Investigating the Influence of Illumination and Temperature on the Impedance Characteristics of Industrial c-Si Solar Cells
S.M.S. Naoom (TU Delft - Electrical Engineering, Mathematics and Computer Science)
P. Manganiello – Mentor (TU Delft - Electrical Engineering, Mathematics and Computer Science)
D.A. van Nijen – Graduation committee member (TU Delft - Electrical Engineering, Mathematics and Computer Science)
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Abstract
Partial shading poses a significant challenge in the photovoltaic (PV) technology sector, potentially leading to a considerable drop in energy production. To address this issue, sub-module Maximum Power Point Tracking (MPPT) devices have been developed and employed. In such devices, capacitors are used at the input to mitigate input voltage ripples. In order to reduce the number of circuit components and potentially increase the reliability of these devices, there is a proposal to substitute the input capacitor of the sub-module MPPT device with the capacitance of the PV cell or PV cell string. However, PV cell capacitances may not be suitable for this purpose due to their sensitivity to temperature and illumination levels. This sensitivity can result in a mismatch between the PV cell and the sub-module MPPT. In addition, PV cell capacitance may also be relevant for applications such as visible light communication and the determination of the minimum required I–V measurement scan time. Therefore, this thesis aims to investigate the influence of temperature and illumination levels on PV cell impedance.
To assess the impedance of the PV cell, impedance spectroscopy (IS) was employed. Experiments were conducted under varying temperature and illumination conditions on two PV cell types: Interdigitated Back Contact (IBC) and Passivated Emitter and Rear Contact (PERC) cells. The Complex Nonlinear Least Squares (CNLS) algorithm, together with an electrical equivalent model for PV cell impedance, was used to fit the impedance data.
In the temperature experiments, both cell types showed a strong increase in capacitance with increasing temperature. For the IBC cell, the capacitance increased from 7.6 mF at 25°C to 22 mF at 60°C under dark conditions at the maximum power point voltage. For the PERC cell, capacitance increased from 0.19 mF to 2.4 mF between 25°C and 55°C. This behaviour is related to changes in intrinsic carrier concentration.
In the illumination experiments, capacitance also increased with higher light intensity. For the IBC cell, capacitance increased from 9.2 mF in dark conditions to 13.8 mF at 500 W/m². For the PERC cell, it increased from 0.3 mF to 0.7 mF at 700 W/m². This is related to changes in the PN-junction voltage.
A combined temperature and illumination study showed that illumination has a stronger impact on capacitance than temperature, as changes in intensity significantly affect the maximum power point voltage. Under 30–60°C and 100–500 W/m², the IBC capacitance varied between 3 and 9.5 mF, while the PERC cell varied between 0.045 and 0.25 mF.
Overall, the results show that PV cell capacitance is highly dependent on both temperature and illumination. The IBC cell exhibits higher capacitance than the PERC cell and therefore shows greater potential for replacing the input capacitor in sub-module MPPT applications. However, PERC cell strings may still achieve sufficiently high capacitance under certain operating conditions.