Fig.1: The Equivalent Circuit of a Solar Cell Design and implementation of Smart Relay Based Two-axis Sun Tracking System Dr. Jawad Radhi Mahmood Haider Muhammed University of Basrah, College of Engineering, Electrical Department Abstract: Solar power is environment friendly power source, but it is characterized by being highly dependent on the irradiation level which is function of the sun position in the sky. To overcome this situation and extract maximum power from the sun, the PV array must be kept nearly perpendicular to the sun during the daytime. In this paper, a smart relay based sun tracking system has been designed and implemented to keep the PV array perpendicular to the sun during the day hours. 1. Introduction Current trends in electric energy generation are moving towards the utilization of the environment friendly sources of energy, represented by the wind and solar energy ones as supplementary energy resources. The solar based one is the more attractive because it is characterized by being free, incur no fuel cost, abundant, limited less, distributed all over the world, pollution free, and require little maintenance. Every day, the sun rises in the east, moves across the sky, and sets in the west. If one could get a solar cell to turn and look at the sun all day hours, then it could receive the maximum amount of sunlight possible and convert it into the more useful energy form ( electricity). The current of the solar module is very sensitive to the isolation of the sun. Small tilt in the solar module makes the current drop rapidly. At each hour, the earth rotates 15 degree about its axis [1], therefore the solar module must be tilt 15 degree every hour to make line of sigh with the sun otherwise the output power of the module decreases. To get the maximum from the sun, many tracking techniques have been developed and still are being developed [2, 3, 4, 5, and 6]. 2-Photovoltaic Cell Operation and Characteristic Photovoltaic cell consists of semiconductor material, which converts sunlight into DC current using the photovoltaic effect. The conversion process passes through two phases. In the first phase, absorption of sun light generates an electron- hole pairs. In the second phase, the electron and holes are separated ( by the internal electric field) to the negative and positive terminal thereby generating electrical power [ 7 ]. 2.1 Photovoltaic Cell Equivalent Circuit & Mathematical Model The most general model is the one so called one diode model . This model is shown in Fig.1 [8]. It consists of current source ( directly proportional to solar irradiation), diode ( stands for the intrinsic P-N junction), shunt resistance (Rsh), and series resistance (Rs). Where I and V stand for terminal current and voltage respectively of the solar cell, I ph is the light generated current, I d is the diode current, R S and R sh are the series and parallel resistances of the cell. The mathematical model which relates the various parameters of the equivalent circuit is given by the following expressions: 2010 1st International Conference on Energy, Power and Control (EPC-IQ), College of Engineering, University of Basrah, Basrah, Iraq, November 30 - December 2, 2010 259