15 ms·
This sounded pretty strong to me, so I did a quick napkin-math check of feasibility (in short - it passed the napkin check). On a sunny day, total solar energy
by mixedmath 3y ago
This sounded pretty strong to me, so I did a quick napkin-math check of feasibility (in short - it passed the napkin check).
On a sunny day, total solar energy hitting the top of the atmosphere gives about 1360 watts per square meter (source - top result from google, something something NASA). It takes about 4.184 joules to raise one ml of water by 1 degree celsius. Let's suppose that the "suitcase" proposed in the article has 1 square meter of space (a pretty large suitcase) and is able to capture 100 percent of solar energy (unreasonably effective). Then pure solar energy-wise, there is enough energy to raise 325 ml of water by one degree of celsius per second in a square meter. If the water is, say, at 16 degrees (the temperature of the ocean near me), in would take 84 seconds to bring 325 ml of water to boiling temperature. To transform boiling water to steam, each ml of water takes an additional 2257 joules. In 540 seconds, the 325 ml of water is now converted to steam.
In total, it has taken 624 seconds to steamify 325 ml of water (under very optimistic circumstances). Morally one could do this almost 6 times in an hour, giving approximately 2 litres of water per hour.
(Edit): Looking a bit closer, I see that the article's description of their work suggests that they recapture some of the energy from condensing the steam back into water. I didn't account for this. Even a moderate amount of recapture changes the math very favorably.
This makes me suspect that their claim of 4-6 liters per hour would require a rather large "suitcase"-sized device, but is within the ballpark of reason.
- grandchild 3y agofurther down the article: > if each stage were scaled up to a square meter, it would produce up to 5 liters of drinking water per hour so 1m², times the number of layers.
- lta 3y agoI think the layers are on to of each other if the total surface is a square meter, it doesn't bring any more solar energy to stack layers, right ?
- o0banky0o 3y agoI don't think they are boiling it. It read to me like they're just evaporating the water in some efficient way, rather than boiling. What would the math look like if they aren't necessarily going to boil?
- scotty79 3y agoThermodynamics can't be cheated. If you want to turn some liquid into gas at given pressure you need to deliver specific amount of energy regardless of how you do it.
- mitthrowaway2 3y agoHowever, you also need to turn the same amount of gas back into liquid, just somewhere else without the salt. It does seem like there's good potential for recovering and reusing that energy.
- Izkata 3y agoFrom some quick searches, it looks like 2257 J the energy used by the phase transition, so that part will probably be the same.
- AlexandrB 3y agoBoiling is just evaporating water rapidly. No matter what you do, you need to put the same amount of energy in to convert some mass of water to vapor. Any efficiency gains would be in how you get this energy into the water (electric element vs. directly heated by the sun) not in the energy required.
- PicassoCTs 3y agoUltrasound would do it without boiling?
- AlexandrB 3y agoI don't think it matters whether you "boil" the water or not, you still need to put in enough energy to cause a state transition in the water you're evaporating. I see some papers on using ultrasonics to increase the efficiency of energy transfer from a heating element[1], but I don't think the second law of thermodynamics allows for a free lunch here. [1] https://www.sciencedirect.com/science/article/abs/pii/S1359431118300383 https://www.sciencedirect.com/science/article/abs/pii/S13594... Edit: If you're thinking of something like an ultrasonic humidifier, I don't think these actually evaporate the water[2]. The mist these produce would still contain salt if you tried to use them for desalination. [2] https://en.wikipedia.org/wiki/Humidifier#Ultrasonic_humidifiers https://en.wikipedia.org/wiki/Humidifier#Ultrasonic_humidifi...
- JackFr 3y ago> On a sunny day, total solar energy hitting the top of the atmosphere Aren't they all sunny at the top of the atmosphere? > gives about 1360 watts per square meter I assume that's very much latitude dependent
- deleted 3y ago[deleted]
- gmm1990 3y agoshould be the same at all latitude
- laurencerowe 3y agoBut only if you point angle it towards the sun, so at 45 degrees latitude a 1x1m panel would take up 1x1.41m of ground area. There’s also more atmosphere in the way the further you go from the equator which will affect how much solar radiation reaches the earth. Overall only about half of solar radiation reaches the surface.
- PaulKeeble 3y agoSolar heaters are remarkably efficient, well above 95%. Solar water heaters are really quite efficient so there is no particular reason why with the right coatings such a device wont get quite close. Its actually surprisingly easy to make a solar water boiler and they aren't expensive. People heat pools with DIY water heaters. If heating water is the goal then going directly to heating the water is more than 4x the power capture of a solar panel. Dropping one solar panel on your roof for a water heater pays off its just that the solar heaters are kind of expensive dealing with all the other aspects, like if the pump fails or its too cold and the water would freeze etc etc. Those other aspects are probably going to dominate the complexity and cost of the devices.
- bertil 3y agoI’ve always wondered how hard it would be to have standard solar panels water-cooled and use that heat for hot water storage.
- azornathogron 3y agoYou can buy such things. Eg, https://dualsun.com/en/products/dualsun-spring/ https://dualsun.com/en/products/dualsun-spring/ (Not affiliated, no idea if that particular one is any good, it was just one of the first results)
- colechristensen 3y agoThe problem is you want your hot water much hotter than your solar panels so there may be limited use for this compared to a traditional solar water heater.
- hinkley 3y agoHeat pump water heaters exist. I would think running a warm water source past one would increase the efficiency since it lowers the delta-T
- PaulKeeble 3y agoThe main problem is the expected lifetime (and payout periods) of the two parts. Cooling solar panels brings more efficiency so it feels like a reasonable fit, cool the panels and get hot water. Its less efficient water heating because the panels aren't the ideal coating but they do still get quite hot and a few of them can definitely do the job. The big mismatch is in life. A solar thermal heater typically lasts 10 years, its a pretty harsh environment and they have pumps and expansion vessels and water in the heat range of -25 through +80 C is just damaging over time. Where as a solar panel has a rated 80% output at 25 years. That mismatch is problematic for Solar panel payoff, 10 years is only just past break even point. Whereas the solar thermal heaters typically pay off within a few years so the reduced lifetime isn't such a big problem (but both are driven by local gas and electrical prices). Companies are trying to do this better and products exist to do it. The added installation complexity and reduced expected lifetime are all a bit of an issue. Currently I suspect most people are better off with a dedicated solar heater and solar panels separately, at least for now until the water channels in the panels at least are reliable enough that they meet the panel lifetime and then the pump is installed in the roof cavity so its cheap and easy to replace. Work still needs to be done to work out the details and make the parts that fail easily swappable.
- Gibbon1 3y agoThere are desalinization systems for boats. They are expensive but everything about boats is. I saw one model that produces 20gph and seems to use about 1200 watts. Doesn't feel like there is a gross thermodynamic reason it wouldn't work.
- yencabulator 3y ago20 gallons = 75.7 liters. 4 to 6 liters is 5-8% of that. 5-8% of 1200 watts is 60-96 watts. If you remember the numbers right, that thing is already roughly in this range. A 100 watt solar panel might fold into a briefcase.
- adrianmonk 3y agoCan't you boost the efficiency by using heat recovery? The steam is very hot, and the condensed fresh water is also pretty hot. You can use a heat exchanger to transfer this heat into your 16°C ocean water, preheating it so the sun doesn't have to do all the work. This wouldn't mean magically getting free energy. Your output fresh water would otherwise be really hot, and that's where the energy comes from. Obviously there are limits. If you have 100°C fresh water coming out and 16°C ocean water going in, a heat exchanger will at best average their temperatures to 58°C. You could in theory overcome this with a heat pump, but that's pretty elaborate. (I'm not trying to design the perfect desalination system here. The point is there may be some easy efficiency wins.)
- Dylan16807 3y agoA multi-stage or continuous heat exchanger can transfer a lot more than half the heat.
- m463 3y agoif you can do it at lower pressure, boiling and evaporation happen at a lower temperature.
- bertil 3y agoGood idea, but it probably makes a very cheap system more expensive — kitchen-equipment expensive.
- e44858 3y agoA countercurrent heat exchanger could transfer up to 99% of the heat from the steam to the ocean water: https://en.m.wikipedia.org/wiki/Countercurrent_exchange https://en.m.wikipedia.org/wiki/Countercurrent_exchange
- bertil 3y agoThat's obviously the way to go, but… Where does the 99% come from?
- japanuspus 3y agoThe fundamental limit to energy expenditure is not the heating and boiling of water (as others have pointed out, that energy can all be captured). Rather, the limit is set by the requirement for the entropy of the universe to not decrease when we reduce the local entropy in the salt-water system. According to GPT4, the numbers come out to 760 J/L for seawater with a salt concentration of 35 g/L. That would mean a limit of around 2 L/s*m2 for full intensity sunlight.
- _8j50 3y agoI had this same idea on an HN comment early this year lol. Part of my idea is to use desalinated water to build artificial lakes and rivers. Specifically, a river system along interstate highways, especially in the US west and alongside that infra build highspeed railways. These two things along with interstate highway will allow new towns and cities to flourish which will help with economic activity recouping some of the cost but also solve homelessness, climate goals and even social unrest and instability. It passed my poorly done napkin math. If hitler had plans to dam the mediterranean and turn the sahara into an fertile land and generate crapton of energy wth are we doing today with all the peace, economic health and insane amounts of technical progress? $300B in subsidizing ev car chargers (car makers should pay for) instead of high speed railways makes me sad.
- lostlogin 3y agoA breakdown like this is why I come to HN. Thank you.