Chemical storage of concentrated solar power

Solar energy is one of the most important sources among renewables to reach the global energy requirements without the concomitant production of greenhouses gases. Concentrating Solar Power (CSP) systems have been recognized as a promising technology thanks to the easy integration with Thermal Energy Storage (TES) devices, which allow to overcome the intrinsic unsteady nature of the solar energy. Gas–solid Fluidized Bed (FB) systems can play a key role when employed as solar receiver/reactor: the high thermal conductivity and diffusivity in such reactors can help in absorbing the concentrated solar energy ensuring low heat losses and in equalizing the heat received avoiding thermo mechanical stresses to both reactor walls and reactive materials. The work performed in this PhD Thesis was devoted to the investigation of gas–solid FBs as solar receiver/reactor in CSP systems. In particular, in the present PhD Thesis an integration between solar energy and conventional fossil fuels sources has been proposed. An integrated scheme of a continuous Calcium Looping (CaL) process for CO2 capture from combustion flue gases of a power plant, based on FB technology and sustained by a CSP, was developed. Storage of the excess incident solar power during the daytime as calcined sorbent, which is eventually utilized in the CaL loop during the nighttime, was proposed to overcome the inherently unsteadiness of CSP systems. The potentialities of a similar integrated scheme were assessed by means of model computations to estimate the main features, performance, storage and energy requirements of the process. Model computations of a base case suggested that in order to couple a CSP with a 100 MWth combustion plant, the use of two storage vessels of nearly 2000 m3 is required, together with the building of an heliostat field of 0.26 sqkm. The heat recovery in the carbonator is of crucial importance and allows to largely increase the overall thermal throughput of the power plant. It was also highlighted that larger inlet Ca/C molar ratio could result into a higher CO2 capture efficiency but at the expense of a larger energy requirement for the heliostat field. In order to study the practical feasibility of the process, directly irradiated FB systems were considered, as the direct heating configuration permits to obtain the high operating temperatures required to perform the calcination reaction. Initial studies were targeted at the comprehension of the heat transfer phenomena in directly irradiated FBs, with the aim of establishing the efficiency of the solar FB receiver. Experimental investigations were performed by mapping through a thermal infrared camera the surface of a fluidized bed subjected to a concentrated solar simulated radiation. Experimental results suggested that collection of incident radiation changes from a surface-receiver to a volumetric receiver paradigm as the gas superficial velocity is increased from fixed to bubbling fluidized bed conditions: this also results in a significant reduction of peak temperature and temperature fluctuations at the focal point of the receiver. A strong reduction of bed surface temperature has been obtained by exploring an uneven fluidization strategy and, in turn, by tailoring the hydrodynamics of the bed through injecting a chains of bubbles from a nozzle located in the proximity of the radiation focal point. A compartmental heat transfer model which provides a simple, though accurate, equation for predicting in-bed dispersion of radiative power and extent of temperature non-uniformity was also developed. Finally, a directly irradiated gas–solid FB reactor suitable for the investigation of thermochemical heat storage processes has been designed and operated to study the solar CaL process. Several calcination carbonation reactions were performed on an Italian Ca based sorbent (Massicci), and its CO2 capture capacity was evaluated over repeated cycling. The operation of the directly irradiated FB reactor resulted into a continuous fluctuation of the bed surface temperature, with a mean over temperature value of over 100 °C. The CO2 capture capacity of the investigated limestone showed a first initial decreasing trend followed by a stabilization at lower values, with a residual CO2 capture capacity of about 6%. According to the obtained results, the solar CaL process appears to be a practicable way toward the integration of conventional fossil fuels and energy sources, with the aim of both taking advantage of the solar energy and reducing CO2 emissions