Different Solar Water Heating System Types
Also called an “active” or “open loop” system, this is the type of system most often installed in the central and southern areas of Florida, and other non-freezing sunbelt climates within the United States.
In the direct system, an electronic control system  compares the temperature of a sensor  located at the solar collector  with the temperature of a sensor  located in the bottom of the hot water storage tank  (where the coldest water is located). When the solar collector temperature is warmer than the water in the bottom of the tank by some predetermined difference (four degrees, for example), the electronic control turns on a small pump , that draws cold water from the bottom of the hot water storage tank and circulates it through the solar collector. Solar heated water is returned to the top of the tank.
The circulating pump is very small and typically uses about the same amount of electricity as a 100-watt lightbulb. Another version of this system uses a small photovoltaic (solar electric) panel to operate a direct current (DC) circulating pump.
Advantages. The direct system typically produces the highest operating efficiency because there is no nighttime heat loss from hot water stored on the roof; nor is any efficiency lost through a heat exchange process. Potable water from the hot water storage tank is circulated directly through the collector.
Disadvantages. The only disadvantage of this system is that freeze protection is provided by circulating warm tank water through the collector. This is not a desirable method of freeze protection in climates that experience more than a day or two of freezing weather each year, because energy loss during freezing weather could be significant. Even more important, freezing weather can coincide with a power outage, preventing the pump from circulating warm water through the solar collector.
Integral collector storage systems, also called “batch” solar heaters, combine the hot water storage tank and the solar collector surface into a single component, eliminating the need for circulating pumps or automatic control systems. In its most simple implementation, a water storage tank painted black and sitting out in the sunlight is a rudimentary ICS system.
This type of system works best as a preheater for a conventional or tankless water heater. The cold water line that feeds the conventional water heater is diverted  and sent first through the ICS solar module . Circulation is provided by utility mains pressure. In other words, when hot water is drawn out for use from the conventional water heater , the storage tank is replenished with solar heated water instead of cold water. This allows the electric or gas heater to work substantially less.
Advantages. The biggest advantage is simplicity: the system has no pumps, no temperature sensors, no electronic controls and no heat exchanger. When combined with a tankless water heater, the system can free up five to six square feet of floor space by eliminating the conventional water heater storage tank.
Disadvantages. The biggest disadvantage is nighttime heat loss. Stored heat is lost through the glass cover plate at night, which of necessity cannot be insulated to prevent heat loss. However, this heat loss is reduced in advanced ICS systems by stretching a thin clear film just underneath the glass cover plate, which creates an insulating air gap. Also, while the greater thermal mass of stored hot water within an ICS solar module makes this type of system more freeze resistant than the direct system (above), ICS systems are not appropriate for climates that experience more than four to five freezing nights per year.
Drain Back System
Drainback systems have a few more components, but are specifically designed to provide fail-safe operation in climates with frequent freezing during the coldest winter weather. The solar collector and control system are the same as the direct circulation system. However, an antifreeze solution is circulated through the solar collector and back into a heat exchanger in the hot water storage tank. The addition of a heat exchanger adds to the cost of the system and creates some degree of heat transfer energy loss, and this combined with the fact that these systems are typically installed at higher latitudes, where incoming solar radiation is reduced, makes indirect solar water heating less viable than its direct circulation cousin.
Like the direct system, an electronic control system  compares the temperature of a sensor  located at the solar collector  with the temperature of a sensor  located in the bottom of the hot water storage tank  (where the coldest water is located). When the solar collector temperature is warmer than the water in the bottom of the tank by some predetermined difference (four degrees, for example), the electronic control turns on a small pump … However, in the drainback system, the fluid circulating throught the solar collector is separated from the potable water in the hot water storage tank .
Either water or a glycol solution is circulated through the solar collector and a drainback tank . When the pump stops, fluid in the solar collector “drains back” into the drainback tank, leaving the solar collector empty whenever it has no fluid circulating through it. A second circulating pump  circulates potable water from the hot water storage tank through a heat exchanger in the drainback tank.
In an alternative design, only one pump is required—in the drainback–solar collector loop. In this arrangement, the heat exchanger is typically “wrapped around” the hot water storage tank.
Advantages. The system is designed to fail-safe and drain the solar collector(s) during freezing weather, even if a power failure should occur.
Disadvantages. The heat exchanger makes this system slightly less efficient than a direct system. And as you might expect, the drainback tank, second pump and heat exchanger make this system a bit more expensive than an ICS or direct system with comparable solar collector area and hot water storage capacity. On the other hand, this system is ideally suited to climates that may experience 10 or more days of freezing weather per year.
This type of system is the most common in Japan, Israel and Australia, which have enjoyed an installed base of millions of residential and commercial systems for the last 30 years. Like the ICS system, the thermosyphon system eliminates the circulation and control system.
However, circulation is provided by the thermosyphoning principle: The hot water storage tank is located higher than the solar collector and circulation flow is induced when the coldest water in the bottom of the storage tank falls by gravity through a circulation line into the bottom of the solar collector panel, where it is heated and rises. Water heated in the solar collector panel rises into a circulation line to a high point in the water storage tank.
Advantages. Unlike the ICS system, which combines hot water storage with energy collection and so can lose heat at night through its glass cover plate, the thermosyphon system optimizes system efficiency by fully insulating a separate storage tank.
Disadvantages. The thermosyphon system’s primary drawback is appearance: The hot water storage tank must be higher than the solar collector, so it becomes a bulky protrusion on the roof. Modern thermosyphon systems place the tank on its side, along the top edge of the solar collector panel (see the photo), but while modern solar water heating collectors look like skylights, many homeowners are resistant to the idea of a tank on their roof.
An additional concern is weight: While the ICS system spreads its hot water storage over a greater roof area, the weight in a thermosyphon system storage tank is usually more concentrated. An older roof structure may not be able to support the added weight of a hot water storage tank.
Why not use solar electric power for water heating?
This is an excellent question. Understanding the answer provides some insight into why, 50 to 100 years from now, solar water heaters will still be the best way to heat water.
And why you should go ahead and install a solar water heater today?
The issue is conversion efficiency. A typical flat plate solar water heating collector transfers about 63 percent of the solar energy that strikes it directly into the water (or heat transfer fluid, in indirect circulation systems).
A typical photovoltaic (PV) solar electric cell converts only 15 percent or so of the energy striking it into electrical energy under ideal conditions: PV cells lose efficiency as operating temperatures rise. Additional inefficiency occurs when an inverter changes direct solar electric current (DC) into alternating current (AC). The end result is that a PV cell with a 15 percent rated peak efficiency only delivers about 10 percent of the energy striking its surface to a demand load.
So here is the problem: The solar electric PV panels would require more than six times the roof area of a flat plate solar thermal collector to meet the same (water heating) demand load.
Meeting the hot water needs of an average Florida family of four requires about 40 square feet of flat plate solar water heating collector surface area, so to do the same job with PV panels would take about 40 x 6 = 240 square feet of PV panel surface area.
References and Notes
The best freeze protection in a sunbelt climate with very rare freezing weather is to manually drain the solar collector: Turn off the gate valves on the circulating lines that connect the solar collector to the hot water storage tank, attach a garden hose to a drain spigot on the solar feed line and open the spigot to drain water out of the solar collector. The solar loop gate valves and drain spigot are usually located just above the hot water storage tank.
Roughly half of all Florida homes had thermosyphon solar water heaters during the 1940s, with an estimated 60,000 systems in Miami.
Thermosyphon system solar collectors were often separated from the hot water storage tank in old Florida “Cracker” homes. The tank was typically installed in the attic in an upright position, often in the center of the space under the peak of the roof. The solar collector panel was located at a lower point on the roof so the collector panel inlet was lower than the bottom of the hot water storage tank