5 ms·

One of the key questions for desalination is how good can it get. It turns out that many desalination plants are probably close to the thermodynamic limit [1, 2

by entee 8y ago

One of the key questions for desalination is how good can it get. It turns out that many desalination plants are probably close to the thermodynamic limit [1, 2], which means that even with new technologies it's likely energy costs aren't going to go down by an enormous amount. The Ars review suggests the best we can do is a factor of 3-4x, which is nice, but if we're expecting this to solve all our water needs we may have another thing coming. For reference, the cost of water is as low as $25 per acre-foot [3] in CA, and the most recent desalination plant in San Diego is on the order of $2000 per acre-foot [4]. Even some of the other estimates in [3] suggest that the average cost of water is something like $7-800 per acre-foot, which would put us right around where the thermodynamic limit suggests the floor of desalinated water would be. Obviously this is great new tech and more water purification technologies are always needed, but given the physical boundaries it's unlikely we'll be able to use desalination as the only solution to water crises. We should still be figuring out ways to be more efficient about water use. [1] https://arstechnica.com/science/2011/08/desalinization-is-this-as-good-as-it-gets/ https://arstechnica.com/science/2011/08/desalinization-is-th... [2] http://science.sciencemag.org/content/333/6043/712 http://science.sciencemag.org/content/333/6043/712 [3] http://www.cpuc.ca.gov/uploadedFiles/CPUC_Public_Website/Content/About_Us/Organization/Divisions/Policy_and_Planning/PPD_Work/PPD_Work_Products_(2014_forward)/PPD%20-%20Production%20costs%20for%20new%20water.pdf http://www.cpuc.ca.gov/uploadedFiles/CPUC_Public_Website/Con... [4] https://www.mercurynews.com/2014/05/29/nations-largest-ocean-desalination-plant-goes-up-near-san-diego-future-of-the-california-coast/ https://www.mercurynews.com/2014/05/29/nations-largest-ocean...

BariumBlue 8y ago
I think the calculus should get a bit better with cheap renewables - desalinating only when more energy is produced than can be reasonably used. I think if we can find a bunch of things that can effectively use excess power but aren't time sensitive, it'll also make renewables much more attractive.
- thinkcontext 8y ago
  The lower the capacity factor of the plant the longer it will take to recover the capital cost of building it. Whether this is economic depends on the ratio of energy cost to capital cost. Do you know what that ratio is?
- pishpash 8y ago
  > only when more energy is produced than can be reasonably used But when would that ever happen. They'd just get used up in e.g. mining $coins.
raz32dust 8y ago
This is assuming the current energy costs though, right? If we can get cheaper energy, say via fusion, it could go down.
- entee 8y ago
  For sure! But cheap energy is hard :( I'm cheering for Solar, renewables and Nuclear Fission to become cheaper and more widespread. Fusion... well it would be nice, but given that it's never been demonstrated in a form suitable for commercialization, it's hard to hold out for that.
  - est31 8y ago
    With renewables due to changing weather and demand, prices for electricity at international markets are falling regularly, sometimes even becoming negative (keeping the frequency is must), only to rise again when there is more demand than supply. Desalination plants have the advantage that, as long as you have overcapacities compared to the average needs around, they can be turned on when energy is cheap and off when it's expensive. While they are operating, they would fill reservoirs that would provide a continuous supply of water. There might be a thermodynamic limit onto how much energy you need to desalinate water, but the limits of harvesting renewable energy are far from being reached. We might make the sahara green, in this very century.
  - bduerst 8y ago
    Anyone know why we don't just use nuclear fission as the heat source in desalination? I mean, other than the implications of your nuclear purified drinking water.
    - gotocake 8y ago
      For the same reason we don’t utilize fission in other areas, people are ignorant and terrified of it, politicians are trying to be re-elected, and as a result real problems such as distributed rather than centralized storage of waste emerge. There are also real issues with processing vast quantities of seawater in a way that doesn’t destroy littoral ecosystems. Then there’s disposal whatever heavy metals and pollutants are concentrated as a byproduct of the process, regardless of your power source. Still, it’s feasible to overcome the last two issues, but I’m pretty sure that ignorance is going to kill us.
gizmo686 8y ago
You are assuming that electricity is the only cost involved. Based on the numbers in the article, the plant runs at 38-megawatts, consumes 100,000,000 gallons of sea-water a day, and produces 1 gallon of drinkable water per 2 gallons of sea-water. Assuming a cost of 15.2 cents/kwh [0] (california seems to be one of the most expensive states), this comes out to an electricity bill of $451.70/acre-foot. A non-trivial portion of the total cost, but there is plenty of room for cost savings without reducing electricity usage. Its also worth keeping in mind that Isreal exists, relies heavily on desalination plants, and is an agricultural exporter so we know the situation is solvable [1] [0] https://www.npr.org/sections/money/2011/10/27/141766341/the-price-of-electricity-in-your-state https://www.npr.org/sections/money/2011/10/27/141766341/the-... [1] Although will likely involve less wasteful agricutlure.
- entee 8y ago
  Fair enough! I didn't know some of those numbers, maybe there's more hope :)
- mabbo 8y ago
  > Isreal exists, relies heavily on desalination plants, and is an agricultural exporter so we know the situation is solvable Israel also has a lot of reclamation systems. Most of their sewage gets filtered and the fresh water reused. In most of North America, we give minimal safe treatment to sewage and then dump it back into our water sources. That is a very expensive process, but cheaper than desalination usually. We just need to get over the ick factor.
  - loeg 8y ago
    "Gray water" use for irrigation is pretty common in dry areas. I don't know if it used for farms producing food for humans, though.
  - atoav 8y ago
    That is interesting to hear, I grew up in the alps, where water plays an important role in my province (90% of the enery comes from water). You can drink the water from the lakes etc. The idea not to perfectly clean the sewage water you created, would sound irresponsible to anybody living here, as the water cycle becomes very apparent even to the craziest right wing nut when you live in the mountains.
Areading314 8y ago
Great point. Especially given how wasteful we are with drinking water there is far more room for efficiency on the demand side than the supply side. This source says we use 80-100 gallons (304-380L) per day (in the US) https://water.usgs.gov/edu/qa-home-percapita.html https://water.usgs.gov/edu/qa-home-percapita.html
- TylerE 8y ago
  Wonder if we'll ever run dual source systems...like salt water is just fine for pooing in.
  - thestepafter 8y ago
    Very interesting concept that I never considered. Shower with salt water and then rinse with regular water. Pay one price for salt, one price for regular water.
    - mbreese 8y ago
      The salt water would probably be too corrosive to pipe significant distances, which would make that difficult to get enough scale to make it feasible. Most of the dual-use water systems I’ve read about were fresh water vs gray water.
      - TylerE 8y ago
        Much piping these days (>75% for major distribution pipes, and increasingly common in residential use) is PVC, which doesn't corrode.
        zikzak 8y ago
        Salt and minerals get into fittings, destroy valves, and coat everything when the water evaporates. Probably not a great solution.
        8y ago
deleted 8y ago
[deleted]
loeg 8y ago
It's worth keeping in mind that residential water use is a tiny number when expressed in acre-foots. The vast majority of drinking water is used by farming. Desalination, even at today's costs, is not ruinously expensive for drinking water; but it might make farming inefficient crops (i.e., almonds) nonviable in the desert. Farming might just move geographically, or use water more efficiently. Neither of these are necessarily bad outcomes.
- samatman 8y ago
  Israel is a leading desalinator and major almond exporter, using less water per bushel than California by a factor of 30. Their secret? Each tree has a pipe about one meter deep. Water goes into the pipe instead of being sprayed on the ground. There are huge amounts of low-hanging fruit in California agriculture. Water is priced too low to make it worth taking advantage of.
- Scoundreller 8y ago
  You’ve touched on another problem: New/increased usage gets charged the average price. Meanwhile, new capacity is the most expensive. So you get subsidized if you switch to a more water-intensive crop.
jillesvangurp 8y ago
Clean energy cost is still going down rapidly and is already cheaper than coal and other fossil fuels. Existing desalination approaches will get progressively cheaper as well simply because of energy price reductions. IMHO, solving cheap, plentiful, clean energy solves a lot of problems as most other problems boil down to being energy problems as well. E.g. food production is bottlenecked on water. Solve cheap energy, you get clean water as well. Solve that, and food production becomes cheaper as well. The water crisis is an energy crisis. I do agree we need to be smarter how we use the land and water. E.g. California draining its natural resources to provide cheap water in LA is not sustainable. There's no excuse for that city to not have extensive desalination as its primary means of supplying water; other places in the world are managing just fine. That 25$ cost is basically about not factoring in the true cost of destroying natural resources. Also, farming practices in California are kind of wasteful and result in lots of water being wasted.
- thucydidesofusa 8y ago
  Renewables can't deliver order-of-magnitude cost reductions like nuclear can. Sure, solar might be marginally cheaper than coal in some climates (and more every day) but not massively so, we can't count on renewables to deliver amazing cost reductions that would drive us up the kardashev scale.

by entee 8y ago

BariumBlue 8y ago

I think the calculus should get a bit better with cheap renewables - desalinating only when more energy is produced than can be reasonably used. I think if we can find a bunch of things that can effectively use excess power but aren't time sensitive, it'll also make renewables much more attractive.

thinkcontext 8y ago
The lower the capacity factor of the plant the longer it will take to recover the capital cost of building it. Whether this is economic depends on the ratio of energy cost to capital cost. Do you know what that ratio is?
pishpash 8y ago
> only when more energy is produced than can be reasonably used But when would that ever happen. They'd just get used up in e.g. mining $coins.

raz32dust 8y ago

This is assuming the current energy costs though, right? If we can get cheaper energy, say via fusion, it could go down.

entee 8y ago
For sure! But cheap energy is hard :( I'm cheering for Solar, renewables and Nuclear Fission to become cheaper and more widespread. Fusion... well it would be nice, but given that it's never been demonstrated in a form suitable for commercialization, it's hard to hold out for that.
- est31 8y ago
  With renewables due to changing weather and demand, prices for electricity at international markets are falling regularly, sometimes even becoming negative (keeping the frequency is must), only to rise again when there is more demand than supply. Desalination plants have the advantage that, as long as you have overcapacities compared to the average needs around, they can be turned on when energy is cheap and off when it's expensive. While they are operating, they would fill reservoirs that would provide a continuous supply of water. There might be a thermodynamic limit onto how much energy you need to desalinate water, but the limits of harvesting renewable energy are far from being reached. We might make the sahara green, in this very century.
- bduerst 8y ago
  Anyone know why we don't just use nuclear fission as the heat source in desalination? I mean, other than the implications of your nuclear purified drinking water.
  - gotocake 8y ago
    For the same reason we don’t utilize fission in other areas, people are ignorant and terrified of it, politicians are trying to be re-elected, and as a result real problems such as distributed rather than centralized storage of waste emerge. There are also real issues with processing vast quantities of seawater in a way that doesn’t destroy littoral ecosystems. Then there’s disposal whatever heavy metals and pollutants are concentrated as a byproduct of the process, regardless of your power source. Still, it’s feasible to overcome the last two issues, but I’m pretty sure that ignorance is going to kill us.

gizmo686 8y ago

You are assuming that electricity is the only cost involved. Based on the numbers in the article, the plant runs at 38-megawatts, consumes 100,000,000 gallons of sea-water a day, and produces 1 gallon of drinkable water per 2 gallons of sea-water. Assuming a cost of 15.2 cents/kwh [0] (california seems to be one of the most expensive states), this comes out to an electricity bill of $451.70/acre-foot. A non-trivial portion of the total cost, but there is plenty of room for cost savings without reducing electricity usage. Its also worth keeping in mind that Isreal exists, relies heavily on desalination plants, and is an agricultural exporter so we know the situation is solvable [1] [0] https://www.npr.org/sections/money/2011/10/27/141766341/the-price-of-electricity-in-your-state https://www.npr.org/sections/money/2011/10/27/141766341/the-... [1] Although will likely involve less wasteful agricutlure.

entee 8y ago
Fair enough! I didn't know some of those numbers, maybe there's more hope :)
mabbo 8y ago
> Isreal exists, relies heavily on desalination plants, and is an agricultural exporter so we know the situation is solvable Israel also has a lot of reclamation systems. Most of their sewage gets filtered and the fresh water reused. In most of North America, we give minimal safe treatment to sewage and then dump it back into our water sources. That is a very expensive process, but cheaper than desalination usually. We just need to get over the ick factor.
- loeg 8y ago
  "Gray water" use for irrigation is pretty common in dry areas. I don't know if it used for farms producing food for humans, though.
- atoav 8y ago
  That is interesting to hear, I grew up in the alps, where water plays an important role in my province (90% of the enery comes from water). You can drink the water from the lakes etc. The idea not to perfectly clean the sewage water you created, would sound irresponsible to anybody living here, as the water cycle becomes very apparent even to the craziest right wing nut when you live in the mountains.

Areading314 8y ago

Great point. Especially given how wasteful we are with drinking water there is far more room for efficiency on the demand side than the supply side. This source says we use 80-100 gallons (304-380L) per day (in the US) https://water.usgs.gov/edu/qa-home-percapita.html https://water.usgs.gov/edu/qa-home-percapita.html

TylerE 8y ago
Wonder if we'll ever run dual source systems...like salt water is just fine for pooing in.
- thestepafter 8y ago
  Very interesting concept that I never considered. Shower with salt water and then rinse with regular water. Pay one price for salt, one price for regular water.
  - mbreese 8y ago
    The salt water would probably be too corrosive to pipe significant distances, which would make that difficult to get enough scale to make it feasible. Most of the dual-use water systems I’ve read about were fresh water vs gray water.
    - TylerE 8y ago
      Much piping these days (>75% for major distribution pipes, and increasingly common in residential use) is PVC, which doesn't corrode.
      - zikzak 8y ago
        Salt and minerals get into fittings, destroy valves, and coat everything when the water evaporates. Probably not a great solution.
        8y ago

deleted 8y ago

[deleted]

loeg 8y ago

It's worth keeping in mind that residential water use is a tiny number when expressed in acre-foots. The vast majority of drinking water is used by farming. Desalination, even at today's costs, is not ruinously expensive for drinking water; but it might make farming inefficient crops (i.e., almonds) nonviable in the desert. Farming might just move geographically, or use water more efficiently. Neither of these are necessarily bad outcomes.

samatman 8y ago
Israel is a leading desalinator and major almond exporter, using less water per bushel than California by a factor of 30. Their secret? Each tree has a pipe about one meter deep. Water goes into the pipe instead of being sprayed on the ground. There are huge amounts of low-hanging fruit in California agriculture. Water is priced too low to make it worth taking advantage of.
Scoundreller 8y ago
You’ve touched on another problem: New/increased usage gets charged the average price. Meanwhile, new capacity is the most expensive. So you get subsidized if you switch to a more water-intensive crop.

jillesvangurp 8y ago

Clean energy cost is still going down rapidly and is already cheaper than coal and other fossil fuels. Existing desalination approaches will get progressively cheaper as well simply because of energy price reductions. IMHO, solving cheap, plentiful, clean energy solves a lot of problems as most other problems boil down to being energy problems as well. E.g. food production is bottlenecked on water. Solve cheap energy, you get clean water as well. Solve that, and food production becomes cheaper as well. The water crisis is an energy crisis. I do agree we need to be smarter how we use the land and water. E.g. California draining its natural resources to provide cheap water in LA is not sustainable. There's no excuse for that city to not have extensive desalination as its primary means of supplying water; other places in the world are managing just fine. That 25$ cost is basically about not factoring in the true cost of destroying natural resources. Also, farming practices in California are kind of wasteful and result in lots of water being wasted.

thucydidesofusa 8y ago
Renewables can't deliver order-of-magnitude cost reductions like nuclear can. Sure, solar might be marginally cheaper than coal in some climates (and more every day) but not massively so, we can't count on renewables to deliver amazing cost reductions that would drive us up the kardashev scale.