The organic Rankine cycle receiver which is presently being developed is described. The receiver employs an integrated cavity/pool boiler which permits the design of a small, lightweight, low cost and efficient moderate temperature receiver for use in a dish-Rankine solar thermal system

Osborn, D. B.

English

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AN ORGANIC UNKINE RECEIVER FOR THE SCSTPE PROGRAM
by
D. B. Osborn
Ford Aerospace & Communications Corporation (FACC)
Aeronutronic Division
Newport Beach, CA 92663
ABSTRACT
This paper presents a brief description of an Organic Rankine Cycle
(ORC) Receiver which is presently being developed for the SCSTPE
Phase II program. The receiver employs an integrated cavity /pool boiler*
which permits the design of a small, lightweight, low cost and efficient
moderate temperature receiver for use in a dish-Ran%ine PFDR system.
INTRODUCTION
Under the SCSTPE (EE#l) Phase II Program, FACC will develop a PFDR
solar thermal electric system employing Rankine-cycle Power Conversion
Assemblies (PCAs) mounted at the focus of Parabolic dish concentrators.
The PCA consists of a receiver, organic Rankine cycle (ORC) engine and
high-speed AC generator (Figure 1). The design of the receiver is a major
technical challenge in the development of this PFDR system. Cavity-type
receivers are generally most efficient for moderate and high-temperature
systems. There are, in general, two types of cavity receivers:
e Direct heated, where the engine working fluid is heated within
the receiver.
• Indirect heated, where a secondary fluid is heated in the
receiver and the primary (engine working) fluid is heated in a
separate heat exchanger.
Each approach has advantages and disadvantages, strongly influenced by
system considerations. For the SCSTPE Phase II Program, FACC has chosen
to try an indirect heated design, based on a concept studied in depth
during the Phase I part of the program. The most important difference
between the Phase I and Phase II efforts, however, is the operating
temperature range. The high temperature (800°C) Stirling engine studied
during Phase I resulted in the selection of sodium for the receiver fluid.
The moderate operating temperature (427°C) of the Phase II ORC requires
the search for a more applicable fluid.
SUBSYSTEM DESCRIPTION
A sketch of the baseline receiver configuration is presented in Fig-
ure 2. The cavity receiver is a simple structure consisting of two
cylindrical shells, one cantilevered inside the other from a connecting
toroidal shell. The two ends are capped with spherical heads forming a
*Patent applied for
37
https://ntrs.nasa.gov/search.jsp?R=19800016278 2020-03-21T18:59:15+00:00Z
sdesign with minimal modifications. For example, canisters of high ther-
mal capacity materials (i.e., eutectic salts) can be inserted within the
receiver annulus, with energy transfer facilitated by the high boiling
heat transfer coefficients of the secondary floi.I. The fully insulated
receiver (12.7 cm of insulation) can accommodate a maximum 104 kWt energy
with.I.Tt ,.t 1 x 1 x 1.2 m envelope at a gross weight of 180 kg including
20 kg of a typical fluid. Based on Phase I vendor quotations, the com-
plete receiver /thermal transport package can be manufactured for under
$2500 when built in production quantities. The operational life of the
optimized, lowest life cycle cost receiver is selected at 15 years,
based on design tradeoffs between material corrosion rate and material
c,:eep-rupture limitations.
BENEFITS OF POOL BOILING RECEIVER
The heat transfer coefficients associated with pool boiling are very
high and generally result in:
• Reduced cavity area (thus, a lighter and lower cost receiver)
• Reduced receiver-to-engine temperature differential (reduces
thermal losses and thus increases system efficiency)
• Reduced sensitivity to the incident flux levels (thus a large
safety factor against temperature overshoot or burnthrough).
Other benefits include a simplified design that does not require
tubes, fins, flux adjusters, pumps or special. controls. By selecting
the right fluid, the vapor pressure at the steady-state operating temp-
erature can be about 1 atmosphere. This virtually eliminates any pres-
sure differential, hence reduces the cavity wall thickness which in turn
reduces the temperature gradients, weight and cost of the receiver. The
thermal inertial of the receiver cavity walls and boiling pool provide
an inherent buffer storage capability during short inclement operating
conditions such as those associated with a cloud passage. The thermo-
syphon also operates at a nearly constant temperature.
The condenser/heat exchanger also has very high film coefficients.
This permits a heat exchanger configuration and surface geometry to
yield a small compact unit without concern about the solar flux distri-
bution and levels.
PRELIMINARY FLUID SELECTION
Some of the more important characteristics of the receiver fluid are
listed below.
• Critical point above the maximum operating conditions of the
engine working fluid.
• High boiling heat transfer coefficients.
• High burnout heat flux.
i
• Statue pool boiling fcr a range of operating conditions.
• Low vapor pressure nt operating temperature.
• Non"corrosive, non-toxic, non-flammable and chemically stable.
• Easy to handle and fill/drain receiver.
• Freeze point below ambient conditions.
• High heat capacity.
• Low cost.
As previously mentioned, sodium was selected for use in the high tem-
perature (N800*C) receiver. At the lower operating temperatures of ORC
engines (^350-425°C), several candidate fluids exist subject to further
investigation. These fluids include the various terphenyls, sulfur,
aluminum bromide, etc.
PERFORMANCE
Figure 3 presents a comparison-of typical boiling and gas -in-tube
heat transfer rates. As shown, the boiling systems have much higher
heat transfer rates than those for forced gas convection; also less
heat transfer surface area is required and lower temperature differen-
tials between the surface and the working fluid are experienced. Refer-
ence 1 indicates that suUur (with a small amount or iodine to reduce
the high viscosity at lower temperatures) has comparable performance to
water in a heat pipe application. In addition, the burnout heat flux
for sulfur is approximately 3000 kW/m2 at a pressure of 1 atm (Reference
2). This gives a safety factor of approximately 3 over the predicted
worst case flux deposition for a typical cavity receiver configuration.
Figure 4 shows the vapor pressures of various heat transfer fluids as
a function of temperature. The vapor pressures of the terphenyls and
sulfur are approximately 1 atm at the upper operating temperature
(-427°C) of ORC engines. The vapor pressure of aluminum bromide is
slightly higher. In the case of the terphenyls and sulfur, the net
pressure loading on the receiver is very low, with obvious structural
and safety advantages.
The temperature gradients throughout the receiver subsystem are pre-
sented in Figure 5. The largest temperature drop occurs through the
receiver wall. The backface wall-to-pool. temperature differential is
typically quite small due to the high beat transfer coefficients charac-
teristic of the pool boiling phenomena.' The temperature drop through
the vapor pipe is predicted to be on t,t, order of a few degrees centi-
grade, due to the pressure difference between the pool and heat
exchanger. A very small temperature drop is also predicted for the
condensation heat transfer to the secondary heat exchanger. The tem-
perature drops through the secondary heat exchanger will be minimized
by utilizing a high performance plate-fin configuration.
Transient Operating Characteristics
An in-depth transient analysis of the receiver./thermal transport sub-
system is necessary in view of the inherent non-steady nature of the
90
E
3
G'.
m
Z
EXCESS TEMPERATURE, 00
FIG. 3. BOILING AND GAS—IN—TUBE
HEAT TRANSFER
ti h0
3a
to 0
to
m W2 yJ
Z	 Jas JF
0.1
0.01
0	 700	 400	 .,.	 Boo	 loon	 1200
rg-4
ur
0.
RECEIVERC80ILER	 VAPOR PIPE _	 SECONDARYHEATEXCHANOER
TRECEIVER
-► j
"TWALL
SOLAR	 ^	 '^TPOOL^RHX
POWER ' p 	
^'TTOTAL
ATFLUID
^00
	 ATFLUID	
^ATWALLTAT_...._
FIG. 5. SUBSYSTEM TEMPERATURE GRADIENTS
ORIGINAL PAGE IS
OF p40R QUALITY
1oa
LAg INPUT PO" I",LA  T
ILA
1
TWALI.	 1	 .—.—
1L	 T2HX.	 /.1.	
/	 1
t
ENGINE SHAFT OUTPUT POWER
1 ,	,,••,/l^._._._.i
^/ f P2HX
Sri	 "IDLE" POW ER -
r
LID OPEN
ENGINE
	
ENGINE	 TIME
STARTUP	 TEIIAP
C014TROL
FIG. 6. SUBSYSTEM RESPONSE DURING
^PERnT^Re."C	
NORMAL STARTiIP
FIG. 4. VAPOR PRESSURES FOR SEVERAL
HEAT TRANSFER FLUIDS
LID	 TIME	 11D
CLOSFp	 .OPENED
FIG. 7. SUBSYSTEM TEMPERATURE
RESPONSE DURING CLOUD
PASSAGE
solar flux. The objective is to characterize the behavior of the sub-
system for all operational modes encountered in a solar application.
During Phase I, a transient thermal model was developed from the basie
equations for conservation of mass, conservation of energy, the equation
of state for vapor, and the vapor pressure equation. It is assumed that
the vapor o saturated (vapor and liquid in quasiequilibrlum), the vapor
volume 15 approximately constant and that the receiver orientation
effects are negligible. The energy balance considered the incoming
solar power, the thermal power to the engine, and the thermal losses
from the subsystem. Pressure drops throughout the entire transport
subsystem were also included. Typical results of the transient analy-
sis are discussed below.
R
NORMAL START-UP. Figure 6 presents the subsystem temperature response
and engine output power during a normal start-up condition. The solar
input power profile is based on the 15 minute Barstow, California solar
insolation data for a typical uncloudy day and includes the effects of
both concentrator size and efficiency, thus representing the total power
entering the receiver. In the morning, after normal operation from the
previous day and nighttime cool down, the receiver pool is still warm.
The receiver lid is opened prior to focusing on the sun. Once the sun
is "on", the pool and heat exchanger temperatures rapidly increase
towards the steady-state operating level. The pool and heat exchanger
temperatures correspond to the saturated vapor pressure at the respec-
tive temperatures. The temperatures, pressures, and vapor mass flow
rate increase as the solar energy continues to enter the system. After
several minutes the engine head temperature has reached the pre-
determined start temperature. The engine is then started and operated
at ,a low or "idle" power level consistent with a previously selected
condition. After several more minutes, the engine working fluid reaches
the steady-state operating temperature (427°C) and the engine tempera-
ture control mode is activated. At this point the temperature control-
ler is modulated to maintain the engine temperature within the control
band as the solar flux varies during the day.
CLOUD PASSAGE. Figure 7 presents
the subsystem temperature response
during a typical cloud passage. As
the cloud passage starts its passage,
the solar input power goes to zero,
the receiver lid closes and the sub-
system temperatures start to decrease.
The engine output power will decrease
until operation is at a low or "idle"
power level. During cloud passage
the receiver pool and engine fluid
temperature decrease while the
engine "idle" power remains rela-
tively constant. When the cloud has
passed, the solar power returns and
the receiver lid is opened allowing
the subsystem temperatures to
increase. Several. minutes are
92
required to heat the subsystem back up to the steady-state operating
temperature.
The results of all of the cases investigated during Phase i of the pro-
gram demonstrate that the performance of the subsystem is stable, well-
behaved, and has long thermal. time constants which facilitate engine
control.
CONCLUSIONS
An ORC receiver utilizing an integrated cavity/pool boiler is techni-
cally feasible at the present time and uses state-of-tbe--art materials.
The use of a pool boiler permits the design of a small, lightweight, low
cost and efficient, moderate temperature receiver for use in a dish-
Rankine PFDR system.
REFERENCES
1. Polasek, F. and Stule, P., "Heat Pipes for the Temperature Range From
200°C to 600 0C," Proceedings of the 2nd International Heat Pipe Con-
ference, sponsored by CNR, co-sponsored by AIAA,_ ESA and Euratom,
Bologna, Italy, 1976.
2. Sawle, D. R., "Sulfur Cooled Power Reactor Study," AEC Report AGN
8015, 1961.
93


An organic Rankine receiver for the SCSTPE program

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