GB2548123A

GB2548123A - Aircraft superconducting electrical propulsion system

Info

Publication number: GB2548123A
Application number: GB1604028.9A
Authority: GB
Inventors: Palmer Joseph; Mark Husband Stephen
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2016-03-09
Filing date: 2016-03-09
Publication date: 2017-09-13
Also published as: GB201604028D0

Abstract

An aircraft superconducting electrical propulsion system comprising an electric motor 104 configured to drive one or more propulsors 102. Each electrical motor is doubly (fully) superconducting, having superconducting stator (106, figure 7) electrical windings and super conductor rotor (108) electrical windings. The system comprises at least one closed reverse Brayton cycle cryocooler (optionally two recuperative cycle cryocoolers, see figure 6) operable on a working fluid (e.g. helium or hydrogen) configured to cool the electric motor 104. The cryo-cooler comprises a thermal accumulator 136 comprising a heat storage medium (e.g. aluminium, lithium, nitrogen or methane) having a higher volumetric heat capacity (VHC) than the working fluid. The cryocooler may comprise in fluid flow series, a compressor, heat exchanger in thermal communication with a heat sink, a turbine and a second heat exchanger configured to exchange heat with at least the electric motor. Figure 3 illustrates alternative thermal accumulator positions A, B, C, D in the cryocooler. The thermal accumulator may meet peak energy demands (e.g. during take-off, see figure 4).

Description

Aircraft Superconducting Electrical Propulsion System

The present disclosure concerns a heat management system for a superconducting aircraft electrical network.

Aircraft comprising superconducting electrical networks have been proposed, for example in Next Generation More-Electric Aircraft: A Potential Application for HTS Superconductors, Luogno et al, published in IEEE transactions on Applied Superconductivity, vol. 19, issue 3. In such aircraft, the superconducting electrical network comprises an electrical interconnector which conducts electricity between an electrical source (comprising, for example, a gas turbine engine or fuel cell), and an electric motor, which in turn drives a propulsor. In a superconducting electrical network, at least one of the electrical components comprises a superconductor. For example, one or more of the electrical interconnector, electrical generator stator windings, electrical generator rotor windings, electrical motor stator windings, and electrical motor rotor windings could be superconducting. Such a “hybrid” propulsion design offer advantages, for example, due to the possibility of the placement of large numbers of propulsors in a “distributed propulsion” arrangement.

Superconducting electrical networks increase the efficiency of the electrical network by reducing resistive losses. This leads to direct efficiency improvement, as well as reducing the amount of heat that must be rejected by the system, thereby providing further efficiency improvements. Furthermore, such networks generally have higher current carrying capacities for a given mass and volume, thereby providing further advantages.

As recognized by Luogno et al, a particularly beneficial arrangement is where the electrical motor is “doubly” or “fully” superconducting, i.e. comprises both superconducting rotors and stator windings. In such an arrangement, the power density of the electrical motor can be increased, without encountering core magnetic saturation. Flowever, currently available superconductors only have superconducting properties below a critical temperature Tc which is generally below the ambient temperature at which aircraft operate. For example. superconductors of the yttrium barium copper oxide (YBCO) family have a critical temperature of approximately 90 Kelvin. Consequently, they require cooling in use. Furthermore, such superconductors become “quenched”, i.e. lose their superconducting properties when a critical current density is exceeded. This critical current density can be increased by reducing the temperature of the superconducting component.

Previously, it has been suggested to use either a store of a cryogenic fluid, or a cryocooler in order to maintain the superconducting components below their critical temperature. Cryostorage is not generally considered to be a viable solution for long distance aircraft, in view of insulation requirements, and the quantity of coolant required, and therefore the additional weight of such a system. Where a cryocooler is employed, the size (i.e. cooling capacity) of the cryocooler is determined by the most demanding operating point of the system, generally takeoff. The size and power requirements of the cryo-cooler generally increases exponentially as coolant temperatures are reduced. Consequently, particularly for long range aircraft, the cryogenic cooling system represents a large proportion of the weight of the overall propulsion system, and accounts for significant losses in view of the relatively low efficiencies of cooling systems with high temperature differentials. Consequently, it is desirable to minimise the size of the heat management system, to thereby reduce total propulsion system weight. A discussion of cryocooler types and design is provided in “Cryocoolers: the state of the art and recent developments” by Ray Radebaugh, published in the Jounral of Physics: Condensed Matter 21 (2009). A further discussion of electrical and cryogenic components of aircraft electric propulsion systems is discussed in “Weights and Efficiencies of Electric Components of a Turboelectric Aircraft Propulsion System” by Gerald V Brown, published in the 49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 4-7 January 2011, Orlando, Florida.

Consequently, the present invention aims to provide an aircraft superconducting network heat management system which seeks to address some or all of the above problems.

According to a first aspect of the invention there is provided an aircraft superconducting electrical propulsion system comprising: an electric motor configured to drive one or more propulsors, each electrical motor having superconducting stator electrical windings and superconducting rotor electrical windings; at least one closed reverse Brayton cycle cryocooler operable on a working fluid configured to cool the electric motor; wherein the cryocooler comprises a thermal accumulator in thermal contact with the working fluid, the thermal accumulator comprising a heat storage medium having a higher volumetric heat capacity than the working fluid.

Advantageously, it has been found that, by providing a thermal accumulator, the working fluid can be maintained at a relatively low temperature while the thermal load is increased, resulting in lower temperatures with a relatively small capacity cryocooler. Consequently, one or both of the cryo-cooler weight and the electric motor weight can be reduced.

The working fluid may comprise helium.

The electric motor may comprise any of a wound field synchronous motor, an induction motor, a permanent magnet motor, a reluctance motor and a switched reluctance motor.

The heat storage medium may comprise one or more of aluminium and lithium.

The cryocooler may comprise, in fluid flow series, a working fluid compressor, a first heat exchanger configured to exchange heat with a heat sink, a working fluid turbine, and a second heat exchanger configured to exchange heat with at least the electric motor.

The electrical propulsion system may comprise first and second reverse Brayton cycle cryocoolers, the thermal accumulator being in thermal contact with the working fluid of one or both of the cryocoolers. The electrical propulsion system may comprise an intermediate heat exchanger configured to exchange heat between the working fluid of the first cryocooler with the working fluid of the second cryocooler. The first cryocooler may comprise the thermal accumulator, and may comprise the second heat exchanger configured to exchange heat with at least the electric motor. The second cryocooler may comprise the first heat exchanger configured to exchange heat with the heat sink.

The thermal accumulator may be provided in thermal contact with the working fluid between any of the compressor and the first heat exchanger, the first heat exchanger and the turbine, the turbine and the second heat exchanger, and the second heat exchanger and the compressor of the at least one cryocooler.

The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects of the invention may be applied mutatis mutandis to any other aspect of the invention.

Embodiments of the invention will now be described by way of example only, with reference to the Figures, in which:

Figure 1 is a schematic of an aircraft propulsion system including an electrical network heat management system in accordance with the present disclosure;

Figure 2 is a schematic of a component of the system of figure 1;

Figure 3 is a schematic illustrating alternative arrangements of electrical network heat management systems in accordance with the present disclosure;

Figure 3 is graph illustrating cooling requirements of an aircraft electrical network cryogenic cooling system in different phases of an aircraft flight cycle;

Figure 5 is a graph illustrating a typical relationship between critical current density and temperature for a superconducting material;

Figure 6 is a schematic illustrating an alternative reverse Brayton cryocooler arrangement; and

Figure 7 is a schematic cross sectional view of an electrical motor suitable for use in the system of the present disclosure.

With reference to Figures 1 2 7 and 8, a schematic representation of a hybrid aircraft electrical propulsion system 100 is shown.

The system 100 comprises at least one propulsor. In this case the propulsors comprise propellers 102, but could comprise ducted fans or other forms of propulsion. Each propeller 102 is driven by a respective doubly superconducting AC electrical motor 104 (described in further detail below) having superconducting stator windings 106 and rotor windings 108. Each AC electrical motor 104 is supplied with electrical power from a superconducting electrical bus 110, typically comprising a high temperature superconductor such as a member of the yttrium barium copper oxide (YBCO) family which is superconducting below a critical temperature Tc. The electrical bus 110 is in turn supplied with electricity from a superconducting electrical generator 112, which is driven by a gas turbine engine 114 comprising a compressor 116, combustor 118 and turbine 119 in fluid flow series. The stator and rotor windings 106, 108 of the electrical motors 104 and, optionally, the electrical bus 110 and the generator 112 form a superconducting electrical network.

The system 100 further includes an electrical network heat management system 120. The heat management system comprises a working fluid such as helium or hydrogen which is configured to cool components of the electrical network via suitable heat exchange mechanisms collectively shown as heat exchanger 122.

The heat management system further comprises a closed reverse Brayton cycle cryo-cooler 124 comprising a compressor 126, a heat sink 128, and a turbine 130 in fluid flow series. The compressor 126 and turbine 130 are connected together by a common shaft 132, which is driven by an electrical motor 134, which may also be superconducting. Such a system may be considered to be of a “reverse Rankine cycle” type, where a phase change occurs in the compressor and / or expansion stages. Such a system is known as a “recuperative” cycle, in which there is a steady flow of working fluid in a single direction in normal use. The heat management system further comprises a thermal accumulator 136, described in further detail below and shown in figure la.

In operation, the compressor 126 is configured to compress the working fluid which has been heated by the heat exchanger 122, thereby increasing its pressure and temperature. The working fluid is then passed to the heat sink 128 where the temperature of the working fluid is reduced. The heat sink 128 comprises a further heat exchanger configured to exchange heat between the working fluid and a further fluid, such as air or gas turbine engine fuel. In some cases, the gas turbine engine fuel may comprise a further cryogenic fuel, such as liquid hydrogen or liquefied natural gas (LNG). Once cooled by the heat sink 128, the working fluid is passed to the turbine 130, which supplies some propulsive power to the shaft 132, and also reduces the temperature of the working fluid. Once cooled by the turbine 130, the working fluid is then passed to the thermal accumulator 136.

Referring to figure 1b, the thermal accumulator 136 comprises a heat exchanger configured to exchange heat between the working fluid and a thermal storage medium such as lithium beads 137 having a higher mass volumetric heat capacity than the working fluid. It will be understood that, in use, where the working fluid has a higher temperature than the thermal storage medium, heat is transferred from the working fluid to the thermal storage medium, while heat flows in the opposite direction when the thermal storage medium is at a higher temperature than the working fluid. Consequently, the thermal accumulator 136 provides additional thermal inertia, which can be used to provide consistently low working fluid temperatures at the inlet to the heat exchanger 122 using a relatively low capacity reverse Brayton cry-cooler, in spite of large transient heat loads. Referring to figure 2, the thermal accumulator 136 comprises a plurality of first pipes 138 containing thermal storage medium in thermal contact with second pipes 140 through which the working fluid flows in use. Where the thermal storage medium is a fluid, the thermal accumulator may comprise a storage vessel (not shown) for storing additional thermal storage medium. In such a case, the thermal storage medium could be arranged to flow through the first pipes 138 (for example in a counter flow direction).

Figure 7 shows a doubly superconducting electric motor 104 suitable for use with the present disclosure. The motor 104 comprises a stator 106 having superconducting electrical windings surrounded by screening 142 to enhance safety. A rotor 108 is provided, also having superconducting electrical windings. The rotor is mounted on a shaft 144 which is in turn mounted by bearings 146 via a shaft seal 148 for rotation about its axis. Working fluid for cooling is provided to the stator 106 and rotor 108 by a cryostat 150 containing working fluid. The motor 104 is in turn typically housed within insulation (not shown) to reduce heat leakage.

Figure 4 shows heat generated by components of the propulsion system during a typical flight cycle for an aircraft having the propulsion system of figure 1. The electrical motors 104 and generator 112 generate heat in operation, which is dependent at least in part on the propulsive power necessary for the current portion of the flight cycle. This heat must be absorbed by the heat management system 120, in order to maintain the components below their respective critical temperatures Tc, in order to prevent quenching. As illustrated in figure 4, the propulsion power required (and therefore the heat generated) varies considerably, and may be particularly high during takeoff, or during a rejecting landing. Typically, heat generation during peak heat load periods can be twice or more than that generated during cruise. A typical flight cycle includes taxiing for several minutes (which generates a small amount of heat), followed by takeoff (which generates peak heat and lasts up to 700 seconds), followed by cruise (which lasts several hours and produces heat loads approximately half those produced during takeoff), followed by descent (which generally produces little heat load) and landing (which generally also generates low heat loads, but may require high heat loads in the event of a rejected takeoff).

In one specific example, an aircraft comprises a pair of gas turbine engines, each driving a respective electrical generator. The generators are electrically coupled to eight electric motors, which each drives a separate propulsor. In this example, the electrical generators, superconducting electrical bus and motors are configured for a power rating of 10 Megawatts (MW) for takeoff, and 5 MW for cruise. The electrical network has a total thermal loss of 0.001% of input power, i.e. produces IkW of heat for each 1MW of electrical input. This heat must be removed by the heat management system in order to maintain temperatures below the critical temperature Tc. An electrical motor having a 10MW power output would be expected to have a mass of approximately 92kg.

The reverse Brayton cryco-cooler is configured to remove approximately 5kW of heat from the working fluid at the required temperature, and so is effectively sized for the cruise condition, rather than the takeoff condition. According to our calculations, a typical estimated weight for a 10 kW cryo-cooler configured to cool a working fluid to 25K using a cryogenic fuelheat sink is approximately 500 kg, and would require an input power of approximately 225 kW. On the other hand, under similar conditions, a 5 kW cryo-cooler would have a weight of approximately 350 kg, and an input power of 112 kW.

In order to maintain the working fluid at the required temperature during the transient takeoff condition for the required 700 seconds (i.e. the overload requirement shown in figure 4), an additional cooling load of 5kW must be absorbed for 700 seconds, i.e. 3.5MJ of heat energy. In the case where the thermal storage medium consists of lithium metal beads (which have a specific thermal capacity of 3.5kJ/kg.K), this heat could be absorbed by 100kg of thermal storage medium, while the temperature would be raised by 10K (e.g. from 15K to 25K). It is estimated that approximately 50kg of thermal insulation may be required, in order to maintain the thermal accumulator at the required temperature, such that the total additional mass of the thermal accumulator in this scenario is 150kg. Consequently, there is a 100kg to 150kg net mass saving in this analysis. This in turn translates into a 2.5% reduction in cruise fuel burn for a typical mission profile, which in turn represents a significant operating cost reduction.

On the other hand, since there is little or no heat load during the taxi phase of operations, the cryo-cooler can be operated at its full power rating during these conditions, thereby cooling the working fluid to well below its critical temperature, e.g. 15K where the critical temperature is 25K. The cryo-cooler could also be operated at high power where situated at the airport terminal, where electrical power is generally cheaper, with this reduced temperature being stored by the thermal storage medium. Consequently, the thermal accumulator can be used to absorb this additional heat, without raising the temperature of the working fluid to a temperature above the critical temperature.

Further weight savings may be realised by the present invention, since the motor weight may be reduced in view of this reduced operating temperature. It will be understood that “quench” is a consideration when designing superconducting electrical motors. Quench is where the superconducting coil enters a non-superconducting (resistive) state. Quench can occur when the temperature rises to above a critical temperature Tc, which is dependent on the material properties of the superconductor, and the internal magnetic field within the coil. As will be understood, in an electrical coil, the internal magnetic field is related to the current density within the coil. Figure 5 shows the relationship between current density and temperature on the critical temperature Tc of a typical member of the YBCQ family of high temperature superconductors. As can be seen, as temperature is reduced, the current density that can be sustained prior to quench occurring is increased. Consequently, where superconducting electromagnetic coils are operated at reduced temperatures, they can support higher current densities, and so can be made lighter and smaller for a given power. Consequently, superconducting electrical machines can be made lighter where they are operated at lower temperatures.

Alternatively, the system can be used to lower the working fluid temperature to a temperature significantly below the critical temperature Tc. Referring to figure 5,

Figure 3 shows alternative arrangements for the disclosed heat management system. The thermal storage medium could be provided in thermal contact with the working fluid at one or more alternative locations within the cycle. Each of these locations would have associated advantages and disadvantages. For example, where the thermal accumulator is provided at location A in the reverse

Brayton cycle loop shown in figure 3, (i.e. after the working fluid has been cooled by the turbine 130, but before it is introduced to the components to be cooled 122), the lower doubly superconducting motor 104 operating temperature at peak load can be utilised by increasing the density of the motor windings 106, 108, thereby reducing the overall weight of the system. Alternatively or in addition, where the thermal accumulator is provided at location B in the reverse Brayton cycle loop shown in figure 3, (i.e. after the working fluid has been warmed by the cooled components 122, but before it is compressed by the compressor 126), the lower compressor inlet temperature enabled by the thermal storage medium at high loads will result in lower compressor power being required in order to compress the working fluid, thereby allowing for reducing compressor power consumption, and therefore reduced overall system power consumption. Alternatively, where the thermal accumulator is provided at location C in the reverse Brayton cycle loop(i.e. after the working fluid has been compressed by the compressor, but before the heat is rejected by the heat exchanger), allows for reduced thermal transients in the heat exchanger, which may reduce thermal stress on this component, thereby reducing component weight or increasing component life.

Figure 6 shows an alternative reverse Brayton cryocooler arrangement 220. The arrangement 220 comprises first 220a and second 220b reverse Brayton cycle cryocoolers, which may increase the efficiency of the system relative to the first system, by reducing the temperature differences at each stage of the cycle.

The first cryocooler 220a comprises a second heat exchanger 222 configured to exchange heat between the superconducting electrical components and the working fluid of the first cryocooler 220a. Downstream of the second heat exchanger 222 is a first compressor 226a configured to raise the pressure of the working fluid, and pump it to a hot side of an intermediate heat exchanger 252 provided downstream. The intermediate heat exchanger 252 is configured to exchange heat between the working fluid of the first cryocooler 220a with the working fluid of the second cryocooler 220b. Downstream of the intermediate heat exchanger 252 is a first turbine 230a, configured to expand and thereby reduce the pressure and temperature of the working fluid. Further downstream is a thermal accumulator 236 similar to the accumulator 136 of the first embodiment. The second heat exchanger 222 is provided downstream, thereby completing the loop.

Similarly, the second cryocooler 220b comprises the cold side of the intermediate heat exchanger 252. Downstream of the intermediate heat exchanger 252 is a second compressor 226b configured to raise the pressure of the working fluid, and pump it to a heat sink heat exchanger 228 provided downstream. The heat sink heat exchanger 252 is configured to exchange heat between the working fluid of the second cryocooler 220b with the gas turbine engine fuel. Downstream of the heat sink heat exchanger 228 is a second turbine 230b, configured to expand and thereby reduce the pressure and temperature of the working fluid. The intermediate heat exchanger 252 is provided downstream, thereby completing the loop. Each of the respective reverse Brayton cycle cryocoolers 220a, 220b comprises a respective electric motor 234a, 234 configured to drive the respective compressor 226a, 226b via a respective shaft 232a, 232b, which is also coupled to the respective turbine 230a, 230b.

Alternative or additional locations for the thermal accumulator are shown in figure 6 at locations 1 a, 2a, 3a, 4a, 1 b, 2b, 3b, 4b. Consequently, one or both of the cryocoolers may comprise a thermal accumulator.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and subcombinations of one or more features described herein.

For example, the superconducting electrical network could comprise further superconducting electrical components cooled by the heat management system. These could include superconducting magnetic energy storage systems, superconducting transformers etc.

Alternative heat storage media could be employed, such as nitrogen or methane, which undergo a phase change around the critical temperature of typical high temperature superconductors. Consequently, such heat storage media could provide additional temperature stability.

The cry-cooler could have multiple compression stages, with a thermal accumulator provided therebetween to provide intercooling.

Claims

1. An aircraft superconducting electrical propulsion system comprising: an electric motor configured to drive one or more propulsors, each electrical motor having superconducting stator electrical windings and superconducting rotor electrical windings; at least one closed reverse Brayton cycle cryocooler operable on a working fluid configured to cool the electric motor; wherein the cryocooler comprises a thermal accumulator in thermal contact with the working fluid, the thermal accumulator comprising a heat storage medium having a higher specific volumetric heat capacity than the working fluid.

2. A system according to claim 1, wherein the working fluid comprises helium.

3. A system according to claim 1 or claim 2, wherein the heat storage medium comprises one or more of aluminium and lithium.

4. A system according to any of the preceding claims, wherein the cryocooler comprises in fluid flow series, a working fluid compressor, a first heat exchanger configured to exchange heat with a heat sink, a working fluid turbine, and a second heat exchanger configured to exchange heat with at least the electric motor.

5. A system according to claim 4, wherein the thermal accumulator may be provided in thermal contact with the working fluid between any of the compressor and the first heat exchanger, the first heat exchanger and the turbine, the turbine and the second heat exchanger, and the second heat exchanger and the compressor of the at least one cryocooler.

6. An aircraft according to claim 4 or claim 5, wherein the electrical propulsion system comprises first and second reverse Brayton cycle cryocoolers, the thermal accumulator being in thermal contact with the working fluid of one or both of the cryocoolers.

7. An aircraft according to claim 6, wherein the electrical propulsion system comprises an intermediate heat exchanger configured to exchange heat between the working fluid of the first cryocooler with the working fluid of the second cryocooler.

8. An aircraft according to claim 6 or claim 7, wherein the first cryocooler comprises the thermal accumulator and the second heat exchanger configured to exchange heat with at least the electric motor, and the second cryocooler comprises the first heat exchanger configured to exchange heat with the heat sink.