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?

The amount of solar radiation is affected by the geographic location, the season, and the climate conditions of the location. For example, the amount of available solar radiation in Phoenix, Arizona is greater than that in New York City. If we install two lighting systems located in these two cities with the same components, the system in New York City will require a larger solar panel to power the system reliably. The National Renewable Energy Laboratory (NREL) has detailed statistical information about the amount of solar radiation at different locations across the U.S. Figure 7 shows two examples of NREL solar radiation data. The first example shows the average daily amount of solar radiation on horizontal flat panels on an annual average. The second example shows the same information for the month of December only.

Figure 7. Examples of NREL solar radiation data

Source: National Renewable Energy Laboratory (NREL) Resource Assessment Program

NREL also provides data on daily total solar radiation for different months in various US cities. Using these data, NLPIP performed a comparison of the relative size of photovoltaic (PV) panels needed for different PV lighting systems in different months of a year for various US cities. The results of this evaluation are shown in Figure 8. These comparisons provide two examples of PV lighting systems. The first system is a parking lot luminaire mounted at a height of 10 ft (3.0 m) that will provide approximately 10 lux or 100 moonlights illuminance on the ground; the second is a post-top luminaire mounted at 8 ft (2.4 m) that will provide about 0.5-lux (5 moonlights) illuminance. The comparisons assume a luminaire that has a single light source and an optical efficiency of 50%, which uniformly distributes all light output on a circular area with a radius equal to the pole height.

The assumptions for both of these PV lighting systems are:

  • The lighting will be turned on for 8 hours of operation at night, per day.

  • PV panel conversion efficiency is 10%.

  • A fixed-direction horizontal (facing up) flat PV panel is used.

  • Electronics efficiency (including charge controller and dc ballast for the compact fluorescent lamp [CFL] or dc driver for the light-emitting diode [LED]) is 80%.

  • The battery is sufficiently sized and has a battery charge/discharge efficiency, including conduit loss, of 60%.

  • The light source efficacy is 65 lumens per watt (LPW) for the CFL and 25 LPW for the LED.

The parking lot luminaire uses an 11-watt CFL powered by a dc ballast, with a total system wattage of 13 watts. The post-top luminaire uses a 1-watt white LED powered by an LED driver, with a total system wattage of 1.5 watts.

The numbers quoted above are typical values for PV-powered luminaires. They assume that the battery capacity of the system is large enough to allow necessary discharge for powering the lamp. Rural roads and parking lots are often unpaved and create a lot of dust, which can block solar irradiance. In northern locations, snow may cover PV panels, blocking much of the solar irradiance. For the purposes of this example, the PV panel was assumed to be cleaned regularly and therefore free of dust and snow, so its conversion efficiency remains stable throughout the year. These two systems were selected for comparison to illustrate the difference in the size requirements for PV systems designed for two different types of lighting applications. The parking lot luminaire, designed to provide 10 lux (100 moonlights) on the ground, uses a CFL because this is the most efficient source currently available that can provide this light level at the selected mounting height of 10 ft. (3.0 m). 10 lux (100 moonlights) is similar to light levels found in many parking areas that are not heavily trafficked at night.

The post-top luminaire is designed to provide a much lower light level, 0.5 lux (5 moonlights), at a mounting height of only 8 ft (2.4 m). This luminaire uses an LED because it is the most efficient source capable of providing this light level. CFLs, for example, are not available in lumen packages small enough to provide a light level this low at this mounting height. This post-top luminaire would be appropriate for pathways and other types of lighting applications in rural or remote areas where the surrounding nighttime conditions are very dark.

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