mcdonald (2003) tor considerations for future higher eff micro turbines

Applied Thermal Engineering 23 (2003) 1463–1487www.elsevier.com/locate/apthermeng

Recuperator considerations for futurehigher efficiency microturbines

Colin F. McDonald *

McDonald Thermal Engineering, 1730 Castellana Road, La Jolla, CA 92037, USA

Received 3 December 2002; accepted 20 March 2003

Abstract

First-generation microturbines are based on the use of existing materials and proven technology, and

with low levels of compressor pressure ratio and modest turbine inlet temperatures, have thermal effi-

ciencies approaching 30% for turbogenerators rated up to 100 kW. For such small machines the goal of

advancing beyond this level of performance is unlikely to include more complex thermodynamic cycles, but

rather will be realised with higher turbine inlet temperatures. Advancing engine performance in this manner

has a significant impact on recuperator technology and cost. In the compact heat exchanger field very

efficient heat transfer surface geometries have been developed over the last few decades but further im-

provements perhaps using CFD methods will likely be only incremental. Automated fabrication processesfor the manufacture of microturbine recuperators are in place, and on-going developments to facilitate

efficient higher temperature operation are primarily focused in the materials area. Based on the assumptions

made in this paper it is postulated that in the 100 kW size the maximum thermal efficiency attainable for an

all-metallic engine is 35%. To achieve this the recuperator cannot be designed in an isolated manner, and

must be addressed in an integrated approach as part of the overall power conversion system. In this regard,

temperature limitations as they impact the recuperator and turbine are put into perspective. In this paper

there is strong focus on recuperator material selection and cost, including a proposed bi-metallic approach

to establish a cost-effective counterflow primary surface recuperator for higher temperature service. If in-deed there is a long-term goal to achieve an efficiency of 40% for small microturbines, it can only be

projected based on the utilisation of ceramic hot end components. Alas, the high temperature component

that has had the minimum development in recent years to realise this goal is the ceramic recuperator, and

efforts to remedy this situation need to be undertaken in the near future.

� 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Microturbine; Turbogenerator; Recuperator; Gas turbine performance; Metals and ceramics

* Tel.: +1-858-459-9389; fax: +1-858-459-6626.

E-mail address: [email protected] (C.F. McDonald).

1359-4311/03/$ - see front matter � 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S1359-4311(03)00083-8

mail to: [email protected]

1464 C.F. McDonald / Applied Thermal Engineering 23 (2003) 1463–1487

1. Introduction

The beauty of current microturbines is their simplicity, particularly the use of single-shaft radialflow turbomachinery. However, after over six decades of development compressor and turbineaerodynamic efficiencies are near plateauing, and further gains even using CFD methodology arelikely to be only incremental. Combustion, mechanical and generator efficiencies are already athigh levels, so in what areas can changes be made to advance microturbine performance? The twoparameters that have potential for efficiency advancement are increased values of turbine inlettemperature and higher recuperator effectiveness. Unlike larger axial flow turbines, the cooling ofsmall metallic radial flow turbines is difficult, and increased temperatures will be paced by ma-terials technology, hopefully leading to the eventual utilisation of ceramic rotors. The twoaforementioned parameters have a strong impact on the recuperator. Even with a modest increasein compressor pressure ratio, a higher turbine inlet temperature means that the recuperator hotgas inlet temperature increases, this necessitating the use of higher cost superalloys for a com-ponent that is already the most expensive in the system.

Increasing the effectiveness is straightforward but results in a dramatic increase in recuperatorsize and weight, again aggravating the cost situation. In light of the above it is clear that inno-vativeness on the part of both materials scientists and heat exchanger designers will play a sig-nificant role, not only for higher efficiency microturbines, but also for other gas turbines that canbenefit from the use of a recuperated cycle for improved efficiency and reduced emissions.

There are no established industrial guidelines to categorise small gas turbines for electricitygeneration and for combined power and heat system operation. It is arbitrarily assumed thatmachines in the 5–200 kW range are classed as microturbines, and in the 200–500 kW range areminiturbines. This paper addresses recuperator considerations for future higher efficiency mi-croturbines (the example selected being a 100 kW unit) that could be realised by increasing theturbine inlet temperature. Background discussions are included on heat exchanger surface geo-metries and construction types, with emphasis placed on primary-surface units. The impact onturbogenerator performance and recuperator cost by utilising superalloys in the heat exchanger,use of a bi-metallic approach, and the eventual use of a ceramic recuperator to enable the per-formance potential of small gas turbines to be fully realised are discussed.

2. Microturbine state-of-the-art technology

2.1. Turbogenerator configurations

There are several engineering and packaging considerations regarding the utilisation of a re-cuperator in small gas turbines with radial flow rotating machinery, and since they have beendiscussed previously [1,2] they are only summarised here. It is not the purpose of this paper torecommend an installation approach, since this is done by the turbogenerator designer, never-theless it is germane to mention this topic in the context of the recuperator. There are basically twomajor types, the first being the close coupling of the recuperator, and the other an arrangementwhere the heat exchanger is positioned behind, above or alongside the rotating machinery.

An example of the coupled approach is shown in Fig. 1. A high degree of integration can berealised with an annular wrap-around recuperator. Advantages of this approach include the

Fig. 1. Microturbine concept with annular wrap-around recuperator (courtesy C. Rodgers ITC).

C.F. McDonald / Applied Thermal Engineering 23 (2003) 1463–1487 1465

following: (1) good aerodynamic gas flow paths resulting in low pressure losses, (2) lower acousticsignature, (3) built-in rotor burst shield, and (4) eliminates the need for external ducts and thermalexpansion devices.

An attractive turbogenerator package can also be achieved using a conventional ‘‘cube-shaped’’recuperator installed behind and inline with the rotating machinery as shown in Fig. 2. This type

Fig. 2. Microturbine concept with rear-mounted recuperator (courtesy C. Rodgers ITC).


of installation is adaptable to either a single can or annular combustor. There are applicationswhere this installation has advantages and these include: (1) ease of recuperator hot gas bypass forcogeneration, (2) use of an external combustor or heat exchanger for burning biomass or lowgrade dirty fuels, and (3) perhaps the most important being the ease of coupling a recuperatedmicroturbine with a high temperature solid oxide fuel cell [3] to give a very high efficiency hybridpower generation system with near zero emissions.

2.2. Performance of first-generation microturbines

Some of the major parameters that impact thermal efficiency and specific power are shown inFig. 3. This performance array is based on assumed component efficiencies [3] for a microturbine

Fig. 3. Performance array for recuperated microturbine.


with a power output of about 100 kW. Based on state-of-the-art technology and existing materialsthe thermal efficiency of representative first-generation microturbines with a compressor pressureratio of about 4 is shown as approaching 30%.

2.3. Higher temperature operation for improved performance

The most important parameter in the quest for higher efficiency is turbine inlet temperature.The shape of the curves and data displayed in Fig. 3 reflect the assumptions made, and it re-cognised that other analysts may show slightly different data, nevertheless the trends are felt tobe representative enough for the scope of this paper.

Superimposed on the basic plot are lines of recuperator hot gas inlet temperature, and thisparameter has a strong effect on heat exchanger material selection.

Based on the assumptions made for a 100 kW turbogenerator it is postulated that a thermalefficiency of about 35% is near to the maximum that could be realised with an all-metallic single-shaft low pressure ratio engine. This point will be addressed in more detail in a later sectioncovering materials selection.

A long-term microturbine efficiency goal of 40% has been mentioned in many papers and ar-ticles, but to achieve this an engine with ceramic hot end components (i.e. combustor, turbine, andrecuperator) would be required, but such technology is not commercially foreseen for severalyears.

3. Gas turbine recuperator technology

3.1. Heat transfer surface geometries

There is a wide variety of efficient surface geometries that can be considered for compact higheffectiveness counterflow heat exchangers, but discussion here is limited to only types that areconsidered representative for small gas turbine recuperators.

3.1.1. Primary-surface geometryThe main attributes of primary-surface types are that the surface geometry is 100% effective (i.e.

no secondary surface fin efficiency effects), and sealing can be accomplished by welding withoutthe need for an expensive and time consuming high temperature furnace brazing operation.

The recuperator that has been produced in the largest quantity (i.e. over 15,000) is the primary-surface heat exchanger for the AGT 1500 engine in the US Army M1 main battle tank. Thisannular recuperator embodies a multiplicity of embossed wavy plates that are laser cut andwelded to give a compact assembly [4].

Caterpillar and Solar Turbines developed a compact primary-surface recuperator that has beenfabricated in platular and annular forms [5]. Produced in significant quantities for several mi-croturbines these recuperators have demonstrated good performance and structural integrity.

Examples of primary-surface recuperators are shown in a later section of this paper, whereemphasis is placed on this type of construction for future higher efficiency microturbines.

Fig. 4. Compact plate-fin microturbine recuperator (courtesy Toyo Radiator Co.).


3.1.2. Plate-fin geometry

Plate-fin heat exchanger technology is well known, and units of this type have been used forseveral decades in many aerospace and industrial applications. The performance and structuralintegrity of plate-fin recuperators have continually improved over the last three decades [6,7] andthey have been used for a variety of different gas turbine applications. A compact plate-fin re-cuperator used in a 75 kW microturbine is shown in Fig. 4. Advancements continue to be made [8]and plate-fin recuperators remain a viable option for future microturbines.

3.1.3. Tubular geometry

While thin-walled small diameter tubes are known to have a high cost, tubular geometries haveexcellent pressure containing capability, and it would be remiss not to mention them since re-cuperators of this type are considered suitable for some future gas turbine applications. Acompact matrix utilising small hydraulic diameter profile tubes is shown in Fig. 5. This type ofconstruction has demonstrated high performance and structural integrity in a cyclic environmentand has been discussed previously for the recuperator in the LV100 gas turbine tank engine [9,10].With its proven high pressure retaining capability this type of light weight construction is beinginvestigated for the recuperator in a high pressure ratio intercooled and recuperated (ICR) tur-bofan aeroengine development project underway in Europe [11].

3.2. Recuperator specific size and weight

Presenting actual recuperator size and weight data are beyond the scope of this paper, but ageneral discussion may be of benefit to non-specialists. The impact of effectiveness on recuperatorspecific size and weight for low pressure ratio gas turbines is shown in Fig. 6. The data portrayed(based on heat exchanger information accumulated over many years by the author) are for the

Fig. 5. Compact profile-tube recuperator core (courtesy MTU GmbH).


recuperator matrix only and do not include casings, connecting ducts or supporting structure thatare all installation sensitive. The bands of data for counterflow units reflect several factors in-cluding variation in engine parameters, material type and thickness, and heat exchanger config-uration (e.g. platular or annular). Discrete points are shown for existing hardware with the scattersurely reflecting the many variables involved.

Since preparing this plot [12] the author has received additional data points from industry on aconfidential basis that give further credence to the general validity of the curve bands. After over ahalf century of compact heat exchanger geometry development it has been suggested that furtheradvancements, perhaps utilising CFD methodology [13] could result in a reduction in recuperatorsize and weight, but these are likely to be only incremental compared with the data shown in asimplistic form in Fig. 6.

3.3. Recuperator performance characteristics

The performance of heat exchangers over a wide range of flow conditions is well understood,and a typical performance map is shown in Fig. 7. The portrayal of data in this form is useful for

Fig. 6. Recuperator specific size and weight.


determining recuperator performance for applications where bleed or bypass flows are consideredon one or both sides of the unit.

Fig. 7. Typical recuperator performance map.


What is less well known is recuperator performance at very low flows. In many heat exchangersthe assumption that heat conduction in the separating walls is only in the direction normal to flowis reasonably valid. However, in a high effectiveness counterflow recuperator there is a longitu-dinal flow of heat from the hot to the cold end of the unit. This heat flow in the matrix metal in thelongitudinal direction is undesirable since it tends to unify the wall temperature. This adverseeffect on thermal performance is dependent on material thermal conductivity, conduction flowpath length, and metal cross sectional area in the matrix. Longitudinal conduction impact oneffectiveness is significant at very low flows. This is shown in Fig. 8 [14], the plot being in aconvenient form for inclusion in systems analysis codes to give engine part load performance.With the trend towards utilising ever increasing surface compactness, consideration must be givento longitudinal conduction effects, although they are likely to be less pronounced in surfacegeometries such as off-set fins, where there are periodic discontinuities in the heat conductionpath.

Fig. 8. Effect of longitudinal conduction on recuperator performance at low flows.


4. Primary-surface recuperator considerations

While differing types of recuperator construction, as discussed above, will find acceptance forgas turbine applications, the remainder of this paper focuses on primary-surface units. The ra-tionalisation for this includes the following: (1) well established technology base, (2) simpleconstruction amenable to different matrix shapes and sizes, (3) welded seals, this obviating theneed for a high temperature furnace brazing operation and the cost of the braze alloy, (4) ame-nable to automated high volume fabrication, (5) potential for minimum cost, and (6) as will beoutlined in the following section, primary-surface geometries are amenable to a proposed bi-metallic approach to establish a cost-effective unit for higher temperature service.

4.1. Multiple element approach

Starting with a thin-foil stock, a semi-automated process consists basically of the followingsteps: (1) folding to form the heat transfer surface geometry, (2) press and trim the individualsheets, (3) welding of the two sheets together to generate the basic air cell, and (4) pressure testingof the basic element for leak tightness.

The cells can be formed in different shapes. Based on an involute form the individual cells canbe welded together to yield an annular core as shown in Fig. 9. Several thousand recuperators ofthis type have been fabricated for microturbine service (30 and 60 kW units) and they have ac-cumulated over a million operating hours without a failure [15].

Fig. 9. Annular primary-surface recuperator core (courtesy Capstone Turbine Corp.).

Fig. 10. Primary-surface recuperator (courtesy Rekuperator Svenska AB).


A platular or cube shaped core can also be fabricated for applications where there is no closecoupling between the rotating machinery and the recuperator. An example of this type for a 100kW microturbine is shown in Fig. 10 [16]. The flat air cell is made of two plates stamped with theheat transfer geometry and laser welded at their periphery. The cells are then stacked and laserwelded together to form the core. The recuperator is then completed by welding the air inlet andoutlet manifolds to the core.

4.2. Spirally wrapped recuperator

Spirally wrapped heat exchangers are used for a variety of industrial applications, andthe merits of extending this technology to primary-surface recuperators have been discussed

Fig. 11. Spirally wrapped recuperator (courtesy Rolls Royce plc).


previously [12]. In its simplest form the automated fabrication process would consist of two spoolsof thin-metal foil that are continuously formed into heat transfer surfaces and seal welded as thematrix is being spirally wrapped.

Spirally wrapped recuperators with different types of headering have been proposed. A spirallywrapped annular recuperator of basically primary-surface geometry, together with a variant thathas secondary surface fins on the gas side has been proposed in the UK [17]. The spiral wrappingoperation for this type of unit is shown in Fig. 11. The completed annular matrix embodies in-tegrally formed and seal welded air inlet and oulet headers. This attractive and compact recu-perator is being developed for a range of gas turbine applications [18].

Development work in Belgium has progressed on an interesting high effectiveness spirallywrapped compact primary-surface recuperator with laser welded seals [19]. The unique headeringof this unit involves locally opening air channels by a spark erosion process, and then welding the

Fig. 12. Spirally wrapped primary-surface microturbine recuperator (courtesy Acte S.A.).


air side manifolds on to the two end faces of the core. Initially developed for a small microturbine(Fig. 12), this type of annular recuperator could be used for multi-megawatt gas turbines with theheat exchanger mounted at the rear of the engine.

4.3. Automated high volume fabrication

With the projection that microturbines, perhaps as small as 5 kW [20], may be produced in largequantities, new recuperator manufacturing methods are needed, and these are being addressed indifferent parts of the world. Minimizing the number of parts in the matrix is mandatory, the idealfeedstock being just two spools of thin-foil material. An automated forming and welding process isneeded to continuously make the matrix in high volume production. The all-welded primary-surface type meets these goals, with the spirally wrapped approach offering potential for low cost.

Recuperator fabrication specialists can perhaps take advantage of high volume fabricationtechnology from the automotive industry where tens of millions of radiators are made in auto-mated facilities each year, and where well established cost learning curves exist.

5. Quest for higher microturbine efficiency

5.1. Thermodynamic cycle options

Current low pressure ratio microturbines up to power ratings of about 100 kW utilise fixed-boundary recuperators to achieve thermal efficiencies approaching 30%. This type of engineconfiguration is likely to be extended for microturbines rated up to say 200 kW. As will be dis-cussed below, advancing to higher levels of efficiency will require increasing the turbine inlettemperature.

There is interest in miniturbines in the power range of 200–500 kW, with an ambitious efficiencygoal of 40% based on using existing materials and state-of-the-art component technology. Withthese restraints it is unlikely that this level of performance can be realised in the near-term withjust a recuperated cycle. For such larger machines, different approaches are being investigated,these including ICR cycles [8], and the use of organic fluid Rankine bottoming cycles. These topicsare beyond the scope of this paper, but it is suffice to say that concepts in this power class willrequire high performance and low cost recuperators to achieve 40% efficiency and meet de-manding cost goals.

5.2. Temperature limitations

The impact of turbine inlet temperature on thermal efficiency is shown in Fig. 3. Using the sameaforementioned cycle data it is convenient to replot it in a different manner (Fig. 13) to illustrateapproximate temperature limitations in two of the major components.

5.2.1. Uncooled metallic radial flow turbine

Unlike in larger axial flow engines where turbine blade cooling is commonplace, the very natureof the geometry in small radial flow turbines makes cooling of the nozzle and impeller difficult. It

Fig. 13. Effect of recuperator temperature limitations on microturbine performance.


is recognised that research is underway on this topic, but today cooled metallic small radial flowturbines are not commercially available for microturbines. In this paper an assumption was madebased on specialist input [21] that the upper temperature limit for a radial flow turbine in a 100kW microturbine is about 1150 �C (2100 �F) and this is shown in Fig. 13.

5.2.2. Recuperator material limitations

In first-generation microturbines the selection of the recuperator material depends on the cycleconditions and user preference. Type 347 austenitic stainless steel is widely used based on itsproperties and cost, and is considered as the base case in this paper. As turbine inlet temperatureis increased in low pressure ratio engines the attendant hot gas temperature entering the recu-


perator reaches a value where heat exchanger life goal (typically 40,000 h) using 347 stainless steelcan no longer be realised. Temperature limits for the recuperator are based on the magnitude ofthe material�s tensile strength and its resistance to corrosion, oxidation, and creep deformation.

The author views the portrayal of data in Fig. 13 as being representative, but recognises thatother specialists may generate different data based on their assumptions. Nevertheless, at least inthis simplistic form it represents a starting point for discussion regarding materials selection.Input was received from a materials specialist [22] on approximate temperature limits for can-didate materials, and these are shown in Fig. 13.

5.3. Existing heat exchanger materials

As mentioned above Type 347 stainless steel is used in some microturbine recuperators, but thishas a temperature limit of about 675 �C (1250 �F). As shown in Fig. 13 an efficiency approaching30% is the highest that can be reached using this austenitic steel in the recuperator.

For increased temperature service a higher nickel content alloy must be used. It is of interest tonote that the primary-surface recuperator produced in the largest quantities for the AGT1500engine in the M1 tank was fabricated from Inconel 625 [23], a much higher cost material thanstainless steel as will be discussed below.

5.4. Proposed customized recuperator materials

In support of advancing microturbine efficiency a materials research programme investigatinghigher temperature recuperator materials is underway at the Oak Ridge National Laboratory.This involves not only an evaluation of existing materials, but the identification of custom alloysfor recuperator service [24–28]. A much more comprehensive version of Fig. 13 could be generatedwhen the properties of these new alloys become definitized and are commercially available in athin foil form in the future. It is projected that an increase in recuperator hot gas inlet temperatureto 750 �C (1382 �F) is possible with a customized super 347 stainless steel. Based on the as-sumptions made, it can be seen from Fig. 13 that this would increase a low pressure microturbineefficiency to perhaps as high as 33%.

By increasing the compressor pressure ratio slightly, and assuming a recuperator effectivenessof 91% it is projected that a thermal efficiency of 35% could be realised with an Inconel 625 re-cuperator. It is interesting that this data point (Fig. 13) also corresponds to the maximum tem-perature limit for an uncooled metallic radial flow turbine. Again, based on the assumptions madeit is projected that the maximum thermal efficiency for an all-metallic microturbine in the 100 kWsize is about 35%. By utilising an even higher grade (and much higher cost) superalloy (Haynes214) in the recuperator, a thermal efficiency of about 38% is projected, but since this necessitates aceramic turbine it is viewed as a rather academic case, but is included for completeness.

5.5. Proposed bi-metallic counterflow primary-surface recuperator

The use of a multi-pass cross counterflow modular recuperator has been suggested for small gasturbine applications [29]. Such a unit could be mechanically assembled using a superalloy in thefirst high temperature pass, and then lower grade materials in the other modules towards the


colder end of the heat exchanger. However, a novel monolithic counterflow heat exchanger withhigh temperature capability and better performance is proposed below as being a more compactand cost-effective solution.

Since the temperature gradient in a counterflow recuperator is in the longitudinal direction assimply illustrated in Fig. 14, the notion of a bi-metallic approach has fascinated the author formore than three decades. As will be discussed below there is a strong economic case for using acostly superalloy only at the hot end of the recuperator core where it is really needed. From theeconomic standpoint this approach could perhaps be considered analogous to the fabrication ofhack saw blades where only a thin strip of high quality steel for the teeth section is used. The bulkof the blade being a lower cost material, with bonding of the two metal strips done by a con-tinuous butt welding operation.

For a compact recuperator, two spools of thin-foil stock would be used with a continuous buttwelding operation done prior to forming of the surface geometry. The two metals must beweldable and have similar coefficients of thermal expansion to avoid raising thermal stresses in thematrix.

At this point it is germane to mention the temperature conditions at the recuperator hot gasinlet face. Materials scientists use maximum metal temperature in their assessments, whereas in

Fig. 14. Simplistic temperature gradient through high effectiveness primary-surface recuperator for microturbine rated

at 100 kW with an efficiency of 35%.

Fig. 15. Microstructure of thin-foil bi-metallic weld sample (courtesy Toyo Radiator Co. Ltd.).


this paper the various curves show the hot gas temperature. Using the latter�s slightly higher valueis felt to be reasonably representative since for a very high effectiveness recuperator the metaltemperature is on the order of 20–25 �C below the maximum gas temperature at the hot end of theunit as shown in Fig. 14.

To investigate the practicality of a bi-metallic approach samples of thin-foil austenitic 347stainless steel and nickel-based alloy Inconel 625 (both materials of which have been used inrecuperators) were laser welded together by Toyo Radiator Company in Japan. A microsection ofthe weld is shown in Fig. 15. From the metallurgical standpoint the welded joint looked satis-factory, namely good fusion, and with no cracks, voids or obvious defects in the microstructure[30].

With the recuperator currently being the highest cost component, an initial part of the inves-tigation to advance microturbine efficiency to say 35% would be to determine the impact onengine life cycle cost for a range of higher temperature recuperator superalloys, including thesuggested bi-metallic approach. Assuming that the results were positive for the latter case, a re-search effort could then be initiated to establish the viability of the bi-metallic approach and thiswould include the following: (1) a survey of the range of higher nickel content alloys [31] andselection of the one most compatible with 347 stainless steel, (2) identify the best welding ap-proach, (3) regarding the material form, would it be better for example to weld together thickersections and then roll down to the required size? (4) determine whether the joint has sufficientductility to be formed in to representative compact primary surface geometries, and (5) fabricateand test a well instrumented counterflow heat exchanger to determine the stability of the weldedcore in a thermally cyclic environment.

In the quest for improved microturbine performance the impact of recuperator gas inlet tem-perature on thermal efficiency for a 100 kW turbogenerator is shown in Fig. 16. The base casecorresponds to the temperature limit of 347 stainless steel, and yields a state-of-the-art thermalefficiency close to 30%. Based on the assumptions made, the upper temperature limit for a long-life Inconel 625 recuperator corresponds to an efficiency of 35%.

Considering a bi-metallic approach, only a portion of the counterflow matrix needs to be madefrom Inconel 625. For the 35% efficiency case it can be seen from Figs. 14 and 16 that a compositematrix would consist of 23% Inconel 625 and 77% 347 stainless steel. Clearly, this would have asignificant beneficial impact on the matrix material cost as discussed below.

Fig. 16. Impact of material selection on primary-surface recuperator matrix cost and performance for 100 kW mi-

croturbine.


5.6. Recuperator matrix cost

Microturbine recuperator cost information within the industry is understandably regarded asbeing proprietary, thus direct comparisons between existing units cannot be made. It is widelyrecognised that the recuperator is the most expensive component, and efforts need to be expendedto reduce its cost. This would include detailed analyses to study the effect of matrix type (e.g.primary surface, plate-fin, tubular geometries), surface compactness, bonding and sealing pro-cesses (e.g. welding, brazing, diffusion bonding), and material types and thicknesses. However,proprietary comprehensive data bases from the heat exchanger industry are needed to accomplishthis. Lacking these, it has been assumed here that in high volume production the recuperator costwill be dominated by the material cost, and this aspect is addressed below.

Data taken from the various curves in this paper are put into perspective in Table 1 for severalcandidate recuperator materials for a 100 kW microturbine. For the 347 stainless steel temperaturelimited base case, the thermal efficiency is 29.5% and the estimated matrix material cost is $984.

Now say for discussion purposes that a near-term goal is to advance the efficiency to 35%. Thiscan be achieved by increasing the recuperator effectiveness, and by increasing the turbine inlet

Table 1

Comparison of recuperator materials for 100 kW microturbine

Material Data source 347 stainless

steel

Super 347

stainless steel

Inconel 625 Haynes 230 Haynes 214 Bi-metallic 77% 347,

23% Inconel 625

Material cost factor Ref. [22] 1 1.5 5 7 9 1.92 equiv.

Maximum metal

temperature, �CRef. [22] 675 750 800 850 900 As for parent metals

Nickel content, % Ref. [24] 11.2 13.0 61.2 52.7 76.5 –

Relative thermal

expansion coefficient

at 675 �C

Ref. [27] 1.0 1.0 0.85 0.82 0.78 –

Approximate

thermal efficiency, %

Fig. 13 29.5 33.0 35.0 36.5 38.0 35.0

Recuperator

effectiveness

Fig. 13 0.87 0.91 0.91 0.91 0.91 0.91

Turbine inlet

temperature, �CFig. 13 954 1070 1150 1205 1271 1150

Compressor

pressure ratio

4 4 4–5 4–5 4–5 4–5

Specific power,

kW/kg s

Figs. 3 and 13 122 156 178 200 220 178

Air flow, kg/s 0.82 0.64 0.56 0.50 0.45 0.56

Matrix specific weight,

kg/kg s

Fig. 6 from

Ref. [12]

100 160 160 160 160 160

Matrix metal weight,

kg

82 102 90 80 72 90

Metal cost, $/kg Ref. [12] for

base case

12 base case 18 60 84 108 23 equiv.

Matrix metal cost, $ 984 1836 5400 6720 7776 2070

Notes Existing

operating

machines

New stainless

steel alloy

under

development

Upper limit

for all-metal-

lic engine

For ref.

ceramic

turbine

required

For ref.

maximum for

superalloy

recuperator

with ceramic

turbine

Proposed lower cost

recuperator

approach

C.F.McDonald/Applied

Therm

alEngineerin

g23(2003)1463–1487

1481


temperature, and having a slightly higher compressor pressure ratio the specific power is alsoimproved. This case is beyond the capability of a stainless steel heat exchanger, and necessitatesthe use of a higher nickel content alloy. If an all Inconel 625 recuperator was used the matrixmaterial cost would be $5400. If a bi-metallic unit was used with Inconel 625 only in the hot end ofthe recuperator where it was really needed the matrix material cost would be about $2070. Thispotential material cost saving of over 60%, while admittedly based on a simplistic model, isconsidered the motivation for an industrial programme to thoroughly investigate the viability of abi-metallic recuperator for advanced higher efficiency microturbines. Other recuperator alloyswith higher temperature capability than Inconel 625 are shown in Table 1, but they are includedfor reference only since for the attendant higher thermal efficiency projected, a ceramic turbine isnecessary.

For production of recuperators in very large quantities a factor of 1.5 times the material costfor the complete matrix cartridge has been suggested [12]. On-going recuperator development andproduction activities will determine if such a target can be realised.

5.7. System integration

The various curve arrays presented in this paper clearly show that the recuperator cannot betreated as an isolated component, and must be included in the overall engine parametric evalu-ation. To facilitate the generation of the recuperator material-related cost curves as shown in Fig.16, it was necessary to utilise previously derived engine and heat exchanger data based on thereferenced sources given in Table 1. The shape of the cost curves for the differing alloys was notobvious at the onset of the study, and are rationalised as follows. As the turbine inlet (and exit)temperature is raised the turbogenerator specific power increases, hence for a given power (say100 kW) the mass flow rate through the machine decreases resulting in a smaller and lighterweight recuperator. For a given effectiveness this manifests itself in a reduction of recuperatormatrix material cost as shown in Fig. 16.

Another interesting point from the parametric study was that with a superalloy recuperator,albeit at high cost, the thermal efficiency potential is 38%, but this could not be realised today withan uncooled metallic radial flow turbine.

6. Long-term goal of a ceramic recuperator

It is understandable that today�s microturbines, and those planned in the foreseeable futureutilise metallic recuperators, since the needed technology is available. However, the need forceramic heat exchangers for a variety of gas turbine applications has been recognised for decades[32]. There has been interest in rotary regenerators for over half a century [33], and interestingcomparisons have been made between rotary and fixed-boundary heat exchangers [34,35]. Insupport of automotive gas turbines several decades of development were undertaken on rotaryregenerators [36], initially operating experience with metallic discs [37], and later use of ceramicdiscs [38,39]. Many of these had materials related problems, and excessive leakage that degraded

Fig. 17. Ceramic plate-fin recuperator module (courtesy Garrett Corp.).

Fig. 18. Compact plate-fin ceramic recuperator (courtesy Ceramique & Composites S.A.).


engine performance. Even after many years of development their questionable durability essen-tially ruled them out for the majority of first-generation microturbines due to life concerns. Itshould be pointed out that work continues on new regenerator concepts [40–42] aimed at over-coming the problems associated with earlier variants.

The ultimate performance potential of microturbines can only be realised with a high tem-perature ceramic heat exchanger [43], and this is apparent from studying Figs. 3 and 13. Alas, verylittle development is underway towards this goal. A compact plate-fin ceramic recuperator modulewas fabricated in the late 1970s [44] and this is shown in Fig. 17. A programme was undertakenfairly recently in Europe [45,46] to develop a counterflow ceramic plate-fin recuperator (Fig. 18) insupport of an automotive gas turbine, but this did not proceed beyond the initial developmentphase.

When the gas turbine industry thinks the time is right to develop a ceramic recuperator for anadvanced microturbine, it should seriously consider the primary-surface type. In the 1980s such aunit (Fig. 19) was partially developed for an automotive gas turbine [47]. Initial results for a cube-shaped matrix showed promise [48], but development was discontinued because of changes in theoverall engine programme. It is the author�s view that this approach should be seriously revisitedbased on using 21st century ceramics technology.

Fig. 19. Elements from compact primary-surface ceramic recuperator (courtesy Golden Technologies).


7. Summary

The microturbine performance data presented in this paper are based on assumptions that maydiffer from work by others, but put into perspective the results show in a reasonable manner howtemperature limitations in the recuperator and turbine impact projected advancement in thermalefficiency for future advanced small turbogenerators.

To advance the efficiency of say a 100 kW microturbine by several percentage points beyond thehigh twenties of first-generation units, will require a multi-faceted effort involving changes to thecycle parameters and increased component efficiencies. With radial flow compressor and turbineaerodynamic efficiencies near plateauing, the most significant factor for improved thermal perfor-mance will be higher turbine inlet temperatures. In retaining simple low pressure ratio single-shaftradial flow turbomachinery this will result in an increase in the recuperator hot gas inlet temperatureto values that will necessitate material changes in the heat exchanger from those that are used today.

For a long-life recuperator, approximate temperature limits have been based on the magnitudeof the material�s tensile strength and its resistance to corrosion, oxidation and creep resistance. Inthis paper focus has been on primary-surface units since they are amenable to high volume au-tomated fabrication, and have the potential for minimum cost. In the evaluation of materials,Type 347 stainless steel was assumed as the base case since in current microturbines (having ef-ficiencies approaching 30%), the recuperator hot gas inlet temperature of 675 �C (1250 �F) is nearto its operating limit.

The results from initial metallurgical research to establish a customized version of this material(i.e. super 347) are encouraging, and its utilisation could raise the thermal efficiency by two tothree percentage points. However, it could be a while before a material of this type is completelydefinitized and commercially available in the necessary thin foil form.

The next significant value of efficiency discussed in this paper is 35%, which represents themaximum achievable with an Inconel 625 recuperator and uncooled metallic radial flow turbine.


If the recuperator was fabricated entirely from this nickel-based alloy the matrix material costwould be $5400, this being a reflection of the much higher cost of Inconel 625 compared withType 347 stainless steel.

Based on the longitudinal temperature gradient characteristic in a counterflow recuperator, abi-metallic approach has been suggested in which only the hot end of the heat exchanger wouldhave the higher temperature superalloy. Based on the assumed limits for the two candidate ma-terials (i.e. Inconel 625 and 347 stainless steel) the primary-surface recuperator matrix materialcost, based on simplistic economic model, was estimated as $2070.

While much development work remains to be done to confirm the viability of the bi-metallicapproach, the potential material cost savings of over 60% compared with an all Inconel 625matrix (for a thermal efficiency of 35%) would suggest that this approach is well worth investi-gating for a compact high effectiveness primary-surface recuperator for an advanced microtur-bine.

This proposed cost-effective approach could provide the means of achieving higher temperatureoperation and bridge the span between first-generation microturbines with austenitic steel recu-perators and the eventual goal of utilising a ceramic recuperator to permit the full performancepotential of microturbines to be realised.

Acknowledgements

The author expresses his thanks to Colin Rodgers (ITC), a good friend for many years, fordiscussions and advice on the performance of small gas turbines. Thanks are also given to Dr.John Mason, Professor David Gordon Wilson (MIT) and Hubert Antione (Belgium) for theirexpertise and insight on heat exchanger technology, and to Dr. Phillip Maziasz (ORNL) and FredStarr (UK) for guidance on various aspects of high temperature materials science. Special thanksare expressed to Kenjiro Takase and Yoichiro Yoshida (Japan) for their interest and pioneeringefforts on the welding of a thin-foil bi-metallic joint. This paper has been enhanced by the in-clusion of hardware photographs, and the author is appreciative to all concerned, with creditsbeing duly noted.

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mcdonald (2003) tor considerations for future higher eff micro turbines

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