I recently installed some of those now-common solar-powered driveway-marker lights ("installed" is perhaps too dramatic; "stuck them in the ground" is more accurate). At around $3 each at the local Home Depot, hey, it's worth seeing how well they work short- and longer-term (see here).
They use a single, user-accessible, easily replaceable AA-size nickel-cadmium (NiCd) rechargeable battery, which is especially convenient. (Ironically, I had to return one unit which failed with an interesting failure mode: it would not turn the illumination LED off, but instead stayed on all day, and so was unable to fully charge itself.)
Since it is summer here, these lights are getting lots of those free photons and running for about seven hours after they turn themselves on at sunset. But I am also thinking about the winter ahead, and I'll be watching how they perform in comparison. I see several possible factors working against them as we shift seasons:
- Most obvious, there will be fewer hours of sunlight, and those rays will be weaker due to the sun's lower angle
- It will be colder, getting down to–and likely below–freezing, which will affect the battery efficiency at storing and returning charge
- The solar cells themselves may not be as efficient in converting photons to current
- Their internal, light-level-based turn-on/turn-off circuit also may be affected by the temperature, as it drops down 0°C or lower
- By design, the NiCd cells will have a daily charge/discharge cycle; how many of these cycles can the batteries tolerate?
While I won’t be setting up a formal data logging station to collect data on mean solar exposure, ambient temperature, after-dark "on" time, and other parameters, I'll keep informal notes on the daily (and nightly) situation. Clearly, this is a experimental set-up that an aspiring, junior scientist or engineer could get involved with, and gain some data-collection and analysis experience at low cost
But I am thinking much bigger. My next step may be to apply for a grant to formally study these solar-powered lamps, and perhaps analyze different models for an in-depth comparative study as well. I'll need a temperature-controlled environmental chamber, including solar-equivalent illumination (think of all those watts!) and calibrated control and measurement equipment. If I get really ambitious, I could even do some total life-cycle environmental costing, since these lights probably have a viable life of only a few years –one or more among the battery, solar cell, charging circuit, or plastic enclosure will likely deteriorate or fail outright.
After all, this topic has all the "hot buttons": solar-powered, rechargeable, green, no emissions, and similar. So, where do I apply? ?