What is photovoltaic lighting?

Volume 9 Issue 3	July 2006

application - The use to which a lighting system will be put; for example, a lamp may be intended for indoor residential applications. ballast - A device required by electric-discharge light sources such as fluorescent or HID lamps to regulate voltage and current supplied to the lamp during start and throughout operation. color rendering index (CRI) - A rating index commonly used to represent how well a light source renders the colors of objects that it illuminates. For a CRI value of 100, the maximum value, the colors of objects can be expected to be seen as they would appear under an incandescent or daylight spectrum of the same correlated color temperature (CCT). Sources with CRI values less than 50 are generally regarded as rendering colors poorly, that is, colors may appear unnatural. compact fluorescent lamp (CFL) - A family of single-ended fluorescent-discharge light sources with small-diameter [16-millimeter (5/8-inch) or less] tubes. high-intensity discharge (HID) - An electric lamp that produces light directly from an arc discharge under high pressure. Metal halide, high-pressure sodium, and mercury vapor are types of HID lamps. grid - The combination of electric power plants and transmission lines operated by an electric utility. lamp - A radiant light source. lumen (lm) - A unit measurement of the rate at which a lamp produces light. A lamp's light output rating expresses the total amount of light emitted in all directions per unit time. Ratings of initial light output provided by manufacturers express the total light output after 100 hours of operation. luminaire - A complete lighting unit consisting of a lamp or lamps and the parts designed to distribute the light, to position and protect the lamp(s), and to connect the lamp(s) to the power supply. (Also referred to as fixture.) correlated color temperature (CCT) - A specification for white light sources used to describe the dominant color tone along the dimension from warm (yellows and reds) to cool (blue). Lamps with a CCT rating below 3200 K are usually considered warm sources, whereas those with a CCT above 4000 K usually considered cool in appearance. Temperatures in between are considered neutral in appearance. Technically, CCT extends the practice of using temperature, in kelvins (K), for specifying the spectrum of light sources other than blackbody radiators. Incandescent lamps and daylight closely approximate the spectra of black body radiators at different temperatures and can be designated by the corresponding temperature of a blackbody radiator. The spectra of fluorescent and LED sources, however, differ substantially from black body radiators yet they can have a color appearance similar to a blackbody radiator of a particular temperature as given by CCT. efficacy - The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt. halogen lamp - An incandescent lamp that uses a halogen fill gas. Halogen lamps have higher rated efficacies and longer lives than standard incandescent A-lamps. illuminance - The amount of light (luminous flux) incident on a surface area. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. luminance - The photometric quantity most closely associated with the perception of brightness, measured in units of luminous intensity (candelas) per unit area (square feet or square meter). glare - The sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted, which causes annoyance, discomfort, or loss in visual performance and visibility. lumen maintenance - The ability of a lamp to retain its light output over time. Greater lumen maintenance means a lamp will remain brighter longer. The opposite of lumen maintenance is lumen depreciation, which represents the reduction of lumen output over time. Lamp lumen depreciation factor (LLD) is commonly used as a multiplier to the initial lumen rating in illuminance calculations to compensate for the lumen depreciation. The LLD factor is a dimensionless value between 0 and 1. footcandle (fc) - A measure of illuminance in lumens per square foot. One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. lux (lx) - A measure of illuminance in lumens per square meter. One lux equals 0.093 footcandle. driver - For light emitting diodes, a device that regulates the voltage and current powering the source. illumination - The process of using light to see objects at a particular location. PN junction - For light emitting diodes, the portion of the device where positive and negative charges combine to produce light. fluorescent lamp - A low-pressure mercury electric-discharge lamp in which a phosphor coating on the inside of the glass tubing transforms most of the ultraviolet energy created inside the lamp into visible light. inverter - Also known as “power inverter.” A device used to convert direct current (dc) electricity into alternating (ac) current. irradiance - The density of radiant flux incident on a surface. light-emitting diode (LED) - A solid-state electronic device formed by a junction of P- and N-type semiconductor material that emits light when electric current passes through it. LED commonly refers to either the semiconductor by itself, i.e. the chip, or the entire lamp package including the chip, electrical leads, optics and encasement. photon - A small bundle or quantum of electromagnetic energy, including light. photovoltaic (PV) - Photovoltaic (PV) cells produce electric current from light energy (photons). PV cells are joined to make PV panels.

Photovoltaic (PV) lighting, or PV-powered lighting, is lighting that is at least partially powered by electricity generated from PV panels (often called solar panels). A popular example of PV lighting is the solar garden or pathway light. Other examples include post-top luminaires and parking lot luminaires carrying a solar panel on top. These PV lighting systems are usually off-grid, or "stand-alone" systems; their only power source is solar energy. Utility-connected and hybrid systems are more complicated because solar power is combined with other power sources such as a utility-generated grid, wind-powered generators, or fossil fuel-powered generators. This Lighting Answers considers only stand-alone/off-grid PV lighting technologies that are intended for nighttime lighting applications.

A PV lighting system collects solar energy using one or more PV panels, stores that energy in a battery or series of batteries, and then releases the energy to power light sources at night. Typically, PV lighting system components include PV panels, batteries, electronics (including battery charge controller, inverter or ballast/driver, and timer or switch), light sources (lamps), and luminaires. The initial purchase cost and maintenance cost for many of these components are high compared to traditional lighting systems powered by a grid. Designing and installing a PV lighting system are also more complicated than traditional grid-powered lighting systems. As a result, lighting applications suitable for PV lighting systems are currently somewhat limited. However, successful PV lighting applications can be developed through a carefully planned design process and educated choices between PV power and grid power. PV lighting applications are generally most successful where low light levels and limited electric power are acceptable and/or where access to a grid is expensive or difficult. Some examples of these types of applications are campgrounds, rural pathways, and parking lots in remote areas.