Solar Hot Water - Glossary of Terms

Absorber: The part of the collector that actively absorbs the light rays.For flat plate finned collectors this is defined as the cross-sectional area of the selective surface parallel riser fins connected to the top and bottom headers.
Active Systems: Active systems use electric pumps, valves, and controllers to circulate water or other heat-transfer fluids through the collectors. They are usually more expensive than passive systems but generally more efficient. Active systems are often easier to retrofit than passive systems because their storage tanks do not need to be installed above or close to the collectors. If installed using a PV panel to operate the pump, an active system can operate even during a power outage.
Aperture: The part of the collector through which light enters. For flat plate collectors this refers to the cross-sectional surface area of the clear glass plate measured using the internal frame dimensions. For double glass evacuated tubes this refers to the cross-sectional surface area of the outer clear glass tube measured using the internal diameter, not the outside diameter. For single glass evacuated tubes this refers to the cross-sectional surface area of the clear glass tube measured using the outside diameter.
Auxiliary Heat: The extra heat provided by a conventional heating system for periods of cloudiness or intense cold when a solar heating system cannot provide enough.
BTU: Stands for British Thermal Units. This is an imperial unit of measurement for heat widely used in the US and also in the UK. In Canada we use gigajoules. Convert as follows:
1 Kw=.0036 gigajoules=3,412.
10,000 BTU's=2.93Kwh=.01055 gigajoules
1 gigajoule=947,817 BTU's=277.8Kwh
Celsius: The metric unit for temperature measurement. Convert as follows:
Fahrenheit=(°C x 1.8)+32.
Celsius=(°F-32)/1.8
Closed-Loop Active Systems: These systems pump heat-transfer fluids (usually a glycol-water antifreeze mixture) through the solar water heater. Heat exchangers transfer the heat from the fluid to the water that is stored in tanks. Double-walled heat exchangers or twin coil solar tanks prevent contamination of household water. Some standards require double walls when the heat-transfer fluid is anything other than household water. Closed-loop glycol systems are popular in areas subject to extended subzero temperatures because they offer good freeze protection. However, glycol antifreeze systems are more expensive to purchase and install and glycol must be checked each year and changed every few years, depending on glycol quality and system temperatures.
Concentrating Collectors: These are usually parabolic troughs that use mirrored surfaces to concentrate the sun's energy on an absorber tube (called a receiver) containing a heat-transfer fluid, or the water itself.
Delta-T Controller: Delta-T refers to the difference in two temperatures. This term is often used in relation to a solar controller. In such case the Delta-T is the difference between the solar collector temperature and the temperature of the water in the in the solar storage tank. A Delta-T controller can be configured to turn on the pump when the Delta-T difference exceeds a certain level (Eg.7°C/12.7°F) and off again when the temperature difference drops below another setting (Eg.2°C/3.6°F). The controller turns on the pump when there is heat potential in the manifold. A Delta-T controller can also be used to provide freeze protection by circulating warm water from the tank through the manifold when the manifold temperature drops below 5°C.