Abstract. But the influence of the temperature

Abstract. This paper proposes an improved generalized
physical model for PV cells, panels and arrays modelling taking into the
account the behaviour in the Direct and the reverse mode biasing. The Enhanced
model, named IDRM model, shows actually how the shot circuit current behaves
according to the variation of both temperature and irradiation. Due to its
easier implementation. This proposed model can be is useful power electronic
systems. Existing PV physical models were no problems for simulating influence
of irradiation changes on the short circuit current but for temperature changes
influences could not be visualized. The developed model was tested on a
marketed PV panel and it gives a satisfactory results compared with those shown
on the manufacturer datasheet.

Keywords.
Photovoltaic
cells, Photovoltaic panels, PV cell/panel simulations, Direct and reverse mode
modeling, IDRM Model, I-V characteristics, Solar Irradiation.

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INTRODUCTION

The efficiency of photovoltaic panels depends on the health
of the basic element that is the PV cell. PV cell can operates in the first
quadrant of the I-V characteristics as generator or in the second one as
receiver. In this last operating mode the PV cell imposes its functioning
against the others PV cells in the same string. Shadow as example conducts the
PV cell when shaded acts as load, consumes power delivered from other cells and
its temperature rises allowing appearance of hot spot heating 1-6. Therefore,
the authors in 7 have proposed a DRM model which is able to produce I-V
characteristics in both direct and reverse operating mode according to the experiments
made on the reverse mode biasing of 60 polycrystalline silicon cells.
References 8-9 were proposed a model based on a transistor model.  

These proposed models, except the DRM model; do not take
into account the reverse operating mode. DRM model reproduce practically  the I-V behaviour of PV cells, PV panel and PV
array according to the irradiation and the temperature changes. But the
influence of the temperature variation is not visible on its I-V
characteristics. In this proposed physical model, we present an improved DRM
model for PV cells/panel/array modelling named IDRM.

The basic idea for thinking about a new proposal named DRM
model for PV modelling is coming from testing what about 60 polycrystalline PV
cells in the reverse mode biasing. Experimental results of current vs voltage
of these 60 PV cells reverse biased were shown in Fig .1.

FigURE 1. Experimental I-V curves of 60 polycrystalline
PV cells in the reverse mode biasing.

 

From this figure, it important is to note the presence of a
huge discrepancy in the behaviour of the PV cells. We note also the appearance
of two avalanche voltages on the reverse mode around -5V and -14V. These
results conduct the authors in 7 to propose the initial DRM model illustrated
in Fig .2 and which is implemented under Orcad/Pspice as shown in Fig .3.

FigURE 2. The DRM model proposed in 7.

 

Fig URE 3. The DRM model of 7 implemented under
orcad/pspice.

 

Improved DRM model for PV cells
modelling

Figure 4 show the layout of the improved DRM model for PV
cells modelling proposed in work. This model take into account simultaneously
of the variation of the photocurrent the short circuit current. The principle contribution
in this model is the adding of the diode (D384) to the previous model presented
in Fig .3. This added diode is able to reproduce perfectly the influence of
temperature variation on the photonic current on the I-V characteristics.

FigURE 4. Layout of the improved physical DRM for PV
cell modelling.

 

To test its effectiveness, this improved model has been  applied to a PV cell modelling, with the
following step variation of temperature: -10, 0, 25, 50, 70, 85 and 100 °C. Fig
.5 shows the obtained simulation.

 

Figure 5. I-V characteristics of a PV
cell under influence of temperature changes by applying the improved DRM model.

 

From Fig .5, one can remark that the increasing of
temperature causes a decrease simultaneously on the open circuit voltage and
the maximum power of the PV cell and causes also  an increasing of the short circuit current,
which represents the photonic current image. Due to the temperature increasing,
it is clear that the short circuit current changes from 1.5 to 1.6 amps for the
entire region that the PV cells behaves as a DC power current supply.

Application of the improved DRM
model on the shaded marketed PV panel

The BP solar module BP350J used in simulation is a 50W PV
panel composed by two parallel of 19 polycrystalline silicon PV cells strings
connected in series. The open voltage value 
is 21.8v under STD operating conditions and produce a short current of
3A. The MPP is about (2.9A, 17.5v) and can produce 50 Watts. Only one terminal
by-pass diode mounted for its protection against hot spots. The I-V
characteristics, under temperature variations, as provided by manufacturer are
shown in Fig .6.

Figure 6. Influence of temperature variations on the
I-V characteristics of PV BP solar BP350J from manufacturer datasheet.

 

In order to show the influence of both shadow and
temperature changes, we have partially shaded the entire surface of one PV cell
in the panel and varied the temperature with the same manner as in Fig .5. The
model of the simulated PV panel implemented under Orcad/Pspice software was
built by associating the PV cell model according to its configuration presented
in its datasheet.

Simulated results of I-V and P-V curves due to the
temperature variations were presented in Figs .(7-8) respectively.

These figures show that the temperature diminution causes an
increase in the peak power and the shadow effect is visualized by the presence
of knees in the I-V curves and a local maxima in the P-V curves. The influence of
the temperature changes on the short circuit current and on the open voltage
circuit is also observed in Figs .(7-8) with a good reproduction of the values
indicated in Fig .6. In the simulated model, the temperature influence is
estimated by a value equal to: 0.04545 mA/°C. So,  the photonic current is affected by a value
equal to: 0.045%/°C which is nearby of the value mentioned by the manufacturer
datasheet. This result proves that the new proposed IDRM model is correct.

 

Figure 7.  Influence of the
temperature variations on the I-V characteristics of the shadowed PV BP solar
BP350J simulated with IDRM and accompanied with a zoom around the short circuit
current.

 

FIGURE 8. Displacement
of the MPP on the p-v characteristics of the BP solar BP350J panel under
partial shadow and temperature changes.

 

Conclusion

In this work, a new general improved physical model for PV
cells, panels and arrays operating in both direct and reverse biasing modes is
proposed. The new model, named IDRM, is based on the use of a conventional one
or two diodes model for PV modelling and experiments performed on
polycrystalline PV cells operate in the reverse biased mode. The IDRM model is
able to reproduce the influence of environmental variables (temperature and
irradiation) on the I-V and P-V characteristics in both the first and the second
quadrant of the operating modes. Really, the photonic current and as a result the
short circuit current are affected by the temperature variations. Noting that this
fact is taken in consideration in the proposed model. To prove the validity of
the proposed model, the degree of the temperature influence on the photonic
current is compared to the I-V curves of a marketed panel provided by a manufacturer.
Noting that, the shadow defect is considered in the IDRM model and applied to
the simulated marketed panel.