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What rocks trap and hold heat best?
November 11, 2011 6:38 AM   Subscribe

What kind of rocks collect, hold and release heat best?

I want to use some rocks in my little pop-up greenhouse to collect heat from the sun during the day and release it at night to keep the plants warmer.

The plants in question live outside most of the year at my Zone 7 home, but I can't bring them inside during the winter because the cats destroy them. There are some geraniums, a spider plant, two big hibiscus [hibisci?] and a few other generally green things that I've inherited but can't identify.

Are there particular kinds of rocks that would work best t for this purpose? Dark ones, right? Beyond that, I have no clue. Rocks aren't my thing, and I'm only barely scraping by with these plants. :)
posted by mccxxiii to Home & Garden (14 answers total) 3 users marked this as a favorite
 
I have seen greenhouses that use water as a "heat sink" One I have seen uses huge floor to ceiling cylinders filled with water at the back of the greenhouse.

You could do something similar, and painting them black would help too! I'm thinking milk jugs, or those plastic water carriers used for camping.
posted by hollyanderbody at 7:22 AM on November 11, 2011


I have seen greenhouses that use water as a "heat sink" One I have seen uses huge floor to ceiling cylinders filled with water at the back of the greenhouse.

Water is ideal for colder temperatures because freezing water is exothermic. That is to say, as water freezes it gives off a good bit of heat to the surrounding environment. There's actually quite a lot of energy stored in liquid water that gets released as it transitions to a solid phase. This works nicely in a greenhouse where the temperature varies, but obviously is not really useful if the temperature is always under freezing.
posted by three blind mice at 7:56 AM on November 11, 2011


IANAPhysicist, but my understanding is that the kind of rocks you seek are easily identified because they are:
  1. Heavy and
  2. Black
I'm not sure there's anything else to it. Perhaps surface area: rough rocks are better than smooth, and many smaller rocks are better than one big one. But as mccxxiii points out, anything that's massive, black, and exposed to the sun at least part of the time, will do what you want.
posted by spacewrench at 7:57 AM on November 11, 2011


Oops, s/mccxxiii/hollyanderbody/. Sorry.
posted by spacewrench at 7:58 AM on November 11, 2011


In terms of a fairly casual design, you're right, dark rocks are what you're looking for. Greater density of the rocks will provide greater thermal mass, but when it comes to most rocks their densities aren't terribly different for this kind of use. You'd probably want to avoid something on the 'lighter' end of the spectrum like pumice or coal, but most common mafic rocks like basalt and andesite would be fine. Equally, black bricks or painted concrete blocks could work just fine.
posted by pappy at 8:01 AM on November 11, 2011


Spacewrench: Really? My (wild-assed) guess is that larger rocks will hold heat longer than gravel if your goal is to keep the heat going longer. I'd think more surface area would result in faster heat dissipation.

Metafilter: massive, black, and exposed to the sun at least part of the time
posted by rmd1023 at 8:43 AM on November 11, 2011


Fire bricks.
posted by weapons-grade pandemonium at 8:47 AM on November 11, 2011


Three blind mice makes an excellent point; water as it freezes gives off more than enough heat to raise that same quantity of liquid water from 0 to 75 degrees C.

As I read over the various answers, I am visualizing the smallest size wading pool for children filled with basalt sand, and with enough water added to it that the water level would be less than an inch from the surface of the sand, with your plants in their pots sitting widely spaced on that sand surface.

This would have the added benefit that water vapor coming out of the warmed sand would condense on the leaves, and water when it condenses gives off more than 5 times the amount of heat it would take to raise that same quantity of liquid water from 0 to 100 degrees C.

An inch of basalt sand over ordinary sand would probably be just as good.

Plus, you could embed a curved sheet of aluminized plastic along one side of the pool to reflect sunlight back onto the sand and the plants-- but you'd have to be careful not to cook the plants, which you could probably avoid by making the reflector no higher than the rims of the pots.
posted by jamjam at 9:07 AM on November 11, 2011


Fire bricks would be ridiculously expensive for this application. Water is excellent. I don't think you'll find a huge difference in performance from different rocks, especially as compared to the time and effort required to seek out and identify them.
posted by ssg at 9:12 AM on November 11, 2011


I don't know anything about gardening, but Basalt is most commonly used for hot stone massage because it holds heat so well.
posted by purpletangerine at 9:20 AM on November 11, 2011


Soapstone is another heat retaining stone.
posted by gudrun at 10:15 AM on November 11, 2011


You can use large black painted buckets/drums filled with water as a thermal mass to retain heat, advantage is that they are easier to get into place and then just fill. Obviously leaving a gap to allow for freezing and thawing of water if needed. Even painted soft drink bottles if you have a small cloche or something.

I think you'd be Ok with pretty much any dark stone what you want though is to fill any empty spaces with thermal mass.
posted by wwax at 12:45 PM on November 11, 2011


Three blind mice, I humbly point out that while the green house I've observed IS in a climate that freezes, the water inside the greenhouse never does, nor does the temperature inside the green house. I still think that water jugs, painted black, sitting in the sun, will absorb, and then emit heat. I am NOT a physicist though.
posted by hollyanderbody at 1:12 PM on November 11, 2011


My (wild-assed) guess is that larger rocks will hold heat longer than gravel if your goal is to keep the heat going longer...

I think that's correct for the heat-emitting part of the cycle, but it works the other way for the heat-absorbing part: gravel will absorb faster than a big rock. Easy come, easy go.

What you probably want is something that's just the right mass so that it can't quite warm up to the daytime temperature. If the whole rock gets as hot as is possible given the ambient temperature and sun, any additional energy that is available after it reaches that temperature is wasted. But that "just right mass" depends on the rock's starting temperature, the ambient temperature and the amount of sun.
posted by spacewrench at 4:31 PM on November 11, 2011


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