Automated system for determining parameters of a solar cell model

Abstract

An automated system is developed, which allows measuring the light and dark current-voltage characteristics (CVC) of a solar cell (SC). It consists of a STM32 based control unit, a digital-to-analog converter, a voltage-current transformation circuit, a current and voltage sensor and a laptop with a LabVIEW graphical programming environment. The digital voltage value from STM32 is transmitted to the AD5624R DAC via the SPI interface. AD5624R is a 12 bit DAC with built-in reference voltage 2,5 V. In this configuration, the accuracy of the voltage control is 1,2 mV and the accuracy of the current setting is 40 μA. INA226 sensor was used to measure the voltage on the sample and the current flowing through it. The current is measured as the voltage drop across the shunt resistor, which is connected to the corresponding sensor inputs, using the built-in 16-bit ADC. A shunt resistor with a resistance of 0.1 Ohm is used, which allows measuring the current up to 800 mA with a resolution of 25 μA. The readings of the sensor were calibrated using the precision voltmeter B7-46. Measurement and data processing is carried out with a virtual device created in the LabVIEW environment. As a microcontroller, the SM32F401RE Nucleo development board was used. This unit provides control of the DAC, the current sensor INA226, connection of the circuit for measuring light or dark CVC, as well as recording and storing the data received from sensors in the computer. By measuring the light and dark current-voltage characteristics it is possible to obtain an approximate solar cell model, the parameters of which depend on the illumination and temperature. Solar cell light CVC is used to determine the photocurrent. By its value at a certain temperature and luminosity, it’s possible to define the photocurrent at different temperatures and luminosity levels. The saturation current and the parameter α of the solar cell are calculated using the dark CVC. With the obtained data, a model of a solar cell is defined, which can be further used to determine the maximum power point at different levels of illumination and temperature. With information about the maximum power point, it is possible to calculate the optimal resistance value for the solar cell and to provide the corresponding control signal to the voltage converter. An approach for calculating the parameters of the SC model from its CVC is described and used to determine the MPP (maximum power point) of the solar cell module. The difference with the experimental data was 6.6%. In further research, the study of methods for calculating the parameters of the SC, which could provide a more accurate model, will be continued.