Solar Pond

[svetz]

In most bodies of water the warmer water is less dense and rises to the top, so the top few inches are warm and cold beneath that. You've probably experienced this while skinny dipping in the lake.

But, a curious thing happens when the water gets really really really salty. The warmer water falls to the bottom. That's the basis of a solar pond, the heat gets trapped and accumulates at the bottom of the pond.

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The largest operating solar pond for electricity generation was the Beit HaArava pond built in Israel and operated up until 1988. It had an area of 210,000 m² and gave an electrical output of 5 MW. (ref)

 

[svetz]

Most of the heat losses to the atmosphere are due to evaporation. Wonder if you could get a higher efficiency with a membrane over the top like those floating anti-evaporation balls? Perhaps floating solar panels whose back's are cooled by the water? But something solar absorbing that prevents evaporation and transfers the heat.

 

[svetz]

"Shade balls" reduce evaporation by 85% (ref), the cooler water is at the top, so there isn't a high thermal driving force. But, they'll be less efficient at transmitting heat, so let's assume the shade balls keep 50% evaporation (good a guess as any ?).

Then the overall efficiency of the heat capture should be .84 x .5 + 14% = 52%. That's twice as efficient as PV.

Wonder if anyone is working on that? If not, bet you could get a grant from the DOE to build one and study it or use it for your masters thesis!

 

[Rider]

"Shade balls" reduce evaporation by 85% (ref)

I've seen this before, but wondered why the balls aren't white. While preventing the sun from directly hitting the water, black would absorb heat and transfer the energy to the top layer of water. White would reflect as well as block. In my mind, anyway.....

 

[eddie1261]

I have never heard of this but I think it's a great topic to research. Which nerd boy here will do!

 

[svetz]

...Which nerd boy here will do!

Free money for digging a hole and filling it up with water and taking temperature readings at different depths. Doesn't sound too bad (except that you just salted some number of acres and nothing is ever going to grow there again...).

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[eddie1261]

It worked for the Romans when they salted the land in Carthage!

 

[svetz]

Which nerd boy here will do!

You know, without evaporation it might not freeze during winter even at cold temperatures - which means you might be able to do it Eddie. And, use the hot water to heat your place.

 

[svetz]

... why[aren't] the balls aren't white[?]

It would trigger global cooling.

 

[eddie1261]

You know, without evaporation it might not freeze during winter even at cold temperatures - which means you might be able to do it Eddie. And, use the hot water to heat your place.

Well, first I need a place where there is an acre to dedicated to a man made lake. In a city that would allow me to dig it. Then wait for rain to fill it.

 

[svetz]

I wonder from @EclecticBadger's thread on focal mirrors if some sort of heliostat panel could take advantage of the analemma shape to focus more sun on the pond? Phase 2 funding?

 

[TKB4]

Line the pond with some non permeable material and the salt wouldn't be there later to harm the land. Better yet dig the pond keeping the dirt nearby and when finished put the dirt back in so the surface 8 feet or more doesn't have high salt content when replaced.

 

[Deleted member 783]

Fascinating. I had no idea that salt water worked that way. I have a small pond on my property. It's lined with a rubber liner, but I use it for irrigation. I have a dump off spot to empty it into an arroyo just in case it becomes contaminated with something, so I'd be up for trying an experiment. My pond only holds about 10,000 gallons though.

Ranchers around here use the shade balls in their drinkers. The black keeps the balls warm enough to prevent the top from freezing in the winter, so the cattle can still drink.

 

[TKB4]

just for general info if I recall correctly the freezing point of salt water I believe is 22 degrees F but with different salt concentration it would be different. Also important physics fact that is one of the keys for life on earth is that water is different fro other compounds in that it becomes less dense between 0 and 4 degrees centigrade so in winter the ice floats and actually more or less keeps more heat in the body of water and prevents any body of water more than several feet deep from becoming a solid ice block therefore water life such as fish etc are not killed.

 

[lowrads]

In terms of salinity, you wouldn't be limited to just salts of sodium. A pond without an earthen bottom that had a lot of monovalent salts, such as sodium, would be very disaggregated, resulting in a lot of turbidity. The current experimental ponds rely on ways of reducing turbidity and algae so that sunlight can reach the deepest layers.

Also, the experimental ponds have a gradient of salinity along the water column to exploit the relative thermal conductivity of different concentrations. It's not clear if this gradient is self-managing due to the effects of the temperature/solubility gradient, or if there are membranes in use.

Simply by choosing divalent member salts, mainly calcium and magnesium bearing minerals by abundance, you would mitigate the negative impacts of sodic waters on surrounding soil, and to a lesser degree on estuaries. A common minerals that might be used would be gypsum of dolomitic limestone, which is commonly used to restore fertility to high sodium soils, or more likely some combination of minerals with higher solubilities and opposed pH effects such as magnesium sulfate and calcium carbonate.

 

[TStamatakis]

Most of the heat losses to the atmosphere are due to evaporation. Wonder if you could get a higher efficiency with a membrane over the top like those floating anti-evaporation balls? Perhaps floating solar panels whose back's are cooled by the water? But something solar absorbing that prevents evaporation and transfers the heat.

Where can you buy these black plastic balls?

 

[dell11]

If we compare to convention water pumps, the solar water pumps require very low maintenance but i am confused which is best?

 

[Sparky]

1) how do they fail?

2) how do they become less efficient / inefficient?

3) what is the payback period?

 

[svetz]

Sparky, I suspect it's an idea that's more of a curiosity now as the original studies found the useful energy is only 14% compared to say PV which is 20% and needs no additional moving parts. If you could increase the efficiency (e.g., like the evaporation balls) there might be an economic incentive; but we won't know until someone with a few acres actually studies it.

Might be a good graduate thesis project. From this ref, the claim is that shade balls reduce evaporation by 85%. From the prior study, 84% energy was lost due to evaporation and 2% to the ground, so if the shade balls could increase the energy output from 14% to (14% + .85x.84=) 85%; well that might make it a whole different ball game. Probably have to re-engineer the shade balls to be transparent on top and black on the bottom so as to not lose solar energy.

If anyone does have some land, they might be able to get a grant (see post #6) to experiment with. If so be sure to keep us informed! Unlike other forms of solar energy, the thermal mass would allow this system to keep producing energy at night.

 

[Zil]

Going off on a tangent, a wide one. I read about in the desert of The Mid East the night thermal loss into outer space alows ice to form in shallow ponds designed for that purpose.