Application of thermal energy storage using a point-focus fresnel lens concentrator -

Abstract

This thesis presents a numerical and experimental analysis to model and design a thermal energy storage system for Lebanon using a point-focus Fresnel refractor lens. For this purpose, a specially designed cavity receiver was built and tested to study thermal performance. A numerical model for the cavity receiver and the entire energy storage system is presented. The geometrical parameters of the receiver were optimized to minimize heat losses and to reach optimal performance. The model was used to predict the thermal efficiency of the receiver, the outlet temperature of the heat transfer fluid and the thermal energy storage profile in Lebanon for each month of the year. Furthermore, a new method to model the refracted solar irradiation using the Discrete Ordinates (DO) radiation model is presented in ANSYS FLUENT. The method combines the optical and thermal simulation for the Fresnel refractor lens using finite volume method. It is useful to predict the absorbed solar irradiation and the temperature distribution profiles over the cavity surface, as well as the air flow profiles. The simulation results were validated experimentally in the outdoor conditions using eight different heat transfer fluid flow rates and four different solar irradiation values ranging from 500-1000 Watts-m2. The results show the ability of the small-scale Fresnel lens to increase the temperature of the heat transfer fluid and to generate a temperature difference of 200 ᵒC in 10-12 minutes which proves the effectiveness of the Fresnel lens for thermal energy concentration. The amount of thermal energy stored varied between 1112 and 3885 Watt-hour per day during January and June respectively with a thermal efficiency of 93.6 – 97.2percent. Simulations also show an almost uniform temperature distribution profile over the cavity surface which proves the high performance of the proposed cavity geometry.