Effective solar heat storage can be the Holy Grail of hobby
greenhouse heating. What could be better? It's cheap, easy, and rumored to be
effective Click here to read one real world example of
successful solar energy storage. Unfortunately it is not a foolproof
system. There are many variables that affect how well a solar energy storage
system will work.
To try to get an idea if the
system will work for you first need to figure out how may BTU's needed to heat
your greenhouse on a given day. Once you
have the BTU's you can determine how much thermal mass you need to make a
difference in your greenhouse. The
calculation used to determine this is: BTU's stored = Pounds of material x specific heat of material x
temperature difference Water is a popular
thermal mass in hobby greenhouses so lets look at an example using a 55gallon
drum of water. Water is approximately 8lbs/gallon so you have 440 lbs. of
water. The specific heat of water is 1 BTU/LB degR. Most other materials have a
lower specific heat. Look at the chart below to get the specific heats of other
materials. So now you have two of the three numbers you need. The temperature
difference is tricky, it can be affected by a great number of variables. In a
true passive solar system the drums will not get significantly hotter than the
surrounding greenhouse. So if your house only gets to 75 degrees your thermal
mass will not be much warmer. For the sake of argument assume your water
temperature changes 25 degrees in a day from 50 degrees to 75.
Your final calculation would look like this:
Btu's = 440 lbs. water x 1 specific heat of water x 25
temperature difference Btu's = 11,000
Keep in mind this works both ways. As your greenhouse
warms from 50 degrees to 75 degrees it will take 11,000 extra Btu's because of
the thermal mass of the water. In some cases this could be an advantage. If you
greenhouse is prone to overheating some thermal mass can slow the heating
process. It can also be a disadvantage. If your thermal mass routinely drops
below your minimum heat you will be paying to heat the thermal mass.
You can convert this to cost by figuring
the 90,000 BTU/gal of propane and 2000 BTU/cu ft of Natural Gas (and knowing
the cost of the fuel)
