Patent Description:
The use of hydrogen as an alternative to liquid hydrocarbon fuels for aircraft propulsion poses technical problems. Given the low density of hydrogen in gaseous form, storing significant quantities requires either high pressures at ambient temperatures or at low pressures in liquid form at cryogenic temperatures. For aircraft applications, cryogenic storage is a more practical solution, given the ability to store larger quantities at low pressures and with reduced overall weight and volume. Providing a supply of gaseous fuel to generate power, whether by way of a gas turbine or a fuel cell, requires the liquid fuel supply to be heated from cryogenic temperatures prior to being reacted or combusted. Additional and differently configured features and control systems are required to control the supply of hydrogen fuel compared to those used for a conventional liquid hydrocarbon fuel such as kerosene.

As well as potentially replacing conventional liquid hydrocarbon fuel for use in gas turbine engines, hydrogen can be used to generate electrical power directly through being oxidised in a hydrogen fuel cell. While gas turbines are advantageous in generating large amounts of power in kinetic form that can be used to provide propulsion, fuel cells can be used to generate smaller amounts of electrical power to support electrically powered services that would otherwise require generation by an electrical machine driven by a gas turbine engine. A combination of a gas turbine with a hydrogen fuel cell may therefore be advantageous, given the need in aircraft for both propulsive thrust and electrical power.

A conventional aircraft gas turbine engine will typically supply a proportion of output shaft power via an electrical generator to service electrical aircraft systems, including those on the engine itself. The proportion of electrical power is currently a small but significant proportion of total engine power, and is likely to be become greater for newer more electrified vehicles. Fuel cells offer the possibility of directly and efficiently converting stored fuel energy into electrical power without the need for combustion and mechanical to electrical conversion beforehand. Fuel cells are, however, limited in power density, making their use for propulsive applications less attractive.

A gas turbine engine is capable of operating more efficiently when used for generating propulsive power alone. When hydrogen is used as a fuel source, the power requirements for pumping fuel from liquid hydrogen in a cryogenic tank to gaseous form at the gas turbine are increased substantially compared to an equivalent conventional fuelled engine. A fuel cell may therefore be able to generate at least some of the electrical supply that the engine would otherwise need to provide, resulting in an overall gain in thermal efficiency. A problem, however, lies in how to implement a system that can operate efficiently using both a gas turbine engine and a fuel cell.

Prior art documents <CIT>, <CIT>, <CIT>, <CIT>, and <CIT> disclose aircraft power systems comprising a gas turbine engine and a fuel cell, both hydrogen-fuelled, with one or more heat exchangers.

In accordance with a first aspect there is provided a hydrogen-fuelled aircraft power system comprising:.

The hydrogen-fuelled aircraft power system may further comprise a first electrical machine connected to the gas turbine engine.

The gas turbine engine may be part of a turboprop engine, the system comprising a propellor connected to the gas turbine engine. The gas turbine engine may alternatively be part of a turbofan engine, the system comprising a fan connected to the gas turbine engine.

The hydrogen fuel supply system may comprise a cryogenic hydrogen storage tank, a pre-heater and a third heat exchanger, the hydrogen fuel supply system configured to provide the flow of gaseous hydrogen fuel from the third heat exchanger heated by the pre-heater.

The cathode air inlet line may comprise a cathode air inlet turbine connected to drive a second electrical machine.

The cathode exhaust line may comprise a cathode exhaust turbine connected to drive a third electrical machine.

The second fluid flow path of the second heat exchanger may for example be between the cathode exhaust and the cathode exhaust turbine.

The gas turbine engine may further comprise a recuperator at an outlet of the turbine, the cathode exhaust line passing through the recuperator between the second fluid flow path of the second heat exchanger and the cathode exhaust turbine.

In accordance with a second aspect there is provided a method of operating a hydrogen-fuelled aircraft power system, the method comprising:.

The method may further comprise driving a first electrical machine with the gas turbine engine.

The method may comprise driving a propellor connected to the gas turbine engine or may comprise driving a fan connected to the gas turbine engine.

The hydrogen fuel supply system may comprise a cryogenic hydrogen storage tank, a pre-heater and a third heat exchanger, the hydrogen fuel supply system providing the flow of gaseous hydrogen fuel from the third heat exchanger heated by the pre-heater.

The second fluid flow path of the second heat exchanger may be between the cathode exhaust and the cathode exhaust turbine.

The gas turbine engine may comprise a recuperator at an outlet of the turbine, the cathode exhaust line passing through the recuperator between the second fluid flow path of the second heat exchanger and the cathode exhaust turbine.

Embodiments of the invention are described below by way of example only and with reference to the accompanying drawings, which are purely schematic and not to scale, in which:.

A hydrogen-fuelled airliner is illustrated schematically 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>. The turbofan engines <NUM> may for example be geared turbofan engines.

A hydrogen storage tank <NUM> located in the fuselage <NUM> for a hydrogen fuel supply is connected with core gas turbines <NUM> in the turbofan engines <NUM> via a fuel delivery system. In the illustrated example, the hydrogen storage tank <NUM> is a cryogenic hydrogen storage tank that stores the hydrogen fuel in a liquid state, i.e. at or below around <NUM>. The hydrogen fuel may be pressurised to between around from <NUM> to <NUM> bar, for example around <NUM> bar.

A schematic block diagram illustrating the flow of hydrogen fuel to a gas turbine engine is shown in <FIG>. Hydrogen fuel is obtained from a hydrogen storage tank <NUM> by a fuel delivery system <NUM> and is supplied to a core of a gas turbine <NUM>. Only one of the gas turbines is shown for clarity. In this illustrated embodiment, the gas turbine <NUM> is a simple cycle gas turbine engine. In other embodiments, complex cycles may be implemented via fuel-cooling of the gas path.

Referring again to <FIG>, the gas turbine <NUM> comprises, in axial flow series, a low-pressure compressor <NUM>, an interstage duct <NUM>, a high-pressure compressor <NUM>, a diffuser <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 fuel injection system <NUM> may be a lean direct fuel injection system. 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>. In alternative examples, the gas turbine <NUM> may comprise more than two shafts.

In a geared turbofan engine the low-pressure turbine <NUM> also drives a fan <NUM> via a reduction gearbox <NUM>. The reduction gearbox <NUM> receives an input drive from the second shaft <NUM> and provides an output drive to the fan <NUM> via a fan shaft <NUM>. The reduction gearbox <NUM> may be an epicyclic gearbox, which may be of planetary, star or compound configuration. In further alternatives, the reduction gearbox <NUM> may be a layshaft-type reduction gearbox or another 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 <NUM> and the fan <NUM>.

In operation, the fuel delivery system <NUM> is configured to obtain liquid hydrogen fuel from the cryogenic hydrogen storage tank <NUM> and provide the fuel to the fuel injection system <NUM> in gaseous form. This requires the amount of liquid fuel from the tank <NUM> to be controlled and a controlled amount of heat provided to the fuel to ensure the fuel in gaseous form is at a required temperature prior to injection into the gas turbine <NUM>, or in alternative arrangements into a hydrogen fuel cell.

<FIG> is a simplified schematic block diagram of a hydrogen fuelled power delivery system <NUM> comprising a gas turbine engine <NUM> and a fuel cell <NUM>, in which air supply and cooling for the fuel cell <NUM> are integrated with the gas turbine engine <NUM> to improve overall efficiency. A hydrogen fuel supply system <NUM> provides a supply of hydrogen fuel to both the fuel cell <NUM> and the gas turbine engine <NUM>. The hydrogen fuel supply system <NUM> provides a flow of gaseous hydrogen to a first fluid path <NUM> of a first heat exchanger <NUM>.

Hydrogen fuel exiting the first heat exchanger <NUM> enters an anode inlet <NUM> of the fuel cell <NUM> and is reacted within the fuel cell <NUM> to generate electrical power. The fuel cell <NUM> may for example be a polymer electrolyte membrane fuel cell (PEMFC) or a solid oxide fuel cell (SOFC). The fuel cell <NUM> comprises a cooling flow path <NUM>, which flows through the fuel cell <NUM> and through a second fluid path <NUM> of the first heat exchanger <NUM>, thereby exchanging heat between fluid in the fuel cell cooling flow path <NUM> and the incoming hydrogen fuel. A radiator to extract excess heat from the fuel cell <NUM> is not required, as cooling of the fuel cell <NUM> can be achieved by transferring excess heat to the incoming hydrogen fuel. Waste heat generated by the fuel cell <NUM> is instead reused to heat the incoming fuel, thereby increasing overall efficiency. Operation of the pre-heater <NUM> can be adjusted according to the temperature of the fuel as measured exiting the first heat exchanger <NUM> so that a desired temperature of fuel entering both the fuel cell <NUM> and gas turbine engine <NUM> can be maintained.

The gas turbine engine <NUM> comprises in series a high-pressure compressor <NUM>, a combustor <NUM>, a high-pressure turbine <NUM> and a low-pressure turbine <NUM>. Hydrogen fuel exiting the first heat exchanger <NUM> enters the combustor <NUM> and is combusted to drive the turbines <NUM>, <NUM>. In the illustrated example, the gas turbine engine <NUM> drives a first electrical machine <NUM> to generate electrical power, for example to drive one or more electrical propulsors. In alternative arrangements, the gas turbine engine <NUM> may drive a fan or a propellor to provide propulsion. The number of compressors and/or turbines may also differ in other arrangements, for example having a low pressure and high-pressure compressor as in the example in <FIG>.

An air supply to the cathode inlet <NUM> of the fuel cell <NUM> is provided from the compressor <NUM> via a compressor bleed line <NUM>. The position of the compressor bleed line <NUM> may be selected according to the temperature and pressure required. For example, in a first position <NUM> the temperature and pressure is lower than in a second position <NUM> downstream from the first position <NUM>. Bleed air exiting the compressor <NUM> via the compressor bleed line <NUM> passes through a first fluid flow path <NUM> of a second heat exchanger <NUM> and through a cathode air inlet line <NUM> connected to the cathode inlet <NUM> of the fuel cell <NUM>. A cathode exhaust <NUM> is connected to a cathode exhaust line <NUM>, which passes through a second fluid flow path <NUM> of the second heat exchanger <NUM> towards an exhaust <NUM>. The exhaust <NUM> may for example be directed towards a bypass path of the gas turbine engine <NUM>.

The hydrogen fuel supply system <NUM> comprises a cryogenic hydrogen storage tank <NUM>, a pre-heater <NUM> and a third heat exchanger <NUM>. The pre-heater <NUM> and third heat exchanger <NUM> ensure that fuel provided to the fuel cell <NUM> and engine <NUM> is sufficiently heated prior to steady state operation of the system <NUM>. Hydrogen fuel from the cryogenic tank <NUM>, which may for example be at a temperature of around <NUM>, passes through the third heat exchanger <NUM> and is heated to ambient temperature levels, for example around <NUM>, through burning a small proportion of the fuel in the pre-heater <NUM>. An air supply to the pre-heater <NUM> may be provided from the compressor <NUM> of the gas turbine engine <NUM>. An exhaust of the pre-heater <NUM> may be connected to a bypass path of the gas turbine engine <NUM>.

Additional turbines <NUM>, <NUM> may be provided in the cathode air inlet line <NUM> and/or the cathode exhaust line <NUM> to recover energy from high pressure air flows passing through the lines <NUM>, <NUM>. A cathode air inlet turbine <NUM> may be provided in the cathode air inlet line <NUM>, the turbine <NUM> connected to drive a second electrical machine <NUM> to generate further electrical power. A cathode exhaust turbine <NUM> may be provided in the cathode exhaust line <NUM>, the cathode exhaust turbine <NUM> connected to drive a third electrical machine <NUM> to generate further electrical power. Air flow through the cathode air inlet and outlet lines <NUM>, <NUM> can be controlled through operation of the turbines <NUM>, <NUM> and the load extracted from the turbines by the electrical machines <NUM>, <NUM>. Heat transferred from air in the compressor bleed line <NUM> to the exhaust air in the cathode air outlet line <NUM> enables a higher energy recovery factor using the cathode air outlet turbine <NUM>.

Each of the electrical machines <NUM>, <NUM>, <NUM> are typically operated as generators during operation of the system <NUM>, but may also operate in reverse as motors, for example to start operation of the gas turbine <NUM>.

In an alternative example hydrogen fuelled power delivery system <NUM> illustrated in.

<FIG>, a recuperator <NUM> may be added to an outlet of the low-pressure turbine <NUM> to further enhance energy recovery. The cathode exhaust line <NUM> passes through the recuperator <NUM> prior to the cathode exhaust turbine <NUM>, gaining further heat from the low-pressure turbine <NUM> exhaust and thereby further enhancing energy recovery through the cathode exhaust turbine <NUM> and third electrical machine <NUM>. Other components of the system <NUM> are as described above in relation to the system <NUM> of <FIG>.

Operating a system as described above may result in a reduction in fuel requirement for the gas turbine <NUM> through use of an appropriately sized fuel cell <NUM>. The fuel cell <NUM> may for example be sized to provide a level of electrical power that corresponds to a peak designed power for the aircraft electrical systems. The proportion of power provided by the fuel cell <NUM> may for example be up to around <NUM>-<NUM>% in terms of overall fuel consumption. A proportion of compressor air passing into the compressor bleed line <NUM> in such an example may be up to around <NUM> to <NUM>%. Overall system efficiency in comparison with a conventional kerosene fuelled gas turbine engine may be increased, with only a small weight addition due to the increase in weight of the fuel cell <NUM> and associated components.

An advantage of the system as described herein is that the gas turbine engine <NUM> can operate as a purely propulsive unit, enabling a significant improvement in efficiency, while the fuel cell <NUM> generates an electrical supply more efficiently than through using an electrical machine for providing electrical power for electrical systems in the gas turbine engine <NUM> and elsewhere in the aircraft. The system <NUM>, <NUM> may be particularly applicable in large aircraft applications where engine off-takes and liquid hydrogen pumping power requirements are significant and may otherwise have a deleterious impact on a pure hydrogen fuelled gas turbine system.

A small portion of bleed air is required to provide an air supply to the fuel cell <NUM>, which can be taken from the gas turbine compressor <NUM>. The second heat exchanger and turbines <NUM>, <NUM> allow power to be recovered from the compressor <NUM> pressure rise which is proportional to the pressure gain itself. With the outlet at the second position <NUM> the power recovery is higher, but the bleed losses are also higher.

Claim 1:
A hydrogen-fuelled aircraft power system (<NUM>, <NUM>) comprising:
a hydrogen fuel supply system (<NUM>) configured to provide a flow of gaseous hydrogen fuel;
a hydrogen fuel cell (<NUM>) having an anode inlet (<NUM>) connected to receive a first portion of the flow of gaseous hydrogen fuel from the hydrogen fuel supply system (<NUM>), a cathode air inlet (<NUM>), a cathode exhaust (<NUM>) and a cooling flow path (<NUM>);
a gas turbine engine (<NUM>) having a compressor (<NUM>), a combustor (<NUM>) connected to receive a second portion of the flow of gaseous hydrogen fuel from the hydrogen fuel supply system (<NUM>), and a turbine (<NUM>, <NUM>);
a first heat exchanger (<NUM>) having first and second fluid flow paths, the first fluid flow path of the first heat exchanger (<NUM>) being between the hydrogen fuel supply system (<NUM>) and the fuel cell anode inlet (<NUM>), the fuel cell cooling flow path (<NUM>) passing through the second fluid flow path of the first heat exchanger (<NUM>); and
a second heat exchanger (<NUM>) having first and second fluid flow paths (<NUM>, <NUM>), the first fluid flow path (<NUM>) of the second heat exchanger (<NUM>) connected between a compressor bleed line (<NUM>) and a cathode air inlet line (<NUM>) connected to the fuel cell cathode air inlet (<NUM>), a cathode exhaust line (<NUM>) from the fuel cell cathode exhaust (<NUM>) passing through the second fluid flow path (<NUM>) of the second heat exchanger (<NUM>) towards an exhaust (<NUM>).