Solar Thermal Glossary
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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 relative to the solar energy striking it. Solar collectors have high absorptive coatings.
Active System: A solar water heating system that requires external mechanical power e.g. pumps, collectors 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.
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.
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British Thermal Unit (BTU): The quantity of heat needed to raise the temperature of one pound of water one degree Fahrenheit.
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Calorie: The quantity of heat needed to raise the temperature of one gram of water one degree Celsius.
Closed-Loop System: A solar system where the transfer fluid does not mix with the potable water and uses a heat exchanger to heat the household water by conduction and/or convection.
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 (Solar): A device that collects solar radiation and converts it to heat. A solar hot water collector is usually an insulated box, with a tempered, low iron glass cover. Solar pool collectors usually do not have insulation or a cover plate.
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
Cover Plate: A sheet of glass or transparent plastic placed above the absorber in a flat plate collector.
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Degree Day - DD: A unit that represents a 1 degree F. deviation from some fixed reference point (usually 65°F.) in the mean daily outdoor temperature. The colder it gets, the more DD available. Degree-day figures quantify how hot or cold the weather has been as a single index number for the region and month (or week). They allow you to account properly for the effect of weather on energy consumption.
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.
DHW: Domestic Hot Water
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.
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Emittance: A measure of the propensity of a material to emit thermal radiation.
Expansion Tank: A tank in a heating system that provides space for the physical expansion of water or a water solution.
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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. It is the most efficient type of collector for use with temperatures between the freezing and boiling points of water and up to about 350 degrees F. when used with air as the working medium. Flat plate collectors are normally used with the flat surface facing south and tilted to an angle appropriate to the intended use.
Forced Convection: The transfer of heat by the flow of fluids (such as air or water) driven by fans, blowers or pumps.
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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.
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Header: 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 that 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.
HTF: Heat Transfer Fluid used in closed loop systems e.g. glycol/water mixtures or pure water.
Hybrid Solar Energy System: A system that uses both active and passive methods in its operation.
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ICS: Integrate Collector and Storage for solar water heater.
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.
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Kilowatt-hour (KWH): a unit of energy equal to 1 kilowatt (1,000 watts) of electric power being used for 1 hour. Equivalent to 3,413 BTU’s of thermal energy or 1 1/3 hp of mechanical energy.
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Langley: A measure of solar radiation; equal to one calorie per square centimeter.
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Murphy’s Law: Anything that can go wrong will go wrong at the worst possible way when you least expect it.
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NABCEP: North American Board of Certified Energy Practitioners
NOAA – National Oceanic & Atmospheric Administration Worldwide Weather Data: Consult for lowest local freezes etc. on record, solar radiation data & local weather data.
Nocturnal Cooling: The cooling of a building or heat storage device by the radiation of excess heat into the night sky.
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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 Loop System: Some part of the System is open to the atmosphere, or system contains fresh or changeable water.
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Passive System: A solar heating or cooling system that uses no external mechanical power e.g. pump or controller, 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.
Potable Water: The household water that comes out of your bathroom and kitchen sinks.
Pyranometer: An instrument for measuring solar radiation.
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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 snow, water or 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.
Risers: The flow channels or pipes that distribute the heat transfer liquid across the face of an absorber.
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Seasonal Efficiency: The ratio, over an entire heating season, of solar energy collected and used as opposed 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. 1 solar constant is 1,000 watts/sq. meter or 1.432 Langleys/min.
SEIA - Solar Energy Industry Association: The national trade association of solar energy manufacturers, dealers, distributors, contractors, installers, architects, consultants, and marketers who work to expand the use of solar technologies in the global marketplace.
Solar Influx: Solar intensity of all diffuse and direct solar radiation measured at the site.
Solar %: An Actual measurement with a BTU or watt-hour meter, or a quick estimate of the solar influx at the site during real time monitoring/testing of the system by the contractor or owner.
Solar Radiation: (Solar Energy) Electromagnetic radiation emitted by the sun.
Specific Heat: The quantity of heat, in BTU, needed to raise the temperature of one pound of a material 1°F.
SRRC - Solar Collection Certification and Rating: In 1980 the Solar Rating and Certification Corporation (SRCC) was incorporated as a non-profit organization whose primary purpose is the development and implementation of certification programs and national rating standards for solar energy equipment.
Standby Heat Loss: Heat lost though storage tank and piping walls, even if no hot water is used.
Sun Path Diagram (Solar Window): A circular projection of the sky vault, similar to a map, which can be used to determine solar positions and to calculate shading. This allows you to evaluate shading at a specific location form 8:00 am to 4:00 pm each month of the year.
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Temperature vs. Energy: Knowledge of temperature is necessary to knowledge of thermal energy, but is not enough. Both the temperature and quantity of material containing the energy must be known. We could not reasonably expect a match flame at 2,000 degrees to be able to heat a swimming pool, but solar collectors at 90 degrees will do it readily if we use enough of them.
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 e.g. the sun or a hot wood stove.
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 from a warm collector or heat exchanger to a cooler storage tank above it. (See Gravity Convection).
Tilt Angle: The angle that a flat plate collector surface forms with the horizontal plane.
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Ultraviolet Radiation: Electromagnetic radiation with wavelengths slightly shorter than visible light.