I would assume that the higher density of the rock would allow for better heat retention than water alone, while the water would increase the thermal transfer of the system.

Seems like a win win to me.

Keep them separate, or just use water.

Their thermal absorption-desorption rates are different, as is their thermal mass.

If you put the rocks in the water, it alters the way energy is taken in and released.

This may not concern you, but given the choice, I'd rather have a small pond and some rocks over plastic jugs of water with rocks inside.

But then, I live in the desert, what do I know?

The overall design of the system must be considered.. Having the water open to the atmosphere with rock mass heating the water would raise the humidity level perhaps to an unhealthy level, promoting fungal growth and such...In a closed system it may slow the release of heat, but this could be accomplished in several ways without the rock....I would think a ground loop of sufficient size would work well, rather than a static system...I would also have a CO² boost system...

Very interesting question you have asked...Ok, I will blither a little.

More water than rock means faster (and more) thermal uptake, and unfortunately outflow. Since you are in the great white north, you may be at equilibrium by midnight (that being a constantly moving point as well).

Your average temperature during your anticipated growing season will play a factor here.

I think you will have to marry a mixture that will give you a lower thermal storage temperature with a longer storage time.

I'm guessing lots of course rock in a water "bath".

The accurate answer to this one might be two years of data collection...maybe you could sell it.

This is what I was thinking:

To come up with a more structured design, real numbers like greenhouse volume, an ambient temperature profile for your area, what materials you have on hand to work with, how much footprint is available for the system, and etc.

Thanks for the comments. The problem of moderating day/night extremes in greenhouses seems to be universal, from what I've been reading. It's too hot in the day, and has to be ventilated to get rid of the heat and then is too cold at night. Everyone has the same problem, more or less.

I have an attached greenhouse which is 12X28 feet on the south side of my home. Not a lot of extra space for pure storage mass, so that is a consideration as well. The plants are in aboveground containers - old fish tubs - which also function as mass and release moisture into the air, so any additional water storage mass has to be closed.

The floor of the greenhouse is stone - big rocks with a flat side up in a bed of chipped rock. The mass in the floor really only functions on a seasonal basis, afaict, making the greenhouse moderately warmer into the fall months as long as we had a decently sunny summer.

Water containers used presently are fairly small - 5 gallons or less. I'm looking to add more water storage to the system (so far haven't identified the ideal container, other than the 5 gallon buckets which are sturdy and have tight lids.) On two cold nights in April, I filled 5 gallon and smaller containers with hot water to serve as radiators - that saved the day, but they were all completely cold by morning, for sure. I used some one and two litre bottles right next to the plants as well.

I also have some bags of rough stone I've been storing out there under a table - I need the space because the plants are getting big now, but I'm reluctant to remove any mass at all at this point in the season. So I am tempted to sort the stone (by colour not size, which varies) into some extra 5 gallon buckets (which can be neatly stacked), cover with water and.... yes, this is where the curiosity about rock/water storage came from. The rough, incidentally, is mostly jasper - not especially heat sensitive, although I wouldn't pour boiling water over em....

More information would be useful.

1. Glass greenhouse?

2. Heated?

3. Insulation ?

I gave up on my plastic greenhouse after the first year, because it was too hot or too cold for plants, even with ventilation. I live in a St. Louis, MO type climate. I have some pavers and a lot of cinder blocks for shelving, but it wasn't enough to keep things warm enough in early spring, and was too hot in the summer. I have been thinking of putting stock tanks in there with fish, for more moderation. If you have all that water maybe add aquaculture. You might want to see The Winter Harvest Handbook:http://www.amazon.com/The-Winter-Harvest-Handbook-Greenhouses/dp/1603580816/ref=sr_1_4?s=books&ie=UTF8&qid=1336617122&sr=1-4

Ron, it is glass, double glazed. Not heated. Side walls are insulated.

Moderating greenhouse temperature extremes does seem to be a universal problem. I don't have room for aquaculture, but if you did, I think you'd need fish that are very tolerant of temperature extremes.

Here is a link that may help you decide.

http://www.engineeringtoolbox.com/sensible-heat-storage-d_1217.html

I read about a greenhouse operator that raised rabbits for market, and used their body heat to heat the greenhouse.By Easter,he had plenty of rabbits for sale, and used the money to restock his greenhouse and buy starter feed for the next generation of rabbits.The rabbit manure was used for fertilizer, and dead plants were used for rabbit feed.

Rabbits have a very high metabolism, and the cages were below the plant beds.Very little additional heat was required during the winter.

Here is another link that may help:

http://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html

You would also be surprised at how much heat a bale of hay can produce while decomposing, as well as being an excellent insulator.This is true green technology.

Good luck.

I've seen these animal systems on the internet, chickens work as well - very good way to use a hoop house.

My attached greenhouse is a bit small for animals. Maybe another hoop house. I would have to deal with the foxes first, though. I'm in a wild kingdom.

Great idea, I never thought of using animals for greenhouses. Rabbits are quiet, and their poop is not too hot for direct use. Fertilizer savings are a big factor too. Great for someone who is comfortable with slaughtering rabbits. Some rabbit pelts have value also. Another idea would be to build a nearby biomethane digester, and use the gas for heating. Lights for the short winter days would supply part of the heat also. Composting is also an excellent way to go.

Water releases heat more instantly than rock. submerging the stane will not help. u keep two thing separate. rock releases heat relavily slowly and for long time. putting water on rock will nullify the advantage of slow heat release as heat transfer from rock to water is too quick.

Why not (instead of venting the heat), pump it into the ground instead? Maybe straight into the bed that has the most heat loving plants?

I am making a greenhouse using a design as I go method. I got 5 sheets of free glass when I moved and the whole thing is a "lean to" with a mish mash of different glass sizes. I am not attaching it to the fence because the fence is so old that it waves in the wind and is to a neighbours anyway. I plan plywood for the back wall and to have reflective mylar on the plywood and maybe shelves or windowfarm type hydroponics there. So all my plant starts would be on that back wall. The glass is so darn big that I leaned the roof back instead of forward. This is not as daft as it sounds. The fence is exactly east west and the summer sun sets slightly north of west so comes in more directly in the evening than if it leaned "normally". I think that that will mean more even temperature during the day. (A bit more light will bounce off the glass at midday than if the roof was normal. It also means that the gutters will not interfere with the light coming in. (Because they are on the back).

http://www.facebook.com/media/set/?set=a.10150925789340767.477288.736625766&type=3&l=05cab04462

I have not put in the door or last sheets on roof yet. (Have not figured out ventilation so don't know where best to put an opening skylight or small hight side opening window.) but maybe a solar powered fan sending the heat into the ground would be a good option. Has anybody done aquaponics? The pond would probably be outside the greenhouse so as not to stress the fish. Brian

I'm very interested in your roof angle design - let me know how that works out. If the design reduces heat capture during the hottest part of the day, it could be a good strategy.

I would love to pump that heat into the ground - into the stone floor if possible. My resources are limited at present but this is definitely a future goal - minimize waste of heat when it is available.

The amount of sunshine required to raise air temperatures inside a greenhouse is ridiculously small.

Hi, artsmith, I added a few pictures to the facebook album and I also did a video to try to explain the roof angle thing. My greenhouse is a lean-to because I got the free glass (BUT not enough to do a full greenhouse). It leans back instead of forwards because the glass panes are so HUGE and I had difficulty handling them on my own. I just looked at two pages of lean-to greenhouse images and not a single one leans back instead of forwards! To my mind, that is daft. the more north you go, the more useful a leaning back lean-to greenhouse will be. AND we have height restrictions on fences here. You would need to be about 4 ft tall to fit in a leaning forward greenhouse (attached to or close to a boundary fence) here in Victoria! Maybe some of the engineers here have software that is sufficiently good and they could look into a performance comparison between a forward leaning lean-to and a backward leaning lean-to? The key things for me is that the roof will reflect or bounce away more of the midday sun (because of the roof angle) but (again because of the roof angle) it will catch more of the morning and evening sun. In theory at least, this should have more stable temperatures and light input than the forward sloping lean-to. https://www.youtube.com/watch?v=Pd9xQiZxKlQ

What do you think?

^{The typical greenhouse is oriented to maximize solar gain.. but I agree, the optimal orientation would be to maximize gain in winter vs in summer, and limiting heat gain in the middle of the day is a good idea. I don't have the expertise to comment on how your design should do, why not post the question and you will get answers from better qualified people.... You're still going to get plenty of heat gain through the front pane at midday, but the north sloping roof glass will take in less afaict... Let us know how it works out.}

Water has the highest volumetric heat capacity of any substance:-

http://en.wikipedia.org/wiki/Heat_capacity#Table_of_specific_heat_capacities

So forget about the rocks.

If you're trying to restrict the speed of heat transfer you could insulate a tank and control water flow to and from heat exchangers: (radiators) above the tank at night and (solar thermal absorption units, OK, coiled black hose pipe) below the height of the tank during the day.

wikipedia has a good artical on "latent heat" that might interest you.

Yep: going through a state change enables you to store vastly more energy than simply utilising the specific heat capacity of any substance, and, using a suitable state change material may be a much better and simpler way to maintain a constant temperature in a green house.

I think some insulation glass wool is coated with a sort of wax that melts at a lowish temperature. So we just have to find what material. Maybe coat the glass wool thickly and let the hot air circulate through it? http://web.mit.edu/invent/iow/telkes.html is a worthwhile link. Maria Telkes was a leader in solar energy research. The Dover house is one of her great achievements. She used sodium sulfate with phase change to store the energy. She also did great work on solar ovens.There are links that go into detail about the Dover house. I have to run off to work so search yourself for other links. The one above is from MIT so it might impress engineers.

They are having a discussion about thermal storage for small sheds right now in the "simply solar " yahoo group. http://tech.groups.yahoo.com/group/SimplySolar/

They are a pretty great group. They gave us the screen collector (black or grey flyscreen as a solar collector for warming air) and probably the downspout collector too (also for air warming). A few members do side by side tests of different solar collector types. Including water heating. Of all the types they have tested the 2 leaders (by quite a margin) are flyscreen and downspout. They could also give good advice on where to put your fan or pump and where to put your snap switch to turn it on and off, etc. Brian