# How Do Solar Ponds Work? **URL:** \ **Category:** In My Humble Opinion\ **Created:** [June 22, 2012, 4:48pm UTC](https://boards.straightdope.com/t/how-do-solar-ponds-work/625958 "2012-06-22T16:48:14Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![ralph124c](https://avatars.discourse-cdn.com/v4/letter/r/8797f3/32.png) [@ralph124c](https://boards.straightdope.com/u/ralph124c)\ **Post date:** [June 22, 2012, 4:48pm UTC](https://boards.straightdope.com/t/how-do-solar-ponds-work/625958/1 "2012-06-22T16:48:14Z")
A solar pond is a way of capturing thermal energy from the sun. It is basically a pool of water, with two layers: -a high density salt water layer at the bottom -a lower density, non-salty layer at the top. The bottom water heats up from the sun’s rays, and the top layer stays cool. You can then run a heat engine on the temperature difference between the layers. My question: why don’t convection currents mix the layers? ---
**Author:** ![Mines\_Mystique](https://avatars.discourse-cdn.com/v4/letter/m/d26b3c/32.png) [@Mines\_Mystique](https://boards.straightdope.com/u/Mines_Mystique)\ **Post date:** [June 22, 2012, 4:55pm UTC](https://boards.straightdope.com/t/how-do-solar-ponds-work/625958/2 "2012-06-22T16:55:19Z")
I am by no means an expert, but some quick googling led to [this](http://en.wikipedia.org/wiki/Solar_pond) page on Wikipedia. According to the article, there are actually 3 layers, the top-most is fresh water, the middle has a salinity that increases as the depth increases, and the bottom layer has a uniform salinity. The middle layer acts as a barrier to keep the convection mixing from occurring. The mixing is prevented because the salinity gradient makes the water denser as the depth increases, while the temperature gradient tries to make the density decrease as the depth increases. These 2 effects cancel each other out, preventing the mixing from occurring. ---
**Author:** ![wevets](https://avatars.discourse-cdn.com/v4/letter/w/e9a140/32.png) [@wevets](https://boards.straightdope.com/u/wevets)\ **Post date:** [June 22, 2012, 5:01pm UTC](https://boards.straightdope.com/t/how-do-solar-ponds-work/625958/3 "2012-06-22T17:01:10Z")
In solar ponds there is also a salinity gradient, but in the opposite direction of what you mention. You want the high salinity water on the bottom, because it is more dense, and under the right concentration of salt and temperature it will _not_ lose enough density to cause a convection current. Your top layer is your low-salinity layer because likewise, it will not sink due to a higher density than your bottom layer. There’s also a third layer between of intermediate salinity, that helps to separate the two. … ETA: yes, what **Mines Mystique** said. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [June 22, 2012, 6:18pm UTC](https://boards.straightdope.com/t/how-do-solar-ponds-work/625958/4 "2012-06-22T18:18:22Z")
Plagiaristically like Sherlock Holmes. August Derleth shoulda been sued. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [June 22, 2012, 7:00pm UTC](https://boards.straightdope.com/t/how-do-solar-ponds-work/625958/5 "2012-06-22T19:00:46Z")
While I understand the basic concept I was always kinda surprised that diffusion wouldn’t render the things unoperable in relatively short order. The perfectionist in me couldn’t help but put a thin layer of clear plastic between the the different layers of water. My theory is they work like magnets 🙂 ---
**Author:** ![DocCathode](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/doccathode/32/18773_2.png) [@DocCathode](https://boards.straightdope.com/u/DocCathode)\ **Post date:** [June 22, 2012, 8:39pm UTC](https://boards.straightdope.com/t/how-do-solar-ponds-work/625958/6 "2012-06-22T20:39:20Z")
> [@CalMeacham](#): > > Plagiaristically like Sherlock Holmes. > August Derleth shoulda been sued. Curse you! I wanted to be the one to make a Solar Pons joke! > **[Solar Pons](https://en.wikipedia.org/wiki/Solar_Pons)** > > Solar Pons is a fictional detective created by August Derleth as a pastiche of Arthur Conan Doyle's Sherlock Holmes. > Robert Bloch wrote of the series, "During a span of a century there have been literally hundreds of Sherlockian imitations, ranging from parody to direct duplication, but no one except August Derleth ever succeeded in capturing the essential charm of Doyle's original concept... To Pons's exploits he brought not only expertise but evident expression of his respect, appreciation, ...