Solar Water Heating Systems 101: An overview of types and parts
Posted by Lisa Carey
Solar water heaters have come a long way, yet many from the 1970s are still in operation and most have some of the same basic parts in common.
The most common parts of a solar water heating system include a solar collector and a storage collection tank. There 
will also be a heat transfer medium and plumbing. There may be additional parts such as an expansion tank and pumps on active solar water heaters (details to follow).
Solar water heating systems can provide half or more of the hot water needed for your home and save you money by reducing water heating cost.
If you’re considering buying, installing, or even building a solar water heater, it is helpful to understand the vocabulary behind solar water heater options.
Temperature and pressure relief valve:
Also called a T & P relief valve, the temperature and pressure valve is often used as a safety measure in case water gets too hot. If the system builds up too much pressure, the valve will open to release water or pressure to safe levels. This is important as some DIY solar water heater makers have learned that the water in a well-working solar heater can become scalding in a very short amount of time.
Glycol:
Glycol is a non-toxic anti-freeze sometimes used in solar water heating systems.
Active and passive solar water heaters:
Active solar water heaters use a pump to move the heat and will have a controller for turning the pump on and off.
Passive solar water heating systems do not have a pump and instead use natural convection, conduction, or even radiation to transfer the heat.
The obvious difference and reason many people choose passive over active designs is that active designs (with controls, motors, relays, and pumps) require electricity. A passive solar water heater requires little or no electricity to operate. Passive designs are also simpler and less expensive.
Active designs are more expensive and have more working parts which means more things can break or go wrong. On the other hand, active water heating systems are more efficient. Active designs are convenient, successful, and very popular.
Open vs. Closed Solar Water Heaters
An open solar water heater is when the water circulating through the system is the very water that will be used in the shower later.
A closed solar water heather system, on the other hand, separates the heated water into a loop with a heat exchanger.
A closed system obviously has added parts, so why would some choose a closed system? One answer is freezing, which has been one of the challenges in the history of solar water heating systems. When water freezes, the collector or tubes can burst, disabling your solar water heater and making a mess just like a busted water pipe elsewhere would. This is important to factor in even if you live in an area with milder weather.
Many solar water heater users have learned that in a condition called night sky radiation, water in an open collector can freeze, even when the ambient air temperature is up to 40 degrees Fahrenheit.
While these are the basic types and components of solar water heaters, look for more articles to come on seven different specific types of solar water heaters. Have you ever used a solar water heating system? Are you interested in one? We’d love to hear your stories in our comments section.
Sources:
http://www.nrel.gov/learning/re_solar_hot_water.html
http://www.treehugger.com/files/2007/08/make_a_solar_wa.php
www.infinitepower.org/newfact/new96-814-No10.pdf
http://www.backwoodshome.com/articles/hackleman65.html
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I have been looking at the possibility of using night sky radiation for cooling during the day. Do yo know anything about this potential and if it is even possible?