Abraham Kogan

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:

Direct solar thermal splitting of water and on-site separation of the products—II. Experimental feasibility study

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

Production of hydrogen and carbon by solar thermal methane splitting. I. The unseeded reactor

2500/kWe for solar/hybrid power plants.

The Tornado Flow Configuration—An Effective Method for Screening of a Solar Reactor Window

Abstract The development of a process of hydrogen production by solar thermal water splitting (HSTWS) presents a formidable technological task. The process has, however, great potential from the thermodynamic point of view and, when combined with fuel cell technology, it can lead to efficient conversion of solar energy to power. In the process under development at the Weizmann Institute of Science, water vapor is partially dissociated in a solar reactor at temperatures approaching 2500 K. Hydrogen is separated from the hot mixture of water splitting products by gas diffusion through a porous ceramic membrane. The paper describes the problems encountered during the development of the HSTWS process. The following topics are discussed in some detail: (a) achievement of very high solar hydrogen reactor temperatures by secondary concentration of solar energy; (b) materials problems encountered in the manufacture of the solar reactor; (c) development of special porous ceramic membranes that resist clogging by sintering at very high temperatures.

Direct solar thermal splitting of water and on-site separation of the products. III.: Improvement of reactor efficiency by steam entrainment

Abstract Solar thermal methane splitting was performed in a series of tests with an unseeded low capacity reactor. Effective screening of the reactor window from contact with carbon particles was achieved by application of the tornado flow configuration (J Solar Energy Eng 124 (2002) 206). The tests were performed at atmospheric pressure and at temperatures up to 1320 K . An extent of reaction of 28% was attained. Most of the carbon generated in the process clang to the irradiated reactor wall and it formed a very hard deposit. In most cases, the tests were terminates when the reactor exit port became choked by the accrued carbon deposit. The results of the tests are discussed and ways to correct the problems encountered with the unseeded reactor are proposed.

Direct solar thermal splitting of water and on-site separation of the products - IV. Development of porous ceramic membranes for a solar thermal water-splitting reactor

The working fluid in solar receivers, utilized for effecting chemical reactions, is usually flown through a sealed enclosure provided with a quartz window. When one of the reactants or products of reaction is a powder, care must be taken to prevent contact of the incandescent powder particles with the window, in order to obviate its destruction by overheating. Attempts made in the past to screen the window against particle deposition by a curtain of an auxiliary gas stream showed that very substantial flow rates of auxiliary gas (30-80% of the main stream flow rate) were necessary for perfect window screening. The heat absorbed by the auxiliary gas stream represented a major loss of energy. In an effort to reduce the auxiliary stream flow rate to a minimum, a certain flow pattern akin to the natural tornado phenomenon has recently been developed in our laboratory. It enabled effective reactor window screening by an auxiliary gas flow rate less than 5% of the main gas flow rate. The tornado effect is discussed and demonstrated by a smoke flow visualization technique.

Direct solar thermal splitting of water and on site separation of the products I. Theoretical evaluation of hydrogen yield

Abstract Hydrogen can be produced by direct solar thermal water splitting. Steam heated by concentrated solar radiation is partially dissociated. Hydrogen is separated from the hot mixture of water splitting products by gas diffusion through a porous ceramic membrane. The reactor thermodynamic efficiency in this process rises with increasing reaction temperature and with decreasing pressure at the downstream side of the gas-separating membrane. This paper discusses two attempts to raise the reactor efficiency, the first — by heat and mass recovery, using a steam injector in a two-stage reactor — and the other — by lowering reactor exit pressure of the hydrogen-enriched gas stream, using a multistage ejector.

Steam flowrate to solar cavity reactor—a simple measurement and control method

Abstract A crucial element in the solar thermal water-splitting (STWS) reactor is the porous ceramic membrane that enables separation of hydrogen from the hot water-splitting reaction products. Zirconia porous membranes are manufactured by powder sintering at about 1800°C. When such a membrane is exposed in the solar reactor to a higher temperature, it loses its gas permeability due to pore closure by continued sintering. Efforts were made to inhibit the membrane sintering process and to postpone its fast occurrence to higher temperatures, by the use of special stabilized zirconia powders consisting of particles with a rounded shape. The fast sintering of membrane samples made of a homogeneous powder of relatively large spherical particles, prepared by the Sol–Gel method, occurred at some 200°C above the normal zirconia sintering temperature. The overall picture gathered from our experiments suggests, however, that it will be hardly possible to bridge the temperature gap between the normal sintering temperature of stabilized zirconia and the STWS reactor temperature, by the use of stabilized zirconia powders of a particular morphology.

Production of hydrogen and carbon by solar thermal methane splitting. II. Room temperature simulation tests of seeded solar reactor

A process of hydrogen production by direct solar thermal water dissociation and in situ separation of hydrogen from the dissociation products by gas diffusion through a porous membrane is simulated by a theoretical model. The mole fraction of the species of dissociation products which leave the solar reactor in two gas streams are related by a system of 12 non-linear equations. An iterative solution of this system of equations enables calculation of the net hydrogen yield of the process. The variation of hydrogen yield with reactor temperature, with pressures on both sides of the membrane and with the fraction of gas flowrate that diffuses through the membrane is presented and discussed.

Solar decomposition of fossil fuels as an option for sustainability

Abstract A method for measurement and control of gas flowrate is described. It is based on the choked nozzle flow phenomenon, in which gas flowrate is independent of nozzle downstream pressure. Monitoring of superheated steam flowrate to a solar cavity reactor for hydrogen production by thermal dissociation of water is analysed as an application.