The organic Rankine cycle engine under consideration for a solar thermal system being developed is described. Design parameters, method of control, performance and cost data are provided for engine power levels up to 80 kWe; efficiency is shown as a function of turbine inlet temperature in the range of 149 C to 427 C

Boda, F. P.

English

NASA Technical Reports Server

,, N80 24773
THE SCSTPS ORGANIC RANKINE ENGINE
BY
F. SODA
FORD AEROSPACE b COMMUNICATIONS CORPORATION (FACC)
NEWPORT BEACH, CA 92660
ABSTRACT
This paper describes the Organic Rankine cycle (ORC) engine under con-
sideration for the PFDR solar thermal system being developed for JPL/DOE
by FACC under Contract 955637. Design parameters, method of control:
performance and cost data are provided for engine power levels up to
80 kWe; efficiency is shown as a function of turbine inlet temperature
in the range of 1490C (3000F) to 4270C (8000F).
INTRODUCTION
Under the SCSTPS Program, FACC will develop a solar thermal PFDR, dis-
tributed generation system employing small Rankine-'cycle Power Conversion
Units (PCUs) mounted at the focus of parabolic dish concentrators. The
baseline PCU is comprised of an organic Rankine cycle (ORC) enginer high
speed AC generator and a ground-mounted AC/DC converter (rectifier).
The Rankine cycle was selected for the SCSTPS Program on the basis of
highest performance for least program risk (compared with other heat
engine cycles). The ORC engine was selected over a steam Rankine engine
on the basis of programmatic and technical factors, although a steam
system is currently under consideration for a parallel development effort,.
The PCU has not yet been selected; proposals are currently being solicited
from a number of engine manufacturers, including Sundstrand f Garrett/
AiResearch,, Barber-Nichols, Thermo-Electron and General Eleetric. During
the Phase I Small Power Systems study, however, FACC contracted with
Sundstrand to provide a preliminary high performance ORC design (Ref
0. The design is an outgrowth of Rankine engine studies carried out
by Sundstrand for NASA (Ref 2)'; it currently serves as the baseline
SCSTPS engine until final ORC engine selection and is described below.
SYSTEM DESCRIPTION
The prototype ORC engine is a regenerative, hermetically-sealed 22 kWe
unit with a single-stage axial-flow turbine; supercritical toluene is
the working fluid, with a turbine inlet temperature (TIT) of 427 0C (8000F)
and a maximum system pressure of 4.52 MPa (656 psia). A high-speed,
4-pole homopolar inductor alternator is direct-coupled to the turbine
and cooled by the working fluid; variable-frequency AC power is con-
verted to DC output by means of the solid state rectifier. Figure 1
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99
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mr
CONDENSER
TUBES
FIG. 1. ORC POWER CONVERSION ASSEMBL;X
i START
W
PUMP
TB00
FROM
RECEIVER
PUMP
nuu
FIG. 2. BASELINE (22 KWe) ORC
POWER CONVERSION UNIT
is a cutaway view of the power conversion assembly, i.e., the PCU and
the receiver*, showing the arrangement of the major engine components;
the assembly is pallet-mounted for ease of installation/removal. A
cross-section of the PCU is shown in Figure 2; the condenser is con-
figured in a cylindrical shape surrounding the regenerator and the com-
bined rotating unit (CRU), i.e., the hermetically-sealed turbine/
alternator/feed pump components. This makes for a very compact PCU,
measuring 0.87m (34 in.) in diameter by 1.55m (61 in.) in length. Forced
draft cooling of the condenser is accomplished with an electrically-
t?riven fan whose speed is varied (in 3 steps) as a function of cooling
load and ambient air temperature to minimize parasitic losses. A dry-
cooling arrangement was selected because of the lack of water in the
high-insolation regions of the U.S. A PCU system schematic diagram
is shown in Figure 3; note that the working fluid is used to cool the
bearings and the controller as well as the alternator. The pertinent
thermodynamic state points are identified on the figure and specified
in Table 1. A pressure-enthalpy diagram for toluene is given in Figure 4,
*The receiver shown is an alternate Garrett/AiResearch-designed unit of
the direct-heated type, based on a steam receiver design currently under
development for JPL/DOE; in the companion systems paper by Pons and
Grigsby the baseline receiver is shown as the indirect-heated type,
designed by FACC.
100
PRESSURE low
StAT[/OINT
W.
IPSIAt CPC 01., F..
tUR.IN[INUT iN IS7.1Y 1171 111001
R 
got 
NlRATOR VAPOR INLET 0411 17071 700 IN71
CONDENSER VAPOR I141,11T 0.017 (157) 711 IIMI
PE[OPUW INLET
AL7[RNATOR COOLINO INLET 0.01160.67 1101ISS►1
117
611
11701
I/MI
R!0[ NTRAIOR L10U101NLST SAi !116.1 111 11M1
7	 VAPOR11tR LIOUID INLET 4.411 15/111 0111 14101
^M 161111 ►7! ISOUIV^AIIOIR,I2,,S``R Ol1il[T
Ol^l+^ llti E.L.
OF POOR
FIG. 3. BASELINE PCU SYSTEM
SCHEMATIC
TABLE 1.	 PCU THERMODYNAMIC STATE POINTS
PRESSURE TEMP.
STATE POINT MPa ('PSIA) °C (°F)
1 TURBINE INLET 3.96 (574) 427 (800)
2 REGENERATOR VAPOR INLET 0.014 (2.07) 308 (587)
3 CONDENSER VAPOR INLET 0.013 (1.92) 76 (168)
4 FEED PUMP INLET 0.069 (10) 52 (125)
5 ALTERNATOR COOLING INLET 4.52 ( 656) 58 (136)
6 REGENERATOR LIQUID INLET 4.51 (654) 63 (146)
7 VAPORIZER LIQUID INLET 4.45 (646) 248 (478)
8 VAPORIZER OUTLET 4.06 (589) 427 (800)
6
1 	 7
R,`NRiR
`ECEIVER
ALTERNATOR	 3.06 MP.
I'
I' I41270C
t
. TIT - 4270C t000r:
W . TIP • 3.96 MPR (674 PSIA)
^LL
g	 (	 1^
'	 FIGURE NOT
6OOST	 TOSCALE
PUMP
2
CONONSR	 y	 REGNRTR
ENTHALPY
FIG. 4. PRESSURE-ENTHALPY DIAGRAM
FOR TOLUENE SHOWING ORC
STATEPOINTS
101
also showing the cycle operating path and the aforementioned state points.
Gross weight of the PCU is estimated at 296 kg (653 lb); a weight break-
down of the 22 kWe unit is given in Table 2.
TAME 2. BASELINE (22 kWe) PCU WEIGHT BREAKDOWN
WEIGHT
COMPONENT	 kg	 (lb)
CONDENSER	 96	 (16)
FAN/MOTOR	 44	 (212)
COMBINED ROTATING UNIT 	 85	 (96)
REGENERATOR	 17	 (37)
START PUMP	 16	 (35)
ELECTRONIC CONTROLLER 	 9	 (20)
FLUID (INVENTORY)	 15	 (33)
BOOST PUMP, VALVES, MISC 	 14	 (32)
TOTAL	 296	 (653)
CONTROL
The PCU control scheme is designed both for stable operation under
varying solar input and for high part-load efficiency, a highly desirable
feature in a solar power system. As the solar input to the receiver
varies, the turbine inlet temperature is held constant by varying the
toluene flow rate; this is achieved by slaving an electro-mechanical
flow control valve (which controls the output of the shaft-driven
toluene feed pump) to a turbine inlet temperature sensor. In addition,
a controller located at the power module provides turbine speed (and
power) control by varying the alternator field excitation, thereby regu-
lating output voltage and delivering variable amounts of power into
the grid in direct proportion to the solar input. The local controller
is under supervisory control by the plant central microprocessor, which
contains command look-up functions to permit operation at the optimum
turbine speed for each load value. As mentioned earlier, condenser
fan speed is also varied with load (under central control) to maximize
system efficiency.
PERFORMANCE
4270C (800 0r) is generally considered to be the upper limit for operating
with toluene and was picked as the baseline TIT to maximize PCU efficiency
and reduce overall system energy cost. At the (rated) ambient temperature
of 270C (800F), overall PCU efficiency, i.e., from thermal input to DC
electrical output, is 28.5%. In consideration of the possibility of
102
toluene breakdown at elevated temperature } PCU performance was also
determined for lower values of TIT; Table 3 summarizes design point
characteristics f,rr TIT values of 371 0C (7000F) and 3990C (7500F) as
well as for the baseline design. Note that the penalty for operation
at 371 0C is two points in efficiency,	 26.5% compared with 28.5%
baseline value; as shown in the compae %ws systems paper by Pons and
Grigsby, this corresponds to about a 71' inc ,tease in (op rational) system
energy cost. Operation at reduced temiser cure may, of course, not be
necessary. Foi example, it is projected that the rate of toluene
degradation will actually be quite low, since the bulk of the inventory
is not at peak temperature most of time. Sundstrand estimates that
the raluene will have to be changed only about every 30,000 hours, or
abok, t twice in the 30 year life of the solar plant. Note further that
TABLE 3. PCU DESIGN POINT CHARACTERISTICS
PARAMETER
	
3710C (700 F)	 3990C (750 F) 4270C (800 F)
DC POWER OUT kW 22 22 22
THERMAL POWER IN kW 83 79.7 77.2
PCU EFFICIENCY* % 26.5 27.6 28.5
FAN POWER kW 1.7 1.8 1.9
PARASITIC POWER'* kW 0.12 0.12 0.12
RECTIFIER EFF'Y % 97 97 97
ALTERNATOR EFF'Y 7 92 92 92
PUMP EFV Y % 54 53 52
TURBINE EFF'Y % 74 73 73
REGENERATOR EFFY o 95 95 95
BEARING LOSSES kW 0.1 0.1 0.1
FLUID FLOW RATE kg/hr 519 494 461
(Lb/hr) (1144) (1090) (1021)
*PO DC OUTPUT
**INCLUDES CONTROLS AND BOOST PUMP
103
the use of a hermetic-sealed system minimizes oxide formation and atten-
dant scale deposition on the plumbing. Figure 5 shows the general trend
in efficiency for small (20-50 We ) ORC power conversion units, over
the range 3000F < TIT < 800°F .
Figure 6 shows the effect of engine size on PCU efficiency for the speci-
fied three values of TIT. There is a substantial drop in efficiency
below about 20 kWe. Figure 7 shows the efficiency of the 22 We PCU as
a function of thermal, input and ambient air temperature. Note the excel-
lent part-load performance, which is the major reason for the system's
high annual capacity factor (ACF = 0.418) at the Barstow, CA site.
COST
For system economic projections, both capital and maintenance costs for
the PCU are essential inputs. Figure 8 is an estimate of these PCU costs
over the power output range of 5.to 80 We, based on annual production
rates of 100 9 000 units/year. At this stage in the development of the
system }
 however, there is considerable uncertainty over the accuracy
of these data; a possible error of +40% is not an unreasonable assumption
at the present time.
CONCLUSIONS
A carefully-designed ORC power conversion unit can achieve good
efficiency at relatively low temperature, i.e., compared with Brayton,
Stirling or steam Rankine cycle engines. The low temperature assures
high solar collection efficiency (which contributes to high overall
system efficiency), lower risk and lower cost, since less critical
materials and lower-stress equipment designs can be used.
Many thousands of operating hours exist on ORC systems/components, albeit
at different sizes than for the components of the baseline system.
Nevertheless, the ORC-PCU should be considered a state-of-the
-art device.
In addition, toluene is a fully characterized substance and its toxicity,
flammability and other environmentally sensitive parameters are quite
well known and safe handling procedures are well established.
REFERENCES
1) Anon, "Phase I of the First (Solar) Small Power System Experiment
(Experimental System No. 1)" Final Report, No. V-6529, Ford Aerospace
& Communications Corp., Aexonutronic Division, Newport Beach, CA,
5 May 1979.
2) Bland, T., "15 We (Nominal) Solar Thermal Electric Power Conversion
Concept Definition Study - Steam Rankine Turbine,System," Sundstrand
Corp., Report DOE/NASA/0061-79/1, NASA CR-159589, AER 1713.
104
30	 TURD. INLET TEMP.
4770C 1000OF1 F)
3119C 1700f)
>	 20R
• SMALL ENGINES 1 . 2060 Mel	 w	 •TOLUENE
10	
. 27°C I80°FI AMD. TEMP
0:30
w
ww
0.70
0,10
W
V.
W
OF -•
0	
300
	 400	 000	 000	 700	 000
100
	 200	 700	 400	 600
EXPANDER INLET TEMPCRATURC, °C
FIG. 5. INFLUENCE OF TEMPERATURE'
ON ORC POWER CONVERSION
UNIT EFFICIENCY
0'	 r--r-	 -*---+
0	 10	 20	 30	 40	 60
PCU RATED POWER OUTPUT, P°p, KW
FIG. 6. EFFECT OF ENGINE SIZE ON
PCU EFFICIENCY
^r
600
AMBIENT AIR
30	 TEMPERATURE 70°F0^ -F 80°F
^	 ^---^ - 116°F
xw
u 20
ur '.	 DESIGN POINTW
• THERMAL INPUT» 77.2 kW1h
• TURBINE INLET-800OF
10	 • WORKING FLUID «TOLUENE
VARIABLE SPEED FAN &TURBINE
• AMBIENT AIR TEMP,-800F
• PCU EFFICIENCY-24,6%
26%. - 	60%.	 76%	 100% THERMAL INPUT
YYI105
K
FIG. 7. PCU PART LOAD PERFOPMANCE tl
u1,,IGINAL PAGE IS
Or, 	 QUALITyl
0
100
PCU OUTPUT. POWER, P
°e KW,
FIG. 3. PROJECTED PCU COSTS
105


The SCSTPE organic Rankine engine

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