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Thermionic generators produce electricity by heating
one surface and cooling and removing heat from another surface. There is a
vacuum between the surfaces or plates. The hot surface emits electrons
from one plate called the cathode and collected on the other plate, the
anode. These devices are governed by the heat engine limitations of Carnot.
Further limitations are caused by the considerable work function of
sending electrons across the vacuum and the poor quality of the electron
emissions from the cathode.
Classically these devices required very high temperatures and has
efficiencies of less than 1%. McDonald Douglas had done considerable work
with thermionic generator at the focus of parabolic dishes.
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Thermo-photovoltaic (TPV) electric power generators : these comprise
a gas burner pre-heated using exhaust heat, with the main flame heating
a SiC emitter, the emitter being surrounded by Gallium Antimonide (GaSb)
or Silicon photovoltaic cells ('solar' cells). TPV electric generators
are used in military and outdoor recreational contexts, for example
recreational vehicles (RVs), and have been proposed as a quiet low
emission power source for electric vehicles. The advantage of the DRAX
burner in the TPV is that it will heat the emitter to a much higher
temperature, emitting more of the near infra-red and visible radiation
that the photovoltaic cells require.
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Further information on thermo-ionic
devices:
A company that has made significant advancements in thermionic
generators is
Borealis Technical Limited. The major breakthrough is in
improving the efficiency and cost reduction of the devices. Their devices
have efficiencies approaching 20%.
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Thermionic Conversion and Related Basic Physics
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Odysseus Mission
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Planck's Linear Oscillator Concept Of Matter Can Explain Thermionic
Direct Conversion Of Heat To Electricity
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Technical background:
DRAX burners currently require access to pressurised air, with at
least a few tens of millibar pressure. Further development could reduce
the pressure requirement, allowing a low power fan to be used, which
could then be powered by the TPV itself without losing significant
output power. Working on the assumption of an achievable radiator
temperature of 1300 C from a conventional burner, 1500 C from an air
pre-heated burner, and 1700 C from a DRAX burner, a simple calculation
gives some idea of the advantage which the DRAX burner would have. With
the simplifying assumption of black body radiation, these are the
figures for both Gallium Antimonide (GaSb) and Silicon photovoltaic
cells:
Gallium Antimonide (band-gap 0.72 eV, 1.75 microns)
Fraction of energy available for conversion
- Ordinary Bunsen burner 21%
- Air pre-heat burner 27%
- DRAX burner 32% (20% improved over Air pre-heat, 50% over
Bunsen)
The DRAX burner might offer further advantages in compactness, in
that its intensity of emission would be increased 50% over Air
pre-heat and
Silicon (band-gap c. 1.2eV, 1.05 microns - the indirect band-gap giving
a less clear cut-off)
- Ordinary burner 2.9%
- Air pre-heat burner 5.4%
- DRAX burner 8.4% (60% improved over Air pre-heat, 190% over
Bunsen)
Clearly the DRAX burner would give spectacular increases in efficiency
using silicon cells. Silicon has the advantage of very low cost,
robustness and lower surface reflection losses which further boost its
performance relative to the idealised GaSb result above.
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