Compressed gas energy storage system and a method for operating a compressed gas energy storage system

Abstract

A compressed gas energy storage system, and a method of operating a compressed gas energy storage system are disclosed. The compressed gas energy storage system (100) comprises: a compressor system (250) comprising at least one compressor (250a, 250b, 250c, 250d) for compressing gas; a compressed gas store (120) configured to receive and store gas compressed by the compressor system (250); and turbine system (260) comprising at least one turbine (260a, 260b) configured to be driven by gas discharged from the compressed gas store (120); wherein the compressed gas store (120) comprises: a plurality of consecutive sections (120_1, 120_2, …, 120_n) of a pipeline; and at least one valve (122_1, 122_2,…) for controlling a flow of gas between two consecutive sections of said plurality of consecutive sections (120_1, 120_2, …, 120_n) of the pipeline; wherein the compressed gas energy storage system (100) further comprises a controller (130) for controlling said at least one valve (122_1, 122_2, …).

Description

[0001] P393079.GB.01

[0002] 1

[0003] COMPRESSED GAS ENERGY STORAGE SYSTEM AND A METHOD FOR OPERATING A COMPRESSED GAS ENERGY STORAGE SYSTEM

[0004] FIELD

[0005] The present disclosure relates to a system and method for compressed gas energy storage. More particularly, but not exclusively, the present disclosure relates to a system for compressed gas energy storage comprising a compressed gas energy store in the form of a pipeline, and a method for operating the system.

[0006] BACKGROUND

[0007] There are multiple modes of electrical energy storage, each with their own advantages and disadvantages in terms of design, environmental impact, potential to scale-up, costs and the availability of any raw materials.

[0008] One promising grid-scale energy storage method is Compressed Gas Energy Storage (CGES), of which one type is Compressed Air Energy Storage (CAES). Energy is stored by converting surplus electrical energy into mechanical potential energy by means of compressing and storing air. At times of high demand, this high-pressure air is fed though an expansion train powering a turbine connected to a generator, feeding power back into an electrical power network.

[0009] It is a relatively mature, grid-scale method of energy storage, with the ability to support the deployment of flexible Renewable Energy Systems (RES), due to its high-power rating and long-term storage potential. A typical CAES system has three building blocks: a compression train (CT), compressed air storage (CAS), and an expansion train (ET). Surplus electricity is used to drive compressors in the CT, allowing it to be stored as mechanical potential energy in the form of compressed air. When there is a deficit in electrical supply, the high-pressure air is used to drive turbines in the ET. This converts the mechanical potential energy back into electricity, supplying power to the grid. Moreover, the technology benefits from fast start-up times (typically 10 to 12 minutes), high ramp rates of about 30%, and flexible load management.

[0010] There are two common categories of CAES that exist: adiabatic (A-CAES) and diabatic (D-CAES). These categories relate to the method of re-heating the air prior toP393079.GB.01

[0011] 2

[0012] entering the ET. During compression, the air is cooled to ambient temperature before storage. This reduces the CT work requirement, increases the energy density, and reduces thermal stresses in the CAS. However, to increase the efficiency and lifespan of the ET, re-heating of the air is required. Whereas D-CAES uses combustion chambers (CC), which are typically supplied with natural gas, A-CAES uses thermal energy storage (TES) to capture heat rejected during compression. This means A-CAES benefits from higher efficiencies, lower environmental impact, and lower operational expenditure (OPEX). However, the requirement of TES increases capital expenditure (CAPEX) for A-CAES and can limit the duration of storage.

[0013] A suitable CAS medium is a key requirement of CAES. Restrictive volume, pressure, and structural integrity needs mean that suitable options are limited. Currently, dissolution mined salt caverns are the most popular choice. Two commercial CAES plants, McIntosh and Huntorf, utilise this option. This is because salt caverns are typically large volume and the walls have a “self-healing” property, which results in low leakage at high pressures. Other options include disused mines and tunnels. These have been the focus of projects in Japan and Europe. However, this often results in high capital expenditure (CAPEX) at the outset and places geographic restrictions on where CAES plants can be located.P393079.GB.01

[0014] 3

[0015] SUMMARY OF THE DISCLOSURE

[0016] Aspects of the disclosure are set out in the accompanying claims. Combinations of features from the dependent claims may be combined with features of the independent claims as appropriate and not merely as explicitly set out in the claims.

[0017] According to an aspect of the disclosure, there is provided a compressed gas energy storage system, comprising:

[0018] a compressor system comprising at least one compressor for compressing gas; a compressed gas store configured to receive and store gas compressed by the compressor system; and

[0019] a turbine system comprising at least one turbine configured to be driven by gas discharged from the compressed gas store;

[0020] wherein the compressed gas store comprises:

[0021] a plurality of consecutive sections of a pipeline; and

[0022] at least one valve for controlling a flow of gas between two respective consecutive sections of said plurality of consecutive sections of the pipeline;

[0023] wherein the compressed gas energy storage system further comprises a controller for controlling said at least one valve.

[0024] Since the compressed gas store comprises a plurality of consecutive sections of a pipeline and at least one valve for controlling a flow of gas between two consecutive sections of said plurality of consecutive sections of the pipeline, the compressed gas store has a flexible size, enabling the compressed gas energy storage system to respond to fluctuating demands for energy storage on a range of timescales. The at least one valve may be controlled to support expansion or reduction of the compressed gas store to support efficient operation of the overall system. A further advantage of the compressed gas energy storage system is that controlling the at least one valve enables control of flow rates and pressure changes along the compressed gas store, thereby reducing frictional losses and improving operational lifetime of the system.P393079.GB.01

[0025] 4

[0026] In some embodiments, the controller is configured to control the at least one valve dependent upon a required storage volume and / or a predetermined quantity of gas to be added to or discharged from the compressed gas store.

[0027] Thus, the active volume of the compressed gas store may be adapted to demand. This may help to protect sections of the compressed gas store from exposure to frequent or extreme pressure changes, thus reducing fatigue events, and / or high flow rates, thus reducing frictional losses.

[0028] In some embodiments, the controller is configured to control said at least one valve prior to said predetermined quantity of gas being added to or discharged from the compressed gas store.

[0029] In some embodiments, the controller is configured to hold said at least one valve closed during charging or discharging of gas into the compressed gas store, and, subsequently, to open said at least one valve to allow gas to flow between at least two sections of the compressed gas store.

[0030] In this way, during charging / discharging of the compressed gas store, sections of the compressed gas store downstream of the valve may be protected from pressure changes and flow rates may be reduced. Subsequent distribution of gas to the downstream section or sections of the compressed gas store may take place over a longer time period and / or at lower flow rates, reducing frictional losses and rates of pressure change. This may also help extend the operational lifetime of the downstream sections, which may be more difficult to access and maintain.

[0031] In some embodiments, the two consecutive sections comprise a first section and a second section, said second section being located further from the compressor system relative to the first section; wherein the controller is configured to control said at least one valve dependent upon a pressure in said first section relative to a pressure in said second section.P393079.GB.01

[0032] 5

[0033] In some embodiments, in response to determining that a pressure in said first section is lower than a pressure in said second section, the controller is configured to hold said at least one valve closed while compressing gas into said first section to increase the pressure in said first section, prior to at least partially opening said at least one valve.

[0034] This may reduce the occurrence or intensity of pressure swings in one or more sections of the compressed gas store during charging of the compressed gas store. The controller may be configured to subsequently hold said at least one valve at least partially open during further charging of said compressed gas store.

[0035] In some embodiments, the controller is further configured to allow the first pressure to increase above the second pressure prior to opening said valve.

[0036] In some embodiments, in response to determining that a pressure in said first section is higher than a pressure in said second section, the controller is configured to hold said at least one valve closed while discharging gas from said first section to decrease the pressure in said first section, prior to at least partially opening said at least one valve.

[0037] This may reduce the occurrence or intensity of pressure swings in one or more sections of the compressed gas store during discharging of the compressed gas store. The controller may be configured to subsequently hold said at least one valve at least partially open during further discharging of said compressed gas store.

[0038] In some embodiments, the controller is configured to at least partially open said at least one valve when a pressure in said first section matches a pressure in said second section.

[0039] Allowing the respective pressures in consecutive sections of the compressed gas store to equalise prior to opening the valve to allow flow between the two sections may further reduce the occurrence or intensity of pressure swings in one or more sections of the compressed gas store.P393079.GB.01

[0040] 6

[0041] In some embodiments, the controller is configured to:

[0042] determine an operational schedule for the compressor system and turbine system based on expected storage demand;

[0043] using a fluid dynamic model of the compressed gas store, determine a valve control schedule for controlling the at least one valve, including opening timings and / or opening percentages of the at least one valve, that minimises expected energy losses in the compressed gas energy storage system during the determined operational schedule; and

[0044] control the at least one valve according to the determined valve timing schedule during operation of the compressor system and turbine system.

[0045] Accordingly, the sequence of valve opening timings and opening percentages may be optimised to reduce losses and thereby maximise system efficiency.

[0046] In some embodiments, said two consecutive sections comprise a first section and a second section, said second section being located further from the compressor system relative to the first section; and:

[0047] the second section of the pipeline is operated as an overflow and / or reserve for the first section of the pipeline, and / or

[0048] the first section of the pipeline is used for short term gas storage and the second section of the pipeline is used for longer term gas storage.

[0049] Operating the system in this way may extend the relative operational lifetime of the downstream section of the compressed gas store, which may be more difficult to access and / or maintain.

[0050] In some embodiments, the controller is configured to control said at least one valve to prevent a flow rate of gas in the compressed energy store exceeding 100m / s.

[0051] This may help to reduce frictional losses due to the walls of the compressed gas store.P393079.GB.01

[0052] 7

[0053] In some embodiments, the compressed gas energy storage system further comprises at least one of the following features:

[0054] (a) wherein the compressor system and the turbine system are coupled to the same end of the plurality of consecutive sections of the pipeline;

[0055] (b) wherein the compressed gas energy storage system further comprises a main valve for controlling flow of gas between the compressor system and the compressed gas store, and for controlling flow of gas between the compressed gas store and the turbine system; and / or

[0056] (c) wherein the compressed gas energy storage system further comprises a terminal valve for terminating the section of the plurality of consecutive sections of the pipeline which is furthest from the compressor system and / or the turbine system.

[0057] In some embodiments, the plurality of consecutive sections of the pipeline have a total length at least 100 times greater than the diameter of the pipeline.

[0058] In some embodiments, at least one of the plurality of consecutive sections of the pipeline has a length at least 100 times greater than its diameter.

[0059] In some embodiments, the plurality of consecutive sections of the pipeline have a total length at least 1000 times greater than the diameter of the pipeline.

[0060] In some embodiments, at least one of the plurality of consecutive sections of the pipeline has a length at least 1000 times greater than its diameter.

[0061] In some embodiments, the pipeline is a decommissioned pipeline previously used for transport of oil or gas. The pipeline may include an offshore pipeline.

[0062] In some embodiments, the compressed gas energy storage system further comprises:

[0063] a cooling system for removing heat from the gas when it is compressed; a thermal energy store, coupled to the cooling system for storing the heat removed from the compressed gas, for providing thermal energy for heating gasP393079.GB.01

[0064] 8

[0065] released from the compressed gas store prior to driving at least one turbine of the turbine system.

[0066] In some embodiments, the compressed gas energy storage system further comprises at least one heat exchanger for transferring heat from the thermal energy store to the compressed gas.

[0067] In some embodiments, the thermal energy store has a capacity for storing sufficient thermal energy for heating gas discharged from a section of the plurality of consecutive sections of the pipeline which is closest to the turbine system.

[0068] In some embodiments, the thermal energy store has a thermal energy capacity that is insufficient for heating gas discharged from all the sections of the pipeline comprised in the compressed gas store.

[0069] This may help to achieve a balance between avoiding redundancy (due to an underused thermal energy store) and improving efficiency (by capturing heat from the gas during compression for re-heating the gas during expansion).

[0070] In some embodiments, the compressed gas energy storage system further comprises a secondary heat source for providing further thermal energy for heating the gas released from the compressed energy store prior to driving at least one turbine of the turbine system.

[0071] In some embodiments, the secondary heat source comprises a combustion heater.

[0072] In some embodiments, during discharge of the compressed gas store, the gas released from the compressed gas store is heated using thermal energy from the thermal energy store prior to using heat from the secondary heat source.

[0073] In some embodiments, the compressed gas energy storage system comprises a multi-stage compression train comprising said at least one compressor.P393079.GB.01

[0074] 9

[0075] In some embodiments, the compressed gas energy storage system comprises a multi-stage expansion train comprising said at least one turbine.

[0076] In some embodiments, the compressor system is conductively coupled to an electrical energy source for providing electricity for powering the at least one compressor. The electrical energy source may include at least one intermittent electrical power generator. In some embodiments, the turbine system is configured to generate electrical power and is conductively coupled to an electrical network for distributing electrical power generated by the turbine system.

[0077] In some embodiments, the gas received and stored by the compressed gas store is air.

[0078] In some embodiments, the gas received and stored by the compressed gas store is hydrogen.

[0079] In some embodiments, the gas received and stored by the compressed gas store is natural gas.

[0080] In some embodiments, the gas received and stored by the compressed gas store is carbon dioxide.

[0081] According to another aspect of this disclosure, there is provided a method of operating a compressed gas energy storage system, comprising:

[0082] charging a compressed gas store by compressing gas into a compressed gas store;

[0083] driving at least one turbine, by discharging compressed gas from the compressed gas store through said at least one turbine;

[0084] wherein the compressed gas store comprises:

[0085] a plurality of consecutive sections of a pipeline; and

[0086] at least one valve for controlling a flow of gas between two respective consecutive sections of said plurality of consecutive sections of the pipeline;P393079.GB.01

[0087] 10

[0088] the method further comprising:

[0089] controlling said at least one valve to control the flow rate of gas between said respective consecutive sections of the pipeline during at least one of charging or discharging of the compressed gas store.

[0090] In the foregoing general description, references to a first section and a second section of the compressed gas store may refer to any two consecutive sections of the plurality of consecutive sections comprised in the compressed gas store.

[0091] It will be appreciated that any features described herein as being suitable for incorporation into one or more aspects or embodiments of the present disclosure are intended to be generalizable across any and all aspects and embodiments of the present disclosure. Other aspects of the present disclosure can be understood by those skilled in the art in light of the description, the claims, and the drawings of the present disclosure. The foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the claims.

[0092] BRIEF DESCRIPTION OF THE DRAWINGS

[0093] Example embodiments of the present disclosure will be described, by way of example only, with reference to the accompanying drawings in which like reference signs relate to like elements and in which:

[0094] Figure 1 schematically illustrates a compressed gas energy storage system according to an example embodiment of the present disclosure;

[0095] Figure 2 schematically illustrates the compressor / expander subsystem 110 (compressed gas energy storage power plant) and compressed gas energy store 120 of the compressed gas energy storage system of Figure 1 ; and

[0096] Figure 3 illustrates a method of operating the compressed gas energy storage system according to an example embodiment of the present disclosure.P393079.GB.01

[0097] 11

[0098] DETAILED DESCRIPTION

[0099] As society transitions toward renewable power supplies, the legacy assets of the oil / gas era may have limited use beyond their original design lifetimes. Hence, there is an opportunity to utilise, recycle and repurpose these assets for other applications.

[0100] One such opportunity is to repurpose a decommissioned pipeline as a compressed gas store in a compressed gas energy storage (CGES) system. Pipelines differ from typical gas storage vessels in that they may be many kilometres in length, with a length to diameter ratio of more than 1000: 1 , and therefore need to be managed in a different way. In addition, to offer compressed gas storage on various different timescales, for example 1 hour, 10 hours and 100 hours, the system needs to be flexible enough to be able to handle a significant amount of redundancy. This is generally a limitation of any form of long-term storage.

[0101] The use of a pipeline as a key component of a long-duration compressed gas energy storage system faces a number of technical challenges. Specifically, these include:

[0102] i. Undertaking an evaluation of the state of the pipeline.

[0103] ii. To protect energy conversion equipment, cleaning and removing unwanted remnants, materials and gases from the pipeline.

[0104] iii. To prevent significant failure, testing of the pipeline (including its welds) for its integrity for carrying other gases.

[0105] iv. To maximise overall system efficiency, minimising losses due to pipeline wall friction.

[0106] v. To maximise the efficiency of the energy conversion processes, improving flexibility of heat management equipment for long duration storage.

[0107] vi. To maximise system operational lifetime, minimising extreme and frequent pressure loading events since the pipeline would have been originally designed to operate with a near constant pressure or small pressure swings.

[0108] vii. Ensuring the energy conversion equipment is suitably sized for the pipeline compressed gas store. The size of the store has to be appropriate to serve the energy storage market. In general, this is unknown at the design stage as the market is still being established.P393079.GB.01

[0109] 12

[0110] The compressed gas energy storage system disclosed herein, and its method of operation, are configured or adapted to address at least some of the above challenges associated with using a pipeline as a compressed gas energy store.

[0111] Figure 1 illustrates a compressed gas energy storage (CGES) system 100 according to an example embodiment of the present disclosure. The compressed gas energy storage system 100 includes a compressed gas energy storage power plant 110, for compressing / expanding gas (e.g. air, hydrogen, carbon dioxide, or another gas), a compressed gas store 120 for storing compressed gas, and a controller in the form of control system 130.

[0112] The compressed gas energy storage power plant 110, in the form of a compressor / expander subsystem (see Figure 2 for details), is arranged to supply compressed gas via at least one compressor (not shown in Figure 1) to the compressed gas store 120, and to expand gas released from the compressed gas store 120 via turning at least one turbine (not shown in Figure 1) to generate electrical power. The compressed gas energy storage power plant 110 is electrically coupled to an electricity network 140 (or an intermittent energy supplier) via an electrical power coupling 142. The electricity network 140 provides a source / demand of electricity requiring energy storage to balance supply / demand intermittency. Surplus electricity provided by the electricity network 140 is used to drive the compressor(s) in the compressor / expander subsystem 110, allowing the electrical energy to be stored as mechanical potential energy in the form of compressed gas stored in the compressed gas store 120 at an elevated pressure. When there is a demand for electrical energy, compressed gas is discharged from the compressed gas store 120 and used to drive the turbine(s) in the compressor / expander subsystem 110, thereby converting the mechanical potential energy back into electricity, supplying power to the electricity network 140. Although a single electricity network 140 is shown in Figure 1, the compressed gas energy storage power plant 110 may be connected to multiple electricity networks and / or suppliers / users. For example, one or more intermittent energy generators (e.g., renewable energy generators) may be connected to the CGES power plant 110 to provide a power source, while one or more other networks or users may be connected to the CGES power plant 110 providing a demand for power.P393079.GB.01

[0113] 13

[0114] The compressed gas energy store 120 comprises a plurality of consecutive sections 120_1 , 120_2, ...120_n of a pipeline. The pipeline itself may have been used to transport gases and is typically one long pipe extending in one length for many kilometres. For use as a CGES storage medium, it is divided into n sections 120_1 , 120_2, ..., 120_n by installing (n-1) two-way valves 122_1, 122_2, ..., 122_(n-1). ... at intervals along the pipeline. Each of the two-way valves 122_1 , 122_2, ... , 122_(n-1 ) is located between two consecutive or adjacent sections of the pipeline and controls flow of gas between those two sections. For example, a first two-way valve 122_1 controls flow of gas between the first section 120_1 and second section 120_2 of the pipeline 120. An (n-1 )th two way valve 122_(n-1 ) controls the flow of gas between the (n-1 )th section 120_(n-1) and the nth section 120_n of the pipeline 120. The ithsection 122_i of the pipeline 120 has an instantaneous pressure Pi and an average flow velocity (averaged across its length) of Vj, which vary with time as gas is charged into or discharged from the compressed gas store 120.

[0115] As an example, the length of the pipeline 120 may be up to 250km or more. The diameter of the pipeline 120, and / or of each pipeline section 120_1, 120_2, ..., 120_n, may be, for example, in the range 0.25m to 1.25m. Each of the pipeline sections 120_1, 120_2, ..., 120_n may have a ratio of diameter to length of 1:100 or more. The length of a single pipeline section 120_1, 120_2, ..., 120_n may be, for example, 100m or more, 100km or more, or several hundreds of kilometres. In some embodiments, the length of a single pipeline section 120_1, 120_2, ..., 120_n may be in the range of 50km to 150km. The lengths of the pipeline sections 120_1 , 120_2, ... , 120_n may be the same or different from each other. Although Figure 1 illustrates an embodiment in which the pipeline compressed gas store 120 comprises three or more sections, the compressed gas store 120 may be divided into a total number n of sections equal to 2 or more.

[0116] In addition, flow of gas between the CGES power plant 110 and the pipeline compressed gas store 120 (in the example embodiment of Figure 1, between the CGES power plant 110 and one end of the compressed gas store 120, that is, the first section 120_1 ), is controlled by a main valve 124, in the form of a two-way valve. The compressed gas store 120 is terminated at its other end, beyond the nth section 120_n, by a terminal valve 126. The terminal valve 126 may seal the end of the pipeline compressed gas store 120 from its original use.P393079.GB.01

[0117] 14

[0118] The two-way valves 122_1, 122_2, 122_(n-1 ), the main valve 124 and the terminal valve 126, are remotely controlled by the control system 130.

[0119] Each section of the pipeline 120_1, 120_2, 120_n is provided with a respective pressure sensor (not shown), arranged and configured for outputting a signal representing a pressure of the compressed gas stored in the respective section 120_1 , 120_2, ..., 120_n. The control system 130 may receive the respective signals output by each pressure sensor.

[0120] The pipeline sections closest to the CGES powerplant would typically be used for shorter term storage whilst those further away (further downstream) from the CGES powerplant would typically be used for longer duration or seasonal storage. The first section 120_1 of the pipeline compressed gas store 120 would typically be used to meet short term energy storage demands. The second section 120_2 of the pipeline compressed gas store 120 would typically be used for overflow from the first section 120_1 or to support longer term storage than can be supported from the first section 120_1 alone. Sections further downstream, including the nth pipeline section 120_n, would typically be used for offering medium to long term storage capacity. Some sections may have long periods during which they are inactive and sealed off from other sections further upstream towards the compressor system. They may be used as overflow for the upstream sections and as reserves for those sections if they are expected to be diminished below their minimum operating pressures.

[0121] Figure 2 schematically illustrates the compressor / expander subsystem 110 (compressed gas energy storage power plant) and compressed gas energy store 120 of the compressed gas energy storage system 100 of Figure 1. For simplicity, the valves of the compressed gas store 120 and the control system 130 are not shown in Figure 2. The compressor / expander subsystem 110 (the CGES powerplant) includes a compression system 250 in the form of a compression train 250 comprising at least one compressor (C) 250a, 250b, 250c, 250d, a turbine system 260 in the form of an expansion train 260 comprising at least one turbine (T) 260a, 260b, a thermal energy store (TES) 270, a cooling system 280 comprising heat exchangers 280a-d for transferring heat from the compressed gas to the thermal energy store 270, a heating system 290 comprising heat exchangers 290a-b, for transferring heat from the thermal energy store 270 to gas released from the compressed gas store 120 for driving at least one of the turbine(s) 260a, b, and a secondary heat source provided by at leastP393079.GB.01

[0122] 15

[0123] one combustion heater (CC) 290a, 290b. A thermocouple located in the thermal store 270 is used to measure the state of charge (stored thermal energy) of the thermal store 270.

[0124] In the embodiment shown in Figure 2, the compression train 250 is a multistage compression train 250 comprising multiple compressors (C) 250a-d. Four compressors 250 a-d are shown in Figure 2, but a different number of compression stages may be used, as required. Similarly, the expansion train 260 is a multi-stage expansion train 260 comprising multiple turbines (T) 260a, 260b. Two turbines 260a-b are shown in Figure 2, but a different number of expansion stages may be used, as required. The maximum sizes of the compression train 250 and expansion train 260 should only be sufficient to provide enough flowrate such that flow velocities in the pipeline do not exceed 100m / s for as long as is necessary.

[0125] Gas enters the compression train 250 at inlet 252, then passes through each of the compressors 250a-d of the compression train 250 in turn. The compressors 250a-d are electrical compressors driven by surplus electricity provided by the electricity network 140. The skilled person will appreciate that different numbers and arrangements of compressors 250a-d and heat exchangers 280a-d may be used, with the compressor arranged in series and / or parallel. After compression by the last compressor 250d of the compression train 250, the gas stream passes into the compressed gas store 120 for storage. During compression, the cooling system 280 cools the gas to ambient temperature before storage, to reduce the work requirement of the compression train, increase energy density and reduce thermal stresses in the compressed gas store 120. In this embodiment, the cooling system 280 comprises heat exchangers 280a-d, in the form of gas-liquid heat exchangers, which are arranged in the gas flow path after each compressor 250a-d. Heat captured from the gas stream by the heat exchangers 280a-d is transferred to the thermal energy store 270. The thermal energy store 270 may comprise, for example, a phase change material, a packed bed, a liquid, a solid, sand, and / or a thermochemical. Coolant may be circulated between the heat exchangers 280a-d of the cooling system 280 as indicated by the arrows in Figure 2.

[0126] When there is a demand for electrical power by the electricity network 140, gas is discharged (released) from the compressed gas store 120 and passes through each of the turbines 260a-b of the expansion train 260 in turn. The expanded gasP393079.GB.01

[0127] 16

[0128] (potentially including some pollutants) is then released to the atmosphere through outlet 262. The gas released from the compressed gas store 120 needs to be reheated to increase the efficiency and lifespan of the expansion train 260. This is at least partially achieved by the heat exchangers 290a, b of the heating system 290. The liquid-gas heat exchangers 290a, b transfer heat from the thermal energy store 270 (i.e., the heat rejected from the gas stream during compression) to the gas stream released from the compressed gas store 120, prior to the gas stream passing through at least one of the turbines 260a, b of the expansion train 260. Heat is circulated between the heat exchangers 292a-c of the heating system as indicated by the arrows. Any residual heat remaining in the expanded gas exiting the final turbine 260b of the expansion train 260 is returned to the gas stream entering the expansion train 260 via one of the heat exchangers 290a. In addition, the heating system 290 includes a secondary heat source 292, in the form of combustion heaters 292a, 292b, which provide additional heating as required. Typically, the combustion heaters 292a, 292b bum combustion fuel in the form of natural gas or hydrogen to provide heat to the expanding gas stream. The skilled person will appreciate that different numbers and arrangements of expanders (turbines) 260a, b, heat exchangers 290a-c and / or combustion chambers 292a, b may be used, with the turbines arranged in series and / or parallel.

[0129] The thermal energy store 270 preferably contains enough storage capacity to meet the thermal demand of discharging the first section 120_1 of the pipeline compressed gas store 120 through the turbine system 260. However, to make efficient use of resources and equipment, the capacity of the thermal energy store 270 is preferably insufficient to meet the thermal demand for discharging all sections of the pipeline compressed gas store 120 through the turbine system 260. In some embodiments, the thermal energy store 270 is sized to meet the thermal demand of discharging the full capacity of the first section 120_1 only. If the demand for electrical power requires that a greater volume of gas is released from the store 120 (i.e., by discharging multiple sections of the compressed gas store 120), then secondary heat source 292a, b (i.e., the combustion heaters 292a, b) is used to provide the additional thermal energy to heat the rest of the discharged gas. Preferably, when discharging more than one section 120_1, 120_2, ...120_n of the compressed gas store 120, the thermal energy store 270 is used first, prior to using the secondary heat source 292.P393079.GB.01

[0130] 17

[0131] When designing the compressed gas energy storage system 100, the size or capacity of the thermal energy store 270 may be adapted to the expected pattern of demand (intensity, duration and frequency of electrical power demands). For example, when the system 100 is used for short term energy storage, the first section 120_1 may be charged and discharged frequently, while other sections are isolated by closing the valve 122_1. The thermal energy store 270 may be sized to store sufficient thermal energy for this usage. Then, the secondary heat source 292 is only required when the system 100 is responding to longer-term energy demands.

[0132] Accordingly, the compressed gas energy storage system disclosed above is a hybrid system, combining the benefits of both adiabatic CGES (A-CGES) and diabatic CGES (D-CGES). In particular, the size of the thermal energy store 270 is selected to provide an optimum balance between operational efficiency and use of resources, adapted to the configuration of the pipeline compressed gas store 120 described above. For example, the thermal energy capacity of the thermal energy store 270 may be the equivalent energy required to discharge the first pipeline section 120-1 of the pipeline 120, or up to around 30% or less of full charge duration. In this way, the efficiency of the energy conversion processes may be increased by the improved flexibility of the heat management equipment for long duration storage.

[0133] To achieve a high operational efficiency and long operational lifetime of the compressed gas energy storage system 100, it is necessary to take into account the specific configuration of the compressed gas store 120.

[0134] Firstly, since the pipeline would typically have been originally designed to operate with a near constant pressure or small pressure swings, extreme and / or frequent pressure loading events may lead to damage or ageing of the pipeline and a reduction in operational lifetime of the compressed gas store 120. Therefore, it is important to operate the pipeline compressed gas store 120 with minimal pressure changes and to reduce the frequency of pressure swings to avoid pipeline stress fatigue. This may be achieved through the location and timing of the opening / closing (including the opening percentages) of the valves 122_1, 122_2, ..., 122_(n-1). For example, relatively rapid or extreme pressure changes in one or more sections (e.g., the first section 120_1) of the pipeline compressed gas store 120 closest to the compressor system 150 may be reduced or avoided by opening one or more of the valves 122_1 , 122_2, ...., 122_(n-1), to enable flow to the downstream sections (e.g.,P393079.GB.01

[0135] 18

[0136] the second section 120_2) of the pipeline compressed gas store 120 during charging / discharging events (i.e., during operation of the compressor system / turbine system).

[0137] Secondly, due to the extreme diameter to length ratio of both the individual sections 120_1, 120_2, ...120_n of the compressed gas store 120 and the compressed gas store 120 as a whole, frictional losses due to pipeline wall friction will be high compared to other configurations of compressed gas store. Frictional losses increase as a function of the speed of the gas flow. Therefore, to improve or maximise overall system efficiency, these frictional losses should be reduced by controlling the flowrates in the individual sections 120_1, 120_2, ...120_n of the compressed gas store 120, in particular through the location and timing of opening / closing (including opening percentages) of the valves 122_1, 122_2, ...122_(n-1). Preferably, the flowrates Vi in each section 120-i of the pipeline compressed gas store 120 should be controlled to 100m / s or less.

[0138] Thirdly, each section 120_1, 120_2, ...120_n of the pipeline compressed gas store 120 is preferably operated between its operational maximum Pmax and minimum P min pressure.

[0139] With reference to Figure 3, a method 300 of operating the compressed gas energy storage system 100 will now be described.

[0140] In a step 302 of the method 300, the current state of charge (pressure) of each section 120_1, 120_2, ...120_n of the compressed gas store 120 is determined using the respective pressure sensor installed in each section.

[0141] In a step 304, the current state of charge (temperature) of the thermal energy store 270 is measured using the thermocouple installed in the thermal energy store 270.

[0142] In a step 306, the estimated electrical supply to the compressed gas energy storage (CGES) system 100 and the estimated electrical demand from the CGES system 100, from the electricity network 140, is determined over short (<30% of full charge duration at maximum discharge power), medium (30-60% of full charge duration at maximum discharge power) and longer (>60% of full charge duration at maximum discharge power) timescales. This results in an anticipated operational charging and discharging schedules.P393079.GB.01

[0143] 19

[0144] In a step 308, the anticipated operational schedule of the CGES powerplant 110 is determined over short, medium and longer timescales, based on the data including the current state of charge of the compressed gas store 120, the current state of charge of the thermal energy store 270, known system loss rates, and the anticipated operational charging and discharging schedules.

[0145] In a step 310, a strategy for delivering the anticipated operational schedule of the CGES powerplant 110 is determined using a thermal / fluid dynamic model of the CGES system 100, running and being optimised in real time. The optimal use of each section 120_1, 120_2, ...120_n of the compressed gas store 120 and the thermal energy store 270 overtime is estimated using the model. This includes calculating the pressure, Pn in each pipeline section 120_1, 120_2, ...120_n, the corresponding flow velocities vnin each pipeline section 120_1 , 120_2, ...120_n, the likely corresponding frictional losses, the temperature of the thermal energy store 270 and electricity / gas consumption rates (for powering the compression train 250 and combustion heaters 292a, b respectively) as a function of time. A multi-dimensional optimisation is then carried out which seeks to minimise an objective function, F(x), of system losses caused by excessive flow velocities (xv), leakages (x(), pipe fatigue (x^), gas and electricity consumption (xe) and other factors (x0). Input variables, to the optimisation include the opening timings (and open percentages) of the two-way valves 122_1, 122_2, ... , 122_(n-1 ) as a function of time and the timing of utilising the thermal energy stored in the thermal energy store 270 and or the secondary heat source 292.

[0146] F(x) = xv+ x(+ x + xe+ x0

[0147] min(F(x))

[0148] In a step 312, the optimised schedule is then implemented by the CGES storage system 100. During this step, the valves 122_1, 122_2, ..., 122_(n-1) are controlled by the control system 130 according to the timings and opening percentages determined in step 310. During this step, heat is supplied to compressed gas released from the compressed gas store 120 from the thermal energy store 270 and / or the secondary heat source 292 based on the timings determined in step 310.

[0149] As new data on demand, supply or other aspects related to system performance and cost effectiveness is made available, the method 300 is re-run to update the schedule implemented by the CGES storage system 100.P393079.GB.01

[0150] 20

[0151] During charging of the compressed gas energy store 120, any excess heat from the compression process is captured and stored in the thermal energy store 270, until the thermal energy store 270 is full. Once the thermal energy store 270 is full, any excess heat from the compression process is vented to atmosphere or to another heat sink. When available, use of heat held in the thermal energy store 270 is prioritised over the consumption of combustion fuel for heating gas released from the compressed gas store 120 into the expansion train 260. When the thermal energy store 270 is empty, combustion of natural gas or hydrogen should be used for preheating.

[0152] Implementation of the method 300 may lead to periods during which the pipeline compressed gas energy storage system 100 is operating most effectively with some upstream valves 122_i closed to isolate the corresponding downstream sections 120_(i+1), ..., 120_n of the pipeline compressed gas store 120 from unnecessary frequent pressure loading and fatigue. For example, valve 122_2 may be closed to isolate sections 120_3, ...,120_n of the compressed gas store 120 downstream of valve 122_2 from the CGES powerplant 110.

[0153] In an example embodiment of a method of operating the pipeline compressed gas store, the required active volume of gas storage over the expected charging / discharging time period is estimated, taking into account the current state of charge (pressure) in each section of the pipeline compressed gas store 120. The required active volume of gas storage can be set by opening / closing valves 122_1, 122_2, ..., 122_(n-1) forone or more sections 120_1 , 120_2, ..., 120_n of the pipeline compressed gas store 120. The active volume of gas storage comprises the first section 122_1 of the compressed gas store 120, together with any consecutive sections of the compressed gas store 120 in fluid communication with the first section 122_1. In other words, the active gas store comprises all sections of the compressed gas store 120 upstream of the first closed valve, that is, between the compressor and the first closed valve nearest to the compressor. These valve open / closing events may take place ahead of any charging / discharging events. This (i) protects sections of the pipeline compressed gas store 120 from being exposed to frequent pressure variance and fatigue events and; (ii) minimises flowrates, v across the pipeline and thus reduces friction losses.P393079.GB.01

[0154] 21

[0155] In some embodiments, implementation of the method 300 may lead to a net reduction of flow velocities (and thus frictional losses) across the pipeline compressed gas store 120, especially during periods of short and high-power charge / discharge, by i) closing one or more of the valves 122_1 , 122_2, ..., 122_(n-1) between sections of the pipeline compressed gas store 120, thus reducing the total active compressed gas storage volume and relatively increasing / reducing local pressures in one or some sections rather than across all pipeline sections; and / or ii) then either simultaneously (or even latterly when discharging is not active) opening valves downstream of the active section(s) to move gas along the pipeline at lower speed.

[0156] Charging and discharging of gas into / from the compressed gas store 120 are intermittent events and can result in significant flows of gas in the compressed gas store 120 through the first section 120_1 of the pipeline compressed gas store 120 during those times. However, with sufficient time available, gas exchange between other sections of the pipeline compressed gas store 120 can take place at a later time, and at lower flow rates. For example, if the anticipated operational schedule of the CGES power plant 110 includes a charging event that will last 2 hours at maximum compressor power, the optimised schedule may include closing the first valve 122_1 between the first section 120_1 and the second section 120_1 of the compressed gas store 120, then charging only the first section 120_1 of the pipeline compressed gas store 120 during that time. In some cases, it may be preferable to ‘overfill’ the first section 120_1 by increasing its pressure relative to sections further downstream. At a later time, the valve 122_1 between the first section 120_1 and the second section 120_2 may be opened to pass the gas further along the pipeline compressed gas store 120. The opening percentage of the valve 120_2 may be controlled to limit the flow rate, e.g., to below 100m / s. By delaying opening the valves to the downstream section(s), for example until after the charging event, the downstream section(s) can be filled over a longer period of time, thereby reducing the flowrates and rates of pressure change in those sections.

[0157] During charging, the active pipeline section(s) may have a high flowrate, v, and thus wall friction losses may be high. If not expecting to exceed the maximum pressure, Pmax, of the active pipeline section(s), the valve to the next section of the pipeline compressed gas store 120 may be closed (or partially closed) and opened later to reduce maximum flowrates.P393079.GB.01

[0158] 22

[0159] During discharging, the active pipeline section(s) may have a high flowrate and thus wall friction losses may be high. If the pressure in the active section(s) is not expected to fall below its minimum pressure, Pmin, the valve to the next section of the pipeline compressed gas store 120 may be held closed (or partially closed) and opened later to reduce maximum flowrates.

[0160] If any active pipeline section (i.e. , any section comprised in the active gas store) is expected to reach its operational maximum pressure, Pmax, then at the point when the pressure in the active gas store matches the pressure of the next pipeline section, the next valve may be opened (or partly opened), thus increasing the pressure of both pipeline sections as charging continues.

[0161] Similarly, during discharge of the compressed gas store 120, If any active pipeline section is expected to reach its operational minimum pressure, Pmin, then at the point when the pressure in the active gas store matches the pressure of the next pipeline section, the next valve may be opened (or partly opened), thus decreasing the pressure of both pipeline sections as discharging continues.

[0162] Due to the nature of the energy market, an operator may wish to only to be active in the short-term market and thus isolate the second and further pipeline sections 120_2, ..., 120_n. As a result, the first section 120_1 may briefly retain a pressure Pi that is higher or lower than in the further downstream sections 120_2, ..., 120_n.

[0163] In general, the sections of the pipeline compressed gas store 120 closest to the CGES power plant 110 would be generally more active than those further away. Operators would seek a balance between frictional losses (caused by high gas flow speeds and yielding less revenue) and fatiguing the pipeline sections (caused by frequent and significant pressure changes and yielding increased maintenance costs). Controlling the valves to temporarily isolate or reduce flow between some sections of the compressed gas store 120 can help to avoid or reduce the frequency of both extremes.

[0164] Although particular example embodiments of the disclosure have been described above, it will be appreciated than many modifications, including additions and / or substitutions, may be made within the scope of the appended claims.

Claims

P393079.GB.0123CLAIMS1. A compressed gas energy storage system, comprising:a compressor system comprising at least one compressor for compressing gas;a compressed gas store configured to receive and store gas compressed by the compressor system; anda turbine system comprising at least one turbine configured to be driven by gas discharged from the compressed gas store;wherein the compressed gas store comprises:a plurality of consecutive sections of a pipeline; andat least one valve for controlling a flow of gas between two respective consecutive sections of said plurality of consecutive sections of the pipeline;wherein the compressed gas energy storage system further comprises a controller for controlling said at least one valve.

2. A compressed gas energy storage system according to claim 1 ,wherein the controller is configured to control said at least one valve dependent upon a required storage volume and / or a predetermined quantity of gas to be added to or discharged from the compressed gas store.

3. A compressed gas energy storage system according to claim 2, wherein the controller is configured to control said at least one valve prior to said predetermined quantity of gas being added to or discharged from the compressed gas store.

4. A compressed gas energy storage system according to claim 1 , wherein said controller is configured to hold said at least one valve closed during charging or discharging of gas into the compressed gas store, and, subsequently, to open said at least one valve to allow gas to flow between at least two sections of the compressed gas store.P393079.GB.01245. A compressed gas energy storage system according to any one of the preceding claims,wherein said two consecutive sections comprise a first section and a second section, said second section being located further from the compressor system relative to the first section;wherein the controller is configured to control said at least one valve dependent upon a pressure in said first section relative to a pressure in said second section.

6. A compressed gas energy storage system according to claim 5, wherein, in response to determining that a pressure in said first section is lower than a pressure in said second section, the controller is configured to hold said at least one valve closed while compressing gas into said first section to increase the pressure in said first section, prior to at least partially opening said at least one valve.

7. A compressed gas energy storage system according to claim 6, wherein said controller is further configured to allow the first pressure to increase above the second pressure prior to opening said valve.

8. A compressed gas energy storage system according to any one of claims 5 to 7, wherein, in response to determining that a pressure in said first section is higher than a pressure in said second section, the controller is configured to hold said at least one valve closed while discharging gas from said first section to decrease the pressure in said first section, prior to at least partially opening said at least one valve.

9. A compressed gas energy storage system according to any one of claims 5 to 8, wherein the controller is configured to at least partially open said at least one valve when a pressure in said first section matches a pressure in said second section.

10. A compressed gas energy storage system according to any one of the preceding claims, wherein the controller is configured to:P393079.GB.0125determine an operational schedule for the compressor system and turbine system based on expected storage demand;using a fluid dynamic model of the compressed gas store, determine a valve control schedule for controlling the at least one valve, including opening timings and / or opening percentages of the at least one valve, that minimises expected energy losses in the compressed gas energy storage system during the determined operational schedule; andcontrol the at least one valve according to the determined valve timing schedule during operation of the compressor system and turbine system.

11. A compressed gas energy storage system according to any one of the preceding claims,wherein said two consecutive sections comprise a first section and a second section, said second section being located further from the compressor system relative to the first section; and wherein:the second section of the pipeline is operated as an overflow and / or reserve for the first section of the pipeline, and / orthe first section of the pipeline is used for short term gas storage and the second section of the pipeline is used for longer term gas storage.

12. A compressed gas energy storage system according to any one of the preceding claims,wherein the controller is configured to control said at least one valve to prevent a flow rate of gas in the compressed energy store exceeding 100m / s.

13. A compressed gas energy storage system according to any one of the preceding claims, further comprising at least one of the following features:(a) wherein the compressor system and the turbine system are coupled to the same end of the plurality of consecutive sections of the pipeline;(b) wherein the compressed gas energy storage system further comprises a main valve for controlling flow of gas between the compressor system and the compressed gas store, and for controlling flow of gas between the compressed gas store and the turbine system; and / orP393079.GB.0126(c) wherein the compressed gas energy storage system further comprises a terminal valve for terminating the section of the plurality of consecutive sections of the pipeline which is furthest from the compressor system and / or the turbine system.

14. A compressed gas energy storage system according to any one of the preceding claims,wherein the plurality of consecutive sections of the pipeline have a total length at least 100 times greater than the diameter of the pipeline.

15. A compressed gas energy storage system according to claim 14, wherein at least one of the plurality of consecutive sections of the pipeline has a length at least 100 times greater than its diameter.

16. A compressed gas energy storage system according to any one of the preceding claims, further comprising:a cooling system for removing heat from the gas when it is compressed; a thermal energy store, coupled to the cooling system for storing the heat removed from the compressed gas, for providing thermal energy for heating gas released from the compressed gas store prior to driving at least one turbine of the turbine system.

17. A compressed gas energy storage system according to claim 16, wherein the thermal energy store has a capacity for storing sufficient thermal energy for heating gas discharged from a section of the plurality of consecutive sections of the pipeline which is closest to the turbine system.

18. A compressed gas energy storage system according to claim 16 or claim 17, wherein the thermal energy store has a thermal energy capacity that is insufficient for heating gas discharged from all the sections of the pipeline comprised in the compressed gas store.

19. A compressed gas energy storage system according to any one of claims 16 to 18,P393079.GB.0127further comprising a secondary heat source for providing further thermal energy for heating the gas released from the compressed energy store prior to driving at least one turbine of the turbine system.

20. A compressed gas energy storage system according to claim 19, wherein the secondary heat source comprises a combustion heater.

21. A compressed gas energy storage system according to claim 19 or claim 20, wherein, during discharge of the compressed gas store, the gas released from the compressed gas store is heated using thermal energy from the thermal energy store prior to using heat from the secondary heat source.

22. A method of operating a compressed gas energy storage system, comprising:charging a compressed gas store by compressing gas into a compressed gas store;driving at least one turbine, by discharging compressed gas from the compressed gas store through said at least one turbine;wherein the compressed gas store comprises:a plurality of consecutive sections of a pipeline; andat least one valve for controlling a flow of gas between two respective consecutive sections of said plurality of consecutive sections of the pipeline;the method further comprising:controlling said at least one valve to control the flow rate of gas between said respective consecutive sections of the pipeline during at least one of charging or discharging of the compressed gas store.

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