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NASA Technical Memorandum 100241

SPRE I Free-Piston Stirling Engine Testingat NASA Lewis Research Center[NASIl-Tfl- m0241) SPRE 1 F R E E - P I S T O N m a - 11747S T I R L I N G ENGINE T E S T I N G AT N A S A LEWISRESEARCH C E l T E B ( N A S A ) 15 p Avail: NTISHC A03/HP 801 CSCL 10B U n c l a sG3/20 0 107322

James E. Cairellih i s Research CenterCleveland, Ohio

I

Prepared for the1988 Solar Energy Conferencesponsored by the American Society of Mechanical EngineersGolden, Colorado, April 10-14, 1988


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SPRE I FREE- PI STON STI RLI NG ENGI NE TESTI NG AT NASA LEW S RESEARCH CENTERJ ames E. Cai rel l iNat i onal Aer onaut i cs and Space Adm ni st r at i onLew s Research Cent erCl evel and, Ohi o 44135

Abstract - As part of the NASA funded portionof the SP-100 Advanced Technology Program theSpace Power Research Engine (SPRE I) wasdesigned and built to serve as a research toolfor evaluation and development of advancedStirling engine concepts. The SPRE I isdesigned to produce 12.5 kW electrical powerwhen operated with helium at 15 MPa and withan absolute temperature ratio of two.facility which was designed and built at NASALeRC specifically to test the SPRE I. Thispaper describes the SPRE I, the NASA testfacility, the initial SPRE I test results, andfuture SPRE I test plans.Introduction - As part of the NASA fundedportion of the SP-100 Advanced TechnologyProgram the Space Power Research Engine (SPREI) was designed and built, on contract byMechanical Technology Inc. (MTI), to serve asa research tool for the evaluation and devel-opment of advanced Stirling engine concepts.These include active dynamic balancing,hydrodynamic bearings, heat pipe heat exchang-ers and power control. The Stirling enginetechnologies developed in the SPRE I ultimate-ly will be used in nuclear and solar spacepower systems. The technologies developed inthis project also apply directly to solarterrestrial power systems.

The engine is now under test in a new test

A new test facility has been designed andbuilt at NASA Lewis Research Center specifi-cally or testing the SPRE I. The SPRE I hasbeen installed in the NASA test facility andis now under test. The following describesthe SPRE I engine. the NASA test facility,initial NASA test results, and future SPRE Itest plans.SPRE I Enqine - The SPRE I is a free-pistonStirling engine coupled to a linear alternatorto produce electrical power. Two SPRE

engines were derived directly from the SpacePower Demonstrator Engine (SPDE ) which wasdesigned, built, and tested by MTI as a twocylinder opposed engine having a commonexpansion space (hot end). The SPDE was cuin two and the hot end of the cylinders weremodified to close off the cylinders; thusforming two separate engines. The SPRE I wasdelivered to NASA LeRC. The SPRE I1 remainsat UTI. Additional description of the SPDEand SP-100 Stirling Program may be found inreferences 1, 2, 3, and 4.Figure 1 is a photo of the SPRE I as i tarrived at NASA. Figure 2 is a later photo

showing the SPRE I installed in the NASA testcell.The SPRE is designed to produce 12-1/2 kWof electrical power with 50 kW thermal powerinput to the heater while operating at 100 Hzwith helium at 15 MPa and an absolute heatexchanger wall temperature ratio, T(heater)/T(cooler), of two. Heat is supplied to theengine by circulating molten salt through thengine heater and waste heat is rejected bycirculating water through the engine cooler.For these tests the electrical power produceby the engine is absorbed by a water cooledresistance load.The SPRE engine is shown in cross sectioin figure 3. The engine has two moving parta power piston and a displacer plston con-tained in a single cylinder. The displacerseparates the hot end of the cylinder from tcold end. The hot end of the cylinder isconnected to the cold end through annular tyheat exchangerscooler).In order toboth the pistonhydrostatic gas

(heater, regenerator, andhave potential for long lifeand displacer are guided bybearings. This eliminates a

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chance of wear due to contact between themoving parts and stationary parts.displacer gas spring is divided into a forwardgas spring and an aft gas spring. The weightsof the pistons and the spring rates aredesigned such that the resulting spring-masssys tems operate with about an 05 degree phaseangle between the piston motions.

Each piston has its own gas spring. The

The motion of the displacer causes thehelium working gas contained in the cylinderto flow through the heat exchangers alter-nately between the hot end of the cylinder(expansion space) and the cold end of thecylinder (compression space). The working gastemperature alternates between hot and cold,as it flows back and forth through the regen-erator, causing a corresponding change inpressure. The power piston motion changes thetotal volume occupied by the working gas.The piston and displacer motions aresynchronized so that the working ga s iscompressed by the power piston while the gasis in the cold end of the cylinder and expand-ed while the gas is in the hot end of the

cylinder. The heat energy extracted from thegas during expansion is replenished by theheater and the energy transmitted to the gasduring compression is extracted from the gasby the cooler.The net power produced by the power pistonis converted to electrical power by a linearalternator which consists of an alternatorplunger, with permanent magnets, attached t othe power piston (the armature); and station-ary electrical windings and core materialattached to the engine case (the stator).Because the single cylinder engine is notinherently balanced, a large mass (about 2000kg.) is attached t o the hot end of the engineto reduce the maximum case motion to less than0.1 mm. The engine and mass are separated bya thin walled stainless steel spool piece t ominimize axial thermal conduction losses.The engine instrumentation used or thesetests are shown in figure 4 . The instrumenta-tion is used to measure steady state surfaceand gas temperatures; dynamic pressures withinthe cycle and in each of the gas springs; andpiston and diSplaCer dynamic positions. Thesemeasurements are used to analyze the engin esthermodynamic cycle and to evaluate thermallosses.-alt Heatina Svstems - The SPRE salt heatingsystem is shown schematically in figure 5.About 2500 kg. of a molten salt mixture ofsodium nitrate, sodium nitrite, and potassiumnitrate, are contained in an insulated storagetank heated by electric heaters. The salt ispumped through the engine heater and thenreturns to the tank where it is reheated. Anautomatic control maintains the salt tanktemperature, over a range of 150C to 450C, byregulating the power to the electric heaters.The temperature range can be extended t o 540C

by changing the salt mixture. The salt flowis maintained at about 1.7 L/sec. by a fixedorifice in the salt return line.The measurements made in the salt heatingsystem are used to determine the thermalenergy input to the engine heater; and tomonitor and control the salt temperature.

thermal shock, steam is used to heat theengine to match the salt temperature beforethe circulating pump is started,Coolinq Water System - The SPRE cooling watersystem Is shown in figure 6. The engine iscooled by circulating water through the enginecooler. The pressure vessel surrounding thealternator, and several electrical loadcomponents are also water cooled. Automaticcontrols are used to maintain constant totalwater flow and inlet temperature to theengine. The flow control range is from 0.5 to1.5 L/sec. The temperature control isadjustable from about 20 t o BOC.

To avoid exposing the engine to severe

The cooling water system is instrumentedto measure the heat rejected by the enginecooler, alternator cooler, and the electricalload; for flow and temperature control; and tomonitor the system.Helium Svstem - Figure 7 is a simplifiedschematic o t the helium system. The systemconsists of a helium bottle su ppl y, meanpressure control with an operating range of 5to 15 MPa to charge the engine with workinggas, a control to center the displacer strokefor starting, and circulating pumps for thedisplacer and power piston hydrostatic bear-ings. Displacer centering is accomplished byadding gas to or venting gas from the aftdisplacer spring. An emergency stop valve isalso included to stop the displacer motion byconnecting the displacer forward and aft gasspring volumes together.

Instrumentation is provided to monitor thengine mean pressure and gas bearing circula-tion circuits.Electrical Load System - In order to absorbthe electrical power produced by thealternator and to control the power pistonstroke, an electrical load system is includedin the test setup. Figure 0 is a simplifiedschematic drawing of the electrical loadsystem. The load system contains an a.c.circuit comprised of the alternator in serieswith manually selected tuning capacitors andan untiltered d.c. circuit made up of afull-wave rectifier, 16 selectable fixedresistors, and two transistor regulatedvariable resistors.

Since the alternator voltage and result.and.c. voltage is directly proportional to thepiston stroke, the power piston stroke controis accomplished by using an automatic voltagecontrol which selects the proper number of

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fixed resistors and amount of variable resis-tance to maintain the desired d.c. voltage.The normal piston amplitude (half stroke)control range is from 5 m m to 10 nun.A v a r i a b l e t r a n s f o r m e r c o n n e c t e d t o t h e 60Hz a.c . commerc ial line power and tied to theload system throu gh a pair of relay contactsis used to start the engine. Thi s arrangementprovides a precise and convenien t way tocontrol piston stroke while the load system i sbeing brought "on line". During startin g thecontact s are closed and the voltage adjustedto drive the alternator/piston. After theengine has started, the load control isa d j u s t e d . s l o w l y , t o a b s o r b a l l of the alter-nator electric al output. At this point, thec o n t a c t s a r e o p e n e d t o i s o l a t e t h e s t a r t i n gpower source f r o m t h e l o a d s y s t e m . T h i sprocedu re provides an absolut ely "bumpless"transfer from the starting system to the loadcontrol system.The load system instrumentation includealernator voltage and current, load voltage,and tuning capacitor voltage. These dynamicmeasurements a re used to analyze the alterna-

tor and load characteristics. In addition,alternator, load, and starting power aremeasured.Safety Systems - In order to protect theengine from damage due to hardware malfunctionor operator error, several engine and facilityoperating parameters are continuously moni-tored. These include piston and displacerpositions, bearing differential pressures,cooling water flow, and engine case accelera-tion. If any of these parameters ar e out ofpreset limits, engine shutdown is initiatedautomatically by opening the displacer springshort circuit valve and connecting all resis-tors into the alternator load (a pseudoelectrical short circuit). This shutdownprocedure has been demonstrated to be smooth ,well controlled, and with no risk to theengine; even at maximum power condition.Data Acquisition - Steady state measurements(up to 256 data channels) are monitored and,upon request, ar e recorded using a centraldata system known as ESCORT. The dynamicmeasurements are recorded and processed by adedicated personal computer (PC ) using fastFourier analysis. Processed data results aredisplayed on the PC's CRT display and aretransmitted by computer link into ESCORT wherethey are merged with the steady state data.To aid in running the test, a limited numberof "on line" calculations are made by ESCORT.The results of these calculations and all datainput to ESCORT, including PC data, can beviewed on two ESCORT CRT displays. Uponcommand, hard copies of the ESCORT displayscan be made on a local printer. Data recordedby ESCORT is transmitted to a data base filein the central computer for further batchprocessing, after the test.Test Results - At this time only about 35hours of SPRE I testing has been done, atNASA,; primarily to check out the facilitysystems and to verify the accuracy of the

data. This process is still under way. Theengine has been operated at temperature ratioof 1.6, 1.8, and 2.0, mean pressures of 5.0,7.5, 10.0, 12.5, and 15 MPa; and pistonamplit udes of 5.0, 6.0, 7.0, 8.0, and 9.0 mm.The following is a sampling of the data takento date at a temperature ratio of two.Figure 9 shows the measured piston P-V poweversus the piston stroke at the variouspressures. The maximum P-V power measured wa11.60 kW. The P-V power tends to vary linearly with both the piston amplitude and the meapressure.

figure 10. The maximum alternator powermeasured was 7.47 kW. The trends ofalternator power are similar to those of theP-V power. The alternator power varieslinearly with piston amplitude. However, therelation to mean pressure appears to be morecomplex.

The measured alternator power is shown in

Figure 11 show s the relation of displaceramplitude to piston amplitude at variouspressures. The displacer amplitude tends tovary linearly with piston amplitude andappears to increase with increasing pressureby a constant which is a function of the meanpressure.

The phase angle of the displacer motionrelative to to piston motion is shown infigure 12. The displacer phase angle variesfrom about 84 degrees to 09 degrees. Theangle appears to decrease slightly (about twodegrees) with increasing piston amplitude fora constant pressure. For each differentpressure, the phase angle appears to shift bya constant angle which is a function of thepressure.Figure 13 shows the relation of enginefrequency to mean pressure. The predictedfrequency is also shown for reference. Themeasured frequency appears to be about 2 to 3Hz higher than the predicted value.

SPRE Test Plans - SPRE baseline performancewill be measured following completion offacility check out. Baseline performancetests will be repeated with a dynamic balanceinstalled to replace the 2000 kg mass.Testing will then turn to component develop-ment primarily to evaluate the use of hydrodynamic displacer bearings and alternate regen-erator materials. Throughout the SPRE testinthere will be an effort made to gather appro-priate engine data for comparison to the NASAcomputer code predictions and to improve thecode to better predict engine performance. Wplan to present the results of these activi-ties in future NASA reports.References -1. Dochat. G. R., "Free-Piston Stirling .Engines for Space Power", Proceedings of

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the Twenty-Second Automotive TechnoiogyDev elom ent Contractor's CoordinationMeeting, Society of Automotive Engineers,Warrendale PA, 1984, pp. 209-213.3


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2. Slaby, J. G., "Overview of the 1985 NASA 4. Slaby, J. G., "Overview of Free-PistonLewis Research Center SP-100 Free-Piston Stirling SP-100 Activities at the NASAStirling Engine Activities", NASA Lewis Research Center", NASA TM-87224,TM-87028, 1985. 1986.3. Slaby, J. G. , "Overview of Free-PistonStirling SP-100 Activities at the NASALewis Research Center", NASA TM-87156,1985.

F IGURE 1 , - SPRE I AS RECEIVED AT NASA .

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ORIGINAL PAGE ISDE POOR QUALITY

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F I G U R E 2. - SPRE I N S T A LL E D I N TH E N AS A F A C I L I T Y .

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FM FLOWMETERP PRESSURE TRANSDUCERPR PRESSURE REGULATORT/C THERMOCOUPLEA P PRESSURE TRANSDUCER DIF FERENTIA LA T DIFFERENTIA L TEMPERATURE

TEMPERATURESET POI NTY

@ SALT RESERVOIRFIGURE 5 . - SALT HEATING SYSTEM.


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FM FLOWMETERP PRESSURE TRANSDUCERPR PRESSURE REGULATORT/C THERMOCOUPLEA P PRESSURE TRANSDUCER DIFFERENTIA LA T DIFFERENTIAL TEMPERATURE

GAS #VENT

TOWERWATERHEAT EXCHANGER STRAINER

FIGURE 6 . - COOLING WATER SYSTEM.

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ALTER-NATOR

208=:ACWATER-COOLLOAD (20OHMS EACH)i

LOAD VOLTAGE CONTROL

..STARTINGPOWER

FIGURE 8. - ELECTRICAL LOAD SYSTEM.

MEANPRESSURE

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4 6 8 10 12PISTON AMPLITUDE, MM

FIGURE 9. - PISTON P-V POWER VERSUS PISTON AMPLITUDE.

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PISTON AMPLITUDE, MMFIGURE 10. - ALTERNATOR POWER VERSUS PISTON AMPLITUDE.

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MEANPRESSURE

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Report Documentation PageI . Report No. 2. Government Accession No.

NASA TM-1002410. Title and Subtitle

3. Recipient's Catalog No.

5. Report Date

SPRE I F re e -P is to n S t i r l i n g En gin e T e s t i n g a tNASA Lewis Research Center7. Author@)

James E . C a l r e l l i

6. Performing Organization Code

8. performing Organization Report No.E-3865

2. Sponsoring Agency Name and Address

Na t ion a l Aeronau ti cs and Space Ad mi n is t ra t io nWashington, D.C. 20546-0001

10. Work Unit No.506-41-319. Performing Organization Name and Address 11. Contract or Grant No.Na t ion a l Aeronau t ics and Space Ad m in is t ra t io n

Technical Memorandum14. Sponsoring Agency Code

Lewis Research CenterC1eve1 and, Oh1o 441 35-31 91

17. Key Words (Suggested by Authoqs))Free-p i s ton S t i r l ngT e s t da taT e s t f a c i 1i ySpace power

13. Type of Report and Period Covered

18. Distribution StatementU n c l a s s i f i e d - U n l i m i t e dSub jec t Ca tegory 20

19. Security Classif. (of this report) 20. Security Classif. (of this page)Unc 1ass i i d U n c l a s s i f i e d 1 421. No of pages

Prepared fo r the 1988 Solar Energy Conference sponsored by the Amer ican Societyo f Mechanica l Engineers, Golden, Colorado, A p r i l 10-14, 1988.

22. Price'A02

6 ' r $ a r t of t h e NASA f u n d e d p o r t i o n of t h e SP-100 Advanced Technology Program the

The SPRE I i s d es ig ne d to produce 12.5 kW e lec t r i ca l power when opera tedThe engineT h i s pape r desc r i bes t he SPRE I, th e NASA

Space Power Research Engine (SPRE I > was des igned and b u i l t to serve as ar esea r ch tool for eva lua t ion and deve lopment o f advanced S t i r l i n g eng ine con-cep ts .w i t h h e l iu m a t 1 5 MPa and w i t h an abso lu te tempera tu re r a t i o o f two.i s now under t e s t i n a new t e s t f a c i l i t y wh ic h was d es ig ne d and b u i l t a t NASAL e w i s s p e c i f i c a l l y to t e s t the SPRE I .t e s t f a c i l i t y , the i n i t i a l SPRE I t e s t r e s u l t s , and f u t u r e SPRE I t e s t p l ans .

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