Experimental and numerical investigations of a small scale double-reflector concentrating solar system with latent heat storage

Abstract

The main aim of this doctoral thesis is to analyze a small scale double- reflector concentrating solar system with latent heat storage in temperature range 230 – 250 oC so that cooking can be done efficiently and effectively in the late evening or at night time. Many solar heat collection systems are based on transportation of heat from the focal point to the storage by a circulating heat transfer fluid. In this study, double-reflector arrangement is designed and tested to heat up the thermal heat storage directly without using any heat transport fluid. This makes the system more simple and easy to fabricate. NaNO3-KNO3 binary mixture is selected as the latent heat storage medium because the melting temperature of around 220oC is in a suitable range.

There are several objectives in this study. First of all, characterization of phase change materials has been carried out using differential scanning calorimeter (DSC). Important information such as heat capacity as a function of temperature, melting temperature, solid-solid phase transition temperature, enthalpy of fusion, and enthalpy of solid-solid phase transition can be obtained and used in the phase change numerical simulations.

After the characterization and selection of a phase change material, a double-reflector system with thermal energy storage was designed and constructed. In order to test the concept of the design, a reflection system using laser diode technique was used in a smoke chamber. Focal point of the primary reflector was determined experimentally and compared with the theoretical calculations. The latent heat storage unit was filled with the NaNO3-KNO3 binary mixture until 90% full. Copper top plate and fin was used to increase the heat transfer rate into the phase change material. With the double-reflector system, thermal charging of the heat storage was carried out under the sun.

Numerical simulations of the thermal charging process have been done using finite element model from COMSOL Multiphysics. 2-dimensional (2D) and 3-dimensional (3D) models with transient analysis were chosen. Effective heat capacity method was used to simulate the solid-liquid phase change problem. Heat capacity as a function of temperature obtained from the previous differential scanning calorimeter test was modified and used. Enthalpy of fusion and solid-solid phase transition were incorporated in the modified heat capacity. The simulated results were validated with the experimental data. Both 2D and 3D models can predict the experimental results quite satisfactory. For simplicity, 2-dimensional model was selected in the following studies. Following the positive results from the thermal charging experiments of heat storage under the sun, a parametric analysis was performed to better understand and optimize the latent heat storage unit. Heat storage unit with higher thermal heat storage capacity was investigated numerically on effects of heat flux, number of fins, fin and top plate thickness, top plate and fin material using the model developed. Based on this analysis, a latent heat storage unit with 7.5 kg of NaNO3-KNO3 binary mixture was fabricated and tested for cooking applications. Aluminum top plate and fins were used. The unit was thermally charged in an oven and practical tests on food cooking applications were made. A potential application of such a system is to supply cooking power in a modular basis. A scaled up system can charge the heat storage modules which can be distributed to individual users for cooking of an evening meal.