Solar pond | Hacker News Reader

Solar pond

en.wikipedia.org

78 pointsColinWright11 comments

If you're curious as to how the thermal energy is extracted:

http://www.sciencedirect.com/science/article/pii/S0038092X10...

> Heat has generally been successfully extracted from the lower convective zone (LCZ) of solar ponds by two main methods. In the first, hot brine from the LCZ is circulated through an external heat exchanger, as tested and demonstrated in El Paso and elsewhere. In the second method, a heat transfer fluid circulates in a closed cycle through an in-pond heat exchanger, as used in the Pyramid Hill solar pond, in Victoria, Australia.

They use the hot brine to vaporize a motive fluid such as liquid pentane in the heat exchanger, when the liquid is converted to gas it creates a high pressure environment and is piped into a turbine which spins the generator. Then they condense the motive fluid back to a liquid so they can vaporize it again. It's the same way binary geothermal power works, with the Organic Rankine Cycle.

http://en.wikipedia.org/wiki/Organic_Rankine_cycle

A do-at-home demo: http://matse1.matse.illinois.edu/energy/b.html

Note: I haven't tried these particular instructions myself, although I did do a similar demo around 20 years ago.

They used this, and maybe still do, at UIUC to heat the Vietnamese potbelly pig barns south of campus.

>This means that the temperature at the bottom of the pond will rise to over 90 °C while the temperature at the top of the pond is usually around 30 °C.

Can that really be right, it seems like a very high gradient.

Depends upon the depth (and geometry) of the pond. Without convection, some of our intuition for thermal conductivity in liquids and gases is lost.

I'm not sold on the numbers, but I'm also not yet convinced of the idea's physical impossibility.

Water has quite low thermal conductivity (~0.6 W/(m*K)). If you exclude convection you can easily have such a temperature difference. For example with heat flux of 100 W/m2 you have 60 K temperature difference over a 36 cm thick layer of water.

Is this cheaper or better then solar panels?

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