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Solar Pond

Way Cool... hadn't heard about this! Thank you for reporting it!
...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.

Ice forms anywhere from thermal loss... but I'm sure this must be occurring above freezing? Found a reference ... (actually a bunch)

A clear night sky has a very low temperature, and on clear nights surfaces facing the sky radiate heat into the sky. In this way the temperature of the water in the pool could drop below the temperature of the air, and on cold nights it would drop below 0°C, thus creating ice.
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I believe the ice, not so sure the real reason is it radiating to a clear night sky.

I bet what's happening is on cold clear nights the desert surface air becomes extremely dry. A shallow pond has a large surface area and low thermal mass. The dry air probably causes rapid evaporation (~1400 BTUs/lb evaporation) which is faster than ambient heat on a cold night can radiate into the pond... and that causes the water to freeze.
 
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The dry air probably causes rapid evaporation (~1400 BTUs/lb evaporation) which is faster than ambient heat on a cold night can radiate into the pond... and that causes the water to freeze
I'll need further convincing to not see that these ponds are simply frozen when air temp drops below 0c just before dawn.
Evaporation in arid climes does cool water but will stop once the first crust of ice is formed.
 
I was reading @pinkeng's reply in @curiouscarbon's thread about heat exchangers, and for some reason it made me think that solar ponds might make cheap/practical energy storage mechanisms.

Imagine a solar farm around a Solar pond (or perhaps pv panels floating on a solar pond?), All by itself, a solar pond is 14-17% efficient and the panels are 21% efficient. So, the PV array could provide power during the day, the pond at night.

A solar-only pond operates at about ~90C. Ideally, you want hotter since the Carnot efficiency is based on Thot/Tcold. If you could raise that bottom temperature you could increase efficiency.

Could a denser bottom layer be added that would allow Thot to be increased more?
If so, you could also pump any excess solar energy into that layer during the day and then take it back out at night.
 
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I'm not sure that's Ponds are great for energy storage. they they can be used to dump waste Heat. Back in the 90s there was a pond I think in Reno. We used the pond to reject condenser water heat from a few 1,000 ton Chillers. it basically saved a couple 150 horsepower fans on a couple cooling towers. It actually worked like a charm. The problem with using it as thermo storage, is it not at all insulated. There is heat loss into the ground as well as evaporation loss and radiant skin loss at the surface
 
...or perhaps pv panels floating on a solar pond?...
Nope. Floating the panels would cool the panels, but a solar pond needs to let the light through to work. The reason the bottom layer gets hot is because the high energy solar radiation makes it to the bottom and the NCZ (Non-Convective Zone) acts as an insulator keeping the 90°C water separate from the cooler upper layer.

I'm not sure that's Ponds are great for energy storage. .... There is heat loss into the ground as well as evaporation loss and radiant skin loss at the surface
"Solar ponds" are specialized ponds. The trick to a "solar pond" is the salinity gradient keeps heat at the bottom rather than letting it rise to the top.... from wikipedia:

High-salinity water at the bottom of the pond does not mix readily with the low-salinity water above it, so when the bottom layer of water is heated, convection occurs separately in the bottom and top layers, with only mild mixing between the two. This greatly reduces heat loss, and allows for the high-salinity water to get up to 90 °C while maintaining 30 °C low-salinity water. This hot, salty water can then be pumped away for use in electricity generation, through a turbine or as a source of thermal energy.

A thin transparent surface (e.g., sort of like #2) might also prevent normal evaporative losses.

The main problem with the technology is Carnot, with a Thot of 80°C and Tcold of 20, the maximum theoretical efficiency is 1−(273+20)/(273+80)=17%. Can't go over the boiling point at the pressure less it disturb the gradient.

The 51 acre Beit HaArava solar pond built in Israel had a 5 MW output, they didn't report significant ground losses, but that could have more to do with the geology. I wonder if an aircrete liner would help in other areas? R value is ~3.9/in.
 
Ah It appears that the Beit HaArava system was using the hot water produced by the solar pond simply to make hot water for domestic / industrial processes. It was not generating electricity. It's a poor mans hydronic solar collector. Ok, that would work for a shallow pond. Everywhere in the world but the USA, uses watts for heat energy as well as electrical energy. The hot water produced in a solar pond is too low quality / low density to produce electrical energy.

Think ivanpah for solar electrical generation, which works great on paper. The problem with Ivanpah is it needs a natural gas boiler to preheat the morning water so the helostat does not waste it's 6 hours of usable sunlight just heating the water up.. 90C water could be used to run an absorption chiller to make chilled water or to produce hot water. It could not be used to make electric power.
 
Ah It appears that the Beit HaArava system was using the hot water produced by the solar pond simply to make hot water for domestic / industrial processes. It was not generating electricity.
Nope...
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
But you're right that 90°C is less than desirable. Now if we could find a way around Carnot (like fuel-cells do) or a fluid whose density increases with temperature and doesn't boil until 1000°C, oh yeah... that would be different.
 
Ah Thermal Rankine cycle using two or three heat exchangers with refrigerant. I'm scratching my head trying to figure out how they get Usable net energy out of the system. My issue is they would have to drive two pumps, one to pump the lower hot water to the evaporator and another taken from the top of the pond to cool the condenser.

Lets say they are using two 30 GPM pumps through a 2" pipe. 30 GPM at 20 degree Delta would provide 8.75kw of energy, before taking in efficiency losses in the turbine and pump energy used to pump the hot and cold solar pond water through heat exchangers. Two 2 hp pumps might use 3.8 kw of power, (once you add power factor and efficiency losses in the motor and pump impeller. So your down to 4.9kw. Next deduct efficiency losses for the turbine and your left with somewhere around 2kw of net energy output.

My point being that you need a lot of equipment (pumps, heat exchangers, turbine pump with generator, piping, storage tanks for Freon, emergency ventilation for the freon, etc). It's a lot of capital cost for 2-3 kwh. Probably a $100,000 or three. Seems you could install 10 kw of solar panels with a power wall with no moving parts, for less.

I see the reference to 5 Mw but counting efficiency losses from such a low energy source, that seems high. Maybe that's a yearly usage, which would make sense. Would love to read the original study.
 
Would love to read the original study.
Agreed! Getting power out of low temp heat is no easy feat. They may have cherry-picked the day they measured 5MW (e.g., a very cold night).
 
I think insulated thermal storage is really interesting. Saline ponds seem really effective if construction is possible. I probably won’t build one due to space constraints, but still will be attempting other water based insulated thermal storage ?

Grateful for this thread educating me about thermal solutions in climates and areas I’m unfamiliar with?

water’s specific heat capacity of ~4J/g really is handy!
 
How much energy could you get from a 1 acre solar pond?

Background
A solar pond uses a unique property of very salty water to sink when it gets hot. At the bottom of the pond the temperature can reach very high temperatures. In practice, a pond is kept under 80°C because if it starts to boil it messes up the salt gradient. Efficiency is low (~15%) when converted to electricity, but if used as heat efficiency is good. The large thermal mass allows the system to work day & night. Fresh rainwater on top isn't a problem, but dirt can make the water cloudy.

Back of the Envelope
1 acre = 4,046 m², so in winter it might see 4000kWh/d and in summer might see 20,000kWh/d. Or, in terms of electricity, 600 kWh/d in winter and 3,000 kWh/d in summer. According to this, costs about $3500 to $11000 per acre to build an artificial lake. Not sure what the generator would cost.

So, if you only wanted 100 kWh/d generation/storage in the winter with an insolation of 1 you'd need about 1/6th that, or a pond with a 48' radius.
 
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If Will had an Olympic sized pool (50x25m) and converted it into a pond, what's the theoretical power out?
(Actually, it's not deep enough at 2m, would need to be around 10'... but otherwise...)

50x25 = 1250m², insolation in Los Vegas is 2.83 to 7.62, so that's 3500 to 9525 kWh/d. At 15%, that's 0.5 to 1.4MWh/d.
 

Fatal flaw?​

In the example above, I started to wonder if there would be enough water in the bottom 3' of a 10' deep pool to hold the megawatt of energy collected - that is it would have to be much deeper to store meaningful energy.

It takes 1 calorie to raise 1 kg of water 1 degree C and a cubic meter of water is 1000 kg. So, a 1 meter depth of the Olympic pool is 1250 cubic meters and a 10-degree drop would be 1250 m3 x 1000 kg/m3 x 1 calorie/kg x 10 degrees - so 12,500 calories or 14 kWh. So, nowhere near enough.

You could probably get around that by putting a phase change material on the bottom of the pool, say something that melts at 80°C. You could use something like ethanol (boils at 79°C) for a higher latent heat change, but that would need to be stored inside tanks at the bottom making the solution more costly.
 
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