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.
How does solar radiation vary by location? (cont'd)


Figure 8. PV panel size requirements in different months for different US cities
(ft2 = 0.09 m2)

As seen in Figure 8, the luminaire providing 10 lux (100 moonlights) on the ground needs a much larger PV panel than that needed by the luminaire providing 0.5 lux (5 moonlights). All locations need larger PV panels in December than they do in June because daylight hours are shorter in winter, giving the system less time to recharge. The sun's energy is also less intense in the winter. This decreased solar irradiance affects the ability of the PV panel to collect sufficient energy to power the lighting throughout the night, thus necessitating a larger panel size.

This analysis also indicates that Seattle needs significantly larger PV panels than nearly all other locations, especially in winter. Using the 10-lux (100 moonlights) luminaire as an example, a PV panel of approximately 29 square ft (2.7 square m) would be needed to power the lighting system reliably for Seattle's winter. This is too large to be practical for a parking lot luminaire. Therefore, other adjustments would be needed to the lighting system to make it more practical and realistic. These adjustments could include lowering the wattage of the light source or downsizing the energy requirements of the system in some other way. Conversely, in Miami, a PV panel of less than 7.5 square ft (0.7 square m) could comfortably power the 10-lux system throughout the year. A solar panel of this size could more easily be mounted on a luminaire pole, making PV lighting a much more viable option in this location.

When the light level requirements are reduced to 0.5 lux (5 moonlights) the size requirements of the system components reduce dramatically across all locations. This will significantly reduce the system's cost. In most areas of the country 0.5 lux (5 moonlights) of illumination could comfortably be provided by a luminaire with a PV panel of 1 square ft (0.09 square m) or less. A PV panel of this size could easily be incorporated directly into the design of a luminaire. For illustrations of these systems, see Figure 6 in "What factors should be considered when selecting a luminaire for PV lighting?."

The solar panel size estimates shown in Figure 8 are based upon average solar radiation for the various months listed. When calculating the size requirements for a solar panel, using average solar radiation data for the month with the least sunshine will ensure that the panel will provide the power required to operate the lighting system, even during the winter months. However, this does not cover a "worst-case-scenario" in which a location might receive little or no sunlight for a higher than average number of consecutive days. If a PV lighting system is installed in a location where it is critical that the system operate fully every night of the year, a system specifier may want to "over design" the size of the PV panels to help to ensure that it will provide the required power under all possible sky and weather conditions.


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