of ~2.4 at 300K and quantum-dot PbTe/PbTeSe superlattices^ (Harman et al. 2002) offering a ZT
of ~2 at 550K. Most of the enhancements have been attributed to lattice thermal conductivity
reduction in nanoscale dimensions. It is anticipated that further reduction is possible with a
comprehensive understanding of phonon transport in low-dimensional systems. There is also
potential for significant ZT enhancement through quantum-confinement effects (Hicks and
Dresselhaus 1993).
Thermophotovoltaics
Significant progress has been made in the TPV cells^ (Coutts et al. 2003). The efficiency of TPV
systems depends critically on the spectral control so that only useful photons reach the PV cells.
Ideally, spectral control should be done at the emitter side, although filters standing alone or
deposited on PV cells are also being developed. However, the temperature of the emitters
exceeding 1,000°C imposes great challenges to the stability of the materials and structures used
in a TPV system, especially for those components that provide spectral control.
Solar Concentrators and Hot Water Heaters
Today’s concentrators generally consist of a precise shaped metallic support structure and silver-
glass reflector elements with an average reflectivity of 88%. They are responsible for more than
50% of the investment costs 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/glass components to integrated systems manufactured using mass production technique,
such as those associated with polymeric materials. Major limitations of currently availably
polymers are outdoor durability (UV; water, oxygen, mechanical stress, thermal stress) for at
least 20 years. Needs include development of thin-film protection layers for reflectors, high
strength, high thermal conductivity polymers; development of materials with high transparency
and durable glazing for heat exchangers, and engineered surfaces that prevent dust deposition on
reflector surfaces.
Thermal Storage Materials
Innovative thermal storage methods must address the need to provide reliable electricity supply
based on demand, which generally does not coincide with the incident sunlight periods, as
demonstrated in Figure 57. Achieving this requires the development of high energy density, high
thermal conductivity, and stable, latent heat materials for thermal storage. One promising
approach is using encapsulated and nanocrystal polymers.
The operating conditions (i.e., temperature and pressure) of the thermal storage must match those
of the power conversion process, and therefore, vary from 80–150°C for low-temperature
systems and 400–1,000°C for high-temperature systems. Solar-derived fuels become the logical
choice for storage at >1,000°C.