This might be an old idea, as far as I know I just made it up, and would love to know if it's been done, if it worked, etc....
I'll be building a greenhouse soon, I have some 55 gallon food grade barrels I am planning to fill with water for thermal mass. I heard that they are slow to heat up, and that smaller works better. I'm thinking make a column of barrels (2 high) and next to it a stack of 5 gallon buckets to the same height, next to that a stack of coffee cans to the same height, then buckets, then barrels, buckets, coffee cans etc to make a wall that is wavy. I think that would make it so the smaller diameter column of water heats up fastest, discharges fastest, the mid size will heat and cool slower, the large slowest to heat, but also slowest to discharge.  I'd probably paint them all black.

Pros and cons of this? Anyone done it? Did it work?

I used a 5 gain bucket in a small green house and seen them in others. Make sure they don't get cold enough to freeze out themselves. Also consider sunlight quantity and thermal mass means slow to heat too. I used to boil a pot of water and dump it in the 5 gal bucket on cold nights I didn't want to bring the seedling in. I think it helped. If your making a tower, make sure it's stable. Those buckets are too heavy to be falling while your planting. Good luck!

The problem with 55-gal drums is that the water in the barrels doesn't heat completely.
Instead. it stratifies into being warm at the top of the barrel and then gets colder as you move
towards the bottom.

Better to stack smaller containers on top of each other.

I would actually move the small containers close to any tender plants you have for some localized protection -- they will provide very very (probably tiny) small amounts of thermal storage and are best used close to plants. I'd also think about how that arrangement affects your overall floor plan.
I just wrapped up a blog post on this topic (water barrels as thermal mass) actually - http://www.ceresgs.com/tips-on-using-water-barrels-in-a-solar-greenhouse/

I don't have a green house, but thermal mass is something I stumbled on a few years ago.  I used 12" X 12" X 2" pavers in my garden to walk on.  All around the pavers the plants grew the best!  Nice warm masses helped warm the soil and speed up growth.

I have since used dark rocks in different areas where I didn't want pavers and they work great.

I think the idea of having several "sizes" of thermal mass makes sense. By having some that can take in heat quickly (and give it up just as fast) along with medium time frame and long time frame storage you come closer to approximating a system which can tend to keep its  temp stable over much longer periods of time. (for the nerds, the idea is to approximate the Fourier transform of a square wave) The key being the ratio of the surface area to the volume of the containers.

That said, other things come into play too, such as the previous comment about 55g containers stratifying and issues of how much growing space you are willing to devote to heat storage. Personally I like the SHCS which pipes hot air underground, heating the soil of the GH. A lot of thermal mass there, which is out of the way, and warmer soil helps the plants.

I have a 3 storey old home well insulated with a foam inner thermal break.  I use thermal mass of various types to maintain a fairly constant 70 degrees F in this house.  Water is an excellent cheap thermal mass and I doubt that it really stratifies much over the height you would stack these.  It would be important to add additional heat to the greenhouse over a period of time to bring the water mass up to a desired temperature and then let it radiate back into the space when you are not wanting to add that additional heat.  Temp variations will not be severe if properly sized for the space and loss and besides, the plants will tolerate some variation naturally especially as they shift from daylight to nighttime.  If you are able to pass some tubing that will carry some heated water from your heating source into the bottom barrels, the heat will rise up into the upper barrels and radiate back out.  Heating the ground is another good idea already proposed because most of that heat will rise back into the space except for the outer perimeter that should be somehow provided with a thermal break for maximum retention.

Hi Pearl, I did some experiments a while back because I was also interested in what sized containers would work best. It was published in Growing Green International (Magazine of the Vegan Organic Network) but I have made it available here as a PDF. Hope this helps. Tony
http://www.what-is-permaculture.co.uk/wp-content/articles/water-temperature-moderation-permaculture-design-greenhouse-GGI-33.pdf

Two problems, first, the thermal lag of water is very short, so it discharges all its heat very rapidly. Secondly, unless you have a means to keep the heat in (i.e. insulation), your heat gain is not worth much. Your best bet is to place some insulating material under dark pavers (thermal mass), in a place where they receive direct sunlight.

Hi Elizabeth,  "the thermal lag of water is very short". The rate of discharge and charge will be effected by the volume of water, container material and thickness of material and the temperature differential between the air and the surface of the container.
To slow down the thermal transfer speed of water whilst retaining it's high specific heat capacity you could add a preservative and a gelling agent then it would act more as a solid, or even add some sponges to reduce convection currents.

Secondly whilst the slabs would store some heat you might be better off adding insulation around the edges of the greenhouse and thermally connecting the slabs to the mass of soil below. You may also get some heat gain in the depths of winter from the ground as per earth sheltered greenhouses.

Thank you all for commenting! I enjoyed reading the links!

The greenhouse in question will be good sized, starting with the frame from a 2 car metal carport, extending off of it a bit bigger to get my south face slope. It will be insulated to hell and back, sealed well to keep out drafts (cold wind is almost more issue than temperature on this site) whole north wall is solid (non-opaque) and heavily insulated. I can't get away with earth sheltering it on the north side (my slope faces north, not south) but I'll probably work animal housing into that side later to further insulate the north wall. Insulation extending down well below the frost line, and I'm hoping to put in air tubes for underneath heat, there's a patio that will be cement right near it (lovely south exposure) that I'm hoping to tube under for house air pre-heat, and I may hook the greenhouse into that system also, so the air intake is at least preheated by the slab, if not the house (my house exhaust air may go into the greenhouse) (and the excess heat off the greenhouse may be venting into the house also, and humidity in winter is a good thing for my health.) The house will also have cooling tubes that come from way down the north slope, I'll hook the greenhouse into them for summer cooling help (as well as openable windows, etc.) Definitely circulation fans to take heat off the top and put it down to the floor. Pavers or something similar on the floor, also adding mass. So there's enough room for 55 barrels if I want to use them. I am planning to put waterbed heaters on the side of the barrels (down low) so I can heat them if I need to, which might do interesting things to the stratification, that would be my major supplemental heat if needed on the place. Main purpose of this structure is to give me a zone change, I have some plants I'd LOVE to grow that want to be a zone or two warmer than the air here, and we want a sunny spot to hang out in when it's cold, etc.

I'm glad the idea of using multiple sizes of mass makes sense to others (thanks Steve Sherman and Tony Paul Martin!) I think it would release heat in a more stable manner (I was trying not to use the words "Fourier transform of a square wave" but I had that idea.) I like the idea of putting smaller ones around the plants (thanks Lindsey Schiller!) I'll probably put a pretty curved line out off my wavy back wall of the small ones around the bases of some of the the plants. Maybe use those for small retaining walls (sort of) or at least that look, putting tile on top of them (just sitting flat on the lids) might add a bit more mass and look snazzy. And I also liked the idea of painting them colors other than black, blue or purple would be my favorite choices, not a fan of the color red, I'll do black before red unless I need it for wavelengths.

Dave Colglazier: My last house had 2 foot thick rock and concrete walls inside and out (old place) took the mass 3 days to go up or down in temperature at season change, but it held stable for a good week after that. During things like winter storm heater failure and air conditioner failure, the temp in the house was still tolerable for that week. Thermal break on the exterior would have been nice, but couldn't happen. The plants I had along the walls were really happy with that mass, both inside and out. It's experience from dealing with that thermal mass that gave me the idea of differing sizes of mass to release slower and faster to stabilize the temperature. The walls there were so bulky that if I had to heat the house fairly quickly (in less than three days) I'd actually face a heater to the wall, heating the air was a waste of time.

Thank you all for the thoughts!! Glad it's not a totally wrong idea!! I'll try to remember to post data on how it works, but it probably wont even be built for a year or more, we are building the house this summer. What I'm trying to do is get my design straight in my head, so I can make sure things like air tubes and vents in the house are installed while the house is being built, so I can put in the greenhouse later and not say "oh DAMN, I didn't leave a connection point for that!!"

Sounds like a lot of effort which should be well rewarded. One thing that will also help. Heat is lost through conduction, air currents but also radiation. Night time reflective blinds automatically drawn over could make a huge difference (even if you are using double glazed panels in your green house)
Also if you get any condensation you may be able to channel it somewhere useful. Exhaust gases from your house will contain a small amount of extra carbon dioxide which may make a small difference. Above all have fun and experiment

Experimentation with different sizes of water containers to be used in solar greenhouses was done back in the 1970's and the results published in 1978 by Rodale Press
to "marry the fine work of formal researchers with the work of many informed backyard researchers."

In the 'Solar Greenhouse Book' edited by James C McCullagh, there is chapters on orientation, glazing, construction and managing--with several pages on heat storage.

Experiments with 55-gallon drums show that there is indeed separation between the top and bottom levels of the drum which causes the top portion of the drum to lose
its heat 15 to 20 percent faster than if the drum water was well-mixed.  It was suggested that the surfaces of the drum not heated directly by the sun should be
insulated--ie: the top of the drum and the bottom half.

The research showed that smaller containers stacked directly on top of each other--not separated by a shelf--eliminated most of the problems associated with 55-gallon drums.

This project is indeed a lot of effort which should be well rewarded.  A copy of this book can be bought used from Amazon for around $20 good condition--or you can experiment
and spend a lot of money to "reinvent the wheel".  

Many useful comments here. I have not been a poster here, but this is an interesting subject.  8-9  years ago I recycled a 35' dome greenhouse built originally  for the high cold areas of Colorado. I then  took inspiration from "passive solar greenhouses" and insulated the north 1/2. I am at lat 32 and  8k feet, so southwest mountains. I studied water as diurnal storage and installed 32 each 55 gallon barrels, ( which became my planting benches) Also a 2500 gallon fish tank. All I can say is that the structure without any heat except the sun has dipped below freezing only one time in that span when a terrific norther blew out a north window ( 70-90 mph, 4' snow)  That one did make mush of the bananna trees, but also helped by beating pests back, and all the remaining vegetation came roaring back.
As I spend 6+ months a year out of state for work, this thing has to run on autopilot a lot. Water has been the salvation. Also go to BUILD IT SOLAR.COM, several good articles, and data on water as diurnal heat storage and temperature tempering.
I tried to work on the stratification issue which is real, too much a pain frankly. Anything with pumps and plumbing is a full time job. I say that as a completely off grid, completely solar, completely handy as hell person.

Sounds like a fun project, good luck with it.

You might look at some of the SHCS designs. While pulling hot air from up high and bringing it down to the floor is good, you will get better heat transfer if you run that hot air in tubes under the soil. The surface area will go up quite a bit making for better heat transfer. I mention it now because it is a lot easier to run the underground tubing before you have your floor and interior in place than later.

I am in zone 6a too. I have 2 55 gallon drums stacked for makeup water in my aquaponics greenhouse. When I fire up the wood heater and the GH warms up the barrels sweat indicating the cold is leaving the barrels. The barrels are on the opposite end of the GH as the heater. Today I was contemplating if they were moved next to the heater they would absorb more heat. You could stack containers all around the heater. My heater is in a closet type area on the side oft the GH but open to the GH. I'm thinking ift the closet were wider and I could put the barrels inside it they would absorb even more heat. I have a design for another option using a RMH to heat the not only the GH but my fish tanks as well. 

I have a greenhouse on my property,too.  It is insulated on three sides--2 x 4 with wood planking outside
and in and fiberglass insulation in between.  Glazing on the front 5-foot knee wall and the slope up to the
rear 11-foot wall is some type of fiberglass panel {the greenhouse was built by the previous owner} and it
encloses approx 12 X 18-20 foot area.  A bench runs along the whole length of the knee wall.

At latitude 54 and at approx 2100ft elevation, it gets fairly warm except during the 4 winter months when
temperatures can drop to 40 below.  This makes it good enough to start transplants and raise some plants
during the fall season.

As a season extender for starting transplants, the greenhouse is ideal.  It is a permanent structure that
increases the value of your property--but is it a more efficient "season extender" compared to hoop houses,
cold frames, or row covers and "caterpillars?"  Add to that soil problems,pest problems,pollination problems,
condensate problems...not to mention the diffferences in cost between these options.

Eliot Coleman's books--"Four Season Harvest" and "The Winter Harvest Handbook" are excellent resources that
discusses in some detail about these other options.

I don't know whether this is available in the States, but Canadian gardeners can borrow many of the organic
and permaculture books that have been published.  The Canadian Organic Growers <cog.ca> has a lending library
where, after you register, will mail you up to 4 books for a month, and pays the mailing cost both ways.

Thanks for more comments!! Keep it up, I'm having fun and learning a lot!!
Tony Paul Martin: Yes, I will have reflective shades. They can also help against the heat in summer, I'm making ... hm.. I don't have a word for it, like how a trombe wall vents top and bottom and hits thermal mass, in summer open vents top and bottom with reflective shades, so the heat gain through the glass stays by the glass and goes back out. I'm doing this as cheaply as I can, so my first shades will be modified old vertical blinds, made to reflect, insulate, and overlap each other more than they usually do. If that is ineffective, I'll work on it from there. I have large amount of insulated drapes to play with too, and lots of styrofoam. Yay dumpster diving!!

R Jay: I have a copy of The Solar Greenhouse Book by James C McCullagh, and got some ideas from it, that's where my head got the idea of coffee cans of water as small storage, in the book they were using, hm, milk jugs maybe? And when one cracked and drained they had stability issues and it was hard to replace out. Plastic coffee cans are easy to find in the local recycle place, and they stack neatly and are pretty stable (even more stable columns if you hot glue lids to bases, so it holds together a bit more. Still won't go above 6 feet high, probably 5, I'm short.)  I loved all that math in that book, you can either skip over it easy, or if you need to know what some angle should be etc, they have it all worked out for you. Some of the ideas have been worked out more since then, so I find it a good base to read as you read stuff off the net that is more current.

Michael S New: What a fun thing to recycle! I envy you that project!! Glad to know enough mass can keep a greenhouse at least above freezing... I have had BuildItSolar bookmarked on my computer for several years, some good info on there. Was there just the other day, using their stuff to explain earth tubes to someone. As far as stratification, I think at this point I'm inclined to not worry about it too much, I don't expect 100% efficiency from the big barrels, and if I need to add heat to the greenhouse, the lower sides of the barrels will be my target, with the waterbed heaters (when I moved I seem to have shook something like 8 of them out of the chaos of my last life.) The sheer size bulk of them will, I think, help stabilize the temperature a lot, if not increase it, at least slow down a decrease.

Steve Sherman: Wanting to design my greenhouse before the house is built was the idea behind this thread I'm hoping (don't know if I can, but I hope) as things are done for the house at the same time do the stuff the greenhouse needs too, like sub dirt tubes, dig it out while there's a backhoe there already, get the ditch witch to do this and that for the house, and get those other things in place for this first greenhouse too. If I manage to tube the house, pretty high odds I can do the greenhouse at the same time. Codes is being interesting here... we'll see what I can pull off....  Pipe dreams is this only my first greenhouse I know where I want to locate more ....

Jeffrey Sullivan: I had to look up how Michigan could be in the same zone, lake effect! Only thing I'd wonder with putting your barrels closer to the RMH is whether you'll degrade plastic faster (ignore if they are metal) and whether a water leak would make a mess of your RMH... Might be worth considering. Good drain or something.

R Jay: I have read Four Season Harvest, Winter Harvest Handbook I think is already on my want list. I hate returning books I like, so I tend to buy them when I can, cool to know they can be checked out! We have just moved to a very small town, and between me and my mom, I think the lady at the library who does the interlibrary loans knows my phone number by heart already Very small library, but happy to order books. As far as general season extenders like hoop houses etc, yes, I'll use those on my main beds, they are more WAY effective for that. My main propagation area is in an old house foundation on the property, it can be hooped really easily, there are walls at a good spacing for stabilizing the hoops, and for holding glazing for cold frames. I want things like pomegranates, citrus and figs, and am just a bit far north for them, as well as some perennial flowers that might not like the winter here (that cold sharp wind on the place is going to be interesting to learn to deal with.) A major thing we want though is a sun space for us, we just moved from the desert and winter is grey here. Also a dry space, it rains here, and is humid, you can't even hang out laundry sometimes. So the structure is somewhat less classic season extender as it is winter solarium/summer rain controlled outdoor space, and year around wind break. With trees.

Hi Pearl, sounds great, of you are blocking the light will the plants get enough? Would instead a thermal chimney help move enough air through to keep the temps down?
This sort of idea http://sustainablesources.com/images/passsolar6.gif or maybe solar powered fan, the more sun the more air. Best of luck with this.

Tony Paul Martin: I'm sorry, should have clarified (there's only so much I can type at once!!) That would be on the west vertical wall, maybe on the east if it needs it, north is non-opaque and insulated heavily, south angled wall would be unobscured for light. West view is not currently blocked by trees etc, and looks like it will be excess heat gain. Solar chimney effect will be in use, the earth tubes from way down the hill, there's a gully down there that seems to be foggy and cool every morning in the summer, will be pulling cool air up from there through the chimney effect, for both the house make up air, and the greenhouse.
As far as fans, maybe someone has heard of this and can point me to info, I can't find anything when I search, but don't have a clue what the correct search term might be... The property always has wind, it's on a ridge. Not always very high speed, but really constant. I am visualizing for both the house and greenhouse a basic cheap turbine fan that has no duct, instead of pulling air out of the house it turns a shaft that comes through the roof, and that turns a fan in a duct  to move heat down from the peak of the ceiling to the floor (or basement). Very low tech, seems like it should exist, can't find one to see how I could do it.  I have seen various versions online of electric fans to do it, and computer fans being used, but I love the idea of using the outside wind to turn a fan. Any ideas?

I know in my area there isn't enough sun in the winter to justify any type of solar.  I cover all the walls with 1" foam board. The roof is double layer greenhouse plastic. I know that I get more benefit from keeping the cold out than letting the sun in. Temps in my GH run 20-25 degrees warmer than outside temps on a sunny day only. I run my grow lights at night and they produce a good amount of heat. Don't overthink your greenhouse build. I did and wish I hadn't. I'm a firm believer in the KISS method. As for fans I just got this neat thing from amazon  https://www.amazon.com/gp/product/B01KEYDNKK/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1  ; Just hang the probe in the ceiling an it will control your fans. There are cheaper ones out there but this one is plug and play.

Sounds an amazing place! Some pics when you have finished would be welcomed.
Worst case scenario. Fan, the trouble is that whilst it may seem always windy, sods law says that on the hottest days when you need the wind it will stop and it doesn't take long to fry plants so I think a windmill would be risky.
At least with the sun, no sun = no heat. You are guaranteed power for the fans without any fancy control circuitry. Yes computer fans are popular and cheap. It might be good to have at least 2 on separate circuits and solar panels in case of failure. A simple protection device for the fans would be a suitably power rated 12volt zener diode. This will stop your fan getting extra voltage and which would reduce their life expectancy or completely burn them out. Hope this helps.

Tony Paul Martin:  At the moment it's only amazing in my head    Right now I have a messy lot with a north slope, old foundations where the previous house burned, a a barn that needs work, zoning codes that won't allow me to build anything until the house is done, and builder issues. The pieces are shuffling into place, I finally got a working brushcutter and cut the other day, I have my first swale in where I need it most, I don't dare do anything too close to where the house will go, in case things have to move, and to avoid damage during the mess....
Fans: Thank you for the link Jeffery Sullivan Those look useful...   I'm definitely putting in electric fans to move air from peak to ground, but I don't trust electric, things crap out when you need them most, the lower the tech, the more I trust it. The wind turbine fan idea is a parallel system, so if one system doesn't work, the other may. Those fans are just for circulation, not for heavy air removal of excess heat. As far as plants frying, some vents will be on bimetal openers, and I can always manually open the rest. I just think the turbine idea is a neat one, and would love to see if anyone knows of it ever being done.
Had a thought on barrel stratification, I wonder if my barrels leak if they are on their sides....

For sure always have backups. That's especially true in my situation where I have fish to support as well. After one episode I realized I needed my water pumps and air pumps on separate circuits (cost me 40lbs of Tilapia). Your less expensive thermostatic fans have to be reset if the power flickers. That's why I use the separate sensor switch and manual fans.

Would a good insulated wall and a soda can solar heater work? 
If the solar is working a fan powered by a small solar panel to move the warm air around the greenhouse.
Just a thought.

Solar only works during the day unless you produce enough power to charge a battery with an inverter if you're looking to run fans at night.