Hybrid compressed air energy storage system

Abstract

A hybrid compressed air energy storage (CAES) system is provided. The hybrid CAES system comprises one or more compressed air reservoirs 140, a compressor system 110, configured to compress air into t

Description

The present disclosure relates to a compressed air energy storage (CAES) system and method of operating a CAES system. Background Compressed air energy storage (CAES) systems can be used to store energy, for example energy harnessed from renewable sources. During periods where energy is available in excess of the amount required to meet an electricity demand, the excess energy can be stored by a CAES system. The stored energy can then be harnessed during periods of low production or high electricity demand to generate electricity. Typical CAES systems use a compressor during periods of excess electricity production to compress air. The compressed air is stored in an underground cavern. When there is a demand, the compressed air is drawn from the cavern and expanded through air expanders, to produce electricity. Some known CAES systems may be referred to as diabatic CAES systems. In these systems, air is compressed via a series of compressors and stored at a high pressure in a reservoir (e.g., an underground cavern). Heat is generated as the air is compressed and this heat is dissipated to the environment. To release energy, the compressed air is extracted from the reservoir and heated, typically achieved through burning natural gas or hydrogen, and then expanded across an air expander system to generate electricity. Other known CAES systems are adiabatic CAES systems. Adiabatic CAES systems work in a similar way to diabatic CAES systems, but the heat generated at compression is stored in a heat storage means. This stored heat is then used to heat up the air leaving the cavern. However, such adiabatic CAES systems are limited by the capacity of the heat storage. When the heat storage is at maximum capacity, no further air can be compressed. Similarly, once the heat storage is empty, no further air can be expanded. Furthermore, the heat available for the air expanders in an adiabatic CAES system is limited in temperature, depending on the temperature generated during the compression process. It is desirable to provide a CAES system with improved efficiency, greater power capability and increased power production duration. Summary of Invention The present invention provides a compressed air energy storage (CAES) system as defined in the appended independent claims, to which reference should now be made. Preferred or advantageous features are set out in dependent sub-claims. A first aspect of the present disclosure relates to a CAES system. The CAES system may comprise one or more compressed air reservoirs; a compressor system configured to compress air into the one or more air reservoirs; and an air expander system configured to generate power using air released from the one or more compressed air reservoirs. The CAES system may comprise a heat storage system for removing heat from the compressor system, storing heat, and supplying heat to the air expander system. The CAES system may comprise a heat source for supplying heat to at least one of the heat storage system and the air expander system. The CAES system may be referred to herein as a hybrid CAES system. The hybrid CAES system has improved flexibility compared to known adiabatic and diabatic CAES systems because the hybrid CAES system is capable of providing heat to the air expander system from either the heat source or the heat storage system, or both. Therefore, the system is capable of being operated using either the heat source, the heat storage system, or both the heat source and the heat storage system. As a result, the expansion capacity of the system may be improved compared to adiabatic systems. Advantageously, the efficiency of the hybrid CAES system may be improved compared to diabatic systems, because the heat removed from the compression system may be used to provide heat to the air expander system, therefore less additional fuel may be required to increase the temperature of the air for air expansion. However, the hybrid CAES system is capable of increasing the amount of heat supplied to the air expander system by duration or by temperature due to the presence of the heat source. As such, the capacity of the hybrid CAES system may be less limited by the capacity of the heat storage system than an adiabatic CAES system and may have a greater maximum power output and power production duration. Preferably, the hybrid CAES system may comprise a cooling system for removing heat from the compressor system and dissipating the heat. Advantageously, the hybrid CAES system has improved flexibility compared to known adiabatic and diabatic CAES systems because the hybrid CAES system is capable of removing heat from the compression system with both the heat storage system and the cooling system. As a result, the hybrid CAES system is not limited to compressing air only until the heat storage system is at maximum capacity. The hybrid CAES system is configured to also be able to dissipate heat using the cooling system, therefore the compression capacity of the system may be improved compared to known adiabatic systems. The hybrid CAES system may be configured to be operated in at least two of a dissipation mode, a heat storage mode, or a combination cooling mode. In the dissipation mode the cooling system is configured to remove heat from the compressor system. In the heat storage mode the heat storage system is configured to remove heat from the compressor system. In the combination cooling mode the cooling system and the heat storage system are both configured to remove heat from the compressor system. The hybrid CAES system may be configured to switch between the at least two of the dissipation mode, the heat storage mode and the combination cooling mode. The hybrid CAES system may be configured to operated in each of the dissipation mode, the heat storage mode, and the combination cooling mode. The hybrid CAES system may be configured to switch between each of the dissipation mode, the heat storage mode and the combination cooling mode. The hybrid CAES system may be configured to be operated in at least two of a heat storage supply mode, a heat source mode, or a heat supply combination mode. In the heat storage supply mode the heat storage system is configured to supply heat to the air expander system. In the heat source mode the heat source is configured to supply heat to at least one of the heat storage system or the air expander system. In the heat supply combination mode the heat storage system is configured to supply heat to the air expander system and the heat source is configured to supply heat to at least one of the heat storage system or the air expander system. The hybrid CAES system may be configured to switch between the at least two of heat storage supply mode, the heat source mode and the heat supply combination mode. The hybrid CAES system may be configured to be operated in each of the heat storage supply mode, the heat source mode, and the heat supply combination mode. The hybrid CAES system may be configured to switch between each of the heat storage supply mode, the heat source mode and the heat supply combination mode. Advantageously, the hybrid CAES system being configured to operate in this way allows the hybrid CAES system to be operated with flexibility to allow operation of the system to react to changes in supply of air or electricity demands. The hybrid CAES system may be configured to operate in an adiabatic operation, wherein the heat storage system is configured to remove heat from the compressor system, store heat, and supply heat to the turbine air expander system during the appropriate stages of operation. In the adiabatic operation, when the compressor system is in operation the hybrid CAES system is configured to operate in the heat storage mode and when the expander system is in operation the hybrid CAES system is configured to operate in the heat storage supply mode. The hybrid CAES system may be configured to operate in a diabatic operation, wherein the cooling system is configured to remove heat from the compressor system and dissipate the removed heat, and the heat source is configured to provide heat to the air expander system. In the diabatic operation, when the compressor system is in operation the hybrid CAES system is configured to operate in the dissipation mode and when the expander system is in operation the hybrid CAES system is configured to operate in the heat source mode. The hybrid CAES system may be configured to operate in a hybrid operation, wherein a combination of the heat storage system and the cooling system may remove heat from the compressor system, the heat source may supply heat to at least one of the heat storage system and the air expander system, and the heat storage system may provide heat to the air expander system. In the hybrid operation, the hybrid CAES system may be configured to operate in a different combination of the described modes. For example, when the compressor system is in operation, the hybrid CAES system may be configured to operate in the heat storage mode and when the expander system is in operation, the hybrid CAES system may be configured to operate in the heat supply combination mode. Alternatively, when the compressor system is in operation, the hybrid CAES system may be configured to operate in the combination cooling mode and when the expander system is in operation, the hybrid CAES system may be configured to operate in the heat storage supply mode. Alternatively, the hybrid CAES system may be configured to operate in the combination cooling mode and the heat supply combination mode. By utilising the different modes, the hybrid CAES system may be more responsive to different demands on the system compared to prior CAES systems. Preferably, the hybrid CAES system is configured to switch between the modes in response to a control signal. In the hybrid operation, the combination of heat storage system and cooling system may operate to remove heat from the compressor system simultaneously. Preferably, in the hybrid operation, the heat storage system and the cooling system may operate to remove heat from the compressor system sequentially. For example, in the hybrid operation the hybrid CAES system is configured to operate in the combination cooling mode and / or switch between the dissipation mode, the heat storage mode and the combination cooling mode. Preferably, the heat storage system is configured to remove heat from the compressor system until the heat storage system is at maximum capacity and after the heat storage system is at maximum capacity, the cooling system is configured to remove heat from the compressor system. Therefore, the hybrid CAES system may be configured to operate in the heat storage mode until the heat storage is at maximum capacity or full and then switch to the dissipation mode. Advantageously, this allows as much heat as possible to be captured from the compressor system so that less heat is wasted but means compression can continue even after the heat storage system has reached maximum capacity. In the hybrid operation, the heat source may be configured to supply heat to the heat storage system and the air expander system simultaneously. Optionally, in the hybrid operation, the heat source may be configured to supply heat to the heat storage system and the air expander system sequentially. Preferably, in the hybrid operation, the heat source provides heat to the heat storage system. Preferably, the heat storage system supplies heat to the air expander system. Optionally, in the hybrid operation, the heat source provides heat to the air expander system. For example, in the hybrid operation the hybrid CAES system is configured to operate in the heat supply combination mode and / or switch between the heat storage supply mode, the heat source mode, or the heat supply combination mode. The hybrid CAES system may be configured to initially supply heat to the air expander system using the heat source and to subsequently supply heat to the air expander system using the heat storage system. Therefore, the hybrid CAES system may be configured to operate in the heat source mode until the heat storage is depleted or empty and then switch to the heat storage supply mode. Alternatively, the hybrid CAES system may be configured to initially supply heat to the air expander system using the heat storage system and to subsequently supply heat to the air expander system using the heat source. Therefore, the hybrid CAES system may be configured to operate in the heat storage supply mode until the heat storage is depleted or empty and then switch to the heat source mode. The heat source may be configured to directly increase the temperature of the air that is fed into, or present in, the air expander system. The heat source may be configured to supply additional heat to the heat storage system. The heat source may be configured to supply heat to the heat storage system to maintain or increase the temperature of the air that is fed into, or present in, the air expander system. The heat source may be configured to provide between 20 percent and 100 percent of the heat required by the air expander system. For example, the heat source may configured to provide 50% of the heat required by the air expander system. The heat source may further be configured to generate power. Optionally, the heat source comprises a gas turbine. The gas turbine may comprise a gas turbine generator that is configured to generate power. The gas turbine may be configured to operate independently of the air expander system. The gas turbine may be configured to respond to a requirement or demand to generate power more quickly than the air expander system. The gas turbine and air expander system may be configured such that the gas turbine generates power first and subsequently the air expander system generates power. The gas turbine may quickly respond to power demands from the grid. The gas turbine will be able to start up more quickly than the air expander portion of a CAES system. For example, the gas turbine may be able transition from a non-operational state to a maximum power generation state in a time of between 2 and 10 minutes. Whereas the air expander system may be able transition from a non-operational state to a maximum power generation state in a time of between 5 and 10 minutes. Advantageously, the gas turbine may allow the hybrid CAES system to generate power in response to demand from the grid before and / or while the air expander system starts up. Advantageously, the gas turbine does not rely on electrical power to generate heat. The gas turbine may be configured to generate power at the same time as the air expander system. Advantageously, the gas turbine may increase the maximum power output of the hybrid CAES system compared to a CAES system without the gas turbine. Heat will be produced during operation of the gas turbine. It is desirable to use this heat elsewhere in the CAES system. The gas turbine may be configured to provide heat to the heat storage system. Alternatively, or in addition, the gas turbine may be configured to provide heat to the air expander system. Advantageously, using heat produced by the gas turbine increases the efficiency of the hybrid CAES system and may increase the duration of power output from the air expander system and boost the power output of the air expanders. The gas turbine may be a natural gas fired gas turbine. Preferably, the gas turbine may be a dual natural gas / hydrogen fired gas turbine. Optionally, the gas turbine may be powered by at least one of biogas, diesel, biodiesel, or ammonia. The gas turbine may have a power output of between 20 and 100 MWe. For example, the gas turbine may have a power output 40 MWe gas turbine. The heat source may comprise more than one gas turbine, for example, the heat source may comprise two, three or four gas turbines. In one example, the CAES system may comprise one gas turbine configured to supply 50 percent of the heat required by the air expander system. Alternatively, the heat source may comprise two gas turbines, which in combination are configured to supply 100 percent of the heat required by the air expander system. The heat source may comprise an electric heater. The electric heater may be configured to supply heat to the heat storage system. The electric heater may be configured to supply heat to the heat storage system at the same time as the air is compressed into the one or more reservoirs. Advantageously, heat may be added to the heat storage system by the electric heater when there is an oversupply of electricity and / or when the price of electricity is low. The hybrid CAES system may comprise a furnace. The heat source may comprise the furnace. The furnace may be co-fired with natural gas and / or hydrogen. The furnace may be configured to carry out oxy-fuel combustion. The furnace may be configured to burn hydrogen with oxygen, preferably high purity oxygen, preferably at least 90% oxygen. Advantageously, burning hydrogen in the presence of oxygen, may avoid the production of NOX that would otherwise occur if the hydrogen were burned with air. The furnace may be configured to provide heat to the heat storage system. Alternatively, or in addition, the furnace may be configured to provide heat to the air expander system. The heat source may comprise one or more heat pumps. The heat pump may comprise a heat intake from ambient air, subsurface heat, solar heat, or heat integration from another system. The hybrid CAES system may comprise a heat recovery unit, for example an economizer, configured to transfer heat from the heat source to the heat storage system. The heat recovery unit may be configured to recover heat from the gas turbine, or from the furnace. The heat recovery unit may be configured to transfer recovered heat from the gas turbine or the furnace to the heat storage system and / or the air expander system. The heat storage system may be configured to remove heat from the compressor system using a heat transfer material. The heat transfer material may be a fluid. Preferably, the heat transfer material is water. The heat storage system may comprise thermal energy storage (TES) for storing heat. Optionally, the heat source may be configured to increase the temperature of the TES. Optionally, the heat source may be configured to maintain the temperature of the TES. The heat storage system may comprise one or more storage tanks containing the TES. A first storage tank may be at a first temperature and a second storage tank may be at a second temperature, wherein the second temperature may be greater than the first temperature. The first tank may be at a first pressure and the second tank may be at a second pressure, wherein the second pressure is greater than the first pressure. The heat source may be configured to supply heat to the second storage tank. The first temperature may be up to 98 degrees Celsius. The heat storage system may comprise a heat storage material for storing heat. The TES may comprise the heat storage material. The heat storage may be the same material as the heat transfer material. The heat storage material may comprise a different material to the heat transfer material. The heat storage material may comprise a solid material. The heat storage material may comprise ceramics (rocks, bricks), sand, or metal. The heat storage material may comprise a phase change material configured to absorb and release heat energy when changing phase from solid to liquid, for example a salt, or aluminium. The heat source may be configured to increase the temperature of the heat storage material in the heat storage system. The heat storage material may receive some heat from the compressor system and some heat from the heat source. The heat source may be configured to provide additional heat to the heat storage system, for example when the compressors are not operating. In this way, the heat source may be configured to prolong the capacity of the heat storage system. The compressor system may be configured to compress air to a pressure of between 5 MPa and 24 MPa, for example between 9 MPa and 20 MPa, for example between 5 MPa and 12 MPa. The compressor system comprises at least one compressor configured to compress air; this may be referred to as an air compressor. The compressor system may comprise more than one air compressor. For example, the compressor system may comprise one, two, three, four, five, or six air compressors. The more than one air compressors of the compressor system may be arranged in series. Advantageously, operating more than one air compressor in series along an air flow path increases the compression achieved by a compressor system. The more than one air compressor of the compressor system may be arranged in parallel. Some of the more than one air compressors may be arranged in series and some of the more than one air compressors arranged in parallel. For example, the compressor system may comprise four air compressors. The four air compressors may comprise a first set of two air compressors arranged in series. The four air compressors may also comprise a second set of two air compressors arranged in series. The first set of air compressors may be arranged in parallel with the second set of air compressors. The compressor system may comprise a compressor train. The compressor system may comprise a plurality of compressor trains. Preferably, each compressor train of the plurality of compressor trains are arranged in parallel and configured to compress air at ambient pressure to a storage pressure for the air to be stored in the one or more compressed air reservoirs. Optionally, any one or more of the plurality of compressor trains may operate simultaneously. A plurality of compressor trains allows for a greater throughput of air than a single compression train of the same size as one of the plurality of compression trains. A plurality of compressor trains arranged in parallel allows a greater flexibility than a single compression train because at least one compressor train could be operating while at least one other compressor train is not operating. This may allow a greater flexibility in the throughput of air through the system. For example, during period when a large amount of air is needed, all of the plurality of compressor trains could be operated, whereas when a small amount of air is needed then, for example, only one compressor train could be operated. Operating one compressor train out of a plurality of compressor trains may be more efficient than operating a single compressor train at a lowered capacity. Multiple compressor trains can also increase the responsivity of the compressor system compared to using a single, large compressor train. The plurality of compressor trains may comprise 2, 3,4, 5, or 6 compressor trains, for example. In particular, the plurality of compressor trains may comprise four compressor trains. Four compressor trains advantageously may provide a preferable capacity of the system. Four compressor trains may also allow a large amount of flexibility in the system. Optionally, each of the plurality of compressor trains may comprise a plurality of compressors arranged in series along an airflow path. Operating a plurality of compressors in series along an air flow path advantageously increases the compression achieved by a compressor train. Optionally, the plurality of compressors may comprise one or more high pressure compressors and one or more low pressure compressors. The plurality of compressors comprising one or more high pressure compressors and one or more low pressure compressor allows the most appropriate compressor to be selected for the amount of compression required. This may lead to more efficient compression and increase the flexibility of the compressor system. Preferably, each of the compressor trains comprise a low pressure compressor and a high pressure compressor. A single motor drives both the low pressure compressor and the high pressure compressor. Preferably the low pressure compressor intakes air at around ambient pressure and compresses the air to around 5 MPa. Preferably, the high pressure compressor intakes the compressed air at around 5 MPa and compresses the air up to 20 MPa. Optionally, each of the plurality of compressor trains is configured to have an operational power consumption range of between 10 MW and 100 MW, and preferably of between 35 MW and 55 MW. Each of the plurality of compressor trains will have a maximum power consumption. Optionally, each of the plurality of compressor trains may be configured to modulate its power consumption at a rate of 10% of the maximum power consumption per minute, preferably at a rate of 20% of the maximum power consumption per minute, more preferably at a rate of 30% of the maximum power consumption per minute. The compressor system may comprise a multi-stage air compressor. If the compressor system comprises more than one air compressor, some or all of the compressors may be multi-stage air compressors. The multi-stage air compressor may comprise an inter-stage cooling system. Advantageously, the inter-stage cooling system reduces the temperature of the air through the multi-stage compressor and may allow more efficient compression. Inter-stage cooling may reduce the total power needed for the air compression. The inter-stage cooling system may be a water-cooled system. The inter-stage cooling system may be water-cooled system where the water is cooled by the ambient air temperature. The inter-stage cooling system may be the cooling system. For example, the interstage cooling system may be a water-cooled system. Alternatively, the inter-stage cooling system may be an air-cooled system. The inter-stage cooling system may be cooled by ambient air. The heat storage system may comprise the inter-stage cooling system. Optionally, the multi-stage compressor may be a 6-stage compressor. Six-stages of compression may achieve a compression of 30 times the ambient air pressure with a high level of efficiency and flexibility. Optionally, a first compressor of the compressor system is configured to compress air from ambient pressure to between 1 MPa and 10 MPa, preferably 3 MPa. Optionally a second compressor of the compressor system is configured to compress air up to between 10 MPa and 100 MPa, preferably 19 MPa. As used herein, supplying heat to the air expander system may comprise supplying heat to compressed air prior to the compressed air entering the air expander system. Supplying heat to the air expander system may, alternatively or in addition, comprise supplying heat to compressed air within the air expander system. Optionally, the air expander system comprises a plurality of air expander trains. Each air expander train may be arranged in parallel. Each air expander train may be configured to generate power as compressed air from the compressed air reservoir passes through the air expander train. Optionally, any one or more of the plurality of air expander trains may operate simultaneously. A plurality of air expander trains allows for a greater throughput of air than a single air expander train of the same size as one of the plurality of air expander trains. A plurality of air expander trains arranged in parallel allows a greater flexibility than a single air expander train because at least one air expander train could be operating while at least one other air expander train is not operating. This may allow a greater flexibility in the throughput of air through the system. For example, during period when a large amount of power is needed, all of the plurality of air expander trains could be operated, whereas when a small amount of power is needed then, for example, only one air expander train could be operated. Operating one air expander train out of a plurality of air expander trains may be more efficient than operating a single air expander train at a lowered capacity. Multiple air expander trains can also increase the responsivity of the air expander system compared to using a single, large air expander train. The plurality of air expander trains may comprise two air expander trains. Two air expander trains advantageously may provide a preferable capacity of the system. Two air expander trains may also allow a large amount of flexibility in the system. Each of the plurality of air expander trains may comprise a plurality of air expanders arranged in series along an airflow path. Operating a plurality of air expanders in series along an air flow path advantageously increases the power generation achieved by an air expander train. The plurality of air expanders may comprise one or more high pressure air expanders and one or more low pressure air expanders. The plurality of air expanders comprising one or more high pressure air expanders and one or more low pressure air expanders allows the most appropriate air expanders to be selected for the amount of power generation required. This may lead to more efficient power generation and increase the flexibility of the air expanders system. The plurality of air expanders may comprise one or more high pressure air expanders and one or more low pressure air expanders. The plurality of air expanders comprising one or more high pressure air expanders and one or more low pressure air expanders allows the most appropriate air expanders to be selected for the amount of power generation required. This may lead to more efficient power generation and increase the flexibility of the air expander system. The one or more high pressure air expanders may have an air input pressure between 5,000 and 20,000 kPa, such as 15,000 kPa. The one or more low pressure air expanders may have an air input at a pressure between 2,000 kPa and 3,000 kPa, such as 2,500 kPa. The plurality of air expanders may comprise a two-stage high pressure air expander and a low pressure air expander. Preferably, the air inlet temperature to the air expander system is between 100 and 1000 degrees Celsius, for example between 400 and 500 degrees Celsius. The CAES system may further comprise one or more hydrogen storage means and one or more water electrolysers configured to produce hydrogen for storing in the hydrogen storage means, the one or more water electrolysers may be configured to be powered at least in part by the air expander system. The hybrid CAES system may comprise a cooling system for removing heat from the compressor system. The cooling system may be configured to dissipate heat to the environment. That is, cooling system may be configured to remove heat form the comprise system and dissipate that heat to the external environment, outside of the hybrid CAES system. The cooling system may comprise a heat exchanger. The heat exchanger may be any suitable type of heat exchanger. The cooling system may comprise an air-cooled heat exchanger. The cooling system may comprise a water-cooled heat exchanger. The cooling system may comprise multiple heat exchangers. For example, if the compressor system comprises multiple compressors, each compressor may have a corresponding heat exchanger of the cooling system to remove heat from the compressor. For example, if the compressor system comprises first, second, third and fourth compressors, the cooling system may comprise first, second, third and fourth heat exchangers. Each of the heat exchangers may be situated after the corresponding compressor configured to remove heat from the compressed air before it passes to the next compressor or to the compressed air storage reservoir. The cooling system may be configured to provide inter-stage cooling. For example, if the compressor system comprises a multi-stage compressor the cooling system may be configured to remove heat between each stage of cooling. The cooling system may comprise a heat exchanger for each stage of the multi-stage compressor. The one or more compressed air reservoirs may be configured to store air at pressures of up to 19,000 kPa. The one or more compressed air reservoirs may include one or more underground caverns. The one or more compressed air reservoirs may comprise at least two air reservoirs. The at least two air reservoirs may be fluidly connected such that they remain at an equal pressure. Air may enter or leave the at least two air reservoirs that are fluidly connected via a shared interface. A second aspect of the present disclosure relates to a method of operating a CAES system. The method comprises compressing air into one or more compressed air reservoirs using a compressor system. The method comprises removing heat from the compressor system using a heat storage system for storing heat, a cooling system for dissipating heat, or a combination of the heat storage system and the cooling system. The method comprises releasing the compressed air from the one or more air reservoirs. The method comprises supplying heat to an air expander system using the heat storage system, the heat source, or combination of the heat storage system and the heat source. The method comprises generating power using the air expander system and the air released from the one or more compressed air reservoirs. The method may be referred to as a method of operating a hybrid CAES system. The method is suitable for operating the CAES system set out in the first aspect of this disclosure. The method may comprise using the heat source to provide heat to the heat storage system. Alternatively, or in addition, the method may comprise using the heat source to provide heat to the air expander system. The method may comprise operating the hybrid CAES system in at least two of: a dissipation mode in which the cooling system is configured to remove heat from the compressor system; a heat storage mode in which the heat storage system is configured to remove heat from the compressor system; or a combination cooling mode in which the cooling system and the heat storage system are both configured to remove heat from the compressor system. The method may comprise switching between at the least two of the dissipation mode, the heat storage mode and the combination cooling mode. The method may comprise operating the hybrid CAES system in each of the dissipation mode, the heat storage mode, and the combination cooling mode. The method may comprise switching between each of the dissipation mode, the heat storage mode and the combination cooling mode. The method may comprise operating the hybrid CAES system in at least two of: a heat storage supply mode in which the heat storage system is configured to supply heat to the air expander system; a heat source mode in which the heat source is configured to supply heat to at least one of the heat storage system or the air expander system; or a heat supply combination mode in which the heat storage system is configured to supply heat to the air expander system and the heat source is configured to supply heat to at least one of the heat storage system or the air expander system. The method may comprise switching between at the least two of the heat storage supply mode, the heat source mode and the heat supply combination mode. The method may comprise operating the hybrid CAES system in each of the heat storage supply mode, the heat source mode, and the heat supply combination mode. The method may comprise switching between each of the heat storage supply mode, the heat source mode and the heat supply combination mode. The method may comprise operating the hybrid CAES system in an adiabatic operation, wherein the adiabatic operation comprises removing heat from the compressor system, storing heat, and supplying heat to the air expander air expander system using the heat storage system. The method may comprise operating the hybrid CAES system a diabatic operation, wherein the diabatic operation comprises removing heat from the compressor system using the cooling system and dissipating the removed heat, and supplying heat to the air expander system using the heat source. The method may comprise operating the hybrid CAES system in a hybrid operation, wherein the hybrid operation comprises supplying heat to the heat storage system and / or to the air expander air expander system using the heat source, and supplying heat to the air expander system using the heat storage system. The hybrid operation may comprise removing heat from the compressor system using the heat storage system, the cooling system, or a combination of the heat storage system and the cooling system. The method may comprise supplying heat to the air expander system using the heat source and subsequently supplying heat to the air expander system using the heat storage system. Alternatively, the method may comprise supplying heat to the air expander system using the heat storage system and subsequently supplying heat to the air expander system using the heat source. The method may comprise removing heat from the compressor system using the heat storage system until the heat storage system is full and subsequently removing heat from the compressor system using the cooling system. Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. Furthermore, any, some and / or all features in one aspect can be applied to any, some and / or all features in any other aspect, in any appropriate combination. It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and / or supplied and / or used independently. Brief Description of the Figures Embodiments of the invention will now be further described by way of example only and with reference to the accompanying figures in which: Figure 1 illustrates a schematic representation of a CAES system according to a first embodiment of the present disclosure; Figure 2 illustrates a schematic representation of a CAES system according to a second embodiment of the present disclosure; Figure 3 illustrates a schematic representation of a CAES system according to a third embodiment of the present disclosure; and Figure 4 illustrates a schematic representation of a CAES system according to a fourth embodiment of the present disclosure. Detailed Description of the Invention Figure 1 illustrates a hybrid CAES system 100 according to a first embodiment of the present invention. The hybrid CAES system 100 comprises a compressor system 110, that comprises a single compressor train comprising three compressors 111,112,113. As shown in Figure 1, the compressor system comprises three compressors. However, it will be understood that the compressor system may comprise a different number of compressors. For example, the compressor system may comprise six compressors. As shown in Figure 1, the compressor system 110 is illustrated having one compressor train, however, it will be appreciated that the compressor system 110 may comprise additional compressor trains. In general, the compressor system 110 may comprise a plurality of compressor trains, for example four compressor trains. If the compressor system 110 comprises more than one compressor train, the compressor trains are preferably arranged in parallel. The compressor system 110 comprises an air inlet 101 that is configured to draw atmospheric air in to the CAES system 100. The air inlet 101 includes a filter to remove contaminants and prevent them from entering other stages of the CAES system 100. For example, the filter may be configured to prevent large objects (such as birds, sticks, leaves, etc.) from entering the CAES system 100. In addition, or alternatively, the filter may filter particulates or other smaller contaminants, such as grit, dust, and so forth. The compressor system 110 comprises first 111, second 112 and third 113 compressors. An electric motor 125 is configured to supply power to the compressors of the compressor system 110. As shown in Figure 1, the first 111, second 112 and third 113 compressors are driven by a single motor 125. Alternatively, the compressor system 110 may comprise two or more motors, wherein each motor may be configured to drive a separate compressor. The compressor system is configured to operate between a minimum setpoint and a maximum setpoint. Suitable minimum setpoints for each compressor train of the compressor system 110 are typically in the range 1-10 MW, whereas suitable maximum setpoints are typically in the range 10-100 MW. For example, the compressor train may have a minimum setpoint of 5 MW and a maximum setpoint of 55 MW. The first compressor 111 is a low pressure compressor. The first compressor 111 is configured to compress ambient air from ambient pressure to an intermediate air pressure (a pressure higher than ambient atmospheric pressure but lower than the pressure at which the air is to be stored). For example, such an intermediate air pressure may be in the range 1,000-10,000 kPa, e.g., 3,000 kPa. The first compressor 111 may comprise a single stage or a plurality of stages. The second compressor 112 is an intermediate compressor. The second compressor 112 is configured to compress ambient air from an intermediate air pressure to a higher intermediate pressure. For example, if the first compressor 111 compresses air to a pressure of 3,000 kPa, the second compressor 112 may be configured to compress air to a pressure of 10,000 kPa. The second compressor 112 may comprise a single stage or a plurality of stages. The third compressor 113 is a high pressure compressor. The third compressor 113 is configured to a high pressure. The high pressure is the pressure at which the air is to be stored. The third compressor 113 may comprise a single stage or a plurality of stages. The third compressor 113 is configured to compress air up to a storage pressure, which is typically in the range of 10,000-100,000 kPa, for example 19,000 kPa. The hybrid CAES system 100 comprises a cooling system 120. The cooling system 120 is configured to remove heat from the compressor system 110. The cooling system 120 shown in Figure 1 includes a cooling system heat exchanger 121, which is an air-cooled heat exchanger in this example. The cooling system 120 comprises a heat transfer material configured to remove heat from the compressor system 110. The cooling system heat exchanger 121 is configured to remove heat from the heat transfer material and dissipate the heat, for example to the atmosphere. The heat exchanger 121 is configured to cool the heat transfer material exiting the third compressor 113. Figure 1 illustrates the cooling system including the air cooled cooling system heat exchanger 121. However, it will be appreciated that the cooling system 120 may comprise any type of suitable heat exchanger. The suitable heat exchanger for the cooling system 120 may be situated adjacent to any one of the compressors. The cooling system 120 may comprise multiple suitable heat exchangers. The hybrid CAES system comprises one or more compressed air reservoirs such as the underground cavern 140. It will be appreciated that while one cavern 140 is illustrated in Figure 1, the one or more compressed air reservoirs may comprise two or more caverns for storing compressed air. During use, the pressure of the stored compressed air within the cavern is maintained between acceptable limits. These limits may be based on a number of factors, such as the physical constraints of the reservoir (e.g., the geology of the caverns), minimum and maximum operating parameters of the turbines to be powered by the compressed air, as well as legal or other administrative limits that may be set by the relevant authorities. For example, the maximum storage pressure in cavern 140 may be between 10,000 and 100,000 kPa, such as 20,000 kPa. The maximum storage pressure in the compressed air reservoir, in general, must be no lower than the pressure to which the air for storage is compressed by the high pressure compressor. The reservoir may also have a minimum storage pressure. This may be, in particular, be determined by the minimum operating pressure of the turbines that are powered from air from the reservoir. The minimum storage pressure may be, for example, between 1,000 and 10,000 kPa. The hybrid CAES system 100 comprises a heat storage system 130 for removing heat from the compressor system. During operation of the compressor system 110, heat is generated by the compression of air. This heat must be removed from the compressor system 110. In the embodiment shown in Figure 1, this heat is removed from the compressor system by a combination of the cooling system 120 and the heat storage system 130. The heat storage system 130 comprises a heat storage tank 132 and a heat storage exchanger 134. The heat storage tank 132, may also be referred to as a thermal energy store. The heat storage tank contains a heat storage material that stores heat removed from the compressor system 110. The heat storage system 130 comprises a compressor heat exchanger 131. The compressor heat exchanger 131 is shown in Figure 1 as situated within heat storage tank 132. The compressor heat exchanger 131 is configured to remove heat from the compressor system 110. Each of the first 111, second 112, and third 113 compressors is coupled to the heat exchanger 131 to remove heat from each of the first 111, second 112, and third 113 compressors. The heat removed from the compressors by the compressor heat exchanger 131 is transferred to the heat storage tank 132 of the heat storage system 130. The heat storage system 130 comprises heat transfer material. The heat transfer material passes through the first 111, second 112, and third 113 compressors to remove heat from the compressor system 110. The heat transfer material passes through the compressor heat exchanger 131 where heat is removed from the heat transfer material and transferred to the heat storage material of the heat storage tank 132. As shown in Figure 1, the heat transfer material of the cooling system 120 and the heat transfer material of heat storage system 130 are integrated. As such, the cooling system 120 may remove heat from the heat transfer material before the heat transfer material passes through the compressor heat exchanger 131. However, it will be appreciated that the heat transfer material of the cooling system 120 may be separate from and not integrated with the heat transfer material of the heat storage system 130. The hybrid CAES system comprises a heat source. In the example shown in Figure 1, the heat source comprises a gas turbine 150. The heat storage system 130 comprises a gas turbine exchanger 134. The gas turbine exchanger 134 receives heat from the gas turbine 150. The gas turbine 150 intakes air and fuel and combusts them to produce heat and rotate a turbine drive shaft. The turbine drive shaft is connected to a generator 155 to generate electricity. The heat generated by the combustion in the gas turbine 150 is received by the heat storage system 130 via the gas turbine exchanger 134. In the example shown in Figure 1, the heat source comprises a gas turbine 150 however, it will be appreciated that alternatively or in addition the system of Figure 1 may comprise a different heat source. For example, a heat source described in relation to any of the second, third or fourth embodiments. For example, the heat source in the first embodiment may be an electric heat pump or a gas turbine comprising an economizer. The hybrid CAES system further comprises an air expander system 160 that extracts the stored compressed air and uses it to generate power. As shown in Figure 1, the air expander system comprises three air expanders. However, it will be understood that the air expander system may comprise a different number of air expanders. For example, the air expander system may comprise six air expanders. As shown in Figure 1, the air expander system 110 is illustrated having one air expander train, however, it will be appreciated that the air expander system 110 may comprise additional air expander trains. In general, the air expander system 110 may comprise a plurality of air expander trains, for example two air expander trains. If the compressor system 110 comprises more than one air expander train, the air expander trains are preferably arranged in parallel. The air expander system in this embodiment comprises first 161, second 162 and third 163 air expanders and first 171 and second 172 expander heat exchangers. The air expander system 160 receives compressed air from the cavern 140 and the air expanders 161, 162, 163 expand the air. To expand the air, heat must be input into the air expander system 160, so the air expander system 160 comprises first 171 and second 172 expander heat exchangers to receive heat from the heat storage system 130 and supply the heat to the air passing through the air expander system 160. The expansion process in the first expander 161 will cause the temperature of the air to drop. So air exiting the first expander 161 will be cooler than the heated air entering the first expander 161. The first expander heat exchanger 171 receives heat from the heat storage system 130 and is configured to heat the air exiting the first expander 161 and / or air in the second expander 162 to a high temperature, typically between 100 and 1,000 degrees Celsius, such as between 150 degrees Celsius and 350 degrees Celsius. The second expander heat exchanger 172 receives heat from the heat storage system 130 and is configured to heat the air exiting the second expander 162 and / or air in the third expander 163 once again to high temperature, typically between 100 and 1,000 degrees Celsius, such as between 150 degrees Celsius and 350 degrees Celsius. The heat transfer material used in the first expander heat exchanger 171 and the second expander heat exchanger 172 may be the same as used in the cooling system 120 and the compressor heat exchanger 131, for example. Alternatively, a different heat transfer material may be used. The expanded air is used to drive a turbine to generate electricity. Each air expander of the air expander system is configured to drive the turbine. Preferably, the first air expander 161, the second air expander 162, and the third air expander 163, are configured to operate over a wide range of power output, for example, having a minimum power output and a maximum power output. For example, the air expander system 160 may provide between 1 and 100 MW (e.g., 5 MW) at minimum power output and between 10 and 1000 MW (e.g., 47 MW) at maximum power output. The air exiting the air expander system 160 is substantially at atmospheric pressure, or close to atmospheric pressure, and may be released to the atmosphere. During operation of the hybrid CAES system 100, when the electricity demand is low and / or electricity production by renewable sources is high, air is drawn into the air compressor system 110. First, air is drawn into the CAES system by the compressor system 110 and passes through the air filter 101 to remove debris from the air. After passing into the CAES system 100 via the air inlet 101, the air enters the compressor system. Atmospheric air enters the first compressor 111 where it is compressed from atmospheric pressure (approximately 101 kPa) to a pressure of an intermediate air pressure (a pressure higher than ambient atmospheric pressure but lower than the pressure at which the air is to be stored). For example, such an intermediate air pressure may be in the range 1,000-10,000 kPa, e.g., 3,000 kPa. The hybrid CAES system 100 is configured to operate in several different modes. The hybrid CAES system 100 is configured to be operated in each of a dissipation mode in which the cooling system 120 is configured to remove heat from the compressor system 110, a heat storage mode in which the heat storage system 130 is configured to remove heat from the compressor system 110, or a combination cooling mode in which the cooling system 120 and the heat storage system 130 heat are both configured to remove heat from the compressor system 110. The hybrid CAES system 110 is configured to switch between the dissipation mode, the heat storage mode and the combination cooling mode. The hybrid CAES system 100 is also configured to be operated in each of a heat storage supply mode in which the heat storage system 130 is configured to supply heat to the air expander system 160, a heat source mode in which the heat source, in this example the gas turbine 150, is configured to supply heat to at least one of the heat storage system 130 or the air expander system 160, and a heat supply combination mode in which the heat storage system 130 is configured to supply heat to the air expander system 160 and the heat source, in this example the gas turbine 150, is configured to supply heat to at least one of the heat storage system 130 or the air expander system 160. The hybrid CAES system 100 is configured to switch between the heat storage supply mode, the heat source mode and the heat supply combination mode. In the example shown in Figure 1, the gas turbine 150 is configured to supply heat to the heat storage system 130. Heat from the heat storage system 130 can then be transferred to the air expander system 160 via heat exchangers 171, 172. However it will be appreciated that alternatively or in addition, the gas turbine 150, or another heat source, may be configured to supply heat directly to the air expander system 160. As described herein in relation to Figure 1, the hybrid CAES system is operating in a hybrid operation. However, it is also possible to operate the hybrid CAES system in an adiabatic operation or a diabatic operation. In the adiabatic operation, the heat storage system 130 is configured to remove heat from the compressor system 110, store heat, and supply heat to the air expander system 160 during the appropriate stages of operation. In the adiabatic operation, when the compressor system 110 is in operation the hybrid CAES system 100 is configured to operate in the heat storage mode and when the expander system 160 is in operation the hybrid CAES system 100 is configured to operate in the heat storage supply mode. In the diabatic operation, the cooling system 120 is configured to remove heat from the compressor system 110 and dissipate the removed heat, and the heat source, in this example the gas turbine 150, is configured to provide heat to the air expander system. In the diabatic operation, when the compressor system 110 is in operation the hybrid CAES system 100 is configured to operate in the dissipation mode and when the expander system 160 is in operation the hybrid CAES system 100 is configured to operate in the heat source mode. In the hybrid operation, a combination of the heat storage system 130 and the cooling system 120 are configured to remove heat from the compressor system 110. The heat source, in this example the gas turbine 150, is configured to supply heat to at least one of the heat storage system 130 and the air expander system 160, and the heat storage system 130 is configured to provide heat to the air expander system 160. In the hybrid operation, the hybrid CAES system 100 is configured to operate in a combination of the described modes. In the hybrid operation of the hybrid CAES system, heat is removed from the first compressor 111 by the heat storage system 130. Then air at intermediate pressure passes through the second compressor 112 where the air is compressed further, to a higher pressure. Heat is removed from the second compressor 112 by the heat storage system 130. The air then passes through the third compressor 113 where it is compressed to a storage pressure which is the pressure at which the air is to be stored. The storage pressure is between 10,000 kPa and 100,000 kPa, preferably 19,000 kPa. Heat is removed from the third compressor 113 by the heat storage system 130 and the cooling system heat exchanger 121. After being compressed to a high, storage air pressure, and cooled to a suitable temperature, the compressed air output from the compressor system 110 is stored in the cavern 140. The air in the cavern 140 is stored at around 45 degrees Celsius. The air in the cavern is typically stored at a pressure of between 9 MPa and 20 MPa. During use, the pressure of the stored compressed air within the cavern is maintained between acceptable limits. These limits may be based on a number of factors, such as the physical constraints of the reservoir (e.g., the geology of the cavern), minimum and maximum operating parameters of the turbines to be powered by the compressed air, as well as legal or other administrative limits that may be set by the relevant authorities. For example, the maximum storage pressure in cavern 140 may be between 10,000 and 100,000 kPa, such as 20,000 kPa. The cavern may also have a minimum storage pressure. This may be, in particular, be determined by the minimum operating pressure of the air expanders that are powered from air from the reservoir. The minimum storage pressure may be, for example, between 1,000 and 10,000 kPa. Each cavern may have a volume in the range 100,00 to 1,000,000 m3, such as 500,000 m3 per cavern. Such caverns may provide in the region of 10 to 100 hours of power generation, and take in the region of 10 to 100 hours to fully fill from empty. For example, a hybrid CAES system 100 comprising two caverns having volumes of 500,000 m3 may take between 70 and 80 hours to fill using a compressor system 110 with a total power consumption of around 220 MW, and this may be capable of providing in the region of 84 hours of power using an air expander system with a total output of 330 MW. In response to an electricity demand, the hybrid CAES system operates to generate power. In response to the demand, the gas turbine 150 is operated to generate power via the gas turbine generator 155. Heat produced during operation of the gas turbine 150 is sent to the heat storage system 130 via the gas turbine heat exchanger 134. The gas turbine 150 can transition from a non-operational state to a maximum power generation state in a time of between 2 and 10 minutes, to respond to quickly to the electricity demand. Simultaneously, in response to an electricity demand, air can be withdrawn from the cavern 140. The air is pumped into the air expander system 160, which uses the air to generate power. The response time of the air expander system 160 to generate power is slower than the gas turbine 150. Upon leaving the cavern 140, the compressed air is relatively cold, typically at ambient air temperature or slightly above (e.g., 45 degrees Celsius). A heat exchanger may be used to pre-heat the air before it passes through the air expander system 160. In order for it to efficiently be used to power the generator 165, the high pressure air is heated as it passes through the air expander system. The heat storage system 130 is configured to heats the air to a high temperature, typically between 100 and 1,000 degrees Celsius, such as in the region 400-500 degrees Celsius. The compressed air passes into the first air expander 161 of the expander system. The air enters the first air expander 161 at the pressure at which it is stored or at a lower pressure. Heat is transferred from the heat storage system 130 to the air expander system 160 by the first heat exchanger 171. The first heat exchanger 171 is configured to transfer heat from the heat storage system 130 to at least one of the air passing through the first air expander 161, exiting the first air expander 161, or the air passing through the second air expander 162. This heats the air to a high temperature, typically between 100 and 1,000 degrees Celsius, such as in the region 400-500 degrees Celsius. The air then passes through the second air expander 162 to be expanded to a lower pressure. Heat is transferred from the heat storage system 130 to the air expander system 160 by the second heat exchanger 172. The second heat exchanger 172 is configured to transfer heat from the heat storage system 130 to at least one of the air passing through the second air expander 162, exiting the second air expander 162, or the air passing through the third air expander 163. The air is therefore once again heated to a high temperature, typically between 100 and 1,000 degrees Celsius, such as in the region 400-500 degrees Celsius. The air is then expanded in the third air expander 163. The air passing through the air expander system generates power in the form of electricity using the generator 165. Once the generator 165 is operating to the meet the electricity demand, operation of the gas turbine 150 can be stopped. The operation of the gas turbine 150 can be tapered to reduce the power output of the gas turbine 150, as the power output of the generator 165 increased. Alternatively, the gas turbine 150 may operate to produce a fixed target power output until the generator 165 reaches a target power setpoint, at which time the gas turbine operation is stopped. The compressor system is preferably designed to recharge the reservoir at a rate not less than the withdrawal rate of the expansion system. For example, the compressor system may be configured to be able to refill the reservoir at a multiple greater than 1.0 of the rate at which the reservoir can be emptied through the air expander system. For example, this ratio may be in the region of 1.3 (e.g., 1.3 ± 0.05). It will be appreciated that the Figure 1 is merely a schematic representation of a CAES system 100, and that many components, such as valves, controllers, sensors and the like have been omitted from the Figure and description for simplicity. Similarly, as also discussed above, additional compressors, compressor trains, air expanders, air expander trains, heat exchangers and reservoirs may be provided, amongst other things. Figure 2 is a schematic illustration according to a second embodiment of the present disclosure. The system of Figure 2 is a hybrid CAES system 200 wherein the heat source is a heat pump 250. The hybrid CAES system 200 is configured to operate in the modes and the adiabatic, diabatic and hybrid operation as described in relation to the first embodiment. In the CAES system 200 shown in Figure 2, the compressor system 210 comprises a first 211, a second 212, and a third 213 compressor. The compressor system 210 shown in Figure 2 comprises an electric motor 225 for powering the compressor system. In Figure 2, the electric motor is illustrated as a single electric motor for powering the first 211, second 212, and the third 213 compressors. However, it will be appreciated the compressor system 210 may comprise additional motors, for example each compressor may have a corresponding motor. The air entering the compressor system 210 is compressed by the compressors in series as described above with reference to Figure 1. Although not shown in Figure 2 for simplicity, the system 200 also comprises a cooling system for removing heat from the compressor system. The cooling system may be implemented as described in relation to the first embodiment. The air exiting the third compressor 213 is cooled by heat storage system. In particular, the heat storage exchanger 234 is configured to remove heat from the compressed air before it enters the storage cavern 240. The heat storage exchanger 234 removes heat from the output of the compressor 213 and transfers the heat to the first heat storage tank 232. Alternatively, heat may be transferred from the compressor system 210 to the heat storage system 230, in the same was as described in relation to the compressor system and heat storage tank of the example shown in Figure 1 above. For, example, heat from compressors, 211, 212 and 213 may be removed by a compressor heat exchanger and transferred to the heat storage tank 232 of the heat storage system 230. The compressed air is transferred from the heat storage exchanger 234 to the storage cavern 240. In the storage cavern the conditions are the same as those described above for the first embodiment. When there is a demand on the system, for example when demand from an energy grid is high, the compressed air is removed from the storage cavern 240 and transferred to the air expander system 260. The air is transferred to the air expander system 260 via the heat storage system 230. Firstly, heat is added to the compressed air from heat stored in the first heat storage tank 232. The heat storage system receives heat from both the compressor system 210, via the heat storage exchanger 234, and a heat source, which in this embodiment is a heat pump 250. The heat pump 250 comprises a heat intake from ambient air, subsurface heat, solar heat or heat integration from another system. For example, the heat pump may be a groundsource heat pump. The heat storage exchanger 234 supplies heat to the first heat storage tank 132. The first storage tank is at around ambient pressure of 101 kPa and comprises a heat storage material, the temperature of which reaches up to 100 degrees Celsius. For example, the heat storage material may be water and the temperature of the water in the first heat storage tank may reach a maximum of 98 degrees Celsius. The heat storage system shown in Figure 2 includes a second heat storage tank 236. The second heat storage tank is at a higher temperature and a higher pressure than the first heat storage tank 232. The second storage tank 236 has a lower volumetric capacity than the first storage tank 232. The heat pump is configured to take heat stored in the first storage tank 232 and increase the temperature of it and supply it to the second storage tank 236. Air exiting the cavern 240 passes through the first heat storage tank 232. In this instance “passes through” is used to describe compressed air that exchanges heat with the heat storage material in the first heat storage tank 232. For example, the compressed air may “pass through” a heat exchanger where the heat transfer material is the heat storage material. The temperature of the compressed air is increased by the first heat storage tank 232, this may be referred to as a pre-heating step, the compressed air exiting the first storage tank may be referred to as pre-heated compressed air. The temperature of the compressed air exiting the first heat storage tank may be at a maximum of 100 degrees Celsius. The preheated compressed air may then pass through the second heat storage tank 236. The heat from the second heat storage tank 236 further increases the temperature of the pre-heated compressed air. The compressed air exiting the second heat storage tank 236 may be at high temperatures of between 100 and 1,000 degrees Celsius, such as in the region 400-500 degrees Celsius. This compressed air may be referred to as high temperature compressed air. The high temperature compressed air then enters the air expander system 260. The air expander system 260 comprises a first 261, a second 262, and a third 263 air expander, as well as a generator 265. The air expander system 260 operates in the same way as described above for the air expander system of the first embodiment, except that the air expander system shown in Figure 2 does not include heat exchangers between each air expander. However, it is to be understood that the air expander system 260 may further comprise heat exchanger between the first 261 and second 262 air expanders and between the second 262 and third 263 air expanders. The heat exchangers between each air expander may receive heat from the heat storage system 230, in particular from the second heat storage tank 236. Alternatively, or in addition, the heat exchangers may receive heat from a heat source. The heat source may be the heat pump 250 or the heat source may be a separate or additional heat source such as an additional gas turbine. The air expander system 260 generates electrical power via the generator 265. Figure 3 shows a schematic illustration of a third embodiment of the present disclosure. The hybrid CAES system 300 shown in Figure 3 shares many features with the second embodiment of the present disclosure, so the description for the second embodiment may equally be applied to the third embodiment, except where described below. The same reference numerals are used for features that are the same for both the second and third embodiments. The hybrid CAES system 300 is configured to operate in the modes and the adiabatic, diabatic and hybrid operation as described in relation to the first embodiment. Although not shown in Figure 3 for simplicity, the system 300 also comprises a cooling system for removing heat from the compressor system. The cooling system may be implemented as described in relation to the first embodiment. The hybrid CAES system 300 of the third embodiment of the present disclosure comprises a heat source which comprises a gas turbine 350 and a generator 355. The gas turbine 350 and generator 355 operate in the same way as the gas turbine 150 and generator 155, described in relation to the first embodiment. The gas turbine 350 may be a natural gas or a natural gas and hydrogen fired turbine. The exhaust gas of the gas turbine 350 is used to supply heat to the heat storage system 230 and the air expander system 260. The heat in the exhaust gas generated by the gas turbine 350 is used to supply heat to the heat storage system and the air expander system via an economizer 352. As shown in Figure 3, the heat from the gas turbine exhaust gas is supplied to the economizer 352. The economizer 352 is configured to supply heat to the compressed air stream that has been heated by the heat storage system 236 and which is being directed to the air expander system 260. The economizer 352 is also configured to provide additional heat to the heat storage system 230, in particular by supplying heat to the second heat storage tank 236. Figure 4 shows a schematic illustration of a fourth embodiment of the present disclosure. The hybrid CAES system 400 shown in Figure 4 shares many features with the third embodiment of the present disclosure, so the description for the third embodiment may equally be applied to the fourth embodiment, except where described below. The same reference numerals are used for features that are the same for both the third and fourth embodiments. The hybrid CAES system 400 is configured to operate in the modes and the adiabatic, diabatic and hybrid operation as described in relation to the first embodiment. Although not shown in Figure 4 for simplicity, the system 400 also comprises a cooling system for removing heat from the compressor system. The cooling system may be implemented as described in relation to the first embodiment. The hybrid CAES system 400, shown in Figure 4, comprises a heat source. The heat source comprises a furnace 450. The fuel for the furnace 450 is a natural gas and / or hydrogen. The furnace 450 is configured to burn the fuel in oxygen. Burning the fuel produces heat in the form of hot exhaust gas. The furnace 450 is configured to supply heat from the hot exhaust gas to at least one of the heat storage system or to the air expander system. The hot exhaust gas passes through the economizer 352 and supplies heat to the heat storage system 230 and the air expander system as described above in relation to the third embodiment. Described above are a number of embodiments with various optional features. It should be appreciated that, with the exception of any mutually exclusive features, any combination of one or more of the optional features are possible.

Claims

1. A hybrid compressed air energy storage (CAES) system comprising:one or more compressed air reservoirs;a compressor system configured to compress air into the one or more air reservoirs; an air expander system configured to generate power using air released from the one or more compressed air reservoirs;a heat storage system for removing heat from the compressor system, storing heat, and supplying heat to the air expander system;a cooling system for removing heat from the compressor system and dissipating heat; anda heat source for supplying heat to at least one of the heat storage system and the air expander system.

2. The hybrid CAES system according to claim 1, wherein the hybrid CAES system is configured to be operated in at least two of a dissipation mode in which the cooling system is configured to remove heat from the compressor system, a heat storage mode in which the heat storage system is configured to remove heat from the compressor system, and a combination cooling mode in which the cooling system and the heat storage system are both configured to remove heat from the compressor system, wherein the hybrid CAES system is configured to switch between the at least two of the dissipation mode, the heat storage mode and the combination cooling mode.

3. The hybrid CAES system according to claim 1 or 2, wherein the hybrid CAES system is configured to be operated in at least two of a heat storage supply mode in which the heat storage system is configured to supply heat to the air expander system, a heat source mode in which the heat source is configured to supply heat to at least one of the heat storage system or the air expander system, and a heat supply combination mode in which the heat storage system is configured to supply heat to the air expander system and the heat source is configured to supply heat to at least one of the heat storage system or the air expander system, wherein the hybrid CAES system is configured to switch between the at least two of the heat storage supply mode, the heat source mode and the heat supply combination mode.

4. The hybrid CAES system according to claim 1 or 2, wherein the heat source comprises a gas turbine configured to provide heat to at least one of the heat storage system and the air expander system.

5. The hybrid CAES system according to claim 4, wherein the gas turbine is a dual natural gas / hydrogen fired gas turbine.

6. The hybrid CAES system according to any preceding claim, wherein the heat source comprises an electric heater configured to supply heat to the heat storage system.

7. The hybrid CAES system according to any preceding claim, wherein the heat source comprises a furnace configured to provide heat to at least one of the heat storage system and the air expander system.

8. The hybrid CAES system according to any preceding claim, wherein the heat source comprises one or more heat pumps.

9. The hybrid CAES system according to any preceding claim, wherein the hybrid CAES system is configured to initially supply heat to the air expander system using the heat source and to subsequently supply heat to the air expander system using the heat storage system.

10. The hybrid CAES system according to any preceding claim, wherein the hybrid CAES system is configured to initially remove heat from the compressor system using the heat storage system, until the heat storage system is full, and subsequently remove heat from the compressor system using the cooling system.

11. The hybrid CAES system according to any preceding claim, wherein the air inlet temperature to the air expander system is between 100 and 1000 degrees Celsius.

12. The hybrid CAES system according to any preceding claim, wherein the heat storage system is configured to remove heat from the compressor system using a heat-transfer material wherein optionally the heat-transfer material is water.

13. The hybrid CAES system according to any preceding claim, wherein the heat storage system comprises thermal energy storage for storing heat.

14. The hybrid CAES system according to claim 13, wherein the heat storage system comprises one or more thermal energy storage tanks.

15. The hybrid CAES system according to claim 14, wherein a first storage tank is at a first temperature and a second storage tank is at a second temperature and wherein the second temperature is greater than the first temperature.

16. The hybrid CAES system according to claim 15, wherein the first tank is at a first pressure and the second tank is at a second pressure, wherein the second pressure is greater than the first pressure.

17. The hybrid CAES system according to claim 15 or 16, wherein the heat source is configured to supply heat to the second storage tank.

18. The hybrid CAES system according to any preceding claim, wherein the compressor system is configured to compress air to a pressure of between 9 MPa and 20 MPa.

19. A method of operating a hybrid CAES system, comprising:compressing air into one or more compressed air reservoirs using a compressor system;removing heat from the compressor system using a heat storage system for storing heat, a cooling system for dissipating heat, or a combination of the heat storage system and the cooling system;releasing the compressed air from the one or more air reservoirs;supplying heat to an air expander system using the heat storage system, the heat source, or combination of the heat storage system and the heat source; andgenerating power using the air expander system and the air released from the one or more compressed air reservoirs.

20. The method according to claim 19, comprising:supplying heat to at least one of the heat storage system and the air expander system using the heat source.

21. The method according to claim 19 or 20, comprising operating the hybrid CAES system in at least two of:a dissipation mode in which the cooling system is configured to remove heat from the compressor system;a heat storage mode in which the heat storage system is configured to remove heat from the compressor system; anda combination cooling mode in which the cooling system and the heat storage system are both configured to remove heat from the compressor system;and comprising switching between the at least two of the dissipation mode, the heat storage mode and the combination cooling mode.

22. The method according to any of claims 19 to 21, comprising operating the hybrid CAES system in at least two of:a heat storage supply mode in which the heat storage system is configured to supply heat to the air expander system;a heat source mode in which the heat source is configured to supply heat to at least one of the heat storage system or the air expander system; anda heat supply combination mode in which the heat storage system is configured to supply heat to the air expander system and the heat source is configured to supply heat to at least one of the heat storage system or the air expander system;and comprising switching between the at least two of the heat storage supply mode, the heat source mode and the heat supply combination mode.

23. The method according to any one of claims 19 to 22, comprising initially supplying heat to the air expander system using the heat source and subsequently supplying heat to the air expander system using the heat storage system.

24. The method according to any one of claims 19 to 22, comprising initially supplying heat to the air expander system using the heat storage system and subsequently supplying heat to the air expander system using the heat source.

25. The method according to any one of claims 19 to 24, comprising initially removing heat from the compressor system using the heat storage system and subsequently removing heat from the compressor system using the cooling system.

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