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.

Case Study: Life Cycle Cost

Various scenarios are provided in this case study that use a life cycle cost (LCC) analysis to compare the cost-effectiveness of photovoltaic (PV)-powered lighting systems to grid-powered lighting systems for a variety of applications.

Assumptions:

  • 10-year analysis period
  • 3% inflation rate
  • 4% discount rate
  • No salvage cost at the end of 10 years
  • $0.10 per kWh electricity cost (unless otherwise specified)
  • Grid-powered systems require a 50-ft extension of the power line at a cost of $800 (unless otherwise specified)
  • Eight hours of operation per night
  • The 10-lux illuminator parking lot luminaire (see Figure 6 in "What factors should be considered when selecting a luminaire for PV lighting?") uses an 11-watt compact fluorescent lamp (CFL) with a system wattage of 13 W that costs $10 and must be replaced every three years. The luminaire hardware (luminaire, pole, and supporting hardware) costs $200.
    When powered by PV, the system requires a $300 50-watt (peak power) PV panel, $100 of electronic components (including charge controller and dc ballast), a $100 rechargeable battery that must be replaced every five years, and $50 of annual maintenance (including lamp and battery replacement and PV-panel cleaning). When powered by the grid, the maintenance cost is $20 per year.
  • The 0.5-lux illuminator post-top luminaire (see Figure 6 as above) uses a 1-watt light-emitting diode (LED) with a system wattage of 1.5 watts that costs $15. The luminaire hardware costs $80. When powered by PV, the system requires a $30 PV panel, $10 of electronic components, a $20 rechargeable battery that must be replaced every five years, and $25 of annual maintenance (including lamp and battery replacement and PV-panel cleaning). When powered by the grid, the maintenance cost is $10 per year.
  • The indicator luminaire (see Figure 6 as above) uses a 0.2-watt LED. When powered by PV, the system uses a $20 self-contained solar LED garden luminaire. When powered by the grid, the luminaires cost $5 each. The maintenance cost is negligible in both cases.
Scenario 1: One 10-lux illuminator: parking lot luminaire

  PV-powered lighting Grid-powered lighting
 

Component Present worth ($) Present worth ($)

PV panel 300 -

Electronics 100 -

Battery 0 yr. 100 -

Battery 5 yr. 95 -

Luminaire hardware 200 200

Lamp 0 yr. 10 10

Lamp 3 yr. 10 10

Lamp 6 yr. 9 9

Maintenance 479 192

Electricity - 31

Grid line extension (50 ft) - 800

TOTAL $1303 $1252

(Note: For all tables in this case study, 'Present worth' represents the value or 'worth' of a future cost, given a prevailing rate of interest and inflation.)

The LCC analysis in the table above shows that the PV and grid power systems are nearly the same. Increasing the electricity cost to $0.15 per kWh only increases the LCC of the grid-powered system by $15, so the options are still comparable in the total cost.


Scenario 2: One 0.5-lux illuminator: post-top luminaire

PV-powered lighting Grid-powered lighting


Component Present worth ($) Present worth ($)

PV panel 30 -

Electronics 10 -

Battery 0 yr. 20 -

Battery 5 yr. 19 -

Luminaire hardware 80 80

Lamp 0 yr. 15 15

Lamp 3 yr. - -

Lamp 6 yr. - -

Maintenance 239 96

Electricity - 4

Grid line extension (50 ft) - 800

TOTAL $414 $995

In this scenario, the cost of the PV-powered lighting system is only half the cost of the grid-powered system. The lower light level requirement makes the PV system a much more viable option. Increasing the electricity cost to $0.15 per kWh only increases the LCC of the grid-powered system by $2, a negligible effect on the LCC.

Scenario 3: Five indicator luminaires

In some residential or rural pathway applications, an 11-watt CFL or a 1-watt LED will produce more light than needed. In addition, it will be difficult to distribute the light uniformly along a pathway. In these environments, it is sometimes only necessary to use indicator lights to outline the path's edge. For this purpose, it may be preferable to use a series of luminaires with 0.2-watt LEDs as the light sources. In this case, the power requirement drops to 1.6 watt-hours per day per luminaire. Although these luminaires provide significantly less light than the 11-watt CFL or the 1-watt LED, the light is not being used to provide illumination, but rather to provide sufficient luminance to mark the edge of the path.


PV-powered lighting Grid-powered lighting
 

Component Present worth ($) Present worth ($)

Luminaire hardware 100 25

Electricity cost - 3

Grid line extension (50 ft) - 800

TOTAL $100 $828

In this case, the solar LED garden lights provide a much less expensive solution. Because the pathway lighting only needs to provide enough light to define the profile of the path, a series of 0.2-watt LEDs provide enough light for this purpose. For more information see "What are the most suitable applications for photovoltaic lighting?"


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