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 are the application issues related to PV lighting systems?

Installation

Installing a solar yard light is not necessarily difficult; however, installing a large-scale photovoltaic (PV) lighting system such as a 40-watt solar street luminaire with 12 square feet of PV panels is not a simple task. A survey of PV installations conducted in 2002 showed that 50% of PV systems (stand-alone, interconnected, hybrid, and multi-mode) in the U.S. were installed improperly. This resulted in deficiencies in safety, durability, and/or performance (Wiles et al. 2002). Stand-alone PV systems are more technically complex than electric grid-connected systems (DOE 2003).

In order to avoid installation problems associated with PV lighting systems, it is important to work with trained and experienced PV designers and installers, and to use well-established PV module technology and reliable equipment, while following the best available information and codes such as the National Electrical Code (NEC).

System integration

All components of PV lighting systems need to be well designed and matched to provide satisfactory system performance. If system components are not well matched, the system will likely be inefficient and the life of the components will be compromised. An automobile battery, for example, is a poor choice for a PV lighting system. An automobile battery is designed to supply high current for a short duration. Taking too much energy out of this type of battery before recharging it is likely to damage the internal components. A PV lighting system will repeatedly remove large amounts of a battery's energy capacity. Therefore, a specialized type of battery, called a deep discharge battery, is better suited to a PV lighting system.

If a PV lighting system uses a fluorescent or high intensity discharge (HID) lamp, a dc ballast may be used to replace two components: the dc-ac inverter and the ac ballast. This will improve the overall efficiency of the PV system, if the efficiency of the dc ballast is higher than the efficiency of the combination of the other two components. However, few dc ballasts exist for these light sources. For light-emitting diode (LED) lamps, a dc driver (current regulator) is a better choice than the combination of a dc-ac inverter and an ac driver. The dc driver for LED lamps in PV lighting systems is sometimes considered unnecessary because the output voltage from PV panels can be designed to match the required driving voltage of the LED lamps. However, it is normally poor practice not to use a dc driver, since unexpected changes in the circuit condition such as voltage or current fluctuations are likely to occur.

Maintenance

PV panels degrade over time, and the cover glass may collect dirt. Although panels are becoming more robust, a study in 2000 warned that PV modules may delaminate due to oxidization, thus resulting in reduced efficiency, and the cover glass could crack due to hail damage or thermal cycling (Quintana et al. 2000). Exposure to atmospheric oxygen, high current density, and elevated temperatures contribute to solar cell degradation (Riesen and Bett 2005). High aluminum content of PV modules also contributes to degradation. Many types of solar cells need high aluminum content in the window layer to ensure transparency, but layers with high aluminum content are prone to oxidation. Layers with lower aluminum content tend to be more stable (Riesen and Bett 2005). Moisture can also cause damage in thin-film PV modules, including material delamination and electrochemical corrosion (Mon et al. 1988).

Batteries also degrade over time. After eight years, battery capacities typically drop to just three to 50% of their initial value (Diaz and Egido 2003). Also, a battery's capacity decreases under lower ambient temperature. For example, a battery's capacity may drop 25% when the ambient temperature drops from 25° to 0° C (Sandia National Laboratories 1995).

Aesthetics

PV lighting sometimes causes aesthetic concerns. The PV panel is usually large compared to the luminaire itself, and may be considered an eyesore by some people. A designer for a PV lighting system should integrate the PV panel into the architectural and environmental surroundings as much as possible. This will help to avoid objections from people who live in the area where the system is being installed. With low-power PV lighting systems, the size of PV panel should be small, and aesthetics may not be as much of a concern because it can be more easily incorporated into the luminaire and/or its supporting structure. Figure 10 shows an example of PV-powered luminaires along a pedestrian pathway located alongside a public street.

Figure 10. PV-powered luminaires used to illuminate a pathway

Conversely, the use of a PV lighting system may illustrate that the system's owner is sensitive to the environment and is interested in the use of "cutting-edge," environmentally sustainable technology. In this case the PV panels may be viewed as a positive feature of the luminaire, which the owner may wish to showcase. However, it is still a good idea to try to integrate the PV panel into the design of the luminaire or mounting structure in a way that is visually pleasing and structurally sound.s

 

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