AU2015354384B2 - Gas turbine with inlet air cooling system - Google Patents

Abstract

An air driven gas turbine power system (1) including a gas turbine (2) having a compressor (3) with a compressor air inlet (4) and a compressed air outlet (5), and a downstream expansion turbine (6), coupled with a selectively operable compressor inlet air cooling system (10). The inlet air cooling system (10) includes a closed circuit volume of relatively high specific heat capacity working fluid, the circuit including a first warm working fluid mass storage region (11) and a second cooled working fluid mass storage region (12). Means (13) are provided to convey working fluid extracted from the first warm mass storage region (11) through a first air cooled heat exchanger region (14) whereby the working fluid is cooled by a source of cooler external ambient air and then conveyed into the second cooled mass storage region (12), and means (17) are provided to convey working fluid from the second cooled mass storage system through a second air cooling heat exchanger region (18) and then return to the first warm mass storage region. The cooling system (10) also includes means to convey ambient air via path (20) over the second air cooling heat exchanger region for cooling via the cooled working fluid therein prior to delivery to the gas turbine compressor air inlet.

Description

[0001] The present invention relates to an improved air driven gas turbine power system for use in arid and semi-arid environments where there are significant diurnal ambient air temperature variations.

Background to the invention [0002] Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

[0003] It is well known that the efficiency of air driven gas turbine power systems, be they direct fired (internal combustion), or indirect fired, using one or more external heat sources, decreases as the ambient temperature of the turbine inlet air increases. This is because the density of the air decreases with increasing temperature, thereby limiting the maximum quantity of air able to be drawn in to the compressor and lowering the airflow through the expansion turbine.

[0004] Gas turbines however are regularly deployed in hot remote regions as centralised, decentralised and off-grid power generation systems, such as in or near mine sites. They are also used in other applications including solar and hybrid solar concentrator power generation systems, which again by their nature are generally set up in remote desert locations. In such areas, daily day/night temperature variations of around 20°C are not unusual. Depending on the type of gas turbine, this can represent efficiency variations of up to 30% which are significant.

[0005] Various gas turbine inlet air (GTIA) cooling solutions are available to improve plant efficiency during the high ambient temperature periods during the day. These cooling solutions fall into two general categories.

[0006] The first category comprises open circuit water based systems which includes those based on fogging, whereby water is pumped at high pressure via fogging nozzles through the ambient air, where the atomised water then evaporates causing the air to cool. While reasonably effective, this type of system has numerous disadvantages in that it consumes

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-2large quantities of demineralised water, requires a water treatment system, and extra measures must be taken to ensure water droplets are removed from the air prior to entry into the compressor.

[0007] Another type of system within this category are known as evaporative coolers which work on the principle of passing the air through a wet porous surface where at least a part of the water is evaporated thereby cooling the air. While this typically consumes less water than fogging, it is also generally less efficient, and the same issues apply with respect to the need to control the humidity of the cooled air prior to entry to the compressor and the need for water treatment.

[0008] The second category comprises refrigerant based systems. These include mechanical vapour compression chillers, in which a closed circuit of coolant fluid is compressed mechanically and then expanded through a coil arrangement within a heat exchanger. The expansion process results in a temperature drop of the coolant, which then indirectly cools the inlet air passing over the coils. These systems can be very effective in terms of cooling capability, but also have high capital and operational costs, as well as considerable maintenance and monitoring requirements.

[0009] Another type of refrigerant based system includes vapour absorption chillers which utilise heat energy instead of mechanical energy to drive the cooling system. While these may be a useful option, in that they can operate on waste heat from another system component, they are generally less efficient than vapour compression chillers and still suffer from the same disadvantages regarding capital and operating costs as well as maintenance and monitoring requirements.

[0010] Importantly, the majority of these systems are instantaneous and accordingly need to be run during the high temperature peak period, further increasing parasitic consumption of the overall installation at that time.

[0011] While it is also known to combine one or more of these particular known cooling systems with each other, and/or with some form of thermal energy storage system, the energy storage system is limited to using heat energy from a part of the process. Further, the disadvantages specific to each system remain, and the overall cost and complexity further increases with the addition of any associated thermal energy storage system of this kind.

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-3Summary of the invention [0012] According to a first aspect of the invention, there is provided an air driven gas turbine power system including:

a gas turbine having a compressor with a compressor air inlet and a compressed air outlet, and a downstream expansion turbine with a heated air inlet and an exhaust outlet; and a selectively operable compressor inlet air cooling system, the inlet air cooling system comprising:

a closed circuit volume of relatively high specific heat capacity working fluid, the circuit including:

a first warm working fluid mass storage region and a second cooled working fluid mass storage region;

means to convey working fluid extracted from the first warm mass storage region through a first air cooled heat exchanger region whereby the working fluid is cooled by a source of cooler external ambient air and then conveyed into the second cooled mass storage region;

means to convey working fluid from the second cooled mass storage system through a second air cooling heat exchanger region and then return to the first warm mass storage region; and means to convey ambient air over the second air cooling heat exchanger region for cooling via the cooled working fluid therein prior to delivery to the gas turbine compressor air inlet.

[0013] The means to convey the working fluid through the first air cooled heat exchanger and the second air cooling heat exchanger may be in the form of one or more pumps and/or include gravity feed systems where appropriate.

[0014] In a preferred form, a first pump means is provided to convey the working fluid to the first air cooled heat exchanger and a second pump means is provided to convey the working fluid to the second air cooling heat exchanger. However, in another form a single pump with associated switching means could be used to perform both functions.

[0015] Regardless of form, the means to convey the working fluid through the first air cooled heat exchanger is generally configured to selectively operate during the low temperature night time periods when the ambient air temperatures are at their lowest, to transfer warm working fluid from the first mass storage region, through the air cooled heat exchanger into the second cooled mass storage region. Similarly, the means to convey the working fluid through the second air cooling heat exchanger is generally configured to

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-4selectively operate during the high temperature day time periods, to pump cooled working fluid from the cooled mass storage region, through the second heat exchanger to cool the ambient air for delivery to the compressor air inlet.

[0016] In this manner embodiments of the invention provide a very simple, low capital cost and low maintenance means of cooling the inlet air to the gas turbine, by using cool ambient air at night to cool a closed circuit volume of high specific heat capacity working fluid, which is mass stored for subsequent use during the day to cool the inlet air when the ambient air temperatures are at their highest. The absolute inlet air temperature reductions achievable may not be as large as achievable by some of the prior art options discussed above, but the system enables automated long term operation to average the inlet air temperature fluctuations throughout the day with potentially significant cost savings.

[0017] Desirably, the system further comprises control means to automatically control operation of the inlet air cooling system in accordance with predetermined operating parameters.

[0018] In one preferred form, the first heat exchanger is of a fin - fan type, where the working fluid is conveyed through high surface area finned conduits and the cool ambient air is mechanically driven over the finned regions to maximise the heat transfer. In other embodiments air-cooled heat exchanger using plain tubes or cooling tower type arrangements could be used.

[0019] In a preferred form, the working fluid is water which is low cost, easy to process and readily available. However, in other embodiments the working fluid may be other suitable relatively high specific heat capacity fluids including refrigerants.

[0020] The first and second working fluid mass storage regions may comprise separate tanks. Alternatively, the warm and cold regions may be provided within a single tank unit having an insulating barrier there between. In one form the insulating barrier is in the form of a floating barrier that separates the two regions, automatically adjusting the capacity of each region as required.

[0021] In embodiments where the working fluid is a refrigerant, the first pump means may include, or consist of, a compressor to increase the pressure and temperature of the warm working media from the first mass storage region prior to cooling to close to ambient

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- 5temperature in the first air cooled heat exchanger. Additionally, a pressure reduction means would be provided downstream of the first heat exchanger to lower the pressure of the working fluid and enable expansion to provide the refrigerant cooling effect prior to transfer into the second cooled working fluid mass storage system.

[0022] In direct fired gas turbine power systems, the compressed air exiting the compressor is fed into a combustion chamber and mixed with fuel, which mixture once ignited creates high energy products of combustion which then expand throughout the various stages of the expansion turbine.

[0023] According to a second aspect of the invention there is provided a method of operating a gas turbine power system according to the first aspect of the invention, the method including the steps of:

operating the gas turbine; and selectively activating the inlet air cooling system to operate in either a first cooler period charging mode or a second warmer period cooling mode;

wherein during the first cooler period charging mode the high specific heat capacity working fluid is conveyed from the first warm mass storage region through the first air cooled heat exchanger region and into the second cooled mass storage region; and during the separate warmer period cooling mode working fluid from the second cooled mass storage region is conveyed through a second air cooling heat exchanger region and then returns to the first warm mass storage regionand ambient air is conveyed over the second air cooling heat exchanger region for cooling via the cooled working fluid therein prior to delivery to the gas turbine compressor air inlet.

Brief description of the drawings [0024] Preferred embodiments of the invention will now be described, by way of example only, with reference to the following drawings in which:

[0025] Figure 1 is a schematic system layout for a first embodiment gas turbine power system according to the invention utilising water or a similar fluid as the working fluid in the inlet air cooling system and a direct fired gas turbine;

[0026] Figure 2 is a schematic system layout for a second embodiment gas turbine power system according to the invention utilising refrigerant as the working fluid in the inlet air cooling system and a direct fired gas turbine;

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-6[0027] Figure 3 is a schematic system layout for a third embodiment gas turbine power system according to the invention utilising water or a similar fluid as the working fluid in the inlet air cooling system and an indirect fired gas turbine;

[0028] Figure 4 is a schematic system layout for a fourth embodiment gas turbine power system according to the invention utilising water or a similar fluid as the working fluid in the inlet air cooling system, and an indirect fired gas turbine utilising heat energy from a concentrating solar power installation;

[0029] Figure 5 is sectional sketch of an optional combined warm and cooled working fluid tank;

[0030] Figure 6 is a graph showing theoretical variations in generation capacity and average ambient temperature during a given hour range of the day; and [0031] Figure 7 is a graph illustrating daily temperature variation for 2005 at Newman Airport, Western Australia taken from the Australian Bureau of Meteorology.

Preferred embodiments of the invention [0032] Referring firstly to Figure 1 there is shown a schematic layout for an improved gas turbine power system shown generally at 1. The system includes a direct fired open cycle gas turbine 2 comprising a compressor 3, having an air inlet 4 and a compressed air outlet 5, and an associated expansion turbine 6, having a heated gas inlet 7 and an exhaust outlet 8. A combustion fuel source 9 is also provided in fluid flow communication with air exiting the compressed air outlet, generally via a combustion chamber (not shown), for subsequent combustion with the compressed air to generate the heated gas delivered to the heated gas inlet 7.

[0033] The system further includes a selectively operable compressor inlet air cooling system denoted generally by the envelope marked 10. This cooling system includes a closed circuit volume of relatively high specific heat capacity working fluid, which circuit includes a first warm working fluid mass storage region 11, and a second cooled working fluid mass storage region 12.

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-7[0034] Means to convey the working fluid include first pump means 13 provided to convey the working fluid from the first warm mass storage region to a first heat exchanger region shown generally at 14. This heat exchanger is preferably a simple air cooled heat exchanger whereby the working fluid conveyed from the warm mass storage region 11 is passed through a tube bundle consisting of plain or finned tubes 15 which are subjected to a flow of ambient air shown schematically at 16 which cools the fluid in the tubes.

[0035] Second pump means 17 are then provided downstream of the cooled working fluid mass storage region 12 to convey the cooled working fluid though a second heat exchanger region 18. This second heat exchanger again includes plain or finned tubes through which the cooled working fluid flows to cool ambient inlet air 19 guided around the tubes via the air cooling path 20.

[0036] In normal use the gas turbine would operate by drawing ambient air into the air inlet 4 of the compressor 3, where the air would be compressed and then mixed with fuel 9, such as natural gas or diesel, for combustion and expansion to generate high energy heated gas which in turn drives the expansion turbine 6. Typically the expansion turbine 6 is coupled to and drives the associated compressor 3. The inlet air to the compressor may be drawn via the path 20, or switched to an alternative more direct feed.

[0037] The air inlet cooling system 10 has two basic operating modes, which in most applications will correspond to night time operating periods when ambient temperatures are generally lower, and day time operating periods when ambient temperatures are generally higher. The first mode is that in which the working fluid is cooled and stored, and the second mode is that in which the cooled working fluid is used to cool the hotter ambient inlet air to the gas turbine.

[0038] In the first night time mode, working fluid from the warm mass storage region 11 is conveyed, via pump means 13, to the air cooled heat exchanger, where cool ambient air is preferably fan driven over plain or finned tubes within the heat exchanger to cool the working fluid there in. Depending on the configuration of the heat exchanger, a low approach temperature may be possible to bring the temperature of the working fluid fairly close to the ambient temperature at that time. Once cooled, the cooled working fluid is stored in the second cooled mass storage region 12. This storage region is ideally insulated, the degree of insulation needed being subject to factors such as the thermal mass of the stored working fluid and the surrounding environmental conditions etc.

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-8[0039] In the second day time mode, the cooled working fluid from the cooled mass storage region 12 is conveyed via the pump means 17 through the second air cooling heat exchanger region 18, which generally comprises a series of plain or finned tubes over, which the inlet air 19 is guided via path 20. This operates to lower the inlet air temperature from an ambient day time temperature to a temperature much closer to an ambient night time temperature which at the extremes could be around 20°C lower.

[0040] It will be appreciated that while the preferred embodiments incorporate means to convey the working fluid in the form of two separate first and second pumps 13 and 17, the system could be configured to use a single pump for both modes and/or include other conveying means such as gravity feeds.

[0041] Detailed configuration of the cooling system in terms of selection of type and volume of working fluid, designed durations of first and second mode operation, and heat exchanger and pump structure and power sources, will require optimisation according to various parameters including site location, gas turbine type, and local diurnal temperature variations as well as to consideration of various additional factors such as whether other existing cooling systems, such as generator cooling systems, may have extra capacity at night to contribute to the inlet air cooling.

[0042] However, details of a modelled example based on a 15 MWe open cycle gas turbine follows. This model is based on temperature variation data relating to inland regions of Western Australia as exemplified in Figure 7 which shows daily temperature variations for 2005 at Newman Airport, Western Australia. Based on a night-time ambient temperature of 20°C and day-time ambient temperature of about 40°C as shown by the highlighted summer temperature range in Figure 7, a GTIA cooling system as shown in Figure 1 would operate as following. At night the water from the warm mass storage region 11 is pumped to the heat exchanger 14 where it is cooled with the night-time ambient air of 20°C. By pushing air past the tube bundle 15 the water in the tubes can be cooled to about 22°C and stored at this temperature in the cooled mass storage region 12. During the higher ambient time daytime of 40°C, when the gas turbine performance decreases, pump 17 transports this 22 °C water from the cooled mass temperature storage 12 region to the heat exchanger 18 where the water cools the gas turbine inlet air 19 from around 40°C to around 27°C. The inlet air 19 cannot be cooled right down to the 22°C water from of the cold mass temperature storage 12, as all heat exchanger have an approach temperature. However, lowering the gas turbine inlet air 19 from 40°C to 27°C increases the gas turbine efficiency and the impact on the generation capacity of

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-9the GTIA cooling system with 4, 6 and 8 hours of cold mass temperature storage is shown in Figure 6. By cooling the inlet air 19 the water temperature increases to 35°C and pump 17 forwards it to the warm water mass temperature storage 11. To cool the gas turbine inlet air during a 40°C day around 35 tons of water at 22°C would be needed per hour of full-load operation. Once the cold mass temperature storage 12 is empty and the warm mass temperature storage 11 full the cooling process stops until the colder night-time temperature can be used to recharge the cold mass temperature storage 12, thus repeating the cycle.

[0043] Referring next to Figure 2 there is shown a second alternative embodiment gas turbine power system according to the invention that utilises refrigerant as the working fluid in the cooling system 10’, and like reference numerals have been used to denote corresponding features.

[0044] As will be apparent, the gas turbine is once again a direct fired open cycle gas turbine having the features previously described, the main changes residing in the cooling system 10’ which includes a change in working fluid and the equipment needed to handle that fluid. In this regard the closed circuit containing the refrigerant also includes a refrigerant compressor 21 and a pressure reduction means 22. The refrigerant compressor 21, which may replace the pump 13 of the first embodiment, acts to compress the working fluid to increase the pressure and temperature of the fluid prior to passing through the tubes 15 of the air cooled heat exchanger 14, and the pressure reduction means lowers the pressure of the cooled compressed working fluid, causing it to cool significantly further.

[0045] Conventional refrigerant cycles compress and cool the refrigerant during the operation of the water/air chiller system, while this embodiment decouples water/air chiller operation and refrigerant compression and cooling. This enables lower parasitic loads during the daytime operation where the value of electricity or mechanical energy is most valuable.

[0046] Turning then to Figure 3 there is shown a third alternative in which the cooling system is again water based as per system 10 of Figure 1, the main changes residing this time in the form of the open cycle gas turbine 2. In this embodiment the gas turbine has an indirect fired configuration, whereby all or some of the heat to be added to the compressed air exiting the compressor for subsequently driving the expansion turbine is provided via indirect heat exchange from an external heat source indicated generally at 23. This source could be generated by some form of external combustion system, which may be useful where it is desirable to use a low cost, external steam, or relatively “dirty” high residue solid fuel sources,

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- 10such as combusted coal or waste materials, or from other sources such as described in more detail with reference to Figure 4. In the event the external heat source 23 is insufficient or inadequate to reach the desired operating temperatures, the systems may also include means to optionally introduce, as marked at 24, a liquid or gaseous fuel to the air heated by the heat source 23 for subsequent combustion as required to achieve the target operating temperatures.

[0047] Referring to Figure 4, the illustrated system is basically the same as that shown in Figure 3 except for the detail in relation to the external heat source 23. In this system, compressed air is extracted downstream of the gas turbine and passed through heat source 23 in the form of a solar receiver 25 which is mounted on a tall tower 26. The solar tower operates by using a field of heliostats 28 to direct and concentrate heat energy from the sun 27 to the receiver 25. Housed within or external to the receiver are means to convey a suitable working fluid that are configured to absorb the concentrated heat energy and heat the fluid. In some cases the heated working fluid can be used to directly drive a turbine, but in the present case the energy is transferred from the working fluid to the air extracted from the gas turbine compressor. As described above, fuel may optionally be added to the heated compressed air at 24 if required.

[0048] It will also be understood that the water based inlet air cooling system 10 could be substituted with the refrigerant based cooling system 10’ in any embodiment of the invention.

[0049] While the warm and cooled working fluid mass storage regions could be discrete and physically spaced separate tanks, another option is shown at Figure 5. In this embodiment the warm and cold regions 11 and 12 may be provided within a single tank unit 29 having an insulating barrier 30 there between. In one form the insulating barrier is in the form of a floating barrier that separates the two regions, automatically adjusting the capacity of each region as required.

[0050] Furthermore, in all embodiments heat recovery systems using, for example, water, thermal oil, molten salts etc, could be installed downstream of the gas turbine 2 to further maximise cycle efficiency.

[0051] It will be appreciated that the system of the invention provides some significant advantages. For example the system is relatively inexpensive to install, is suitable for retrofitting to all gas turbine systems, does not consume any water or other working fluid, is not

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- 11 directly connected to air passing through the gas turbine, and is ideally suited to arid areas where temperature reductions of up to around 20°C may be achieved.

[0052] Furthermore, the low complexity enables unattended operation, simple maintenance and facilitates local construction and manufacturing content where possible. Additionally, the system is very flexible, as the night time cooling operation can be run independently of the gas turbine, and selectively utilised by the gas turbine during the day.

[0053] In summary, while the invention has been described with reference to specific examples, it will be appreciated that the invention is not so limited and may be embodied in many other forms.

Claims (17)

1. An air driven gas turbine power system including:

a gas turbine having a compressor with a compressor air inlet and a compressed air outlet, and a downstream expansion turbine with a heated air inlet and an exhaust outlet; and a selectively operable compressor inlet air cooling system, the inlet air cooling system comprising:

a closed circuit volume of relatively high specific heat capacity working fluid, the circuit including:

a first warm working fluid mass storage region and a second cooled working fluid mass storage region;

means to convey working fluid extracted from the first warm mass storage region through a first air cooled heat exchanger region whereby the working fluid is cooled by a source of cooler external ambient air and then conveyed into the second cooled mass storage region;

means to convey working fluid from the second cooled mass storage system through a second air cooling heat exchanger region and then return to the first warm mass storage region; and means to convey ambient air over the second air cooling heat exchanger region for cooling via the cooled working fluid therein prior to delivery to the gas turbine compressor air inlet.

2. A gas turbine power system according to claim 1 wherein the second air cooling heat exchanger is in the form of a fin-fan type heat exchanger.

3. A gas turbine power system according to any one of the preceding claims wherein the first warm working fluid mass storage region and the second cooled working fluid mass storage region are housed within a single vessel separated with an insulating barrier.

4. A gas turbine power system according to any one of the preceding claims wherein at least one of said means to convey working fluid is in the form of a pump.

5. A gas turbine power system according to any one of claims 1 to 4 wherein a single switchable pump is used to convey working fluid from the first warm mass storage region through the first air cooled heat exchanger and from the cooled mass storage system through the second air cooling heat exchanger.

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6. A gas turbine power system according to any one of claims 1 to 4 wherein the means to convey working fluid through the first air cooled heat exchanger and the second air cooling heat exchanger includes some form of gravity feed.

7. A gas turbine power system according to any one of the preceding claims wherein the working fluid is water.

8. A gas turbine power system according to claim 1 wherein the working fluid is a refrigerant and the first pump means includes, or consists of, a refrigerant compressor.

9. A gas turbine power system according to claim 8 wherein a pressure reduction means is provided downstream of the first heat exchanger to lower the pressure of the refrigerant and enable expansion to provide the refrigerant cooling effect prior to transfer into the second cooled working fluid mass storage system.

10. A gas turbine power system according to any one of the preceding claims wherein the gas turbine is a direct fired open cycle gas turbine.

11. A gas turbine power system according to any one of claims 1 to 9 wherein the gas turbine is an indirect fired gas turbine whereby some or all of the heat to be added to the compressed air exiting the compressor is provided via indirect heat exchange from an external heat source.

12. A gas turbine power system according to claim 11 wherein the external heat source is generated by some form of external combustion system.

13. A gas turbine power system according to claim 11 wherein the external heat source comes from a solar tower.

14. A gas turbine power system according to any one of claims 11 to 13 including means to introduce a fuel to the external heat source to achieve the target operating temperatures.

15. A gas turbine power system according to any one of the preceding claims wherein a heat recovery system is installed downstream of the gas turbine.

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16. A gas turbine power system according to claim 15 wherein the heat recovery system is in the form of any of a recovery system based on any one of thermal oils, molten salts or water.

17. A method of operating a gas turbine power system according to any one of claims 1 to 14, the method including the steps of:

operating the gas turbine; and selectively activating the inlet air cooling system to operate in either a first cooler period charging mode or a second warmer period cooling mode as required;

wherein during the first cooler period charging mode the high specific heat capacity working fluid is conveyed from the first warm mass storage region through the first air cooled heat exchanger region and into the second cooled mass storage region; and during the separate warmer period cooling mode working fluid from the second cooled mass storage system is conveyed through a second air cooling heat exchanger region and then returns to the first warm mass storage region, and ambient air is conveyed over the second air cooling heat exchanger region for cooling via the cooled working fluid therein prior to delivery to the gas turbine compressor air inlet.