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    Japan has opened its first osmotic power plant

    (www.theguardian.com)
    301 points pseudolus | 16 comments | 25 Aug 25 02:53 UTC | HN request time: 0.402s | source | bottom
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    elil17 ◴[26 Aug 25 17:55 UTC] No.45029999[source]▶
    They really fail to explain a key point here. The reason you colocate this with a desalination plant is because you use the super-salty wastewater from desalination as the salty side of the osmosis power plant. Then you find some wastewater which is low in salt (such as semi-treated sewage), and use that as the fresh side of the osmosis power plant.

    The end result is that the salty wastewater is partially diluted, which means it has a lower environmental impact when it is discharged to the ocean.

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    1. tempestn ◴[26 Aug 25 20:42 UTC] No.45032060[source]▶
    Yeah, this is the coolest part. The leftover brine from desalination is generally just a problem. It's harmful to the marine habitat if you just put it back into the ocean, and there isn't a lot else good to be done with it. (Basically you have to dilute it first.) But this way you get useful work out of the dilution!

    The article also doesn't say if it produces more power than the attached desalination plant requires. I doubt it as you'd be getting close to a perpetual motion machine if so. In which case basically what you've got is a very energy efficient desalination plant, more than a power plant.

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    2. blix ◴[26 Aug 25 21:48 UTC] No.45032755[source]▶
    Fukuoka's desalination plant treats about 16400 m^3 of water per day. Assuming 3kWh per m^3 of water, this works out to a time-averaged power consuption of ~2000kW.

    The osmotic power plant generates about 100kW, so it's about 5% of the total desalination energy requirement.

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    3. wavemode ◴[26 Aug 25 22:34 UTC] No.45033187[source]▶
    Why are we assuming 3kWh per cubic meter of water?
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    4. blix ◴[26 Aug 25 22:41 UTC] No.45033238{3}[source]▶
    I did some cursory research and that seems to be a common estimate for modern osmosis-based desalination energy costs.

    If you have a better estimate, feel free to supply it.

    5. tempestn ◴[26 Aug 25 23:59 UTC] No.45033826[source]▶
    Ah, so a slightly more efficient desalination plant then.
    replies(1): >>45034295 #
    6. rrrrrrrrrrrryan ◴[27 Aug 25 01:09 UTC] No.45034295{3}[source]▶
    Slightly more efficient, with less waste.
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    7. mrexroad ◴[27 Aug 25 03:00 UTC] No.45034994{4}[source]▶
    Pretty solid win-win
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    8. rileyphone ◴[27 Aug 25 03:07 UTC] No.45035039[source]▶
    According to this delightful overview [1] of the desalination plant, the capacity overall is 12000kW so that's definitely close enough.

    1. https://www.niph.go.jp/soshiki/suido/pdf/h21JPUS/abstract/r9...

    9. fragmede ◴[27 Aug 25 03:18 UTC] No.45035105[source]▶
    Yeah it's like regenerative braking vs losing that energy as waste heat.
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    10. jalk ◴[27 Aug 25 05:10 UTC] No.45035682[source]▶
    Great analogy. This is similar to producing gas from sewage to feed back into the gas network, which is used to cook food, which produces sewage. (i.e. https://www.hofor.dk/english/hofor-utilities/town-gas/)
    11. kvgr ◴[27 Aug 25 09:11 UTC] No.45037141[source]▶
    Can't we just process it into salt/lithium and whatever is there? Since its already concentrated?
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    12. sl-1 ◴[27 Aug 25 09:54 UTC] No.45037446{5}[source]▶
    Depends on the CAPEX and OPEX requirements. If it is cheap to do, it could be a solid win, but if the plant requires a lot of capital, it might be cheaper to just take the hit on efficiency
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    13. azepoi ◴[27 Aug 25 12:08 UTC] No.45038544{6}[source]▶
    Yes the brine could just be diluted wih gray water to reduce the environnemental impact without the energy recovery of the osmotic plant and the capital can be invested in other renewable with better efficiency.

    That being said it's a first so it's a pilot project needed to have feedback on a real plant in operation and not just back of the enveloppe calculations and suppositions. Sometime you need to just build the thing to encounter problems, issues or non-issues.

    14. legacynl ◴[27 Aug 25 13:39 UTC] No.45039524[source]▶
    > The article also doesn't say if it produces more power than the attached desalination plant requires. I doubt it as you'd be getting close to a perpetual motion machine if so.

    Not really. Even if it would generate enough to power the plant, it would still rely on work being done outside of the plant, i.e. the flow of semi-treated waste-water and possibly the brine itself.

    15. elil17 ◴[27 Aug 25 14:32 UTC] No.45040267[source]▶
    If you want that much salt, then yes. But you probably don't
    16. mapt ◴[27 Aug 25 14:49 UTC] No.45040503[source]▶
    It's harmful to a tiny watershed of marine habitat immediately downstream of the discharge pipe, and dilutes rapidly. With that said - if you can harvest a meaningful amount of energy from desal anything helps. I don't know that 5% is a meaningful amount, however.