3 ms·

If you electrolyze seawater, to make hydrogen when the grid is over producing...and then you burn it to produce electricity when the grid is in need....can you

If you electrolyze seawater, to make hydrogen when the grid is over producing...and then you burn it to produce electricity when the grid is in need....can you capture the waste water from the exhaust... and call the entire process desalination?

loufe 3y ago
The energy conversion losses in the cycle you describe are enormous.
- thsksbd 3y ago
  Not enormous, but not insignificant either.
- jimnotgym 3y ago
  But are they more or less than reverse osmosis?
- fbdab103 3y ago
  If energy becomes "free" does it matter? If solar continues on its current trajectory, there needs to be some kind of sink to utilize the energy or it is all going to turn into bitcoins.
  - cordenr 3y ago
    +1 The "fully charged show" sometimes have videos on this and similar topics. This [0] video goes through the pros and cons of using hydrogen in different ways My takeaway is that as much of the energy as possible should be used without converting it. This avoids the substantial losses converting to and from hydrogen. But storage is one of the viable uses. [0] https://youtu.be/MgmBkvrO0Dg?si=8c-y3naRRH65O4Lb https://youtu.be/MgmBkvrO0Dg?si=8c-y3naRRH65O4Lb
thsksbd 3y ago
I worked on this a little bit. You still need to energetically pay the entropic cost of separating salt from water, and the kinetic over-potential (on the electrodes). Also these cells hate running in reverse. Ruins their lifespan. Overall it is still not viable. Your idea to use it to iron out the economics, though, might have some merit if the price fluctuations are high enough (which is what motivated us to begin with)
- jimnotgym 3y ago
  Is the cost more or less than reverse osmosis, that is the real question.
  - thsksbd 3y ago
    I dont know, but probably more. The cells dont like running in the reverse direction so you either research bidirectional cells or double the capital costs making cells optimized for each direction. Also, over potentials are pretty high and the catalysts foul easily but for the cleanest of inputs. Also, is RO the cheapest for DS? I thought modern distillation is back on top, especially if you have waste heat
calamari4065 3y ago
In theory, sure. Hydrogen fuel cells will spit out water as a waste product, that's probably the easiest way. You could directly burn the captured hydrogen or hydrox gas and power a conventional steam turbine. You'd then have to condense the exhaust gas, which is mainly steam, but that's not terribly complicated. The problem is that the amount of energy you get back would be fairly small in comparison to the input energy. It might not be economically practical. The amount of energy you can reasonably store per unit of input energy might not ever surpass the cost of the equipment to extract the energy and put it back into the grid. There's a minimum energy cost in separating water from salt. Salt dissociated into water has a lower energy state than separate salt and water molecules. You have to put that much energy back into the salt ions to pull them out of solution, and they will keep that energy until dissociated again. So you have to pay the cost of entropy, then pay for all the energy loss in the combustion, capture, and conversion stages. I'm not sure if you'd get enough energy out to ever be worth the trouble.
- jimnotgym 3y ago
  But is the cost any higher than reverse osmosis?
  - calamari4065 3y ago
    The thermodynamic cost of separating salt from water is the same per molecule no matter what. It's a fundamental physical limit. RO can be more efficient than electrolysis. The major factor is that electrolysis wastes a good fraction of the input energy on heating the water. On the other hand, RO requires high pressures, which is energy intensive. RO is generally more efficient overall, but either way you have to pay the cost of reversing entropy.
    - jimnotgym 3y ago
      But the electrolysis technique gives you the opportunity to store the energy as hydrogen. This means you can use cheap surplus energy for the electrolysis and burn the hydrogen later when energy is scarce. RO does not have that property
senectus1 3y ago
doesn't the water used to create hydrogen need to be "clean" and desal'd first?
- jimnotgym 3y ago
  Why is that? Does the salt foul the electrodes? I thought the salt might improve conduction through the fluid?

loufe 3y ago

The energy conversion losses in the cycle you describe are enormous.

thsksbd 3y ago
Not enormous, but not insignificant either.
jimnotgym 3y ago
But are they more or less than reverse osmosis?
fbdab103 3y ago
If energy becomes "free" does it matter? If solar continues on its current trajectory, there needs to be some kind of sink to utilize the energy or it is all going to turn into bitcoins.
- cordenr 3y ago
  +1 The "fully charged show" sometimes have videos on this and similar topics. This [0] video goes through the pros and cons of using hydrogen in different ways My takeaway is that as much of the energy as possible should be used without converting it. This avoids the substantial losses converting to and from hydrogen. But storage is one of the viable uses. [0] https://youtu.be/MgmBkvrO0Dg?si=8c-y3naRRH65O4Lb https://youtu.be/MgmBkvrO0Dg?si=8c-y3naRRH65O4Lb

thsksbd 3y ago

I worked on this a little bit. You still need to energetically pay the entropic cost of separating salt from water, and the kinetic over-potential (on the electrodes). Also these cells hate running in reverse. Ruins their lifespan. Overall it is still not viable. Your idea to use it to iron out the economics, though, might have some merit if the price fluctuations are high enough (which is what motivated us to begin with)

jimnotgym 3y ago
Is the cost more or less than reverse osmosis, that is the real question.
- thsksbd 3y ago
  I dont know, but probably more. The cells dont like running in the reverse direction so you either research bidirectional cells or double the capital costs making cells optimized for each direction. Also, over potentials are pretty high and the catalysts foul easily but for the cleanest of inputs. Also, is RO the cheapest for DS? I thought modern distillation is back on top, especially if you have waste heat

calamari4065 3y ago

In theory, sure. Hydrogen fuel cells will spit out water as a waste product, that's probably the easiest way. You could directly burn the captured hydrogen or hydrox gas and power a conventional steam turbine. You'd then have to condense the exhaust gas, which is mainly steam, but that's not terribly complicated. The problem is that the amount of energy you get back would be fairly small in comparison to the input energy. It might not be economically practical. The amount of energy you can reasonably store per unit of input energy might not ever surpass the cost of the equipment to extract the energy and put it back into the grid. There's a minimum energy cost in separating water from salt. Salt dissociated into water has a lower energy state than separate salt and water molecules. You have to put that much energy back into the salt ions to pull them out of solution, and they will keep that energy until dissociated again. So you have to pay the cost of entropy, then pay for all the energy loss in the combustion, capture, and conversion stages. I'm not sure if you'd get enough energy out to ever be worth the trouble.

jimnotgym 3y ago
But is the cost any higher than reverse osmosis?
- calamari4065 3y ago
  The thermodynamic cost of separating salt from water is the same per molecule no matter what. It's a fundamental physical limit. RO can be more efficient than electrolysis. The major factor is that electrolysis wastes a good fraction of the input energy on heating the water. On the other hand, RO requires high pressures, which is energy intensive. RO is generally more efficient overall, but either way you have to pay the cost of reversing entropy.
  - jimnotgym 3y ago
    But the electrolysis technique gives you the opportunity to store the energy as hydrogen. This means you can use cheap surplus energy for the electrolysis and burn the hydrogen later when energy is scarce. RO does not have that property

senectus1 3y ago

doesn't the water used to create hydrogen need to be "clean" and desal'd first?

jimnotgym 3y ago
Why is that? Does the salt foul the electrodes? I thought the salt might improve conduction through the fluid?