Electric heaters and other sources of Btu's may work just as effectively,
but this system operates for free and doesn't have to be switched on every
evening. It also stores excess daytime solar energy for release later
during cooler nights resulting in lower daytime highs and higher nightly
lows. This water-based temperature stabilizing system is in both of my
greenhouses and it works well except for drastic low temperature drops like
the one that caught my Nepenthes last fall.
The system is referred to as "passive solar energy storage" and is described
in most greenhouse books. These books often fail to mention the added bonus
of a water-based heat storage system is that water will resist the
temperature drop below freezing as it requires additional energy in order to
freeze.
I don't have be old physics text book handy but if I recall correctly, to
cool 1 millilitre of water by 1 degree Ceslius, it requires the removal of 1
calorie of energy. A Litre would then require 1000 calories to cool it only
1 degree. For a gallon, well, I need a conversion calculator for that one.
The actual freezing of water, and the alignment of the molecules to form
ice, requires even greater amounts of energy than to just cool it. This
"latent heat of freezing" is what allows large water resevoirs to act as a
greenhouse anti-freezing backup safeguard system.
What all of this amounts to is that if the temperature outside of your
greenhouse drops to minus 2 Celsius or 30 degrees farenheit, and for
example you have ten 50 gallon drums of water inside of it, then your
plants won't freeze until after those 500 gallons of water are cooled
degree by degree down to freezing, and then absorb the additional energy
required to form ice.
Of course you also need some air circulation going on to prevent any cold
pockets of air forming in the corners and allowing early freezing there. A
small fan on the floor will usually keep things moving enough for the system
to be effective.
During the week I'll sit down and try to figure out exeactly how much energy
is actually required to freeze this much water (ie. from 3 degrees celsius).
I'll post the value by next weekend, unless some other science-buff wants to
figure it out and post it for me in the meantime.
It may seem a lot of trouble to put all of those drums into your greenhouse
but they are often available free at landfills or recycling plants and I use
them as the supports for my plant trays. I use them under all of my tomato,
cucumber and CP beds/trays so that I don't need to buy lumber to build
tables and table-legs. If the former contents were smelly, I seal the top
holes up with the original plugs or use gasket silicone and plastic yougurt
lids for the job.
Always tinkering,
Randy