Glossary
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.
Angle of Incidence
The angle that a ray of sun makes with a line perpendicular to the surface. For example, a surface that directly faces the sun has a solar angle of incidence of zero, but if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90°.
Alternating Current (AC)
A type of electrical current, the direction of which is reversed at regular intervals or cycles. In the United States, the standard is 120 reversals or 60 cycles per second. Electricity transmission networks use AC because voltage can be controlled with relative ease.
Array
Array Current
Produced by a photovoltaic array when it is exposed to sunlight.
ASHRAE
Abbreviation for the American Society of Heating and Air-Conditioning Engineers.
Azimuth
The angular distance between true south and the point on the horizon directly below the sun.
BTU (British Thermal Unit)
The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.
Concentrated Solar Power (CSP) System
Uses lenses or mirrors and tracking systems to focus a large area of sunlight onto a small area. The concentrated light is then used as a heat source for a conventional power plant or is concentrated onto photovoltaic surfaces.
Distributed Generation
A popular term for localized or on-site power generation.
Direct Current (DC)
A type of electricity transmission and distribution by which electricity flows in one direction through the conductor, usually relatively low voltage and high current. To be used for typical 120 volt or 220 volt household appliances, DC must be converted to alternating current, its opposite.
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.
Electrical Grid
An integrated system of electricity distribution, usually covering a large area.
Energy Audit
A survey that shows how much energy used in a home, which helps find ways to use less energy.
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.
Flat-Plate Photovoltaics (PV)
A PV array or module that consists of non-concentrating elements. Flat-plate arrays and modules use direct and diffuse sunlight, but if the array is fixed in position, some portion of the direct sunlight is lost because of oblique sun-angles in relation to the array.
Gigawatt (GW)
A unit of power equal to 1 billion Watts; 1 million kilowatts, or 1,000 megawatts.
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.
Hybrid Solar Energy System
A system that uses both active and passive methods in its operation.
Inverter
A device that converts direct current electricity to alternating current either for stand-alone systems or to supply power to an electricity grid.
Junction Box
A photovoltaic (PV) generator junction box is an enclosure on the module where PV strings are electrically connected and where protection devices can be located, if necessary.
Junction Diode
A semiconductor device with a junction and a built-in potential that passes current better in one direction than the other. All solar cells are junction diodes.
Kilowatt (kW)
A standard unit of electrical power equal to 1000 watts, or to the energy consumption at a rate of 1000 joules per second.
Kilowatt-Hour (kWh)
1,000 thousand watts acting over a period of 1 hour. The kWh is a unit of energy. 1 kWh=3600 kJ.
Liquid Type Collector
A collector using a liquid as the heat transfer fluid.
Maintenance-Free Battery
A sealed battery to which water cannot be added to maintain electrolyte level.
Ohm
A measure of the electrical resistance of a material equal to the resistance of a circuit in which the potential difference of 1 volt produces a current of 1 ampere.
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).
Orientation
Placement with respect to the cardinal directions, N, S, E, W; azimuth is the measure of orientation from north.
Passive System
A solar heating or cooling system that uses no external mechanical power to move the collected solar heat.
Peak Demand/Load
The maximum energy demand or load in a specified time period.
Photovoltaic(s) (PV)
Pertaining to the direct conversion of light into electricity
Photovoltaic (PV) Array
An interconnected system of PV modules that function as a single electricity-producing unit. The modules are assembled as a discrete structure, with common support or mounting. In smaller systems, an array can consist of a single module.
Photovoltaic (PV) Cell
The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (direct current voltage and current). Also called a solar cell.
Photovoltaic (PV) Module
The smallest environmentally protected, essentially planar assembly of solar cells and ancillary parts, such as interconnections, terminals, [and protective devices such as diodes] intended to generate direct current power under unconcentrated sunlight. The structural (load carrying) member of a module can either be the top layer (superstrate) or the back layer (substrate).
Photovoltaic (PV) Panel
Often used interchangeably with PV module (especially in one-module systems), but more accurately used to refer to a physically connected collection of modules (i.e., a laminate string of modules used to achieve a required voltage and current).
Photovoltaic (PV) System
A complete set of components for converting sunlight into electricity by the photovoltaic process, including the array and balance of system components.
Photovoltaic-Thermal (PV/T) System
A photovoltaic system that, in addition to converting sunlight into electricity, collects the residual heat energy and delivers both heat and electricity in usable form. Also called a total energy system.
Qualification Test
A procedure applied to a selected set of photovoltaic modules involving the application of defined electrical, mechanical, or thermal stress in a prescribed manner and amount. Test results are subject to a list of defined requirements.
Rectifier
A device that converts alternating current to direct current. See inverter.
Resistance, or R Value
The tendency of a material to retard the flow of heat.
Sacrificial Anode
A piece of metal buried near a structure that is to be protected from corrosion. The metal of the sacrificial anode is intended to corrode and reduce the corrosion of the protected structure.
Seasonal Efficiency
The ratio, over an entire heating season, of solar energy collected and used to the solar energy striking the collector.
Solar Energy
Electromagnetic energy transmitted from the sun (solar radiation). The amount that reaches the earth is equal to one billionth of total solar energy generated, or the equivalent of about 420 trillion kilowatt-hours.
Solar Thermal Electric Systems
Solar energy conversion technologies that convert solar energy to electricity, by heating a working fluid to power a turbine that drives a generator. Examples of these systems include central receiver systems, parabolic dish, and solar trough.
Stand-Alone System
An autonomous or hybrid photovoltaic system not connected to a grid. May or may not have storage, but most stand-alone systems require batteries or some other form of storage.
String
A number of photovoltaic modules or panels interconnected electrically in series to produce the operating voltage required by the load.
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.
Thermophotovoltaic Cell (TPV)
A device where sunlight concentrated onto a absorber heats it to a high temperature, and the thermal radiation emitted by the absorber is used as the energy source for a photovoltaic cell that is designed to maximize conversion efficiency at the wavelength of the thermal radiation.
Thin Film
A layer of semiconductor material, such as copper indium diselenide or gallium arsenide, a few microns or less in thickness, used to make photovoltaic cells. This technology is designed for specialized applications as it does not yet yield an rate of conversion equal to Flat-plate arrays.
Tilt Angle
The angle at which a photovoltaic array is set to face the sun relative to a horizontal position. The tilt angle can be set or adjusted to maximize seasonal or annual energy collection.
Tracking Array
photovoltaic (PV) array that follows the path of the sun to maximize the solar radiation incident on the PV surface. The two most common orientations are (1) one axis where the array tracks the sun east to west and (2) two-axis tracking where the array points directly at the sun at all times. Tracking arrays use both the direct and diffuse sunlight. Two-axis tracking arrays capture the maximum possible daily energy.
Transformer
An electromagnetic device that changes the voltage of alternating current electricity.
Two-Axis Tracking
A photovoltaic array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal).
Utility-Interactive Inverter
An inverter that can function only when tied to the utility grid, and uses the prevailing line-voltage frequency on the utility line as a control parameter to ensure that the photovoltaic system's output is fully synchronized with the utility power.
Varistor
A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.
Voltage Protection
Many inverters have sensing circuits that will disconnect the unit from the battery if input voltage limits are exceeded.
Watt
The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).
Zenith Angle
The angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).
