3 rd International Conference on Renewable Energy Sources and Energy Efficiency, 2008 1 Optimal Power Management for PV Systems Κonstadinos Nikolau and Dimitrios Bargiotas Technological Educational Institute of Chalkida, Dep. of Electrical Engineering, Greece Abstract : The electrical power that is produced from a PV system is marked by variability, because it depends on the unstable parameters of power density of the solar irradiation and the temperature of the solar cells. This fact, in combination with the unusual (non linear) behavior of PV systems, as power sources, make the management of the pro- duced power, complicated. Under these circums- tances, the use of a load input voltage and current control method is required, which can achieve quick and efficient load adjustment, in order to maintain the desired maximum efficiency of the PV system. In this paper, the conditions that ensure the Maximum Power Transfer from the PV source to the load are explained. Additionally, a new, im- proved control algorithm is introduced, which is based on the principles of an already existing MPPT methods family, but is expected to produce better results. Keywords and Terminology: Maximum Power Point Tracking - M.P.P.T, Pulse Width Modulation - PWM, Electromotive Force - EMF, Duty Cycle – D.C, p-n Junction, Photovoltaic - PV, Ripple, MOSFET, flyback diode, Lowpass Filter, Buck (step-down) DC/DC Converter, Perturbation & Observe - P&O «Hill Climbing» method I. INTRODUCTION Solar PV generators are the purest ecological alternative power sources the uses of which show, as the years go by, an always upward tendency. However, the purchase and installation cost of a PV system, in combination with their relatively low efficiency factor, make them yet non competitive, compared with the power sources that use conven- tional fuels. However, in specific occasions which the use of PV systems is absolutely necessary, is aspired to be extracted the maximum power, in or- der to reduce the ratio of the cost per KWh of pro- duced energy. [1] II. ELECTRICAL CHARACTERISTICS OF PV CELLS The electrical equivalent of a PV cell that is shown in fig 1, [2] consist of a current source, a p-n Junction that is flowed by the reverse thermal cur- rent (I R ), the serial resistor (R s ) of the diode materi- al that is flowed by the load current (I c ) and a shunt resistor (R p ) that is flowed by the surface reverse leakage current of the diode (I L ). fig 1 The characteristic equation that connects the output quantities (voltage V c and current I c ) of a PV cell, is described in Eq. 1 [2],[3]. Eq. 1   0 1 e c Vc Rs Ic q c s c AkT c ph p V R I I I I e R                            or in Eq. 2 : Eq. 2 0 0 ln c s c ph c c p c s c e V R I I I I kT R V A R I q I                        where q e is the electrical charge of electron   19 1.602 10 C   , k is the Boltzmann constant   23 1.38 10 J K   , I c [Α] the load current that the PV source supply, I ph [Α] the maximum current (photocurrent or short-circuit current) of the PV source (depended on power density of the solar ir- radiation), I 0 the reverse saturation current of the PV cell diode   4 2 10 A   ฀ , R s the serial and R p the shunt resistor of the PV cell, T c [C] an operation temperature factor of the PV cell and V c the output voltage of the PV cell. The «Α» factor is a non di- mensional constant that adjusts the numerical result with the true values of the PV source. From the previous equations, the non linear relation between output quantities (Voltage vs Cur- rent, Power vs Voltage or Power vs Current) is ob- vious as also are the factors that affect them. In the reference [2] is explained that the increase of the R s R p I ph I c V c I R I L