Power system

Abstract

A power system 1100 for an aircraft includes a plurality of fuel cells having at least a first fuel cell and a second fuel cell arranged such that, in use, the first fuel cell receives a coolant 116 f

Description

Field and Background The present invention relates to power systems for aircraft, such as electrically powered aircraft. More particularly, the present invention relates to cooling of fuel cells in a power system for an 5 aircraft. Aircraft that use fuel cells, for example electrically powered aircraft that use fuel cells as a power source, are not widespread. Such aircraft are not at present used in commercial contexts (e.g. 50 or 100 or more passenger aircraft and / or CS-25 related aircraft). Indeed, the use of fuel cells in aircraft is an area of cutting-edge research. Existing power systems 10 and existing approaches towards cooling of such power systems are not suitable for the complexities and considerations involved with the use of fuel cells in aircraft power systems. The systems and arrangements described herein may be directed toward use of green energy options with any sized aircraft that uses at least a plurality of fuel cells, for example as a power source option, and in particular with at least partially electrically powered aircraft. 15 The power systems discussed herein provide improved cooling performance while reducing sizing and / or coolant flow requirements for cooling. Summary Aspects of the invention are set out in the accompanying claims. Viewed from a first aspect, there is provided a power system for an aircraft, the power system comprising: a plurality of fuel cells having at least a first fuel cell and a second fuel cell arranged such that, in use, the first fuel cell receives a coolant for cooling the first fuel cell and provides the coolant to the second fuel cell for cooling the second fuel cell. Hence, the fuel cells are cooled by the coolant in series, whereby coolant is used to cool a first fuel cell, and then the coolant is used to cool a second fuel cell. This series cooling arrangement provides significant advantages, which include at least: a reduction in the required coolant flow rate, a reduction in the size and mass of the power / cooling system, resulting in a more compact and more lightweight arrangement that may be more efficiently integrated into an aircraft. These and other advantages will now be discussed further. The present inventors have realised that fuel cells may only be able to accept a certain temperature difference between incoming and outgoing coolant, for example up to about 45 degrees Celsius (more preferably up to about 20 degrees Celsius), without overly negatively affecting fuel cell performance and fuel cell lifetime. For particular fuel cell technologies, this is to maintain a temperature gradient across the fuel cell, such as a membrane electrode assembly, that prevents damage or a reduction in performance of the fuel cell. The negative effect of this temperature difference on fuel cell performance and fuel cell lifetime becomes smaller with a smaller temperature difference, and hence the present inventors have realised that it is desirable to reduce the temperature difference. As a consequence, cooling systems (also known as Thermal Management Systems, TMS) may be adapted to reduce the maximum temperature difference across a fuel cell and / or account for the maximum temperature difference that a given fuel cell may be able to accept without adversely affecting performance or lifetime. Such adaptations can include increasing the required mass flow rate of coolant, for example by increasing frontal area size and / or increasing air duct size to increase air flow at a given aircraft speed in an air-cooled implementation or by including pumps increasing sizing of pipe diameters for example in a liquid-cooled implementation. This increases the mass, size, and power draw of the cooling system, and results in installation and packaging challenges. The present inventors have counter-intuitively identified that by using multiple fuel cells / fuel cell stacks with their coolant flow in series, each fuel cell / fuel cell stack may only experience a comparatively small coolant temperature difference, while the accumulated coolant temperature difference across the whole arrangement of fuel cells / fuel cell stacks is comparatively large. Hence, the temperature difference across a given fuel cell / fuel cell stack is reduced. As a result, 2 a significant reduction in required coolant flow rates is realised, thereby reducing the size and mass of the TMS (such as the frontal area). Furthermore, by supporting a lower coolant temperature difference experienced by each fuel cell, the likelihood of damage or a reduction in the performance of a fuel cell can be decreased by ensuring that excessive coolant temperature differences do not damage the fuel cells. Also, a greater increase in the overall temperature difference of the coolant can be realised, because a number of fuel cells can be provided in a series cooling arrangement while still satisfying a fuel cell’s individual requirement for maximum coolant temperature difference. This is particularly advantageous because this effectively reduces the operating temperature requirement (and input coolant temperature requirement) for the first fuel cell in the series, thereby reducing a heating power requirement of a heating arrangement that may be provided to pre-heat the coolant to a temperature suitable for the first fuel cell. For instance, in examples where the coolant is ambient air, the ambient air may not be at a suitable temperature for the first fuel cell and can in some cases cause freezing of the first fuel cell or at least a significant reduction in performance. Thus, in some examples, a heating arrangement (such as a heating device) may be used to heat the ambient air before the air is provided to the first fuel cell as a coolant. With the present power system, a power requirement of the heating arrangement can be reduced. In a parallel cooling arrangement, each fuel cell / fuel cell stack may be provided with a high power heating arrangement (i.e. a large duty heating device) to heat the coolant to a suitable coolant inlet temperature for the fuel cell / fuel cell stack. Whereas, in a series cooling arrangement as discussed herein, the amount that a heating arrangement is required to increase a temperature of the coolant can be reduced (and thus the size / mass of the heating arrangement may be reduced) as explained above. This also reduces a parasitic power demand of the TMS. For example, a power of a heating arrangement required to pre-heat the air is proportional to the amount of air flow in the duct. Hence, if the air flow is doubled, the required power of the heating arrangement is also approximately doubled. Further, the power to pre-heat the air is proportional to temperature difference. The present techniques are counter-intuitive for a number of reasons. For example, typically, there is a desire to reduce complexity in aircraft cooling systems. However, the present inventors have counter-intuitively identified that while the series cooling arrangement may add complexity, for example compared to a parallel cooling arrangement, such a system is nevertheless particularly advantageous because airflow rate, frontal area, and power of a heating arrangement to pre-heat air, can all be reduced. Furthermore, counter-intuitively, in some examples, the present fuel cells are intentionally being operated away from their nominal operating temperature. For example, when operating the same type of fuel cells in a series arrangement below their nominal operating temperature, the polarisation curve of the fuel cells is affected and the efficiency is impacted. The present inventors have realised that, by deliberately operating a fuel cell away from its nominal operating temperature (such as below its nominal operating temperature), a coolant bleed arrangement can be provided (i.e. to remove a portion of air between fuel cells). This coolant bleed can then advantageously be used for other aircraft systems, as discussed further herein. The further the fuel cells are operated from their nominal operating temperature, the more coolant bleed can be provided. Further, in a parallel arrangement, the pressure drop across the fuel cells / fuel cell stacks is minimised and thus recirculation of air is minimal (such as by a fan). However, in a series arrangement of fuel cells / fuel cell stacks, the pressure drop is increased because each fuel cell / stack introduces a pressure drop and this accumulates over the series arrangement. This is a further reason why the present series arrangement appears counter-intuitive. As a result of the increased pressure drop, recirculation of air may be increased (i.e. with a recirculation fan) to counteract this increased pressure drop. However, the present inventors have realised that in the series arrangement there is a significantly reduced air flow rate, so that a recirculation fan may operate with a higher pressure drop but a reduced air flow rate, and the air flow rate is the predominant factor affecting fan power. Hence, present inventors have identified that the adverse effect of the increased pressure drop caused by the series arrangement is countered by the reduced air flow rate provided by the series arrangement. The coolant may be a fluid such as air or another gas or gas mixture, or may be liquid, such as oil, water, water-glycol etc. In implementations where the coolant is air, significant reductions in the frontal area required to cool the fuel cells can be realised. For example, compared to a parallel arrangement of fuel cells, where coolant is not shared in a series manner from a first fuel cell to a second fuel cell, the frontal area (i.e. coolant flow rate) required to reach the desired temperature difference between incoming and outgoing coolant for a given fuel cell (so as to prevent damage / not reduce performance as discussed above) in a series arrangement discussed herein is significantly reduced. Thus, compared to a parallel arrangement, the series arrangement discussed herein reduces a frontal area requirement and allows for a more compact arrangement of fuel cells in an air flow direction. This also reduces drag. Further, as discussed above, a power of a heating arrangement (such as a duty of a heating device) to pre-heat air before the air is introduced to the first fuel cell / fuel cell stack can be reduced in the present series arrangement compared to a parallel arrangement. In implementations where the coolant is liquid, similar reductions in the size and mass of the TMS can be realised compared to parallel fuel cell cooling arrangements. Further, the power and number of pumps required to pump the coolant can be reduced compared to a parallel fuel cell liquid cooling arrangement, as the same pump / a reduced number of pumps can be used to circulate the coolant. Thus, by providing fuel cells having a series coolant arrangement, advantages over existing approaches are provided which include at least: a reduction in the required coolant flow rate, leading to a reduction in the size and mass of the TMS and the drag associated with the TMS, resulting in a more compact and more lightweight arrangement that may be more efficiently integrated into an aircraft. It will be appreciated that reference herein to ‘fuel cell’ may be replaced by ‘fuel cell stack’. That is to say, in some examples, a number of fuel cells may be grouped together into a group or stack, and series cooling may be provided within the stack itself, or between separate fuel cell stacks. For example, a fuel cell stack may produce 100kW of power, and so to produce 1MW of total power, ten such fuel cell stacks may be provided. In some examples, the first and second fuel cells are each arranged in use to progressively increase the temperature of the coolant. Thus, the coolant is higher in temperature after the coolant has been output / provided by a respective fuel cell compared to the temperature of the coolant when the coolant is input / received by the respective fuel cell. Hence, the coolant is progressively heated by the fuel cells as the fuel cells are cooled by the coolant in a corresponding way. The fuel cells may be cooled by the transfer of heat from the fuel cell into the coolant. In some examples, the first and second fuel cells are fuel cells of the same type. As referred to herein, fuel cells of the same type means fuel cells that have the same polarisation curve at the same operating temperature or that have the same operating temperature range. In a PEM fuel cell implementation, fuel cells being the same type means that the fuel cells have the same membrane electrode assembly. In some examples, a type of fuel cell refers to an operating temperature range or ideal operating temperature of the fuel cell (such as low temperature (LT), intermediate temperature (IT), and high temperature (HT)). This is discussed further below. By providing first and second fuel cells of the same type, a required coolant flow rate, frontal area of the arrangement and power of a heating arrangement (e.g. duty on a heating device) for heating the coolant for the first fuel cell (if provided) can be reduced. In some examples, the first and second fuel cells are fuel cells of a different type. As referred to herein, fuel cells of a different type means fuel cells that have different polarisation curves at the same operating temperature or that have different operating temperature ranges. In a PEM fuel cell implementation, fuel cells being a different type means that the fuel cells have a different membrane electrode assembly. In some examples, a type of fuel cell refers to an operating temperature range of the fuel cell (such as low temperature, intermediate temperature, and high temperature). By providing first and second fuel cells of different types (such as first and second fuel cells selected from LT, IT, or HT fuel cells), the type of fuel cell can be selected such that the fuel cell is suited to the temperature of the coolant at that particular point in the series arrangement. This is particularly advantageous in the series arrangement where the coolant is progressively heated by the fuel cells. For example, the first fuel cell may be a fuel cell with a lower nominal operating temperature than the second fuel cell, as the coolant will have a lower temperature when the coolant is cooling the first fuel cell compared to the second fuel cell (because by cooling the first fuel cell the temperature of the coolant will be increased). This approach can significantly reduce the required coolant flow rate and also duty on a heating device (if provided) to initially heat the coolant before input to the first fuel cell. In some examples, the first fuel cell is arranged in use to increase the temperature of the coolant to a temperature corresponding to an operating temperature range of the second fuel cell. Hence, the second fuel cell (i.e. a type of the second fuel cell) is selected to correspond to an expected temperature of the coolant after the coolant exits from the first fuel cell. This means that the coolant, by heating action of the first fuel cell, is already at a temperature suitable for use in the second fuel cell to cool the second fuel cell. As a result, a separate / dedicated heating arrangement to heat the coolant for the second fuel cell can be omitted, reducing mass of the system and a power demand of the system. In some examples, the first and second fuel cells are arranged in use to provide maximal operating efficiency at different temperatures. In some examples, the first fuel cell is arranged to provide a maximal operating efficiency at a lower temperature than the second fuel cell. Hence, the fuel cells are arranged in an order that is suited for the progressive heating of the coolant, ensuring that the fuel cell at a given point in the power system is matched to the operating temperature that will actually be experienced by that fuel cell. This increases overall efficiency. As described earlier, when fuel cell stacks of the same type are operated in series, the stacks operating at lower temperature than their nominal operating temperature will have a different polarisation curve and lower operating efficiency. Thus, in some examples, one or more of the fuel cells are arranged to operate at a temperature lower than their associated respective nominal operating temperatures. As also described herein, this is advantageous because a coolant bleed arrangement may then be provided to bleed a portion of coolant for other uses. In some examples, the plurality of fuel cells further comprises a third fuel cell arranged such that, in use, the third fuel cell receives the coolant from the second fuel cell for cooling the third fuel cell. Hence, a third fuel cell may also be provided with the first and second fuel cell such that there is series cooling of the first, second, and third fuel cells. It will be appreciated that the number of fuel cells is not particularly limited, and may take any value greater than two (two being the minimum number of fuel cells to provide a series cooling arrangement). In some examples, each fuel cell of the plurality of fuel cells is arranged in use to increase the temperature of the coolant by approximately 5 to 40 degrees Celsius. The present inventors have identified that this range of temperatures provides an advantageous balance between coolant flow rate and overall coolant temperature increase, in addition to a reduction in the required power of a heating arrangement to pre-heat air for the first fuel cell in the series arrangement. In some examples, this may be 10 to 30 degrees Celsius. In other examples, this may be 5 to 50 degrees Celsius. This may be referred to as a coolant delta T, or dT. Thus, each fuel cell may progressively increase the temperature of the coolant, while maintaining a coolant temperature difference across the fuel cell within predetermined operating limits to increase the lifetime and performance of the fuel cell. The present approach applies to a variety of fuel cell technologies and fuel cell configurations (like electrolyte substance, membrane type and composition, fuel type, anode and cathode catalysts, gas diffusion layers, different ion transfer mechanisms etc.). For example, protonexchange membrane fuel cells, phosphoric acid fuel cells, solid acid fuel cells, alkaline fuel cells, solid oxide fuel cells, molten-carbonate fuel cells, etc. However, the present approach is particularly well-suited to polymer electrolyte membrane (PEM) fuel cells, also known as protonexchange membrane fuel cells, due to their high power / energy density being particularly useful for transportation implementations, such as in aircraft and particularly electrically powered aircraft. Thus, in some examples, the plurality of fuel cells is a plurality of PEM fuel cells. In some examples, the plurality of fuel cells comprises one or more PEM fuel cells. In some examples, the plurality of fuel cells is a plurality of PEM fuel cells having two or more different membrane electrode assemblies. In some examples, the plurality of fuel cells comprises one or more of: a low temperature (LT) polymer electrolyte membrane (PEM) fuel cell; an intermediate temperature (IT) PEM fuel cell; and a high temperature (HT) PEM fuel cell. The LT, IT, and HT refers to operating temperature ranges for the fuel cell. In examples, a nominal operating temperature refers to an operating temperature in the operating temperature range for a given fuel cell type that corresponds to a temperature at which the fuel cell is designed to operate / has maximum membrane conductivity. An LT PEM fuel cell may refer to a fuel cell having an operating temperature range of approximately 50 to 90 degrees Celsius. An IT PEM fuel cell may refer to a fuel cell having an operating temperature range of approximately 90 to 120 degrees Celsius. An HT PEM fuel cell may refer to a fuel cell having an operating temperature range of approximately 120 to 250 degrees Celsius, for example 120 to 200 degrees Celsius. It will be appreciated that the LT, IT, and HT fuel cells may be operated outside of their operating temperature ranges in principle, but that their efficiency and performance will be reduced. It will be appreciated that the internal configuration (e.g. membrane electrode assembly, primary membrane material, bi-polar plate material, plate sealing, etc.) of LT, IT and HT fuel cells may differ so as to provide the different operating temperature ranges and is not particularly limited. Example compositions of LT, IT, and HT PEM fuel cells will now be described. An example LT PEM fuel cell may comprise a perfluorosulfonic acid (PFSA) membrane (such as a Nafion-based membrane or electrolyte). As a result of the hydration requirement for Nation electrolyte, Nation membrane-based fuel cells (i.e. an LT PEM fuel cell) may have an upper operating temperature of approximately 100 degrees Celsius. An example IT fuel cell may comprise Nation and a hydroscopic oxide (e.g. SiO2 or TiO2) to increase water retention at higher temperatures in the IT PEM operating temperature range. Alternatively, solid-acid membranes may be used as an electrolyte for an IT PEM fuel cell. An example HT PEM fuel cell may comprise phosphoric acid doped polybenzimidazole (PBI). PBI exhibits a proton conduction mechanism that does not require the presence of liquid water and thus operates effectively above 100 degrees Celsius and higher. Further, as discussed above, different fuel cell types may have different coolant input / output temperature requirements so as to prevent an excessive temperature gradient across the fuel cell (such as the membrane of the fuel cell) that can cause a reduction in performance and lifespan. For an LT PEM fuel cell, a coolant input temperature may be approximately 60 degrees Celsius and a coolant output temperature may be approximately 80 degrees Celsius. For an IT PEM fuel cell, a coolant input temperature may be approximately 100 degrees Celsius and a coolant output temperature may be approximately 120 degrees Celsius. For an HT PEM fuel cell, a coolant input temperature may be approximately 160 degrees Celsius and a coolant output temperature may be approximately 180 degrees Celsius. In some examples, the first fuel cell is an LT PEM fuel cell and the second fuel cell is one of an IT PEM or HT PEM fuel cell, or the first fuel cell is an IT PEM fuel cell and the second fuel cell is an HT PEM fuel cell. Hence, the fuel cells may be ordered in a coolant flow direction from the first fuel cell to the second fuel cell (and to the third fuel cell if present), in an order from LT to IT to HT. That is to say, the fuel cells are arranged such that the fuel cells have different operating temperature ranges and the operating temperature ranges increase from the first fuel cell to the second fuel cell (to the third fuel cell, etc.). It will be appreciated that the plurality of fuel cells may comprise fuel cells of a different type and fuel cells of the same type. For example, in a coolant flow direction from left to right, the fuel cells may be ordered as follows: LT - LT - IT - IT - HT - HT, or LT - IT - IT - HT - HT - HT. However, it will be appreciated that the present techniques are not particularly limited in this respect and a variety of configurations, orders, and numbers of different fuel cell types are envisaged. In some examples, the power system further comprises a coolant bleed arrangement arranged in use between the first fuel cell and the second fuel cell to bleed a portion of the coolant before the coolant is received by the second fuel cell. The coolant bleed arrangement may be provided between each fuel cell, or between a subset of fuel cells of the plurality of fuel cells. In order to maintain a certain coolant temperature difference across a fuel cell (dT) (which is useful for maximising fuel cell performance and lifespan, but also useful for intentionally operating a fuel cell below its nominal operating temperature), it can be advantageous to provide a coolant bleed arrangement to remove a portion of coolant before the coolant is received by a next fuel cell in the arrangement. An equation for heat transfer (i.e. waste heat), Q in J / s, is as follows: Q = m cn dT , where m is the mass flow rate in kg / s, cp is the specific heat capacity in J / kgK, and dT is the temperature difference in Kelvin. Hence, by operating a fuel cell away from its nominal operating point and thus intentionally operating the fuel cell at a less efficient point, the amount of waste heat produced by the fuel cell is increased. This is counter-intuitive because typically it is desirable to maximise fuel cell efficiency and minimise waste heat production. However, by increasing the amount of waste heat produced by a fuel cell, a coolant bleed arrangement can be provided. In some examples, the fuel cells may be arranged to provide a constant (non-changing) dT across each respective fuel cell stack. A constant (i.e. non-reducing) dT across the fuel stacks is advantageous because it maximises the overall dT across the whole series arrangement, thus minimising the initial preheat required. In other examples, the fuel cells may be arranged to provide a reducing dT across the fuel cell stacks, as a lower dT results in a higher fuel cell efficiency and less waste heat. This will now be described in more detail. Without a coolant bleed arrangement (i.e. to remove air between fuel cells), the mass / airflow rate through the series of fuel cell stacks would be the same. By the above equation, this would result in the dT for a fuel cell stack progressively reducing across the series arrangement by the same proportion as the increase in Q. For example, the first fuel cell stack may have a dT of 20 degrees Celsius, and the last fuel cell stack may have a dT of 10 degrees Celsius. This also means that fuel cell stacks with a lower dT will operate at a higher efficiency. However, operating with a lower dT then requires additional fuel cell stacks in the series to achieve the same overall dT across the whole series arrangement as compared to an example where constant dT is realised with a bleed arrangement. The present inventors have identified the presence of this trade-off, and have further realised that the coolant bleed is particularly advantageous because it can be used in other aircraft systems and indeed to further pre-heat the air input to the first fuel cell. Hence, a coolant bleed arrangement may be provided between the first and second fuel cells to bleed a portion of the coolant. It will be appreciated that a coolant bleed arrangement may additionally or alternatively be provided between a second and third fuel cell etc. Indeed, a plurality of coolant bleed arrangements may be provided between various pairs of fuel cells. In some examples, a coolant bleed arrangement is provided between each fuel cell (and so in a three fuel cell arrangement there would be two coolant bleed arrangements, in a four fuel cell arrangement there would be three coolant bleed arrangements, etc.). As described above, providing a coolant bleed arrangement is counter-intuitive because The portion of coolant bled by the coolant bleed arrangement(s) can advantageously be used for a variety of purposes within the power system or an aircraft. In some examples, the coolant bleed arrangement is arranged in use to provide the portion of the coolant to at least one of: a coolant mixing arrangement; a pre-heater arrangement; an intercooler arrangement; an anti-icing arrangement; an environmental conditioning arrangement; and an air delivery arrangement for providing air as fuel to a cathode of a fuel cell. Thus, the portion of the coolant can be used to provide useful cooling / heating functions for other aircraft systems. In some examples, the coolant bleed arrangement is arranged in use to provide the portion of the coolant to the pre-heater arrangement to heat cryogenic fluid and then provide the portion of the coolant to the intercooler. The present techniques may be used with hydrogen fuel cells, where the hydrogen fuel is stored as cryogenic hydrogen. Thus, the portion of the coolant may be used to pre-heat the fuel to a temperature more suitable for the fuel cells. Further, by using the portion of coolant with the intercooler, the size and mass of the intercooler can be reduced. In some examples, the coolant bleed arrangement comprises a manifold arrangement to bleed the portion of the coolant. In some examples, the power system comprises a bleed control arrangement configured to control the coolant bleed arrangement to provide variable coolant bleed. Hence, the power system can bleed coolant in a variety of ways depending on implementation. In some cases, a fixed manifold may be used which has been sized for a desired portion of coolant bleed, and in other cases the coolant bleed may be variable and thus can be tuned to an aircraft operating state or requirement. In some examples, the coolant is air. Thus, in these examples, the fuel cells can be air-cooled. The series cooling arrangement described herein is particularly advantageous when the coolant is air because the air flow rate required to effectively cool fuel cells in a parallel cooling arrangement can be excessively high. In some examples, the air is ambient air. In some examples, the first fuel cell is arranged in use to receive the air from an atmosphere external to the aircraft. Thus, the fuel cells are air-cooled by ambient air. In use, relative motion of an aircraft and the atmosphere provides the air for cooling 10 the fuel cells. In such an air-cooling example, if a parallel cooling arrangement were to be used, the frontal area required to air-cool the fuel cells can be prohibitively large. However, with the series arrangement, a much more compact arrangement can be realised, as the frontal area requirement is significantly reduced. This allows for improved integration of the power system into an aircraft, and can increase the space for other aircraft components. In some examples, the power system comprises a shroud arranged in use to direct the air from the first fuel cell to the second fuel cell. Hence, air may be directed to flow over / around / through the first fuel cell, and then flow over / around / through the second fuel cell etc. by the shroud. The shroud may substantially encapsulate the fuel cells. The shroud may be an elongate tube (of a variety of cross-sectional shapes) or substantially cylindrical. The shroud may form part of a nacelle. The present technique is not particularly limited in this respect. In some examples, the first fuel cell is arranged in use in an upstream airflow direction to the second fuel cell. Hence, the first fuel cell is arranged to receive the air (coolant) first, and then the first fuel cell is cooled by the air, and then the second fuel cell receives the air. In some examples, the power system further comprises a heating arrangement arranged in use to heat the coolant before the coolant is received by the first fuel cell. As mentioned herein, it can be advantageous to increase the coolant temperature before the coolant is supplied to the first fuel cell. Indeed, on a particularly cold day, outside / ambient air temperature can be as low as minus 50 degrees Celsius, and if this air were to be introduced into a fuel cell, the fuel cell may be damaged and / or the fuel cell may freeze resulting in poor operating efficiency. Thus, a heating device may be provided to heat the air to a temperature suitable for input as coolant to the fuel cell (the coolant temperature depending on the type of fuel cell as discussed above). In some examples, the heating arrangement comprises an electric heater. Hence, the temperature of the coolant can be increased before introduction to the first fuel cell in a simple manner. In some examples, the heating arrangement comprises a regenerative heat exchange. For example, the regenerative heat exchange may be configured to use energy recovered from an exhaust of the fuel cells to heat the coolant before the coolant is received by the first fuel cell. This can increase the efficiency of the heating of the coolant and reduces a power demand of the heating arrangement. In some examples, the heating arrangement comprises a compressor arranged in use upstream of the plurality of fuel cells. The compressor may be arranged in use in an upstream coolant / air flow direction. The compressor may be powered by an electric motor. By arranging a compressor upstream of the first fuel cell, the compressor can compress incoming air and thus increase the temperature of the incoming air. This can reduce the power demand of the heating arrangement. 11 In some examples, the power system further comprises a turbine, and the compressor is arranged in use to be powered by the turbine. The turbine may be arranged downstream of the plurality of fuel cells (i.e. in an airflow / coolant flow direction in use). This arrangement can be particularly advantageous because the power system can be balanced to produce zero drag. In some examples, the power system further comprises a power turbine arranged in use to generate power. The power turbine may be arranged downstream of the plurality of fuel cells (i.e. in an airflow / coolant flow direction in use). The power turbine may be connected to an electric generator. This system can produce net power as a result of the power turbine. In some examples, the power system further comprises a nozzle arranged in use downstream of the plurality of fuel cells (i.e. in an airflow / coolant flow direction in use). As a result of expanding air in the nozzle, this power system can produce net thrust. In some examples, the power system further comprises a propulsive fan arranged in use upstream of the plurality of fuel cells. Hence, this system can provide propulsion. In some examples, the power system is arranged within a nacelle. Hence, the present techniques may be efficiently integrated with existing aircraft structures and arrangements. In some examples, the heating arrangement comprises a coolant mixing arrangement, the power system further comprising a coolant recirculation arrangement arranged in use to provide a portion of coolant exhausted from the plurality of fuel cells to the coolant mixing arrangement to heat the coolant before the coolant is received as input by the first fuel cell. Thus, in these examples, the hot exhaust (for example the coolant exhausted from the plurality of fuel cells) can be used to heat the coolant before the coolant is received by the first fuel cell by mixing the hot exhausted coolant with the incoming cold / unheated coolant. This is a particularly efficient way to heat the coolant to a suitable temperature for the first fuel cell. Furthermore, by providing a coolant recirculation arrangement, coolant already present in the system can be used during start-up. Hence, the amount of coolant external from the power system (i.e. ambient air) required to be drawn through the power system can be reduced, reducing the power demand on start-up. Further, the amount of coolant heating required can be reduced as the coolant already in the system may be at a more suitable temperature for introduction into the first fuel cell (i.e. it may be warmer than ambient air due to the air in the system being insulated from the outside atmosphere). In some examples, the coolant is a cooling liquid. For example, the cooling liquid may be water, water-glycol, oil, etc. Thus, the present series cooling arrangement can be used with a variety of coolants, including ambient air, but also liquid coolants. In some examples, the power system further comprises a radiator arrangement for cooling the coolant exhausted from the plurality of fuel cells. In these examples, the temperature of the coolant, i.e. cooling liquid, can be reduced by the radiator arrangement before being recirculated to the first fuel cell. The extent to which the coolant is cooled before being recirculated can be 5 varied depending on the radiator configuration. Viewed from a second aspect, there is provided a nacelle comprising the power system described herein. Viewed from a third aspect, there is provided an aircraft comprising the power system described herein. The aircraft may be an at least electrically powered aircraft. 10 Viewed from a fourth aspect, there is provided a method for cooling fuel cells in the power system described herein, the method comprising: receiving coolant by the first fuel cell of the plurality of fuel cells; cooling the first fuel cell with the coolant; providing the coolant from the first fuel cell to a second fuel cell of the plurality of fuel cells; and cooling the second fuel cell with the coolant. Other aspects will also become apparent upon review of the present disclosure, in particular upon 15 review of the Brief Description of the Drawings, Detailed Description and Claims sections. Brief Description of the Drawings Examples of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1A shows a schematic view of an example power system as discussed herein; Figure 1B shows a schematic view of an example power system as discussed herein; Figure 2A shows an example relationship between membrane conductivity and operating temperature for a fuel cell; Figure 2B shows a schematic view of an example power system as discussed herein; Figure 3 shows a schematic view of an example power system as discussed herein; Figure 4 shows a schematic view of an example power system as discussed herein; Figure 5 shows a schematic view of an example power system as discussed herein; Figure 6 shows a schematic view of an example power system as discussed herein; Figure 7 shows a schematic view of an example power system as discussed herein; Figure 8A shows a schematic view of an example power system as discussed herein; Figure 8B shows a schematic view of an example power system as discussed herein; Figure 8C shows a schematic view of an example power system as discussed herein; Figure 8D shows a schematic view of an example power system as discussed herein; Figure 9 shows a schematic view of an example power system as discussed herein; Figure 10 shows a schematic view of a nacelle and an example power system as discussed herein; Figure 11 shows a schematic view of an example power system as discussed herein; Figure 12 shows a schematic view of an example power system as discussed herein; Figure 13 shows a schematic view of an example system comprising a plurality of power systems (i.e. channels) as discussed herein; Figure 14 shows a schematic view of an example aircraft comprising a power system as discussed herein; Figure 15 shows a schematic view of an example power system integrated into an aircraft as discussed herein; and Figure 16 shows example steps for cooling fuel cells as discussed herein. While the disclosure is susceptible to various modifications and alternative forms, specific example approaches are shown by way of example in the drawings and are herein described in detail. It should be understood however that the drawings and detailed description attached hereto 5 are not intended to limit the disclosure to the particular form disclosed but rather the disclosure is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the claimed invention. It will be recognised that the features of the above-described examples of the disclosure can conveniently and interchangeably be used in any suitable combination. 10 Detailed Description An invention described herein relates to power systems for aircraft, and in particular electrically powered aircraft. The power system described herein may be used with any vehicle or power requiring arrangement, however it is a particularly advantageous arrangement for use in aircraft. A particular use for this invention may be in an aircraft with an electrically drivable motor or a drivable motor that is at least partially electrically driven. For example, the propulsion in the aircraft in which the power systems disclosed herein are used may be fully or partially electrically powered. Indeed, it will be appreciated that the term ‘electrically powered aircraft’ refers to an aircraft that is at least partially electrically powered (and may also be at least partially combustion powered). Partially powered aircraft may use thrust provided in part by electrical means and in part by combustion means. This invention may be used in a fully or partially combustion powered aircraft. The electrical and combustion aspects may be provided by one or by a few fuels. In some examples a cryogenic fuel may be used. For example, a high energy density power source arrangement, such as the fuel cells discussed herein, may comprise a cryogenic fuel arrangement arranged to provide fuel for power generation. Various example power systems will now be described with reference to the figures. It will be appreciated that the example power systems may include additional components not shown in the figures, such as other power generating arrangements and fuel arrangements for providing fuel (such as hydrogen and air) to the fuel cells for power generation. Discussion of like-labelled features is not repeated throughout the figures but applies in a similar way. It will be appreciated that various features of the power systems shown in the figures may be omitted, and indeed that various features from the power systems of the figures may be combined. Figure 1A shows an example power system 100 for an aircraft according to the present teachings. Power system 100 includes a plurality of fuel cells 102 having a first fuel cell 104 and a second fuel cell 106 (FC refers to fuel cell). The plurality of fuel cells 102 are arranged to provide power for the power system. For example, the plurality of fuel cells 102 may use hydrogen and air as fuel to generate power (i.e. electricity). The power / electricity may then be used in the aircraft for powering one or more propulsive motors, and / or for use in other aircraft systems. Fuel cells using other fuels and fuel types (such as methanol, ethanol, ammonia, etc.), are also envisaged. While the present teachings refer to a plurality of fuel cells 102 having first and second fuel cells, it will be appreciated that the present teachings apply similarly to a plurality of fuel cell stacks having first and second fuel cell stacks. A fuel cell stack may refer to a plurality of fuel cells. Series cooling as discussed herein may be provided between fuel cell stacks in a similar way to how series cooling may be provided between individual fuel cells in a single stack. For example, a single fuel cell stack may comprise one or more fuel cells of different types (e.g. LT, IT, HT PEM) and these individual fuel cells may be cooled in series as described herein. As shown in figure 1A, the first fuel cell 104 receives a coolant 108 for cooling the first fuel cell 104 (as shown by the arrow). At a point before the coolant 108 is received by the first fuel cell 104, the coolant has a temperature, T = X. The coolant 108 is then used to cool the first fuel cell 104. For example, the coolant 108 may flow over / through / around the fuel cell 104 and components thereof to cool the fuel cell 104. Alternatively, or additionally one or more conduits or channels may be provided within or surrounding the first fuel cell 104 through which the coolant 108 may circulate to draw heat from the fuel cell 104 into the coolant 108. As a result of the cooling action of the first fuel cell 104, the temperature of the coolant 108 increases. The coolant 108 is then provided to the second fuel cell 106. The coolant 108 may be provided to the second fuel cell 106 as a result of a physical arrangement of the fuel cells (i.e. being linearly arranged with respect to a coolant flow direction) or as a result of being directed by a shroud or similar physical arrangement that directs the coolant flow. Additionally, or alternatively, the coolant 108 may be provided to the second fuel cell 106 by one or more conduits. The coolant 108 then cools the second fuel cell 106 in a similar manner as described above in relation to the first fuel cell 104. Before the coolant 108 is provided to the second fuel cell 106 and after the coolant has cooled the first fuel cell 104, the temperature of the coolant 108 increases to T >X. In other words, the temperature of the coolant 108 has increased as a result of the cooling action provided to the first fuel cell 104. In this way, the coolant 108 is used to cool the first fuel cell 104 and then cool the second fuel cell 108. Hence, a series cooling arrangement is provided by the power system 100 whereby the same coolant is used to cool both the first and second fuel cells, the second fuel cell being cooled by the coolant 108 after the first fuel cell has been cooled by the coolant 108. The plurality of fuel cells 102 may include a variety of fuel cell technologies. In some examples, the first fuel cell 104 and the second fuel cell 106 are fuel cells of a different type. For example, the first fuel cell 104 and the second fuel cell 106 may have different operating temperature ranges or different polarisation curves at the same operating temperature. The first fuel cell 104 and the second fuel cell 106 may have different ideal operating temperatures or provide maximum performance or efficiency at different operating temperatures. In some examples, the first fuel cell 104 may be a LT PEM fuel cell and the second fuel cell 106 may be an IT or HT PEM fuel cell. Alternatively, the first fuel cell 104 may be an IT PEM fuel cell and the second fuel cell 106 may be an HT PEM fuel cell. Alternatively, the first fuel cell 104 may be an IT PEM fuel cell and the second fuel cell 106 may also be an IT PEM fuel cell. As discussed herein, this progressive heating of the coolant 108 in a series cooling arrangement means that each fuel cell may only experience a comparatively small coolant temperature difference, while the accumulated coolant temperature difference across the whole arrangement of fuel cells is comparatively large. As a result, a significant reduction in required coolant flow rates is realised, thereby reducing the size and mass of the TMS. Further, by supporting a lower coolant temperature difference experienced by each fuel cell, the likelihood of damage or a reduction in the performance of a given fuel cell can be decreased by ensuring that excessive coolant temperature differences do not damage the fuel cells. It will be appreciated that the temperature difference, dT, for each fuel cell in the series arrangement may not be constant. In some implementations, a suitable dT may depend on the operating efficiency, coolant bleed mass flow rate, and nominal operating temperature of the fuel cell (e.g. its membrane electrode assembly). A greater increase in the overall temperature difference of the coolant can also be realised, because a number of fuel cells can be provided in a series cooling arrangement while still satisfying a fuel cell’s individual requirement for maximum coolant temperature difference. This is particularly advantageous because this effectively reduces the operating temperature requirement (and input coolant temperature requirement) for the first fuel cell in the series, thereby reducing a heating power requirement of a heating arrangement that may be provided to pre-heat / heat the coolant to a temperature suitable for the first fuel cell. The number of fuel cells in the power systems discussed herein is not particularly limited. This is shown with reference to figure 1B. Figure 1B shows an example power system 150 for an aircraft according to the present teachings. Power system 150 may correspond to power system 100 of figure 1A. In addition to first fuel cell 104 and second fuel cell 106, power system 150 includes a third fuel cell 110. The temperature of the coolant 108 after having cooled the second fuel cell 106 and before being received by the third fuel cell 110 (and before cooling the third fuel cell 110) is: T »X. That is to say, the temperature of the coolant 108 at this point is greater than the temperature of the coolant between the first and second fuel cells 104, 106 discussed above. In this way, the number of fuel cells provided in the series arrangement is not particularly limited, and may be two or more fuel cells. As shown by figures 1A and 1B, the temperature of the coolant 108 is progressively increased by the use of the coolant to cool each of the fuel cells in turn. This progressive heating of the coolant is advantageous as discussed herein and in relation to figure 1A. The first, second, and third fuel cells 104, 106, 110 may be fuel cells of the same type, fuel cells of a different type, or some may be fuel cells of the same type and another fuel cell may be a different type. As mentioned above, the plurality of fuel cells of the power system may advantageously include fuel cells of various types. This is explained with reference to figure 2A. Figure 2A shows an example relationship between membrane conductivity and operating temperature for an example fuel cell, which in this case is an LT PEM fuel cell, for example comprising a Nation membrane (although this applies generally to fuel cells of other types). As shown in figure 2A, the LT PEM fuel cell has a nominal operating temperature of 80 degrees Celsius (shown by point A), where membrane conductivity is greatest. The most performant operating temperature is largely dictated by mitigation of electrode flooding, hydration, and material degradation. A higher operating temperature (i.e. towards and beyond point C) causes membrane dryness and consequently a large ionic resistance and ohmic loss. A higher operating temperature also increases material degradation. A low operating temperature (i.e. towards and beyond B) increases the likelihood of water product existing in its liquid state, in turn increasing the likelihood of electrode flooding, resulting in oxygen starvation, and a large mass concentration polarisation. An operating temperature that is too low may also be unfavourable for PEM proton conductivity and electrochemical reaction kinetics, which are sensitive to temperature. As can be understood from figure 2A, operating the fuel cell at a temperature too low or too high compared to its nominal operating temperature can result in adverse effects as discussed above. Hence, in some examples, it can be advantageous to provide a heating arrangement to pre-heat the coolant before it is received by the first fuel cell in the series arrangement, to ensure that the initial coolant temperature is not at a temperature that will negatively affect the fuel cell (i.e. damage as a result of freezing the fuel cell, or resulting in a significantly reduced membrane conductivity). It will be appreciated that the relationship shown in figure 2A is specific to fuel cell type (such as LT, IT, HT), but that the general shape remains similar. Further, the present inventors have identified that this behaviour can be used to inform the selection of the fuel cell types in the present power systems. In particular, the present inventors have identified that deliberately operating a fuel cell at a temperature other than its nominal temperature at which it was designed for can be beneficial. That is to say, for a given position in the series cooling arrangement, it can be beneficial to select a fuel cell type (such as LT, IT, or HT) that has a higher nominal operating temperature than the temperature at which the fuel cell will actually operate in that given position, because the fuel cell performance drop for a fuel cell operating at less than its ideal operating temperature is less than a fuel cell operating at greater than its ideal operating temperature (see e.g. 60 vs. 100 degrees Celsius in figure 2A). This is explained with reference to figure 2B, which shows the power system of 2B, where the first fuel cell 104 has an operating temperature range 1, the second fuel cell 106 has an operating 19 temperature range 2, and the third fuel cell 110 has an operating temperature range 3. In this example, operating temperature range 1 is less than operating temperature range 2 and 3, and operating temperature range 2 is less than operating temperature range 3. The nominal operating temperature of the first fuel cell 104 is within the operating temperature range 1, and is the temperature at which the first fuel cell 104 is designed to operate (and may in some examples correspond to a temperature where the first fuel cell 104 has maximum membrane conductivity). Similarly, the nominal operating temperature of the second fuel cell 106 is within the operating temperature range 2, and is the temperature at which the second fuel cell 106 is designed to operate. Similarly, the nominal operating temperature of the third fuel cell 110 is within the operating temperature range 3, and is the temperature at which the third fuel cell 110 is designed to operate. Hence, the first cells are arranged in an order of increasing nominal operating temperature in a coolant input to coolant output direction (i.e. left to right in figure 2B). Hence, the first, second and third fuel cells 104, 106, 110 are fuel cells of different types, wherein a fuel cell operating range of each fuel cell increases from the first fuel cell 104 to the third fuel cell 110. Further, in some examples, the nominal (i.e. rated) operating temperature of a given fuel cell may be greater than a temperature at which the given fuel cell is operated in use. In some examples, the nominal operating temperature of a given fuel cell may be greater than a temperature of the coolant at input to the given fuel cell in use. As a result, the membrane conductivity and thus efficiency of the fuel cells can be increased. It will be appreciated that nominal operating temperature means an operating temperature at which the fuel cell was designed to operate. The coolant used in the present power systems is not particularly limited, but various advantages can be realised when the coolant is air. Further, some example power systems may include a heating arrangement as discussed below. Figure 3 shows an example power system 300 according to the present techniques. The power system 300 may correspond to power system 100, 150, or 250. Previous discussion of like-labelled features is not repeated but applies to the present figure. In the example of figure 3, the coolant 108 is ambient air 108, for example air that in use moves relative to the power system 300 as a result of relative motion of the aircraft and the atmosphere (i.e. due to motion of the aircraft). As described above, ambient air may be at a temperature unsuitable for introduction to the first fuel cell of the plurality of fuel cells 102. For example, the ambient air may be so cold that it would cause freezing and / or damage to the first fuel cell (or a reduction of fuel cell performance) if it were to be used to cool the first fuel cell. An example of such a temperature would be minus 50 degrees Celsius. Indeed, as discussed herein, a fuel cell may have an associated predetermined 20 acceptable coolant inlet temperature range (which corresponds to a temperature that does not negatively impact fuel cell performance (by being outside that fuel cell’s operating temperature range)) or by causing damage / freezing to the fuel cell, as discussed in relation to figure 2A. This may vary depending on the fuel cell type. Hence, power system 300 also includes a heating arrangement 112 for pre-heating the coolant 108 (in this example, air) before the coolant is received by the plurality of fuel cells 102 (i.e. the first fuel cell). As described further herein, the heating arrangement may take a variety of forms. The heating arrangement may be integrated with the fuel cells or provided as a separate arrangement. The heating arrangement may also be sized such that the power system provides zero drag (i.e. the heating arrangement may comprise a compressor and fan), provides thrust (with a nozzle), and / or provides electrical power (with a power turbine). In some examples, the heating arrangement 112 may be omitted. In this example, the plurality of fuel cells 102 are HT PEM fuel cells. The nominal operating temperature of the HT PEM fuel cells in this example is 180 degrees Celsius. In this example, where dT is 20 degrees Celsius, an acceptable coolant inlet temperature for an HT PEM fuel cell may be approximately 160 degrees Celsius, as shown in figure 3. Hence, the heating arrangement 112 may be configured to increase the temperature of the coolant 108 from an ambient temperature (for example minus 50 degrees Celsius) to 160 degrees Celsius, before the coolant 108 is provided to the plurality of fuel cells 102. As shown in figure 3, the temperature of the coolant 108 increases to 180 degrees Celsius at exhaust from the plurality of fuel cells 102 as a result of being used to cool the plurality of fuel cells 102). It will be appreciated that the type of fuel cells 102 that may be provided can vary depending on implementation and reference above to HT PEM fuel cells is provided only as an example. By providing a series air-cooling arrangement, advantages over parallel air-cooling arrangements can be realised. For example, in an air-cooling example having fuel cells in parallel, a significant air flow rate may be required, which can in turn lead to large frontal area requirements for the power system (so as to be able to introduce enough air), resulting in aircraft installation challenges and a significant power draw for the pre-heating of the air by the heating arrangement. A worked example is provided as follows for an example of fuel cells producing 2MW of power and air-cooled in parallel. The fuel cells in this worked example have nominal efficiency and operating temperature, of 50% and 180 degrees Celsius, respectively. This is an arrangement with fuel cell stacks in parallel. If stacks are operated with dT=20 degrees Celsius, then the air mass flow required is 100 kg / s (assuming cp is constant). The power of the heating arrangement (i.e. a duty on the heating device) is then 21 MW. One way to reduce the flow rate and the power of the heating 21 arrangement / duty on the heating device would be to increase dT across the fuel cell stacks. If this is increased to 60 degrees Celsius, the flow rate reduces to 33 kg / s and heating arrangement / device power / duty to circa 5.61 MW. However, individual fuel cell stacks will suffer significant performance loss if they are operated with such large dT. Hence, it can be advantageous to keep dT as low as possible, and the present technique provides a way to do this while still achieving benefits of reduced air flow rate and reducing the required power of the heating arrangement. The required power of the heating arrangement shown in the table below provides a baseline against which the performance benefit of the current technique can be shown. The values in the table are calculated as if the heating arrangement is an ideal electric heater (i.e. 100% efficiency) to pre-heat the air from ambient to the temperature of the first FC. Ideal electric heater meaning its efficiency is 100% as a hypothetical case to define the baseline. Temperature ambient (degC) Temperature of fuel cell inlet (degC) Fuel cell dT (degC) Required air flow rate (kg / s) Heating arrangement power (kW) Case-A -50 160 20 100 21,000 Case-B -50 140 40 50 9,500 Case-C -50 120 60 33 5,610 The power system of the present teachings may achieve 50 kg / s of flow rate (CASE-B above) without compromising dT while keeping it at dT=20 and having two stacks in series instead. In this arrangement there are two stacks operating at dT=20, and achieving the 50 kg / s and 9,500kW of air flow rate and heating arrangement required power respectively. The power systems of the present teachings may be provided with one or more coolant bleed arrangements. This is discussed in relation to figure 4. Figure 4 shows an example power system 400 according to the present techniques. The power system 400 may correspond to power system 100, 150, 250, or 300. Previous discussion of like-labelled features is not repeated but applies to the present figure. In this example, the first and second fuel cells 104, 106 are HT PEM fuel cells (as labelled in the figure), although it will be appreciated that other fuel cell types may be provided instead or in addition. As shown, the efficiency of the first fuel cell 104 is slightly lower than the efficiency of the second fuel cell 106 (Eta refers to efficiency). This is because the second fuel cell 106 is operating at a temperature more aligned with its nominal operating temperature range than the first fuel cell 104. In this example, the fuel cell dT for each of the first and second fuel cells 104, 106 is 20 degrees Celsius, although it will be appreciated that this may be varied depending on implementation. Power system 400 includes a coolant bleed arrangement 114 arranged in use between the first fuel cell 104 and the second fuel cell 106 to bleed a portion of the coolant 116 before the coolant 108 is received by the second fuel cell 106. Doing so helps to maintain a fuel cell target dT (20 degrees Celsius in this example). The coolant bleed arrangement 114 may be integrated with the fuel cells or may be provided as a separate arrangement. The coolant bleed arrangement 114 bleeds (i.e. splits / siphons / removes) the portion of coolant 116 from the coolant 108, and this portion of coolant 116 can advantageously be used for a variety of purposes. For example, the portion of coolant 116 may be used in one or more of a coolant mixing arrangement; a pre-heater arrangement; an intercooler arrangement; an anti-icing arrangement; an environmental conditioning arrangement; and an air delivery arrangement for providing air as fuel to a cathode of a fuel cell. Hence, the portion of the coolant 116 can be used to provide useful cooling / heating functions for a variety of other aircraft systems, and / or to provide conditioned air at a temperature suitable for the cathode air-side of a fuel cell (for the electrochemical reaction in the fuel cell itself). In some examples, a fuel cell may have an open cathode. In some examples, the ambient air may include both reactant and coolant air (stoichiometry » 1), i.e. air may be diverted for cooling, and other air may be diverted for use as cathode air (for the electro-chemical reaction of the fuel cell). Figure 4 shows example numbers for air flow rate, coolant temperature, fuel cell efficiency, and waste heat load (Q). In the example of figure 4, a relatively small portion of air bleed is provided by the coolant bleed arrangement 114. This may counter-intuitively increase with the number of fuel cells in series. Figure 5 shows a power system 500 corresponding to power system 400 of figure 4, and illustrates an example use of the portion of the coolant 116 that is bled from the coolant 108 by the coolant bleed arrangement 114. In this example, the coolant bleed arrangement 114 provides the portion of the coolant 116 to a pre-heater arrangement 118. Air may be advantageously used as the fluid for heating the hydrogen because dry air or air that has a low humidity does not freeze when it comes into contact with cryogenic hydrogen. The pre-heater arrangement 118 is arranged in use to heat cryogenic hydrogen using the portion of the coolant 116, and then provide the heated hydrogen to the fuel cells (such as 104, 106). This can reduce a heating requirement or avoid the inclusion of a dedicated cryogenic hydrogen heater. As a result of the heating of the cryogenic hydrogen, the temperature of the portion of coolant 116 reduces (the heat being transferred from the portion of the coolant 108 to the cryogenic hydrogen 23 to heat the hydrogen). This cooled portion of coolant 116 is then provided to an intercooler 120. Cooling of the intercooler is advantageous because the intercooler heat exchanger sizing is reduced and performance improved the lower the temperature of the coolant. The intercooler 120 is configured to receive hot, compressed cathode air and cool the compressed cathode air using the portion of the coolant 116 and then provide the cooled compressed cathode air to the fuel cells. By using the portion of coolant 116 in the intercooler, the size and mass of the intercooler 120 can be reduced. This is because the temperature of the coolant is suitably low when it comes from the hydrogen pre-heater. Various arrangements of fuel cells and coolant bleed arrangements are envisaged. An example arrangement is shown in figure 6. Figure 6 shows a further example power system 600 according to the present teachings. The power system 600 provides a series cooling arrangement as described herein. In this example, the plurality of fuel cells 102 comprise four HT PEM fuel cells, i.e. fuel cells of the same type (as labelled in the figure). It will be appreciated that the number and type of fuel cells may be varied. With four identical HT PEM fuel cells in series (i.e. fuel cells of the same type), each fuel cell operates at slightly worse efficiency with lower operating temperature (i.e. away from their respective nominal operating temperatures, and thus ‘off design’). Thus, overall there is more waste heat produced (shown as Q in figure 6, which increases with decreasing fuel cell efficiency), but this arrangement provides substantial advantages in reducing air flow rate (which is reduced to 29 kg / s compared to 50kg / s in figure 5 while keeping dT across stacks at 20 degrees Celsius), frontal area requirements, and the required heating power of the heating arrangement 112. This also provides coolant bleed that can be used with other arrangements, such as the hydrogen preheater and intercooler discussed in relation to figure 5. Compared to the arrangement of figure 4, in figure 6 with more fuel cell stacks in series, the coolant bleed is greater, from 0.2 kg / s to 4 kg / s. This is beneficial because more coolant bleed is available for other purposes. In this example, the coolant bleed arrangement 114 comprises a plurality of coolant bleeds resulting in a plurality of portions of coolant bleed 116a, 116b, and 116c. These may be combined into a mixed bleed to produce a desired temperature of coolant as shown, or may be provided separately to other aircraft components that are suited to that temperature of coolant. The values provided throughout the figures, including temperature, flow rate, efficiency, and waste power, are provided as a worked example for a 2 MW fuel cell system, and it will be appreciated that various example arrangements are possible and that the specific values will change accordingly. The present teachings are not limited in this respect. Figures 4, 5, and 6 have shown the fuel cells as being HT PEM fuel cells as an example arrangement. The number and type of fuel cells may be varied, and in some examples the power system comprises fuel cells of different types, e.g. fuel cells having different nominal operating temperatures. This is shown in figure 7. Figure 7 shows a further example power system 700 according to the present teachings. Power system 700 provides a series cooling arrangement as described herein. In this example, the plurality of fuel cells 102 comprise seven fuel cells, one LT PEM, two IT PEM, and four HT PEM fuel cells, i.e. fuel cells of different types (e.g. different fuel cell technologies and different nominal designs, LT, IT, HT) (as labelled in the figure). It will be appreciated that the number and type of fuel cells may be varied. The plurality of fuel cells 102 are arranged in series cooling as discussed herein, and each fuel cell is operating at a different temperature. By arranging in series fuel cells of different types, the required heating power of the heating arrangement 112 can be further lowered by lowering the lowest temperature at the inlet to the first fuel cell in the series (i.e. the left-most fuel cell as shown). Also, as shown, the required coolant flow rate is further reduced at the air inlet (i.e. compared to figures 5 and 6). Indeed, figure 7 again shows fuel cell stacks achieving dT of 20 degrees Celsius while reducing coolant flow rate even further, and reducing the required power of the heating arrangement even further. Further, in figure 7, with an air flow rate of 13 kg / s and first FC coolant inlet temperature of 60 degrees Celsius, the duty / required power of the heating arrangement 112 becomes 1,430 kW. This is nearly 10x smaller than the 21,000 kW shown in the table above for the same dT=20. As shown in figure 7, the efficiency of a fuel cell depends on its type (LT, IT, or HT) and the temperature at which it is operating. Hence, for both the IT PEM and HT PEM fuel cells, the operating efficiency of a given fuel cell increases the closer the fuel cell is operating to its nominal operating temperature (or operating temperature range) for that fuel cell. A number of coolant bleeding arrangements are provided in this example, each arranged to bleed a portion of coolant 116 from the coolant 108. It will be appreciated that the number and location of these coolant bleed arrangements may be varied. Various forms of the heating arrangement to pre-heat the air before being received by the first fuel cell are envisaged, and examples of the heating arrangement will now be discussed. Figures 8A to 8D show example power systems where the heating arrangement comprises a compressor 122. The power systems shown in figures 8A to 8D provide a series cooling arrangement as described herein. As shown in each of figures 8A, 8B, 8C, and 8D, a compressor 122 is arranged upstream (in an airflow / coolant flow direction in use) of the plurality of fuel cells 102. The compressor 122 heats the coolant (such as air) 108 before the coolant 108 is received by the plurality of fuel cells 102. The compressor 122 may be powered by an electric motor. Hence, a high power electric heater may be omitted from these examples, reducing the power draw of the heating arrangement. In the example of figure 8B, the power system also includes a turbine 124. The compressor 122 may be powered by the turbine 124, such that the power system is balanced and provides zero drag. This may correspond to a Brayton cycle, were the additional heat comes from the fuel cell stacks rather than a combustion chamber. In the example of figure 8C, the power system also includes a power turbine 126. The power turbine 126 may be arranged to generate power, for example the power turbine 126 may be connected to an electric generator. This power system can therefore produce net power in the power turbine 126. For example, the expansion in the turbine may be reduced such that the pressure ratio across the power turbine is suitable to cause the power turbine to spin and produce power. In the example of figure 8D, the power system also includes a nozzle 128. This power system can therefore produce net thrust by expanding air in the nozzle 128. This may correspond to a Brayton cycle, were the additional heat comes from the fuel cell stacks rather than a combustion chamber. The various components of figures 8A to 8D may be combined with the power systems and plurality of fuel cells 102 having series cooling arrangements described herein. Figure 9 shows a further example power system 900 that includes a compressor and turbine according to the present techniques. Power system 900 provides a series cooling arrangement as described herein. In this example, the compressor 122 is powered by the turbine 124 and the plurality of fuel cells 102 are arranged in a series cooling arrangement as discussed herein. In this example, the power system may create zero drag because the mass flow rate, pressure, and the velocity at the exhaust (right of the turbine 124) is such that the system is balanced. Power system 900 includes a coolant mixing arrangement 130, and portions of coolant 116 that are bled from the coolant 108 between the first and second, and second and third fuel cells, are provided to the coolant mixing arrangement 130. This advantageously increases the flow rate of the air (i.e. coolant) entering the turbine 130, as shown by the relative increase in flow rate of the air before and after the coolant mixing arrangement 130. When the flow rate increases while other parameters stay constant like inlet / outlet conditions and efficiency, the power output of the turbine increases proportionally. This is because the turbine is converting more energy from the increased mass of air passing through it into mechanical work. Hence, increasing air flow rate can increase power produced by the turbine. The present power systems may be integrated into aircraft structures in various ways. In some examples, the present power systems may be integrated in an aircraft engine nacelle. Figure 10 shows a cross-section of an example nacelle 1000 comprising a power system 1050 according to the present techniques. Power system 1050 provides a series cooling arrangement as described herein. Power system 1050 includes a propulsive fan 132 at an intake side of the nacelle 1000 to direct airflowthrough the nacelle 1000. A shroud 134 is arranged to split the airflow into a bypass stream 136 for producing thrust, and a core stream 138 that is approximately drag neutral. In this example, the power system comprises a plurality of compressors, namely a low pressure compressor 140 and a high pressure compressor 142. This can help optimise performance across a wider range of operating conditions. This can also improve the throttle response, reduce a risk of compressor stall or surge, and de-couple rotational speeds of the turbines. The low pressure compressor 140 is upstream of the high pressure compressor 142 and initially compresses air of the core stream 138 and increases the temperature of the air, for example from minus 22 degrees Celsius to 32 degrees Celsius, and then the high pressure compressor 142 further increases the temperature of the air, for example to 150 degrees Celsius. The low and high pressure compressors 140, 142 may correspond to the heating arrangement as described herein. The air from the output of the high pressure compressor 142 is then provided to the plurality of fuel cells 102. In this example, the plurality of fuel cells 102 are provided radially around a central longitudinal axis of the nacelle 1000. The plurality of fuel cells 102 are arranged to provide series cooling as discussed herein (not shown). A cathode air bleed 144 may also be provided from the low pressure compressor 140 to a cathode air intake (i.e. a fuel intake rather than a coolant intake) of the plurality of fuel cells 102. The cathode air is used as one of the reactants for the electrochemical reaction inside the fuel cell (this air being different from the air provided for cooling the fuel cell). It will be appreciated that channels may be provided inside a fuel cell to route cathode air, and separately a different channels are provided to route cooling air. In this example, the dT of the plurality of fuel cells 102 is 30 degrees Celsius, but it will be appreciated that this may vary depending on implementation and desired dT. In particular, this may vary because for compressors to provide this 150 degrees Celsius, the pressure may be increased to 9 bar (as shown on the figure). This means that fuel cell stacks are to withstand 9 bar pressure of the cooling air inside their structure. If this pressure is too excessive, it can be lowered by increasing the number of fuel cell stacks in series, as this would effectively reduce compressor delivery temperature and the resulting pressure. The power generated by the plurality of fuel cells 102 is then used to power a motor / generator 148, for example a propulsive motor, as shown by dotted line 146. The air coolant exhausted from the plurality of fuel cells 102 is provided to a high pressure turbine 150 and then a low pressure turbine 152, and is then used for cooling of a motor / generator 148 and / or power electronics (not shown), before being exhausted. Recirculation of air may additionally or alternatively be used to heat the air at the intake before being received by a first fuel cell of the plurality of fuel cells. Figure 11 shows an example power system 1100 that uses air recirculation according to the present teachings. Power system 1100 provides a series cooling arrangement as described herein, and now shows a power system 1100 that produces 667 kW of power (not2MW) with four FCs each producing 167 kW of power. Power system 1100 may be combined with other similar power systems. For example, three power systems may be combined to produce 2MW total power. This can advantageously increase redundancy and thus safety of the power system. In this example, the heating arrangement comprises a coolant mixing arrangement 154a. A recirculation fan 154b draws hot exhaust air exhausted from the plurality of fuel cells 102 at a coolant recirculation arrangement 154c. This hot exhaust air is mixed with incoming ambient air in the coolant mixing arrangement 154a, to increase the overall temperature of the air before the air is received by the first fuel cell of the plurality of fuel cells 102 (i.e. the left-most fuel cell). This arrangement is particularly advantageous because a substantial decrease in the required intake flow rate is realised (as shown by the reduction to 3.3kg / s in the present example) as a result of the air recirculation. That is to say, in this 667kW system with four fuel cells in series, would require an air intake of 9.8kg / s but with the recirculation this can be reduced to 3.3kg / s. In this example, the power system 1100 also includes a diffuser 156, which may be omitted in some examples depending on implementation. The diffuser may be provided to reduce the airflow speed of air before the air is received by the fuel cells. For example, fuel cells may act as heat exchangers and have fin structures to enhance heat transfer to the air. To prevent adverse viscous effects, the air speed may be sufficiently low in this case (i.e. less than Mach 0.2). However, as the aircraft speed increases to higher Mach numbers, it may be advantageous for the air to be slowed before the air enters the fuel cells and so a diffuser may be provided. Figure 11 also shows the fan pressure ratio (fan PR), efficiency, and power, for this example configuration. Power system 1100 also includes air bleed arrangements as discussed previously. Again, it will be appreciated that this may be varied or omitted depending on implementation. While in some examples the coolant may be air, in other examples the coolant may be liquid, such as water-glycol, oil, or the like. Figure 12 shows an example power system 1200 where the coolant is liquid according to the present teachings. In a similar way to the air coolant examples discussed above, the cooling of the fuel cells is arranged in series to realise similar advantages as discussed herein. In particular, the coolant 108 is progressively heated by each of the fuel cells of the plurality of fuel cells 102 to progressively increase the temperature of the coolant 108. Power system 1200 provides a liquid coolant 108 that circulates the coolant circuit which passes in a series manner through each of the plurality of fuel cells 102 in turn (i.e. from left to right as shown). In this example, the power system 1200 also includes a radiator 158 to cool the coolant after the coolant temperature has increased as a result of cooling each of the plurality of fuel cells. The size of the radiator 158 may vary depending on implementation and cooling requirements. The radiator 158 may be air-cooled, for example by ambient air. The radiator 158 may be designed so as to maximise a surface area to promote air-cooling of the radiator 158. In some cases, if the dT of the coolant before / after the radiator 158 (with a fixed maximum coolant temperature) is increased, the coolant gets much colder before leaving the radiator and the average temperature of the coolant, and therefore the temperature differential between the coolant and the air, is reduced. At sufficiently large dT, a heat transfer limitation can move to the coolant rather than the air-cooling of the radiator 158. The coolant will thus flow more slowly due to the reduced flow rate. This can become a bottleneck because each unit of coolant carries more heat (since dT is higher), but the radiator must be efficient enough to transfer this heat to the air despite the lower flow and temperature differential between fluids. Hence, an internal structure of the radiator may be optimised to increase the likelihood that the coolant efficiently releases its heat to the air despite the lower coolant flow rate. Thus, in some examples, one or more baffles or turbulence generating arrangements may be provided in the radiator or coolant circuit to increase the turbulence within the coolant flow to enhance heat transfer, or the internal surface area of the radiator 158 in contact with the coolant may be increased, for example by providing one or more fin structures in the coolant conduits of the radiator 158. Power system 1200 also includes a pump 160 to circulate the coolant 108 around the coolant circuit. Again, power system 1200 may optionally include a coolant bleed arrangement 114 to bleed portions of the coolant between fuel cells. The coolant bled in this manner may be used for cooling / heating of other aircraft systems as discussed previously. Multiple power systems as described herein may be provided as part of a larger system, where each power system may be provided as a separate channel. This increases redundancy and thus safety of the power systems. This is shown with reference to figure 13. Each power system / channel provides a series cooling arrangement as described herein. System 1300 includes three channels, channel 1, 2, and 3, each of which comprises a plurality of fuel cells 102 having series cooling arrangements. Each channel may correspond to a power system as described herein. Each channel is separate from each other channel in that the series cooling is provided on a per-channel basis. As shown, coolant 108 such as air is received by each of the channels and the air is exhausted to the right-hand side of the channels. As described before, the air cools each of the fuel cells in each plurality of fuel cells 102 in turn, progressively increasing in temperature. As also shown, the air recirculation arrangement discussed in relation to figure 11 is provided for each channel, such that hot exhaust air is recirculated and provided to the intake air, to increase the temperature of the intake air before the air is received by the first fuel cell of each plurality of fuel cells 102. The exact arrangement of fuel cells and channels may vary depending on implementation. For example, it will be appreciated that a different number of channels and different number of fuel cells per channel may be provided. The power systems described herein may be provided with or integrated with an aircraft in a variety of ways. For example, a power system may be integrated as part of a nacelle as described in relation to figure 10, or a wing or fuselage assembly of an aircraft. One possible way to integrate the power systems described herein is shown in figure 14. Figure 14 shows an example aircraft 1400 having a power system 1450 arranged on a top-side of the fuselage, between the wings and the tail fin (i.e. vertical stabiliser). Power system 1450 provides a series cooling arrangement as described herein. In operation, ambient air (i.e. coolant) may enter the power system 1450 from the left-hand side (shown by the arrow), and be exhausted from the power system 1450 from the right-hand side (shown by the other arrow). The power system 1450 may correspond to any power system as described herein. Figure 15 shows a further example of how the present power systems may be integrated with an aircraft. Figure 15 shows a cross-section of an aircraft. As shown, in this example, a standard fuselage cross-sectional shape 164 has been modified to create a modified fuselage shape 166, which has increased dimensions and one or more protruding portions, which correspond to an air scoop 162 and an ambient air intake 168. As shown by the air / coolant 108 arrows, air is collected by the air scoop 162 (as a result of its physical shape and the relative motion of the aircraft and the ambient air) and the air 108 is directed to the plurality of fuel cells 102 (corresponding to the power systems as described herein). The plurality of fuel cells 102 are arranged as discussed herein to provide series cooling of the fuel cells. In this case, the air is directed between the fuel cells as a result of the physical arrangement of the modified fuselage shape 166. An ambient air intake 168 may be provided on an opposite side of the fuselage to the air scoop 162 to balance the air intake, and to provide additional air for recirculation. A recirculation fan 170 is provided to recirculate the air around the periphery of fuselage and within the modified fuselage 166. For example, the modified fuselage shape 166 may provide a channel containing the recirculating air that recirculates external to the standard fuselage shape 164. In this way, the modified fuselage shape 166 may correspond to an external channel / skin that surrounds the standard fuselage. For example, a rectangular air channel may be attached to the standard fuselage to provide the modified shape. This arrangement provides a number of advantages, such as: suitable coolant flow length for flow mixing against cold ambient air and hot fuel cell exhaust; additional cooling with skin cooling against ambient air; and fuselage skin integrated fuel cells in one row around the circumference of the fuselage, and to the required depth, providing a compact integration of fuel cells with the aircraft. The present invention facilitates such installations as Figure 15 because airflow is significantly reduced, and thus air intakes and ducting can also be reduced in size. Figure 16 shows an example process 1600 for cooling fuel cells in a power system as described herein. Process 1600 includes steps 1602, 1604, 1606, and 1608. Step 1602 includes receiving coolant by the first fuel cell of the plurality of fuel cells, for example the plurality of fuel cells 102 described herein. Step 1604 includes cooling the first fuel cell with the coolant. This cooling may be performed by the first fuel cell transferring heat to the coolant, and then relative motion between the coolant and the first fuel cell moving the heated coolant away from the first fuel cell (for example as a result of relative motion of the power system and ambient air, or liquid coolant being pumped through conduits proximal to the first fuel cell). Step 1606 includes providing the coolant from the first fuel cell to a second fuel cell of the plurality of fuel cells. The coolant may be provided as a result of a shroud or physical arrangement that directs the coolant from the first fuel cell to the second fuel cell, or for liquid coolant examples, may be a conduit. Step 1608 includes cooling the second fuel cell with the coolant. This cooling may be performed in a similar manner as for step 1604. Although illustrative teachings of the disclosure have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise teachings, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims. Examples are set out in the following numbered clauses. 1. A power system for an aircraft, the power system comprising: a plurality of fuel cells having at least a first fuel cell and a second fuel cell arranged such that, in use, the first fuel cell receives a coolant for cooling the first fuel cell and provides the coolant to the second fuel cell for cooling the second fuel cell. 2. The power system of clause 1, wherein the first and second fuel cells are each arranged in use to progressively increase the temperature of the coolant. 3. The power system of any preceding clause, wherein the first and second fuel cells are fuel cells of the same type. 4. The power system of any of clauses 1 or 2, wherein the first and second fuel cells are fuel cells of a different type. 5. The power system of clause 4, wherein the first fuel cell has a lower nominal operating temperature than the second fuel cell. 6. The power system of clause 5, wherein the first fuel cell is arranged in use to increase the temperature of the coolant to a temperature corresponding to an operating temperature range of the second fuel cell. 7. The power system of any preceding clause, wherein the plurality of fuel cells further comprises a third fuel cell arranged such that, in use, the third fuel cell receives the coolant from the second fuel cell for cooling the third fuel cell. 8. The power system of any preceding clause, wherein each fuel cell of the plurality of fuel cells is arranged in use to increase the temperature of the coolant by approximately 5 to 40 degrees Celsius. 9. The power system of any preceding clause, wherein the plurality of fuel cells comprises one or more of: a low temperature (LT) polymer electrolyte membrane (PEM) fuel cell; an intermediate temperature (IT) PEM fuel cell; and a high temperature (HT) PEM fuel cell. 10. The power system of clause 9, wherein the first fuel cell is an LT PEM fuel cell and the second fuel cell is one of an IT PEM or HT PEM fuel cell, or the first fuel cell is an IT PEM fuel cell and the second fuel cell is an HT PEM fuel cell. 11. The power system of any preceding clause, further comprising a coolant bleed arrangement arranged in use between the first fuel cell and the second fuel cell to bleed a portion of the coolant before the coolant is received by the second fuel cell. 12. The power system of clause 11, wherein the coolant bleed arrangement is arranged in use to provide the portion of the coolant to at least one of: a coolant mixing arrangement; a preheater arrangement; an intercooler arrangement; an anti-icing arrangement; an environmental conditioning arrangement; and an air delivery arrangement for providing air as fuel to a cathode of a fuel cell. 13. The power system of clause 12, wherein the coolant bleed arrangement is arranged in use to provide the portion of the coolant to the pre-heater arrangement to heat cryogenic fluid and then provide the portion of the coolant to the intercooler. 14. The power system of any of clauses 11 to 13, wherein the coolant bleed arrangement comprises a manifold arrangement to bleed the portion of the coolant. 15. The power system of any of clauses 11 to 14, further comprising a bleed control arrangement configured to control the coolant bleed arrangement to provide variable coolant bleed. 16. The power system of any preceding clause, wherein the coolant is air. 17. The power system of clause 15, wherein the first fuel cell is arranged in use to receive the air from an atmosphere external to the aircraft. 18. The power system of any of clauses 16 or 17, wherein the power system comprises a shroud arranged in use to direct the air from the first fuel cell to the second fuel cell. 19. The power system of any of clauses 16 to 18, wherein the first fuel cell is arranged in use in an upstream airflow direction to the second fuel cell. 20. The power system of any preceding clause, further comprising a heating arrangement arranged in use to heat the coolant before the coolant is received by the first fuel cell. 21. The power system of clause 20, wherein the heating arrangement comprises an electric heater. 22. The power system of any of clauses 20 or 21, wherein the heating arrangement comprises a regenerative heat exchanger. 23. The power system of any of clauses 20 to 22, wherein the heating arrangement comprises a compressor arranged in use upstream of the plurality of fuel cells. 24. The power system of clause 23, wherein the compressor is arranged in use to be powered by an electric motor. 25. The power system of any of clauses 23 or 24, further comprising a turbine, and the compressor is arranged in use to be powered by the turbine. 26. The power system of clause 25, wherein the turbine is arranged in use downstream of the plurality of fuel cells. 27. The power system of any of clauses 20 to 26, further comprising a power turbine arranged in use to generate power. 28. The power system of any of clauses 20 to 27, further comprising a nozzle arranged in use downstream of the plurality of fuel cells. 29. The power system of any of clauses 20 to 28, further comprising a propulsive fan arranged in use upstream of the plurality of fuel cells. 30. The power system of any of clauses 20 to 29, wherein the heating arrangement comprises a coolant mixing arrangement, the power system further comprising a coolant recirculation arrangement arranged in use to provide a portion of coolant exhausted from the plurality of fuel cells to the coolant mixing arrangement to heat the coolant before the coolant is received as input by the first fuel cell. 31. The power system of any of clauses 1 to 15, wherein the coolant is a cooling liquid. 32. The power system of clause 31, further comprising a radiator arrangement for cooling the coolant exhausted from the plurality of fuel cells. 33. A nacelle comprising the power system of any of clauses 1 to 31. 34. An aircraft comprising the power system of any of clauses 1 to 31. 35. A method for cooling fuel cells in the power system of any of clauses 1 to 31, the method comprising: receiving coolant by the first fuel cell of the plurality of fuel cells; cooling the first fuel cell with the coolant; providing the coolant from the first fuel cell to a second fuel cell of the plurality of fuel cells; and cooling the second fuel cell with the coolant.

Claims

1. A power system for an aircraft, the power system comprising:a plurality of fuel cells having at least a first fuel cell and a second fuel cell arranged such that, in use, the first fuel cell receives a coolant for cooling the first fuel cell and provides the coolant to the second fuel cell for cooling the second fuel cell.

2. The power system of claim 1, wherein the first and second fuel cells are each arranged in use to progressively increase the temperature of the coolant.

3. The power system of any preceding claim, wherein the first and second fuel cells are fuel cells of the same type.

4. The power system of any of claims 1 or 2, wherein the first and second fuel cells are fuel cells of a different type.

5. The power system of claim 4, wherein the first fuel cell has a lower nominal operating temperature than the second fuel cell.

6. The power system of claim 5, wherein the first fuel cell is arranged in use to increase the temperature of the coolant to a temperature corresponding to an operating temperature range of the second fuel cell.

7. The power system of any preceding claim, wherein the plurality of fuel cells further comprises a third fuel cell arranged such that, in use, the third fuel cell receives the coolant from the second fuel cell for cooling the third fuel cell.

8. The power system of any preceding claim, wherein each fuel cell of the plurality of fuel cells is arranged in use to increase the temperature of the coolant by approximately 5 to 40 degrees Celsius.

9. The power system of any preceding claim, wherein the plurality of fuel cells comprises one or more polymer electrolyte membrane (PEM) fuel cells.

10. The power system of any preceding claim, wherein the plurality of fuel cells comprises one or more of: a low temperature (LT) polymer electrolyte membrane (PEM) fuel cell; an intermediate temperature (IT) PEM fuel cell; and a high temperature (HT) PEM fuel cell.

11. The power system of claim 10, wherein the first fuel cell is an LT PEM fuel cell and the second fuel cell is one of an IT PEM or HT PEM fuel cell, or the first fuel cell is an IT PEM fuel cell and the second fuel cell is an HT PEM fuel cell.

12. The power system of any preceding claim, further comprising a coolant bleed arrangement arranged in use between the first fuel cell and the second fuel cell to bleed a portion of the coolant before the coolant is received by the second fuel cell.

13. The power system of claim 12, wherein the coolant bleed arrangement is arranged in use to provide the portion of the coolant to at least one of: a coolant mixing arrangement; a pre-heater arrangement; an intercooler arrangement; an anti-icing arrangement; an environmental conditioning arrangement; and an air delivery arrangement for providing air as fuel to a cathode of a fuel cell.

14. The power system of claim 13, wherein the coolant bleed arrangement is arranged in use to provide the portion of the coolant to the pre-heater arrangement to heat cryogenic fluid and then provide the portion of the coolant to the intercooler.

15. The power system of any preceding claim, wherein the coolant is air, and the first fuel cell is arranged in use to receive the air from an atmosphere external to the aircraft.

16. The power system of any of claims 14 or 15, wherein the power system comprises a shroud arranged in use to direct the air from the first fuel cell to the second fuel cell.

17. The power system of any of claims 14 to 16, wherein the first fuel cell is arranged in use in an upstream airflow direction to the second fuel cell.

18. The power system of any preceding claim, further comprising a heating arrangement arranged in use to heat the coolant before the coolant is received by the first fuel cell.

19. The power system of any of claim 18, wherein the heating arrangement comprises a compressor arranged in use upstream of the plurality of fuel cells, wherein optionally: the power system further comprises a turbine, and the compressor is arranged in use to be powered by the turbine.

20. The power system of any preceding claim, further comprising a power turbine arranged in use to generate power, a nozzle arranged in use downstream of the plurality of fuel cells, and a propulsive fan arranged in use upstream of the plurality of fuel cells.

21. The power system of any of claims 18 to 20, wherein the heating arrangement comprises a coolant mixing arrangement, the power system further comprising a coolant recirculation arrangement arranged in use to provide a portion of coolant exhausted from the plurality of fuel cells to the coolant mixing arrangement to heat the coolant before the coolant is received as input by the first fuel cell.

22. The power system of any of claims 1 to 13, wherein the coolant is a cooling liquid, and wherein the power system further comprises a radiator arrangement for cooling the coolant exhausted from the plurality of fuel cells.

23. A nacelle comprising the power system of any of claims 1 to 22.

24. An aircraft comprising the power system of any of claims 1 to 22.

25. A method for cooling fuel cells in the power system of any of claims 1 to 22, the methodcomprising:receiving coolant by the first fuel cell of the plurality of fuel cells;cooling the first fuel cell with the coolant;providing the coolant from the first fuel cell to a second fuel cell of the plurality of fuel cells; andcooling the second fuel cell with the coolant.Application No: GB2416703.3 Examiner: Chris BennettClaims searched: 1-5, and (10-14, 16-21 in part) Date of search: 12 May 2025Patents Act 1977: Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance X X X X X X 1-11,15, 22,and 25 1-11,15, 22,and 25 1-11,15, 22,and 25 1-11,15, 22,and 25 1-11,15, 22,and 25 1-11,15, 22,and 25 US 6794068 B2 RAPAPORT et al - See figure 2A references 56, 40, 50, and 52. US 2005 / 0064257 Al ISODA et al - See figure 3 references 32A, 32B, and 32C. US 2004 / 0038103 Al WARIISHI et al - See figure 5 references 14, 16, and 48. US 2009 / 0042069 Al SUGIURA et al - See figure 5 references 14, 16, and 48. US 4080487 A REISER - See figure 1 references 68, 70, and 72. US 7354670 B2 ENJOJI et al - See figure 1 reference 36.Categories: X Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if combined with one or more other documents of same category. P Document published on or after the declared priority date but before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:Search of GB, EP, WO &US patent documents classified in the following areas of the UKCX :Worldwide search of patent documents classified in the following areas of the IPC_____________B64C; B64U; F41G; H04K________________________________________The following online and other databases have been used in the preparation of this search report SEARCH - PATENTInternational Classification:Subclass Subgroup Valid From B64D 0027 / 355 01 / 01 / 2024 B64D 0033 / 08 01 / 01 / 2006

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