Comment by newyankee | Hacker News Reader

Comment by newyankee | Hacker News Reader

Does anyone working in the field know if a 100-1000x reduction in energy consumption of desalination is theoretically possible - using so called promised supermaterials based on graphene/nanotubes etc. or some kind of economies of scale ? Or are we limited by the law of physics here ?

I am really skeptical of promise of materials like graphene because although i have heard about magical properties for so many years now i have never heard of practical mass manufacturing completely upending an old school method for something. May be i am not knowledgeable enough.

This is one of the technologies that is increasingly important for the future of the earth especially in water poor countries with heavy population.

In the large scheme of things it might turn out cheaper to move population closer to the coasts, have desalination plants instead of other solutions to water scarcity assuming renewable energy costs reduce significantly as well along with a lot of surplus.

The current state of the art (reverse osmosis) is already fairly good. The first link I found [1] demonstrates the increase in efficiency - we're talking about roughly 3 kWh/m3. I don't recall exactly, but I think that is in an order of 5 of the theoretic maximum.

At an average, rather immodest level of water consumption per capita (~1580 m3/annum), then this is about ~4740kWh per year, or 13 kWh per day. I'd say this is significant, but then:

For comparison, current energy use per capita in the USA is 7000kg-oil-equivalent [2] (or 81000 kWh [3]), so an additional 4740 kWh is about 5%.

The real problem is that people use absolutely indecent amounts of energy and water and other resources, usually for no good reason at all...

[1] https://s3.amazonaws.com/academia.edu.documents/45145253/The... [2] https://data.worldbank.org/indicator/EG.USE.PCAP.KG.OE?locat... [3] https://en.wikipedia.org/wiki/Tonne_of_oil_equivalent

Thanks for the stats -- I actually just installed a solar array on my house which should produce around 8000kWh/yr which is in the ballpark of how much energy our 2-person household would need to desalinate our own water, so it's nice to have a tangible metric (and to know it could be done on our own roof space)... ~$24k installed, does not include cost of desalination plant :D

Are those numbers right? 1580 m3/year is 4.3 m3 or 1100 gallons per day. http://www.theworldcounts.com/stories/average-daily-water-us... says 100 to 175 gallons per day per person.

I got that number from here - tried to find a more 'official' source, but water isn't as well documented as energy.

Note that those numbers when I last looked into it typically include industrial and agricultural water use - residential water consumption is usually approximately 15% of total water use, so that fits reasonably.

One problem is that in many countries, water consumers (e.g. farmers) are able to withdraw water from their own soil, and so nobody really knows how much water that is, although it's possible to make educated guesses.

[1] https://www.statista.com/statistics/263156/water-consumption...

Seems plausible within an order of magnitude. 1 pound of beef takes ~1,800 gallons of water.

Please do not spread this twisted truth.

1. Those numbers are from all the rain hitting the pasture divided by by the cows mass (that is eatable)

1.a including whatever it eats

2. The way it's told makes it seem that none of the water is reused which is not true Though beef is a water intensive 'crop' it is not as bad as certain people makes it out to be.

Edit: formatting

Are they taking into consideration the water it takes to produce the food that you eat in that 1100 gpd figure? Also, the amount of water that you use to shower/wash dishes/flush/water your lawn?

Wonder what level of energy is needed for water transportation and pumping inside cities. Btw for reference the water consumption per capita is much lower and very skewed in developing countries. It is all about access and fights over that. I hope for many coastal regions desalination might become the economically viable option

There is a theoretical minimum energy required to desalinate water, because an entropy cost is involved. At room temperature, the cost works out to about 0.8 kWh per cubic meter, and our best systems seem to reach approx. 25% efficiency.

Thanks. It's quite a bit better than I expected.

Anything close to a 100-1000x reduction in energy consumption is unlikely in the next 10-20 years. That said filters have steadily gotten better over the years (better throughput, more durable, etc.)

Also, the main thing to focus on is cost of water produced. If you design a new filter that is much more energy efficient but costs 100x more than the standard plastic filter, it may not provide much advantage.

Water evaporated and leaves brine behind, at any temperature above 0C (even below through sublimation). Theoretically I guess you can imagine the sun evaporating the water that you capture, so you could have close to 0 energy input. So there is no physical limit I suppose. But you don't want to wait for the sun, you want to speed up that process, I don't know what the limit is there, when the speed matters (how much does it matter?).

Maybe with enough surface area, waiting for the Sun wouldn't be such a time-consuming task.

Any site doing this is more likely to use a mirror array for concentration, but I'm not sure offhand which shape(s) might be optimal for different reasons.

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