Dry rock geothermal methods are currently being used in parts of the world for energy production, but Missimer says that the heat can be used in more efficient ways, especially with desalination. Details of the new research constructing a "geothermal energy-water campus" will be presented on Monday at the 2018 Geological Society of America Annual Meeting in Indianapolis, Indiana.
Usually, geothermally heated water from the ground is converted to steam, that steam powers a turbine to create electricity, then the heated water is vented to the atmosphere while it's still hot—still over 100 degrees Celcius[sic] in most cases. Instead of venting, the team wants to use that hot steam in their desalination processes.
The first desalination process is multiple effect distillation (MED), which requires hot water (above 100 degrees Celsius), but the second process, adsorption desalination (AD), can be run on cooler water, says Missimer. As the steam moves through the system and cools, it is still effective for powering desalination. "Now you have an efficient system where you have conserved the latent heat that you've captured in the ground through three processes: turbine electricity generation, MED and AD."
At the end of the desalination process, Missimer says that distilled water and chilled water (from the AD process) are the final products. While the distilled water can be consumed, even the chilled water is reused—the cool water can be recycled through the plant to help with air conditioning.
(Score: 4, Insightful) by VLM on Tuesday November 06 2018, @12:27PM
You have to balance the cost of desal with the cost of moving the people away from a desert and toward somewhere there's too much fresh water to use, vs the third option of passive devices like pipelines to abundant freshwater vs fourth option of merely decreasing consumption via efficiency.
So... that balance being complicated, and desal being incredibly capital intensive, from the outside it looks chaotic compared to something like the energy biz where life is pretty cut n dried and as such pretty well organized.
(Score: 2) by The Shire on Tuesday November 06 2018, @04:48PM (5 children)
I don't get it. Desalination requires energy. One form of energy is geothermal therefore geothermal can be used for desalination. This isn't revolutionary. The real problem is suitable geothermal fields are relatively rare and finding one near where you need to desalinate water is even rarer.
You want carbon neutral desalination? Try nuclear, wind, or solar.
(Score: 0) by Anonymous Coward on Tuesday November 06 2018, @05:19PM
Yes, TFA pretty much just says that geothermal energy sources (specifically "dry rock geothermal") are not currently being used for desalination. Then the authors propose that this would be a reasonable approach in several areas, specifically mentioning southern California, northern Africa and the Red Sea region.
They suggest reusing waste heat from the process as an energy source instead of venting it. This is not a new idea, this is basically how closed-cycle heat-pump dryers work: instead of venting hot, wet air outside you can use a heat pump to condense the water out of the air, giving you some of the thermal energy back which is then used to heat the dryer more (plus some waste water). In principle this would work regardless of how you power the desalination system but, as always, whether or not it is a good idea depends entirely on the cost of such a system compared to, say, putting up a couple more solar panels.
(Score: 3, Interesting) by ElizabethGreene on Tuesday November 06 2018, @07:53PM (3 children)
>You want carbon neutral desalination? Try nuclear, wind, or solar.
Geothermal is as carbon neutral as any of those three. i.e. the carbon freed after the construction is completed is a rounding error.
TFA's topic of note is they found a way to get utility out of waste heat. Good on them. Nuke plants have a lot of waste heat too*, they should look to see if the same methods will apply there.
* The big concave funnel that people associate with nuke plants is a cooling tower, and they shed a bunch of heat into the local waterway too.
(Score: 0) by Anonymous Coward on Tuesday November 06 2018, @09:30PM
It's not at all the same thing: the heat from a nuclear plant you are talking about isn't "wasted". Nuclear power plants generate electricity by operating a heat engine. In order to operate a heat engine you need a hot side and a cold side -- the bigger the temperature difference between the two sides the more energy you get out of it. The hot side of this engine is the reactor, and the cold side is the cooling towers (or more typically, it is a large body of water). Thermal energy moves from the hot side to the cold side, producing useful work.
To do the reverse, taking that energy back out of the cold side, requires work input. If everything was perfectly efficient, to take all the energy you dumped into the cold side back out requires, at minimum, the same amount of work you got out of the generator in the first place.
(Score: 2) by The Shire on Wednesday November 07 2018, @04:07AM (1 child)
TFA is about doing as much work as possible while RETAINING heat which is then pumped back into the geothermal pocket. Remember this is DRY ROCK geothermal they're talking about - the water has to be pumped into the pocket and allowed to heat before extraction. They're just trying to reduce the amount of reheating needed by the geothermal source and still extract work. And the amount of desalinization work they can get from such a low temperature differential is extremely small. We're talking single digit efficiency gains.
That kind of heat preservation is totally pointless with something like nuclear. Unlike geothermal plants which operate at relatively low steam temps to begin with, a nuclear reactor already extracts far more energy from the high pressure high temperature steam it produces. This is a basic engineering concept - the higher the temperature differential, the more work can be extracted. So there is nothing to be gained in a nuclear reactor by trying to keep the waste water hot - the efficiency gains compared to the original work extracted is far smaller than even the small gains geothermal sees. Vastly more energy producing work has already been extracted and there is a near infinite source of cool water already at hand - the ocean.
There's a good reason the authors are focused on geothermal, it's because that's the only place it's at all useful. And lets face it, the number of productive geothermal sites is tiny, and the number of those sites which also have need of desalinization is even smaller.
And lastly, there's the problem of contaminating the aquifer. We're talking about pumping large quantities of ocean water into the rock bed. This will not only have the same effect as fraking (increasing the chances of fault rupture in places like California), but can also run the risk of getting into the fresh water aquifer and contaminating it with brine.
(Score: 2) by ElizabethGreene on Wednesday November 07 2018, @09:25PM
Are you certain?
Dry Rock Geothermal works by drilling (at least) two wells into dry rock and using pressure to fracture the rocks between the two wells. That fractured rock works like a big heat exchanger. Then the wells remain pressurized so the rock doesn't collapse. Cold water is pumped into the injection well and hot water (still a liquid because it's under pressure) is recovered from the extraction well. Then hot water is allowed to expand into steam to drive a turbine, condensed, and piped back to repeat.
My read of TFA is they are taking the hot-but-not-hot-enough-for-turbines steam and using that waste heat for desalination. They do talk about using the man-made aquifer for storage, but I understood that to be freshwater storage.
(Score: 2) by stretch611 on Tuesday November 06 2018, @08:08PM (1 child)
Ok, I'm a software developer... not a chemist or engineer... So I am not sure if I am missing something in my reasoning here or not, but I think I have an obvious answer.
If you have geothermal turning heated ground water into steam vapor, wouldn't the easy way to produce desalinated water be to just cool down the steam vapor? A metal tube with cool air inside inserted into the flow of the hot steam vapor should allow water to condense on the outside and drip down into a gathering pool of fresh clean water.
If you need more desalinated water, just let some salt water drip down into the geothermal vent for it to heat up and create more steam vapor. The only issue I foresee with this approach is that if too much salt accumulates at the bottom, heating efficiency can be reduced, but if a simple method can be used to remove the salt buildup you should be ok.
Now with 5 covid vaccine shots/boosters altering my DNA :P
(Score: 0) by Anonymous Coward on Tuesday November 06 2018, @09:38PM
Yes, this works in principle. Instead of cooling the vapor as you describe, though, it would be more efficient to use a heat pump to move energy from hot steam into the water-to-be-vaporized. Such a heat pump could be powered by some clean energy source such as geothermal or solar.