Instructions

Understanding the Data
So, why is all this data important? Remember, the numbers you calculate here will
be unique to your environment and lighting conditions.
The duty cycle of a system, refers to how long you are charging for, and how long
you are discharging for. So a duty cycle of 50% spends half of its time charging,
and half of its time discharging.
When designing an energy harvesting system, you have to store enough energy in
the charge cycle, in order to power your equipment for long enough in the discharge
cycle. It is also important to balance the time it takes to charge, against the time it
takes to discharge. If it takes 4 hours to charge a system and you can then only use
the fan for 2 seconds, that is not a very well designed system.
Think back to the garden light project, as long as there are enough sunlight hours in
the day to store enough energy and the bulb is low power enough, the garden light
will light the garden path for the whole night time.
When designing a system, the designer can choose bigger solar panels, a bigger
energy store, or a more energy efficient piece of attached equipment that uses the
stored energy slower. They can also analyse the timing of the system so that there
is always enough time to charge the energy store sufficiently, to power the attached
equipment for long enough to be useful, before it can be recharged.
Understanding Duty Cycle
Duty Cycle is a term that describes what percentage of time is spent charging and
discharging, and it is important to design a system with the correct duty cycle, so
that it always works. If you don't store enough charge in your Solar Store, then your
Bulb or fan won't run for long enough. Below is a graph that shows the duty cycle of
two different systems:
In the left hand graph the duty cycle is 50%, which means it takes as much time to
charge as it does to discharge. In the right hand graph, the duty cycle is 25%, which
means it spends 3 quarters of its time charging, and a quarter of its time
discharging. Any energy harvesting system can be fine tuned into a well balanced
system by changing the rate of harvestable energy, the size of the store, how
energy efficient the attached device is, how long it is expected to be charging and
how long it is expected to be discharging.
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