How It Works?

Solar water heaters use thermal panels to collect heat from the sun's rays. These panels, which require an unshaded south-facing location, are usually installed on a rooftop.
The heat from the sun is used to warm a liquid that circulates through pipes connecting the panels to the water heater. The liquid can be either the home water itself, or a heat-transfer fluid like an environmentally safe antifreeze.
If an antifreeze mixture is used, the heat is extracted from the fluid through a heat exchanger inside the water heater, just like a standard natural gas or electric model.

The sun heats a non-toxic, freeze-proof solar fluid in a solar collector on the roof.
The collector detects that the solar fluid is hotter than the water in the hot water tank.
A pump in the Solar Station pumps solar fluid from the collector to the hot water tank.
The solar fluid flows through a pipe coiling inside the hot water tank, and heats the water in the tank.
The hot water is available for use in the house.
The solar fluid is pumped back up to the collector, where it is once again heated by the sun.

There are two types of solar water heating systems:
active, which have circulating pumps and controls, and passive, which don't.

Images from EERE Consumer's guide

Solar water heating systems include storage tanks and solar collectors. Solar storage tanks have an additional outlet and inlet connected to and from the collector. In two-tank systems, the solar water heater preheats water before it enters the conventional water heater. In one-tank systems, the back-up heater is combined with the solar storage in one tank.

Three types of solar collectors are used for residential applications:
• Flat-plate collector
Glazed flat-plate collectors are insulated, weatherproofed boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers. Unglazed flat-plate collectors—typically used for solar pool heating—have a dark absorber plate, made of metal or polymer, without a cover or enclosure. Unglazed, or pool collectors such as those manufactured by TechnoSolis, are also called low temperature collectors. These collectors are efficient when the difference in temperature between ambient and desired water temperature is low (around 10-15 degrees). Glazed flat plate collectors are medium temperature collectors. They are efficient when the difference in temperature between ambient and desired water temperature is up to 60 degrees above ambient.

Glazed Solar Collectors

Unglazed Solar Collectors

•Integral collector-storage systems
Also known as ICS or batch systems, they feature one or more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector, which preheats the water. The water then continues on to the conventional backup water heater, providing a reliable source of hot water. They should be installed only in mild-freeze climates because the outdoor pipes could freeze in severe, cold weather. These systems are not durable choices in the southwest region of the U.S. where freezing temperatures are common.

• Evacuated-tube solar collectors
They feature parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin's coating absorbs solar energy but inhibits radiative heat loss. These collectors are used more frequently for U.S. commercial applications. These collectors are efficient when the temperature difference between the desired temperature and the ambient temperature is high, and consequently are also called high temperature collectors. They are efficient when the difference in temperature between ambient and desired water temperature is over 60 degrees above ambient.

Evacuated Tube Solar Collectors

El Paso’s yearly weather has several days of freezing temperatures. The solar hot water systems must have freeze protection.
SSL has chosen an active, indirect, closed loop system for this reason. A closed loop solar hot water system consists of a solar collector, a hot water tank with a back up heating system and a heat exchanger that either circulates an antifreeze solution or pure distilled water as a heat transfer fluid.
American Energy Technologies markets a drain back system with electric or gas backup. While Schuco international markets a steamback system. The differences are mainly in the freeze protection offered by the two.

Take a look at the animated demonstration of how this system works by clicking on the image. It will open in a new window.