Studies on Solar Cell AC Parameters (Instrumentation, Measurements and Applications)

Abstract

Photovoltaic (PV) conversion of solar energy appears to be one of the most promising ways of meeting the increasing energy demand. In space, photovoltaic power source is the only safe alternative. Conventional silicon solar cell technologies have seen several improvements and off late GaAs/Ge and multijunction solar cells are developed to improve conversion efficiency. Demand for higher power, smaller size, lesser weight and higher efficiency has necessitated the use of high frequency switching power conditioners, which requires a better understanding of the AC characteristics of the solar cell, especially its capacitance. Solar cell is large p-n junction diode, whose AC parameters (capacitance and resistance) varies nonlinearly with its operating voltage, temperature and depend on the method (frequency or time domain) of measurement.Hence, studies on AC parameters of solar cells is taken up involving development of instrumentation, measurements on various types of solar cells and applications of AC parameters on switching shunt regulators.

In the present research work a measurement set-up to measure the solar cell AC parameters using impedance spectroscopy technique is established first with the commercial instruments. Here a small AC voltage (<VT) is applied about the operating voltage (DC bias) and its complex impedance is measured from the resultant current over a wide range of frequencies. Cell capacitance, parallel resistance, series resistance and inductance are estimated from the impedance spectrum, which is plot of the cell impedance in a complex plane. The principle of measurement, details of measurement set-up with calibration, testing and limitations observed when applied to solar cells, are presented. To over come the limitations in the measurement set-up, a dedicated userfriendly instrument called Solar Cell Impedance Analyser is developed to measure solar cell AC parameters. It is a personal computer based virtual instrument, which has a power amplifier, a high-speed data acquisition card and an arbitrary function generator card with a custom built micro controller based hardware with an application specific software developed using graphical programming language. A novel concept of software range extender is introduced, which virtually increases the dynamic range of the power amplifier.