![]() ![]() Volume 9 Issue 3
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July 2006
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IntroductionThe sun is an energy resource like no other, providing the sustaining power for life on earth. Harnessing the sun's power through the use of photovoltaic (PV) collection systems can create a sustainable form of clean energy and reduce demand for utility-generated electric (grid) power. PV collection systems have the potential to power lighting systems. However, the effectiveness of a PV system for a given lighting application depends upon a complex set of environmental, technical, and cost variables. Performance expectations and aesthetics also must be taken into consideration when deciding whether to use a PV system to power lighting. Although solar cells and PV collection systems were introduced in the 1950s (Messenger and Ventre 2004), to date these systems have been used for lighting in only a very limited number of applications. Some of the reasons for their restricted use include high initial purchase and maintenance costs, unreliable light production, and complexities in designing and installing PV systems. Although several field demonstrations have showcased PV lighting systems in parking lot and roadway applications, PV lighting systems are generally not economically and practically feasible when the lighting for these applications is designed according to existing illumination guidelines. Therefore, this Lighting Answers focuses on outdoor applications such as those in rural or remote areas, designed for use where light levels similar to moonlight may be sufficient for the tasks being performed. Not covered in this report are PV systems that power indicator lights such as lights on ocean buoys, blinking traffic lights, and construction signage, although these are viable applications. Also, hybrid PV systems that combine small generators or grid power with PV panels are not included in this Lighting Answers. In the early part of the twentieth century, when electric street lighting was beginning to be installed in many areas of the United States, moonlight levels were commonly used as a standard or reference point for outdoor lighting. In many instances, the visual quality of a street lighting design was measured against moonlight. The closer the illumination provided by electric street lighting came to moonlight, the more highly it was rated, even though electric lighting could provide significantly higher light levels (Blood 1907). Moonlight was also believed to provide better visibility than electric lighting. In some areas of the United States, electric street lighting was actually turned off when the moon was full, because it was believed that the electric lighting detracted from the illumination provided by moonlight (Hyde 1910). When considering the needs of an outdoor lighting installation, it is still helpful to think in terms of moonlight levels. Under very dim visual conditions (i.e., those found in rural areas), moonlight, with an illuminance of approximately 0.1 to 0.5 lux, or 0.01 to 0.05 footcandles (fc) on the ground, often provides enough lighting for people's basic needs such as walking, or finding a house or car. NLPIP proposes that moonlight illuminance can be used as a reference value for setting expectations of suitable light levels in rural or remote lighting applications. For the purposes of this Lighting Answers, one moonlight is defined as 0.1 lux (0.01 footcandle). This is equivalent to the average illuminance on the earth's surface provided by a full moon. In order to assist the reader to compare the requirements and cost-effectiveness of PV lighting systems for various applications, this Lighting Answers uses three sizes of lighting systems as examples:
These lighting conditions were selected because they represent a range over which PV lighting systems might be considered practical.
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