ro

a

Cas

e, S

Received 15 October 2006; accepted 5 January 2007Available online 7 February 2007

Currently ICEs, burning both gasoline and diesel fuel,dominate the small gen-set market. Other variants includ-ing Stirling engines and fuel cells are being developed forthis growing market. Technological and economic aspectsof these approaches are being addressed by other special-ists. Only microturbines are covered in this paper, where

* Corresponding author. Tel.: +1 858 459 9389; fax: +1 858 459 6629.E-mail addresses: kmcdona1@san.rr.com (C.F. McDonald),

rodgersc@cox.net (C. Rodgers).

Applied Thermal Engineering1. Introduction

First generation microturbines that entered servicearound 1995 were based on proven technology, conserva-tive operating parameters, and the use of existing materials.The motivation for this was to produce turbogeneratorswith high reliability, and requiring minimum maintenance,

and this was achieved. Based on the above criteria,machines rated in the 30100 kW power range operate withmodest levels of eciency on the order of 30%. This,together with their high cost, and some distributed genera-tion institutional issues, has likely contributed to the smal-ler number of units being manufactured annually than hadbeen projected at the onset of microturbine deployment.Abstract

It has been about a decade since microturbines rst entered service in the distributed generation market, and the eciencies of theseturbogenerators rated in the 30100 kW power range have remained essentially on the order of 30%. In this time frame the cost of fuel(natural gas and oil) has increased substantially, and eorts are now underway to increase the eciency of microturbines to 40% orhigher.

Various near-term means of achieving this are underway by utilizing established gas turbine technology, but now based on more com-plex thermodynamic cycles. A longer-term approach of improving eciency is proposed in this paper based on the retention of the basicrecuperated Brayton cycle, but now operating at signicantly higher levels of turbine inlet temperature. However, in small low pressureratio recuperated microturbines embodying radial ow turbomachinery this necessitates the use of ceramic components, including theturbine, recuperator and combustor.

A development approach is proposed to design, fabricate and test a 7.5 kW ceramic microturbine demonstrator concept, which for therst time would involve the coupling of a ceramic radial ow turbine, a ceramic combustor, and a compact ceramic xed-boundary higheectiveness recuperator. In a period of some three years, the major objectives of the proposed small ceramic microturbine R&D eortwould be to establish a technology base involving thermal and stress analysis, design methodology, ceramic component fabrication tech-niques, and component development, these culminating in the assembly and testing to demonstrate engine structural integrity, and toverify performance. This would provide a benchmark for more condently advancing to increased size ceramic-based turbogeneratorswith the potential for eciencies of over 40%. In addition, the power size of the tested prototype could possibly emerge as a viable prod-uct, namely as a natural gas-red turbogenerator with the capability of meeting the total energy needs of an average house. 2007 Elsevier Ltd. All rights reserved.

Keywords: Ceramic microturbines; Gas turbine; Turbogenerator; Recuperator; RegeneratorSmall recuperated ceramic mic

Colin F. McDonalda McDonald Thermal Engineering, 1730

b ITC, 3010 N. Arroyo Driv1359-4311/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.applthermaleng.2007.01.020turbine demonstrator concept

,*, Colin Rodgers b

tellana Road, La Jolla, CA 92037, USA

an Diego, CA 92103, USA

www.elsevier.com/locate/apthermeng

28 (2008) 6074

focus is on increasing the eciency of small low pressureratio recuperated turbogenerators by utilizing ceramiccomponents to facilitate operation at temperatures higherthan can be accommodated by non-internally cooledmetallic radial ow turbines and existing superalloyrecuperators.

Several decades of ceramic development work have beenundertaken particularly in support of vehicular gas tur-bines rated in the 10s and 100s of kilo Watts. However,

ceramic technologies for the size of the components neededin these engines (namely the turbine and heat exchanger)have so far not demonstrated the necessary integrity andreliability for commercial service in the vehicular or powergeneration markets. In taking advantage of existing cera-mic technology the proposed recuperated ceramic micro-turbine concept is, by virtue of its small size (i.e.,7.5 kW), viewed as a reduced cost and lower risk modestrst step towards the demonstration of a structurally viable

Nomenclature

APU auxiliary power unitAGATA advanced gas turbine for automobilesCHP combined heat and powerDG distributed generationhp horsepowerHX heat exchangerICE internal combustion engineICR intercooled and recuperatedkW kilo Watt

LOM laminated object manufacturingNTU number of transfer unitsORBC organic rankine bottoming cyclePMG permanent magnet generatorR compressor pressure ratioSFC specic fuel consumptionSOFC solid oxide fuel cellTIT turbine inlet temperatureUAV unmanned aerial vehicle

C.F. McDonald, C. Rodgers / Applied Thermal Engineering 28 (2008) 6074 61Fig. 1. Eciency comparison of candidate small power conversion systems.

Thceramic based turbogenerator. The lower risk projection isbased on two factors: (1) the use of a ceramic radial owturbine about the same size as those used in turbochargersthat have been in service since the mid 1980s in high perfor-mance cars in Japan; and (2) the emergence of a compacthigh eectiveness ceramic micro-channel recuperator. Thecondence gained from the successful demonstration of asmall recuperated ceramic gas turbine would contributetowards the technologies needed for the eventual deploy-ment of increased size ceramic based microturbines witheciencies of 40% and higher.

For the proposed ceramic recuperated microturbinedemonstrator concept, germane technology bases, compo-nent design considerations, performance data, turbogener-ator conguration layout, cost considerations, and atentative development schedule to accomplish the projectin less than some three years, are addressed in this paper.

2. Small power generation systems overview

In the power range from a few kilo Watts to say overabout 500 kW a variety of power generation systems willnd niches in the marketplace. It is not the purpose of thispaper to discuss in detail dierences between options sincethe emphasis here is on microturbines; however, it was feltto be meaningful to include an overall comparison of thetype shown on Fig. 1. This plot is by no means comprehen-sive, and since there are so many variables involved, thedata shown is regarded as tentative, and specialists in thevarious elds could well show diering boundaries. Indeed,performance data for the various power generation systemsshown, and numerous others, have been included in manypublished papers and books. In the foreseeable future ICEswith established manufacturing, user and service infrastruc-tures will continue to be dominant in the small gen-set eld.Diesel and gasoline engines have beneted from over a cen-tury of continuous evolution, and development continueswith emphasis on emissions reduction. Stirling enginesand fuel cells and other hybrid systems are expected to ndgradual acceptance in power generation and CHP markets.These prime-movers are not discussed here since they areviewed as being beyond the scope of this paper. However,the portrayal of data in the form shown on Fig. 1 is includedhere primarily to put into perspective the relative ecienciesof rst generation state-of-the-art metallic microturbines,and projected future generation advanced ceramic variants,and this topic is discussed in Section 3.

3. Small gas turbine developments

3.1. State-of-the-art metallic engines

Several thousand microturbines, dominantly naturalgas-red, are currently in service in the 30100 kW powerrange. These microturbine turbogenerators have ecien-

62 C.F. McDonald, C. Rodgers / Appliedcies on the order of 30%. With interest now being expressedin somewhat larger size turbogenerators, the eciency ofmachines rated at 100 kW and above can be increased byoperating with higher turbine inlet temperatures, but stillretaining simple radial ow turbomachinery. Increasingthe compressor pressure ratio and turbine inlet temperatureto values that can be accommodated by a non-internallycooled metallic radial turbine, and using a superalloy recu-perator, an upper eciency of about 35% is felt to be apractical limit for todays fairly low pressure ratio all-metallic turbogenerators operating at moderately hot dayconditions [1].

3.2. Higher eciency metallic engines

For higher pressure ratio engines the realization of closeto 40% eciency necessitates the use of more complex ther-modynamic cycles. One approach is to use an ICR cycle.The engine is more complex, but this approach has theadvantage that the additional metallic components are ofproven technology. Recent studies [24] have shown thatall-metallic ICR microturbines in the 300400 kW powerrange have the potential for an eciency close to 40% usinga metallic turbine with an inlet temperature of 1100 C.

Again using proven technology, another approach is toadd a bottoming Rankine cycle power conversion system[57]. The temperature of the gas turbine exhaust is su-cient to heat a low boiling point liquid in a heat exchangerto drive a vapor turbine generator. This is regarded as astate-of-the-art solution since ORBC technology is wellunderstood. On the basis of the same eciency, it is notclear at this stage how the cost of these two state-of-the-art gen-sets would compare with a more advanced technol-ogy ceramic microturbine.

3.3. Ceramic gas turbine developments

Prior to initiating the proposed ceramic microturbinedemonstrator development project it is germane to takeadvantage of the extensive R&D eorts that have beenundertaken over the last ve decades on ceramic gas tur-bine components. Two recently published books give verycomprehensive accounts of ceramic engine design and testexperience [8] and component development and character-ization [9]. Additional valuable data on materials and cera-mic component development can be gleaned from recentpapers [1015]. We would be remiss if we did not mentionthe signicant accomplishments made in the small ceramicgas turbine eld with details of the engines highlighted onTable 1, and briey discussed as follows.

The European AGATA program was focused on thedevelopment of three critical ceramic components; a cata-lytic combustor, a radial turbine, and a xed boundaryrecuperator for a 60 kW turbogenerator in a future hybridvehicle. After ve years of work the AGATA programachieved signicant accomplishments in the areas of cera-mic component design, fabrication and full scale testing.

ermal Engineering 28 (2008) 6074The feasibilility testing concluded that it was possible tomeet the design specications [16].

d ThTable 1Operated small ceramic gas turbines

Application AGATA CCGT 302

Project/activity Ceramic component development for60 kW automotive gas turbine

300 kWCeramicengine

Year 19931998 19881998Thermodynamic

cycleRecuperated Recuperated

Pressure ratio 4.0 8.0Turbine inlet

(C)1350 1350

Eciency goal(%)

40 42

Rotorarrangement

Single shaft Two shaft

Compressortype

Radial Radial

C.F. McDonald, C. Rodgers / ApplieWhile a larger machine embodying an axial ow turbine,a 300 kW ceramic engine development project carried outover a 10 year period in Japan is noteworthy because ofthe high eciency demonstrated. The ceramic componentsincluded a can combustor liner, turbine nozzle and axialow turbine and turbine scroll. The lower heat exchangergas inlet temperature associated with the selection of ahigher compressor pressure ratio allowed the use of ametallic recuperator. The engine tested, designated CCGT302 demonstrated an exceptional thermal eciency of42.1% (without a generator) at a turbine inlet temperatureof 1350 C [17]. If the generator and frequency converterwere included with say a 90% overall electrical eciency,the thermal eciency would have been 37.4%.

The Japan Automotive Research Institute undertook aseven year program to develop a 100 kW automobile sin-gle-shaft ceramic engine with radial ow turbomachinery.The components included a ceramic can combustor, radialow turbine and nozzle, and twin ceramic high eectiveness

Rotor speed(rpm)

125,000 48,000

Turbine type Radial AxialTurbine

materialCeramic Ceramic

Turb tip speed(m/s)

650 480

Combustor type Catalytic CanCombustor

materialCeramic Ceramic

Heat exchangertype

Platen Platen

HX material Ceramic MetallicHX eectiveness 0.90 0.82Generator type

ResultsachievedTurbine inlet(C)

1350 1350

Power (kW) NA 312Eciency NA 42.1a

a Excludes generator eciency.Automotivegas turbine

Modied J-850 Modelaircraft turbojet

Proposed ceramicmicroturbine demonstrator

100 kWCeramicengine

Ceramic turbine test Small 7.5 kW recuperatedceramic turbogenerator

19901997 2005 2009Regenerated Simple cycle Recuperated

5.0 3.0 3.01350 1280 1170

40 30

Single shaft Single shaft Single shaft

Radial Radial Radial

ermal Engineering 28 (2008) 6074 63rotary regenerators. A very creditable eciency of 36.5%(non-electrical) was demonstrated with a turbine inlet tem-perature of 1350 C [18]. As in other small gas turbine pro-jects the sealing and durability of the ceramic rotaryregenerator presented a major challenge.

4. Small recuperated ceramic microturbine demonstrator

concept

4.1. Genesis

The eciency of very small gas turbines (i.e., in the 10sand 100s of kilo Watts) is impacted by factors whichinclude small blade heights, low Reynolds number eects,tip clearance eects, manufacturing tolerances, surfacenish, and engine-to-engine variations. This was apparentin a study done by the authors over ve years ago on arecuperated microturbine rated at 5 kW [19]. Based onstate-of-the-art technologies the projected eciency for this

110,000 140,000 160,000

Radial Radial RadialCeramic Ceramic Ceramic

665 410 585

Can Annular CanCeramic Metallic Ceramic

Regenerator Micro-channel

Ceramic Ceramic0.93 0.92 PMG

1350 1280

NA 36.5 NA

conservatively designed radial ow machine with a turbineinlet temperature of 900 C was 21.5%.

With the eciencies of the major components (i.e., com-pressor, turbine and generator) near plateauing after sev-eral decades of development it was concluded that asignicant gain in eciency could only be realized byincreasing the turbine inlet temperature. In such a smallmachine size there was not an existing technology base,and this was the motivating factor for proposing a smallceramic microturbine concept with the power increasedto 7.5 kW.

The authors are encouraged that the proposed essen-tially ground-up demonstrator approach has alreadystarted in Japan in a very unique way. A small turbojetengine used in model aircraft was modied to incorporatea ceramic radial ow turbine [20]. The advantage of start-ing with such a small demonstrator included fabrication,facilitation, lower cost, and decreased probability of fail-ure. The small test engine embodied a 55 mm diameterradial ow turbine (about the same size as ceramic turbinesused in automobile turbochargers in Japan), and for thisapplication was fabricated in SN235 ceramic material by

ceramic xed-boundary recuperator, and extracting powervia a high speed generator.

4.2. Thermodynamic cycle

Fig. 2. 2.6 kW Micro gas turbine generator set (courtesy IHI).

64 C.F. McDonald, C. Rodgers / Applied Thermal Engineering 28 (2008) 6074Kyocera [21]. While some problems were encountered arotational speed of 140,000 rpm was demonstrated at a tur-bine inlet temperature of 1280 C [22]. Continuing develop-ment led to a 1-h test run at a temperature above 1000 Cwithout severe damage [23], and further duration tests areplanned. It was this innovative approach in Japan toexplore the capability of a small ceramic turbine in a gasturbine environment that also encouraged the authors tosuggest the proposed demonstrator concept. The proposed7.5 kW ceramic demonstrator is viewed as an obvious nextstep, and made much more meaningful by incorporating aFig. 3. Small ceramic microturbIt is dicult for small simple-cycle gas turbines toachieve eciencies much above 20% [24], thus from theonset the inclusion of a high eectiveness recuperator ismandatory for small microturbines. In small low pressureratio radial ow turbomachinery the impact of componenteciency is very signicant, and compared with existingmicroturbines this is aggravated by the very small size ofthe components in the proposed ceramic demonstrator.

It is of interest to note that two very small turbogenera-tors with turbocharger-based turbomachinery were builtand operated in Japan. With a metallic radial ow turbinediameter of 52 mm a small gas turbine engine for aine concept cycle diagram.

mini-cogeneration system was built and tested using natu-ral gas fuel in Japan [25]. Utilizing an exhaust-heated cyclethe target electrical power was 3 kW at 20% eciency.

The other small microturbine (shown on Fig. 2) rated at2.6 kW, again based on turbocharger technology, was pro-duced as a commercial product [26] and several hundredunits were sold. While the target eciency when burningkerosene was 20%, it fell far short of this, and an investiga-tion was undertaken to determine the performance

5. Major component design considerations

order of 0.25 mm. Moderate pressure ratios are thereforeto be preferred for microturbines with power ratings lessthan approximately 10 kW.

To keep the rotating assembly simple, a back-to-backaluminum centrifugal compressor and ceramic radial tur-bine rotor assembly is proposed. With this arrangementcareful design attention must be given to the interfacebetween the two, plus adjacent back shrouds to minimizethe heat soak back from the hot ceramic turbine to thecold metallic compressor. The topic of heat transferbetween the turbine and compressor has been addressedpreviously [29], and could either be a turbine cooling assetor a performance penalty. Its eect on engine performancecan be minimized by designing the compressor near opti-

jets, and an example of a small turbojet with a proposed

C.F. McDonald, C. Rodgers / Applied Thermal Engineering 28 (2008) 6074 655.1. Compressor

A single stage centrifugal compressor is the inevitablechoice for microturbines as a consequence of its cost, sim-plicity, compactness and performance characteristics suchas wide surge margins with high inlet ow distortion toler-ance. Typical eciency characteristics have been discussedpreviously [28]. The attainable eciencies for small massow compressors with impeller tip diameters on the orderof 60 mm, as would be the case for the proposed demon-strator, would be about 75% (see Fig. 4) and largely depen-dent on design choice of specic speed and Mach number.

Specic speed is a function of rotational speed, volumeow, and adiabatic head. Mach number is a function ofpressure ratio and is particularly critical for small compres-sor entry blading, necessitating very thin blades on thedeciency.This detailed investigation of the 2.6 kW turbogenerator

focused on the complex interfaces between the small com-ponents, and the eects of uid and heat leaks, and sealbypass ows were analysed. To a large extent the eciencydiscrepancy between estimated performance and test datawas rationalized, and simple changes such as thermal isola-tion and leak sealing were suggested as a means of restor-ing the target eciency [27]. Lessons learned from thisinvestigation would be taken advantage of in the designof the proposed 7.5 kW turbogenerator.

The simplistic nature of the proposed microturbine dem-onstrator system is shown in the cycle diagram on Fig. 3.While the major goal of the ceramic-based engine is todemonstrate the integrity and performance of the completeturbogenerator, a possible waste heat recovery module forCHP service is shown.Fig. 4. Size eects on eciency of radial ow compressor and turbine.application as a 7 kW turbogenerator embodying a ceramicaxial turbine [30] is shown on Fig. 5. Small gas turbineswith turbine expansion ratios above 3.0 can result in over-loading the single stage axial turbine, and for highermum specic speed (small surface area/mass ow) , but itis aggravated by higher compressor exit to turbine inlettemperature dierentials. The long-term stability of theceramic-to-metallic interface must also be demonstrated.

5.2. Turbine

Inward ow radial and mixed ow turbines have estab-lished a prominence in small turbomachines because oftheir simplicity, low cost, relatively high performance,and ease of installation. The predominant application ofthese turbines are in small gas turbines and turbochargersin the 0.052.0 kg/s ow range.

Extensive development of both radial and axial ow tur-bines with outputs of 100 kW or less has shown the superi-ority of the radial type particularly including eciency andmanufacturing cost. Small ceramic turbocharger radial tur-bines have proven reliability records in the automobileindustry as a consequence of small rotor mass extendingWiebull stress related life.

Single stage axial turbines are being used in small turbo-Fig. 5. Small gas turbine (7 kW equivalent rating) with ceramic axial owturbine (courtesy Innotech Europe BV).

eciencies two stages would be necessary. Design studieshave indicated that for the same overall turbine eciencythe single stage radial can oer lower cost, but requires alarger diameter, higher tip speed rotor, concomitant withincreased stresses.

The attainable eciencies for rotor tip diameters on theorder of 70 mm are about 83% as shown on Fig. 4. Thisradial turbine eciency is representative of levels demon-strated in gas turbine applications with optimum bladesolidities and thickness, together with vane nozzles. Smallturbochargers utilize more robust blades to cater forincreased blade loads during pulse blow down operation,and coincidentally benecial for ceramic casting processes.Automobile turbocharger rotor tip speeds rarely exceed450 m/s. Turbine entry volutes are being superseded byvariable nozzles, some with ceramic vanes, to improveacceleration response and transient emissions.

In the automobile turbocharger eld various technolo-gies have been exploited to reduce the characteristic

rotational speed for a microturbine rated at 7.5 kW witha turbine inlet temperature of 1170 C, and pressure ratiosof 2.53.5, as portrayed on Fig. 7 was computed using acycle optimization computational technique described pre-viously [41]. The optimum rotational speed is shown toencompass the range of 150,000200,000 rpm. A rotationalspeed of 160,000 rpm with a pressure ratio of 3.0 was cho-sen to meet the target eciency of 30%. The compressorand turbine tip diameters are on the order of 60 and70 mm, respectively. The turbine tip speed of 585 m/swould be higher than current state-of-the-art automobileceramic turbines requiring progressive advancements inaerodynamic design to optimize the degree of reaction

66 C.F. McDonald, C. Rodgers / Applied Thermal Engineering 28 (2008) 6074response dwell time during rotor spool-up. Reducing theturbine rotor inertia by utilizing a ceramic radial ow tur-bine has been adopted for some applications, particularlyin the last two decades mainly in Japan [3138].

Ceramic radial ow turbines manufactured by Kyocerafor small turbochargers (as shown on Fig. 6) have beendeployed in thousands of premium market performancecars in Japan since the mid 1980s [39], and are still in pro-duction today [40]. The ceramic rotor for the proposed7.5 kW ceramic microturbine concept would be similar insize to the one shown.

The design of microturbines with single stage radialcompressors and radial turbines is focused towards optimi-zation of overall engine thermal eciency, thus the selec-tion of rotational speed becomes a compromise betweenboth compressor and turbine aerothermodynamic designcriteria. The predicted thermal eciency variation withFig. 6. Ceramic radial ow turbine from automobile turbocharger(courtesy Kyocera International Inc).Small gas turbines currently operate with rotationalspeeds from 60,000 to 150,000 rpm with both conventionalImprovements in magnetic materials have resulted inlighter and more ecient permanent magnet generators(PMGs) than wound eld generators. The eld excitationis provided by permanent magnets that are capable of oper-ating at temperatures up to 250 C. Power output is deter-mined by the attainable generator tip speed, diameter andlength-to-diameter ratio. Approximate generator sizing isillustrated on Fig. 8 as limited by a rotor tip speed limitof 250 m/s. Generator tip speed, diameter, and lengthparameters would therefore bracket rotating speedsbetween 150,000 and 200,000 rpm for the 7.5 kW rating,ideally matching the aerothermodynamic optimum speed.

Generator cooling and heat rejection is a major consid-eration and may incur additional parasitic power losses.An air-cooled generator with ambient air being drawn overexternal ns surrounding the generator casing is conceptu-alized with a conservative overall eciency of 88%, includ-ing frequency conversion.

5.4. Bearingstogether with improved ceramic materials and brittle stressanalysis techniques.

5.3. GeneratorFig. 7. Optimal rotating speed for 7.5 kW microturbine concept.

thrust bearing, which may be as large in diameter as the

d Thcompressor impeller.

5.5. Combustor

Scaling techniques for the design of mini combustors areless well dened due in part to the eects of the following.Surface area/volume changes with size, increased eects ofwall quenching, low fuel ows resulting in tiny injectors,and increased eect of leakage gaps on pattern factor.

In an earlier 5 kW turbogenerator design concept [19] anannular combustor was selected from the gas ow pathantifriction and air bearings. Some microturbines in servicetoday have demonstrated trouble free operation with airbearings, and this type is proposed for the 7.5 kW ceramicmicroturbine. Air bearings require no lubrication or associ-ated lubrication cooling system, plus minimal parasiticdrag during starting. Air bearings however possess lowthrust bearing load capacity, and are sensitive to thermalgradients, and shock loading under high g accelerations.Air bearings do incur reduced power losses, especially the

Fig. 8. Permanent magnet generator sizing.

C.F. McDonald, C. Rodgers / Appliestandpoint since it was symmetrically compatible with anannular prime-surface metallic recuperator. As will be dis-cussed in a Section 6, having a rear-mounted cube-shapedceramic modular recuperator in the proposed 7.5 kWengine design concept has a major impact on the enginearchitecture. A compact overall engine arrangement, withacceptable gas ow paths, was established with the selec-tion of a single can combustor with a ceramic liner. Thedesign and fabrication of this component can take advan-tage of an established ceramic combustor technology base[11]. Emissions and carbon dioxide release are rapidlybecoming a dominant criterion in the design of smallmicroturbines, and a development eort on this small com-bustor is foreseen. The proposed ceramic microturbinedemonstrator would be natural gas-red, but could beengineered to operate with a variety of liquid fuels, includ-ing biodiesel and ethanol which are currently receiving a lotof attention. However, the use of liquid fuels can increasethe likelihood of build up of carbon deposits in the com-bustor. When they become dislodged from the combustoror fuel injectors they could cause damage to the ceramicnozzle and rotor blades. Radial impellers are more suscep-tible to damage because the interspace centrifugal forcestend to reverberate the particles back against the nozzletrailing edge causing erosion. One remedy to this problemis the use of axial type turbine nozzles.

Natural gas is a fuel of choice for small business anddomestic microturbines, but most probably would requirecompression from essentially atmospheric pressure to levelsexceeding microturbine compressor delivery pressure.Selection of microturbine cycle pressure ratio thereforerequires consideration being given to the gas supply sys-tem. The design of the very small gas compressor wouldtake advantage of units currently operating and the useof emerging technologies.

6. Ceramic recuperator

6.1. Background

A separate section is devoted to the recuperator since inthe microturbine eld it is the component that has receivedthe least development attention, and because the proposedheat exchanger represents a major ceramic recuperatortechnology advancement. For several decades there hasbeen interest in ceramic recuperators and regenerators fora variety of small gas turbine applications, and the meritsof each type have been discussed previously [42,43]. Devel-opment activities were primarily focused on vehicular gasturbines and a wide range of surface geometries and typesof construction were tested [44,45]. Organizations world-wide have undertaken ceramic recuperator development,but over the years there has been no coordinated eortsor continuity. In general, technical progress has been mod-est, and today it is recognized in the microturbine eld thatfurther ceramic recuperator development activities are war-ranted. In their current metallic form recuperators areexpensive and represent about 30% of the overall turbogen-erator cost.

Until recently the authors had felt that ceramic recuper-ators would have as their genesis the same types of surfacegeometry and construction as metallic heat exchangers(e.g., platen, primary surface, tubular, etc.), but anemerging ceramic heat exchanger technology has made thisthinking essentially obsolete as discussed below.

6.2. Ceramic micro-channel recuperator

Utilizing laminated object manufacturing (LOM) meth-ods a compact ceramic recuperator for use in high temper-ature microturbines is being developed by Ceramatec [46].The selection of silicon carbide was based on the followingconsiderations; cost, fabrication and compatibility with thegas turbine environment. The LOM process begins with the

ermal Engineering 28 (2008) 6074 67blending of ceramic powders and organic binders. Thisblend (i.e., ceramic slip) is then cast onto a mylar lm that

dries and cures into a exible form, and in the green-statemicro-channels are formed. These are then sintered to formmonolithic structures amenable to mass production. Forgas turbine recuperator applications these micro-channelstypically have air side and gas side hydraulic diameters of1 and 2 mm, respectively. With such a compact surfacegeometry, the engine would be run initially with naturalgas as the fuel. In forming the matrix a thin dense layerof silicon carbide that acts as the primary surface is posi-tioned between the channels.

The micro-channels are designed into a planar or platethat can be installed as part of the shell-and-plate type ofconstruction. An example of the basic ceramic plate isshown on Fig. 9, and these are the primary building blocksof the recuperator and include features for gas manifold-ing, and macro features to be integrated into the gas head-ers to give the counterow conguration as shown by thearrows on Fig. 9. By stacking and bonding these plates

bine demonstrator is viewed as the rst step towards this

Fig. 10. Ceramic micro-channel gas turbine recuperator module (courtesyCermatec Inc).

68 C.F. McDonald, C. Rodgers / Applied Thermal Engineering 28 (2008) 6074with spacers, narrow gaps are formed and dene the gaschannels for the turbine exhaust gas. The plates are thenstacked to form the recuperator matrix as shown onFig. 10. This cube-shaped module is manifolded for instal-lation on the rear of the microturbine.

To achieve the targeted engine performance goal ademanding requirement is put on the recuperator in termsof eectiveness. The impact of recuperator eectiveness onengine eciency is shown on Fig. 11. For the selected tur-bine inlet temperature, an eectiveness of 0.92 is requiredto meet the turbogenerator eciency goal of 30%. The highsurface compactness of the silicon carbide micro-channelrecuperator results in a very small heat exchanger size (vol-ume of approximately 10,000 cubic centimeters) that inte-grates well with the rotating machinery to give a compactand light weight turbogenerator package. In establishingthe size of the counterow recuperator the eectiveness-NTU relationship for both metallic and ceramic variantsis the same. The part load characteristics would be slightlyFig. 9. Ceramic micro-channel recuperator elementgoal.

7. Proposed engine design layout concept

The layout of the 7.5 kW microturbine concept is shownin a simple form on Fig. 12. The compact turbogeneratorhas the following major features: (1) single stage centrifugalcompressor; (2) can combustor with a ceramic liner; (3)ceramic radial ow turbine and nozzle; (4) ceramicmicro-channel recuperator; (5) ceramic interconnectingdierent since longitudinal conduction eects are related inpart to the matrix material thermal conductivity.

The utilization of such a compact high temperaturerecuperator with the type of construction amenable tolow cost high volume production is regarded as a key fac-tor for the future deployment of the next generation ofhigher eciency microturbines. The design, fabrication,development and testing of the proposed ceramic microtur-and ow conguration (courtesy Cermatec Inc).

d ThC.F. McDonald, C. Rodgers / Appliehot gas ducts and scrolls; (6) high speed rotor supported onhydrodynamic air bearings; and (7) direct-drive air-cooledpermanent magnet generator.

The basic turbogenerator design concept shown onFig. 12 is very compact, with a height of about 300 mm

Fig. 11. Impact of recuperator eectiveness on

Fig. 12. Ceramic microturbine dermal Engineering 28 (2008) 6074 69and an overall length on the order of 500 mm. Severalcomponents not shown on the layout include the controlsystem, gas compressor, instrumentation, other smallaccessories, electrical equipment and thermal insulation.When assembled within a packaged enclosure the envelope

small ceramic microturbine performance.

emonstrator concept layout.

of the unit would be about the same size as a domestic dishwasher.

8. Projected performance of ceramic microturbine

demonstrator

The proposed 7.5 kW turbogenerator concept is a fol-low-on to a 5 kW machine design suggested previously bythe authors [19]. The design of this small turbogeneratorwas based on state-of-the-art component technology andthe use of existing materials. For this all-metallic enginedesigned with conservative parameters (i.e. pressure ratio3.0, turbine inlet temperature 900 C, and recuperatoreectiveness 0.85) the estimated eciency was 21.5%.While this concept was aimed at meeting the total energyneeds of an average house [47] this natural gas-red micro-turbine did not attract any interest. The low eciency com-pared with existing ICEs and emerging SOFCs was felt tobe a factor. Since focus in this paper is on a ceramic micro-turbine concept, in-depth comparisons with other systemsincluding ICEs and fuel cells are not really appropriateat this stage.

Using the component eciencies discussed in a previoussection the power rating of the aforementioned all-metallic

perator and combustor. The design overall cycle pressureloss selected was 10%. The overall generator eciencyselected was 88% including power conditioning and bear-ing losses. Engine output would be reduced approximately6% if an electrically driven fuel gas compressor wasrequired. A representative performance array relating thesalient parameters for the 7.5 kW machine is shown onFig. 13. Superimposing the eciency of the earlier 5 kWall metallic turbogenerator on this gure is not validbecause of the dierent levels of recuperator eectivenessused for the two machines.

The ceramic turbine inlet temperature is essentiallydetermined by the turbine material Wiebull strength, highcycle fatigue strength, oxidation resistance, and duty cycle.Another important cycle parameter is the recuperator hotgas inlet temperature, which in the past has been limitedby metal oxidation considerations together with liferequirements for the selected material. The pressure ratiois determined by the compressor type and the materialused. The rotational speed is initially selected during cycleoptimization studies as described in Section 5.2 and subse-quently rened by rotor dynamics, bearing loads, and gen-erator considerations, together with rotor stress limitationscommensurate with the machine life requirement.

70 C.F. McDonald, C. Rodgers / Applied Thermal Engineering 28 (2008) 6074machine was increased to 7.5 kW and means of improvingthe eciency by raising the turbine inlet temperature wasinvestigated. Advantage was taken of previous work doneon the eects of engine size on performance [48], particu-larly an assessment of system losses. Typical overall cyclepressure losses for small recuperated microturbines rangefrom 5% to 11% basically dependent on the type of recu-Fig. 13. Small microturbinBased on various aerodynamic, thermal, and stress load-ing criteria an ambitious eciency goal of 30% was tar-geted for such a small turbogenerator. The selection of30% eciency was based on what was felt to be realizablewith a very small ceramic machine, and to avoid confusionis not meant to be compared with existing metallic micro-turbines in the 30100 kW power range that are operatinge performance array.

Thermodynamic cycle Recuperated

d ThTurbomachine type Single-shaft rotorRotational speed (rpm) 160,000Bearing type Air bearingsTurbine type Single stage radial

(70 mm diameter)Turbine inlet temperature, C (F) 1170 (2137)Turbine eciency 83Turbine material Monolithic ceramicCompressor type Single stage radial

(60 mm diameter)Compressor inlet temperature, C (F) 30 (86)Compressor pressure ratio 3.0Compressor eciency (%) 75System pressure loss (%) 10.0Generator type PMG air-cooledGenerator eciency (%) 88Combustor eciency (%) 98Recuperator type Compact counterow

micro-channelTable 2Salient features of small recuperated ceramic microturbine demonstratorconcept

Component/parameter Feature

Turbogenerator type Advanced small ceramicmicroturbine

Power (kW) 7.5 (11 hp for mechanical drive)Demonstrator eciency target/goal

(%)30

C.F. McDonald, C. Rodgers / Appliewith eciencies on the order of 30%. From Fig. 13 it can beseen that this is projected with a pressure ratio of 3.0 and aturbine inlet temperature of 1170 C. To achieve this alarge demand is placed on the recuperator, necessitating ahigh eectiveness value of 0.92. Compared with the previ-ous 5 kW design [19] the temperature of the hot gas enter-ing the recuperator mandates the use of a ceramic heatexchanger. The major parameters and features of the7.5 kW ceramic microturbine demonstrator concept arehighlighted on Table 2. At this conceptual stage it is recog-nized that these parameters are tentative and a comprehen-sive trade study involving the inuence of individualcomponent eciencies and duty cycle on engine perfor-mance would be undertaken as part of the conceptualdesign.

9. Cost considerations

There have been many microturbine papers published inthe open literature in recent years, but because of the pro-prietary nature of the business very little cost data has beenreported by the gas turbine industry. This is disappointingto researchers since cost data to be really meaningful mustcome from the microturbine manufacturing industry. Thefew numbers available for microturbines range from$1000/kW for fully equipped units being produced in lim-

Recuperator material Silicon carbideRecuperator eectiveness 0.92Recuperator gas inlet temperature,

(C) (F)906 (1663)ited quantities [49], to a projected cost of $400/kW for a30 kW unit with an annual production of 100,000 units[50].

In a previous paper by the authors a preliminary costbreakdown of a 5 kW turbogenerator was reported [19].In updating this cost generated six years ago a cost targetof $250/kW for a metallic turbogenerator in the powerrange of 510 kW has been reported [51]. This estimate isbased on turbogenerators being mass produced in verylarge quantities like automobile turbochargers that are fab-ricated in automated factories in Europe.

The above cost data can be regarded only as a target fora small ceramic turbogenerator since an established database does not exist to permit a realistic cost estimate. Sim-ilarly, more in-depth studies are required to establish theoverall project cost, involving the analysis, design, fabrica-tion, test facilities, component development and testing ofa small recuperated ceramic microturbine demonstratorof the type proposed in this paper.

10. Ceramic microturbine demonstrator tentative schedule

In an eort to put into perspective the time-frame for aprogram leading to the demonstration of a 7.5 kW recuper-ated ceramic-based turbogenerator prototype, a tentativeand simplistic schedule was prepared and is shown onFig. 14. The development of the metallic componentswould draw heavily on technologies already deployed inexisting microturbines, particularly experience gained inthe fabrication of very small gas turbine components [52].The deployment of air bearings in such a small machineis recognized to be a formidable task, and a developmenteort would be initiated right from the start of the project.

The pace of the program would be determined by thefabrication and integrity of the ceramic components. Pro-totype ceramic fabrication would benet greatly by theadvent of green and bisque machining which would reducelead and iteration time over production methods used forautomotive turbocharger rotors. Ducting, sealing andattachments will also be key to successful deployment.While full advantage would be taken of established ceramictechnology bases, there would be a learning process andinevitable failures along the way. The design and fabrica-tion of the radial ow turbine would have as its genesisthe know-how from similar sized turbines used for almosttwo decades in automobile turbochargers in Japan. It beingrecognized from the onset, of course, that there is a largedierence in operating conditions between that of a turbo-charger and a gas turbine. The ceramic micro-channel recu-perator is new, and the extensive development and testingrequired could well be the pacing item, since assurance ofits structural integrity and performance is a key factortowards the success of the proposed demonstration project.

From the onset it has to be recognized that the 30%eciency goal is ambitious for such a small turbogenera-

ermal Engineering 28 (2008) 6074 71tor. If at the end of the envisioned three year program,the prototype has demonstrated mechanical integrity with

era

Than eciency close to 30% it will have been judged a success.Further nessing of the prototype into the fourth yearcould be foreseen to fully explore its eciency potential,and for it to meet demanding emission goals, together withrigorous thermal cycling to prove durability.

Fig. 14. Tentative schedule for c

72 C.F. McDonald, C. Rodgers / Applied11. Summary

In this paper a ceramic microturbine demonstrator con-cept has been presented, the development and operation ofwhich could provide the basis for the eventual developmentof microturbine eciency to 40% or higher. While fulladvantage would be taken of earlier gas turbine develop-ment activities based on the use of ceramic components,a new approach based on a much smaller turbogeneratoris suggested. The rationalization for this was based on fac-tors which included low risk, modest cost eort, plus agood chance of being successful to get a complete recuper-ated ceramic microturbine operational in less than aboutthree years.

A conventional turbogenerator conguration layout hasbeen discussed this being based on a single shaft approachusing radial ow turbomachinery, a can combustor, rear-installed recuperator, direct drive air cooled generator withthe high speed rotor supported by air bearings. For the rsttime it represents a microturbine involving the coupling ofa ceramic combustor, ceramic radial ow turbine, and aceramic xed boundary recuperator. The selection of the7.5 kW machine rating was strongly inuenced by two ofthe major ceramic components, namely the radial turbineand the counter-ow modular recuperator. The size ofthe ceramic turbine is similar to those used in automobileturbochargers that have been in service in Japan since themid 1980s. The ceramic micro-channel recuperator is newand initial development has been encouraging for gas tur-bine service. Such a high eectiveness heat exchanger con-tributes signicantly towards the goal of demonstrating an

mic microturbine demonstrator.

ermal Engineering 28 (2008) 6074eciency of 30%.Two of the major objectives of the proposed R&D eort

are to demonstrate the performance potential and struc-tural integrity of a small microturbine embodying ceramiccomponents. Additional data will be acquired in the fol-lowing areas; verication of analytical and design method-ologies, application of new ceramic manufacturingprocesses, the ability of small ceramic components to with-stand thermal transients, observe the impact of foreignmaterial ingestion, materials-related eects when runningon dierent gaseous and liquid fuels, characterization ofemissions, and means of acoustic attenuation.

The ultimate goal of the proposed microturbine demon-strator unit will be to establish a technology base thatwould provide a benchmark for the eventual realizationof microturbines rated up to about 100 kW with ecienciesof over 40% based on the use of ceramic components.

In closing, it is germane to project beyond the successfuloperation of the demonstrator to potential applications ofsuch a small recuperated microturbine including: (1) anatural gas-red CHP system to meet the total energyneeds of an average house; (2) small energy systems usingindigenous fuels in the developing nations; (3) it could becoupled with a SOFC to give a very high eciency andlow emissions hybrid power source [53] and (4) perhapsmeet propulsion needs for future extended enduranceUAVs.

d ThAcknowledgements

In preparing this paper the authors would like to expresstheir thanks to the following for helpful discussions,providing material, and for their constructive reviewcomments: Merrill Wilson (Ceramatec Inc.), Jay Scovie(Kyocera Intl. Inc.), Takero Fukudome (Kyocera, Japan),Bryan Seegers (M-Dot), Dr. Hiro Yoshida (AIST), PeterKuijpers (Innotech Europe BV), and consultants Dr. JohnMason, David Carruthers and David Richerson. Thispaper has been enhanced by the inclusion of hardwarephotographs and the authors are appreciative to all con-cerned, with credits being duly noted.

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74 C.F. McDonald, C. Rodgers / Applied Thermal Engineering 28 (2008) 6074

Small recuperated ceramic microturbine demonstrator conceptIntroductionSmall power generation systems overviewSmall gas turbine developmentsState-of-the-art metallic enginesHigher efficiency metallic enginesCeramic gas turbine developments

Small recuperated ceramic microturbine demonstrator conceptGenesisThermodynamic cycle

Major component design considerationsCompressorTurbineGeneratorBearingsCombustor

Ceramic recuperatorBackgroundCeramic micro-channel recuperator

Proposed engine design layout conceptProjected performance of ceramic microturbine demonstratorCost considerationsCeramic microturbine demonstrator tentative scheduleSummaryAcknowledgementsReferences