Glossary of Common Solar Energy Terms

Active Solar Thermal – technologies, which use fans or pumps to convert solar energy into usable heat or cooling (thermal energy). Active solar technologies use electrical or mechanical equipment, such as pumps and fans, to move the heat transfer fluid, air, water, anti-freeze, etc. from the surface where it is collected to the location where it is used to heat, cool, or dry something else.

Balance of System (BOS) – All the non-solar collector components needed to harness the solar energy that falls on the collector surface. For a solar air system, this usually includes a fan, ducts, and a simple thermostat. For water based heating systems this usually includes a pump, piping, antifreeze, a storage tank, thermostat and heat exchangers to transfer the heat to domestic water or space heat. For a solar electric system, it usually includes wiring, an inverter to convert direct current to alternating current, electrical switch gear to connect to the existing electric grid or building power panels or both, and sometimes batteries and battery charging and monitoring equipment to store electricity.

Btu – British thermal unit, a measure of heat. One Btu is the amount of heat energy needed to raise the temperature of one pound of water by one degree F. This is the standard measurement used to state the amount of energy that a fuel gives off when burned as well as the amount of output of any heat generating device.

You might be able to imagine it this way. Take one gallon (~8 pounds) of water and put it on your stove. Assume that the water starts at 60 degrees F and you want to bring it to a boil (212 degrees F.). To reach the boiling point, you will need about 1,200 Btu’s.
Here’s the math: You need to raise each pound of water 152 degrees F. (212-60 = 152) and you have to raise the temperature for 8 pounds, requiring 1,216 Btu’s (152 x 8 = 1216).

To give some examples,

  • It takes about 50 BTU to take a cup of cold tap water to hot coffee temperature.
  • It takes about 600 BTU to warm a gallon of cold water to hot water for sinks and showers.
  • A typical home furnace in the central United States produces about 100,000 BTU in one hour.
  • One square foot of well oriented solar roof (or wall, or collector) can produce about 100,000 BTU in one winter and 200,000 BTU in one year.

Concentrated Solar Thermal Power Systems – Technologies using the sun’s heat to produce electricity. These systems typically concentrate the sun’s heat to boil water or another liquid at high temperatures. The steam from the boiling water rotates a large turbine, which spins a generator that produces electricity.

Heat Engine – A device that converts thermal energy to mechanical output. The mechanical output is called work and the thermal energy input is called heat. Heat engines typically run on a specific thermodynamic cycle (Rankine, Stirling, etc.). Heat engines are often named after the thermodynamic cycle by which they are modeled. They often pick up alternate names, such as gasoline/petrol, turbine, or steam engines.

Passive Solar – Technologies capture sunlight for heat energy without the use of active mechanical systems such as fans or pumps (as contrasted to active solar). Such technologies convert sunlight into usable heat (water, air) or store heat for future use, with little or no use of other energy sources. A common example is a solarium on the sunny side of a building. Passive cooling is the use of the same design principles (no fans or pumps) to block solar heating to reduce summer cooling requirements.

R-Value – A measure of thermal resistance used in the building and construction industry. A bigger number means the building insulation is more effective. The units of R-value are (hr * degree F*ft squared)/Btu. You can interpret R-Value units as the number of hours it takes for heat to flow across one square foot of assembly, where there is a 1 degree F temperature differential between the inside and outside temperatures. If an assembly of several components has a higher R-value it means that it poses more resistance to the flow of heat. A solar re-roof creates a warm solar air space above the old roof, which helps to increase the R value of the overall roof assembly.

Solar Cell or Photovoltaic Cell – An electronic device that converts solar energy into direct current electricity by the photovoltaic effect. Photovoltaics is the field of technology related to the application of solar cells for solar energy. Assemblies of cells are used to make solar modules, or photovoltaic arrays, which generate electricity.

Solar Energy – The radiant energy from the Sun that influences Earth’s climate and weather and sustains life. It includes visible light and several non-visible frequencies such as ultraviolet light. All of these frequencies can be used to produce heat using solar thermal techniques and many can be used to produce electricity using photovoltaics.

Solar Energy Technologies – Convert solar energy to useful purposes. They can provide electrical generation by heat engine or photovoltaic means; space heating and cooling in active and passive solar buildings; potable water via distillation and disinfection; day lighting; hot water; thermal energy for cooking; and high-temperature process heat for industrial purposes.

Solar Heat Gain – Measurement of the amount of solar heat that is captured for use by a solar collector surface including a solar roof, solar siding or ground-mounted solar system. A typical measure is Btu per hour per square foot.

Solar Heated-Air – Air heated by the use of solar energy.

Solar Hot Water – Water heated by the use of solar energy.

Solar Power – Sometimes used as a synonym for solar energy or more specifically to refer to electricity generated from solar radiation.

Solar Thermal Power Systems – Technologies using the sun’s heat to produce electricity or mechanical power. There are two kinds of systems. The most common are Concentrating Solar Thermal Power. A new system uses low-temperature solar heat (without concentrating sunlight) to boil a special liquid that passes through a heat engine driving a generator to produce electrical power.

Transpired Solar Collector – A specialized solar collector that passes air through holes or openings in metal panels. Sunlight heats the metal panels. The outside air is heated as it moves through the openings in the panels. Other systems such as solar roofs of rustic shakes, shingles or tiles provide a similar effect but are not usually referred to as transpired collectors.

Solar Ventilation Preheating – Solar preheating outdoor air before it enters the ventilation system of a building. Most large buildings require a large volume (thousands of cubic feet per minute) of outdoor air to be continuously pulled into the building to maintain acceptable indoor air quality. During the long heating season in most of North America, the air needs to be heated to comfort levels before being distributed through the building. Many buildings do this with expensive gas, oil, or electric heating systems. Even efficient modern buildings often use expensive electric reheat systems at the ventilation terminals.

Large buildings often have large roofing or siding areas. These solar roofing or siding surfaces can provide 1-2 cubic feet per minute of solar preheated air for every square foot of roof or siding. This approach to solar ventilation preheating uses existing building components to produce low-cost solar heat and reduces the need for expensive electric or fossil fueled heating, saving energy and money.

U-Value – Measurement of the heat flow that occurs during one hour through one square foot of material, when there is a 1 degree F temperature difference between the inside and outside air temperatures. The units are Btu/(hr *degreesF*ft squared ). If an assembly of several components has a higher U-value, it means that it loses more heat than one with a lower U-value.