Solar water heaters can be either active or passive. Passive systems are cheaper because they're simpler and have fewer parts. But they're also less versatile. Active systems, on the other hand, can put out more energy and work under a wider range of weather conditions.
In addition to choosing between active and passive systems, you have a couple of other decisions to make:
Direct systems heat the water right in the collector. Open-loop systems are all direct. Water is fed into the loop and taken out of the loop after it's heated.
Indirect systems use heat exchangers. A secondary fluid (water, glycol, or antifreeze) collects the heat, which is then transferred to the water via a heat exchanger. These systems are used in cold climates, where water would freeze if it were exposed to the elements. Closed-loop systems have isolated circuits with a constantly recirculating fluid. They're all indirect and active (they require pumps). They require entirely different engineering than open-loops and are generally much more expensive because of their increased parts count and complexity.
Every passive solar water heating system has several basic functions and components:
Collectors: Sunlight must be collected and transformed into usable heat.
Flow: A flow system channels the heated water to where you plan to use it.
Controller: A controller makes judicious decisions on when and how to move the water or antifreeze liquids.
Mounting: You mount the collector to optimize the amount of sunlight received .
Active systems add a pump and an active (electrical) means of controlling the pump.
In systems with copper and metal parts, using softened water is essential because hard water will calcify and corrode some pipes. You may need to find out whether your water is soft enough. If not, you may need a water softener.
Freeze damage is a major concern. If you have no danger of freezing, you can use any type of system you want. If your climate freezes a lot, you're limited, although your options are still good. Here's how to deal with the risks:
In a process called recirculation, some (active) systems turn the pump on when the temperature gets low enough. Moving liquid will not freeze nearly as easily as stationary liquid. This method works well, but it's inefficient; the basic goal of a solar system is to collect energy, not use it to preserve the system.
Drain valves, either manual or automatic, may purge the collector and exposed pipes of all fluids. This works, but in this context, all means all because if any trace amounts linger, freeze damage can still occur. This works well, but once again, the process takes energy, and this implies inefficiency.
Closed-loop systems use antifreeze and a heat exchanger. Water in the system can never freeze. These are the most common types of systems installed on houses in North America because they're the most versatile and reliable.
Any solar water heating system presents a danger of scalding. Water over a temperature of 160 degrees F can burn you badly enough to require medical attention. You need to understand exactly what your system is doing and where the dangers lurk. If you're going to install any kind of system, even if you don't do it yourself, you should understand what's going on inside and why. Well-designed systems account for these dangers, and the systems are perfectly safe. County codes all require the use of a temper valve, which mixes hot water with supply water to ensure that water temperatures that reach a user (faucet) are safe.
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Source:http://www.dummies.com/how-to/content/solar-water-heating-systems.html
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