Parameter Extraction Of Solar Cell Equivalent Circuit Model

As a clean and promising technology for alternatives to fossil fuels,PV system utilises solar cells to convert sunlight directly into electricity and has been identified as the key areas of research given our society’s impending demands of electric power.As the heart of PV system,solar modules may be connected in series or parallel,while itself comprises of series and parallel solar cell combinations.Therefore,accurate physical modeling and parameter extraction that closely represent the nonlinear current vs.voltage(I–V)characteristics of solar cell are essential for not only evaluating the performance,fabrication process and quality control of a solar cell,but also the design calculation,performance analysis,and real time optimal control of PV system.Over the past decades,several models were developed to describe the behavior of solar cell and module,only two main equivalent circuit models are used practically: implicit single diode model(ISDM)and implicit double diode model(IDDM).The ISDM equation contains five unknown parameters,whereas the IDDM equation has seven unknown parameters,which are not always available in datasheets.Moreover,both the ISDM and IDDM equations are inherently implicit and transcendental in nature,this increases the complexity and difficulty of parameter extraction.This Ph D thesis focuses on the evolution and parameter extraction of ISDM and IDDM equations,and the main contributions are as follows:1.Based on reasonable simplification and several key points on the I-V characteristic curve,i.e.the short-circuit current point,open circuit voltage point,maximum power point,and slopes of I-V curve at the axis intersections,a new analytical method is developed in Section Two for extracting the five parameters of ISDM from experimental data.The main strength of the proposed analytical method are free of solving d P/d I=V+Id V/d I=0 and more accurate analytic expression for series and parallel resistances.To evaluate the performance of the proposed analytical method,other four outstanding analytical methods recorded in literature were compared to extract the parameters from experimental data of various soalr cells and modules.Comparison results indicate that the proposed analytical method has wider applicability,higher accuracy,and better stability than other four analytical methods.2.Theoretically,explicit single-diode model(ESDM)equation in terms of Lambert W function is exactly derived from ISDM equation,both the equations should be fully equivalent to each other.Actually due to the introduction of Lambert W function,ESDM equation is closely linked with but different from ISDM equation.To demonstrate the distinction between ESDM and ISDM equations,in Section Three,a lot of parameter sets extracted by analytical and numerical methods are back substituted into ESDM and ISDM equations to quantify their respective fitness to experimental data of various soalr cells and modules.Comparison results show that ESDM equation can present smaller individual absolute errors(IAEs)and root-mean-square error,and always has better fitness in representing the I–V characteristics of solar cell and module.Also,this superiority of ESDM equation is independent of cell technology and the condition of irradiation and temperature,which can be further used to extract more accurate maximum power points for solar array.3.To further evaluate the difference between ESDM and ISDM equations,a restart-based bound constrained Nelder-Mead simplex method(rbc NM)is proposed in Section Three to extract their optimal parameters for various soalr cells and modules.Results indicate that:(1)Compared to the state-of-the-art methods recorded in literature,the proposed rbc Nm method is computationally efficient and presents fast convergence and high accuracy.The accuracy of the optimal parameters extracted by rbc NM for ISDM equation outperforms those obtained by other methods,the optimal parameters extracted by rbc NM for ESDM equation have a much better accuracy than those reported in the literature,and this improvement is almost one order of magnitude.(2)Under the same conditions,the optimal parameters extracted from ESDM equation are much more accurate than those from ISDM equation,and the I–V characteristics reconstructed by the optimal parameters of ESDM equation are in better accordance with the experimental data of various soalr cells and modules.(3)The optimal parameters extracted from ISDM equation can be further optimized in ESDM equation by using any numerical methods,and the reader is encouraged to identify this viewpoint.4.Inspired by the advantage of ESDM equation,in Section Four,a Lambert W function based double diode model(LDDM)equation is derived exactly from the physics-based IDDM equation.Under the same parameter sets,the fitness of the proposed LDDM equation,IDDM equation,and the reported alternative explicit double diode model(AEDDM)equation to experimental data are compared.Comparison results indicate that the proposed LDDM equation exhibits smallest IAEs and best fitness to experimental data,and always provide the closest representation of the nonlinear I–V characteristics of various solar cells and modules.AEDDM equation is only a rough approximation of IDDM equation,and has the largest IAEs and worst fitness for most of solar cells and modules.5.To further evaluate the difference between the proposed LDDM equation and IDDM equation,the rbc NM method is revised in Section Four and used to compare their parameter extraction performance for various solar cells and modules.Results indicate that:(1)The optimal parameters extracted by rbc NM for IDDM equation has the highest accuracy when compared with the existing methods reported in the literature.(2)Despite more computational effort and time consuming,the optimal parameters extracted from LDDM equation are significantly more accurate and robust than those from IDDM equation.(3)In some circumstances,IDDM equation and the proposed LDDM equation might be degenerated into ISDM equation and ESDM equation respectively,and this degradation is more obvious for soalr modules.(4)In the process of iteration,despite the proposed LDDM equation can evolve into explicit double diode model equation,it essentially does not belong to the latter and still has some implicit features.