Glossary of Solar Heating Terms Absorber The blackened surface in a collector that absorbs the solar radiation and converts it to heat energy. Absorptance The ratio of solar energy absorbed by a surface to the solar energy striking it. Active System A solar heating or cooling system that requires external mechanical power to move the collected heat. Air System Solar domestic hot water systems employing air-type collectors are available. Hot air generated by these collectors is fan forced through an air-to-liquid heat exchanger with the potable water being pumped through the liquid section of the exchanger. The heated water is then circulated through the storage tank in a similar fashion to the liquid collector system. Air does not need to be protected from freezing or boiling, is non-corrosive, and is free. However, air ducts and air handling units require greater space than piping, and air leaks are difficult to detect. Air-Type Collector A collector that uses air as the heat transfer fluid. Altitude The angular distance from the horizon to the sun. Ambient Temperature The temperature of the surrounding air. ASHRAE Abbreviation for the American Society of Heating and Air-Conditioning Engineers. 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. Azimuth The angular distance between true south and the point on the horizon directly below the sun. British Thermal Unit (BTU) The quantity of heat needed to raise the temperature of one pound of water one degree Fahrenheit. Calorie The quantity of heat needed to raise the temperature of one gram of water one degree Celsius. Coefficient of Heat Transmission The rate of heat loss in BTU per hour through a square foot wall or other building surface when the difference between indoor and outdoor air temperatures is one degree Fahrenheit. Collector A device that collects solar radiation and converts it to heat. Collector Efficiency The ratio of usable heat energy extracted from a collector to the solar energy striking the cover. Concentrating Collector A device which concentrates the sun's rays on an absorber surface which is significantly smaller than the overall collector area. Conductance The rate of heat flow (in BTUs per hour) through an object when a 1° F. temperature difference is maintained between the sides of the object. Conduction The flow of heat due to temperature variations within a material. Conductivity A measure of the ability of a material to permit conduction of heat flow through it. Convection The motion of fluid such as gas or liquid by which heat may be transported. Cover Plate A sheet of glass or transparent plastic placed above the absorber in a flat plate collector. Degree Day A unit that represents a 1 degree F. deviation from some fixed reference point (usually 65°F.) in the mean daily outdoor temperature. Design Heat Load The total heat loss from a house under the most severe winter conditions likely to occur. Design Temperature The temperature close to the lowest expected for a location, used to determine the design heat load. Diffuse Radiation Indirect sunlight that is scattered from air molecules, dust and water vapor. Direct Radiation Solar radiation that comes straight from the sun, casting shadows on a clear day. Drain down System Potable water is circulated from the storage tank through the collector loop. Freeze protection is provided by solenoid valves opening and dumping the water at a preset low temperature. Collectors and piping must be pitched so that the system can drain down, and must be assembled carefully to withstand 100 psi. city water line pressures. Pressure reducing valves are recommended when city water pressure is greater than the working pressure of the system. Drain back System The solar heat transfer fluid automatically drains into a tank by gravity. Drain back systems are available in one or two tank configurations. A heat exchanger is necessary, because the city inlet pressure would prevent draining. The heat transfer fluid in the collector loop may be distilled or city water if the loop plumbing is copper. If the plumbing is threaded galvanized pipe, inhibitors may be added to prevent corrosion. Most inhibitors are non-potable and require a double wall heat exchanger. The pump used must be sized to overcome static head. Emittance A measure of the propensity of a material to emit thermal radiation. Eutectic Salts A group of materials that melt at low temperatures, absorbing large quantities of heat. Flat Plate Collector A solar collection device in which sunlight is converted into heat on a plane surface without the aid of reflecting surfaces to concentrate the rays. Forced Convection The transfer of heat by the flow of fluids (such as air or water) driven by fans, blowers or pumps. Galvanic Corrosion A condition caused as a result of a conducting liquid making contact with two different metal which are not properly isolated physically and/or electrically. Getters A column or cartridge containing an active metal which will be sacrificed to protect some other metal in the system against galvanic corrosion. Glaubers Salt Sodium sulfate a eutectic salt that melts at 90°F. and absorbs about 104 Btu per pound as it does so. Gravity Convection The natural movement of heat that occurs when a warm fluid rises and a cool fluid sinks under the influence of gravity. Headers The pipe that runs across the edge of an array of solar collectors, gathering or distributing the heat transfer fluid from, or to the risers in the individual collectors. This insures that equal flow rates and pressure are maintained. Heat Capacity A property of a material denoting its ability to absorb heat. Heat Exchanger A device, such as a coiled copper tube immersed in a tank of water, that is used to transfer heat from one fluid to another through a separating wall. Heat Storage A device or medium that absorbs collected solar heat and stores it for use during periods of inclement or cold weather. Heat Storage Capacity The amount of heat which can be stored by a material. Heating Season The period from early fall to late spring (in the northern hemisphere) during which additional heat is needed to keep a house comfortable for its occupants. Heat Pump A mechanical device that transfers heat from one medium to another, thereby cooling the first and warming the second. Heat Sink A medium or container to which heat flows. Heat Source A medium or container from which heat flows. Hybrid Solar Energy System A system that uses both active and passive methods in its operation. Indirect System A solar heating or cooling system in which the solar heat is collected exterior to the building and transferred inside using ducts or piping and, usually fans or ducts. Infrared Radiation Electromagnetic radiation from the sun that has wavelengths slightly longer than visible light. Insolation The total amount of solar radiation direct, diffused and reflected-striking a surface exposed to the sky. Insulation A material with high resistance (R-value) to heat flow. Langley A measure of solar radiation; equal to one calorie per square centimeter. Liquid Type Collector A collector using a liquid as the heat transfer fluid. Natural Convection See Gravity Convection. Nocturnal Cooling The cooling of a building or heat storage device by the radiation of excess heat into the night sky. One-Tank Closed-Loop System A conventional DHW tank, usually electrically heated, is converted to a solar DHW storage tank by installing an external heat exchanger coil. The lower electrical element is removed, leaving the uppermost of the usual two elements to provide auxiliary water heating and to achieve good stratification (layering of hotter water over progressively colder water). Open System Some part of the System is open to the atmosphere, or system contains fresh or changeable water. Passive System A solar heating or cooling system that uses no external mechanical power to move the collected solar heat. Percentage of Possible Sunshine The percentage of daytime hours during which there is enough direct solar radiation to cast a shadow. Photosynthesis The conversion of solar energy to chemical energy, by the action of chlorophyll in plants and algae. Photovoltaic Cells Semi conductor devices that convert solar energy into electricity. Pyranometer An instrument for measuring solar radiation. Radiant Panels Panels with integral passages for the flow of warm fluids, either air or liquids. Heat from the fluid is conducted through the metal and transferred to the rooms by thermal radiation. Radiation The flow of energy through open space via electromagnetic waves, such as visible light. Reflected Radiation Sunlight that is reflected from surrounding trees, terrain or buildings onto a surface exposed to the sky. Refrigerant A liquid such as Freon that is use in cooling devices to absorb heat from surrounding air or liquids as it evaporates. Resistance, or R Value The tendency of a material to retard the flow of heat. Retrofitting The application of a solar heating or cooling system to an existing building. Risers The flow channels or pipes that distribute the heat transfer liquid across the face of an absorber. Seasonal Efficiency The ratio, over an entire heating season, of solar energy collected and used to the solar energy striking the collector. Selective Surface A surface that absorbs radiation of one wavelength (for example, sunlight) but emits little radiation of another wavelength (for example, infrared); used as a coating for absorber plates. Shading Coefficient The ratio of the solar heat gain through a specific glazing system to the total solar heat gain through a single layer of clear double-strength glass. Solar Constant The average intensity of solar radiation reaching the earth outside the atmosphere; accounting to two langleys or 1.94 gram-calories per square centimeter, equal to 442.4 BTU/hr/ft.², or 1395 watts/m². Solar Radiation (Solar Energy) Electromagnetic radiation emitted by the sun. Solar Rights A legal issue concerning the right of access to sunlight. Specific Heat The quantity of heat, in BTU, needed to raise the temperature of one pound of a material 1°F. Standby Heat Loss Heat lost though storage tank and piping walls. Sun Path Diagram A circular projection of the sky vault, similar to a map, that can be used to determine solar positions and to calculate shading. Thermal Capacity The quantity of heat needed to warm a collector up to its operating temperature. Thermal Mass or Thermal Inertia The tendency of a building with large quantities of heavy materials to remain at the same temperature or to fluctuate only very slowly; also the overall heat storage capacity of the building. Thermal Radiation Electromagnetic radiation emitted by a warm body. Thermistor Sensing device which changes its electrical resistance according to temperature. Used in the control system to generate input data on collector and storage temperatures. Thermosyphoning The process that makes water circulate automatically between a warm collector and a cooler storage tank above it. (See Gravity Convection). Tilt Angle The angle that a flat plate collector surface forms with the horizontal plane. Trickle Type Collector A collector in which the heat transfer liquid flows through metal tubes which are fastened to the absorber plate by solder, clamps or other means. (See Collector). Tube-in-Plate-Absorber A metal absorber plate in which the heat transfer fluid flows through passages formed in the plate itself. Tube-Type Collector A collector in which the heat transfer fluid flows through metal tubes that are fastened to the absorber plate with solder, clamps or other means. (See Collector). Ultraviolet Radiation Electromagnetic radiation with wavelengths slightly shorter than visible light.