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Simple, solar-powered water desalination (2020)

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Re: Simple, solar-powered water desalination (2020)

#131

Post by dsr_ »

squarefoot wrote: post #107

FTA: "Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of. In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater, according to the researchers." The capabilities are freaking interesting, but let's say someone builds a big enough settlement on a…

This chunk from the article is, unfortunately, an outright lie.

By definition, removing salt from seawater leaves you with water and salt. The article handwaves the salt away by supposing that the desalinator will float on top of the ocean.

In actual production configurations other than towing it behind your sailboat, you will end up with a brine pool that needs to be disposed of, or a concentration in the wicking material that prevents low-concentration seawater from entering, or a pumping system.

This is not just a common negative externality, but a casual lie about it.

Re: Simple, solar-powered water desalination (2020)

#132

Post by pfdietz »

An issue with desalination systems of this type (that have water vapor in a carrier gas, so called HDH or Humidification-Dehumidification desalination systems) is the retardation of mass transfer to the cold surface by the carrier gas. Multieffect distillation (MED) systems usually operate without a carrier gas for this reason, but this means they operate below atmospheric pressure, so they have to be sturdy to resist pressure loads.

A few years ago, some people at MIT solved this problem with a bubble heat exchanger: the humidified carrier gas is bubbled up through trays of water (at progressively lower temperatures). The surfaces of the bubbles provide a very large surface area for heat/mass transfer.

https://news.mit.edu/2013/produced-water-cleanup-0205 https://dspace.mit.edu/handle/1721.1/86334 http://web.mit.edu/lienhard/www/papers/reviews/HDH-Desalinat...

Some of those involved went on to form a company to commercialize the technology, which has been used to recycle/purify brine from natural gas wells and other industrial tasks. I think they ended up being bought out after several years.

Re: Simple, solar-powered water desalination (2020)

#133

Post by kortex »

londons_explore wrote: post #104
jeroenhd wrote: post #67

Earlier quoted context omitted.

> In production, they think a system built to serve the needs of a family might be built for around $100. I think this is much more important to a great many people than the theoretical efficiency of reverse osmosis. Percentages greater than 100% always do well in media reports about these topics, but they're not necessarily the point of the exercise. If the goal was to produce a system that's as efficient as possibl…

Nothing about reverse osmosis can't be scaled down in size or cost. The actual membranes are $9 for enough for 100 gallons per day (retail prices, [1]). High pressure pumps and hoses scale linearly. Solar panels scale linearly. Filters scale linearly. In fact, using this solar fountain [2] as the basis for the design, and switching the pump impeller for a high pressure version, and the nozzle for an RO membrane and h…

I have first-hand experience that a) you need positive displacement pumps of a certain quality and power for RO and the cheap Ali ones self-destruct in minutes/hours b) you still need housing, plumbing, receptacles, etc c) fouling is a non-trivial issue.

Expect to pay around $300 minimum for a small RO solar plant, and a couple bucks a month for upkeep. And logistics. Granted that's USA prices (upstate NY, not SV) but that prices out a lot of developing regions.

Re: Simple, solar-powered water desalination (2020)

#134

Post by amn79 »

squarefoot wrote: post #107

FTA: "Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of. In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater, according to the researchers." The capabilities are freaking interesting, but let's say someone builds a big enough settlement on a…

Why release the salt back into sea water? This is salt production alongside desalination.

Re: Simple, solar-powered water desalination (2020)

#135

Post by tonyhb »

mianos wrote: post #114
ghshephard wrote: post #87

Earlier quoted context omitted.

Back of the envelope - say that you could get 7-8 hours of sunlight a day that could destill water. That would mean ~50 liters/day. ~20 Days to get 1m^3. Reverse Osmosis costs approx $0.50/m^3 water, so your payback on a $100 system would be ~200 * 20 Days or 4,000 Days to equal what you could get for spending $100 on buying water from a reverse osmosis system. The objective here isn't large scale economics, but self…

There are a lot of quite liveable islands that are deserted only due to lack of fresh water. This could change that.

Or maybe we don't have to spread humans everywhere like a malignant cancer, leaving at least these Islands to nature.

Re: Simple, solar-powered water desalination (2020)

#136

Post by Someone »

dystroy wrote: post #115
squarefoot wrote: post #107

FTA: "Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of. In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater, according to the researchers." The capabilities are freaking interesting, but let's say someone builds a big enough settlement on a…

Salt release is already a huge environmental concern with exiting desalinization systems. Energy consumption and installation costs aren't the sole (and probably not the main) problems. Desalinization is inherently a pollution.

A small part of it can be avoided by combining this with a salt evaporation pond (https://en.wikipedia.org/wiki/Salt_evaporation_pond)

Problem will be that we’ll get way too much salt (a few grams of salt per liter of water), so it won’t help much.

Re: Simple, solar-powered water desalination (2020)

#137

Post by londons_explore »

kortex wrote: post #133
londons_explore wrote: post #104

Earlier quoted context omitted.

Nothing about reverse osmosis can't be scaled down in size or cost. The actual membranes are $9 for enough for 100 gallons per day (retail prices, [1]). High pressure pumps and hoses scale linearly. Solar panels scale linearly. Filters scale linearly. In fact, using this solar fountain [2] as the basis for the design, and switching the pump impeller for a high pressure version, and the nozzle for an RO membrane and h…

I have first-hand experience that a) you need positive displacement pumps of a certain quality and power for RO and the cheap Ali ones self-destruct in minutes/hours b) you still need housing, plumbing, receptacles, etc c) fouling is a non-trivial issue. Expect to pay around $300 minimum for a small RO solar plant, and a couple bucks a month for upkeep. And logistics. Granted that's USA prices (upstate NY, not SV) bu…

Fouling can be dramatically reduced by putting a small electrical current through the saltwater, freeing Chlorine ions which quickly kill biological things before they can adhere to your membrane.

That combined with reverse flow flushing will probably last many years. And it can all be controlled by a 10 cent microcontroller, one pump, one valve, and a pressure sensor.

I don't doubt that high pressure pumps are bad... but that's just a design issue - there is nothing theoretically expensive about them.

Re: Simple, solar-powered water desalination (2020)

#138

Post by betwixthewires »

tonyhb wrote: post #135
mianos wrote: post #114

Earlier quoted context omitted.

There are a lot of quite liveable islands that are deserted only due to lack of fresh water. This could change that.

Or maybe we don't have to spread humans everywhere like a malignant cancer, leaving at least these Islands to nature.

Maybe don't spread your Malthusian bullshit like a malignant cancer. Some of us like being human.

Re: Simple, solar-powered water desalination (2020)

#139

Post by einpoklum »

6nf wrote: post #100
einpoklum wrote: post #80

> The system delivered pure water That's toxic! We can't drink pure water. You have to maintain _some_ of the minerals in there, you just want to take most of the NaCL out. Does the system avoid this over-distillation? i.e. is the water properly potable, or is it just not-salty?

I drink distilled water all the time, it's not toxic in normal quantities.

Ok, fair enough, but if that's all the water you drink than you will have to get a bunch of minerals elsewhere in your diet.

Here: https://naturalhealthfundamentals.com/is-distilled-water-saf...

it says:

Those who may need to be cautious when drinking it:

* Anyone who is already deficient in minerals. You may be someone who would benefit from the little extra minerals water can give you. Although if you are already deficient in minerals, and have health issues, make sure to get your water from a good source to avoid all the chemical additives from most tap waters.

* There also isn’t much information on how well we absorb inorganic minerals from water. There is also the possibility that the minerals present in the water are not doing us much good. But since it could be helpful it probably doesn’t hurt to have the minerals there.

* Someone who has health issues or malabsorption problems.

* Anyone who has an extremely poor diet and doesn’t get many minerals from their food should remineralize distilled water if they are drinking it.

Re: Simple, solar-powered water desalination (2020)

#140

Post by msandford »

kortex wrote: post #133
londons_explore wrote: post #104

Earlier quoted context omitted.

Nothing about reverse osmosis can't be scaled down in size or cost. The actual membranes are $9 for enough for 100 gallons per day (retail prices, [1]). High pressure pumps and hoses scale linearly. Solar panels scale linearly. Filters scale linearly. In fact, using this solar fountain [2] as the basis for the design, and switching the pump impeller for a high pressure version, and the nozzle for an RO membrane and h…

I have first-hand experience that a) you need positive displacement pumps of a certain quality and power for RO and the cheap Ali ones self-destruct in minutes/hours b) you still need housing, plumbing, receptacles, etc c) fouling is a non-trivial issue. Expect to pay around $300 minimum for a small RO solar plant, and a couple bucks a month for upkeep. And logistics. Granted that's USA prices (upstate NY, not SV) bu…

Even these numbers are too small if you're going to RO seawater. I suspect what you have in upstate NY isn't access to seawater but access to some kind of less than amazing mostly-not-saltwater.

The osmotic pressure you need to overcome to perform desal is directly proportional to the concentration. So 500-1000ppm water that you don't love is a lot less challenging than seawater which is about 3.5% or 35000ppm.

You can do "I want extra pure drinking water from 'normal' water RO" for a few hundred, sure because the pressures are in the range of 20-60 psi. Reasonably efficient seawater starts somewhere around 400-600psi and I've heard of plenty of systems that work at more like 800-1000psi. Different pumps, membranes, membrane housings, piping, all of it. It's a couple thousand dollars plus a fair amount of energy unless you buy even more expensive energy recycling pumps that use the pressure in the brine output to help on the input.

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