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Comment by Animats | Hacker News Reader

If you only run the desalination plant when energy is cheap, plant cost per liter of water goes up. May or may not be a win. Probably a win for small, isolated plants, say for powering and watering a small island. Large grid-connected plants, don't know.

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At least in the US it is not a win.

The Carlsbad desalination plant [1] cost $1 billion to build, and it produces 190000 tons of desalinated water per day, or 5.7 million tons per month. Assuming a 6% financing cost, and 30 years amortization, you pay $6 million per month, or about $1 per ton of fresh water. That's a cost you incur whether you desalinate the water or not. If you decide to run the plant at 50%, then that's $2 per ton.

It takes 3.6 kWh to desalinate a ton of water. The cost of electricity in California is about $0.12/kWh for industrial users, so that's about $0.44 per ton of desalinated water.

Let's say you switch from the current electricity providers to solar generation that is absolutely free. Then you save $0.44 for each ton, but incur an additional capital cost of $1.

[1] https://en.wikipedia.org/wiki/Claude_%22Bud%22_Lewis_Carlsba...

OK, that's useful. Not a win at current plant and power costs.

All water infrastructure in mainland America is stored in large reservoirs / lakes and/or water-towers for future use.

If it takes 1GW-hr to make I dunno, a billion gallons of water or whatever, then it might be cheaper to store a billion-gallons of water + build a 2nd desal plant rather than trying to build a 1GW-hr Li-ion battery.

That's the real question that's being discussed here. What's cheaper? Additional Desal plants + water storage? Or Giant-batteries?

Presumably, pumped-hydro is off the table (which is true in some geographies). Pumped-hydro is an effective form of GW-hr sized energy storage but is somewhat frustrating at how limited the geography can be for it in practice.

I don't think the geography at the Ludington pumped hydro storage plant is particularly uncommon.

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

Water probably isn't grid connected like electricity. Nor does the supply need to perfectly match the demand every second. There is quite a bit of water storage in the system.

There might be pumps, in addition to just gravity assisted transportation of water. When energy is cheap, we can do both desalinate + transport.

You're missing the point. If you need 24 m^3/day of fresh water, you can have a desalination plant producing 1 m^3/hour and a water tank to match supply to demand. But if you only want to run the plant for 3 hours/day when electricity is cheap, you now need a plant which can produce 8 m^3/hour.

There's a tradeoff -- spending more building the desalination plant makes it possible to run at a lower duty cycle so you can spend less on the electricity.

But mainland desalination plants are connected to the electricity grid.

The problem with only running them part of the time is that the fixed capital and other operational costs of the plant are now split over a smaller amount of water produced so that water is more expensive than if you ran the plant all the time paying more for electricity at other times of the day.

There is certainly a trade off there, does the cheaper electricity compensate for the fact that you need to add say 20% extra capacity and the accompanying costs to your desalination plants as they will now be idle during low renewable production/times of high electricity costs.

If it's solar powered, more like 60% extra capacity to compensate for that annoying daily power down.

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