Solar technology is just barely past its infancy, analogous to automotive technology in the early part of this century. Therefore, previous articles in this series have explained basic concepts while ignoring refinements, just as an article about cars in 1900 might have shown different types of engines and motors, while explaining that they are either directly or indirectly connected to the wheels to make the car move.
When the automotive industry was developing, there were scores of small companies selling cars which differed radically from each other. There were cars powered by steam engines and electric motors, as well as by internal combustion engines. Some had three wheels, and some had four, but all were rudimentary by today's standards. For most people then, there was no direct economic benefit to purchasing a car. Consider how this situation changed by the 1950's. Owning a car became an economic necessity in the United States (e.g. to get to work), and most people had one. The variety had virtually disappeared, leaving fewer than five U.S. companies producing four-wheelers powered by an internal combustion engine at the front.
For solar water heating, the technological variety is still here, represented by a multitude of companies, each producing collectors, tanks, pumps, or systems in rather small numbers. Each company emphasizes the advantages of a particular system, and fills one market niche. For example, people in warm climates have the option of saving money by avoiding the extra complexity required for freeze protection. If you have no space indoors for a solar storage tank, a thermosiphon system with a rooftop tank is advantageous. Given an old house near an earthquake fault, it is instead better to install the tank in the basement or garage to avoid costly reinforcement of the roof structure.
If and when solar water heating moves into the mainstream due to economic forces (i.e. rising cost of fossil fuels), new houses will be designed specifically with either a strong roof or a larger utility room, to accept the preferred kind of solar water heating system, whatever it may be. Cost-effective, highly reliable solar water heating systems will be commonly sold at building supply stores. Since they will be mass-produced by a few major manufacturers, most of the variations may disappear. For example, freeze protection could become a standard feature, just as anti-freeze in cars is now standard even in warm climates.
Recently, I talked with two professionals, George Leone and Mike Cochran, who work with solar water heating systems in different parts of California. They independently indicated that there is a trend toward one preferred system, using the "drainback" scheme, illustrated below. The solar collector is on the roof and the hot water storage tank is down below. This is a "closed system," with a few gallons of water in a small tank which flows through pump number 1 and the collector to bring the heat down to the heat exchanger. The second pump circulates the water to be heated through the other side of the heat exchanger, and the hot water flows into the solar storage tank.
The drainback feature is the key to automatic freeze protection without requiring liquids. When the collector cools and the pumps turn off, the water in the collector falls back to the small tank on the left, and the air in the tank ends up in the collector. Therefore, the collector cannot be damaged by freezing temperatures. When the collector is heated by the sun, pump number 1 lifts the water back up to the collector and circulates it. Note that the second pump could be eliminated by building the heat exchanger into the solar storage tank, but such special tanks are presently scarce and expensive.
The drainback system is immune to a number of potential problems in addition to freezing. The distilled water cannot clog collector tubes, and any leaks in attic or rooftop pipes can only spill a few nontoxic gallons at low pressure. The price for these advantages is complexity, with more components than any of the systems I illustrated previously, including possibly 2 pumps which need electricity. If a significant quantity of these systems are produced and installed, the complexity should not prevent them from being cost effective and reliable, however. Returning to the automobile analogy, my modern Chevrolet has many more parts and features than a 1923 Model T, but it is more reliable and quite cost effective.
No one knows for sure what the preferred type of system will be when solar water heating becomes widespread. The drainback system is one candidate for the "mainstream" solar water heater of the next century. There are other recent concepts for reliable solar water heating, including thermosiphon systems with built-in freeze protection, and other systems which eliminate pumps without requiring rooftop tanks. Most installations presently cost roughly five thousand dollars, so it remains to be seen whether the price will come down through mass production or whether lower-cost alternatives will be developed.
Dr. John Whitehead is a rocket scientist 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.