thought, id say dry, because doesn't evaporation of water occur at its greatest in dry air? However on second thought Id say as humid as possible because I want the water to condense in another container. See I really don't know about this point)
- Air pressure? (This wouldn't be too practical, but say I could put the container undre a partial vacuum. Would this increase evaporation and to what extent?)
- Container material - Whats the best possible material that is heat absorbant?
- Plant design - Anyone know an ingenious design that would be very effective? I need to come up with something that will give me the edge over the other students.

Any thoughts on these points and or other suggestions would be much appreciated.

Thank you!

 

[Clausius2]

what is the osmosis effect? Would this RELLY be a practical solution?

Here are some of things I've thought of to maximize the evaporation of the water. (assuming I am going to deslinate via distillation)
- Solar cells to heat water
Maybe, you could use the DC current to heat up the water, but surely you will have a loss of available energy during the conversion process. This loss is less than if you buld a heat exchanger air-water to intercept radiation. The efficiency of this last solution is about 20%. The efficiency of the first option (solar cells) is about 90%.
- Base of container that has the water in it is painted black (to absorb heat)
Great idea. Thermal solar panels are designed with a bottom plate painted on dark colour.
- Maximize the surface area of the water that is exposed to the sun (maybe have multiple containers on top of each other at different levels so perhaps the sun is able to penetrate more surface area)
Great idea.

- Humidity - should I attempt to have the air above the sea water as humid or as dry as possible. From first thought, id say dry, because doesn't evaporation of water occur at its greatest in dry air? However on second thought Id say as humid as possible because I want the water to condense in another container. See I really don't know about this point)
The ambient must be as dry as possible. Therefore it will be able to absorb more vapor molecules. Once it is evaporated, you have to cool the humid stream. Some easy condenser could be to provoke the stream to crash into a cooled plate, and collect the liquid water stream.
- Air pressure? (This wouldn't be too practical, but say I could put the container undre a partial vacuum. Would this increase evaporation and to what extent?)
The less air pressure, the less temperature you need to boil the water. You could save energy, but this energy would be employed in mantaining the vacuum. You're right, it's not a first hand solution.
Any thoughts on these points and or other suggestions would be much appreciated.

Thank you!

Good luck.

 

[quark]

You can have any container which is compatible with salt water. Make sure that this is well insulated. Focus the sun light from solar panels on the fluid in the container. Water starts evaporating when the container pressure is the vapor pressure at the given temperature. Get the steam tables to check the steam pressure w.r.to the acheivable temperature via solar panel. Cool the water vapor with the incoming saline water so that you can preheat the saline water and condense the steam as well(mind that there is no one to one relation, to condense one lb of steam you require about 200lbs of water if the temperature difference is 5F)

Vacuuming should be done to the container in which you are collecting the condensate and not the container in which you are boiling the saline water.

Don't worry about the atmospheric humidity. That doesn't matter as this is forced evaporation.

Solar heat is an excellent catalyst for algae growth and it may trouble you if you are restricted from using any chemicals (ideally you should use NaOCl)

Good luck,

 

[Astronuc]

I am not aware of solar photovoltaic cells achieving 90% efficiency. Earlier generations are about 10-15% efficient, and more recent advances may more than double that.

Using a vacuum to reduce the pressure in the boiler and increasing the pressure in the condenser would be good.

Consider forced convection, air-cooling in the condenser, but compared that to using the incoming seawater - which can be preheated or 'recovered'.

 

[Clausius2]

I am not aware of solar photovoltaic cells achieving 90% efficiency. Earlier generations are about 10-15% efficient, and more recent advances may more than double that.

Maybe I am a bit confused and wrong here. The conversion of solar energy (in form of radiation) to electrical energy has higher efficiency than the conversion of solar energy into internal energy in a fluid, hasn't it?. I see, my data of 90% is something stupid. I really don't know too much about solar photovoltaic panels. Fortunately, my course on Renewable energies only covered thermal solar power.

 

[Astronuc]

Maybe I am a bit confused and wrong here. The conversion of solar energy (in form of radiation) to electrical energy has higher efficiency than the conversion of solar energy into internal energy in a fluid, hasn't it?. I see, my data of 90% is something stupid. I really don't know too much about solar photovoltaic panels. Fortunately, my course on Renewable energies only covered thermal solar power.

Clausius, I wasn't quite sure what you meant by 'solar cells'. I assumed PV, which is the common meaning, but perhaps these days it is becoming more generic.

You are right that solar thermal is much less efficient than 90%, but I don't know of any solar system, thermal or PV which approaches 90%, although I have heard of recent developments in PV or something similar which might achieve much greater than 30%.

I'll have to dig around for the articles.

Meanwhile, a solar thermal system using a solar concentrator on a heat exchanger (boiler) might be the best option. Enclosing (inside a transparent (e.g. glass vessel) the boiler in a vacuum can reduce heat loss. The tradeoff is how much sunlight would be absorbed (lost) because of the glass.