Heat transfer characteristics of a novel fluidized bed for concentrating solar with thermal energy storage

Abstract

A novel fluidized-bed particle receiver concept has been proposed and tested for concentrating solar applications with thermal energy storage. The receiver transfers heat received from solar radiation on the receiver external walls into a near-wall fluidized bed in a narrow vertical passage. The fluidized bed has a unique net-downward flow of particles against a counterflow of fluidizing gas. Fluidizing gas exits the near-wall region through a mesh-covered opening in the receiver back wall. Understanding the bed hydrodynamics and the functional dependencies of the wall-to-bed heat transfer coefficient are paramount to receiver design, modeling, and scale up. The wall-to-bed heat transfer coefficient in fluidized beds has been studied for other bed geometries, but existing correlations may not apply to the unique design or conditions of this receiver concept. This thesis presents an experimental study of the fluidized bed hydrodynamics and heat transfer, using an experimental rig, that replicates the flow and heat transfer conditions of the concept receiver. Experiments were conducted both in batch mode with a fixed mass of fluidized particles and in continuous-flow mode which had steady addition of particles at the top of the bed and removal of particles from the bottom of the bed. The batch-mode experiments showed very similar heat-transfer performance as continuous flow experiments and allowed easier control and testing for a wide range of experimental conditions. In continuous mode, particles entered from a centered orifice and instantly redistributed due to fluidization, which allowed for the particles to exit readily from a central orifice such that the particle flow out the bottom of the bed matches the inlet flow. Batch mode experiments were conducted over ranges of bed temperature (100 C < T < 650 C), of superficial gas velocity (0.25 < U < 1.7 m/s), and of solids fraction (0.1 < alpha < 0.45). For these ranges of conditions, wall-to-bed heat transfer coefficients varied from 400-1000 W/m2-K. Wall-to-bed heat transfer coefficients increased with T due to increases both in the effective radiative heat transfer coefficient and in the gas thermal conductivity with T. The fractional contribution of the radiative heat transfer coefficient to the total increases with T, from negligible contribution for T < 100 C up to almost 15% for T = 650 C. The wall-to-bed heat transfer coefficient decreased with increases in U above the minimal fluidization velocity in part due to decreases in alpha. Increases in U created significant gas bubbles spanning the thickness of the bed, which locally reduced the particle-enabled convection from the wall into the bed. For a volumetric receiver with a fixed solar flux spreading angle, the higher values of wall-to-bed heat transfer coefficient around 1000 W/m2-K would allow solar concentrations up to 2 times greater than the lower values of 400 W/m2-K and the doubling in solar concentrations would not only reduce receiver size but also increase solar receiver efficiency by approximately 14%. The work presented here shows that this concept provides the high values of wall-to-bed heat transfer coefficient as is needed for concentrating solar applications and further proves the feasibility of the proposed fluidized bed configuration.