JAPMED06 Bucharest, 27-29 July 2009 Page 1 / 4 EFFECTS OF PARTIAL SHADING ON THE PV MODULE CHARACTERISTIC CURVES Evagelia PARASKEVADAKI Stavros PAPATHANASSIOU Georgios A. VOKAS NTUA, Electric Power Division - Greece TEI Piraeus, Dep. of Electronics Engineering, evaparask@yahoo.gr gvokas@teipir.gr Abstract Photovoltaic system performance is influenced by a variety of factors such as irradiance, temperature, shading, degradation, mismatch losses, soiling, etc. Especially shading, complete or partial, can have a significant contribution to the reduction of power output, depending mainly on the PV array configuration, the shading pattern and the existence of bypass diodes incorporated in the PV module design. In order to obtain the maximum power from a PV generator, it is of great importance to evaluate the complex effects of shading on the P-V and I-V curves. Key- words: Photovoltaic cell, Photovoltaic module, Partial shading, Maximum power point. 1. Introduction Photovoltaic systems are one of the leading technologies in the field of alternative energy sources. They are currently employed in various applications, from large PV plants to small residential systems, interconnected to the utility grid. PV energy is also utilized in stand - alone systems, combined or not with other energy sources. These systems can be a reliable source of energy, as one comes to understand the nature of their performance. One of the main considerations when it comes to PV systems is the ability to track the maximum power available, for the given irradiance and temperature conditions. Multiple techniques performing the MPP tracking have been developed over the last 20 years, reflecting the fact that the efficiency of a solar system depends greatly on the efficiency of the MPPT. Under partial shading conditions, the irradiance differs between PV cells in the same string, resulting in a mismatch of PV module characteristics. The P-V curves become complex, with multiple peaks, thus confusing the MPPT process. The respective power loss, when a local instead of the global maximum is tracked, can be significant, leading to the deterioration of the overall system performance. In order to improve the performance of a PV system under partial shading, conclusions must be drawn on the impact of shading on the PV array basic curves. In this paper, the effect of various partial shading patterns on different PV modules and array configurations is examined. The evaluation is performed for a multitude of shading levels. 2. Modelling of the photovoltaic cell The behaviour of each PV cell is modelled based on the two diode equivalent circuit (Fig.1). Circuit based methods allow better understanding of the cell behaviour as part of an electrical circuit, especially in the case of shading effects. The ideal solar cell can be modelled by means of a current source anti-parallel to a diode. The dc current produced by the source is a linear function of irradiance at the surface of the cell. In order to refine the cell model, shunt and series resistances are added. The second diode contributes to the accuracy of the I-V curve fitting, accounting for the difference in the current flow due to charge recombination in the semiconductor depletion region at low current values. The electrical PV model shown in Fig. 1 has been implemented in Pspice. Fig. 1. Equivalent electrical circuit of a PV cell based on the two – diode model Equation (1) describes the two diodes model. The first diode (D 1 ) is assumed to be ideal, setting the ideality factor n 1 to 1. The ideality factor n 2 of the second diode is set to 2. The photocurrent I G depends on irradiance as well as temperature, as described in equation (2). sh s Vt IRs V s nVt IRs V s G L R IR V e I e I I           ) 1 ( ) 1 ( 2 2 1 (1) ) ( 2 1 T c c E I G   (2) Two types of PV cells have been modelled. Both cell types are multi - crystalline silicon based, having different characteristics, thus resulting in the two – diode model parameters shown in Table 1[1]. Table 1. PV cell model parameters for two cell types. The I-V and P-V characteristics of the two cell types are presented in Fig. 2, for constant temperature of 25 o C and different insolation levels. The electrical characteristics of each PV cell type are shown in Table 2. PV cell c1 c2 Is1 Is2 n1 n2 Rs Rsh (1) (2) pA μΑ - - mΩ Ω Type A 1.8 2.00 797 9.0 1 2 15.0 7.5 Type B 2.2 2.29 299 6.4 1 2 13.6 35 1. mV - 1m 2 2. μV -1 K -1 m 2