Gabriel Olalde

A 300kW Solar Chemical Pilot Plant for the Carbothermic Production of Zinc

In the framework of the EU-project SOLZINC, a 300-kW solar chemical pilot plant for the production of zinc by carbothermic reduction of ZnO was experimentally demonstrated in a beam-down solar tower concentrating facility of Cassegrain optical configuration. The solar chemical reactor, featuring two cavities, of which the upper one is functioning as the solar absorber and the lower one as the reaction chamber containing a ZnO/C packed bed, was batch-operated in the 1300–1500 K range and yielded 50 kg/h of 95%-purity Zn. The measured energy conversion efficiency, i.e., the ratio of the reaction enthalpy change to the solar power input, was 30%. Zinc finds application as a fuel for Zn/air batteries and fuel cells, and can also react with water to form high-purity hydrogen. In either case, the chemical product is ZnO, which in turn is solar-recycled to Zn. The SOLZINC process provides an efficient thermochemical route for the storage and transportation of solar energy in the form of solar fuels.

Towards the Industrial Solar Carbothermal Production of Zinc

Based on the experimental results of a 300 kW solar chemical pilot plant for the production of zinc by carbothermal reduction of ZnO, we performed a conceptual design of a 5 MW demonstration plant and of a 30 MW commercial plant. Zinc can be used as a fuel for zinc-air batteries and fuel cells, or it can be reacted with water to form high-purity hydrogen. In either case, the chemical product is ZnO, which in turn is solar recycled to zinc. The proposed thermochemical process provides an energy efficient route for the conversion, storage, and transportation of solar energy in the form of solar fuels.

Experimental Determination of the Extinction Coefficient for a Packed-Bed Particulate Medium

The extinction coefficient of a powder mixture of ZnO and beech charcoal is determined at ambient temperature using two experimental set-ups: (a) radiation in the range 500–1000 nm is captured by a 200 μm-diameter fiber optic and sent to a spectrometer equipped with a Si/PbSe detector; (b) radiation in the range 350–1100 nm is captured by a 5 mm-diameter Si-photodiode. Both set-ups measure the attenuation of intensity from a 3273 K blackbody source. The experimental results are implemented in the numerical solution of the equation of radiative transfer, using the Monte-Carlo ray-tracing technique. The extinction coefficient is determined to be 10103 ± 615 m−1 at 1000 nm using set up (a), and 7850 ± 337 m−1 for the range 350–1100 nm using the more accurate set-up (b).

TRANSMITTANCE ENHANCEMENT OF PACKED-BED PARTICULATE MEDIA

The optical thickness of highly attenuating packed-bed particulate media can be significantly reduced and, consequently, the radiation heat transfer enhanced, by the addition of large (> 100μm) semi-transparent SiO 2 particles. The monochromatic transmittance of packed-bed mixtures of SiO 2, ZnO, and C particles of various relative mass fractions is experimentally measured as a function of the packed-bed thickness using a He-Ne laser/fiber optic/spectrometer system. Two functions, one derived from the general solution of the equation of radiative transfer for an absorbing-scattering-non emitting medium, and a second one derived from Bouguers law, were fitted to the experimental data and used to elucidate the effect of the incoming scattering and optical thickness on the medium transmittance. The augmenting contribution of the incoming scattering diminishes with increasing content of highly absorbing carbon particles, and, when it becomes negligible, the extinction coefficient is directly determined by applying Bouguers law for attenuation of incident radiation along its path.

Radiative heat transfer modelling in a concentrated solar energy bubbling fluidized bed receiver using the Monte Carlo Method

According to recent studies in the solar energy community, a promising ways of solar energy conversion seems to be the beam down concentration technology associated to a fluidized bed receiver. The advantage of this system is its ability to heat air at temperature reaching 1000 K in a receiver directly exposed to a concentrated solar beam and integrated in a thermodynamic cycle. This paper focus on the modelling of radiative heat transfer from the optical concentrator to the receiver by taking into account absorption and multiple scattering of light in the particles bed. To achieve this objective, we develop in this work an efficient tool based on an algorithm solving the integral formulation of the Radiative Transfer Equation by the Monte Carlo Method. This algorithm is implemented in EDStaR environment where computer graphics libraries, parallel computing and specific functionalities to produce statistical quantities and their associated derivatives are available. One of the main advantage of the proposed radiative transfer modelling is the determination of the sensitivities (derivatives) of the physical quantities to any physical or geometrical parameter without significant additional CPU time.

Solar Carbothermic Production of Zinc From Zinc Oxide: Solzinc

In late 2004, the pilot Solzinc solar reactor was commissioned. The European Union and the Swiss Federal Office of Science and Education are funding this project to demonstrate the technical feasibility and the economical potential of producing Zn by reducing zinc oxide with the aid of concentrated solar energy and a small amount of carbon at a close to industrial scale. The zinc can be used as a means to store solar energy in a chemical way, e.g. suited to release electricity in Zinc-air fuel cells. This allows on demand use, boosting the availability of solar energy. Furthermore, as the Zinc-air fuel cells’ waste is ZnO, we get a cyclic process by reducing this ZnO in the Solzinc solar reactor. Numerous lab tests and numerical studies of the chemical and thermal behavior of the solar carbothermic ZnO reduction process were conducted by the Swiss Paul Scherrer Institute, the Swiss Federal Institute of Technology, the Israeli Weizmann Institute and the French CNRS Processes, Materials and Solar Energy laboratory. An indirectly heated beam-down reactor concept was chosen and influencing parameters, such as the type of carbon, the stoichiometry of the ZnO-C mix and the process temperature were explored. Based on these findings the technology was scaled up for the pilot plant for about 0.25 MW solar input leading to a designed zinc production rate of 50kg/h. The Swedish company ScanArc Plasma Systems AB developed a special quench system to produce zinc dust directly from the gaseous zinc exhausted from the solar reactor. The dust’s characteristics were adapted to the requirements of the Zn-air fuel cells developed by the German company ZOXY Energy System AG. The resulting zinc can be easily stored and transported for generating electricity as needed. In 2004, the pilot reactor, the quench system and extensive instrumentation were installed at the Weizmann Institute’s solar facilities to process batches of up to 500 kg of ZnO-C mixture. After cold testing of the installation and fulfilling all safety requirements, the first batches were processed. This paper explores the results of the commissioning to show the technical feasibility of this process to produce zinc and to store solar energy.Copyright

Design and proof of concept of an innovative very high temperature ceramic solar absorber

Hybrid solar gas-turbine (HSGT) is an attractive technology to foster market penetration of CSP. HSGT offers some major advantages like for example high solar-to-electric conversion efficiency, reduced water requirement and low capital cost. A very high temperature solar receiver is needed when elevated solar share is claimed. A few research works, as reported by Karni et al. [8] and by Buck et al. [1], have been dedicated to solar receiver technologies able to deliver pressurized air at temperature above 750°C. The present work focuses on research aiming at developing an efficient and reliable solar absorber able to provide pressurized air at temperature up to 1000°C and more. A surface absorber technology is selected and a modular design of receiver is proposed in which each absorber module is made of BOOSTEC® SiC ceramic (silicon carbide) as bulk material with straight air channels inside. Early stage experimental works done at CNRS/PROMES on lab-scale absorbers showed that the thermo-mechanical behavi...

Experimental design of a fluidized bed solar receiver with direct absorption of concentrated solar radiation

The aim of this work concerns the study of the silicone carbide fluidized bed in order to release a fluidized bed solar receiver with direct absorption of the concentrated solar radiation and to optimize the quantity of collected energy. For that, we tested several geometries in order to optimise the distribution of the particles during fluidization. For that, we built and tested several transparent columns with different geometries and dimensions and analyse several cold experiments. An analysis of test results and images enabled us to determine the height and the porosity of different layers in the fluidized bed, to optimise the geometry of the column and its dimensions.