xiSOLAR THERMAL SYSTEMS
The key challenge in solar thermal technology is to identify cost-effective methods to convert
sunlight into storable, dispatchable thermal energy. Reactors heated by focused, concentrated
sunlight in thermal towers reach temperatures exceeding 3,000°C, enabling the efficient
chemical production of fuels from raw materials without expensive catalysts. New materials that
withstand the high temperatures of solar thermal reactors are needed to drive applications of this
technology. New chemical conversion sequences, like those that split water to produce H 2 using
the heat from nuclear fission reactors, could be used to convert focused solar thermal energy into
chemical fuel with unprecedented efficiency and cost effectiveness. At lower solar concentration
temperatures, solar heat can be used to drive turbines that produce electricity mechanically with
greater efficiency than the current generation of solar photovoltaics. When combined with solar-
driven chemical storage/release cycles, such as those based on the dissociation and synthesis of
ammonia, solar engines can produce electricity continuously 24 h/day. Novel thermal storage
materials with an embedded phase transition offer the potential of high thermal storage capacity
and long release times, bridging the diurnal cycle. Nanostructured thermoelectric materials, in
the form of nanowires or quantum dot arrays, offer a promise of direct electricity production
from temperature differentials with efficiencies of 20–30% over a temperature differential of a
few hundred degrees Celsius. The much larger differentials in solar thermal reactors make even
higher efficiencies possible. New low-cost, high-performance reflective materials for the
focusing systems are needed to optimize the cost effectiveness of all concentrated solar thermal
technologies.
PRIORITY RESEARCH DIRECTIONS
Workshop attendees identified thirteen priority research directions (PRDs) with high potential
for producing scientific breakthroughs that could dramatically advance solar energy conversion
to electricity, fuels, and thermal end uses. Many of these PRDs address issues of concern to more
than one approach or technology. These cross-cutting issues include (1) coaxing cheap materials
to perform as well as expensive materials in terms of their electrical, optical, chemical, and
physical properties; (2) developing new paradigms for solar cell design that surpass traditional
efficiency limits; (3) finding catalysts that enable inexpensive, efficient conversion of solar
energy into chemical fuels; (4) identifying novel methods for self-assembly of molecular
components into functionally integrated systems; and (5) developing materials for solar energy
conversion infrastructure, such as transparent conductors and robust, inexpensive thermal
management materials.
A key outcome of the workshop is the sense of optimism in the cross-disciplinary community of
solar energy scientists spanning academia, government, and industry. Although large barriers
prevent present technology from producing a significant fraction of our primary energy from
sunlight by the mid-21st century, workshop participants identified promising routes for basic
research that can bring this goal within reach. Much of this optimism is based on the continuing,
rapid worldwide progress in nanoscience. Powerful new methods of nanoscale fabrication,
characterization, and simulation — using tools that were not available as little as five years ago
— create new opportunities for understanding and manipulating the molecular and electronic
pathways of solar energy conversion. Additional optimism arises from impressive strides in