Potential Applications of Concentrated Solar Photons (1991)

In 1989 the National Research Council formed a committee, upon the request of the Department of Energy (DOE), to assess potential applications of concentrated solar photons beyond the production of electricity. The committee interpreted the term applications to be those of commercial value, that is, applications in which the use of concentrated solar photons leads to (1) a new product or process, creating a new market; (2) cost reduction for an existing product or process; (3) improvement in a product or process; or (4) provision of a technical service.

The impetus for the existing research could be traced to the oil crisis of the 1970s, when extensive efforts were initiated in the use of light energy to drive useful chemical reactions. Reactions of interest included those that produce possible fuels (mainly hydrogen), interesting synthetic reactions, and reactions for decomposition of waste materials in water. Aqueous and nonaqueous systems involving semiconductor materials as electrodes were optimized for the production of electricity and for the use of semiconductor particles, such as TiO₂, for the decomposition of low concentrations of waste materials in aqueous solutions.

The goal of this study was to determine whether special advantages might result when concentrated solar photons are the source of energy for photochemical, photoelectrochemical, and thermal processes. The study undertook to assess the state of the art of potential applications, such as water and waste treatment. Other possible applications of solar photons, such as materials processing and solar pumping of lasers, also were considered.

In assessing potential applications of solar energy, the particular advantages and limitations must be considered. Although solar energy is an environmentally benign, inexhaustible resource that can be used in both small-and large-scale configurations, it is a diffuse energy source; its collection requires large areas. Solar energy is also an intermittent and unreliable source of energy. Moreover, solar radiation itself is not directly storable. These considerations become important in comparisons of solar-energy-based technologies with competitive ones. Finally, the energy distribution of solar radiation is largely in the visible and near-infrared regions, regions generally not suitable for most photochemical or semiconductor photoelectrochemical reactions of interest. For example, terrestrial sunlight has only about 4 percent of its spectrum in the ultraviolet region (energies greater than about 3.0 eV). Solar energy is most useful in locations remote

from other available power sources (e.g., space applications). This report, however, deals mainly with potential terrestrial applications.

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 up to 20 years to develop and that solar energy will be more widely used as fossil fuels are depleted, the committee strongly encourages basic research in solar photo-driven and thermal processes.

The committee's recommendations are as follows.

DOE should broaden the disciplinary distribution of its technical staff and program management to reflect the breadth of the potential application fields. Lacking a substantive expansion in the technical base, it is unlikely that the recommendations of this report could be successfully implemented.

DOE should continue to support fundamental research in diverse areas. With a full knowledge of academic research in this field, activities at the Solar Energy Research Institute and Sandia National Laboratory should be oriented to areas in which they can establish scientific leadership. The proposed fundamental research should be followed by bench-scale experiments in order to both strengthen the knowledge base and evaluate innovative technologies.

DOE should proceed with business analysis of the proposed applications prior to scale-up or pilot plant. The analysis of markets, competition, and cost 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.

DOE should establish a user program at existing solar concentrator facilities. This 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 photo-chemistry).

Research on the knowledge base for water and wastewater treatment via photocatalysis is recommended in the following areas: new photocatalysts, catalyst immobilization and surface modification, catalyst stabilization, reaction mechanisms and kinetics, reaction promoters and quenchers, photoreactor design, and process integration. DOE should routinely include substantive market analyses and cost studies to identify and prioritize applications to specified contaminated water sources. Pending such studies, large-scale demonstration projects are not encouraged.

Research on high-temperature photochemistry is needed to define the waste streams and experimental conditions for which solar-based processes can compete with commercial alternatives.

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 research and development 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 utilizes existing solar furnaces should be established.

Although the committee was unable to identify immediate applications in which concentrated solar photons can compete with current industrial processes, continued innovative fundamental research is encouraged in diverse photon-based fields, including biosynthesis, spectroscopy, dynamics, and theoretical, inorganic, and surface chemistry, with the long-term goal of identifying novel solar applications. Most of this research can be performed in academic and industrial laboratories around the world.

Efforts to specifically design and develop terrestrial solar-pumped lasers should benefit and build on the existing knowledge base at the National Aeronautics and Space Administration. The most productive avenues for research should 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 scalability issues of terrestrial solar-pumped lasers.

Production of solar fuels must be regarded as a very long-term goal, with novel concepts requiring periodic and systematic examination. The high temperature and photon flux realized in the solar furnace would enable testing on the laboratory scale of thermochemical, photochemical, and photoelectrochemical routes toward fuel production using only research of the most fundamental nature.