Patent Description:
In order to limit emissions of carbon dioxide, use of hydrogen as an alternative to hydrocarbon fuel in gas turbine engines has historically only been practical in land-based installations. However, liquid hydrogen fuelled airliners have recently been proposed. The liquid for fuel such aircraft however must be heated prior to combustion. Doing so in a manner which is efficient from an overall propulsion system perspective is a significant challenge.

United States patent application <CIT> discloses a hydrogen fuel vaporiser for vaporising cryogenically-stored hydrogen fuel prior to injection into a gas turbine engine. The vaporiser comprises a fuel offtake configured and arranged to divert a portion of hydrogen fuel from a main fuel conduit, a burner configured and arranged to burn the portion of hydrogen fuel diverted from the main fuel conduit, and a heat exchanger configured and arranged to transfer heat produced by the burner to hydrogen fuel in the main fuel conduit.

United States patent application <CIT> discloses a hybrid engine including features to meet aircraft thrust, passenger airflow, and fuel cell requirements. The engine includes a combustor burning the same fuel as the fuel cell. The engine has electric motors to utilize the power output of the fuel cell. The engine shafts have sprags to allow motors to drive the compressors and over run the turbines. The engine has variable flowpath geometry to bypass the combustor.

United States patent application <CIT> discloses a gas turbine engine system including a gas turbine engine and a fuel turbine system. The gas turbine engine includes a heat exchange system configured to transfer thermal energy from a first compressed air flow and an exhaust gas flow to a fuel to produce a gaseous fuel. The fuel turbine system includes a fuel turbine fluidly coupled to the heat exchange system and a combustor of the gas turbine engine, and a fuel pump fluidly coupled to the heat exchange system and configured to be driven by the fuel turbine. The fuel turbine is configured to extract energy from expansion of the gaseous fuel to produce the gaseous fuel at a lower pressure for delivery to the combustor.

European patent application <CIT> discloses a cryogenic fuel auxiliary power system for an engine which includes a cryogenic fuel supply, a first valve in fluid communication with the cryogenic fuel supply and configured to control a fuel flow, a first heat exchanger, configured to receive the fuel flow, in fluid communication with the first valve and a combustion chamber of the engine, and a fuel cell in fluid communication between the first valve and the first heat exchanger.

The invention is directed towards a combined gas turbine engine and hydrogen fuel cell system.

In an aspect, one such combined gas turbine engine and hydrogen fuel cell system comprises:.

Advantageously, a system that provides both propulsive and electrical power is provided, in which a common source of temperature controlled hydrogen is provided for both systems. The provision of a hydrogen fuel cell utilising hydrogen from the same source enables the reduction or elimination of gas turbine driven electrical generators, which both improves gas turbine engine specific fuel consumption, and reduces operation restrictions (such as minimum operating rotational speeds) of the.

In an embodiment, the closed cooling loop comprises a second fuel cell heat exchanger arranged to transmit waste heat from the coolant of the closed cooling loop to hydrogen fuel. Advantageously, waste heat from the fuel cell is utilised within the combined engine and fuel cell fuel cycle, thereby increasing overall efficiency.

In an embodiment, the second fuel cell heat exchanger is provided upstream of the fuel cell and downstream of the second fuel offtake in fuel cell fuel flow. Advantageously, waste heat from the fuel cell cycle is maintained within the fuel cell thermodynamic cycle, thereby increasing overall thermal efficiency.

In an embodiment, a third fuel offtake is provided downstream of the second fuel cell heat exchanger in fuel cell fuel flow, and is configured to offtake a portion of the hydrogen fuel diverted from the second fuel offtake to fuel the hydrogen fuel cell, and divert the remaining portion of the hydrogen fuel to a bypass line.

In an embodiment, a third fuel cell heat exchanger is provided in the bypass line downstream of the third fuel offtake, and is configured to exchange heat between hydrogen fuel in the bypass line and a fuel cell air intake line. Advantageously, air input to the fuel cell is cooled, while the hydrogen fuel is further heated.

In an embodiment, the bypass line comprises an outflow which exhausts into the main fuel conduit downstream of the second fuel offtake in gas turbine engine fuel flow. Advantageously, waste heat from the fuel cell is returned to the main fuel conduit via the heat exchangers and bypass line, such that waste heat is returned to the thermodynamic cycle of the gas turbine engine. As such, the overall thermodynamic efficiency of the combined gas turbine engine and hydrogen fuel cell is increased.

In an embodiment, the system comprises a burner exhaust turbine arranged to be driven by exhaust from the burner. Advantageously, the waste heat and pressure in the burner exhaust flow can be used to power additional equipment via the exhaust turbine.

In an embodiment, the burner exhaust turbine is configured to drive a fuel cell air compressor configured to provide air to the hydrogen fuel cell. Advantageously, compressed air for the hydrogen fuel cell is provided from power generated from waste heat from the burner. As such, overall system thermodynamic efficiency is improved. in an embodiment, the gas turbine engine comprises a bleed air system configured to bleed compressed air from a gas turbine engine core compressor and supply it to the burner to burn with the portion of hydrogen fuel diverted from the main fuel conduit.

In an embodiment, the bleed air system is configured to supply air to the hydrogen fuel cell. As such, the fuel cell air compressor can optionally be omitted.

A hydrogen-fuelled airliner is illustrated in <FIG>. In this example, the airliner <NUM> is of substantially conventional tube-and-wing twinjet configuration with a central fuselage <NUM> and substantially identical underwing-mounted turbofan engines <NUM>. In the present embodiment, the turbofan engines <NUM> are geared turbofan engines comprising a combined gas turbine and hydrogen fuel cell system.

A hydrogen storage tank <NUM> is located in the fuselage <NUM>. In the present embodiment, the hydrogen storage tank <NUM> is a cryogenic hydrogen storage tank and thus stores the hydrogen fuel in a liquid state.

A block diagram of one of the combined gas turbine engine and hydrogen fuel cell system turbofan engines <NUM> is shown in <FIG>.

The turbofan engine <NUM> comprises a core gas turbine <NUM>.

The core gas turbine <NUM> comprises, in fluid flow series, a low-pressure compressor <NUM>, a high-pressure compressor <NUM>, a fuel injection system <NUM>, a combustor <NUM>, a high-pressure turbine <NUM>, a low-pressure turbine <NUM>, and a core nozzle <NUM>. The high-pressure compressor <NUM> is driven by the high-pressure turbine <NUM> via a first shaft <NUM>, and the low-pressure compressor <NUM> is driven by the low-pressure turbine <NUM> via a second shaft <NUM>. It will be appreciated that in alternative embodiments, the core gas turbine could be of three-shaft configuration.

As described previously, in the present embodiment, the turbofan engines <NUM> are geared turbofan engines. Thus, in operation the low-pressure turbine <NUM> drives a fan <NUM> via a reduction gearbox <NUM>. The reduction gearbox receives input drive from the second shaft <NUM> and provides output drive to the fan <NUM> via a fan shaft <NUM>. In an embodiment, the reduction gearbox <NUM> is an epicyclic reduction gearbox. In a specific embodiment, it is a planetary reduction gearbox. Alternatively, it may be a star reduction gearbox, or a compound epicyclic reduction gearbox. As a further alternative, the reduction gearbox <NUM> could be a layshaft-type reduction gearbox or any other type of reduction gearbox. It will also be appreciated that the principles disclosed herein may be applied to a direct-drive type turbofan engine, i.e. in which there is no reduction gearbox between the low-pressure turbine and the fan.

In operation, hydrogen fuel is pumped from the hydrogen storage tank <NUM> by a pump <NUM> and into a main fuel conduit <NUM> which ultimately delivers fuel to the fuel injection system <NUM>. In the present embodiment, the pump <NUM> is high-speed centrifugal pump. In a specific embodiment, the centrifugal pump comprises an axial inducer to minimise the required inlet pressure and to accommodate multiphase flow in addition to the centrifugal impeller for developing the majority of the required pressure rise. In an alternative embodiment, a piston-type pump could be used.

In an embodiment, the pump <NUM> is located inside the hydrogen storage tank <NUM>. In this way leakage of hydrogen fuel past pump seals etc. is accommodated.

In an embodiment, the pump <NUM> is driven by an electrical machine. In an embodiment, the drive means for the pump <NUM> are also located in the hydrogen storage tank <NUM>.

As will be appreciated, it is desirable to increase the temperature of the fuel from the cryogenic storage condition to a temperature much closer to the firing temperature of the core gas turbine <NUM>; of course this is subject to the constraint of not exceeding the autoignition temperature of the hydrogen fuel prior to admission into the combustor <NUM>. In an example, the injection temperature is from <NUM> to <NUM> kelvin, for example <NUM> kelvin. In some cases, it may be desirable to increase the fuel temperature to above an icing temperature, such as <NUM> kelvin.

In the present embodiment, a pre-heater <NUM> is therefore provided for heating of the hydrogen fuel. In some cases, this may provide a phase change from a liquid to either a supercritical fluid or a gas. In the present embodiment, this takes place between the pump <NUM> and the fuel injection system <NUM>. The pre-heater <NUM> is configured to raise the temperature of the hydrogen fuel to an intermediate temperature less than the injection temperature. This could for example be from <NUM> to <NUM> kelvin, for example <NUM> kelvin. Further heating is subsequently achieved by additional heating means as will be described further.

In a simple cycle configuration, it has been determined that due to the significant heat capacity of the hydrogen fuel, even if it is utilised as a heatsink for engine waste heat, it will still not reach the required injection temperature without implementation of the pre-heater <NUM>. Further, even in a complex cycle configuration in which the heat of combustion products is recuperated into the hydrogen fuel, it has been determined that at certain points in the operational envelope there will be insufficient heat output from the engine to raise the fuel temperature to the injection temperature. Such occasions may include, for example, ground start, in-flight relight, end of cruise idle, etc..

The pre-heater <NUM> comprises a first hydrogen fuel offtake <NUM> to divert a portion of the hydrogen fuel from the main fuel conduit <NUM>. The amount of hydrogen bled from the main fuel conduit <NUM> is controlled by a valve (not shown). In an embodiment, the valve is controlled actively, for example in response to the temperature of the fuel at the fuel injection system <NUM>. Alternatively, the valve may be passively controlled. In operation, of the order of around <NUM> percent of the hydrogen fuel flow through the main fuel conduit <NUM> is bled for use in the pre-heater <NUM>.

As described previously, hydrogen has very high specific and latent heat capacities; however as a gas or supercritical fluid it has a very low molecular weight and may have a low density, and thus it can be challenging to exchange heat in a compact way. However, these properties may also be beneficial, as described later herein. Thus the pre-heater <NUM> heats the hydrogen fuel in the main fuel conduit <NUM> by combustion of the bled fuel in a burner <NUM> located in heat exchange relationship with the main fuel conduit <NUM>.

In some embodiments, the burner <NUM> is concentric around the main fuel conduit <NUM> and hence the burner <NUM> itself comprises the heat exchanger for transferring heat to hydrogen fuel in the main fuel conduit <NUM>. It will of course be appreciated that other arrangements are possible. For example, the burner <NUM> could be positioned separately from the main fuel conduit <NUM> and exhaust gases therefrom directed through a dedicated heat exchanger unit <NUM>. Such a unit may comprise a first pass for the flow of hot exhaust products from the burner <NUM>, and a second pass for the main fuel flow which then heats as it flows through the heat exchanger unit.

In the present embodiment, air for combustion with the bled hydrogen fuel is bled from the high-pressure compressor <NUM> from a compressor bleed <NUM> via a bleed air line <NUM>. Alternatively, it may be bled from the low-pressure compressor <NUM>. It will be appreciated that the air for combustion could be obtained from any other suitable location.

In the present example, the air and the bled hydrogen fuel are mixed in a pre-mixer <NUM> prior to supply to the burner <NUM>, although in alternative embodiments it may be directly co-injected into the burner with the hydrogen fuel instead.

It should be understood that, in the present example, the products of combustion from the burner <NUM> are not mixed with the fuel in the main fuel conduit <NUM>. In this respect, the pre-heater <NUM> therefore differs from a pre-burner system as used in staged combustion cycle rocket engines.

In steady state, there is enough heat emanating from the burner <NUM> to ensure heating of the small amount of bled hydrogen fuel to a supercritical state. At engine start or other cold conditions for example, the pre-heater <NUM> comprises a pre-heater <NUM> to ensure that the bled hydrogen fuel boils prior to mixing with air in the pre-mixer. In a specific embodiment, the pre-heater <NUM> comprises an electric heating element, for example an inductive coil. Alternatively, the pre-heater <NUM> could be simply configured as a boil volume, in which the ambient conditions therein contain sufficient enthalpy to boil the initial flow of bled hydrogen fuel prior to delivery to the pre-mixer and the burner <NUM>.

As described previously, it is envisaged that the fuel delivery system <NUM> and fuel injection system <NUM> may be used in an embodiment of the core gas turbine <NUM> implementing a simple cycle as described with reference to <FIG>, possibly with fuel cooling of engine or gearbox oil or cooling air. Alternatively, the core gas turbine engine <NUM> may implement a complex cycle.

Combustion products from the burner <NUM> are exhausted into an exhaust line <NUM> provided downstream of the heat exchanger <NUM>. The temperature through the exhaust is relatively low, in view of the heat transfer to the hydrogen fuel via the heat exchanger <NUM>. Similarly, mass flow through the exhaust is relatively low, in view of the relatively small amount of fuel used by the burner. However, the available pressure is relatively high.

As such, energy from the exhaust flow can be recovered by a turbine <NUM> configured to extract power from exhaust gasses of the burner downstream of the heat-exchanger.

The turbine <NUM> is of conventional construction, and is configured to receive a hot, high pressure exhaust fluid input from the exhaust line <NUM>, and exhaust spent, low pressure fluid into the core nozzle or bypass stream.

The turbine <NUM> is coupled to a load in the form of a hydrogen fuel cell air compressor <NUM> via a turbine shaft <NUM>, which is described in further detail below.

A surprisingly high power output is available from the turbine <NUM>. As such, aircraft shaft power extraction for can be significantly reduced. In some cases, a mechanical coupling may be provided for re-introducing shaft power into the gas turbine engine.

As noted above, the combined gas turbine and fuel cell system comprises a fuel cell, which is indicated generally at <NUM>. Hydrogen fuel cells are well known, and comprise an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. In this disclosure, the fuel comprises hydrogen, and oxygen for the reaction is provided from air. As will be understood, such systems generate waste heat, which needs to be removed from the fuel cell to ensure efficient operation, and which represents inefficiency. The present disclosure reintroduces this waste heat to the combined cycle, to thereby increase overall system efficiency.

The combined gas turbine hydrogen fuel cell system comprises a second fuel offtake <NUM> configured to divert a portion of fuel from the main fuel conduit <NUM> to the hydrogen fuel cell <NUM> via a hydrogen fuel cell fuel conduit <NUM>. The second fuel offtake <NUM> is provided downstream of the heat exchanger <NUM> and upstream of the fuel injector <NUM> in main gas turbine engine hydrogen fuel flow.

The hydrogen fuel cell <NUM> is cooled using a closed-loop cooling loop <NUM>. The cooling loop <NUM> is configured to remove waste heat from the hydrogen fuel cell <NUM> via a first hydrogen fuel cell heat exchanger <NUM>, and transfer the heat to a cooling fluid of the cooling loop <NUM>.

A second hydrogen fuel cell heat exchanger <NUM> is provided, which is in thermal communication with the coolant of the cooling loop <NUM> and the hydrogen fuel of the hydrogen fuel cell fuel conduit <NUM>. The second heat exchanger <NUM> is provided between the second fuel offtake <NUM> and a hydrogen fuel cell inlet in hydrogen fuel cell flow. Consequently, heat from the fuel cell is transferred to the hydrogen fuel in the conduit <NUM>, before being reintroduced to the hydrogen fuel cell from the hydrogen fuel.

Consequently, the temperature of the hydrogen fuel is maintained at a required temperature for efficient operation. In some cases, hydrogen fuel cells may require a relatively high fuel input temperature, which may be above the temperature of the hydrogen fuel downstream of the pre-heater heat exchanger <NUM>. As such, waste heat from the fuel cell is used to preheat hydrogen fuel for the fuel cell, thereby allowing operation using the same low temperature hydrogen fuel as that used by the gas turbine engine <NUM>.

In some cases, an auxiliary air heat exchanger <NUM> may also be provided, in cooling fluid flow between the first and second heat exchangers <NUM>, <NUM>, downstream of the first heat exchanger <NUM>. If the cooling capacity of the hydrogen fuel in the line <NUM> is insufficient to provide adequate cooling for the hydrogen fuel cell <NUM>, additional air flow over the auxiliary air heat exchanger from ambient air may be provided to ensure proper temperature control. In general however, the hydrogen fuel cell is sized such that adequate cooling is provided by the hydrogen fuel in most operational conditions.

A third fuel offtake <NUM> is provided downstream of the second heat exchanger <NUM> in fuel cell hydrogen fuel flow. The third fuel offtake is configured to offtake a portion of the heated fuel flow from the hydrogen fuel conduit <NUM>, and direct this to the hydrogen fuel cell <NUM> for reaction with oxygen to generate electricity. A pressure regulator <NUM> may be provided to reduce the pressure of fuel delivered to the hydrogen fuel cell, which typically requires a lower fuel inlet pressure than the gas turbine engine <NUM>. The remainder of the hydrogen fuel in the hydrogen fuel conduit <NUM> is directed to a bypass line <NUM>.

The air from the hydrogen fuel cell air compressor <NUM> is directed to the fuel cell <NUM> to be reacted with hydrogen fuel to generate electricity via a fuel cell air intake line <NUM>. However, this high-pressure air may be at a relatively high temperature, and may need cooling prior to being introduced to the hydrogen fuel cell. Furthermore, by cooling the inlet air, the air density can be increased, which may allow for a higher density fuel cell. As such, a third hydrogen fuel cell heat exchanger <NUM> is provided downstream of the compressor <NUM> and upstream of the fuel cell <NUM> in compressor air flow. The third heat exchanger is configured to exchange heat with hydrogen fuel in the bypass line <NUM>, thereby cooling the compressor air and further warming the hydrogen fuel in the bypass line <NUM>. As such, the third hydrogen fuel cell heat exchanger <NUM> is provided downstream of the third fuel offtake <NUM> in fuel cell hydrogen fuel flow.

The bypass line <NUM> comprises a fluid connection <NUM>, which fluidly connects the bypass line <NUM> and main fuel conduit <NUM> downstream of the third hydrogen fuel heat exchanger <NUM> in fuel cell fuel flow, and downstream of the second fuel offtake <NUM> but upstream of the fuel injector <NUM> in main gas turbine engine fuel flow. As such, the heated hydrogen fuel is combined back into the main gas turbine engine fuel flow prior to introduction to the gas turbine engine injector <NUM>, thereby warming the flow.

Hydrogen fuel downstream of the fluid connection <NUM> is arranged to be at a temperature and pressure sufficient to meet the requirement of the fuel injector <NUM> for combustion in the gas turbine engine <NUM>. Consequently, fuel is heated to an adequate extent to ensure efficient gas turbine engine combustion using waste heat from a hydrogen fuel cell cycle. Meanwhile, electrical output from the hydrogen fuel cell is provided at high efficiency, since waste heat is reintroduced to the overall system cycle. Consequently, electrical requirements are met in a highly efficient manner.

It will be understood that air for the hydrogen fuel cell <NUM> can be provided by other means.

For example, <FIG> illustrates an alternative arrangement, in which the turbine <NUM>, shaft <NUM>, and air compressor <NUM> are omitted.

In place of these items, a main compressor bleed offtake <NUM> is provided, which diverts air from the main engine compressor <NUM> bleed offtake line <NUM>. The remainder of the gas turbine engine <NUM> and fuel cell <NUM> is the same as the first embodiment, with the same heat exchangers and offtakes being employed.

In some cases, the airflow and pressure requirements of the pre-burner <NUM> and hydrogen fuel cell may differ from one another. For example, the hydrogen fuel cell <NUM> typically requires a relatively low pressure air supply (at around <NUM> to <NUM> bar of pressure for example), i.e. lower than the air pressure required for the burner <NUM>. As such, separate compressor air bleeds may be provided for the burner <NUM> and fuel air cell <NUM>, rather than the single feed shown. Typically, a low-pressure compressor stage (such as an earlier stage of the high-pressure compressor <NUM>, or a stage of the low-pressure compressor <NUM>) would be employed for the hydrogen fuel cell <NUM>, while a higher pressure stage would be utilised for the burner <NUM>.

It will be understood that aspects of these two embodiments could be combined, for example, the turbine <NUM> and air compressor <NUM> could be retained, with air from the air compressor <NUM> being used for aircraft pneumatic systems such as passenger air conditioning systems. Alternatively, the turbine <NUM> could power a load such as an electrical generator. Similarly, the bleed offtake <NUM> could be provided in the system of the first embodiment for provision of additional hydrogen fuel cell intake air where insufficient air is provided by the compressor <NUM>.

For example, the system is suitable for use with other gas turbine engine types.

In one example, the gas turbine engine could be of a "complex cycle" type, comprising one or more recuperator configured to transfer heat from a turbine outlet to a combustor inlet, and an inter-cooler configured to cool compressed air between compressor stages of the core compressor.

In such a complex cycle, specific fuel consumption is typically reduced, and as such, the fuel cell would have to be sized to accommodate the reduced fuel flow.

Claim 1:
A combined gas turbine engine and hydrogen fuel cell system comprising:
a hydrogen fuelled gas turbine engine (<NUM>);
a cryogenic liquid hydrogen fuel tank (<NUM>);
a first fuel offtake (<NUM>) configured and arranged to divert a portion of hydrogen fuel from a main fuel conduit (<NUM>);
a burner (<NUM>) configured and arranged to burn the portion of hydrogen fuel diverted from the main fuel conduit (<NUM>);
a heat exchanger (<NUM>) configured and arranged to transfer heat from exhaust gasses produced by the burner (<NUM>) to hydrogen fuel in the main fuel conduit (<NUM>);
a second fuel offtake (<NUM>) arranged to divert a portion of hydrogen fuel from the main fuel conduit (<NUM>) downstream of the heat exchanger (<NUM>); and
a hydrogen fuel cell (<NUM>) configured and arranged to produce electric power using hydrogen fuel diverted from the second fuel offtake (<NUM>), the hydrogen fuel cell (<NUM>) comprising a cooling system comprising a closed cooling loop (<NUM>); characterised in that:
the closed cooling loop (<NUM>) comprises a first fuel cell heat exchanger (<NUM>) configured to transmit waste heat from the hydrogen fuel cell (<NUM>) to a coolant of the closed cooling loop (<NUM>).