Fluidized bed reactors for thermochemical storage of concentrated solar power

Extensive R&D is in progress to exploit the huge amount of solar energy falling on Earth (105 TW). A fast growing technology developed to pursue this goal is Concentrating Solar Power (CSP): a field of optical sun tracking mirrors is used to concentrate the solar energy onto a receiver to eventually produce electricity or to drive an endothermic chemical reaction. The key to success of CSP is the integration with thermal energy storage systems, which provide a mean to overcome the inherent limitations associated with the variable/seasonal availability and spatial non uniformity of the primary energy source. Incident fluxes in CSP can reach and exceed 3000 kW m–2. For this reason, the receiver has a crucial role in CSP systems, as it must collect and transfer the incident solar energy while ensuring low heat losses and minimizing local overheating. Gas−solid Fluidized Beds (FB) can be conveniently applied to CSP thanks to their large heat transfer coefficients and effective thermal diffusivities. The interaction between the incident radiative flux and the FB can occur in an indirect way, focusing the radiation onto an exposed surface which transfers the heat to the FB, or in a direct way, providing the FB with transparent walls or windows. Direct absorption of solar energy permits operating temperatures high enough to perform thermochemical storage processes with high energy density. A critical issue in FB receivers is the bed surface overheating −induced by high concentrated solar radiations− that can cause sintering and/or degradation of the fluidized particles, hence a strong reduction of the thermochemical cycles efficiency. In the first part of the present work, the dynamics of a directly irradiated FB exposed to a highly concentrated simulated solar radiation has been investigated. Analysis of local temperature fluctuations in time and frequency domains has been performed. Conditioning of bed hydrodynamics close to the surface has been investigated as a mean to improve the interaction between the incident radiative flux and the bed. In the second part of the work, the application of a solar irradiated reactor to endothermal reactions has been demonstrated with reference to solar driven limestone calcination, followed by autothermal recarbonation of lime. Solar driven calcination has been investigated with the twofold perspective of: a) accomplishing thermochemical energy storage by a reversible high enthalpy and high temperature chemical reaction; b) performing solar aided CO2 capture from flue gas to be embodied in carbon capture and sequestration schemes such as calcium looping.