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Simple, solar-powered water desalination

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Re: Simple, solar-powered water desalination

#131

Post by MayeulC »

ebg13 wrote: post #54
zamalek wrote: post #40

Earlier quoted context omitted.

> 1 billion gallons of water per day * How much of that water need be drinkable? * How much of that water can be salinated (e.g. for flushing)? * How much of that water can be partially desalinated (e.g. for showering)?

Indeed. If you do only the drinking water, 2L per person per day for 20 million people is only 10566000 gallons or 132 football fields.

And for those who wonder like I did, I counted around fifty of those on that page: https://www.nycgovparks.org/facilities/football

Re: Simple, solar-powered water desalination

#132

Post by WrtCdEvrydy »

AnimalMuppet wrote: post #128
gpm wrote: post #74

Earlier quoted context omitted.

Say solar panels cost $2 per watt, 6 liters/hour is about 20 watts of power, is about 40 dollars of solar panels. Plus whatever the reverse osmosis equipment costs. This system gets approximately 6 liters/hour/sq m... so it makes sense if we can manufacturer it for less than 40 dollars per square meter and space is cheap.

And, in places without usable water, space tends to be cheap.

I wonder if there is a formula for how cheap a place is based on how easy it is to have usable water and usable crops.

I have had these feelings since I saw the Egypt map showing how much of it is just not usable.

Re: Simple, solar-powered water desalination

#133

Post by ngold »

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

So they took the technology of the revolver and applied it to pressure relief. Fascinating stuff.

>One particularly efficient type of pressure exchanger is a rotary pressure exchanger. This device uses a cylindrical rotor with longitudinal ducts parallel to its rotational axis. The rotor spins inside a sleeve between two end covers. Pressure energy is transferred directly from the high pressure stream to the low pressure stream in the ducts of the rotor. Some fluid that remains in the ducts serves as a barrier that inhibits mixing between the streams. This rotational action is similar to that of an old fashioned machine gun firing high pressure bullets and it is continuously refilled with new fluid cartridges. The ducts of the rotor charge and discharge as the pressure transfer process repeats itself.

Re: Simple, solar-powered water desalination

#134

Post by newnewpdro »

kragen wrote: post #129
newnewpdro wrote: post #113

Earlier quoted context omitted.

> I'm surprised boats are already moving to lithium. When the alternative is a bank of lead-acid batteries with effectively ~50% usable capacity and poor charge efficiency, at least for cruisers effectively living at sea off solar power, it's not surprising at all.

The lead-acid batteries still cost a lot less per joule of capacity than the lithium batteries do. They just weigh more. Or maybe charge efficiency is the reason?

Space tends to be at a premium for cruisers, and modern cruisers are increasingly embracing electricity for powering everything.

Charge efficiency is a big one, but it's also just maximizing energy capacity in the available volume.

Re: Simple, solar-powered water desalination

#135

Post by lovemenot »

adamcharnock wrote: post #126
DenisM wrote: post #96

Earlier quoted context omitted.

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.

I made a comment with regards to this a while ago [1]. I'll repeat it here as I think it is relevant: --- I’ve heard on the grapevine that facilities are now being designed which work at under 100% capacity in order to soak up excess/cheap power. I believe this is coming about because: 1) the high cost of power and presence of sporadically cheap power makes it economically viable, and 2) designing equipment with lowe…

It'd be a boon for renewables if energy consuming plant costs were less dominated by investment in equipment.

Imagine doing shift planning at such a site though. Eight days from now, there's a 60% chance of wind in excess of 5m/s between 22.00 and 06.00. But labour costs would be 130% higher than 06.00 to 14.00 on that day. On that morning shift there's a 50% chance of clear skies ...

Re: Simple, solar-powered water desalination

#136

Post by gnaritas »

newnewpdro wrote: post #113
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?

> I'm surprised boats are already moving to lithium. When the alternative is a bank of lead-acid batteries with effectively ~50% usable capacity and poor charge efficiency, at least for cruisers effectively living at sea off solar power, it's not surprising at all.

Lead-acid is still better, it's 1/10 the cost for equivalent usable amp-hours. Most boaters use deep cycle golf cart batteries, they're produced at such scale that they're cheap as hell. Lithium is a waste of money and dangerous to boot. They only make sense if you're extremely space limited, which most bigger boats aren't.

Re: Simple, solar-powered water desalination

#137

Post by caf »

lovemenot wrote: post #135
adamcharnock wrote: post #126

Earlier quoted context omitted.

I made a comment with regards to this a while ago [1]. I'll repeat it here as I think it is relevant: --- I’ve heard on the grapevine that facilities are now being designed which work at under 100% capacity in order to soak up excess/cheap power. I believe this is coming about because: 1) the high cost of power and presence of sporadically cheap power makes it economically viable, and 2) designing equipment with lowe…

It'd be a boon for renewables if energy consuming plant costs were less dominated by investment in equipment. Imagine doing shift planning at such a site though. Eight days from now, there's a 60% chance of wind in excess of 5m/s between 22.00 and 06.00. But labour costs would be 130% higher than 06.00 to 14.00 on that day. On that morning shift there's a 50% chance of clear skies ...

If only there was some form of programmable electronic computation device that could be used to solve such complex sets of constraints modelled as equations :)

Re: Simple, solar-powered water desalination

#138

Post by colechristensen »

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.

The question is if you could build and operate a smaller plant to run at 100% capacity at all times which includes energy storage for less money.

You either have losses in:

* Underutilized solar generation

* Underutilized plant capacity

* Overhead of energy storage

Design your system to minimize that cost. Engineering systems of all sorts are full of this kind of optimization problem: "Should I add a subsystem to recover loss x?" There is always more you can do to recover losses, but each extra system you add suffers diminishing returns a little more.

Re: Simple, solar-powered water desalination

#139

Post by fghorow »

It's a solar thermal Multi-Effect-Distillation (MED) plant!

I was part of a team that suggested using geothermal heat for desal with MED technology in Perth, Western Australia a bit more than a decade ago!

The advantage over work based (i.e. electrical/Reverse Osmosis) systems is that you do not lose heat via Carnot (or real world, for that matter) efficiency. You utilize more of the original heat, and via the multiple stages ("effects") you can re-use the latent heat of vaporization over and over.

MEDs are really cool gizmos!

Re: Simple, solar-powered water desalination

#140

Post by caf »

jfengel wrote: post #118
logfromblammo wrote: post #75

Earlier quoted context omitted.

Given the increasing popularity of 500 mL and 1 L units of bottled water, and the ubiquity of the 2 L HDPE beverage bottle, I think it would have been safe to measure the potable water output in liters. About the only things still sold by whole gallons any more are milk, water, iced tea, and lemonade, in the 1 gal jugs, or water in the 5-gal water-cooler jugs. I'm more confused by the efficiency numbers exceeding 100…

Servings of beverages in the US are still mostly in imperial measures. A can of soda is 12 ounces; so are most bottles. Beer in restaurants is usually a pint; if not, they'll specify ounces. A Starbucks venti is so called because it's 20 ounces; a tall is defined as 12 ounces. And American recipes still use cups and tablespoons (where the rest of the world has gone almost exclusively to grams, and not volume at all).…

Recipes elsewhere in the five eyes still use cups, tablespoons and teaspoons as well, although they're not the same size everywhere (eg. a metric cup is 250ml).
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