Figure 4

Photo-separatory Nozzle reactor

A computer program, called Iontrack, calculates the elevation and azimuth of the sun based on the time of day, and the latitude and longitude of the mirror on the planet's surface. Iontrack reads the actual elevation and azimuth positions of the mirror from the two analog sensors, and determines the movement required for each axis to reduce the error to zero. Iontrack directs the two stepper motors to make the necessary movements to keep the tracking error under 0.10 degrees.

The Photo-separatory Nozzle reactor was constructed from concentric quartz tubes and surrounded by a convection and conduction barrier with radiant energy reflecting properties. See Figure 5.

Figure 5

Reactor Containment Vessel

An aluminum housing protects the quartz tubes, aligns and holds the silicone "O" ring seals in place, and serves as a mounting fixture on the stage of the searchlight concentrator. An optical viewing slit in the reactor containment vessel was provided for studying the transonic flow region, between the dissociator-nozzle and the skimmer.

A quartz window admits the concentrated solar beam to the interior of the reactor. Inside the reactor, the gas feed-ring/electrode is positioned so that the converging solar beam passes through the feed-ring and the solar image "fireball" lands in the center of the trumpet-shaped dissociator-nozzle. See Figure 6.

Figure 6

Solar Beam Entrance Window with Feed-Ring

The feed-ring electrode was made from a loop of stainless steel tubing. The tubing was welded closed at one end and drilled with feed holes 0.508 mm(0.020 inches) diameter chamfered 1 mm (0.040 inches) periodically at 1.27 cm (0.5 inch) intervals along its length. The tubing was then rolled around a mandrel to form the feed-ring electrode. A quartz tubing sleeve was used as a feed-through insulator at the reactor window retainer wall to supply reactant gas and electrical power. The electrical power supply connection to the feed-ring electrode is made outside the reactor on the stainless steel gas feed tube.

It is important to operate the reactor when it is "on-sun" with water vapor flowing to the feed ring. In one experiment water vapor feed was interrupted and the stainless steel feed-ring was evaporated. The feed-ring turns black after operation on-sun for a few hours, but is not damaged by the beam as long as the water vapor feed is continuous.

The dissociator-nozzle is a sonic nozzle with a trumpet-shaped converging entrance section made from a cylindrical quartz tube. A sealing flat flange was fused to the opposite end of the tube. A conductive thin film of platinum was applied to the outside of the quartz cylinder to provide an electrical connection (negative ground) for the dissociator-nozzle. The quartz tubing (General Electric Type 204 clear fused quartz) was coated with an yttria and calcia stabilized zirconium oxide refractory (Aremco Products Inc. Ultratemp 516 high temperature ceramic adhesive, Osining, New York) for the reaction zone and solar beam target.

The sonic quartz dissociator-nozzle is shown in Figure 7.

Figure 7

Sonic Quartz Dissociator-Nozzle with Ceramic Zirconia Coating

After 10 to 20 hours of on-sun operation, samples of a broken zirconia-on-quartz dissociator-nozzle were analyzed by x-ray diffraction. Zircon (zirconium-silicon oxide) micro crystals were detected, indicating a new conductive interface layer had been formed between the quartz tube and zirconia coating. Better thermal-expansion matching of materials seems to be obtained with a zircon layer. A glassy surface finish is produced on the heat-affected portion of the dissociator-nozzle coating after "break-in".

The skimmer nozzle/electrode is a conical nozzle that is mounted on the end of a cylindrical stainless steel tube which fits coaxially inside the cylindrical dissociator-nozzle tube. See Figure 8.

The conical skimmer-nozzle/electrode entrance faces the dissociator-nozzle's exhaust jet. The conical skimmer-nozzle was fabricated from a sharp-edge truncated cone made of quartz tubing and ground to a knife-edge using a diamond abrasive wheel. The quartz skimmer-nozzle was coated with platinum for electrical conductivity and connected (via a feed-through) to an electrical power supply (PS-2 in Figure 3) on the outside of the reactor.

Figure 8

Conical Skimmer Nozzle/Electrode

A scissors jack is used to move the skimmer in and out with respect to the dissociator-nozzle exhaust axis. The scissors jack counteracts the forces on the skimmer caused by atmospheric pressure exerted on one side of the skimmer and the lower process pressure on the inside of the skimmer. An electronic proximity sensor and a dial gauge are used to provide axial position information. The spacing between the dissociator-nozzle and skimmer is of the order of one nozzle diameter (about 1mm.) The skimmer can be moved from about 0.1mm to about 2.0mm downstream of the dissociator-nozzle. Depending on the pressure in the region between the dissociator-nozzle and the skimmer this distance is approximately 1 mean-free-path (between atom/molecular collisions).

An eccentric is used to move the skimmer from right/left and up/down around its pivot point. The pivot point is located about 22 cm downstream from the dissociator-nozzle’s exit orifice. The pivot point is made from 3 dimples inside the dissociator-nozzle tube. Two more proximity detectors and dial gauges provide up/down and right /left skimmer position information.

Sampling valves are located in the two product discharge lines. Each sampling valve can admit product gas into the mass spectrometer. The spectra from each product line are alternately taken; then the mass spectrometer background gas pressure spectrum is subtracted from each. The mass ratios for the light and heavy products are calculated and used to obtain a separation factor.

The mass-flow rates of the water vapor feed and the skimmer (heavy product) and annulus (light product) were measured with Teledyne Hastings-Raydist model ST and model HFM-200H mass flow meters.

Using the three mass-flow meters allowed us to perform a mass balance across the reactor to obtain measurement closure with the flow meters (as a calibration and leak check.)

A Pyro Micro-Optical Pyrometer made by the Pyrometer Instrument Company Inc. (Northvale N.J.) was used to measure the temperature of the dissociator-nozzle surface under different operating conditions. We used an EC-6 high temperature range filter and a 90 degree M-14 prism to view the dissociator-nozzle. Corrections to the indicated temperature were made for the prism, the filter, the range selection, and the reactor window transmission loss for any given temperature.

We normally measure temperatures in the 2300 K to 2800 K range at the nozzle surface, depending on the water vapor feed pressure and the solar beam flux. Stagnation temperatures as high as 3250 K have been measured on the nozzle surface (somewhat higher than the published melting point of zirconia.)

Pumping the products from low pressure to atmospheric pressure or higher requires compression work, which is a parasitic loss in the overall efficiency of the process. The losses for the oxygen-rich stream compressor and the hydrogen-rich stream compressor must be minimized by selecting reactor pressures no lower than necessary, and by the use of efficient compressor designs.

The net power required to pump the process fluids in this reactor was about 18 watts.

To compress the products adiabatically (no cooling) from 1 Torr to one atmosphere with an inlet temperature of 300 K would require about 50 kJ/g-mole (11.9 kcal/g-mole). Isothermal compression (with intercooling) would require only about 1/3 as much compression work.

The optical losses can be reduced through:

• Use of a higher quality solar concentrator mirror.
• Use of a dome shaped solar-beam entrance window instead of a flat window.
• A receiver geometry resembling a cavity.

The pumping losses and glow-discharge losses are, in reality, electrical power requirements, which, if met by photo-voltaic means, can be considered a capital cost.

Experimental solar-to-hydrogen efficiencies to date are shown in Figure 10.

Figure 10

Efficiency of Solar-Hydrogen Processes

The first bar on the left(labeled DTH) is the direct thermal hydrogen conversion efficiency obtained by analyzing the dissociator-nozzle jet flow-field without the use of a glow discharge or skimmer. About 1.1% solar-to-hydrogen (higher heating value) conversion was obtained.

The second bar (labeled DTH Glow) shows an improvement in solar to hydrogen conversion to about 2.1% when a glow discharge was applied and the skimmer and annulus hydrogen production was added together. The maximum and minimum measured values are shown about the mean.

The third bar (labeled PVE1) shows the solar-to-hydrogen efficiency obtained using silicon photovoltaic cells and an alkaline electrolyzer. The fourth bar (labeled PVE2) shows the results of another photovoltaic electrolysis system.

The conversion efficiency for a solar dish Stirling generator combined with an alkaline electrolyzer [75% efficient] is labeled SSE in Figure 10.

The next bar (labeled NREL Goal) shows the long term solar-to-hydrogen efficiency goal established by the National Renewable Energy Laboratory, for reference purposes.

Multi-junction single crystal gallium arsenide solar cells with future 37% solar-to-electrical efficiency combined with a 75% efficient alkaline electrolyzer is labeled PVE Ga As.

The bar (labeled DTH Future) indicates that the future direct thermal hydrogen process may compete with other known methods on an efficiency basis. It may have the further advantage that it does not use expensive single crystal materials.

Micro-nozzle arrays may allow scale-up of the process for a wide area solar reactor receiver. With smaller nozzles themean-free-path dimensions will also be smaller, and higher operating pressures will be allowable in principle. The higher operating pressure will result in a reduction of parasitic compression work required .

The arrays can be arranged in matrices or linearly. We can envision arrays of solar-hydrogen reactors along two parallel pipe lines, running from an area with high insolation, to supply cities. Existing natural gas pipelines could be used to transport the hydrogen and oxygen products separately to their point of use. Alternatively, one pipeline could be used to transport hydrogen to the point of use and the oxygen could be vented.

We have seen that some direct thermal hydrogen production can be obtained in this reactor simply by passing water vapor through a hot ceramic nozzle (1.1%). We have also found that impressing a glow-discharge across the dissociator-nozzle almost doubles the conversion (2.1%). The separation factor for the skimmer has not yet been optimized. More work remains to obtain accurate spatial calibration of the skimmer, and investigate the effect of pressure, flow separation, and glow discharge on the separation efficiency. At this time the amount of recombination which occurs after dissociation is unknown. However, we feel there are very exciting possibilities for improvement of the process.

Further improvements in product gas analysis (mass spectrometer)calibration and skimmer nozzle position calibration are needed to remove uncertainty from the measurements and provide real-time data during "on-sun" experimental runs.

We thank J.D. Healy, E.C. Petersen, and R. Cortez for their assistance in constructing and trouble-shooting the apparatus for these experiments.

Becker, E.W. et al, 1955, "The Separatory Nozzle a New Device for Separation of Gases and Isotopes", Z. Naturforschg, Part A, Vol.10a, pp565-572.

Bockris, J.O.M. et al, 1985, "On the Splitting of Water", International Journal of Hydrogen Energy, Vol. 10 No. 30, pp.179-201

Lede, J., Lapique, F., Villermaux, J., Cales, B., Baumard, J.F., and Anthony, A.M., 1982, "Production of Hydrogen by Direct Thermal Decomposition of Water, Preliminary Investigation", International Journal of Hydrogen Energy, Vol.7, No.12, pp.939-950.

Melik-Asloanova, T.A., Abbosov, A.S., Shilnikov,V.I., 1978, "Dissociation of Water Vapor in Supersonic Flow of Non-equilibrium Plasma", Proceedings World Hydrogen Energy Conference 2, pp.1063-1069.

Pyle, W.R., 1983, "Photo-separatory Nozzle", U.S. Patent No.4,405,594.

Pyle, W.R., Hayes, M.H., Healy, J.D., Petersen, E.C., Spivak, A.L., Cortez, R., 1994"Direct-Thermal Solar Hydrogen Production from Water Using Nozzles/Skimmers and Glow Discharge in the Gas Phase at Low Pressure and High Temperature", H-Ion Solar Company, NREL Task No. HY413801, Subcontract No. AAP-4-14240-01, Richmond, California, pp.1-63.