I'm one of the skeptics and you just proved my point.

There is a difference in the utility of water as thermal mass in a GH which you are just trying to keep above freezing and a GH which needs to be at 55F for citrus, bananas, orchids, or whatever.

In your case you are taking advantage of the latent heat for energy. The water freezes and as it does so, puts out lots of BTUs.

In the other case, the water in the will never freeze, so the GH owner can never take advantage of the latent heat. This GH owner is better off leaving the barrels out, downsizing the GH (by building smaller, partitioning off a part, or whatever) and heating less space.

I can see your point there. I still have lots to learn. I had never intended to sustain a minimum temp. other than freezing. You would have to find a way to heat the water more efficiently than the air to sustain any higher temps. Which would not be an easy task. The good side is they would have to help alot with cooling, I wont get to see how much since they're already in there.

Again, it is all about what your purpose is. If you are simply growing greens and extending the season, dark barrels can prevent freezing. If you want orchids and bananas, you have to pay. One of the old-timey arguments from well over 100 years ago.

Dan

Back in March 09, goodgreen posted a story about his home within a greenhouse. Here is a link to a page in goodgreens website showing an earth oven. http://www.goodgreenfarm.com/earth_oven_photos.htm

I built one at home, then we built another last summer at the farmers market. It is a permanant structure. At the market, we got creative and embeded a 4foot length of 1/4inch od coppertubing to heat water for coffee, etc. Since the earth oven gets up over 1000degrees inside, we were afraid to embed a larger piece of copper for fear of scalding folks when the water comes out the bottom. Some kids in Cal have heated a hot tub in this manner with very little wood. It would be a great way to keep numerous barrels of water hot in a greenhouse.

Our costs were $36 for 18 firebricks and $32 for builders sand from the HW store. You could put the earth oven right in the greenhouse to increase the heat, but I would not recommend it. We were cooking pizzas in the oven in three minutes and it is a great thing to have outside in summer for cooking pizza and bread. And maybe heating a hot tub...

Check on Amazon for a book entitled Earth Oven by Kiko Denzer to learn how to build the oven. For info on embeding the copper, just email me.
Carol

Thanks for the sight, it is very interesting. I don't think I would put it in the greenhouse either. I have looked some at water heating with woodstoves, quite similar concept, the previous owner of my house used to heat their water with wood, but that was long before I got here. I've actually speculated creating a sort of solar water heating system. Like I said at this point I am just wanting to sustain above freezing, but I'm sure the more I get into it the more I'll try to heat it. I would really like to get the GH to the point of electrical independence, which is why I'm not too worried about minimum temp. I'm actually thinking about a micro-hydro project in the nearby creek, to run ventilation and inflation fans, but that's way in the future. I'm just tired of the elec. rates and that little heater I mentioned cost me an extra $100 bucks or so in Dec. Thank again for the info, I hope your greenhouse works out good for you.

Check this out...
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm#ActiveWater

You could use that type of collector to heat the water in your barrels nicely. You would just need a pump. Some people even run their pumps from a small solar panel.

That is assuming of course that you have a decent amount of sun during the winters and the skill to build one of those collectors yourself.

I wonder if you had the right kind of foundation if you could also install an underfloor type heating system using pex.

Being in central Texas (very mild winters), I plan to experiment with using this kind of system to heat my green house whenever I get it setup.

I am doing it a bit differently than you, passive all the way. I only have one 50 gallon plastic barrel, partially buried, for extra heat. When the sun shines it is great but it takes a bit for it to warm up enough to help.

I would think adding another layer of poly would help you more to keep the heat in. That would make more sense than paying an electric bill.

Its easy to figure the value. Temp of the water when at its highest and then when at lowest, times the quality of water will give total BTUs actually delivered. When i did a quick trial I found that even a small electric heater was better for me, unless great quantities of water and using radiators to heat and cool it. YMMV.

When judging the utility of water as thermal mass, it helps to clarify the GH situation and setting.

First, there are passive and active systems. Killer-D refers to an active system. An active system could, in fact, be very effective. Lots of people talk the active system talk. Having watched this forum now for about 6 years, I can tell you that virtually nobody walks the walk. I cannot recall ever seeing a pic of a finished, functional system using active management of water as thermal mass.

Regarding passive systems, the efficacy depends on whether or not you can take advantage of water's latent heat, and that in turn depends on the min temp of your house. If you need a min temp of just ~32F, you can take advantage of the latent heat. Go for it.

If you need a warmer house, then it doesn't work so well. The absolute difference in water temp x the quantity of water will yield the BTU delivered, true. But one of the problems is WHEN those BTU are released. Consider a GH with a min temp of 50F. First thing in the morning, the water temp is 50F. The sun is out and the GH temp starts to rise. As soon as the GH temp is 51F, heat differential favors gain and the water starts to warm as well. The process continues until the GH is 80F at 1pm. Let's assume that the water has risen to 80F as well, storing the maximal amount of heat. Then by 3pm, the temp starts drop, now it is 79F. You don't need any heat because your min temp is 50F, but the water is already giving up it's BTU. By 6pm it is dark and the GH is 55F and the water temp is 60F. It has given up 2/3 of it's BTU and you aren't even down to your min temp yet, the temp in the GH when you actually need the BTU. By the time the temp drops to 50F, you only have a small fraction of the stored BTU left in the water for when you actually need it. This is what my water storage experiments show.

One other factor is the location of your GH. Anecdotally, thermal mass seems to be somewhat more effective or efficient, even in a passive situation, at altitude.

Spogarden I do have a double poly w/inflation which helps trmendously, I've actually been contemplating adding another layer on the inside to make a flat ceiling basically.
Stressbaby I am at 3500' which probably makes some difference. And I have a W-E orientation for optimal exposure. I also painted the N side and tops of my barrels white, but I'm kind of skeptical as to wether that is helping. I'm not exactly scientific with all my measurements, but the barrels are definitely helping at night. I do have my thermometer directly above them prob not the best place. It would seem to me that barrels would hold heat longer than your experiments showed, but there are many variables.
The temps. have been rising here and we now only get about two nights a week below freezing. The GH is doin great. The temps. at 7am outside can be as low as 28F and it stays above 40F inside. I also added 2' wide paver walkway beside the barrels and I'm sure it's doing its part too. I've been doing lots of research and I'm thinking of adding a compost tumbler for the winter.
I had put thought into in ground heating before but my location is actually on fill that is comprised mostly of asphalt thanks to the previous owner. I figured it would have good drainage.
Something interesting I read about was Phase Change Materials (PCM), such as Glauber's salt. Quite interesting stuff, but there's not enough information out there. Alot of the info goes back to the 40's, when green efforts such as this were ignored.
Thanks to all for your help and opinions. This is such a great site for learning and sharing!

Ah! Glauber's salt. Neat stuff. I had looked into that a few years ago when i was looking at all the alternatives. Right now, I don't think i need to go through with the expense. Still playing with less expensive methods.

I had a bunch of links, including one for a company that manufactures a series of different materials that absorb and releases the heat in varying temperature ranges. unfortunately that computer has recently bitten the dust and i have not tried to recover the hard drive(s) yet.

From what I understood, from reading, the salt was not that expensive, but I never actually looked at prices to buy it. I'm sure the MIT experiment was costly with 21 tons and air circulation equipment. But I would think a downscaled version could be built quite easily for a GH if one was more familiar with the concepts.

My unheated greenhouse is maintaining a 3-4C edge over the outside temperature. This is the same spread I get after sunset above 0C with water in the greenhouse. As the temperature approaches 0C, with 600 litres of water, the inside temperature stabilizes.

There's another problem with the water barrel idea.

Someone conducted an experiment and found that the temperature of a 7-gallon container of water dropped at a rate of about 7 degrees F per hour. (Albeit the water temperature started at 87 and the ambient temperature was much cooler at 53. Obviously the rate of heat loss is going to be slower if the temperature differential is smaller)

http://www.jc-solarhomes.com/COLLECTORS/Solar_Hot_Water_Calculations.htm

What that means is that as the ambient inside of the greenhouse begins to go down, the water is going to be releasing its heat even though you don't need it at that time. For example, the temperature inside, without the water barrels, normally reaches 82 and then an hour later it goes down to 70. Suppose a heat loss rate of 4 degrees per hour. Now instead of being 70 degrees, with the water barrels the ambient inside temperature may be kept up at 74 degrees, while the temperature of the barrel will have already lost some heat, falling from 82 to 78 degrees. If this pattern keeps up, the water inside the barrels is always only going to stay a little bit warmer than the temperature inside the greenhouse, so that's not going to be warming things much when it comes to 8 hours later at 2am in the morning.

You might need to use some insulation around the barrels to slow the heat release. (Perhaps the sides not facing the sun)

What you would really need to do is to make sure the rate of heat loss from the barrels is substantially less than the normal rate of heat decrease in the greenhouse from the warmest part of the day to sunrise the next morning.

Let's assume a 3pm temperature of 85 F and a temperature of 35 F the next morning 14 hours later. That would mean the rate of heat loss from the barrels would need to be less than about 3.6 degrees per hour if you wanted any heat to be saved up, from the warmest part of the day to the coldest.

(At a 2.6 degree/hour heat loss rate from the barrels, the greenhouse could be 14 degrees warmer than it otherwise would be by 5am the following morning)