One problem with the modern classroom is how to power computers on the desktop, but allow for flexible arrangement of the room. Replacing desktop computers with laptops and tablets removes the restriction of a large box on the desk. Use of WiFi removes the need for a data cable. Laptops can run from batteries for a few hours and tablet computers for longer, but power may still be needed. Could power be provided wireless to the desktop?
Some calculators use a small photovoltaic cell ("solar cell" for supplying power. So could a larger cell power a laptop or tablet computer?
The Australian standard for lighting in a classroom is 240 Lux (AS 1680.2.3, as quoted in Guide to school lighting: Resources for school energy managers, 2008).
One Lux is equal to 0.001496 watts/square meter of light energy. So if all this energy was harvested with a photoelectric cell ("solar cell") it would produce 0.35904 Watts per square meter. The photovoltaic cells are about 15% efficient in converting sunlight to electricity, so they would produce about 0.054 Watts per square meter in a classroom.
An Apple iPad uses about 2.5 Watts. So it would need about 70 square meters of solar cells for running continuously. This makes the idea infeasible.
To check the figures, I tried a Jaycar Amorphous 10 Watt 12 Volt Solar Panel (CAT. NO. ZM9030). The panel has two cells, in total 0.2976 square metres (930mm x 320mm). In full sunlight at the rated output, this should give 33.6 Watts per square meter. In a classroom it should provide about 0.011 Watts per square meter.
I placed the Jaycar panel 2m below a ceiling with two 36 Watt florescent tubes fitted with diffuser and reflector. This produced 1 mAmp at 8 volts, or about 0.008 Watt, or 0.027 Watt per square meter. This is less than expected, but a realistic figure.
Assuming each student has a free desk area of 0.9 x 0.6 meters, that is an ear of 0.54 square meters, which could hold a solar cell generating 0.012 Watt. This is not enough to operate a tablet compuer with a conventional back-lit LCD display, or even the OLPC with its display operating with the back-light off (420 mW).
The Amazon Kindle Touch eReader with its low power eInk screen lasts 1,440 hours on a 5.25Wh battery, indicating a power use of about 0.004 Watt. This could be recharged from a solar desk. But with such a long operation time from one battery charge there would be little need for desktop power.