Phlogiston
Enlightened
I have the Anker IQ 14W foldable solar panel, as a source for emergency-charging Li-Ion USB powerbanks. It has a built in 5V regulator and two USB outputs, so it's particularly convenient for that. I've been experimenting with the panel since January, so I thought I'd post some interesting points I came across in the process.
In early January, I tried using it indoors with a double-glazed window between it and the sun, just to see what would happen. Under perfect weather conditions, with the panel angled for maximum collection, the best I could get out of it was about 1W. I live at a fairly high northern latitude, so I wasn't particularly surprised by that
I tried it again in mid-March, and I got about 5W. It also occurred to me to try spreading a sheet of aluminium kitchen foil on the floor as a booster reflector, which got me up to 7W.
I did another test in mid-April, but I still only got about 5W. Although the sun in April was higher in the sky and more energy would have made it through the shorter atmospheric path, the higher sun angle also meant that the sunlight was arriving at a more grazing angle to the window glass, so more of the energy was being reflected away to the ground outside.
In other words, the limiting factor in the winter half of the year is the weaker sunlight, just as you'd expect, but there's also a hard limit to what one can ever get out of a panel indoors because of the interaction between the sun angle and the reflectivity of the window glass.
Unfortunately, I don't have any skylights to experiment with, but I imagine a skylight in a suitably angled roof would be a good place to park an indoor-mounted solar panel during the summer half of the year.
Of course, one can always put the panel outside, but that's not always practical. For example, the weather might be showery with sunny spells, or there might not be anywhere outside to put the panel, or there might be a risk of theft.
Indoor operation - perhaps with improvised reflectors using stiff wire, duct tape and kitchen foil - can be an acceptable substitute. Dual reflectors at optimal angles, one each side of the panel, should be able to get it up to about 3W at midwinter and full power for something like eight months of the year.
I did a final test in early June, and I only got about 5W indoors again, as I expected. To confirm the panel's actual capacity, I took it outside this time and got about 11.5W. On that basis, I imagine that the panel would meet its rating in ideal conditions, i.e. full tropical sunlight from clear skies at noon.
I'm happy with the Anker panel's overall performance, because it can still achieve usable output even in quite seriously suboptimal conditions. Even 1W for 4 hours will get you a quarter charge on a 3200mAh Li-Ion 18650 or a full charge on a pair of AAA Eneloops (after conversion losses and charger inefficiency).
In a Fenix UC35, that quarter-charged 18650 should manage 15lm all the way through the longest night of the winter. Alternatively, the two AAA Eneloops will buy me 25lm for 5 or 6 hours in an LD02, or 8lm all night.
Notes:
[1] All wattages given above are the peak outputs I saw, calculated as snapshots using KCX-017 USB meters (HKJ review) with USB power banks as loads (unfortunately, I don't have automated logging equipment). The power banks were Anker 2nd Gen Astro E4 13000mAh and RavPower RP-PB16 7800mAh models.
Usually, one meter and power bank was enough to fully load the panel, but the Anker power bank only draws 10W maximum, so the June outdoor test required a second meter and the RavPower power bank (capable of drawing 3.5W maximum) to get the most out of the panel. The panel's two USB outputs proved convenient for that part!
[2] I noticed that extracting the most power from the panel required a careful selection of load, especially in suboptimal conditions. I had to use the RavPower power bank for the January test, because the Anker one tried to draw so much current that the resulting voltage sag rendered the panel unusable. Think of it as manual Maximum Power Point Tracking (MPPT)
This implies that there are times when you're better off using a little 0.5A charger for your Li-Ion cells, because a 1A charger would drag the panel voltage down too much. It's definitely a good idea to experiment with your particular setup in the actual conditions you need it to work in.
In early January, I tried using it indoors with a double-glazed window between it and the sun, just to see what would happen. Under perfect weather conditions, with the panel angled for maximum collection, the best I could get out of it was about 1W. I live at a fairly high northern latitude, so I wasn't particularly surprised by that
I tried it again in mid-March, and I got about 5W. It also occurred to me to try spreading a sheet of aluminium kitchen foil on the floor as a booster reflector, which got me up to 7W.
I did another test in mid-April, but I still only got about 5W. Although the sun in April was higher in the sky and more energy would have made it through the shorter atmospheric path, the higher sun angle also meant that the sunlight was arriving at a more grazing angle to the window glass, so more of the energy was being reflected away to the ground outside.
In other words, the limiting factor in the winter half of the year is the weaker sunlight, just as you'd expect, but there's also a hard limit to what one can ever get out of a panel indoors because of the interaction between the sun angle and the reflectivity of the window glass.
Unfortunately, I don't have any skylights to experiment with, but I imagine a skylight in a suitably angled roof would be a good place to park an indoor-mounted solar panel during the summer half of the year.
Of course, one can always put the panel outside, but that's not always practical. For example, the weather might be showery with sunny spells, or there might not be anywhere outside to put the panel, or there might be a risk of theft.
Indoor operation - perhaps with improvised reflectors using stiff wire, duct tape and kitchen foil - can be an acceptable substitute. Dual reflectors at optimal angles, one each side of the panel, should be able to get it up to about 3W at midwinter and full power for something like eight months of the year.
I did a final test in early June, and I only got about 5W indoors again, as I expected. To confirm the panel's actual capacity, I took it outside this time and got about 11.5W. On that basis, I imagine that the panel would meet its rating in ideal conditions, i.e. full tropical sunlight from clear skies at noon.
I'm happy with the Anker panel's overall performance, because it can still achieve usable output even in quite seriously suboptimal conditions. Even 1W for 4 hours will get you a quarter charge on a 3200mAh Li-Ion 18650 or a full charge on a pair of AAA Eneloops (after conversion losses and charger inefficiency).
In a Fenix UC35, that quarter-charged 18650 should manage 15lm all the way through the longest night of the winter. Alternatively, the two AAA Eneloops will buy me 25lm for 5 or 6 hours in an LD02, or 8lm all night.
Notes:
[1] All wattages given above are the peak outputs I saw, calculated as snapshots using KCX-017 USB meters (HKJ review) with USB power banks as loads (unfortunately, I don't have automated logging equipment). The power banks were Anker 2nd Gen Astro E4 13000mAh and RavPower RP-PB16 7800mAh models.
Usually, one meter and power bank was enough to fully load the panel, but the Anker power bank only draws 10W maximum, so the June outdoor test required a second meter and the RavPower power bank (capable of drawing 3.5W maximum) to get the most out of the panel. The panel's two USB outputs proved convenient for that part!
[2] I noticed that extracting the most power from the panel required a careful selection of load, especially in suboptimal conditions. I had to use the RavPower power bank for the January test, because the Anker one tried to draw so much current that the resulting voltage sag rendered the panel unusable. Think of it as manual Maximum Power Point Tracking (MPPT)
This implies that there are times when you're better off using a little 0.5A charger for your Li-Ion cells, because a 1A charger would drag the panel voltage down too much. It's definitely a good idea to experiment with your particular setup in the actual conditions you need it to work in.
