2002), left-handed materials, and doping with lanthanides are concepts from the optics
community which can be exploited for the spectral control components required in TPV systems.
Key research needs are as follows:
- Nanostructured metallic and dielectric materials with low diffusion and
evaporation rates: The major challenge of spectral control for TPV systems is
given by the high operating temperatures of ∼1,200°C in the case of fuel-
powered TPV and ∼2,000°C in the case of solar TPV. Diffusion processes and
evaporation of material may limit the durability of the components
significantly. Suitable concepts of material engineering to reduce theses
effects have to be developed and fully understood by using multiscale models. - Scalable manufacturing processes applicable to various geometries: The
optical approaches mentioned are based on materials properties and on precise
nanostructuring of the materials. Currently, the techniques for producing
nanostructures are top-down approaches that are limited to small
homogeneously structured areas and to flat surfaces. They are also not cost-
efficient. Thus, novel manufacturing techniques need to be developed, and
these very likely will incorporate self-organization processes. - Novel device concepts: Novel device concepts such as microgap TPV and
device structures should be explored. Tunneling of evanescent and surface
waves can lead to devices with higher power density and efficiency.
Technological challenges to maintain the gap in the range of 10 nm to
submicrons in high-temperature systems must be solved for such concepts to
be useful in practical systems.
Solar Concentrators and Hot-water Heaters
Today’s concentrators generally consist of a precisely shaped metallic support structure and
silver-glass reflector elements with an average reflectivity of 88%; they are responsible for more
of 50% of the investment cost of concentrating solar systems. Likewise, the primary challenge
for widespread implementation of nonconcentrating solar thermal systems is to substantially
reduce the initial cost of installed systems. Future research should aim at a paradigm shift from
metal and glass components to integrated systems manufactured by mass production techniques,
such as those associated with polymeric materials. The major limitations of currently availably
polymers are associated with their inability to withstand outdoor elements, such as ultraviolet
radiation, water and oxygen exposure, and mechanical and thermal stresses, for at least 20 years.
Needs include development of thin-film protection layers for reflectors; high-strength, high-
thermal-conductivity polymers; materials with high transparency and durable glazing for heat
exchangers; and engineered surfaces that prevent dust deposition on reflector surfaces.
Heat transfer surfaces for water heaters call for polymer and composites with high mechanical
strength, ultraviolet degradation resistance, and high thermal conductivity. Concentrator support
structures require polymers with high mechanical strength and low thermal expansion