Michael Epstein

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.

The kinetics of hydrogen production in the oxidation of liquid zinc with water vapor

Thermochemical cycles where metal oxides are reduced and the elementary metal is reoxidized with water, thus generating hydrogen has been proposed for conversion of solar energy to chemical energy and fuels. The two steps of the cycle can be separated in time and place, thus providing also means for storage of solar energy in chemical form. The second step of these cycles involves the oxidation of the elementary metal in water vapor where hydrogen is generated and the metal oxide is recovered and recycled. Reduction of zinc oxide is a typical reaction used for such cycles. Therefore, the oxidation of liquid zinc with water, specifically, becomes an important part of such a cycle. The kinetics of oxidation of bulk of liquid zinc at 450–500°C, with water vapors bubbled through the liquid, was studied. The water vapor was introduced into the liquid using argon as a carrier gas. The water vapor partial pressure was in the range of 65–560 mb. The gas mixture was fed through a tube dipped in the liquid. The diameter of the tube and its depth were varied as parameters of the experiments. The results of the experiments show that the specific reaction rate (hydrogen production per cm2 of surface area of the bubble), Wsp, increases, as the water partial pressure is increased. The order of the reaction, with respect to water partial pressure, is 0 < n < 1. As a result of the experiments, the kinetic expression Wsp=kPH2O/(1+ b·PH2O) was found: k = 1.86×10−3·EXP(−40376/RT) mole·cm−2·s−1bar−1, b = 1.55×10−10·EXP(146330/RT) bar−1. The main stages of the reaction are interaction between elementary zinc and water vapor on the interface between the gas and the solid oxide layer, and the diffusion of zinc atoms through the film of solid zinc oxide.

Solar “tower reflector” systems: A new approach for high-temperature solar plants

Abstract During the last few years, considerable research efforts have been directed at the Weizmann Institute towards development of high-concentration, high-temperature solar energy systems. This included optical methods and devices, thermal receivers for solar thermal electricity generation, and thermo-chemical processes for solar energy storage and solar fuel production. Some of these efforts are now mature enough for transfer to industry, and programs are starting to affect the transfer and upscale the new technologies to commercial levels. Feasibility studies carried out during 1995 in cooperation with industry have shown the advantage of the new high-concentration system approach. The costs of high-quality solar energy are attractive, even before application of government subsidies:

Solar Upgrading of Fuels for Generation of Electricity

800/kWth for high-temperature process heat applications, and

The optics of the solar tower reflector

2500/kWe for solar/hybrid power plants.

The solar thermal decarbonization of natural gas

This paper presents a novel process comprising solar upgrading of hydrocarbons by steam reforming in solar specific receiver-reactors and utilizing the upgraded, hydrogenrich fuel in high efficiency conversion systems, such as gas turbines or fuel cells. In comparison to conventionally heated processes about 30% of fuel can be saved with respect to the same specific output. Such processes can be used in small scale as a stand-alone system for off-grid markets as well as in large scale to be operated in connection with conventional combined-cycle plants. The complete reforming process will be demonstrated in the SOLASYS project, supported by the European Commission in the JOULE/THERMIE framework. The project has been started in June 1998. The SOLASYS plant is designed for 300 kW el output, it consists of the solar field, the solar reformer and a gas turbine, adjusted to operate with the reformed gas. The SOLASYS plant will be operated at the experimental solar test facility of the Weizmann Institute of Science in Israel. Start-up of the pilot plant is scheduled in April 2001. The midterm goal is to replace fossil fuels by renewable or non-conventional feedstock in order to increase the share of renewable energy and to establish processes with only minor or no CO 2 emission. Examples might be upgrading of bio-gas from municipal solid waste as well as upgrading of weak gas resources.

Ambiguities in the interpretation of proton magnetic relaxation data in water solutions of Gd3+ ions

Abstract The concept of the reflective solar tower is based on inverting the path of the solar rays originating from a heliostat field to a solar receiver that can be placed on the ground. This system is based on the property of a reflective quadric surface to reflect each ray oriented to one of its foci to its second focus. Two shapes of surfaces can be used: one is concave — the ellipsoid, and the other is convex — the hyperboloid (with two sheets). This study analyzes the achievable optical performances of these types of reflectors and proves that the hyperboloidal surface is a superior reflector. The optics of the hyperboloid are analyzed in detail.

Solar Gasification of Biomass: A Molten Salt Pyrolysis Study

The endothermic decomposition of natural gas into a carbon-rich condensed phase and a hydrogen-rich gas phase, using concentrated solar energy as the source of high-temperature process heat, is considered as a model reaction for conducting a 2nd-law analysis of a solar decarbonization process in which carbon is removed from fossil fuels prior to their use for power generation. The theoretical maximum closed-cycle exergy efficiency, defined as the ratio of the Gibbs free energy change of the reaction to the solar power input, can be as high as 35% for a black-body solar cavity-receiver/reactor operating at 1500K and under a mean solar flux concentration ratio of 1000, and decreases to 21% if the products exiting the solar reactor are quenched without recovering their sensible heat. Four technically viable routes are examined for extracting power from the chemical products of the solar decomposition of CH4: (1) carbon is sequestered and only H2 is used in a fuel cell; (2) carbon is used to fuel a conventional Rankine cycle and H2 is used in a fuel cell; (3) carbon is steam-gasified to syngas in a solar gasification process and the syngas further processed to H2, which, together with H2 from the CH4-decomposition reaction, is used in a fuel cell; and (4) carbon serves as a reducing agent of ZnO in a solar carbothermic process for producing Zn and CO that are further converted via water-splitting and water-shifting reactions to H2 for use in a fuel cell. The open-cycle energy efficiency, defined as the ratio of electric power output to the thermal energy input (solar + heating value of reactants), exceeds 65% for the 3rd and 4th power generation routes. Both of these routes offer a net gain of 40% in the electrical output and, consequently, an equal percent reduction in the corresponding specific CO2 emissions, vis-a-vis the direct use of CH4 for fueling a 55%-efficient combined Brayton–Rankine cycle. For route nr. 1, the energy penalty for avoiding CO2 emissions amounts to 30% of the electrical output. Nomenclature C mean solar flux concentration ratio (dimensionless) EGF ratio of the electric output of the process to that obtained when using same amount of CH4 to fuel a 55%-efficient combined Brayton–Rankine cycle HHV high heating value (kJmol−1) I normal beam insolation (kWm−2) Irrheatexchanger irreversibility associated with heat exchanger (kWK−1) Irrquench irreversibility associated with quenching (kWK−1) Irrreactor irreversibility associated with solar reactor (kWK−1) LHV low heating value (kJmol−1) ṅ molar flow-rate (mols−1) p pressure (bar) Qheatexchanger heat transferred by heat exchanger (kW) QFC heat rejected to the surroundings by fuel cell (kW) QHE heat rejected to the surroundings by heat engine (kW) Qquench heat rejected to the surroundings by quenching (kW) Qreactor,net net power absorbed by solar reactor (kW) Qreradiation power re-radiated through reactors aperture (kW) Qsolar solar power input through reactors aperture (kW) WFC work output by fuel cell (kW) WHE work output by heat engine (kW) ΔG Gibbs free energy change (kJmol−1) ΔH enthalpy change (kJmol−1) ΔS entropy change (kJmol−1K−1) ηabsorption solar energy absorption efficiency ηCarnot efficiency of a Carnot heat engine operating between TH and TL ηheatexchanger heat recovery factor by heat exchanger ηHE efficiency of heat engine ηexergy,max maximum exergy efficiency of an ideal system ηexergy,Closed−Cycle exergy efficiency of closed-cycle system ηexergy,Open−Cycle exergy efficiency of open-cycle system σ Stefan–Boltzmann constant (5.6705×10−8Wm−2K−4)

Towards the Industrial Solar Carbothermal Production of Zinc

We report measurements of the solvent proton spin–lattice relaxation rate 1/T1 in a water solution of GdCl3, pH 5.5, at 5, 15, and 25 °C. The data were obtained as a function of magnetic field, from 5 Oe to 12 kOe, corresponding to proton Larmor frequencies in the range 20 kHz to 50 MHz. The data are compared with theory, and it is shown that a satisfactory fit may be obtained for a range of values of the parameters of the theory. We give the results of a least squares fit of the theory to the data for two extremes of the range, one limit corresponding to the condition that the residence lifetime τM of a proton in the first hydration shell of the Gd3+ aquoion be comparable to the spin–lattice relaxation time T1M of the proton on the ion, the other limit corresponding to τM≪T1M. It is shown that knowledge of the temperature dependence of 1/T1 cannot distinguish between the two limits, and indeed, that unless a value for τM is obtained from independent experiments, the ambiguous results cannot be resolved. ...

Experimental study of solar reactors for carboreduction of zinc oxide

A novel solar process and reactor for thermochemical conversion of biomass to synthesisgas is described. The concept is based on dispersion of biomass particles in a molteninorganic salt medium and, simultaneously, absorbing, storing and transferring solarenergy needed to perform pyrolysis reactions in the high-temperature liquid phase. Alab-scale reactor filled with carbonates of potassium and sodium was set up to study thekinetics of fast pyrolysis and the characteristics of transient heat transfer for celluloseparticles (few millimeters size) introduced into the molten salt medium. The operatingconditions were reaction temperatures of 1073–1188 K and a particle peak-heating rateof 100 K/sec. The assessments performed for a commercial-scale solar reactor demon-strate that pyrolysis of biomass particles dispersed in a molten salt phase could be afeasible option for the continuous, round-the-clock production of syngas, using solarenergy only. @DOI: 10.1115/1.1753577#Keywords: Biomass, Pyrolysis, Molten salt, Kinetics, Solar reactor, Thermal storage