Patent Publication Number: US-2023159176-A1

Title: Motor drive system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage of, and claims priority to, Patent Cooperation Treaty Application No. PCT/GB2021/051030, filed on Apr. 29, 2021, which application claims priority to Great Britain Application No. GB 2006278.2, filed on Apr. 29, 2020, which applications are hereby incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     The present disclosure is concerned with motor drive systems. The efficiency of motor drives systems is of interest due to the environmental impact of inefficient drive systems. Motor drive systems may be used in the operation of both land-based and airborne vehicles. 
     According to most estimates, airline traffic is set to double every fifteen years providing a significant increase in the operation of airborne motor drive systems. Inefficient drive systems lead to greater usage of resources to generate the drive required to account for the inefficient system. As such, usage of resources (such as kerosene) can be reduced by the development of more efficient systems. Emissions from many drive systems are known to be harmful whether produced at ground level or at altitude. 
     Though use of alternative fuels is known and these provide advantages over present fuels, there are areas of motor drive systems which can be improved upon. As there are a number of elements within even the most simplistic motor drive system, any attempt to improve the efficiency of a motor drive system has a large number of possible starting options. 
     Therefore, despite a number of advances in the improvement of efficiencies of motor drive systems, there remains the desire for a more efficient system. The present disclosure described a motor drive system which has a wide range of previously unavailable advantages which are described herein. 
     SUMMARY OF THE INVENTION 
     Viewed from a first aspect there is provided in this disclosure a motor drive system comprising: a fuel cell; a motor, electrically connected to the fuel cell; and, a cryogenic system arranged to contain a cryogen, wherein the fuel cell is arranged to output current to the motor, and wherein the cryogenic system is arranged to communicate a cryogen from the cryogenic system to the fuel cell. 
     Viewed from a second aspect there is provided an aircraft comprising the motor drive system of the first aspect. 
     Viewed from a third aspect there is provided a method of operating a motor, the method comprising: providing a fuel cell; providing a motor; providing an electrical connection between the fuel cell and the motor; providing a cryogen to the fuel cell; providing a cryogen to the fuel cell; and, outputting electrical power direct from the fuel cell to the motor. 
     Viewed from a fourth aspect there is provided an aircraft propulsion apparatus comprising: a propeller arranged to generate thrust on rotation in air; and, a motor drive system arranged to cause rotation of the propeller, the motor drive system comprising: a fuel cell; a motor, electrically connected to the fuel cell; and, a cryogenic system arranged to contain a cryogen, wherein the fuel cell is arranged to output current to the motor, and wherein the cryogenic system is arranged to communicate a cryogen from the cryogenic system to the fuel cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more embodiments of the disclosure will now be described, by way of example only, and with reference to the following figures in which: 
         FIG.  1    shows a schematic of a motor drive system; 
         FIG.  2    shows a schematic of a motor drive system according to an example of the present disclosure; 
         FIG.  3 A  shows a schematic of a portion of a motor for a motor drive system according to an example of the present disclosure; 
         FIG.  3 B  shows a schematic of a portion of a motor for a motor drive system according to an example of the present disclosure; 
         FIG.  4    shows a schematic of a portion of a motor for a motor drive system according to an example of the present disclosure; 
         FIG.  5    shows a schematic arrangement of a motor drive system according to an example of the present disclosure; and, 
         FIG.  6    shows a schematic of a motor drive system according to an example of the present disclosure arranged within a portion of an aircraft. 
     
    
    
     Any reference to prior art documents in this specification is not to be considered an admission that such prior art is widely known or forms part of the common general knowledge in the field. As used in this specification, the words “comprises”, “comprising”, and similar words, are not to be interpreted in an exclusive or exhaustive sense. In other words, they are intended to mean “including, but not limited to”. The disclosure is further described with reference to the following examples. It will be appreciated that the invention as claimed is not intended to be limited in any way by these examples. It will also be recognised that the invention covers not only individual embodiments but also combination of the embodiments described herein. 
     The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and/or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and/or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the spirit and scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc, other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in future. 
     DETAILED DESCRIPTION 
     The present disclosure is concerned with motor drive systems and specifically to aircraft motor drive systems which can provide improvements on current efficiencies. 
       FIG.  1    shows a schematic of a motor drive system  10 . The modern motor drive system  100  may have an electricity source  12  which produces electrical power. The electrical power is passed along electrical conduit  14 . The electrical conduit  14  has a transformer  16 ,  18  arranged at either end (electricity source  12  end and motor  20  end). The electrical power is transformed so as to reduce the losses of the electrical power along the length of the electrical conduit  14 . Such losses may come from eddy currents or the like. Indeed, the transformers  16 ,  18  reduce losses due to the joule effect associated with the transfer of high current, where high voltage is preferred to high current in conventional systems  10 . 
     The electrical power from the electricity source  12  is stepped up (the voltage is increased and the current lowered) by the first transformer  16  prior to travelling along the electrical conduit  14 . This travel stage may be across a relatively large distance and so lowering the current prior to travelling along this distance reduces electrical losses. The electrical power is then stepped down (voltage is lowered and current increased) by the second transformer  18  prior to being supplied to the motor  20 . The motor  20  shown comprises a stator  22  and a rotor  24 . 
     This present system  10  enables electrical power to be supplied to a motor  20  so that propulsive power may be generated. In an example, the electricity source  12  may be a fuel cell. In a specific example, the system  10  shown in  FIG.  1    may have a system portion  11  which includes the electrical conduit  14 , the transformers  16 ,  18 , and the motor  20 . This system portion  11  has an efficiency which can be assessed. The electrical conduit  14  typically has an efficiency of around 98%, the transformers  16 ,  18  typically have an efficiency of around 98%, and the motor typically has an efficiency of 97%. This system portion  11  therefore has an overall efficiency of around 92%. 
     In place of the transformers  16 ,  18 , the arrangement  10  may have the fuel cell  12  arranged so as to produce sufficiently low current so as to avoid the above joule heating, however this still results in a reduced overall system efficiency, as the fuel cell  12  preferentially produces high current. This arrangement  10  may use a balance of a plant controller and some power conditioning at the output. Such power conditioning may be partially controlled using an intermediate electrical storage element, such as a battery or the like. The inclusion of these features again reduces the overall efficiency of the system portion  11 . 
       FIG.  2   a    shows a schematic of a motor drive system  100  according to an example of the present disclosure. The motor drive system  100  shown has an electricity source  110 . The electricity source  110  may be a fuel cell  110 . The motor drive system  100  has a motor  120  which is electrically connected to the fuel cell  110 . The motor drive system  100  has a cryogenic system  130  which is arranged to contain a cryogen. , the cryogenic system  130  is arranged to communicate a cryogen from the cryogenic system  130  to the fuel cell  110 . This cryogen may reduce the temperature of the fuel cell  110 . 
     The motor drive system  100  shown also has an electrical conduit  140  to electrically connect the fuel cell  110  to the motor  120 . The motor drive system  100  shown also has a switch  150  arranged on the electrical conduit  140  which can controllably, and reversibly, break the electrical connection between the fuel cell  110  and the motor  120 . The motor drive system  100  may have a plurality of switches  150 . The switch  150  may be used for safety in shutting down the system  100 . The switch  150  may in an example be a contactor. Different architectures (shown in  FIGS.  2   a  and  2   b   ) show different arrangements which enable safety in different ways. In  FIG.  2   b   , an inverter  118  is shown between the fuel cell  110  and the motor  120 . 
     In an embodiment, the inverter  118  (and/or a converter) may be used to change the voltage or current of the output from the fuel cell  110 . This output may be provided to the motor  120 . The inverter  118  (or power electronics) for the motor  120  or the stator coil may be built into the same unit. For example, the inverter  118  may be integrated or built into the motor  120 . In an example wherein the inverter  118  is built into the motor  120 , the inverter  118  and the stator windings may share the same cryocircuit. A cryocircuit may be a circuit that is cryogenically cooled or may be the arrangement of conduits carrying the cryogen to allow cryogenic cooling of components. Such components may be the motor, inverter and stator windings for example. The inverter  118  may allow a controllably changing magnetic field in the motor  120 . The switch  150  and/or the inverter  118  can act to isolate back EMF from stator coils. This advantageously provides a safety function against energy going back into the system. 
     The cryogenic system  130  may be arranged to contain a cryogen. The cryogen may be a liquid or a gas. The cryogen may be any of liquid hydrogen (LH 2 ) or liquid nitrogen (LN) or Liquid Helium (LHE) or Liquid Natural Gas (LNG) or the like. The cryogenic system  130  may supply a liquid cryogen to the fuel cell  110  for generation of electrical power. By “supply...”, “provide...” or “communicate a cryogen” to various elements, it is meant herein that the cryogen is moved, or allowed to move, into some proximity of the elements so as to be in thermal communication with the elements resulting in the transferral of thermal energy away from the element and into the cryogen. This communication of the cryogen to the element causes a reduction in a temperature of the element. 
     Herein terms such as “cryogen”, “cryogenic substance” and “cryogenic source” may be used interchangeably to refer to the actual substance that is of a cryogenic temperature. Such a substance would in most arrangements be contained within a tank or container or the like. A cryogenic temperature clearly depends on the substance in question however cryogenic behaviour has been observed in substances up to -50° C. Therefore, cryogenic temperature is taken herein to refer to temperatures below -50° C. 
     As used herein, the term cryogenic source or cryogen is deemed to be a non-restricting term and so may refer to any of liquid hydrogen, liquid natural gas, liquid nitrogen, liquid helium, and the like. The cryogen need not necessarily be only one of the above list. In an example, H 2  may be used as a fuel source, while cryogenic cooling is supplied by, e.g., liquid nitrogen. 
     The electrical conduit  140  along which the electrical power is conducted from the fuel cell  110  to the motor  120  may be supercooled by cryogen supplied by the cryogenic system  130  to reduce transmission losses. It may be advantageous to avoid freezing of the stacks. An option for preventing freezing is to utilise the heat released in the system and route it to prevent freezing. The temperature of the water may be controlled so that water formed is below the dew point but above the freezing point. This will prevent the water freezing upon contact with a surface. Other methods to prevent freezing include arranging any surface which the water may contact to be above the freezing point of water. The thermoelectrical design of the system can be controlled to ensure that heat produced in the system is routed within the cells and stacks to prevent freezing. 
     A further technique involves controlling the inlet properties as well as the expansion of air in the system. Such expansion can drop the temperature of the walls which can lead to water freezing on the cold surfaces. As such, air expansion should be controlled and limited. Control should be exacted over quite how hydrogen and oxygen are allowed to pass through plates of the fuel cell  110 . Such control can be provided by a series of valves and conduits or the like for controlling the air flow in the system. 
     The cryogenic system  130  may also communicate cryogen to the motor  120  so as to cool the motor  120 . Cooling of the motor  120  may entail passing the cryogen within thermal communication of the motor  120  so as to lower the temperature of the motor  120 . In particular the cryogen may pass within thermal communication of the motor  120  to cool the stator coils/windings. The cryogen may be passed in a conduit or the like to enable recycling of the cryogen once some heat has been removed from the motor  120  and/or the electrical conduit  140 . As the cryogen is heated, some cryogen may become suitable for usage as fuel in the fuel cell. This is an efficient method of providing cooling and fuel to a fuel cell  110 . 
     In the arrangement shown in  FIG.  2   , operation of the fuel cell  110  generates electrical power which, subsequently, provides drive to the stator  122  of the motor  120 . In the example shown, the fuel cell  110  operates to provide electrical power. This electrical power is at a high current and travels along the electrical conduit  140  at high current to the motor  120 . The losses, which would be significant and, therefore, preventative for this method to be used in the system of  FIG.  1   , are mitigated by virtue of the cryogen that is communicated to the electrical conduit  140 . The electrical conduit  140  may be made superconducting by the communication of cryogen to the electrical conduit  140  by the cryogenic system  130 . The electrical conduit  140  may be an electrical bus. 
     The motor  120  in the arrangement  100  converts the electrical energy from the fuel cell  110  through the movement of charge (current) into a magnetic field where the current density is the limiting factor on magnetic field strength and hence torque. In order to increase the torque, the current density and therefore cooling is increased. With greater cooling of the electrical bus  140  therefore a more efficient arrangement  100  is provided. In an embodiment, the voltage is very low and the current is very high. The voltage may be used to modulate the field. This can be supported by a cryogenically cooled or a high temperature superconducting arrangement of the electrical bus  140 . 
     The electrical bus  140  may be a high power electrical bus  140  so as to allow passage of high current electrical power produced from the fuel cell  110 . The electrical bus  140  may have to carry very high current density, i.e. a high number of amps per unit area sent directly into the fuel windings. The system  100  may utilise a form of voltage control on the field windings of the motor  120  so as to be able to control the activation and deactivation of the motor  120 . Materials that may be used for the electrical bus include conductive elements such as copper, aluminium, graphene or superconducting (and high temperature superconducting) bus bar, cables, wires or litz e.g. magnesium Boride (MgB2) or the like. 
     The system  100  shown in  FIG.  2    combines, in a synergistic manner, the function of a fuel cell  110  and a motor  120 . This is such that the high current output of the fuel cell  110  may be directly fed into the field windings of the motor  120 . Furthermore, the cryogen contained in the cryogenic system  130  may be communicated to the fuel cell  110 , the motor  120  and the electrical bus  140  to improve electrical efficiencies and the like. There is no need for transformers to step up or down the electrical power produced by the fuel cell  110 . 
     As such, the electric current produced, or output, by the fuel cell  110  is substantially the same as the current that enters, or is input into, the motor  120 . Further, the electric current produced, or output, by the fuel cell  110  is substantially the same as the current that is carried along the electrical bus  140 . 
     “Substantially” has been used here, as the electrical bus may not, in practice, be perfectly superconducting, but rather, in an example, any imperfections in the electrical bus  140  may lead to some degradation in the current (even if minor), in the form of thermal losses along the length of the electrical bus  140 . Such imperfections may stem from e.g. defects in manufacturing or impurities or the like. In another example, imperfections in the cryogenic system  130  and delivery of cryogen may lead to the electrical bus  140  not being perfectly superconducting during the entire duration of its use. As such, some small low level change in the current may occur in practice between the fuel cell  110  and the motor  120  due to these minor losses. However, “substantially” should be interpreted as in contrast to the modern system which involves an active step (e.g. via transformers) of altering the current prior to passing along the electrical conduit  140 . Such an active alteration step has, in an example, been rendered redundant due to the novel arrangement disclosed herein. In an example, an inverter  118  may be placed between the fuel cell  110  and the motor  120 , see  FIG.  2   b   . 
     In this way, over the modern arrangement  10  shown in  FIG.  1   , the present disclosure, an example of which is shown in  FIG.  2   , provides a series of advantages. In particular, the disclosed arrangement  100  reduces the complication and expense of inclusion of these elements. Furthermore, this in turn improves the reliability and efficiency of the system  100 . 
     The fuel cell  110  may be a fuel cell stack  110  containing a plurality of fuel cells. The plurality of fuel cells  110  in a stack may be optimised for high current transfer and share the same structure as (i.e. integrated with) the motor stator  122  housing. The output of the fuel cell stack  110  may be DC, and the field windings may also be DC. 
     The motor  120  may have a power electronic motor drive which is a controller to control the machine such as an inverter, allowing for control of current (more or less or none etc.) put into the stator coils. The shaft output may be used for electrical propulsion or to drive compressors and /or turbines or the like as part of an environmental control system. The stator  122  of the motor  120  may be a cryostator. This may result from a cryogenically cooled stator coil or windings, or a cooling of the stator as a whole. The advantage in cooling just the coils is a reduced use of cryogen to provide the cooling. Therefore, there is a saving in the amount of cryogen used. Furthermore, the cooled coils can be thermally isolated from e.g. the rotor to further reduce requirement of cryogen to maintain a cryocooled coil set. The rotor  124  of the motor  120  may be a permanent magnet as this can be a cost effective way of producing the arrangement  100  shown in  FIG.  2   . In another example, the magnet may be a superconducting magnet (with cryogen provided by the cryogenic system  130 ) or a normal magnet (e.g. rare earth or ferrite core or the like). 
       FIG.  3 A  shows a schematic of a portion  200  of a motor for a motor drive system according to an example of the present disclosure.  FIG.  3 A  in particular shows an axi-symmetrical arrangement of a stator  210  and a rotor  220 . The rotor  220  is an outer rotor  220 . The stator  210  is arranged inwardly of the outer rotor  220 . The cryogen in the cryogenic system may be pumped by a pumping device on the rotor  220  of the motor. 
       FIG.  3 B  shows a schematic of a portion  200  of a motor for a motor drive system according to an example of the present disclosure.  FIG.  3 B  in particular shows an axi-symmetrical arrangement of a stator  230  and a rotor  240 . The rotor  240  is an inner rotor  240 . The inner rotor  240  is arranged inwardly of the stator  230 . A direction of rotation of the inner rotor  240  is shown by arrow A. The cryogen in the cryogenic system may be pumped by a pumping device on the rotor  240  of the motor. 
     The present disclosure can be used with either of the arrangements of  FIGS.  3 A and  3 B , but it may be preferable with 3B (rim-driven). This may be operated with a hollow and cylindrical structure or the like. In another arrangement, not shown, there is a singular rotor and a plurality of stators. In an example, the rotor is arranged inwardly of an outer stator and outwardly of an inner stator. In another arrangement, there may be a series of rotors arranged inwardly and outwardly of a series of stators, in an extension of the arrangements shown in  FIGS.  3 A and  3 B . 
       FIG.  4    shows a schematic of a portion  300  of a motor for a motor drive system according to an example of the present disclosure.  FIG.  4   , in particular, shows an asymmetrical arrangement of a stator  310  and a rotor  320 . The arrangement in  FIG.  4    is a rim-driven motor  300 . The rotor  320  is an inner rotor  320 . The stator  310  is arranged outwardly of the inner rotor  320 . Also shown in  FIG.  4    are blades  330  of a rim driven fan. 
     The motor drive system arrangement described herein may optionally have a cryocooler for performing heat exchange to condense vaporised liquid cryogen back into liquid cryogen. Use of a cryocooler may reduce the amount of cryogen that is ultimately lost during a particular flight, and as such can reduce the running costs of the arrangement. In an example of the arrangement where there is no cryocooler present, vaporised cryogen may be returned to the bulk source to condense back to liquid form. It may alternatively or additionally be used as fuel for the fuel cell. 
       FIG.  5    shows a schematic arrangement of a motor drive system  400  according to an example of the present disclosure. The motor drive system  400  shown has a fuel cell  410 , a branching electrical bus (or series of electrical buses)  420 ,  422 ,  424 ,  426  and a plurality of motors  442 ,  444 ,  446 . The electrical bus  420  that projects from the fuel cell branches into three different branches (or different buses)  422 ,  424 ,  426 . Each of these branches  422 ,  424 ,  426  joins a respective motor  442 ,  444 ,  446 . In this arrangement, the fuel cell  410  may provide electrical power to a plurality of motors  442 ,  444 ,  446 . In this way, propulsion may be generated at a number of different locations within the vehicle in which this motor drive system  400  is arranged. This may enable an efficient distribution of propulsion and therefore more efficient propulsion which, in turn, may lower the requirement on resources provided to the fuel cell to generate propulsion. 
     Furthermore, the cryogen provided to the electrical bus  420 ,  422 ,  424 ,  426  ensures that electrical power may be carried over long distances without incurring high electrical losses. This enables the use of one fuel cell  410  to provide power to a plurality of motors  442 ,  444 ,  446  alongside those motors being distributed in advantageous locations within the vehicle. This therefore reduces the cost of using a plurality of fuel cells and increases the reliability of the system as a whole. 
     In an example, each of the bus portions  420 ,  422 ,  424 ,  426  may have a switch for controlling passage of electrical power. Alternatively or additionally, some combination of the bus portions  420 ,  422 ,  424 ,  426  may have switches for controlling passage of electrical power to a specific motor  442 ,  444 ,  446  or specific combination of motors  442 ,  444 ,  446 . In this way, motors may be selectively and controllably activated based on the required propulsion. E.g. a user may need full power to be provided across all motors and therefore select all switches to be closed. However, for more precise movement, the user may opt to have only certain motors activate since propulsion will be generated from the specific location of that those certain motors. 
     In a specific example, the motor drive system disclosed herein may be used in an aircraft.  FIG.  6    shows a schematic of a motor drive system according to an example of the present disclosure arranged within a portion of an aircraft  500 . The portion of the aircraft  500  contains part of a nacelle  560  and an inner shaft  570 , which may be a centre sting. The motor drive system has a fuel cell  510 , a motor  520 , a cryogenic system  530  and an electrical bus  540 , as described in detail above. The motor drive system components interact substantially as described in earlier examples. The fuel cell  510  is located in the nacelle and away from the gas path as this is a thermal advantageous location for the fuel cell  510 . Airflow indicated by arrow B passes over and through the nacelle leading to locations of different temperatures which can advantageously be used for locating elements of the motor drive system described herein. 
     The electrical bus  540  is shown passing through a guide vane  552 . The electrical bus  540  may pass through any of the plurality of guide vanes  552 ,  554  in the nacelle  560 . In the specific arrangement shown, the cryogenically cooled electrical bus  540  may cool the outlet guide vanes from the exhaust gases that may pass through the nacelle. In a similar manner, thermal control to prevent icing occurring may be provided in the form of water channels through the outlet guide vanes, with water flowing through said channels. A de-icing function may also be provided by such water channels. 
     The motor  520  may be supplied with cryogen from the cryogenic system  530 . The motor  520  therefore may be a fully superconducting motor. As described above, this may improve the efficiency of the motor  520  during use. 
     In the example of  FIG.  6   , the fuel cell  510  is arranged in a nacelle  560 . The fuel cell  510  may also be arranged within the fuselage of an aircraft. It may be beneficial to locate the fuel cell  510  in a nacelle  560 , as heat can be input to the nacelle  560  itself from the fuel cell  510 . This may be via heat exchangers or the like. This may increase the thermodynamic energy of the air flow and therefore provide more thrust from the nacelle  560 . This heat energy may also or alternatively be used for de-icing or the like of the nose. There may also be a beneficial impact on the pressure distribution of the nacelle  560 , and this also provides a use of the low quality heat from fuel cell  510 . 
     The fuel cell may instead be located in centre body  570 . Location of fuel cell  510  after fan blades may be advantageous, this will confine noise output and improves aerodynamics over use in fuselage. 
     The fuel cell may be located in the fuselage of an aircraft. This may be advantageous as it provides the benefit of full integration of fuel cell  510  and motor  520 . This may reduce transmission losses but there may then be a need to transmit fluids from one to the other. As mentioned above, in relation to the nacelle, similar advantages can be obtained by location in the fuselage of the fuel cell. For example, heat exchange into the boundary layer may improve aerodynamic efficiency as well as leading to added efficiencies in downstream propulsors, such as a boundary layer ingestor. 
     As fuel cells require a relatively large amount of volume to be stored, fuel cells may be advantageously placed in nacelle or large fuselage area or the wing, where there is a large volume to accommodate the fuel cells. 
     There may be a ballast tank located in the aircraft to prevent water being deposited to nearby inhabited locations. Therefore, holding the water in the ballast tank enables an option for the water to be stored in the ballast tank and not released. Release of water can be controllably selected so as to be appropriately removed from the aircraft. 
     In the example described herein, the field windings of the motor can be part of a cryogenically cooled conventional, or high temperature superconducting, asynchronous machine. This has an advantage of not requiring expensive magnets but also uses the high magnetic field density capability of cryogenic or high temperature superconducting windings. The stator of the motor may then be driven at constant or stepped (depending upon power demand) current densities reflecting different fuel cell operating conditions. Such conditions may be e.g. nominal power for all conditions except takeoff or emergency power for takeoff or one engine inoperative (OEI) conditions. Use of an asynchronous machine with a variable frequency drive means the rotor torque and velocity vectors can be modified to suit the vehicle demands. This could support also energy recovery and reverse speed operation. This arrangement therefore provides a significant amount of control to a user of the motor drive system disclosed herein. 
     In the present solution, rather than a use of air cooling for the stack, an oxidant is used as the cooling mechanism. This results in a less complex, smaller and lighter stack. As the oxidant is used as a coolant mechanism, the propulsion gas path is not interrupted and therefore the arrangement operates at higher efficiency. The cryogenic fuel is used to allow that one or both of the reactant (gas from the cryogenic fuel) and oxidant may provide a cooling function. 
     In the present solution, skin heat exchangers are used on the boundary of the gas path to the cowling in order to dissipate heat into the flow. This will have an effect in increasing the flow energy but without any associated aerodynamic losses. Once the cryogen is heated up from cooling various elements of the motor drive system disclosed herein, the resulting non-cryogen may be used as a fuel for the fuel cell (or fuel cell stack). This further increases the overall efficiency of the arrangement. 
     The efficiencies provided by the system as described herein, in contrast to the 92% for the arrangement of  FIG.  1   , are as high as 99.8%. As such, this is a significant improvement on modern systems. This directly leads to a drop in the resources required to produce propulsion and therefore has a direct improvement on the environment through which the motor drive system passes. Similarly, the arrangement shown has significant financial benefits for the user of the motor drive system. 
     As such, there is provided herein a motor drive system comprising: a fuel cell; a motor, electrically connected to the fuel cell; and, a cryogenic system arranged to contain a cryogen, wherein the fuel cell is arranged to output current to the motor, and wherein the cryogenic system is arranged to provide a cryogen contained in the cryogenic system to the fuel cell. 
     Components of the system may be arranged in various locations in the aircraft. For example, it is advantageous to avoid freezing in the drive system. As such, location of the elements of the drive system should be considered to prevent freezing. 
     Arrangement of the motor drive system within the aircraft may allow advantage to be taken of other effects, such as the hot thermal areas and the cold thermal areas of the aircraft. For example, the hot areas can be used to provide thermal energy to portions of the system (e.g. to the fuel cell to avoid freezing) while the cooler areas can be used to remove thermal energy from the system (e.g. to assist in cooling electronic components to increase efficiencies). 
     Furthermore, the use of a fuel cell to provide electrical power results in only the emission of H 2 O, as opposed to harmful gaseous emissions produced by standard combustion engines. This H 2 O may be captured and used within the aircraft as potable or non-potable H 2 O. 
     The arrangement as described herein may be part of an aircraft propulsion apparatus which may include for example a propeller or propeller arrangement or the like to generate thrust.