The original Solar Shrub was a great success from the standpoint that it did what it was supposed to do (charge small devices using nothing but energy from the Sun) . Plus winning the grand prize in the Instructables Green Tech Contest was definitely an ego boost!
But if I really want to bring Solar Shrubs to market, I’ll have to do some redesigning and re-engineering. There are several specific problems with the current design that could greatly effect its viability as a salable product:
1. Low Current: The current produced is too low to charge some devices in a reasonable amount of time. I had originally calculated a much higher current (around 300mA) based on bad information about the specs of the solar cells. What I wound up with was about 130mA.
My solution is to use higher power cells and create three or more parallel banks instead of two. This should give me a good useable current around 240-300mA for the small shrubs and more for the larger ones.
2. Intermittent Charging: Because it’s generating power real-time and feeding it directly to the USB port, the charging capacity is only at its maximum when the Solar Shrub is in direct sunlight. If a cloud passes over, or the cells are shaded even temporarily, the device stops charging.
My solution is to add lithium batteries and charging/boosting circuits. When the Solar Shrub is not being used to charge a device, it will change its internal battery. Then, the battery will act as a booster or supplement whenever the cells are not in full sunlight. This will even allow consumers to carry the Solar Shrub indoors (after some time out in the Sun) for charging devices in the comfort of their homes.
3. Difficult to Assemble: My strategy for the original Solar Shrub was to build “stems” with shrink-wrapped, small-diameter, solid-conductor wires attached to “leaves” (round solar cells). These stems were then pushed though holes in a base, bent to 90 degrees, and tie wrapped into place. These turned out to be somewhat difficult to assemble and would certainly create problems for packaging and shipping.
My solution is to build “pluggable” stems using audio-type phone plugs and jacks. A round base will contain holes for the jacks in a circular pattern. The stems will have a solar cell on one end and a plug on the other. So stems could be plugged and unplugged as needed for quick and easy assembly.
4. Vulnerable to Weather: One of the first questions I got about the Solar Shrub on Instructables was, “Is it waterproof?” My answer was no, but it got me thinking. If your Solar Shrub is outside charging and it starts to rain, what are you to do? So the new design will definitely have to be waterproof.
My solution is to build the Solar Shrub into a self-contained plastic “capsule” with all the electronics inside, stem jacks on the top, and a USB cable with female connector coming out of the enclosure. This design will also be much more portable. Consumers will be able to move the Solar Shrub to any flower pot they wish.
So, as you can see, I have a lot of work ahead of me. I’ve actually already started building a new prototype with the specs listed here. I’ll post an update with pictures as soon as I’ve made some progress.