The present invention relates to electrical generation systems for aircraft and methods
of generating electrical power. In particular, the present invention relates to electrical
generation systems for aircraft that utilise waste heat generated by aircraft systems.
In the prior art, the electrical power needs of aircraft are met by generators that are
driven mechanically by the engines. Generally, each engine may have two generators
attached to it. Backup sources, such as an auxiliary power unit (APU) and a ram air
turbine (RAT) are also known to be provided. Each of these power sources requires
aviation fuel to be burnt in order to generate power. In the case of the generators
^ ^ attached to the engines, the increase in fuel burn is caused by the extra load on the
engine over and above the requirements for moving the aircraft. In the case of the
RAT, the increase in fuel burn is caused by the additional aerodynamic drag caused by
the exposition of the RAT to the external air stream. An APU on the other hand
generally comprises an engine that directly burns its own supply of fuel. In each case,
fuel must be burned to generate electrical power for the aircraft.
The present invention provides an electrical power generation and distribution system
for an aircraft, comprising one or more heat sources, a thermoelectric generator
arranged to receive waste heat from at least one heat source and to convert the waste
heat to electrical current, the thermoelectric generator being in electrical connection
^ with an electrical distribution bus which is operable to supply power to aircraft
electrical systems.
There follows a detailed description of embodiments of the invention by way of
example only with reference to the accompanying drawings, in which:
Fig. 1 is a schematic representation of a power generation and distribution system
embodying the invention;
Fig. 2 is a cross-sectional view of a thermoelectric generation device;
Fig. 3 is a cross-sectional partial view of a thermoelectric generation device installed
on an engine exhaust;
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Fig. 4 is a plan view of a panel of thermoelectric generation devices;
Fig. 5 is a graph of current against temperature difference for a thermoelectric device
embodying the invention; and
Fig. 6 is a graph of voltage against temperature difference corresponding to the graph
of Fig. 5.
In Fig. 1, there is shown an electrical generation and distribution system 1 comprising
a plurality of sources of heat 2,3,4,5 each connected to a respective thermoelectric
generator 6,7,8,9. Primary sources of heat 3,4 are shown in the middle of the
arrangement, and comprise first and second engines for driving the aircraft. The
engines are the greatest producers of heat on the aircraft. A secondary source of heat
w comprises an environmental control system (ECS) 2 which is provided on the left of
the arrangement shown in Fig, 1, the ECS being responsible for maintaining
temperature and pressure conditions in the aircraft. The ECS 2 includes a number of
heat exchangers which output heat usable by the thermoelectric generator 6 connected
thereto. A further secondary source of heat comprises an APU 5 which is connected to
another thermoelectric generator 9. The connection between the APU 5 and the
thermoelectric generator 9 is formed preferably by incorporating the generator into a
thermal management system of the APU. The generator 9 therefore advantageously
provides cooling to the APU.
Each thermoelectric generator 6,7,8,9 is connected to an individual heat source
^ 2,3,4,5, although in an alternative embodiment, the heat sources may each have a
plurality of generators, arranged about the heat source to maximise the capture of heat
from that source 2,3,4,5. Each generator 6,7,8,9 comprises a plurality of solid state
thermoelectric elements.
The thermoelectric generators 6,7,8,9 are connected to the electrical power
distribution system 10 of the aircraft, which distributes the electrical power to the
various loads of the aircraft. In a preferred embodiment, the thermally derived power
is used to operate the cabin systems and other non-essential loads.
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An additional source of heat can be provided by fuel cells used to generate electricity
for the aircraft. Fuel cells may operate at an efficiency of around 50%, whilst the
efficiency of thermoelectric systems generally approaches 37% thus leading to an
overall efficiency approaching 68.5% for a combined system of fuel cells and
thermoelectric devices to capture their waste heat.
Fig. 2 is a cross-sectional view of a portion of a thermoelectric generator 6,7,8,9
comprising a plurality of solid state thermoelectric elements 13,14,15,16,17,18. The
thermoelectric elements are doped alternately with n-type and p-type material and are
sandwiched between thermally conductive layers 11 and 12. Thus the elements
numbered 13,15 and 17 are n-type material, whilst those numbered 14,16 and 18 are
^ * p-type material. In one embodiment, the thermally conductive layers 11 and 12
comprise ceramic wafers. A heat source is situated close to the ceramic wafer 12,
which is shown at the top of the generator in Fig.2. In practice, the generator 6,7,8,9
can be oriented at any suitable angle to harness heat 26 from the heat source.
Electrically conductive interconnection members 19, 20, 21,22,23,24,25 are situated
alternately between the ends of the thermoelectric elements 13-18 to allow a flow of
current. Heat 26 from the heat source causes charge carriers to diffuse from the hot
end of the thermoelectric elements to the cooler end thereof in accordance with the
Seebeck Effect. This leads to a flow of current through the device.
Fig. 3 shows a cross-section of part of a thermoelectric generator 3,4 affixed to an
^ exhaust section 31 of an aircraft engine. The engine can be any type of aircraft engine.
In practice, the thermoelectric generator 3,4 can extend completely around the exhaust
section. In use, hot exhaust emissions pass through the exhaust section of the engine,
and heat 26 is transferred from the emissions via the exhaust section 31 to the heat
receiving surface 12 of the thermoelectric generator 3,4. It is believed to be possible to [
generate 70kW of electrical energy per engine. Even higher temperature differentials !
can be found in the combustor region of the engine, and if suitably temperature
resistant materials are developed, this could also be used as a thermal power source.
The outer surface of the generator 3,4, ie the thermally conductive layer 11 is '
advantageously exposed to the ambient air outside the aircraft, thus ensuring
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continued cooling of the outer surface and hence maintenance of the temperature
difference across the generator that is required to generate electricity.
Fig. 4 is a plan view of a form of the thermoelectric generator 3,4 that may be used in
accordance with an embodiment of the invention. The thermoelectric generator 3,4
comprises a panel, which can have dimensions of 3m by lm. In the manufacture of the
thermoelectric generator, the panel 3, 4 is constructed of a plurality of semiconductor
elements which are connected together serially whereby the potential difference across
each element is summed in operation. Fig. 5 shows a graph of current against temperature difference for a thermoelectric
^ generator. With increasing temperature difference across the generator, the current
produced rises non-linearly. According to Fig.6, the voltage across the generator rises
abruptly at an activation temperature difference TA and then levels off, increasing little
for subsequent rises in temperature difference. The generator produces a DC voltage
which is useful for many aircraft loads that run on DC power.
The generators 6-9 can advantageously be operated in a cooling mode. If an external [
current is applied to the generators cooling occurs in accordance with the Peltier
effect.
In an alternative embodiment, the thermoelectric generator can comprise an Infra-Red
^ photovoltaic generator. The thermoelectric elements 13-18 form part of an array
which can extend both along and across the array, to form a lattice of elements. j
Advantageously, embodiments of the present invention reduce fuel usage, by I
exploiting waste heat rather than throwing it away. The invention therefore reduces
environmental damage and reduces costs. Further, the present invention has the
advantage of allowing greater flexibility to the distribution of the energy around the
aircraft. For example, energy can be used closer to the location at which it is generated. Also rectification of AC currents can be avoided by generating DC currents
as needed.

We Claim:
1. An electrical power generation and distribution .system for an aircraft
comprising one or more heat sources,
a thermoelectric generator arranged to receive waste heat from at least one heat source
and to convert the waste heat to electrical current, the thermoelectric generator being
in electrical connection with an electrical distribution bus which is operable to supply
power to aircraft electrical systems.
2. A system according to claim 1, wherein the one or more heat sources include
an engine exhaust section.
3. A system according to claim 1 or 2, wherein the one or more heat sources
^ ^ include an auxiliary power unit.
4. A system according to any of the preceding claims, wherein the one or more
heat sources include an environmental control system.
5. A system according to any of the preceding claims, wherein the one or more
heat sources include a ftael cell.
6. A system according to any of the preceding claims, wherein the thermoelectric
generator comprises one or more solid state thermoelectric elements.
7. A system according to any of the preceding claims, wherein the thermoelectric
^ 1 generator comprises alternating elements of n-type and p-type semiconductor material.
8. A system according to claim 7, wherein the semiconductor elements are
electrically connected to adjacent elements by electrically conductive cormectors.
9. A system according to claim 8, wherein the semiconductor elements are
electrically connected in series.
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10. A system according to claim 8 or 9, wherein the semiconductor elements are
thermally connected in parallel.
11. A system according to any of the preceding claims, wherein in use a DC
voltage is produced by the generator.
12. A system according to any of the preceding claims, wherein the thermoelectric
generator comprises an infra-red photovoltaic device.
13. An electrical power generation and distribution system substantially as herein
described with reference to the accompanying drawings.