VOL. 11, NO. 24, DECEMBER 2016 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences © 2006-2016 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com 14308 OPTICAL ANALYSIS OF VARIOUS REFLECTORS APPLIED IN SOLAR BEAM DOWN TOWER Ayad Kadhim Khlief, Syed Ihtsham- ul-Haq Gilani, Hussain H. Al-Kayiem and Basil H. Ali Department of Mechanical Engineering, Universiti Teknologi Petronas, Bandar Seri Iskandar, Perak, Malaysia E-Mail: kadhimayad73@yahoo.com ABSTRACT The concept of the reflective solar tower is based on the reflection of the solar radiations from heliostats toward reflective mirror at the top of the tower and then redirected the radiation to one of its foci on the ground. This new arrangement would allow setup the energy collection at the ground level; hence the heavy tower top setup is avoided. For the suggested beam down solar tower located at Universiti Teknologi Petronas, three reflective shapes are examined, i.e. ellipsoidal/concave, hyperboloid/convex and a flat surface. A comparative study is carried out using lightXlab simulation platform to investigate the effect of focal point distance on the radiation spread at the ground level and the effect of angle change of the second reflector on the radiation concentration as well as the beam travel. The results of the simulation showed that the flat surface reflector is having superiority and better control over the other two reflectors, in terms of beam concentration and beam travel. Keywords: concentrating solar power, reflective solar tower, ellipsoidal and hyperboloid mirror, heliostat, beam down mirror. INTRODUCTION Concentrated solar power (CSP) systems use thermal energy storage (TES) [1, 2 and 3] at a cost much lower than that of the electrical storage options currently available. However, opportunities exist to improve TES implementations in terms of transport energy and heat exchanger exergy losses. As for the conventional concentrating solar tower, solar energy from a large group of heliostats is concentrated onto a tower which picks up the heat by a receiver, and then using a steam generator and a steam turbine converts that heat to electricity. However, it is expensive in terms of constructing tower. Besides, the thermal and optical losses are considerable in order to overcome these shortcomings, it has been proposed a beam-down optical system [4], where a second set of beam-down concentrator directs the solar radiation back to a central receiver (CR) placed on the ground. Very few teams in the world were interested in the concept, such as Weizmann Institute of Science [5] and Masdar Institute (Abu Dhabi), developed by the Tokyo Institute of Technology [6]. This is a multi-ring reflector developed under the Tokyo Tech-Cosmo-Masdar project. This reflector is made multiple hyperboloids of having the same focal points, represents a multiple reflector cup rings. The advantages put forward are numerous [7], (A lighter structure, better ventilation, reduction in wind resistance, better cooling of mirrors and Lower cost). These studies give their performance comparable to conventional power tower plants [8, 9] for a construction cost well below. Segal and Epstein [10, 11 and 12], described the effect of using two types of secondary mirror reflectors, i.e. elliptical and hyperbolical for a comparative performance analysis. Their analysis revealed that, to reach a similar level of radiation concentration, elliptical reflectors require higher tower and larger size then the hyperboloid reflectors. The objective of this study is to examine the suitability of various reflector shapes and height of the tower for the existing heliostat field, at UTP solar site, for a minimum radiation spread and low beam travel through air. UTP heliostat field layout To perform this study, solar research site at the University Technology PETRONAS was selected. The solar field is in the city of Bandar Seri Iskandar, Perak, Malaysia and is at global coordinates of 4ᵒ N-latitude, 100ᵒ E-longitude. Initial investigations of the heliostat field led to a determination of 12 heliostats to be utilized in simulating the facility. The reflectivity established for the heliostat is the same as that specified for the down beam reflector, i.e. 89% specular. Heliostats of collective reflective surface area 45 m 2 were selected for modeling of the facility, distributed as shown in Figure-1, the distance between the tower and first row of heliostats is 10 meters, where is, the inter-row distance is 3.5m. The reflection area of each mirror is 3.75 m². Figure-1. Schematic distribution of heliostats in the field (UTP Heliostat field) sketch not to scale. Optical simulation approach A number of simulation software are available to simulate the optical phenomena, i.e. Soltrace, lightXlab, etc. In this study, the lightXlab optical simulation platform