A Novel Fluidized Bed Thermochemical Battery for Chemical Energy Storage in Concentrated Solar Power Systems

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 the 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. This study deals with the development of a novel fluidized bed device finalized to TCES which lumps in a single unit all the basic tasks required for solar energy collection and exploitation. The main goal of the device is to smartly exploit the unique properties of fluidized bed reactors and in terms of flow ability of granular medium, of heat exchange mechanisms and of switching mode of operation by finely tuning the fluidization conditions in order to simultaneously perform: i) collection of solar energy; ii) endothermic chemical reactions for solar energy storage; iii) storage of chemically charged material; iv) exothermic chemical reactions for release of stored solar energy; v) storage of chemically uncharged material. An outline of the proposed device is reported in Figure 1. It basically consists of two interconnected fluidized beds a riser and a fixed/bubbling fluidized bed. The bed material is upward transported in the riser (R) towards a conical section, which acts, at the same time, as gas-solid separation unit; solar receiver and high-temperature reactor. The gas-solid separation takes place, as in the sedimentation chambers, owing to the reduction of gas velocity and to the action of the gravity force on the solid particles. As consequence, the particles fall down onto the lateral surface of the conical section and are collected in an external tube concentric (annulus tube, AT) with the riser tube. As a granular medium, the solid particles descend the annular gap between the two tubes until to reach four holes made on the outer surface of the external tube in order to be conveyed towards a fixed/bubbling fluidized bed. The fixed/bubbling fluidized bed (F/BFB) is hosted by a third tube also concentric with the other two tubes and can act as thermochemical energy storage and/or low temperature reactor. The fluidizing gas is fed to the F/BFB through an annular sparger located at pre-set distance from the bottom of the F/BFB tube in order to form a bottom region of not fluidized particles. The bottom region of the F/BFB is again connected to the riser by means four holes made on the riser tube. The fluidizing gas is fed to the riser through a tube nozzle whose exit is located at level well below the holes which connect the F/BFB to the riser. The geometrical characteristics of three concentric circular tubes are: OD=12 mm, ID=10 mm, length=90 mm for riser tube, OD=21 mm, ID=16 mm, length=80 mm for the annulus tube and OD=48 mm, ID=40 mm, length=130 mm for F/BFB tube. The conical section is characterized by an internal angle of 30° and a height of about 120 mm. The device operation is based on the heating-up of rising dense particle suspension in the riser owing to the heat exchange due to the descending particles in the annulus section. Then, at the outlet of the riser, the dense particle suspension directly interacts with the solar concentrated radiation and increases its temperature at the final stage. Here, course of endothermic chemical reactions is fostered by direct interaction with concentrated solar energy. The fluidization conditions in the riser are critical to promote or limit bed solid circulation and respond to possible instabilities/variations of the incoming concentrated radiation. The descending particles along the annulus section are cooled until to reach the F/BFB section for thermochemical energy storage and/or lower temperature release of heat for the inverse exothermic reactions. Hydrodynamic patterns in the two reactors and in the connections between them are critically tuned and controlled to have the correct circulation of solid particles and prevent gas cross flow between the reactors. In order to extract the released heat, the storage section is also equipped with heat exchanger. The above describe device is designed to perform any chemical reaction suited to TCES and/or solar fuels production. In this work, the proof of concept of the device has been demonstrated by exploiting CaCO3 calcination and subsequent carbonation as model reaction. Short arc Xe lamps coupled with elliptical reflector were used to simulate the concentrated solar radiation, achieving peak flux in the order of MW m–2 in the focal point. Air was used as fluidizing gas during the endothermic calcination, and a proper mixture of air/CO2 during the exothermic step. Device was equipped with several thermocouples to measure the temperature in the different sections of the reactor during the charging/discharing reactions, and with gas analysis to assess the extent of CaCO3 calcination and carbonation. Overall bed inventory of the device is of approximately 120 g. Considering a mean carbonation degree of 30%, which is representative of a limestone samples looped for 15 reaction cycles, the chemical energy stored is of about 65 kJ, which corresponds to the amount energy stored by a typical lithium battery of a modern smartphone device.