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A solar water heater can provide from 50-100% of the home's hot water needs. That will save you money and help save the environment too. Some of the systems are sold as kits that can be added onto existing storage water heaters while others include a storage tank and in some cases a boiler for a backup heater.
The two main types of systems are active and passive systems.
In an active system, a pump is used to circulate water through a solar collector where the water is heated by the sun. The water is then pumped to the storage tank. The pump is usually controlled by a controller that senses the temperature in the collector as well as the storage tank. It compares the two temperatures and when the temperature of the water in the collector gets a certain amount higher than the temperature in the storage tank, the circulating pump is started.
In a passive system the water circulates by natural convection. This is a process where warm water rises and cold water falls. For that reason the storage tank must be mounted above the collector. These systems are sometimes referred to as thermosiphon systems or batch water heaters. The hot water is stored in the tank on the roof with the collector.
Active systems are used in moderate to cold climates. Passive systems are suitable for hotter climates where there is no danger of freezing.
In areas where the outside temperature drops below freezing, the collector is connected to an indirect fired water heater. This is a standard water tank with a heat exchanger inside. This is also referred to as a closed loop system.
The water in the solar water heater system has an antifreeze solution in it and circulates inside the heat exchanger. The heat exchanger transfers the heat from the solar system to the home's hot water system while keeping them separate.
Another option for cold climates is to drain the system during the cold months. Some systems automatically drain the water as required by the temperature and are referred to as auto drainback systems. In this case, the water that is normally inside the collectors drains down into the home and is stored in atank while the system is off.
The solar collectors are the part of the solar water heater that actually absorb the heat from the sun. There are three main types which are flat plate collectors, concentrating collectors, and evacuated tube collectors.
Flat plate collectors are the simplest and usually the least expensive. They are also the least efficient . They are basically a waterproof box with a dark absorber plate under a transparent cover. Fluid passes through copper pipes under the absorber material and absorbs heat.
Concentrating collectors are also referred to as parabolic troughs. They use mirrored surfaces to concentrate the solar energy onto the receiver (absorber plate). Although these systems are the most efficienct they are also fragile and the most expensive type. They are not typically used in residential water heating applications.
There are two basic kinds of evacuated tube solar collectors. The first type are commonly called twin-glass tube collectors. They are basically a glass tube inside of another glass tube with a vacuum between the two. This vacuum keeps them from losing heat and therefore makes them more efficient. The inner tube is coated to absorb the solar radiation. The other kind of evacuated tube collector is usually called a heat pipe. This system uses a copper pipe that is evacuated and a small quantity of water with some additives is put inside. Because of the vacuum, the water's boiling point is lowered therefore when the tube absorbs the solar radiation, the water is boiled. This boiling water rises to the top of the tube where it transfers some of its heat to the water around the header pipe. This cause the water inside the copper pipe to condense and fall back to the bottom of the pipe where the process is repeated. This copper pipe is also placed inside of a glass tube and a vacuum is pulled betwenn the two tubes. Once again, this minimizes the heat loss from the collectors. Many of these pipes are connected together at the top to form a panel.
Determining the proper size of solar water heater starts in the same way as sizing a conventional water heater system. You need to figure the number of people that are routinely using water in the home and the number of hot water outlets that will be used at the same time. This will tell you what size storage tank is required. Once you know the amount of hot water you require, you can match the solar collector system to the size of the storage tank.
**HOT TIP** Most manufacturers have packaged all of the components into a kit. This makes it easy to get the proper size because all you need to know is how many people are in the home.
If you are considering purchasing a solar water heater, you can check out our active system page for a comparison of systems that use an electrical pump to circulate the water or our passive system page for systems that do not have a pump.