I like this article. Energy density for portable applications is a real limiting factor for many otherwise worthwhile or essential products. This is a good leadin to a look at advances in battery technology and capacitor technology.
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In this article there is the wrong assertion:
"...the supercapacitor will draw short circuit current from the solar cell when it is at 0V. As it charges, the current reduces according to the solar cell V-I characteristic, but at all times, the supercapacitor is drawing the maximum current it can, so it is charging at the fastest possible rate..."
This is wrong because the power delivered from source is very small when voltage is near to 0V (see magenta line on Fig.4).
If source work at MPPT then the power delivered to supercap is max and charge time is minimum.