Power unit

Abstract

A power unit for power generation in aerospace and other systems. The power unit has a power generation device (101) coupled to a liquid hydrogen reservoir (102) via a first conduit (105) and to an oxygen supply via a second conduit (107). A heat exchanger (106) is provided between the two conduits, and a separator (109) is present in the second conduit for separation of gaseous nitrogen from liquid oxygen. The arrangement provides an efficient, lightweight and compact power generation unit.

Description

[0001] Power Unit

[0002] The present invention relates to power unit, associated propulsion systems, vehicles, devices, apparatus and methods. The invention is particularly useful in the aerospace field but is also useful in maritime, land-based and other vehicles or devices which are powered by power generation devices which utilize hydrogen and oxygen as a fuel.

[0003] The aerospace industry is currently developing net zero carbon emission propulsion systems. A potential fuel for future commercial net zero aviation is liquid hydrogen. Liquid hydrogen fuel may be stored onboard the aircraft and used in a fuel cell to produce electricity which powers electric motors. Typically, these fuel cell systems have an overall specific fuel consumption of around 70 g / kWh (equivalent to a brake thermal efficiency of between 40% to 45%). This figure includes the parasitic power losses associated with the balance of plant and DC-DC converters etc. The remaining 50% to 60% of unused energy is rejected as heat. This is a significant amount of heat to remove and requires heavy and large heat exchangers which also add notable amounts of aerodynamic drag to the parasitic losses. Fuel cell systems typically have an overall specific power of around 0.5 kW / kg. The specific power of a typical gas turbine aircraft is two orders of magnitude greater. Therefore, two primary challenges exist in powering aircraft with hydrogen fuel cells. Significant improvements are needed in both the specific power and the specific fuel consumption. Hydrogen and oxygen can also be used as fuels in gas turbines or internal combustion engines.

[0004] It is amongst the objects of the invention to provide a power unit with improved power and / or reduced weight and / or reduced size and / or improved specific fuel consumption. It is also amongst the objects of the invention to provide a safer power unit, vehicle, propulsion system or device, without an increase in size or weight. It is also amongst the objects of the invention to provide a more environmentally friendly power unit which has low NOx emissions.

[0005] In a first aspect the invention provides a power unit comprising; a power generation device, a reservoir for containing liquid hydrogen, a first conduit for delivering hydrogen from the reservoir to the power generation device, a second conduit for delivering oxygen to the power generation device, a heat exchanger, and a separator, wherein the heat exchanger is adapted to exchange heat between hydrogen flowing through the first conduit and oxygen flowing through the second conduit, and wherein the separator is adapted to separate gas from liquid oxygen in the second conduit and remove the gas from the second conduit.

[0006] The power generation device may be a hydrogen fuel cell, a gas turbine or an internal combustion engine.

[0007] Fuel cells according to the prior art are fuelled with atmospheric air and gaseous hydrogen at a pressure of around 3 bar and a temperature of approximately 50°C. At altitude this atmospheric air has to be compressed by electrically driven compressors, then cooled with heat exchangers. This parasitic loss consumes between 10% and 30% of the electrical power produced by the fuel cell, significantly impacting its specific power and fuel consumption. Around 79% of the air being compressed is unusable nitrogen gas. The present invention allows cold liquid hydrogen to be used to liquify the oxygen component of air (in the second conduit as the gas which it contains passes through the heat exchanger), whilst allowing the nitrogen component of air to remain as a gas. The separator then separates the liquid oxygen from the gaseous nitrogen. Only the liquid oxygen continues down the second conduit to the fuel cell. This increases the power of the fuel cell, because it is being supplied with pure (or at least enriched) oxygen. It also avoids the need to compress the atmospheric oxygen because the oxygen is already a liquid after the heat exchange which it undergoes with hydrogen according to the invention. The avoidance of the need to compress air entering the power unit is also useful in embodiments in which the power generation device is an internal combustion engine or a gas turbine. The enriched oxygen supply which is generated from air is also useful in these embodiments.

[0008] The separator may comprise a sump with a volume into which liquid oxygen collects by gravity and a drain to drain the liquid oxygen from the sump. The liquid oxygen may be drained from the sump and fed into the remainder of the second conduit, for delivery to the power generation device. The drain may be provided with a flow regulator or valve arrangement to ensure that a desired flow rate of liquid oxygen is achieved. The gas may be diverted from the separator into a third conduit. Other types of liquid / gas separator will also be useable. Preferably the separator is downstream of the heat exchanger in the second conduit. That is, the separator is preferably positioned between the heat exchanger and the power generation device.

[0009] In fuel cell embodiments, the increased partial pressure of oxygen at the cathode of the fuel cell increases the specific power of the fuel cell. This partial pressure may be further increased by the addition of a back pressure regulator which enables the fuel cell to operate at a higher internal pressure than normal, without over-stressing the fuel cell’s membrane.

[0010] The invention also reduces the size, or in some embodiments entirely eliminates, the need for a compressor and / or an associated heat exchanger, thereby reducing the electrical consumption, mass, size and / or drag of the power unit.

[0011] The separator may be provided with a third conduit for directing gas which is removed from the second conduit to an auxiliary device. This allows a byproduct of the invention (the gas separated from the liquid oxygen) to be used for other purposes. Preferably the separated gas is a dry and warm gas which is predominantly nitrogen.

[0012] The auxiliary device may be a cooling device in which the gas removed from the second conduit is used to cool an electrical or mechanical component.

[0013] The auxiliary device may be a fire prevention device which vents gas in the third conduit around a flammable leakage zone. The gas is preferably rich in nitrogen, which is inert. This is particularly good for inerting flammable leakage zones around an aircraft or other vehicle or device.

[0014] The auxiliary device may be a de-icing system. The gas is preferably warm and may therefore be used to de-ice other parts of a vehicle or device. This is particularly useful in aircraft.

[0015] The liquid oxygen may be pumped through a low power pump to increase its pressure as required by the fuel cell. Preferably the pump is a liquid fixed displacement type pump. The cold hydrogen, oxygen and nitrogen may be used to cool the rest of the system as required en route to the power generation device and any vent system. The heat exchanger may be adapted to exchange heat between hydrogen or oxygen in the first and second conduits to cool the power generation device.

[0016] The unit may further comprise a second heat exchanger which is adapted to exchange heat between hydrogen and / or oxygen in the first and / or second conduits and the power generation device to cool the power generation device. This is particularly useful when the power generation device is a fuel cell. The second heat exchanger may be adapted to exchange heat between the power generation device and a gas in the third conduit, which gas has been separated from the second conduit. Preferably the second heat exchanger is downstream of the first heat exchanger.

[0017] The second heat exchanger may comprise a circuit which comprises a further heat exchange fluid. Preferably that fluid comprises, or is, glycol.

[0018] The unit may further comprise a pre-cooler which is adapted to cool oxygen (usually in the form of an air mixture) passing through the second conduit before the oxygen passes through the heat exchanger. The pre-cooler may be a heat exchanger.

[0019] The pre-cooler may be adapted to exchange heat between hydrogen and / or oxygen in the first and / or second conduits (after the hydrogen and / or oxygen have passed through the heat exchanger) with the oxygen (usually in the form of an air mixture) passing through the second conduit before the oxygen passes through the heat exchanger.

[0020] The pre-cooler may be adapted to exchange heat between gas which is present in the third conduit and oxygen (usually in the form of an air mixture) passing through the second conduit before the oxygen passes through the heat exchanger.

[0021] The heat exchanger may be provided with a temperature monitoring device and a flow regulator.

[0022] The power unit may further comprise a back pressure regulator on an outlet of the power generation device when the power generation device is a fuel cell. This enables the fuel cell to operate at an increased but balanced pressure. This increased pressure also reduces the surface area required inside the fuel cell stack and therefore further increases the specific power. This is particularly useful for operating the power unit in high or low pressure external environments, such as in aircraft or aerospace applications.

[0023] A hot and / or relatively high pressure water and / or steam output of the power generation device may be connected to a turbine which is adapted to drive an electricity generator such that electrical power can be generated from the output of the power generation device. This is particularly useful when the power generation device is a fuel cell.

[0024] The second conduit may comprise a duct for the intake of atmospheric air. Preferably the duct is a NACA type of duct.

[0025] The power unit may further comprise a compressor to compress a portion of the gas in the second conduit, or gas in a further conduit, which gas is supplied to the power generation device. The portion of gas to be compressed by the compressor may be removed from the second conduit before the remainder of the gas in the second conduit passes through the heat exchanger. The gas which is removed from the second conduit for compression by the compressor may already have undergone a pre-cooling step. That is, the gas may be removed from the second conduit after passing through a pre-cooler and before passing through the heat exchanger.

[0026] The power unit may further comprise a condenser. When the power generation device is a gas turbine or an internal combustion engine, the condenser may be arranged so that a portion of the exhaust from the gas turbine or the internal combustion engine is directed through the condenser and fed back into the gas turbine or internal combustion engine. Preferably the power unit comprises a pump to compress condensed exhaust before it is fed back into the gas turbine or internal combustion engine. Preferably the pump is a liquid fixed displacement-type pump. Preferably the condensate is, or comprises, water. This can reduce the temperature of the gas turbine or internal combustion engine to lengthen the working life of its parts. This additional water, which absorbs heat during the combustion process, turns into steam which can also increase the power of the system. It avoids NOx emissions (or at least reduces them significantly) because the gas turbine or internal combustion engine is run on only oxygen and hydrogen (or at least on a gas mixture which has proportionally less nitrogen and more oxygen than atmospheric air) rather than on atmospheric air. It therefore provides a more environmentally friendly power unit. The liquid condensate is easy to pressurise (to pressures which are workable with gas turbines or internal combustion engines) with a pump and therefore no gas compressor is required. There is therefore no need to provide large compressors to compress nitrogen or any other gas which might be introduced into the turbine to reduce its temperature. This reduces the weight and drag of the propulsion system. The reduction of the temperature of exhaust which his vented to the atmosphere is also useful for certain aerospace applications.

[0027] In a second aspect the invention provides a propulsion system comprising a power unit as described herein, connected to a propulsion device such as an electric motor such that the power unit drives the propulsion device. The propulsion system may be a turbo propeller (‘turbo-prop’) system or an internal combustion engine.

[0028] In a third aspect, the invention provides a vehicle comprising a power unit or propulsion system as described herein, wherein the vehicle is selected from the group consisting of an aircraft, a ship, a submarine, a spacecraft, and a land based- vehicle. The vehicle need not be a vehicle for transporting people or objects. The vehicle may be a satellite. The vehicle or device may be manned or unmanned.

[0029] In a fourth aspect the invention provides a device comprising a power unit or propulsion system as described herein. Such devices or power units may be mobile / portable powerpacks or may be electrical generators.

[0030] Preferably the vehicle or device utilises atmospheric air as a source of the oxygen required by the power generation device.

[0031] In a fifth aspect the invention provides a method of using a power unit, propulsion system, device or vehicle as described herein, comprising the steps of supplying a mixture of oxygen and a second gas which has a lower boiling point than oxygen to the power generation device via the second conduit, and supplying hydrogen to the power generation device via the first conduit.

[0032] In a further aspect the invention provides a method of using a power unit, propulsion system, device or vehicle as described herein comprising the steps of; supplying a mixture of oxygen and a second gas which has a lower boiling point than oxygen to the second conduit, and transferring liquid hydrogen from the reservoir for containing liquid hydrogen into the first conduit and through the heat exchanger such that hydrogen is supplied to the power generation device via the first conduit, wherein, in the heat exchanger, the mixture of oxygen and the second gas in the second conduit is cooled to a temperature which is below the boiling temperature of oxygen but above the boiling temperature of the second gas, wherein the separator separates the second gas from liquid oxygen in the second conduit and removes the second gas from the second conduit, and wherein oxygen in the second conduit is supplied to the power generation device.

[0033] In these method aspects, the boiling point relationships above are defined using the same pressure for each of the oxygen and the second gas.

[0034] In these method aspects, the heat exchanger may cool the mixture of oxygen and the second gas in the second conduit to a temperature of at or below the boiling temperature of oxygen but above the boiling temperature of nitrogen.

[0035] In these method aspects, preferably the heat exchanger may cool the mixture of oxygen and the second gas in the second conduit to a temperature of between 90K and 77K. At standard pressure of 101 ,000 Pa, these temperatures correspond to the boiling temperatures of oxygen and nitrogen. When the power unit is operating in a lower pressure environment, the heat exchanger may cool the mixture of oxygen and the second gas in the second conduit to a temperature of between 80K and 67K.

[0036] In these method aspects, preferably, the second gas is, or comprises, nitrogen. Preferably, the hydrogen turns from a liquid to a gas as it flows from the reservoir for containing liquid hydrogen to the power generation device. Preferably, the hydrogen turns from a liquid to a gas as it flows through the heat exchanger.

[0037] A standard fuel cell operates with a partial pressure of oxygen at around 9Psi. In these method aspects, when the power generation device is a fuel cell, preferably the partial pressure of oxygen entering the fuel cell is greater than around 9 Psi. Preferably the partial pressure of oxygen entering the fuel cell is greater than 10, 11 , 12, 13, 14, 15, or 16 Psi or higher. Preferably the purity of oxygen entering the fuel cell at the cathode is significantly higher than the atmospheric purity of oxygen (which is usually around 20%). In these method aspects, preferably the oxygen in the second conduit is supplied to the power generation device as a gas mixture which comprises between around 20% and around 100% oxygen. Preferably the gas supplied to the power generation device via the second conduit is 100% oxygen. Preferably the gas comprises between 20% and 100% oxygen, and more preferably 30%, 40%, 50%, 60%, 70%, 80% or 90% to 100% oxygen. These percentages are percentages by volume.

[0038] These oxygen percentages are particularly useful when the power generation device is a fuel cell. Preferably the gas mixture is supplied to the fuel cell’s cathode.

[0039] In these method aspects, preferably the mixture of oxygen and the second gas is atmospheric air.

[0040] In these method aspects, when the power generation device is a fuel cell, preferred temperature ranges for the gases and liquids at various points are as follows:

[0041] Entry to the fuel cell: around 323K.

[0042] Entry and exit to the ‘main’ heat exchanger (106 in the embodiments):

[0043] Liquid hydrogen entry temperature around 26K.

[0044] Liquid hydrogen exit temperature around 80K.

[0045] Air entry temperature around 288K.

[0046] Air exit temperature around 80K.

[0047] Entry and exit to the pre-ch iller heat exchanger 201 :

[0048] Air entry temperature: around 288K.

[0049] Air exit temperature around 180K.

[0050] In a further aspect, the invention provides a method of running a power generation device in which relatively cold hydrogen is used to liquefy an oxygen containing portion of a relatively warm gas mixture, followed by separation of the liquid oxygen portion from the remainder of the gas mixture and supply of the separated oxygen portion to the power generation device. This method is particularly useful when the power generation device is a fuel cell, a gas turbine or an internal combustion engine.

[0051] In a further aspect, the invention provides an apparatus for running a power generation device, comprising; a heat exchange device arranged so that liquid hydrogen fuel can exchange heat with an oxygen containing portion of a gas mixture, to liquefy the oxygen containing portion, and a separator for separating a liquified oxygen portion from a remainder of the gas mixture and directing the separated oxygen portion (whether in liquid or gaseous form) towards the power generation device. This method is particularly useful when the power generation device is a fuel cell, a gas turbine or an internal combustion engine.

[0052] Features of any aspect of the invention described above, or features in the following specific description, may be incorporated into any other aspect of the invention as described herein.

[0053] A non-limiting description of preferred embodiments of the invention follows, with reference to the drawings, in which:

[0054] Figure 1 shows a first embodiment of the invention.

[0055] Figure 2 shows a second embodiment of the invention, in which a device to pre-cool ambient air is included.

[0056] Figure 3 shows a third embodiment of the invention, in which gas separated from the second conduit is used to cool an electric motor.

[0057] Figure 4 shows a fourth embodiment of the invention, in which only part of the atmospheric air is liquified. The remaining gaseous air is compressed via a traditional compressor technique to provide oxygen for the fuel cell.

[0058] Figure 5 shows a fifth embodiment in which the power unit comprises a gas turbine or internal combustion engine rather than a fuel cell.

[0059] Figure 1 shows a power unit comprising a hydrogen fuel cell 101. The power unit is contained within an aircraft. Liquid hydrogen is stored in a feeder tank 102. The feeder tank is pressurized by a high-pressure hydrogen gas tank 103. Between the hydrogen gas tank and the liquid hydrogen tank there is a pressure regulating valve 104 for maintaining pressure in the feeder tank. Some embodiments of the invention do not have a tank for gaseous hydrogen and the associated pressure regulating valve. Instead, they may use heat to maintain pressure in the feeder tank and / or may use a liquid hydrogen compatible pump. Liquid hydrogen leaves the liquid hydrogen tank and travels through the first conduit 105 at a temperature of around 25 K. The conduit in this embodiment is a tube.

[0060] Atmospheric air is drawn into a second conduit 107 via an NACA air inlet duct. The air inlet duct is formed in the body of a vehicle, which in this embodiment is an aircraft. The air enters the duct at an ambient temperature which, for commercial aircraft, can typically range from 218K up to 328K.

[0061] The ducts 105 and 107 enter a heat exchanger 106. In the heat exchanger, the ambient temperature air in the conduit 107 and the cold liquid hydrogen in the conduit 105 exchange heat. This cools the incoming air to a temperature of between about 90K and 77K. Preferably the air in the second conduit is chilled to about 80 K. During this air chilling process, the liquid hydrogen is converted to gaseous hydrogen in the conduit 105. The gaseous hydrogen exits the heat exchanger at a temperature of around 80K. The air which is cooled by this heat exchanger is converted into a liquid component and a gaseous component in the conduit 107. This happens because different components of the air liquify at different temperatures. Oxygen boils at around 90K whereas nitrogen boils at around 77K. Since the air is chilled to around 80K, the oxygen will form a liquid phase and the nitrogen will remain in the gaseous phase.

[0062] A separator 109 is positioned in the conduit 107, downstream of the heat exchanger. The separator is a cryogenic separator. The separator comprises a sump which has a volume into which liquid oxygen collects under gravity. The separator separates a gaseous component in the conduit 107 from the liquid component in the conduit 107. It does so by draining liquid oxygen from the sump via a drain valve. The gaseous component is directed into a third conduit 110. The liquid component, which is predominantly oxygen, continues down the conduit 107 towards the fuel cell 101 . The liquid oxygen is pressurized and moved down the conduit 107 by the pump 111 (which may be a fixed displacement type pump). As the liquid oxygen travels along the conduit 107 it is allowed to evaporate so that by the time it reaches the fuel cell, it is in the gaseous form. The oxygen is also allowed to warm up as it travels along the conduit 107 such that, by the time it arrives at the fuel cell, it is at a temperature of around 340 K. An inlet pressure regulator 112 is provided in the conduit 107 close to the fuel cell 101 . This regulates the pressure of the oxygen gas which enters the fuel cell. The gaseous component which is separated by the separator is predominantly nitrogen and other low boiling point components of air. This component travels along the conduit 110. When it leaves the separator its temperature is around 80 K. The gas is allowed to warm up as it travels along the conduit 110. At the end of the conduit the conduit 110, the gas is vented to the atmosphere or an area at a high risk of fire, or an area in need of heat for de-icing, at a temperature of around 340K. The gas is predominantly nitrogen (with some Argon) and it is therefore inert. It therefore has good fire prevention properties, when vented into an area such as a compartment which may otherwise contain flammable vapours and be at risk of ignition.

[0063] As mentioned above the hydrogen, other gas, and liquid oxygen are, when leaving the heat exchanger at a temperature of around 80K in their respective conduits. They are allowed to warm up as they travel through their respective conduits. In the present embodiment the conduits all run through a second heat exchanger 114 which is thermally linked to the fuel cell itself. The cold gases flowing through the conduits can therefore chill the fuel cell to prevent it from overheating. In other embodiments, only one or only two of the conduits may run through a further heat exchanger which is thermally coupled to the fuel cell. The heat exchanger which is coupled to the fuel cell uses a glycol-type coolant as a heat transfer fluid. The glycol is contained in its own closed cooling loop and is pumped into thermal contact with the fuel cell by the pump 115.

[0064] In the present embodiment, the temperature of the hydrogen by the time it reaches the fuel cell is around 340K. It is therefore in the gaseous form. An inlet pressure regulator 113 is provided in the first conduit 105 close to the fuel cell 101. This regulates the pressure of the hydrogen gas which enters the fuel cell.

[0065] The fuel cell provides an electrical output 119 which can be used to power aircraft systems, including propulsion devices. The numeral 119 may also schematically represent an electrical motor which drives a propulsion device, such as a propeller or a crank shaft. The fuel cell 101 also provides a hot water / vapour output which flows through the conduit 116. A back pressure regulator 117 is provided in the conduit 116. This enables the system to be run at an increased internal pressure, in external environments which are at a relatively low pressure. This is particularly useful in the present embodiment where the power unit is installed on an aircraft which operates in a low-pressure environment. Increasing the pressure inside the fuel cell 101 further increases the partial pressures inside the fuel cell 101 and hence the power density. The hot water / vapour could be simply exhausted overboard. However, in the present embodiment the hot water / vapour circuit is used to generate power in an electrical generator 118. This additional power can be combined with that of the main fuel cell 101 or used separately to run aircraft auxiliary systems. The hot water / vapour is vented 120 from the system at a temperature of around 340K.

[0066] Figure 2 shows a power unit which is similar to the embodiment shown in figure 1 . Similar components are indicated by the same reference numerals which are used in figure 1. In this embodiment the hydrogen gas, the predominantly nitrogen gas, and the liquid oxygen exit the heat exchanger 106 at around 80K before the liquid oxygen and nitrogen are separated by the separator 109. However, the ambient air which enters the conduit 107 via the NACA duct 108 is pre-chilled before it enters the heat exchanger 106. The pre-chilling takes the ambient air from a temperature of around 288K to about 180K. The pre-chilling is achieved in this embodiment by passing the conduit 107 through a heat exchanger 201 . This heat exchanger 201 permits heat transfer between one or more of the conduits 107, 110 and 105 containing the cold gases or liquids (which have already passed through the heat exchanger 106) and the ambient air in the conduit 107. The benefit of the pre-cooling in heat exchanger 201 is that it utilises more of the cooling available from the cold source of liquid hydrogen for the process of liquifying the oxygen.

[0067] Figure 3 shows power unit which is similar to the embodiment described in figures 1 and 2. Similar components are indicated by the same reference numerals which are used in figures 1 and 2. A difference between figure 3 and figure 2 is that the conduit 110 in figure 3 which carries the gas (separated by the separator 109) does not pass through the heat exchanger 114 which is thermally coupled to the fuel cell 101. Furthermore, it does not pass through the heat exchanger 201 which is used to prechill the ambient air entering the conduit 107. Instead, the cold gas flowing in the conduit 110 is used to cool an electric motor. In this embodiment, the electric motor is a superconducting electric motor 301 which drives a propeller 302 of the aircraft. After the cold gas has absorbed heat from the superconducting electric motor, it is vented at a temperature of around 340K. Optionally the warmed gas in conduit 110 is vented to a de-icing device or to flammable leakage zones which is shown schematically as box 303. This functionality is particularly useful in aircraft. Figure 4 shows a fourth embodiment of the invention. The power unit is identical to the embodiment shown in figure 2, except for the presence of the fourth conduit 401 . After the gas (which is usually atmospheric air) has entered the second conduit 107 via the NACA duct 108, it passes through the pre-cooler heat exchanger 201 . After it has been pre-cooled by the pre-cooler heat exchanger a portion of the gas is directed down the fourth conduit 401 . This gas passes through a compressor 402 which compresses the gas. The compressed gas is then reunited and mixed with the oxygen rich gas in the second conduit 107 before the mixture is delivered to the fuel cell 101 via the inlet pressure regulator 112. The advantage of this system is that the fuel cell is not wholly reliant on the liquification and separation process to provide fuel to the fuel cell. The inclusion of the compressor 402 can therefore provide gas to the fuel cell in the traditional way in order to smooth out peaks and troughs in the supply generated by the heat exchanger 106 and the separator 109. It can also assist in dealing with abnormal circumstances such as when the air entering the NACA duct is very hot and therefore requires a high degree of cooling by a limited amount of cold hydrogen in the heat exchanger 106. Even though a compressor is present in this embodiment which inevitably provides some bulk to the system, a much smaller compressor 402 can be used than the compressors which are required in conventional fuel cell arrangements, in which all of the air supplied to the fuel cell must be compressed. That is, due to the presence of the oxygen liquification and its separation in the second conduit 107, a smaller compressor may be used than in fuel cell systems according to the prior art. This provides a power unit which has a low bulk and weight, but which may also be used in a wide range of conditions.

[0068] Figure 5 shows a fifth embodiment of the invention. The power unit is contained within an aircraft and is used to drive a propeller 501 . The power unit is similar to the arrangement shown in figure 1 . A difference is that the hydrogen and oxygen in the first and second conduits are not fed into a fuel cell. Instead, those fuel components are supplied to a gas turbine 502 which drives the propeller 501 . This embodiment therefore comprises a turbo-prop propulsion unit. Steam is exhausted from the gas turbine. A portion of the exhaust steam is vented to the atmosphere via the exhaust vent 504. A portion of the steam exiting the turbine is diverted before it reaches the exhaust vent into the fifth conduit 503. This portion of the steam is condensed by flowing the steam through a condenser 505. This condenser exchanges heat between the hot steam and one or more of the cold gases in the first 105 second 107 or third 110 conduits. This cools and condenses the steam from the exhaust. The condensed water from the steam is compressed by a liquid fixed displacement type pump 506. It is pressurized to a pressure of around 1380 kPa (around 200 psi) or greater.

[0069] The compressed liquid is then fed back into the gas turbine. The liquid may be fed directly back into the gas turbine or premixed with the hydrogen and oxygen from the first 105 and second 107 conduits before it is fed back into the gas turbine. This has several benefits. It can reduce the temperature of the gas turbine 502 to lengthen the working life of its parts. It also increases the power of the turbine because the water comprises heavy molecules and enhances the momentum transfer between the contents of the combustion chamber and the turbine blades. The gas turbine avoids NOx emissions (or at least reduces them significantly) because the turbine is run on only oxygen and hydrogen (or at least on a gas mixture which has proportionally less nitrogen and more oxygen than atmospheric air) rather than on atmospheric air. It is the nitrogen from atmospheric air which, in prior art arrangements, leads to the formation of NOx compounds. Enriching the oxygen content of the fuel relative to the nitrogen content, by using the liquification of oxygen and separation of nitrogen which is achieved the by the heat exchanger 106 and the separator 109 therefore provides a more environmentally friendly turbo-prop (or other gas turbine or internal combustion engine) propulsion system. Furthermore, there is no need to provide large compressors to compress nitrogen or any other gas which might be introduced into the turbine to reduce its temperature. This reduces the weight, drag and parasitic losses of the propulsion system. The reduction of the temperature of exhaust which his vented to the atmosphere is also useful for certain aerospace applications.

[0070] In other embodiments an internal combustion engine can be present instead of the gas turbine 502.

Claims

Claims1 . A power unit comprising; a power generation device, a reservoir for containing liquid hydrogen, a first conduit for delivering hydrogen from the reservoir to the power generation device, a second conduit for delivering oxygen to the power generation device, a heat exchanger, and a separator, wherein the heat exchanger is adapted to exchange heat between hydrogen flowing through the first conduit and oxygen flowing through the second conduit, and wherein the separator is adapted to separate gas from liquid oxygen in the second conduit and remove the gas from the second conduit.

2. A power unit according to claim 1 , wherein the power generation device is a hydrogen fuel cell, a gas turbine or an internal combustion engine.

3. A power unit according to either of claims 1 or 2 wherein the separator is provided with a third conduit for directing gas which is removed from the second conduit to an auxiliary device.

4. A power unit according to claim 3 wherein the auxiliary device is a cooling device in which the gas removed from the second conduit is used to cool an electrical component.

5. A power unit according to either of claims 3 or 4 wherein the auxiliary device is a fire prevention device which vents gas in the third conduit around a flammable leakage zone.

6. A power unit according to any of claims 3-5 wherein the auxiliary device is a deicing system.

7. A power unit according to any preceding claim wherein the heat exchanger is adapted to exchange heat between hydrogen or oxygen in the first and second conduits to cool the power generation device.

8. A power unit according to any preceding claim further comprising a second heat exchanger which is adapted to exchange heat between hydrogen or oxygen in the first or second conduits with the power generation device to cool the power generation device.

9. A power unit according to claim 8 wherein the second heat exchanger comprises a circuit which comprises a further heat exchange fluid.

10. A power unit according to any preceding claim further comprising a pre-cooler which is adapted to cool oxygen passing through the second conduit before the oxygen passes through the heat exchanger.11 . A power unit according to claim 10 wherein the pre-cooler is adapted to exchange heat between hydrogen or oxygen in the first or second conduits after the hydrogen or oxygen have passed through the heat exchanger, with oxygen in the second conduit before the oxygen passes through the heat exchanger.

12. A power unit according to any preceding claim wherein the heat exchanger is provided with a temperature monitoring device and a flow regulator.

13. A power unit according to any preceding claim wherein the second conduit comprises a duct for the intake of atmospheric air.

14. A power unit according to any preceding claim wherein a hot liquid or vapour output of the power generation device is connected to a turbine which is adapted to drive an electricity generator such that electrical power can be generated from the output of the power generation device.

15. A power unit according to any preceding claim wherein the power generation device is a fuel cell and the power unit further comprises a back pressure regulator on an outlet of the fuel cell.

16. A power unit according to any of claims 1-14 further comprising a condenser, wherein the power generation device is a gas turbine or an internal combustion engine, and wherein the condenser is arranged so that a portion of the exhaust from the gas turbine or the internal combustion engine is directed through the condenser and fed back into the gas turbine or internal combustion engine.

17. A propulsion system comprising a power unit according to any preceding claim connected to a propulsion device, such as an electric motor, such that the power unit drives the propulsion device.

18. A vehicle comprising a power unit or propulsion system according to any preceding claim wherein the vehicle is selected from the group consisting of an aircraft, a ship, a submarine, a spacecraft, and a land based-vehicle.

19. A vehicle according to claim 18 wherein the vehicle uses atmospheric air to provide oxygen to the power generation device.

20. A device comprising a power unit or propulsion system according to any preceding claim.21 . A method of using a power unit, propulsion system, device or vehicle according to any preceding claim comprising the steps of; supplying a mixture of oxygen and a second gas which has a lower boiling point than oxygen to the second conduit, and transferring liquid hydrogen from the reservoir for containing liquid hydrogen into the first conduit and through the heat exchanger such that hydrogen is supplied to the power generation device via the first conduit, wherein, in the heat exchanger, the mixture of oxygen and the second gas in the second conduit is cooled to a temperature which is below the boiling temperature of oxygen but above the boiling temperature of the second gas, wherein the separator separates the second gas from liquid oxygen in the second conduit and removes the second gas from the second conduit, and wherein oxygen in the second conduit is supplied to the power generation device.

22. A method according to claim 21 wherein the oxygen in the second conduit is supplied to the power generation device as a gas mixture which comprises between 20% and 100% oxygen.

23. A method according to either of claims 21 or 22 in which the heat exchanger cools the mixture of oxygen and the second gas in the second conduit to a temperature of between 90K and 77K.

24. A method according to any of claims 21 to 23 wherein the second gas is nitrogen.

25. A method according to any of claims 21-24 in which the hydrogen turns from a liquid to a gas as it flows from the reservoir for containing liquid hydrogen to the power generation device.

26. A method according to claim 25 wherein the hydrogen turns from a liquid to a gas as it flows through the heat exchanger.

27. A method according to any of claims 21-26 wherein the mixture of oxygen and the second gas is atmospheric air.

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