2
Review and Evaluation of Various Application Areas

WATER AND WASTEWATER TREATMENT*

Brief Description

Since the late 1970s, a series of process concepts based largely on aqueous phase hydroxyl radical chemistry have been demonstrated to completely destroy (mineralize), under appropriate conditions, a wide spectrum of organic, haloorganic, and metallo-organic compounds found as dilute contaminants of well water, aquifers, and surface waters or as components of industrial and domestic wastewater effluents or waste disposal site leachates. These chemical processes, collectively referred to as advanced oxidation processes (AOPs) [1], include water treatment with the mixed oxidants ozone and hydrogen peroxide [1] as well as photolytic ozonation (ultraviolet [UV] light plus ozone [13]); photolytic peroxidation (UV light plus hydrogen peroxide) [1,2]; and photocatalysis (near-ultraviolet light plus photocatalyst and oxidant [oxygen and/or hydrogen peroxide]) [4,5]. Simpler processes involving only photolysis or ozonation or peroxide alone typically are not able to achieve complete contaminant mineralization, whereas AOPs can accomplish this task. This achievement is singularly important in that it demonstrates simultaneous water purification and destruction of the hazardous contaminant. The latter eliminates the legal liability associated with discharge of a hazardous material, which is a very attractive characteristic.

The characteristic chemical composition of these waters varies according to water type:

  1. Well water—dissolved minerals and trace substances, trace microbial contamination.

  2. Aquifers—dissolved minerals, trace contaminant organics.

  3. Surface waters—humic substances (low to high), suspended particulate, solutes from agricultural activity or urban runoff.

  4. Industrial wastewaters—organic pollutants from process raw materials (reactants, contaminants), products or by-products, or products of prior water treatment.

*  

References for this section appear on pages 17–18.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 11
Potential Applications of Concentrated Solar Photons 2 Review and Evaluation of Various Application Areas WATER AND WASTEWATER TREATMENT* Brief Description Since the late 1970s, a series of process concepts based largely on aqueous phase hydroxyl radical chemistry have been demonstrated to completely destroy (mineralize), under appropriate conditions, a wide spectrum of organic, haloorganic, and metallo-organic compounds found as dilute contaminants of well water, aquifers, and surface waters or as components of industrial and domestic wastewater effluents or waste disposal site leachates. These chemical processes, collectively referred to as advanced oxidation processes (AOPs) [1], include water treatment with the mixed oxidants ozone and hydrogen peroxide [1] as well as photolytic ozonation (ultraviolet [UV] light plus ozone [1–3]); photolytic peroxidation (UV light plus hydrogen peroxide) [1,2]; and photocatalysis (near-ultraviolet light plus photocatalyst and oxidant [oxygen and/or hydrogen peroxide]) [4,5]. Simpler processes involving only photolysis or ozonation or peroxide alone typically are not able to achieve complete contaminant mineralization, whereas AOPs can accomplish this task. This achievement is singularly important in that it demonstrates simultaneous water purification and destruction of the hazardous contaminant. The latter eliminates the legal liability associated with discharge of a hazardous material, which is a very attractive characteristic. The characteristic chemical composition of these waters varies according to water type: Well water—dissolved minerals and trace substances, trace microbial contamination. Aquifers—dissolved minerals, trace contaminant organics. Surface waters—humic substances (low to high), suspended particulate, solutes from agricultural activity or urban runoff. Industrial wastewaters—organic pollutants from process raw materials (reactants, contaminants), products or by-products, or products of prior water treatment. *   References for this section appear on pages 17–18.

OCR for page 11
Potential Applications of Concentrated Solar Photons Domestic wastewater effluent—"recalcitrant" or nonbiodegradable organics, trace microbial and virus levels. Industrial wastewaters are expected to show the most variability of composition, given the large numbers of extant industrial production processes. In such wastewaters, contaminants that are potentially suitable among the 129 priority pollutants by the Environmental Protection Agency. Techniques for removing these organic compounds from water have included air or steam stripping, adsorption on activated carbon or porous polymeric resins, and biological (microbial) treatment [36]. Advanced oxidation technologies for this full range of compound structures are only now beginning to be explored. An individual process chemistry (e.g., ozonation or aerobic biological treatment) is termed a unit operation. A process may be a sequence of such unit operations (e.g., ozonation followed by biological treatment). In wastewater treatment the last step in the process is often used to effect removal of trace residuals; such "polishing" operations typically involve feed streams of clear or nearly clear water with only trace contaminant levels and may be regarded as ideal streams for advanced oxidation technologies, applicable after most contaminants have been removed by the cheaper, larger-scale proven technologies such as carbon adsorption or biological treatment. Among these AOPs, photocatalysis requires solar photons in the 300-to 360-nm range (i.e., the near UV-spectrum), constituting about 1 to 2 percent of the sea-level solar irradiance (6). Using this portion of the solar spectrum, a potential exists for a water decontamination process based on solar-driven photocatalytic oxidation. Heterogeneous photocatalysis uses light to activate the catalyst (rather than the contaminant) and thus exemplifies indirect photochemistry. In the open literature, this phenomenon has been explored primarily by using a single, largely crystalline form (anatase) of the white paint pigment titanium dioxide, the illumination of which permits use of dissolved oxygen to achieve contaminant destruction by a catalyzed wet combustion process. The best currently available treatment processes are carbon adsorption (general) and air stripping (volatile contaminants only). These widely practiced technologies are encountering increased costs (more expensive carbon disposal) or even outright ban (air stripping in some locations). The decreasing availability of best-proven technologies and the slowly growing use of dark- (ozone-peroxide) and light-driven AOP chemistry signals that this area represents a "market pull" opportunity for photochemical processing. Status of Technology Ozonation is widely practiced in Europe for drinking water pretreatment, particularly France, where it is preferred to the various chlorination processes widely used in North America. Artificial UV light is also used on a large scale to sterilize and disinfect water. About 2000 water treatment plants in Europe currently use UV light, primarily for disinfection. The largest plant processes 14.5 million gpd [21]. Thus, there is appreciable, large, process-scale experience with the use of an oxidant or light alone for water treatment. AOPs typically use a combination of elements, either a double oxidant (ozone-peroxide) or light plus at least one oxidant. UV ozonation is the oldest AOP in commerce; it is found at water remediation sites on the scale of 20 to 200 gpm, as are UV/peroxide and UV/ozone-peroxide [3,7] installations. The largest-scale ozone-peroxide installation is about 104 gpm or 107 gpd for treatment of pesticide-contaminated surface waters [8].

OCR for page 11
Potential Applications of Concentrated Solar Photons Heterogeneous photocatalysis is not yet in commerce, although a few companies have initiated development of water purification devices or catalysts based on the use of near-UV light with a titanium dioxide photocatalyst. Suspended titanium dioxide photocatalyst particles have been used to mineralize (completely combust) dilute organic and haloorganic contaminants of aqueous solutions, including two of the most common chlorinated hydrocarbons encountered in water supplies (trichloroethylene and trichloromethane) [9]. Catalyst immobilization is attractive here, as in other areas of industrial catalysis, because it allows an essentially free, continual recovery of catalyst from the stream being treated. Such easy catalyst recovery is gained at the expense of synthesis of the immobilized configuration and an increased possibility of both mass transfer limitations [33] and decreased quantum yield [31]. Titanium dioxide has been demonstrated to be photocatalytically active in the presence of sunlight [9–15]. Both photocatalyst slurries [9,11–15] and photocatalyst immobilized inside glass tubes [10] or on fibrous glass [11–15] exhibit appreciable degradation in sunlight. Like the slurry, the immobilized catalyst can yield complete mineralization under sufficient residence time with illumination; a total organic carbon analyzer based on the complete mineralization of organic solutes using synthetic illumination and immobilized photocatalyst has been patented and reported [34,35]. Parabolic trough concentrators, developed originally for capturing direct solar photons for thermal energy storage, have recently been examined as a solar photoreactor configuration. Scales of collector trough area in published reports include 0.45 m2 in Australia [10], several square meters at the Solar Energy Research Institute (SERI) [11,13], and a 465-m2 system at Sandia National Laboratories (SNL) [12–15]. Each of these concentrators has demonstrated contaminant degradation activity with both suspended photocatalyst and immobilized photocatalyst. The allowed concentration of residual photocatalyst in the treated water will be application specific, with lowest allowed residual expected for drinking water or ultra-pure water (microelectronics, biotechnology) and higher levels allowed for waters from polishing of domestic or industrial wastewater effluents. Neither the stability of current immobilized catalyst configurations (tubes, beads, mesh) or recovery techniques for suspended photocatalyst has yet been seriously studied. Surface waters may contain natural organic solutes (humic and fulvic acids) as well as contaminants from agricultural activities (pesticides, pesticide byproducts, fertilizers) or urban runoff (hydrocarbons, chlorinated solvents, metals, particulates). The photooxidation (via any AOP technology) of multiple solute feed streams of these micropollutants has not been widely explored. Recent examples of binary solute studies include photolytic peroxidation of benzene/trichloroethylene mixtures [37] and photocatalytic: mineralization of benzene/perchloroethylene solutions [38]. Additionally, surface waters may contain appreciable levels of ions (especially carbonate and bicarbonate) or molecules that act as radical scavengers and that deleteriously affect the rate of oxidation reaction, as illustrated by inhibition of tetrachloroethylene photolytic ozonation by solutes of lake water [39]. The drinking water chlorination practices in the United States routinely lead to conversion of trace organics in feed water with resultant formation of trace chlorinated organics, especially chloromethanes and chloroacetic acids. As a result, interest has arisen in diminishing the concentration of these trace organic chemicals prior to chlorination operations, for example. Whether photocatalysis can destroy or reduce the level of such precursors, and whether any by-products remain or are generated that could raise new problems, has not yet been seriously examined.

OCR for page 11
Potential Applications of Concentrated Solar Photons Academic studies using a variety of artificial light sources, designed to act as solar simulators, have established that nearly all common chloroalkanes, bromoalkanes, and haloalkenes as well as haloaromatics can be completely destroyed, and major, though not always complete, conversions have been reported for common pesticides (DDT, bentazon, atrazine) as well as dioxins [16–19]; gasoline components (e.g., benzene) [20,21]; and phenol, the major coal-processing waste component [22]. Conclusion 1: There is a valid scientific base for the complete photocatalytic destruction of common organic contaminants of water supplies and aquifers, but there is need for further research to determine if low-level by-products are produced, particularly at realistic photon and/or oxidant doses. Conclusion 2: Water decontamination using solar photon concentrations of 3 to 10 or more sun equivalents has been studied in laboratory, pilot, and full-scale concentrator installations. While no commercial photocatalysis installations using solar illumination are known to the committee, a pilot unit is currently being tested by SERI at a groundwater remediation site at Lawrence Livermore Laboratory (as an upstream insert prior to a permitted UV-peroxide treatment system) [23]. Likewise, no commercial installations are yet known that use photocatalysis for water treatment with artificial illumination, although more than a dozen patents for such processes have been filed to date in the United States, Japan, Australia, and Europe. Conclusion 3: Photocatalysis represents an emerging process chemistry, but it is not currently an industrial water treatment practice. Economics Large-scale water treatment by advanced oxidation processes has been estimated to cost as little as $0.10 ptg in a very large, hypothetical, ozone-peroxide water treatment process operating on an "easy" challenge, the mineralization of 0.2 ppm trichloroethylene [24]. Commercial installations for UV/ozone are in place at reported operating and maintenance costs of $0.45 to $1.50 ptg for ppm levels of conventional contaminants [trichloroethylene; BTX (benzene, toluene, xylenes); ketones; biological oxygen demand] and as high as $5.00 ptg for difficult, small-scale situations [3]. Inclusion of capital costs could increase these estimates by 50 to 80 percent [25]. Most of these cost examples are in the vicinity of $1.00 ptg, typical of processing costs for municipal wastewaters, and are below treatment costs for appreciably more concentrated industrial wastewaters [26]. However, as with other photon-driven AOPs, photocatalysis requires a clear or nearly clear feed water, so that most photons are used for the desired photochemistry. Thus, it is directly applicable to aquifer and well water, as well as source leachate streams. Aqueous waste streams that have particulate content or high optical density in the near-UV range will require one or several pretreatment steps to render a clear stream suitable for photon-based AOP. Processes found at the end of a series of treatment steps are polishing operations. The use of solar-driven photocatalysis involving parabolic trough concentrators at a small scale has been estimated very recently by SERI [27] to be $16.00 ptg, a value about four to six times higher than for other applicable technologies (e.g., carbon adsorption) operating at the same scale. In this estimate, the trough concentrator was about 90 percent of the major capital cost (uninstalled). The committee heard from two SERI speakers [27,28] that these high estimates, taken with the large cost contribution of the concentrator device as well as the peculiar influence of concentrated light on catalyst kinetics (see Appendix D), suggest that the unexplored, nonconcentrating photoreactor configuration might be a useful, more optimal configuration. The committee agrees.

OCR for page 11
Potential Applications of Concentrated Solar Photons Conclusion 4: Nonconcentrating configurations should be tested in parallel to establish their technical and cost (dis)advantages. The number of water stream candidates for detoxification is large and diverse in both composition and contaminant concentrations. Streams that may be decontaminated via sunlight should be prioritized through market analysis to identify those that might best be treated by concentrated or nonconcentrated solar photons. Conclusion 5: Market and cost analyses have not yet been performed for specific streams. In the absence of this specific information, development, engineering, or demonstration projects should be deferred. Most of the work in this field has been carried out with titanium dioxide as the photocatalyst. While this material has the advantages of low cost and high stability, its large bandgap allows only a small portion of the solar spectrum to be used. Improvements in the photocatalyst made by finding a stable and useful material with a smaller bandgap or suitable sensitizers (dyes) for titanium dioxide might be possible. Note, however, that there have already been a large number of studies of different photocatalytic materials in the United States, Japan, and Europe, mainly in the search for materials appropriate in the photosplitting of water to hydrogen and oxygen and in the design of liquid junction photovoltaic cells. New materials for the photodestruction of wastes must be stable in the presence of strong photogenerated oxidants and must promote rapid reaction of photogenerated electrons with oxygen or other oxidants. Fundamental research in this area appears to be worthwhile; but given the background in this field, the discovery of useful new catalysts will probably be difficult. Conclusion 6: Catalyst improvement studies should consider new materials or sensitizers that will allow better utilization of the solar spectrum and higher efficiencies. They should also consider other enhancement mechanisms, such as enhancement of electron transfer to oxygen or other electron acceptor or increased contaminant adsorption or integration of the photocatalyst with an adsorbent [32]. Assessment of Knowledge Base For solar-driven photocatalysis, applied to water treatment, the committee finds the knowledge base to be as follows: Solar concentrator design is well developed, to the point where commercial-scale, off-the-shelf parabolic concentrators are conveniently available, and engineering models for predicting the usable flux of concentrated solar photons are in use at SERI [11]. Catalyst photoactivation for semiconductor oxides, especially titanium dioxide, is well understood with respect to wavelength dependence and fundamental photophysics but not with respect to surface chemistry and the feasibility of modifying the surface chemistry or with respect to understanding the kinetics of photooxidation. A knowledge base in these areas will result in better photocatalysts and would likely result in a competitive process for photooxidation of trace impurities in water. Catalyst preparation in various immobilized forms (on tubes, beads, meshes, membranes, etc.) has not been well studied with respect to optimal configurations for large-scale applications, catalyst attrition (particulate loss), or deactivation (physical or chemical site blockage).

OCR for page 11
Potential Applications of Concentrated Solar Photons Understanding of the photocatalyzed chemistry and kinetic networks of contaminant degradation and mineralization is fair at best. While hydroxyl radical generation is generally accepted as an early, possibly initial, reaction network event in photocatalyzed degradations, the full range of reaction intermediates appearing and disappearing as the degradation proceeds, ultimately to CO2, is not well known even for the simplest contaminant structures. For example, the aromatic compounds generate a range of detectable intermediates, a number of which may have appreciable toxicity (there is a need to add toxicological assays to current Department of Energy photodegradation analyses). The influence of rate enhancers, such as hydrogen peroxide, deserves further study. Finally, the role of quenchers needs to be defined. The influence of common anions (chloride, sulfate, phosphate, nitrate) in water supplies is only modestly inhibitory [30]. The influence of irradiance level on catalyst rate and the apparent quantum yield has only recently (1990) been confirmed to be a half-order at or above 1 sun near-UV equivalents for titanium dioxide (anatase form) acting on typical contaminant structures (chloromethane, chloroethylenes, phenols). This characteristic half-order behavior leads to a substantial rate and quantum yield penalty associated with the use of concentrated photons, solar or otherwise [31] (see Appendix D). An unexplored area is that of solution additives, especially electron acceptors. Through these, one might extend the intensity at which the transition from first-order to half-order intensity dependence to higher intensities occurs. Photochemical reactor design has been substantially covered in the literature for low optical density systems associated with industrial photohalogenations and direct photolysis of water contaminants. The open design literature for heterogeneous photoreactors, in which bubbles and/or solid phases are present in a liquid phase, is much weaker, and very few papers exist that include comparisons of photoreactor analytical models and corresponding measurements of particle absorption, scattering, and reflectance. Process integration of photocatalysis, or other competitive photoprocesses (UV/O3 or UV/H2O2), with established water treatment technologies (e.g., adsorption, air stripping, membrane separations, ion exchange) is nonexistent. Thus, there is a need to survey current integrated water treatment systems to determine whether and where photooxidation processes, and solar-driven photoprocessing in particular, could be usefully inserted. The need for a market survey for emerging photo-driven processes is emphasized again in a later section of this report. Conclusion 7: The knowledge base requires deepening and broadening to include substantial conceptual models for (1) reactant degradation mechanisms, including the influence of oxidant (e.g., oxygen, hydrogen peroxide, ozone) and temperature, and (2) reactor design, including coupling between intrinsic kinetics and mass transfer (especially for immobilized catalysts) and illumination supply level and spatial distributions within the reactor from direct, diffuse, and concentrated solar sources. RECOMMENDATIONS The committee makes the following general recommendations, in addition to those cited previously. 1. For the study of water treatment with solar-driven photocatalysis at various process scales:

OCR for page 11
Potential Applications of Concentrated Solar Photons LABORATORY-SCALE: Expand the knowledge base. Include studies of reaction intermediates arising in solar-assisted conversions, reactor designs and operating conditions that more effectively utilize solar photons, and the economics of concentrating and nonconcentrating configurations as well as studies to establish whether concentration is justified. Expand the study of photocatalyst surface modifiers. Include broader assays for photochemical and dark intermediates to test for toxicity, mutagenicity, and so forth. BENCH-SCALE: Develop nonconcentrating concepts; compare performance with concentrator configurations. Improve photocatalyst through surface modification. PILOT-SCALE: Pending outcome of the proposed bench-scale studies (see above). FULL-SCALE: No full scale studies are currently recommended. 2. Study of solar-driven photocatalysis is an interdisciplinary activity, requiring substantive collaboration of mechanical engineers (concentrators or reflectors and solar resource characterization); surface scientists (contaminant surface concentrators on TiO2 and O2 reduction catalysis); materials scientists (catalyst support, semiconductor photocatalyst synthesis and attachment to support, and optical characterization); physical chemists (reaction kinetics); analytical chemists; and chemical engineers (reactor design). As no single discipline can credibly cover this range, a substantive interdisciplinary team is required to competently execute any development program in this area. REFERENCES 1. Glaze, W. Workshop on Potential Applications of Concentrated Solar Photons, NRC, Nov. 7–8, 1990, Golden, Colo. 2. Peyton, G. Workshop on Potential Applications of Concentrated Solar Photons, NRC, Nov. 7–8, 1990, Golden, Colo. 3. Zeff, N., Ultrox International. Workshop on Potential Applications of Concentrated Solar Photons, NRC, Nov. 7–8, 1990, Golden. Colo. 4. Serpone, N., and Pelizzetti, E. Photocatalysis, Wiley-Interscience, New York (1989). 5. Ollis, D.F. Environ. Sci. Technol., 19, 480 (1985). 6. Blake, D. (SERI). Advanced Oxidation Processes Symposium, June 1990, Ontario, Canada. 7. Hager, D.G., et al. ''On-site Chemical Oxidation of Organic Contaminants in Groundwater Using UV-Catalyzed Hydrogen Peroxide." American Water Works Association Annual Conference and Exposition, June 19–23, 1988, Orlando, Fla. 8. Glaze, W. (University of North Carolina). Personal communication (1991). 9. Ahmed, S., and Ollis, D. Sol. Energy, 32, 597 (1984). 10. Matthews, R.W. Sol. Energy, 38, 405 (1987). 11. Mehos, M. Workshop on Potential Applications of Concentrated Solar Photons, NRC, Nov. 7–8, 1990, Golden, Colo. 12. Pacheco, J., Prairie, M., Evans, L., and Yellowhorse, L. (SNL). 25th Intersociety Energy Conversion Engineering Conference, Aug. 12–17, 1990, Reno, Nev. 13. Pacheco, J.E., Magrini, K., Mehos, M., and Carville, C. Waste Management '90, Tucson, Ariz., March 1990. 14. Pacheco, J.E., and Tyner, C.E. Pp. 163–166 in 1990 ASIDE International Solar Energy Conference, April 1990, Miami, Fla. 15. Tyner, C.E., Pacheco, J.E., and Holmes, J.T. Hazardous Material Management Conference, March 1990, Rosemont, Ill. 16. Borello, R., et al. Environ. Toxicol., 8, 997 (1989). 17. Pelizzetti, E., et al. Chemosphere, 18, 1437 (1989). 18. Pelizzetti, E., et al. Environ. Sci. Technol., 24, 1559 (1990). 19. Pelizzetti, E., et al. Chemosphere, 17, 499 (1988).

OCR for page 11
Potential Applications of Concentrated Solar Photons 20. Turchi, C., and Ollis, D.F. J. Catal., 119, 483 (1989). 21. Matthews, R. Water Res., 20, 569 (1986). 22. Okamoto, K., et al. Bull. Chem. Soc. Jpn., 58, 2023 (1985). 23. Link, H. Workshop on Potential Applications of Concentrated Solar Photons, NRC, Nov. 7–8, 1990, Golden, Colo. 24. Aieta, M., et al. J. Am. Water Works Assoc., 80, 64 (1988). 25. Haag, W., Jr. Stanford Research Institute, private communication, Nov. 7, 1990. 26. Daley, P., Chemical Waste Management. Workshop on Potential Applications of Concentrated Solar Photons, NRC, Nov. 7–8, 1990, Golden, Colo. 27. Link, H. and Turchi, C. "Cost and Performance Projections for Solar Water Detoxification Systems." Presented at the Workshop on Potential Applications of Concentrated Solar Photons, NRC, Nov. 7–8, 1990, Golden, Colo., and ASME International Solar Energy Meeting, Reno, Nev., March 17–22, 1991. 28. Turchi, C. Workshop on Potential Applications of Concentrated Solar Photons, NRC, Nov. 7–8, 1990, Golden, Colo. 29. Wolfe, R.L. "Ultraviolet Disinfection of Potable Water." Environ. Sci. Technol., 24, 768 (1990). 30. Abdullah, M., et al. J. Phys. Chem., 24, 682 (1990). 31. Ollis, D.F. "Solar-Assisted Photocatalysis for Water Purification: Issues, Data, Questions." Eighth International Meeting of the Photochemistry Society, Palermo, Italy, (1990). (E. Pelizzetti and M. Schiavello. eds.) Kluwer Publishers. 32. Matthews, R. J. Catal., 113, 549 (1988). 33. Turchi, C., and Ollis, D.F. J. Phys. Chem., 92, 6852 (1988). 34. Matthews, R., Abdullah, M., and Low, G.K.C. Anal. Chim. Acta., 233, 171 (1990). 35. Matthews, R.W. PCT/AU88/00059 (Australian patent). 36. Patterson, J.W. Pp. 303–360 in Industrial Wastewater Treatment Technology, 2nd ed. Butterworth Publishers, Boston (1985). 37. Weir, B., Sundstrom, D., Weir, B.A., and Redig, K.A. Emerging Technologies in Hazardous Waste Management. Pp. 67–76 in ACS Symposium Series #422 (D.W. Tedder and F.D. Pohland, eds.). American Chemical Society, Washington, D.C. (1990). 38. Turchi, C., and Ollis, D.F. J. Catal., 122, 178 (1990). 39. Peyton, G., Wang, F., Burleson, J., and Glase, W. "Destruction of Pollutants in Water with Ozone in Combination with Ultraviolet Radiation," I. General Principles and Oxidation of TCE. Environ. Sci. Technol., 16, 448–453 (1982). WASTE TREATMENT* Brief Description Thermal destruction processes are used for the treatment of a variety of wastes, including municipal, hazardous, medical, and sewage sludge. The discussion in this section is restricted to hazardous wastes, since high cost of their destruction makes them a more attractive candidate for treatment by solar processes. The volumes of hazardous wastes considered suitable for destruction by thermal processes include 50 million tons/year of industrial hazardous wastes, 0.7 million tons/year of organic fumes, and 10 million to 30 million tons/year of materials from the cleanup of Superfund sites [1]. A variety of treatment technologies are available. Classified by the temperature ranges in which they operate, these technologies include thermal desorption of soils in the range of 350° to 500°C, incineration at 1000° to 1200°C, and plasma processing at higher temperatures. The atmosphere *   References for this section appear on pages 26–27.

OCR for page 11
Potential Applications of Concentrated Solar Photons for the waste destruction can be oxygen rich (incineration), H2O rich or CO2 rich (reforming), H2 rich (reduction to a potential product stream), or inert (pyrolysis). The potential applications of solar energy for waste treatment are those in which solar collectors can supply the temperatures needed for destruction at a cost competitive with other energy sources or in which the enhancement of destruction by photolytic reactions is sufficient to compensate for differentials in the energy cost. Added benefits of using solar energy in lieu of fossil fuels to supply the energy for thermal destruction are a reduction in the gas volume to be treated by air pollution control devices and elimination of the incremental emissions resulting from fuel oxidation. The public may find a solar detoxification process more environmentally acceptable than conventional incineration, but the stigma associated with any hazardous waste treatment process is hard to eliminate. Moreover, the permitting procedures required for a hazardous waste treatment facility are time consuming and costly and will not be averted by the use of solar energy. Although there may be a public demand for alternative technologies for the disposal of hazardous wastes, the market penetration of a number of novel technologies has been small up to this time because of the difficulty of competing with the versatility, reliability, and cost of existing processes [2]. Status of Technology At present, the prevalent commercial processes involve the use of rotary kiln incinerators for sludges, solids, and containerized wastes and direct injection units for flammable solvents and fumes. Major factors in the selection of these processes are system reliability and versatility in handling wastes of widely varying chemical composition and physical state. Incineration and pyrolysis of wastes, however, can generate toxic by-products, such as the chlorinated polychlorinated dibenzodioxins and dibenzofurans, emissions of which in relatively high concentrations from poorly operated incinerators have been responsible for much of the public opposition to the siting of incinerators. Special consideration should be given to minimizing such by-products in the development of alternative technologies. Representative values of hazardous waste treatment costs are given in Table 2-1. Examination of the breakdown of the cost of a typical process, shown in Figure 2-1, indicates that energy costs TABLE 2-1 Commercial Hazardous Waste Treatment Costs Materials $/Gallon $/Ton Organic solids, liquids and sludges   >5,000 BTUs/lb, "low tox." -0.50 to +2.00 -100 to +500 <5,000 BTUs/lb. "low tox." 0.80 to 2.00 200 to 800 PCBs and other "toxics" 2.00 to 3.00 650 to 1000 Aqueous liquids   Deep Well 0.04 to 1.00 10 to 250 Incineration 0.40 to 2.00 100 to 500 Large commercial treaters 0.06 to 1.00 15 to 250 small specialty treaters 0.20 to 2.00 50 to 500 Inorganic solids and sludges   100 to 1000 NOTE: Negative costs refer to the credit for energy generated when burning nontoxic wastes of high heating value. SOURCE: [2]

OCR for page 11
Potential Applications of Concentrated Solar Photons CAPITAL: OPERATIONS: FIGURE 2-1 Typical hazardous waste treatment costs. SOURCE: [2]. are a small part of the total costs. Energy savings through the use of a solar detoxification process, although valuable, may therefore not be a decisive factor in selecting a treatment technology. Capital costs are typically much larger than those for energy consumption and therefore may pose a significant deterrent to the use of a solar detoxification unit, operated only a small fraction of the time or involving a backup heating system. Applications in which solar energy may be most economical are those in which intermittent operation is inherent to the process. Examples are capture of organic contaminants on activated carbon sorbents with periodic regeneration of the sorbent, and treatment of the organics stripped from a pump-and-treat decontamination of an aquifer where the pumping is at intervals selected to permit the hydraulic head to be reestablished or at sites of small-volume generators where waste is stored until it reaches volumes adequate for treatment or off-site disposal. Current studies of the use of solar energy for waste treatment have been restricted to processing of gaseous streams composed of model waste compounds and mixtures at the bench-and pilot-scale levels. Activities at or sponsored by SERI include photolysis [3] and photocatalysis [4] of wastes using solar simulators and evaluation of the destruction efficiencies and products of incomplete destruction of the pyrolysis and steam reforming of wastes [5]. Studies [6,7] at SNL concern steam reforming of wastes in a field test reactor heated by a 15-kW solar furnace, supported by catalyst development studies at the University of Houston [8]. Preliminary photothermal (mostly thermal) treatments of chlorobiphenyls and chlorobenzenes in inert atmospheres have been conducted at the Hahn-Meitner Institute, yielding a carbonaceous inert residue and HCl [9]. Assessment of the Knowledge Base The kinetics of thermal and photochemical reactions are needed for process evaluation, design, and optimization. Homogeneous Thermal Reactions The detailed kinetics of the homogeneous gas-phase pyrolysis and oxidation reactions are key to evaluating of the use of solar energy in either purely thermal destruction processes or those in which such processes are accelerated by photochemical generation of free radicals. The detailed chemistry of the breakdown of organic chemicals during incineration is complex [10]. Nevertheless, by building on what is known about hydrocarbon combustion processes, the problem can be rendered more tractable [11,12].

OCR for page 11
Potential Applications of Concentrated Solar Photons In a system containing carbon, hydrogen, and oxygen, the three principal pathways that affect the destruction of organics during combustion are radical attack by OH, radical attack by H, and unimolecular decomposition [13, 14], with their relative importance determined by the fuel/air ratio and combustion temperature. The effect on the combustion kinetics of the chlorine present in many waste streams is varied. The carbon-chlorine bond is weaker than carbon-carbon bonds and carbon-hydrogen bonds, so any reduction in the ease of destruction of chlorinated compounds is more through the reduction of the H and OH radical concentrations in the presence of chlorine [15] than in any enhancement of the stability of chlorinated molecules [16]. Chlorine in a waste stream also affects the composition of combustion by-products by the chlorination of any residual organic compounds as the temperature of the reaction products is reduced to levels at which the chlorinated compounds are thermodynamically stable and the chlorination reactions are still facile [17]. One of the disadvantages of the thermal destruction of wastes is the large number of by-products that can be produced, including the controversial chlorinated dibenzodioxins and dibenzofurans, albeit in very low concentrations. An extensive data base [18–20] has been developed for the elementary reactions involved in combustion, and it can be used to assess the impact of solar enhancement of reactions by means of the generation of free radicals by photolysis. High-Temperature Photochemistry The temperature at which selected hazardous compounds decompose has been shown by Dellinger and co-workers [3] to be reduced by amounts ranging from a few tens up to several hundred degrees centigrade when the gas mixture is exposed to a simulated solar flux. For example, Dellinger et al.'s results on the decomposition of 3.3',4,4'-tetrachlorobiphenyl heated for 10 s at various temperatures in a flow reactor with and without exposure to a laser flux of 9.5 W/cm2 (95 suns) are shown in Figure 2-2. The fraction of the tetrachlorobiphenyl recovered after 10 seconds residence time in the reactor decreases with increasing gas temperature, with the amount remaining at a given temperature always being lower in the presence of irradiation. The solar flux was simulated by filtered radiation from a xenon lamp in these experiments. The temperature required to achieve a given destruction level is reduced by varying amounts, from a modest 30°C for a destruction efficiency of 99.9 percent (0.001 fraction remaining) to 250°C at a destruction efficiency of 70 percent (0.3 fraction remaining). More significantly, production of the tetrachlorinated dibenzofuran by-product is reduced significantly by the simulated solar irradiation for temperatures above 480°C. At higher temperatures the fractional conversion of tetrachlorobiphenyl to tetrachlorodibenzofuran was as high as 0.01 in the absence of irradiation but below detection limits for the run with 95 suns. Solar processes have a potential benefit over thermal processes of a reduction of the reaction temperature required to achieve a given destruction efficiency, with a reduction in the concentration of undesirable by-products as well. Our understanding, however, of high-temperature photochemistry is in its infancy compared with that of thermal processes, so the conditions for carrying out such reactions at useful rates cannot be defined at present. A highly simplified rendition of an energy-bond coordinate is presented in Figure 2-3 to illustrate the relevant fundamental processes. For the ground electronic state (So), the rate constant kr(So) is determined by passage over the relatively high energy barrier, Ea(So). The rate of the photochemical process is related to (1) the rate of energy absorption, kab, to, say, the first singlet state S1; (2) the rate kr(S1) of passage over the smaller activation barrier Ea(S1); (3) the rate of the competing deactivation process, kf, and (4) the rate of equilibration of the vibrational levels in the excited state, governed by kexc and kvr. Possible complicating factors are the presence of other

OCR for page 11
Potential Applications of Concentrated Solar Photons FIGURE 2-6 Solar pumped laser system. Source: [11]. Assessment of Knowledge Base The basic technology of lasers, lasing concepts, and applications of laser light are well established and understood. While many lasers have been demonstrated, few have proven to be practical, and even fewer have been used in commercial applications. Often lasers have poor efficiency and high capital equipment costs. Thus, conventional laser development continues, and new lasers continue to be introduced commercially, even after 30 years of research and development. Solar-pumped lasers are now being explored internationally as alternatives, including gases, liquid dyes, and solid-state lasers. Gases are interesting because in principle they are scalable to large powers, so that they may, in turn, be used for applications such as materials processing. While CO2 lasers have been the most efficient electrically pumped lasers used for materials processing, they have not been efficiently pumped by solar energy. For space applications, considerable emphasis has been applied to the iodine laser, which in recent years has been tailored to achieve absorption bands matching the solar spectrum. The solar-pumped iodine laser uses either a trough or an imaging concentrator, which is adequate for pumping gases of molecules containing iodine to above their laser threshold [4-6]. The utility of such a laser for space-based applications, such as communications and power transfer to space vehicles, is being explored. A laser output power of 0.6 W with a trough using a 2 x 3 m solar concentrator and a solar input of 5.3 kW have been demonstrated [11]. A scaling study shows that a 100 W laser is feasible at a 1000-sun concentration [11]. However, the anticipated efficiency of this type of laser (about 0.1 percent) is rather low for prospective terrestrial applications in competition with other technologies. One terrestrial application, not needed in space, is tunable lasers pumped by the sun. To date, only the dye laser has been investigated [12]. The lasing threshold for a favorable dye such as rhodamine 6G has been shown to be about 10,000 solar constants, not very far below the maximum pumping intensity that can be achieved with advanced concentrators, and the lasing threshold must be exceeded by a large factor to give high overall efficiency. Thus, there may be practical limits to the efficiency. Questions about the performance achievable with solar-pumped dye lasers will soon be addressed by experiments to be performed by R. Winston's group at the University of Chicago [13].

OCR for page 11
Potential Applications of Concentrated Solar Photons An important consideration for dye lasers, even if they can be solar pumped, is their potential for practical applications. Photochemical applications that would use this laser need to be identified. One technology discussed was laser isotope separation [13]. Winston's cost estimates for this technology assume an overall dye laser efficiency of 5 percent. However, implementation of laser isotope separation in its current form requires laser pulses with a fairly low duty cycle. This is a mode of operation in which solar-pumped lasers are very inefficient, because the low duty cycle wastes much of the collected solar power. This question must be considered, and, if necessary, its impact on the cost estimates should be included. Solar-pumped solid-state lasers have been demonstrated [14,15] and will undoubtedly have many space applications, but it is unclear what their terrestrial applications will be. A solar-pumped Nd:YAG laser has been demonstrated but with relatively low efficiency. Lower laser threshold and higher efficiency appear to be possible with Nd:Cr:GSGG. Such lasers can provide tens (perhaps hundreds) of watts near 1-μm wavelength. The recently developed tunable solid-state laser, Ti:sapphire, uses laser pumping; its potential for solar pumping has not yet been investigated. However, initiation of SERI-based research in solar-pumped solid-state lasers should wait until practical terrestrial applications are identified. An innovative pumping scheme based on blackbody cavities has been discussed [8-10]. The idea is to pump lasers with the solar radiation trapped in a blackbody cavity. In principle, recycling of the radiation trapped in the blackbody cavity might provide more efficient utilization of the collected solar flux than direct solar pumping. It remains to be demonstrated, however, that this advantage can be achieved in practical blackbody cavities. The loss of thermal energy from the blackbody must not be much larger than, and preferably smaller than, the laser output, in order to achieve the predicted efficiencies. A potential advantage of blackbody pumping is that the collected solar energy is stored in the blackbody during periods of interruption of the solar flux, during which laser operation might continue, perhaps with the infusion of substitute forms of heating of the blackbody during long interruptions. Blackbody pumping of YAG lasers has been proposed. However, the severity of cooling requirements on YAG crystals in the near proximity to high-temperature blackbodies may make it difficult to obtain stable and efficient operation. Conclusions and Recommendations The unique attributes of solar pumping require that several issues be addressed [16-20]. The recommendations listed here address the key issues. Dye lasers are important because the laser emission can be varied in wavelength. Plans for laser isotope separation use dye lasers, and a natural question arises as to the usefulness of solar pumping for this application. It would appear that because of high thresholds, efficient operation requires the high solar intensity of advanced nonimaging concentrators. The University of Chicago has an ongoing program to demonstrate the feasibility of a dye laser system in a nonimaging concentrator. The feasibility of scaling up such a system to practical levels necessary for commercial application is unclear. Recommendation: The feasibility of practical solar-pumped dye lasers should not be addressed by SERI until after a satisfactory demonstration has been accomplished at the University of Chicago. After the limits to their performance in this prototype system are understood, based on measurements of a working system, scaling studies should be able to predict whether practical isotope separation or other materials processing technologies make sense commercially.

OCR for page 11
Potential Applications of Concentrated Solar Photons The scalability of certain laser designs encourages thoughts about high-power terrestrial systems pumped by the sun. For example, the simplicity of trough concentration matches the linear geometry of lasers, with scalability to long lengths and high power. Uniformly optically pumped lasers of long linear dimensions cannot be constructed any other way. Recommendation: Research programs in solar-pumped lasers should emphasize those aspects unique to solar terrestrial applications. However, unless fundamentally new laser processes are discovered, enough is known about presently available lasers that careful design studies should precede the building of prototypes. The broadband nature of the solar spectrum represents a very different means of laser pumping compared with conventional laser pumping. To date, mostly conventional lasing materials have been examined for solar-pumped lasers. Optimal performance will be achieved only when materials are developed to specifically use the solar spectrum in the pumping of atoms and molecules into the appropriate excited states [21]. Recommendation: The development of materials designed for lasing using the solar spectrum, but operating at wavelengths and scalable to power levels that have commercial potential, represents a productive area for research. Very high temperatures can be achieved in concentrated solar systems, suggesting the idea of infrared lasers based on blackbody cavity pumping [5], which takes advantage of the wide-wavelength region of the solar spectrum. The National Aeronautics and Space Administration (NASA) has funded a program in this area for space-related applications, but earth-based utilization of this concept has not been demonstrated. Recommendation: A design study to ascertain the merits of solar blackbody cavities for trough pumping for earth-based lasers, utilizing as much information from previous NASA studies as possible, could answer the questions about this scheme raised in the assessment of the knowledge base. No experimental work at this time is justified. Development of solar-pumped gas lasers and solid-state lasers is under way, with promising space-based applications but without well-defined terrestrial applications. Recommendation: SERI should continue to assess the technological developments of space-based solar lasers as they become available. However, major programs in these gas or solid-state lasers should not be initiated by SERI until practical terrestrial applications are identified. Applications and Economics A wide variety of applications of high-power, earth-based Lasers can be imagined. Many of the applications, such as isotope separation, micro-electronic processing, and welding/machining use lasers with intensity levels and measures of control that are difficult to expect from solar-pumped lasers. Implicit in suggesting research into solar-pumped lasers are their possible economic merits. The National Research Council meeting produced some speculation about the relative capital saving of solar-pumped lasers, methods of scale-up, the ability to put modules together to increase power output, and the expectations for very high power lasers. These ideas, as well as the commercial viability of any proposed use, must be put on a firmer basis before any large-scale programs are proposed. Once sufficiently efficient lasers are available, perhaps by solar pumping, their use in photochemistry may become more attractive [22]. Identification of possible processes and the wavelengths required is encouraged.

OCR for page 11
Potential Applications of Concentrated Solar Photons At this stage of their development, it might be said that solar-pumped lasers are a possible solution in search of possible problems. While judging the merits of solar pumping of lasers for any application, it is, of course, important to keep in mind the competing technologies that imply either solar or nonsolar energy sources. For example, semiconductor diode laser-pumped solid-state lasers powered by solar-pumped photoelectric cells may provide stiff competition in the future for direct solar pumping in prospective laser applications. At this time too little is known about the performance and cost of solar-pumped lasers to make meaningful assessments of their suitability for applications or comparisons with other technologies. It is therefore important to shape research and development programs on the various solar pumping schemes with a view toward clearly establishing the limits to their performance. At the same time, efforts should be made to identify promising applications of solar-pumped lasers and to make credible assessments of their suitability for those applications. If progress is not made in parallel in these two areas, it may be difficult to come up with either the ''solution" or the "problem." Overall Recommendation: The area of solar pumping of lasers looks attractive for space applications, but competitive and cost-effective earth-based systems have not yet been clearly identified. Continued effort to identify applications along the lines of this report is encouraged. The center of knowledge for solar-pumped lasers clearly resides with NASA, and future programs should build upon this existing knowledge base. REFERENCES 1. Kiss, Z.J., Lewis, H.R., and Duncan, R.C. "Sun-pumped CaF2: Dy optical maser." J. Appl. Phys. Lett., 2, 93 (1963). 2. DeYoung, R.J., Lee, J.H., Williams, M.D., Schuster, G., and Conway, E.J. "One-Megawatt Solar Pumped and Electrically Driven Lasers for Space Power Transmission." Pp. 709–714 in Proceedings of the 23rd Intersociety Energy Conversion Engineering Conference, Denver Colo., July 1988. 3. Arashi, H., Oka, Y., and Ishigame, M. "Solar Pumped Laser on the Space Station." Space Sol. Power Rev., 5, 131–133 (1985). 4. Heinbockel, J.H. "Theoretical Studies of Solar Lasers and Converters." NASA Report CR186194 (Jan. 1990). 5. Han. K.S. "Direct Solar Pumped Iodine Laser Amplifier." NASA Report CR-181391 (Oct. 1987). 6. DeYoung, R.J., and Conway E.J. "Progress in Solar Pumped Laser Research." Pp. 467–474 in Lasers '85, Proceeding of the Eighth International Conference, Las Vegas Nev., Dec. 1985. 7. Gleckman, P., O'Gallagher, J., and Winston, R. "Concentration of Sunlight to Solar-Surface Levels Using Non-imaging Optics." Nature, 339, 198–200 (1989). 8. Christiansen, W.H. "New Concept for Solar Pumped Lasers." Prog. Astronaut. Aeronaut., 61, 346–356 (1978). 9. Christiansen, W.H., and Chang J. "Scaling Studies of Solar Pumped Lasers." NASA Report CR-176240 (Aug. 1985). 10. DeYoung, R.J. "Solar Pumped Lasers." NASA Report TM-100621 (Aug. 1988). 11. Lees, J.H. "Solar Pumped Lasers and Their Applications." Submitted at the Workshop on Potential Applications of Concentrated Solar Photons, NRC, Nov. 7–8, 1990, Golden, Colo. 12. Lee, J.H., Kim, K.C., and Kim, K.H. "Threshold Pump Power of a Solar-Pumped Dye Laser." Appl. Phys. Lett., 53, 2021 (1988). 13. Winston, R. "Solar Pumped Lasers: Work in Progress at the University of Chicago." Submitted at the Workshop on Potential Applications of Concentrated Solar Photons, NRC, Nov. 7–8, 1990, Golden, Colo. 14. Benmoir, R.M.J., Kagan, J., Kalisky, Y., Noter, Y., Oron, M., Shimony, Y., and Yogev, A. "Solar-pumped Er, Tm, Ho: YAG laser." Opt. Lett., 15(1), 36–38 (1990). 15. See also, Weksler, M., and Shwartz, J. "Solar-Pumped Solid State Lasers." IEEE J. Quantum Electron., 24, 1222–1228 (1988).

OCR for page 11
Potential Applications of Concentrated Solar Photons 16. DeYoung, R.J. "Long Gain Length Solar Pumped Box Lasers." U.S. Patent 4,594,720 (June 10, 1986). 17. Lee, J.H., Hohl, F., and Weaver, W.R. "Solar Pumped Laser." U.S. Patent 4,424,592 (Jan. 3, 1984). 18. Taynai, J.D. "Solar Pumped Laser." Report AFAL-TR-75-191 (Sept. 1976). 19. Golger, A.L., and Klimovskii, I.I. "Lasers Pumped by Solar Radiation." Sov. J. Quantum Electron., 14, 164–179 (1984). 20. Arashi, H., Oka, Y., Sasahara, N., Kaimai, A., and Ishigame, M. "Solar Pumped CW 18W Nd:YAG Lasers." Jpn. J. Appl. Phys., Pt. 1, 23, 1051–1053 (1984). 21. Shiner, C.S. "Design and Chemical Synthesis of Iodine-Containing Molecules for Application to Solar-Pumped I* Lasers." NASA Report CR-176854 (1986). 22. Yogev, A., Shwartz, J., Levy, I., and Shapiro, M. "Solar Pumped Lasers for Energy Intensive Photochemistry in a Solar Central Receiver." Pp. 745–755 in Solar Thermal Central Receiver Systems, Proceedings of the Third International Workshop 2 (1986). 23. "Power Solar Pumped Laser a Success." Chem. Eng. News, p. 12, Nov. 24, 1986. SOLAR FUELS AND BIOMASS CONVERSION* Background A long-sought goal of energy research has been a method to produce hydrogen fuel economically by splitting water using sunlight as the primary energy source. Several methods make use of sunlight to produce fuels from available and abundant raw materials. Unfortunately, their large-scale implementation generally involves significant energy costs and capital outlay. After the initial capital investment, sunlight is an attractive means of providing a renewable source of energy to drive the process(es). However, the combination of capital costs to provide the concentrated solar energy and the elaborate and expensive plants required to carry out the chemical processes places a heavy financial burden on this approach to a clean and renewable energy economy. Alternatively, if sunlight were used in nonconcentrated systems, the cost per unit area of the converter would have to be very low to make a system viable. The interrelationships between classes of solar energy conversion and hydrogen production technologies are illustrated in Figure 2-7 [1]. Nonconventional energy sources (i.e., direct solar energy, wind energy, ocean thermal energy, nuclear breeders, fusion reactors, and geothermal energy) do not possess the attributes of fossil fuels. However, solar, wind, and ocean thermal energy sources are environmentally compatible (not an insignificant factor), and the supply is nearly limitless [2]. Shortcomings in nonconventional energy sources point to the need for an intermediary energy system to link primary energy sources to the consumer sectors of the economy. Many synthetic fuels (hydrogen, methane, methanol, ethanol, ammonia) meet the criteria of abundance, storability, transportability, and environmental compatibility, but H2 best meets these criteria [2-5]. A solar hydrogen energy system represents the only potential closed energy system (H2O → H2 + O2 and H2 + O2, → H2O). *   References for this section appear on pages 46–47.

OCR for page 11
Potential Applications of Concentrated Solar Photons FIGURE 2-7 Flow diagram for hydrogen production technologies. Source: [1]. Status of Technology Solar-Driven Methods of Hydrogen Production Solar-driven fuel-processing methods include thermal decomposition and thermochemical, photochemical, electrochemical, biochemical, and hybrid reactions. Appendix F gives a brief description of each. Feedstocks include inorganic compounds (water and carbon dioxide) and organic sources (oil shales, coal, biomass, and methane). The approaches to carry out these processes run the gamut from well-established chemical engineering practices with near-term predictable costs to long-term basic photochemical processes, the details of which are still speculative. Thus, the goal remains elusive because near-term systems are costly, while the costs of advanced long-term systems are not well defined. The economics notwithstanding, past research efforts have ranged from pure theoretical studies of potential reaction sequences to investigation of specific chemical reactions to fabrication and testing of pilot plants for investigating the total process. In the past two decades, thermochemical water decomposition by closed cycle processes has received considerable attention. In a thermochemical process, thermal energy is transformed into chemical energy (hydrogen) without the need to first convert heat to mechanical energy and then to electrical energy, as is the case in water electrolysis. Only a few of the processes examined have practical applications [6]; some of these are elaborated on in Appendix F: the sulfuric acid/iodine cycle (General Atomics); the hybrid sulfuric acid cycle (LASL, Westinghouse, Euratom); the hybrid sulfuric acid/hydrogen bromide cycle (Euratom); the bismuth sulfate/sulfuric acid cycle (LASL);

OCR for page 11
Potential Applications of Concentrated Solar Photons the zinc selenide cycle (Lawrence Livermore National Laboratory); the calcium bromide/iron oxide cycle—UT-3 (University of Tokyo, Japan); and the magnesium/iodine cycle (National Chemical Laboratory, Japan). A combination (hybrid) of several reactions (photochemical, thermochemical, electrochemical) presents certain advantages with respect to solar energy utilization as it embraces both the heat generated in a solar reactor and the high-energy photons available. Reversibility of the photochemical step still presents some difficulties. The hybrid systems are promising devices for technical solar energy utilization, although little is known about eventual corrosion problems and stability, among other factors [7]. Most hydrogen-generating cycles may require a membrane for separation of hydrogen at a high temperature. Fundamental materials research aimed at such a membrane is to be encouraged. Biomass as a Source of Fuels and Chemicals Hydrogen can also be produced from the gasification of biomass. A significant advantage of using biomass as a hydrogen feedstock is that the CO2 produced in biomass conversion does not constitute a net release of CO2 [8]. Unfortunately, the biomass resource base, while renewable, is several times smaller than the coal resource base. Production of biofuels is constrained by requirements of careful soil management and by considerations of dedicating land to fuel production rather than food production. At 1989 levels of energy consumption in the United States, biofuels cannot supply half of the U.S. transportation energy demand, let alone all sectors of a hydrogen economy. The more abundant hydrogen feedstocks are coal, a low-cost resource base for the near to medium term, and solar water electrolysis (low environmental impact) in the long term [8]. Additionally, biomass conversion to hydrogen is less efficient (1 to 5 percent) than direct solar chemical conversion (25 percent) and is significantly more costly [9]. Conversion of biomass to bulk chemicals, with or without solar photons, could prove profitable [10,11]. Recent efforts at SERI, with government and industry support, have focused on biomass (wood chips, sawdust, bark) pyrolysis to produce valuable oils. Phenols derived from the process are significantly cheaper ($0.10 to $0.27/1b vs. $0.42/1b) than those currently used in the plastics industry. Solar thermal energy could be used to heat the reactor (650°C), but in this particular process it was not needed [12] as the conversion process supplied its own energy. Research and Development of Solar Fuels Production Technologies in Various Countries Data as of 1987 show that several countries [the United States (SNL), Germany, Israel, Australia, the USSR, and Spain] have had active, ongoing research and development (R&D) programs on thermochemical energy systems utilizing solar collectors and concentrators [13]. All of these research and development efforts have focused principally on thermochemical energy transport systems based on CO2 and steam reforming of methane. International collaborations, under the auspices of the International Energy Agency (IEA), have been fruitful. The CAESAR (CAtalytically Enhanced Solar Absorption Receiver) project between the SNL and German Aerospace Research Establishment (DLR) in Stuttgart has successfully demonstrated the technical feasibility of CO2 reforming of methane with a direct absorption/receiver/reactor on a parabolic dish of approximately 150 kW of solar power [14]. This joint venture is being phased out in FY 1990; the German DLR establishment is continuing the project.

OCR for page 11
Potential Applications of Concentrated Solar Photons Competitive Technologies to Solar-Driven Fuels Hydrogen is produced primarily by steam reforming of hydrocarbon feedstocks. For ammonia and methanol it is produced by steam reforming of natural gas, while hydrogen supplies for petroleum refining derive from octane-enhancement reformers, fuel-gas and naphtha reformers, and residual partial oxidation gasifiers [15]. Steam reforming is expected to continue since it requires both low capital investment and low production costs. Energy and exergy* process efficiencies for steam/methane reforming are 80 percent and 78 percent, respectively, and 59 percent and 49 percent, respectively, for coal gasification. The cost of steam reforming of natural gas (approximately 10 percent of natural gas consumption) is approximately <33 percent as much as when produced from coal or from water electrolysis. Production costs are expected to increase as the feedstock shifts from natural gas to oil, coal, and other sources (e.g., solid wastes) [5,15]. Gasification of solid wastes and sewage [16] to a synthesis gas, on reforming, yields hydrogen. This solid waste concept can solve two problems: disposal of urban refuse and sewage and is a source of hydrogen fuel. Certain H2 production processes have reached maturity to possible commercial exploitation: (1) steam reforming of natural gas, (2) catalytic decomposition of natural gas, (3) partial oxidation of heavy oils, (4) coal gasification, and (5) steam-iron coal gasification. Others, such as thermochemical, photochemical, photoelectrochemical, and photobiological processes are early in the R&D stage. Economic Analyses Costs of fuels computed by using figures on energy efficiencies are misleading [4]. Other consequences of the use of each fuel, particularly health hazards and environmental damage, and its monetary value have not been considered and added to the cost of fuel production. Cost and economic comparisons are particularly difficult because these costs are too often based on certain economic premises and on guessed future costs of raw materials. These factors cannot be estimated to any degree of reliability since market, supply and demand, and economic forces along with political decisions often dictate the real costs. Exergy analyses of solar/thermochemical hydrogen production show that the ratio of the actual rate of hydrogen production to the theoretical maximum rate decreases linearly with the system irreversibilities and exergy losses [17]. There are capital costs attached to exergy losses, and production costs depend on both capital cost and efficiency. For example, for the General Atomics S/I2 process, exergy losses occur mainly in the receiver/collector system and in the thermochemical process. Exergy efficiencies in these two areas are, respectively, 53 percent and 56 percent; energy efficiencies are 71 percent and 50 percent. This leads to overall 35.5 percent and 29.6 percent efficiencies, respectively [17]. Table 2-2 shows a comparison (in 1987 dollars) of costs for H2 production of various processes noted above. *   Exergy analyses take into account both the first and second laws of thermodynamics. The exergy of a stream is the maximum work obtainable as the stream is brought to equilibrium with its environment; it is a measure of the quality of an energy stream.

OCR for page 11
Potential Applications of Concentrated Solar Photons TABLE 2-2 First and Second Law Efficiencies (n1,n2), Plant Size, and Typical Costs of Solar Hydrogen (capital charge rate = 0.10) Solar Process n1 n2 Plant Size (GJ of H2/yr) Operation $(1987)/ GJ of H2 Thermolysis 4.11 3.39 300 Modular 68 Hybrid thermolysis 5.39 4.44 600 Modular 46 GA cycle 25.60 21.03 1.35 x 106   62 Mark 13 21.50 17.67 106   39 Cristina 21.50 17.67 106   60 Mark 11-A 22.00 18.08 106   34 Mark 11-B 68.71a 56.46a 106     Thermo electrolysis 18.90 15.53 106   48 PV electrolysisb –11 9.04 1 Modular 57-125 a Daily efficiency. b $1 to 3/W (peak) installed. Source: [18]. Most costs and economic analyses of fuel production have accounted for only costs of production; no account has been taken of other costs inherent down-scale to consumption and its effects on the quality of life. A recent (1989) comprehensive comparative analysis for hydrogen production costs taken with the use of hydrogen in various sectors, particularly transportation, has been made by DeLuchi [8]. Another cost/efficiency analysis reported in 1987 [19] indicates that the costs of hydrogen production from improved thermochemical processes and advanced water electrolysis are similar and in the range of $14 to $17/GJ of hydrogen. In calculating the costs of fuels to society, their environmental effects and damages must be considered [20]. Effects of fossil-fuel-produced acid rain and CO2 on the greenhouse effect are but two examples, together with the effects of pollution on human health. An Industry's Experience in Solar Fuel Research The Gas Research Institute (GRI) has evaluated the cost of solar-produced fuels, assuming a hypothetical, nonconcentrating pond system capable of making hydrogen or methane using membrane separation techniques [21]. Costs were based on land area, membrane requirements, and various other costs to arrive at a fuel price. Methane costs for the pond were estimated at $30/GJ and for systems that use concentrated solar photons at $82/GJ. Methane production for mature thermochemical processes has been estimated at $40/GJ. These costs are to be contrasted with less than $2/GJ for current wellhead natural gas prices. Process efficiencies and material costs have a strong influence on solar methane production costs. The cost of photovoltaic-operated electrolysis to produce hydrogen was estimated at $20/GJ for mature plants. By comparison, 1988 prices for hydrogen produced by the reforming of natural gas were set at about $7/GJ [21]. Development and Commercialization of New Technologies As in the cases of coal, oil, and even nuclear energy, whose technical and economic development started some 35 years ago and which nonetheless contribute only 4 percent of the primary energy consumption worldwide, solar energy and solar fuels will require time for R&D, and for introduction to the marketplace and adaptation to the worldwide established energy system

OCR for page 11
Potential Applications of Concentrated Solar Photons before its growth dynamics can no longer be interrupted [22]. Seen in this light, solar energy and solar fuels have since the mid-1970s (just over 10 years of R&D), perhaps 20 percent to 25 percent of their development history behind them. A continuous positive political attitude plus appropriate support and a wide consensus among energy users will be required if solar energy is to make any headway. Conclusions Conclusion: Utilization of concentrated solar photons for the production of solar fuels is a long-term prospect, requiring further multidisciplinary R&D at all levels. Conclusion: Little is known about high-temperature photochemical processes that might be photothermally activated by solar photons (many suns). Conclusion: Biomass conversion to bulk chemicals using concentrated solar photons has not been widely studied. Conclusion: The feasibility of steam reforming and CO2 reforming of methane has been examined using solar concentrators in several countries. Recommendations Recommendation: Research in high-temperature photochemical processes might explore the use of high solar fluxes in presently energy-demanding industrial processes, such as biomass conversion and hydrogeneration of biomass using H2O as the hydrogen source followed by thermal cracking to obtain bulk chemicals and fuels. Also, potential multiphotonic processes under high photon flux might be explored. Recommendation: Basic laboratory-scale studies are needed in biomass conversion to expand the knowledge base, as appropriate. Recommendation: The United States should foster international collaborations in the area of renewable energy resources (e.g., under the auspices of the IEA). REFERENCES 1. Hanson, J.A., Foster, R.W., Escher, W.J.D., and R.R. Tison. "Solar/Hydrogen Systems for the 1985-2000 Time Frame: A Review and Assessment." Int. J. Hydrogen Energy, 7, 3 (1982). 2. Nejat Veziroglu, T. "Hydrogen Technology for Energy Needs of Human Settlements." Int. J. Hydrogen Energy, 12, 99 (1987). 3. Winter, C.-J. "Hydrogen Energy — Expected Engineering Breakthroughs." Int. J. Hydrogen Energy, 12, 521 (1987). 4. Sastri, M.V.C. "Hydrogen Energy Research and Development in India — An Overview." Int. J. Hydrogen Energy, 12, 137 (1987). 5. Dinga, G.P. "Hydrogen: The Ultimate Fuel and Energy Carrier." Int. J. Hydrogen Energy, 14, 777 (1989). 6. Yalcin, S. "A Review of Nuclear Hydrogen Production." Int. J. Hydrogen Energy, 14, 551 (1989). 7. Getoff, N. "Photoelectrochemical and Photocatalytic Methods of Hydrogen Production: A Short Review." Int. J. Hydrogen Energy, 15, 407 (1990). 8. DeLuchi, M.A. "Hydrogen Vehicles: An Evaluation of Fuel Storage, Performance, Safety, Environmental Impacts, and Cost." Int. J. Hydrogen Energy, 14, 81 (1989).

OCR for page 11
Potential Applications of Concentrated Solar Photons 9. Nozik, A.J. "Solar Energy Conversion." In Advances in Solar Energy Technology, Vol. 3, (W.H. Bloss and F. Pfisterer, eds.), Pergamon Press, Oxford (1987). 10. Lede, J., Villermaux, J., Royere, C., Blouri, B., and Flamant, G. "Utilisation de l'energie solaire concentree pour la pyrolyse du bois et des huiles lourdes du petrole." Entropie, 113, 57–69 (1983). 11. Hunjan, M.S., Mok, W.S.-L., and Antal, M.J., Jr. "Photolytic Formation of Free Radicals and Their Effect on Hydrocarbon Pyrolysis Chemistry in a Concentrated Solar Environment." I&EC Res., 28, 1140 (1989). 12. Chum, H. "Inexpensive Phenol Replacement from Biomass: An ongoing Technology Transfer Effort." Workshop on Potential Applications of Concentrated Solar Photons, NRC, Nov. 7–8, 1990. Golden, Colo. 13. Fish, J.D. "Application of Solar Technology to Fuel Production Chemical Processing and Thermochemical Energy Transport: Status and Future." In Advances in Solar Tech-nology, Vol. 3, (W.H. Bloss and F. Pfisterer, eds.), Pergamon Press, Oxford (1987). 14. Tyner, C. Rapporteur Report of Session 6 by A. Hunt in Workshop on Potential Applications of Concentrated Solar Photons, NRC, Nov. 7–8, 1990, Golden, Colo. 15. Baird, H.A. "Hydrogen Energy: An Engineer's Perspective." Int. J. Hydrogen Energy, 8, 867 (1983). 16. Private communication (1979) to G.P. Dinga from the Hydrogen Energy Corp., 6030 Connecticut Ave., Kansas City, Mo. (previously the Billings Energy Corp., Provo, Utah). 17. Funk J.E., and Knoche, K.F. "Hydrogen by Thermochemical Water Splitting." P. 2894 in Advances in Solar Energy Technology, Vol. 3, (W.H. Bloss and F. Pfisteier, eds.), Pergamon Press, Oxford (1987). 18. Bilsen, E. "Thermal, Thermalchemical, and Hybrid Solar Hydrogen Production." Submitted at the Workshop on Potential Applications of Concentrated Solar Photons, NRC, Nov. 7–8, 1990, Golden, Colo. 19. Engels, H., Funk, J.E., Hesselmann, K., and Knoche, K.F. "Thermochemical Hydrogen Production." Int. J. Hydrogen Energy, 12, 291 (1987). 20. Bockris, J.O'M. "The Economics of Hydrogen as a Fuel." Int. J. Hydrogen Energy, 6, 223 (1981). 21. Krist, K. "Gas Research Institute Experience in Solar Fuel Research." Submitted at the Workshop on Potential Applications of Concentrated Solar Photons, NRC, Nov. 7–8, 1990, Golden, Colo. 22. Nitsch, J., and Winter, C.J. "Solar Hydrogen Energy in the Federal Republic of Germany: 12 Theses." Int. J. Hydrogen Energy, 12, 663 (1987). SOLAR AIR CONDITIONING Brief Description Solar concentrators are prominent components of absorption-cycle-based and desiccant-cycle-based air conditioners. When concentrator development brings heating unit technology to a point where concentrated sunlight competes, at least in areas where electrical power is expensive, with the gas units now used in absorption or desiccant coolers, solar concentrators will have a niche in a market. DOE has had, at least since 1974, programs in solar air conditioning. As soon as SERI and DOE perceive that the needed solar concentrators are manufacturable at appropriate performance and cost, business analysis on how to bring the technology to market will be appropriate. Uses of solar coolers in applications other than air conditioning (e.g., in chemical processing and separations, might also be considered). Recommendation With the assistance of the business analysis community, the status of solar concentrators in the context of solar coolers should be periodically reviewed.