Active Solar Heating Systems

Select an active solar heating system and collection medium appropriate for the building's heating and cooling system. A solar heating system should be designed to be compatible and interactive with conventional HVAC systems in the building. Water-based systems tend to be most compatible with HVAC systems that also use water as a distribution mechanism, though some interface with air distribution systems. Air-based systems tend to be most appropriate when the building uses a large, centralized air-distribution system. A central heating system has sophisticated controls and centralized ducts that can interface well with a central solar thermal-storage source.

Evaluate water-based collectors. A water-based system typically uses heat exchangers to move heat from the collection medium to the heat-storage or distribution medium. Heat exchanges can transfer heat to water-storage, water-distribution, and also air-distribution systems. (See also Active Solar Hot Water for additional issues to consider.)

Consider air-based collectors. Air-based systems are the least complex of active systems; therefore, they avoid many of the problems of water collectors. Air collectors are typically simple, flat-plate collectors with plastic covers. They are easily serviced, and have less extreme and costly failures. While safe from freezing or boiling, they do take up considerably more surface area, and their ducts and fans require more space than water pipes and pumps. In addition, sealing an air system against leakage and finding and repairing leaks are more difficult than repairs in water-based systems.

When considering air collectors:

• Determine the use of the system. A very simple air system can provide preheated air for a mechanical system. This is basically a heating economizer, and it can use control logic similar to that of a cooling economizer. The resulting energy savings are significant if sunny weather typically coincides with the hours when the building needs heat.



• Determine heat storage needs. Heating requirements in commercial buildings are greatest in the early morning and evening, when solar heat is not available. These buildings require a thermal-storage system to provide solar heat, on an as-needed basis, after it has been collected.

Consider ventilation air preheat systems. This space heating system uses solar energy to preheat ambient air and bring it into a building's ventilation systems. The system utilizes a dark-colored, perforated, unglazed collector, integrated into the building structure, to preheat the air. These systems have efficiencies as high as 75 percent, require low maintenance, and can be installed economically, depending on the building type, climate, and fuel costs.