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Simple, solar-powered water desalination

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Re: Simple, solar-powered water desalination

#61

Post by steamer25 »

pfdietz wrote: post #31
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…

One fascinating (well, to me) component of modern RO desalination systems is the rotary pressure exchanger. Such a simple device, but so important. It looks like something from the 19th century, but the base patent was issued in 1988. https://en.wikipedia.org/wiki/Pressure_exchanger

I found this video which seems to animate the concept nicely:

https://www.youtube.com/watch?v=udffed4Pq3g

Re: Simple, solar-powered water desalination

#62

Post by larkeith »

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.

Solar watt-hours are not converted to BTUs at a rate > 1, however, the device effectively uses each watt-hour multiple times, as some of the energy is recovered after each cycle. Needless to say, if the goal were to produce energy, this would be impossible, but much like with heat pumps (which can exceed 100% efficiency by moving heat instead of converting electricity), the device is not producing energy from solar power. The baseline 100% efficiency they are comparing against is that of convertible the water to steam, i.e. the maximum efficiency of a solar desalinator that does not reuse any waste heat.

As a reminder, "Be kind. Don't be snarky." [1]

[1] https://news.ycombinator.com/newsguidelines.html

Re: Simple, solar-powered water desalination

#63

Post by hinkley »

kragen wrote: post #53
SCAQTony wrote: post #34

A lot of lithium batteries are making their way onto both sailboats and yachts. With significant advantages come equal disadvantages, and those disadvantages are the risk of potential battery fires, which only a Class D fire extinguisher can snuff out. The Dive boat fire in San Diego, where 34-persons perished, is one example.

You don't need batteries to run a reverse osmosis desalinator from solar panels; you can run the desalinator when the sun is shining and store the water. I'm surprised boats are already moving to lithium. I wonder if it would be worthwhile to tow the batteries in a separate buoy behind the boat?

Or to put it another way, any device with intermittent power only requires one storage reservoir, not multiple. Water jugs are close to the oldest technology we have.

Re: Simple, solar-powered water desalination

#64

Post by kragen »

hinkley wrote: post #63
kragen wrote: post #53

Earlier quoted context omitted.

You don't need batteries to run a reverse osmosis desalinator from solar panels; you can run the desalinator when the sun is shining and store the water. I'm surprised boats are already moving to lithium. I wonder if it would be worthwhile to tow the batteries in a separate buoy behind the boat?

Or to put it another way, any device with intermittent power only requires one storage reservoir, not multiple. Water jugs are close to the oldest technology we have.

Right. You still need batteries on your boat, just not for desalination.

An interesting aspect of the history of water jugs is that the humans were cultivating them for thousands of years before they started cultivating food crops: https://phys.org/news/2014-02-mystery-bottle-gourd-migration...

Re: Simple, solar-powered water desalination

#65

Post by brudgers »

VikingCoder wrote: post #47
101404 wrote: post #28

Earlier quoted context omitted.

What's that in bathtubs per football field?

EDITED: bathtub ≈ 302 liters football field = 5351 square meters They achieved 5.78 L/m^2/h So 5351 m^2/football field * 5.78 L/m^2/h = 30928.78 L/h/football field 30928.78 L/h/football field / 302 L/bathtub = 102.4 bathtubs/football field/hour h/t to brudgers for the correction about football field surface area!

A football field is 5351m^2. Futbol pitches vary.

Re: Simple, solar-powered water desalination

#66

Post by einpoklum »

jfengel wrote: post #39
worik wrote: post #9

"than 1.5 gallons of fresh drinking water per hour for every square meter of solar collecting area." Units, people. Units. Be imperial. Be metric. Hey! Be both!

I guess it's not really surprising for American engineers. Food and drink are measured in Imperial units. Technical doodads are measured in metric. You don't really need them to be compatible in this case: even though it appears that you're talking about multiples of units of length, you don't really compare them. You're not going to do the calculation that this is "1.58 micrometers per second" or ".447 feet per day"…

> 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.

Re: Simple, solar-powered water desalination

#67

Post by philipkglass »

SCAQTony wrote: post #34

A lot of lithium batteries are making their way onto both sailboats and yachts. With significant advantages come equal disadvantages, and those disadvantages are the risk of potential battery fires, which only a Class D fire extinguisher can snuff out. The Dive boat fire in San Diego, where 34-persons perished, is one example.

Class D fire extinguishers are for extinguishing flammable metal fires, involving e.g. lithium, magnesium, titanium, aluminum, and so on. Disposable (primary) lithium batteries contain metallic lithium that can ignite.

Lithium ion batteries contain lithium in an ionic (non-metallic) form. Lithium ion batteries contain flammable electrolytes and polymers, so they can burn, but these are not metal fires calling for a Class D fire extinguisher.

Re: Simple, solar-powered water desalination

#68

Post by [deleted] »

hinkley wrote: post #60
riazrizvi wrote: post #48

Earlier quoted context omitted.

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.

So that's how this is gonna be. Good luck.

[deleted]

Re: Simple, solar-powered water desalination

#69

Post by gpm »

numbsafari wrote: post #51
numbsafari wrote: post #32

Earlier quoted context omitted.

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-...

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.

Re: Simple, solar-powered water desalination

#70

Post by markdown »

kragen wrote: post #45
asdfman123 wrote: post #30

Earlier quoted context omitted.

That means for every square rod of surface area, this unit produces over 11,000 hogsheads of water! Impressive.

What, per millifortnight? cf. https://news.ycombinator.com/item?id=22270160

How much water can I get in a moon?
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