19 05 2014

Another question – ‘I was wondering if you could possibly do an example of an example of calculating Pmax for a solar cell please‘.

This one needs a bit more explanation than a one liner – so here goes.  Starting with the circuit schematic for the solar cell with a load resistor, which can be found in figure 11.3 Hence, the current through the load resistor is the photo-current (IL) and a reverse direction p-n junction, which is described by Shockleys equation (see the second year notes!).  This gives equation 11.1. From this, it is possible to calculate the short circuit current, by setting the voltage term in Shockley’s equation to zero, giving equation 11.3 – but realise that a lot of the terms at the front of the equation are in fact Is – the saturation current! You can also balance the two currents (IL and the p-n junction) to get Voc (i.e. the voltage when no current flows) and so this gives equation 11.2 Having got all this, the last step is to look at the maximum power.  Because you get zero power at Isc (because V=0) and at Voc (because I=0) there is a voltage and current at which the maximum occurs and this is given by Voc x Isc x FF.  Here the fill factor (FF) is an experimentally extracted value (quoted to you if required)