Investigation of the heat transfer inside a fluidized bed at high temperatures

Abstract

To combat climate change effectively renewable energy sources must be more integrated into the electric/ heat grid. This poses the risk of creating an instability to the grid since most renewable energy sources, like wind and solar, are not controllable in their available output. Thus it is necessary to generate enough electricity/ thermal energy when the resources are available and then store it until it is needed. Such a storage possibility has been developed by the Institute of Energy Systems and Thermodynamics at the Vienna University of Technology, based on heat transfer via a fluidized bed.The objective of this thesis was to investigate the influence of a variety of factors on the heat transfer from a heated tube into a fluidized bed. The investigated factors included the particle properties, the heated tube geometry, the tube bundle spacing around the heated tube, degree of fluidization, the horizontal particle flux in the fluidized bed and the bed-temperature. For an effective heat storage system the heat transfer coefficient must be as high as possible while keeping the required utilities to a minimum.For this purpose a test rig was designed and built that allowed for a bed-temperature of up to 400°C, which was compatible with two different tube bundles and measurement tube geometries, could accommodate more than 150kg of particles and ensured a homogeneous distribution of the fluidization air over the entire base area of the fluidized bed chamber of 913mmx200mm. Three different particle classes, quartz sand with a mean diameter of 146μm and two aluminium silicate ceramics with a mean diameter of 163μm and 352μm, were tested.