Identification of PV solar cells and modules parameters using the genetic algorithms: Application to maximum power extraction
Introduction
The algorithms that we use to determine parameters of PV generators (solar cells, modules and arrays) must be efficient and sufficiently accurate for process optimization and photovoltaic systems design tasks. These algorithms are of two types: those that use selected parts of the current–voltage (I–V) characteristic (Charles et al., 1981, Charles et al., 1985, Laplaze and Youm, 1985, Chan and Phang, 1987) and those that exploit the whole characteristic (Easwarakhanthan et al., 1986; Phang and Chan, 1986; Ikegami et al., 2001, Jervase et al., 2001). The first group of algorithms involves the solution of five equations, derived from considering selected points of the I–V characteristic, i.e. the open-circuit and short-circuit points, the maximum power points and the slopes at strategic portions of the characteristic for different level of illumination and temperature. Although, the exact solution of these equations requires iterative techniques, this method is often much faster and simpler in comparison to curve fitting. The disadvantage of this approach is that only selected parts of the I–V characteristic are used to determine the parameters. The second group of algorithms is based on curve fitting and offers the advantage of taking all the experimental data in consideration. Conversely, it has also the disadvantage of artificial solutions. In fact, the fitting techniques with several parameters are, generally, based on non-convex optimization criterion, and using traditional deterministic optimization algorithms leads to several sets of local minima solutions, and none one of them can describe the physical reality.
In this paper, we present a non-linear least-squares optimization algorithm for the identification of the five electrical solar cell and module parameters from experimental data. This fitting is based on the Genetic Algorithms (GAs) strategy. These algorithms are recently applied in several domains, such as in optimization of large solar hot water systems (Loomans and Visser, 2002), in design and control strategies of PV–Diesel systems (Dufo-Lopez and Bernal-Agustin, 2005), in sizing optimization of hybrid solar–wind system (Yang et al., 2008), etc. For the estimation of the electrical parameters of PV generators, we found that GAs increase the probability of obtaining the best minimum value of the cost function in very reasonable time, and more accurate solution in comparison to the approaches reported in the literature (Ikegami et al., 2001). The identified parameters are then used to extract the working maximum power point (MPP).
Section snippets
The one diode model
The theoretical expression of the current crossing a photovoltaic cell versus the applied voltage results from the Schottky diffusion model in a PN junction, and is given by (Charles et al., 1985):where Iph and Is are the photocurrent and the saturation current, respectively. Rs is the series resistance, Rsh is the shunt resistance, n is the ideality factor and Vth is the thermal voltage.
The electrical parameters Rs, Gsh = 1/Rsh, Iph, n and Is are computed from the I
The genetic algorithms
To numerically treat the I–V curves, we performed a fitting procedure based on the genetic algorithms (GAs). The error criterion which used in the non-linear fitting procedure is based on the sum of the squared difference between the theoretical and experimental current values. Consequently, the cost function to be minimized is given by (Easwarakhanthan et al., 1986, Phang and Chan, 1986):where is the measured current at the Vi bias, θ = (Iph, Is, Rs, Gsh, n) is the set
Identification of the electrical parameters
We use a homemade GAs program developed on Matlab environment for both PV cell, module and array. For flexibility, we choose to develop this program instead of using Genetic Algorithms and Direct Search Toolbox of Matlab.
Maximum power point extraction
In order to extract the maximum available power from PV cell, it is necessary to operate it (the cell) at its maximum power point (MPP). Several MPP methods, such as perturbation, fuzzy control, power–voltage differentiation and on-line method have been reported (Bahgat et al., 2004, Yu et al., 2004, Enrique et al., 2007). These control methods have drawbacks in stability and response time in the case when solar illumination changes abruptly. A direct MPP method using PV model parameters was
Conclusion
In this work, we applied the genetic algorithms to characterize PV solar cells and modules, particularly for the determination of electrical parameters namely such as the photocurrent, the saturation current, the series resistance, the shunt resistance and the ideality factor. Determination of these parameters starting from the experimental data is formulated in the form of a non convex optimization problem. The resolution of this problem by conventional techniques of non-linear programming,
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