Last week's illustrations showed the basic concept of pumping water or an anti-freeze solution through a rooftop solar heat collector while the sun is shining. Assuming that the freeze of December 1990 was a rare event rather than a sign of coming climatic change, we were able to conclude that the extra expense of circulating anti-freeze can usually be avoided in California. For the lowest cost, the actual water destined to fall from your shower head runs up to the roof to receive its solar heat. This monthÕs illustration on the left shows this open system concept again, with a few additional complexities to make it represent a real solar hot water system.
In particular, there is a controller, represented by a capital "C" which turns the pump on and off. Typical controllers have sensors for the temperatures in the collector and in the solar storage tank. Whenever the collector is hotter than the water in the tank (i.e. when the sun is shining), the controller turns the pump on. Unfortunately, "C" also stands for cost, both for the controller's electronics and for the electricity to run the pump. Electricity is a more specialized and therefore a more expensive form of energy than heat (e.g. from natural gas), so the pump must be efficient in order to avoid replacing your gas bill savings with an electric bill increase when you install solar water heating.
Notice that the circulation loop through the pump and collector does not directly affect flow into and out of the solar storage tank through the other two pipes. Whether the pump is running or not, the water in the tank will be replenished by fresh cold water, whenever hot water is being used.
Ideally, the tank stores enough hot water for evening, night, and early morning use, and the sun shines every day to provide 100% of the required energy. Extra hot water may be needed during some evenings, and the sun doesn't shine every day, so most households will choose to have a backup gas water heater shown, in case the water flowing from the solar tank to the shower is not hot enough. If you have a solar hot water system installed, your existing water heater may become the "backup" heater. A properly designed solar hot water system will greatly reduce your energy bills because the backup heater will only run occasionally.
The pipes entering and leaving the solar tank are arranged to take advantage of the fact that "heat rises." Water expands when heated, so a gallon of cold water is heavier than a gallon of hot water. Thus, cold water enters at the bottom of the tank and stays there, while the shower receives hot water from the top of the tank. During the day, the pump takes cold water from the bottom of the tank and returns solar hot water to the top, where it remains until needed.
The circulation loop can also take advantage of the fact that cold water sinks below hot water, and hot water floats on top of cold water. The illustration of the thermosiphon system on the right shows how the controller and pump can both be eliminated, by placing the storage tank in the attic or on the roof. When the tank is above the level of the solar collector, the density difference between hot and cold water forces water to flow through the pipes and the collector. Heavy cold water in the bottom of the tank and in the pipe to the left of the collector "sinks" toward the bottom of the collector, while the water being heated in the collector becomes lighter and floats upward, whereupon it returns to the tank. Another way to think of this is that the pressure from a falling gallon of heavier cold water easily lifts a gallon of lighter hot water.
Given the principle of the thermosiphon, now consider what happens after sunset. The collector cools, until the water inside it has the same temperature as the water in the pipe to the left of the collector. There is no longer any driving force, so the flow stops. The opposite happens after sunrise, so the thermosiphon system's circulation turns on and off just like the system with the pump and the controller. The difference is that no electricity or moving parts are required whatsoever, so a thermosiphon system can be cheaper and more reliable. In addition, it is not necessary to squeeze the storage tank into an already crowded garage next to the existing water heater, for example.
Thermosiphon systems have potential disadvantages which must be considered in their design. If a tank in the attic leaks water, it will probably cause more damage to the house than if a tank in the garage or basement leaks. A leaky tank above the roof only dumps water on the roof, impervious to water. Rooftop tanks can be an eyesore, so manufacturers have built the collector with an attached tank into one attractive, visually-appealing package. If you see a solar collector with a bulge at the top on someone's roof, this is a thermosiphon system with a built-in solar storage tank. Another consideration is that water tanks are heavy, so the hot water storage capacity may have to be reduced, or the roof structure may have to be reinforced. This is especially important in California, where heavy objects high up can cause trouble during earthquakes.
NEXT WEEK: There are as many different solar water heating systems as there are different climates, different kinds of houses, different hot water use habits, and different budgets.
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.