For this module measurement method we use the variable resistance to graph the power IV curve. By changing the module resistance load and measuring voltage and current, we can generate the panel’s IV curve. The power IV curve can determine the maximum power point (Mpp) of the module, which can be compared with the manufacturer’s specifications. Also this method can help identify the shunt and series resistance in a module.
Module Measurement with Load
Ideally, we should operate the module at the maximum power point. Then, module voltage is VMP and the module current is IMP. The max power output of the module is known, but we also need the load resistance, Rload, so we can use ohms law:
Rload = VMP / IMP
The resistors used should be near the Vmp and Imp. If the Vmp and Imp are unknown, Voc and Isc is a good approximation. Normally, the back of the module will have a label listing VMP and the IMP. VOC and ISC can be measured using a multimeter.
Modules produce hundreds of watts, so high wattage power resistors are required. Yet, the measurements are only for short periods so we won’t need power resistors of the full power rating. Water cooling the resistors helps as well.
- Two Multimeters – One for voltage and the other for current – with Two or more alligator clip cords
- Water for cooling the resistors
- Protective gloves – as modules and resistors get hot
- Solar Module
- Power Resistors
- Place the module in a position where it can have full sunlight and no shading.
- Set one multimeter to the voltage scale and one to the current scale.
- Measure VOCand ISC
- Now add the resistors using the schematic below and record the current and voltage for each resistor.
- Change the values of the resistor to cover the IV curve.
- After measuring with all the resistors, measure VOCand ISC again
After measuring VOC and ISC, resistors can be swapped in and out to gather results. Connect the black lead from the multimeter reading current to one end of a resistor. Using an extra black alligator cord, connect one clip to the positive end of the panel. There will be one leftover free clip which will will be used to connect to the resistor, and thus completing the circuit.
Results and Discussion
Measure the cell voltage and current for each resistor. The power is P = I V. The resistance is also calculated from R = V/I but it is not necessary.
Graph the current against the voltage and the power against the voltage
The module PMAX is the maximum in the graph. The fill factor (FF) is calculated from the equation:
FF = (PMAX / VOC )*ISC
Safety precautions should be strictly followed when working with Solar modules as they can represent a Hazard due to the generation and presence of live electrical current which can cause shocks and serious damage if not handled adequately. Also use protective gloves and cool the resistors with water.