3

This is mostly a mental exercise. Now that I've thought about it I feel a need to know what the issues are with this idea. I hope that's alright.

  • You create a ship that is a floating surface structure with an impermeable reinforced cloth bag folded underneath, ready to deploy. The larger this can be the better. It doesn't need speed, so it can be a sailing ship.
  • You take it to a polar area and deploy the bag, allowing it to fill with seawater
  • You allow the environment to freeze the water
  • Take your sweet time getting to a coastal city that lacks fresh water. The market for water is constant, there's no hurry.
  • Salt is naturally excluded from crystals of ice. In natural sea ice, it's present in young ice in pockets of brine, and much less so in old ice. Natural processes that cause the brine to drain down through the ice can be imitated as the ship moves to warmer regions
  • Purge the briny water that drains out until you are carrying a load of fairly pure water ice.
  • Let it melt
  • As long as you can keep the cloth intact, so that salt water doesn't mix with the fresh water, once you get to a city that's a viable market, you can sell the water.
  • Once it has been pumped out, the cloth is stowed and the ship returns to the Arctic (or Antarctic) and does it again.

I don't know if there are engineering issues with this, but now that I've watched some videos on water shortages and the costs of desalination, I can't leave this alone. The most efficient reverse osmosis desalination plants use 3 kWh per cubic meter of fresh water produced(check page 243 near bottom). As long as the time is taken to do this passively, the energy cost seems like it would be lower. Ice is never hauled, the ship has no motors. There's some pumping, to get rid of the brine that collects and to empty the bag, that's all. Might it work?

It doesn't seem like it would need a large crew, or very expensive tech, or have big running expenses or repair expenses. Where are the flaws?

kim holder
  • 419
  • 3
  • 11
  • 5
    Very obviously, the main flaw is that the entire process is very overcomplicated: the Antarctic is already full of an inexhaustible amount of ice. No need for the membrane etc. Just go to the Antarctic, cut as much ice as you want, and bring it home. – AlexP May 29 '21 at 23:10
  • @AlexP Cutting the ice and loading it onto a ship is a lot of work. This eliminates that. – kim holder May 29 '21 at 23:28
  • 3
    Lifting a lot water on the shore to let it freeze is an even greater amount of work... And then anyway you need to load it on the ship. (The water won't freeze in water, because otherwise the sea itself would have frozen.) – AlexP May 30 '21 at 02:47
  • 1
    @kim holder: You don't cut up the ice and load it on a ship. You tow (or push) the large, flat iceberg to wherever you want it. Fairly serious proposals to do just that. – jamesqf May 30 '21 at 03:49
  • @AlexP That's actually not the case, sea ice freezes in the sea, but it does so slowly because cold water sinks as it becomes denser. You don't have to lift the water, you just have to prevent it from sinking, and it will freeze. – kim holder May 30 '21 at 04:14
  • @jamesqf That's interesting. I'll search for that, I like that kind of stuff. – kim holder May 30 '21 at 04:14
  • actually water gets less dense close to freezing, water one of the few material in which the coldest liquid form ( ~4 degrees above freezing and below) and solid form or less dense than the normal liquid form. this is why ice form on the top of water and not from the bottom. you should also look into ice shipping which was a thing before electrical refrigeration. – John May 30 '21 at 04:34
  • I don't understand the reason for the membrane, why use a membrane. A membrane is permeable don't you want the exact opposite? Just a non-permeable piece of plastic that you can fold out to catch the water? – D.J. Klomp May 30 '21 at 14:47
  • @D.J.Klomp you're right, membrane is the wrong word. I'm editing, for that and a couple of other things. – kim holder May 30 '21 at 20:31
  • @John it was in the linked article that I got the thing about cold salt water sinking. It does that, unlike fresh water. It's in the 2nd paragraph of the section 'What is the difference between sea ice and icebergs, glaciers, and lake ice?' at https://nsidc.org/cryosphere/seaice/index.html – kim holder May 30 '21 at 21:08
  • @kimholder good catch I hadn't considered the effects of salinity change on density. Something to think about however, how to put the membrane on the ice without it getting contaminated with saltwater. – John May 31 '21 at 06:58
  • 1
    Note freezing sea ice thick enough to survive long distance travel takes a LONG time, and much of that thickness is from snow falling on the ice. Icebergs are not made of sea ice. – John Jun 01 '21 at 00:21

3 Answers3

7

The problem is one of scale.

Current container ships can usually carry loads of up to 24,000 TEU. That's a unit for a standard container, but such containers are usually loaded with up to 24 (metric) tons per TEU.

So a cargo ship could transport... Carry the three... 576,000 metric tons of cargo, which in pure water would give you that same amount in cubic meters.

Now Egypt is a pretty desertic place[citation needed], so you think they could benefit from this, but the Nile discharges 2,830 cubic meters per second to the Mediterranean. That amounts to 244,512,000 cubic meters per day. That's three orders of magnitude more than you could carry in a weeks long trip with a modern cargo ship, with the advantage that the Nile does not charge for that discharge.

In ancient times, you would find a river and proceed to do your civilizing around it after you unlocked the irrigation technology. In modern times it is possible to desalinize the sea at your doorstep like Israel does, so the whole point of mining ice for water becomes moot. The only places that need water transported from afar are deserts which are far from surface water sources, but then there is always an lake or river that is closer than the closest geographical pole.

The Square-Cube Law
  • 141,440
  • 29
  • 264
  • 586
  • 1
    I am also willing to bet transporting the water requires more energy than desalinating on site. – John May 30 '21 at 00:37
  • 1
    @John the Portuguese have been desalinating on site since the 1100's. That is actually how they got enough precious salt to bankroll the foundation of their country. Judging by the process it does seem to be less costly in energy than importing from the poles. – The Square-Cube Law May 30 '21 at 01:37
  • 5
    @TheSquare-CubeLaw: No they haven't. Nobody was desalinating sea water on any significant scale before the 19th century. What the Portuguese were doing (and the Romans before them, and the Greeks before the Romans) was not desalinating water, it was dewatering salt. – AlexP May 30 '21 at 02:49
  • There are a number of places looking into desalination. Places with rivers are not the places to look at to figure the market here, clearly they aren't the customers. Also, I don't think a comparison to a typical container ship is appropriate. You are slowly moving a membrane filled with water - its max practical size would be calculated quite differently than a cargo ship. – kim holder May 30 '21 at 04:19
  • @John Desalination plants expend 3 to 10 kilowatt hours per cubic meter of water, using modern reverse osmosis tech. I should add the reference for that to the question. Their main drawback is how energy intensive they are. This doesn't need to use much energy at all, it only needs to use time. – kim holder May 30 '21 at 04:21
  • its need quite a bit of energy to move it, water is heavy. It also needs to be cut and moved onxe on site. also consider solar desalinization is possible. – John May 30 '21 at 04:24
  • @John: But if you were moving large icebergs, you could get the energy on-site, as it were, with sails, solar panels, &c. Plus an adroit use of wind and ocean currents would minimize the energy needed. – jamesqf May 30 '21 at 18:28
  • 1
    If the position here is any system that could do this with sea water in freezing regions could just fill up near the mouth of a river and skip the freezing and thawing part, then that point I can see. I was actually thinking it would take such a huge ship to be worthwhile that it would be a problem in a river - but then that would mean it's also a problem to empty it at a coastal port. – kim holder May 30 '21 at 21:13
  • @jamesqf it is certainly possible and would likely depend on the distance you needed to travel, for instance moving and melting sea ice for water in Greenland is probably easier, in Ecuador probably not so much. – John May 31 '21 at 06:48
  • 2
    @jamesqf it is certainly possible and would likely depend on the distance you needed to travel, for instance moving and melting sea ice for water in Greenland is probably easier, in Ecuador probably not so much. Keep in mind as well no simple membrane is holding a iceberg, a cracking iceberg can tear steel. Icebergs are also not made of sea ice. – John May 31 '21 at 07:01
  • @John: You would have to discuss engineering details with the people who are actively proposing this. However, Greenland icebergs don't work all that well, being relatively small and irregular compared to Antarctic ones. AFAIK, no membrane is involved in these schemes: they just depend on the iceberg's thermal mass. – jamesqf May 31 '21 at 16:17
4

Economics

The main problem is in the economics. Whether a solution is possible or not does not make it profitable. In this case you have a lot of issues hampering your profitability.

  1. Speed is of the essence. All machines have owning costs and operation costs. Since you only get profit once shipment is delivered you want as many deliveries as possible from a single ship, so that means speed. So your assumption that speed does not matter is wrong here. As soon as ships go above a certain speed the fuel cost and maintenance cost become more expensive. So there is some optimal speed calculated in profitability, this can even change with market circumstances.
  2. Competition. There are more profitable ways of carrying water to dry regions. Just buy water from a wet region and transport it by ship to a dry region would probably already be one. Desalination is another option that is probably cheaper. As soon as your competitor is way cheaper you will be out of business whether you solution works or not.
  3. Supply chain demands. The size of the operation needs to fit the demands somewhat. All consumers (and retailers) want steady reliable supply chains that can produce the amount they want. This method does not seem to provide a stable enough supply chain to carry enough water for consumers. At that point it becomes much harder to sell even if it is cheaper or you need a big organisation to get a stable supply chain which cost money making the system less profitable.
  4. Law. At large scale I think international law will have something to say about sailing a lot of ships into a nature reserve and taking water from it.
D.J. Klomp
  • 2,425
  • 9
  • 14
1

You're trying to put a sail on an iceberg

There's no need to freeze water in a bag - we find ice on the sea all the time, and towing icebergs to thirsty cities has been a perennial proposal for at least half a century. The problem with trying to do this "free", with a sail, is simply that you'd need a lot of sail to move that ice. Also, you can't be entirely unhurried since it will be melting on the way.

For decades folks on Web forums have been suggesting enhancements to this idea - Pykrete shaping and insulation of the iceberg, for example. There may well be a way to get it to work. (Not this, apparently) But any capital or labor-intensive scheme you can think up should be weighed against the option of roofing the target city in solar panels to power the desalination plant.

Mike Serfas
  • 21,774
  • 17
  • 79