Thermoelectric, thermionic and thermophotovoltaic energy conversion

Abstract

Key characteristics of thermoelectric, ballistic thermionic and quasi diffusive thermionic energy converters are compared. First, the main assumptions used to derive the linear Boltzmann transport equations for electrons are examined and the possibility that a higher order transport coefficient may become relevant is discussed. In the linear transport regime, there is a fundamental trade off between high Seebeck coefficient and high electrical conductivity for bulk materials and for many multilayer structures due to the interplay between electronic density-of-states (DOS) and electron group velocity and also due to the shape of DOS versus energy curve deep inside a band. While low dimensional structures alter the density-of-states, a similar trade off still exists. If large barrier heights and high doping concentrations could be achieved solid-state thermionic energy converters would be able to alleviate this trade off, thereby achieving a very high thermoelectric power factor. For this to occur, the electron transverse momentum perpendicular to heterostructure barriers must not be conserved. This can be achieved with non-planar structures or with embedded nanostructures. Finally, a comparison between thermoelectric/ thermionic devices and thermophotovoltaic energy converters shows a difference in the average energy of the emitted hot carriers due to the difference between electronic and photonic density-of-states in the reservoirs. The use of both electrons and photons from a hot reservoir or the engineering of the reservoir density-ofstates may provide additional means to achieve higher efficiency in energy conversion devices and to approach the limit given by the entropy generation more easily.

Nanostructured Thermoelectrics: Big Efficiency Gains from Small Features

“…[ 30 ] The early motivation for these investigations was based on the prediction from Hicks and Dresselhaus [ 1 ] that quantum confi nement of in-plane carrier transport could substantially enhance the power factor over that of homogeneous materials, leading to ten-fold increases in ZT . Such enhancement could occur because sharp features in the electronic density of states of quantum-confi ned structures [ 31 ] (see Figure 2 ) enable a doping-level-tunable increase in the asymmetry between hot and cold electron transport, resulting in a large average transport energy and a large number of carriers moving in the material (i.e., a large Seebeck coeffi cient and electrical conductivity).…”

Solar electron source and thermionic solar cell

“…1,2 On the other hand, thermionic conversion of heat to electricity has been of interest for a century. [3][4][5] The fact that the thermionic emission current depends on the cathode temperature exponentially allows one to increase the device efficiency significantly using a small increase in cathode temperature. Naturally, it is highly desirable to have a solar thermionic electricity generator -a device where the cathode is heated using sunlight.…”

Solid-State and Vacuum Thermionic Energy Conversion

“…Of course the fractional coverage only works when there is good heat spreading at the hot and cold surfaces, so F can not be very low. With 1% coverage, one can produce 10's W/cm 2 with a heat sink requirement of 1W/cm 2 K. Assuming that the hot and cold side temperatures of the thermoelectric leg are constant, it is instructive to note that the expression of the conventional power generation density depends only on the thermoelectric power factor, and it increases as the thickness is reduced: 2 /d. On the other hand, if we assume a heat sink with finite thermal resistance, the expression of power generation density will also depend on material's thermal conductivity and there is an optimum thickness that gives the maximum power.…”

“…However, highly degenerate semiconductors and metals are not good bulk thermoelectric materials due to their low Seebeck coefficient. In reference [ 2 ], we analyzed the trade off between electrical conductivity and the Seebeck coefficient and explained that highly degenerate semiconductors and metallic structures can have high thermoelectric power factors (Seebeck coefficient square times electrical conductivity) if there is an appropriate hot electron filter (potential barrier) that selectively scatters cold electrons. Here, in the near linear transport regime, hot electrons denote carriers that contribute to electrical conduction with energies higher than the Fermi level and cold electrons have energies lower than the Fermi level.…”