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Greenhouses & Garden Structures

Water as thermal mass for a very small low tunnel/cold frame

10 years ago

Hello everyone,

In the past I have build a very small cold frame over my tomatoes, and it worked great. This is about 10ft long, maybe about 5 ft wide and 5 ft tall... it's just three 10ft .5" diameter PVC pipes anchored by rebar with cheapo clear plastic stretched over the top. My tomatoes have loved it and grow much better than they would outside.

Anyway, I've always put a lot of 2 liter soft drink bottles filled with water to help keep the tent warm at night. Since I more or less gave up soda pop recently I don't have those bottles anymore. I was wondering if getting from free plastic buckets from the local bakery or restaurants and filling those with water would work as well. The buckets would not be transparent, so I don't know if they'd heat up in the sunlight enough to be useful. Would dark colored buckets work better than white ones?

Also, if you think that the thermal mass this would ad is not enough to help at all, I won't be upset if you can demonstrate this is the case. :)

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Comments (8)

oakhill (zone 9A, Calif.)
10 years ago
Here is a link with a calculation for the amount of heat that can be stored with water in a greenhouse.

Here is a link that might be useful: greenhouse heating with storage water
myfamilysfarm
10 years ago
The darker buckets would absorb faster than the lighter colored ones.

cuestaroble, thanks for the link, I was wondering how much water storage I needed. The math is easy.
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ljpother
10 years ago
If water has worked for you in 2 litre bottles, the same volume in other containers should work. I put my ice cream pails in dark plastic garbage bags.

In my green house after a sunny day, water holds the lows at 0C. This year I am supplementing with an electric oil heater (looks like an old steam type radiator). Not everyday is sunny :( It's giving me 2-3C when the outside lows are
SalmanBah
10 years ago
Can you advise me of the steps I need to build a green house in a very hot wether like Bahrain
kudzu9
10 years ago
SalmanBah-
In a climate like yours, the main issue is killing the plants from too much heat. I live in a climate where the summer, outdoor temperatures only get to about 25C, but, even with venting, the greenhouse can easily get to 45C...which is a threat to many plants. In your country, you could get to much higher temperatures if the greenhouse was not well-designed.

I don't have firsthand knowledge of greenhouses in Bahrain, but any greenhouse you would build there should have: 1) automatic roof vent openers, 2) one or more large volume fans operating on a thermostat, 3) a misting system on an automatic timer; and 4) shade cloth on the roof.

What size greenhouse are you talking about, and what would you want to grow in it?
kudzu9
10 years ago
Salman-
Also, it would be a good idea, and you might get more responses, if you posted your own thread about this matter, rather than adding it to the end of someone else's unrelated issue.
thinman
10 years ago
Small math-physics point:

One advantage the 2 liter bottles have over buckets is that they have more surface area per liter of water. Since surface area is what absorbs the sun's energy, the water in the bottles will heat up more than the same amount of water in buckets. By the same token, the same amount of water in a bunch of small buckets will absorb more energy in an hour than a couple of big ones would, assuming they're all the same shape. Also, square buckets can absorb more than round ones.

If the buckets are plastic and you wanted to go to the extra trouble, you could try painting them with Krylon flat black paint to get more absorption.

ThinMan
gjcore
10 years ago
In my cold frames I use used gallon water jugs and then add a variety of food dyes to the water to get it almost black.

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