DOE/NASA/33408-3

NASA TM-102085

\

I j

Comparison of Conceptual Designs

for 25 kWe Advanced Stirling

Conversion Systems for Dish

Electric Applications

[NASA-T_-I02085) COIIPARISON OF CONCEP2UAL

DESIGNS FOR 25 kWe ADVANCED STIRLiNG

CONVERSION SYSTEMS FOR DISH ELECfI£1C

APPLICATION Final Report (NASA.. Lewis

Research Center) 16 p CSCL 10B G3/85

N89-2678

Unclas

0212643

Richard K. Shaltens and Jeffrey G. Schreiber

National Aeronautics and Space Administration

Lewis Research Center

t

Work performed for

U.S. DEPARTMENT OF ENERGY

Conservation and Renewable Energy

Office of Solar Heat Technologies

Prepared for the

24th Intersociety Energy Conversion Engineering Conference

cosponsored by IEEE, AIAA, ANS, ASME, SAE, ACS, and AIChE

Washington, D.C., August 6-11, 1989
DISCLAIMER

This report was prepared as an account of work sponsored by an agency

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endorsement, recommendation, or favoring by the United States

Government or any agency thereof. The views and opinions of authors

expressed herein do not necessarily state or reflect those of the United

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DOE/NASA/33408-3

NASA TM-102085

Comparison of Conceptual Designs

for 25 kWe Advanced Stirling

Conversion Systems for Dish

Electric Applications

Richard K. Shaltens and Jeffrey G. Schreiber

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

Work performed for

U.S. DEPARTMENT OF ENERGY

Conservation and Renewable Energy

Office of Solar Heat Technologies

Washington, D.C. 20545

Under Interagency Agreement DE-AT04-85AL33408

Prepared for the

24th Intersociety Energy Conversion Engineering Conference

cosponsored by IEEE, AIAA, ANS, ASME, SAE, ACS, and AIChE

Washington, D.C., August 6-11, 1989
Trade names or manufacturers' names are used in this report for identification

only. This usage does not constitute an official endorsement, either expressed or

implied, by the National Aeronautics and Space Administration.
COMPARISON OF CONCEPTUAL DESIGNS FOR 25 kHe ADVANCED STIRLING CONVERSION

SYSTEMS FOR DISH ELECTRIC APPLICATIONS

Richard K. Shaltens and Jeffrey G. Schreiber

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

ABSTRACT

The Advanced Stifling Conversion System (ASCS)

Project is managed by NASA Lewis Research Center

through a cooperative Interagency Agreement with

DOE. Conceptual designs for the ASCS's were completed under parallel contracts in 1987 by Mechanical Technology Inc. (MTI) of Latham, NY, and

Stifling Technology Company (STC) of Richland, HA.

Each design features a free-piston Stifling engine,

a liquid metal heat pipe receiver, and a means to

provide about 25 kH of electric power to a utility

grid while meeting DOE's long term performance and

cost goals. An independent assessment showed that

both designs are manufacturable and have the potential to easily meet DOE's long term cost goals.

INTRODUCTION

The Department of Energy's (DOE) Solar Thermal Electric Technology Program, Sandia National

Laboratories Albuquerque (_NLA) is evaluating heat

engines for terrestrial Solar Distributed Heat

Receivers. The Stifling engine has been identified

by SNLA as one of the most promising engines for

terrestrial applications. Recent studies by SERI

and EPRI have concluded that dish systems utilizing

reflux receivers integrated with Stifling engines

are a promising candidate as a future power source

for _he utilities industry. The free-piston

Stifling engine has the potential to meet DOE's

long term goals for performance and cost.

The Stifling Technology Branch at NASA Lewis

Research Center is responsible for a variety of

projects, including both the free-piston and kinematic Stifling engines. The Stifling cycle has

been pursued as a candidate dynamic power source

for future high-power space conversion systems at

NASA Lewis since the 1960's. Current projects for

space power requirements are shown in Fig. I.

Space power requirements include high efficiency,

very long life, high reliability, and low vibration. In addition, system weight and operating

temperatures are important. The free-piston Stifling shown schematically in Fig. 2, has the potential for a highly reliable engine with long life

because it has only a few moving parts, noncontacting gas bearings, and can be hermetically sealed.

A discussion of free-piston Stifling technology

for the space power application is contained in

Ref. I. Although both applications, terrestrial

and space power, appear to be quite different,

their requirements complement each other.

The development of alternative powerplants for

the Department of Energy (DOE) in the late 1970's

initiated the DOE's Automotive Heat Engine Program, which included the development of both the

gas turbine and Stirling engine for automobiles.

These programs were planned and implemented by

NASA Lewis. As a result of these programs, a

unique technology base and Stifling expertise was

gradually developed at NASA Lewis during the

1970's. In 1981, the applicability of both the

gas turbine and Stirling automotive heat engines

for solar thermal power applications, was evaluated. 2 Although, both the gas turbine and Stirling

have the potential to meet the high efficiency

goals for the solar program, it was determined that

a number of modifications were needed to adapt

these automotive engines for the solar application.

The duty cycles for these applications are quite

different, the automotive application requiring

frequent speed and power changes, while the solar

application has a constant speed and power requirement. Areas primarily of concern with these

engines were the life and reliability requirements,

along with issues of initial and life cycle costs

for the solar application. In summary, this report

concluded, that it was more desirable to start with

a clean sheet, and design an engine specifically

for the solar thermal application, rather than

adapt one of the automotive heat engines.

A comparison of the kinematic and free-piston

Stifling engines for the solar application is

shown in Table I. The kinematic engine was originally designed for the automotive duty cycle, while

the free-piston Stirling engine is being developed

for the long life space power application. Major

differences shown are in the drive and gas systems.

The drive system components (drive shaft, bearings,

oil pump, piston rings, crossheads and shaft seals,

etc.) and the gas system (the power control system)

components (compressor, check valves, storage tank

and power control valve, etc.) have wear potential

which would reduce engine life in the kinematic

configuration. Design life of between I0 000 and

20 OOO hr should be easily obtained for reciprocating engines such as the kinematic Stifling, when

modified for operation with a constant speed and

load. The free-piston Stirling engine, however,

has no significant side loads, which minimizes wear

mechanisms and allows for long life. The use of

noncontacting gas bearings during operation should

permit the free-piston to exceed the 60 000 hr life

requirement for the solar application without difficulty. In addition, the free-piston Stirling can
1o00 MW

100 MW

I0 MW

I MW

100 KW

10 KW

I KW

I00 W

10 W

m

MARS -, /

LUNAR CAMP-_, //.x\\\

\J _

-- SPACE STATION_ ¢_\\

\\\ I \\ 2002'

\l, \\ , J

CURRENT I\\ _.-_,_

SPACE- .-_J _X"'"

CRAFT-_ _" d_ )v 20011

\ SKYLAB _

_, _ I 199_ .....

I'

, I _ I , I

197o 199o 2olo

CALENDAR YEAR

FIGURE I. - FUTURE SPACE POWER REQUIREMENTS.

PISTON

GAS

SPRING -,

- PERMANENT MAGNET - REGENERATOR

/-LINEAR ALTERNATOR STATOR ,--DISPLACER

_r / GAS SPRING

.-PISTON I .... _ ,'_,--.

COMPRESSIONSPACE-L//' // LCOOLER ' i LEXPANSION

/ SPACE

STATIONARYROD_'

HEATER] LDISPLACER

FIGURE2. - FREE-PISTONSTIRLINGENGINEWITH LINEARALTERNATOR.

TABLE I. - COMPARISON OF KINEMATIC AND FREE-PISTON STiRLiNG ENGINES

COMPONENTS FOR SOLAR APPLICATIONS

Heat source

Receiver

Drive system

Lubrication system

Working fluid (pressure)

Working gas system

Cooling system

Auxiliaries

Control system

aFor stand-alone applications.

Kinematic Free-piston

(automotive)

Solar

Liquid metal heat

transport system

Shaft(s)

Bearings

Piston rings

Crossheads

Shaft seals

Oil pump

He (15 MPa)

Compressor

Check valves

Storage tank(s)

Power contro] system

Water pump

Radiator

Blower fan

Starter motor a

Microprocessor

Solar

Liquid metal heat

transport system

Gas bearings

Gas springs

Gas bearings

He (I0 MPa)

Hermetically sealed

Hater pump

Radiator

Blower fan

Starter motor a

Microprocessor
behermeticallysealedthereforeeliminatingthe

needfor a gasmakeupsystem.Analysiscanshow

that thesamehighbrakeefficiencyasa percentageof Carnotefficiency is expectedfromeither

thefree-pistonor kinematicStir]ing configurations.3,4 Theexpectedhighefficiency, alongwith

the inherentlysimp]erdesign,makethefreepistonStir]ing theengineof choicefor the long termso]atapplication. It shou]dbenotedthat

whencomparisonsof dataandinformationfromsimilar typeenginesaremade,whichweredesignedfor

different dutycycles(i.e., automotiveand

so]at), theycanbemisleadingandmayleadto

wrongconc]usions.

ADVANCED STIRLINGCONVERSIONYSTEM

DOEsigneda cooperativeInteragencyAgreement

(IAA)in 1985withNASALewisto providetechnical

management for theAdvancedStifling Conversion

System(ASCS)Project. Underthis IAA,theASCS

ProjecthasprovidedDOE/SNLA with twoconceptual

designsof free-pistonStifling enginesystems, whichwerecompletedin 1987.Bothconceptshave

thepotentialof meetingDOE'systemandengine

performanceprojections(Figs. 3 and4) whi]e

appearingto meetor exceedtheDOElongtermcost

goals. TheDOEcostgoalsareshownin Table11.5

Thereceiverandconversioncostshavebeen

combinedto providea total systemcostfor the

ASCS,excludingtheconcentrator.TheASCStotal

costgoalis basedonco]lectingconcentrated

solar energyinto a receiverfroman11mparabolic

dishwhileproviding25kNor moreof electrical

powerto theutility grid. ,Both the MTI and STC

ASCS concepts were evaluated independently by

Pioneer as stand a]one systems for the cost study,

which we be]ieve to be a more meaningfu] comparison. The manufacturing rate for the cost study is

I0000 units/year. Based on the Pioneer study,

both systems (in 1984 dollars), the MTI ASCS at

$363/kNe, and the STC ASCS at $308/kNe can easily

meet %he DOE long term goal of $452/kNe.

Each ASCS consists of a solar energy receiver,

a liquid metal heat transport system, a free-piston

Stirling engine, the engine heat rejection system,

an alternator or generator either directly or

indirectly coupled the engine to the utility grid,

and the appropriate controls and power conditioning. The proposed concepts and the preliminary

conceptual designs are discussed in Refs. 6 and

7. The final conceptual design completed by MTI 8

features a heat pipe receiver integrated with a

free-piston Stifling engine/linear alternator conversion system. The STC 9 final concept.ual design

features a reflux boiler receiver integrated with

a Stirllng engine/hydraulic conversion system.

Objectives of the conceptual design tasks for

the ASCS Project are as follows:

(I) Define the ASCS configuration

<2) Predict ASCS performance over a range of

solar inputs

(3) Estimate system and major component weight

(4) Define engine and electrical power conditioning and control requirements

.40 --

.35 --

z .30 --

la.a

u_ .25 --

.20-

.15 --

_ .10 --

.05 --

0

300

%7

[] STIRLING

V ORC

0 BRAYTON

I I I I I I I I I I

400 500 600 700 800 900 I000 1100 1200 1300

TEMPERATURE,°C

FIGURE 3. - SOLAR SYSTEM PERFORMANCE PROJECTIONS,

.G --

.S --

#I --

.3 --

.2 --

.I --

O,

3OO

[] STIRLING

V ORC

0 BRAYTON

I I _L__L I I I I I I

400 500 600 700 800 900 I000 II00 1200 1300

TEMPERATURE,°C

FIGURE 4. - PEAK ENGINE PERFORMANCEPROJECTIONS.

TABLE 11 - DOE COST a GOALS

Receiver, dollars/m 2

Conversion, dollarslkNe

ASCS total, b dollars/kNe

Energy cost, do]lars/kNe

Current

technology

70

380

646

0.13

I Long-term

goals

4O

3OO

452

I 0.O5

a1984 Dollars.

bReceiver (11-m concentrator) and conversion

(nominal 25 kNe) combined.
(5) Define keYitechnology needs not ready by

the late 1980's in meeting goals

_6) Provide a manufacturability and cost evaluation for the engine power conversion

system

The approach for designing the 25 kN solar

conversion system for DOE/SNLA was to design with

a clean sheet as recommended in earlier studies.

The method used was to start from a system point

of view which featured the free-piston Stirling

engine along with the appropriate subsystems. This

allowed for the proper optimization of the engine

within the system to maximize both efficiency and

power for the solar application. Proper evaluation

of the va_ious subsystem losses at an early stage

of the design process, eliminate the need for compromise later on. The system is divided as shown

below.

Receiver

Heat transport system

Conversion system:

Stirling engine

Power output device

Auxiliaries

Power conditioning and controls

The MTI and STC conceptual designs are considered unique yet complementary to earn ether. Each

ASCS design features the Stirling engine, a _iquid

metal heat transport system as oar_ _f the so]or

receiver, a means to generate electric power, a

cooling system, and the power conditioning and control system. Nherever possible, both conceptual

designs include the use of existing technology

which is expected to be available in the late

1980's. The MTI concept utilizes a neat pipe

receiver integrated with the free-piston Stirling

conversion system to directly convert the solar

energy to electricity. The STC concept uses a

reflux boiler receiver integrated with a Stirling

hydraulic conversion system geoerating eiectricity

indirectly using the hydraulic .output to a hydraulic pump/motor coupled to a rotating alternator.

The ASCS is being designed to mount on, and to

receive concentrated solar energy from an II m test

bed concentrator (TBC) located at the SNLA test

facilities in Albuquerque, NM. The ASCS Project

design requirements and the SNLA .TBC characteristics are given in Ref. 7.

During the conceptual design process the specification for the operating temperature far the

ASCS measured at the hot end (Stifling heater

TABLE Ill. - COMPARISON OF THE ASCS CONCEPTUAL OESIGNS

MTI STC

Heat supplied peak), kNt

Electric power (peak), kNe

Heat supplied (nomin#1), kNt

Electric power (nominal), kNe

Efficiency engine (Qin to PNRout), percent

Efficiency (solar to electric), percent

Annual energy, kwh

Annual efficiency, percent

Receiver

Receiver efficiency (vertical), percent

86.8

2_7.5

75

23.2

38.5

30.9

59 200

28.8

Heat

Liquid metal Sodium

pipe

91.8

(No)

86.8

Z9.6

75

25.2

37.3

33.6

65 200

31.8

Reflux boiler

90.4

Potassium (K)

Heater head temperature, °C (K)

Cooler temperature, °C (K)

Engine frequency, Hz

Pressure, MPa (psi)

Temperature ratio, Th/T c

Norking f]uid

Power conversion

Electric efficiency, percent

Electrical output

Power conditioning

Controls

Neight on TBC, kg (lb)

ASCS tota] cost, 1984 do]lars

ASCS, dollars/kNe, 1984 dollars

700 (973)

60 (333)

6O

I0.5 (1540)

2.9

He

Linear alt

94.0

240 V,

one phase

Autotransformer

Automatic

680 (1500)

8429

363

700 (973)

50 (323)

46

17.9 (2630)

3.0

He with Freon

buffer

Hydraulic pump

with Ind. Gen.

96.0

240 V,

three phase

None

Automatic

869 (1914)

7670

3O4
ORiG|NAL PAGE IS

OF POOR QUALITY

head) wasdecreased From 800 to 700 °C. This temperature reduction will increase material life by

increasing the creep rupture and fatigue ]ire and

should minimize, where practical, potential liquidmetal problems such as corrosion. The predicted

ASCS performance (solar to electric) at operating

temperatures of 700 °C is about 3] percent for the

MTI ASCS while the STC ASCS is about 33 percent.

The estimated annual energy for both concepts is

about 60 000 kNh The specification for the nominal insolation of 950 N/m 2 provides about 75 kW of

thermal energy to the receiver absorber surface.

However, both concepts have been designed to operate continuously when the peak insolation of

.,1100 N/m 2 is available. This results in about

87 kW of thermal energy at the receiver. Designing

for the peak insolation allows for an increase in

annual energy production. A comparison of the MTI

and STC concepts is provided in Table Ill. The

MTI and STC conceptual designs are discussed in

detail in Refs. 8 and 9, respectively.

MTI ASCS CONCEPTUAL DESIGN

MTI, the prime contractor, was responsible for

project management, the design of the Free-piston

linear alternator conversion concept and system

integration. MTI was teamed with Thermacore, Inc.,

Lancaster, PA, For the heat transport system and

with Sanders Associates, Nashua, NH, For the solar

receiver design. Pioneer Engineering and Manufacturing Co., Warren, MI, provided the independent

manufacturing and cost analysis for the MTI design.

At the design review held in June 1987 MTI recommended an ASCS configuration, shown in Fig. 5,

which includes a heat pipe receiver integrated

with a single piston Stifling conversion system

with a passive (with no active feedback) vibration

absorber for the baseline design. The ASCS is controlled with an autotransformer with a tuning

capacitor in the load circuit to the utility grid.

The estimated annual energy For the MTI ASCS is

shown in Fig. 6. MTI divided the ASCS into the

following subsystems:

(l) Receiver/heat transport system

(2) Engine/linear alternator

(3) Vibration absorber

(4) Engine mounts

(5) Cooling system

(6) Power control system

All the subsystems, except the power control

system, can be mounted directly on the focal point

of the l] m concentrator. Each of the major systems are discussed below.

Receiver/Heat Transport System

The receiver/heat transport system is a single

heat pipe on a hemispherical she]l shown in

Fig. 7. The solar energy is collected from an

II m parabolic dish which focuses the solar energy

through an 203 mm (8 in.) diameter aperture in the

receiver face. The receiver has been designed to

operate continuously with a nominal insolation of

950 N/m 2 providing about 75 kN of thermal energy

to the absorber surface. The surface of the evaporator is covered with a powdered metal wick with a

primary and a redundant circumferential/radial

artery system. Sodium was chosen as the _iquid

metal for the heat pipe because _he superior surface tension properties allow for easier wicklng.

The heat pipe cavity behind the absorber/evaporator

surface is designed so that the condensed !_quid

drains by gravity to the arteries at the base of

the evaporator shell. The use of the heat pipe fori

the heat transport system should minimize local hot

spots and provide more uniform heating to ensure

longer life for the Stifling heater head tubes.

Engine/Linear Alternator System

The single cylinder Stifling engine is integrated with a linear alternator to directly convert

the energy to electricity to the grid. Figure 8

shows a cross section of the engine/alternator conversion system. This configuration uses enly two

moving parts, a displacer and a power piston. The

heater head ]s designed to operate at a temperature

of 700 °C (973 K). The working fluid is Helium at

a mean pressure of 10,5 MPa (1520 psi). An annular configuration is proposed for the regenerator

and cooler. Hydrodynamic gas bearings have been

chosen for the conceptual design which allows the

use of close tolerance, noncontacting seals which

eliminate wear mechanisms during operation. A

spin motor is conceptually shown to drive the

power piston and displacer to create the hydrodynamic effect. The engine and the linear alternator

are contained in a common vessel which allows for

the conversion system to be hermetically sealed.

A permanent magnet concept was selected for the

linear alternator, using Neodymium-Boron-Iron magnets. The linear alternator is connected to a

series tuning capacitor and to the grid through an

autotransformer. A cooling system is incorporated

for the stator to assure the proper temperature of

the magnetic materials. The single phase linear

alternator has been designed to provide 23.2 kNe

(nominal) and 27.5 kNe (peak) to the utility grid.

Vibration Absorber

A passive (with no active feedback) vibration

absorber was chosen for the single piston Stir]ing

conversion system configuration. The mass and

spring stiffness was selected so that the resonant

frequency of the vibration absorber is the same as

the nominal operating frequency of the engine,

60±0.6 Hz.

Cooling System

The heat rejection system is a closed system

which is divided into two parallel loops: (l) for

the engine cooler, and (2) for the aiternator stator. MTI recommended automotive type cooling'system components which include a radiator, a blower

fan, and a coolant pump.

Power Control System

The linear alternator is connected to the

utility grid through an autotransformer and a

series tuning capacitor. The frequency and the
I

HEATER

TUBES--_

WICK_

ENGINE/

ALTERNATOR _

FLEX

MOUNTS_

\

\

\

FALTERNATOR COOLING

LINE

/-VIBRATION

/ ABSORBER

/-SUPPORT

/

/

/

/

I_NA VAPOR

rRECEIVER

SHELLS

NA LIQUID

r COOLING

LINE

FIGURE 5. - MTI'S SINGLE PISTON STIRLING ENGINE ASCS CONFIGURATION.

140 --

SOLAR 0 DESIGN POINT

120 -- INSOLATION 7 1100 W/M2 INSOLATION

/

/ RECEIVER 86.6 KW TO RECEIVER

I00 _--_ // HEAT 27.5 KW TO GRID

/ INPUT7

80(_ _// Q NOMINAL POINT

__ //_ 950 W/M2 INSOLUTION

GO l_ __ _ 75 KW TO RECEIVER I ENGINE__ _3.2 _wTOGR,D

I HEAT _

40I- iNPUt .... -

2°_1

0 1000 2000 3000 4000

HOURSPER YEAR

FIGURE G. - MTI'S ANNUAL ENERGYPROJECTION.
HEATER _

HEAD

INSULATION __

_ APERTURE _

(A) HEAT PIPE RECEIVER.

FIGURE 7. - HEAT TRANSPORT SYSTEM.

_ _ _ EVAPORATOR SURFACE

WITH WICK

_-LIQUID

- >,_ SODIUM

_>C_-I

--A RyTsE REYM

--AsRyTsEFREVM

RADIAL

I

// ARTERY

POWDERED

METAL WICK

"L-CIRCUMFERENTIAL

ARTERIES

(B) HEAT PIPE EVAPORATOR.

FIGURE 7. - CONCLUDED.

HEATER

TUBES_

!!iiiio. 

EXPANSION //

SPACE J_

GAS

SPRING_

\

\

_ _/_ DI SPLACER R

_ // _-REGENERATO

.,../

/":'_ _COOLER

/

_ COOLANt

// PORT

COMPRESSION \L_ .... _POST AND

SPACE_\\_\ I _ /_ FLANGE

/ _ _'_ % .'_\ _POWER

I _ I__ _PI_T_ETS

_ _ _/_COIL

_STATOR

PISTON _ _ _/_II(----_SPIN MOTOR

GAS SPRING----_____

FIGURE 8. - MTI'S FREE-PISTON/LINEAR ALTERNATOR DETAILS.
autotransformer output voltage are essentially constant and established by the grid. The voltage at

the alternator terminals is adjusted to match the

changes in power while keeping the heater head temperature at its design point. The series capacitors used to compensate for the internal inductance

of the linear alternator assure stable operation of

the Stirling conversion system.

Controls

Fully automatic, unattended operation is

planned for the ASCS.

STC ASCS CONCEPTUAL DESIGN

STC, the prime contractor, was responsible for

project management, the design of the free-piston

Stirling/hydraulic conversion concept and system

integration. STC was teamed with Thermacore, Inc.,

Lancaster, PA, for the heat transport system and

with Sanders Associates, Nashua, NH, for the solar

receiver design. Pioneer Engineering and Manufacturing Co., Warren, MI, provided the independent

manufacturing and cost analysis for the STC design.

Consultants used during the conceptual design process were: Gedeon Associates, Athens, OH, for the

thermodynamic simulation; Saaski Technologies,

Inc., Seattle, NA, as a the heat transport consultant; and Nestinghouse Hanford Co., Hartford, WA, for

the high temperature materials expertise. At the

design review held in July 1987, STC recommended

an ASCS configuration, shown in Fig. 9, which

includes a reflux boiler receiver integrated with

the Stirling/hydraulic conversion system. A commercial hydraulic pump/motor and an induction generator were identified to convert _he hydraulic

output to electricity for the grid. The estimated

annual energy for the STC ASCS is shown in Fig. I0.

The ASCS is divided into the following subsystems:

(I) Receiver/heat transport system

(2) Engine/hydraulic output

(3) Hydraulic pump/induction generator

(4) Cooling system

(5) Power control system

(6) Controls

The STC study included both a stand alone

ASCS and an array of up to 20 ASCS's. The Pioneer

evaluation concluded that the array configuration

was actually more expensive and would increase the

capital cost about 10 percent; therefore, only the

stand alone ASCS will be discussed. All of the

subsystems can be mounted directly on the focal

point of the II m concentrator. Each of the major

systems are discussed below.

Receiver/Heat Transport System

For the receiver/heat transport system a

reflux boiler (Fig. 11) was chosen over a number of

candidates. The energy is collected from an 11 m

solar concentrator which focuses the energy through

an 203 mm (8 in.) diameter aperture in the receiver

face. The receiver has been designed to operate

continuously with a nominal insolation of about

950 N/m 2 providing about 75 kN of thermal energy

to the absorber surface. Potassium was chosen as

the liquid metal for the reflux boiler because the

higher vapor pressure at 700 °C allows for easier

boiling. The potassium is evaporated, similar to

the operation of a double boilerl to provide vapor

which results in a uniform temperature at the

Stifling heater head tubes. The system is configured so that the heater head tubes are never submerged in the potassium poo ! , regardless of the

concentrator elevation angle. A disadvantage of

this boiler configuration is the large amount of

liquid potassium inventory. The use of the reflux

boiler for the heat transport system should provide

a simple and uniform method for heating the heater

head tubes and also minimize any local hot spots

ensuring long life.

Engine/Hydraulic Output

The free-piston Stifling/hydraulic engine

(STIRLIC) is hermetically sealed with the use of

metal bellows. The engine delivers high pressure

hydraulic fluid to a commercial hydraulic pump/

motor which is connected to a commercial induction

generator to convert the energy to the utility

grid. Figure ]2 shows a cross section of the

engine/hydraulic conversion system. The heater

head is designed to operate at a temperature of

700 °C (973 K). The working fluid is Helium at a

mean pressure of 17.9 MPa (2630 psi). An annular

configuration is proposed for the regenerator and

cooler. The displacer employs metal bellows seals

which allow the working fluid to be hermetically

sealed and is connected hydrau]ically to a stabilizer mechanism. The engine working gas is sealed

from the hydraulic fluid by use of the metal bellows, which are pressure balanced to provide very

long life. The power pistons are arranged in an

opposed piston configuration and also employ metal

bellows seals. The power pistons, the displacer

rod and the stabilizer/controller are immersed in

hydraulic fluid which provides full film hydrodynamic lubrication of all sliding parts. Conceptually the STIRLIC utilizes a completely automatic

internal lubrication and makeup system.

Hydraulic Pump/Induction Generator

The hydraulic motor selected is a commercial

variable displacement motor coupled to a commercial

rotary induction generator. Control is achieved

through the automatic variation of the hydraulic

motor displacement. The three phase induction generator has been sized to provide 25.2 kNe (nominal)

and 29.6 kNe (peak) to the utility grid.

Cooling System

The engine cooling system requires industrial

grade components which include a radiator, cooling

fan and cooling pump.

Power Contro] System

Automatic regulation of the inlet pressure

for the hydraulic pump is a factory option. No

control is required for the induction generator or

the power output circuit when operating on a grid.

Frequency and voltage are established by the grid.

Under a majority of loading conditions, the power
RECEIVER

AND REFLUX

BOILER -_

\

\

\

\

\

\

STIRLICTM

ENGINESTABILIZER/1

CONTROLLER J

/"

APERATUREI I

PLATE -/

VAPOR

POTASSIUM \

\

\--HEATER TUBES

T

APERTURE --

_ ABSORBER

FIGURE 9. - STC'S STIRLING/HYDRAULIC ASCS CONFIGURATION.

,C

lqO

12o

lOO

80 (

6O

4o

2o

SOLAR 0 DESIGN POINT

INSOLATION7 1100 W/M2 INSOLATION

I RECEIVER 86.8 KW TO RECEIVER

I HEAT 29.6 KW TO GRID

INPUT7 [] NOMINAL 950 W/M2POINT INSOLATION

75 KW TO RECEIVER

_ 2K.TOGR,D

ENGINE /_"_.,_

HEAT INPUT-" _

-- .-NET_ECTR_C_\\

1000 2000 3000 4000

HOURS PER YEAR

FIGURE I0. - STC'S ANNUAL ENERGY PROJECTION.
HEATER

HEAD

TIIUt S -

APERTURE

!

APERTURE

I

PLATE_ "

LEQUID

POT ASS [ UM

_VAPOR

._ STAMPED ALUMINUM

CLAMSIIELL COVER

/

/

/

BEANKEI .-"

[NSIILAIION --/

FIGURE 11. - REFLEX BOILER HEAT TRANSPORT SYSTEM.

HEATER

TUBES--..

BUFFER

HOUSING_

\

\\

DISPLACER_

COOLANT

PORT--..

POWER

BELLOWS_

DRIVE / /

PISTON_ _

SPEED

CONTROL

VALUE _

STABILIZER

MECHANISM ._

_-IIEATER HEAD

_ HOUSING

/

_- REGENERATOR

J

..PCOOLER

/-- POWER PISTON

I

/

/ _BUFFER

/

\

_-PUMPING

PISTON

_BOUNCE

CHAMBER

\

POWER\\

PISTON \

BELLOWS _

BOUNCE

PISTON

_HYDRAULIC

_ STARTER

MOTOR

FIGURE 12. - STC'S FREE-PISTON/HYDRAULIC DETAILS.

lO
factor is greater than 0.81" therefore, no power

factor correction is required.

Controls

Fully automatic, unattended operation is

planned for the ASCS.

MANUFACTURING AND COST ANALYSIS

Pioneer Engineering and Manufacturing Company,

Marten, MI, provided an independent manufacturing

evaluation and cost analysis for both the MTI and

STC conceptual designs. Pioneer maintained dialogues with both contractors throughout the conceptual design process to assure the proposed concepts

were manufacturable. The methodology for the evaluation of the conceptual designs utilized Pareto's

Law to forecast both system and subsystem cost.

The manufacturing rate used for this study was

10 000 ASCS units/year. Pareto's Law states that

20 percent of the major components constitute

80 percent of the total cost. This methodology

was believed to be an accurate predictor for the

comparison of both conversion concepts, the freepiston Stifling linear alternator and the freepiston Stirling/hydraulic. The ASCS was divided

by functional groups or subsystems for the detailed

comparison. Components selected for detailed costing are believed to be functionally representative

for both the system and each subsystem. The production volume was given as 10 DO0 units/year.

Total costs reflect the use of Michigan labor and

material costs for 1986 and were extrapolated to

reflect the DOE goals in 1984 dollars. Material

costs are a major portion of the ASCS projected

costs. The ASCS projecte_ costs are shown in

Table IV. Detailed information is provided in

Refs. 8 and 9.

TABLE IV. - ASCS PROJECTED COSTSa

t

i MTI STC

Receiver/heat transoort system, dollars

Stirling engine, dollars

Power generation, dollars

Power conditioning, dollars

Cooling system, dollars

Total ASCS, dollars

ASCS, dollars/kNe

aln 1984 dollars.

bHeat pipe.

CReflux boiler.

dCommercial equipment.

eNominal rating 23.2 kNe.

fNominal rating 25.2 kNe.

727 b

4843

1304

542

1014 d

8430

363 e

_55 c

3259

3323 d

I014 d

7761

308 f

The independent assessment by Pioneer was completed in December 1987. The results indicate

that both free-piston Stifling concepts are manufacturable and can easily meet the DOE long term

cost goals of less than $450/kNe.

CONCLUDING REMARKS

Both conceptual designs meet all of the ASCS

Project objectives. The Stirling engine has been

identified by DOE as one of the most promising

heat engines for terrestrial applications. Freepiston technology currently being developed at

NASA Lewis for space and terrestrial applications

has the potential to easily meet DOE's goals for

performance and cost. Common requirements for the

terrestrial and space applications include high

efficiency, and very long life with high reliability. Both the MTI and STC conceptual designs

which feature the free-piston Stifling in an

Advanced Stifling Conversion System are unique and

complementary to each other. The MTI concept

incorporates a heat pipe receiver integrated with

a Stirling/linear alternator conversion system to

convert electricity directly to the grid. The STC

concept incorporates a reflux boiler receiver integrated with a Stirling/hydraulic conversion system

to a hydraulic pump/motor coupled to a rotating

induction generator to provide electricity to the

grid. An independent evaluation by Pioneer indicates that both concepts are manufacturable and

can easily meet DOE's long term cost goals.

REFERENCES

I. Slaby, J.G.: Free-Piston Stirling Technology

for Space Power. NASA TM-lO1916, 1989.

2. Beremand, D.C.; Evans, D.C.; and Alger, D.L.:

Applicability of Advanced Automotive Heat

Engines to Solar Thermal Power. NASA TM-81618,

DOE/NASA/I060-4, 1981.

3. Nest, C.D.: A Historical Perspective on Stirling Engine Performance. Proceedings of the 23rd

Intersociety Energy Conversion Engineering Conference, Vol. 1, ASME, ]988, pp. 11-]4.

4. Schreiber, J.G.; Geng, S.M.; Lorenz, G.V.:

RE-IOO0 Free-Piston Stifling Engine Sensitivity

Test Results. NASA TM-88846, 1986.

1. Five Year Research and Development Plan 1986-

1990. U.S. Department of Energy, DOE/CE-OI60,

Sept. 1986.

6. Shaltens, R.K.: Comparison of Stirling Engines

for Use Nith a 21-kN Dish-Electric Conversion

System. NASA TM-IO0111, DOE/NASA/33408-2, 1987.

7.

8.

Shaltens, R.K.: Advanced Stirling Conversion

Systems for Terrestrial Applications. NASA

TM-88897, 1987.

Conceptual Design of an Advanced Stirling Conversion System for Terrestrial Power Generation.

DOE/NASA/D372-1, NASA CR-180890, 1988.

9. 25 kNe Solar Thermal Stirling Hydraulic System.

DOE/NASA/0371-1, NASA CR-180889, 1988.

11
Report Documentation Page NationalAeronautics and

Space Administration

1. Report No. NASA TM-102085 2 Government Accession No. 3. Recipient's Catalog No.

DOE/NASA/33408-3

4. Title and Subtitle 5. Report Date

Comparison of Conceptual Designs for 25 kWe Advanced Stirling

Conversion Systems for Dish Electric Applications

7. Author(s)

Richard K. Shaltens and Jeffrey G. Schreiber

9. Performing Organization Name and Address

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

12. Sponsoring Agency Name and Address

U.S. Department of Energy

Office of Solar Heat Technologies

Washington, D.C. 20545

6. Performing Organization Code

8. Performing Organization Report No.

E-4806

10. Work Unit No.

776-81-63

11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Memorandum

14. Sponsoring Agency Code

15. Supplementary Notes

Final Report. Prepared under Interagency Agreement DE-AT04-85AL33408. Prepared for the 24th Intersociety

Energy Conversion Engineering Conference, cosponsored by IEEE, AIAA, ANS, ASME, SAE, ACS, and AIChE,

Washington, D.C., August 6-11, 1989.

z

16. Abstract

The Advanced Stirling Conversion System (ASCS) Project is managed by NASA Lewis Research Center through

a cooperative Interagency Agreement with DOE. Conceptual designs for the ASCS's were completed under parallel

contracts in 1987 by Mechanical Technology Inc. (MTI) of Latham, NY, and Stirling Technology Company

(STC) of Richland, WA. Each design features a free-piston Stirling engine, a liquid metal heat pipe receiver, and

a means to provide about 25 kW of electric power to a utility grid while meeting DOE's long term performance

and cost goals. An independent assessment showed that both designs are manufacturable and have the potential to

easily meet DOE's long term cost goals.

17. Key Words (Suggested by Author(s))

Free-piston Stirling engine

Advanced Stirling Conversion System

Solar dish electric

18. Distribution Statement

Unclassified- Unlimited

Subject Category 85

DOE Category UC-96

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No of pages

Unclassified Unclassified 14

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