How much energy does desalinisation use? Is it “absurdly cheap”? | Hacker News Reader

How much energy does desalinisation use? Is it “absurdly cheap”?

sustainabilitybynumbers.com

28 pointsphilipkglass48 comments

> Getting 50 litres of domestic water per day from desalinisation would eat up more than half of a month’s income every year.

Not getting enough water kills you and these people earn so little because they live somewhere not developed. I have the feeling that if you solve one of these problems the other will solve itself.

The better question is.. which one do you solve first?

> Let’s take an example of wheat. It takes around 650 litres to produce one kilogram of wheat.14 It would take around 2.3 kWh of electricity to produce the water for this. Take electricity costs in the UK and the water costs $0.66.

I would not calculate the cost of crops in terms of the cost of potable water. These are two very different water sources.

I imagine this would be a great use case for solar - desalination plants are mostly needed in hot desert places where the sun is strong. I imagine you could use energy at off peak times, since clean water can easily be stored.

Exactly. This what Israel has been doing, as I understand it. We don’t need to use desalination for everything, but for places like California which have water shortages, it could certainly help.

The desert is actually not great for solar, it's too hot, will decrease performance of the panels

That's why you do thermo solar in hot regions.

E.g. https://marquesadosolar.com/plant-andasol-3/

I'm not an energy engineer and know only basic high school/college physics and chemistry (and not very well).

Other than corrosion/filtering concerns, what's stopping thermal desalination being done at the same time as power generation? We pump water into a power plant, make it hot, and use the steam to turn a turbine and generate electricity. The steam just goes up and away. What if you collected it and let it condense back into water? Would it still be too salty? Would it change the characteristics of the steam too much? Just a logistics problem of what to do with the brine/particulates? It just seems silly to burn natural gas to make steam to make electricity just to use the electricity to pull the salt out in some other way. Seems almost guaranteed to be less efficient.

My understanding is that these sorts of plants do exist, but that they’re not common for the reasons you brought up. Some plants do generate power and desalinate at the same time, but they usually use excess heat for desalination and not the same water stream that they use for power generation. Corrosion, dealing with brine, keeping purity, etc, are all super challenging issues.

Brine management is the most difficult part of the entire process, which the author skips over entirely.

Few notes:

- reverse osmosis does not give good water, it's way less salty, but not much good to drink;

- thermal desalinization gives PURE water, so we need to mineralize it before drinking, and that's demand typically clean sand, witch demand water to be cleaned... Something actually very energivore.

Long story: we can get not-to-drink except on occasional basis "cheap water" or very expensive good one.

Aside there are some notes on how much time it take to get a certain amount of drinkable water, because well, for a small boat it's a thing, for agriculture it's definitively another. Another point: as an EV owner with domestic p.v. so with anything monitored, I definitely do not consume more to makes hot water than to power my car, I can't imaging how that's so much different in the USA. Ok, I have no pool, which is ENORMOUSLY energivore, but how many household respect of the total population have a pool at home?

It also sound a bit strange to me that hot water consumption is greater than home heating, ok, USA have little population in Alaska compared to the south and they are pretty at south European latitudes and below, but... I doubt so many homes are new well insulated ones... Even more strange that cooling consume more than heating. For Texas I've no doubt, but for the average USA?

Desalination releases brine, and would even if you had free energy.

It's a disaster for local sea life. You can either pump it back out to disperse it over a large area, or release it and kill all local marine life. Pumping it back out to disperse it is extremely expensive because the brine is corrosive. You could evaporate the brine but that takes land area plus you have to have the infra to actually use the salt, purify it, etc.

This all is a far bigger problem than the energy cost.

Too bad we don’t pump sea water to the desert, fill up a basin somewhere we don’t care so much about, let all the brine sit in evaporation pools, and power it all with local solar. Input from somewhere in Gulf of California and send the water to somewhere in imperial county away from the crops. We process Tijuana’s sewage free of charge so it seems reasonable we could ask Mexico to help us with a pipeline.

Desalinization at scale produces a LOT of brine. Like, way more than anyone needs. The evaporation pools would need to be gargantuan to service SoCal. State of California uses about 5maf of water (non-ag; with agriculture, much much more). That's is 6 cubic km of brine, per year. Say we service SoCal, that's 3 km^3 of brine per year... The entire salton sea is 7km^3. And that is only residential/commercial use like drinking water.

Which basin don't we care about? The desert is not "outside the environment" especially at that scale. There are critters living even in the most godforsaken areas of Anza Borrego. We are talking an environmental catastrophe, something like the Aral Sea.

plus california has the literal example of the salton sea ecological disaster here to draw upon. like, brine is a well known and difficult problem to deal with at scale. it isn’t just the massive amounts of salt, it’s all the concentrated metals as well.

Isn't the whole reason it releases brine is because energy isn't free?

You'd just run thermal desalination to a 100% reaction of salt water = salt + water and then have no brine and a bunch of salt to sell no?

Well, the energy isn't free, so you'd definitely be doing osmosis and making brine (osmosis is like 1/5 the energy compared to distillation). But even if it were and you could run thermal desalinization.

It's a LOT of salt. And the salt is not pure -- it's "salt" in a chemical sense, not a table salt sense, which you'd then have to purify, removing heavy metals, pollutants, etc. At rates of desalination that would service an appreciable amount of drinking/household water, you would no longer be selling the salt, you'd be disposing of it. Because there would be too much of it to sell.

The honest to god ONLY place you can naturally dispose of a lot of salt is the ocean, for the same reason that its there in the first place.

I live in Paraguay, electricity is 5c / kWh, it would cost 15 cents to desalinate 1000 liters of water. That is absurdly cheap even in Paraguay.

Malawian's seem to be able to afford corruption, I'm sure if they took the corruption money and put it towards desal they could afford it.

This article sounds pretty dumb, there's no point in desalinating every drop of water when the earth already does it for us, and especially not for farming.

If people want to live in an area that is a desert then they need to pay for desal. Most people don't live in those areas because you can't farm there without building greenhouses / aquaculture.

I wonder if the leftover brine can be used further after drying it. Like as part of the mix for a building material and maybe for the construction of roads. Like part of the mix that goes right under asphalt.

> A few caveats to start: I’ll focus on energy use and costs here. I’m not analyzing the amount of brine water that’s generated and how to manage that in an environmentally-sustainable way. Not because that’s not important, but because this post would '“absurdly” long.

Also, because maybe, just maybe, it would ruin the point the author is trying to confirm here. This is a very expensive and resource intensive part of the process. It feels completely disingenuous to ignore those costs in the context of what this article is trying to do, because these costs are significant:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/200...

> The average American household uses around 1135 litres of water per day.

What the fuck are y'all doing with over 1 cubic meter of water a day?! Germans are at 126 liters a day and head, about 10% of the average American [1].

[1] https://www.umweltbundesamt.de/daten/private-haushalte-konsu...

A few things here - 1) TFA is referencing household use, and you're referencing individual use, and 2) the linked source from TFA doesn't actually give the 1135L/day stat. What it gives is this: "Each American uses an average of 82 gallons of water a day at home" which is ~310 liters.

Also, it's not clear if your source is including water use for home irrigation and recreation (lawns and pools?), whereas the US source is.

I don't think the distinction between indoors or outdoors usage is relevant, in the end we are still talking about the volume of drinkable water people use. Whether they use it for drinking, cooking, washing, or watering the lawn.

My guess is that the average is very different than the median here. In general there are a few considerations:

- Green lawns are considered a status symbol in the US and in many higher end neighborhoods houses run irrigation multiple times a day.

- Same goes for pools.

- Baths are common (especially for kids) and use a fair amount of water.

- Our toilets and showers are not efficient.

- We tend to do laundry and dishes with machines (though I'm not sure if that would really use more or less water).

I also suspect that number may include industrial and commercial. Since even considering the above it still seems high.

Washing dishes with the machine actually tends to save quite a lot of water, as long as you actually fill the machine.

> as long as you actually fill the machine

That's more or less what I suspected. Though... and I'm not sure if this is an American thing, an international thing, or just a weird quirk of the specific people I know... I know several people who run their machines every day even if they aren't even close to full.

A dishwasher is so much more efficient than handwashing dishes that it's going to save water even without being anywhere close to full.

I'm not a fan of letting dirty dishes sit. Dried on food doesn't come off as easily.

> - Our toilets and showers are not efficient.

You can't make blanket claims about the US as a whole for things that are driven by the states - many states have highly restrictive shower/toilet efficiency requirements.

You are right, I cannot generalize. In fact our main flush function in newer toilets is the same flow as almost all countries (Australia and Japan being the exceptions) but we also have far fewer "Duel Flush" toilets (apparently, I'm just learning this myself) and the low flow on duel flush toilets uses much less water.

Whenever I’m in the states and see the urinal “American Standard” 1 gpf (gallon per flush) I cry in Australian.

American Standard sells urinals with water use as low as 1/8 (0.125) gallons per flush.

https://www.americanstandard-us.com/urinals-list

(I only thought of this because the urinals at the companies I've worked at over the past decade proudly displayed these 1/8-gallon numbers.

.. and plenty of urinals are completely water free...

And they uniformly stink...

How much is the gpf or lpf in Australia?

In Australia, they're measured in pints

> - We tend to do laundry and dishes with machines (though I'm not sure if that would really use more or less water).

Dish washers save water, also those are machines that Germans also use.

Fair enough, that was the one that seemed most questionable even as I wrote it.

I could not believe that number but indeed its 300 gallons per day: https://www.epa.gov/watersense/how-we-use-water

That is about 1350 (!) liters per day

70% is used inside the house, meaning 945 liters

there is something of a difference between 'per household' (US) and 'per head (Germany). So maybe introduce a factor of 4.something and the US doesn't look _quite_ as extravagant, or Germany _quite_ as frugal, depending on which way you look at it

As other posters have pointed out, that is the household number. The individual number is ~300 liters per day. But, ignoring that point, even if it were 1000 extra liters per person, 1000 liters is only ~0.40$ of desalinated water.

These Americans spending 40 more cents a day, what profligacy, what waste! Why, if they scrimp and save their water, they could buy one extra Starbucks drink every 10 days. 10 days!

And that is desalinated water which massively more expensive and is completely non-viable economically compared to regular sources of water.

It should be no wonder why the average American does not have different habits with regard to water usage. There is no real freshwater crisis in wealthy countries with enough lead time to build out desalination capacity.

You could argue there is still a environmental crisis if we allocated that much additional energy to desalination, but I leave that as a separate discussion.

America is a big country and the numbers vary greatly by state. Individuals in Connecticut on average use 35 gallons per day, not as great as Germans but close. Compared to states like Wyoming or Idaho with 176 gallons and 186 gallons per person, respectively.

From "Estimated Use of Water in the United States in 2015" which is the source cited by the EPA page. https://pubs.usgs.gov/circ/1441/circ1441.pdf

I'm assuming that some of that number is mixing in industrial uses of water?

That is, I'd love to say it is something silly like US people shower/bathe far more, but I have a hard time seeing how personal uses of water can add up to that in any way.

If you find a good read on this, please share it!

No, this is the domestic usage, not including industrial processes. And industrial processes exist everywhere in the world anyway.

That tracks with what I would largely expect, but https://www.worldometers.info/water/ has Germany at 856? That has the US at 3732. Such that I don't think I can just correct up.

Sounds like a lot of ambiguity on what different people are measuring, which is obnoxious. Understandable, mind. Still obnoxious.

Bathing, pooping, washing things, and watering lawns.

And you're comparing individuals vs. households.

Watering our yards instead of living in cramped comblock apartments?

> The average American family uses more than 300 gallons of water per day at home. Roughly 70 percent of this use occurs indoors.

70% of the usage is from inside homes [0], not watering yards. No fucking idea how.

[0] https://www.epa.gov/watersense/how-we-use-water

Reading about levels of energy, water and other resources used by Americans makes it much harder to sympathise when they complain about cost of living.

Liberty costs 1,009 gallons of water per day :P

Reply on news.ycombinator.com