Solar buildings technologies use the non-polluting power of the sun to help heat, cool, and power our buildings. Because buildings now use one-third of the energy currently consumed in this country, the potential markets for using solar buildings technologies in place of conventional ones is substantial.
Whether you're a home owner, architect, builder, business owner, or industry or utility representative, discover the many available technologies to harness the sun to heat water and provide space heating and cooling for your buildings.
Solar collectors are at the heart of most active solar energy systems. The collector absorbs the sun's light energy and changes it into heat energy. This thermal energy can then be used to provide heated water for residential or commercial use, to provide space heating or cooling, or for many other applications where fossil fuels might otherwise be used.
Solar thermal collectors are the key component of active solar systems, and are designed to meet the specific temperature requirements and climate conditions for the different end-uses. There are several types of solar collectors:
- Flat-plate collectors
- Evacuated-tube collectors
- Concentrating collectors
- Transpired air collectors
Residential and commercial building applications that require temperatures below 200°F typically use flat-plate or transpired air collectors, whereas those requiring temperatures greater than 200°F use evacuated-tube or concentrating collectors.
Flat-plate collectors are the most common collector for residential water-heating and space-heating installations. A typical flat-plate collector is an insulated metal box with a glass or plastic cover (called the glazing) and a dark-colored absorber plate. These collectors heat either liquid or air at low temperatures (less than 180°F).
Liquid flat-plate collectors heat liquid as it flows through tubes in or adjacent to the absorber plate. The simplest liquid systems use potable household water, which is heated as it passes directly through the collector and then flows to the house.
Swimming pool–heating systems use liquid flat-plate collector technology. The pool's existing filtration system pumps water through the solar collectors, and the collected heat is transferred into the pool. Because solar pool-heating collectors operate just slightly warmer than the surrounding air temperature, these systems typically use inexpensive, unglazed low-temperature collectors made from specially formulated plastic materials. Glazed (glass-covered) solar collectors usually are not used in pool-heating applications, except for indoor pools, hot tubs, or spas in colder climates. In some cases, unglazed copper or copper-aluminum solar collectors are used.
Air flat-plate collectors are used primarily for space heating. The absorber plates in air collectors can be metal sheets, layers of screen, or non-metallic materials. The air flows past the absorber by using natural convection or a fan. Because air conducts heat much less readily than liquid does, less heat is transferred from an air collector's absorber than from a liquid collector's absorber.
Integral collector storage (ICS) collectors (also called 'batch' or 'breadbox' water heaters) combine the collector and storage tank into an insulated box with a glazed side facing the sun. The sun shining into the collector strikes the storage tank, directly heating the water. In colder climates, the use of double glazing and selective surfaces will prevent freeze damage to the collector. In even mildly cold climates, installation and maintenance of insulation is needed to prevent supply and return pipes from freezing.
Evacuated-tube collectors are typically more efficient at higher temperatures than flat-plate collectors. In an evacuated-tube collector, sunlight enters through the outer glass tube and strikes the absorber, where the energy is converted to heat. The heat is transferred to the liquid flowing through the absorber. The collector consists of rows of parallel transparent glass tubes, each of which contains an absorber covered with a selective coating. The absorber typically is of tin-tube design, although cylindrical absorbers also are used.
When evacuated tubes are manufactured, air is evacuated from the space between the two tubes, forming a vacuum. Convective and conductive heat losses are eliminated because there is no air to convect or conduct heat.
As a result, evacuated-tube collectors are more efficient at higher temperatures than flat-plate collectors. They perform well in both direct and diffuse solar radiation. Evacuated-tube collectors are more appropriate for most commercial and industrial applications due to the extremely high temperatures they can achieve (170°-350°F). However, evacuated-tube collectors are more expensive than flate-plate collectors.
Concentrating collectors use curved mirrors to concentrate sunlight on the receiver at up to 60 times its normal intensity. These high-temperature systems are used primarily in commercial and industrial applications.
Parabolic trough collectors use trough-shaped reflectors that concentrate sunlight on a receiver tube running along the reflector's focal line, achieving much higher temperatures than flat-plate or evacuated-tube collectors. These systems usually include a mechanical control system that keeps the trough reflector pointed at the sun throughout the day. Parabolic-trough concentrating systems can provide hot water and steam, and are generally used in commercial and industrial applications.
Compound parabolic concentrating collectors (CPCCs) use mirrored surfaces to concentrate the sun's energy on an absorber called a receiver, similar to parabolic trough collectors. CPCCs achieve moderate concentration and moderately high temperatures but, unlike parabolic trough collectors, they can collect both direct and diffuse sunlight and don't require an automated sun-tracking system. CPCCs are being investigated for use in commercial applications where higher temperatures are required.
Transpired air collectors are made of dark, perforated metal. The sun heats the metal, and a fan pulls ambient air through the holes in the metal, which heats the air. They have been used for pre-heating ventilation air and for crop drying.
Transpired air collectors have achieved efficiencies of more than 70% in some commercial applications. Because they require no glazing or insulation, transpired air collectors are inexpensive to manufacture. All these factors result in a very cost-effective source of solar heat. In fact, R&D Magazine recognized transpired air collectors as one of the 100 most important technology innovations in 1995!
Residential and Commercial Water Heating
Solar water heaters use the sun to heat either water or a heat-transfer fluid in collectors. That water is then stored for use as needed, with a conventional system providing any necessary additional heating. A typical system will reduce the need for conventional water heating by about two-thirds, minimizing the cost of electricity or fossil fuel and the environmental impacts associated with their use.
Active systems use electric pumps, valves, and controllers to circulate water or other heat-transfer fluids through the collectors. There are three types of active systems:
- Open-loop active systems use pumps to circulate water through the collectors. These systems are appropriate in areas that do not freeze for long periods and do not have hard or acidic water.
- Closed-loop active systems pump heat-transfer fluids such as a mixture of glycol and water antifreeze through collectors. Heat exchangers transfer the heat from the fluid to the water stored in the tanks.
- Drainback systems use pumps to circulate water through the collectors. Because the water in the collector loop drains into a reservoir tank when the pumps stop, this is a good system for colder climates.
Systems that move water or heat-transfer fluid through the system without pumps are called passive systems. Because they contain no electric components, passive systems are generally more reliable, easier to maintain, and possibly longer lasting than active systems.
- Batch heaters or integral collector storage systems consist
of one or more storage tanks placed in an insulated box with a glazed side facing the sun. During the winter, they must be protected from freezing or drained.
- Thermosiphon systems rely on natural convection warm water rising to circulate water through the collectors and to the tank, which is located above the collector. As water in the solar collector heats, it becomes lighter and rises naturally into the tank above. Meanwhile, the tank's cooler water below flows down pipes to the bottom of the collector, causing circulation throughout the system.
The U.S. Department of Energy estimates that Americans consume about 2.5 quads of end-use energy annually to produce hot water at a cost of more than $20 billion. Yet solar energy provides only a tiny fraction of that demand. Today's solar water-heating systems can provide 40%-80% of a typical household's hot water demand, depending on the local climate, system size, and type.
The current residential water heater replacement market represents a market potential of 27 million systems within the United States. Potential exists for an estimated 700,000 additional systems per year from new home construction in areas suitable for solar.
Homeowners and business owners have installed more than a million solar water-heating systems in the United States. These consumers have realized the many benefits of using the sun to heat their water—for both domestic purposes and for swimming pools. Solar water heating is clean, safe, affordable, and easy to install. Plus, because solar water heating takes advantage of a plentiful energy resource, the use of electricity and fuel is lower, meaning a lower utility bill!
Space Heating and Cooling
Medium-temperature solar collectors are used for space heating and operate in much the same way as indirect solar water-heating systems, but they have a larger collector array area, larger storage units, and more complex control systems. They can also be configured to provide solar water heating and typically provide 30%-70% of the residential heating—or combined heating and hot water—requirements. Active space-heating systems require more sophisticated design, installation, and maintenance techniques.
Transpired air collectors, mounted as an exterior cladding on a building's south-facing wall, are used for ventilation preheating. These collectors are unglazed. A blower or fan is used to draw air through perforations in the wall to deliver ventilation air into the building. Solar ventilation air preheating systems are generally used in commercial and industrial applications that require large quantities of ventilation air, including warehouses, large manufacturing plants, and airplane maintenance hangars.
Solar process heating systems are designed to meet the need for large quantities of hot water or space heating at commercial/industrial/institutional buildings. A typical system consists of several thousand square feet of ground-mounted collectors, combined with pumps, heat exchangers, controls, and one or more large-volume storage tanks. Solar process heating systems have successfully developed niche markets in federal and state governments, where facilities such as schools, military bases, office buildings, and prisons use hot water for bathing, cooking, laundry, and space heating.
Cooling and refrigeration using active solar cooling systems can provide for year-round utilization of collected solar heat, thereby significantly increasing the cost effectiveness and energy contribution of solar installations. These systems are sized to provide 30%-60% of the building's cooling requirements. Solar-driven absorption systems utilize an active cooling technology option that currently appears to have the greatest potential.
A solar absorption system uses the thermal energy from the solar collector to separate a binary mixture of an absorbent and a refrigerant fluid. The refrigerant is condensed, throttled, and evaporated to yield a cooling effect, after which it is reabsorbed to continue the cycle. Due to the high-temperature requirements of absorption cooling systems, evacuated-tube or concentrating collectors are typically used.
Several advanced concepts in solar thermal energy show sufficient promise for future markets. Research and development efforts sponsored by DOE could lead to cost-competitive energy options for several applications.
Low-Cost Solar Water Heating
This concept involves replacing copper and glass in current solar domestic hot water system designs with lower-cost materials, especially polymers and elastomers. Potential polymer solar components include unpressurized integral collector storage and a flat-plate collector. Researchers are examining the durability of thin films and other polymer components.
- Collectors for Crop Drying
The transpired air collector is a fundamentally new type of air-heating collector with significant advantages in cost, efficiency, simplicity, and durability. While these collectors have been successfully used to preheat building ventilation air, researchers are finding that transpired collectors may be applicable to other commercial uses such as crop drying. The international market potential for low-cost transpired air collectors is great especially in developing countries, where vast quantities of coffee, grains, fruits, vegetables, and other crops are harvested and conventional fuels are expensive or unavailable.
- Solar Absorption Cooling
Solar absorption cooling shows promise for commercialization. DOE recently issued a solicitation for proposals to significantly reduce the life-cycle energy cost of solar absorption cooling technology; improve the technical feasibility of advanced-cycle solar absorption cooling; and bring the technology closer to commercialization.
In addition to the concepts above, other solar thermal applications such as water disinfection or desalination could be researched and developed for future markets, especially in developing countries.