The sandTES technology utilizes a fluidized bed counter current heat exchanger for thermal energy storage applications. Its main feature is an imposed horizontal flow of sand (SiO₂) particles fluidized by a vertical air flow across a heat exchanger consisting of several horizontal rows of tubes. Past international research on heat transfer in dense fluidized beds has focused on stationary (stirred tank) systems, and there is little to no information available on the impact of longitudinal or helical fins. Previous pilot plant scale experiments at TU Wien led to the conclusion that the currently available correlations for predicting the heat transfer coefficient between the tube surface and the surrounding fluidized bed are insufficient for the horizontal sand flow imposed by the sandTES technology. Therefore, several smaller test rigs were designed in this study to investigate the influence of different tube arrangements and flow conditions on the external convective heat transfer coefficient and possible improvements by using finned tubes. It could be shown that helically finned tubes in a transversal arrangement, where the horizontal sand flow is perpendicular to the tube axes, allows an increase in the heat transfer coefficient per tube length (i.e., the virtual heat transfer coefficient) by a factor of 3.5 to about 1250 W/m²K at ambient temperature. Based on the literature, this heat transfer coefficient is expected to increase at higher temperatures. The new design criteria allow the design of compact, low-cost heat exchangers for thermal energy storage applications, in particular electro-thermal energy storage.

Particle-based systems have immense potential for combining thermal energy storage (TES) with renewable energy sources. The so-called sandTES system, which is an active TES system, utilizes sand or other small particles as a storage material and consists of a hot tank, a cold tank, and a reversible fluidized bed heat exchanger. In the preferred design, the tubes are arranged in horizontal serpentine tube bundles; thus, the headers are positioned vertically, for one phase subcritical, two-phase and supercritical water/steam conditions. So far, no design principles have been published for macro-sized vertical headers and horizontal tube bundles. This paper investigates the unbalance in flow distribution and other flow instabilities of such an unusual heat exchanger and its scale-up using APROS simulations. The steady-state simulations of the tallest investigated heat exchanger show a strong unbalance in the mass flows between individual heat exchanger tubes: the smallest mass flow was only 55% of the largest one when generating steam. Smaller heat exchangers experience less pronounced unbalance factors in the order of 88%. When steam is condensed, the unbalance is insignificant. Dynamic simulations of start-up and shutdown revealed instabilities and that the flow maldistributions even lead to flow reversals in most investigated cases although neither last for stationary operation. Part-load cases show higher unbalances than their respective full-load case by 9 to 17 percentage points. Header design modifications - secondary headers with submodules and orifices - improve the unbalance factor from 55% to 88%.

While there are many semi-empirical correlations for estimating the wall-to-bed heat transfer coefficient in a fluidized bed, most are only usable in a limited range of operating conditions. The correlation developed by Molerus and Wirth (1997) provides the most expansive and successful model; however, it does not consider the influence of the properties of an immersed tube bank or a horizontal movement of particles (cross-flow). This study expands Molerus and Wirth’s correlation to include these additional influencing factors by identifying and introducing new dimensionless factors using dimensional analysis. Collected secondary data and measurements from a test rig were used to evaluate the extended model. The model’s estimates largely align with the collected secondary data and previously published models describing the influence of tube diameter and tube packing density on the wall-to-bed heat transfer coefficient. The model also provides new insight into the conditions under which a particle cross-flow contributes significantly to the wall-to-bed heat transfer coefficient. Future research should use these findings to conduct targeted measurements and further improve the model’s predictions.