Superefficient solar desalination (2020) | Hacker News Reader

Superefficient solar desalination (2020)

cen.acs.org

73 pointsabrax314149 comments

This is just as exciting as when it came up in 2020.

One of the nice features of this system is that, overnight, the wick self-cleans, diffusing accumulated salt back to the sea water.

Another way to recycle the heat of condensation, in a bigger system not relying on capillary action, would be to circulate incoming brine in the condenser on its way to the evaporator, pre-warming it while cooling the vapor; and run outgoing brine through a heat exchanger to further warm incoming brine.

A key observation in design of low-cost desalination is that there is no need to minimize the amount of brine circulating in the system. So long as heat collected in the brine is not wasted, you are better off circulating more brine. The less you concentrate the brine, the easier it is to get water out.

> diffusing accumulated salt back to the sea water.

A big issue of desalinisation is the increase in salt in water: brine streams coming out of osmosis plants are dead zones in the sea. I'm not sure this is a good idea for the shore.

As I noted, there is no need to concentrate the brine to such a degree that discharging it could be a problem. But even if you did, discharging the brine through a long, leaky pipe or hose would avoid creating any dead zones.

How do you create a long leaky pipe without bio-fouling clogging it up? I guess the high salinity itself could prevent fouling.

With positive pressure and small holes, it would be hard for bio to get in and foul it.

Generally you are more worried about biofouling on water intakes, with good reason.

It's only seems like a big issue to people who don't know a lot about the ocean. I grew up surfing in a town at "sewer peak". The effluent from a sewage treatment plant was nearby. It was too close to shore, so they moved the discharge further out into deeper water. Discharging water with a higher salt content than the ocean wouldn't cause any harm at the discharge point, so this concern always seemed like a canard or unfounded theory to me. Some people don't want desal plants near them, so they throw this out there for NIMBY reasons.

I gather San Diego discharged sewage into the sea forever. When they finally got to cleaning it up, maybe to appease surfers, they were forbidden because an ecosystem had grown up dependent on it the way it was.

Santa Cruz didn't clean it up because of surfers, who didn't care because it wasn't very noticeable, although you knew you were taking a chance of getting mildly sick. It's really difficult to resist going out when the waves are good.

It was because the fecal bacteria count was over the new environmental standards that came to be in the last two decades of the 1900's. Many people don't realize there's an acceptable amount of mouse dung and insect carcasses in the food they buy, so it may sound gross to surf in gray water, but it really wasn't.

It may be a bigger problem in San Diego because the water is much warmer than the cold currents the come down the coast from Alaska to Santa Cruz (along with some good waves)

> Discharging water with a higher salt content than the ocean wouldn't cause any harm at the discharge point

Sewage and hot, concentrated brine are not the same. Plenty of studies show that there is harm to ecosystems, that's often not properly addressed.

- https://www.ccc.tas.gov.au/wp-content/uploads/2018/11/Apx-22...

- https://phys.org/news/2019-01-brine-discharge-desalination-g...

Again, this is a problem only if you just drain concentrated brine straight into the sea. But there are cheap, easy alternatives. Easiest is a long, leaky pipe leasing way out offshore. Or you could exhaust it into freshwater right before that enters the sea. Or you could dilute the brine with lots of seawater before draining it.

Yes, but out past the kelp beds and sea otters it's all flat sand until it hits the Monterey Bay canyon. The Bay is a marine sanctuary so we'd probably put the desal plant up the Coast on open ocean to serve mostly San Francisco, which now gets it's water from the Sierras. That may not last.

Also interesting to me is that they turned the local fossil fuel electric plant that I grew up with into a giant battery:

Moss Landing Battery Storage Project

https://www.nsenergybusiness.com/projects/moss-landing/

A few decades ago we lost power for a few days when the ELF (Earth Liberation Front) cut a main power line from Moss Landing to Santa Cruz.

edit: I can't find the story and it wasn't the ELF so it must have been a sister group

https://en.wikipedia.org/wiki/Timeline_of_Earth_Liberation_F...

Edit: The first link you shared says it's historically been a concern but seems to dismiss the concern by saying it's all in where you put the plant and how you do the discharge, which is obvious:

Environmental impacts associated with concentrated discharge have historically been considered as a major environmental concern to marine life with desalination plants. The environmental impact of desalination plants will vary depending on several factors:

i. The location of desalination plant.

ii. The location of the inlet and outlet.

iii. The method used in the desalination facility and the outlet (water channel and pipeline).

The second link says nothing was harmed but regulation limits were exceeded, so they need to be considerate of that concern:

The bad news in the study is that the salinity level in the discharge zone exceeded the permitted level, and the salinity plume extended much farther offshore than permitted under the California Ocean Plan. Senior author Adina Paytan, a research professor in the Institute of Marine Sciences at UC Santa Cruz, said the study provides valuable information for planners considering where to locate future desalination plants and what discharge technologies to use.

My dad is a Marine biologist and was the Chief Science Officer of the largest sea research institute in the world.

So he probably knows all about how to mitigate problems with brine outflows.

He knows that people like to say it's possible, promise mitigation, and he knows what it feels like to come back later and see the devastation left by not implementing it to save costs.

This is surprisingly poor production?

Peak insolation varies widely, but 1000W/m^2 is a typical value. 5.8L/hr/m^2 means that it's using something like 180kWh/m^3 on raw solar insolation.

For comparison, reverse osmosis is around 3kWh/m^3. This means that 20% efficient solar panel would produce around 67 L/hr/m^2 (aka ~11x more).

Obviously this is passive versus active, but it's still a surprisingly large difference.

Converting water to steam is an extremely inefficient way to separate water from salt.

If reverse osmosis desalination only takes 3 kwh/m^3 then you could produce 4 million acre-feet for 14.9 billion kWh which would cost about $1.5 billion per year. That is pocket change for replacing a big portion of Colorado river water.

I guess Californian environmentalists love Hetch Hetchy reservoir and making the desert stay dry.

Reverse osmosis desalination does have some capital and operational costs, though, but you're not far off. 5 billion m^3 or 4 million acre-feet would still only be about 2.5 B per year. The problem is that if you fill up the Colorado River the farmers that own the water rights will just drain it down again because they don't pay based on the quantity they use. Or if they do pay based on quantity the price is much lower than the cost of desalination. Desalinating large quantities of water without fixing the water rights situation would be like minting pennies out of gold.

sure, there would be capital costs but those could be financed, plenty of people would lend 30 year money for such an enterprise.

So to be clear you hate agriculture so much you are willing to end it with drought? I like agriculture personally. I would like more of it! There is plenty of desert in the west that could be made to bloom.

This device is literally some 3D printed nylon frames with papertowels!

https://sci-hub.se/10.1039/C9EE04122B

Given the simplicity and cheapness of the device... has anyone replicated their results? Seems like a fun weekend project!

There's also an aerogel layer. That's probably what would keep most people from building it at home.

What’s the aerogel doing anyway? And I’d that something that can be bought? How expensive is it?

Painfully expensive.

Mke your own

It's not too expensive, you can buy a 50cm x 50 cm sheet of silica aerogel from AliExpress for about $20... But it's hard to say what this device uses, it might be something more exotic.

They're using aerogel for its transparency & thermal insulation properties. Are there any other good contenders that are easier to procure -- e.g. if you're shooting for practicality rather than record-setting?

Multi-pane windows would probably be the easier homebrew option. You could even make the panes out of plastic film (e.g., ETFE).

I had the same thought - an off the shelf double-paned window module would be great, otherwise a homebrewed version with two panes and a partial-vacuum pumped out between them.

Just out of curiosity, does anyone know the theoretical minimum energy required for desalination? Is it the energy released when salt is dissolved in water?

By analogy, Vaclav Smil says the best iron smelting and ammonia synthesis plants are within 100% of the theoretical minimum energy required for the chemical reactions to take place - within 50% in the case of the very best iron smelters. Aluminum smelting is slightly worse than these two IIRC.

Minimum is around 1 kWh/m^3 for sea-water levels of salt concentration. (It varies a fair bit depending on salinity, the actual salts involved, the temperature etc etc).

Thank you!

Do you have a reference on Vaclav Smil?

I’d be interested in reading the book that discusses this sort of thing.

> can produce 5.8 liters of fresh water per hour in full sun

All this from a 10 x 10cm panel. Sounds too good to be true.

> On the roof, the device produced 72 mL of water in 4.5 hours.

That sounds more in line with what I'd expect from Massachusetts sunshine.

Why does the article describe the difference between 5.8l/h and .072l/4.5h as 50% lower? It looks to me like this ratio is more than 300x.

I wonder why seaside towns don't all have 1m2 of this per 1000 residents. Seems like a no brainer if it really works but I can't believe it does.

This is incorrect. The statistic of 5.8 liters per hour is actually liters per hour per square meter. A 10x10cm panel would produce 58ml per hour in full sun. If you instead put a solar panel with 20% efficiency and used the electricity for reverse osmosis you would produce about an order of magnitude more water per square meter. This may have lower capital or operational costs, though, and it is certainly more decentralized, so it could be useful in some situations.

What’s the capital/op cost of r.o.? Isn’t it just a pump and a membrane?

Just for clarity, is that a 10x10cm solar panel?

What I mean is that a solar panel of any given size will produce electricity that can be used to desalinate an order of magnitude more water than a device like this of the same size. The device in this article desalinated 5.8 liters per square meter per hour in full sun, which means this 10x10cm device actually desalinated 58ml of water in an hour. A 10x10cm solar panel would produce enough electricity in an hour in full sun to desalinate approximately 600ml of water using reverse osmosis. A solar panel that was 1 square meter would generate enough electricity to desalinate about 60 liters of water in an hour in full sun.

The linked article's production rate is off by two orders of magnitude. If you read the actual paper [1], the quoted production is 5.8L/hr/m^2 -- i.e. 5.8L/hr would require a 1 m^2 system, not the 10cm x 10cm system under test (0.01m^2).

https://sci-hub.se/10.1039/C9EE04122B

I posted this link to a BBC World Service podcast about a year ago of using a solar still in Somalia to produce water for a greenhouse AND salt:

https://www.bbc.co.uk/programmes/w3cszv2l

Just thought I’d mention it heat again. Different technology, but pointing out there is demand for the salt.

Not to detract from this article which is amazing. The actual paper of which is here: https://pubs.rsc.org/en/content/articlelanding/2020/EE/C9EE0...

There is a Finnish company doing something like that. https://solarwatersolutions.fi/en/

Hope plans for this will get released so that others can build their own. Would be great to use while sailing or doing costal hiking/camping

Someone, a YouTuber, has made a YouTube video about the work:

https://youtu.be/bTa6QnNIzsQ

Wow, they'll have enough salt to last forever.

I wonder what they could do with all that salt, as releasing it in the environment isn't exactly safe. Would be there any uses for the huge quantities involved?

(2020)

Added. Thanks!

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