June 29, 2008 at 10:29 pm #638
Does anybody know what the current state-of-the art for fresh water extraction is for sea going boats. For example, if you own an ocean going sail boat, is there some sort of reverse osmosis or vacuum desalination technology that you buy to help top off the fresh water tanks. How about for cargo ships? Do that just have large fresh water tanks that they fill up at port, or do they have some sea water to fresh water technology that they use. Are there off-the-shelf desalination solutions that can be purchased that are sized appropriate to a boat?June 29, 2008 at 10:42 pm #3400
This will let you see some of the stuff boat people are using:
http://www.westmarine.com/webapp/wcs/stores/servlet/SiteSearchView?catalogId=10001&Ntx=mode+matchallpartial&keyword=reverse+osmosis&Ntt=reverse+osmosis&N=377+710&y=0&x=0&storeId=10001&Ntk=Primary+Search&ddkey=SiteSearchJune 29, 2008 at 11:34 pm #3401
I just found that page on my own. I kept typing “reverse osmosis” and “desalination” into Google and kept getting inappropriate hits. If I had typed “watermaker” I would have found them all. I should have looked at the west marine catalog from the start. They are sort of the the definition of “off the shelf” when it comes to marine products. Thanks for the link.July 1, 2008 at 4:46 pm #3405
Reverse osmosis (RO) is the practical choice for a vessel that does not have waste heat to produce steam or the capability of using cold, deep ocean water. Generally, RO is pretty power hungry. Spectra Watermakers is most likely the most efficient you’ll find in the 1,000 gallon per day (GPD) range. Using multiple units for the desired capacity is probably the best way to go for redundancy, maintenance, and power conservation. Energy recapture technology for larger units (>5,000GPD) is not quite off the shelf. Spectra’s technology is.
(Note: I have 4 years experience installing, servicing, and building watermakers.)July 2, 2008 at 1:56 am #3408
What we do in Anguilla is collect water from the roof. To do this you want a cistern that is almost big enough to hold a years worth of water. If you have salt water handy, the cost of this tank might make solar panels and reverse osmosis not look so bad. In some seastead designs it might be possible to use part of the structure without needing to spend extra for a tank and then storage could be very reasonable.
You have to take life as it happens, but you should try to make it
happen the way you want to take it.
– German ProverbJuly 23, 2008 at 3:58 am #3486
Essentially desalination is just turning seawater to steam, separating water vapors from salt, then letting the vapor cool into a separate place sounds simple… wouldn’t it be cheaper to just build something ourselves?August 26, 2008 at 10:49 pm #3642
Just out of interest, does anyone have information on the methods and their efficiency of gathering air humidity and converting it into liquid water?
The Wikipedia article on the closest topic seems to be more of a commercial resource, offering some comparisions, but a more conclusive source would be welcome.August 27, 2008 at 10:46 am #3645
Id guess that would require a lot of surface area, which we do not have.August 27, 2008 at 10:50 am #3646
I knew big ships use RO, but i didnt know RO equipment scaled all the way down like that. Neat.August 27, 2008 at 6:56 pm #3649
Water is everywhere. In the ocean, in the air, in the ground. Producing potable water is just a matter of energy. If you have plenty of energy it is a non-issue.August 28, 2008 at 2:13 pm #3673
I’d like to point out that the concept of “energy” is often misunderstood by the layman. What you really need in order to produce useful work is really disentropy (negative entropy), which is an unbalance of something that tends to balance itself. Temperature differential rather than raw heat, altitude differential, speed differences rather than raw velocity, pressure or density or concentration differentials (partial pressure in gases and osmotic pressure in liquids count, too), lighting differentials (between day and night, even), all these can be harnessed.August 28, 2008 at 2:59 pm #3675
With what Jesrad is stating, we will have plenty of oppurtunities to harness disentropic sources out on the surface of an ever-changing seascape.October 29, 2008 at 6:27 pm #4080
That is correct, you need disentropy. That is why I think pulling that 40F water from the deep may still be worthwhile even if you use no OTEC at all. That cold water can be used for airconditioning, improving the efficiency of refrigeration and, most importantly, can be used in the making of fresh water by condensing water from saturated air. Solar can be used to heat surface ocean water, even if it is just passive. Then use the cold sea water to condense freshwater out. Using a vacuum pressure can make the process faster/more efficient. The only question then is the economical feasiblity of pulling the deep cold water to the surface and the cost of the pipe.
Reverse osmosis is good, but you will need a LOT of fresh water and I don’t think it would be cost effective for the amount needed. It would be nice to get some numbers on this.
I understand that this deep water may have other valuable uses, as it is mineral rich.March 11, 2009 at 3:45 am #5143
OK…I conceptually designed a truncated icosohedron (aka soccer ball, or buckyball) vessel with a desal membrane at each vertex (59), except one. The membranes’ (which are cylindrical) effluent points into the vessel, and a hose (reinforced with cable) is connected to the one vertex that remains. Inside the vessel is a submersible pump (like that in a groundwater well) connected to the hose to deliver the water and keep the pressure head at the top of the vessel, maintaining the pressure. If the pump ever turns off (power failure), teh pressure head continues to push water through the membranes until equilibrium is reached.
The vessel is charged with fresh water, it sinks (because we weighted it to overcome the lesser density of fresh water w.r.t. saline water) and once it gets to about 150 fathoms, the pressure is great enough to operate the membranes (i.e. push water into the vessel) at near optimum flow rates. The only energy needed is to power the pump inside the vessel. Maintenance is compartmentalized.
I haven’t done much R&D, but a sphere (work with me, here) with a 5m radius holds 138,320 gallons, and with 30 membranes (the kind I chose…there are others more expensive) operating at 50% efficiency, it would take 61 days to fill (closer to a month with 59).
I write water appropriation permits for WA State, and I typically write 0.3 acre-feet per year per 2.3 peeps (>260 gallons/day). The 5-meter vessel could serve 17 people for a year if it’s filled 12 times.
Don’t know how the energy exchange would work, per se (joules for watts for amps, for HP, for lift…whatever…I’m no electrician) for powering the pump…but the free pressure might make it worth it.
NoelJune 19, 2009 at 6:38 pm #6639
If there is a hose that is used to pump the water up there is no need to have this sphere. Any shape will do. The hose will need to be reinforced to withstand the pressure at depth so it doesn’t collapse, but assuming this is done, the pressure in the hose will be atmopheric pressure. As the pressure indide of the desal membrane equalizes with the outside pressure, the water should rise up the hose. We should only need to expend energy to pump the water up from the depth where the column of water in the hose is exerting enough pressure to prevent the continued passage of water through the membrane.
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