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Solar hot water systems

Most people would be surprised to know that 12-17% of their total home energy use goes towards heating hot water according to the U.S. Department of Energy. The reason is simple, most of us, at least in North America, use storage tank type water heaters which waste a lot of energy heating hot water when no one is using it. Granted newer units have a bit more insulation than the old ones did but for the most part conventional natural gas or electric storage water tanks are very inefficient systems for heating hot water.

Solar thermal hot water systems can serve as an effective supplement or alternative to tank-style hot water heating systems and can significantly reduce your energy costs. During the late 1970's and early 1980's there was a big surge in their use because the Carter administration provided incentives for using them. However, these early systems were not as efficient as they could be and were often installed by some less than trustworthy people. All of this served to give this emerging solar technology a bad name.  Fortunately there is now a new generation of solar water heating systems which are far more efficient and reliable and which are installed by experienced and trustworthy contractors.

 

There are three primary types of solar systems for heating water; batch collectors, flat plat collectors and evacuated tube collectors.  Each has specific advantages and disadvantages depending upon the climate you live in and the demands you have for hot water. These will be explained below.

Batch Collectors

Batch collector systems are very popular in tropical or warm climates. They usually consist of one or more water tanks that sits on your roof instead of your basement or garage and they use the heat of the sun directly to warm the water. Most batch systems use one or two hot water tanks of between 20 and 40 gallons.  These sit in glass enclosed containers which capture the heat of the sun. The plumbing for these types of systems is pretty simple.  Cold water enters the tank on the roof.  It is heated by the sun and then is drained off as hot water is needed. They are sometimes referred to as direct or open-loop systems.

Batch type solar water heaters tend to be used only in warmer climates because they are susceptible to freezing.  One option is to drain your tanks during the winter but getting on your roof in the middle of winter to drain your tanks is not high on most people's favorite things to do.

If you live in a very warm climate such as California or Arizona, you can actually find that batch type collectors can heat the water to too high a temperature (over 160 degrees Fahrenheit). In these situations you might also need a mixing or tempering valve which allows the overly heated water to be mixed with cool water in order to obtain an optimal temperature. Most commercial batch hot water systems come be purchased with sensor driven valves of this type.

Flat Plate Collectors

A more common type of solar hot water system are those that use flat black metal plates to collect heat.  The heat can then be transferred to heat the hot water.  early solar hot water systems used an electric pump to circulate hot water over the plate to collect the heat.  The problem with this approach is that the water in the tubes can freeze in winter. To solve this problem some newer systems circulate a freeze proof liquid, think antifreeze, over the solar plates and then use a heat exchanger to transfer that heat to your hot water tank. Such systems are a bit more expensive but have a long life cycle.

Some solar hot water systems are actually hybrid systems in that the use the sun directly to heat the water, but also indirectly by adding photovoltaic panels to run the pumps that pump the fluid over the collector plates. These can add even greater efficiency to the system.

Hot water systems generally require sensors.  The simple reason being that you don't want your pump running if the water in the collector is cooler than the water in your hot water tank.

Evacuated Tube Collectors

Evacuated tube collects are the newest of the technologies for collecting hot water. Evacuated tube collectors consist of a series of long transparent glass tubes. Inside each tube is a copper pipe called the absorber tube. The absorber tube is covered with an absorbent material to collect the heat from the sun. Inside the inner tube there is a fluid designed for heat transfer, usually methanol.  When the tube is manufactured the air in the outer tube is pumped out creating a vacuum (as in evacuated). Vacuums are very poor conductors of heat.  Therefore the heat in the inner tube once captured cannot radiate out again and therefore a higher percentage of the captured heat is retained making them more efficient then conventional flat plat collectors.

The great thing about evacuated tube collectors is that because of their greater efficiency they can work well on cloudy days and in colder climates. Some manufacturers claim they will capture as much as 80% of the available radiant energy. Another advantage of these types of systems is that the vacuum in the tube prevents condensation from collecting, which sometimes happens in flat plate collectors.  The disadvantage is that these systems are more expensive, though they are rapidly becoming cost competitive with more traditional flat plate collection systems. Being a fairly new technology reliability data is rather limited but this is definitely a technology to keep your eye on.

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