June 26, 2011
June 26, 2011
June 29, 2011
Energy Conversion and Conservation
22.59.1 - 22.59.13
A Low-cost Laboratory Experiment to Generate the I-V Characteristic Curves of a Solar CellSolar cells can be characterized by their current-voltage (I-V) characteristic curves. This curveshows how the voltage of the solar cell varies with the current drawn from it. The I-V curve canshow how the solar cell will operate under varying parameters such as light intensity andtemperature. It can also be used to identify the maximum power point of the solar cells whichspecifies the current that should be drawn from the cell in order to achieve its maximumefficiency.The I-V curve can be generated using a voltmeter, ammeter, and a resistive load. However, for alarge number of points or parameters, this method is tedious and time consuming. Instrumentsexist that can generate these curves automatically, however, they are too expensive for thetypical laboratory experiment. As an alternative, a low-cost system to generate the I-V curvewas developed. This was funded in part by an NSF grant funding undergraduate research. Thelow-cost system consists of three parts: a laptop, a data acquisition (DAQ) device and a currentsink circuit. The system was also designed to be portable and to be powered by the laptop’sUSB port. This was done so that the system could be carried outside to use the sun to power thesolar cell as light sources accurately simulating the spectrum of the sun are expensive.The DAQ used was the National Instruments USB-6008. This DAQ connected via a USB portand was programmed with LabVIEW. The DAQ output an analog voltage to the current sinkthat controlled the amount of current drawn from the solar cell. The DAQ acquired two inputvoltages: one which measured the voltage across the solar cell and another which, through theuse of a current-sensing resistor, measured the actual current drawn from the solar cell. TheLabVIEW program then graphed the I-V and power curves and saved the data. The LabVIEWprogramming was challenging as the DAQ had to output a voltage, wait a specified time, obtaintwo voltage measurements, wait, and repeat. This was solved by the use of NI-DAQmx Baseblocks.The current sink circuit used was a simple current regulator. It consists of a single-supplyoperational amplifier which controlled the base current of a transistor based on the differencebetween the output voltage from the DAQ and the voltage across the current-sensing resistor.The collector of the transistor was connected to the solar cell, thus controlling the current drawnfrom it.The USB-6008 is low-cost and when packaged with LabVIEW cost $169. This cost can beoffset by its use in other laboratory experiments as it can be programmed for variousapplications. Alternatively, if a different DAQ capable of running LabVIEW is already availablein the laboratory, it may be used instead. The parts for the current sink circuit will cost about $5-$20, depending on the source and whether a solderless breadboard or printed-circuit board isused. These parts and appropriate substitutes are common and may be already be present in thelaboratory.
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