I have one more lingering criticism that I don’t think they addressed (I reread it twice).
What happens to the salt after it’s removed? If they don’t have a filter, where does it go? Do they pump brine water back?
This particular passage is suggestive of their device being more efficient than RO at small scales, although no figures are given for comparison:
> Commercially available portable desalination units typically require high-pressure pumps to push water through filters, which are very difficult to miniaturize without compromising the energy-efficiency of the device, explains Yoon.
Sure, that might be boring for some readers but they could throw a couple of sentences about it at the bottom.
I thought the metal plates were collecting the salt but the article and video don’t make it clear.
Some comment in the article left me thinking they end up trapping them in the media and have to reverse charge to release them so I assume it would operate pulsed and presumably have a valve to dump the output during regen.
Though I'm a little confused in that normally CEDI is used after RO because the CEDI media is pretty sensitive to fouling and also doesn't work well when the water conductivity is highly variable. Maybe they solve the fouling with charge reversal.
...who knows, because popular coverage of this stuff never hits on the important parts and almost never links the relevant publications. There are many ways to make drinkable water from seawater-- making them some useful mixture of energy efficient, cost effective, portable, waste-water efficient, and reliable is the actual hard part.
And I agree, at ounces per hour there's no need to dilute the brine, just dumping it is fine.
Let me rephrase and use watt/hour units.
I was assuming it was a 50 watt panel that would only produce 30wh from the weak winter sunlight.
Divide 30wh by two (since it was only ru Ning for half an hour) and get 15 watts generated to purify 5 ounces.
Had they let it run for the full hour, they’d get 10 ounces from my assumed 30wh of energy the panel produced.
30wh divided by 10 ounces would be 3wh per ounce (or unit of water produced).
How many units of water can be produced by 1kwh? Let’s divide 1,000wh by 3wh to get the result of 333.3 units (ounces) produced.
In your original post, the number was 15 watts, and you figured it produced 5 ounces of water in 30 minutes. So you then figured that 3 watts can produce 1 ounce of water in 30 minutes. So far so good; that's correct. If the panel continuously produces 3 watts of power (remember: watts are a rate of energy) over a period of one hour, that's 3 watt-hours of energy. For 30 minutes, it's 1.5 watt-hours of energy. Thus, it took 1.5 watt-hours of energy to produce 1 ounce of water. (1000 watt-hours) * (1 ounce / 1.5 watt-hours) = 666 ounces.
It may make more sense to translate this into distance, which I think is more intuitive for most people. Watts (power) are like miles-per-hour (velocity), and watt-hours (energy) are like miles (position). The panel runs at 15 MPH. After 30 minutes it has run 7.5 miles and produced 5 ounces of water. Thus, it could produce 1 ounce of water by running just 1.5 miles. It still runs at a speed of 15 MPH no matter what.
They meant 15 watt-hours in both cases, which would be half of 30 watt-hours — or a 50 watt panel for 30 minutes at 60% efficiency. (Which is the stated working premise.)
15 watt-hours -> 5oz would imply that 3 watt-hours -> 1oz, and hence 1 kWh is 333oz.
I generally try to assume best intentions — so if they repeated otherwise the same correct math, but for misnaming a unit, I try to assume they’re correct and typo’d.
However, I think I’d still make the same mistake in the future because I don’t understand how to express what the (assumed) 50 watt rated panel is generating over half an hour.
If it’s running at 60% efficiency then it’s generating at 30 watts per hour.
After an hour of running, we’ve accumulated 30 watts of energy.
If we only ran it for half hour, we’d have 15 watts.
Dividing that out tells me that 3 watts of energy will provide 1 once of water.
Does it matter if those 3 watts are provided over an hour via a 3wh power source or over 10 minutes via a 16wh power source?
“Watt” is the unit of power which is the rate of change in “joules”, which is the unit of energy. In the way that “velocity” is the rate of change in “position”.
1w = 1J/s
A “watt-hour” is another way to represent joules/energy by integrating watts/power over time.
30 watts * 30 minutes = 15 watt-hours or 54 kJ.
30 miles/hour * 30 minutes = 15 miles or 24km
> Does it matter if those 3 watts are provided over an hour via a 3wh power source or over 10 minutes via a 16wh power source?
You have the units backwards:
You can get 3wh from 1 hr @ 3w or 10 min @ 18w.
So let’s create one: the “walk” is the speed a person can walk — 3mph.
Then I can say the store is “3 walk-hours away”, even if I normally drive the 9 miles with my car.
3 walk-hours = 3 hours @ 1 walk = 18 minutes @ 10 walk (or 30mph)
Batteries are rated in “watt-hours” for the same reason:
3wH = 3 hours @ 1 watt = 18 minutes @ 10 watt
I’m assuming what I’m saying is wrong, I just can’t spot it. Thanks for the help.
This is 15Wh/33 oz, or 2200 oz per kWh.
That's roughly one order of magnitude greater than your estimate.
The video specifically shows (at 1:34) that the machine operates at 15Wh/L (for the larger version, the smaller version is 20Wh/L).
This is 15Wh/33 oz (since we seem so set on using customary units), or 2200 oz per kWh.
So my remarks were only about the article text.
But seriously the main reason I write these comments is to work through my thoughts as I try to articulate them and get feedback on what I’m thinking.
I’m actually happy when someone points out a mistake because I learned something.
When I’m right, I haven’t gained anything. Sure, my ego enjoys the satisfaction of being right but my already inflated ego doesn’t need anymore stroking.
However, they’re not trying to be more efficient than RO devices, they’re trying to be more compact, portable, and avoid the need for filters.
They’re solving for a different problem than I was measuring them against.
In my race to criticize, I overlooked those key details.
I still think my comment is worth reading to learn about the efficiency differences but I guess that measurement is not important for the viability of this technology.
Original criticism below: ———————
I’m skeptical about how practical this actually is.
While it’s refreshing to see a drinking water solution that isn’t based on inefficient dehumidifier technology, this still seems impractical.
From the demonstration video, their portable ~50 watt solar panel took 30 minutes to get a few ounces of drinking water.
It looked like a cold winter day, so let’s be generous and assume the panel only produced 15 watts of power during those 30 minutes.
Let’s also be generous and say they got 5 ounces of water.
That means they produce 1 ounce of water for every 3 watts.
That’d be 333 ounces per kWh.
To put that into perspective, a desalination plant produces more than 100x that much.
Yes, there is lots of additional infrastructure to consider with a traditional desalination plant but It’s not like this solution wouldn’t require the same complexity to scale up.
I don’t remember the cost of dehumidifier drinking water “solutions” but I think this is slightly more efficient than those horrifically inefficient solutions.