Irina Vishnevetsky

Closed loop control of heliostats

Tracking control in current heliostats is performed with an open loop, without any verification that the radiation is actually arriving at the desired target. Errors due to open-loop tracking control are often around 1–2 mrad and can accumulate during operation. A significant reduction of tracking error by closing the control loop is presented. The method includes a dynamic measurement of the actual radiation incident around the receiver’s aperture (spillage), detection of aiming errors, and feedback of a correction signal to the tracking algorithm. The measurement does not interfere with the receiver operation. The detection method can distinguish among different heliostats in the field, producing individual corrections to each heliostat. The closed loop control system was developed and successfully operated at the Weizmann Institute heliostat field. Both large errors and gradual drift errors were detected and corrected automatically. Resolution of the closed loop detection algorithm can reach 0.1 mrad, which is insignificant in the overall heliostat beam quality.

Vacuum Carbothermic Reduction of Alumina

The current industrial production of aluminum from alumina is based on the electrochemical Hall-Héroult process, which has the drawbacks of high-greenhouse gas emissions, reaching up to 0.70 kg CO2-equiv/kg Al, and large energy consumption, about 0.055 GJ/kg Al. An alternative process is the carbothermic reduction of alumina. Thermodynamic equilibrium calculations and experiments by induction furnace heating indicated that this reaction could be achieved under atmospheric pressure only above 2200°C. Lower required reaction temperatures can be achieved by alumina reduction under vacuum. This was experimentally demonstrated under simulated concentrated solar illumination and by induction furnace heating. By decreasing the CO partial pressure from 3.5 mbar to 0.2 mbar, the temperature required for almost complete reactant consumption could be decreased from 1800°C to 1550°C. Deposits condensed on the relatively cold reactor walls contained up to 71 wt% of Al. Almost pure aluminum was observed as Al drops, while a gray powder contained 60–80% Al and a yellow-orange powder contained only Al4C3, Al-oxycarbides and Al2O3.

Boron Hydrolysis at Moderate Temperatures: First Step to Solar Fuel Cycle for Transportation

Boron hydrolysis reaction can be used for onboard production of hydrogen. Boron is a promising candidate because of its low molecular weight and relatively high valence. The oxide product from this process can be reduced and the boron can be recovered using known technologies, e.g., chemically with magnesium or via electrolysis. In both routes solar energy can play a major role. In the case of magnesium, an intermediate product, magnesium oxide, is formed, and its reduction back to magnesium can exploit solar energy. The boron hydrolysis process at moderate reactor temperature up to 650°C, potentially suitable for use in vehicles, has not been sufficiently studied so far. This paper addresses the operational requirements using an experimental setup for investigating the hydrolysis reaction of metal powders exposed to steam containing atmosphere. The output hydrogen is measured as a function of temperature in reaction zone, steam partial pressure, and the different steam to metal ratio. Test results obtained during the hydrolysis of amorphous boron powder in batch experiments (with 0.1―2 g of boron, water mass flow rate of 0.1―1 g/min, carrier gas flow rate of 100 cm 3 /min at total atmospheric pressure with steam partial pressure of 0.55―0.95 bar abs) indicate that the reaction occurs in two different stages, depending on the temperature. A slow reaction starts at about 300°C and hydrogen output increases with reactor temperature and steam partial pressure. The fast stage starts as the reactor temperature approaches 500°C. At this temperature, the reaction develops vigorously due to higher reaction rate and its strong exothermic nature. The fast stage is self-restrained when 50―60% of the loaded boron is reacted and 1.5—1.8 SPT L H 2 per 1 g of boron is produced. Raising the temperature before the steam flow starts during the preheating period above 500°C increases the hydrogen yield at the fast stage. Then, the reaction continues for a long time at slow rate until the hydrogen release is terminated. The duration of the fast step decreases sharply with the increase of the steam to boron ratio.

Tin as a Possible Candidate for Solar Thermochemical Redox Process for Hydrogen Production

The feasibility to produce hydrogen in the Sn-H2 O/SnO2 -C thermochemical water splitting redox process depends mainly on the efficiency of the tin hydrolysis step which has not been studied adequately so far, whereas the carboreduction step was investigated because of the industrial production of tin from its mineral casseterite using charcoal or anthracite as reducing agents. The present work deals with the hydrolysis process of different kinds of tin powders at different experimental conditions at moderate temperature range of 180–620°C. In spite of the fact that the rate of hydrogen production is lower compared to other metals e.g. zinc, at the same reactor temperature, high conversion and hydrogen yield were obtained in a controllable reaction. Consequently, tin can be a promising candidate considering the advantage of significant lower temperatures required for the solar carboreduction of its oxide.Copyright

The SnO2/Sn Carbothermic Cycle for Splitting Water and Production of Hydrogen

The carboreduction in SnO 2 to produce Sn and its hydrolysis with steam to generate hydrogen were studied. The SnO 2 /ClSn system has several advantages compared with the most advanced cycle considered so far, which is the ZnO/C/Zn system. The most significant one is the lower reduction temperatures (850―900°C for the Sn0 2 versus 1100―1150 ° C for the ZnO). The rate of carbothermal reduction was studied experimentally. SnO 2 powder (300 mesh, 99.9% purity) was reduced with beech charcoal and graphite using a thermogravimetric analysis apparatus and fixed bed flow reactor at a temperature range of 800―1000°C. Optimal temperature range for the reduction with beech charcoal is 875―900°C. The reaction time needed to reach conversion of Sn0 2 close to 100% is 5―10 min in this temperature range. The transmission electron microscopy results show that after cooling, the product of carboreduction contains mainly metallic Sn with a particle size of 1―3 μm. The hydrolysis step is crucial to the success of the entire cycle. Reactions between the steam and solid tin having as powder structure similar to the reduced one were performed at a temperature range of 350―600°C. Results of both the reduction and hydrolysis reactions are presented in addition to thermodynamic analysis of this cycle.

Continuous Tracking of Heliostats

Tracking motions in current heliostats are usually performed in discrete steps, even though the motion of the sun is continuous. Aiming errors due to the discrete steps are often about 1 mrad or more. A significant reduction of tracking error by smooth continuous tracking is presented. The implementation uses an electronic speed control unit to modify the rotational speed of the two AC motors on an existing heliostat. The continuous tracking system was implemented and successfully operated at the Weizmann Institute heliostat field. Measurements of heliostat motion show that aiming error due to tracking intervals was practically eliminated. A comparison of heliostat motions and flux on the target in step-tracking and continuous tracking modes is reported.

Simulation of Thermal and Chemical Processes in Annular Layer of ZnO–C Mixtures

A special setup, electrically heated, enabling the simulation of the process conditions encountered in a solar chemical reactor, is described. The setup allows us to study the thermal and chemical processes in different solid (powder or granules) reactant layers from the beginning of the heating until the reaction is completed, in a heating condition typical for indirectly, externally heated solar reactors. The particular case of the ZnO carboreduction process is analyzed in this paper as an example. Tests were executed using different powder mixtures of ZnO-C to demonstrate the layer-wise nature of the process. The results show that the reactivity and the behavior of mixtures strongly depend on their components structures, impurities, and stoichiometry. This method can be generally applied for studying endothermic chemical reactions involving other solid reactants.

Preventing Conglomeration of Reduced Fine Powder in Solar Thermochemical Redox Cycles Based on Metals With Low Melting and High Boiling Points

This paper describes a new method that enables avoiding conglomeration of small metal drops after reduction from their oxides. These solidified metal micro drops can be hydrolyzed with steam to generate hydrogen and close a solar redox cycle. Metals with low melting points and low vapor pressure at the reduction temperature do not evaporate and their drops tend to agglomerate. This phenomenon severely reduces the yield of the hydrogen production in the hydrolysis reaction. To avoid this agglomeration the mixture of metal oxide and carbon is blended with an additional small amount of inert ceramic powder that does not take a part in either reaction. This inert powder minimizes the amount of the reduction agent necessary for high conversion of the oxide and allows producing micron and submicron metal particles which are suitable for the hydrolysis step when performed in a fixed bed flow reactor. Experimental results with tin dioxide powder as a representative material are presented. Tests were done with different proportions of tin dioxide powder, charcoal and alumina powder as agglomeration retardant. Product morphology as a function of the components content was analyzed by TEM and SEM. Conversion was controlled by measuring of weight losses, amount of oxygen in output gases and X-ray Diffraction Quantitative Analysis.Copyright

Simulation of Thermal and Chemical Processes in Annular Layer of ZnO-C Mixtures

A special setup, electrically heated, is described, enabling the simulation of the process conditions encountered in a solar chemical reactor. The setup allows studying of the thermal and chemical processes in different solid (powder or granules) reactant layers from the beginning of the heating until the reaction is completed, in heating condition typical for indirectly, externally heated solar reactors. Tests were executed with ZnO carboreduction process, using different powder mixtures of ZnO-C to demonstrate the layer-wise character of the reaction. The results show that the reactivity and the behavior of mixtures strongly depend on their components structures, impurities and stoichiometry. This method can be generally applied for studying endothermic chemical reactions involving other solid reactants.Copyright

Solar-pumped dimmer gas lasers

Many attempts were made in the past to convert Solar light to Laser light. To date, only two systems were demonstrated successfully: Photo-Dissociation Lasers and Solid State Solar lasers. The absorption spectrum of many dimmer molecules posses a broad structural spectrum overlapping with the solar spectrum and can be good candidates for direct solar pumping. In the gas phase, the emission spectrum of the active medium offers tunability and high beam quality without significant thermal lensing or thermal induced birefringence. Optical characterization and initial pumping experiments of a few selected systems will be discussed.