These solar collectors provide 80% of the hot water for the residence.

Solar Hot Water Heating Types

An estimated one million residential and 200,000 commercial solar water-heating systems have been installed in the United States. Although there are a large number of different types of solar hot water heating systems, the basic technology is very simple. Sunlight strikes and heats an "absorber" surface within a "solar collector" or an actual storage tank. Either a heat-transfer fluid or the actual 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 include a heat exchanger that then heats the potable water.) The heated solar hot water is stored in a separate preheat solar hot water tank or a conventional water heater tank until needed. If additional heat is needed, it is provided by electricity or fossil-fuel energy by the conventional water-heating system. By reducing the amount of heat that must be provided by conventional water-heating, solar hot water heating systems directly substitute renewable energy for conventional energy, reducing the use of electricity or fossil fuels by as much as 80%.

Today's solar hot water heating systems are well proven and reliable when correctly matched to climate and load. The current market consists of a relatively small number of manufacturers and installers that provide reliable equipment and quality system design. A quality assurance and performance-rating program for solar hot water heating systems, instituted by a voluntary association of the solar industry and various consumer groups, makes it easier to select reliable equipment with confidence. Building owners should investigate installing solar hot water-heating systems to reduce energy use. Before sizing a solar hot water system, water-use reduction strategies should be put into practice.

There are five types of solar hot water systems:

• Thermosiphon Systems





• Direct-Circulation Systems





• Drain-Down Systems





• Indirect Solar Hot Water Heating Systems





• Air Systems

Thermosiphon solar hot water heaters are shown on employee housing at Yosemite National Park.

Thermosiphon Systems. These systems heat solar hot water or an antifreeze fluid, such as glycol. The fluid rises by natural convection from collectors to the storage tank, which is placed at a higher level. No pumps are required. In thermosiphon systems fluid movement, and therefore 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 solar hot water from storage to collectors during sunny hours. Freeze protection is obtained by recirculating solar hot water from the storage tank, or by flushing the collectors (drain-down). Since the recirculation system increases energy use 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 generally indirect solar hot water heating systems. Treated or untreated solar hot water is circulated through a closed loop, and heat is transferred 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 reduces the temperature of the stored solar hot water.

Indirect Solar Hot 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% efficiency. The most commonly used fluids for freeze protection are water-ethylene glycol solutions and water-propylene glycol solutions.

Air Systems. In this indirect 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 needs. The efficiency of the heat exchanger is in the 50% range. Direct-circulation, thermosiphon, or pump-activated systems, require higher maintenance in freezing climates. For most of the United States, indirect air and solar hot water systems are the most appropriate. Air solar systems, while not as efficient as solar hot water sytems, should be considered if maintenance is a primary concern since they do not leak or burst.

Types of Collectors

There are basically three types of collectors: flat-plate, evacuated-tube, and concentrating.

A close-up view of flat-plate collectors.

A flat-plate collector, the most common type, is an insulated, weatherproofed box containing a dark absorber plate under one or more transparent or translucent covers.

Evacuated-tube collectors are made up of rows of parallel, transparent glass tubes. Each tube consists of a glass outer tube and an inner tube, or absorber, covered with a selective coating that absorbs solar energy well but inhibits radiative heat loss. The air is withdrawn (evacuated) from the space between the tubes to form a vacuum, which eliminates conductive and convective heat loss. The vacuum also helps them achieve extremely high temperatures (170°-350° F); so they are appropriate for commercial and industrial uses.

Concentrating collectors are usually parabolic troughs that use mirrored surfaces to concentrate the sun's energy on an absorber tube (called a receiver) containing a heat-transfer fluid. They provide solar hot water and steam, usually for industrial and commercial applications.

Parabolic-trough collectors use curved mirrors to focus the sunlight on a receiver tube (sometimes encased in an evacuated tube) running through the focal point of the mirrors and can heat their transfer fluid to as much as 570°F (299°C). Because they use only direct-beam sunlight, parabolic-trough systems require tracking systems to keep them focused toward the sun and are best suited to areas with high direct solar radiation. Because they are particularly susceptible to transmitting structural stress from wind loading and require large areas for installation, parabolic-trough collectors are usually ground mounted. For electrical generation or industrial uses that require very high temperatures (greater than 392°F [200°C]), a heat-transfer fluid such as oil is used, but depending on the degree of danger of freezing, antifreeze or water is used in the heat-transfer loop for domestic water-heating systems. Parabolic-trough collectors generally require greater maintenance and supervision and particularly benefit from economies of scale, so are generally used for larger systems.

Parabolic-trough solar hot water heating system at a prison

Low-, Mid-, and High-Temperature Collectors

The collectors can be low-temperature, mid-temperature, or high-temperature. The glazed, flat-plate collectors most commonly used for commercial or residential domestic hot water are classified as "mid-temperature" collectors, generally increasing water temperature to as much as 160ÐF (71°C). Flat-plate collectors consist of an insulated, weather-tight housing or box, a clear glass or plastic cover glazing, a black absorber plate, and a system of passages for the heat-transfer fluid to pass through the collector. Special coatings on the absorber maximize absorption of sunlight and minimize re-radiation of heat. Gaskets and seals at the connections between the piping and the collector and around the glazing ensure a watertight system.

Low-temperature collectors, which generally increase water temperature to as much as 90°F (32°C), are less expensive because they consist simply of an absorber with flow passages and have no covering glass (glazing), insulation, or expensive materials such as aluminum or copper. These collectors are less efficient in retaining solar energy when outdoor temperatures are low, but are quite efficient when outside air temperatures are close to the temperature to which the water is being heated. They are highly suitable for swimming pool heating and other uses that require only a moderate increase in temperature and are most commonly used in warmer areas. For the last several years, they have been the most frequently installed collectors. In warm climates, low-temperature collectors are sometimes used in hybrid systems that heat a pool in the winter and supplement domestic water heating in the summer, when pool heating is not needed.

Solar Hot Water Pool Heating

Solar pool heating was used at the 1996 Summer Olympics in Atlanta. Such heating systems are one of the most cost-effective applications of solar energy. It is relatively simple to integrate a solar water heater since most pools require a pump, filter, and plumbing. With a solar energy system, the pool's water is pumped through the filter and then through a solar energy collector(s) instead of directly back to the pool. The sun heats the water in the collector(s) before it returns to the pool. Solar pool heating can be used for residential, commercial, or community swimming pools..

Swimming pool water heating is a popular use for solar collectors.

Large facilities or ones with quasi-industrial operations such as laundries may be able to efficiently use more sophisticated high-temperature collectors. Although they are also used in mid-temperature systems, evacuated-tube collectors can be designed to increase water/steam temperatures to as much as 350°F (177°C). They may use a variety of configurations, but generally encase both the absorber surface and the tubes of working fluid in a tubular glass vacuum for highly efficient insulation. Evacuated-tube collectors are the most efficient collector type for cold climates with low-level diffuse sunlight. They can be mounted either on a roof or on the ground, but they need to be protected from vandalism and can be damaged by hail or hurricanes.