The previous article in this series relied on calculations and concepts from basic science, to show how much energy can be obtained from the sun for various purposes. For this week's column, I did some field research by talking with people who have years of experience at building and installing solar water heating systems, namely the folks at Sun Light and Power (SL&P) in Berkeley.
The illustrations above show the operational principles for two basic methods of solar water heating. To collect the solar heat, both systems use a rooftop collector containing a surface which absorbs sunlight and is in direct contact with many small tubes through which the water or other fluid flows to receive the heat energy. Both systems need circulation through the collector to retrieve the energy being collected. In the examples shown, a controller turns on the pump whenever there is enough sunlight to collect energy, and hot water is stored in the tank until needed. In the "Open system," the water to be heated flows through the rooftop solar collector to receive its heat directly. However, outdoor water pipes and the collector are vulnerable to damage by winter freezing. It is possible to install drain valves and simply not use solar hot water in the winter, but the ideal solar water system can be installed and ignored for 20 years of reliable operation, just like your regular gas water heater. Where freezing is expected, the alternative is to have an anti-freeze solution flowing through the collector. In this case, the collector fluid flows in a "closed" loop through a coiled heat exchanger tube inside the hot water tank, and the water receives its heat indirectly through the wall of the coiled tube. This kind of solar water heating is more expensive, since heat exchanger tanks are limited-production items costing up to $1000.
The people at SL&P told me that most of the solar water heaters in California are open systems, because freezing is rarely a problem. Since the freeze of December 1990 was fresh in my mind, I prompted them to say that 70 phone calls were received from customers with solar freeze damage. Since it rarely gets that cold in California, it may be worth risking an occasional repair (probably covered by homeowner's insurance) to avoid the extra expense of a system with circulating anti-freeze.
Freezing is not the only consideration. In an open system, many tens of thousands of gallons of water must flow through the small tubes of the collector in its lifetime. One open system in Sebastopol had water with a lot of minerals, resulting in a hopelessly clogged collector. Since a new 4 by 8 foot solar collector costs about $700, damage is costly.
Speaking of cost, let's consider how much you should be willing to pay for a solar water heating system. A gas water heater costs a few hundred dollars, but a solar water heating system costs thousands. Therefore, the economic tradeoff is that for a large initial investment, the part of your gas bill devoted to water heating can be nearly eliminated for many years In Part 1 of this series, those of us with gas water heaters and no other gas appliances running during the summer, looked at our PG&E bills to determine how much energy is used per month to heat water. As an example, let's assume your household uses 1 therm of natural gas per day for water heating, at an average cost of 65 cents per therm. This works out to about $20 per month or $240 per year for the energy to heat water. Suppose you think natural gas prices are not going to change over the next ten years. Therefore, you would be willing to pay $20 per month to repay a l0-year loan used to install a solar water heating system. At 10% interest, the original loan amount would be about $1500 for the solar collector, pipes, and installation.
Of course, your own price threshold would depend on how mu}ch hot water you use, on your method of financing the project, and on how many years of reliable operation you can expect. Given today's solar technology and costs, and the present price of natural gas, there is no overwhelming economic incentive to install solar. Some households are saving money with solar hot water, and some are losing money but investing in the environment by reducing pollution and the consumption of natural resources. It is impossible to predict what will happen to the price of natural gas, b,ut there is no doubt that the world's supply continues to dwindle.
Next week: Why do some solar hot water systems have tanks on the roof?
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.