Solar Energy Harvesting: No Such Thing as a Free Lunch?

Article By : Bill Schweber

An experiment with solar-powered outdoor lamps was conducted to see if solar energy harvesting works well and what limits the lifetime of these outdoor lamps...

Like many people, I was attracted by the convenience of those small solar-powered outside lights that you can use to illuminate a walkway. After all, they seem to be a simple, headache-free solution to a minor issue. They are solar-based, it’s free energy harvesting, and no wiring. Just stick them in the ground and the problems is solved.

These widely available solar-powered lights are now quite common as they solve a problem (at least for a while) and are trivial to install and set up. (Source: The Home Depot)

Well, not quite. I’m actually on my third set of these inexpensive lights, which typically run from about $10 to $30 for a set of five or six, and are widely available at home centers, discount stores, department stores, and many other places. (For all I know, the countless varieties available are all made in the same few factories but with different trims, shapes, and packaging.) After three years, the latest set has followed the path of the previous ones and no longer provides light when the sun sets.

The obvious culprit is the weakening and demise of the low-cost, low-quality, rechargeable battery in these inexpensive units. So, I replaced those (in my first set) with better, more expensive batteries from a presumably reliable source (though you often can’t tell, there are so many counterfeits on the market).

These better batteries lasted about four years. Actually, that’s pretty good, considering the rough life these batteries face: extremes of temperature, repeated thermal cycles, and an erratic, far-from-optimal energy-collection/power-use cycle that is hard to manage efficiently. (In the summer, when there are more hours of sunlight and the sun is stronger, there are also fewer dark hours needing illumination; in the winter, the situation is the reverse.)

There are other issues, too. The contacts inside the battery case corrode due to the moisture while the plastic of the units starts to crack from solar exposure. The repeated thermal cycles also stress the solder joints, contacts, materials, and their mating surfaces.

These solar-harvesting lights are OK as a short-tern solution, but are not good as longer-term, consistent, many-year solution. I realize it is unrealistic to  expect otherwise from low-cost units like this. I also looked at some more expensive ones but frankly, I think that while they looked better — more trim, more apparent substance — I suspect the end result would be the same or similar.

Even better original batteries would only help somewhat, as even better ones are typically good for only a few thousand charge-discharge cycles before they drop to 80% and then less of their nominal capacity — and that is under benign stress conditions. So, we’re looking at perhaps a maximum five years of life, which is not bad, but it is not the “forever” that many harvesting applications need.

The lessons from this obviously non-cortical application are the usual ones: when you see something for almost nothing, be wary, and carefully go through the details and assumptions. Experience shows us that there are very few free lunches even in the world of energy harvesting. Using solar-powered energy harvesting as a power source that needs to last for many years needs careful, thoughtful design, component selection, and fabrication.

You can’t just throw together a solar cell, harvesting IC, and rechargeable battery and say “done.” Perhaps you need a more complicated circuit with a supercapacitor, for example. There are even external issues to worry about, as natural growth of plants and foliage which may block the solar cell over the years, or someone unintentionally moving a box and blocking the light path.

This easily mounted AXDP–10-Watt monocrystalline solar panel costs about $100 and also needs a power controller and regulator. (Source: Ghost Controls LLC

This 120VAC/12VAC 60-W outdoor-rated transformer can be used to drive 12-V outdoor lamps; special connectors offered with the complete kit simplify completing the parallel-wiring connections. (Source: Sunvie via Amazon)

As for my modest problem, I am considering my options:

  1. accept the reality and simply replace the low-cost units every few years (seems wasteful);
  2. go with a solar panel on its own stand, with a higher-capacity “better” battery and harvesting power-conversion subsystem; or
  3. forget about harvesting and use line-power for a set of low-voltage lights via a 12 V outdoor transformer?

For options 2 and 3, wiring from power source to the lights will be needed, which in contrary to the optional “easy and simple” concept of no wires and localized harvesting in each light. As usual, it’s a tradeoff, as are nearly all engineering decisions.

What has been your experience with these inexpensive solar-power outdoor lamps? Have you been satisfied, disappointed, or maybe you didn’t expect much? Have you found higher-quality ones, and did their price reflect this? Have you found other points of failure in addition to the battery?

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