Pergamon PIh S0038-092X (96) 00135-1 Solar Energy Vol. 57, No. 6, pp. 465-469, 1996 Copyright© 1997 Publishedby ElsevierScienceLtd Printed in Great Britain. All rights reserved 0038-092X/96 $15.00+ 0.00 DESIGN AND CONSTRUCTION OF A ONE-AXIS SUN-TRACKING SYSTEM SOTERIS A. KALOGIROU t Higher Technical Institute, Department of Mechanical Engineering, P.O. Box 423, Nicosia, Cyprus (Received 28 December 1995; revised version accepted 13 September 1996) (Communicated by Lorin Vant-Hull) Abstract--This paper describes a tracking system which can be used with single-axis solar concentrating systems. The position and "status" of the Sun are detected by three light-dependent resistors (LDRs), one of which detects whether the collector is focused, whilst a second resistor determines if there is cloud cover, and the third senses whether it is day or night. The resultant signals are fed to an electronic control system which operates a low-speed 12-V d.c. motor which rotates the collector via a speed-reduction gearbox. The tracking system accuracy depends on the magnitude of the solar irradiance. The deviation from zero is 0.2 and 0.05 ° with solar radiation of 100 and 600 W m -2, respectively. Both values suggest that the mechanism can be used satisfactorily for parabolic trough collectors of medium to high concen- tration ratios. Copyright © 1997 Published by Elsevier Science Ltd. 1. INTRODUCTION Concentrating solar collectors can provide sig- nificant advantages over the usual fiat plate collectors especially for medium- to high-tem- perature applications, although the Sun must be very accurately tracked to ensure good ther- mal performance. Various forms of tracking mechanisms, vary- ing from complex to very simple, have been proposed. They can be divided into two broad categories, namely mechanical (Kupta et al., 1978; Cope et al., 1981 ) and electrical/electronic systems. The electronic systems generally exhibit improved reliability and tracking accuracy. These can be further subdivided into: (1) mechanisms employing motors controlled electronically through sensors which detect the magnitude of the solar illumination (Hession and Bonwick, 1984; Zogbi and Laplaze, 1984; Mori et al., 1977; Rumala, 1986); (2) mechanisms using computer-controlled motors with feedback control provided from sensors measuring the solar flux on the receiver (Briggs, 1980; Boultinghouse, 1982). Inevitably, these latter systems employing computer control are more expensive. Due to the fact that the Sun always moves in one direction, it is believed that the refinement of rISES member. the feedback control does not justify the extra cost. 2. THE TRACKING SYSTEM A tracking mechanism must be reliable and able to follow the Sun with a certain degree of accuracy, return the collector to its original position at the end of the day or during the night, and also track during periods of intermittent cloud cover. The required accuracy of the tracking mecha- nism depends on the collector acceptance angle. This angle is defined as the range of solar incidence angles, measured relative to the normal to the tracking axis, over which the efficiency varies by less than 2% from that associated with normal incidence (ASHRAE, 1986). The results of tests, carried out on the parabolic trough collector model constructed as part of a research program, are presented in Fig. 1. The acceptance angle of this collector is approximately 1 ° (Kalogirou, 1991). It thus appears that it is sufficient for the present mechanism to track the Sun with an accuracy of 0.5 ° (i.e., a value equal to half the acceptance angle). The employed mode of tracking is the E-W horizontal, i.e., the parabolic trough col- lector axis is orientated in a N-S direction and the collector tracks the Sun in an E-W direction. 2.1. System description The final system, which was designed to oper- ate with the required tracking accuracy, consists 465