Solar Cell Capacitance Determination Based on an RLC Resonant Circuit

Abstract

The capacitance is one of the key dynamic parameters of solar cells, which can provide essential information regarding the quality and health state of the cell. However, the measurement of this parameter is not a trivial task, as it typically requires high accuracy instruments using, e.g., electrical impedance spectroscopy (IS). This paper introduces a simple and effective method to determine the electric capacitance of the solar cells. An RLC (Resistor Inductance Capacitor) circuit is formed by using an inductor as a load for the solar cell. The capacitance of the solar cell is found by measuring the frequency of the damped oscillation that occurs at the moment of connecting the inductor to the solar cell. The study is performed through simulation based on National Instruments (NI) Multisim application as SPICE simulation software and through experimental capacitance measurements of a monocrystalline silicon commercial solar cell and a photovoltaic panel using the proposed method. The results were validated using impedance spectroscopy. The differences between the capacitance values obtained by the two methods are of 1% for the solar cells and of 9.6% for the PV panel. The irradiance level effect upon the solar cell capacitance was studied obtaining an increase in the capacitance in function of the irradiance. By connecting different inductors to the solar cell, the frequency effect upon the solar cell capacitance was studied noticing a very small decrease in the capacitance with the frequency. Additionally, the temperature effect over the solar cell capacitance was studied achieving an increase in capacitance with temperature.Energies 2018, 11, 672 2 of 13 be determined by using the fitting method (like the series and parallel resistance and the capacitance in the case of solar cells). The IS method has the advantage of offering the information about three important dynamic parameters and can be implemented with dedicated instruments or LCR meters. The disadvantage of this method is the need for an external signal injection into the DUT which can increase the price of the used instruments, especially if the tests are conducted under light conditions. These conditions involve DC power electronics in the instruments in order to bias the PV.Other methods are based on the time response of the PV modules. These methods are based on the transient effect of the Resistor Capacitor (RC) circuit (discharging effect of the PV capacitance [19,20] or time constant of the PV circuit [21]). The reported time response-based methods require external signal injection into the DUT and are used only in dark conditions.The dynamic parameters can be used as tools to characterize and diagnose the quality and the degradation status of the PV modules. One of the key dynamic parameters is the electrical capacitance of the PV modules. Oprea et al. [22] found that the PV panel capacitance and parallel resistance are affected by the PV panel degradation state when the potential-induced degradation was studied. Bhat et al. [23] found that t...

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“…Details of the derivation can be found in [49] or [61]. The parameter values of the PV cell/module model are not constant, but depend on irradiation, temperature, and cell/module voltage bias [40,43,54,59]. Generally, increasing voltage bias increases parallel capacitance and decreases parallel resistance [40,42,54].…”

“…Regarding parallel capacitance, transition capacitance (C T,PVc ) is voltage-dependent whereas diffusion capacitance (C D,PVc ) is voltage-and frequency-dependent [43,45,46,50,59]. Therefore, the mathematical model of parallel capacitance (C p,PVc ) at various frequency f and voltage bias of the PV cell V PVc is identified by curve fitting using [49]…”

“…Focusing on low irradiation conditions (about 100 W/m 2 ), reference [42] describes that the parallel capacitance at MPP and OC conditions could achieve a 294% and 340% value, respectively, compared to that of the SC condition. The resulting parallel capacitance is also irradiance-dependent [42,46,51,59,60]. Thus, for different voltage biases, this capacitance could decrease in the range 15% to 70% when irradiance decreases from high to low irradiation [42,60].…”

“…The most widely used method to study the PV dynamic behavior is the impedance spectroscopy [40,43,[54][55][56][57][58]. Alternative methods are based on time domain techniques [59,60]. Based on these or other methods, the AC parameters of a PV cell/module, such as capacitance and parallel and series resistance, can be accurately characterized.…”

Photovoltaic System Design and Performance

“…Degradation of solar cells as a result of various causes is known to affect the capacitance of solar cells. Cotfas et al [13] demonstrated a simple technique to measure solar cell capacitance using an inductor, thus forming a resistor/inductor/capacitor (RLC) circuit. When connecting the inductor to the cell, oscillations in voltage and current appear, which are damped after some time.…”