3 ms·

Desalination is "solved". Main problem is the huge energy requiered.

• iancmceachern 3y ago
  So, not solved?
  • Avshalom 3y ago
    I'm not a physical chemist but there's probably a computable minimum energy necessary. A quick search popped up https://pubs.acs.org/doi/10.1021/acs.jchemed.0c01194 https://pubs.acs.org/doi/10.1021/acs.jchemed.0c01194 For example, desalinating seawater with a typical seawater salt concentration of 35 g L–1 (corresponding osmotic pressure of 29.7 bar) and 50% water recovery (i.e., 50% of the feed stream becomes purified water and 50% becomes brine) requires at least 1.1 kWh per cubic meter of purified water. Regardless of the desalination technology, it is impossible to desalinate water using less energy than that determined by eq 12. which a couple other searches seems to be within order unity of the current energy use
    • iancmceachern 3y ago
      With current membrane technology, yes. There are so many examples in our collective technical history of overcoming these types of limitations not with brute force, but with finesse. In this case, I expect it will be a combination of improved pumps, improved RO membrane technology, and finding synergies like making sea salt from the brine, collecting other useful minerals front the brine,etc. All of these things help to pay for the energy and development costs. It's not a hard physics limit like the rule if squares or the speed of light. It's a complex engineering system that has may different dynamics and interactions between those dynamics, all opportunity for improvement.
      • Avshalom 3y ago
        Gibbs Free Energy is pretty much a hard physics limit. Trying to pull gold out of the ocean to pay for pulling salt out of the ocean is --again not a chemist but-- probably thermodynamically worse.
        • iancmceachern 3y ago
          That's my point. It's not a single thing, it's not about making it add up trying to do one thing, but if you batch them together and say, use thr waste heat from one process to power the next, etc. One can find new efficiencies. Of course I'm not saying we need to break the laws of thermodynamics, in my house we obey the laws of thermodynamics. In this case, the work needed is defined by the features of the RO membrane. It's conceivable that we could develop a RO membrane that requires less pressure or energy to operate. In fact they have been. In that case, we would gain a more efficient process, while still obeying the laws of thermodynamics
          • daedrdev 3y ago
            I see this view a lot on HN, but this kind of wish for great new development can be unrealistic simple because there are no more low hanging fruit for these very physical processes. It could easily be the case that RO is going to see only marginal improvements for the next decade or two (except perhaps some test membranes that are too expensive), and modern RO already uses energy recovery in the process.
            • iancmceachern 3y ago
              This seems highly speculative
      • kergonath 3y ago
        > With current membrane technology, yes. Equation 12 is the energy you need to counteract the effect of the entropy of the ions in solution and separate the initial solution into one that does not have the ions. It’s pretty much a thermodynamic limit and does not depend on process or technology. They explicitly assume that they have a perfect membrane when they introduce the equation. The floor will never be zero, it is a physical limit.
      • rocqua 3y ago
        You can produce energy by disolving salt in fresh-water. That means there has to be a hard phyisical energy limit to desalinating water, otherwise you get a free energy producing machine. The energy required happents to go into separating the bonds between the salt ions and the water molecules. Those bonds are quite strong, so it takes some energy to break them. The engineering problem is more one about capital investment. The price of water, vs the price of electricity are not what block desalination. It is the cost (and maintenance) of the machines that you need to earn back. That cost is what tends to make desalination un-economical. And that is an area where engineering has a lot of space to improve.