EP1206634A4

EP1206634A4 - Supercharging system for gas turbines

Info

Publication number: EP1206634A4
Application number: EP00968293A
Authority: EP
Inventors: William L Kopko
Original assignee: Enhanced Turbine Output Holding LLC
Current assignee: Enhanced Turbine Output Holding LLC
Priority date: 1999-06-10
Filing date: 2000-06-09
Publication date: 2005-10-12
Also published as: AU775318B2; EA200200007A1; CN1355869A; AU7823200A; BR0011468A; JP2003529701A; EA005393B1; EP1206634A1; MXPA01012754A; BR0011468B1; CA2376788A1; KR20020038921A; KR100874508B1; CN1304740C

Abstract

A supercharging system for gas turbine power plants (11). The system includes a supercharging fan (30, 32) and a controller (50) for limiting turbine power output to prevent overload of the generator (28) at lower ambient temperatures. The controller can limit power output by burner control, inlet temperature control, control of supercharging fan pressure and other options. The system can be retrofit on an existing turbine without replacing the generator and associated parts.

Description

SUPERCHARGING SYSTEM FOR GAS TURBINES

Applicant claims the benefit of co-pending U.S. provisional patent application Serial

No. 60/138,848 filed June 10, 1999; Serial No. 60/139,894 filed June 22, 1999; Serial No.

60/152,277 filed September 3, 1999; Serial No. 60/159,207 filed October 13, 1999; and

Serial No. 60/195,302 filed April 10, 2000. This application is a continuation-in-part of co-

pending patent application Serial No. 09/388,927, filed September 2, 1999.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates generally to gas turbine power generation systems and more

particularly relates to a supercharging system for improving the capacity of gas turbine

power plants at high ambient temperatures. Specifically, the system uses a supercharging

fan combined with a controller to pressurize inlet air to the turbine to allow operation

with existing turbines; the supercharging fan is preferably combined with an inlet air

cooling system.

Background and Prior Art

It has long been recognized that the capacity of gas turbines declines with

increasing inlet air temperature; the typical penalty is on the order of 0.4% per degree F,

and this relationship is illustrated in Figure 1. This characteristic is especially

troublesome for gas turbines used in electrical power generation since the peak electricity

demand usually coincides with the highest ambient air temperatures. Gas turbines and associated generators and power distribution systems are usually rated based on turbine

capacity at 40 to 50 °F inlet air temperature. This low rating temperature means that the

capacity reduction at summer-peaking conditions can amount to approximately 20 to 40%

of turbine capacity, depending on the design, local weather conditions, and the

characteristics of the particular turbine.

Many different approaches for cooling inlet air to the turbine in order to reduce or

eliminate this capacity penalty are known in the prior art. A summary of these

approaches is described in the ASME paper, "Options in Gas Turbine Power

Augmentation Using Inlet Air Chilling," Igor Ondryas et al., presented at the Gas Turbine

and Aeroengine Congress and Exposition, June 11-14, 1990, Brussels, Belgium. Among

the alternatives for cooling are direct and indirect evaporative cooling, electric vapor-

compression, absorption, and thermal storage systems.

Of these many alternatives, direct evaporative cooling is the only approach that

has seen any significant commercial application. Direct evaporative cooling has the

advantage of low cost and simplicity, but the ambient wet-bulb temperature limits the

possible temperature reduction. For locations in the eastern U.S., direct evaporative

cooling can reduce inlet air temperatures by 10 to 20 °F, depending on the local climate.

Larger reductions are possible in warm, dry climates such as those of the southwestern

U.S. While direct evaporative cooling is helpful, it does not allow the turbines to run at

their full design capacity. After over 50 years of intensive research and development in

gas turbines, a better approach for dealing with high ambient temperatures has not been

produced. An interesting but virtually unused approach to address these problems is

described in the paper "Supercharging of Gas Turbines by Forced Draft Fans With

Evaporative Intercooling" by R.W. Foster-Pegg, ASME 1965. This paper describes the

use of a high-pressure fan to increase the inlet pressure to a gas turbine combined with an

evaporative cooler downstream of the fan as a way of increasing turbine capacity. This

approach could give large theoretical advantages, but it required special sizing of the

generator which limited its use to new turbines. In addition, the systems used a single fan

with inlet vanes for control purposes, which reduced the efficiency of the system.

Kolp et al. show the economics of the supercharging and cooling systems in the

ASME paper "Advantages of Air Conditioning and Supercharging an LM6000 Gas

Turbine Inlet," Journal of Engineering for Gas Turbines and Power, July 1995, vol. 117,

p. 513-527. This paper shows that while evaporative cooling is extremely attractive, the

economics of supercharging are not very attractive, with payback periods of over 10 years

for simple cycles. Supercharging is more attractive in combined-cycle plants, but its

economics are still marginal. The supercharging systems described in this paper are

virtually identical to those from the 1960s, so supercharging has not advanced

significantly despite decades of turbine development.

One significant problem in the prior art is that the supercharging arrangements

require increasing the size of the associated generator and other auxiliary equipment. The

cost of replacing the generator and other auxiliaries is so large that it effectively

eliminates this possibility for existing plants. Even in new installations, supercharging

may not be a practical option except at the very beginning of the project since the basic requirements of the generator, power distribution system, and associated hardware would

have to change. As Kolp et al. state in their paper (page 520), "in contrast to

supercharging, it is not necessary to increase the size of gas turbine plant equipment when

adding evaporative cooling." Thus the conventional wisdom is that evaporative cooling

may be added to an existing power plant, but supercharging is not a retrofit option.

Another method for controlling turbine capacity involves a variable-speed

compressor. Relevant patents include U.S. Patent Nos. 3,853,432 and 2,693,080. These

systems would provide a large range of control and usually were intended for use in

aircraft applications. A major problem with these systems is the cost and complexity of

the variable-speed compressor. Related problems are the reliability and maintenance

related to the large gearing required for these systems. These systems have not seen

significant use in power-generation applications.

The use of fogging for cooling inlet air for gas turbines is an additional related

technology in the prior art. For example, see Meher-Homji and Mee, Gas Turbine Power

Augmentation by Fogging of Inlet Air, Proceedings of the 28th Turbomachinery

Symposium, 1999. In addition to the benefit associated with cooling the inlet air, fogging

can further improve turbine performance by cooling air inside the compressor. This

intercooling effect is roughly a 5% increase in capacity for a water mass flow equal to 1%

of the air mass flow. This paper shows that fog intercooling may also improve

compressor efficiency.

One limitation of fogging is the amount of water mist that compressor may

safely ingest. Excessive water can cause problems with corrosion, erosion, or other damage to the compressor section of the turbine. At least one turbine manufacturer has

expressed concern about the effect on the compressor and will not guarantee its turbines

with fogging systems in many cases. While fogging offers some additional advantages

compared to evaporative pads, concerns about potential adverse affects on compressor

performance limit the capacity benefits of fogging to roughly 10% or less for many

turbines in humid coastal climates.

SUMMARY OF THE INVENTION

In accordance with one preferred embodiment of the present invention, a system

for improving gas turbine capacity at high ambient air temperatures includes at least one

supercharging fan that pressurizes turbine inlet air and a controller which limits

maximum turbine capacity to that of the turbine without supercharging. The system

preferably includes an air cooler for reducing turbine inlet air temperature. The system of

the invention may be added to an existing gas turbine or designed for a new,

supercharged, gas turbine system.

One of the principal objectives and advantages of the invention is the ability to

increase turbine capacity under summer-peaking conditions while reducing the installed

cost of the system based on capacity at summer-peaking conditions. Related objectives

of the invention are to create a relatively compact system that is not excessively complex

and that does not create unacceptable reliability or maintenance problems. An additional

objective is to develop a supercharger that allows practical retrofit of existing turbines. In

accordance with another aspect of the present invention, a supercharger for a gas turbine

comprises a fan and a fogger located in the inlet air stream of the turbine. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph illustrating the relationship between turbine capacity and

ambient temperature;

Figures 2A-2C are schematic diagrams of one preferred embodiment of the

invention that includes an air cooler downstream of a supercharging fan which

pressurizes air entering a gas turbine;

Figure 3 shows another preferred embodiment of the invention that uses an

indirect evaporative cooler;

Figure 4 shows an alternate embodiment of the invention that uses an axial-flow

fan;

Figure 5 shows another alternate embodiment of the invention that uses series

fans;

Figure 6 is a graph comparing the temperature- variant turbine capacity of several

different turbine systems;

Figures 7 A and 7B are schematic compressor maps that show the principles of

operation of the system according to the invention;

Figures 8A and 8B are fan curves that show how multiple fans can be used to vary

inlet pressure to a gas turbine;

Figure 9 is a graph of maximum supercharging pressure as a function of turbine

inlet air temperature;

Figure 10 shows an alternate embodiment of the invention in which heated

compressor output air is fed back into the compressor inlet air stream to regulate gas turbine performance;

Figure 11 shows an alternate embodiment of the invention in which turbine output

is controlled by regulating burner output;

Figure 12 shows an alternate embodiment of the invention in which turbine output

is controlled by regulating compressor inlet temperature;

Figure 13 illustrates an embodiment of the invention employed in a combined-

cycle (gas turbine/steam turbine) power plant;

Figure 14 illustrates a preferred embodiment of the invention which uses a

cooling tower and a cooling coil to cool air leaving the supercharging fan;

Figure 15 is a perspective view illustrating the preferred construction

configuration of high-pressure ductwork for use with the present invention;

Figures 16A and 16B are an end view and a section view, respectively, illustrating

construction of a diffuser/flow accelerator section of ductwork constructed as shown in

Figure 15;

Figures 17A and 17B are perspective views illustrating application of the

ductwork configuration shown in Figures 15, 16 A, and 16B to a supercharging fan

assembly; and

Figure 18 is a schematic diagram of another preferred embodiment of the

invention, wherein a supercharger for a gas turbine contains a fan and a fogger located in

the inlet air stream of the turbine.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 2A shows a preferred embodiment of the invention. A gas turbine power plant 11 contains a gas turbine system 10 and a generator 28. Gas turbine system 10

includes a compressor 12, a burner 14, and a turbine 16. The turbine 16 shares a common

shaft 24 with the compressor 12 and the generator 28. The compressor 12 receives

compressor inlet air stream 18 and pumps it to a higher pressure to create pressurized

burner inlet air stream 20 which is supplied to burner 14. Burner 14 heats the air from

stream 20 and supplies the resulting heated turbine inlet air stream 22 to the turbine 16.

Turbine 16 rotates in response to the received heated inlet air stream 22, thereby rotating

shaft 24 which in turn rotates generator 28 to generate electric power. Exhaust air stream

26 exits the turbine 16. This exhaust stream 26 may go directly into the atmosphere, or it

can enter a steam boiler in the case of a combined cycle plant as described below in

connection with Figure 13.

While Figure 2A depicts a simple turbine arrangement for the purpose of

explaining the invention, it will be recognized that more complicated arrangements (such

as a dual-spool configuration) also may be employed and do not change the principle of

operation of the invention. Of course, the gas turbine will normally include filters,

controls, safety devices, etc. as is known to those skilled in the art, and as such no

illustration or detailed explanation of such components is provided or necessary for

purposes of explanation of the present invention.

The inlet air stream 18 is provided to the compressor 12 as follows. A first

supercharging fan 30 and a second supercharging fan 32 draw ambient air 40 and supply

pressurized air to plenum 38. The air then goes through air cooler 34 which cools the air,

thereby forming a compressor inlet air stream 18 that enters compressor 12. The air cooler 34 is preferably a direct evaporative cooler that cools and humidifies the air

stream. Examples of possible configurations for evaporative coolers are well known in

the prior art. Other alternatives for the air cooler include a direct-expansion evaporator, a

chilled water coil or direct-contact heat exchanger, an indirect evaporative cooler, or other

device for lowering the temperature of the air stream. For the case of a chilled- water coil,

cold water can be provided by a vapor-compression chiller, an absorption chiller, or from

naturally occurring sources of cold water such as groundwater or bottom water from deep

lakes or seas. In the case of an absorption chiller, waste heat from the turbine exhaust

may be used as a heat source to drive the chiller. The air cooler is preferably located in

the air stream between the supercharging fans and the turbine, but it also could be located

upstream of the supercharging fans. The advantage of locating the air cooler downstream

of the supercharging fans is that it can remove any heat added by the supercharging fans.

A bypass damper 36 allows air to enter the plenum 38 without going through the

supercharging fans when the supercharging fans are not operating. First supercharging

fan 30 has a corresponding first damper 42 on its discharge end. Likewise, second

supercharging fan 32 has a second damper 44 on its discharge end. The controller 50

receives an input control signal from sensor 52 and controls operation of supercharging

fans 30 and 32 in accordance with such signal. Possible control inputs include air stream

temperature, compressor outlet pressure, generator output power, and ambient air

temperature. The controller can be as simple as a thermostat; alternatively, it may include

computer (i.e.. microprocessor) control and other associated electronics that also may

control and monitor turbine performance. Dampers 42 and 44 and bypass damper 36 act as check valves to prevent reverse

air flow from the gas turbine. The dampers preferably open in response to a pressure

gradient across the damper, with a gravity return to the closed position in the absence of a

pressure gradient. Figures 2B and 2C show how the dampers operate in response to

different fan operating modes, as explained more fully below.

The first and second supercharging fans 30 and 32 preferably are belt-driven

centrifugal fans or direct-drive axial fans. For centrifugal fans, the preferred design uses

backwardly inclined airfoil blades to maximize efficiency. Fans of this type can supply a

design static pressure of about 60 inches of water, which is approximately the required

value for most commercial applications. Electric motors — preferably three-phase

induction motors ~ normally would provide the power to drive the supercharging fans,

although engines or mechanical connection to the turbine itself are possible alternatives.

While Figures 2A-2C show two supercharging fans, three or more supercharging

fans can be used, or even a single supercharging fan may be used. Multiple

supercharging fans allow for staging of fans to adjust turbine inlet pressure in increments,

whereas a single supercharging fan does not provide this control option.

Preferably, the two supercharging fans have approximately equal pressure

capability but are of unequal flow sizing. The lead supercharging fan has a larger flow

capacity and preferably operates at a fixed speed to reduce cost. The lag supercharging

fan has variable flow capacity and adjusts turbine inlet pressure as is described more fully

below. Variable-speed drives or, in the case of axial fans, variable-pitch blades are

preferred means for adjusting supercharging fan flow. Inlet vanes are another alternative, but not preferred because of their relatively poor efficiency.

Figure 3 shows an alternate embodiment that uses an indirect evaporative cooler

that can approach the ambient dewpoint temperature. The configuration is similar to that

of Figure 2 except that an indirect evaporative cooler 60 is located in the air stream

between the plenum 38 and the gas turbine 10. As with the previous embodiment, the gas

turbine system 10 and generator 28 form a gas turbine power plant 11. Likewise, the gas

turbine system 10 includes a compressor 12, a burner 14, and a turbine 16. The indirect

evaporative cooler 60 uses a secondary air stream 62, which is taken from a portion of the

primary air stream 64 that exits from an optional direct evaporative cooler 68 located

between the indirect evaporative cooler 60 and the turbine 10 to optionally further cool

the air entering the turbine. A turbine inlet air stream 66 is formed by the remaining

portion of the primary air stream 64 and enters turbine 10. The air from the secondary air

stream 62 is indirectly heated and humidified by the air flow from plenum 38 inside the

indirect evaporative cooler 60 and exits as exhaust air stream 65.

Figure 4 shows another alternate embodiment that uses a motor-driven axial-flow

supercharging fan. Supercharging fan 116 includes a motor 100 that drives impeller 102,

both of which are contained in housing 106. The motor 100 is preferably a three-phase

induction motor and is connected to a utility power line by conductors 110, 112, and 114

through switching contactor 104. When the contactor 104 closes, the motor 100 is

energized, thereby driving impeller 102 which increases the pressure of discharge air

stream 108 entering the turbine system 10. When the contactor 104 is open, the motor

100 is de-energized and the impeller 102 is driven solely by the air stream 108, thus acting as a pressure reducer which reduces the pressure of air stream 108.

The contactor 104 may be a simple, manually operated device, in which case an

operator will determine when extra turbine capacity is appropriate and close the switch to

operate the supercharging fan. The preferred arrangement includes a thermostat 109

— preferably located in contact with the air stream 108 — which controls the contactor 104

and allows the supercharging fan 116 to operate when the temperature of the air stream

108 exceeds a predetermined value. Thermostat 109 thus functions to limit turbine

output power and therefore prevents overloading of the gas turbine power plant 11.

More sophisticated controls are possible. For example, the supercharging fan can

have variable-pitch blades, which are adjusted by a controller that senses pressure and

temperature conditions and varies the pitch of the blades to maximize turbine output.

Very sophisticated control is currently possible with microprocessor-based systems.

Additional mechanical hardware can be added to improve performance. For example, a

bypass damper can reduce the pressure drop through the supercharging fan when it is not

energized. A direct evaporative cooler or other cooling means could be added to reduce

turbine inlet temperatures.

One advantage of the system shown in Figure 4 is its low cost and simplicity,

which are especially important in small turbines. The optimum configuration requires

careful evaluation of the commercial application. While not preferred, the switch 104

may be eliminated in special applications. For example, if a turbine is moved from a cold

climate to a tropical climate, it may be desirable to add a supercharging fan that runs

continuously whenever the turbine is running. (Essentially, the supercharging fan would adjust the design conditions of the turbine to match the higher ambient temperatures.)

Another, less preferred option would be to drive the supercharging fan directly

from the turbine. The simplest configuration would be a direct mechanical connection

using a shaft that is attached to the turbine. However, normally this arrangement would

require a reduction gear to allow the supercharging fan to run at a speed that is much

slower than that of the turbine. An eddy current clutch or mechanical clutch could be

used to allow changes in fan speed. Another option would be to use a differential gearing

to a brake, a generator, or other reactor to reduce the speed of the fan by controlling the

speed of a second shaft. These arrangements would be difficult (or perhaps impossible)

to retrofit onto existing turbines, and the gearing and other mechanical components

necessary for this approach would require regular maintenance. Accordingly, this

alternate embodiment may not be as reliable or desirable as others.

Figure 5 shows another alternate embodiment, this one having two supercharging

fans arranged in series. A first supercharging fan 216 includes a first impeller 202 and a

first motor 200 in a first housing 206. The first fan is located in an inlet air stream 208

which enters compressor 12. Conductors 210, 212, and 214 connect the first motor 200

through switch 204 to the utility power line. A second fan 236 is located upstream of the

first fan 216. The second fan comprises a second impeller 222 and a second motor 220 in

a second housing 226. The second motor 220 is connected through switch 224 and

conductors 230, 232, and 234 to a utility power line.

Operation and benefits of the system of the invention will now be described in the

context of Figures 2A, 2B, and 2C, which show a system that operates with two supercharging fans arranged in parallel. In Figure 2A, both the first and second

supercharging fans 30 and 32 are operating, and the bypass damper 36 is closed to

prevent backward flow away from the turbine. First and second damper 42 and 44 are

both open to allow air flow from the fans.

Figure 2B shows operation with the first supercharging fan 30 off and the second

supercharging fan 32 on. The bypass damper 36 is again closed. The second damper 44

remains open, while the first damper 42 is closed. In Figure 2C, both supercharging fans

30 and 32 are off and their corresponding dampers 42 and 44 are closed. The bypass

damper 36 is open to allow air to go around the supercharging fans 30 and 32.

Figure 6 illustrates the benefits of such a system. This figure is based on

published performance data for gas turbines. The base system without any supercharging

or inlet cooling has a peak power output around an ambient temperature of 40°F, and

performance drops rapidly at higher temperatures. The figure shows that the capacity of

the present invention is essentially flat, while the capacity for conventional systems drops

rapidly at high ambient air temperatures. The base system is a simple-cycle turbine that

gives a maximum output power of 100 MW below 40 °F, but much lower capacity at

higher ambient temperatures. The present invention also has the same 100 MW capacity

but maintains it at high ambient temperatures.

The bigger base and bigger base with evaporative cooler are simple-cycle turbines

designed to match the present invention at high ambient temperatures. The bigger base

also can be a supercharged turbine from the prior art. The bigger base with evaporative

cooler takes advantage of the lower inlet temperatures available with evaporative cooling to reduce the size requirements of the turbine for a given capacity at high ambient

temperatures. The performance of the bigger base with evaporative cooling is also

similar to that of supercharged turbines from the prior art.

Conventional evaporative inlet cooling helps the performance of the turbine at

higher ambient temperatures without increasing the maximum output power at 40°F. A

conventional supercharger, which includes an evaporative cooler, further increases

turbine output power at all ambient temperatures, which would undesirably overload the

gas turbine power plant at lower ambient temperatures.

The present invention limits turbine output to allow the benefits of supercharging

at high ambient temperatures while preventing overloading of the power plant at low

ambient temperatures. This new feature results in a power output that varies very little

with ambient temperature changes.

Table 1 shows a cost comparison (adapted from Kolp et al.) for the supercharger

of the present invention compared to conventional systems. This table shows that adding

the new supercharger is less than half of the cost of adding peaking turbine capacity. The

incremental cost per kW for adding the supercharger to a system with an evaporative

cooler is about $300 per kW, whereas the cost of a new peaking turbine power plant is

$700 per kW. The combination of an evaporative cooler and the supercharger increases

turbine capacity at summer-peaking conditions by over 30%. The controls in the present

invention eliminate the need for the bigger generator and related hardware, since the peak

power output of the turbine at low ambient temperatures is unchanged. This analysis

shows that the present invention has a significant advantage in new installations. In retrofit situations, the cost of adding a conventional supercharger would be at least an

order of magnitude larger since it would require replacement of the generator and related

equipment in order to handle the increased output at low-ambient-temperature conditions.

Table 1 : Cost comparison

Table 2 shows how adding a supercharging fan can improve power plant

performance. For the example in the table, a supercharging fan would increase the turbine output by just over 4 MW while consuming only 1.24 MW of power. The result

is an increase in net power output of almost 2.8 MW. This simple analysis shows that a

supercharging fan can significantly improve peak power output for gas turbines.

Table 2: Power Comparison for a 100-MW plant

(Assumptions: 8 CFM/kW, 95°F DB, 75°F WB, 80% fan efficiency, 95%

efficient fan motor, 0.4% change in kW/(F, 0.6% change in kW/inch H₂O, 90% cooler

effectiveness.)

This table also shows that, in combination, a supercharging fan and an evaporative

cooler work synergistically to increase turbine power output. As stated before, adding the supercharging fan to the turbine without a cooling mechanism would increase power

output by 2.80 MW. On the other hand, adding the same fan to a turbine with a direct

evaporative cooler adds 4.06 MW of capacity. The effect of the supercharging fan is thus

almost 50% greater with the cooler than without the cooler. This analysis shows that the

effect of the combination is greater than the sum of the parts and thus is especially

desirable.

Table 2 shows that a significant improvement is possible with a supercharging fan

with only 10 inches of static pressure. Additional improvement is possible for

supercharging fans with higher static pressures. In many installations the optimum

supercharging pressure may exceed 60 inches of static pressure.

Figures 7A and 7B are compressor maps illustrating the improvement in turbine

capacity which can be obtained with the present invention. The vertical axis is turbine

pressure ratio, which is the turbine inlet pressure divided by the atmospheric pressure.

The horizontal axis is the mass flow parameter, which is given by the equation:

mass flow parameter - ^wv° , δ where:

m is the turbine mass flow rate,

δ is the compressor inlet pressure divided by the standard atmospheric pressure,

and

θ is the compressor inlet absolute temperature divided by design absolute

temperature.

The pressure ratio is the compressor discharge pressure divided by the atmospheric pressure. For purposes of this analysis, the effect of burner pressure drop

and other minor factors are lumped with the compressor and turbine performance. (For

background information in component matching, see B lathe, Fundamentals of Gas

Turbines, chapter 10.)

Compressor curve 300 shows the performance of the compressor at design

conditions, while compressor curve 301 shows the performance at peak inlet temperature.

Turbine line 302 shows the performance of the turbine at design conditions, and turbine

line 303 represents the turbine performance at peak inlet temperature. The intersection of

compressor curve 300 and turbine line 302 defines the design operating point 304. The

intersection of the compressor curve 301 and turbine line 303 is operating point 305 at the

peak inlet temperature. The operating line 306 shows possible turbine operating points at

different inlet temperatures. Surge line 307 is the limit of stable operation for the

compressor.

At operating point 305, the turbine capacity is significantly reduced from that at

the design operating point 304. The higher inlet temperature increases the speed of sound

of the air, which reduces the compressor Mach number and moves the compressor curve

to the left as shown in the figure. In addition, the higher temperature reduces air density

which further reduces mass flow rate. These changes reduce the compressor pressure ratio

and mass flow rate, which reduces the energy available for driving the turbine. Cooling

the air can restore the turbine capacity.

Figure 7B illustrates how the new system can improve turbine capacity at peak

ambient temperatures. Pressurizing the air entering the compressor increases the turbine pressure ratio and compressor inlet temperature to create a compressor curve 310. A new

turbine line 312 reflects the slightly higher temperature. The intersection of the turbine

line 312 and the compressor curve 310 defines an operating point 314. This operating

point corresponds to the operation with a supercharging fan and no inlet cooling.

Operating line 318 shows possible operating conditions with different operating

pressures. An additional benefit of higher pressure air is increased air density, which

further enhances the capacity improvement.

Another compressor curve 316 and a turbine line 320 correspond to a lower

compressor inlet temperature that can be achieved using an evaporative cooler. An

operating point 322 at the intersection of the compressor curve 316 and turbine line 320

corresponds to an operating condition for a supercharging fan and inlet cooling. This

analysis indicates that it is possible to approximate the original design capacity of a gas

turbine through a combination of inlet cooling and pressurization. The ultimate limits of

the turbine capacity are the operating pressures and power output that are acceptable for

the turbine, generator, etc. These factors would normally prevent the supercharging fan

and inlet cooling from creating a turbine output that is significantly above the turbine

design output.

Figure 8A plots fan curves showing how parallel supercharging fans can work

together to create a range of turbine inlet pressures. For parallel operation, the pressure

across the two fans is the same and the flows add together. A lead fan curve 350 is for

the lead fan. The gas turbine line 356 is nearly vertical since the flow through the turbine

varies only slightly with inlet pressure. The operating point 358 is at the intersection of the lead fan curve 350 and the turbine line 356. This operating point 358 corresponds to

operation with one fan.

A first lag fan curve 352 corresponds to the performance of the lag fan at full

speed. Fan curve 364 corresponds to running both fans together. The intersection of the

fan curve 364 and the turbine line is an operating point 366 that corresponds to operation

with both fans. A second lag fan curve 354 represents fan performance at low speed, and

fan curve 360 represents the corresponding two fan operation. An operating point 362

represents the operating condition with both fans operating and the lag fan at low speed.

Figure 8B illustrates operation with two similar fans in series. Fan curve 372

corresponds to operation with only one fan. The intersection of the fan curve and the

turbine line 370 represents operation point 378 for one fan. Fan curve 374 corresponds to

both fans running. Operating point 376 represents turbine operation with both fans

running.

Figure 9 shows a simple relation between maximum supercharging pressure and

inlet temperature that may be used to control the supercharger. The controller can use the

temperature entering the compressor to adjust the supercharger pressure. The result is a

very simple control system for maximizing turbine performance. This approach may be

very useful in controlling a supercharger in retrofit situations since it would require little

or no changes to existing controls for the gas turbine power plant.

More sophisticated control and operation are also within the scope of the

invention. For example, another preferred embodiment having a recirculation

arrangement around the compressor to control capacity of a gas turbine power plant 421 is shown in Figure 10. The gas turbine power plant comprises a gas turbine system 432

and a generator 426. The gas turbine system 432 includes burner 422, compressor 420,

and turbine 424, the latter two of which share a common shaft 430. Compressor inlet air

stream 440 enters the compressor 420 and is compressed to form burner inlet air stream

442. The burner 422 heats the air stream 442 to form burner outlet air stream/ turbine

inlet air stream 444 that enters turbine 424. The turbine 424 extracts power from the air

stream, which exits as exhaust 446. The gas turbine power plant also includes structure,

bearings, controls, and other components which are known in the art and therefore not

shown. The gas turbine power plant also may include a bottoming steam cycle system or

multiple shaft arrangements.

The turbine 424 drives the compressor 420 and generator 426, which also shares

the shaft 430. The generator supplies electric power to the utility grid through conductors

436 and a transformer 428.

Supercharging fan 423 pressurizes fan inlet air stream 453 to form a pressurized

air stream 455 that enters a first evaporative cooler 425. The first evaporative cooler 425

includes a pump 437 and an evaporative pad 435. The first evaporative cooler 425 cools

the pressurized air stream 455 to form compressor inlet air stream 440.

The supercharging fan supplies a static pressure on the order of 60 inches of water

and preferably may be a centrifugal fan or an axial fan. The pressurized air stream 455

and the compressor inlet air stream 440 are confined in ducts that can handle this high

pressure; preferred duct configurations are described below in connection with Figures

15, 16A, 16B, 17A, and 17B. A second evaporative cooler 470 is provided upstream of the supercharging fan

423. The evaporative cooler includes an evaporative pad 434 and a pump 436, which

circulates water over the pad to create a wet surface for cooling the ambient air stream

451 by means of evaporation before it is supplied to supercharging fan 423 as fan inlet air

stream 453. The evaporative coolers each also may include a sump with a float valve to

control water level and a means for bleeding off a small portion of the circulated water to

prevent build-up of salts.

Numerous evaporative coolers are available commercially from a variety of

manufacturers, so details of the cooler design are not included. Additionally, while direct

evaporative coolers are illustrated, indirect evaporative coolers or indirect-direct

evaporative coolers also may be employed.

A key feature of the present invention is the relative sizing of the supercharged

gas turbine and the generator and associated equipment. In particular, the generator and

the supercharged turbine are sized so that the generator operates at nearly full capacity at

summer-peaking conditions. (In contrast, according to the prior art designs, the generator

and auxiliaries would be sized based on full supercharged output at winter conditions,

which would typically be inlet temperatures of 40 °F or lower, and performance would

"fall of from these as ambient temperatures increased.) Accordingly, the present

invention includes means for controlling turbine capacity at low ambient temperatures so

as to prevent overloading the generator.

Preferably, a controller 460 receives a current signal 468 from a current sensor

462 that senses generator current. The controller 460 preferably includes the normal capacity and safety functions of the gas turbine power plant, but it may alternately be a

stand-alone unit. The current sensor is preferably a current transformer, in which case the

signal is in the form of an AC current. Other possible sensors may be such as to provide

a voltage, optical, or radio frequency output signal or some other type of signal.

The controller 460 provides a damper control signal 464 to damper 450 to control

the flow of a heated air stream 452, which is drawn from the burner inlet air stream 442

and circulates through the damper 450 to the compressor inlet. The controller 460 also

provides pump control signals 466 and 467 to pumps 436 and 437, respectively, and a fan

control signal 465 to the supercharging fan 423. Pump and fan control for this

embodiment may be as simple as on/off control, although variable control with variable-

speed drives, inlet vanes, or variable-pitched blades also might be employed.

Because the system is sized to provide maximum turbine/generator performance

at summer-peaking temperatures, as ambient temperature drops and the turbine output

increases, generator output will start to rise and exceed the maximum design output.

Therefore, as the ambient air temperature drops, the current sensor 462 senses a

correspondingly higher generator current signal 468 and communicates this to the

controller 460. The controller responds first by turning off the pump 436, thereby

deactivating the second evaporative cooler 470 and allowing the fan inlet air stream 453

to approach the ambient dry-bulb temperature. If the ambient temperature drops further,

the preferred control response is to start to open the damper 450 to allow heated air 452 to

flow from the compressor outlet to mix with the compressor inlet air stream 440. As

ambient air temperature drops even further, the damper will open even further so as to limit the generator current to that produced at summer-peaking performance.

If the ambient temperature drops even further, the controller 460 sends a fan

control signal 465 and a pump control signal 467 to turn off the supercharging fan 423

and the pump 437 for the first evaporative cooler 425; these signals terminate

supercharging entirely. The controller 460 may then send a damper control signal 464 to

close the damper 450.

Another preferred alternate embodiment of a turbine system according to the

invention is shown in Figure 11. Similar to the embodiment shown in Figure 10, a

controller 474 receives a current signal 468 from a current sensor 462 that senses

generator current, and the controller 474 provides a pump control signal 466 to pump 436

in response thereto. Rather than regulating a damper to provide heated compressor output

air back into the compressor inlet, however, the controller 474 also provides a burner

control signal 472 to the burner 422, which regulates the burner output and hence turbine

output. Preferably, the normal operating and safety controls for the gas turbine power

plant are integrated into the controller 474, but the controller also can be a stand-alone

controller.

The control approach used with the embodiment shown in Figure 11 is similar to

that used with the embodiment shown in Figure 10. As the ambient temperature drops

and causes a resultant increase in output generator current, controller 474 responds by

turning off pump 436 to deactivate the second evaporative cooler 470. If the generator

current still exceeds the summer-peaking design output, the controller 474 responds by

adjusting the burner control signal 472 to reduce the output of burner 422. At still lower ambient temperatures, the controller will turn off the supercharging fan 423 and the first

evaporative cooler 425 altogether so that the power plant operates without any

supercharging at all.

Figure 12 shows another alternate embodiment of the invention in which turbine

output is controlled by regulating compressor inlet temperature using a heater.

Controller 484 receives a temperature signal 490 from a temperature sensor 488 that is

located in the compressor inlet air stream 440. The controller 484 provides a heater

control signal 482 to a heater 480 located upstream of the compressor inlet, and the heater

provides a heated air stream 486 to the compressor 420. (While the preferred location for

the heater is upstream of the evaporative cooler, the heater alternatively may be located

between the evaporative cooler 470 and the compressor 420 if the heater is made of

materials that can handle high relative humidity without excessive coπosion.)

There are a number of options for implementing the heater. One simple option is

to use a gas burner. A second option is to use a boiler with a separate liquid-to-air heat

exchanger. A third option is to use a heat exchanger that recovers heat from the turbine

exhaust 446. (This option should provide the best efficiency and therefore is preferred if

installed cost is not excessive.) Electric heaters are a fourth option, although they are not

preferred because of their poor efficiency. Finally, blowing a portion of the exhaust 446

into the compressor inlet air stream 440 is a low-cost fifth option, but that may cause

corrosion problems in the compressor 420 or other components. Regardless of the

specific type, the heater should be capable of modulating its output so as to maintain the

heated air stream 486 at an approximately constant temperature. The control approach utilized with the embodiment shown in Figure 12 is to

maintain a minimum temperature of the supercharging fan inlet air stream 453.

Therefore, as the ambient air temperature drops, the controller 484 responds by turning

off the pump 436 to deactivate the evaporative cooler 470. If the ambient temperature

drops further, the controller 484 provides a heater control signal 482 to turn on heater 480

and adjust its output to maintain the required temperature of the fan inlet air stream 453.

Figure 13 shows another, similar alternate embodiment that is especially suitable

for use with a combined-cycle power plant. A combined-cycle gas turbine power plant

506 includes an additional steam cycle system 498 that utilizes exhaust heat from the

turbine 424 to generate additional power. The steam cycle 498 includes a boiler 504, a

steam turbine 500, a condenser 502, and a feed water pump 503, which are all connected

together in a circuit. A liquid-to-air heat exchanger 491 is provided in a fluid loop with

pump 492 and heat recovery heat exchanger 496. The pump 492 circulates heat transfer

liquid 512 through the heat recovery heat exchanger 496, where it absorbs heat from

exhaust stream 508. The pump receives a signal 494 from controller 484, which regulates

flow of the heat transfer liquid 512, thereby controlling the temperature of heated air

stream 486 entering the evaporative cooler 470. As with the embodiment in Figure 12,

the controller also can turn off the second evaporative cooler 470 as a first step in

controlling turbine inlet temperature, and then ultimately may shut down supercharging

altogether.

(One concern with using the exhaust stream 508 as a heat source, however, is

corrosion, and corrosion-resistant materials are required to handle the presence of nitric acid and possibly sulfuric acid. One possibility is to use plastic materials for the various

conduits, but plastics usually have relatively low temperature limits. Therefore, it may be

desirable to mix the exhaust with ambient air to reduce the maximum temperatures,

thereby permitting use of plastics. Direct contact liquid-to-gas heat exchange is also an

option. The liquid would contain a suitable neutralizing agent (such as sodium

bicarbonate) to prevent problems with acidic condensate. Here again it may be desirable

to mix the exhaust air stream with ambient air to reduce maximum operating

temperatures.)

In addition to an increased overall generating capacity at high ambient

temperatures, the system configuration shown in Figure 13 also should provide enhanced

efficiency, since combined cycle power plants actually increase their efficiency slightly at

higher compressor inlet temperatures. Moreover, whereas Figure 13 illustrates a heat

transfer loop for recovering waste heat, many other configurations are possible. For

example, heat from the condenser 502 could be used to warm the inlet air stream.

Providing an air-to-air heat exchanger between the exhaust and inlet air streams is also an

option.

The illustrated embodiments described above represent possible configurations of

the present invention. While these embodiments use either generator current or

compressor inlet temperature as feedback control parameters, other parameters may be

used to achieve similar results. For example, control inputs may include generator

power; ambient dry-bulb temperature; ambient wet-bulb temperature; shaft torque; or

other inputs. Furthermore, while Figures 10 through 13 show a second evaporative cooler

located upstream of the supercharging fan, this feature is optional and can be eliminated

without a major change in performance. The second evaporative cooler does enhance the

pressure capability of the supercharging fan at high ambient temperatures, however, and

thus adds some additional output power.

The use of the first evaporative cooler is also optional to some extent, but

performance will suffer significantly if it is eliminated. Therefore, only in cases where

water and/or space is/are severely limited would it be desirable to eliminate the first

evaporative cooler.

Finally, with respect to the various embodiments described above, simple on/off

control is the preferred method of control for the supercharging fan. More sophisticated

control using variable-speed drives or variable-pitch fan blades is an option, however, and

may result in some energy savings, but controlling the burner or inlet temperatures

already provides a means for modulating power output.

Figure 14 illustrates a preferred embodiment that incorporates an indirect

evaporative cooling system and a variable-speed drive for the supercharging fan. As

illustrated in Figure 14, a variable-speed drive 550 receives electrical power from the

utility grid and supplies variable-frequency AC power to an induction motor 522. The

induction motor 552 drives a shaft 554 that drives a supercharging fan 556. The

supercharging fan draws ambient air and supplies a pressurized air stream 558 to a

cooling coil 560. The cooling coil is a water-to-air heat exchanger that cools the air

without adding moisture to it. A cooled air stream 570 exits the coil and enters an evaporative cooler 425, which is configured as described above.

The cooling coil 560 is connected, by means of piping 562, to a pump 564 and a

cooling tower 566 to form a circuit, which circuit acts as an indirect evaporative cooler.

The cooling tower is preferably a forced-draft wet tower which can cool water to

temperatures approaching the ambient wet-bulb temperature.

(In areas where suitable water supplies are limited, an alternative would be to use

a dry tower and eliminate the evaporative cooler. In that case, the dry cooling tower and

coil assembly would act as a simple heat exchanger between the ambient air and the air

leaving the supercharging fan.)

Other heat exchanger and cooling configurations are contemplated. For example,

the cooling tower and cooling coil could be replaced with an air-to-air heat located

exchanger between an ambient air stream and the air exiting the supercharging fan. In

that case, a direct evaporative cooler could be placed upstream of the heat exchanger, on

the ambient air side, to provide additional cooling. While these configurations are

possible, they are not preferred because the large required heat exchanger area and the

large required pressure differential between the two air streams would create the need for

a large, expensive heat exchanger.

In climates where freezing presents a potential problem, the piping, cooling coil,

and cooling tower would require suitable freeze protection. Possible protection

alternatives include insulation and heaters, drain-down provisions, and the use of brine in

the cooling coil with a water-to-brine heat exchanger. (A water-to-brine heat exchanger

would provide the additional benefit of isolating the cooling coil from the fouling dirt associated wit the cooling tower; an appropriate filter could be included to reduce fouling

in systems without a secondary loop.)

In this embodiments, controller 568 prevents overload of the gas turbine at lower

ambient temperatures. The controller 568 receives a temperature signal 572 from a

temperature sensor 573 located at the compressor inlet, in the compressor inlet air stream

440. The controller also receives a pressure signal 574 from a pressure sensor 575

located in the pressurized air stream 558. The pressure sensor 575 can be located

anywhere in the air stream between the supercharging fan 556 and the gas turbine system

432, since the pressure changes are small in this section.

The controller 568 provides a speed control signal 578 to the variable-speed drive

550. This signal modulates the speed of the supercharging fan to maintain the optimum

supercharging pressure. The controller also provides an output signal 576 to pump 564 in

the cooling tower loop and an output signal 580 to the pump 437 in the evaporative

cooler. These two output signals would normally provide a simple on/off control.

While this embodiment uses a variable-frequency drive to control fan speed, there

are many other possible ways of modulating fan output that could be used. Examples

include electromechanical variable-speed drives such as described in U.S. Patent No.

5,947,854 and co-pending provisional application serial number 60/164590; eddy current

clutches; DC motors; variable pitch fan blade; variable inlet vanes; etc.

An important advantage of this embodiment is its ability to supply relatively

cooler air to the gas turbine system. The wet bulb temperature of the air entering the

cooling tower is unaffected by the energy input from the supercharging fan. Therefore, the cooling coil can cool the airstream 558 to temperatures approaching the ambient wet-

bulb temperature without adding any moisture. The evaporative cooler 425 can then

further cool and humidify the air stream. As a result, the airstream entering the turbine

system can be 10°F cooler than in the case where a simple direct evaporative cooler is

employed. This reduction in temperature provides roughly an additional 2.5% increase in

power output from the gas turbine. Because the cooling tower fan and pump typically

consume only 10% of this capacity increase, a net capacity improvement of over 2% can

be obtained with this system.

More complicated cooling systems are also possible. For example a mechanical,

desiccant, or absorption cooling systems may replace the direct evaporative cooler.

Cooling systems using ground water or cold lake or ocean water are another option.

These systems can achieve much lower air temperatures, which can further enhance the

capacity improvement. The disadvantage of this approach is the additional complexity

and cost of the additional cooling system.

As noted above, the pressurized air streams and the compressor inlet air stream

are confined within ducts able to withstand relatively high pressure, and Figure 15 shows

details of the preferred embodiment of such ductwork. An interior duct 610 is disposed

inside a round, outer duct 611, and a flow passage 612 equalizes the pressure of a fluid

flowing inside the interior duct and a space 615 that is located between the two ducts.

(For the present application, the fluid would be air, but the ductwork assembly may be

used to convey any high pressure fluid.) Preferably, the space 615 is filled with fluid or

may include porous material such as fiberglass, open-cell foam, etc. One advantage of a porous fill material is that it can help to reduce noise and also helps to support the ducts.

The flow passage 612 for equalizing pressure across the interior duct may be

simply an aperture or it can be a series of apertures or constituted by a porous surface.

The flow passage may also be provided by cracks or other small openings that are

common in unsealed ductwork. The critical requirement for the design of the flow

passage 612 is that it must be able to permit sufficient fluid flow rates through it to ensure

that if a leak develops between the round outer duct and the ambient atmosphere, an

excessive pressure drop across the interior duct will not result. Advantageously, the flow

path may include a pressure relief valve to reduce the risk of damage to the interior duct if

such a leak does occur.

While Figure 15 shows a rectangular interior duct, the interior duct may have

virtually any shape other than a round shape, and it may include large, flat surfaces

without concern for excessive strength requirements. The interior duct need only handle

any pressure drops, velocity pressure, turbulence, etc. associated with fluid movement

through the duct.

The outer duct preferably has a circular cross-section to minimize material

requirements. Other shapes such as oval or elliptical may be used, but they are not

preferred since they would increase the strength requirements significantly. Large round

ducts may include corrugations or other reinforcements to improve rigidity and reduce

risk of damage to the duct from wind or accidental loading.

Typical materials for constructing the ducts include metals such as steel or

aluminum. Other possible materials include plastic, wood, ceramics, etc. Material selection depends on factors such as strength, cost, and compatibility with the particular

fluid flowing through the ducts.

Figure 16A and 16B show a front and a cross-sectional view, respectively, of a

transition duct which may be used to connect two different sized ducts. A circular,

conical outer duct 620 encloses a tetrahedral or pyramidoidal interior duct 621. A flow

passage 622 is provided in the wall of the interior duct 621 to equalize fluid pressure

between the inside of the interior duct 621 and the space 623 that is located between the

two ducts. This transitional duct assembly can be used either as a diffuser or a flow

accelerator, depending on the direction of fluid flow. Moreover, although Figures 15,

16 A, and 16B illustrate basic configurations of the high pressure duct according to the

invention, this invention can be applied to practically any particular duct geometry,

including elbows, tees, transitions, etc.

Figures 17A and 17B illustrate such ductwork employed specifically with a gas

turbine supercharger. Figure 17A shows the cooler, without a supercharger, with a direct

evaporative cooler 630 at the end of a rectangular duct 631 that supplies air to the inlet of

a gas turbine. The rectangular duct 631 is designed to carry a pressure difference (as

compared to ambient) of just a few inches of water or less.

Figure 17B shows the corresponding supercharged configuration. The fan 635

increases the air static pressure to very high levels — typically about 60 inches of water.

The fan 635 is connected to an end piece 639 of a diffuser duct 636. The diffuser duct is

connected to a straight duct 637 that encloses the evaporative cooler and a contraction

duct 638. Each of these ducts includes a round duct on the exterior that encloses a rectangular interior duct.

This ductwork arrangement provides several significant advantages over the prior

art:

• Lower material weight and cost compared to conventional rectangular ducts;

• Lower pressure drop than conventional round ducts used with rectangular

components;

• Simple geometry for easy construction; and

• Ability to retrofit lower-pressure ductwork to withstand higher pressures.

These advantages are especially desirable for use in gas turbine superchargers.

The high pressures ( approximately 60 inches of water) and the large duct dimensions

(duct diameter of 30 feet or more) create the need for an extremely strong rectangular

duct that requires huge amounts of material for reinforcement. The new duct

configuration of the invention eliminates the need for the rectangular duct to withstand

the high gas pressure, which greatly reduces the material cost and weight. It also allows

the use of existing evaporative coolers and associated ducts, which greatly reduces

installation cost of the supercharger. While the invention is especially attractive for use

in gas turbine superchargers, it can be used in air-conditioning systems and other

industrial and commercial applications that require moving gases or liquids.

Fig. 18 illustrates an alternate preferred embodiment of the invention, using a

fogger in the inlet air stream of the gas turbine. A gas-turbine power plant 121 comprises

a compressor 120 and an expander 124 that are rigidly attached to a shaft 130 that drives

a generator 126. An air stream 191 enters the compressor, which pressurizes the air and supplies it to a combustor 122. The combustor heats the air and supplies it to the

expander 124. The expander extracts work from the expanding gas to drive the

compressor and the generator.

A supercharger 190 is located upstream of the gas-turbine power plant. The

supercharger comprises a fan 140, a first fogger 149, and a second fogger 169 that are

located inside a duct 147. The first fogger is located upstream of the fan, while the

second fogger is located between the fan and the turbine.

The fan 140 comprises a hub 141 and fan blades 142. The fan is rigidly attached

to a motor shaft 144. A motor 146 drives the motor shaft 144 and thereby drives the fan

140. The fan is preferably a variable-pitch axial flow fan. The hub 141 includes a

mechanism for adjusting the pitch of the fan blades 142 to adjust fan output pressure and

flow.

The motor is preferably a three-phase induction motor or other electric motor.

Another option is to drive the fan directly from the main turbine, which eliminates the

need for the motor. Separate prime mover for the fan, such as a second gas, a steam

turbine, or an internal-combustion engine, is also an option, though not preferred. An

important advantage of an electric motor is that it is relatively easy to install on an

existing gas turbine.

The output of the fan is on the order of 60 inches of water static pressure. The

optimum pressure depends on the availability of a suitable fan, generator capacity, turbine

capacity, and other factors.

A multistage, axial- flow fan, as shown in Figure 18, can achieve this static pressure. Centrifugal fans or single-stage axial fans are also an option. If centrifugal fan

is used, variable-pitch blades are not normally an option so a variable-speed drive is the

preferred means for controlling fan capacity. Other options include variable inlet vanes

or dampers, but they are less efficient. Variable-speed is also an alternative for axial fans.

The first fogger 149 comprises a first manifold 156, second manifold 158, and a

third manifold 160. Each manifold has spray nozzles that create mist 162. The first

manifold receives pressurized water from a first pump 150. Likewise a second pump 152

and a third pump 154 supply pressurized water to the second and third manifolds 158 and

160 respectively. The pump outlet pressure is preferably roughly 1000 to 3000 psi. A

stream of water 164 feeds the pump inlets. The water is preferably filtered,

demineralized water. An air stream 148 is drawn into the duct 147 through the first

fogger 149.

The second fogger 169 is located downstream of the fan. Like the first fogger,

the second fogger is comprised of multiple manifolds and pumps. The fourth, fifth and

sixth manifolds, 176, 178, and 180 are connected to fourth, fifth, and sixth pumps 170,

172, and 174 respectively. The action of pressurized water in the nozzles in the

manifolds creates a mist 182.

There is a large degree of flexibility in the design of the foggers. For example the

number of manifolds in each fogger is somewhat arbitrary. A larger number allows for

easier control over the amount of fog produced and provides additional redundancy. On

the other hand, fewer manifolds simplify installation and may reduce cost. In addition

manifolds are not necessarily of equal capacity. As far as the capacity of the foggers, the first fogger is preferably sized to ensure

nearly saturated air on the outlet of the fan. The second fogger would further saturate the

air and provide extra moisture for cooling inside the compressor 120. The total mass of

water added to the air stream would preferably be the amount for saturation at the inlet to

the compressor plus roughly 0 to 2% of the air mass flow rate.

A controller 161 controls the operation of the supercharger 190. The basic

approach is to reduce fan pressure and the amount of fogging at lower ambient

temperatures to prevent overload of the generator and other components in the gas-

turbine power plant. A fan inlet temperature sensor 182 and a fan outlet temperature

sensor 184 provide input to the controller. As the ambient wet-bulb temperature drops,

the fan inlet temperature provides a signal to the controller to reduce fan capacity by

providing an output signal to reduce the pitch of the fan blades. In addition, the lower

temperatures mean that less water is required to saturate the air, so the controller 160 can

turn off some of the pumps for the foggers.

As temperatures approach freezing, the pumps for the first fogger can be turned

off to prevent ice formation. The second fogger may still operate at this condition, if the

capacity of the generator and other components is adequate.

At very low temperatures when no supercharging is possible, the fan and the

foggers may be turned off and the fan may be allowed to free rotate in the air stream. A

bypass damper around the fan may be provided to reduce pressure drop to the turbine

under these conditions.

There are numerous variations on this basic embodiment. For example the second fogger may be eliminated in cases where the turbine compressor is especially sensitive to

the droplets of water. In this case the controller could modulate the amount of fog from

the first fogger to ensure complete evaporation of the water droplet before they reach the

turbine.

Another option, though not preferred would be to eliminate the first fogger. This

change would cause only a relatively small penalty in performance if the capacity of the

second fogger were increased to compensate.

Many options for the fan selection are possible. For example, multiple fans may

be desirable for some applications. Multiple fans can provide redundancy to improve

system reliability. They may also reduce cost through the use common parts and may

allow for more sophisticated control options.

Fixed fan output is another simple control alternative. This approach is an option

for cases where the generator is sized to handle the full output of the turbine at the normal

operating conditions. This option is also possible in cases where another means of

turbine capacity control, such as modulation of the combustor output or means for

heating the inlet air stream, can prevent overload of the generator and other components.

As with conventional gas-turbine power plants, filters and silencers are normally

provided with this system. The operation of the fan and the foggers is not normally

affected by dust so the location of the filter is primarily a matter of convenience. Fog

droplets do somewhat increase the pressure drop through filter, so the preferred location

is normally upstream of the fogger. A silencer is preferably located upstream of the fan

to prevent radiation of noise. This system can supply a large capacity increase. For a conventional supercharger

with a conventional evaporative pad with a 90% effectiveness and 60 inches of water

supercharging, the capacity increase is between 20 and 30 percent. Fog intercooling can

provide up to 10 percent additional capacity in additional capacity. In addition the fog

can effectively provide 100% evaporative cooling effectiveness and reduces the

theoretical fan power requirements by several percent. The result is a system that can

readily achieve a capacity increase of 20 to 40% or more, depending on the climate and

specific design limit of the gas-turbine power plant.

Advantages of This Embodiment

This preferred embodiment of the present invention has several important

advantages:

1) A large increase in capacity: The system can achieve a capacity increase

of 20-40 percent with most gas turbines.

2) Low cost: The cost of the fan and the fogger system is much less than that

for new turbine capacity.

3) Compact: The elimination of the large evaporative pad required for

conventional superchargers greatly reduces the size and cost of evaporative cooler and

associated ductwork.

4) Easy retrofit: The small size and the ability to use the existing generator

allow the system to be installed on existing turbines.

5) Improved controls: The controls allow the system to match the maximum capacity of the gas-turbine power plant at a wide range of ambient temperature

conditions.

6) Reduced fan power: The first fogger creates a mist that cools the air as it

goes through the fan, which reduces the theoretical fan power required for a given

pressure increase and mass flow rate.

7) Simplicity: The invention uses only a few simple components.

8) Reliability: The system uses proven components with good reliability. In

addition the gas-turbine power plant can continue to operate without the supercharger in

case of a component failure or other problem.

The advantages of the supercharging system for gas turbines, as a whole, are

significant and numerous. Among the most important ones are:

• Large increase in turbine capacity at high ambient temperatures;

• Ability to achieve a large capacity increase even in humid climates;

• Low installed cost;

• Simple design;

• Compact design;

• Flexible control possible;

• Can be retrofitted on existing gas turbines;

• Bypass damper allows turbine operation without supercharging fan;

• Multiple fans and dampers provide redundancy for reliable operation; and

• Blower and cooling means work synergistically to give large capacity

improvement. Overall, this system represents a major breakthrough in gas turbine technology.

Its simplicity and low-cost make it extremely desirable for power-generation applications

that now face major performance penalties at high ambient temperatures. The invention

having been thus described, it will become apparent to those skilled in the art that the

same may be varied in many ways without departing from the spirit and scope of the

invention. Any and all such modifications are intended to be covered within the scope of

the following claims.

Claims

What is Claimed Is:

1. A supercharged, power-producing gas turbine system, said system comprising: a gas turbine subsystem and an electrical generator, said gas turbine subsystem comprising a compressor, a burner, and a gas turbine, wherein a gas turbine subsystem input airstream is compressed by said compressor, heated by said burner, and expanded through said turbine to cause said turbine to rotate, whereby said turbine drives said generator to generate electrical power; a supercharging subsystem comprising at least one supercharging fan which increases the pressure of said gas turbine subsystem input airstream, whereby power output of said turbine and hence electrical output of said electrical generator may be increased; and a system controller; wherein said system controller monitors at least one system parameter and controls operation of at least one system component such that as ambient temperature decreases, turbine power output, which otherwise would increase with decreasing ambient temperature, does not exceed maximum supercharged summer-peaking power

output.

2. The supercharged, power-producing gas turbine system of claim 1, wherein said system controller monitors temperature of said turbine subsystem input airstream.

3. The supercharged, power-producing gas turbine system of claim 2, wherein said system controller controls operation of said at least one supercharging fan as a function of the temperature of said turbine subsystem input airstream.

4. The supercharged, power-producing gas turbine system of claim 1, wherein said system controller monitors pressure of said turbine subsystem input airstream.

5. The supercharged, power-producing gas turbine system of claim 4, wherein said system controller controls operation of said at least one supercharging fan as a function of the pressure of said turbine subsystem input airstream.

6. The supercharged, power-producing gas turbine system of claim 1, wherein said system comprises two or more supercharging fans arranged in parallel, said supercharging fans pressurizing a plenum from which said turbine subsystem input airstream is drawn, said plenum having a bypass damper to permit operation of said system without supercharging fan-pressurization of said turbine subsystem input

airstream.

7. The supercharged, power-producing gas turbine system of claim 1, wherein said system comprises two or more supercharging fans arranged in series.

8. The supercharged, power-producing gas turbine system of claim 1, wherein said supercharging subsystem comprises a first air cooler disposed between said at least one supercharging fan and said gas turbine subsystem so as to cool said gas turbine subsystem input airstream.

9. The supercharged, power-producing gas turbine system of claim 8, wherein said first air cooler comprises a direct evaporative cooler.

10. The supercharged, power-producing gas turbine system of claim 8, wherein a secondary airstream is drawn from said gas turbine subsystem input airstream and passed back through said first air cooler to enhance cooling performance of said first air cooler.

11. The supercharged, power-producing gas turbine system of claim 8, wherein said system controller controls operation of said first air cooler as a function of said at least one monitored system parameter.

12. The supercharged, power-producing gas turbine system of claim 11, wherein said first air cooler is an indirect evaporative cooler, said indirect evaporative cooler being disposed in a circuit including a pump and a cooling tower.

13. The supercharged, power-producing gas turbine system of claim 12, wherein said cooling tower is a forced-draft wet tower.

14. The supercharged, power-producing gas turbine system of claim 11, further comprising a second air cooler disposed upstream of said at least one supercharging fan, wherein said system controller also controls operation of said second air cooler as a function of said at least one monitored system parameter.

15. The supercharged, power-producing gas turbine system of claim 1, wherein said system controller monitors ambient air temperature.

16. The supercharged, power-producing gas turbine system of claim 15, wherein said system controller controls operation of said at least one supercharging fan as a function of the ambient air temperature.

17. The supercharged, power-producing gas turbine system of claim 1, wherein said system controller monitors electrical output of said electrical generator and controls operation of said at least one system component as a function thereof.

18. The supercharged, power-producing gas turbine system of claim 1, wherein a recirculation flow system feeds a portion of a compressor outlet airstream back into said gas turbine subsystem input airstream and said system controller controls operation of said recirculation flow system as a function of said at least one monitored system parameter.

19. The supercharged, power-producing gas turbine system of claim 1, wherein said controller controls operation of said burner as a function of said at least one

monitored system parameter.

20. The supercharged, power-producing gas turbine system of claim 1, wherein said system further comprises an airstream heater located upstream of said compressor and said system controller controls operation of said airstream heater as a function of said at least one monitored system parameter.

21. The supercharged, power-producing gas turbine system of claim 20, wherein said system controller monitors the temperature of said gas turbine subsystem input airstream and controls operation of said airstream heater as a function thereof.

22. The supercharged, power-producing gas turbine system of claim 20, wherein said airstream heater comprises a first heat exchanger through which circulates a heat- exchanging fluid, said heat-exchanging fluid circulating through a second heat exchanger in which said heat-exchanging fluid absorbs exhaust heat from said gas turbine.

23. The supercharged, power-producing gas turbine system of claim 22, wherein said system comprises a combined-cycle power plant and further includes an auxiliary steam cycle subsystem, said auxiliary steam cycle subsystem utilizing the exhaust heat from said gas turbine to generate additional electrical power.

24. The supercharged, power-producing gas turbine system of claim 1, wherein said supercharging subsystem comprises a variable drive which drives said at least one supercharging fan.

25. The supercharged, power-producing gas turbine system of claim 1, wherein said gas turbine subsystem and said generator are pre-existing and said supercharging subsystem and said system controller are provided by means of a retrofit.

26. A supercharging subsystem for use in a supercharged, power-producing gas turbine system, said supercharged, power-producing gas turbine system comprising a gas turbine subsystem and an electrical generator, said gas turbine subsystem comprising a compressor, a burner, and a gas turbine, wherein a gas turbine subsystem input airstream is compressed by said compressor, heated by said burner, and expanded through said turbine to cause said turbine to rotate, whereby said turbine drives said generator to generate electrical power, said supercharging subsystem comprising: at least one supercharging fan which increases the pressure of said gas turbine subsystem input airstream, whereby power output of said turbine and hence electrical output of said electrical generator may be increased; and a system controller; wherein said system controller monitors at least one system parameter and controls operation of at least one system component such that as ambient temperature decreases, turbine power output, which otherwise would increase with decreasing ambient temperature, does not exceed maximum supercharged summer-peaking power output.

27. The supercharging subsystem of claim 26, wherein said system controller monitors temperature of said turbine subsystem input airstream.

28. The supercharging subsystem of claim 27, wherein said system controller controls operation of said at least one supercharging fan as a function of the temperature of said turbine subsystem input airstream.

29. The supercharging subsystem of claim 26, wherein said system controller monitors pressure of said turbine subsystem input airstream.

30. The supercharging subsystem of claim 29, wherein said system controller

controls operation of said at least one supercharging fan as a function of the pressure of said turbine subsystem input airstream.

31. The supercharging subsystem of claim 26, wherein said system comprises two or more supercharging fans arranged in parallel, said supercharging fans pressurizing a plenum from which said turbine subsystem input airstream is drawn, said plenum having a bypass damper to permit operation of said system without supercharging fan- pressurization of said turbine subsystem input airstream.

32. The supercharging subsystem of claim 26, wherein said system comprises two or more supercharging fans arranged in series.

33. The supercharging subsystem of claim 26, wherein said supercharging subsystem comprises a first air cooler disposed between said at least one supercharging fan and said gas turbine subsystem so as to cool said gas turbine subsystem input airstream.

34. The supercharging subsystem of claim 33, wherein said first air cooler comprises a direct evaporative cooler.

35. The supercharging subsystem of claim 33, wherein a secondary airstream is drawn from said gas turbine subsystem input airstream and passed back through said first air cooler to enhance cooling performance of said first air cooler.

36. The supercharging subsystem of claim 33, wherein said system controller controls operation of said first air cooler as a function of said at least one monitored system parameter.

37. The supercharging subsystem of claim 36, wherein said first air cooler is an indirect evaporative cooler, said indirect evaporative cooler being disposed in a circuit including a pump and a cooling tower.

38. The supercharging subsystem of claim 37, wherein said cooling tower is a forced-draft wet tower.

39. The supercharging subsystem of claim 36, further comprising a second air cooler disposed upstream of said at least one supercharging fan, wherein said system controller also controls operation of said second air cooler as a function of said at least one monitored system parameter.

40. The supercharging subsystem of claim 26, wherein said system controller monitors ambient air temperature.

41. The supercharging subsystem of claim 40, wherein said system controller controls operation of said at least one supercharging fan as a function of the ambient air

temperature.

42. The supercharging subsystem of claim 26, wherein said system controller monitors electrical output of said electrical generator and controls operation of said at least one system component as a function thereof.

43. The supercharging subsystem of claim 26, wherein a recirculation flow system feeds a portion of a compressor outlet airstream back into said gas turbine subsystem input airstream and said system controller controls operation of said recirculation flow system as a function of said at least one monitored system parameter.

44. The supercharging subsystem of claim 26, wherein said controller controls operation of said burner as a function of said at least one monitored system parameter.

45. The supercharging subsystem of claim 26, wherein said subsystem further comprises an airstream heater located upstream of said compressor and said system controller controls operation of said airstream heater as a function of said at least one monitored system parameter.

46. The supercharging subsystem of claim 45, wherein said system controller monitors the temperature of said gas turbine subsystem input airstream and controls operation of said airstream heater as a function thereof.

47. The supercharging subsystem of claim 45, wherein said airstream heater comprises a first heat exchanger through which circulates a heat-exchanging fluid, said heat-exchanging fluid circulating through a second heat exchanger in which said heat- exchanging fluid absorbs exhaust heat from said gas turbine.

48. The supercharging subsystem of claim 26, wherein said supercharging subsystem comprises a variable drive which drives said at least one supercharging fan.

49. A method of operating a supercharged, power-producing gas turbine system, said method comprising monitoring at least one system parameter and controlling operation of at least one system component such that as ambient temperature decreases, power output of said gas turbine system does not exceed maximum supercharged summer-peaking power output of said system.

50. A duct for conveying a high-pressure fluid, said duct comprising an interior conduit disposed within an exterior conduit, said interior conduit having a polygonal cross-section and said exterior conduit having an arcuate cross-section, said interior conduit and said exterior conduit defining a space therebetween and said interior conduit having a flow passage in a wall thereof to provide fluid communication and equalize pressure between the interior of said interior conduit and said space.

51. The duct of claim 50, wherein said interior conduit has a rectangular cross-

section.

52. The duct of claim 51 , wherein said duct is a flow diffuser or flow accelerator duct and said interior conduit has a tetrahedral or pyramidoidal cross-section.

53. The duct of claim 50, wherein said exterior conduit has a circular cross-

section.

54. The duct of claim 53, wherein said duct is a flow diffuser or flow accelerator duct and said exterior conduit has a conical cross-section.

55. The duct of claim 50, wherein said flow passage is provided by means selected from the group consisting of an aperture, a series of apertures, and a porous surface.

56. The duct of claim 50, wherein said space is filled with fluid or a porous

material.

57. The duct of claim 50, wherein said interior conduit is supported within said exterior conduit by means of vertices of said interior conduit engaging inner surfaces of said exterior conduit.

58. A supercharged, power-producing gas turbine system, said system comprising: a gas turbine subsystem and an electrical generator, said gas turbine subsystem comprising a compressor, a burner, and a gas turbine, wherein a gas turbine subsystem input airstream is compressed by said compressor, heated by said burner, and expanded through said turbine to cause said turbine to rotate, whereby said turbine drives said generator to generate electrical power; a supercharging subsystem comprising at least one supercharging fan which increases the pressure of said gas turbine subsystem input airstream, whereby power output of said turbine and hence electrical output of said electrical generator may be increased; a system controller; wherein said system controller monitors at least one system parameter and

controls operation of at least one system component such that as ambient temperature decreases, turbine power output, which otherwise would increase with decreasing ambient temperature, does not exceed maximum supercharged summer-peaking power output; and a duct for conveying a high-pressure airstream through said system, said duct comprising an interior conduit disposed within an exterior conduit, said interior conduit having a polygonal cross-section and said exterior conduit having an arcuate cross- section, said interior conduit and said exterior conduit defining a space therebetween and said interior conduit having a flow passage in a wall thereof to provide fluid communication and equalize pressure between the interior of said interior conduit and said space.

59. A supercharged gas-turbine power plant, comprising: a gas turbine power system including a compressor, a burner and a gas turbine, wherein an inlet air stream fed to said power system is compressed by said compressor, heated by said burner, and expanded through said turbine to cause said turbine to rotate, whereby said turbine drives an electrical generator to generate electrical power; and a plurality of supercharging fans located upstream of said power system for increasing the pressure of said inlet air stream fed to said power plant.

60. The supercharger of claim 59, wherein said plurality of supercharging fans are in a parallel flow configuration.

61. The supercharger of claim 60, further comprising means for preventing flow away from said gas-turbine power system to prevent flow away from said gas turbine when the supercharging fans are not running.

62. The supercharger of claim 61, wherein said plurality of supercharging fans have different flow capacities.

63. The supercharger of claim 62, further comprising a controller that controls the operation of said supercharging fans so as to limit the gas-turbine capacity at low ambient temperatures.

64. The supercharger of claim 63, further comprising means for varying capacity of at least one of the supercharging fans in response to a signal from said controller.

65. The supercharger of claim 64, wherein said means for varying capacity comprises means for varying the speed of said at least one of said supercharging fans.

66. The supercharger of claim 59, further comprising an air cooler located in the air stream between said supercharging fans and said gas-turbine power system.

67. The supercharger of claim 59, wherein said plurality of supercharging fans are in a series flow configuration.

68. The supercharger of claim 67, wherein said supercharging fans are axial-flow

fans.

69. The supercharger of claim 68, further comprising a controller that controls the operation of said fans so as to limit gas-turbine capacity at low ambient temperatures.

70. The supercharger of claim 67, wherein said supercharging fans have approximately equal flow capacity.

71. The supercharger of claim 69, further comprising means for varying capacity of at least one supercharging fan in response to a signal from said controller.

72. The supercharger of claim 71, wherein said means for varying capacity comprises means for varying fan speed.

73. The supercharger of claim 72, wherein said means for varying capacity comprises means for varying fan-blade pitch.

74. The supercharger of claim 59, further comprising a bypass damper that allows air flow around said supercharging fans when they are not operating.

75. The supercharger of claim 59, further comprising a round duct that provides a flow path between said supercharging fans and said gas-turbine power plant.

76. In a supercharged gas turbine power plant having a gas turbine power system including a compressor, a burner, and a gas turbine, wherein an inlet air stream fed to the system is compressed by the compressor, heated by the burner, and expanded through the turbine to cause the turbine to rotate, whereby the turbine drives an electrical generator to generate electrical power; and having a supercharging system for increasing the pressure of the inlet air stream fed to the compressor to provide a pressurized inlet air stream to the compressor, the improvement comprising: a duct having a round cross sectional area that provides a flow path between said supercharging system and said gas-turbine power system for said pressurized inlet air stream.

77. A supercharged, power-producing gas turbine system, said system comprising: a gas turbine subsystem and an electrical generator, said gas turbine subsystem comprising a compressor, a burner, and a gas turbine, wherein a gas turbine subsystem input airstream is compressed by said compressor, heated by said burner, and expanded through said turbine to cause said turbine to rotate, whereby said turbine drives said generator to generate electrical power; a supercharging subsystem comprising at least one supercharging fan which increases the pressure of said gas turbine subsystem input airstream, whereby power output of said turbine and hence electrical output of said electrical generator may be

increased; and at least one fogger located upstream of said gas turbine subsystem input airstream, for providing a source of mist to humidify and cool said input airstream before it is inputted to said compressor.

78. The supercharged, power-producing gas turbine system of claim 77, wherein said at least one fogger is located upstream of said fan.

79. The supercharged, power-producing gas turbine system of claim 77, wherein said at least one fogger is located between said fan and said compressor.

80. The supercharged, power-producing gas turbine system of claim 77, further comprising a second fogger, wherein said at least one fogger is located upstream of said fan, and said second fogger is located between said fan and said compressor.

81. The supercharged, power-producing gas turbine system of claim 77, further comprising: a system controller; wherein said system controller monitors at least one system parameter and controls operation of said at least one fogger such that as ambient temperature decreases, turbine power output, which otherwise would increase with decreasing ambient temperature, does not exceed maximum supercharged summer- peaking power output.