The fluidized bed air heat exchanger in a hybrid Brayton-cycle solar power plant

Using group A particle suspensions as heat transfer fluid in concentrated solar power plants leads to higher efficiency and lower costs. Combined cycle power generation becomes possible with e.g. a topping Brayton air turbine cycle and an advanced steam power block as bottoming cycle. This hybrid combined cycle solar tower power plant will be tested on a 3 MWth pilot-scale at the CNRS-Themis solar tower (France) with the receiver, hot powder storage and air Brayton turbine. The suspension will exit the receiver at a nominal outlet temperature of 750-800°C. Hot powders will be stored and will subsequently exchange heat with the turbine air. The outlet temperature of the air heat exchanger (625 to 700 °C) will considerably determine the hybrid operation (reducing the possibly used fossil fuel boost) and the heat exchanger design is of paramount importance. The air heat exchanger will be a baffled cross-flow fluidized bed. Air will be heated in an in-bed finned-tube bundle. Air will be fed at 5.8 bar and∼270°C. The hydrodynamics and heat transfer characteristics of the air heat exchanger were experimentally investigated towards bubble properties and heat trasnfer coefficient. The bed to tube heat transfer coefficient was measured for different pipe geometries at bed temperatures up to 700 °C, exceeding 2 kW/m2K for a twin-bore finned tube but only about 650 W/m²K for the bare tube of equal outside diameter. The heat transfer coefficient from the tube wall to the turbulent air flow inside the tube (∼ 325 W/m2K) determines the design. NEPTUNE_CFD software was used to perform 3D-numerical simulations of the fluidized bed hydrodynamics via an Eulerian n-fluid approach. Simulation and experimental results were in very fair agreement, stressing the capability of mathematical models to predict the behaviour of a cross-flow bubbling fluidized bed.