How useful would underground saltwater be?

[farmerben]

Many parts of the world suffer a shortage of water, and are too high elevation to make conventional aquaducts practical.

Where irrigating deserts is practical it results in at least one bad side effect.  Freshwater brings forth salts from the soil.  And where evaporation is much greater than drainage brine and salt builds up at the surface.  The ground becomes alkali.

My idea is to use salinity as an advantage by siphoning it to the bottom of the sea.  

A sea water canal that drops 1 ft per mile could carry seawater hundreds of miles inland through underground tunnels.  Industry above the canal could tap seawater with wells.  For each well we need a weir in the seawater canal to prevent back flow and back mixing.  Industry can take seawater from the well, and dump all their brine into the lower levels of the system.  

 At the two extreme ends of the system we have one unbroken siphon that runs from -500 ft below sea level on the land side to -5000 ft at the bottom of the sea.  The outlet of the siphon is in balance with the sea.  If we add brine to the top, it flows out on the bottom of the sea.  The input canal is something like 5 times larger than the siphon.  We can juggle those numbers quite a bit either way by changing certain parameters.  

In California it is possible to bring so much seawater into the Salton Sea to bring it's salinity way down.  In Death Valley it is possible to create a new sea very similar to what the Salton Sea is today.  We could make either sea slightly larger or smaller than what the Salton Sea is today depending on simple engineering choices.  

If we ever need to get the siphon really moving, we can spike it with calcium chloride which is way denser than sodium chloride, but abundant and natural in the sea.

[Scotius]

A lot of work for dubious gain. Are you planning to irrigate potential fields with seawater? If not, it would be simpler and cheaper to build nuclear desalinization plant on the shore and pump fresh water to hydroponic farms built in the desert. Less water losses and no problem with ground salinity.

[farmerben]

1 hour ago, Scotius said:

A lot of work for dubious gain. Are you planning to irrigate potential fields with seawater? If not, it would be simpler and cheaper to build nuclear desalinization plant on the shore and pump fresh water to hydroponic farms built in the desert. Less water losses and no problem with ground salinity.

Probably a dubious investment.  

There would be specific brine fields, because just letting the normal desert environment evaporate seawater is the cheapest way to desalinate it.  Seawater could serve many cooling purposes without being pre-desalinated.  

If you pump freshwater out onto the desert, you still get a salinity problem.  Only land where some of the water flows to the sea before fully evaporating avoids buildup of alkali.  This setup could reduce the salinity of the Salton Sea.  

Losing water only at the well heads is the idea.  Losing all the salt and about 1/5 the original water at the bottom of the sea.

 

Theoretically it costs the same amount of electricity to operate wells as it does a pump at the coast and pipes that run uphill.  But actual costs will be completely different.  A pipe that runs uphill requires a certain high capacity pump that does not scale with demand.  The wells can use as much or as little electricity as they like at any time.  The underground flow requires no electricity.  

 

[kerbiloid]

Why shouldn't the water reach the sea level in the wells eliminating any flow?

(If I get this right, the maroon line means a sewer pipe?)

Edited October 27, 2019 by kerbiloid

[Nuke]

5 hours ago, Scotius said:

A lot of work for dubious gain. Are you planning to irrigate potential fields with seawater? If not, it would be simpler and cheaper to build nuclear desalinization plant on the shore and pump fresh water to hydroponic farms built in the desert. Less water losses and no problem with ground salinity.

presuming you do this in the desert, where its solar thermal is most optimal, you can run a desalination plant purely on solar power. you can therefor support food production in places where it would have otherwise been impossible. 

[Nuke]

3 hours ago, kerbiloid said:

Why shouldn't the water reach the sea level in the wells eliminating any flow?

(If I get this right, the maroon line means a sewer pipe?)

im curious if the change in salinity is enough to provide the pumping action. assuming the intake is below sea level and output significantly deeper. assuming at each tap the salinity will increase and increase the unit mass of the water.  so long as the salinity gradient in the system is steeper than that of the ocean, you will see a positive flow rate. you might even be able to generate power from the flow, though this power is a secondary product, you need the surface industry to ramp up the salinity which powers the whole thing.

[Nightside]

4 hours ago, Nuke said:

im curious if the change in salinity is enough to provide the pumping action. assuming the intake is below sea level and output significantly deeper. assuming at each tap the salinity will increase and increase the unit mass of the water.  so long as the salinity gradient in the system is steeper than that of the ocean, you will see a positive flow rate. you might even be able to generate power from the flow, though this power is a secondary product, you need the surface industry to ramp up the salinity which powers the whole thing.

Would you also end up with a giant battery?

[kerbiloid]

Spoiler

With similar salinity gradient in the sea and in the wells. Seems to me, that's what would happen.

Edited October 28, 2019 by kerbiloid

[Nuke]

1 hour ago, Nightside said:

Would you also end up with a giant battery?

when the sun goes down and desalinated water production tapers off, the brine dumped into the system will want to work its way back to the ocean. how long this takes will depend on the length of the tunnels. so it would have some energy storage capability. i dont expect it to be huge and dependant on the quantity of waste brine produced. 

you also are going to need to have to store a lot of desalinated water somewhere. pump it to an uphill tank in the day time when your solar thermal plant is operating, and run it downhill through a turbine at night. between these storage sources and the solar thermal plant, you have enough infrastructure for a rural farming settlement. 

[kerbiloid]

Why at all put the brine back to the ocean instead of drying it in bricks?

Edited October 28, 2019 by kerbiloid

[farmerben]

The seawater supply can drop 1 ft per mile due to gravity.  Back flow is not allowed.  

If 1/5 of the water is used to flush the brine it is about 10% denser than seawater.  Therefore you could run a siphon from -500 ft to -5000 ft.  That is to say 5000 ft of ocean has the same pressure as 4500 ft of brine in a pipe.  If none of the water is evaporated from the wells then the system would stagnante, but not be damaged.  

In terms of being economical, a rural farming settlement could afford to build and operate its own well.  The entire thing could be justified if you think centuries from now Nevada could have an entirely different future.  Where millions of people would benefit who are not here to pay for it now.  

Once we desalinate water we want to be able to move it long distances.  Gravity does that the best.  The location of Death Valley is ideal with 14,000 ft mountains nearby.

In terms of raising water, it takes minimal energy to lift steam to high altitudes.  The problem is the area of steam piping required to get condensed water at the top is astronomical.  The practical thing is just discharge steam on the east side of the Sierra Nevada.

I was messing with figures like 100 tons of seawater per second... 3.5 tons of salt per second is a lot.  It's a lot of steam too.