Hacker News Reader

Comment by sacred_numbers | Hacker News Reader

Efficiency in desalination is measured by comparing the energy expenditure to the enthalpy of vaporization of water (basically the energy required to distill water by boiling it). The current state of the art reverse osmosis desalination plants use about 3.2 KWh of electricity per cubic meter of fresh water (https://pdfs.semanticscholar.org/d4d7/821d585699719289dddd10...). This technology uses about 173 KWh of solar energy per cubic meter of fresh water. The advantage of this method is lower capital costs and not having to convert solar energy to electricity. For large-scale desalination, however, it is almost certainly more cost-effective to use solar panels, batteries, and large scale reverse osmosis systems. This is still a useful project for making drinking water in remote locations, though.

View on HN

One fascinating (well, to me) component of modern RO desalination systems is the rotary pressure exchanger. Such a simple device, but so important. It looks like something from the 19th century, but the base patent was issued in 1988.

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

I found this video which seems to animate the concept nicely:

https://www.youtube.com/watch?v=udffed4Pq3g

Bloody Hell I did not know that. I have parts of broken Katadyn PUR-06. I was thinking of rebuilding it from better materials. The pressure recovery is the worst part. I can totally fathom now a hand-held desalinator with only rotating parts running from solar or Li-On batteries.

PUR-06 lasts 300 days, not "indefinitely": https://youtu.be/dNjdgfGgPdE?t=208

I don't know how well this would scale down. Viscosity would work against you.

Thank you, that was fascinating.

Awesome video. One of the better uses of Youtube, and what we should be striving for in terms of "online content".

Awesome. The video made it crystal clear.

That is such an incredibly smart piece of design. A tip of the hat to whom ever thought it up.

Leif J. Hauge

https://www.powershow.com/download/3b604b-ZGQwN/Pressure_Exc...

Read the history, including how the then-current prototype (out of series of improved versions developed over the 12 years it took to get to a market ready design) and all the inventor's personal belongings were lost in the invasion of Kuwait.

So they took the technology of the revolver and applied it to pressure relief. Fascinating stuff.

>One particularly efficient type of pressure exchanger is a rotary pressure exchanger. This device uses a cylindrical rotor with longitudinal ducts parallel to its rotational axis. The rotor spins inside a sleeve between two end covers. Pressure energy is transferred directly from the high pressure stream to the low pressure stream in the ducts of the rotor. Some fluid that remains in the ducts serves as a barrier that inhibits mixing between the streams. This rotational action is similar to that of an old fashioned machine gun firing high pressure bullets and it is continuously refilled with new fluid cartridges. The ducts of the rotor charge and discharge as the pressure transfer process repeats itself.

Thanks for pointing this out. I knew that recovering pressure from the outgoing brine was an important efficiency measure but I assumed it was done using a hydraulic motor coupled directly to a pump arrangement.

very similar in principle to 10 years younger https://en.wikipedia.org/wiki/Pressure_wave_supercharger

That sounds like it would be a great way to eliminate the EGR system on vehicles as well if there was a way to vary the relative speed of the rotor to the crankshaft.

Regarding remote locations- they usually lack water altogether.

A long time ago, as a kid i read about a experiment in a GEO-magazine harvesting water directly from air. The used idea was a hygroscopic chemical (Silicagel) collects the moisture from the air. Sunlight heats the silica gel- which releases the moisture as steam- which then condenses in a destil. It was very little water for a lot of effort, but the complete lack of moving parts and self-containedness of such a system deeply impressed me, being young and with Frank Herberts Dune on my mind.

To drop such a contraption into the deep dessert, where it could keep a plant alive, become a oasis with no basis ..

You can make a solar still without desiccants. Dig a hole somewhere that will get sunlight most of the time, drop a cup in the bottom to catch water, and create a funnel with a plastic sheet. (If you don't have a plastic sheet, branches can work.) There are various ways to optimize it, but a single water trap isn't going to keep a person alive.

But if they're cheap and easy, and a person could make a dozen, that changes things a bit. But still, I think we're still talking about short term survival only. If that's your environment, the local flora & fauna are unlikely to be prolific enough to fill other needs without extensive effort. Unless, you know, you've got a Still suit and can call giant worms and stuff.

The thing is- this could work without intervention - and without the intention of keeping humans alive for years, thus creating a ecosystem over time. Somewhere in the middle of nowhere- watering cacti, then hardened other plants, reclaiming the moisture they loose, until bushes and trees take over these functions.

Its hard describing this fascination with self-contained or only slowly expanding pockets of life.. imagine a glass bubble , filled with small plants, beatles and life, thriving in the midst of a frozzen over wasteland like mars or a dessert like the death valley.

Its like a https://en.wikipedia.org/wiki/Bottle_garden but without the surrounding walls..

To what extent would systems like this extract moisture from the air. siphoning it off from flora/fauna that already make use of it?

There's a newer technology called a metal organic framework that does this at a reasonable rate for the effort.

https://www.youtube.com/watch?v=XPSYzLZ7xKU

https://www.sciencemag.org/news/2019/09/crystalline-nets-har...

http://wahainc.com/

Not so fast.

From the panel to the outlet, you only get out from 10 to 15% of input solar power, while thermal desal can use nearly all of it right away, and it leaves more concentrated brine. Multiple effect desal will probably move the efficiency up a bit. It's nowhere near clear cut.

Also if you have low potential reject heat from power plants, it's idea for desal use.

If you have nuclear as your heat source, then economy-wise thermal desal will beat electric many times over.

I recently read about thermochemical, and electrochmical desalination methods which can be added atop a thermal plant, further increasing thermal desalination efficiency.

>more cost-effective to use solar panels, batteries, and large scale reverse osmosis systems

I can't imagine why you'd use batteries at all. Storing fresh water efficiently is cheap, storing power efficiently is not.

To make twice as much water you may need to either install a set of batteries to run at night, or install twice as much RO hardware to run during the day. Whichever's cheaper.

I made a comment with regards to this a while ago [1]. I'll repeat it here as I think it is relevant:

---

I’ve heard on the grapevine that facilities are now being designed which work at under 100% capacity in order to soak up excess/cheap power.

I believe this is coming about because: 1) the high cost of power and presence of sporadically cheap power makes it economically viable, and 2) designing equipment with lower duty cycles can actually provide substantial cost savings. Ie a facility which only runs 50% of the time is much cheaper to build & run than once which runs 100% of the time.

Sorry I cannot cite sources right now.

[1]: https://news.ycombinator.com/item?id=18929542

It'd be a boon for renewables if energy consuming plant costs were less dominated by investment in equipment.

Imagine doing shift planning at such a site though. Eight days from now, there's a 60% chance of wind in excess of 5m/s between 22.00 and 06.00. But labour costs would be 130% higher than 06.00 to 14.00 on that day. On that morning shift there's a 50% chance of clear skies ...

If only there was some form of programmable electronic computation device that could be used to solve such complex sets of constraints modelled as equations :)

It's not the calculation part that's the problem, it's the investment part (and perhaps also labor part).

I think the savings in planning for low duty cycle probably result from reduced redundancy i.e. installing one pressure pump instead of two. Which naturally means when there is a low chance of excess energy is a good time to work on maintaining the systems rather than just taking one off line and working on the other.

This means that you just staff it at a consistent level and plan the work around the energy levels.

I would be sad to be the worker subject to that shift planning

Just make a water tower to store provide pressure at night.

Because you're likely going to be able to collect more energy than you can immediately use. That makes it effective to do so you can run the desalination even when the sun is down.

The question is if you could build and operate a smaller plant to run at 100% capacity at all times which includes energy storage for less money.

You either have losses in:

* Underutilized solar generation

* Underutilized plant capacity

* Overhead of energy storage

Design your system to minimize that cost. Engineering systems of all sorts are full of this kind of optimization problem: "Should I add a subsystem to recover loss x?" There is always more you can do to recover losses, but each extra system you add suffers diminishing returns a little more.

You actually don't even really need the batteries. Israeli utility scale desal is designed to operate in variable mode, in their case they run about 10% capacity during summer days and 100% during summer nights. Their grid peaks are air conditioning dominated which is why that pattern.

It does mean much larger service water tanks as you need to buffer demand but if you have lots of "free" solar power then it may well be worth it.

You're comparing both systems using watthours of energy expended, so mentioning enthalpy of water vaporization is unnecessary and confusing.

Sorry, I meant to say that the enthalpy of water vaporization is about 630 KWh per cubic meter for context to compare to the energy use of this method and reverse osmosis. I guess I forgot to actually include the number!

Say solar panels cost $2 per watt, 6 liters/hour is about 20 watts of power, is about 40 dollars of solar panels. Plus whatever the reverse osmosis equipment costs.

This system gets approximately 6 liters/hour/sq m... so it makes sense if we can manufacturer it for less than 40 dollars per square meter and space is cheap.

> Say solar panels cost $2 per watt

Not for a large desalination project. You will pay close to the spot price. Which is 30-45 cents.

Your biggest cost will be labor anyway. And the osmosis membrane, which needs to be regularly replaced, unlike the panels.

And, in places without usable water, space tends to be cheap.

I wonder if there is a formula for how cheap a place is based on how easy it is to have usable water and usable crops.

I have had these feelings since I saw the Egypt map showing how much of it is just not usable.

If the capital costs of such a system are lower and they can convert the design into something floating on the sea on continental shelf with tethers mooring it to the floor i see a lot of potential for water, land and electricity (competitive) stressed cities. A case in example is Chennai, India.

I wonder if RO based desalination has any learning rate associated with it and whether a lot of CAPEX costs are related to some proprietary IP. I know that a lot of OPEX is energy costs and hopeful that solar will get even more cheaper with time and even more scale. However if the same can be done with scaling of RO technologies and reducing costs to say ~20 % of existing a great many opportunities will arise.

Really hope it will be lower than 0.1 $ / m3 before 2030.

You seem to know a bit about this: would you have a similar comparison on the capital cost issues, e.g., cost per cubic meter for the at-scale version vs. this local small scale? I mean, that's probably unimportant even if there's a big difference given that remote locations simply wouldn't be practical to have a large plant. But if the costs are even only twice as much per cubic meter, the rapidity of rollout for the small-scale solution could provide rapid relief, especially in some of the areas of the world with acute shortages that need rapid remediation even if it's only a short term solution. (like Cape Town recently)

> For large-scale desalination, however, it is almost certainly more cost-effective to use solar panels, batteries, and large scale reverse osmosis systems.

For large scale desalination, you need to attach tiny magnetic particles (think nano-scale) to the salt, and then extract it with magnets. And as a byproduct, you'll get Lithium.

Is thata dangerous byproduct? Or is the lithium isolated and of usable quality as feed stock for the battery industry?

For a sodium ion in solution it's more a question of pico-scale. And since most sodium salts are quite soluble you'll need some pretty exotic glue to attach that nano-magnetite.

How much space would this thing take up for say, a remote oil rig?

Reply on news.ycombinator.com