A square meter of area (about 10 square feet) pointed toward the sun receives 1.4 kilowatts of sun power. Given that the earth is rotating, a fixed solar collector will not always be pointed directly toward the sun, so it's fair to assume that 10 sqsuare feet of your roof receives 1 kilowatt on the average during the middle hours of the day. This is convenient because this power level for one second is almost enough energy to make one BTU (British Thermal Unit), a unit of energy commonly used when dealing with heating appliances. A typical solar collector panel is the same size as a standard sheet of plywood, namely 4 x 8 feet, for 32 square feet of area. From the above information, arithmetic tells us that a collector panel during the day receives 3 BTU's per second, which is roughly 10,000 BTU's per hour. In our sunny climate, we can count on five hours of bright sunshine each day, for 50,000 BTU of solar heat per day.
A quick trip into the garage tells me that my 50 gallon natural gas water heater (typical for a 3 bedroom house) is rated at 50,000 BTU's per hour, which is the power level when the flame is on. A direct comparison to the solar collector energy leads to a useful rule of thumb: A single solar collector can replace one hour per day of gas water heater operation, roughly 50,000 BTU. A temperature probe connected to my home computer was placed near my water heater burner to produce the graph accompanying this article. The temperature goes off scale whenever the flame is on. Monitoring these graphs has taught me a great deal about our hot water use patterns. Note that the heater operates for a total of roughly one hour during the day, indicating that a standard solar collector could provide most of the hot water used in my household.
If you're serious about spending money on a solar water heating system, it is worthwhile to find out how much energy your household actually uses to heat water. Fortunately, there may be an easier way than the above method. If, like me, you have a gas water heater with no other gas appliances operating during the warm months, simply I look'at your gas bills for these months when you are not using gas to heat the house. The amount of energy is given in "therms," which is 100,000 BTU. My May bill, for example, shows a value of seven tenths of a therm per day, or 70,000 BTU per day for a three person household. This information cconfirms that one solar panel would be able to provide most of our hot water.
Saving. water is a concern on everyone's mind, in spite of the recent rains, so let's work out how many gallons of hot water we are talking about. The British conveniently chose the BTU as the amount of energy needed to raise one pound of water one degree Fahrenheit, so you can readily determine how much hot water can be obtained from the amount of energy you are purchasing from PG&E, or obtaining from a solar collector. Suppose your cold water is at 60 degrees, and you want to raise its temperature to 140 degrees, enough for dishwasher operation. Dividing 50,000 by the 80 degree temperature rise tells us that 625 pounds, or about 75 gallons at 8.3 lb/gal, of hot water could be obtained per day from a solar panel. That's up to an hour of showers with a water saving showerhead, assuming some cold water is mixed in to avoid scalding.
It is important to realize that not all of the energy reaching the solar collector or released by burning gas finds its way into the water. Also, heat is lost from water which is stored hot for hours until used. These and other topics will be addressed next week, when we consider the economics and practicalities of choosing a solar water heating system.
Dr. John Whitehead is a rocket engineer at Lawrence Livermore laboratories who has discovered that the same scientific and technological expertise (heat transfer, fluid flow, plumbing, valves, etc.) needed for rocket propulsion is directly applicable to solar energy engineering.