Scientific Bulletin of the Electrical Engineering Faculty – no. 2 / 2009 COMPUTER-AIDED DESIGN AND SIMULATION OF PHOTOVOLTAIC SYSTEMS Adela Gabriela HUSU 1 , Traian IVANOVICI 2 , Florin Mihail STAN 1 , Daniel NECULA 3 , Nicolae OLARIU 1 , Valentin DOGARU-ULIERU 2 , 1 „VALAHIA” University of Târgovişte, Electrical Engineering Faculty, Automatics, Informatics and Electrical Engineering Department 2 „VALAHIA” University of Târgovişte, Electrical Engineering Faculty, Electronics, Telecommunications and Energetic Engineering Department 3 „TERMOSERV” Doicesti, Romania E-mail: adelahusu@yahoo.com; traian_20@yahoo.com; flo.stan@gmail.com; daniel.necula@ymail.com; nolariu@valahia.ro; dogaruvalentin@gmail.com Abstract − In the preliminary stages of a project, PV integrators often have to answer highly detailed questions from customers about how to achieve the best possible system yield. In these cases, simulation programs can be helpful tools, allowing suppliers to better plan their customers‘ demands. This is especially true for stand-alone power systems. 1. INTRODUCTION Before a solar electric system is built, system planners and installers should simulate the system using a computer program. For a higher rate for electricity fed into the grid, the customer would like to know what yield is expected and how cost-effective the various systems are. With simulation programs, installation companies can also demonstrate the productivity of an solar electric system to their customers. Due to their appealing graphical interface, several programs are especially suitable for this purpose. Calculations of the projected energy yield, the economic feasibility and the reduced emissions demonstrate the advantages of the proposed system and offer good reasons for its purchase. The choice of the correct input values for each system concept is a decisive factor in the accuracy of the simulation results. In several programs, the input value undergoes an automatic plausibility test. This assists in avoiding gross mistakes in dimensioning the system. Several programs aid in the choosing the optimum system dimensions by use of optimisation functions and by automatically comparing the variations simulated. In addition to the simulation processes used in the program, the application, the program scope and range of applicability are decisive factors. Both the type of system and the system configuration to be simulated play an essential role. Here it is important to select the most suitable program for the given system and application. 2. MATHEMATICAL RELATIONS UNDERLYING THE CALCULATION PERFORMANCE OF PHOTOVOLTAIC CELLS A photon who falls in the p-n junction has the effect occurrence of electrons and holes. Electrons move to the n-type silicon layer and holes to the p-type silicon layer. Thus appears a current I ph through the junction, determined by photovoltaic conversion of incident radiation. This current moving through the junction from the n-type material to the p-type material, leads to a direct voltage on external load R connected to terminals. Voltage cause through the junction a direct current I D , opposite of photovoltaic current. The current through the junction will be given by the following relation: I = I ph – I D = I s – I 0 [exp(V/V T )-1] (1) where: I 0 is the intensity of saturation current at polarization reverse of the junction, and V T is thermal voltage equivalent with operating temperature of junction, defined by the relation V T = kT/e (2) where: k is Boltzmann's constant; T - absolute temperature; e - electron charge. At the equation (1) of current-voltage characteristic of a photocell it corresponding an equivalent scheme represented in Figure 1. Figure 1: Equivalent diagram of simplified photovoltaic cell 41