A novel concept of a fluidized bed “thermochemical battery” for concentrated solar power applications

Extensive R&D is in progress to exploit the huge amount of solar energy falling on Earth. The potential advantage of Concentrating Solar Power (CSP) over its direct photovoltaic (PV) competitor relies on easier integration with energy storage. CSP coupled with Thermal Energy Storage (TES) systems is as yet more convenient than PV with batteries. Sensible heat storage, the less complex technology among TES systems, is already available at commercial scale. TES based on molten salts is well assessed and commercially available, but suffers from fairly small density of energy storage and relatively low upper temperature limit (560 °C) which hampers the efficiency of the associated energy conversion thermodynamic cycles. The use of granular solids as radiative flux collection and TES media is gaining much interest, as it enables to overcome temperature limitations of molten salts. Thermochemical energy storage (TCES) is more ambitious than TES, striving for much higher energy density storage and improved stability over long time-scales. Reversible chemical reactions such as hydration/dehydration, carbonation/calcination, oxidation/reduction of inorganic compounds are being considered for TCES. Many processes entail gas-solid reversible reactions and are performed in multiphase chemical reactors, frequently of the Fluidized Bed (FB) type. A fluidized bed “thermochemical battery” is presented in this study, based on a novel design of a fluidized bed solar collector. The peculiar design of the reactor enables to lump in a single unit all the basic goals of a solar collector with TCES: collection of solar energy by direct radiation of the granular solid; course of the endothermic reversible heterogeneous chemical reaction U→C promoted by solar radiation; storage of both the thermochemically “charged” solid compound C, as well as of the original “uncharged” solid compound U, in the very same vessel. Energy recovery associated with the reverse reaction C→U (i.e. the thermochemical battery discharge) is accomplished in a separate reactor, where heat extraction is optimized during the course of the exothermal energy production step. An outline of the proposed design is reported in Figure 1. The thermochemical battery is reported on the right side of Figure 1. The battery consists of four sections: i) storage of the uncharged compound U at the bottom of the bed; ii) storage of the charged compound C at the top of the bed; iii) a disengagement section where collection of solar energy takes place, supporting the course of the “charge” endothermal reaction U→C; iv) a draft tube where the uncharged material U is conveyed into the disengagement section and to the top of the bed. Solids segregation along the bed is promoted by density/size differences between U and C particles. Operational conditions of the fluidized bed are set so as to maximize solids segregation, while preserving the required mobility of granular solids. Operation of the central draft duct is directed to ensure the required solids recirculation rate which, in turn, is strictly related to the incoming solar flux. Altogether, operation of the thermochemical battery is a dynamic process where the relative inventories of the charged and uncharged materials vary, and the interface between U and C moves during the charge and discharge phases much like a thermo/chemo-cline. The system is complemented by another fluidized bed reactor, where the exothermal “discharge” reaction C→U takes place. The proof-of-concept of the fluidized bed thermochemical battery is given with reference to the CaO-CaCO3 cycle. A directly irradiated 0.1 m ID FB reactor exposed to a 12 kWel simulated solar furnace (Figure 2) has been used to demonstrate operation of the fluidized bed thermochemical battery. Key variables of the thermochemical battery (temperatures, inventories of CaO and CaCO3, quality of fluidization and extent of segregation) have been recorded during operation to verify and support further optimization of the proposed concept.