Simple, solar-powered water desalination

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Simple, solar-powered water desalination

#1  Postby Macdoc » Feb 11, 2020 11:07 am

Remarkable bit of design/engineering :thumbup:

Simple, solar-powered water desalination
System achieves new level of efficiency in harnessing sunlight to make fresh potable water from seawater
Date:
February 7, 2020
Source:
Massachusetts Institute of Technology
Summary:
A completely passive solar-powered desalination system could provide more than 1.5 gallons of fresh drinking water per hour for every square meter of solar collecting area. Such systems could potentially serve off-grid arid coastal areas to provide an efficient, low-cost water source.


more
https://www.sciencedaily.com/releases/2 ... 124456.htm

snip

The team estimates that a system with a roughly 1-square-meter solar collecting area could meet the daily drinking water needs of one person. In production, they think a system built to serve the needs of a family might be built for around $100.
:what: :clap:
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Re: Simple, solar-powered water desalination

#2  Postby GrahamH » Feb 11, 2020 11:57 am

The key to the system's efficiency lies in the way it uses each of the multiple stages to desalinate the water. At each stage, heat released by the previous stage is harnessed instead of wasted. In this way, the team's demonstration device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation.

Whenever vapor condenses on a surface, it releases heat; in typical condenser systems, that heat is simply lost to the environment. But in this multilayer evaporator the released heat flows to the next evaporating layer, recycling the solar heat and boosting the overall efficiency.
"When you condense water, you release energy as heat," Wang says. "If you have more than one stage, you can take advantage of that heat."
Adding more layers increases the conversion efficiency for producing potable water, but each layer also adds cost and bulk to the system. The team settled on a 10-stage system for their proof-of-concept device, which was tested on an MIT building rooftop. The system delivered pure water that exceeded city drinking water standards, at a rate of 5.78 liters per square meter (about 1.52 gallons per 11 square feet) of solar collecting area. This is more than two times as much as the record amount previously produced by any such passive solar-powered desalination system, Wang says.
Theoretically, with more desalination stages and further optimization, such systems could reach overall efficiency levels as high as 700 or 800 percent, Zhang says.


What? :?

That doesn't sound right at all. To condense water you need a cold surface so you have to get rid of the heat. The condenser plate can't directly heat the next evaporator.

They could use heat pumps, but they are far from over unity efficiency and more heat is lost than transferred. And there is no mention of heat pumps

I'd expect something credible from MiT so what's the real story?

Their demonstration unit was built mostly from inexpensive, readily available materials such as a commercial black solar absorber and paper towels for a capillary wick to carry the water into contact with the solar absorber. In most other attempts to make passive solar desalination systems, the solar absorber material and the wicking material have been a single component, which requires specialized and expensive materials, Wang says. "We've been able to decouple these two."
The most expensive component of the prototype is a layer of transparent aerogel used as an insulator at the top of the stack, but the team suggests other less expensive insulators could be used as an alternative.


It takes a lot of energy to evaporate water and a big thermal gradient has to be maintained somehow.

http://news.mit.edu/2020/passive-solar- ... ation-0207

They show a diagram, with transparent insulator, solar absorber, wicking layer to evaporate and a cold plate to condense, but no indication how the cold plate is cooled or how energy is recovered from the cold plate to heat a following stage. Anyone have any ideas on that?
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Re: Simple, solar-powered water desalination

#3  Postby zulumoose » Feb 14, 2020 7:00 am

I find it rather odd that 1.5 gallons PER HOUR is apparently just sufficient to meet the needs of one person for drinking? Both figures from the OP quotes
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Re: Simple, solar-powered water desalination

#4  Postby GrahamH » Feb 14, 2020 12:10 pm

This may be a form of
Multiple-effect distillation
Multiple-effect distillation (MED) works through a series of steps called "effects".[10] Incoming water is sprayed onto pipes which are then heated to generate steam. The steam is then used to heat the next batch of incoming sea water.[10] To increase efficiency, the steam used to heat the sea water can be taken from nearby power plants.[10] Although this method is the most thermodynamically efficient among methods powered by heat,[11] a few limitations exist such as a max temperature and max number of effects.
https://en.wikipedia.org/wiki/Desalinat ... onsumption

At much lower temperature. Inflowing seawater could cool the condensers and be pre-heat ahead of the next stage of evaporator. If solar heating is added at each stage, and each condenser is cooled by inflow, you might get some efficiency gains compared to a single stage solar distillation system. Still such a series of stages must have a lower thermal gradient in each stage so there would be diminishing returns. Each stage, input to output, will be less efficient with a warm condenser than if the condenser was at the lowest possible temperature (seawater temperature).

The most efficient technique seems to be reverse osmosis at 3 - 5.5 kWh.m3 which might be run using a solar powered electrical pump.

https://en.wikipedia.org/wiki/Desalinat ... onsumption

Assuming PV generation of 300W/m2 (a crude guestimate)

0.3/5.5 -> 0.0545m3/h or ~12 gallon/m2/h in full sun.

So, ignoring the silly efficiency numbers, it doesn't seem too far fetched to run desalination plant on solar energy

Although RO is not maintenance free, according to Wikipedia.
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Re: Simple, solar-powered water desalination

#5  Postby Macdoc » Feb 14, 2020 12:19 pm

Does seem a bit of something for nothing but then evaporative cooling is very powerful and you have constant solar input.

Something here is the key. The energy in the phase change seems to be involved

However, the solar-to-vapor conversion efficiency is below 100%, if the vaporization enthalpy is lost to the surrounding environment.8,27,28 Thus, harvesting and reusing the vaporization enthalpy is the key to enhance energy conversion efficiency.29


released heat flows to the next evaporating layer,


HOW???!!

Here is the paper

Image
https://pubs.rsc.org/en/content/article ... ivAbstract
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Re: Simple, solar-powered water desalination

#6  Postby GrahamH » Feb 14, 2020 12:21 pm

https://en.wikipedia.org/wiki/Membrane_ ... stillation



Billions of people still have no access to safe drinking water or adequate sanitation. Especially inhabitants of rural areas are affected.
If drinking water is needed far away from any infrastructure, SolarPurification can help. The system can even produce up to 200,000 liters of clean drinking water a day from heavily polluted surface and ground water.

https://solarspring.de/en/solar-purification/
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Re: Simple, solar-powered water desalination

#7  Postby GrahamH » Feb 14, 2020 12:29 pm

Macdoc wrote:
released heat flows to the next evaporating layer,


HOW???!!

Here is the paper

Image
https://pubs.rsc.org/en/content/article ... ivAbstract


That's the question. Searching "recyking enthalpy" turned up this:
https://www.sciencedirect.com/science/a ... 511830388X

Results and Discussion
The storage and recycling of interfacial solar steam enthalpy for simultaneous generation of clean water and electricity is shown in Figure 1B (the real setup is shown in Figure S1). This high-temperature steam generated by an interfacial solar steam generator (as shown in Figure S1A) flows into the area of thermal storage and water generation, with an aluminum chamber wrapped by a polyurethane foam (as shown in Figure S1B) for thermal insulation. Because of the heat exchange between high-temperature steam and the chamber, steam will be condensed to produce clean water at the outlet of the chamber, while the energy in the steam is reserved in the chamber to maintain it at a high temperature. The temperature gradient formed between this chamber and the room-temperature environment can then be used for electricity generation by using the thermoelectric modules (based on Bi2Te3 materials; for more details see Experimental Procedures).


But I'm still confused how you can condense steam on a hot plate. Don't you need to keep the condenser cold for the system to work?

I'm assuming that it does work, serious institutions are involved so it seem more likely my ignorance than anything else.
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Re: Simple, solar-powered water desalination

#8  Postby laklak » Feb 14, 2020 12:51 pm

Problem with reverse osmosis is the membranes, they're expensive and require regular replacement. I'm quite intereseted in this idea, been toying with the idea of putting a water maker on the boat. They're reverse osmosis systems driven by an electric pump. They're a bit pwer hungry, though, small units draw over 30 amps at 12v, so you're looking at pretty heavy cabling. On the plus side, they produce a shitton more water, the smallest units will do 10 gallons an hour. They're also quite compact and easy to install, but at $8000 for an entry level system it hast to stay on the wish list for now.
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Re: Simple, solar-powered water desalination

#9  Postby GrahamH » Feb 14, 2020 1:54 pm

laklak wrote:the smallest units will do 10 gallons an hour..


Here's a 25-30 l/h unit
http://www.sailfishmarine.co.uk/Ecosis- ... HAQAvD_BwE

And you can get smaller hand-operated devices, 20l/day
https://www.spectrawatermakers.com/us/u ... urvivor-06
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Re: Simple, solar-powered water desalination

#10  Postby Macdoc » Feb 14, 2020 2:28 pm

Somehow they are recapturing the heat extracted from a phase change and yet still using the condensing in a small form factor.

Image

I'd guess it is multiple layers of this ...and perhaps the aerogel has a role

Aerogel is a synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas. The result is a solid with extremely low density and extremely low thermal conductivity.


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Re: Simple, solar-powered water desalination

#11  Postby laklak » Feb 14, 2020 2:46 pm

GrahamH wrote:
laklak wrote:the smallest units will do 10 gallons an hour..


Here's a 25-30 l/h unit
http://www.sailfishmarine.co.uk/Ecosis- ... HAQAvD_BwE

And you can get smaller hand-operated devices, 20l/day
https://www.spectrawatermakers.com/us/u ... urvivor-06


That's a good price on the electric model, under $5000 US. I've seen reviews of the hand models, they're OK for a survival situation but wouldn't work well on a daily basis. They also make propeller driven ones that you tow behind the boat but they don't appear to work very well from what I've read. If we ever put a hard top on the flybridge I'll cover it with solar panels and charge the batteries from there, but that's more wish list stuff. One small Lotto win, that's all I need...

On big boats they drive the pumps directly off the engines, but that's a major refit.
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Re: Simple, solar-powered water desalination

#12  Postby GrahamH » Feb 14, 2020 3:49 pm

Macdoc wrote:Somehow they are recapturing the heat extracted from a phase change and yet still using the condensing in a small form factor.

Image

I'd guess it is multiple layers of this ...and perhaps the aerogel has a role


I think the aerogel is insulation to keep heat in the thermal storage material. They mention keeping the heat engine running from stored heat after dark, which makes sense in itself.

What I don't get is that if you insulate the condenser to keep the heat in then it will get hot and stop working as a condenser.

Likewise, if you thermally connect the condenser to the evaporator of a next stage you have a condenser and an evaporator at similar temperatures which can't be good. You need the hot bit hot and the cold bit cold, and condensation keeps moving heat from hot to cold so you have to keep putting heat into the hot bit and getting rid of from the cold bit or the system drifts toward thermal equilibrium and stops working.
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Re: Simple, solar-powered water desalination

#13  Postby Macdoc » Feb 14, 2020 4:48 pm

I think you are forgetting evaporative cooling
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Re: Simple, solar-powered water desalination

#14  Postby GrahamH » Feb 14, 2020 8:07 pm

Macdoc wrote:I think you are forgetting evaporative cooling
Can you elaborate?
Evaporation from the condenser? The thermal storage?
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Re: Simple, solar-powered water desalination

#15  Postby Macdoc » Feb 14, 2020 10:10 pm

The anwer is in here but I can't quite get my head around the heat flow

https://en.wikipedia.org/wiki/Latent_heat
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