Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Simple, solar-powered water desalination

news.mit.edu

Post reply on HN
Print view View original

171–180 of 205 posts

Re: Simple, solar-powered water desalination

#171

Post by markdown »

i_am_nomad wrote: post #164
jvanderbot wrote: post #114

Earlier quoted context omitted.

There's no difference. Except that the processing and shipping of ions is passive / much easier. Contrasted with large permeable membrane plants that have to burn power to push all that brine out into the ocean.

But the processing and shipping isn’t the problem with desalination. The problem is that dumping bribe or salt into the ocean can kill local wildlife.

They dump the salt outside the environment. https://www.youtube.com/watch?v=3m5qxZm_JqM

Seriously though, far out to sea away from any landmass it's pretty much a desert. Any salt dumped there would disperse very quickly and not pose a problem to wildlife.

Re: Simple, solar-powered water desalination

#172

Post by newyankee »

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…

If the capital costs of such a system are lower and they can convert the design into something floating on the sea on continental shelf with tethers mooring it to the floor i see a lot of potential for water, land and electricity (competitive) stressed cities. A case in example is Chennai, India.

I wonder if RO based desalination has any learning rate associated with it and whether a lot of CAPEX costs are related to some proprietary IP. I know that a lot of OPEX is energy costs and hopeful that solar will get even more cheaper with time and even more scale. However if the same can be done with scaling of RO technologies and reducing costs to say ~20 % of existing a great many opportunities will arise.

Really hope it will be lower than 0.1 $ / m3 before 2030.

Re: Simple, solar-powered water desalination

#173

Post by taneq »

ebg13 wrote: post #167
MayeulC wrote: post #131

Earlier quoted context omitted.

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

Ok, but if you use those then you don't have anywhere to play. I think more importantly, 132 football fields is about the same as the total floor area of the Pentagon.

Install the solar panels over the football fields as roofing.

Re: Simple, solar-powered water desalination

#175

Post by darksaints »

gnaritas wrote: post #136
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.

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.

Most marine lithium batteries are LiFePO, not lithium ion. They're far more stable and safe to have on a boat. Compared to lead acid, they have better charge efficiency, deeper cycles, longer lifespans, higher energy density and specific capacity. Furthermore, I've never found lead acid batteries that come close to 1/10th the price...1/4 is more like it.

Re: Simple, solar-powered water desalination

#176

Post by quickthrower2 »

asdfman123 wrote: post #30
DanTheManPR wrote: post #23

Earlier quoted context omitted.

It comes out to about 0.4 liters per square foot, for those of you who are confused.

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

Firkin hell

Re: Simple, solar-powered water desalination

#177

Post by gnaritas »

darksaints wrote: post #175
gnaritas wrote: post #136

Earlier quoted context omitted.

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.

Most marine lithium batteries are LiFePO, not lithium ion. They're far more stable and safe to have on a boat. Compared to lead acid, they have better charge efficiency, deeper cycles, longer lifespans, higher energy density and specific capacity. Furthermore, I've never found lead acid batteries that come close to 1/10th the price...1/4 is more like it.

Fair point on the LiFePO, but none of those other things really matter as they're easily made up for by a larger bank, what matters is the cost per amp/hour and lithium isn't remotely there yet. The exact fraction varies, but lithium isn't going to touch some Costco golf cart batteries at a $100 a pop. You can buy an entire house bank for the cost of 1 lithium LiFePO battery (nearly 1k on Amazon). Unless you just like wasting money, deep-cycle lead-acid is a far better choice.

Re: Simple, solar-powered water desalination

#178

Post by timonoko »

steamer25 wrote: post #61
pfdietz wrote: post #31

Earlier quoted context omitted.

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

Bloody Hell I did not know that. I have parts of broken Katadyn PUR-06. I was thinking of rebuilding it from better materials. The pressure recovery is the worst part. I can totally fathom now a hand-held desalinator with only rotating parts running from solar or Li-On batteries.

Re: Simple, solar-powered water desalination

#179

Post by TeMPOraL »

caf wrote: post #137
lovemenot wrote: post #135

Earlier quoted context omitted.

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 :)

It's not the calculation part that's the problem, it's the investment part (and perhaps also labor part).

Re: Simple, solar-powered water desalination

#180

Post by VBprogrammer »

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 ...

I think the savings in planning for low duty cycle probably result from reduced redundancy i.e. installing one pressure pump instead of two. Which naturally means when there is a low chance of excess energy is a good time to work on maintaining the systems rather than just taking one off line and working on the other.

This means that you just staff it at a consistent level and plan the work around the energy levels.

Post reply on HN