The invention relates to a compressed air energy storage (CAES) system. In particular, the invention relates to a CAES system having a compressor system and a turbine system that are not coupled.

Renewable energy sources such as wind and solar power are increasingly relied upon to supply energy to the grid due to the need to reduce fossil fuel emissions. These renewable sources are variable by nature. For example, during periods of high winds wind turbines can produce large amounts of power, which can be in excess of the demand at that time.

During periods of low winds, which may or may not coincide with periods of high demand for electricity, wind turbines may not produce enough power to meet the demand.

Energy storage systems can be used to store energy harnessed from renewable sources. During periods of high electricity production excess energy can be stored. During periods of low electricity production, or high demand, the stored energy can be used to generate further electricity to supply to the grid.

Various energy storage solutions can be implemented to store surplus energy. Many of these solutions can only store the scale of energy required to balance the grid for a relatively the small period of time. For example, battery storage is useful but can be difficult to scale up to the size required for grid balancing. It is desirable to increase the duration of the energy storage, known as long duration energy storage (LDES). LDES can allow energy to be stored and then released over a prolonged period, advantageously allowing supply to be provided even during several hours of decreased production.

Compressed air energy storage (CAES) is one solution that can be used to store the excess energy available during periods of high production and to harness that energy during periods of low production or high demand to produce electricity. Typical CAES systems use a compressor during periods of excess production to compress air. The compressed air is stored in an underground cavern. When there is demand, the compressed air is drawn from the cavern and expanded through expansion turbines, to produce power for the grid. Typically CAES systems may have a slow response time to demands from the market. It would be desirable to provide a CAES system that is able to react quickly to changes in the supply or demand. It is desirable to achieve this flexibility whilst also achieving longer duration energy storage.

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.

The present invention provides a CAES system comprising one or more compressed air reservoirs, a compressor system configured to compress air into the one or more air reservoirs; and a turbine system configured to generate power using air released from the one or more compressed air reservoirs. The compressor system and the turbine system are not mechanically coupled such that the compressor system can operate in isolation, the turbine system can operate in isolation, and both the compressor system and the turbine system can operate at the same time.

Advantageously, because the compressor system and the turbine system are not mechanically coupled there may be greater flexibility in the operation of the CAES system. Due to the compression system and the turbine system being uncoupled, the operation of the compressor system may be controlled separately to the control of the turbine system. Therefore, a higher efficiency of the CAES system can be achieved from a partial load up to a full load operation. The CAES system has different operation modes. For example, during standard operation, both the compressor system and the turbine system may operate at the same time. When there is an increase in demand from the grid the compressor system may stop operating and the turbine system may operate in isolation. This may advantageously lead to a rapid increase in the net supply of power to the grid. Conversely, turbine system may stop operating and the compression system may operate in isolation if the demands on the grid are low or there is an excess of power supply. This may rapidly use excess power and lead to an increased amount of air stored in the one or more compressed air reservoirs.

Optionally, the compressor system comprises a plurality of compressor trains, each compressor train arranged in parallel and configured to compress air at ambient pressure to a storage pressure 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. 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. Optionally, the one or more low pressure compressors may comprise a multi-stage low pressure compressor and/or optionally the one or more high pressure compressors may comprise high pressure compressor.

The multi-stage compressor may comprise an inter-stage cooling system. The inter-stage cooling system may be a water cooled system. The inter-stage cooling system may be cooled by the ambient air temperature. The inter-stage cooling system may be water cooled system where the water is cooled by the ambient air temperature. Advantageously, the inter-stage reduces the temperature of the air through the multi-stage low pressure compressor and may allow more efficient compression. Inter-stage cooling may reduce the total power needed for the air compression.

Optionally, the multi-stage low pressure 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, air being compressed may pass first through each stage of the low pressure compressors and then through the high pressure compressor.

Optionally, the low pressure compressor is configured to compress air from ambient pressure to between 1 ,000 kPa and 10,000 kPa, preferably 3,000 kPa. Optionally the high pressure compressor is configured to compress air to between 10,000 kPa and 100,000 kPa, preferably 19,000 kPa.

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 may 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.

Each of the plurality of compressor trains may further comprise a condenser. The condenser may be configured to remove excess moisture form the compressed air. The condenser may be situated between the outlet of a low pressure compressor and an inlet of the high pressure condenser.

Optionally, the turbine system comprises a plurality of turbine trains, each turbine train arranged in parallel . Each turbine train may be configured to generate power as compressed air from the compressed air reservoir passes through the turbine train.

Optionally, any one or more of the plurality of turbine trains may operate simultaneously.

A plurality of turbine trains allows for a greater throughput of air than a single turbine train of the same size as one of the plurality of turbine trains.

A plurality of turbine trains arranged in parallel allows a greater flexibility than a single turbine train because at least one turbine train could be operating while at least one other turbine 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 turbine trains could be operated, whereas when a small amount of power is needed then, for example, only one turbine train could be operated. Operating one turbine train out of a plurality of turbine trains may be more efficient than operating a single turbine train at a lowered capacity. Multiple turbine trains can also increase the responsivity of the turbine system compared to using a single, large turbine train.

The plurality of turbine trains may comprise two turbine trains. Two turbine trains advantageously may provide a preferable capacity of the system. Two turbine trains may also allow a large amount of flexibility in the system.

Each of the plurality of turbine trains may comprise a plurality of turbines arranged in series along an airflow path. Operating a plurality of turbines in series along an air flow path advantageously increases the power generation achieved by a turbine train.

The plurality of turbines may comprise one or more high pressure turbines and one or more low pressure turbines. The plurality of turbines comprising one or more high pressure turbines and one or more low pressure turbines allows the most appropriate turbines to be selected for the amount of power generation required. This may lead to more efficient power generation and increase the flexibility of the turbines system. The one or more high pressure turbines may have an air input pressure between 5,000 and 20,000 kPa, such as 15,000 kPa.

The one or more low pressure turbines may have an air input at a pressure between 2,000 kPa and 3,000 kPa, such as 2,500 kPa.

The plurality of turbines may comprise a two-stage high pressure turbine and a low pressure turbine.

The two-stage high pressure turbine may comprise one or more air exchangers used to cool and/or mix air for secondary air uses, including for cooling the turbines as turbine cooling air. A control mechanism can allow for part or no-load operations.

Optionally, compressed air may pass first through each stage of the high pressure turbine and then through the low pressure turbine.

The turbine system may further comprise one or more bypasses, each of the one or more bypasses allowing air to bypass one or more of the plurality of turbines or turbine stages. In this manner, the turbine trains within the turbine system can be efficiently operated over a wider range of operational power output. One or more stages of the high-pressure turbine may be partially of fully bypassed, for example using control valves, to vary the power production. If all high pressure turbines and high pressure turbine stages are fully bypassed, air will go directly into the low pressure turbines.

Optionally, each turbine train may comprise a fuel combustor situated on an airflow path prior to the one or more low pressure turbines to heat air before it enters the one or more low pressure turbines.

The fuel combustor may comprise a pre-mixing system. The pre-mixing system may allow air to be pre-mixed with a fuel prior to combustion. This pre-mixing may allow a lower flame temperature and therefore lower emissions, for example lower NOx production and lower carbon monoxide production. The low pressure combustor may be a dry low emission combustor.

The fuel combustor may be configured to burn natural gas, hydrogen, or both natural gas and hydrogen alone or as a mixture. Burning hydrogen may be more environmentally friendly than burning natural gas, whilst the ability to burn a mixture of the two increases the flexibility and adaptability of the system.

After air heated by the fuel combustor has passed through the one or more low pressure turbines it may be used to heat air before it enters one or more of the high pressure turbines via one or more heat exchangers. Such pre-heating of the air prior to entering a turbine or turbine stage can increase the efficiency of the turbine and of the turbine system as a whole.

For each of the plurality of turbine trains, each of the plurality of turbines in the turbine train may be connected via a common shaft.

Each of the plurality of turbine trains may be configured to have an operational power generation range of between 10 MW and 200 MW, and preferably of between 16 MW and 165 MW.

Each of the one or more low pressure turbines may be configured to have an operational power generation range of between 5 MW and 150 MW, preferably between 6 MW and 126 MW. Each of the one or more high pressure turbines may be configured to have an operational power generation range of between 10 MW and 100MW, preferably between 12 MW and 47 MW.

The CAES system at full capacity may be configured such that each turbine train generates 165 MW of electricity. The CAES system may comprise two turbine trains that can generate 330 MW at full capacity and the CAES system may be configured to consume 220 MW of electricity at maximum consumption using four compressor trains, which individually may consume up to 55 MW.

Each of the turbine trains may be configured to change from outputting no power to outputting maximum power within 20 minutes, preferably within 15 minutes, more preferably with 10 minutes. This may be achieved using multiple turbine trains and multiple turbines within each turbine train, and allows for increased flexibility and applicability of the CAES system.

Each of the plurality of turbine trains may have a maximum power output. Each of the plurality of turbine trains may be configured to modulate its power output at a rate of 10% of the maximum power output per minute, preferably at a rate of 15% of the maximum power output per minute, more preferably at a rate of 20% of the maximum power output per minute.

Each of the plurality of turbine trains may have a maximum power output, and each of the plurality of turbine trains may output power in a range of between 20% and 100% of the maximum power output, preferably in a range of between 10% and 100% of the maximum power output, more preferably in a range of between 5% and 100% of the maximum power output.

The one or more compressed air reservoirs may be configured to store air at pressures 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.

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 turbine system.

The fuel combustor of the CAES system may be configured to burn at least in part hydrogen from the hydrogen storage means.

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 the invention.

Figure 2 illustrates a representation of an energy grid comprising a CAES system according to the invention.

Like reference numbers are used for like elements throughout the description and figures.

Detailed Description of the Invention

Figure 1 shows a diagram of a compressed air energy storage (CAES) system 100 according to the invention. The CAES system 100 includes an air inlet 101 that is configured to draw in atmospheric air. The air inlet 101 may include a filter to remove contaminants and prevent them from entering other stages of the CAES system. For example, the filter may be configured to prevent large objects (such as birds, sticks, leaves, etc.) from entering the CAES system. In addition or alternatively, the filter may also filter particulates or other smaller contaminants, such as grit, dust, and so forth.

After passing into the CAES system 100 via the air inlet 101, the air enters a compressor train. The compressor train comprises a low pressure compressor 102 and a high pressure compressor 105. As shown in Figure 1 , the CAES system 100 is illustrated having one compressor train, however, it will be appreciated that the CAES system 100 may comprises additional compressor trains. In general, the CAES system 100 may comprise a plurality of compressor trains, for example four compressor trains. If the CAES system 100 comprises more than one compressor train, the compressor trains are arranged in parallel. That is, they each take in atmospheric air and output compressed air for storage - an air parcel can flow through any one of the compressor trains but not more than one. The low pressure compressor 102 illustrated is a multi-stage compressor with an interstage cooling system 103. The low pressure compressor 102 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. In the example shown in Figure 1 , the low pressure compressor 102 comprises five stages, with a heat exchanger providing cooling between each stage. For example, heat exchanger 103a cools the air output from low pressure compressor stage 102a, prior to that output air entering low pressure compressor stage 102b.

It will be appreciated that a low pressure compressor 102 having five stages is merely exemplary, and that another number of stages may be used, including just a single stage. For example, the low pressure compressor 102may comprise four stages or six stages. The inter-stage cooling system 103 may be any suitable cooling system, but as illustrated comprises a plurality water-cooled heat exchangers (e.g., heat exchanger 103a). In order to reduce the energy consumption of the system (that is, the energy lost when compressing air, rather the energy stored in the compressed air), ambient temperatures can be used to cool the heat-exchange water to minimise the energy requirements of the system. That is, the water used as coolant may be at an ambient temperature (typically around sixteen degrees Celsius at European latitudes). The requirements of the inter-stage cooling system 103 will in part depend on the type of low pressure compressor 102 used, and in particular the number of stages of the low pressure compressor 102. For example, if more stages are used then each individual stage will generate less heat and so the cooling needed between each stage will be less than if fewer stages are used, whereby each stage will generate more heat.

The CAES system 100 also comprises a high pressure compressor 105. Air enters the high pressure compressor 105 at the intermediate air pressure, after passing through the low pressure compressor 102, and is compressed to a high pressure by the high pressure compressor 105. This high pressure is the pressure at which the air is to be stored.

The high pressure compressor 105 may comprise a single stage or a plurality of stages. The high pressure compressor 105 is coupled to the outlet of the low pressure compressor 102. The high pressure compressor 105 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. As shown in Figure 1 , the high pressure compressor 105 comprises two stages 105a, 105b with an inter-stage cooling system 106. As for the low pressure inter-stage cooling system 103, the inter-stage cooling system 106 for the high pressure compressor 105 may be any suitable cooling system (and may or may not be of the same type as the low pressure cooling system 103). For example, the inter-stage cooling system 106 may comprise water- cooled heat exchangers. Ambient temperatures can be used to cool the heat-exchange water to minimise the energy requirements of the system, as discussed above for the low pressure inter-stage cooling system 103.

The compressor train has an electric motor 104 that is configured to supply power to both the low pressure compressor 102 and the high pressure compressor 105, and, depending upon the precise low pressure compressor 102 and high pressure compressor 105 used, one or more gearboxes may also be present. The compressor train is configured to operate between a minimum setpoint and a maximum setpoint. Suitable minimum setpoints 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 set point of 55 MW.

One or more of the compressor trains (for example, two out of four compressor trains) can have variable speed drive systems (VSDS), which allows greater flexibility in the compressor system to operate at low power consumption values.

After being compressed to a high, storage air pressure, the compressed air output from the high pressure compressor 105 is stored in one or more compressed air reservoirs, such as underground caverns 107. During use, the pressure of the stored compressed air within the caverns 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 107 may be between 10,000 and 100,000 kPa, such as 20,000 kPa. The maximum storage pressure in the 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 compressors and associated components described above provide compressed air that can be stored by the CAES system 100, in this case in caverns 107. Additionally, the CAES system 100 further comprises a turbine system that extracts the stored compressed air and uses it to generate power. Figure 1 shows a turbine system with a single turbine train, although it will be appreciated that more than one turbine train may also be used. For example, the turbine system may comprise two turbine trains. If multiple turbine trains are used, they should be arranged in parallel. That is, each turbine train takes air from the reservoir (e.g., cavern 107) and expands it across the turbines of the turbine train - an air parcel can flow through any one of the turbine trains but not more than one.

Upon leaving the reservoir, illustrated as cavern 107, the compressed air is relatively cold, typically at ambient air temperature or slightly above (e.g., 30 degrees Celsius). In order for it to efficiently be used to power the turbines, the high pressure air is preferably heated prior to entering any turbines. This can be done using a heat exchanger to transfer heat from the exhaust air to the high pressure air (illustrated with heat exchangers 114a, 114b). This typically 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 heat exchangers may be any suitable heat exchangers, and may be selected based on the source of the heat (which may not necessarily be the exhaust air), amongst other considerations.

After being pre-heated in such a manner, the compressed air passes into the turbines of the turbine train. The turbine train shown in Figure 1 comprises a high pressure turbine 108, which uses relatively high pressure air. As illustrated, the high pressure turbine 108 comprises two stages, though it will be understood that more or fewer (e.g., one) stages may be used. The air may enter the high pressure turbine 108 at the pressure at which it is stored or at a lower pressure, for example, between 1 ,000 and 10,000 kPa, such as 5,000 kPa. Generally speaking, the maximum pressure at which the air can enter the high pressure turbine 108 will be dependent on the pressure at which the air is stored, whereas the actual pressure used may be based upon the optimal operating parameters of the high pressure turbine 108. In particular, when the air enters the high pressure turbine 108 at a lower pressure than at which it can be stored (e.g., at 5,000 kPa compared to a maximum storage pressure of 19,000 kPa), the system may be configured to maintain the lower pressure for entry into the turbine across a wide range of storage pressures above that pressure (e.g., to account for a decrease in pressure as the reservoirs empty). After passing through the first stage 108a of the high pressure turbine 108, the air may once again be heated before passing through any subsequent stages 108b of the high pressure turbine 108. This heating can be in a similar manner to the heating that take place before air enters the first stage 108a of the high pressure turbine 108. For example, air exiting the first stage 108a of the high pressure turbine 108 can pass through heat exchanger 114c to take heat from the exhaust before entering the second stage 108b of the high pressure turbine 108.

The power generated by the high pressure turbine may be dependent on a number of factors, but in particular will depend upon the pressure at which the air enters the high pressure turbine 108 and the volume of air passing through the high pressure turbine 108 in a given time (i.e. , the flow rate of air through the high pressure turbine 108), as well as the number of stages. Preferably, the high pressure turbine 108 is configured to operate over a wide range of power output, for example, having a minimum power output and a maximum power output, with the minimum power output being around one tenth of the maximum power output. For example, the high pressure turbine 108 may provide between 1 and 10 MW (e.g., 5 MW) at minimum power output and between 10 and 1 ,00 MW (e.g., 47 MW) at maximum power output.

Subsequent to passing through the high pressure turbine 108, air then passes through the low pressure turbine 111. However, the air is again preferably heated to a high temperature prior to entering the low pressure turbine. This can be achieved using a low pressure combustor 110. In Figure 1 , a low pressure combustor 110 is coupled between an outlet of the high pressure turbine 108 and an inlet of the low pressure turbine 111. The low pressure combustor 110 may be mounted onto the low pressure turbine 111. Air in the low pressure combustor 110 is heated by combustion of a fuel. The low pressure combustor 110 uses a pre-mixing systemin which the air is pre-mixed with the fuel. This premixing allows a lower flame temperature and therefore lower emissions, for example lower NOx and lower carbon monoxide production. The low pressure combustor may be a dry low emission combustor.

The low pressure combustor 110 may use fossil fuels, such as natural gas, as fuel to heat the air from the high pressure turbine 108. Alternatively, other fuels may be used, such as hydrogen. In some cases, a mixture of hydrogen and natural gas may be used as fuel. The use of hydrogen as fuel may be preferred over other fuels such as natural gas because it is more environmentally sustainable. Once heated in this manner, if a low pressure combustor 110 is used, air passes through low pressure turbine 111. The low pressure turbine 111 expands the heated air to drive a turbine, with the output air being substantially at atmospheric pressure, or close to atmospheric pressure. The low pressure turbine 111 may have a maximum power generation of between 10 and 100 MW, for example, it may be capable of generating 125 MW. Preferably, the low pressure turbine 111 is configured to operate over a wide range of power outputs. For example, the low pressure turbine may be configures to have a minimum power output of between 10 and 100 MW and a maximum power output of between 100 and 1 ,000 MW. Whilst the illustrated low pressure turbine 111 comprises a single stage, it will be appreciated that multiple stages may be used in the low pressure turbine 111.

From the low pressure turbine, the air enters the exhaust system. Within the exhaust system, the hot air that has passed through the low pressure turbine 111 may be used to pass heat to air entering into the high pressure turbine 108, or between stages 108a, 108b of the high pressure turbine 108. This may be achieved using heat exchangers 114a, 114b, 114c as previously discussed. Furthermore, in order to better preheat the air using the heat exchanges 114a, 114b, 114c, the CAES system 108 may further comprise a duct burner 112 prior to at least one of any heat exchangers 114a, 114b, 114c (illustrated in Figure 1 as prior to all three heat exchangers 114a, 114b, 114c that are present). The duct burner is configured to burn fuel, which may be the same type of fuel or a different type of fuel to that used in the low pressure combustor 110, and heats the exhaust air such that it then provides more heat through the heat exchangers 114a, 114b, 114c.

The high pressure turbine 108 and the low pressure turbine 111 may both be connected to the same generator 109. The high pressure turbine 108 and the low pressure turbine 111 may be on the same shaft which passes through the generator 109. Having the high pressure turbine 108 and low pressure turbine 111 on the same shaft increases the moment of inertia of the turbine system which helps to stabilise the power output. On the other hand, it can make it more difficult to balance the system over a range of loads. However, this can be compensated by using multiple turbine trains which can operate independently, using one or more gearboxes, and uncoupling the compressor and turbine trains, all of which are discussed in more detail subsequently. One or more gearboxes may be present in the turbine train. In particular, a gearbox may be present between either or both of the high pressure turbine 108 and the low pressure turbine 111 and the generator 109. The presence and type of gearbox required will generally depend on the properties of the high pressure turbine 108, the low pressure turbine 111 , and the generator 109, and in particular the speeds of rotation at which they are configured to operate. A gearbox can also enable the generator 109 to operate efficiently over a wide range of air pressures output from the reservoir and power outputs.

The compressor system and the turbine system are not mechanically coupled such that the compressor system can operate in isolation, the turbine system can operate in isolation, and both the compressor system and the turbine system can operate at the same time. This is achieved by not having the turbines and compressors on the same shaft or having any other mechanical coupling between them. In this manner, each of the compressor system and the turbine system can operate independently of the other.

By operating the compressor system and the turbine system independently, the net power output of the CAES system 108 can be quickly increased or decreased. The net power output can de defined as the difference between the power consumed by the compressor system to compress air and store it in the reservoirs and the power generated by the turbine system as the stored air is released. In particular, it is helpful to consider the difference between the electrical power used and generated (that is, not including other forms of power used, for example, by burning fuel in the low pressure combustor 110 or the duct burner 112). A positive net power output corresponds to a net generation of power (i.e., the compressor system is consuming less power than the turbine system is generating) whereas a negative net power output corresponds to a net consumption of power (i.e., the compressor system is consuming more power than the turbine system is generating). For example, when the turbine system is operating but not the compressor system, the net power output will be positive. On the other hand, when the compressor system is operating but not the turbine system, the net power output will be negative. Considering the net electrical power output in this way, it can be seen that the CAES system 108 can be effectively used as part of a wider electrical power distribution network. The CAES system 108 can be used to store power at times where power generation exceeds demand (e.g., from wind turbines during a windy night when most people are asleep or from solar cells at midday when the light is at maximum intensity but electricity demand is low whilst people are working). That is, excess electricity can be used to drive the compressor which converts at least a portion of that electrical power into stored potential energy in the form of compressed air in the reservoirs. Then, when electricity demand exceed generation (e.g., if there is little wind or sunlight for wind turbines or solar cells respectively), at least a portion of the stored potential energy can be converted back into electricity by using the compressed air to drive the turbines, which in turn drive the generator.

The turbine system and the compressor system each take a certain period of time to change their power output and consumption respectively. For example, the compressor system may be able to transition from a non-operational state to a maximum power consumption state in a time of around four minutes. In some cases, it may be desirable for the net electrical power output of the CAES system 108 to increase faster than the rate at which the turbine system can increase its power output, of to decrease faster than the rate at which the compressor system can increase its power consumption (decreasing the net power output). If such rapid changes in net power output are envisaged to be required, for example in frequency control applications in an electricity grid, then the uncoupled compressor system and turbine system can both be run simultaneously. This may give a small net power output (positive or negative), because a portion of the power generated and consumed by the CAES system 108 will “cancel out”, but it allows very rapid changes in the net power output. This is because both the rate of power consumption of the compressor system and the rate of power generation by the turbine system can be changed simultaneously. For example, to increase the net power output rapidly, the power consumed by the compressor system can be decreased as well as the power generated by the turbine system being increased. Similarly, to decrease the net power output rapidly, the power generated by the turbine system can be decreased as well as the power consumed by the compressor system being increased. In this manner, the net power output can be varied at a greatly increased rate compared to systems having coupled turbine and compressor systems.

To provide additional flexibility, the compressor system and/or the turbine system can comprise multiple trains in parallel, whereby each of these trains can operate independently, as discussed above. For example, if the compressor system comprises four compressor trains, only one train may be in operation while the other three trains may not. Alternatively, two, three or all four trains may be in operation simultaneously. Similarly, if the turbine system comprises two trains, either just one of the trains may be in operation while the other is not or both trains may be in operation simultaneously. By having multiple trains in this manner, both the range of power output or consumed by the turbine system or the compressor system respectively can be increased. This is because, whilst each train only has a certain operational power range, the number of trains in use can be varied. This can give a large power range from the lowest operating power of a signal train to the maximum operating power of all of the trains combined. For example, this means that whilst each individual compressor train may be able to operate at no less 65% of the maximum power consumption, with four compressor trains this means that the compression system can operate at a minimum of around a quarter of the maximum power consumption. Furthermore, an increased number of trains can also contribute to the ability to rapidly change the net power output. This is because multiple trains can have their power consumption or generation (depending on whether the train is compressor or turbine train respectively) decreased or increased simultaneously. This can increase the rate of power change by a factor equal to the number of trains, compared to a single train system. In addition, if using multiple trains, when compared to using a single train for the same power consumption or generation, the individual turbines or compressors will likely be more reactive due to their smaller size providing increased rate of change benefits. The ability to switch on and off smaller compressors and/or turbines also makes it possible to vary power consumption by a discrete amount in a few minutes, which might be needed for providing ancillary services like automatic Frequency Restoration Reserve (aFRR) to an electricity network. A further advantage of using multiple trains in the compressor and/or turbine systems is the increased redundancy that such multiple trains provide.

It will be appreciated that at different powers the compressor system and turbine system may be varied. With respect to the compressor system, at different power consumptions some stages or compressors may be missed from the airflow path. With respect to the turbine system, at different power outputs one or more of the compressor stages or compressors may be missed from the airflow path. Additionally, the low pressure combustor 110 and the duct burner 112 may or may not be used, as may one or more of the heat exchangers 114a, 114b, 114c. Furthermore, other mixing of relatively hot and cold air from different points along the turbine system may be utilised to achieve the desired power output.

It will also 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, turbines, turbine trains, heat exchangers and reservoirs may be provided, amongst other things.

The compressor system is also 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 turbine system. For example, this ratio may be in the region of 1.3 (e.g., 1.3 ± 0.05).

Turning now to Figure 2, an electrical power distribution network, or energy grid, 200 is shown, illustrating how a CAES system 100, such as that of Figure 1 , may be integrated into an energy grid 200. The energy grid 200 comprises a CAES system 100, as well as upstream power generation 201 and downstream power distribution and consumption 211. The upstream power generation 201 may include any electrical power generation means, such as fossil fuel power plants (e.g., utilising coal, oil or natural gas), nuclear power plants, as well as renewable energy generation means (e.g., wind, solar, hydroelectric, tidal, biomass). As illustrated in Figure 2, wind turbines 203 and solar cells 205 provide electrical power which can be utilised by the CAES system 100, for example via power cable 207. The CAES system 100 may utilise this electrical power from the upstream power generation 201 to compress air and store it the reservoirs of the CAES system 100, in caverns 107a, 107b as described with respect to Figure 1.

To generate electricity, the CAES system 100 may utilise the compressed air from caverns 107a, 107b, as described with respect to Figure 1. This may then be transmitted to downstream power distribution and consumption 211 , for example via power cable 215. As illustrated, the electricity generated by CAES system 100 may be provided back to the grid at substation 213.

Figure 2 also illustrates the use of caverns 107a, 107b as the reservoirs for the CAES system 100 in which the compressed air is stored. Storing the compressed air in underground caverns 107a, 107b can provide a safe, cost-effective means to store large volumes of compressed air. For example, each cavern may have a volume in the range 100,00 to 1 ,000,000 m ³ , such as 500,000 m ³ 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 CAES system 100 comprising two caverns 107a, 107b having volumes of 500,000 m ³ may take between 70 and 80 hours to fill using four compressor trains 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 two turbine trains with a total output of 330 MW. When multiple caverns 107a, 107b (or reservoirs more generally) are used, it is preferable to have them in fluid connection such that the pressure across all of the caverns 107a, 107b (or reservoirs) is equalised. This is achieved in Figure 2 by connecting the caverns 107a, 107b via pipelines 221a and 221b which join at connection 223. Air is then taken from this equalised cavern system via pipeline 225 to power the turbine system, or is input into the caverns also via pipeline 225 from the compressor system. Preferably, pipeline 225 comprises separate routes for air to be simultaneously input and withdrawn from the cavern system for simultaneous operation of the compressor system and the turbine system.

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.