Solar heating- Gary W. Hickman, www.cuestaroble.com

Using the sun to provide heat to greenhouses during the day is, by definition, why they work. Night time heating, when it is usually most needed, is more problematic. Many attempts have been made to design some type of effective, low cost, method of storing the daytime solar heat for use at night. Very few of these systems have proved useful.

   One very simple solar heating system can work, especially in areas that receive ample daytime sunlight. The main component of this passive design is water storage. Large water- filled bags running along the inside walls of the greenhouse, or rows of 5 gallon (20 liter) water bottles, can provide nighttime heat if adequately warmed during the day. These storage units do take up valuable greenhouse space, but properly placed in otherwise unused areas inside the greenhouse, they can provide relatively free heat.

 The basic science of a greenhouse water container design involves the fact that water absorbs heat in the day and releases it at night. The amount of heat energy cost savings that can be gained depends on the amount of water storage provided.

 A sample calculation:

 1 BTU = 1 lb. water raised 1^o F.

60 ^o F to 80 ^o F solar heat gain = 20 BTU per lb. water

10- five gallon water bottles = 400 lbs. X 20 = 8000 BTU

50- five gallon water bottles = 2000 lbs. X 20 = 40,000 BTU

This absorbed heat is then available for release at night when the inside temperature drops below 80 ^o F.  Using the example of 40,000 BTU, this is equivalent to 40 cubic feet of natural gas, 0.43 gallons of propane or 25.6 kwh of electricity.

 (see Table 13 for calculating equivalent energy cost savings depending on the fuel source).

Metric  Solar Water Collector Sample Calculation:

1 KW hour =860 Kcal.

15^o C to 27^o C solar heat gain = 11 Kcal/Kg water.

10 –twenty Liter water bottles = 200 kg X 11 = 2200 Kcal/day

100 –twenty Liter water bottles = 2000 kg X 11 = 22,000 Kcal/day

This absorbed heat is then available for release at night when the inside temperature drops below 27^o C.  Using the example of 22,000 Kcal, this is equivalent to 2.5 M³ of natural gas or 25.6 kwh of electricity savings per day.