Lighting Research Center Lighting Research Center
    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 the cost of PV lighting systems compare to grid-powered lighting systems? (cont'd)

Because light level requirements will affect the overall costs of both PV-powered and grid-powered lighting systems, a life cycle cost analysis was performed for a 0.5 lux (5 moonlights) illuminator (a post-top luminaire designed to provide approximately 0.5 lux on the ground) using a one-watt white light-emitting diode (LED) as the light source. For this example the use of a one-watt LED is based on the assumption that the luminous efficacy of this white LED is 25 lumens per watt (LPW). However, with the rapidly improving efficacy of LEDs, the wattage of the LED lamp can be lowered significantly in the near future. For example, with today's premium LED products at 40 lumens per watt, only a 0.6-watt LED would be needed to provide this light level.

Table 3 summarizes the life cycle cost comparison of one PV-powered luminaire and one grid-powered luminaire located 50 ft (15.2 m) from an electric grid connection. This comparison assumes 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.

Table 3. Life cycle cost comparison of luminaire
(located 50 ft [15.2 m] from grid)

System Type  Illuminance LCC Capital Costs Maintenance Costspw Energy Costspw Replacement Costspw

PV–powered 0.5 lux $414 $155 $239 $0 $19

Grid–powered 0.5 lux $995* $895* $96 $4 $0

* Includes cost of power line extension

In this example, the life cycle costs of the PV lighting system are less than that of the grid-powered system. This is due to the added costs of extending the power line from the grid as well as a substantial decrease in the capital cost associated with the PV lighting system. The lower light level requirements allowed these costs to be reduced significantly because smaller system components (e.g., PV panel, battery) could be used. Table 4 summarizes the life cycle cost comparison of a PV-powered lighting system providing approximately 0.5 lux (5 moonlights) on the ground containing 10 luminaires and a grid-powered lighting system containing 10 luminaires located one mile (1.6 km) from an electric grid connection.

Table 4. Life cycle cost comparison of lighting systems with 10 luminaires
(located one mile [1.6 km] from grid )

System Type  Illuminance LCC Capital Costs Maintenance Costspw Energy Costspw Replacement Costspw

PV–powered 0.5 lux $4,135 $1,550 $2,395 $0 $191

Grid–powered 0.5 lux $31,950* $30,950* $958 $42 $0

* Includes cost of power line extension

The cost of the PV lighting system in this example is significantly lower because it avoids the costs associated with extending the power grid one mile (1.6 km). However, the lower light level requirements have also reduced the costs of the PV lighting system by over $8,800 compared to the previous 10-lux (100 moonlights) system. In contrast, the costs of the grid-powered lighting system were reduced by just over $2,600 due to the lower light level requirements.


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