The most costly part of many outdoor lighting installations is getting power to the luminaires. In addition to the underground trenching and conduit between poles, there is the cost of the service drop, a place for the electrical meter and/or disconnecting devices, and the long-branch circuit connection to the first light. With the installed cost of conduit and wire including trenching and backfilling typically costing more than $20 per foot, a mere 100-foot home run easily could cost more than a pole and luminaire.
Meanwhile, there sits the pole light, fully exposed to nature and, if properly designed, far enough away from trees and buildings to have decent all-day solar exposure. Consequently, it is not a long leap to consider operating the lighting from solar energy. In fact, from very simple residential steplight systems to serious major lighting installations, there are a number of products designed for grid-free installations. But are these systems good enough for professional work, where dusk-to-dawn lighting at Illuminating Engineering Society (IES) recommended light levels is typically provided? Can designers really depend on it?
Other than luminaires and light poles, the standard solar lighting system requires four major components:
The challenge of a solar lighting system is being able to generate and store enough power to run the lights. Daily energy use must be matched by a PV system capable of generating several times the daily use, and a battery capable of storing it. The reason is simple: Solar collection is poor on cloudy days and the battery may have to carry the system for several days during periods of inclement weather. The larger the PV panel and battery, the more likely that the system will provide acceptable performance, even in some winter daylight-challenged locations such as Seattle and other cities in the Pacific Northwest.
Using Energy The first step in designing a lighting PV system is to determine the light source and energy use. The key to a successful design is to use the lowest wattage light source that will meet the project requirements. Some of the best installations of this type employ light-emitting diode (LED) or compact fluorescent lamps. You also might consider low-wattage high-intensity discharge lamps, but issues of warm-up time and restrike limit control options. Among the few practical integrated PV luminaires on the market, typical lamp choices are fewer than 50W, with 26W and 32W compact fluorescents being the most common. As you will discover later, a PV system using ordinary street lighting lamp wattage (100W or more) is not practical because of the physical size of the PV and storage battery.
Collecting And Storing Energy In most of these systems, a PV cell or array is atop the pole of the luminaire. It is pointed at the sun, where the sun will be located at noon on the equinox. Polycrystalline panels currently are the most common and efficient PV collectors, and they generate roughly 15 peak watts per square foot. Peak watts occur when the sun's direct rays are “normal” or perpendicular to the plane of the PV panel, with solar irradiation of 1kW per square meter, or approximately 94,200 lux (8,870 footcandles) on the face of the PV panel. But at best, peak watts occur only once a day; the rest of the time, the PV output is less than peak because of the following:
The output of the PV is stored in deep-cycle batteries, a special type of battery designed to be frequently drained of charge and then recharged. Most lighting systems use 12-volt sealed gel battery systems. Batteries are rated in amp-hours. The battery system's watt-hours (Wh) is determined by multiplying the amp-hours by the voltage. Batteries have reduced light output at low temperatures and they lose some capacity with each cycle, so adding a maintenance factor is recommended. Also, keep in mind that batteries have a useful life of five to 10 years, so at some point battery replacement costs must be expected.
System Engineering Taking all of this into account, along with a little healthy skepticism, the size of the system components can be determined. Planning for worst-case conditions is essential, which for most of North America means winter. Short days for energy collection, long nights of lighting, and extended periods of poor weather easily can stress a system's capabilities. Remember, when the battery is dead, so is the lighting system. The storage battery should be designed for at least five nights of normal operation, and the PV collector should be sized to generate enough power every day, on average, to operate the system. It also should be capable of fully recharging the system 50 percent in one clear winter day.
Take for example a recently installed solar photovoltaic lighting installation for a pedestrian and bike path on the University of Wisconsin campus in Madison, Wis. Funded by the local utility, Madison Gas and Electric Co., the project employs self-powered lighting systems and claims to be one of the largest solar lighting installations in the United States. Each pole is equipped with a 250W peak solar collector and two 12-volt, 100-amp-hour batteries driving a 32W compact fluorescent lamp all night. The storage capacity of the batteries is 2400Wh. The battery system can power a single 35W light for 68 hours (about 5 nights) before being depleted. Assuming there are about 2,500 clear hours in Madison per year, the approximate average daily winter output of the PV will be about 25 percent of peak for eight hours or 500Wh, just barely enough to replace the daily use of 35W for 14 hours. Moreover, the output on a sunny winter day easily could be 50 percent of peak for eight hours, generating at least 1000Wh, almost a 50 percent charge to the battery.
Add Brains The secret ingredient to making solar lighting possible is adding programmable controls that operate lights on reduced schedules or at reduced power. The system in Madison is a good example as its operation can change according to a number of considerations including battery charge level. For instance, the system can be programmed to operate only between dusk and midnight if the battery level drops below 50 percent charge, or cut back to an earlier time, say 10 p.m., if the charge level drops to 25 percent. This will stretch the battery life and minimize outages during periods of need. In the future, LED systems will permit light levels to be dropped and motion sensors to be used, allowing significantly longer battery life and perhaps, lowering collection capacity as a result of the more efficient source.