As fossil fuel reserves available to us for our energy needs continue to dwindle, other energy sources need to be utilized. One of these alternative energy sources is energy from the sun, called solar energy. Enough solar energy hits the United States every twenty minutes to supply our needs for an entire year. An active solar heating system is one way to utilize this energy. Active systems are available in two categories, liquid-based and air-based . The type of system it is depends on what fluid is circulated through the solar collector to be heated.
Liquid-based solar heating systems are typically used for space heating, water heating, and heating pool water. Freezing is the principle cause of failure because the collector radiates heat to the cold night sky and the water in the collector can freeze at temperatures above 32°F. A liquid system is further classed as direct or indirect. A direct liquid system runs city or well water through the solar collector, thus requiring fewer heat exchangers. The indirect liquid system has the collector isolated from the water supply system. This type of system offers several advantages over a direct system. An isolated collector can use fluids that will not freeze at normal temperatures, such as ethylene or propylene glycol, hydrocarbon oils, or common refrigerant oils. When these liquids are used, mineral buildup in the piping is not a problem.
Space heating is the most common use for an air-based solar heating system since the heat does not have to be transferred to another medium before being used. Air can also be used to preheat domestic hot water. Using air for the collection fluid requires no freezing, overheating, or corrosion protection. The drawbacks of an air system are that ducts require more space than pipes, and air has poorer heat transfer qualities than liquids. Poor heat transfer means that more collector surface is required for the same amount of heat collected.
While a solar heating system is cost efficient during the life of the system, its initial costs are much higher than a conventional heating system. This is because a standard heating system must be installed with the solar heating system. A solar heating system cannot provide all the heating needs since heat is not collected at night or during days with overcast skies.
The control system of a solar heating system causes the solar radiation to be collected, stored, and distributed. The main components of a solar heating system are the solar collectors for collecting solar radiation, the solar storage system for storing the heat, and heat exchangers for distributing the heat throughout the building.
Flat plate solar collectors are typically used for solar energy collection. This type of collection system is capable of developing fluid temperatures of 150-200°F. Flat plate collectors have a simple design, are easy to repair, are low cost, and require no tracking mechanisms.