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 kinds of batteries are used in PV lighting systems?

A battery is a device that converts chemical energy contained in its active materials directly into electrical energy by means of an electrochemical reaction. Batteries used in photovoltaic (PV) lighting systems must be rechargeable. Lead-acid batteries are the most common type of batteries used in PV systems, due to their wide availability in many sizes, their low cost, and their well understood performance characteristics. Lead-acid batteries are also commonly recycled. The most common types of lead-acid batteries used in PV systems are lead-antimony batteries, lead-calcium batteries, lead-antimony/lead-calcium hybrid batteries, and captive electrolyte lead-acid batteries, which include gelled batteries and absorbed glass mat (AGM) batteries (Dunlop 1997). Nickel-cadmium cells are used in some applications, but their high initial cost limits their use.

Batteries are the costliest and weakest components in stand-alone PV systems, if battery replacement is considered (Diaz and Lorenzo 2001; Diaz and Egido 2003). Battery capacity degrades over time, so batteries must be replaced at regular intervals. Providing an estimate for a "typical" battery life is difficult because of many factors, including battery type, correct sizing of the PV system, local climate, and proper operational management such as charge controllers and maintenance procedures. Generally, PV lighting systems store solar energy in the batteries during the day and release that energy as lighting at night. This is known as the battery's cycle (i.e., one charge and one discharge period). Batteries are sometimes specified in terms of cycle life. The cycle life of a battery denotes the number of cycles it is expected to last before being reduced to 80% of its rated capacity. Replacement should be considered when a battery reaches this point.

When selecting or specifying a PV lighting system, it is important to check that the battery capacity is sufficient to provide the energy needed to power the lighting system for the required amount of time. A battery's capacity is a measure of the amount of energy that a battery can store. This capacity is measured in ampere hours and indicates the amount of energy that can be drawn from the battery before it is completely discharged. A battery rated at 100 ampere hours, for example, should ideally provide a current of one ampere for 100 hours, or two amperes for 50 hours (or any combination of amperes and hours that give a product of 100).

The optimal type of battery for a PV lighting system is a deep-cycle (or deep discharge) battery that can be repeatedly drained of much of its energy and recharged (EREC 2001). The maximum depth of discharge for low-maintenance (sealed) batteries is 30% (Diaz and Egido 2003). The maximum depth of discharge of a battery is a measure (in percentage) of the amount of energy that can be removed from the battery during a cycle, without damaging the battery. Batteries should generally be located in a weather resistant, nonmetallic enclosure in order to prevent corrosion (Sandia National Laboratories 1995). Batteries should be maintained according to manufacturers' instructions, and it is important to keep them clean to ensure maximum performance over time.


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