Outdoor savings from PVs
Outdoor savings from PVs
Even in urban areas of industrialized countries, photovoltaic outdoor lighting could be a cost-effective alternative to grid-connected streetlights in many cases. The main reason for this is that the costs of digging up asphalt and pavement for the electric cables can be prohibitively high.
In a solar outdoor luminaire, the PV array transforms the energy from the sun into electrical energy and stores it in a battery. A microprocessor unit controls the charging and uses the PV array itself as a photo sensor to decide when lights are to be switched on and off. The number of sunless days that the batteries can last obviously depends on how the system is specified.
Current products for southern US conditions, however, could store enough energy to provide lighting for a period of 5-7 consecutive days without sun. By contrast, at latitudes far to the north or south, the long winter nights pose a serious problem with regard to the use of solar outdoor lighting: Namely, there are many dark hours that require lighting, but, at most, only a few hours of sunlight per day are available for charging the batteries.
In cold climates, there is also another complication. Today, the typical ballast on the market will only drive fluorescent lamps or low-pressure sodium lamps. However, fluorescent lamps do not perform well at low ambient temperatures, and the color quality of low-pressure sodium lamps has much to be desired.
This may change, however. The introduction of ballasts for high-intensity discharge (HID) lamps seems imminent. During 1996, the US National Renewable Energy Laboratory tested a DC-compatible ballast for high-pressure sodium lamps, and a UK company has developed a high-performance, low-loss unit for metal-halide lamps, specified by the UK lighthouse authority (See HID ballast for PV applications . HID lamps would make streetlighting more cost efficient in sunny climates and make it possible, although still expensive, in colder regions.
SOME US EXAMPLES
PV-based outdoor lighting has been used by several local authorities in the US. The US Center for Study of Responsive Law's publication Energy Ideas reports on a number of such projects: The City of Orlando Transit Authority equipped 55 bus shelters with solar lights. This saved money immediately since, otherwise, they would have had to dig trenches and cut up the asphalt in order to install underground cables. In addition, any future costs for moving bus shelters in the case of route changes will be reduced.
In 1989, the city of Albuquerque, New Mexico, chose to install 21 PV lights, instead of grid-connected lights, along a new bike and walking path. The up-front savings were about US$500 per light, compared with the cost of underground cables. Moreover, no energy bills will have to be paid. Energy Ideas also gives an example of the reliability of PV streetlighting.
When Hurricane Andrew hit Dade County in Florida about a year ago, all grid-connected streetlights were down for two weeks. But the solar-powered units continued to operate flawlessly, supplying much needed light at night in the devastated area.
Nils Borg
Contact:
Hal Post
PV Design Assistance Center
Sandia National Laboratories
Tel: +1 505 844 2154
Fax: + 1 505 844 6541
E-mail: hpost@sandia.gov
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