Patent Application
20180058320
Some embodiments relate to a hybrid combustion turbine power generation system, and a method of modulating the power output of the same. In particular, some embodiments are concerned with a hybrid system in which a conventional combustion turbine is integrated with a compressed air energy storage (CAES) system.
CAES systems utilizing thermal energy storage (TES) apparatus to store heat have been known since the 1980's. Adiabatic compressed air energy storage (ACAES) systems store the heat of compression of the compressed air in thermal stores for subsequent return to the air as it leaves a compressed air store before undergoing expansion. The TES apparatus may contain a thermal storage medium through which the compressed air passes, releasing heat to the storage medium, thereby heating the store and cooling the air. The thermal storage medium may be in the form of a porous storage mass, which may be a packed bed of solid particles through which the air passes exchanging thermal energy directly, or, it may comprise a solid matrix or monolith provided with channels or interconnecting pores extending therethrough, or, the fluid may pass through a network of heat exchange pipes that separate it from the storage mass, such as a packed bed of particles (e.g. rocks).
Alternatively, some of the compressed air may pass through a heat exchanger that is coupled to a separate thermal store, such that heat is transferred indirectly to the latter via a heat transfer fluid, in which case the thermal store need not be pressurised and could include a thermal storage medium such as a molten salt or high temperature oil. However, thermal energy stores based on indirect thermal transfer (indirect TES) have much lower efficiencies than ones that store heat directly (direct TES) as mentioned above. In addition the heat exchanger of an indirect TES normally takes a finite amount of time to reach equilibrium conditions and hence, if left inactive, needs warming to temperature before use. Furthermore, for large heat transfer rates this is likely to be quite an expensive heat exchanger.
Air injected power augmentation of combustion turbines is used to increase the power output of a gas turbine up to its normal maximum allowable power where, for example, the power has dropped due to high altitude or high ambient temperatures reducing the density of inlet air.
U.S. Pat. No. 5,934,063 to Nakhamkin proposes a hybrid combustion turbine power generation system (CTPGS) in which a gas turbine is integrated with an ACAES system and pressurised air from the air storage is injected at the combustor to augment the air flow through the gas turbine and hence increase the power output when it would otherwise be below its maximum allowable level. In U.S. Pat. No. 5,934,063, the returning stored air is heated with waste heat from the turbine or from a downstream steam turbine. WO2013/116185 relates to another hybrid CTPGS which instead proposes the use of various heat exchanger stages during the storage mode to store the heat of compression for subsequent return.
There have also been proposals to integrate a combustion turbine (GT) system with an ACAES system, whereby the compressor may be selectively coupled and decoupled from the turbine to allow their independent operation such that the gas turbine can operate in multiple modes; selector valve arrangements may be disposed within the combustion turbine flow path to divert the airflow into and out of the combustion turbine in these multiple modes. However, to date no commercial systems exist due to the cost and complexity of developing such a decouplable gas turbine system.
Integration of an ACAES into a gas turbine system can allow such a hybrid system to turn up or turndown its power output in response to a changing grid requirement. However, where the hybrid system is based on a gas turbine of a conventional arrangement, in which the compressor and turbine are always coupled together for simultaneous operation and air flow is always passing successively downstream through the compressor, combustor and turbine, then any power modulation is limited by the inherent limits and characteristics of the gas turbine. Prior art has focussed upon extending the respective limits of turn-up and turndown and upon improving the efficiency of such modes of operation. However, there is also a need to improve the speed of response.
Current grid requirements require a power plant to be able to increase output by 10% within ten seconds. This is used by the TSO (Transmission System Operator) to supply additional rapid power increases to the network in the case that there is a large loss of generation—for example a power stations trips off or an interconnector is lost. The TSO normally requires or pays for sufficient capacity that it can manage any normal events.
When there is the loss of a large generator, the system frequency immediately starts to drop. The rate at which the frequency drops is a function of how much generation has been lost, how much generation remains, and how much rotating inertia there is on the system. Large thermal power plants have grid synchronised machinery that is directly coupled to the grid. This spinning plant has inertia and it slows the rate at which the frequency falls.
Traditionally on most large grids ten second response times were adequate to deal with unexpected events.
However, wind turbines and solar photo-voltaic (PV) are being adopted on a large scale and they have a double impact on the system. They provide no inertia to the system and they have no marginal cost of production. Consequently when there is a large amount of renewables generating the thermal power plant switches off. This means that the amount of inertia on the system is reduced and the rate of change of frequency increases. There is a further potential issue in that the size of generating units is getting larger. For example, proposed new nuclear power stations are approximately 1600 MW per unit and this means that the grid operator needs to allow for a larger loss of generation and a much faster changing frequency. Consequently, it is desirable that power plants can increase power in a matter of seconds, rather than ten seconds, preferably over a larger range than a 10% increase. This requirement for rapid power is normally only required for short periods of time as other generating assets can normally be brought on line over periods of minutes to replace the lost generation.
Methods of increasing or decreasing power output involve altering the mass flow rate within the gas turbine. For example, power output may be increased by injecting more air from storage but there are limitations on the rate at which air can be injected. For example, thermal stresses need to be managed: as the pressure ratio changes the temperature in the compressor and turbine sections both change and this can lead to thermal stresses that are potentially damaging to the gas turbine and can lead to increased maintenance and likelihood of unpredicted enforced outages. Another concern is combustor stability. DLN combustors normally operate on a very lean mixture and it is possible to ‘blow’ them out if the air fuel ratio is changed to quickly. Accordingly, to avoid these issues, it is usually necessary to limit the rate of change of the mass flow rate within the combustion turbine so that the air fuel ratio within the combustor does not alter too quickly.
Some embodiments are directed towards providing an improved hybrid combustion turbine power generation system and, in particular, one in which the system can modulate its power output within a very short period of time (e.g. within a few seconds).
In accordance with a first aspect of the present invention, there is provided a method of modulating the power output of a hybrid combustion turbine power generation system (CTPGS) comprising:
a combustion turbine (GT) system comprising a compressor, a combustor and a turbine fluidly connected downstream of each other; and,
a compressed air energy storage (CAES) system integrated with the GT system via one or more fluid connections to the GT system so as to allow air to be extracted from, or injected into, the GT system;
wherein the CAES system comprises an airflow passageway network and associated valve structure leading from the one or more fluid connections to a compressed air store;
the method comprising modulating the power output whilst air is passing respectively downstream through the compressor, combustor and turbine by increasing or decreasing the power output by, respectively, selectively reducing or increasing the mass flow rate of the air through the compressor by altering its configuration, and simultaneously selectively adjusting how much air to transfer as a compensatory mass flow between the CAES system and the GT system via the one or more fluid connections, in order partially or fully to compensate for the reduction or increase in mass flow rate through the compressor, thereby minimising or preventing any change in mass flow rate through the combustor and turbine at least for a selected time period.
A reduction or increase in the mass flow rate through the compressor will lead to a commensurate reduction or increase in the power it draws, and hence, to a corresponding increase or reduction in (overall) power output, respectively. By injecting and/or bleeding some, or more, air as a compensatory mass flow at a selected mass flow rate (for example, at a mass flow rate less than or roughly equal to the change in the compressor mass flow rate) from or to the integrated CAES system, by means of fluid connections suitably located in the GT, it is possible to minimise or prevent any change in mass flow rate through the combustor and turbine, and hence, any change in the pressure and temperature conditions there, for a selected time period. By proactively partially or fully balancing mass flow rate in this way, the power can be changed at a faster rate than usual methods involving a more significant change in the mass flow rate through the GT with the associated (time sensitive) change in GT operating conditions.
Currently, power plants may be called upon to operate in a “Frequency Response” mode (FR Mode) i.e. an initial power generation mode from which they can increase output by 10% within ten seconds to meet grid fluctuations. The present invention may allow a power plant to offer an “Improved Frequency Response Mode” or IFR Mode, i.e. an initial power generation mode from which they can modulate power to the second power output in under 10 seconds, for example, within a response time of 7 seconds or less, or preferably, even 5 seconds or less, or even 3 seconds or less. The hybrid system may be configured to operate in both an IFR mode and a less time critical FR mode.
In a preferred embodiment, the CAES system is integrated with the GT system such that it charges and discharges via the GT system, air being both extracted from it, and injected into it, via the one or more fluid connections. While the CAES system will usually both send air to, and receive and store compressed air from the GT system, other hybrid systems in which, for example, the CAES system is additionally charged by, or only charged by, separate power machinery, are not excluded.
For improved efficiency, the compressed air energy storage system will usually comprise an adiabatic compressed air energy storage system (ACAES) that stores and returns thermal energy (i.e. heat of compression) to the compressed air. Thus, the airflow passageway network may comprise a first thermal energy store (TES) that removes and returns thermal energy to the compressed air upon charging and discharging the air store, disposed between the latter and the one or more fluid connections.
In a preferred embodiment, the first TES is a direct TES. A store based on direct thermal transfer contains a thermal storage medium through which the compressed air passes, releasing heat to the storage medium, thereby heating the store and cooling the air. It can return the heat stored in it to a gas flow efficiently and without delay. The thermal storage medium may be in the form of a porous storage mass, which may be a packed bed of solid particles through which the air passes exchanging thermal energy directly, or, it may comprise a solid matrix or monolith provided with channels or interconnecting pores extending therethrough, or, the fluid may pass through a network of heat exchange pipes that separate it from the storage mass, such as a packed bed of particles (e.g. rocks).
A direct TES may also have a significant volume of air at all times which is advantageous because it can provide some damping to pressure fluctuations within the TES when valves open and close rapidly. In addition, if further damping is required it may be advantageous to provide one or more additional compressed air buffers (volumes of air at the same pressure as the TES and linked by open fluid connections) that are directly linked to the TES. These will normally be connected to the ambient temperature side of the TES. The addition of these further volumes of compressed air will further reduce any pressure fluctuations within the TES from the rapid opening and closing of valves (either into or out of the TES). One or more buffers may provide an additional volume of free air that is at least 2× the free volume in the TES, or at least 3× the free volume, or at least 4× the free volume in the TES.
The CAES system may comprise an indirect first TES, that is, a thermal energy store based on indirect thermal transfer. This may comprise a heat exchanger that is coupled to a separate thermal store, such that heat is transferred indirectly to the latter via a heat transfer fluid, in which case the thermal store need not be pressurised and could include a thermal storage medium such as a molten salt or high temperature oil. However, such stores are less efficient that a direct TES and if left inactive need rewarming.
As discussed further below, a heater system may be provided instead of, or, in addition to, a direct or indirect TES, for example, where the heat is supplied either directly or indirectly by a fossil fuel.
In a preferred embodiment, the compensatory mass flow ensures that the rate of change of mass flow rate within the combustor and turbine does not exceed 6% per second (with respect to that flow rate) for the selected time period. However, more preferably, it is limited to changing by not more than 4% per second, or even not more than 2% per second.
In a highly preferred embodiment, the compensatory mass flow ensures that the mass flow rate through the combustor and turbine remains substantially unchanged for the selected time period. Thus, the compensatory mass flow may be selectively adjusted exactly to match the change in mass flow rate through the compressor. By balancing the change in this way, a substantially unchanged mass flow rate may be maintained within the combustor and turbine (e.g. varying by +/−2% of the previous mass flow rate there). Hence, the temperature and pressure conditions within the combustor and turbine remain broadly unchanged. As a result, the rate at which the power is modulated is not restricted by the usual considerations associated with protecting the gas turbine, such as avoiding thermal stresses or destabilising the combustor. In this way, power may be modulated within a certain range within a very fast response period.
In a preferred method, the power output is modulated from an initial power output (e.g. in an initial power generation mode) to a second power output (e.g. in a second power generation mode) within a response time of 5 seconds or less.
In one embodiment, the CAES system is operating before the power modulation in a mode in which it maintains air for injection into the one or more fluid connections at a pressure upstream thereof of at least 0.5 bar higher, or even 1 bar higher or even 5 bar higher than the gas turbine operating pressure (e.g. in combustor). Thus, when the power needs to be modulated, air (or more air) can be injected rapidly into the gas turbine by a fast responding valve operating across this pressure drop. A larger pressure drop also means that transient pressure changes on either side (particularly upstream of the valve ie between the valve and the TES) will have less impact and hence, the mass flow rate can be accurately adjusted. Preferably, the CAES system is configured so that this pressure drop may be sustained once the power modulation step has started for the selected time period, such as, for example, at least 3 seconds, or preferably, at least 5 seconds.
In one embodiment, the CAES system comprises at least one flow regulating device to regulate the mass flow rate of air being injected into, or extracted from the one or more fluid connections, optionally positioned between any TES or heater system that is present in the network, and the one or more fluid connections.
The flow regulating device may be the mechanism that maintains a constant or varying (but controlled) flow through it (this may be actively controlled to allow a constant or varying flow with a varying pressure difference across the flow regulating device). The pressure may vary upstream of the flow regulating device as referenced above, for the reasons given above. The device should selectively (e.g. preferably, finely) adjust flow rate such that the compensatory mass flow is carefully controlled. When a fast response time is required, further opening of the valve may allow rapid flow (injection) of air into the GT system. The flow regulating device will usually be operatively associated with a controller and any required sensors in the flow network (e.g. taking measurements, such as, for example, pressure and temperature) from which mass flow rate data may be derived. A simple (e.g. on/off) valve may also be provided upstream or downstream of any flow regulating device, which may allow the latter to be adjusted beforehand to a new desired setting.
A further flow regulating device will usually be required between any TES or heater system that is present in the network and the compressed air store. In addition it is likely that there will be additional valves, bypass valves or vent valves (so as to protect any power machinery from the high pressure of the air store when not operating or during start up and/or shut down.
In one embodiment, the compensatory mass flow between the CAES system and the GT system is provided via one or more fluid connections provided in ancillary passageways of the GT system containing airflow that bypasses the combustor (“bypass airflow”).
The one or more fluid connections may be any port (e.g. bleed port or injection port) or opening that allows air to be extracted from, or injected into, the GT system, including ones controlled by valves; different fluid connections may be used for bleed and injection, respectively. The connections may be so located as to allow air to be directly or indirectly extracted from, or injected into, the main airstream(s) passing down through the gas turbine. Fluid connections may also be provided in ancillary passageways in the GT system containing airflow that bypasses the combustor (“bypass airflow”), including ducting that ducts cooling air (from compressor) to different parts of the gas turbine, since adjusting air in such ducts can be used indirectly to provide the afore-mentioned balancing of the mass flow in the combustor and turbine.
In one embodiment, the configuration of the compressor is altered by altering the angle of variable inlet guide vanes. The GT compressor may be provided with any of the following which may be used alone or in combination to vary the mass flow rate through it: variable inlet guide vanes (IGV's), variable exit guide vanes (EGV's), or other variable compressor geometry or a compressor inlet restrictor or other inlet equipment associated with the compressor (e.g. filter).
The change in compressor configuration (e.g. setting) may involve a change in compressor geometry and will preferably only change the mass flow rate of air drawn in (as opposed for example to other characteristics of the intake air e.g. such as its temperature). For large industrial gas turbines the normal control of the compressor is the variable inlet guide vanes. These can generally vary the mass flow through the compressor from 70% (fully closed) to 100% (fully open). There may also be some additional blow off valves (vents) that are used when starting the gas turbine.
Thus, the mass flow rate of the air through the compressor may be reduced by making the guide vanes less open, or increased by making the guide vanes more open. In an IFR mode, the guide vane setting should obviously be selected such that it has the capacity to change the vane setting by the amount required for the next power generation mode. It will usually be set (in a partially open position) such that the vanes can open further and can close further, allowing modulation in both directions, but operation in an IFR mode where the inlet guide vanes are fully open is not excluded when it is desirable to have only the ability to increase power rapidly while operating at 100% power output. This is normally the most efficient mode of power generation and the ability to increase power rapidly is normally more valuable than the ability to decrease it, since it is more common to lose a large generating asset than lose a large load (i.e. most power stations are much bigger than the customers that they serve).
In one embodiment, the CAES system further comprises air depressurisation apparatus in fluid communication with the one or more fluid connections for depressurising compressed air extracted from the GT system (rather than storing that air in the CAES).
Air depressurisation apparatus may allow increased power modulation as it may increase the range over which the compressor mass flow can be varied. Thus, if a gas turbine can only safely inject 50 kg/s of air (to avoid compressor surge or stall), then if injection flows are only used for power modulation then 50 kg/s is also the limit on the amount of rapid variation in the compressor mass flow. Hence, the strategic use of bleed (i.e. an extraction mode) in an initial power generation mode can increase the apparent range over and above that allowed by injection during a subsequent power modulation step. Bleed flow through air depressurisation apparatus will however involve an efficiency penalty and additional machinery cost, with larger bleed flows requiring more modification to the GT (e.g. larger ports).
The air depressurisation apparatus may comprise a hot air expander or combined combustor/turbine that extracts useful work.
Air depressurisation apparatus for depressurising extracting compressed air may comprise a hot gas expander that extracts useful work, such as a hot air expander with its own ambient air outlet, or a combined combustor/turbine optionally connected to a main HRSG (for the GT system) or its own HRSG and steam turbine. The hot air expander or turbine should normally be able to operate over the same maximum pressure ratio as the main gas turbine. Such apparatus could not respond from a cold start if it is desired to undertake a rapid power modulation, but can be used to produce useful work when the GT system is operating in a mode (e.g. an initial power generation mode) where it expects to be called upon to provide a rapid power modulation. Such a combustor/turbine could have the compressed air to the combustor pre-heated by the exhaust from the turbine, rather than its exhaust providing additional power via an HRSG.
Similarly to the GT, the system may be configured such that the mass flow rate remains unchanged in the hot air expander or combined combustor/turbine, or only varies a small amount (e.g. varying by less than +/−6%/second of the previous mass flow rate there or 4% or 2%) after a power modulation, in particular where a combustor/turbine arrangement is used. This may even be the case where the source of air (being expanded) changes after the power modulation (e.g. air bled from the GT being replaced with air from storage).
It will be realised by one skilled in the art that the hot air expander or combined combustor/turbine are directly connected to the gas turbine, which processes a much larger quantity of air. Consequently the gas turbine will have a strong influence on the rate that this can change. For example if the mass flow to the depressurisation apparatus is controlled by a choke valve then the pressure upstream will only change as the pressure in the main gas turbine changes. Consequently, this will limit the changes experienced to a similar level seen by the gas turbine. Alternatively, where there is variable flow control to the depressurisation apparatus it may be possible to vary the mass flow rate through the air depressurisation apparatus significantly without there being any significant temperature changes. (This is likely to be less challenging for a hot air expander as it operates at temperatures where variable guide vanes can function more easily.)
Alternatively, the air depressurisation apparatus may be a device that does not extract useful work, such as, for example, a venting valve/throttle valve to atmosphere (i.e. a lower cost device able to operate at a higher mass flow rate). The latter (unlike power machinery) can respond rapidly from a cold start, so does not need to be operating when it is desired to undertake a rapid power modulation, and can provide very rapid bleeding during the power modulation, by venting to atmosphere, with a pressure drop optionally in the 10 to 20 bar range. This may allow good control of the compensatory mass flow rate, as well as a significant reduction in power, more so than a connection to a hot gas expander, but this mode would obviously only be used as a transient mode, given its inefficiency.
In one embodiment, the air depressurisation apparatus is connected by its own separate respective airflow passageways to the one or more fluid connections (for operation independent of the status of the CAES system). Thus, the air depressurisation apparatus may share at least some of the airflow passageways in which air usually flows to storage, or may comprise separate respective (e.g. direct) airflow passageways to the one or more fluid connections such that they may operate independently of the operational status of the CAES system.
In one embodiment, the hybrid system comprises a controller and associated sensors to (i) alter the configuration of the compressor in order to obtain a desired modulation of the power output, and to (ii) selectively adjust how much air to transfer as a compensatory mass flow between the CAES system and the GT system.
Immediately prior to the power modulation, the GT system may be operating in an initial power generation mode where no air is transferring between the CAES system and the GT system, or air may be being injected from the CAES system into the GT system, or air may be being extracted to the CAES system from the GT system. Thus, prior to modulating the power, the CAES system may be in any of an inactive mode, a charging mode, or discharging mode. Usually, in order to meet surges of demand during peak periods, a power modulation will be needed when the hybrid system is operating at or near full (peak) load (e.g. within 15% or even within 10% or within 5% of 100% load).
For a fast response, all apparatus needed for the power modulation should be able to respond rapidly. Accordingly, apparatus should be able to respond from a cold start (e.g. a venting or pressure reducing valve), or alternatively, for apparatus that cannot do so, the initial power generation mode should be one in which the apparatus in question is already operating (e.g. hot air expander) or otherwise held in readiness (e.g. on a minimal setting). For example, where the CAES system comprises a direct first TES, which holds its heat, the CAES system may respond from an inactive status to provide stored hot compressed air from storage. If the CAES system comprises an indirect first TES, in which heat is transferred via a heat exchanger to other stores, then such an arrangement may need to be operating in the initial power generation mode so that the heat exchanger was already active and up to temperature.
In one embodiment, the compensatory mass flow is provided for a selected time period of no more than 20 seconds (or no more than 30 seconds, or even no more than 1 minute) before the GT system alters to a different power generation mode.
Where the power is modulated from an initial power output in an initial power generation mode to a second power output in a second power generation mode, that mode is likely to be used temporarily merely to provide a very rapid response i.e. as a transitional mode. Hence, the selected time period in which the compensatory mass flow is provided in that second mode may be no more than 1 minute (or no more than 30 seconds, or even no more than 20 seconds) before altering to a different power generation mode.
Further modulation of the power may then be carried out in a conventional manner in slower time (e.g. in next 5-15 seconds) with the usual associated change of downstream mass flow rate within the combustor and turbine.
However, the hybrid system may remain operating in the second power generation mode for a significant period of time (e.g. for more than 10 minutes, or more than 30 minutes), for example, if it is energy efficient.
In one embodiment, at least one further stage of power machinery is provided between the GT system and the air store, optionally between any TES or other heat removal system that is present in the network, and the air store. The at least one further stage of power machinery and a pressure reducing device (e.g. throttle valve) may be provided in alternative passageways between the GT system and the air store. Usually the further stage of power machinery will comprise an intercooled compressor disposed in a parallel passageway to the pressure reducing device which does no useful work and may comprise a throttle valve.
In one embodiment, the airflow passageway network comprises a heater system that transfers thermal energy to compressed air that is discharging from the air store.
Such a heater system (i.e. that is not returning stored heat) may be provided in the airflow passageway network instead of a first thermal energy store (TES), or, in addition to the latter. If it is provided in addition, this may be in series for example, downstream disposed between the TES and the GT fluid connections (e.g. so as to provide “top-up heat”). Alternatively, it may be provided in an alternative (e.g. parallel) passageway, for example, to provide additional heat (for additional mass flow) or to provide heat at a faster rate or to provide heat at a different temperature. A heater system may be configured such that the air returning from storage can be heated to a desired temperature having regard to the expected GT system conditions (e.g. to match them or exceed them by a selected amount). For example, a control system may selectively adjust the temperature of any newly injected flow upon a rapid power modulation (e.g. where that starts to involve an injected flow) to ensure the injected flow is less than 50° C., more usually less than 30° C., or less than 20° C. different from the current GT compressor outlet temperature, so as to minimise a significant temperature change in the combustor. A direct or indirect TES is less flexible in that it will return heat at roughly the same temperature that it was charged (inevitably slightly degraded). For this reason, where there is no additional heater system, it may be desirable to charge an ACAES hybrid system that includes a direct or indirect TES with compressed air with the GT system running at the similar operating conditions (or slightly raised with respect thereto) as the mode (i.e. “Initial Mode”) from which it offers an Improved Frequency Response, so that air returning via the TES receives heat of a suitable temperature.
Such a heater system may comprise a direct combustor that heats the air that is discharging from the air store.
While less efficient than a TES, a direct combustor (i.e. based on internal combustion within a gas flow path usually with a fossil fuel) is convenient and may respond rapidly. It will use less fuel than any heating system based on indirect combustion and exhaust gases do not require a separate exhaust device. Such a direct fired additional combustor may easily be kept in readiness to respond to a rapid power modulation requirement.
Alternatively, the heater system may comprise a heat exchanger that heats the air that is discharging from the air store. Thus, the heater system may comprise an indirectly heated heat exchanger that heats the air that is discharging from the air store, usually one heated indirectly by combustion (e.g. of a fossil fuel). Such an arrangement is less efficient than a direct combustor and may be more difficult to keep in readiness to respond.
When a heater system is provided to heat air discharging from the air store, the GT hybrid system may also be configured to charge the air store and in that case the airflow passageway network may further comprise a cooling system (heat extraction system) that removes thermal energy from compressed air being extracted from the GT system.
Where heat is not being stored in the CAES system (diabatic CAES) for subsequent return, some form of cooling system (e.g. cooling heat exchanger) to remove (and discard) heat of compression from the extracted air is desirable. Any further downstream compressors will require coolers (e.g. intercoolers, aftercoolers) again to remove heat prior to storage.
Alternatively, when a heater system is provided, power machinery other than the GT system may be provided to charge the air store with compressed air, either via the airflow passageway network or a separate airflow passageway network.
For example, a small (e.g. ambient air fed) intercooled compressor, or series of compressor stages, could charge a compressed air store such as a pipe store at a small mass flow rate, where the hybrid system is only required to provide (e.g. rapid) power modulation from storage on relatively rare occasions, such that recharging can be accomplished slowly using small power machinery.
The hybrid system may comprise a simple cycle gas turbine systems (OCGT), or form part of a combined cycle gas turbine system (CCGT), or any other suitable derivative combustion turbine plant.
There is further provided, in accordance with the first aspect, a hybrid combustion turbine power generation system (CTPGS) comprising:
a combustion turbine (GT) system comprising a compressor, a combustor and a turbine fluidly connected downstream of each other; and,
a compressed air energy storage (CAES) system integrated with the GT system via one or more fluid connections to the GT system so as to allow air to be extracted from, or injected into, the GT system;
wherein the CAES system comprises an airflow passageway network and associated valve structure leading from the one or more fluid connections to a compressed air store;
wherein the CTPGS comprises a controller and associated sensors to (i) alter the configuration of the compressor in order to obtain a desired modulation of the power output, and to simultaneously (ii) selectively adjust how much air to transfer as a compensatory mass flow between the CAES system and the GT system, via the one or more fluid connections, in order partially or fully to compensate for the reduction or increase in mass flow rate through the compressor.
The hybrid CTPGS may further comprise any one or more other features as outlined previously above.
In a further aspect, there is provided a method of modulating the power output of a hybrid combustion turbine power generation system (CTPGS) comprising:
a combustion turbine (GT) system comprising a compressor, a combustor and a turbine fluidly connected downstream of each other; and,
an adiabatic compressed air energy storage system (ACAES) integrated with the GT system via one or more fluid connections to the GT system so as to allow air to be extracted from, and/or injected into, the GT system;
wherein the ACAES system comprises an airflow passageway network and associated valve structure leading from the one or more fluid connections to a compressed air store via a first thermal energy store (TES) that removes and returns thermal energy to the compressed air upon charging and discharging the air store, respectively; and, wherein at least one further stage of power machinery is provided between the first TES and the air store; and,
the method comprising modulating the power output whilst air is passing respectively downstream through the compressor, combustor and turbine by increasing or decreasing the power output by, respectively, selectively reducing or increasing the mass flow rate of the air through the compressor by altering its configuration, and simultaneously selectively adjusting how much air to transfer as a compensatory mass flow between the ACAES system and the GT system via the one or more fluid connections, in order partially or fully to compensate for the reduction or increase in mass flow rate through the compressor, thereby minimising or preventing any change in mass flow rate through the combustor and turbine at least for a selected time period.
In a yet further aspect, there is provided a method of modulating the power output of a hybrid combustion turbine power generation system (CTPGS) comprising:
a combustion turbine (GT) system comprising a compressor, a combustor and a turbine fluidly connected downstream of each other; and,
an adiabatic compressed air energy storage system (ACAES) integrated with the GT system via one or more fluid connections to the GT system so as to allow air to be extracted from, and/or injected into, the GT system;
wherein the ACAES system comprises an airflow passageway network and associated valve structure leading from the one or more fluid connections to a compressed air store via a first thermal energy store (TES) that removes and returns thermal energy to the compressed air upon charging and discharging the air store, respectively; and,
wherein at least one further stage of power machinery is provided between the first TES and the air store;
the method comprising operating the GT system in an initial power generation mode in which air passes respectively downstream through the compressor, combustor and turbine of the GT system to generate power; and modulating the power to achieve a second power generation mode (e.g. with a second power output) by at least one of the following steps:
(i) increasing the power output by reducing the mass flow rate of the air through the compressor by altering its configuration, and simultaneously adjusting how much air to transfer between the ACAES system and the GT system, via the one or more fluid connections, such that the mass flow rate through the combustor and turbine remains unchanged;
(ii) decreasing the power output by increasing the mass flow rate of the air through the compressor by altering its configuration, and simultaneously adjusting how much air to transfer between the ACAES system and the GT system, via the one or more fluid connections, such that the mass flow rate through the combustor and turbine remains unchanged.
The following embodiments may be used in any of the aspects outlined above.
In one embodiment, the method comprises operating the GT system in an initial power generation mode in which air passes respectively downstream through the compressor, combustor and turbine of the GT system to generate power; and modulating the power to achieve a second power generation mode (e.g. with a second power output) by at least one of the following steps:
(i) increasing the power output by reducing the mass flow rate of the air through the compressor by altering its configuration, and simultaneously adjusting how much air to transfer between the CAES system and the GT system, via the one or more fluid connections, such that the mass flow rate through the combustor and turbine remains roughly unchanged;
(ii) decreasing the power output by increasing the mass flow rate of the air through the compressor by altering its configuration, and simultaneously adjusting how much air to transfer between the CAES system and the GT system, via the one or more fluid connections, such that the mass flow rate through the combustor and turbine remains roughly unchanged.
In step (i) the amount of air being extracted (e.g. to the CAES system or ancillary depressurisation apparatus) may be reduced, or the amount of air being injected from the CAES system may be increased, or, both of those occur. In step (ii) the amount of air being injected from the CAES system may be reduced, or the amount of air being extracted to the CAES system or ancillary depressurisation apparatus may be increased, or, both of those occur. In step (i) or (ii) the CAES may switch from operating in any of an inactive mode or discharging mode or charging mode, to any other such mode, providing the CAES system can be configured with the appropriate responsiveness to meet the Improved Response time period.
In one embodiment, the power output is initially increased by conducting a step (i) whereby the mass flow rate within the combustor and turbine remains unchanged, and is then further increased by conducting a subsequent step (I) whereby the mass flow within the combustor and turbine is increased. For example, the mass flow rate of the air through the compressor may be reduced in a fast initial step (i) by making the guide vanes less open so as to increase the GT power output while the CAES compensates to keep the downstream (i.e. downstream of the flow connections) GT conditions unchanged. The overall power output may then be further increased by making the guide vanes more open and allowing the downstream mass flow rate (and pressure and temperature) in the GT system rise in a slower timeframe. Alternatively, or in addition to re-opening the guide vanes, the CAES may inject some air at a chosen mass flow rate into the GT system such that the downstream GT mass flow rate now rises and the overall power output increases.
Alternatively, in a further embodiment, the GT power output is initially decreased by conducting step (ii) whereby the mass flow rate within the combustor and turbine remains unchanged, and is then further decreased by conducting a subsequent step (II) whereby the mass flow within the combustor and turbine is decreased.
Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:
In the case of an open cycle gas turbine (OCGT), the cooled air is exhausted from the turbine 6 well above ambient temperature (e.g. 450° C., 1 bar). However, in the case of a combined cycle gas turbine (CCGT), the turbine 6 operates with an exhaust temperature that is slightly hotter, either by operating at a lower pressure ratio or by combusting to a higher turbine inlet temperature. The exhaust gas 12 from the turbine 6 then enters a steam turbine system (passing through a heat recovery steam generator or HRSG) where further power is extracted in a steam bottoming cycle.
In the Figures that follow, all embodiments are depicted as simple cycle gas turbine systems (OCGT) for simplicity, but may instead form part of a combined cycle gas turbine system (CCGT), or any other suitable derivative combustion turbine plant. Furthermore, all embodiments relate to a conventional combustion turbine arrangement in which the compressor, combustor and turbine are permanently fluidly connected downstream of each other, so that whenever the gas turbine is operating at least some air flow passes successively downstream through all those components in turn, regardless of whether or not a portion of the flow is being extracted or augmented at the one or more fluid connections, and in that the turbine is non-detachably coupled to the compressor so that both operate together when power is being generated by the turbine.
As also shown in
For example, there is normally a fluid connection or duct 3 that feeds hot compressed air 3a from the compressor discharge plenum (between the compressor exit and the turbine inlet) back to a discharge valve located near the compressor inlet. Hot air is isenthalpically expanded back to atmospheric pressure and added to the inlet air 8 to increase the temperature. This is normally used to prevent ice formation at the entrance to the compressor, but can also be used to reduce the power output of the gas turbine. This connection 3 is known as the anti-icing line or inlet bleed heat line (IBH), and it is possible to connect to this line to either inject or bleed air from the gas turbine, as shown by dotted line 3A.
A further series of fluid connections or ducts 7, 9, 11 from different stages of the compressor may be used to keep the turbine blades cool by providing Turbine Cooling Air (TCA) 7a, 9a and 11a (at different pressures). As much as 15% of the air passing through a gas turbine can be used as TCA and does not pass through the combustor 4. The air 11a for the high pressure stages is taken from the compressor discharge plenum or the later stages of the compressor 2, while cooling air 7a, 9a for the later stages of the turbine is normally taken from an intermediate pressure stage. There may be a number of different pressure supplies to both the rotating and static turbine blades. Again, air can be injected into or extracted from the TCA ducts (e.g. at points 9A, 11A) rather than directly into the gas turbine main passageway 1. If a quantity of air that is less than the normal TCA supply is injected (normally in the higher pressure lines), then the TCA flow from the compressor will be reduced by a similar amount. If a quantity of air that is greater than the normal TCA supply is injected (normally in the higher pressure lines), then the TCA flow will be forced to reverse and enter the GT back through the normal inlet to the TCA duct. Injected airflow will thus start to displace and even completely replace the TCA. The actual injected air will not pass through the combustor 4 (unless it exceeds the normal quantity of TCA as mentioned above), but instead will pass directly to the turbine section of the gas turbine. However, the air that would have passed through the TCA line will now pass through the combustor instead with the same result as if the air had been directly injected into the main passageway 1.
Likewise bleeding air from the TCA line will increase the amount that is withdrawn by the TCA line; however, it is important that the amount of cooling air is not reduced by the bleed such that it fails to provide adequate cooling to the turbine section.
In addition to such ports, it is also possible to adapt the casing or casings to allow for additional injection points.
A gas turbine usually has inlet guide vanes used to control the mass flow entering the compressor, which on a large industrial gas turbine can reduce mass flow by about 30%, thereby usually leading to a reduction in the GT power output.
Referring first to
There are two limitations on the rate at which air can be injected. The first is related to thermal stresses. As the pressure ratio changes, the temperature in the compressor and turbine sections both change and this can lead to thermal stresses that are potentially damaging to the gas turbine and can lead to increased maintenance and likelihood of unpredicted enforced outages. The second concern is combustor stability. DLN (Dry Low NOx) combustors normally operate on a very lean mixture and it is possible to ‘blow’ them out if the air fuel ratio is changed too quickly. Using a less lean mixture can help the stability of the combustor, however this is not suitable for normal operation as it leads to an increase in NOx production.
Systems like this have been proposed by Powerphase, Nakhamkin and the Applicant. The aim in such systems is to increase the mass flow through the combustor and turbine without passing through the compressor. This means that for an injection of, say, 50 kg/s of air in to a GE 9FA gas turbine, the gas turbine power can increase to 116% of the rated power at that ambient condition. On the basis that in normal Frequency Response mode the mass flow rate can be increased at a rate of 10 kg/s2, then a 50 kg/s increase (from air injection) can be achieved over a 5 second period.
In
It is preferable that the thermal store 14 and connecting pipe to the fast acting valve 22, located close to the gas turbine, is pressurised slightly above the operating pressure of the gas turbine 2/4/6. The advantage of this is that the response time of the air injection will be faster and it allows for more accurate control of the flow rate that is being injected, which is desirable to protect the gas turbine. In addition there may also be an additional pressure let-down valve 20 or other pressure reducing device from the compressed air store to drop the pressure of the air to that of the pressure within the heat store. For example, the compressed air store may be at 250 bar, the air in the TES at 20 bar and the operating pressure in the gas turbine 17 bar. In this way when additional power is required, the fast acting valve 22 opens at a controlled rate (determined by the type of gas turbine) to inject additional air into the gas turbine and the additional valve 20 opens to ensure that the pressure in the TES stays at approximately 20 bar. It should be noted that where there is a direct thermal store it is likely that there is a significant buffer of residual air in this store and hence the pressure variation in the store will be relatively slow if the additional valve 20 is used to control the rate of air injection. This is also the reason why it is preferable to use a fast acting valve close to the gas turbine where the supply is above the operating pressure of the gas turbine.
Referring to
The change from the system operating in the mode of 4a to the mode of 4b can be achieved by closing the IGV's at the same time as additional air is injected into the gas turbine to compensate for the reduction in mass flow through the compressor. That process can also be rapidly reversed from mode 4b) back to mode 4a) by re-opening the IGV's, whilst reducing the amount of air injected. As combustor stability is no longer an issue, the one remaining technical constraint is compressor surge. If too much air is injected then it is possible to stall or surge the compressor. This limit will vary from gas turbine to gas turbine, but the upper limit is normally around 10% additional mass flow.
This method allows the power output of a gas turbine to be varied by around 10% within as little as 1-2 seconds without imposing any thermal stresses on the gas turbine or risking combustor instability. Note the actual change will be a function of the ability of the compressor to deal with the additional mass flow. At 10% air injection, the increase in power would be around 13% to the GT output.
The system operating in
As regards suitable control of the system, there are a number of factors that affect how much the mass flow through a compressor will change as the position of its IGV's is changed including, for example, the ambient air density (i.e. ambient temperature and pressure), the pressure ratio over which the compressor is operating, and its relative age. For a given gas turbine, there will usually be a compressor map that takes into account these factors and makes it possible accurately to calculate how much the mass flow through the compressor will change as the position of the IGV's is changed. If the control law and response characteristics of the actuator that controls the IGV position are also known then it is possible to calculate accurately by how much the mass flow will change over time as the IGV is moved from one position to another.
The ACAES system will also need to rely on rapidly responding valves to give accurate mass flow control either into or out of the gas turbine, that broadly match the change in mass flow from the compressor. In general, flow is the result of a pressure difference between two spaces and the discharge coefficient of the interconnecting duct system. If the spaces are close together and have large connecting pipes (high discharge coefficient) then a high flow rate can be achieved with a small pressure drop. If the spaces are a long distance apart and the connecting pipe is smaller (low discharge coefficient) the result will be a higher pressure drop to achieve the same flow. In addition for fast acting systems there is a certain amount of energy required to accelerate or decelerate the flow that can lead to transient pressure drops when valves open.
In the hybrid system it is desirable to have rapidly responding valves that are able to deliver accurate amounts of mass injection or bleed from the gas turbine. Furthermore the location of the thermal stores and compressed air system may be some distance from the gas turbine. Consequently it is preferable when the system is in a frequency response mode (i.e. where it is able to inject air) that it is kept at a pressure that is above that in the gas turbine. This might be 0.5 bar higher, 1 bar higher or even 5 bar higher than the gas turbine operating pressure. The advantage of having this at a significantly higher pressure is that it is easier to provide an accurate mass flow rate if there is a higher pressure drop as any transient pressure changes to either side will have less impact on the actual mass flow through the device. In addition the size of the orifice is also reduced as it is possible for a greater mass flow to pass through a fixed sized hole if the pressure drop is increased.
In terms of the valve, there are two functions taking place. The first is to seal the higher pressure supply from the gas turbine. The second is to control the mass flow rate and how it varies over time. This means that it is possible to replace a valve that also controls the flow with a simple valve and a flow controller within the passageway. Such a flow controller can be either upstream or downstream of the valve. Either solution is acceptable and by using a combination of inlet temperature and pressure and outlet pressure it is possible, for example, to calibrate the device so that the mass flow at different settings is known. To ensure stable inlet temperatures (and flow rates) it is preferable to insulate and heat the connecting pipework to the gas turbine and any valves so that they are maintained at a temperature that is close to that of the thermal store and hence the air exiting the thermal store. Again if the actuator characteristics are known then it is possible to calculate accurately by how much the mass flow will change over time as the valve is opened and the size of the opening provided.
In this way it is possible for both the IGV's and valves to be changed simultaneously so that the variation in mass flow through the combustor and turbine is kept minimal. Whilst it would be possible to use sensors and feedback loops to adjust the flow rates, for a faster acting system, it is preferable if this can be avoided as it is likely to lead to delays.
For fine tuning and recalibration while operating it is likely that such sensors will be useful. For example it may be possible simply by measuring the change to the gas turbine to estimate whether the model of either system is changing with time due to wear and tear. For example if both the IGV's and the valve are moved to positions that reduce the mass flow through the compressor and increase the rate of air injection and the pressure in the gas turbine rises more than expected then one option could be that the gas turbine compressor has degraded slightly. To compensate for this either the IGVs need to be closed slightly further or the rate of air injection (i.e. flow controller open area) needs to be reduced. As it is likely that changes will occur over time this should allow for accurate remapping of the relative performance of each system over time.
Whilst a mapping and calibration approach may be used to limit the mass flow variation through the combustor during power modulation, it may also be feasible that a model based controller could also be used to achieve the same end result. Such a controller might contain equations or code that describe the physical models of the compressor, IGV hardware, gas turbine and the ACAES system with associated control valves etc. Such an approach might result in a control system that is able to cope with the complex interactions, non-linearity and delays that would otherwise have to be extensively mapped to achieve a stable and fast-responding system.
Turning now to
In
The change from the system operating in the mode of
The change from the system operating in the mode of
The gas turbine used in these figures has a mass flow through the compressor at ISO conditions with IGV's fully closed of 450 kg/s and with IGV's fully open of 650 kg/s.
In
Referring to
The use of a hot air expander 28 means that the compressed air does not actually need to be stored (or re-injected) and hence the benefit of the increased upside in power range is provided without a large capital cost. Furthermore, the hot air expander 28 can add to the rated power capacity of the plant as long as there is stored hot compressed air. There are obviously losses associated with compressing and then re-expanding air, however the ability to vary power rapidly is valuable and may well outweigh the disadvantage of these higher losses. This is potentially a very low cost form of extra capacity. In this mode of operation the 50 kg/s is re-expanded from 16 bar back to ambient pressure. The compression work is around 20 MW and the expansion work around 16 MW, hence the losses are in the region of 4 MW for this example. The hot air expander 28 can be sized to match the mass flow being bled or it could have a different capacity to allow some optimization to be carried out between storing the air and having greater or lesser capacity at certain periods. For example the hot air expander could have a capacity of 100 kg/s and 33 MW. In normal bleed operation the flow through the expander is 50 Kg/s and then at peak periods this can be increased to 100 kg/s with supplemental air from the compressed air storage. It may also be preferable to insert a flow control valve upstream of the hot air expander to regulate the flow into this machine or to use some form of variable geometry to allow mass flow to be varied in a controlled manner.
In
In the
In the
Thus, referring to
In
According to the invention, to obtain an Improved Frequency response of merely a few seconds, the mass flow rate through the GT, and hence the combustor and turbine operating conditions, should be proactively controlled such that there is either no change, or, only a minimal change in GT mass flow rate during the time period of the power modulation. This is achieved by selectively changing the compressor mass flow rate and by proactively partially or fully balancing the GT mass flow rate using a compensatory mass flow rate from a compressed air system. While the examples of
Turning now to
In the above Figures, a direct TES 14 is proposed for storing and returning heat because such stores transfer heat more efficiently and hold the heat in readiness such that it can be returned without delay. However, an indirect TES may also be used (e.g. a heat exchanger coupled to liquid stores which do not need for example to store the heat at high pressures) within the hybrid system, although it may not be able to provide a rapid response injection mode unless steps are taken to keep it up to temperature.
In the case of either an indirect TES or direct TES, it may be advantageous for the ACAES system to include a gas buffer storing gas (preferably that has not yet been heated by the TES), and linked to the TES, so that when rapid power modulation is required gas is instantly available.
Usually, the GT system will be integrated with an adiabatic compressed air energy storage (ACAES) system in which the heat of compression is stored and returned. However, other CAES systems are not excluded.
Alternatively,
Such alternative heater systems may be provided in the airflow passageway network instead of a TES, or, in addition to the latter, in which case it may be provided in series or in an alternative (e.g. parallel) passageway, for example, to provide additional heat (for additional mass flow), or provide heat at a faster rate, or provide heat at a different temperature. A heater system may be configured such that the air returning from storage can be heated to a desired temperature having regard to the GT system conditions (e.g. to match them or exceed them by a selected amount), whereas a TES will only return heat at roughly the same temperature that it was charged.
Turning to
The system as depicted has minimal equipment and hence would be a simpler refit to an existing GT system. To that end, a small (e.g. ambient air fed) intercooled compressor 108, or series of compressor stages, is provided to charge a compressed air store such as a pipe store at a small mass flow rate, where the hybrid system is only required to provide (e.g. rapid) power modulation from storage on relatively rare occasions, such that recharging can be accomplished slowly using small power machinery. When a rapid response is required, pressure reducing valve 20 acts to let compressed air out of the store at a suitable flow rate. It the passes through direct combustor 214 which heats it to a suitable selected temperature (e.g. matching the GT conditions), before pressure reducing valve 22, with finer mass flow rate control, allows it to enter the GT system across a pressure drop at a desired flow rate.
Note to allow direct combustor 214 to be operational it may be necessary to have a small feed of air through both valve 20 and 22 as previously explained.
The system as shown is unable to extract air from the GT system, and hence, its functionality in terms of providing a fast response is correspondingly limited to providing for increasing air injection only. However, a further modification could allow this if, for example, a hot gas expander arrangement as in
Lastly,
Thus, the system may operate in an initial power generation mode “Initial Mode” where it is ready to provide an Improved Frequency Response in 5 seconds or less and the turbine mass flow rate is M and GT power is W0. This may involve selecting a suitable initial compressor configuration (for example, ensuring the IGV's have the capacity to be altered by the expected amount needed for a desired change of power ΔW1), and optionally, any bleed or injection mass flow rate in or out of the GT system for this mode. The bleed mode may be a bleed to the compressed air store and/or a bleed to air depressurisation apparatus that extracts useful work, for example, a hot gas expander or combined combustor and expander (optionally with downstream apparatus to extract further power), or to depressurisation apparatus that does not extract useful work such as a vent valve.
When an Improved Frequency Response is required, a control system (with associated sensors) selectively adjusts the compressor to a new Compressor Configuration for a new power setting W1 (=W0+ΔW1) for a second power generation mode or “Balanced Mode”, so-named because simultaneously, the anticipated change in mass flow rate through the compressor is balanced by the control system adjusting the mass flow being transferred in or out of the GT system via the fluid connections so as to keep M roughly constant in the combustor and turbine, within the limits that it may change by up to +/−6%×M per sec.
From
More usually, the Balanced Mode will not be an ideal long term running mode, and that mode will only be used as a transient mode (e.g. held for may be no more than 10 seconds, or up to 30 seconds, or up to a minute). Hence, the system may then revert back to the Initial Mode in readiness for another Improved Frequency response
Alternatively, it may switch from the Balanced Mode to a new (e.g. more efficient or sustainable) Running Mode 1 that has the same power W1, but this is now achieved by alteration, in a slower paced (Normal Frequency Response e.g. taking up to 10 seconds) change, to a new mass flow rate M1 through the combustor and turbine usually by resetting the compressor configuration;
Larger power modulations may require a switch from the GT system operating in a bleed mode to an injection mode (or vice versa), and may be needed to match or nearly compensate for an IGV alteration from at or near fully open to at or near fully closed. While the present invention relates to the operation of a hybrid power generation system comprising a compressed air system, it should be appreciated that some bleed modes need not necessarily involve the compressed air store. For example,
For the avoidance of doubt, the present invention relates to the operation of a hybrid power generation system based upon a conventional gas turbine design in which the compressor and turbine are always (mechanistically) coupled and fluidly connected downstream of one another. This is in contrast to prior art proposed gas turbine designs in which the compressor and turbine can be coupled together and decoupled at will and where flow connectors (e.g. with multi-direction valves) are required to allow or prevent air flow passing successively downstream from the compressor to the combustor and turbine.
Furthermore, whilst a hybrid system based on an ACAES system that stores and returns heat, and that comprises only power machinery that extracts useful work, may be the most efficient storage/generating solution, the present invention is more focussed upon providing a hybrid GT system that can respond in a matter of seconds to a grid requirement. Hence, it encompasses broader system arrangements as detailed above, with alternative components or sub-systems. (For example, power machinery either provided as a second stage in a CAES, or as ancillary depressurisation apparatus, are constrained by the flow rates they can handle and (slower cold start-up), whilst direct and indirect TES systems may be less flexible or responsive than heater systems.)
| Number | Date | Country | Kind |
|---|---|---|---|
| 1503848.2 | Mar 2015 | GB | national |
| 1522742.4 | Dec 2015 | GB | national |
This application is a national phase filing under 35 C.F.R. §371 of and claims priority to PCT Patent Application No. PCT/GB2016/050547, filed on Mar. 2, 2016, which claims the priority benefit under 35 U.S.C. §119 of British Patent Application Nos. 1503848.2 and 1522742.4, filed on Mar. 6, 2015 and Dec. 23, 2015, respectively, the contents of each of which are hereby incorporated in their entireties by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2016/050547 | 3/2/2016 | WO | 00 |
