Having established that the sun’s energy can be usefully harvested by a small single solar cell and converted to a useful voltage the next problem is how to store the energy. A similar, commonly available product, the solar powered garden light uses NiCad rechargeable batteries. I have three objections to using these; they are not environmentally friendly from the point of disposal, they have a lifetime limited by a fairly low number of charge/discharge cycles and they are too expensive for a key stage 3 project. Perhaps at last, this is a suitable application for the super capacitor. I know that the super capacitor has appeared in a number of situations in schools – mostly for powering motors – but we have yet to see it used on a wide scale as a component in student’s projects. Fig 3 was used to explore its suitability as a storage device for the nightlight.
Capacitor C1 is the super capacitor that stores the pulses of energy generated by the ‘Joule Thief’. Zener diode D3 is included to prevent C1 from over charging to the point where the voltage at its terminals exceeds its maximum rated working voltage of 5.5V. D2 prevents the charge stored in C1 from discharging via TR1 during the off-portion of the joule thief switching cycle. A shottky barrier diode was used because it has a slightly lower forward voltage drop (typically 0.4V or less) than the more commonly used 1N4148 (Vf 0.6V). This allows C1 to reach a slightly higher fully charged voltage before D3 shunts surplus energy (see section on energy stored). C1 takes around 1 hour to fully charge on an overcast summer’s day when it reaches a terminal voltage of 5.29V.
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