Numerical simulation of a high temperature thermal storage unit for solar gas turbine applications

Abstract

The implementation of thermal storage systems allows concentrating solar power plants to generate predictable power delivery to the grid by mitigating the variability of solar energy supply. However, while thermal storage solutions exist for Rankine cycles, in the form of molten salts and thermal oil reservoirs, the high temperature nature (>800°C) of Brayton cycles requires a different approach. One solution is to make use of a high temperature packed bed. The transient heating and cooling of a packed bed of ceramic pebbles is investigated in the current work through a numerical analysis, with experimental validation. The technique of orthogonal collocation on finite elements is used to solve the governing heat transfer equations for the fluid, solid and wall temperature distributions in two dimensions. Radiation effects are incorporated in the model through the Zehner-Schlünder effective thermal conductivity model and are verified against the SANA test cases with good agreement. Non-uniform radial porosity and velocity distributions are also incorporated and are found to have a notable effect on the temperature profiles. Numerical results are compared to preliminary test results from a packed bed test rig. The heating tests show a good correlation between predicted and measured temperature profiles, while the cooling of the bed is not accurately represented at present. Modifications to the test setup are proposed.