Solar Space Heating
This prototype home employs active and passive solar technologies, including a hidden, roof-top solar water heater, reflective window coverings, and overhangs.
Just as solar energy can heat the water for a building, it can also heat and cool the air.
A solar space-heating system can consist of a passive system, an active system, or a combination of both. Passive systems are typically less costly and less complex than active systems. However, when retrofitting a building, active systems might be the only option for obtaining solar energy.
Passive Solar Space Heating
Passive solar space heating takes advantage of warmth from the sun through design features, such as large south-facing windows, and materials in the floors or walls that absorb warmth during the day and release that warmth at night when it is needed most. A sunspace or greenhouse is a good example of a passive system for solar space heating.
Passive solar design systems usually have one of three designs:
- Direct gain (the simplest system) stores and slowly releases heat energy collected from the sun shining directly into the building and warming materials such as tile or concrete. Care must be taken to avoid overheating the space.
- Indirect gain (similar to direct gain) uses materials that hold, store, and release heat; the material is located between the sun and living space (typically the wall) and warming materials such as tile or concrete. Care must be taken to avoid overheating the space.
- Isolated gain (collects solar energy remote from the location of the primary living area. For example, a sunroom attached to a house collects warmer air that flows naturally to the rest of the house.
Active Solar Space Heating
Active solar space-heating systems consist of collectors that collect and absorb solar radiation combined with electric fans or pumps to transfer and distribute that solar heat. Active systems also generally have an energy-storage system to provide heat when the sun is not shining. The two basic types of active solar space-heating systems use either liquid or air as the heat-transfer medium in their solar energy collectors.
Liquid-based systems heat water or an antifreeze solution in a hydronic collector. Air-based systems heat air in an air collector. Air-based solar heating systems usually employ an air-to-water heat exchanger to supply heat to the domestic hot water system, making the system useful in the summertime. Both of these systems collect and absorb solar radiation, then transfer the solar heat directly to the interior space or to a storage system, from which the heat is distributed. An auxiliary or backup system provides heat when storage is discharged. Liquid systems are more often used when storage is included.
Here is a summary of the many different types of active solar space-heating systems:
Medium-temperature solar collectors are generally used for solar space heating. Solar space heating systems operate in much the same way as indirect solar water-heating systems, but they have a larger collector area, larger storage units, and more complex control systems. They are also usually configured to provide solar water heating and typically provide 30% to 70% of the residential heating, or combined heating and hot water, requirements. Active solar space-heating systems require more sophisticated design, installation, and maintenance techniques.
A very economical, but specialized space heating system is based upon use of transpired air collectors, mounted as an exterior cladding on a south-facing wall. These systems are used for ventilation preheating. This system heats only outdoor air. These collectors are unglazed, and 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
a) Buildings that require much outdoor ventilation, such as warehouses, large manufacturing plants, and airplane maintenance hangars;
b) Crop Drying
c) Pre-heating of boiler combustion air.
Cooling and refrigeration can be accomplished using thermally activated cooling systems (TACS) driven by solar energy. These systems can provide 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% to 60% of building cooling requirements using solar, with the remainder usually dependent on TACS fueled by natural gas. The TACS available for solar-driven cooling include absorption systems and desiccant systems. Generally, solar cooling is not used because of the high initial costs of TACS and the solar fields needed to drive them.
Solar absorption systems use the thermal energy from a 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, which is then re-absorbed to continue the cycle. Double-effect absorption systems (which use the heat twice in series) are about twice as efficient as single-effect systems but require significantly higher input temperatures. Because of the high-temperature requirements of absorption cooling systems, evacuated-tube or concentrating collectors are typically used.
Solar desiccant systems use thermal energy from the solar collector to regenerate desiccants that dry ambient air; they then use that dry air in indirect and/or direct evaporative stages to provide cooled air to the load. The solar heat is used to regenerate the desiccant, driving off the absorbed water. Some systems use flat-plate collectors at intermediate temperatures.