Small Gas Turbines

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WARNING:Please read the License Agreementon the back cover before removingthe Wrapping Material

Small Gas Turbines forDistributed Generation Markets

Technology, Products, and Business Issues

A Product of EPRIsolutions, Inc., and GTI

1000768

GTI-00/0219

Effective December 6, 2006, this report has been made publicly

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Small Gas Turbines for Distributed Generation Markets

Technology, Products, and Business Issues

1000768

GTI-00/0219

Technical Progress, December 2000

EPRIsolutions Project Manager

K.R. Amarnath

GTI Project Manager

W.E. Liss

EPRIsolutions 3412 Hillview Avenue, Palo Alto, California 94304 PO Box 10414, Palo Alto, California 94303 USA800.313.3774 650.855.2121 [email protected] www.epri.com

Gas Technology Institute 1700 South Mount Prospect Road, Des Plaines, IL 60018-1804847.768.0500 www.gastechnology.org


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CITATIONSThis document was prepared by

Steven I. FreedmanEngineering Consultant410 Carlisle AvenueDeerfield, IL 60015

S. Freedman

Onsite Energy Corp.701 Palomar Airport Road, #200Carlsbad, CA 92009

P. Bautista

This document describes research sponsored by EPRIsolutions and GTI.

The publication is a corporate document that should be cited in the literature in the followingmanner:

Small Gas Turbines for Distributed Generation Markets: Technology, Products, and Business

Issues, EPRIsolutions, Palo Alto, CA, and GTI: 2000. 1000768. GTI-00/0219.


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EPRIsolutions Licensed Material

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ABSTRACT

Small gas turbines (300 kW to 5 MW) offer an attractive way for utilities and energy servicecompanies to generate electric power within distribution grids and for consumers to generate

their own power. Distributed generation also benefits utilities by deferring or avoiding costlyexpansion of the power transmission and distribution system, which could allow them to offercustomers lower cost power.

Gas turbines process more power-generation cycle air per unit size and weight of machine thando reciprocating engines and, consequently, are lighter weight and more compact. This size andweight advantage could reduce gas turbine cost relative to reciprocating engines if businessconditions warrant production of small gas turbines at volumes equal to or greater thanreciprocating engines. The characteristics of gas turbines include clean emissions, goodefficiency in some models, high reliability, low noise and vibration, compact installed footprint(sq ft/kW), a good match with exhaust-fired steam generation boilers, and infrequentmaintenance and overhaul.

However, small gas turbines are not generally sufficiently economically attractive at the presenttime for intermediate-duty, daily cycling applications in competition with deregulated grid pricesor reciprocating engine generators. The high technology of the newest, very large gas turbines both heavy-duty industrial and aeroderivative machines I has not yet found its way into thesmaller sizes. The cost of applying known technology is still unjustifiably high considering thelimited size of the current market and the manufacturing difficulties involved. As deregulationprogresses and more commercial and light industrial businesses and electric utilities takeadvantage of distributed generation options, additional sales of small gas turbines should result inmanufacturing economies of scale and price reductions.

Very small gas turbines, 3 MW or less, are noticeably higher priced per kilowatt than competingreciprocating engines, due principally to low production volumes and low commonality of partsamong multiple turbine models. Reciprocating engines benefit from their modularity of pistonand cylinder configuration, allowing multiple power capacities to be delivered from the samebasic engine family or design. In the 3-5 MW size range, gas turbines are manufactured ingreater volumes, and their compactness and high power density begin to make them moreeconomically competitive. In sizes above 5 MW, gas turbines have an inherent economicadvantage over reciprocating engines in that they process more air per unit volume of machine,and their power generation efficiencies begin to approach those of reciprocating engines.

As distributed generation applications increase, manufacturing economies of scale could makesmall gas turbines an economic source of power or combined heat and power to commercial and

small industrial customers with high intermediate power prices.


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CONTENTS

1 INTRODUCTION ...............................................................................1-1Market Background................................................................................................. 1-1

Competitive Positioning of Small Gas Turbines ...................................................... 1-1

Report Objective ..................................................................................................... 1-4

2TECHNOLOGY DESCRIPTION ........................................................2-1Basics .....................................................................................................................2-1

Types of Gas Turbines ...................................................................................... 2-2

Ratings .............................................................................................................. 2-3

Fuels..................................................................................................................2-3

Fuel Pressure Requirements............................................................................. 2-4

Operational Characteristics ...............................................................................2-6

Thermodynamic Cycles .......................................................................................... 2-7

Simple Cycle...................................................................................................... 2-7

Recuperated Cycle............................................................................................2-8

Intercooled Cycle...............................................................................................2-8

Intercooled Recuperated Cycle ......................................................................... 2-9

Design Considerations..........................................................................................2-12

Recuperated Cycle Design.............................................................................. 2-12

Intercooled Cycle Design................................................................................. 2-13

Combined Intercooled and Recuperated Cycle Design................................... 2-15

Size Considerations..............................................................................................2-16

Gas Turbine Systems......................................................................................2-16

Gas Turbine Component Selection.................................................................. 2-17

Example of a Tradeoff ..................................................................................... 2-18

The Use of Steam to Increase Efficiency..............................................................2-19

Steam Injection................................................................................................ 2-19

Combined Cycles ............................................................................................2-19

High-Temperature Materials and Component Engineering...................................2-20

Turbine Blade Cooling ..................................................................................... 2-21

Competing Technology Reciprocating Engines .................................................2-22


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Heat Recovery and Cogeneration ...................................................................2-23

Noise ...............................................................................................................2-23

Emissions and Environmental Concerns ..............................................................2-24

Advanced Gas Turbine Cycles ............................................................................. 2-25

Recent Developments in Advanced Cycles..................................................... 2-25Wet Compression............................................................................................2-27

Investment in the Development of New Products............................................ 2-28

Alternative Fuels..............................................................................................2-29

3EMISSION CONTROL & CATALYTIC COMBUSTION.....................3-1Introduction ............................................................................................................. 3-1

Emission Control Technology .................................................................................3-1

Catalytic Combustion for Gas Turbines ..................................................................3-2Catalytic Combustion System Developers..............................................................3-3

Alzeta ................................................................................................................ 3-3

Catalytica...........................................................................................................3-4

PCI ....................................................................................................................3-4

Low Emission Gas Turbine Projects .......................................................................3-5

Economics .............................................................................................................. 3-6

4PROVIDERS .....................................................................................4-1

Company Profiles.................................................................................................... 4-8

Alstom Power .................................................................................................... 4-9

Dresser-Rand .................................................................................................... 4-9

Ebara............................................................................................................... 4-10

GAS Power Systems.......................................................................................4-11

Honeywell ........................................................................................................ 4-12

Kawasaki Gas Turbine ....................................................................................4-13

Nuovo Pignone................................................................................................4-14

OPRA Optimal Radial Turbine......................................................................... 4-15

Orenda ............................................................................................................ 4-16

Pratt & Whitney Canada.................................................................................. 4-16

Ramgen Power Systems ................................................................................. 4-17

Rolls-Royce ..................................................................................................... 4-18

Schelde Heron B.V..........................................................................................4-19

Solar Turbines ................................................................................................. 4-20


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Stewart & Stevenson ....................................................................................... 4-21

Turbomeca ...................................................................................................... 4-21

Vericor ............................................................................................................. 4-21

Yanmar Diesel Engine Co. Ltd. .......................................................................4-22

5ECONOMICS & BUSINESS ISSUES................................................5-1Introduction ............................................................................................................. 5-1

Regulated Utilities The Previous Era ..............................................................5-1

Deregulation The Present Era ........................................................................5-2

Applications ............................................................................................................5-3

Combined Heat and Power ...............................................................................5-3

Standby Generation........................................................................................... 5-4

Intermediate Duty ..............................................................................................5-4Competitive Positioning of Small Gas Turbines ...................................................... 5-5

Capital Costs .......................................................................................................... 5-7

Generator Set Equipment..................................................................................5-8

Switchgear and Utility Interconnection............................................................... 5-8

Heat Recovery Equipment................................................................................. 5-9

Emission Control Equipment ............................................................................. 5-9

Total Installed Costs........................................................................................5-11

Operation & Maintenance Costs ...........................................................................5-11

Non-Fuel O&M................................................................................................. 5-11

Fuel .................................................................................................................5-12

Cost of Electricity .................................................................................................. 5-12

Market Outlooks.................................................................................................... 5-13

CHP Market ..................................................................................................... 5-13

Standby Market ...............................................................................................5-14

Intermediate-Duty Market ................................................................................ 5-14

POWER GENERATION HISTORY......................................................... A-1

Gas Turbine History ................................................................................................A-1

Early Development ............................................................................................A-1Small Gas Turbines for Automotive and Other Vehicle Applications .................A-2

Microturbines.....................................................................................................A-3

BIBLIOGRAPHY ..................................................................................... B1


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1-1

1INTRODUCTION

Market Background

Continued market growth is expected for natural gas-fueled prime movers, primarily turbinesand reciprocating engines. Gas turbines in both simple- and combined-cycle systems haveaccounted for the vast majority of power generation capacity added in the last five years in bothinternational and U.S. markets. These are predominantly central station power plants greaterthan 50 MW. In short, large gas turbines have become the power generation technology ofchoice. This trend is expected to continue over the foreseeable future. Several factors contributeto the strong position of gas turbine-based power generation:

1. An optimistic outlook for the supply and price of natural gas

2. Technology advances that produced substantial improvements in efficiency and emissions

3. Emissions regulations that could favor gas turbine projects over traditional fossil-fueledsteam turbines

4. Attractive initial capital costs and reduced time and cost for power plant permitting andinstallation, compared to traditional power plants.

While the market and business climate is quite favorable for large gas turbines, gas turbines inthe distributed generation market face greater challenges. Fundamental market drivers favorlarge gas turbine power plants because of lower capital costs and shorter construction andpermitting lead times than traditional fossil-fueled steam turbines. Large combined-cyclesystems have efficiencies in the 50-58% range, based on the fuels lower heating value (LHV).

The environmentally clean nature of the these plants is evidenced by their ability achieve 9 ppmof nitrogen oxides (NOx) emissions without exhaust treatment and lower than 3 ppm with post-combustion control technologies.

Competitive Positioning of Small Gas Turbines

Gas turbines start to lose their clear advantage at smaller sizes. In the industrial market segment(3.5-30 MW), worldwide sales of gas turbines and reciprocating engines are about equal. Thesize range below 5 MW is dominated by reciprocating engines, both natural gas and dieselfueled. As gas turbines decrease in size, they exhibit higher specific capital costs ($/kW) andlower efficiencies. For example, in the 2-5 MW size range, gas turbines and reciprocating

engines have comparable capital costs, but the reciprocating engines have substantially betterefficiencies, between 37% and 42% (LHV), versus 32% or lower typical of comparably sized gasturbines (Figure 1-1). The new Mercury 50 gas turbine, a recuperated 4-MW machinemanufactured by Solar Turbines Incorporated (San Diego), is the exception to this relationship;this unit has an efficiency of about 40%. In the 2-MW and smaller range, reciprocating enginesare priced lower than gas turbines and are considerably more efficient. Figure 1-2 shows theprice and efficiency of simple-cycle gas turbine products up to 25 MW. Figure 1-3 compares the


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Introduction

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capital costs of reciprocating engines and gas turbines (also called combustion turbines, CT inthe figure) up to 7 MW.

The distinct advantages of small gas turbines over reciprocating engines include higher qualityrecoverable energy, lower emissions, lower maintenance requirements, and higher power density(kilowatt or horsepower per unit of air flow and machine volume). Recoverable energy refers to

the amount of energy that can be recovered from the turbine exhaust stream, usually in the formof clean, high-temperature heat. Higher quality recoverable energy allows for a wider range ofthermal energy (for example, high-pressure steam) to be generated if needed. Consequently,many small gas turbines are currently deployed in combined heat and power configurationswhere this recoverable energy can be used and higher total fuel efficiency can be achieved. Inaddition, gas turbines often have less frequent requirements for routine maintenance compared toreciprocating engines, which need periodic oil and spark-plug maintenance.

Figure 1-1Small Gas Turbine Product PositioningSource: GTI


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Introduction

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Gas Turbine ProductsSimple Cycle Units

Actual purchase prices may vary due to market conditions and other factorsSource: Gas Turbine World/GTI

,

,

,,

,,

,

,

,

, ,

,,,,

,,,

,,

,,

,

,,

,

0 5 10 15 20 25

Size (kW, thousands)

200

400

600

800

1000

Price ($/kW)

20

25

30

35

40

Efficiency (%, LHV)

Price

Efficiency

,

Figure 1-2Small Gas Turbine Products

Actual purchase prices may vary due to market conditions and other factors.Prices do not include gas compressors (if required).Source: Gas Turbine World/SFA Pacific/GTI

0 1 2 3 4 5 6 7

Size (kW, thousands)

200

400

600

800

1000Price ($/kW)

CT Price RE Price

Figure 1-3Capital Cost Comparison of Small Gas Turbine Products with Reciprocating Engine Products


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Introduction

1-4

Manufacturers and providers of small gas turbines have long recognized the competitivepositioning of their products and have initiated developments to enhance their position in theevolving power market. R&D and technology improvements for small gas turbines have focusedprimarily on the testing and integration of new components including recuperators, advancedhot-section materials, and low emission combustion systems.

Report Objective

This report is intended to serve as a primer on small gas turbine technology, products,manufacturers/suppliers, and business issues affecting the use of gas turbines in power markets.The report focuses on the status of technology, potential performance enhancements, productsavailable from key companies, small gas turbine applications, and market and business issues.

The size range investigated for this report is 300 kW to 5 MW. The next four chapters of thereport cover:

1. Technology characteristics and developments (Chapter 2)

2. Catalytic combustion systems and developers (Chapter 3)3. Key companies and providers (Chapter 4)

4. Market and business issues (Chapter 5).


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2TECHNOLOGY DESCRIPTION

Basics

Gas turbines vary greatly in size, from 25-kW microturbines to heavy duty, 250-MW machinesin central power applications. Gas turbines serve a wide variety of purposes including electricpower generation; aircraft, helicopter, and boat propulsion; water pumping; and other mechanicaldrive applications such as compressor drives.

A gas turbine, also referred to as a combustion turbine, is a continuous flow machine thatcompresses the working gas (typically air), heats it, and expands it in an expansion turbine to itsoriginal ambient pressure. (Turbine can refer to the entire machine or just to the expansionturbine component.) Between the compressor and the expansion turbine, the working gas is

heated, raising its temperature enough to increase the systems efficiency and power toeconomically attractive levels. The usual method of raising the working gas temperature is toburn fuel in the compressed air stream. The fuels used most frequently today are natural gas andseveral types of liquid fuels. These fuels must be pumped or compressed to higher pressuresthan the compressor discharge pressure to promote rapid fuel/air mixing in the combustor.

Net power is generated when the expansion turbine produces more power than the compressorconsumes. This occurs when greater volumes of gas pass through the expansion turbine thanthrough the compressor, due to the higher temperature of the expansion gas. Because gasvolume increases with temperature, all other things being equal, gas turbines with higher turbineinlet temperatures have higher efficiency and produce more power per pound of air throughput

than gas turbines with lower turbine inlet temperatures.

Figure 2-1 shows a cutaway of a typical industrial gas turbine machine. Gas turbines have threekey subsections the compressor, combustor, and expander or power turbine.

Heat exchangers can be used in several ways to reduce gas turbine fuel consumption andincrease power output. For example, fuel consumption can be with recuperators preheatingcombustion air, while intercooling can reduce compressor power consumption. Gas turbineexhaust is frequently used to raise steam, which can either supply heat or boost the systemspower generation capacity. For additional power, the steam can be injected into the expansionturbine or passed through a separate steam turbine in a combined cycle. In large sizes, suchcombined gas and steam turbine cycles are the highest efficiency power generation systems in

commercial service today.


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Technology Description

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Figure 2-1

Gas Turbine CutawaySource: Solar Turbines

Types of Gas Turbines

Small gas turbines, ranging in size from 300 kW to 5 MW, are frequently categorized as eitherindustrial or aeroderivative. Aeroderivative turbines are engineered for flight services includingpropulsion via turbojet, turbofan, or turboprop and for helicopter power. Typically,aeroderivative turbines have more high-temperature and high-technology components, such asblades and vanes with complex internal cooling passages, hot-section materials capable of highertemperature operation, and blade and vane profiles that are optimized aerodynamically formaximum performance. These features, along with higher pressure ratios, give aeroderivative

engines higher efficiency and power density (kilowatt or horsepower per unit of air flow andmachine volume) than their industrial competitors. Because of these design features,aeroderivatives typically cost more than industrial turbines with equal continuous-duty ratings.

Because a turbines optimum pressure ratio increases with increasing turbine inlet temperature,pressure ratios of aeroderivative gas turbines are somewhat higher those of industrial types of thesame vintage. In the 300 kW to 5 MW size range, aeroderivative turbine pressure ratios rangefrom below 6.5 to above 14. Both aeroderivative and industrial models offer pressure ratiosfrom 6.8 to 12.5; most units of both types have pressure ratios in the 8-10 range. Small gas


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turbines on the market today were designed over the past 40 years, as turbine inlet temperatureswere advancing appreciably; thus these turbines have pressure ratios that cover a correspondingrange. The high-technology components of older aeroderivatives are less advanced and lessexpensive compared to newer models, due in part due to the dissemination of R&Daccomplishments. A new industrial gas turbine could have a higher pressure ratio than an older

aeroderivative machine. When comparing gas turbines for stationary applications, many featuresvary from product to product over the wide range of machine vintages, which complicatesgeneralizations regarding cost and performance.

Engine weight per unit of power is the most important variable for aeroderivative turbines, evenmore important than efficiency, in their primary market applications. Aeroderivative gasturbines enter service in aeronautical applications, after which the manufacturer modifies themfor stationary power generation and markets units built around the aeronautical core engine.Aeroderivative turbines also seek small frontal area to minimize drag. For stationary powerapplications, turbojet and fan-jet engines require fan removal and the addition of a power turbineto drive the generator at proper rotational speed.

In contrast, heavy-duty industrial gas turbines are designed for long life, high durability, and lowcost. Industrial turbines, which have the longest lives of all gas turbines, have been used sincethe 1950s and have served as natural gas pipeline compressor drives. Some heavy-duty pipelinecompressor turbines have accumulated more than 300,000 operating hours, which wouldcorrespond to 10,000,000 miles of service at an average speed of 33 mph. Turbine life anddurability in this application are increased by operating continuously at a set point, whichminimizes the injurious effects of thermal cycling.

Ratings

Gas turbines are rated for either continuous duty or emergency service (standby or backup). Thedifferent power ratings reflect the number of operating hours before major overhaul. Emergencyturbine generators, with their higher turbine inlet temperatures and correspondingly greaterpower and higher efficiency, are designed for a small number of operating hours betweenoverhauls. Continuous-duty ratings apply to operations with longer overhaul intervals and lowerpower and efficiency. Ratings correspond to differences in the allowable combustor and turbineinlet temperatures, reflecting the deleterious effects of high temperature on the creep of turbineblades, thermal distortion of the combustor liner, and degradation of other hot-section parts.

Ratings for gas turbines assume certain ambient conditions that users should be aware of.Standard ISO rating conditions are 59

oF (15

oC), sea-level elevation, 50% relative humidity, and

no inlet or exhaust pressure losses. Higher ambient temperature or elevation, for example, willresult in derating.

Fuels

Both gaseous and liquid fuels are burned in gas turbines today. Gaseous fuels include naturalgas, propane, landfill gas, several types of refinery off-gas, and gasified solid fuels of severalcompositions, while liquid fuels include kerosene, jet fuel, and No. 2 distillate oil. Nuclear heatsources have been considered, as well as solar concentrators for both terrestrial and spaceapplications. Externally fired primary heaters fueled by the combustion of coal, biomass, and


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2-4

other fuels (which usually contain ash, sulfur, alkalis, and other impurities) have also been usedhistorically for research or demonstration purposes.

Many gas turbines have dual-fuel capabilities. Some can switch fuels after a brief shutdown forchangeover of combustor nozzles, while others require that combustor parts be replaced tooperate on a second fuel. Some turbines have two sets of nozzles in the combustor and can

switch fuels while operating. Not all of these dual-fuel features are available on combustorsdesigned for low NOx emissions, in particular with lean premixed designs.

Industrial gas turbines of commercial interest for distributed generation applications are expectedto burn only clean fuels including natural gas, various clean petroleum liquids, and purifiedalternative gaseous fuels. Gas turbines require clean fuels to avoid the deleterious effects ofcorrosion and deposits. Combinations of sulfur and alkalis (sodium and potassium) in the fuelcreate sodium and potassium sulfate, which chemically attack (corrode) the gas turbine blades attypical surface operating temperatures and form deposits on the blades. Gas turbine fuelspecifications now limit the permissible concentration of these chemicals in the fuel and instructoperators to avoid sites and inlet configurations that result in the intake of salt and dust, which

contribute to corrosion and deposits.

On an experimental basis, solid fuels such as coal have been burned in gas turbines for briefperiods. Even with the best available ash separation systems, sufficient ash entered the turbinethat rapid erosion of the blade profile occurred. Large gas turbines have been operatedsuccessfully for extended periods of time on gasified coal after the fuel is cleaned of particulateand gaseous contaminants. Special combustors have been developed to burn gasified coal thathas been mechanically and chemically cleaned. This technology is applicable in an integratedgasification-combined cycle (IGCC) plant. At present, however, natural gas fueled combinedcycles can produce lower cost electric power than coal gasification-combined cycle plants. Gasturbines of all sizes can use gasified coal technology to burn dilute fuel gas with heat contents aslow as 90-100 Btu/SCF.

Fuel Pressure Requirements

The combustors of gas turbines operate at elevated pressures, typically 75-200 psig (517-1379kPa) or higher depending on machine pressure ratio. The fuel must be at pressures adequatelyabove combustor pressure for it to flow into the machine through control valves, enter thecombustor through nozzles, and retain enough momentum to mix properly with the combustorair. This typically requires fuel pressures of 120-350 psig (827-2410 kPa). Table 2-1 listsrepresentative examples of small gas turbines and booster compressors. These machines aretypically twin-screw, reciprocating, or centrifugal compressors. Figure 2-2 shows a typicalbooster compressor skid, based on a Kobelco twin-screw machine. Pressure is not a problem

with liquid fuels, because available pumps have the appropriate capacity and pressure capability.However, gas compressors for this application (low flow and somewhat high pressure) are not inhigh demand. Typically they must be made to order and thus are not inexpensive.


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2-5

Table 2-1Example Gas Turbine Booster CompressorsSource: GTI*

CompressorManufacturer

CompressorType

OutletPressure

(psig)

Fuel Rate(scfm)

Prime Moverand Type

Prime Mover Size

Gardner Denver Centrifugal 360 625 Orenda turbine 2500 kW

Sullair, others Twin Screw 120 360-460 Dresser-Rand turbine 1450kW/1850 kW

General Electric(Nuovo Pignone)

Recip &Centrifugal

200 490 General Electric(Nuovo Pignone)

turbine PGT-2 - 2000 kW

Mycom/Frick Co.

Twin Screw 200 - 220 331 511 Kawasaki turbine Model M1A1235 - 2065 kW

Mycom/Frick Co.

Twin Screw 200 - 220 768 - 1,039 Kawasaki turbine Model M1T2950 - 4022 kW

Mycom Twin Screw 140 - 180 305 Solar Turbines turbine Saturn - 1210 kW

Mycom Twin Screw 205 - 300 780-975 Solar Turbines turbine Centaur3515-4600 kW

Mycom Twin Screw 205 300 1060-1315 Solar Turbines turbine Taurus52006890 kW

* Data are representative examples. Gas turbine suppliers work with packagers and customers to accommodate theirpreferences on booster compressor suppliers.

Figure 2-2Gas Turbine Booster Compressor SkidSource: Kobelco


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2-6

Gas turbine packagers sometimes install two fuel gas compressors to ensure system reliability isnot compromised by fuel compressor failure or associated long delivery time for replacement.When high-pressure fuel gas is available, it is used directly. In the broad distributed generationmarket, however, power generation engineers cannot assume the availability of high-pressurefuel at sites with power demand in the 300-kW to 5-MW range. The cost and reliability

considerations of the fuel gas booster compressor might need to be considered in projecteconomic and performance analyses.

Operational Characteristics

In the small gas turbine size range, most products have efficiencies ranging from 25% to 40%,with a corresponding range in heat rate of 13,650-8,530 Btu/kWh (LHV). Note thatreciprocating engine and gas turbine efficiency is rated on an LHV basis, whereas natural gas issold on a higher heating value (HHV) basis. The difference between these fuel heat contentratings is the heat of condensation of the water vapor in the combustion products. This heat isunavailable for power generation in reciprocating engine and gas turbine machines, hence its

exclusion in the machine rating. Economic analyses should distinguish between LHV and HHVin fuel price and engine performance. Factors for converting HHV to LHV vary slightly withfuel composition. Typical values are:

Natural gas: HHV = 1.106 x LHV

Diesel fuel: HHV = 1.06 x LHV

In addition to the effects of derating caused by high ambient temperatures or other factors (seethe previous section,Ratings), users need to account for pressure losses and the impact ofparasitic system loads such as pumps, fans, and compressors.

The exhaust of industrial gas turbines typically ranges from 932 to 1112F (500 to 600C),

enabling steam to be generated at medium pressures. The exhaust of recuperated turbines (seeRecuperated Cycle below) and aeroderivative gas turbines is lower in temperature due,respectively, to the use of recovered heat for preheating combustion air and to higher pressureratios. When generating steam, these lower exhaust temperatures result in a somewhat loweramount of heat recovered and a lower heat recovery efficiency.

During periods of peak power demand, operators may choose to overfire a gas turbine (fire atslightly higher temperature than continuous-duty design point) to generate more power andimprove efficiency. However, overfiring reduces the interval between scheduled maintenance.The tradeoff is between the higher energy prices being paid during peaking periods and the costof shortening the maintenance interval.

Gas turbine power can also be increased by inlet air cooling. Lower compressor inlettemperature means that less power is needed by the compressor, resulting in greater output andhigher efficiency. Gas turbine components, including the electric generator, are usually sized foroperation in the expected service temperature range, so the extent of power augmentation has apractical upper limit because the generator output cannot safely exceed its rating. As mentionedearlier, the standard temperature for rating gas turbine output is 59F (15C). Gas turbine outputand efficiency are quite sensitive to compressor inlet temperature, so appreciable derate occurswhen the ambient temperature rises. During peaking periods, increased output is often valued


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2-7

higher than increased efficiency. As air-conditioning loads increase during hot weather, gasturbines and other peaking equipment are called upon to meet the load. At these times, the valueof electricity is greatest, so it often pays to augment turbine output by cooling inlet air.

Inlet air cooling can be accomplished either by a heat exchanger and a cooled fluid or byevaporative cooling. Using a heat exchanger can be expensive, in both capital and operating

cost, because it requires a source of cooling, typically a chiller (electric, turbine exhaust-drivenabsorption, or natural gas-fired absorption). Water at a temperature appreciably cooler than aircan also be obtained by operating a small cooling tower at night, or whenever the ambienttemperature is adequately low, and storing the cooled water for later use during hot-weather peakdemand.

Inlet air evaporative cooling uses an array of spray nozzles in front of the air inlet to create a mistin the flow before it enters the compressor. The spray system should not leave sizable drops inthe air entering the compressor, or compressor blade erosion can result. Adequatelydemineralized water must be used to avoid deposits in the gas turbine that, over time, wouldreduce its output power and efficiency.

Thermodynamic Cycles

The thermodynamic cycle on which the gas turbine operates is called the Brayton cycle. Severalvariations in the Brayton cycle are described below. The basic cycle, referred to as the simplecycle, consists of compression, temperature rise by heat addition, and expansion. Variations inthe cycle involve adding components for efficiency improvement and power increase. In variouscompound cycles, the gas turbine exhaust is used as heat input to a second cycle, often referredto as the bottoming cycle. In such cases the gas turbine becomes the topping cycle.

Simple Cycle

The simple cycle consists of a compressor, combustor, and expansion turbine. Thisconfiguration is most commonly known as a gas turbine. In a jet engine, the gas turbine exhaustis discharged at high velocity by an exhaust nozzle. Simple cycles may consist of one or morerotating shafts with expansion turbines and compressors on them. Jet engines are made intostationary power turbines by adding either a generator and an additional turbine stage to the shaftor adding a second shaft with an expansion power turbine and a generator on it. The simplecycle is shown schematically in Figure 2-3.

Figure 2-3Schematic of Simple Cycle


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Recuperated Cycle

In the recuperated cycle (Figure 2-4), turbine efficiency is raised by adding a recuperative heatexchanger, which uses the hot exhaust gas of the expansion turbine to preheat the air flowinginto the combustor, thereby reducing the fuel required. This cycle is also sometimes referred toas a regenerated cycle. There is no difference between these two designations from a

thermodynamic viewpoint. A recuperator is a heat exchanger with passage walls through whichheat flows by virtue of the temperature difference between the two fluids on either side of thewall. The fluids in a recuperator do not mix at all. A regenerator is a periodic heat exchanger inwhich hot and cold gas flow alternately in opposite directions through a matrix of fine passages.In a regenerator, the two fluids mix to a small degree, and leakage can occur from the high-

pressure, compressor discharge side to the low-pressure, expansion turbine exhaust side.

Figure 2-4Schematic of Recuperated Cycle

Conventional (solid boundary) recuperative heat exchangers are used most frequently in heatingand air-conditioning applications and for industrial heating. Periodic (rotary wheel) regenerativeheat exchangers have been tested since the 1950s for use on automotive gas turbines.

Regenerators have been researched because they could be compact enough for the gas turbine tofit under the hood of a car. However, the high-pressure seals required in the regenerator have notyet achieved adequate life for this application.

The recuperated turbine cycle produces about 10% less power than a simple cycle of the samecompressor pressure ratio and turbine inlet temperature. This is because an inherent pressuredrop is associated with the recuperator and with its connections to the engine and gas turbineexhaust. The design of a practical recuperated cycle involves balancing the tradeoffs among theparameters of efficiency, power, and cost. This is accomplished by analyzing various heatexchanger sizes, dimensions, and configurations to obtain a desired level of pressure drop oneach side of the recuperator and interconnecting ducting, as well as analyzing recuperator cost.

Similar tradeoffs apply to the regenerative cycle.

Intercooled Cycle

The intercooled cycle (Figure 2-5) splits the compressor into two or more sections and usesexternally cooled heat exchangers to cool the air flowing between the sections. Intercoolers arein widespread use for boosting the power of turbocharged diesel and spark-ignited reciprocatingengines. Intercooling reduces compressor work by lowering the temperature of the air in later


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stages of the compressor, thereby enabling them to act on a cooler gas and reducing powerconsumed in the compressor. This increases net machine output and results in a higher powerdensity.

Figure 2-5Schematic of Intercooled Cycle

However, the compressor discharge temperature is lower with an intercooled cycle, so additionalfuel must be burned in the combustor to reach the expansion turbine inlet temperature. In spite

of its added power, the intercooled cycles efficiency is basically the same as that of a simplecycle with the same pressure ratio, due to the increased fuel required and the losses associatedwith the intercooler pressure drop. Intercooled cycle design involves pressure drop tradeoffssimilar to those of recuperated cycles (seeIntercooled Cycle Design).

Intercooled Recuperated Cycle

In the intercooled recuperated cycle (Figure 2-6), both a recuperator and an intercooler are used,achieving the high efficiency advantage of the recuperated cycle and the high power of theintercooled cycle. The U.S. Navy is currently sponsoring development of an intercooled,recuperated gas turbine with higher fuel efficiency and turbine power. Tradeoffs exist involving

heat exchanger pressure drop, with the increases in cycle performance being balanced against thelosses caused by the heat exchanger pressure drop and corresponding sacrifices in power andefficiency. In the range near optimum conditions, these sacrifices are typically quite small.

Figure 2-6Schematic of Intercooled Recuperated Cycle


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Figures 2-7 and 2-8 show the efficiency and specific power (another term for power density) forthe four cycles of concern: simple, recuperated, intercooled, and intercooled and recuperated.The data assume typical component performance (turbine efficiency of 88%, compressorefficiency of 83%, recuperator and intercooler heat exchanger effectiveness of 85%, and

recuperator and intercooler pressure drop of 2% on each side) and a turbine inlet temperature of2100F (1150C), typical of new gas turbines in the 3-5 MW size range. The graphs cover thepressure ranges of interest to the point where the cycle reaches a diminishing return or crossoverpoint.

Figure 2-7Efficiency of Selected Cycles

Figure 2-8Specific Power of Selected Cycles

The gains in efficiency found with recuperators are apparent for both the recuperated cycle andthe intercooled and recuperated cycle, as are the gains in specific power with intercooling alone


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and with intercooling and recuperation. The slightly diminishing advantage at very low and veryhigh pressure ratios is due to the compromising effects of the heat exchanger pressure drops.

The efficiency and specific power of the simple cycle rise with pressure ratio. Increase inefficiency diminishes at pressure ratios above 12, which is the point of maximum specific power.The efficiency of the recuperated cycle reaches a maximum at a pressure ratio of 5; however, the

specific power continues to increase up to a pressure ratio of 10. At pressure ratios between 5and 10, only a small drop in efficiency occurs, so a practical machine would balance the benefitsof efficiency and power by operating in the 5-10 range, perhaps at a pressure ratio where theturbine exhaust temperature becomes low enough to allow the use of a lower grade stainless steelin the recuperator.

At low and moderate pressure ratios, the intercooled cycles efficiency is not up to that of thesimple cycle because of losses due to pressure drop in the intercooler. However, the specificpower of the intercooled cycle is head and shoulders above that of the simple cycle for allpressure ratios. The intercooled, recuperated cycle has the best of both improvements highefficiency and high specific power once the pressure ratio is high enough to overcome the

consequences of heat exchanger pressure drop.

Figures 2-9 and 2-10 show the efficiency and specific power of these cycles as a function ofturbine inlet temperature for pressure ratios that represent a balance between high efficiency andhigh specific power (12-17 for the simple cycle, 8-12 for recuperated, 20-30 for intercooled, and15-30 for intercooled, recuperated. As expected, these pressure ratios increased with turbineinlet temperature.

Figure 2-9Locus of Efficiency for Optimized Cycles


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Figure 2-10Locus of Specific Power for Optimized Cycles

Design Considerations

Recuperated Cycle Design

Recuperators are heat exchangers used for preheating the compressor discharge air to reduce fuelconsumption in the combustor, thereby increasing efficiency. The technology, economics,status, and industrial suppliers of gas turbine recuperators are reported in Gas TurbineRecuperators: Benefits and Status (EPRI TR-113745, GRI-99/0263, December 1999).

Practical considerations in the design of recuperated gas turbines include flow ducting, diffusionof flow to low velocity, flow turning, and viscous pressure drop in heat exchanger cores.Recuperators by themselves reduce net power due to the pressure drop incurred in the heatexchanger. The pressure drop in the recuperator core is related to the heat exchangereffectiveness in a fundamental manner.*

Recuperated cycle efficiency is highest when the gas turbine pressure ratio is low, resulting inhigh temperature differences between the expansion turbine exhaust and the compressordischarge. This enables the recuperator to have a high fuel savings and high heat exchangereffectiveness with somewhat modest surface area (and cost and pressure drop).

* Readers interested in the relationship between heat transfer and pressure drop in heat exchanger passagesare referred to the sections on Reynolds analogy in standard college texts on heat transfer. The PrandtlNumber (diffusivity of momentum divided by the diffusivity of heat), cp/k for air and all simple gases, isnear unity (0.7 in the case of air), so that experimental confirmation of Reynolds analogy in recuperatorsis not surprising. To reduce pressure drop losses, the intercooler should be operated at low velocity(actually low Mach Number) which, however, adds cost and size to the gas turbine and incurs additionalpressure drop in the diffuser. The optimum velocity in the intercooler involves consideration of cost andperformance, which are application-specific.


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Microturbines and the Solar Turbines new Mercury 50 are the only gas turbines currently inproduction that are designed with recuperators and intended for service with high annualcapacity factors. The TF-1500 recuperated gas turbine, which serves as the main engine on theM1A1 Abrams tank, is an expensive engine whose recuperator (and resulting fuel economy) wasjustified by the limited space for fuel onboard a battle tank and by the need to operate under

extreme military service conditions.Many natural gas pipeline compressor stations employ recuperators, which were retrofits onolder, low pressure ratio gas turbines. Pipeline compressor stations typically have high annualcapacity factors on the most efficient units, so the economics of recuperation works outfavorably. Recuperators in pipeline compressor service are known to develop problems withleaks, believed to be associated with differential thermal expansion accompanying thermalcycling. Whether new recuperators will develop leaks after experiencing significant thermalcycling is not known yet, due to their limited experience in the field. Recuperator manufacturers,learning from prior experience, have designed and manufactured their products for cycling duty.However, durability under repeated cyclic operation can be determined only on the basis ofexperience, because it is largely affected by manufacturing quality control and details (especially

the welding or brazing of metal joints). The long-term durability and performance ofmicroturbines and Mercury 50 units are of interest to intermediate-duty gas turbine supplierswho want to offer reliable, high-performance, low-cost products.

Intercooled Cycle Design

Typically two-thirds of the gross power produced by gas turbines is used to drive thecompressor. Consequently, reducing compressor power consumption can significantly increasenet power production and efficiency. Thermodynamically, intercooling appears very attractivefor increasing net power by cooling the air flowing in the middle of the compressor and therebyreducing the work absorbed in later stages of compression. The optimum pressure level for

intercooling an ideal gas turbine is at the square root of the pressure ratio, essentially halfwaythrough the compressor. This balances the advantage of having a substantial amount of heat toremove (which was put into the air by the first part of the compression process) and having asubstantial extent of compression work remaining to be done in the second half of thecompressor. Detailed studies are required for actual machines in specific cases.

Intercooling becomes increasingly attractive as the overall pressure ratio increases. Intercoolingby itself has a substantial effect on increasing net power from the machine, but only a modesteffect on efficiency, because the lower temperature air flowing into the combustor requiresadditional fuel to deliver the combustion products at design temperature. Also, cycle losses areincurred by pressure drop in the heat exchanger and connections to the cycle.*

A price in terms of performance and economy must be paid for the power gained by intercooling.Intercooling heat exchangers are of appreciable size, especially ones designed for ambient andlow-pressure air. Consequently the intercooler, with one side handling air at modest pressureand the other side often handling ambient air, is large compared to the compressor. For the airbeing cooled to flow through the intercooler with reasonable pressure drop, it has to be taken offthe

* Refer to previous footnote, page 2-12.


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compressor and diffused to low velocity, but diffusers create pressure loss. The flow passagesremove air from the compressor and then return it, which changes the direction of flow andcreates pressure loss. The intercooling heat exchanger itself incurs pressure drop, because theheat transfer and wall shear forces are coupled by the diffusion of heat and momentum from thewall into the air stream.

Intercooling can also be accomplished by spray cooling with water at an intermediate conditionin the compressor. Injected water must be fully evaporated to avoid carryover of drops into latersections of the compressor. Droplet carryover could damage compressor blades upon impact.Also, the spray water cannot contain substantial mineral matter, which could result in deposits ineither the compressor or the expansion turbine, either of which would reduce output power andefficiency.

General Electric (GE) offers spray intercooling (Sprint) on its LM 6000, which increases outputpower by 20%. The LM 6000 is a two-shaft aeroderivative gas turbine with adequate space for aspray intercooler between the low-pressure and high-pressure compressors. The spray iscontrolled to very fine droplet size so that most of the evaporation occurs between the

compressor stages. Large droplets lose enough mass that the risk of carryover and blade erosionis acceptable.

Independent engineers have added inlet spray cooling with chilled water. Some systems chill thewater during peak periods for power augmentation, using ice that was made and harvested duringoff-peak hours. The chilled-water spray cooling technique is used on other large, new gasturbines, such as the W 501 B5 and the GE Frame 7EA.

Inlet air cooling and spray intercoolers are new technologies being applied first to large machineswhere the payoff is highest, but they could be applied to small gas turbines as well. The samecare must be taken to avoid large, massive droplets (over 20 microns) and excessive mineralmatter in the water, which would deposit in the compressor and turbine sections. With these

engineering precautions, the power output of small gas turbines can be increased on hot dayswith inlet air cooling and on a more routine basis with wet compression. Water quality for inletair cooling and wet compression are based on manufacturers guidelines for the total amount ofsodium plus potassium, lead, vanadium, calcium, and total dissolved and undissolved solidspermissible for introduction into the gas turbine, whether they are introduced by the water or thefuel.

To increase power with little increase in net engine cost, intercoolers are frequently usedcommercially on turbocharged reciprocating engines in off-highway vehicles, boats, trucks, andautomobiles. However, the only commercial gas turbine with an intercooler was the Stahl-LavalGT120, which is not in production anymore. This machine, made in Sweden during the 1950s-

60s by a predecessor of ABB, took advantage of the low-temperature cooling water available inSwedish lakes and rivers. Its unusual configuration consisted of three shafts. The machine had asplit compressor, with the low-pressure compressor on one end of the machine and the high-pressure compressor on the other end, leaving plenty of space for the diffuser, interconnectingductwork, and intercooler. The added cost of the intercooler and turbine design was presumablyjustified by the systems economic benefits, which resulted from the turbines intermediate-dutyapplication (high number of operating hours per year) and from high oil prices in interiorSweden at the time.


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Most likely, intercooling will be applied first on gas turbines with high pressure ratios, becausethe benefits of intercooling increase with increasing pressure ratio. However, gas turbinepressure ratio must not be so high that, despite the lowering of the compressor dischargetemperature by intercooling, the turbine exhaust temperature is not above the compressordischarge temperature. Studies of advanced cycles by three international manufacturers of large

aeroderivative gas turbines were performed as part of the Collaborative Advanced Gas Turbineprogram in the early 1990s, sponsored by a group of international electric and gas utilities (seeRecent Developments in Advanced Cycles). These studies examined the use of new, high-technology, high-performance, large aeroderivative gas turbines in combined cycles and withintercooling, as possibilities for developing new products for the commercial power generationsector. The program focused on an intercooled aeroderivative product, which incorporated anintercooler and appropriate gas turbine modifications to result in a moderate-cost product ofmedium efficiency (not quite as high as a combined cycle). However, development never beganbecause the manufacturers did not see an adequate market for the product to justify theirinvestment.

Combined Intercooled and Recuperated Cycle DesignThe combination of intercooling and recuperation is not used on commercial stationary gasturbines at the present time. No gas turbine has yet been developed for an application requiringthe performance features of such a machine. Currently, the power generation business is focusedon two applications, baseload and peaking power. The industry is interested in using gasturbines for intermediate-duty applications, but few new products (other than the Mercury 50 andmicroturbines) have been developed specifically for the distributed generation market. High-efficiency, recuperated and intercooled industrial gas turbines that are attractively priced couldprovide power to intermediate-duty customers at lower cost than they are currently paying toregulated distribution companies and could also reduce greenhouse emissions appreciably.

One project is underway to develop an intercooled, recuperated gas turbine. This research isbeing conducted for the U.S. Navy (with some European involvement and interest) by NorthropGrumman Marine Systems (Sunnyvale, CA). The turbine would serve as the main propulsionengine on ships requiring about 30,000 hp of shaftpower. In power generation service thisengine, the WR-21, would produce 24.4 MW at its continuous-duty ISO operating condition.The Navy is interested in fuel conservation to extend the ships range with limited onboard fuelstorage. Fuel cost savings and extended fuel supply during periods of international crisesprovide incentive for applying the intercooled, recuperated cycle. The systems high efficiencyoffers the major advantage of reducing fuel consumption while cruising at part power, yetretaining the ability to go to high power when necessary. The poor efficiency of conventionalgas turbines at part power caused the U.S. Navy to seek a better gas turbine cycle for performing

longer range missions without refueling.Figure 2-11 compares the part-load efficiency of a simple-cycle gas turbine, a diesel engine, andan intercooled, recuperated gas turbine. As shown, little efficiency is sacrificed by theintercooled, recuperated turbine at part load in contrast to the efficiency loss of a simple cycle.The intercooled, recuperated gas turbine is smaller than a diesel engine of comparable power.The cost, durability, and commercial fate of this gas turbine are unknown at this time.


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Figure 2-11Comparison of Part-Load Efficiency of Diesel Engines and Gas Turbines

Size Considerations

Gas Turbine Systems

Historically, both industrial and aeroderivative gas turbines have increased in size from the smallunits built in the 1950s-60s to the larger ones of recent vintage. Several small gas turbines areold models that have recently found a market niche. As sales of gas turbines increased over theyears, manufacturers had cash to develop new products, and larger units were built to competefavorably with diesel engines. The newer, larger turbines employ advanced technology to reachhigher efficiency and specific power and lower cost per kilowatt. The progression from small tolarger sizes illustrates a combination of three phenomena:

Increasing turbine inlet temperature (via improved high-temperature materials and theintroduction of internal blade and vane cooling) with corresponding increases in power andefficiency

Increasing compressor and turbine component efficiencies (due to research on the flow of airin turbomachinery passages) over time

Increasing turbine efficiency as a consequence of an increase in physical size.

This latter improvement is due to a more favorable ratio of volume to surface phenomena, suchas decreased percentage of blade tip leakage, the decreasing effects of Reynolds number-relatedphenomena such as wall frictional pressure drop, and the ability to produce blade and vaneprofiles with less aerodynamic flow losses in larger sizes.


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Gas Turbine Component Selection

Very small gas turbines, typically below 250 kW in size, use centrifugal (radial-outflow)compressors and radial-inflow turbines, while larger sized gas turbines, typically over 2 MW, useaxial-flow compressors and turbines. These designs are most efficient at their respective sizesfor a number of technical and economic reasons. At low volumetric flow rates, the blade heights

of axial-flow compressors and turbines become very small, incurring large viscous surfacepressure loss, proportionately higher tip leakage, and other size-related phenomena affectingcompressor efficiency. Additionally, the relative cost of a single-stage radial-flow compressor orturbine is much less than the cost of a multi-staged, multi-bladed, axial-flow compressor orturbine of comparable overall pressure ratio.

In the transition size range, from 250 kW to 2 MW, the performance differences of thesecomponents compete, and designers must consider numerous factors including manufacturingand development cost, output shaft speed, and the cost and weight of gearboxes for turboshaftand helicopter engine applications. Component selection is also affected by experience withspecific components and by part availability. In this transition size range, centrifugal

compressors are often combined with axial-flow expansion turbines, which can then operate withhigher pressure ratios per stage than possible with axial-flow compressors. Other combinations,such as a few stages of axial-flow compression followed by a centrifugal or mixed-flow stage,have also been used in this size range.

Commercial users should focus mainly on the cost, efficiency, and durability required of aturbine in their application. Gas turbine flow path design tradeoffs are complex, andmanufacturers evaluate these tradeoffs based on long-term, broad market considerations, not onsales of a few units. Numerous texts exist on the topic of turbomachinery and turbine systemdesign.

Gas turbines have an optimum pressure ratio for maximum efficiency and another optimum

pressure ratio for maximum specific power. The specific power maximum occurs at a higherpressure ratio than that for maximum efficiency. These optimum pressure ratios increase withincreasing turbine inlet temperature, which, in terms of gas turbine products on the market,typically increases over time as technology advances.

Figure 2-12 shows efficiency as a function of machine power rating for commercially availablegas turbines in the 300 kW to 5 MW size range. Most of the plot points fall in a broad, centralband sweeping upward and to the right, to increased efficiency with increased size. Points abovethe band are modern aeroderivative engines (signified by an A) and the recently introducedrecuperated industrial gas turbine (R). Points below the band are low-cost, typically older, gasturbines made for emergency power, where efficiency is not a major product selection criteria.For each gas turbine presented in the figure, the manufacturer presumably has optimized thepressure ratio for the turbine inlet temperature and business application.


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Figure 2-12Efficiency of Small Gas TurbinesSource: Turbomachinery International Handbook

Users concerned with the most advantageous gas turbine for their application should focus ontheir particular economics. Generally, higher efficiency equipment costs more to manufactureand carries a premium for the increased benefits to the user. In selecting a small gas turbine, avery important parameter is the annual full power capacity factor. As the power generationmachine gets more use per year, it accumulates more hours over which to spread its fixedcarrying charges and is better able to pay a higher price for a higher efficiency.

Example of a Tradeoff

As a simple tradeoff, a decrease in heat rate of 1000 Btu/kWh results in the same electricity costreduction as a decrease of $112.50/kW of gas turbine cost. This calculation assumes a fuel costof $4.50/million Btu (MMBtu), 5000 hr/yr of operation, and a capital charge factor of 20%(combined interest, taxes, insurance, and depreciation). This 1000 Btu/kWh heat rate decrease,or increase in efficiency from 28.4% (12,000 Btu/kWh) to 31.0% (11,000 Btu/kWh), and thecorresponding variation in gas turbine cost straddle the price and performance range of manycompetitive products.

The formula for this tradeoff is:

dC = d(HR) x (FP) x (D)/i

Where

d = differential operator

C = Cost ($/kW)

HR = heat rate (Btu/kWh)


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FP = fuel price ($/Btu)

D = annual duration of use (hr/yr)

i = capital charge factor (as a decimal, not as a percentage).

Fuel prices are usually quoted in $/MMBtu, so care must be taken to convert the fuel price to$/Btu, or the results will be off by a factor of one million.

The Use of Steam to Increase Efficiency

Steam Injection

Gas turbine exhaust heat can be used to generate steam in a boiler, often called a heat recoverysteam generator (HRSG). The steam can be injected into the combustor or turbine, generatingadditional power by increasing mass flow through the turbine. However, steam injection into thecombustor requires additional fuel to maintain turbine inlet temperature. Using the unfired gasturbine exhaust heat source, larger amounts of steam can be raised in a humidifier than in anHRSG, which is more constrained by the heat transfer pinch point consideration. Inhumidification, compressed air and hot, pressurized water are passed in opposite directions in acombined heat and mass exchanger called a humidification tower (seeRecent Developments inAdvanced Cycles).

Using such large amounts of steam, however, is not possible in existing gas turbines becausethey are designed for approximately equal mass flows in the compressor and expansion turbinesections. Such a turbine could be built, with much larger mass flow in the expansion turbinethan through the compressor, but to date, manufacturers have not invested in it because it lacksthe power generation industrys interest.

Combined Cycles

Combined cycles use the turbine exhaust to raise steam in a separate boiler, or HRSG.Additional power is then produced by a steam turbine. More complex cycles, such as steamaddition by adiabatic saturation (humidification) and reaction of fuel and steam in a catalytic heatexchanger (chemical recuperator) have been considered in analytical studies but have notresulted in commercial development. Due to their complexity and cost, chemical recuperatorsand humidifiers are generally considered by gas turbine manufacturer and power generationbusiness executives to be candidates first for large, central station systems before theircomplexity and cost is reduced enough for them to be economically advantageous in small gasturbines for distributed generation and industrial cogeneration.

Today the vast majority of new baseload power plants use the combined cycle. With its veryhigh efficiency and modest cost, it is the most economic system for large units. Combined-cycleplants are being installed in great numbers in the United States, Europe, and Japan not only foreconomic reasons, but also for their environmental advantages (extremely low emissions of NOx,carbon monoxide [CO], and carbon dioxide [CO2] per MWh generated). These turbine plants areexpected to help industrial nations to meet the Kyoto protocol regarding CO2 emissions.Switching from coal-fired steam to natural gas-fueled combined-cycle plants reduces CO2emissions by more than a factor of two.


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Combined-cycle plants use large, modest pressure ratio gas turbines (the same as those used forpeaking applications, only as continuous-duty machines), with the exhaust ducted into HRSGs,which provide steam to a separate steam turbine. The turbines modest pressure ratio results in ahigh enough gas turbine exhaust temperature that the steam is generated at high enough pressureand temperature to yield very low-cost electricity. High pressure ratio, aeroderivative gas

turbines have been considered for combined-cycle applications, but because of their substantiallylower gas turbine exhaust temperature, they generate less steam at lower pressures, with loweroverall combined-cycle efficiency. Simply put, in the temperature range of 1050F (570C)down to 750F (400C), the Rankine (steam) cycle is more efficient in generating electricity thanis the Brayton (gas turbine) cycle. Steam cycles cannot economically utilize cycle temperaturesover 1050F (570C), while the gas turbine can, so each is preferred in its own mostadvantageous temperature range.

High-Temperature Materials and Component Engineering

Both power output and efficiency increase with turbine inlet temperature. In the early days of

gas turbines (1950s and 60s), much effort was directed at increasing turbine inlet temperature.Advances included materials with high strength and creep resistance at increasing temperaturesand the development of internal cooling of the turbine blades and vanes. Internal coolingtechnology permits higher gas temperatures in the expansion process while keeping the bladematerial below the levels dictated by its metallurgy.

Performance increases have been obtained with compressor blade coatings that provide a smoothsurface, reducing wall shear forces and pressure drop. These coatings prevent or delay fouling ofthe compressor blades and losses in compressor and gas turbine efficiency. Turbine bladecoatings for protection against metal degradation and overheating were developed and are in useon blades of newer stationary gas turbines. High-temperature oxidation and corrosion also limitturbine durability. Fuel chemical purity specifications regarding fuel sulfur and alkalis have

been getting tighter as more is learned about the details of oxidation, diffusion in metals,sulfidation, and grain boundary phenomena.

Alloys used for gas turbine blades and vanes include IN 100, Udimet 500, 700 and 710, INCO713 and 713 LC (low carbon), and Waspaloy. These materials, in one form or another, havebeen around for over 30 years. The U.S. Department of Defense (DOD) and jet enginedevelopers have been pushing materials temperature capabilities higher and higher. Extensiveresearch has been sponsored by the Air Force and the Navy for military jet engines, as well asdecades of proprietary development by gas turbine manufacturers and blade and vane suppliers.Gas turbine users should be aware of the extent to which they should specify low sulfur and lowalkali content in their fuel purchases and the need to select sites and filter inlet air to reduce salt

and dust ingestion into the gas turbine.Recent progress in high-temperature capability has been made in the field of controlledsolidification of gas turbine blades, using current metallurgy and blade coatings. Directionallysolidified alloys and single crystal blades, each produced by a different controlled solidificationtechnique, are obtaining increased creep strength at slightly higher temperatures from availablematerials. Controlled solidification avoids the weaknesses that exist at grain boundaries due tothe polycrystalline nature of conventionally produced materials. Note that such high-temperatureblades become more expensive as the technology for their manufacture becomes more complex


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and sophisticated. Directionally solidified and single crystal blades appear to be too expensivefor todays industrial gas turbines, because manufacturers are aware of the technology but havenot yet offered such products.

Thermal barrier and corrosion protection coatings have been developed for turbine blades andvanes. Thermal barrier coatings provide a thin insulating layer on top of the blade and vane

surfaces, permitting higher temperatures in the gas stream without exceeding the temperaturelimitations of the blade material itself. Thermal barrier coatings are most effective with thecombination of high firing temperatures and internally cooled blades, because the coatingsreduce temperature more effectively in the underlying material when high heat fluxes occurthrough the blade surfaces. Corrosion-resistant coatings provide a ceramic layer on the turbineblade, which drastically reduces the rate at which alkali metal sulfates and other corrosivespecies attack blades. Ceramic coatings must be kept thin, however, because of their much lowerthermal expansion than blade metal. If made too thick, the differential thermal expansion cancause the coating to flake off.

Turbine Blade Cooling

The high-temperature sections of current aeroderivative gas turbines, as well as medium to largestationary gas turbines, use internal cooling. Compressor discharge air is passed through thebase of the blade and through the core of the active portion of the blade to cool the bladematerial. The cooling air flows through carefully engineered internal passages inside the bladesand is discharged into the exhaust from that stage. These large gas turbines typically use axial-flow expansion turbines, whose turbine blades are large and basically straight in the radialdirection, which facilitates their manufacture with internal flow passages. High-temperaturecapability also entails internal cooling of the turbine vanes or nozzles and combustor liners.These cooling flows reduce the amount of air available for power generation via expansion in theturbine section. This reduction in power is the price paid, in terms of performance, for the higher

temperature capability. Gas turbine manufacturers are always trying to develop more effectiveinternal heat transfer passages inside the blade core to reduce the consumption of cooling air andincrease the flow through the turbine, thereby obtaining more power and higher efficiency.

Many small gas turbines use radial-inflow expansion turbines with a more complex internalflowpath. Internal cooling involves casting cooling passages in the blades, some as fine as 0.050inch in heavy-duty and industrial gas turbines and 0.020 inch in aeroderivative models.Designed surface roughness features are contained inside these coolant passages to augment heattransfer. To date it has remained impractical to use this technology in blades for radial-inflowturbines; however, small, aeroderivative gas turbines exist that have axial-flow expansionturbines rather than radial-flow turbines and have been equipped with cooled blades.

The economic tradeoffs in developing internal blade cooling for small gas turbines includehigher manufacturing costs and low production volumes, insufficient to recover investment indeveloping, testing, and tooling cooled blades and accompanying disk and vane cooling. Themanufacture of turbine blades, vanes, and disks with internal cooling passages for very small gasturbines with radial-inflow turbines would require new passage configurations, finer holes, andmore precise manufacture than is considered to be commercially worthwhile today. Theadditional manufacturing cost, the limited market at present, the cost competitiveness of theproduct, and the difficulty in recovering development costs for gas turbines with only modest


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levels of sales mitigate against investment in such technology. In contrast, the development costof technology advances in large machines can be written off against many more kilowatts. Theseadvances, particularly in the turbine hot section, have resulted in lower machine cost per kilowattdelivered and increased machine efficiency.

Competing Technology Reciprocating Engines

Reciprocating engines compete against gas turbines in the small to intermediate size ranges.Both natural gas-fueled, spark-ignited and diesel oil-fueled, compression heat-ignited engines arefound in this market. Reciprocating engine manufacturers typically use the same basic engineblock for both fuel types. However, as the natural gas goes into the cylinder as a gas at inletmanifold pressure, it displaces its own volume in air and leaves less air for combustion. Liquidfuel is injected in diesel engines after the inlet valve has closed, so the fuel does not displacecombustion air. The reduced air charge per cylinder stroke with gaseous fuel, along withderating due to knock limitation concerns (premature ignition), reduces the power capability ofthe engine and consequently increases the specific cost ($/kW) of spark-ignited engines over that

of diesel engines.Reciprocating engines are available in sizes from under a kilowatt up to about 35 MW. Belowabout 25 kW, engines are typically used as emergency generators. Intermediate sizes, 25 kW toabout 300 kW, are often low-cost, high-volume automobile and truck engines modified foremergency and portable power and for commercial and industrial cogeneration. From about 300kW to 1 MW, reciprocating engines are typically large versions of truck engines. From 1 to 35MW, they are uniquely designed stationary engines. The largest bore cylinders are just over 3feet (1 meter) in diameter. Large ship engines technically are not stationary; however, theirdesign and purchasers selection criteria favor high durability, long life, and high efficiencyperformance, so they resemble stationary engines more than they resemble vehicular engines.

Reciprocating engines operating on the diesel cycle with oil as fuel employ higher compressionratios than can be used in spark-ignited engines, because premature ignition (knock) is not aconcern. Diesel cycle engines consequently have somewhat higher efficiencies than spark-ignited engines operated at wide-open throttle condition and built with the same engine blockand cylinder size. When operating at part load, most natural gas-fueled and other spark-ignitedengines have lower efficiency than diesels of the same size, because output is reduced bythrottling the air intake rather than by delivering less fuel. Such throttling creates an irreversiblepressure drop in the cycle and dissipates available work, which lowers efficiency. Diesel enginestypically have greater emissions of NOx and particulates than do spark-ignited engines due toseveral factors, including the higher pressure and temperature of combustion and their need forrapid burning to avoid emission of unburned hydrocarbons. Diesel emissions are being regulated

to a greater degree as time progresses, making siting is more difficult than with spark-ignitednatural gas engines. Diesel engine manufacturers have been developing staged combustion toreduce NOx emissions.

Industrial gas turbines have to compete with both natural gas and diesel oil-fueled reciprocatingengines. In sizes below about 300 kW, reciprocating engines have the advantage of beingmanufactured in large volumes, with corresponding economies of scale. Their productionequipment and tooling is amortized over large numbers of engines, and their spare parts arewidely available, on a highly competitive basis, from competing aftermarket manufacturers.


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Reciprocating engines also benefit from a broad technology base resulting from their widespreaduse over the last 100 years. Reciprocating engine repair and maintenance personnel are morereadily available than are gas turbine repair and maintenance personnel. These factors present aseverely competitive environment for industrial gas turbines.

In intermediate sizes, up to a few megawatts, the shared technology and production system

continues to benefit reciprocating engines through economies of scale, but at sizes of severalmegawatts, reciprocating engines become purpose designed and manufactured in modestquantities. This results in higher cost per unit of power and more competitive pricing ofreciprocating engines and gas turbines in the 3-15 MW size range. Although larger reciprocatingengine are being built today, their market niche appears to be based on the use of low-cost fuel,including residual oil, which gas turbines cannot use without compromising reliability or addingcost for fuel cleanup.

Typically intermediate and large gas turbines process more air through a smaller volume ofmachine and are more compact and lighter weight than reciprocating engines. Also, enginesconsume oil for cylinder lubrication, while gas turbines do not directly consume oil. Large,

marine cathedral diesel engines have separate systems for bearing lubricating oil and pistonring sealing oil, which increases their scheduled maintenance interval. Long-life, platinum-tipped spark plugs are now available commercially for spark-ignited engines. In automotiveservice, these plugs have lives in excess of 100,000 miles, or about 3000 hours of operation.

Reciprocating engines, particularly stationary diesel engines, have operated with grossly oversizeoil sumps and timed additive injection systems so as to permit long service intervals, possibly asinfrequently as twice or even once per year. Both gas turbines and reciprocating engines aregetting to the point where maintenance intervals could become dictated by the need to inspect fordegradation of parts due to inherent wear and reliability considerations

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Small Gas Turbines