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.
What information is needed to specify a PV lighting system?

In many ways, specifying a photovoltaic (PV)-powered lighting system is similar to specifying a grid-powered lighting system. However, expectations and design parameters must be altered if PV systems are to provide illumination effectively and economically. Additional information is also necessary to ensure that the PV system will supply the power needed to meet the requirements of a particular lighting application reliably. Some of the necessary information is provided in manufacturers' literature. The remainder can usually be obtained from a manufacturer's representative or distributor.

Determining application lighting requirements

The first step in specifying a PV lighting system is to determine if a PV system can realistically and economically meet the lighting needs of the application. In order to make this determination it is important to determine the application's lighting requirements. If the application requires relatively high nighttime light levels (greater than 1 lux on the ground) that is distributed uniformly over a large area for a long period of time (e.g., throughout the entire night), and is located in an area where the electric power grid is readily and easily accessible, a PV lighting system will most likely not be able to meet the lighting requirements reliably and cost effectively. An example of this type of application might be a parking lot or roadway that is located in an urban area and is used throughout the night.

A PV lighting system will most likely be able to meet the needs of a lighting application if one or more of the following points apply:

  • Lower light levels, no more than 0.5 lux (5 moonlights), are considered appropriate for the application.
  • The lighting system needs to operate for only a few hours (no more than eight) per night.
  • Lighting needs to be provided only in limited areas of a site (e.g., to outline a path or pedestrian walkway).
  • Small-scale luminaires are desirable for the application.
  • The site is located in an area of the country where solar irradiance is plentiful throughout the year, or the site is only used when the solar irradiance is plentiful (i.e., summer).
  • The electric power grid is not readily accessible and/or it would be costly to bring power to the site.
  • The luminaires can be located in an area that will have non-shaded access to the sun for a majority of daylight hours and in which dirt is not likely to accumulate quickly on the PV panels.
  • Financial subsidies are available through an electric utility, state energy office, or other entity, which would offset a significant portion of the capital costs for the PV lighting system.

If several of these points apply to the site under consideration, a lighting specifier may want to determine if a PV lighting system can meet the needs of the particular application cost effectively.

Selecting appropriate PV lighting equipment

When selecting lighting equipment for a particular application, it is important to understand the light output and distribution requirements of each luminaire. Once these requirements have been determined, a lighting specifier can select PV lighting equipment that will meet these specifications. Usually PV lighting equipment is purchased as a system that has already been assembled by a manufacturer, which contains the PV panel, batteries, electronics, luminaire, and other necessary components. In this case, a lighting specifier may want to check that the system will meet the lighting requirements of the application under consideration. The following information will be helpful.

Systems capacity
The information listed in the process below is needed in order to determine the system capacity or size requirements of a PV lighting system. The various system components (i.e., lamps, PV panels, electrical components) will degrade over time; therefore, a lighting specifier may need to over-design a PV lighting system (design it to exceed the minimum requirements of an installation) to ensure that it will continue to operate reliably over a long period of time.

Before determining energy requirements, first determine appropriate light levels for the application under consideration. For example, a high activity area may require a 10-lux system. A rural pathway, on the other hand, may need only a system with 0.5 lux illumination.

Step 1 - Determine energy requirements of the lighting system
The proper sizing of a PV lighting system depends on the watt-hours needed to operate the system each night. This is a function of the optical efficiency of the luminaire and the total number of lamp lumens (lm) needed, which will in turn determine the number of watts needed to provide those lumens. Once the total wattage of the lighting system (lamp and ballast or driver) is determined, this must be multiplied by the number of hours the system will operate each night. Therefore, the following information is needed:
  • Luminaire efficiency
  • Total number of lumens the lamp must produce (over time)
  • Total wattage of the lighting system (lamp plus ballast or driver)
  • Number of hours the lighting system will operate each night
Step 2 - Determine efficiency of system electronics
To ensure that the PV panels and batteries can provide the required energy for the lighting system, it is necessary to account for any losses that will occur in the system's electronics, such as charge controllers or dc-ac inverters. The efficiency of system electronics is usually given as a percentage. This will allow estimating how much of the battery's capacity will actually be available to power the lighting system. Therefore, it is necessary to know the efficiency of all system electronic components throughout their useful life.
Step 3 - Determine required battery storage capacity
Batteries are typically specified by the number of ampere- (amp) hours they are able to provide. To determine the number of amp-hours needed to provide the number of watt-hours required by the lighting system, it is necessary to know the voltage of the system. To convert watt-hours to amp-hours, divide the watt-hours by the voltage of the system. This will provide a rough estimation of the battery storage capacity needed for a particular system. It is also important to know how much of a battery's capacity is actually available for use. This is sometimes listed in manufacturer's literature as the battery's "usable amp-hours." Many batteries should only discharge 30% of their total storage capacity before being recharged. To check if the battery provided by the manufacturer is sized appropriately to meet the needs of a particular application or to specify battery capacity, the following information is needed:
  • System voltage
  • Usable amp-hours capacity of the battery (over time)
  • Number of cycles for different discharge percentages
Step 4 - Determine the size of the solar panel needed
A crucial step in specifying a PV lighting system is to determine the size of the solar panel that will be needed to power the system reliably. This will vary based upon the area of the country in which the application is located (see "How does solar radiation vary by location?") and whether the design is for "worst case" or average solar radiation availability. Information avsailable on solar radiation charts typically specifies the number of kilowatt hours (kWhs) that can be produced per day in a particular location by a solar panel of one square meter. To determine if the solar panel you are considering is large enough to provide the energy required by a particular system, the following information is needed:
  • The location of the lighting application
  • The conversion efficiency of the solar panel being considered (over time)


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