|Fig. 1: Simplified picture of a solar water heating system. (Source: Phoebe Morgan).|
Solar water heating is used across the United States as a low-cost and effective way to produce hot water. Even my family has used this technology before to heat our pool, and many of our neighbors used similar technology at their houses to lower energy costs. Solar domestic hot water systems can be used in any climate, and the systems can have efficiencies up to 70%. A basic solar water heater consists of a collector, to collect the solar energy, and an insulated storage tank to store hot water. Figure 1 to the right shows a very simplified representation of a typical solar water heating system. There are two main types of solar water heating systems that I will discuss below: active, which include circulating pumps and controls, and passive, which do not have these controls. 
Within the category of active solar water heating systems, there are two main subtypes: direct and indirect circulation systems. Direct systems pump household water directly through the collectors to be used throughout the home, working well in climates where it does not reach freezing temperatures. Indirect systems use a heat-transfer fluid, that does not freeze, through the collectors. In addition, this fluid is pumped through a heat exchanger which helps facilitate the transfer of solar energy gathered in the collector to the liquid or air that is being heated.  Passive heating systems, though often not as efficient, are usually more reliable than active systems. There are also two main subtypes of passive solar water heating system, and these are integral collector-storage passive systems and thermosyphon systems.  Passives stems work best in areas where temperatures are not regularly falling below freezing. In these systems, warm water rises and cooler water sinks, so the collector must be installed below the storage tank so that the warm water will rise into the tank. 
Collectors are an integral part of the solar water heating system. For residential use, there are three main types of solar collectors: flat-plate, integral collector-sttorage systems (ICS), and evacuated-tube solar collectors. Flat-plate collectors are the most popular option, and are essentially just weatherproofed boxes that contain a dark absorber plate under glass or plastic polymer covers. There are glazed and unglazed variations of flat-plate collectors, the difference being a darker absorber plate without enclosure in unglazed collectors.  Next, Integral collector-storage systems, also called batch systems, have black tubes in an insulated, glazed box. Water passed through the solar collector when it is still cold, and then continues on to the backup water heater which allows a reliable source of hot water. ICS systems cannot be used in colder climates because of the potential for the outdoor pipes to freeze. Finally, evacuated-tube solar collectors have parallel rows of transparent glass tubes, each containing another glass outer tube with a metal absorber tube attached to a fin. This fin absorbs solar energy an inhibits radioactive heat loss.
Different types of systems, and even different climates, are going to dramatically effect the costs and benefits of different solar water heating systems. Even in locations with high solar fractions, solar water heaters take a significant amount of time to pay for themselves.  Solar water heaters have a very high initial investment, which often dissuades people from buying them and converting to solar energy. As natural gas prices continue to remain low, we are most likely not going to see a shift toward solar power throughout the United States.
© Phoebe Morgan. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 "Heat Your Water With the Sun," U.S. National Renewable Energy Laboratory, DOE/GO-102003-1824, December 2003.
 B. Fairbanks, "U.S. Solar Water Heating," Physics 240, Stanford University, Fall 2010.