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A comparison with reverse osmosis: 2.46kWh/m^3 (energy consumed per unit of water produced) is claimed for reverse osmisis [1]. This equates to 8.86MJ/m^3.

by femto 5y ago

A comparison with reverse osmosis: 2.46kWh/m^3 (energy consumed per unit of water produced) is claimed for reverse osmisis [1]. This equates to 8.86MJ/m^3. Output for this still is 7L/(m^2.h) Assume a solar flux of 1kW/m^2. Energy consumed per unit of water is 1kW/m^2 / [7L/(m^2.h)] = 3.6MJ/0.007m^3 = 514MJ/m^3. Assuming the above is correct (check anyone?), the still uses 58 times more energy than reverse osmosis. The solar energy may be "free" but with a 20% PV cell efficiency a reverse osmosis system would produce about 11 times more water per unit area of solar collector? [1] http://www.ijesd.org/papers/243-B20001.pdf http://www.ijesd.org/papers/243-B20001.pdf

jeroenhd 5y ago
> In production, they think a system built to serve the needs of a family might be built for around $100. I think this is much more important to a great many people than the theoretical efficiency of reverse osmosis. Percentages greater than 100% always do well in media reports about these topics, but they're not necessarily the point of the exercise. If the goal was to produce a system that's as efficient as possible, the researchers wouldn't have used household-style supplies but more expensive, advanced materials. Reverse osmosis is great for a central area such as a large city in a place with reliable distribution, but in many places around the world, an independent, affordable system that can turn seawater into drinking water for a family or two has much more value.
- londons_explore 5y ago
  Nothing about reverse osmosis can't be scaled down in size or cost. The actual membranes are $9 for enough for 100 gallons per day (retail prices, [1]). High pressure pumps and hoses scale linearly. Solar panels scale linearly. Filters scale linearly. In fact, using this solar fountain [2] as the basis for the design, and switching the pump impeller for a high pressure version, and the nozzle for an RO membrane and hose, you immediately have a drinking water machine for a few people for $20. The fountain already has a pre-filter built in. [1]: https://www.aliexpress.com/item/32669709750.html https://www.aliexpress.com/item/32669709750.html [2]: https://www.aliexpress.com/item/1005002883892948.html https://www.aliexpress.com/item/1005002883892948.html
  - SiempreViernes 5y ago
    Ah, so what you are saying is that this is already deployed on a large scale to solve distributed water issues?
    - SV_BubbleTime 5y ago
      What he’s actually saying is he found some residential grade nonsense from China that only seems economically feasible because of the slave labor and subsidized transport.
  - kortex 5y ago
    I have first-hand experience that a) you need positive displacement pumps of a certain quality and power for RO and the cheap Ali ones self-destruct in minutes/hours b) you still need housing, plumbing, receptacles, etc c) fouling is a non-trivial issue. Expect to pay around $300 minimum for a small RO solar plant, and a couple bucks a month for upkeep. And logistics. Granted that's USA prices (upstate NY, not SV) but that prices out a lot of developing regions.
    - londons_explore 5y ago
      Fouling can be dramatically reduced by putting a small electrical current through the saltwater, freeing Chlorine ions which quickly kill biological things before they can adhere to your membrane. That combined with reverse flow flushing will probably last many years. And it can all be controlled by a 10 cent microcontroller, one pump, one valve, and a pressure sensor. I don't doubt that high pressure pumps are bad... but that's just a design issue - there is nothing theoretically expensive about them.
      - kortex 5y ago
        Heh, this is reminiscent of the infamous HN dropbox comment. Free chlorine (technically assorted chlorine oxides like hypochlorite) attacks RO membranes, so now you have to deactivate your reactive species first. Usually UV lamps, you use sun, but now you need UV-clear tubing, not easy. Reverse flow flushing can be done with the components you mention, that's another $20/50 plus sourcing logistics. Yes, the theoretical expense in quality pumps is quality. There are tighter tolerances, beefier components, better polymers, and more QA. It all adds up. It's still all fairly cheap by Western standards, but it's a tall ask for a lot of places that barely have potable water.
TeMPOraL 5y ago
That's a good point, but! I wonder if it couldn't be more efficient overall, if you look at it globally. Is an equivalent reverse osmosis installation cheaper or more expensive to produce, in terms of embodied energy that goes into manufacturing and maintaining all equipment, including control equipment? Also, I wonder which system would be easier to slap together in a garage out of leftover junk. Not everyone can rely on access to commercial-grade, turn-key solutions to a problem. A design that could be reasonably DYIed could be better in certain contexts, even if less efficient.
londons_explore 5y ago
The MIT system appears to output clean water from each 'stage' of the system at variable temperatures. Thats energy lost - if a counterflow heat exchanger could be devised such that the output water was all cool, the final efficiency would be much higher. They also don't seem to keep good control of salinity within the stages - I suspect after a few hours operation the efficiency will drop as the salinity gets higher and higher within the paper towels.

by femto 5y ago

jeroenhd 5y ago

> In production, they think a system built to serve the needs of a family might be built for around $100. I think this is much more important to a great many people than the theoretical efficiency of reverse osmosis. Percentages greater than 100% always do well in media reports about these topics, but they're not necessarily the point of the exercise. If the goal was to produce a system that's as efficient as possible, the researchers wouldn't have used household-style supplies but more expensive, advanced materials. Reverse osmosis is great for a central area such as a large city in a place with reliable distribution, but in many places around the world, an independent, affordable system that can turn seawater into drinking water for a family or two has much more value.

londons_explore 5y ago
Nothing about reverse osmosis can't be scaled down in size or cost. The actual membranes are $9 for enough for 100 gallons per day (retail prices, [1]). High pressure pumps and hoses scale linearly. Solar panels scale linearly. Filters scale linearly. In fact, using this solar fountain [2] as the basis for the design, and switching the pump impeller for a high pressure version, and the nozzle for an RO membrane and hose, you immediately have a drinking water machine for a few people for $20. The fountain already has a pre-filter built in. [1]: https://www.aliexpress.com/item/32669709750.html https://www.aliexpress.com/item/32669709750.html [2]: https://www.aliexpress.com/item/1005002883892948.html https://www.aliexpress.com/item/1005002883892948.html
- SiempreViernes 5y ago
  Ah, so what you are saying is that this is already deployed on a large scale to solve distributed water issues?
  - SV_BubbleTime 5y ago
    What he’s actually saying is he found some residential grade nonsense from China that only seems economically feasible because of the slave labor and subsidized transport.
- kortex 5y ago
  I have first-hand experience that a) you need positive displacement pumps of a certain quality and power for RO and the cheap Ali ones self-destruct in minutes/hours b) you still need housing, plumbing, receptacles, etc c) fouling is a non-trivial issue. Expect to pay around $300 minimum for a small RO solar plant, and a couple bucks a month for upkeep. And logistics. Granted that's USA prices (upstate NY, not SV) but that prices out a lot of developing regions.
  - londons_explore 5y ago
    Fouling can be dramatically reduced by putting a small electrical current through the saltwater, freeing Chlorine ions which quickly kill biological things before they can adhere to your membrane. That combined with reverse flow flushing will probably last many years. And it can all be controlled by a 10 cent microcontroller, one pump, one valve, and a pressure sensor. I don't doubt that high pressure pumps are bad... but that's just a design issue - there is nothing theoretically expensive about them.
    - kortex 5y ago
      Heh, this is reminiscent of the infamous HN dropbox comment. Free chlorine (technically assorted chlorine oxides like hypochlorite) attacks RO membranes, so now you have to deactivate your reactive species first. Usually UV lamps, you use sun, but now you need UV-clear tubing, not easy. Reverse flow flushing can be done with the components you mention, that's another $20/50 plus sourcing logistics. Yes, the theoretical expense in quality pumps is quality. There are tighter tolerances, beefier components, better polymers, and more QA. It all adds up. It's still all fairly cheap by Western standards, but it's a tall ask for a lot of places that barely have potable water.

TeMPOraL 5y ago

That's a good point, but! I wonder if it couldn't be more efficient overall, if you look at it globally. Is an equivalent reverse osmosis installation cheaper or more expensive to produce, in terms of embodied energy that goes into manufacturing and maintaining all equipment, including control equipment? Also, I wonder which system would be easier to slap together in a garage out of leftover junk. Not everyone can rely on access to commercial-grade, turn-key solutions to a problem. A design that could be reasonably DYIed could be better in certain contexts, even if less efficient.

londons_explore 5y ago

The MIT system appears to output clean water from each 'stage' of the system at variable temperatures. Thats energy lost - if a counterflow heat exchanger could be devised such that the output water was all cool, the final efficiency would be much higher. They also don't seem to keep good control of salinity within the stages - I suspect after a few hours operation the efficiency will drop as the salinity gets higher and higher within the paper towels.