Solar-Powered Desalination

[Airbrush]

You can create your own desalination pit in your back yard.  Just put a clear plastic sheet over a kiddy pool and have the plastic deliver condensation into a central bucket.

Why cannot humanity build desalination containers to float off-shore, like a fleet of a hundred (or thousand, or million) canoes covered with clear plastic.  They float on the ocean allowing just enough water into the container so the sun can heat it and the condensation can flow through pipes or hoses towards shore.

These containers can even be connected by "wigglers" that generate power from wave action.

We could irrigate great deserts to grow trees to capture greenhouse gasses and defeat climate change.

Edited August 19, 2019 by Airbrush

[swansont]

15 minutes ago, Airbrush said:

 Why cannot humanity build desalination containers to float off-shore, like a fleet of a hundred (or thousand, or million) canoes covered with clear plastic.  They float on the ocean allowing just enough water into the container so the sun can heat it and the condensation can flow through pipes or hoses towards shore.

You don't see any obvious structural issues with this? or logistical ones?

[Airbrush]

1 hour ago, swansont said:

You don't see any obvious structural issues with this? or logistical ones?

No because these are modules that flex with the ocean waves.  As they flex they can generate power to assist the fleet of desalination modules.  They could cover square miles, off shore, for all I know (in my limited structural and logistical knowledge.)  If they get damaged in a storm the damaged modules can be easily replaced.

[swansont]

33 minutes ago, Airbrush said:

No because these are modules that flex with the ocean waves.  As they flex they can generate power to assist the fleet of desalination modules.  They could cover square miles, off shore, for all I know (in my limited structural and logistical knowledge.)  If they get damaged in a storm the damaged modules can be easily replaced.

If they exist, how about a link to their description?

[OldChemE]

I recall from many years ago similar projects being discussed.  My recollection is that there were no structural issues-- but there were efficiency issues.  Solar power already evaporates water for the entire surface of the ocean and delivers it to land in the form of precipitation.  This, of course, is of limited efficiency because some of the precipitations falls on the oceans, or in places it cannot be efficiently used.  However, since floating desalination systems detract from the natural evaporation, they would have to be correspondingly more efficient to make a net benefit.   The challenge is in making the floating system, and the energy required to retrieve the water thus produced, sufficiently efficient to make it financially superior to capturing the natural precipitation.

[swansont]

10 hours ago, OldChemE said:

I recall from many years ago similar projects being discussed.  My recollection is that there were no structural issues

i find that surprising. Waves will cause the whole thing to want to contract. If it's rigid, you put a lot of stresses in place, but if it's flexible, then what keeps the "canoes" from collecting together? If the structure is vibrating, you will lose water from it before it can be collected as drops will tend to shake loose. 

Perhaps these are solvable problems, but that does not mean that they aren't an issue. And solutions cost money, affecting the viability. If something is not cost-effective, one reason can be that the cost of solving the engineering issues is too great.

[OldChemE]

9 hours ago, swansont said:

Perhaps these are solvable problems, but that does not mean that they aren't an issue.

I don't disagree-- but I think we have different tolerance levels as to what kind of engineering challenge counts as an issue.

[Airbrush]

14 hours ago, swansont said:

…..Waves will cause the whole thing to want to contract. If it's rigid, you put a lot of stresses in place, but if it's flexible, then what keeps the "canoes" from collecting together? If the structure is vibrating, you will lose water from it before it can be collected as drops will tend to shake loose. 

It is flexible and the canoes are connected in a giant net or web.  Straps keep them from collecting together.  They can also be anchored to the sea bottom in places.  Vibration won't matter, only problem is big waves tip them at extreme angles, but when weather gets rough, the net of tubes running from the canoes shut down and go into survival mode.  They are very tough plastic with clear plastic top for sun light.

Edited August 21, 2019 by Airbrush

[npts2020]

One of those engineering problems swansont referred to is how much water you can expect to get from a given surface area. (keep in mind, the following is only approximations but will demonstrate the scale we are talking about) A good solar distiller will give you about a gallon per day per square meter of surface area. Compare that to a distilling unit on an aircraft carrier that produces about 100,000 gallons/day using waste steam from the propulsion system in a unit that measures about 3.5 meters square by 5 meters high. The comparable solar unit would have to cover around 10 hectares and 100,000 gallons is only enough for about 500 households. Just one small city of 100,000 households would require 20,000 hectares of surface area. For comparison, the state of Rhode Island has about 400,000 households on around 300,000 hectares of land so the scale is not impossible, just very large. It seems doubtful to me that people will tolerate filling the oceans with that many distillers, though (look at how much opposition offshore wind farms got).

Edited August 21, 2019 by npts2020
clarity

[Curious layman]

Maybe they could use the same technology as this for the canoes, wave energy:

Instead of using the electricity to power homes, they could use it to collect and pump the desalinated water to the shore. 

Edited August 21, 2019 by Curious layman

[swansont]

7 hours ago, Airbrush said:

It is flexible and the canoes are connected in a giant net or web.  Straps keep them from collecting together. 

Straps only work under tension (i.e. they keep things from moving apart)

 

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They can also be anchored to the sea bottom in places.

I imagine they have to be moored in at least one spot. But not too tightly.

 

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  Vibration won't matter

Have you ever shaken something wet? Do all (or most of) the drops cling to the material?

 

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, only problem is big waves tip them at extreme angles

No, not the only problem. Basic trig should show you problems.

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, but when weather gets rough, the net of tubes running from the canoes shut down and go into survival mode. 

The tubes probably aren't much of an issue, as they would be subsurface and are the one part of the system that can be flexible without compromising performance.

 

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They are very tough plastic with clear plastic top for sun light.

Flexible or rigid? 

 

[Airbrush]

5 hours ago, swansont said:

Straps only work under tension (i.e. they keep things from moving apart)

I imagine they have to be moored in at least one spot. But not too tightly.

Have you ever shaken something wet? Do all (or most of) the drops cling to the material?

No, not the only problem. Basic trig should show you problems.

The tubes probably aren't much of an issue, as they would be subsurface and are the one part of the system that can be flexible without compromising performance.

Flexible or rigid? 

Some members are flexible and others are rigid.  Separate the modules using poles forming tetrahedrons, with flexible connectors, to keep all canoes distributed evenly, but allowing them a little latitude of motion.

Right, some mooring is appropriate.

Most of the time I don't think the modules will be vibrating.  When you are in a boat on the water, you don't feel vibration.   If the condensation is channeled down a steep slope, any vibration or other motion would speed up the flow of water down to a collector.

What are some problems that trig would reveal?

 

13 hours ago, npts2020 said:

One of those engineering problems swansont referred to is how much water you can expect to get from a given surface area. (keep in mind, the following is only approximations but will demonstrate the scale we are talking about) A good solar distiller will give you about a gallon per day per square meter of surface area. Compare that to a distilling unit on an aircraft carrier that produces about 100,000 gallons/day using waste steam from the propulsion system in a unit that measures about 3.5 meters square by 5 meters high. The comparable solar unit would have to cover around 10 hectares and 100,000 gallons is only enough for about 500 households. Just one small city of 100,000 households would require 20,000 hectares of surface area. For comparison, the state of Rhode Island has about 400,000 households on around 300,000 hectares of land so the scale is not impossible, just very large. It seems doubtful to me that people will tolerate filling the oceans with that many distillers, though (look at how much opposition offshore wind farms got).

This is very interesting.  What is a "good solar distiller" that could produce about a gallon per day per square meter?  What if the water is heated very hot by the sun, plus you can input more power to heat the water from other means of energy generation, such as wind mills, solar cells, or flexers?  The hotter you can make the water, the faster the desalination.

Edited August 21, 2019 by Airbrush

[swansont]

17 minutes ago, Airbrush said:

Some members are flexible and others are rigid.  Separate the modules using poles forming tetrahedrons, with flexible connectors, to keep all canoes distributed evenly, but allowing them a little latitude of motion.

Right, some mooring is appropriate.

Most of the time I don't think the modules will be vibrating.  When you are in a boat on the water, you don't feel vibration.   If the condensation is channeled down a steep slope, any vibration or other motion would speed up the flow of water down to a collector.

You must be one of the few who have never experienced wind or waves while out on the water.

 

17 minutes ago, Airbrush said:

What are some problems that trig would reveal?

That the distance between the canoes must change when there are waves. Adjacent canoes will be at different heights, changing the distance between them, or forcing them to move laterally. This puts stress and torque on rigid components, or will make flexible components go slack and then taught.

 

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This is very interesting.  What is a "good solar distiller" that could produce about a gallon per day per square meter?  What if the water is heated very hot by the sun, plus you can input more power to heat the water from other means of energy generation, such as wind mills, solar cells, or flexers?  The hotter you can make the water, the faster the desalination

One obvious question: is it cheaper/easier to put a device on the ocean, or to move the salt water to a ground-based plant (passive or active)

[Airbrush]

On 8/21/2019 at 9:38 AM, swansont said:

One obvious question: is it cheaper/easier to put a device on the ocean, or to move the salt water to a ground-based plant (passive or active)

How about digging a shallow canal near the ocean and allowing the salt water to flow inland where it can be desalinated in controlled conditions?  Then bury the excess salt?

[swansont]

8 minutes ago, Airbrush said:

How about digging a shallow canal near the ocean and allowing the salt water to flow inland where it can be desalinated in controlled conditions?  Then bury the excess salt?

Or just pump the water someplace (where land is cheaper than on the coast), and pump slightly saltier water out. Or sell the salt.

 

[Airbrush]

10 hours ago, swansont said:

Or just pump the water someplace (where land is cheaper than on the coast), and pump slightly saltier water out. Or sell the salt.

That sounds like a good idea.  Just pump huge quantities of sea water to an inland desert where it fills miles and miles of desalination ponds.  The ponds could be covered with clear plastic that has a lensing effect that magnifies the sunlight.  The water is used to create an oasis and humans can live there too.  Then more ponds are built, and more and more trees are planted over time in deserts.  The trees capture CO2.

And sell the salt!

Edited August 23, 2019 by Airbrush

[Airbrush]

In the US there are thousands of migrants from Central America coming to the US.  I would assume they are looking for work, though I never hear that mentioned.  They are passing through vast deserts to get to the US.  Would they be interested in working on a desert solar desalination farm, especially if they knew they were helping to save the world?  Saving the world is a job most people would be proud of.

Edited August 23, 2019 by Airbrush

[Enthalpy]

While sunlight can make heat and evaporate seawater to obtain sweet water, this is a waste. Separating water from salt takes very little energy, but evaporating water takes much.

At least at identical sunlight collecting area, it's better to run water pumps from sunlight, possibly over electricity, and have reverse osmosis remove the salt.

Then you have to compare the costs, where replacing a plastic sheet by solar cells isn't obvious. The current heavy trend in favour of reverse osmosis tells it's globally cheaper.

Reverse osmosis works so nicely that it does provide sweet water on city scale. It's used in Canarias (Spanish isles) and Israel at least. I believe it's powered by sunlight at least in Canarias, but it could use any power source.

A big project exists to let seawater flow from the Red sea to the lower Dead Sea to avoid its disappearance, turbine the water to make electricity, and use a part of the electricity to desalinate some water.