Thermoelectrics: Direct Solar Thermal Energy Conversion
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3. C.E. Kennedy, “Review of Mid- to High-Temperature Solar Selective Absorber Materials” (NREL Report TP-520-31267, NREL, Golden, CO, 2002; www.nrel.gov/csp/troughnet/pdfs/31267.pdf ) (accessed January 2008). 4. C.E. Kennedy, H. Price, “Solar Engineering 2005: Proceedings of the 2005 International Solar Energy Conference” (ISEC2005), 6–12 August 2005, Orlando, Florida (NREL Report CP-520-36997, American Society of Mechanical Engineers, New York, 2006; www.nrel.gov/csp/troughnet/ pdfs/36997.pdf) p. 749 (accessed January 2008).
5. C. Kennedy, K. Terwilliger, M. Milbourne, “Development and Testing of Solar Reflectors” (NREL Report CP-520-36582, NREL, Golden, CO, 2005; www.nrel.gov/docs/fy050stify05osti/36582.pdf) (accessed January 2008). 6. L. Moens, D. Blake, “Advanced Heat Transfer and Thermal Storage Fluids” (NREL Report CP-510-37083, NREL, Golden, CO, 2005; www.nrel.gov/docs/ fy050stify05osti/37083.pdf) (accessed January 2008). 7. R.G. Reddy, Z. Zhang, M.F. Arenas, D.M. Blake, High Temp. Mater. Processes 22 (2), 87 (2003).
Thermoelectrics: Direct Solar Thermal Energy Conversion Terry M. Tritt (Clemson University, USA), Harald Böttner (Fraunhofer Institut für Physikalische Mebtechnik, Germany), and Lidong Chen (China Academy of Sciences, China)
Introduction
The field of thermoelectricity began in the early 1800s with the discovery of the thermoelectric effect by Thomas Seebeck.1 Seebeck found that, when the junctions of two dissimilar materials are held at different temperatures (∆T ), a voltage (V ) is generated that is proportional to ∆T. The proportionality constant is the Seebeck coefficient or thermopower: α = −∆V/∆T. When the circuit is closed, this couple allows for direct conversion of thermal energy (heat) to electrical energy. The conversion efficiency, ηTE, is related to a quantity called the figure of merit, ZT, that is determined by three main material parameters: the thermopower α, the electrical resistivity ρ, and the thermal conductivity κ. Heat is carried by both electrons (κe) and phonons (κph), and κ = κe + κph. The quantity ZT itself is defined as α 2 σT k e + k ph
(
)
(1)
where σ is the electrical conductivity. In addition, the thermoelectric efficiency, ηTE, is given by
hTE
1 + ZT − 1 = hC 1 + ZT + TC TH
2
(2)
where ηC is the Carnot efficiency, ηC = (TH–TC)/TH and TH and TC are the hot and cold temperatures, respectively. Thus, a significant difference in temperature (large ∆T ) is also needed to generate sufficient electrical energy, and the infrared (IR) region of the solar spectrum can supply the needed hot temperature, TH. This is important because IR radiation generates only waste heat in conventional semiconductor-based solar photovoltaic cells. It was not until the mid-1900s, when semiconductor materials research became prevalent, that thermoelectric materials and devices became more important.2,3 Semiconducting materials permit band tuning and control of the carrier concentration, thus allowing optimization of a given set of materials. A thermoele
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