EP1259991A1

EP1259991A1 - Thermoelectric power generator for an aircraft

Info

Publication number: EP1259991A1
Application number: EP01914334A
Authority: EP
Inventors: Daniel Scott Marshall; John Edward Summers, Jr.
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2000-02-18
Filing date: 2001-01-25
Publication date: 2002-11-27
Also published as: AU2001239730A1; JP2003523298A; WO2001061768A1

Abstract

A thermoelectric power generator for an aircraft including a heat source or hot air exhaust air flow (30), characterized as providing thermal energy, at least one thermoelectric module (32), including a plurality of thermoelectric elements, positioned in communication with the heat source, and a heat extractor, or a cool air flow (18), positioned in communication with the thermoelectric module so that thermal energy flows through the thermoelectric elements thereby generating electrical power. The thermoelectric module is typically comprised of silicon, boron carbide, a silicon/germanium alloy, geranium, or skutterudite which optionally includes a quantum well structure.

Description

THERMOELECTRIC POWER GENERATOR FOR AN AIRCRAFT

Field of the Invention

The present invention relates to thermoelectric modules as a source of power in aircraft, jet engines, and spacecraft and more particularly, to the use of thermoelectric modules as a power generator in aircraft, such as those including jet engines, to reduce overall weight, reduce maintenance, and increase the fuel efficiency of the jet engine.

Background of the Invention

Airline jet engines typically use fan-jet engines which provide in-flight electrical power by driving a power take-off shaft that drives an alternator, including a transmission. This method of generating in-flight electrical power not only represents a significant weight that must be carried everywhere the airplane flies and requires a significant amount of maintenance, but also is becoming a limiting factor for the total electrical energy needs of modern planes. Some aircraft including future high speed aircraft and virtually all spacecraft have no spinning parts in their engines and therefore need a source of significant electrical power. Thermoelectrics can be applied to these aircraft and spacecraft to generate their electrical needs.

Typically a jet engine operates by compressing cool air that enters its front opening, adding fuel to this compressed air and then igniting the fuel air mixture.

The combustion of this mixture heats the gasses tremendously and a resulting expansion of the hot gasses out the back of the engine provides thrust for the aircraft.

A variation of the jet engine is a turbojet engine. In a turbojet engine, the incoming air is compressed by a spinning compression turbine. A small amount of this expansion energy is returned to the compression turbine by a turbine in the exhaust flow via a central shaft connecting the two turbines. A fan-jet engine is a turbojet engine where a turbine in the exhaust flow returns an additional portion of the energy from the expanding gasses into rotational motion of a large frontal fan via a central shaft. Some designs use a separate central shaft for the fan and the compressor turbine and therefor use a second turbine to extract energy from the exhaust flow to drive the fan. Another design uses a single turbine in the exhaust flow to extract power to drive both the compressor and the fan via a single central shaft. The rotation of the frontal fan acts as a first compression stage to the turbojet engine compressor and accelerates additional cool incoming air around the turbojet engine and out the back of the fan-jet engine to provide additional thrust and efficiency as compared to a turbojet engine without a fan. The additional air is called bypass air or ducted air. Present fan-jet and turbojet engines generate electrical power for the aircraft by tapping into rotational motion of one of the central shafts via a variable input speed, constant output speed transmission. This transmission is large, weighing approximately 1000 lbs. in a typical engine. The transmission in turn rotates an alternator that generates an alternating current, thus generating electrical power at a several hundred hertz electrical output.

Passenger aircraft rely on weight limitations and maintenance limitations for cost control. These weight limitations include aircraft weight, and passenger and luggage weight. These maintenance limitations include fluid levels and fluid change maintenance and worn part replacement maintenance. The less weight structurally included within the aircraft, provides for an increase in allowable passenger/luggage weight, thus more passengers and increased revenue, or alternatively a decrease in fuel costs and increased revenue. Also, reducing moving parts that wear against each other and require fluid for lubrication, provides for reduced operating costs and increased revenue. Typical airline power generation, due to the requirement for a generator and transmission, as previously described, includes a great amount of mechanics, thus weight, moving parts and lubricating fluid. Therefore, it is a purpose of the present invention to provide for a means of decreasing the mechanics and thus weight and maintenance of an aircraft, by providing alternate means for generating power that decreases the mechanics required and thus decreases the weight and maintenance of the jet engine and overall aircraft .

It is another purpose of the present invention to provide for a thermoelectric power generator having included as a part thereof thermoelectric modules for the generation of electrical power.

It is still another purpose of the present invention to provide for a thermoelectric power generator including thermoelectric modules that provide sufficient power to replace the alternator and transmission that is typically used for electrical power generation.

It is a still further purpose of the present invention to provide for a thermoelectric power generator including thermoelectric modules that provides for a decrease in the size of structural members supporting the wings of an aircraft, thereby decreasing the overall weight of the aircraft.

It is still a further purpose of the present invention to provide for a thermoelectric power generator, such as for use in a jet engine, including thermoelectric modules that removes the obstruction to gas flow in a jet engine resulting in a more streamlined, powerful, and efficient engine.

It is still a further purpose of the present invention to provide for a thermoelectric power generator for a jet engine including thermoelectric modules that advantageously utilizes the extreme temperature differences generated within the jet engine to generate power.

It is still a further purpose of the present invention to provide a thermoelectric power generator for a jet engine including thermoelectric modules that advantageously utilizes the extreme temperature differences between that generated within the jet engine and the outside air to generate power. It is still a further purpose of the present invention to provide a thermoelectric power generator including thermoelectric modules that advantageously utilizes the extreme temperature differences between various portions of the aircraft to generate power. It is still a further purpose of the present invention to provide a thermoelectric power generator including thermoelectric modules that advantageously utilizes the extreme temperature differences between portions of the aircraft and the fuel or oxidant to generate power. It is still a further purpose of the present invention to provide a thermoelectric power generator including thermoelectric modules that advantageously utilizes the extreme temperature differences between gas flows inside the engine and the fuel or oxidant to generate power.

It is still a further purpose of the present invention to provide an aircraft or spacecraft thermoelectric power generator including thermoelectric modules that advantageously utilizes the extreme temperature differences between gas flows inside a rocket engine and the fuel or oxidant to generate power.

It is still a further purpose of the present invention to provide an aircraft or spacecraft thermoelectric power generator including thermoelectric modules that advantageously utilizes the extreme temperature differences between gas flows inside a rocket engine and radiation to dark space to generate power.

It is still a further purpose of the present invention to provide an aircraft or spacecraft thermoelectric power generator including thermoelectric modules that advantageously utilizes the extreme temperature differences between concentrated solar energy and radiation to dark space to generate power.

Summary of the Invention

These needs and others are substantially met through provision of a thermoelectric power generator for an aircraft including a heat source or hot air exhaust air flow, characterized as providing thermal energy, at least one thermoelectric module, including a plurality of thermoelectric elements, positioned in communication with the heat source, and a heat extractor, or a cool air flow, positioned in communication with the thermoelectric module so that thermal energy flows through the thermoelectric elements thereby generating electrical power. The thermoelectric module is typically comprised of silicon, boron carbide, a silicon/germanium alloy, geranium, or skutterudite which optionally includes a quantum well structure.

Brief Description of the Drawings

FIG. 1 illustrates a simplified cross-sectional view of a spacecraft included as a part thereof at least one thermoelectric module according to the present invention;

FIG. 2 illustrates a simplified cross-sectional view of turbofan engine having included as a part thereof at least one thermoelectric module according to the present invention;

FIG. 3 illustrates a simplified cross-sectional view of thermoelectric module positioned as a freestanding module in a jet engine according to the present invention; FIG. 4 illustrates a simplified cross-sectional view of thermoelectric module having one surface mounted to a cold plate in a jet engine according to the present invention;

FIG. 5 illustrates a simplified cross-sectional view of thermoelectric module having opposed surfaces sandwiched between a cold plate and a hot plate in a jet engine according to the present invention; and

FIG. 6 illustrates a simplified cross-sectional view of thermoelectric module in conjunction with a heat pipe in a jet engine according to the present invention.

Detailed Description of the Preferred Embodiments

During the course of this description, like numbers are used to identify like elements according to the different figures that illustrate the invention.

Thermoelectric devices are known in the art which offer considerable flexibility, in amongst other things, power generation. Thermoelectric modules, when serving as power generators, operate by tapping into heat available from warm body or flow and transferring it to a cool body or flow. In transferring heat through the thermoelectric modules, electrical power is generated by the Seebeck effect in the material that composes the modules. Typically a thermoelectric device is constructed of an N- type and P-type semiconductor material, such as bismuth telluride. The N-type and P-type semiconductor material are electrically connected in series and thermally connected in parallel . When heat is passed through the material, electricity is generated between the N-type and P-type semiconductor material.

A preferred embodiment of a thermoelectric power generator for an aircraft including a jet engine, such as a turbojet engine, operates by tapping into the hot jet exhaust gasses and transferring it via thermoelectric modules to the cool bypass air, the cool air rushing by the skin of the plane, or into a cool reservoir. By mounting thermoelectric modules between the bypass air flow stream and the jet exhaust flow stream, or hot exhaust air flow, in the jet engine, a substantial temperature difference is established and thus heat flux can be forced through the thermoelectric modules and therefore generating significant electrical energy for the airplane. This arrangement will allow the thermoelectric modules to produce electrical power that could replace (or supplement) the present method of generating power. This method of producing power could produce hundreds of kilowatts of power with far less weight. Factors involved in the positioning of the thermoelectric modules within the jet engine include the extremely high temperature of the exhaust (approximately 1000C) as a heat source, the extremely low temperature of the cold outside air (approximately -30C) , the limited amount of heat available for conversion, the efficiency of conversion of this heat into electricity, the reliability of the thermoelectric modules in this environment, and the fact that more heat will be pumped into the hot side of the thermoelectric modules, than out of the cold side. Therefore, proper choice of thermoelectric material and positioning or mounting of the thermoelectric module provides for the achievement of near optimum design in the generation of electrical power. Bismuth telluride decomposes at 300C and therefore is unsuitable for use in this application. Silicon germanium alloys can take the temperature extremes of this application, but have such poor efficiencies that they would be extremely bulky and would result in no net benefit. Only a high temperature material or structure with an efficiency approximately three times that of silicon germanium alloy will result in a significant net benefit in this application. As illustrated in FIG. 1, in a preferred embodiment, a spacecraft 1 power generator uses concentrated solar energy 2 in the form of electromagnetic radiation that is converted to thermal energy as it is absorbed by a body in communication with a thermoelectric module 5. Solar energy 2 warms one side 3, the hot side, of thermoelectric module 5. The other side 4 , the cold side, of thermoelectric module 5 must expel the thermal energy that passes through thermoelectric module 5 from hot side 3. This can be accomplished by facing cold side 4 of thermoelectric module 5, or a body in communication with cold side 4 of thermoelectric module 5 toward deep space. Because deep space is very cold, only a few kelvins above absolute zero, it readily absorbs radiation from any surface at a higher temperature such as cold side 4 of thermoelectric module 5. Even though cold side 4 of thermoelectric module 5 is cold relative to hot side 3 of thermoelectric module 5, it is very much warmer than deep space. Another possible heat extractor in communication with cold side 4 is cold liquid that is either warmed or vaporized by the heat emitted by cold side 4 of thermoelectric module 5. Yet another possible heat extractor is cool gas or a cool solid body. The cool solid body could be the spacecraft 1 itself if the external heating of spacecraft 1 is so low that excess heat from the thermoelectric modules 5 can be used to warm spacecraft 1. Another heat source is combustion of fuel and oxidant from spacecraft 1. As illustrated in

FIG. 1, solar energy 2 is directed toward thermoelectric module 5 by a solar concentrator 6 and a directing or focusing mirror 7. Referring now to FIG. 2, illustrated is a simplified cross-sectional view of turbofan engine 10 having included as a part thereof at least one thermoelectric module (discussed presently) according to the present invention. It should be understood that disclosed is the inclusion of thermoelectric modules for power generation in an aircraft, spacecraft, rocket engine, and any jet engine, including all of a fan-jet engine, a turbo fan engine, a turbo prop engine, a turbo jet engine, a ram jet engine and a jet engine. For purposes of illustration, a turbo fan engine is described with reference to the illustrations.

Turbofan engine 10 is generally comprised of a compressor turbine 12 and a fan 14. During operation fan 14 pulls ambient air 16 into engine 10. Ambient air 16 enters engine 10 with a portion of the air, referenced 18, entering compressor turbine 12. Air 18 is compressed by the spinning action of compressor turbine 12. After the air is compressed by compression turbine 12, fuel is injected into combustion chamber 24 and ignites with compressed air 18. This combustion of the fuel and air mixture within combustion chamber 24 heats the gasses tremendously and results in the expansion of the hot gasses through internal turbines 20, 21 and 23 and out the rear of engine 10 as exhaust gasses 30, thereby providing a portion of the thrust for the aircraft. During operation, a small amount of expansion energy is returned to compression turbine 12 by an internal turbine 20 positioned in the exhaust path. This small amount of expansion energy is returned to compression turbine 12 via a central shaft 22 connecting compression turbine 12 and internal turbine 20.

The remaining thrust for the aircraft is provided by fan 14 via second internal turbine 21 which extracts a further amount of expansion energy from the combustion gasses and accelerates air 16 through an internal duct 28, as illustrated, defined by an internal housing 26. Air 16 moves around internal housing 26 and out the rear of the engine 10 through bypass duct system 28 to provide additional thrust and efficiency. This additional air 16 is generally referred to as cool bypass air or ducted air.

Conventional fan-jet and turbojet engines generate electrical power for the aircraft by tapping into the rotational motion of one of the central shafts, such as central shaft 22, via a variable input speed, constant output speed transmission (not shown) . The transmission rotates an alternator (not shown) that generates an alternating current having a several hundred volt electrical output.

It is disclosed in this invention that at least one thermoelectric module 32, preferably a plurality of thermoelectric modules, as illustrated in FIG. 2, each including a plurality of thermoelectric elements, operates by tapping into the excess heat available from the jet exhaust gasses 30 and transferring it to the cool bypass air 16 or into liquid fuel or oxidant (not shown) that feeds the engine or into outside air that is moving past the skin of the aircraft . In the transferring of the heat through the thermoelectric modules 32, electrical power is generated in the material that composes the modules 32 by the Seebeck effect. Traditionally, the Seebeck effect ^' is defined as the ability to convert a temperature gradient from thermal energy into electrical voltage. By tapping into this voltage, electrical energy can be provided by the thermoelectric modules 32. This ability to convert the excess heat into electrical power eliminates the need for the transmission and alternator as found in conventional engines .

Quantum structures have demonstrated higher thermoelectric performance, including higher efficiency, than traditional bulk materials. Quantum structures include compositional or doping variations smaller than approximately 100 angstroms. In this application, efficiency is extremely important and quantum structures designed for this application must be durable enough to remain intact after many hours at elevated temperatures. Therefore it is our intention to design these modules and quantum wells from materials where they can reliably be subjected to the temperature extremes in this application for many hours .

As stated, the thermoelectric power generator for a jet engine, including a turbojet engine and a fan-jet engine, operates by tapping into the hot jet exhaust gasses 30 and transferring it via thermoelectric modules 32 to the cool flow, such as to the cool air 16 rushing by the skin of the plane 34, or rushing through bypass duct system 28 or into a cool reservoir (not shown) . This cool reservoir may be represented by liquid fuel including cryogenic fuel, and thus provides a cool fuel flow, or by oxidant or cryogenic oxidant, and thus provides a cool oxidant flow, that may include vaporizing the fuel or oxidant from its liquid state to a gaseous state. In addition, this cool reservoir may be represented by a cold radiation sink.

During operation, electricity is generated by the thermoelectric modules. This electricity is generally fed into a power conditioning circuit that modifies that voltage and current to a preset, smooth level. The output of this power conditioning circuit then provides the electrical power needs of the aircraft . During fabrication of jet engine 10, optimum designs allowing for the maximum generation of electrical power are achieved by proper positioning of thermoelectric modules 32 relative to air outflow 16 and exhaust flow 30. Accordingly, illustrated in FIGS. 3-6 are various embodiments for achieving this optimum design. It should be understood that thermoelectric modules are anticipated that include a heat pipe, a thermal siphon or a heat sink. More particularly, FIG. 3 illustrates in cross- sectional view a first embodiment of thermoelectric module 32 positioned relative to jet engine 10 according to the present invention. Illustrated in FIG.3, is a first embodiment of the thermoelectric module 40, similar to thermoelectric module 32 of FIG. 2. In this particular embodiment, thermoelectric module 40 is freestanding, more particularly positioned within internal housing 26, thereby providing for the free flow of air 16 and air 30 about thermoelectric module 40. Structural/electrical support 42 is utilized to mount thermoelectric module 40 to internal housing 26.

Referring now to FIG. 4, illustrated in cross- sectional view is a second embodiment of a thermoelectric module 32 positioned relative to jet engine 10 according to the present invention. More particularly, illustrated in FIG. 4, is a second embodiment of a thermoelectric module 50, similar to thermoelectric module 32 of FIG. 2. In this particular embodiment, thermoelectric module 50 is mounted to a plate 52 thereby providing for the free flow of air 30 about thermoelectric module 50 and the flow of air 16 about plate 52. The inclusion of plate 52 aids in the mechanical support of and electrical connection to thermoelectric module 50. Plate 52 is illustrated as mounted within a portion of internal housing 26. It should be understood that thermoelectric module 50 can be mounted on the cold side of hot plate 52, thereby providing for the free flow of air 16 about thermoelectric module 50 and flow of exhaust air 30 next to plate 52. Referring now to FIG. 5, illustrated in cross- sectional view is a third embodiment of a thermoelectric module 60 positioned relative to jet engine 10 (of FIG. 2) according to the present invention. More particularly, illustrated in FIG. 5, is a third embodiment of a thermoelectric module 60, generally similar to thermoelectric module 32 of FIG. 2. In this particular embodiment, thermoelectric module 60 is mounted between a first plate 62, a cold plate, and a second plate 64, a hot plate, which allow for the transfer of heat from exhaust air 30 to cool air 28. More particularly, thermoelectric module 60 is sandwiched between plate 62 and plate 64. As disclosed with reference to FIG. 5, the inclusion of plates 62 and 64 aids in the electrical connection to and mechanical support of thermoelectric module 60. Also either of plates 62 or 64 can be shaped as heat sinks having fins protruding into airflows 28 or 30 respectively, allowing for better heat transfer from airflow 28 or 30 into plates 62 or 64. Plates 62 and 64 would be called heat sinks when shaped with fins. Referring now to FIG. 6, illustrated in cross- sectional view is a fourth embodiment of a thermoelectric module 70 positioned relative to jet engine 10 (of FIG. 2) according to the present invention. More particularly, illustrated in FIG. 6, is a fourth embodiment of a thermoelectric module 70, similar to thermoelectric module 32 of FIG. 2. In this particular embodiment, thermoelectric module 70 is mounted or positioned relative to a heat pipe 72. Heat pipe 72 as illustrated aids in transporting heat from the surface of heat pipe 72 connected to the warm body or flow to the cooler body or flow connected to the opposing side of heat pipe 72. Heat pipe 72, as illustrated, includes a shell or skin 73, having formed on an interior surface a wicking material 74, comprised of a porous substance.

Heat pipe 72 has contained therein a working fluid, designated 76, which produces a vapor 78. In a preferred embodiment, heat pipe 72 is placed between hot exhaust flow 30 and thermoelectric module 70. The inclusion of heat pipe 72, as described allows for better heat transfer between airflow 30 and thermoelectric module 70. Heat from exhaust flow 30 would be carried more efficiently through heatpipe 72 to thermoelectric module 70 than through heat sink 64 made entirely of metal as illustrated in FIG. 5.

In each of the described embodiments, only a single thermoelectric module, namely 32, 40, 50, 60, and 70, is illustrated, but it should be understood that in a preferred embodiment, a plurality of thermoelectric modules is anticipated. In that FIG. 2 is a cross- sectional view, only a limited number of thermoelectric modules 32 are illustrated positioned about internal housing 26.

According to this disclosure, what is sought to be achieved is a means for generating electrical power in a jet engine by including thermoelectric modules capable of generating electricity in response to heat flux and eliminating bulky, heavy, and maintenance intensive mechanical equipment, such as the standard transmission and alternator. With the inclusion of thermoelectric modules, a streamlined, more operating cost efficient means of generating electricity is disclosed. The thermoelectric modules are positioned to maximize the benefit of the exhaust air temperature and the ambient or cool air temperature utilizing the Seebeck effect in the material that composes the thermoelectric modules. It is disclosed that various positions exist for mounting or including the thermoelectric modules in conjunction with a jet engine, hot and cold sections of aircraft and spacecraft skin, or rocket engine for generating power and accordingly, such instances are intended to be covered by this disclosure.

Claims

What is claimed is:

1. A thermoelectric power generator for an aircraft comprising: a heat source characterized as providing thermal energy; at least one thermoelectric module, including a plurality of thermoelectric elements, positioned in communication with the heat source; and a heat extractor, positioned in communication with the thermoelectric module so that thermal energy flows through the thermoelectric elements thereby generating electrical power.

2. A thermoelectric power generator as claimed in claim 1 wherein the heat source is at least one of a jet exhaust, a frictionally heated aircraft surface, and a combustion source.

3. A thermoelectric power generator as claimed in claim 1 wherein the heat extractor is at least one of a ducted air flow, a fuel, an oxidant, a cryogenic liquid, a cryogenic gas, a cold gas flow, and ambient air.

4. A thermoelectric power generator for an aircraft as claimed in claim 1 wherein the at least one thermoelectric module includes one of silicon, silicon germanium alloy, germanium, skutterudite, and boron carbide .

5. A thermoelectric power generator for an aircraft as claimed in claim 1 wherein the at least one thermoelectric module includes a quantum structure.

6. A thermoelectric power generator as claimed in claim 1 wherein the thermoelectric module includes at least one of a heat pipe, a thermal siphon, and a heat sink.

7. A thermoelectric power generator as claimed in claim 6 wherein the heat pipe is positioned adjacent a portion of a frictionally heated aircraft.

8. A thermoelectric power generator as claimed in claim 1 wherein the thermoelectric module is positioned to provide for direct exposure to at least one of the heat source and the heat extractor.