4
Conclusions and Recommendations

INTRODUCTION

Photochemical processes are widely applied, for example, in photography, microcircuit manufacturing, printing, textiles, paper making, and polymer processing. They are also environmentally relevant, for example, in atmospheric photochemistry (smog formation, ozone formation and loss) and essential to life through photosynthesis. These photon-based processes do not involve concentrated sunlight. Although solar concentrators have been known for a long time, they have been engineered and optimized with respect to manufacturability and cost only in the past two decades. Improvements in design and lower cost may open doors to novel applications. Their possible use in photon-and chemical-based applications is still in the early stages. Extensive research in areas outside mechanical and thermal engineering of concentrators will have to be done, however, for concentrated sunlight to penetrate industrial niches. In the case of lasers, their eventual use resulted not from the work of their inventors (spectroscopists), but from that of materials scientists, chemists, weapons specialists, and electrical and civil engineers. Concentrated sunlight is of potential use in applications where photons are of the essence—involving state-to-state or band-to-band excitation—and in applications where they compete with other heat sources.

Considering potential applications, the committee paid close attention to competing technologies, particularly when looking at near-term applications. On the other hand, recognizing that long-term applications may require as much as 20 years to develop and that solar energy will be more widely used as fossil fuels are depleted and environmental considerations become increasingly important, the committee strongly encourages basic research in solar photo-driven and thermal processes.

GENERAL RECOMMENDATIONS

Personnel

Water and wastewater treatment, waste treatment, materials processing and synthesis, photochemical and photobiological syntheses, solar pumping of lasers, and biomass and fuel



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Potential Applications of Concentrated Solar Photons 4 Conclusions and Recommendations INTRODUCTION Photochemical processes are widely applied, for example, in photography, microcircuit manufacturing, printing, textiles, paper making, and polymer processing. They are also environmentally relevant, for example, in atmospheric photochemistry (smog formation, ozone formation and loss) and essential to life through photosynthesis. These photon-based processes do not involve concentrated sunlight. Although solar concentrators have been known for a long time, they have been engineered and optimized with respect to manufacturability and cost only in the past two decades. Improvements in design and lower cost may open doors to novel applications. Their possible use in photon-and chemical-based applications is still in the early stages. Extensive research in areas outside mechanical and thermal engineering of concentrators will have to be done, however, for concentrated sunlight to penetrate industrial niches. In the case of lasers, their eventual use resulted not from the work of their inventors (spectroscopists), but from that of materials scientists, chemists, weapons specialists, and electrical and civil engineers. Concentrated sunlight is of potential use in applications where photons are of the essence—involving state-to-state or band-to-band excitation—and in applications where they compete with other heat sources. Considering potential applications, the committee paid close attention to competing technologies, particularly when looking at near-term applications. On the other hand, recognizing that long-term applications may require as much as 20 years to develop and that solar energy will be more widely used as fossil fuels are depleted and environmental considerations become increasingly important, the committee strongly encourages basic research in solar photo-driven and thermal processes. GENERAL RECOMMENDATIONS Personnel Water and wastewater treatment, waste treatment, materials processing and synthesis, photochemical and photobiological syntheses, solar pumping of lasers, and biomass and fuel

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Potential Applications of Concentrated Solar Photons conversions are the six applications that were considered by the committee. These areas draw on chemistry, materials science, chemical engineering and civil engineering. The past focus of the Department of Energy's (DOE) solar thermal effort was solar-based electric power generation, and it logically required program personnel in mechanical engineering. The committee finds that the new topic of nonelectric applications of solar energy requires broader expertise in the above disciplines. Therefore, the committee recommends that the disciplinary balance of technical staff and program management be adjusted accordingly. Without a substantive broadening in the disciplinary distribution, it is unlikely that the recommendations of this report could be successfully implemented. An institutional culture to create a more effective collaboration among personnel in various Solar Energy Research Institute (SERI) programs and between SERI and the Sandia National Laboratories (SNL), as well as among university, industrial, and other national laboratories, will be of the essence; even with such collaboration, personnel will have to be added to cover disciplines not adequately represented at this time at SERI. Long Term Fundamental Research The maintenance of a fundamental and diverse solar research program, free of short-term pressures, is of the essence. If new practical concepts are to emerge, such a program is likely to be their source. In contrast to development, engineering, or demonstration projects, fundamental research projects are not costly. The committee sensed that management was applying pressure on researchers to demonstrate technologies within a few years, with a consequent over expenditure of effort on costly large-scale experiments at the expense of more fundamental studies. The review of current knowledge, while identifying only a few short-term applications of concentrated solar photons, revealed potential applications in the long term in each area surveyed. Thus, the committee recommends that fundamental research in diverse areas, such as high-temperature photochemistry (at atmospheric or higher pressures), be supported, with activities at SERI and SNL being devoted to areas in which each can establish scientific leadership. The proposed fundamental research should be followed by bench-scale experiments and, where appropriate, large-scale experiments, in order to both strengthen the knowledge base and evaluate innovative technologies. Market Analysis A recurring theme throughout this report is the need for a reliable, comprehensive analysis of the market opportunities and business-related issues of the proposed applications of concentrated solar photons. The analysis should be slated as early as possible in the project and be completed as part of the justification procedure for scale-up or pilot plant proposals. The analysis must include information on competing technologies, market size and timing, product cycle lifetime, barriers to commercial implementation, assessment of technical risks, and perceived benefit (and value of benefit). Some of the potential applications of concentrated solar photons are to meet a long-term, enduring need (e.g., waste disposal), while others represent business opportunities (e.g., materials processing). These cases need to be considered differently for their impacts. From the detailed market analysis and accompanying economic assessment of the new proposed technology should come definition of specific technical objectives and milestones of pilot plant and demonstration projects.

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Potential Applications of Concentrated Solar Photons User Facilities The establishment of a user program at existing solar concentrator facilities would promote early involvement of, and collaboration by, members of the industrial, academic, and national laboratory communities in potential project areas (e.g., materials processing and photochemistry). SPECIFIC RECOMMENDATIONS Water and Wastewater Treatment Photocatalytic mineralization of trace organic contaminants in water has been widely studied in laboratories over the past decade; only very recently has it been examined at pilot and field scales. A valid science base now proves the theoretical, but not the practical, feasibility of contaminant destruction chemistry. Serious gaps exist, though, in the knowledge required to assess and, if appropriate, implement this knowledge in an industrially relevant technology. The committee recommends that DOE routinely include substantive market analyses and cost studies to identify and prioritize applications to specific contaminated water sources. Pending such studies, large-scale demonstration projects are not encouraged. Expansion of the knowledge base to cover nonconcentrating solar reactors and improvements in photocatalyst performance (via electron transfer enhancement, catalyst surface concentration of the contaminants, and integration with other treatment operations, such as carbon adsorption) is recommended. Waste Treatment New waste treatment technologies in general have had difficulty competing with existing processes because of the wide variability of waste streams and the versatility, reliability, and low cost of existing processes. Solar-energy-driven processes will face a similar difficulty, and developments should be targeted at specific applications, as in the following examples. The cleaning of exhaust airstreams with small amounts of organic contaminants is a pervasive problem in industry well suited to treatment by heterogeneous photocatalysis at near-ambient temperatures. This provides an exciting near-term potential application for solar energy, possibly utilizing trough-type collectors. Small-scale studies are needed on the kinetics of the oxidation, for different catalysts, of the compounds, mostly solvents, of industrial significance and on the identification of any toxic reaction by-products. Such studies will provide the basis for the design, economic evaluation, and testing of prototype reactors. For waste incineration, the enhancement of destruction by solar photons can reduce the temperatures needed and provide an opportunity for using photons for both excitation and heating. Research on high-temperature photochemistry is needed to define the waste streams and the experimental conditions for which solar-based processes can compete with commercial alternatives. Regeneration of activated carbon is an application of solar radiation that is not adversely affected by the intermittency of the energy source. Assessment of a solar-driven process is needed, first with paper studies to determine feasible configurations for a regeneration scheme, followed by bench-scale testing and economic evaluation. Materials Synthesis and Processing Materials synthesis and processing applications have generally been directed toward large-scale, established, conventional processes for which concentrated solar photons currently have a

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Potential Applications of Concentrated Solar Photons significant economic disadvantage. Research should be directed toward new and improved applications that emphasize the special characteristics of concentrated solar photons in contrast to applications using more conventional, well-established thermal and radiative sources. Advanced R&D should consider high-value-added applications as well as more innovative ones for concentrated solar photons. A user program for research, development, and testing that uses existing solar furnaces should be established. Market cost analyses and user involvement are necessary early in the applied "research to proof of concept" development cycle. Photochemical Synthesis Photochemistry is a very old human endeavor. While many chemical and biochemical reactions can be initiated by light and many are involved in vital natural biochemical and atmospheric cycles, no immediate applications have been found in which concentrated solar photons can compete with current industrial processes. Nevertheless, existing new discoveries can be envisioned, and continued innovative fundamental research is encouraged. For example, research aimed at identifying organisms, such as beta-carotene-producing marine algae, that survive under high irradiance and produce high-value biochemicals is recommended. Fundamental research on diverse photon-based processes, including biosynthesis, spectroscopy, dynamics, theoretical, and inorganic and surface chemistry is recommended, with the long-term goal of identifying novel solar applications. Most of the necessary research can be done by using laboratory light sources of 1 sun or less. Consequently, research in the diverse photon-based areas relevant to this field can be performed in academic and industrial laboratories around the world. Solar Pumping of Lasers There have been several successful efforts to demonstrate the feasibility of solar-pumped lasers. In the committee's opinion, their first applications will come in space in systems developed by the National Aeronautics and Space Administration. Efforts to specifically design and develop terrestrial solar-pumped lasers may effectively build on this existing knowledge base. The most productive avenues for research will focus on the special needs and opportunities of terrestrial laser systems. These include identification of lasing media tuned to optimally use the solar spectrum, demonstration of solar-pumped tunable lasers, and evaluation of the unique (and potentially advantageous) scalability issues of terrestrial solar-pumped lasers. Solar Fuels and Biomass Conversion Work has been undertaken to convert sunlight to a useful fuel (i.e., a convenient form of chemically stored energy). Inevitably, much of the effort has been directed at producing hydrogen from abundant and renewable raw materials (water, biomass), with the advantage of being environmentally compatible. The knowledge base in this area has been slowly evolving over the past two decades. While splitting of water to hydrogen and oxygen has been demonstrated, the results are too limited to indicate technological feasibility. It is premature to expect that a hydrogen economy based on present knowledge could materialize. Production of solar fuels must be regarded as a very long term goal, with novel concepts requiring periodic and systematic examination. Concentration of solar photons would yield high temperatures and fluxes up to about 1022 photons/cm2, providing the means to test, on the laboratory scale, thermochemical, photochemical, and photoelectrochemical routes toward fuel production using only research of the most fundamental nature.