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Simple, solar-powered water desalination

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Re: Simple, solar-powered water desalination

#91

Post by hinkley »

r-bryan wrote: post #27
hinkley wrote: post #20

Earlier quoted context omitted.

It takes 1 watt-hour to produce about 3.5 BTUs. With a heat pump, for instance, you might be able to produce 5BTUs per watt-hour, because you're extracting heat from the environment (a very large heat source), which would be 140% efficient. The paragraph you excerpted has your answer: > 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…

You mean BTU/hour.

Yeah yeah yeah.

If we're talking about distillation, the civil engineer probably wants to think about rates (liters per minute per $1000), but for a civilian just trying to figure out how this could work in a laboratory situation?

I find quantities easier to comprehend (and relate). Tell me how many AA batteries or days of full sun it would take to convert a big beaker of salt water into a smaller beaker of distilled water.

Re: Simple, solar-powered water desalination

#92

Post by shkkmo »

gpm wrote: post #69
numbsafari wrote: post #51

Earlier quoted context omitted.

Per the original comment about 5 sq mi: Based on this random article I found from 2015 [1], the average cost per build able sq ft in Manhattan was getting to $1200. So, if you magically found 5 sq miles in NYC, it’d cost around $167.3B just for the land [2]. I’m guessing you’d want to build it outside the city and pump it in via aqueduct. Following the article, an array of off-shore desalinated that pumped water back…

New york city is 468.484 sq mi in size... so we're talking about covering 1 to 2% of it in these panels... I'm pretty sure that much roof space is available. Of course it probably makes more sense economically to do it offshore or just have a giant field somewhere outside of NYC and pump the water in, but you could find the space inside the city if you really wanted to.

I think you'd lose a bit of efficiency have to pump circulated seawater to the roof tops to not only provide water to desalinate, but also to flush out the accumulated brine overnight.

Re: Simple, solar-powered water desalination

#93

Post by simcop2387 »

pytester wrote: post #89
sacred_numbers wrote: post #26

Efficiency in desalination is measured by comparing the energy expenditure to the enthalpy of vaporization of water (basically the energy required to distill water by boiling it). The current state of the art reverse osmosis desalination plants use about 3.2 KWh of electricity per cubic meter of fresh water ( https://pdfs.semanticscholar.org/d4d7/821d585699719289dddd10... ). This technology uses about 173 KWh of sola…

>more cost-effective to use solar panels, batteries, and large scale reverse osmosis systems I can't imagine why you'd use batteries at all. Storing fresh water efficiently is cheap, storing power efficiently is not.

Because you're likely going to be able to collect more energy than you can immediately use. That makes it effective to do so you can run the desalination even when the sun is down.

Re: Simple, solar-powered water desalination

#94

Post by quotha »

spikels wrote: post #21
quotha wrote: post #7

A big question is what do they do with the salt?

Kinda explained in the 9th graf: > Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of. In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater, according to the researchers.

So the salt is just going back into the ocean. How is that different from other systems, they end up dumping the salt somewhere.

Re: Simple, solar-powered water desalination

#95

Post by sacred_numbers »

Amarok wrote: post #72
sacred_numbers wrote: post #26

Efficiency in desalination is measured by comparing the energy expenditure to the enthalpy of vaporization of water (basically the energy required to distill water by boiling it). The current state of the art reverse osmosis desalination plants use about 3.2 KWh of electricity per cubic meter of fresh water ( https://pdfs.semanticscholar.org/d4d7/821d585699719289dddd10... ). This technology uses about 173 KWh of sola…

You're comparing both systems using watthours of energy expended, so mentioning enthalpy of water vaporization is unnecessary and confusing.

Sorry, I meant to say that the enthalpy of water vaporization is about 630 KWh per cubic meter for context to compare to the energy use of this method and reverse osmosis. I guess I forgot to actually include the number!

Re: Simple, solar-powered water desalination

#96

Post by DenisM »

pytester wrote: post #89
sacred_numbers wrote: post #26

Efficiency in desalination is measured by comparing the energy expenditure to the enthalpy of vaporization of water (basically the energy required to distill water by boiling it). The current state of the art reverse osmosis desalination plants use about 3.2 KWh of electricity per cubic meter of fresh water ( https://pdfs.semanticscholar.org/d4d7/821d585699719289dddd10... ). This technology uses about 173 KWh of sola…

>more cost-effective to use solar panels, batteries, and large scale reverse osmosis systems I can't imagine why you'd use batteries at all. Storing fresh water efficiently is cheap, storing power efficiently is not.

To make twice as much water you may need to either install a set of batteries to run at night, or install twice as much RO hardware to run during the day. Whichever's cheaper.

Re: Simple, solar-powered water desalination

#97

Post by logfromblammo »

J5892 wrote: post #82
logfromblammo wrote: post #41

Earlier quoted context omitted.

Given the agricultural issues in that movie, perhaps the purification device was intended more to collect and concentrate the urea than to purify the water. But that assumes the scriptwriter would have thought of that, which may be a stretch, given that Earth has insufficient water to cover all landmasses to a depth of 7 km. That would require about 3.6 billion cubic kilometers more water than already exists in the o…

So the only possible explanation of the cause of the Waterworld apocalypse is earth being hit by millions of giant ice meteors.

The era of the movie is 2500 AD. To add nearly 3 oceans worth of water in that time frame by cometary bombardment would boil the existing ocean.

The only hard sci-fi explanation is that someone towed Europa to Earth and slowly lowered it down via a cable or pipe. Terraforming by an alien aquatic species. Hyperintelligent space whales.

Re: Simple, solar-powered water desalination

#98

Post by mateo411 »

numbsafari wrote: post #32
symplee wrote: post #24

>> 1.5 gallons of fresh drinking water per hour for every square meter of solar collecting area. So a football field could collect around 8,000 gallons of water per hour. Let's say you can get 5 hours of good sunlight per day = 40,000 gallons of water. >> 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. >> Theor…

If I am reading [1] correctly, NYC needs approx. 1 billion gallons of water per day. So, they would need an array of 12,500 football fields to meet their needs. [1] https://data.cityofnewyork.us/Environment/Water-Consumption-...

Yes, but they get the majority of their water from the Catskills. Water is delivered using the force of gravity.

Re: Simple, solar-powered water desalination

#99

Post by kragen »

riazrizvi wrote: post #48
hinkley wrote: post #20

Earlier quoted context omitted.

It takes 1 watt-hour to produce about 3.5 BTUs. With a heat pump, for instance, you might be able to produce 5BTUs per watt-hour, because you're extracting heat from the environment (a very large heat source), which would be 140% efficient. The paragraph you excerpted has your answer: > 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…

I really enjoy artful displays of mental gymnastics, so thank you. A watt is a unit of power , a BTU is a unit of energy , and a watt hour is also a unit of energy . If you can show me how solar watt hours are converted into BTU's at an equivalent rate of 3.85 to 1, I will short sell all the energy stocks using my children as collateral.

A watt hour is actually 3.41 BTUs, not 3.85. A BTU is the energy to raise a pound of water by one degree Frankenstein. (An avoirdupois pound, not a Tower pound, a troy pound, an apothecaries' pound (which happens to be equal to the troy pound), a merchant's pound, an Imperial Standard pound, or a pound sterling or pound of paper, which aren't even units of weight.)

The specific heat of water is one calorie per gram per kelvin, so a BTU works out to one pound, times a calorie per gram, times a degree Frankenstein per kelvin.

A calorie has been defined as having various different values, since water's specific heat varies with temperature (and pressure!) but they're all about 4.18 to 4.19 joules, except for the food "calorie", which is actually a kilocalorie.

Pounds have also been defined as having many different values, even avoirdupois pounds; the values used include 6992 grains, 7000 grains, 7002 grains, and 6999 grains. (Troy grains, not metric grains, which are different.) The currently most popular pound is 7000 grains, but by international agreement it is now defined as 453.59237 grams, previous definitions in terms of the metric system having included 453.59265 grams and 453.59243 grams.

Finally, one degree Frankenstein is precisely defined as 5/9 of a kelvin, although Dr. Fahrenheit's original definition was rather different.

Working all of this out, a BTU turns out to be about 454 g · 4.18 J/g · (5/9)K/K, which is about 1054.3 J. A watt hour is of course 1 W hour · 60 s/min · 60 min/hour = 3600 W s = 3600 J. 3600/1054.3 is about 3.41.

I'll be here all week. Don't forget to tip your servers.

Re: Simple, solar-powered water desalination

#100

Post by einpoklum »

dr_dshiv wrote: post #83
einpoklum wrote: post #66

Earlier quoted context omitted.

> Food and drink are measured in Imperial units. Maybe if you don't mind losing your global Mars orbiter you don't: http://edition.cnn.com/TECH/space/9909/30/mars.metric.02/ _everything_ is measured in metric, period.

Don't forget, 10 hours in a day, 100 minutes in a hour and 100 seconds in a minute (1 metric second = .864 imperial seconds)

... and that is why computers traditionally saved 32-bit seconds since the epoch ("Unix time"). These days we have 64-bits typically but the principle is similar. Only use the funny human units when setting stuff that's visible in the UI; in the back we avoid it.
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