Types of Solar Water Heaters

Approximately one million residential and 200,000 commercial solar water-heating systems have been installed in America. Although there are many different types of solar water heaters available on the market, the basic technology is very simple.

Sunlight strikes and heats an “absorber” surface within a “solar collector” or an actual storage container. These roof-mounted solar heaters supply around 80% of the hot water for the home. Either a heat-transfer fluid or the real potable water to be used flows through tubes attached to the absorber and picks up the heat from it. (Systems with a separate heat-transfer-fluid loop comprise a heat exchanger that then heats the potable water.) The heated water is stored in a separate preheat tank or a conventional water heater tank until required.

If additional heat is required, it is provided by the conventional water-heating system for electricity or gas water heaters.

How a Solar Water Heater Works

By lowering the amount of heat that must be provided by conventional water heating, solar water-heating systems directly substitute renewable energy for traditional energy, reducing the use of electricity or fossil fuels by as much as 80%.

The current types of solar pool water-heating systems available on the market are proven reliable when correctly matched to climate and load. The present market consists of a relatively small number of manufacturers and installers that provide dependable equipment and quality system design.

Quality assurance and performance-rating program for solar water-heating systems, instituted by a voluntary association of the solar industry and various customer groups, makes it easier to select reliable equipment with confidence.

Homeowners should investigate the intricate details of installing solar hot water heating systems to reduce energy use. However, before sizing a solar system, water-use reduction strategies should be practiced.

Different Types of Solar Hot Water Heaters

There are five types of solar thermal water heaters:

• Thermosiphon Systems. These systems warm water or an antifreeze fluid, such as glycol. The fluid rises by natural convection from collectors to the storage container, which is placed at a higher level. No pumps are needed. In thermosiphon systems, fluid movement, and hence heat transfer, increases with temperature, so these systems are most efficient in areas with high levels of solar radiation.
• Direct-Circulation Systems.These systems pump water from storage to collectors during sunny seasons. Freeze protection is attained by recirculating hot water from the storage tank, or by flushing the collectors (drain-down). Since the recirculation system raises the use of energy, while flushing reduces the hours of operation, direct-circulation systems are used only in areas where freezing temperatures are infrequent.
• Drain-Down Systems.These systems are usually indirect water-heating systems. Treated or untreated water is circulated through a closed-loop, and heat is moved to potable water through a heat exchanger. When no solar heat is available, the collector fluid is drained by gravity to avoid freezing and convection loops in which cool collector water lowers the temperature of the stored water.
• Indirect Water-Heating Systems.In these systems, freeze-protected fluid is circulated through a closed-loop, and its heat is transferred to potable water through a heat exchanger with 80 to 90 percent efficacy. The most generally used fluids for freeze protection are water-ethylene glycol solutions and water-propylene glycol solutions.
• Air Systems. In the air system, the collectors heat the air, which is moved by a fan through an air-to-water heat exchanger. The water is then used for domestic or service requirements. The efficacy of the heat exchanger is in the 50% range.

Direct-circulation, thermosiphon, or pump-activated systems need higher maintenance in freezing climates. For most in the United States, indirect air and water systems are the most suitable. While not as efficient as water systems, solar air systems should be considered if maintenance is a significant concern since they do not leak or burst.

Resources and References:

• Water Heating Systems
• Types of Solar Water Heaters