International audienceSolar thermal energy represents an increasingly attractive renewable source.However,to provide continuous availability of this energy,it must be stored. This paper presents the state of the art on high temperature(573-1273K)solar thermal energy storage based on chemical reactions,which seems to be the most advantageous one for long-term storage. The paper summarizes the numerical,experimental and technological studies done so far. Each system is described and the advantages and drawbacks of each reaction couple are considered

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author-deposited version published in : http://oatao.univ-toulouse.fr/ Eprints ID : 15980To link to this article : Michel Ca(OH)2/CaO reversible reaction in a fluidized bed reactor for thermochemical heat storage. AbstractThermal energy storage (TES) is a key factor for increasing the efficiency of concentrated solar power plants. TES using a reversible chemical reaction appears to be a promising technology for high energy density thermal storage (100-500 kW h m À3 ), at high temperature (up to 1000°C) and during a long period (24 h to several months). This paper details an experimental study to carry out the reversible reaction Ca(OH) 2(s) + DH r () CaO (s) + H 2 O (g) in a fluidized bed (FB) reactor. The 4 lm Ca(OH) 2 powder fluidization has been performed with an appropriate proportion of inert easy-to-fluidize particles. Then, Ca(OH) 2 dehydration and CaO hydration have been implemented in a FB reactor and 50 cycles have been reached. The mean energy density obtained is 60 kW h m À3 solid_mixture which amounts to a promising energy density of 156 kW h m À3 Ca(OH) 2 -bulk if the reactants and the easy-to-fluidize particles are separated. The results demonstrated the feasibility of the implementation of the Ca(OH) 2 /CaO thermochemical heat storage in a fluidized bed reactor.

The CaO/Ca(OH) 2 hydration/dehydration chemical loop has long been recognized as a potential candidate for application in energy storage systems for concentrated solar plants. However, the technology still remains at a conceptual level because little information has been published on the performance of the key reactors in the system. In this work, we experimentally investigate the hydration and dehydration reactors in a 5.5 kW th batch fluidized bed reactor, in conditions relevant to larger systems (superficial gas velocities of up to 0.53 m/s, temperatures of up to 500ºC for dehydration, input H 2 O (v) fractions between 0 and 0.8 etc.). Furthermore, to assist in the interpretation of the experimental results, a standard 1D bubbling reactor model has been formulated and fitted to the experimental results by including kinetic information at particle level independently measured in a thermogravimetric apparatus. The results indicate that the hydration reaction is mainly controlled by the slow kinetics of the CaO material tested while significant emulsion-bubble mass-transfer resistances were identified during dehydration due to the much faster dehydration kinetics.

Novel thermochemical energy storage systems that employ fluidized beds of CaO/Ca(OH) 2 for hydration/dehydration reactions are under development because of the inherent advantages of the low cost of the materials and their relatively high temperature operation windows (450ºC-550ºC). We report in this work the results of the first steady state experiments conducted in a new pilot plant designed to test the concept under realistic reactor conditions. The pilot has a fluidized bed reactor with an internal diameter of 0.108 m and a height of 780 mm fed continuously with gas and solids as well as heat exchangers to supply/extract the required reaction heat. The experimental results during dynamic and steady state periods were fitted to a KL reactor bubbling bed model, using kinetic parameters from thermogravimetric studies and a single crossflow factor. The resulting continuous reactor model will serve as useful tool for the continued scaling up of this technology.

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A characterization study has been performed of the French regional resources that may be used in Biomass-to-Liquid plants based on gasification in entrained-flow reactor. It is based on about 90 representative samples of wood chips from forestry, Short Rotation Coppice (SRC) and Short Rotation Forestry (SRF) and of agricultural biomass, including straws and energy crops. Results show that there is not much variability in properties inside the different families. The majority of properties do not seem to be problematical for the process. However, some properties may be questionable: (i) wood chips size distribution, with many small particles ( below 2 mm) and very large particles (above 70 mm), (ii) bulk density, which is very low in agricultural products (about 100 kg.m-3), (iii) sulphur, fluorine and chlorine contents, which are high, especially chlorine in agricultural straws (1000-8000 mg/kg) (iv) other impurities amounts, such as Ni and B, which are in relatively high amounts in some samples (v) ash content, which is high in SRC/SRF (3 wmf%) and very high in most agricultural raw materials (5 wmf%). Based on these statements, first conclusions on the suitability between feedstock and process may be drawn. Wood chips from forestry appear as the most suitable resource for the process and should be the first resource to be used in industrial plants. SRC/SRF also seem quite suitable for the process and may be seen as a short-term alternative. Agricultural raw materials seem to be more problematical and may be seen as a mid-term option.

A Phase Change Material (PCM) thermal energy storage module has been built and tested successfully at CEA on the LHASSA experimental facility. The test campaign aimed at validating the thermo-hydraulic behavior of the storage module under operating conditions similar to those of commercial Direct Steam Generation CSP plants. The measured performances have been compared to the simulation results given by a dynamic model developed at CEA. Tests gave very satisfactory results, with a measured storage capacity meeting the specifications and a very small degradation of temperature and pressure levels in discharge mode. The storage module tests allowed to validate the modeling approach chosen to assess system performances and dynamics. The simulation model developed within the Dymola platform was proven to accurately reproduce the tests results and therefore can be effectively used for performance predictions and for the definition of operating strategies of commercial CSP plants.

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