# Study on the Performance of Optical Lenses under High Fluxes of Solar Radiation
## 1 Introduction  
It is a fact that at the present time solar energy is primarily associated to the production of electricity from photovoltaic (PV) cells or modules and, also, in a different way, to the use of \"solar thermal panels\" which, contrary to PV modules, turn sunlight into heat instead of electric power. Nowadays, either for residential, commercial, or industrial applications, solar heating and cooling technologies are available to provide hot water, space heating and cooling, pool heating, water desalination, etc. Also, for most of the common citizens, a \"solar furnace\" is a way to heat food or things without spending electricity or gas. Therefore, we may say that the potential of solar thermal energy is relatively well-known for applications in the low temperature range (80-150 \({}^{\circ}\)C). The so-called high-concentration or high-flux solar applications, which pertain to attain much higher temperatures, are much less used despite the fact that the origin of high-temperature solar furnaces dates back to the beginning of the 20 th century, when [PERSON] (nicknamed \"[PERSON]\") created and patented an apparatus \"for making industrial use of solar heat, particularly in the metallurgical and chemical arts which necessitate the use of temperatures higher than those of ordinary furnaces\" [1,2]. More than 100 years later, modern solar furnaces--equipped with reactors or process chambers and specially designed accessories--are used for physical and chemical processes requiring high (>400 \({}^{\circ}\)C) and very-high (>1500 \({}^{\circ}\)C) temperatures. Different review works [3, 4, 5, 6, 7, 8] demonstrate the immense potentialities of the usage of solar heat for thermochemical processes involving fluids, but also for processing of (solid) materials. Concentrated solar energy can be competitive with energy-intensive technologies (laser [9,10] or plasma [11]) and has a great potential in the treatment of wastes [12,13]. Nonetheless, many studiesreveal the necessity to further develop the solar-driven high-temperature technologies that are needed to displace the use of fossil fuel combustion furnaces or electric furnaces that require limited and pollutant-conventional energy sources. In particular, it is necessary to ameliorate the temperature homogeneity conditions inside reaction chambers for materials processing when using solar heat. One of the main current shortcomings is to reach a homogeneous temperature distribution on the processed materials, due mainly to the unidirectionality of the radiation pattern and the non-homogeneous distribution of the highly concentrated radiant flux. Thus, there is a need for innovative modular systems for capture, concentration, control, and conduction of concentrated solar radiation. As presented in earlier works [6; 14], optical waveguide transmission lines made of optical fibers can direct the concentrated solar radiation to the place of utilization of the high-flux solar energy. As the fiber optical cables are flexible (see Figure 1), they can be used to illuminate (irradiate) the target not only from one single direction, but by using several cables it is possible to illuminate the target with radiation that is coming from different directions, thus circumventing one of the major problems with traditional high-flux concentrators: the unidirectionality of the radiation pattern. In the case of using a Fresnel lens as the primary concentrator of the direct solar radiation, optical diverging systems may be needed to be inserted at the entrance/inlet of each optical fiber cable to satisfy its acceptance angle (see Figure 2). Figure 1: (**a**) Photo of a 7 m optical fiber bundle/cable specially designed to transmit concentrated sunlight (up to 1000 W); (**b**) detail of the terminals (inlet and outlet) of the optical fiber bundle/cable. Figure 2: Scheme showing a concentration system (Fresnel lens) and a diverging lens to satisfy the acceptance angle of a possible conducting optical cable (not shown in the figure).