What Is It and How Does It Work?
Solar thermal systems use energy from the sun to heat water in a solar collector. The water can then be used either to heat your home or in your hot water system. A solar thermal system (also sometimes called a solar water heater) can be active (using mechanical pumps) or passive (operating solely by thermal properties and gravity). The system includes solar collectors on the roof, a circulation system, a storage tank, and a system to control the flow of water. Many solar thermal systems also include a backup mechanical hot water heater.
Solar thermal systems work by collecting solar energy in the solar collector on your roof. Water passes through the solar collector until it gets hot. The water is then circulated to a tank, where it either can be used directly as hot water or used indirectly to heat water that is used in shower and sinks. In climates where freezing temperatures are common, it is best to use an antifreeze solution in place of water. In this case, when the solution is heated, it enters the tank in pipes to indirectly heat the water.
Why Should I Consider It?
Solar thermal systems can be effective tools for heating water for a variety of uses in your home. They use the energy of the sun to heat water, and although they typically cost more to install than traditional mechanical hot water systems, they are very cheap to operate and can help you save money. They can be implemented in different ways, including as stand alone systems that require no backup mechanical heating, or in conjunction with a traditional hot water heater. Even installing a solar thermal system that works with a traditional hot water heater can save money in the long run by preheating water so the hot water heater does not need to work as hard or use as much energy.
What Are the Options?
There are two types of active solar thermal systems that use mechanical pumps to circulate water or an antifreeze fluid through the system. In the first type of system, direct circulation, water from your home is pumped into the collector where it is heated. Then the water drains into a tank where it can be used as hot water in your shower and sinks. Indirect circulation solar thermal systems typically use an antifreeze fluid to transfer heat to water stored in a tank. The fluid is pumped to the solar collectors on your roof, where it is heated before draining into a pipe that circulates through the water tank, transferring heat to the water that is stored in the tank.
There also two types of passive solar thermal systems that use thermal properties and gravity to circulate heated water through the system. Integral collector-storage combines the functions of the storage tank and collector into one. There are two pipes attached to the collector-storage unit—one low pipe supplies cold water and one high pipe draws heated water from the collector-storage as it rises within the tank. The hot water then circulates to a second tank from which the house's hot water system can draw the heated water. In a thermosyphon system, the water (or an antifreeze fluid) is heated in the collector and rises to an outlet pipe at the top. In this system, the tank must be located above the collector (in all of the other systems the tank can be below the collector), where the water is stored after being heated in the collector.
Each type of active and passive solar thermal system makes use of one of a series of types of solar collectors. The most common type of collector is the flat-plate collector, which is a thin, insulated box with a translucent glass cover and a dark absorber plate inside. Small tubes containing either water or an antifreeze solution pass through the box. The absorber plate gets very hot and then transfers its heat to the fluid in the pipes. Flat-plate collectors can be used with direct, indirect, or thermosyphon systems.
Evacuated tube collectors include an array of parallel transparent glass tubes. The tubes contain an absorptive surface on the inside and a selective coating to promote absorption and reduce emissivity. The tubes also contain an antifreeze fluid that is heated and circulated throughout the rest of the system. This type of collector is only used with indirect circulation systems.
Batch systems (also known as integral collector-storage systems) are solar thermal collectors made up of an insulated box with a large tank or tube inside, essentially combining the roles of collector and tank into one. The interior absorptive surfaces are painted black and the face of the box is glazed. This type of system is not as common as flat-plate collectors or evacuated tubes, but it is an option to use with direct circulation and thermosyphon systems.
How Is It Installed?
Solar thermal systems can be integrated into the design of a new house or, in many cases, added on as a retrofit to an existing building. It is best to work with a qualified solar thermal system contractor for installation. Once installation is complete, your installer should provide you with a maintenance guide to keep your system operating efficiently.
What Are the Potential Benefits?
The benefit of installing a solar thermal system is that, once it is installed, you can generate much of your hot water needs for just the minor cost of occasional maintenance and with no need, or greatly diminished need, to pay for electricity or natural gas to heat water.
How Much Does It Cost?
Solar thermal water heating systems cost more to install than a standard mechanical hot water heating system. However, they can also save money in the longer term. Solar thermal systems can reduce water heating bills by 50 to 80 percent. If you include the cost of installing a solar thermal system in a 30-year mortgage, the cost should be no more than $13 to $20 per month. After subtracting federal income tax deductions and incentives, the additional cost per month on your mortgage will likely be less than the amount you will save per month on utility bills.
What Else Should I Know?
The first thing to do is to evaluate the solar resources at your house. You can take advantage of both direct and diffuse solar energy in order to operate a solar thermal system, but your system will produce more hot water if it receives more sunlight.
Your installer should calculate how big your system will need to be, but you can also get a head start by estimating the size. For collector area, assume that you will need 20 square feet for two people. For each additional person, you should add 8 square feet to your estimate if you live in a sunny climate and 12 to 14 square feet if you live in a cool, northern climate. If you have a small household of two or three people, you will need a 50 to 60 gallon storage tank. The more people you have in your home, the larger your tank will need to be.
Where Can I Get More Information?
