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?

The best analysis of the cost of owning a photovoltaic (PV) lighting system compared to a grid-powered lighting system can be made through a life cycle cost (LCC) analysis. LCC analysis is an evaluation method that takes into account all of the costs of owning a product or system over a period of time (normally over its lifetime). LCC analysis includes the time value of money and calculates the present value (or present worth) of all costs expected to occur.

The costs of owning a lighting system include acquisition or capital costs (C), maintenance costs (M), energy costs (E), and replacement costs (R). This is expressed as a formula:

LCC = C + Mpw + Epw + Rpw
where the "pw" subscript refers to the present worth of each cost.
(Sandia National Laboratories 2002; Messenger and Ventre 2004)

When comparing PV lighting systems to grid-powered lighting systems in applications where the electric grid is readily accessible, where the lighting must operate reliably throughout the night at all times of year, and where relatively high nighttime light levels are required (e.g., 1 lux or greater), it is generally difficult to justify the higher life cycle cost of the PV lighting system. This is primarily because the higher capital cost of the PV system cannot be offset by the energy cost savings attained by the system.

Both a PV lighting system and a grid-powered lighting system will require a luminaire, a pole or other mounting system, and a lamp. It is difficult to make a simple comparison of these particular costs because in some cases, they may be the same for each system and in other situations, the costs may differ. For example, in the case of a large PV lighting system, the pole and foundation may need to be significantly stronger to handle the weight and wind load on the PV panel. This will add to the system's cost. However, the cost comparison shown in Table 1 considers these particular costs to be the same for each system.

In addition to a pole, luminaire, and lamp a PV lighting system will also require the purchase and installation of a PV panel, a battery (which generally must be replaced every five years), and other electronic equipment such as a controller. The grid-powered lighting system will require wiring to connect the system to the grid. The maintenance costs of the PV lighting system will also generally be higher because of battery replacement labor, and because the PV panel is an extra surface that needs cleaning. A comparison of the life cycle costs for a 10 lux (100 moonlights) illuminator (a parking lot luminaire designed to provide approximately 10 lux on the pavement) over a period of 10 years is summarized in Table 1 below. 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 1 summarizes a 10-year 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. The luminaire in this example uses an 11-watt compact fluorescent lamp (CFL), operating eight-hours per night. A more detailed analysis can be found in the Case Study: Life Cycle Cost.

Table 1. 10-year life cycle cost comparison of luminaires
(located 50 ft [15.2 m] from grid)

System Type  Illuminance LCC Capital 
Maintenance Costspw Energy Costspw Replacement Costspw

PV–powered 10 lux $1303 $710 $479 $0 $114

Grid–powered 10 lux $1252* $1010* $192 $31 $19

* Includes cost of power line extension

In the example above, the luminaire was located close to the electric grid. However, if the lighting application were in a remote area that required extending the electric grid for one mile (1.6 km) or more, the life cycle costs of the grid-powered lighting system would increase dramatically. The cost of extending the power grid is estimated to be $30,000 per mile (EERE 2005). A comparison of the life cycle costs for a parking lot lighting system located one mile (1.6 km) off-grid, using 10 luminaires that contain a 11-watt CFL in each, and owned over a period of 10 years is summarized below. A more detailed analysis can be found in the Case Study: Life Cycle Cost.

Table 2 summarizes the life cycle cost comparison of a PV-powered lighting system containing 10 luminaires and a grid-powered lighting system containing 10 luminaires located one mile (1.6 km) from an electric grid connection.

Table 2. 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 10 lux $13,033 $7100 $4789 $0 $1144

Grid–powered 10 lux $34,571* $32,100* $1916 $364 $192

* Includes cost of power line extension

In Tables 1 and 2, the lighting systems were designed to provide about 10 lux (100 moonlights) on the ground and were operated eight hours per night, on average. The power requirements per luminaire were relatively modest, at 13 watts including lamp and ballast. Any changes in system requirements will also change the economic comparison between the PV and grid-powered lighting options. For example, if the application parameters were changed to require that the lighting systems provide higher light levels over a wider area, for a longer period of time, the grid-powered lighting system may become a more attractive option due to its ability to power higher wattage light sources. The costs of the PV lighting system, on the other hand, may rise substantially due to the need for larger PV panels and increased battery capacity. The capital costs associated with PV lighting systems are the most likely costs to push these systems out of consideration when application requirements (e.g., high light levels and long operating times) begin to stretch a PV system's capacity to meet these requirements economically. Conversely, higher expenses for extending the grid such as for easements, difficulty terrain, or repaving, will favor PV systems.

In some areas of the United States, subsidies may be available to help offset the cost of the purchase and installation of PV lighting systems. These subsidies will often help to bring down the initial costs of a PV lighting system, making it a more attractive option for end users. A system specifier or end user will want to consider all of the parameters discussed above when performing an economic comparison of lighting system options.


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