Inspired to continue “greening” my new house with renewable and energy efficient technologies, I purchased a set of solar-powered walkway lamps that use light emitting diodes (LEDs) as luminaires. The lights are a generic brand similar to Malibu One-Light Outdoor Solar-Powered Walk Lights, and according to the manufacturer, they can stay illuminated for more than 10 hours on a full charge from 8-hours of direct sunlight. I have yet to test this claim through personal observation.
Outdoor walkway lighting represents an obvious marriage between solar energy and LEDs due to the superior lighting efficiency and compact form factor provided by LEDs compared to competing illumination technologies. Efficient energy use is the paramount constraint driving solar walkway light design because the lights must remain illuminated all night using only energy stored during the day.
However, there is another reason that LEDs are appearing first in walkway lights. A recent Physics Today article, “LED luminaires—a whole new light”, touts LEDs as the lighting technology poised to replace incandescent and compact florescent lamps. However, the same article points out many technical hurdles still to be overcome. For example, multiple LEDs must be run in tandem to provide illumination levels equal to conventional bulbs. A close-packed array of point light sources creates optical problems such as multi-shadowing and the so-called Lite-Brite effect that cannot be tolerated in conventional lighting applications. These issues can be solved with diffusers or optics, but with loss of illumination level. For walkway illumination, the quality of light is less important than simply pointing out the edges of the path. So, LEDs are adequate.
Due to these unresolved limitations, LEDs are appearing first in niche applications like walkway lights and Christmas tree lights (see my recent pots, “Do LED Lamps Signal the End of Christmas Magic?”). As the technology improves, we will certainly see LEDs infiltrating more into the conventional lighting market.
The control scheme of my new walkway lights is sophisticated but elegant. These lights use a small array of conventional silicon solar cells embedded in the top the lamp. The cells charge a battery, which stores collected energy for overnight operation. A small photo-sensor, also embedded in the top of the lamp, activates the unit in response to lowering ambient light levels at dusk. Provided the battery receives enough charge to keep the LED blazing all night, the lamp shuts off when the sun rises, and the unit returns to battery charging mode.
The luminary assembly contains a pair of white-light LEDs, protected within a plastic housing. Both LEDs light up when the unit turns itself on. The resulting illumination level is not as bright as the conventional, hardwired walkway lights my neighbors utilize, but it is certainly adequate to point the way down the walkway.
When I get all the lights set up outside and fully charged, I will post an image on my blog to demonstrate how well they are functioning.