General Formula for On-Axis Sun-Tracking System

Abstract

Sun-tracking system plays an important role in the development of solar energy applications, especially for the high solar concentration systems that directly convert the solar energy into thermal or electrical energy. High degree of sun-tracking accuracy is required to ensure that the solar collector is capable of harnessing the maximum solar energy throughout the day. High concentration solar power systems, such as central receiver system, parabolic trough, parabolic dish etc, are the common in the applications of collecting solar energy. In order to maintain high output power and stability of the solar power system, a high-precision sun-tracking system is necessary to follow the sun’s trajectory from dawn until dusk. For achieving high degree of tracking accuracy, sun-tracking systems normally employ sensors to feedback error signals to the control system for continuously receiving maximum solar irradiation on the receiver. Over the past two decades, various strategies have been proposed and they can be classified into the following three categories, i.e. open-loop, closed-loop and hybrid sun-tracking (Lee et al., 2009). In the open-loop tracking approach, the control program will perform calculation to identify the sun's path using a specific suntracking formula in order to drive the solar collector towards the sun. Open-loop sensors are employed to determine the rotational angles of the tracking axes and guarantee that the solar collector is positioned at the right angles. On the other hand, for the closed-loop tracking scheme, the solar collector normally will sense the direct solar radiation falling on a closed-loop sensor as a feedback signal to ensure that the solar collector is capable of tracking the sun all the time. Instead of the above options, some researchers have also designed a hybrid system that contains both the open-loop and closed-loop sensors to attain a good tracking accuracy. The above-mentioned tracking methods are operated by either a microcontroller based control system or a PC based control system in order to trace the position of the sun. Azimuth-elevation and tilt-roll tracking mechanisms are among the most commonly used sun-tracking methods for aiming the solar collector towards the sun at all times. Each of these two sun-tracking methods has its own specific sun-tracking formula and they are not interrelated in many decades ago. In this chapter, the most general form of sun-tracking formula that embraces all the possible on-axis tracking approaches is derived and presented in details. The general sun-tracking formula not only can provide a general mathematical solution, but more significantly, it can improve the sun-tracking accuracy by tackling the