About Greenhouses: your greenhouse questions answered here.

Solar Energy Storage

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 55-gallon 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)

Material	Density	Specific Heat Btu/lb-F
Water	8.3 lb/gal or 62 lb/cu ft	1.0
Clay Bricks	135 lb/cu ft	0.2
Sand	95 lb/cu ft	0.2
Rock (3/4"-3" diameter)	100 lb/cu ft	0.2
Concrete (solid)	150 lb/cu ft	0.2

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