Chemists Work to Desalt the Ocean for Drinking Water, One Nanoliter at a Time
utexas.edu
utexas.edu
From their paper it is mentioned they have a 40 nanolitre/min flow rate (25% desalination rate, 99% is considered safe to drink water).
They use an electric pole to generate an ion depletion zone, their problem right now lies in the severely limited flow rate and desalination rate. From my understanding the 25% desalination rate does not compound linearly and decreases in efficiency as the concentration of salt ions decreases.
Additionally their flow rate is 0.4 microlitres per minute, this would equate to needing 625 000 channels for one pass only to get 250mL / min. Scaling for microfluidics isn't simply using a larger pipe size, microfluidic devices largely operate with minimal forces and a Reynolds number of 1, that doesn't hold as you get larger.
The other big problem here is that this requires pressure driven flow, to do that they made two reservoirs of uneven height on opposite ends to drive flow. Again this is very electrically cheap (picowatts) when dealing with such small and perfect flow conditions.
The biggest problems they need to address are the low filtration rate and the low flow rate, it does not seem like there is a simple answer to the first, they do state that they are conducting a larger scale experiment that they will publish later.
I largely suspect that the larger scale experiment will fail in efficiency as another group (doi:10.1038/nnano.2010.34) who also uses a ion depletion zone (albeit by a different mechanism) found after publishing that they were orders of magnitude (10^3) too low in their predictions.
If anything this will perhaps be better than small, portable RO setups but not replace large factories which are actually pretty efficient.
What about parallelization? Are there any obvious downsides to simply having lots of the active units, aside from the (what seems to me largely manageable) increase in micro fluidic chip complexity?
The problem comes from the number of inlets. You can:
a) a common inlet to all of your channels, or b) independent inlet for each channel, or for several channels
a) might seem intuitive, the problem is limitations on channel width, the inlet would have to feed a single channel which would then split (they would split to the inlet channel from the paper), structural limitations of PDMS and manufacturing have maximum widths and heights in millimetres at best. Which would not be enough to achieve the throughput required. The original 'master' inlet would probably have to be at least in the 10-20 cm range to achieve flow rates that could make this a household filter.
Even with another, stronger material, and perhaps manufacturing techniques I'm not aware of you still have limitations of pressure, their channels are very small in the paper (for a reason), pressure becomes a limiting factor to prevent their failure.
The problem with b is sheer complexity, you're talking about 10 000 tubes and connectors and holes punched into a chip if you even do a 1 - 60 split. The chip with just the channels would be about the size of a tissue box, this wouldn't be able to accomodate the channels so now you're talking about something tens of metres x tens of metres. This is hugely cost prohibitive and the channels from connectors would have to be really long, really long channels need more pressure to drive the liquid requiring more energy which reduces the efficiency significantly.
The reason parallelizaton works in pharmaceutical / DNA testing applications is you're going from microlitres of DNA/samples to nano, or picolitres in the channels, a single inlet can sufficiently provide that throughput (you're talking 10 uL / hour perfusion rates)
Edit: TL;DR version: Construction constraints would make this have worse efficiency than reverse osmosis and cost a lot more to manufacture as well.
The advantage being reverse osmosis is more efficient when the salinity difference is smaller. Still I don't think the added complexity is going to be worth slightly lower energy costs.
What a silly measurement. I can buy a laptop battery or a car battery at a store. Also it does not give any idea of how much water could be produced from the battery. A teaspoon per battery probably isn't too great. (Although at the moment they get only nanoliters of salty water from it of course).
[1] http://onlinelibrary.wiley.com/doi/10.1002/anie.201302577/ab...
For that last number, the authors cite ref [1], which does an entropic analysis of unmixing solutions and looks at the minimum (reversible) work required therefore.
[1] Y. A. Cengel, Y. Cerci, B. Wood, Proc. ASME Adv. Energy Syst. Div. 1999, 39, 537–543;
Even if we give it the benefit of the doubt and say we're overestimating the current by a factor of 100, 32 MJ/L still a ridiculous amount of energy, and ~10x more than just distilling water through evaporation, which is 2.23 MJ/L.
Where did you get that 0.7A number?
To make this technology energetically favourable to boiling, it must not exceed ~700μA in current, and it has to be far less than that to be worth its complexity.
Solving for current, Ohm's law is I = V/R. You only deliver .7 amps of current into a 4.285 ohm load.
If you connect a 3 volt battery to a 1,000 ohm resistor, then only 0.003 amps of current will flow, which is 9 milliwatts.
Connect a 3 volt battery to a .01 ohm load (a dead short, almost) and 300 amps of current will flow, (900 watts!) very briefly, until the battery voltage sags under load.
1: Or any voltage source in general, barring some trickery that you can't do with a simple electrochemical battery.
Except it won't, of course, because all real batteries have internal resistance. (How much depends on the battery chemistry and size.) Probably the 700mA rating for the AA battery is into a dead short.
Can you please define the Current Push Theory?
http://en.wikipedia.org/wiki/Hydraulic_analogy
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir.h...
E.g., if you have a resistive load and an inductive power source, the power source will keep upping its voltage until the load accepts the current that's being shoved down its throat.
However I suspect that you'll never actually desalinate water for that amount of energy in a portable system.
The very best commercial systems manage to do it with about 3 times the energy of the minimum. More common ones are about 10 times the minimum.
The absolute minimum energy you need is 2.2kJ per liter.
[1]a microcontroller similar to Arduino which operates on 3.3v and was intended to cost $4.30 for the entire development kit. http://www.ti.com/msp430
Shouldn't we be making more efforts on 1) Growing more trees 2) Reducing environmental pollution 3) Controlling human population
???
The only way to ensure long-term survival is to develop science and technology quickly enough to solve problems (both natural and artificial) as they come up.
This includes solving problems created by past generations, who necessarily did not have access to the knowledge needed to avoid creating those problems in the first place.
It's certainly not guaranteed that we will do so fast enough.
As a humanity we can (and should) multitask, especially when it comes to science and inventions, because you never know which seemingly pointless path can lead you to a great breakthrough.
Imagine everybody since antiquity worked only on the greatest humanity problems as they've seen it at the time. No useless stuff like music, so no irrational numbers for example :)
A floating desalination plant powered by an x000-acre floating solar platform, pumping clean water back to shore, and dispersing slightly saltier water across a wide area, doesn't seem like science fiction to me, and doesn't seem very destructive. The process doesn't even have to be super efficient, we may even end up using old-school techniques like distillation or electrolysis to keep the mechanisms simple.
(granted this helps coastal regions more than inland ones, but moving population-heavy coastal areas to this system will preserve more of the natural water supplies for the inland needs).
The process doesn't even have to be super efficient
California uses 46 billion gallons of water per day.
Supplying 25% of this via desalination would require 36 GW of
thermal-equivalent power. California runs on 30 GW of
electricity, and a total energy budget of 262 GW (thermal;
from oil, gas, coal, hydro, nuclear, etc.—according to the
EIA). That’s a substantial amount for 25% of our water needs.
I wonder what the water-used-to-calories-produced ratio is for something like lettuce compared to something like corn or beans. I imagine it's a good number of orders or magnitude higher.
In other words, an increase in the price of fresh water does not necessarily significantly increase the price of feeding yourself, it significantly increases the price of water-intensive crops and likely increases the demand for less water intensive crops.
However, his estimates for the energy required were very conservative, and even if these numbers don't work out with today's water prices, they do make sense at a certain point that is likely lower than one encouraging violence. That effectively provide a cap where we start to go this route if necessary, and the vast supply means that we shouldn't go above that for very long.
Wars - technological ones of the type major powers would fight - are expensive. Far more expensive then any amount of desalinization we would need to build.
Not to say it will happen, I believe and hope it wouldn't happen, but it could.
very nice job, expect to see it available soon
Something like: "Is the water from the ocean clean enough to drink? Hope to see this product available to buy soon" might be better
No, it usually comes from a river. And guess where the waste water from a sewage treatment plant goes?
Right back into the same river, only downstream. And that's where the next city down the line picks it up.
That water gets recycled multiple times on the way to the ocean, by each city on the river.
That's not a bad thing! But the water can hardly be called distilled.
We currently have techniques that require no additional power (just using the sun), but almost every desal operation uses a powered method.
Power consumption isn't the only factor here. A low power technique that has throughput per area equivalent to solar stills is just worse than all available alternatives.
If you made a device that turns the briny sludge byproduct into fuel or some other consumable, that would be amazing.
I remember a video that shows some kind of filter (it looked like a plastic sheet). They were simply pushing water against it and you get clean water from salty water.
I don't think it gets easier than that. However if I remember correctly the filter gets unusable after a while
It's called reverse osmosis if you want to look it up.