Solar Hot Water (Solar-thermal) Systems
Flat Plate or Tube Collectors Save Energy With Solar Power
Nov 4, 2009
Solar hot water systems (or solar-thermal systems) use the radiant energy of the sun to heat a liquid, usually glycol, that is then used to heat domestic water, radiant floors, towel warmers, pools, spas, and even to preheat air for a heat pump. They are very cost effective systems providing between 65 and 70 percent of a typical household’s hot water needs according the the U.S. Department of Energy. Return on investment can range from five to seven years. The more hot water used, the quicker the payback period.
Difference Between Solar Hot Water (Solar Thermal) and Photo-Voltaic (PV)
When most people think of solar panels, they think of photo-voltaic or PV panels that generate electricity from the sun. Solar hot water systems do not generate electricity. They generate heat.
Although they won’t make the meter run backward, solar thermal systems can save a great deal on energy costs because domestic hot water (used for showers, dishwashing and laundry) is one of the largest home energy expenses. When solar hot water is also used for space heating, the savings is even greater.
Depending upon the application, solar hot water systems can be less costly and require less space for the solar collectors than photo-voltaic systems. With lower initial costs, payback periods are often shorter for solar-thermal than for photo-voltaic.
How Do They Work?
Glycol is circulated through the system between a solar collector on the roof or lawn and a heat exchanger at the hot water tank. The heat exchanger can be inside a special solar hot water tank or adjacent to a conventional hot water tank. In either case, hot glycol flowing through the heat exchanger heats the water inside the tank. The tank water is tied into the household water supply and is used for sinks, showers, laundry, dishes, etc. If the system is also used for space heating, there may be a second heat exchanger that delivers hot glycol or water to the space heating applications (radiators, floors, heat pump for pre-heating). (Illustration of home system)
In locations where the temperature never drops below freezing, water can be substituted for glycol. Another alternative eliminates the heat exchanger. The glycol or water flowing through the collectors is circulated directly through the tank. These systems are only applicable in warm climates and may or may not be allowed by local code.
Different Kinds of Systems: Glass Vacuum Tubes vs. Flat Plate Collectors
Solar hot water systems all function in basically the same way. But there are two different types of collectors that can be used on the roof to gather solar heat. Vacuum tube systems use an array of glass tubes varying in size, but roughly six feet long by 2.5 to 4 inches in diameter. Aluminum fins inside the glass tubes absorb and transfer solar heat to a condenser pipe that sits inside the manifold. Inside the manifold, glycol circulates past the condenser pipes, absorbing heat for transfer to the water tank.
Flat plate collectors work in a similar fashion, except that the fluid is circulated through a length of tubing within the flat glass plates. The sun heats the fluid as it passes through the tubing after which it is sent to the water tank.
Some people prefer the aesthetic of the flat plates because they resemble skylights when installed on a roof. Tube systems have a distinct appearance that some feel are more conspicuous. Flat plate collectors tend to be less expensive than evacuated tubes, but in colder climates, evacuated tubes are more efficient than flat plates. The vacuum inside the tube prevents the surrounding cold air and wind from reducing the temperature at the collector. Flat plate collectors, lacking a vacuum, can lose heat to the surrounding environment.
How Is The System Controlled?
Systems vary, but basically, a series of sensors and pumps control the glycol (or water) circulating through the system. A heat sensor switches on the glycol pump when the temperature at the solar collector is higher than the return temperature by a fixed amount (often factory set to 10 degrees Fahrenheit but can usually be adjusted as needed.)
This pump circulates the glycol solution from the collector to a heat exchanger either within the solar hot water tank, or installed adjacent to conventional water heaters. (When used in conjunction with a conventional hot water tank, a second pump circulates domestic water through the heat exchanger and to the hot water tank.) The cycle continues until the tank water reaches a pre-determined maximum temperature (maximum temperature is usually adjustable).
When the pre-set temperature is reached, the controller switches off the pumps. When the temperature in the tank falls below a preset minimum, or when the collector’s sensor temperature rises above the return temperature by a fixed amount, the cycle will begin again.
The check valve (non-return valve) prevents heat in the tank from rising toward the collector (at night). A tempering valve (required on solar hot-water systems) is located at the hot water discharge and regulates the hot water supply. This prevents scalding water from circulating in the building.
A back up system such as an electric element within the solar tank, a gas-fired on demand heater or a conventional water tank must remain in service for use during prolonged periods of rain or heavy cloud cover.
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