Particle flow and heat transfer in fluidized bed-in-tube solar receivers

This work is part of the European project “Next-CSP” which aims to develop a next generation of concentrated solar power plants using the particle technology and, particularly, the fluidized particle-in-tube technology working at high temperature (>700°C). A 3MWth pilot unit including a solar receiver, storage tanks, a heat exchanger and a gas turbine is under assembly at the top of a solar tower (Themis-France) to demonstrate this technology. The unit will use the fluidized particle-in-tube solar receiver concept. The scaling up of this concept needs researches on the gas-particle flow structure evolution along the tube and on wall-to-fluidized particles heat transfer. Therefore, several experimental set-ups were implemented to study the particle flow and heat exchanges in order to define the best operational conditions for the full-scale 3MW test unit. The first one is a cold experiment with three 3m-long transparent tubes implemented to study the stability of dense particle suspension (DPS) flow in tube and the flow distribution between the different tubes. 3m is the length of the solar receiver tubes. The second one is an on-sun experiment equipped with a one meter-long finned tube to collect data on the distribution of wall surface and particles temperature, thermal exchange and thermal performance useful for further modelling and up scaling. Experiments with the cold mockup indicate that stable particle flowrate ranging from 10 to 340 kg/m².s (0.015 to 0.53 g/s) can be obtained per tube with mean particle volume fraction in the range 0.29-0.36. Solar experiments with finned tube designed to increase the heat exchange between the particle suspension and the irradiated tube result in rather constant values of the heat transfer coefficient at about 1200 ± 400 W/m².K for particle mass flux between 40 and 110 kg/m².s.