Numerical-experimental evaluation of FRESNEL lens heating dynamics

Abstract

Fresnel lenses allow the concentration of sunlight that can be employed in different thermal unitary operations or endothermic chemical transformations. A system with Fresnel lenses should also take into account vessel or reactor surface thermal losses (reflection, convection and emission), as well as the thermal energy required to keep the vessel or reactor structure warm. In this paper, a numerical and experimental analysis indicated that the solar radiation incident on the lens, the Fresnel lens area and the specimen mass can be represented by a grouping called Heating Factor (31 ≤ Ψ (W/kg) ≤ 3347). The Heating Factor (Ψ) was strongly correlated with the maximum temperature conductive specimens could reach when inserted at the focal point of a Fresnel Lens. Equilibrium Temperatures were predicted by a physical-mathematical model and validated by experimental tests. The Fresnel lens system has been shown to be able of providing Equilibrium Temperatures from 345 to 1600 K for specimens with Heating Times between 3 and 85 min and Initial Thermal Rates from 1.16 to 312.00 K/min. Numerical-experimental analyses also showed that the heating dynamic was strongly influenced by the nature of the materials that constituted the specimens (specific mass and the specific heat).