For a good many years, there have been periodic articles about “solar powered lighting,” a supposed creative way to bring lighting to where connection to the grid is next to impossible. But a little cost/benefit analysis has always seemed to put a damper on things. Although there are now dozens of firms in the US which offer such products made in the US or China, there has so far been little traction.
So, what’s the problem here? In a basic system, a solar panel charges a battery during the day and the battery then powers an LED light fixture at night. Sounds simple enough, right? The fact that an LED requires so much less electricity than an incandescent lamp makes the idea worthy of discussion. Furthermore, the inherent directivity of LED emitters makes them much better than any HID lamp in actually delivering lumens to a desired surface, no matter what the specs seem to say for any HID luminaire.
But the devil’s in the details. First of all, a solar panel rated at X watts only delivers those watts at noon in a cloudless sky. In reality, if you do a little 5-minute test yourself with a simple ammeter connected right across any solar panel output to measure the short circuit current (yep, that’s what you do to measure the panel’s efficiency in changing light levels), you will find that any solar panel (they’re all “fundamentally” the same no matter what you’ve been told) with an overcast sky will only deliver 20-30% of its rating, and only 50-70% in a cloudless sky in the morning or late afternoon.
This really means that unless you are located in Phoenix or the Sahara, you need a solar panel 2-3 times what you might think to be sure a battery bank will always be adequately charged. The good news, however, is that solar panel pricing, like LEDs, has dropped like a rock in the last 3-4 years. Panels are now readily purchasable for a dollar a watt.
Now that we know the solar panels need not be a major cost issue, we need some healthy batteries. While you might see some systems using lithium or gel cell batteries, the best bang for the buck (cost, performance, and compatibility with simple charging techniques) for higher power is likely the popular Absorbent Glass Mat (AGM) sealed lead acid types, which have a lot of capacity and are probably less than $2 per AH. But you don’t want to let the battery discharge down to less than 30-40%—meaning you really want to use twice as much battery. Still not a big deal on cost, but the size has historically started to get onerous if we wanted a parking lot luminaire of more than 100W to be on all night long.
However, things are changing. LEDs at 5000K have gone from 50 LPW (lumens per watt) in 2006 to more than 150 LPW in 2015. And not just in some company’s R&D lab, but actually for sale off the shelf from Nichia, Cree, Lumileds, and others. This means reduced size and cost for batteries and solar panels, making that tiny niche market a little larger.
The killer in all this is when we need those lights on all night at full brightness. Yes, many firms say you can greatly reduce battery demand by dimming for most of the night or using motion-detection to keep the luminaire at greatly reduced power most of the time. However, that sales pitch has generally been a non starter in mainstream applications. If somebody wants a parking lot light on for security purposes, they want it on at full brightness all night.
Let us jump to an area where solar powered lighting in fact offers surprising benefits. There are now more than 25,000 non-stadium-type youth athletic fields in the US which have no lighting at all. Many of these are also used for youth lacrosse and football practice. If you want to bring lighting to such a field, where no AC mains power has previously been present, it will cost you (the community) between $150K-200K—best case—to dig long trenches, bury high voltage cables per code, bring in overhead power from a distant utility pole transformer, install various service panels per code, and undergo perhaps a 12 month aggravating process with a utility company.
The major cost is not in the LED luminaires and poles but in bringing in that AC power—an eye-opening challenge. The massive task of doing all that trenching, HV-cable laying, and utility connection is totally eliminated with solar. A small field can be lighted in a week via solar powered LED lighting for a fraction of the cost and time. This is made uniquely possible by the fact that youth athletic fields need only be fully lighted for no more than 2 hours a night. The batteries can be charged “poorly” for 10 hours, even on a cloudy day, but still generate enough charge for the limited time needed for a game or practice several times a week. Philips has already installed some systems like this in Africa and several have now been put in place in the US by other companies, including activity in San Marcos, Texas.
The bottom line is the sharp increase in LED efficacy and drop in LED prices, coupled with drops in solar panel pricing, are making it possible to use LED lighting for surprising new applications, matched to need and driven by economics—just when we thought there was not much new going on in LED lighting!
Ed Rodriguez is a veteran of the power semiconductor, power supply and LED lighting technology industries.