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Passive offshore desalination plant

  1. Apr 3, 2015 #1
    I'm looking for ways to help mitigate California's drought. I've come up with two ideas.

    The first is a pipeline from Washington. The pipeline could be built with similar construction principles to an oil pipeline, which is a pretty mature process. The water could be sold to California municipalities for a tidy profit.
    But the time and cost of building such a pipeline would be prohibitive.

    My second idea is a passive offshore desalination rig. This is what I want to focus on right now. Desalination seems to be a pretty popular topic, here.

    The design would be a large geodesic dome made entirely of glass. Lensed glass, such as fresnel lenses, to focus more light towards the center of the dome. The center of the dome would consist of a basin which fills with sea water with the tide. The basin would need to be insulated (so it doesn't boil the surrounding seawater) and the filling mechanism would need to be filtered (so fish etc. don't get caught in it). The inside rim of the dome would have a catch basin above high tide sea level. This would lead to pipes and a pumping system to pump the water to land. I'm picturing them being somewhere between 40 and 100 feet in diameter, but that depends on a lot of math.

    In simple terms, the dome focuses the sunlight to heat and evaporate the water while acting as a heat sink to reclaim the steam and condense it for use.

    Potential hurdles:
    - Cost. Building an offshore rig, materials for building the rig, manpower to manage filling and cleaning the basins.
    - Energy. Energy cost to pump the water to shore and to its final destinations.
    - Location. Would need a lot of room on a continental shelf. Would also need a beachside pumping station to receive and distribute the distilled water.
    - Efficiency. How much water can a single platform distil? How big will the rig have to be? Can the process be adjusted to be more efficient? Can it be modified to generate electricity? How many rigs would be needed to make a difference to California's water needs?
    - Comparability. How does the cost and efficiency compare to pumping coastal groundwater?

    Possible solutions:
    - Cost. Concrete and composite lenses are fairly inexpensive compared to other materials. Existing oil rig designs could be modified to suit the design.
    - Energy. I need math. It's possible that the rig could produce electricity by heating the water into steam and running it through a turbine. The excess steam is then reclaimed as distilled water. Based on my limited understanding of existing solar thermal power plants, each rig would have to be fairly large to capture enough solar power to make this effective.

    The idea is that these could be built relatively cheaply, that they could be operated with minimal cost, minimal moving parts, minimal maintenance. That they may not be able to produce as much water as an active desalination plant through boiling or osmosis, but the cost difference would make up for it. That dozens or hundreds of these could be built offshore along California's coastline and begin providing fresh water within the next couple of months. That the low cost and simple construction could make them available fairly quickly.

    Where I run into trouble is efficiency and math. I suspect someone's already done the math on ideas similar to this and just couldn't get them to distill enough water to be cost effective. I'd like some help to determine how inefficient this would be - it's possible that we can make them more efficient - like an array of mirrors such as those found at a typical solar thermal plant.
  2. jcsd
  3. Apr 3, 2015 #2


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    To get a cost estimate: concentrated solar power to produce electricity is used commercially, but the costs are still quite high even in deserts where you have sunshine all day.
    Building something on water is more expensive than building it on land. Using salty water makes the boiling process harder and leads to various other issues. Getting energy this way will cost more than getting it with land-based systems, and getting water will cost more than using electricity to boil water (although you can get back some energy with heat exchangers).
    Boiling water needs ~.5 kWh/liter, even with optimistic energy prices this is several dollars per cubic meter. I don't know how much water costs in the US, but that does not look promising (costs for water transportation and distribution infrastructure and waste water treatment are not yet included in that estimate).
  4. Apr 6, 2015 #3
    Water is not easily transported. That's why communities tend to form around fresh water supplies. The problem in the state of California is that both farming and population activities have exploded over the last several decades. And even if there weren't a drought, they'd still be in trouble.

    Moving water from Washington State to California would require pipelines that dwarf the massive pipelines feeding New York City (look up the New York City water tunnel #3). With concerted effort and assuming nothing goes wrong, one might get a tunnel in service in about three decades. Maybe. Furthermore, I'm not sure where you'd get the power to move that much water over that kind of distance. Let me reiterate, petroleum pipelines would be at least an order of magnitude smaller.

    Passive solar evaporate systems would have to be extraordinarily large to supply that much water to the state and it would have a significant ecological effect on the ocean. Furthermore, evaporative desalination is not particularly efficient. Membrane desalinization plants would be significantly more efficient, and probably more practical. But there are still issues, not the least of which is finding the energy to power them.

    Using offshore wind and solar energy to power a membrane desalinization plant is probably the easiest, most practical method out there. Onshore wind or solar farms are often a source of discontent because of their size and impact upon the terrain. The nice thing about desalinization plants is that they can be comparatively small, close to the consumer, and they can store finished water during times when the energy is abundant.
  5. Apr 6, 2015 #4


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    Building pipelines and tunnels in seismically active zones like California is problematic, even if they are carrying fresh water. One earthquake, and a major portion of your fresh water system is gone. :nb)

    I suppose you could build the desalination plant in San Francisco Bay and then distribute the fresh water to other parts of California by bullet train. :eek:

    If Californians didn't want to build offshore platforms to extract oil and gas from the continental shelf, I don't see them suddenly deciding to build massive wind farms offshore instead. WWBSS (What Would Barbra Streisand Say?) :rolleyes:

    You might build a pipeline from Washington State to California, but Oregon is in the way, and I don't see getting all the permits, environmental impact studies, right of way acquisitions, etc., etc. done for many years. The feds, three state governments, the EPA, the USACE, and all the other alphabet agencies can each throw a monkey wrench in the works and bring the whole project to a standstill. :sorry:

    At one time, California had a long-term plan to provide adequate water supply using in-state resources, but politics derailed this plan long ago. o0)

    One thing is certain: the solution will not be easy, cheap, or quick. The people living in California now have some tough choices to make, and business as usual won't cut it anymore. o_O
  6. Aug 7, 2015 #5
    Iam no scientist but a lens on top would have to be bigger then the area your focusing on since a lens only focus light and does not create it. So a lens 20 feet in diameter focused on a 20 foot diameter area is a pain of glass or window.
    But my idea taken from yours would be a round barge with no bottom lets say floating with 20 feet above the water and sumerged 20 feet below the water. on top of the barge would basically a tee-pee shaped greenhouse made from clear plastic. When the water evaporates it will condense on the plastic and flow down the walls to collection gutter. as far as the heat goes you are not generating any more heat then would of been created by the sun hitting the surface of the water, the only thing would be you would be not allowing the heat to be reflected back into the sky. And since the water is not taken anywhere the salt concentrate would be distributed back in the ocean exactily where it came from. Also since salt water and fresh water don't mix readily without some form of agitation and fresh water is lighter then salt any runoff from the desalination process would leave fresh water within the confines of the barge area to be desalinated again. Or if the sides extended farther below the water no gutter just pump fresh water from the top of what would be a great water tank in the ocean?
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