Maximizing heat transfer for energy storage application – design of a continuous fluidized bed cold model

The current research on energy storage systems at the Chair of Energy Systems focusses on the development of a large-scale concept for a continuous fluidized bed reactor. Heat transfer between the fluidized bed and immersed heat exchangers has been previously determined to be the predominant factor for the storage power. An important objective of the current research is to maximize the heat transfer by (1) characterizing the heat transfer coefficient and (2) maximizing the available heat exchanger area in a continuously operating fluidized bed reactor. Therefore a cold fluidized bed model is currently being commissioned to evaluate different heat exchanger geometries and their influence on the degree of fluidization and consequently on the heat transfer coefficient. The main focus herein is to evaluate entire tube bundles. The fluidized bed cold model has a bed volume of 80 L, divided into 4 sections separated by baffles to optimize the residence time. Ca(OH)2-particles with a diameter of 250 to 400 μm are fluidized using dry air with temperatures up to 50 °C. The design of the cold model is modular, allowing for the investigation of different heat exchanger configurations. The measurement of the heat transfer coefficient is conducted with several overall-perimeter heat transfer probes at various positions in the tube bundles. The heat exchangers are tube bundles immersed either horizontally or vertically in the fluidized bed. Aspects of interest are upsides of staggered or aligned configurations, the influence of the clearing distance between the tubes, tube diameters and potentially occurring particle deposition on the tubes. Initial experiments in the fluidized bed cold model focus on (1) characterization of the overall heat transfer coefficient of the entire tube bundle and (2) optimization of the heat exchanger area while maintaining a sufficient degree of fluidization. The long-term goal is to establish correlations for reactor scale-up.