Engineers boost output of solar desalination system by 50%
phys.org
phys.org
The process described in this article ("Membrane Distillation") seems like a hybridisation of the two techniques and the researchers have found a way to improve efficiency by concentrating heat using lenses. Why can't this concentration of heat be used in straightforward distillation and why does distillation always seem to come in second best in terms of efficiency compared to the use of membranes? Is it because of the large surface area required?
If the sun can provide 1 kW/m2, the in 1 hour we can produce a maximum of 1 kWh/m2. That's 3600 kJ. That means we can boil 1 kg (or 1 litre) of water per hour per square meter of sunlight at maximum efficiency. So it's just not feasible.
I welcome fixes to the inevitable errors in the above :-P
That's the problem with heat. It is not a source of energy. It is a store of energy, often one that simply needs to be disposed of.
The major use of evaporative distillation (based on a few minutes of research, though some general familiarity) is in chemical processing, including but not limited to petroleum distillation. There's well over a century of history and R&D here (the first commercial petroleum distillation occurred in the 1830s, though larger-scale plants date from the 1870s and onward, as kerosene was used for lighting). Optimising for energy use and production volumes, controlability, predictability, safety/risk, etc., has been going on for a while.
Keep in mind that in evaporative desalination you need to both heat the input supply and cool the output, and that processing is rate-limited at both ends. So yes, you could pre-heat the inflow via a condensing jacket, but heat in must equal heat out, and if there's an insufficiency in cooling from native inflow, you'll need an additional cooling working fluid.
Other options include partial vacuum on the salty side, overpressure on the pure side (reducing and increasing boiling and condensation points, respectively), though PV = nRT (Boyle's Law) says that that isn't free either.
And better minds than mine have looked at this for a while.
The issues of input purity and filtration (a major problem with RO desal) is another major factor -- you don't want to just dump raw seawater over your osmosis membrane, but need to pre-filter that through other materials (I'm largely assuming gravel, sand, and possibly clay filters), which will foul with time and require replacement, as well as increase the water-handling energy costs (filtration isn't free).
Reverse osmosis has also seen energy recovery research, e.g., https://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=4...
Possibly interesting reading:
https://duckduckgo.com/?q=energy+recovery+evaporative+desali...
https://scholar.google.com/scholar?q=energy%20recovery%20eva...
Thermal desalination plants on other hand, are often as powerful as powerplants.
It is a commercially successful process. For example Hitachi have build 8 plants delivering a total of 8,600m3/day https://www.hitachizosen.co.jp/english/products/products011....
Sun does it but it does so over large surface area and the distillation per unit area that naturally happens is much less than what we require.
Offhand I think two or three reasons.
1. You need a temperature differential to drive distillation. Multistage distillation helps somewhat. That temperature differential limits the ultimate efficiency you can achieve.
2. You need to create a phase change for all the water molecules. And then recover as much of the heat as possible. Vs membrane which operates directly on the sodium/chloride ions.
3. Distillation uses really low grade (low temperature) heat. Turning high grade energy sources to low grade heat tends to result in a dead loss. Vs membrane which uses pressure. Pressure is high grade energy.
Feel free to fact check the above: I'm not an expert and may have been reading marketing materials that hide something important.
Energy intensity (and the cost thereof) are a rate limiting factor for thermal desalination:[0]
"Thermal processes are robust, durable and, once in operation, require less maintenance. However, these processes, mainly MSF [Multi-Stage Flash], require large amount of energy for their operation. In fact, up to 50 percent of the operating cost may be attributed to providing energy for the operation of the plant. As a result, many thermal plants are constructed in conjunction with power plants to utilize exhausted steam from the latter. The associated costs of high-energy consumption can have particularly strong restraining effects on small and medium sized plants. However, thermal processes can utilize low grade, low cost source of heat, which would otherwise be lost and discarded to the environment like stack gases, cooling water streams or low pressure exhaust steam. Heat from solar energy can also be used in such processes."
>It surprises me that we haven't worked out a more efficient solar distillation process for desalination.
I'm often surprised at how long it takes to move from discovery to implementation and adoption. The photovoltaic effect was discovered in 1839.[1] Yet 180 years later solar is only 2.4% of the world's installed electricity capacity; transportation and heating/cooling are still overwhelmingly gas/oil.
On an optimistic note, that means humanity probably has tonnes of undeveloped science and engineering that has been discovered but not yet built. It would be infinitely worse if we had already exhausted our collective capacity for invention, yet still had not solved the problems and existential threats we face.
[0] https://pdfs.semanticscholar.org/3f8c/147635ef7dae3050075c2c...
They state 24% effiency there, so the new result implies 36% overall efficiency? I find articles which only state '50% improvement' without stating the end result, frustrating.
Then again how are they even measuring efficiency, is there a theoretical minimum energy required to remove the salt from water?
This is interesting as well. Ammonia is much easier to handle than hydrogen and the infrastructure is already there.
On top of that liquid ammonia by volume contains more hydrogen than liquid hydrogen.