Ashmore Mawire

Characterisation of a Thermal Energy Storage System: a Research Work Submitted to the North West University in South Africa for the Degree of Doctor of Philosophy in Physics Ashmore Mawire

An oil/pebble?bed thermal energy storage (TES) system to enhance the performance of a solar cooker is characterized by simulation and experiment. Mathematical models to describe the thermal parameters of the solar cooking and TES system are developed using energy balance equations. The models are validated with experimental data and reasonable agreement between experiment and model is achieved. Two charging methods of the TES system are proposed and implemented using a Simulink block model in the Matlab environment. The constant flow rate (CFC) method charges the storage at a constant flow rate while the constant temperature charging (CTC) method maintains the outlet charging temperature at a constant value by making the flow rate variable. Simulation results indicate a greater degree of thermal stratification, a greater energy stored and a greater exergy stored when using constant temperature charging. The performance of three solid material (fused silica glass, alumina, stainless steel) for the pebbles is simulated and compared in terms of the axial temperature distribution, the energy stored and the exergy stored.