Robert Palumbo

Solar-Processed Metals as Clean Energy Carriers and Water-Splitters

Abstract Two-step solar thermochemical cycles and processes for the production of hydrogen, hydrocarbons, and synthesis gas are considered. The first step is based on the thermal, electrothermal, or carbothermal reduction of metal oxides, producing metals, metal nitrides, metal carbides, or lower-valence metal oxides. These are hightemperature highly endothermic reactions that can be driven by concentrated solar energy, reducing the consumption of fossil fuels and their concomitant emissions. The second step involves hydrolysis reactions. The thermodynamics of both reaction steps are examined and relevant experimental studies conducted using solar energy are reviewed.

DESIGN ASPECTS OF SOLAR THERMOCHEMICAL ENGINEERING—A CASE STUDY: TWO-STEP WATER-SPLITTING CYCLE USING THE Fe3O4/FeO REDOX SYSTEM

We describe a methodology for the initial development of solar thermochemical reactors for converting concentrated solar energy into chemical fuels. It consists of determining the implications that the thermodynamics and kinetics of the chemical transformation have on the initial reactor design. The method is applied for a specific case study: the decomposition of iron oxide above 1875 K, as part of a two-step thermochemical cycle for producing hydrogen from water. We demonstrate that the chemistry of the reaction places important constraints on various engineering design aspects, and we present two reactor concepts that satisfy these constraints. This study addresses the initial steps necessary for the design and development of solar chemical reactors.

The production of zinc by thermal dissociation of zinc oxide—solar chemical reactor design

We describe the design, fabrication, and preliminary test of a novel solar chemical reactor for conducting the thermal dissociation of ZnO into zinc and oxygen at above 2000 K. The reactor configuration features a windowed rotating cavity-receiver lined with ZnO particles that are held by centrifugal force. With this arrangement, ZnO is directly exposed to high-flux solar irradiation and serves simultaneously the functions of radiant absorber, thermal insulator, and chemical reactant. The reactor design respects the constraints imposed by both the chemistry of the decomposition reaction and the transitory nature of solar energy. A 10 kW prototype reactor, made from conventional reliable materials, was tested at PSI’s high-flux solar furnace and exposed to peak solar radiation fluxes exceeding 3500 kW m−2. The reactor system proved to have low thermal inertia and resistance to thermal shocks.

Design and experimental investigation of a horizontal rotary reactor for the solar thermal production of lime

We designed and tested a 10-kW solar rotary kiln reactor to effect the calcination reaction: CaCO3 → CaO+CO2. The reactor processes 1–5 mm limestone particles, producing 95% or higher purity lime with a t60 reactivity ranging from 14 s to 38 min. The degree of calcination and the reactivity both depend on the reactant’s decomposition temperature (1323–1423 K), residence time (3–7 min), and feed rate (10–50 g/min). The reactor’s efficiency, defined as the enthalpy of the calcination reaction at a specified temperature divided by the solar energy input, reached 20% for solar flux inputs of about 1200 kW m−2 and for quicklime production rates of about 1.3 kg/h. The solar lime reactor operated reliably for more than 100 h for a total of 24 sunny days, withstanding the thermal shocks that occur in solar applications.

Solar thermal decomposition kinetics of ZnO in the temperature range 1950-2400 K

A concentrating solar furnace was used to study the thermal decomposition of ZnO at a nominal nitrogen pressure of within the temperature range of 1950–. Flash assisted multi-wavelength pyrometry was used to establish both the hemispherical emissivity of the ZnO and its irradiated surface temperature for a given solar flux. We found that the decomposition rate is described well by the equation, . The uncertainty in the equation depends on temperature, but for temperatures near it is ±70% at a 95% confidence interval. The emissivity is 0.9 for temperatures above . Furthermore, a one-dimensional unsteady and steady-state heat transfer model that includes the physical processes radiation, conduction, and chemical decomposition was developed using the above expression for the reaction rate. The model predicts the measured steady-state ZnO surface temperatures and the time to reach steady state. For an average solar flux of and after of transient conditions, the calculated temperature profiles within the ZnO solid as a function of time are within of measured profiles.

Reflections on the design of solar thermal chemical reactors: thoughts in transformation

We illustrate a process for designing solar thermal chemical reactors for industrial applications. The process is iterative and involves developing a numerical model of the reactor that links the radiation heat transfer to the other modes of heat transfer and the kinetics of the chemical reaction. Reactors that effectively convert solar energy to chemical energy match well the solar flux entering the reactor to the rate of the reaction being effected in the reactor. The design parameters controlling this match include the reactor’s geometry, the reactant feed condition, and the form of the reactants.

Thermodynamic analysis of the co-production of zinc and synthesis gas using solar process heat

We present a solar thermochemical process that combines the reduction of zinc oxide with the reforming of natural gas (NG) for the co-production of zinc and syngas. The overall reaction may be represented by ZnO+CH4 = Zn+2H2+CO. The maximum possible overall efficiency is assessed for an ideal, closed cyclic system that recycles all materials and also for a more technically-feasible open system that allows for material flow into and out of the system. Assuming that the equilibrium chemical composition is obtained in a blackbody solar reactor operated at 1250 K, 1 atm, and with a solar power-flux concentration of 2000, closed-cycle efficiencies vary between 40 and 65%, depending on recovery of the product sensible heat. Under the same baseline conditions, open-cycle efficiencies vary between 36 and 50%, depending on whether a Zn/O2 or an H2/O2 fuel cell is employed. Compared to the HHV of methane for generating electricity, the proposed solar open-cycle process releases half as much CO2 to the atmosphere. The process modelling described in this paper establishes a base for evaluating and comparing different solar thermochemical processes.

The development of a solar chemical reactor for the direct thermal dissociation of zinc oxide

A solar chemical reactor was designed, constructed and tested for the direct thermal decomposition of zinc oxide at temperatures as high as 2250 K using concentrated sunlight. Along with the reactor, a 1-dimensional numerical model was developed to predict the reactors thermal performance under various solar flux levels and to identify the physio-chemical properties of ZnO that are critical for designing the reactor. An experimental study was also conducted to ascertain how best to employ a curtain of inert gas to keep the reactors window clean of Zn and ZnO. The reactor proved to be a reliable research tool for effecting the decomposition reaction and it possesses many features characteristic of a reactor scale-able to an industrial level: it is resilient to thermal shock; it has a low effective thermal inertia, and it can operate in a continuous mode when ZnO as a powder is fed to the reactor. Furthermore, experimental work led to insight on how best to keep the window clean in the course of an experiment. Also, comparisons between output from the numerical model and experimental results show that the solar flux and the ZnOs thermal conductivity and emissivity are the most critical variables affecting the exergy efficiency of the reactor and the mass flux of product gases. The comparison further reveals the need to investigate whether or not the magnitude of the published pre-exponential term in the decomposition rate equation used in the numerical model should be reduced for improving agreement between the model and the experimental results.

High Temperature Solar Electrothermal Processing-III. Zinc from Zinc Oxide at 1200-1675K Using a Non-Consumable Anode.

The electrolytic decomposition potential of ZnO has been studied in a solar furnace in the temperature range 1200–1675 K. The electrolyte consisted of various mixtures of CaF2 and Na3AlF6. The measured potentials were close to the thermodynamically predicted values for the reaction ZnO(s) → Zn(g) + 0.502(g). The zero current overvoltages, surprisingly, increased with increasing temperature and the concurrent change in composition. The specific conductances of the electrolytes were estimated in the temperature range 1200–1500 K. They increased with increasing temperature and the concurrent change in composition. Various materials were tested for use as electrodes and crucibles. Some of our experiences and our experimental techniques are described.

A two-cavity reactor for solar chemical processes: heat transfer model and application to carbothermic reduction of ZnO

A 5 kW two-cavity beam down reactor for the solar thermal decomposition of ZnO with solid carbon has been developed and tested in a solar furnace. Initial exploratory experiments show that it operates with a solar to chemical energy conversion efficiency of about 15% when the solar flux entering the reactor is 1300 kW/m2, resulting in a reaction chamber temperature of about 1500 K. The solid products have a purity of nearly 100% Zn. Furthermore, the reactor has been described by a numerical model that combines radiant and conduction heat transfer with the decomposition kinetics of the ZnO–carbon reaction. The model is based on the radiosity exchange method. For a given solar input, the model estimates cavity temperatures, Zn production ra4tes, and the solar to chemical energy conversion efficiency. The model currently makes use of two parameters which are determined from the experimental results: conduction heat transfer through the reactor walls enters the model as a lumped term that reflects the conduction loss during the experiments, and the rate of the chemical reaction includes an experimentally determined term that reflects the effective amount of ZnO and CO participating in the reactor. The model output matches well the experimentally determined cavity temperatures. It suggests that reactors built with this two-cavity concept already on this small scale can reach efficiencies exceeding 25%, if operated with a higher solar flux or if one can reduce conduction heat losses through better insulation and if one can maintain or improve the effective amount of ZnO and CO that participates in the reaction.