Apparatus

Abstract

The present invention relates to a thermal energy management system 100 for use in an electrically powered aircraft, the system comprising: a passive cooling system 110, an active cooling system 120, and, a controller 130 arranged to control the active cooling system based on detected characteristics of flight, wherein the passive cooling system operates at all stage of flight; and wherein the active cooling system is controllably operable by the controller for all stages of flight. The controller may be arranged to receive requirement signals for the thermal energy management system, and the active cooling system is controllably operable in response to the received requirement signals. The active cooling system may comprise a liquid spray arrangement. At least part of the active cooling system may be a consumable cooling agent. The passive cooling system may comprise a thermal exchanger and an air inlet arrangement to inlet air to the thermal exchanger and the active heat cooling system may comprise a thermal exchanger and a controllable air inlet arrangement to inlet air to the thermal exchanger, wherein the controller is arranged to control the inlet of air to the active cooling system thermal exchanger.

Description

Technical Field The present invention is concerned with environmentally friendly propulsion systems for aircraft. According to most estimates, airline traffic is set to double every fifteen years providing a significant increase in the operation of land-based and, subsequently, airborne propulsion systems and therefore the production of associated emissions. Emissions are known to be harmful whether produced at ground level or at altitude. In order to meet targets for reduction of emissions set by the International Air Transport Association, the use of alternate fuels has been identified as a possible avenue of exploration. Alternate fuels include biofuels, synthetic kerosene, compressed natural gas. In addition, the ACARE roadmap for 2050 identifies the need and sets objectives for significant reductions for a range of emissions. It is widely recognised that the opportunities to come close to or achieve these targets are limited. To solve these issues a number of propulsion systems have been employed in different aircraft. Most systems use fossil fuel sources for economic reasons and also due to their very high energy density and specific energy. The prevalence of the gas turbine has also led to fossil fuels being a desirable propulsion mechanism for aircraft. This has led to developments for improving the performance of fossil fuel burning gas turbines. Some aircraft use electrical systems to provide propulsion, whether partially (i.e. alongside other mechanisms) or entirely (i.e. electrically driven only). For example, electrical energy can be provided to a propulsor from an energy store or energy source and used to generate thrust. Energy sources such as fuel cells can be used to provide electrical energy for propulsion. Use of such fuel cells leads to waste heat that can be a safety consideration for balance of plant for such systems. Developments exist that use coolant liquids circulating between heat sources and ambient air as primary heat sinks. The developments used for typical combustion powered aircraft are not suitable for partially or fully electrically powered aircraft. The developments used for typical combustion powered aircraft are not suitable for larger aircraft such as commercial aircraft that are partially or fully electrically powered. Improvements are disclosed herein. Summary of the Invention Aspects of the invention are set out in the accompanying claims. Viewed from first aspect there is provided a thermal energy management system for use in an electrically powered aircraft, the system comprising: a passive cooling system; an active cooling system; and, a controller arranged to control the active cooling system based on detected characteristics of flight, wherein the passive cooling system operates at all stage of flight; and wherein the active cooling system is controllably operable by the controller for all stages of flight. Electrically powered aircraft (which may be fully or partially) are benefitted from the scalable thermal energy management system present herein. The thermal energy management system (TMS) addresses concerns regarding mass, drag, scalability, installation and packaging. During stages of flight, requirements for power vary. Typically, at take-off there is a greater need for power than at cruise. As such, the present system provides a system that varies the output of cooling based on the requirement of cooling. In this way, by providing a slightly more complex and intelligent system, the requirements of the aircraft are far better handled than by other modern systems. Indeed, the system is more space and weight efficient. In some examples, the controller is arranged to receive requirement signals for the thermal energy management system, and the active cooling system is controllably operable in response to the received requirement signals. The present arrangement uses a controllably operable active cooling system that is controlled to operate in response to requirements from the aircraft. The requirements vary over the flight stage and ambient conditions etc. Each of these can be considered and accounted for so that the system provides a suitable cooling as required to provide high performance from an associated power / thrust provider. The requirement signals may stem from the thrust provider or a controller associated with the thrust provider. In some examples, the active cooling system is controllably operable for high requirement stages of flight wherein the high requirement stages of flight include: taxi, take off; climb; and, landing. High requirement stages of flight may include taxi, take off; climb; and, landing as these are instances wherein a passive heat exchange system operates outside of its manufactured sizing point, which is cruise. High requirement relates to any stage of flight where the passive cooling system is incapable of solely providing all cooling needs and an active cooling system must be engaged to make up the balance. Similarly, there are high requirements during e.g. safety events irrespective of the stage of flight in which the safety event occurs. High requirements may also be a relative term, such that a sudden change in requirements from e.g. low to medium need may be deemed a high requirement as the change is a relevant factor in the amount of cooling and the time over which that cooling is to be delivered. By understanding when the need for cooling changes and accounting for it, the present system provides a reactive and reliable system for cooling. The system herein is suitable for powers up to the MW scale, specifically up to around 8 MW, up to 9 MW and / or up to 10 MW. This may be around 10x (or 20x) larger than the conventional gas turbine engines. In some examples, the active cooling system comprises a liquid spray arrangement. Use of liquids are advantageous in thermal energy removal as liquids often have a higher specific heat capacity than gases. In specific examples this liquid may be water which is particularly advantageous as having a large heat capacity. Furthermore, the use of a spray for the liquid enables a large area to be cooled in a more uniform manner. In some examples, the liquid spray arrangement comprises a liquid spray nozzle, wherein the liquid spray nozzle is controllably retractable. Control over the position of the liquid spray nozzle allows the system to reduce the frontal area and therefore drag of the liquid spray arrangement during operation in flight. One of the many considerations while providing a thermal energy management system for an aircraft is the impact of any proposed solution on drag of the aircraft and size within the aircraft. A retractable nozzle allows for great user control over these considerations. In a specific example, this allows the user to operate the liquid spray during ground operations, but would allow a user to retract the nozzle during e.g. normal cruise (i.e. non safety event cruise). In some examples, at least a part of the active cooling system is a consumable cooling element. Such an arrangement is highly advantageous in weight considerations. In consuming the cooling element during use, the weight of the full TMS reduces during flight and is therefore increasingly efficient over the flight. This has synergies with an aircraft in particular as the fuel of an aircraft may reduce over the flight (e.g. if some portion is combustible) and so too reduces the weight of the TMS. In some examples, the system further comprises a sensor arrangement connected to the controller, the sensor arrangement arranged to provide measurements of: temperature; requested thrust; altitude; travel speed. The sensor arrangement provides information to the controller. The controller forms an understanding of the flight condition, flight demands (thrust etc.) and the air condition. Each of these can be processed to form an understanding of the requirements on the TMS. Sensors may be arranged around the aircraft as suitable, e.g. on the fuselage or the wing. In some examples, the passive cooling system comprises a thermal exchanger and an air inlet arranged to inlet air to the thermal exchanger, and the active cooling system comprises a thermal exchanger and a controllable air inlet arranged to inlet air to the thermal exchanger, wherein the controller is arranged to control the inlet of air to the active cooling system thermal exchanger. Such an arrangement aligns with the above examples wherein there is a controllable inlet for providing controlled cooling. In this arrangement there is also a base level of cooling that is provided from the passive cooling system. The cooling from the passive cooling system can therefore be set at the requirement for cruise (compared to the maximum requirement for modern systems) and the active system can be applied as and when it is required based on the sensor arrangement and controller. In some examples, the controllable air inlet has a variable inlet area through which air can pass, wherein the controller controls the inlet area to be open during take-off and closed during cruise. Such an arrangement allows the inlet to provide minimal drag (i.e. be closed) during periods of non-use. In examples, that period of non-use may be normal operating cruise conditions. The inlet may be opened during safety-event cruise flight to provide additional cooling for high requirement manoeuvres or the like. This arrangement has a particular synergy in not having a retractable mechanism for the spray nozzle as the spray nozzle may be contained behind the inlet and therefore once the inlet is closed the spray nozzle also provides no drag. Small optimizations such as this allow for a highly efficient system when considered in total. In some examples, wherein the passive cooling system thermal exchanger is made of microtube fin geometries and the active cooling system thermal exchanger is made of fin-and-tube geometries, which could include micro tube geometries etc. In examples, the passive cooling system thermal exchanger and the active cooling system thermal exchanger are made of varying fin geometries or heat exchanger configurations optimised for their own requirements. In some examples, the active cooling system thermal exchanger is controllably retractable. Introducing a retractable mechanisms for the thermal exchanger increases the mass of the system and may be seen as a disadvantage. In the present arrangement however the retraction allows the thermal exchanger to be removed from drag implications (such as by retracting into e.g. the fuselage) during flight conditions such as normal cruise. The saving across the extensive cruise period of commercial aircraft may account for, in the present arrangement, the gain in mass from the introduction of the retraction mechanism. The system may be retractable or rotatable or the like. Relative movement between the thermal exchanger and other aspects of the arrangement (or the aircraft as a whole) is all that is required. E.g. the exchanger may be rotated into the fuselage. The present arrangement is, unusually, a solution for TMS in an electrically powered aircraft. Modern systems that operate TMS for combustion aircraft do not have the considerations of electrical power aircraft and therefore the gain considerations herein are different to those in the modern combustion propelled solutions. In some examples, the TMS further comprises an electrical energy store arranged to provide electrical energy during high requirement stages of flight. In such examples, the arrangement may comprise e.g. a battery for use during particular stages of high need. This may be for example during take-off or during safety events or the like. In particular, this arrangement allows for “load shaving” wherein the requirement on the fuel cell (or fuel cell stack or stacks, etc.) can be reduced and some power can be provided by the more efficient electrical energy store. In some examples, the controller comprises an operational control module arranged to control aspects of flight, wherein the operational control module is arranged to operate during high requirement stages of flight, wherein the operational control module is arranged to at least one of: restrict altitude; restrict climb rate; and, restrict speed. Sizing of heat exchangers is affected by the cooling air temperature at that sizing point. If the ambient temperature is higher, the air cooling capacity in the air is reduced and sized heat exchanger needs to be larger. Where the ambient temperature is lower, then there is more cooling capacity and the heat exchanger can be sized smaller. Typically, a thermal system may be sized at 35°C above standard atmosphere on the ground which is circa 40°C on the ground. In this hot atmosphere, the temperature of air at high altitude will also be warmer. Aircrafts statistically spend very little time in those extreme conditions, so it can be beneficial to size heat exchangers to lower ambient conditions. This allows the load requirement to not exceed a certain level such that the cooling requirement is not to exceed an amount that cannot be provided. By curtailing the power requirement, the heat load can be constrained. This in turn reduces the impact on the cooling and the cooling can be constructed to provide that which is required. In some examples, the TMS further comprises a coolant arrangement for providing coolant to the passive cooling system and the active cooling system, wherein the coolant arrangement comprises a coolant store for storing liquid coolant and a coolant loop providing fluid communication between the coolant store, the passive cooling system and the active cooling system. In examples, the coolant may be water-glycol, oil based, or phase-changing refrigerant type coolant. Viewed from a second aspect there is provided an aircraft comprising the thermal energy management system of any above example. In some examples, the passive cooling system is arranged within a housing within a fuselage and near a fuselage skin of the aircraft and wherein the active cooling system is arranged within the fuselage. By placing the active cooling system near the surface of the aircraft, the active system can be moved in and out of the surface of the aircraft as necessary to provide cooling when needed and then not provide drag when cooling is not required. As such, this is a highly space conscious and energy-saving location for the active cooling system. Furthermore, this arrangement is more mechanically straightforward to construct alongside desirable features such as being retractable. 5 In some examples, the aircraft further comprises propulsors for generating thrust, wherein the exhaust airflow from the propulsors is arranged to be incident on the passive cooling system. Such exhaust airflow would otherwise be wasted. This is a further environmentally friendly development within an already very environmentally friendly invention. This synergistically 10 decreases the requirement for air and therefore improves the overall efficiency of the system. Viewed from a third aspect there is provided a method of providing electrical power for aircraft propulsion comprising: providing cooling from a passive cooling system; detecting, by a controller, a change in a characteristic of flight requiring cooling; altering, by the controller, an 15 activation status of an active cooling system; detecting, by the controller, a change in a characteristic of flight not requiring cooling; and, altering, by the controller, an activation status of the active cooling system. Brief Description of the Drawings One or more embodiments of the invention will now be described, by way of example only, and with reference to the following figures in which: Figure 1 shows a schematic of a thermal energy management system according to examples disclosed herein; Figure 2 shows a schematic of a thermal energy management system; Figure 3a shows a schematic of a passive cooling system according to examples disclosed herein; Figure 3b shows a schematic of an active cooling system according to examples disclosed herein; Figure 4 shows a schematic of a graph showing stage of flight against waste heat load; Figure 5a shows a schematic of a retractable radiator arrangement for a cooling system according to examples disclosed herein; Figure 5b shows a schematic of a retractable radiator arrangement for a cooling system according to examples disclosed herein; Figure 6 shows a schematic of an active cooling system according to examples disclosed herein; Figure 7 shows a schematic of a thermal energy management system according to examples disclosed herein; Figure 8a shows a schematic of a thermal energy management system according to examples disclosed herein; Figure 8b shows a schematic of a thermal energy management system according to examples disclosed herein; Figure 9a shows a schematic of a thermal energy management system according to examples disclosed herein; Figure 9b shows a schematic of a thermal energy management system according to examples disclosed herein; and, Figure 9c shows a schematic of a thermal energy management system according to examples disclosed herein. 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 invention 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 An invention described herein relates to a scalable thermal management system (TMS) for use in aircraft, in particular aircraft that are least to some extent electrically powered. Operation of electrically powered aircraft may use fuel cells. Fuel cells differ from combustion engines in a number of ways, one is in the generation of waste heat. As such, the removal of this waste heat requires consideration. Specifically, examples herein provide solutions for waste heat generated for large-scale aircraft which, to date, have not operated on electrical power. Examples herein are suitable for use in commercial aircraft, i.e. aircraft that may be around from 10 to around 100 passenger aircraft. Examples herein are suitable for regional aircraft or single aisle aircraft. As such, disclosed herein is a large scale (suitable for e.g. 10, 50 and 100 passenger aircraft) electrically operated electrical aircraft propulsion arrangement that is highly effective at handling the waste heat and overall conditioning of the propulsion arrangement. The system herein is characterised by being highly efficient in terms of mass and drag, being scalable and having benefits to installation ease and packing size. Referring to Figure 1, there is shown an example of a thermal energy management system (TMS) 100. The TMS 100 is for use in an electrically powered aircraft. The TMS 100 comprises a passive cooling system 110. The TMS 100 comprises an active cooling system 120. The TMS 100 comprises a controller 130 arranged to control the active cooling system based on detected characteristics of flight. The passive cooling system 110 operates at all stage of flight. The active cooling system 120 is controllably operable by the controller 130 for all stages of flight. The TMS 100 therefore provides a base level of cooling from the passive cooling system 110. The passive cooling system 110 may be arranged accordingly to provide a suitable level of cooling based on expected use of the aircraft. The active cooling system 120 in contrast may be activated or deactivated as desired / required. The control over the functioning of the active cooling system 120 is provided by controller 130. The controller 130 may note characteristics of flight and control activation of the active cooling system 120 accordingly. The electrically powered aircraft in which the TMS 100 disclosed herein may be used in a partially electrically powered aircraft in that at least some of the propulsion occurs from electrical energy. The electrically powered aircraft may be a fully electrically powered aircraft, in that all the propulsion occurs from electrical energy. In specific examples, the passive cooling system 110 may be a radiator, thermal exchanger or the like for providing a thermal cooling interface. In specific examples, the active cooling system 120 is used following an increase in requirement for cooling in flight or transit. For example, during ground operations there is an increase in the requirement for cooling as such the active cooling system 120 may be activated by the controller 130 based on the detected characteristic of flight being the beginning of the taxi stage. Stage of flight relates to any stage experienced by an aircraft during a flight, including ground based aspects such as taxiing. The controller 130 is arranged to receive requirement signals for the TMS 100. This may be from sensors and / or detectors or the like for conveying the cooling requirement from the TMS 100 for a given moment in operation (e.g. during take-off or during cruise etc.). In response to such received requirement signals, the active cooling system 120 is controllably operated. In this way, the active cooling system 120 can be operated in response to a requirement detected or calculated by the controller 130. In examples, the flight stage of take-off would provide a demand for a high cooling requirement. The controller 130 receives the signal from the user demanding thrust and provide a detection or calculation of a need for cooling. The active cooling system 120 can be operated in response to the need for cooling. The active cooling system 120 may be operated such that the operation of the system 120 provides a magnitude of cooling related to the magnitude of the demand. In this way, while the active cooling system 120 may be a simple on or off arrangement, preferably the active cooling system 120 can be activated across a range of activation magnitudes (e.g. a little cooling, some cooling, or a lot of cooling). In a low period of cooling requirement, the active cooling system 120 may be operable but may not be operated. In this way, the active cooling system 120 can provide a vast range of cooling including, using the language above, no cooling, a little cooling, some cooling or a lot of cooling. These relative terms are used to show that the active cooling system 120 has a wide range of cooling operation levels. The controller 130 receives thrust demands or cooling demands or the like, a value representative of the cooling required. The controller 130 may be linked to sensors or detectors or to e.g. the throttle. As noted above, the performance of the active cooling system 120 may therefore vary based on the requirement and this may vary based on the stage of flight. High requirement stages of flight may include taxi, take off, climb, and, landing inter alia. In this arrangement therefore, a base level of cooling may be provided by a passive cooling system that is not controllably changeable during use (of course the system could be redesigned during downtime but this is not controllably changeable during use) and an active cooling system that is controllably changeable during use (i.e. responsive to demand and therefore reactive and updateable). Referring now to Figure 2, there is shown a thermal management system with a radiator and a variable inlet. Referring now to Figure 3a there is shown a schematic of a passive cooling system according to examples disclosed herein. Referring also to Figure 3b there is shown a schematic of an active cooling system according to examples disclosed herein. Figure 3a and 3b therefore show, combined, a TMS 300. The TMS 300 comprises a passive cooling system 310 (shown in Figure 3a) and an active cooling system 320 (shown in Figure 3b). The passive system provides cooling via a radiator and this system is not changeable or controllable during use. In contrast, the active system is changeable and controllable during use. Referring first to Figure 3a, the passive cooling system 310 has a thermal exchanger 311 (e.g. a radiator). The passive cooling system 310 also has a constant duct inlet area 312. Unlike the arrangement of Figure 2, therefore, the passive cooling system 310 has a predetermined inlet area 312 that is not variable. The system 310 of Figure 3a is a passive heat sink without variable inlet area or water spray. The geometry of the heat exchanger fin and heat exchanger architecture, mass and drag are designed for cruise (in examples low drag cruise) and the installation and packaging design choices make it scalable to any aircraft platform. The active cooling system 320 also has a thermal exchanger 327 (e.g. a radiator). The active cooling system 320 has a controllable air inlet arranged to inlet air to the thermal exchanger 327, wherein the controller (not shown) is arranged to control the inlet of air to the active cooling system thermal exchanger 327. The inlet has a series of inlet areas, inlet area for cruise 321, inlet area for top of climb 324, inlet area for beginning of climb 322 and inlet are for take off 323. The ambient cooling air is able to flow into radiators via inlets and this air is used in the radiator to provide cooling. The active cooling system 320 in Figure 3b also has a water spray subsystem. The subsystem shown has a water spray nozzle 3262 though there is no retraction mechanism shown. The may be case as the cruise inlet area 321 for the inlet may be zero. There may be no requirement for additional cooling during cruise. As such, when designed in tandem with the passive cooling system 310, the requirement for cooling at cruise may be satisfied entirely by the passive cooling arrangement 310. At this point, the active cooling system 320 may be “off during cruise and therefore, when the inlet area is closed 321, the spray nozzle 3262 provides no drag for the overall system 300 and therefore the nozzle 3262 needs not be retractable to avoid drag. The system 320 of Figure 3b is an active ducted radiator assembly with variable inlet area and water spray. The ducted radiator is optimised for take-off only to provide balance in the cooling needs between the required heat dissipation and the cooling provided by the passive system 310. The inlet area is completely closed during cruise to reduce the drag of the system 320 to zero. Drag here refers to core momentum drag. The installation drag may be non-zero depending on location of installation. This external drag may be due to skin friction or flow separation if air needs to flow externally around the active TMS. Installation in the fuselage is therefore advantageous to reduce this drag. The TMS 300 may be seen to have two subsystems 310, 320 each of which may be referred to as a TMS herein, though, in actuality, these operate in tandem (though occasionally with TMS 320 turned off or not activated). The TMS of Figure 3a is optimised for low drag at cruise. The TMS 320 of Figure 3b is optimised for a short duration spike in waste heat load during take-off. While the TMS 310 is designed for low drag and mass, and operates throughout the whole mission, the TMS 320 is un-used for most of the mission so it is designed for low mass, high heat transfer and higher drag that can be tolerated at take-off. The different desired functions from the active cooling system 320 and the passive cooling system 310 allows each to be well designed to perform their specific function and mitigate against the drawbacks of each on the other. The controller (not shown) controls the performance of the active cooling system 320 and can control the inlet area to be open during take-off 323 (and other stages, e.g. 324, 322) and closed during cruise 321. As noted, this is highly efficient in terms of reducing drag impact and mass impact of the system 320. The different roles played by the thermal exchangers 311, 327 allow design choice selection to vary between the heat exchangers used in the passive system 310 from the active system 320. In an example, the passive cooling system thermal exchanger 311 is optimised to minimise drag and weight whereas the active cooling system thermal exchanger 327 is optimised to maximise heat transfer and cooling at the high requirement stages of flight and permits higher drag to achieve this. Maximising cooling may be referred to as maximising heat transfer. The passive cooling system thermal exchanger 311 may be sized, designed, and / or optimised to a cruise stage of flight and the active cooling system thermal exchanger 327 may be sized, designed and / or optimised to a take-off stage of flight. The passive cooling system thermal exchanger 311 is sized, designed and / or optimised to provide substantially all thermal energy during a cruise stage of flight, and the active cooling system thermal exchanger is sized, designed and / or optimised to provide substantially all surplus / outstanding thermal energy during the high requirement stages of flight. In an example, the heat exchangers have different configurations and fin geometries. In a specific example, the passive cooling system thermal exchanger 311 comprises micro-tube fin geometries to maximise thermo-hydraulic properties with low air-side pressure drop, low drag, and low mass. In a specific example, the active cooling system thermal exchanger 327 comprises fin-and-tube geometries where higher air-side pressure drop, higher drag is allowed for the benefit of low mass and improved heat transfer. In a specific example, the passive system, the radiator, is sized for cruise and capable of providing some but not all cooling (alongside the variable inlet area) needs from beginning of climb to landing. The passive system does not change the cooling, rather the cooling is changed by virtue of the controller altering the active system. Varying an inlet area, by a controller, increases the amount of cooling. The active cooling system comprises a spray subsystem. The spray subsystem may spray fluid to provide additional heat capacity for thermal energy removal. In particular, on the ground during taxi and during take-off, the required cooling is often very high. As such, the additional cooling provided by the spray subsystem is highly advantageous. The spray subsystem may be used with liquid such as water, which advantageously has a high specific heat capacity and is non-reactive. The spray subsystem may be controlled by the controller. The cooling of the system can be increased by spraying, e.g., water into the air received into the air inlet. This may provide the additional cooling for high requirement stages of flight (e.g. taxi and take off). The spray subsystem comprises a liquid spray. This spray may be a nozzle or the like. The subsystem comprises a reservoir for cooling liquid such as water. The nozzle may be a liquid spray nozzle. The water may be atomized into fine droplets and sprayed into the duct at high pressure which lowers the temperature of the cooling air and provides additional cooling capacity from evaporating the water droplets inside the radiator. The subsystem may (or may not) further comprise a controllable retraction mechanism. The retraction mechanism controls the position of the spray nozzle in the arrangement. The water spray may be considered as a consumable, which means that the need for it is dependent on the ambient temperature. The TMS may therefore comprise a consumable cooling element. In particular, the water for the spray subsystem 226 may be deemed a consumable heat sink as it receives thermal energy and then is evaporated (i.e. consumed). It may be that, on a hot day ‘full’ water spray required, whereas on a standard or cold day operation little or no water spray is required. This information can be provided to the controller via a heat sensor or the like located on the aircraft. The mass penalty of the water is removed as the water is consumed in performing a cooling function in the TMS 200. When atomised water with sufficiently small droplet diameters is sprayed into an air duct, a number of physical processes take place. First, as the water droplets travel with the air flow, they begin to evaporate due to the heat transfer between the droplets and the surrounding air, which leads to cooling of the air flow. This cooling effect is a result of the water latent heat of vaporisation, which is the energy required to convert the water from liquid to gas. In a specific example, in which a full evaporation rates takes place and all droplets evaporate within the duct, spraying one kilogram of water has a cooling capacity of 2,477 kW. In reality, it is unlikely that all the droplets will evaporate, and the design should be driven to achieve the smallest possible droplet size to maximise the heat transfer surface area and consequently improve the rate of vaporisation. As the droplets continue to travel with the airflow, they may collide and smaller droplets of water join together to form a larger droplet, a process known as coalescence. Increased coalescence in turn leads to reduced evaporation rates, and hence reduced effectiveness of water spray heat sink. Larger droplets may also introduce a risk of heat exchanger fin impingement that could lead to loss in heat exchanger performance, which may be avoided by developing a design that minimises the probability of coalescence. Utilising the thermal inertia of the cooling system during initial slow moving ground operation provides a thermal delay between the time when the heat sources begin generating waste heat load (e.g. when pilot pulls on the throttle) and the time when this heat load reaches the heat sinks. During this time the thermal load is absorbed by the structure and fluids within it, and enables heat exchangers to begin operating when the aircraft is already moving at sufficiently high speed providing required cooling airflow across heat exchanger surfaces. As a result of thermal inertia, the amount of water spray needed at peak power may advantageously be reduced. The thermal inertia of the fuel cell and thermal management system (TMS) is used to determine the water injection requirements. At the very start of the take-off roll, the pilot pushes the engines to full power the fuel cell begins producing heat. As the aircraft is stationary there is negligible airflow over the heat exchangers, meaning the heat dissipation capacity and water evaporation capacity are both close to zero. This implies that the fuel cells will overheat and shut down. Thermal inertia in the system delays the transfer of heat between the fuel cell and the heat exchangers. The result is that by the time that initial fuel cell heat has travelled through the system the aircraft is moving at significant speed down the runway and therefore has much improved heat transfer capacity, reducing the amount of water needed to cover the excess heat load. It also improves evaporation efficiency by increasing the flow volume and velocity of the air in the TMS. The result is that larger thermal inertia figures reduce the water injection requirement, as they produce longer time lags between heat generation and dissipation and therefore allow the aircraft to gain more speed before the TMS is required. It may be preferable to control the airflow Mach number at the face of the radiator to circa M<0.3. Thus, at cruise, a variable duct inlet area may be used to slow down and diffuse the airflow before entering the radiator. Closing the area during cruise additionally (and advantageously) serves as benefit to the reduction in cruise core momentum drag. Similarly, during take-off the inlet airflow is already at circa M=0.2, which allows to increase the inlet duct area and draw more cooling air into the radiator improving its cooling capacity, and by doing so also reducing the amount of water spray needed. As such, controlling the inlet area provides control over the amount of air coming into the system for cooling but also the speed of flow of that air, which is controlled to provide preferential cooling characteristics from the air and avoid complexities associated with viscous and compressibility effects if the air speed is too high. At high enough flight Mach numbers the heat exchanger wall temperature may be considerably higher than free-stream temperature even though no heat transfer takes place due to viscous dissipation effects that both have adverse impact on heat exchanger performance. Thus, current designs prefer to have the flow to enter the heat exchanger at Mach number no greater than M2 = 0.3 to reduce excessive compressibility effects. Duct designs that lead to diffusion angles greater than around 7 degrees and / or e.g. with a shape that avoids internal flow separations often trade a shorter duct length for higher pressure drop and flow non-uniformity at the radiator inlet that adversely affects its performance. Higher pressure drop and flow non-uniformity in the inlet duct also reduces the dynamic head available in the radiator and lead to a heavier and larger radiator. The low pressure duct may be overly long making the system harder to install and less compact for packaging. There may be a retraction mechanism for the spray that would contribute to the weight and size of the arrangement. Such a mechanism would allow the spray nozzle to be fully retracted during stages of flight where no water spray is required. In particular, a retraction mechanism therefore improves the drag of the arrangement as a whole. The water spray can be used during ground and take off and then retracted when not needed reducing unnecessary drag at e.g. cruise. The TMS may comprise a sensor arrangement connected to the controller, the sensor arrangement arranged to provide measurements of: temperature; requested thrust; altitude; travel speed. This sensor arrangement provides data to the controller on requirement for cooling or the data from which the controller can calculate the requirement for cooling. The controller then activates the active cooling system accordingly. Referring now to Figure 4, there is shown a graph 400 showing phase of flight (take off, climb, cruise, descent, landing, taxi) against waste heat load in kW and how that waste heat load is accounted for the passive cooling system (shown in green, 410), the active cooling system thermal exchanger (shown in red, 427) and the active cooling system liquid spray 4262. The graph 400 is for a commercial aircraft and therefore is highly representative of a commercial aircraft that is propelled by electrical energy generation. The present system may be used in a partially electrically propelled aircraft or a fully electrically propelled aircraft. As can be seen, a rapid change in waste heat load is where the present system is particularly advantageous. While other systems can provide high thermal removal, the ability of the system to respond to changes in requirement of the present system is highly advantageous over those. A taxi and take-off the actual required heat removal rapidly jumps up. To handle this, the passive cooling and active cooling thermal exchangers each provide some cooling, however the vast majority of the cooling is provided by the water spray system. After this point, during climb, the thermal exchangers can be used to provide the required cooling without use of the water spray system. Advantageously, the water may be consumed during take-off and early climb thereby removing the weight penalty of the water. Later in flight, the passive cooling thermal exchanger provides the full cooling required and the active system can be turned off. The passive system, as noted above, is sized to provide sufficient cooling from cruise to landing. The active system thermal exchanger is sized to provide cooling from take off to cruise and provides balance cooling capacity from beginning of climb to cruise where the active system is then shut off by, for example, closing the inlet duct. The spike for passive TMS from top of climb to cruise is caused by the aircraft accelerating to required cruise Mach number. As shown, the system provides all cooling needs from take-off to landing while also improving overall TMS mass and drag and rendering installation and packing easier. Referring now to Figures 5a and 5b there are shown schematic views of retractable radiator arrangements for a cooling system according to examples. In the examples of Figure 5a and 5b, the arrangement 500 has a thermal exchanger 527 in the active cooling system (remaining elements not shown). The thermal exchanger 527 is shown in relation to a fuselage 5000 of an aircraft in which the TMS 500 may be installed and used. The examples show an active cooling system thermal exchanger 527 that is controllably retractable. In Figure 5a, the active cooling system thermal exchanger 527 is controllably retractable into the fuselage moving via translation. In Figure 5b, the active cooling system thermal exchanger 527 is controllably retractable into the fuselage moving via rotation. The movement mechanism provides the advantage of removal of the exchanger 527 from creating drag irrespective of the form of the movement and therefore any relative movement between the fuselage 5000 and the thermal exchanger 527 is advantageous. Retracting the exchanger 527 into the fuselage 5000 is an option though moving the exchanger 527 into other locations may provide the same advantages of reducing drag from the active cooling system. In both examples shown, the exchanger 527 is moved out of the air stream. In the examples, the movement is to render the exchanger as lying flush with the fuselage for zero drag. This movement, as noted above, is controlled by a controller. The retractable exchanger may be installed behind a propulsive fan or in the free-stream. The exchanger when moveable may have a larger frontal area (as this drag is mitigated by the movability) and be thinner (as it need not be so robust to withstand the drag). This in turn leads to a low pressure drop heat exchanger design and also an easier way of capturing the incoming airflow. The retractable heat exchanger in Figure 5b has a larger area and a lower pressure drop (than earlier examples). The heat exchanger may therefore be flexibly shaped around its frame in such a way to enhance and improve the captured air and minimise spillage. The exchanger shown in Figure 5a, 5b is at 90 degrees to the fuselage. This perpendicular arrangement is not required. The exchanger may be arranged at an angle to increase the size while decreasing the frontal area causing less drag. The design complexity may increase but there are additional efficiencies in the system from this arrangement. The present invention relates to optimising cooling for the heat from power production in at least partially electrically propelled aircraft. This power production may come from fuel cells or the like. Alternatively or additionally, this power may come from a source of electrical energy such as a electrical energy store (EES). Such an EES may be a battery or the like. Including a battery or the like will increase the mass of the system, negatively impacting the efficiency. However, it has been found that introduction of a battery allows the system to operate in a flexible manner in terms of the required output from the electrical generation elements. In this way, a further option for user control termed “load shaving” can be performed. Load shaving allows the fuel cell efficiency to be controlled and lower the waste generated therefrom. In this way, the requirement for cooling at a particular stage in flight can be reduced. Referring now to Figure 6 there is shown a TMS 600 further comprising an electrical energy store 650 arranged to provide electrical energy during high requirement stages of flight. Specifically, the system 600 may be an active cooling system 600 that comprises an electrical energy store 650. The example shows an arrangement similar to that shown previously in earlier figures. The active cooling system 620 has a variable inlet area and a thermal exchanger connected to a coolant loop that links to the fuel cell stacks. The controller controls the fuel cell stack energy generation. This in turn controls the waste heat generation. In moments of high requirement, the controller can activate the energy store 650 reducing the demand on the fuel cell stacks and therefore reducing the requirement for cooling from the system. Such an aspect may be combined with any above embodiment for further control over the handling of waste heat. In another example of control over the fuel cell output, the controller may limit the altitude, climb rate, Mach number and / or de-rate the aircraft if ambient temperature is too high, or to prevent the waste heat requirement exceeding a predetermined value. This is termed “mission shaving”. Such an aspect may be combined with any above embodiment for further control over the handling of waste heat. Referring now to Figure 7, there is shown a schematic of a thermal energy management system according to examples disclosed herein. The TMS 700 has a passive system 710 and an active system 720 (as per examples disclosed in detail above) with constant and variable size air inlet ducts (as per the above). The arrangement of Figure 7 further comprises a priority based control for coolant flow split between the passive 710 and active systems 720. Varying the amount of coolant flow rate improves the sizing of the active thermal exchanger and may reduce the overall dimensions, which therefore reduces the mass and drag. This is preferably designed so that the amount of coolant into the active exchanger is selected to enable a lowest mass and drag design for the inlet area for the exchanger in the active system 720. This system is shown with numeral 770 showing the coolant arrangement comprising a variable valve, the fuel cell stacks and a manifold. The coolant loop and priority based control are also shown. In this example, the passive and active systems are on a single cooling circuitry and the active exchanger can be sized to avoid large negative implications on mass and drag. The system disclosed herein is for use in an aircraft where such concerns about stages of flight are most relevant. Referring to Figures 8a and 8b there are shown schematic views of thermal energy management systems according to examples disclosed herein. In particular, the TMSs shown are in an aircraft. In an aircraft, the passive system may be slim-line and may be installed in a pod in the free-stream near the fuselage skin. In minimising the diffuser duct it may be preferable for the installation to benefit from the thick boundary layer developed at the skin of the fuselage where the airflow velocity is slower than free-stream. The active system may preferably be installed in the fuselage with a variable inlet area that can be completely closed off to provide zero drag at cruise. This further benefits the active system installation where the airflow velocity is similar to the free-stream, the airflow velocity is in the boundary layer. The arrangements 800 have propulsors shown as 880. The air flow from the propulsors may be directed into the air inlets of the passive cooling system 810a (and in practice 810b though not shown). This recycling of air benefits the efficiency of the arrangement overall. Indeed, it is preferable to use existing airflow from propulsive fans on the ground when the aircraft is stationary. As noted above, ground operation is challenging as limited cooling air flows into the TMS as a whole. This solution therefore overcomes, to some extent, that significant challenge. In Figure 8a, the air flow from the propulsors is directed to the passive air inlets. In the arrangement of Figure 8b, the air flow from the propulsors is into the air inlets as the passive system is located on structural extenders to extend the system into the propulsor air flow stream. In both cases, gains in efficiency are possible. The gains are possible from the exhaust airflow from the propulsors being arranged to be incident on the passive cooling system. The arrangements in Figure 8a, 8b are for ground and stationary operation. Referring now to the arrangements in Figures 9a, 9b, 9n there are examples of the system in an aircraft during take-off (Fig. 9a), beginning of climb (Fig. 9b) and at cruise (Fig. 9c). As noted in the graph of Figure 4, at take-off (Fig. 9a), both the passive 910 and the active 920 cooling systems of the TMS 900 are operational. When the aircraft transitions into the beginning ofclimb (Fig. 9b) a small portion of the active system 920 can be closed off to reduce drag (as per Figs 2, 3b, 6, and 7). During cruise (Fig. 9c), the active system 920 can be completely closed off and the total cooling required is provided by the passive system 910a. Each of these stages are shown correspondingly in Figure 4. In Figure 9a. 9b, 9c only one side of the passive system is shown 910a, this arrangement is as per Figure 8a, 8b where both sides 910s, 910b combine to provide the full passive system. The above systems are noted as being controlled via a controller that may have sensors located in suitable places to detect the conditions around the aircraft alongside being connected to e.g. the throttle to understand the thrust demands on the aircraft. This is a reactive approach that uses up-to-date, real-time data. This provides immediate feedback to adjust control actions accordingly. There has been disclosed a highly efficient TMS that reduces mass and drag as well as providing benefits in installation and packaging. 5 In order to address various issues and advance the art, the entirety of this disclosure shows by way of illustration various embodiments in which the claimed invention(s) may be practiced and provide for a superior thermal energy management system (TMS) for an at least partially electrically propelled aircraft. The advantages and features of the disclosure are of a representative sample of embodiments only, and are not exhaustive and / or exclusive. They 10 are presented only to assist in understanding and teach the claimed features. It is to be understood that advantages, embodiments, examples, functions, features, structures, and / or other aspects of the disclosure are not to be considered limitations on the disclosure 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 scope and / or spirit of the 15 disclosure. Various embodiments may suitably comprise, consist of, or consist essentially of, various combinations of the disclosed elements, components, features, parts, steps, means, etc. In addition, the disclosure includes other inventions not presently claimed, but which may be claimed in future.

Claims

1. A thermal energy management system for use in an electrically powered aircraft, the system comprising:a passive cooling system;an active cooling system; and,a controller arranged to control the active cooling system based on detected characteristics of flight, wherein the passive cooling system operates at all stage of flight; and wherein the active cooling system is controllably operable by the controller for all stages of flight.

2. A thermal energy management system according to claim 1, whereinthe controller is arranged to receive requirement signals for the thermal energy management system, andthe active cooling system is controllably operable in response to the received requirement signals.

3. A thermal energy management system according to claim 1 or 2, wherein the active cooling system is controllably operable for high requirement stages of flightwherein the high requirement stages of flight include: taxi; take off; climb; and, landing.

4. A thermal energy management system according to any previous claim, wherein the active cooling system comprises a liquid spray arrangement.

5. A thermal energy management system according to claim 4, wherein the liquid spray arrangement comprises a liquid spray nozzle, wherein the liquid spray nozzle is controllably retractable.

6. A thermal energy management system according to any previous claim, wherein at least a part of the active cooling system is a consumable cooling element.

7. A thermal energy management system according to any previous claim, further comprises a sensor arrangement connected to the controller, the sensor arrangement arranged to provide measurements of: temperature; requested thrust; altitude; and travel speed.

8. A thermal energy management system according to any of claims 1-7, whereinthe passive cooling system comprises a thermal exchanger and an air inlet arranged to inlet air to the thermal exchanger, andthe active cooling system comprises a thermal exchanger and a controllable air inlet arranged to inlet air to the thermal exchanger, wherein the controller is arranged to control the inlet of air to the active cooling system thermal exchanger.

9. A thermal energy management system according to claim 8, wherein the controllable air inlet has a variable inlet area through which air can pass,wherein the controller controls the inlet area to be open during take-off and closed during cruise.

10. A thermal energy management system according to claim 8 or 9, wherein the passive cooling system thermal exchanger comprises micro-tube fin geometries and the active cooling system thermal exchanger comprises fin-and-tube geometries.

11. A thermal energy management system according to any of claims 8-10, wherein the active cooling system thermal exchanger is controllably retractable.

12. A thermal energy management system according to any preceding claim, further comprising an electrical energy store arranged to provide electrical energy during high requirement stages of flight.

13. A thermal energy management system according to any preceding claim, wherein the controller comprises an operational control module arranged to control aspects of flight, wherein the operational control module is arranged to operate during high requirement stages of flight,wherein the operational control module is arranged to at least one of:restrict altitude; restrict climb rate; restrict speed; and, restrict ambient conditions at sizing.

14. A thermal energy management system according to any preceding claim, further comprising a coolant arrangement for providing coolant to the passive cooling system and the active cooling system,wherein the coolant arrangement comprises a coolant store for storing liquid coolant and a coolant loop providing fluid communication between the coolant store, the passive cooling system and the active cooling system.

15. An aircraft comprising the thermal energy management system of any of claims 1-14.

16. The aircraft of claim 15, wherein the passive cooling system is arranged within a housing within a fuselage and near a fuselage skin of the aircraft and wherein the active cooling system is arranged within the fuselage.

17. The aircraft of claim 15 or 16, further comprising propulsors for generating thrust, wherein the exhaust airflow from the propulsors is arranged to be incident on the passive cooling system.

18. A method of providing electrical power for aircraft propulsion comprising:providing cooling from a passive cooling system;detecting, by a controller, a change in a characteristic of flight requiring cooling;altering, by the controller, an activation status of an active cooling system;detecting, by the controller, a change in a characteristic of flight not requiring cooling; and,altering, by the controller, an activation status of the active cooling system.28

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