In recent years, the wide variety of solar concentrators has been developed, including Fresnel linear reflectors. This concentrator consists of long narrow flat reflectors. This system is positioned on a flat base; each reflector is tilted at an angle such that all incident solar rays falling on them are reflected to a common focus line. The absorber is placed on the focus line to absorb the concentrated radiation. The absorber is generally a tube or a number of tubes which contains a heat traort fluid. Considerable attention has been paid to develop linear Fresnel reflecting concentrators for thermal and photovoltaic conversion of the solar energy. The Absorber of the solar concentrating device plays a very important role in the collection of solar energy. In the operation of the solar collectors, the absorber pipe gets heated up due to the incident radiation and it begins to emit long wavelength radiation. This radiation leads to heat loss from the absorber pipe and results in decrease in thermal efficiency of the collector. Heat losses from the absorber tube also occur through convection and conduction modes. The main advantage of Fresnel systems is their capability to use flexible reflectors which are cheaper in comparison to glass reflectors. Two different approaches have been proposed for designing linear Fresnel reflector solar concentrators. In the first, the concentrator is manufactured at a pre specified size of the absorber. In this approach, the width and the angle of inclination of each reflector with respect to the base of the concentrator are determined, so that incident solar radiation after reflection is intercepted by the absorber tube. An appropriate space, called shift, is introduced between two consecutive reflectors so as to avoid the blocking of radiation reflected from any reflector by its adjacent reflector. In the second approach, the concentrator is produced at any desirable width size and an appropriate size of the absorber tube diameter that will intercept all the solar reflected rays from the reflectors is determined. In this study, the effects of using parabolic reflectors instead of flat reflectors on Fresnel solar concentrator have been studied. For this new system by ray trace statistical method, local concentration ratio distribution has been determined on the surface of the absorber pipe. These results are compared with LFR system results. Also, the effect of geometrical parameters, such as absorber height and reflectors width, on the concentration ratio is studied. The results have shown that using the parabolic reflectors with low curvature instead of linear reflectors has a pronounced effect on the absorber pipe diameter. Comparison of the performance of linear and parabolic systems with identical parameters such as reflectors width, reflectors number and absorber pipe height showed that the absorber tube diameter in a parabolic system is smaller than the absorber tube diameter in a linear system, thus decreased rate of heat loss from the pipe surface. Comparison of the two systems under equal solar energy radiation and absorber pipe diameter showed that in parabolic system wider reflectors can be used, thus reduced number of reflectors and simpler control system. Keywords: Solar concentrator, Fresnel, Concentration ratio, Ray traces method, Linear reflector, Parabolic reflector, Absorber pipe.
