Active solar technologies are employed to convert solar energy into another more useful form of energy. This would normally be a conversion to heat or electrical energy. Inside a building this energy would be used for heating, cooling, or off-setting other energy use or costs. Active solar uses electrical or mechanical equipment for this conversion. Solar energy collection and utilization systems that do not use external energy, such as a solar chimney, are classified as passive solar technologies. Passive solar relies on the inherent thermo-dynamic properties of the system or materials to operate. They do not need external energy sources.
Solar hot water systems, except those based on the thermosiphon, use pumps or fans to circulate fluid (often a mixture of water and glycol to prevent freezing during winter periods) or air, through solar collectors, and are therefore classified under active solar technology.
The basic benefit of active systems is that controls (usually electrical) can be used to maximize their effectiveness. For example a passive solar thermal array which does not rely on pumps and sensors will only start circulating when a certain amount of internal energy has built up in the system. Using sensors and pumps, a relatively small amount of energy (i.e. that used to power a pump and controller) can harvest a far larger amount of available thermal energy by switching on as soon as a useful temperature differential becomes present. Controls also allow a greater variety of choices for utilising the energy that becomes available. For example a solar thermal array could heat a swimming pool on a relatively cool morning where heating a domestic hot water cylinder was impractical due to the different stored water temperatures. Later in the day as the temperature rises the controls could be used to switch the solar heated water over to the cylinder instead.
The downside to Active Solar systems is that the external power sources can fail (probably rendering them useless), and the controls need maintenance.
Most solar collectors are fixed in their array position mounting, but can have a higher performance if they track the path of the sun through the sky (however it is unusual for thermal collectors to be mounted in this way). Solar trackers, used to orient solar arrays may be driven by either passive or active technology, and can have a significant gain in energy yield over the course of a year when compared to a fixed array. Again passive solar tracking would rely on the inherent thermo-dynamic properties of the materials used in the system rather than an external power source to generate its tracking movement. Active Solar Tracking would utilise sensors and motors track the path of the sun across the sky. This action can be caused by geographical and time data being programmed into the controls. However, some systems actually track the brightest point in the sky using light sensors, and manufacturers claim this can add a significant extra yield over and above geographical tracking.
Famous quotes containing the words active and/or solar:
“How often must I repeat, that I know or am conscious of my own being; and that I myself am not my ideas, but somewhat else, a thinking, active principle that perceives, knows, wills, and operates about ideas?”
—George Berkeley (1685–1753)
“Senta: These boats, sir, what are they for?
Hamar: They are solar boats for Pharaoh to use after his death. They’re the means by which Pharaoh will journey across the skies with the sun, with the god Horus. Each day they will sail from east to west, and each night Pharaoh will return to the east by the river which runs underneath the earth.”
—William Faulkner (1897–1962)