the utilization of recuperated and regenerated engine cycles for high-efficiency gas turbines in the...

Pergamon Applied Thermal Engineering Vol. 16, Nos S/9, pp. 635453, 1996

Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved

1359-431 l/96 $15.00 + 0.00

REVIEW PAPER

THE UTILIZATION OF RECUPERATED AND REGENERATED ENGINE CYCLES FOR HIGH-EFFICIENCY

GAS TURBINES IN THE 21ST CENTURY

Colin F. McDonald* and David Gordon Wilsont *McDonald Thermal Engineering, 1730 Castellana Road, La Jolla, Catifornia 92037, USA; and

YDepartment of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

(Receiued 15 October 1995)

Abstract-In the gas-turbine field ‘simple-cycle’ engines (compressor + burner + expander) have been dominant across almost the full spectrum of power-generation and mechanical-drive applications. Paced by aerodynamic and materials-technology advancements, efficiency values have progressed significantly over the last five decades. However, to reduce specific fuel consumption further (by say a step change of 30-&l%) and to reduce emissions significantly, more-complex thermodynamic cycles that include the use of exhaust-heat-recovery exchangers are necessary. Clearly, there are discrete applications where the use of recuperators or regenerators will tind acceptance on a large scale, an example being for gas turbines rated at less than about 100 kW for hybrid automobiles and small generator sets. The role that recuperators and regenerators can play in future gas turbines is put into perspective in this paper. Innovative engineering concepts will be required to meet the demanding high-temperature operating environment and low-cost requirements, and these will essentially necessitate the utilization of ceramic-composite heat-exchanger configurations that are amenable to large-volume manufacturing methods. Copyright 0 1996 Elsevier Science Ltd

Keywords-Gas turbines; recuperators; regenerators; high-efficiency cycles.

INTRODUCTION

To date, the use of gas turbines embodying internal heat recovery within the thermodynamic cycle has been limited. However, reducing specific fuel consumption is necessary if we are to meet the demanding environmental requirements that will be introduced before or soon in the 2 1 st Century. One requirement will be a reduction in carbon-dioxide emissions, which is most easily attained by increasing engine thermal efficiency. One way to do this is to increase compressor and turbine efficiencies. However, these have reached a near plateau, and future advancements will be small. Increased cycle efficiency is also brought about by increased turbine inlet temperature, which is enabled and encouraged by improved materials technology, including ceramics, and better blade cooling (for metallic turbines), including steam cooling. We expect that incremental improvements in these areas will continue. However, to provide a significant step change in performance, more complex thermodynamic cycles will be needed. These include the use of heat exchangers to recover exhaust heat. It is projected that recuperators and regenerators, the two possible forms of two-fluid heat exchanger, will play a significant role for high-efficiency gas turbines entering service after the turn of the century. In this paper recuperators are defined as stationary, fixed-boundary units, and regenerators as having a matrix that moves between the cold-air and hot-gas streams.

While heat exchangers can be used for a wide range of applications (e.g. those shown in Fig. l), the focus of this paper is on applications where early introduction of recuperators and regenerators on a large scale is likely. These applications include small gas turbines for automotive applications and for small generator sets. A discussion is also included on larger engines for marine propulsion and power generation that include intercoolers and exhaust heat exchangers.

The different and unique features of recuperators and regenerators are characterized in terms of the different configurations available and of potential applications. Performance is presented in terms of major parameter selection for several different cycles. It is projected that within two

636 C. F. McDonald and D. G. Wilson

decades heat-exchanged engines will be in the main stream of gas-turbine applications, and will be produced in large volumes (particularly for engines of less than 100 kW) rather than in their current role of meeting specialized needs and of being fabricated in very small quantities. For these small engines, that could see service in, for instance, hybrid automotive applications involving gas-turbine engines and some form of energy storage, the necessary very low projected heat-exchanger cost (of say $150) will mandate the use of high-temperature ceramic-composite materials.

BACKGROUND

Evolutionary development of recuperator technology over the last four decades has resulted in several types of configuration that have overcome many of the early impediments (particularly structural integrity). Today the engine user can select between prime-surface and plate-fin designs, both of which can give high effectiveness and low pressure loss. These units have found limited acceptance for commercial application, but their high cost is not attractive in the current era of low-cost fossil fuels where simple-cycle engines are dominant. The only recuperator to have been produced in large quantities (i.e. over 11,000) is for military use in a battle tank [I]. Development work on rotary regenerators is still in progress some 30 years after the initial ceramic disks were first demonstrated. This work has been primarily focused on the automobile gas turbine, the

I Regenerated Engines

Hybrid Automobile Gas

Turbines and Small Generator

NOTE: Pie diagram is illustrative only, proportions are not necessarily representative

Fig. 1. Projected applications for recuperated and regenerated gas turbines.

Engine cycles for 21st Century gas turbines 631

marketplace of which has always been elusive, and always seemingly 10-20 years away. Major challenges still include materials selection, seal leakage and fabrication cost.

There are several advanced thermodynamic cycles being investigated that have the potential for very high efficiency. These include the humid-air turbine, the intercooled cycle with exhaust-heat recovery, this same cycle with, additionally, reheat combustion, and cycles that incorporate chemical recuperation. These cycles are aimed at large machines for electrical power generation. They require the use of large regenerators or recuperators and are unlikely to see service for at least 10 years. They will be produced in only small quantities.

The authors feel that a market with large volume potential is the small gas turbine (say 65-100 kW) for hybrid and direct-drive automobiles, and a smaller market for larger engines, 150-500 kW, for trucks and buses. For the automobile engine (with a manufacturing cost of at most $25/kW) the exhaust heat exchanger (either a recuperator or regenerator) must be produced for about $150. Such a heat exchanger must have high-temperature capability (indicating the use of ceramic or ceramic composite), must be amenable to low-cost production methods, and must be in a modular form for ease of removal/replacement. Such an engine, fabricated using technology developed for the production of low-cost turbochargers, could well find acceptance as a compact low-cost generator set.

While the authors have different views regarding selection of cycle parameters and heat-exchanger types, one favoring higher-specific-power recuperated variants [2] and the other low-pressure-ratio highly regenerated machines [3,4], these differences are rationalized in this paper, and there is strong agreement regarding the potential that heat-exchanged cycles offer to a variety of users [5]. With emission considerations now being a dominant factor in both cycle selection and in engine feature definition, the exhaust-heat-recovery exchanger will play an important role in future gas turbines.

GAS-TURBINE EFFICIENCY TRENDS

Turbine-inlet temperature

Over the last five decades. the most significant advancement in gas-turbine efficiency is attributable to increased turbine-inlet temperature. This increase has been brought about initially by the development of better materials, and latterly more by increasingly sophisticated blade-cooling technologies. These have advanced to the point where a large combined-cycle gas turbine with metallic turbine blades (steam cooled) will enter service soon with an efficiency of 60% [6]. For large utility-size machines this trend will increase with the next generation of engines having turbine-inlet temperatures above 1500°C (2732°F). For small gas turbines (of say less than 100 kW), the turbine-blade geometry makes cooling very difficult, and for these units it will be necessary to use ceramic components. Turbine-inlet temperature will continue to increase, but an additional factor may impact this, namely ever-more-demanding emission requirements (particularly NO, reduction) that will require improved combustion. This may be the limiting technology.

Compressor and turbine ejiciencies

In the last 50 years intensive research and development has resulted in very significant technology advancements to the point where today’s large axial-flow engines have compressor and turbine efficiencies of over 90%, together with very high aerodynamic loading. Further small improvements will continue from the use of increasingly sophisticated CFD codes, leading to advanced blading geometries, and from the development of passive means for closer tip-clearance control.

In the case of small engines with radial-flow compressors and turbines, the levels of efficiency attained are significantly less than 90%. This is a result of the smaller blade heights where Reynolds-number effects, tip-clearance effects, and surface roughness influence the blading efficiency.

(A principal reason for the espousal of the low-pressure-ratio cycle by one of the authors is that the blade heights of the high-pressure stages in the compressors and turbines are typically more than half those in the low-pressure stages, instead of being of the order of one-quarter or less in the higher-pressure-ratio cycles. Blading efficiency is therefore significantly increased.)

C. F. McDonald and D. G. Wilson

0.50

0.45

0.40

0.35

0.30

025

0.20

0.15

KM 60 RECUPERATED, lW0 STAGE INTERCOOLED GAS NRBINE (1953) FOR COMPARISON

LARGE REDUCllON IN SFC REFlECTlNG OVER 4 DECADES OF GAS TURBINE TECHNOLOGI ADVANCEMENT

DIESEL ENGINE

0.00

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0.50 !!i - % E

0.55 3 z

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Fig. 2. Representative part-load performance of gas-turbine variants.

Future role of exhaust-heat-recovery exchangers

Only modest increases in the efficiency of small gas turbines can therefore come from the traditional means of improved aerodynamic efficiencies and blade-cooling technologies. To attain thermal efficiencies over 40% (i.e. specific fuel consumptions of under 0.35 lbm/hp-hr), the use of a high-effectiveness recuperator or regenerator is mandatory. As will be outlined in the following sections, it is projected that heat-exchanged cycles will find acceptance for small (i.e. less than 100 kW) to medium size (i.e. up to 15 MW) gas-turbine applications.

PERFORMANCE OF HEAT-EXCHANGED CYCLES

The quest for low specific fuel consumption (SFC)

The aforementioned technology advancements have resulted in a dramatic decrease in SFC over the last 50 years. However, improvements in both spark- and compression-ignition engines, particularly from electronic controls and from increased volumetric efficiency (by use of multiple valves), have kept the gas turbine out of large-volume markets, such as automobiles and small generator sets.

Representative SFC data for simple-cycle and heat-exchanged gas turbines are she vn on Fig. 2. The simple-cycle engine has a steep part-load SFC curve. The intercooled/recuperated gas turbine most closely approaches the very flat curve exhibited by compression-ignition engines. For applications such as battle tanks, naval propulsion and direct-drive automotive engines, where much time is spent at part power, a flat SFC curve is important, and the performance of recuperated/regenerated cycles is acceptable and, for the larger engines, the intercooled-heat-exchanged engine is preferable. Substantial improvements in the efficiency and power/weight ratio


of electric motors and generators and fly-wheel energy storage have revived interest in hybrid gas turbine/electric drives. In these, the small gas turbine operates for its full life at full power and constant speed, making a low SFC at this condition paramount. A similar requirement exists for small generator sets that must produce power at a constant frequency. The requirement for low SFC at full power imposes fewer demands on the design of recuperated and regenerated engines than are made by direct-drive systems.

Recuperated and intercooled cycles

Since the computation of the efficiencies of gas-turbine engines is well understood [7] it will be only briefly covered in this paper. It is germane, however, to put into perspective the role that heat exchangers have in enhancing plant performance. The generalized data portrayed in Fig. 3 are representative in terms of understanding the relationship between the compressor pressure ratio and efficiency for different thermodynamic cycles, three of which utilize fixed-boundary heat exchangers. Other major parameters that would make these curves more comprehensive would include turbine inlet temperature, compressor and turbine efficiency, and heat-exchanger effectiveness. The data have been kept simple so that the major types of gas turbine may be compared based on today’s technology.

Recuperated variants are well suited to fairly low-pressure-ratio cycles, but with pressure ratios above about 10, the increasing compressor-discharge temperature and decreasing turbine-exit temperature essentially negate the use of a heat exchanger. Recuperators have been successfully used on earlier low-pressure-ratio industrial gas turbines [S] and there may be a market for retrofitting these engines with higher-performance recuperators. With an abundance of low-cost natural gas there is essentially no interest today in heat-exchanged industrial gas turbines. As will be outlined in a following section, the need for heat exchangers early in the next century will be for automotive applications, marine propulsion and advanced industrial gas turbines.

The introduction of intercooling increases the temperature difference between the compressor

INITIAL ICR ADVANCED INTERCOOLED

HEAVY DUTY INDUSTRIAL GAS TURBINES

0 4 8 12 16 26 24 28 32 36 46

COMPRESSOR PRESSURE RATIO

Fig. 3. Representative performance of recuperated gas turbines.


COMPRESSOR

PERFORMANCE POTENTIAL FOR ADVANCED ICR ENGINE

250 300 350 400 450

SPECIFIC POWER, KW, KG/SEC

500 550

Fig. 4. Representative parameters and performance for large ICR gas turbines.

and turbine discharges and facilitates the use of heat exchange at higher pressure ratios. Based on today’s level of turbine-inlet temperature, intercooled and recuperated (ICR) gas turbines have an efficiency potential of about 44% (see later for comments on the possibility of considerably higher efficiencies). Current aeroderivative engines with pressure ratios in the range of 16 to 20 are ideally suited to ICR operation, although they require reduced-capacity high-pressure compressor spools to match the lower-temperature air. As pressure ratio and turbine-inlet temperature continue to increase, future high-specific-power ICR variants have the potential for an efficiency of 50%, as shown in Fig. 4.

Regenerative cycles

The performance of regenerative cycles has been well documented [7]. Some representative results are shown in Fig. 5. While a regenerative cycle is ideally identical to that used in a recuperated cycle, regenerators have different characteristics that encourage the use of different cycle parameters. A positive characteristic is that the cost in money and weight and volume for producing a very-high-effectiveness regenerator is much less than for a recuperator. If, then, a high effectiveness is used, the cycle pressure ratio for the best thermal efficiency is reduced, from 16:l to 2: 1 in extreme circumstances. While the economic optimum would be higher than this low value (because the specific power is also reduced, meaning that a larger machine would be required), the low pressure ratio has a dramatic effect on the range of blade lengths in the compressors and turbines. The very short blades that wear fast and are the site of a large proportion of the blading losses are not found in low-pressure-ratio machines, so that the design-point efficiency is increased.


Moreover, the mismatch of inlet to outlet areas in the compressors and turbines is much reduced, so that the off-design efficiencies are much improved.

The negative characteristics of regenerators are: the seal and matrix ‘carryover’ leakage, which together can reach high levels-eg. 15% of the compressor mass flow, a substantial penalty; the high wear and poor reliability record of many rotary regenerators; and the impracticability hitherto of being able to manufacture regenerators for engines producing more than about one megawatt. Some developments recently patented at M.I.T. (developments towards which one author must confess favorable bias) are aimed at overcoming all three problem areas.

The discontinuous-rotation regenerator. One development is the discontinuously rotating regenerator with clamping seals. Whereas the normal rotary regenerator rotates continuously with rubbing seals contacting the matrix face (and becoming worn in the process), in the new technology the matrix is rotated incrementally, through steps of, for example, 30”. During the short periods when the matrix is moved, the seals are withdrawn from the matrix face a distance of the order of 0.05 mm (0.002 in.), thus allowing only small seal leakage and almost completely eliminating the possibility of wear. During the dwell times the seals are clamped to the matrix faces, reducing leakage almost to zero.

The modular ceramic regenerator. The matrix is composed of an array of long rectangular-section modules that are passed endwise through close-fitting seals from the low-pressure exhaust face to the high-pressure compressor-delivery face (Fig. 6). Thus seals are very short while being extensive in the flow direction. They may additionally be clamped during the dwell times for the modules if this is found desirable. Regenerators for large engines could be produced from such modules. The modules could be mass-produced by extrusion from simple nozzles. They could be removed from the circuit and replaced while the engine is under load should there be signs of wear or deposition or failure.

T’S Absolute total temperature at turbine inlet Absolute total temperature at compressor infet

r I compressor total+ressure ratio

.,

ririaaat-1

(Heat-exchanger effectiveness 95%; total cycle pressure losses 72%; compressor and turbine efficiencies at state-of-the-art v&m)

0.400 0.800 1.20 1.60 2.00 2.40

NONDIMENSIONAL SPECIFIC POWER

(f&yes produced by T. P. Korakianitis, Washington University)

Fig. 5. Representative performance of regenerated gas turbines.


Matrix module arriving from hot-exhaust heat exchanger

Fig. 6. Overall arrangement of a modular ceramic regenerator.

POTENTIAL APPLICATIONS FOR HEAT-EXCHANGED CYCLES

While a wide variety of gas turbines can benefit from the use of heat exchangers (as shown in Fig. 1) it is unlikely in present circumstances that many of them will reach the market place, since with an abundance of low-cost natural gas the simple cycle gives acceptable operating economics, with a reduced capital cost. This situation could completely change should the price of fuel be increased (including the movement to replace the income tax with a consumption tax). Selected applications where an exhaust-gas heat exchanger is mandatory are highlighted in Table 1. The biggest challenge to heat-exchanger designers is in the very-small-engine field, where high operating temperatures and low cost necessitate the use of ceramics, as emphasized below.

Hybrid automotive gas turbine

Development of automotive gas turbines has been underway since the early 1950s [9] but has been facing a moving target since the fuel economy and emissions of spark- and compression-ignition engines have continually improved. Many engines have been built and demonstrated using both regenerators and recuperators [lo]. in recent years, with emphasis on very-high-temperature operation, the development work has focused on the use of ceramic rotary regenerators. To realize a specific fuel consumption of, say, 0.30 lb/hp.hr (i.e. 46% efficiency) requires a very-high- effectiveness heat exchanger and extensive use of ceramics in the hot section of the engine. Much work has been documented in the open literature on automotive gas-turbine R & D, and only one is referred to here [l 11. An example of a compact unit utilizing a single-disk regenerator is shown in Fig. 7. As mentioned previously, emissions are the dominant factor in the selection of cycle parameters and engine design features, and this is particularly true in the field of automotive gas turbines. While work continues on direct-drive systems [12], there is now great interest in the development of small gas turbines (c 100 kW) for hybrid electric vehicles. Many engine variants are in the development phase including those with an intercooler [13], ceramic rotary regenerator [14], metallic recuperators [15-191, and ceramic recuperator [20]. This renewal of interest in the automobile gas-turbine field [21] provides the designer with many challenges, foremost being cost, with a goal of the order of $25/kW. A compact engine will probably include the following features: (1) constant speed, (2) ceramic catalytic combustor (with multi-fuel capability), (3) low-cost non-metallic composite compressor [22], (4) ceramic turbine, (5) air bearings and (6) ceramic heat exchanger. A target cost for the heat exchanger is of the order of $150, and the aforementioned programs will identify the most cost-effective solution (i.e. ceramic recuperator or

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Fig. 7. Example of a regenerated automotive gas turbine (courtesy AlliedSignal).

regenerator). The key issues in the general design of motor-vehicle gas-turbine engines have been reported previously [23].

Small generator sets

There is a need for small, compact, lightweight generator sets (for military, industrial and recreation use) that have multifuel capability, low emissions and low acoustic signature. Low cost is paramount for the gas turbine to find acceptance in this marketplace, and technology fallout from the hybrid automobile gas turbine could make this application a reality.

Advanced tracked vehicles

With its high power, ability to tolerate a wide range of fuels, quick engine-start characteristics (down to low temperatures) and low acoustic and IR signatures, the gas turbine is an ideal prime-mover for modern battle tanks. The AGT 1500 engine shown in Fig. 8 has been in service for several years in the U.S. Army Ml main battle tank and to date over 11.000 engines have been manufactured. Progress continues on gas-turbine technology for future tank propulsion and programs have been discussed previously [2&27]. The power level of about 1 MW is near the limit of the capability of current regenerators, and for this application recuperators are currently dominant. The recuperator is a key component to realize acceptable fuel economy, particularly at part-power operation, bearing in mind that a modern battle tank could consume of the order of 2000 1 (500 gallons) of fuel per day. The prime-surface recuperator for the AGT 1500 engine is important, since it represents the first unit to be manufactured in large quantities. This recuperator, together with other types under development, is discussed in a later section.

Marine propulsion

For applications where there are volume constraints on the fuel-storage space, high engine efficiency is paramount. The dominance of simple-cycle gas turbines for marine propulsion is a reflection of early recuperator impediments (particularly structural integrity). Today the situation


.

Tank Engine

Cylindrical prime-surface recuperator Multi-waveplate construction Welded assembly Effectiveness - 80% More than 10,000 units fabricated Over 6 million hours of operation

Fig. 8. Recuperated gas turbine for the U.S. Army Ml battle tank (courtesy Lycoming).

is different and recuperator technology has matured to the point where existing units can meet the performance and reliability required for marine propulsion [28]. The intercooled-recuperated (ICR) engine offers a 3040% reduction in specific fuel consumption compared with contemporary gas turbines. The ICR engine is attractive for naval applications, its main attributes being (1) high efficiency, (2) excellent part-load efficiency, (3) acceptable specific power, (4) compact power plant, (5) reduced thermal, IR, and acoustic signatures and (6) potential for high reliability.

Work is well underway on the development of an ICR propulsion engine (WR-21) [29]. This engine (shown in Fig. 9) is based on an aeroderivative gas turbine and utilizes a metallic plate-fin recuperator. The power plant is rated at about 20 MW and has an efficiency of over 42%. The

Fig. 9. WR-21 ICR marine-propulsion gas turbine (courtesy Westinghouse Rolls Royce)


INDUSTRIAL PLATE-FIN UNIT FOR LARGE GAS TURBINES

WAVY-PLATE PRIME SURFACE RECUPERATOR FOR TANK ENGINE

ADVANCED CERAMIC COMPSITE PRIME SURFACE UNITS FOR ULTRA HIGH TEMPERATURE SERVICE

INITIAL PLANT OPERATED WITH RECUPERATOR

PROJECTED REGIME FOR COMPACT LIGHTWEIGHT, DIMPLEO AND PROFILE- TUBE GEOMETRIES FOR PROPULSION APPLICATIORS

POSSIBLE REGIME FOR ENHANCED IFLUTED. FINNED) TUBULAR GEOMETRIES

TUBULAR RECUPERATORS FOR IWDUSTRIAL GAS TURBINES INITIAL TUBULAR UNITS BUILT TO BOILER STANDARDS

I I I I I I I I

1940 1950 1960 1970 1980 1990 2000 YEAR

Fig. 10. Gas-turbine recuperator evolution.

success of this power plant is important, not only for marine propulsion, but for the technology transfer to the power-generation industry.

Electrical power generation

Today’s industrial gas turbines rated below about 15 MW have efficiencies in the low 30s. Compared with large industrial gas turbines, these engines are characterized by the following: (1) pressure ratios less than 15, (2) turbine-inlet temperature of less than 1100°C (2012”F), and (3) compressor and turbine efficiencies below 90%. Increasing their efficiency into the high 30s by recuperation has been achieved only on a limited basis. For 21st Century service, where efficiencies of over 50% are projected, new gas turbines must be developed, and these will have a strong reliance on heat exchangers [5]. This level of efficiency will be realized with engines embodying much higher firing temperatures (with ceramic hot-end components) and intercooled-recuperated cycles. Initially these engines will utilize existing metallic recuperators, but to exploit the cycle’s performance potential, ceramic-composite recuperators or regenerators will be necessary.

The summary-level data for the above engines given in Table 1 are viewed as tentative and are included for comparative purposes for potential gas-turbine applications that will utilize regenerators and recuperators shortly after the year 2000.

HEAT-EXCHANGER TECHNOLOGY READINESS

Metallic recuperators

Many different types of recuperators have evolved over the last half century, as shown in Fig. 10. Recuperator state-of-the-art has been discussed previously [30,31] and, since it is not the purpose


of this paper to recommend a particular type of recuperator, only a brief discussion is included on the several types that are available.

Tubular geometries. Development work continues on compact tubular units embodying enhanced surface geometries. A compact profile-tube design has demonstrated high performance capability and structural integrity in a gas-turbine cyclic environment. This type of unit has been discussed previously for vehicular gas-turbine applications [32].

Prime-surface geometries. The ultimate goal of recuperator designers has been to engineer a configuration based simply on a stack of formed plates that embody the heat-transfer matrix, headering and manifolds. Obviating the need for secondary surfaces and furnace-brazing operations, this class of unit is known as a prime-surface heat exchanger. After many years of investigation, modern manufacturing methods (e.g. laser cutting and welding) have made this concept a reality, and the following two types of units are in service. As mentioned previously, the first prime-surface recuperator to enter service and be produced in large quantities was for the engine in the U.S. Army Ml battle tank (Fig. 8) [33,34]. The cylindrical recuperator consists of an assemblage of multi-waveplates with integral air and gas headers formed to give a counterflow configuration. Over 6 million hours of operation have been accumulated with this recuperator. For industrial gas-turbine applications a multi-year development effort by Solar Turbines culminated in the development of a very compact and lightweight primary-surface recuperator [35,36]. Engineered in a low-profile recuperator for integration with the turbomachinery, this unit has demonstrated very high effectiveness (above 90%) and low pressure loss. The surface compactness of this prime-surface unit is of the same order as some plate-fin units. With over 1.5 million hours of accumulated time on this prime-surface recuperator, the structural integrity is proven and this type of unit can meet a variety of users’ needs in the 1990s.

Plate-& geometry. The major problem with early plate-fin units was their inability to remain leaktight in the cyclic gas-turbine environment. In the early 1970s a concerted research and development effort was undertaken by the Garrett Corporation to resolve the problems, particularly the issue of structural integrity. Major innovations included: (1) seal formed by the thin-section tube plate to give thermal-inertia compatibility and ensure integrity during engine transients, (2) integrally formed header manifolds, (3) drastic reduction in number of parts, (4) modern manufacturing processes, including laser cutting of the tube sheets and (5) improved brazing process to allow large cores to be manufactured, with attendant reduction in heat-exchanger cost. This type of recuperator may be used for many applications. Those in service, on over 60 gas turbines, have accumulated over 3 million hours. These compact recuperators have demonstrated effectiveness levels up to 90% with low pressure loss 8.

Ceramic recuperators

The recuperators in current engines are fabricated mainly from stainless steel. The gas-inlet temperature typically varies from 593°C (1100°F) at full power to perhaps as high as 816°C (1500°F) at part power. The latter temperature represents an upper value for the capability of stainless steels. For operation at higher temperatures, use could be made of super alloys, such as Inconel 625, Inconel 617 or Haynes 230, but their cost would be prohibitively high for most applications. It is generally agreed, particularly for military applications, such as an advanced- battle-tank engine recuperator [37], that ultimately ceramic heat exchangers will be necessary. In the case of the small hybrid automobile gas turbine, cost considerations mandate the use of ceramic heat exchangers. In support of various high-temperature systems, development work on ceramic heat exchangers has been in progress for more than 25 years [38]; examples of early units are shown in Fig. 11. An excellent example of a compact ceramic recuperator module developed by AlliedSignal in the late 1980s for a cruise-missile application [39] is shown in Fig. 12. Ceramic heat exchangers that have been fabricated and tested include plate-fin, prime-surface and tubular geometries. Many of these have been presented in the open literature by the following: Parker and Coombs [40], Forster [41], Bakker [42], Kleiner [43], Avran [44], Yoshimura [45]. This continuing work provides a substantial technology base for efforts now focused on a ceramic recuperator for an automobile hybrid gas turbine. Challenges facing the designer include the following: (1) selection of compact and efficient heat-transfer surface and flow-path geometries to meet high-effectiveness

Engine cycles for 2 1 st Century gas turbines 649

Fig. 12. Example of a compact plate-fin ceramic recuperator (courtesy AlliedSignal).

and low-pressure-loss requirements, (2) leaktightness, (3) structural integrity, (4) construction amenable to high-volume-production methods (e.g. extruded, injection molded or roll formed) and (4) perhaps the greatest challenge relates to meeting cost goals (say $150 heat-exchanger cost for a 60-kW engine).

Conventional rotary regenerator

Ceramic rotary disk regenerators were introduced more than 30 years ago [46] and have operated in many development engines. A particularly noteworthy early program was in support of engines developed by Rover [47], and later vehicular engines [48]. Heat-transfer and performance aspects of regenerators have been well documented [49] and development efforts over the years have focused on materials selection, fabrication and reliability [50,51]. Today, development work is still in progress, particularly in support of automotive gas turbines. One of the major concerns is still that of seal leakage, and in one of the most recent publications [12] a value of 7% (viewed as the goal) has been demonstrated. The sealing system and drive mechanism add complexity and cost to the heat exchanger, and it remains to be seen whether cost goals can be realized with a regenerator for the automobile gas turbine. If the sealing problem can be solved for rotary heat exchangers, then perhaps increased compresser pressure ratios could be considered. This would open up the possibility of using an intercooled-regenerative cycle giving higher specific power. Early automotive gas turbines used a symmetrical two-disk-regenerator configuration and an adaptation of this is shown in Fig. 13. This concept has two rotary heat exchangers: (1) a ceramic regenerator of high effectiveness and (2) a metallic intercooler to cool the air before it enters the second centrifugal compressor. This engine cycle has the potential for over 50%. By mixing the intercooler discharge air and the regenerator exhaust a very low IR signature could be realized for military applications.

Advanced regenerator concepts

To date development efforts have focused on the disk regenerator, but there may be alternative approaches that offer solutions to the remaining problems. A novel concept based on the utilization of foil slats has been proposed as an approach to reduce the regenerator size and weight [52]. Two new forms of regenerator [53,54] designed to reduce leakage and seal wear have been described above. The concept shown in Fig. 14 embodies a number of flexible annular disks engineered in a manner to make the leakage negligible. This type has potential applications to intercoolers and to regenerators in ICR engines, where the higher pressure ratio reduces the turbine-outlet temperature to levels that could be withstood by stainless steel.

Heat-exchanger comparison

Increased engine efficiency by utilizing an exhaust-heat-recovery exchanger, particularly for small gas turbines, has been the major theme of this paper. A comparison of the two major types is given ATE 168&--8


ROTARY INTERCOOLER

RbTARY CERAMIC REGENERATOR

Fig. 13. Concept of a small gas turbine with rotary regenerator and intercooler.

in Table 2, and the selection of a regenerator or recuperator is a user option. While the recuperator effectiveness is normally lower, the overall efficiency of both engine variants (in the low-power range) is about the same, since the higher regenerator effectiveness is offset by the leakage and carryover losses. An argument can be given, and in fact has been expressed previously [55] that the ceramic recuperator offers a simpler approach (e.g. freedom from seals and drive mechanism) for very small gas turbines. This statement must, however, be tempered with the fact that the industry has not yet demonstrated the reliability and durability of a compact ceramic recuperator, of a

duct (from turbine)

Fig. 14. Flexible-disk regenerator concept.


Table 2. Comparison of heat exchanger salient features

Heat exchanger type Gas turbine applications

Thermal Thermodynamic cycle Effectiveness, % Pressure loss, % Leakage/carryover loss, % Engine size, kWe Engine efficiency. %

Fabrication Matrix material

Flow configuration Fabrication method Construction cost Amenable to high volume production

Size/installation Integrated with engine Ease of removal/replacement Accessories Size and weight

Deployment Technology base

Regenerator Hybrid auto Small generator sets

Low pressure ratio (< 5) up to 97 4 I -500 45

Counterflow Extruded Cellular Currently high Yes

Yes Complex Drive mechanism Compact

Rotary disk unit has over Metallic units available three decades of R & D (high cost, limited temperature capability) New concepts being evaluated Composite ceramic technology in its infancy

Reduced leakage Low cost fabrication methods Lower cost Leaktightness

- 2005 - 2005

Recuperator Hybrid auto Small generator sets Tracked vehicle Power generation

Up to 12 (higher with intercooling) 9&93 5 0 15,000 (modular Hx) -45 (can match regen.)

Metallic and ceramic Composite Counterflow TBD for ceramics Compact plate-fin Metal units very high Yes

Yes Cartridge/modular None Larger than regenerator

Major areas of development

Service year

construction type that would yield a low cost when manufactured using high-volume-production methods.

SUMMARY

The benefits of recuperation and regeneration have long been recognized, but early heat-exchanger impediments (particularly structural integrity), together with market forces (e.g. abundance of low-cost fossil fuels), have negated widespread use of this class of gas turbine. For engines currently being planned for service in the first decade of the 21st Century, low emissions will be a dominant factor in the selection of thermodynamic cycle and engine features. For a significant reduction in SFC (say a step-change reduction of 30%) exhaust-heat-recovery exchangers will be required.

The biggest challenge to the designer of heat exchangers is in the small engine class (for hybrid automobiles and small generator sets), where very high temperature will require ceramic-composite heat exchangers, with a cost of the order of $150 for a 60-kW engine. For these applications, where very high production quantities will be required, both ceramic recuperators and regenerators could meet the requirements. While ceramic recuperator work is in its infancy, it may have, because of its simplicity, an advantage in terms of low cost and ease of integration (as a module) for small engines. Development work on rotary ceramic disk regenerators has been ongoing for over three decades and significant advancements have been made in materials technology and manufacturing. Demonstration of long-term durability and the reduction of seal leakage below about 7% remain to be demonstrated. Complex integration with the rotating machinery, together with the seal system and drive mechanism, aggravate the already challenging cost goals.

To realize all of the challenging goals for small engines, it is concluded that new thinking may be required for the regenerator and some novel concepts have been discussed. In the case of the recuperator (for larger engines) there are viable metallic units (e.g. prime-surface, plate-fin and profile-tube) available to meet users’ needs. These units are expensive when produced in only small quantities and, in times of low-cost natural gas, there is little perceived interest for industrial gas turbines. In the case of initial deployment of the ICR engine for marine propulsion, metallic


recuperators will be used, but in later variants ceramic-composite units will be used as the technology becomes available. Ceramic-composite recuperator technology is in its infancy, and it is the potential for high-volume production for small engines that will stimulate development. Renewed thinking is also required in this area since the recuperator matrix will be produced using high-volume-production methods (e.g. as for oil filters). Since the recuperator will have a very high ratio of surface area to volume it is likely that it would be coated with a catalyst, making extended life a strong requirement. Advantage will be taken of heat-exchanger technology transfer from the small-engine field to benefit larger engines. It is projected that advanced industrial gas turbines (up to about 15 MW) entering service after about 2005 will have efficiency levels of 50% based on the use of intercooling and high-effectiveness ceramic-composite recuperators.

The authors feel strongly that the first decade of the 21st Century will be the era for recuperated and regenerated gas-turbine engines. While not well recognized today, these engines will then be in the mainstream of the gas-turbine industry.

1. 2.

3.

4.

5.

6.

7. 8. 9.

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24. 25. 26. 21. 28.

29. 30.

31. 32. 33. 34. 35.

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the utilization of recuperated and regenerated engine cycles for high-efficiency gas turbines in the...

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