Ok, so you have hot rocks. Now what?

April 14, 2009 10:56 AM

This weekend, I saw a green renovation show that used a large amount of gravel in the basement to hold heat gathered from a solar-collecting area in the roof. My dad, a builder, says this method doesn't work well. Help me understand if someone has devised a "better mousetrap" or if this is just something that has never worked and is being done for greenwashing on TV...

The premise: attic space with solar glass panels in the roof (which are exceptionally clear and transmit the most UV rays through) also features double-insulated windows on the inside wall to let light in. Space heats up, presumably like a big solar cooker, and using a 12" PVC tube + small fan, the heat is transmitted to a large, concrete-block-walled room in the basement filled with a few tons of gravel.

Show host says the heat from the attic space warms the rocks, whose thermal mass then distributes the heat for up to 48 hrs. But HOW?

My dad's a carpenter, and worked for a gravel company in the 70s. He said he's seen this done before but it's not very effective.

Effective/efficient are two different things... presuming the heat makes it down and warms up the rocks, sure, they'll stay warm for a while, but what's making my head hurt is HOW can this heat be used to warm up the rest of the house? Even as an adjunct heat source, it just seems as if you'd have a nice warm roomful of rocks in the basement and not any real effect elsewhere, no?

I'm interested in building a straw bale house with an insulated foundation someday and am wondering if putting a system like this would help warm the area under the first floor, but can't quite wrap my head around how this works, or would work best... thanks!

posted by bitter-girl.com to home & garden (14 comments total)

I saw that show a couple of times on planet green. What your missing is the ducts from the basement room that go to around the rest of the house.
posted by majortom1981 at 11:01 AM on April 14

I've seen designs for systems like that before, but they were based on using water instead of gravel as the thermal storage medium. It's a variant on solar hot water heating; the solar panels mounted on top of the roof are used to heat water in a tank under the house.

One of the problems with the approach you're talking about is that air has really low thermal mass, so heating up gravel that way would be extremely inefficient. And that means it wouldn't be effective, either, since the gravel would never become hot. Going from ice cold to only somewhat warmer is useless for home heating.

The water system didn't have that problem; the water was being heated directly.

One of the most clever systems of that kind I have ever heard of had only one moving part. A guy built a house in Arizona or New Mexico somewhere, and the south face of the house was built out of barrels filled with water. There was a wall outside those barrels which was hinged at the bottom, and in the winter during the day the owner would lower that wall to the ground, exposing the barrels to sunlight. At night he'd close it up again, and the barrels would help keep the house warm at night.
posted by Chocolate Pickle at 11:06 AM on April 14 [1 favorite]

Not sure how well it works, but if warm air is being continuously blown into the room full of rocks, then the air (sans some of its heat) must also be leaving that room. I'd assume that the air leaving the rock room is circulating back into the house, completing a circuit of some sort. Later, cool air (at night or on a cloudy day) being blown along the same pathways would absorb heat from the rocks and carry it wherever it goes.

I'm under the impression that systems like this can work, but must be carefully engineered / designed to work well.
posted by jon1270 at 11:08 AM on April 14

It can act as a thermal buffer assuming the gravel is in a well insulated area (insulated from the outside). The gravel heats up slowly during the day, and cools down slowly at night. It works due to the fact that air heats up and cools down much more quickly, so at night the air is cold, the gravel is warm, and the air is warmed by the gravel. Whether it works efficiently would depend on how the air can circulate around the building so you might have to force the hot air down during the day, and re-circulated it at night (I suppose it could passively circulate at night as hot air rises, but I'm not sure if the temperature difference is great enough).

I haven't heard of gravel being used before, but I have read about designs that use large rocks. Masonry, or concrete are used in buildings frequently ( a trombe wall is one term used for this) with a similar effect.

I think building straw-bale with an insulated foundation is an excellent idea. Passive houses use this approach, light-weight, insulating walls with a super-insulated floor slab. You need to ensure that the insulated wall lines up with the edges of the insulated slab though.
posted by a womble is an active kind of sloth at 11:10 AM on April 14

majortom1981 -- you're right, there were ducts-to-be on the show but it wasn't clear to me how the actual heat itself would move...even with ducts, you'd have to somehow transfer All That Heat That's In The Rocks out and around the house somehow, it's not just going to snake its way around all nicely without some form of propulsion, even if heat does generally rise...

Seems to me, a womble is an active kind of sloth, that this might work really nicely as a means of warming up a nicely-insulated foundation more so than it would for any kind of active heating...

Chocolate Pickle, I've seen some of those "water walls" (usually made of old glass jugs or winebottles or something) but not with the movable wall cover -- that's cool! would work nicely somewhere supersunny like the southwest, maybe not so much in the northeast where I want to find land...
posted by bitter-girl.com at 11:22 AM on April 14

Sorry - I may not have exactly answered the last part of your question as I meant to. If you install an insulated floor slab at ground level (and the passive houses I have seen/read about frequently do that) you would not need gravel also. What you could do, is install ducts/piping to transfer your heat directly to the slab (if it is from solar collectors on the roof) and/or for recirculation within the house. All of this working though, is dependent on a nice air-tight house, typically with mechanical ventilation (this is different from traditional 'air-conditioning').
posted by a womble is an active kind of sloth at 11:30 AM on April 14

I am not an engineer, but it seems to me that this wouldn't work very well. In pondering it, the variable my mind keeps coming back to is the surface area of the gravel. It has a much larger surface area per volume of rock than one large boulder of the same mass. This means that gravel will heat faster than a large boulder. However, I think the tradeoff is that the gravel will *lose* the heat much more quickly as well. I think about walking barefoot in my yard after the sun goes down on a hot summer day. Concrete radiates heat for quite some time, but gravel-paths cool off much faster.

Also, there is probably an upper limit to how hot you can get the rocks. Yes, you're blasting them with heat all day, but they're also radiating heat OFF as well during that time. Its like filling a bathtub while the drain is open; there's an equilibrium level the bathwater will reach, but it won't be a full bath. So "charging" the gravel with 12 hours of sunlight heat will not result in 12 hours of radiated heat. The only way to increase the heat-radiation time of the gravel is to pump more and more heat in than what is being lost to natural radiation (like turning up the water nozzle that is putting water into the bath), but that would be far more than redirected solar energy could provide.

Again, not an engineer, and as a scientist, i reserve judgment until I see valid experimentation. It's a fascinating problem to consider, though, Id love to debate it more.
posted by CTORourke at 11:34 AM on April 14

My first question was whether it would work better to capture, store, and spread the heat in water piped through the floors of the house.

Another question I have is whether it would work better if your thermal mass captured heat directly from the sun, e.g., just inside a bank of south facing windows.
posted by salvia at 11:39 AM on April 14

I actually think it would work just out of the fact that heat rises. Convection would bring the warm air up through the ducts, the air would cool in the house and be pushed back down to the basement to repeat the process over again.

I'm looking at this Environmental Science textbook, and there's a picture of passive solar heating. Air moves into a sort of greenhouse-type room, full of windows, gets hot, moves out of the room through a high vent, circulates in the house until it gets cool and sinks back down to a low vent which takes it back to the collecting room. Just replace the collecting room and sun with a basement full of hot rocks, and it works conceptually. I'm not sure about efficiency though.
posted by papayaninja at 11:40 AM on April 14

On lack of preview, the water being piped through the house is another fine idea, but it's best to use antifreeze, particularly in the cold northeast. Try looking up "flat-plate collectors."
posted by papayaninja at 11:43 AM on April 14

If you are considering this, you might be better-served by the more expensive category of phase change materials. Something like a modified and stabilized Glaubert's salt, packed in black tubes, can suck up heat. At 90 F, it melts. It takes a lot of energy to make it melt. At night, your house gets cold, the temperature drops below 90 F, and the melted salt freezes, thus dumping all of its heat back.

How much does freezing dump out? 83 calories per gram, versus 1 calorie per gram per degree Celsius using pure water.

Just as a bottle in well-mixed icewater stays at roughly 32 F until all of the ice melts, so that area stays hot until all of the salt freezes.

Other phase change materials exist, with different temperatures at which they change phase.
posted by adipocere at 11:45 AM on April 14

There's a little bit about Glauber's salt's thermal properties in its Wikipedia entry, but not enough to sate my interest there... I wonder how you'd position such a thing -- say, the salt packed into black tubes -- in a strawbale house to achieve maximum heating-up effect and heat-dumping once things cool down...

The mass of the strawbale walls, covered in limestone plaster, maintains a fairly steady temperature indoors if it's sitting on an insulated foundation, even in winter. The rub comes in how to raise said indoor temp effectively in winter for extra warmth... could the salt-tubes transmit heat to a floor-based heating system somehow?
posted by bitter-girl.com at 12:19 PM on April 14

If you're interested in phase-change materials, commercial products are available.

However, they're not as cost-effective as careful & attentive design at the outset (lots of attention to sun angle throughout the year, prevailing winds, etc. etc.). They're mainly aimed at very low-thermal-mass structures which cannot simply be retrofitted & insulated due to historic concerns (and the like).

...I mean, I'm sure DuPont would be happy to sell you their products, but at least the last time I looked they really didn't make financial sense except for very particular circumstances (that was about 2yrs ago, so YMMV).
posted by aramaic at 1:41 PM on April 14

A major drawback to air/gravel heat storage that came to light is that living spaces are moist and that moisture condenses on the gravel during the gravel-heating stage when the gravel is cold, and eventually grows mold. The mold then gets distributed throughout the house by the circulating air. Cleaning the gravel is next to impossible, other than by replacement, which is only possible if it were planned for upfront.
I lived in a later design-rev of one of these - the revision was to not circulate the air directly over the gravel, but through closed ducts laid among the gravel, which limits the heat transfer (in both directions) and thus, the storage density. Water collectors, a storage tank, and a water to air heat-exchanger in the cold return probably end up being more effective.
The above mentioned house had both systems, plus passive exchange by heating s stone floor behind the south glass.
It was a physicist's wet-dream, but there was near no one familiar with those kinds of systems to maintain them. It can work if you're prepared to GC every job involving any part of solar. On the other side, on a sunny winter day the house was bright and cheery and 70F most of the day and burned no fuel from 10am till after dinner!
posted by TruncatedTiller at 3:07 PM on April 14