There are a number of problems associated with fossil fuels, most of which stem from the by-products created when they are burned to create energy. Chief among those byproducts are carbon dioxide and nitrous oxide, greenhouse gases that are major contributors to global warming. Renewable energy is far cleaner than fossil fuels. Many of the safety concerns surrounding fossil fuels, such as explosions on oil platforms and collapsing coal mines do not exist with renewable energy. Most renewable energy comes either directly or indirectly from the sun. Sunlight, or solar energy , can be used directly for heating and lighting homes and other buildings, for generating electricity, and for hot water heating, solar cooling, and a variety of commercial and industrial uses. One way to produce electricity from the sun is by using the concentrating solar thermal system. This is done by focusing the heat from the sun to produce steam. The steam will drove a generator to produce electricity. Many different types of concentrators are being used in solar industries, including parabolic linear concentrators, parabolic dishes, central receivers, Fresnel concentrators, etc. Fresnel concentrators are simpler and easier to manufacture compared with parabolic concentrator collectors though their efficiency might be slightly lower. In these systems the parabolic reflectors which are expensive are replaces by flat reflectors. Sun rays which are reflected by flat reflectors are partly lost because they do not coincide on the absorber tube; instead they pass near the absorber tube. To increase the efficiency of Fresnel concentrators a second reflector is devised above the absorber tube. This second reflector diverts the otherwise lost rays towards the absorber tube. The other advantage of the second reflector is that it distributes the incident rays more uniformly on the absorber tube. This is especially important in the systems which use a molten salt as the thermal traort medium. In the present study six different designs for the second reflector are proposed and their performances are evaluated. Each design has been simulated in the MATLAB software and the effective parameters on its performance have been investigated. To analyze and compare the performance of different proposed designs, first the system performance is evaluated without considering the secondary reflector. Prior to this, the tilt angle of each of the primary reflectors are adjusted to the optimum value in such a way that most of the reflected rays are diverted towards the absorber tube. Then by adding the secondary reflector to the system its performance is evaluated. The results show that for the secondary reflector, a flat mirror reflector combined with flat wings on both sides is the best choice. The results show that by devising this secondary flat reflector, about 87% of the sun rays incident on the primary reflector are received by the absorber tube. It collects more rays and distributes them more evenly on the absorber tube. Keywords: Solar energy, Fresnel reflectors, secondary reflector, solar power .
