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WORKING FLUIDS FOR SOLAR, RANKINECYCLE COOLING SYSTEMS EZZAT WALES University of Petroleum & Minerals, Dhahran, Saudi Arabia (Received 30 August 1979) AbstractData are presentedon the selection of appropiate working fluids suitable for solar cooling of buildings. Safety operation, system reliability, fluid thermal stability, pressure drop,, heat transfer rates, and maximum allowable heat ffux have been investigated for halogenated and fluorinated compounds in several prototype developments that are presently under construction. The results indicate that refrigerant Rl 13, followed by fluorinert fluid FC88, are potential candidate working fluids for this type of application. NOTATION A BO co D Fr g & h
b
L
heattransfer surface area of the tube boiling number convection number inside tube diameter Froude number gravitational acceleration conversion factor heattransfer coefficient latent heat of vaporization length of the tube mass flow rate Nusselt number pressure drop pressure drop per unit lenght Prandtl number rate of heat rransfer fullydeveloped, liquidphase average velocity vapor quality
NT
AP APlAL Pr ; x
Greek letters ti dynamic viscosity p density u surface tension 6 dimensionless axial tube lenght Subscripts g gas I liquid TP twophase flow I. INTRODUCTION
Several research and development activities are presently in progress for application of the Rankine cycle in operating vaporcompression refrigeration cycIes for the solar cooling of buildings.‘4 Few operational experimental installations have been made for the development of low power level Rankinecycle systems for cooling, pumping of fluids and power generation. The performance of the Rankine cycle depends on the working fluid, its operating temperatures and pressures, as well as on the expander, condenser, pump, and piping losses. The Rankinecycle efficiency can be made more efficient by lowering the fluid exhaust pressure and/or increasing its pressure during heat addition and/or superheating its vapor. In the screening process for the appropiate fluids, one is certainly tempted to select steam as the Rankinecycle working fluid for a low cost unit, since it has the advantage that its technology is well developed.‘‘However, for low power level systems and moderate temperature drops across the turbine, the nozzleet&x velocities will greatly increase and result in excessive turbine shaft speeds that will decrease the turbine reiiabihty. The components of such small power units must be designed with tight tolerances and the result is a delicate tOrt leave of absence, at the University of California, Lawrence Berkeley Laboratory, Energy & Environment Division, Berkeley, CA 94720, 197g79 631
EZZAT WALI
632
machine, which needs complex mechanical manufacturing processes and accordingly does not lend itself to low cost mass production. Introducing a high degree of superheat, to temperatureG’ of about 6OO”C,in order to improve the cycle performance and to avoid exhausting wet steam that may cause turbineblade erosion, will increase significantly the turbine disc stresses, especially at high shaft speeds. Moreover, a twophase flow could be experienced in the steam turbine bearings, even with considerable superheat added. This occurance would complicate the bearing design and might cause erratic operation.‘s9 Among the various organic fluids, which have been considered for the Rankinecycle systems, are the halogenated compounds Rl l,‘*” Rl 13,‘szo R114~‘~Z2the fluorinated compounds FC88tLz6 FC75.” toluene CP25.‘“” dowtherm A;” thiophene CP34;” caloria HT43,34monochlorobenzene ~CB,“T~ and mokoisopropylbiphenyl MIPB.” 2. SAFETY AND EFFICIENCY
CONSIDERATIONS
The Underwriter’s Laboratories Classification System3’ and the safety test results of DuPont co.39 and the 3M
[email protected] the hologenated compounds Rl 1, Rl 13, and Rl 14 and the fluorinated compounds FC88 and FC75 as nonflammable and their liquids or vapors as nonhazardous to life. These fluids may therefore be considered as candidates for further evaluation. However, it must be noted that thermal degradation of the fluorinert fluids at temperatures higher than 200°C may generate hightly toxic decomposition products. For environmental reasons and to meet the safety requirements, all other fluids should be excluded from being used for solar cooling of buildings.4’“3 Figures 1 and 2 present the theoretical Rankinecycle efficiency when using the resulting five candidate fluids and the ideal Carnot efficiency of watercooled systems at 30°C and of 4o.c
Experlmental
data:
Fluid
t
V 0
Condensing temperature
:t.z
RII Rl 13 R114 FC75
25.6 30.0
(“cl
Investigator
Source
6arber aDillard Pigmore88arber Kmetics Corp. Bronicki
& 3 36
I
3D.C
8
. 6 .E .u Z 0 d
20.0
” ._ i
4 10.0

A
/
0.c 50
100 Expander
Fig. 1.
l5D inlet temperature,
200 ‘C
Theoreticaland experimental Rankinecycle efficiency variations with expander inlet for a 30°C watercooled condenser.
Working fluids for solar Rankine cycle cooling systems
633
40.0
30.0
z? _ E .o p 5 20.0 0 5 e ‘3 5 a 10.0
0.0 50
150
100 Expander
inlet
200
trmperaturs,°C
Fig. 2. Theoretical and experimental Rankinecycle efficiency variations’with expander inlet temperature for a 46°C aircooled condenser.
aircooled systems at 44“C, respectively. Saturation conditions and isentropic expansion were assumed in evaluating the efficiencies as a function of the expander inlet temperature. The figures do not show the theoretical efficiency of FC75 because the thermodynamic properties at low vapor pressures are not known. The available experimental data of some Rankinecycle systems developed by other investigators’* I ‘*20,36are also indicated on the figures, Saturation vaporpressure curves are shown in the temperatureentropy plane in Fii. 3. It may be seen from this figure that the slope of the saturation vapor curves of R113, R114, FC88, and FC75 on the temperatureentropy diagram are positive while that for R11 is approximately infinite. The molecular weights of the candidate fluids FC75, FC88, R113, Rl 14, and Rl 1 are 420,290,187.39, 170.38,and 137.28respectively. High molecular weight fluids tend to have large positive slopes. The expansion of such fluids therefore occurs into a region of increasing superheat. Dry expansion eliminates erosion in the expander and a regenerative heat exchanger is often incorporated to transfer thermal energy from the expander exhaust to the boiler feed. 3. THERMAL
STABILITY
The resistence of working fluids to thermal decomposition in the presence of lubricants and container materials is a highly important factor. The lubricant can be miscible or immiscible with the cycle fluid, but for minimum system complexity a miscible oil is desirable. Heating a candidate fluid in a sealed capsule is an acceptable screening approach for evaluating the
EZZAT WALI
634 Entropy
of
saturated
vapor for flourlnert
compounds,
cal/gaK 0.16
0.10 300
250 FC75
200
9
_ 150 i?!
f f
100
50
0 0.18
0.16
0.15 Entropy
of
saturated
vapor
for
halogen
0.20 compounds, Cal/g‘K
Fig. 3. Saturation vapor curves of candidate fluids plotted on a temperatureentropy diagram.
approximate temperature limitations of the combined fluid/metal or fluid/lubricant/metal combinations. However, the final evaluation must be conducted in a dynamic test loop where boiling, condensing and other stresses are imposed. In the absence of oil, static stability tests of Rl 14, Rl 13, and Rl 1 at 149’C, over a period of 2 years, showed a rate of decomposition in copper containers of 0.18, 0.13, and 60.0% respectively, and decreased in steel to 0.013,0.042, and 0.72%, respectively.” In the presence of an equal volume of oil and after 52 days of tests at 149°C the rate of decomposition of R113 and Rl 1 in copper increased to 0.42 and 0.77%, respectively, while in steel containers the rate increased sharply to 23.0% for R113 while the R11 container burst. DuPont fluid/lubricant tests at 121°C also indicate a rate of 0.76% for R113/topper/lubricant and 4.4% for R113lsteeUlubricant after 1.8 years of tests, and showed a rate of 0.36% for Rll/copper/lubricant and 16.0% for Rl l/steel/lubricant after 52 days. During a recent prototype development program,2’ 60 static thermal stability tests were conducted with several cycle fluids/lubricant/material combinations. The data collected over a test period of 6 months showed excellent results for FC88/20% Krytox oil in copper and steel capsules at 176.7”C.The static test results at the 3M CO.~ on FC88 and FC75 indicated that a small amount of decomposition may occur at temperatures above 200°C. Dynamic stability tests were conducted at 250°C for 1OOhrusing dummy loops of steel, copper and aluminum while reproducing the temperature cycling effects on FC75/lubricant. BronickY reported that satisfactory results were obtained.
Working fluids for solar Rankine cycle cooling systems 4. PRESSURE
63.5
DROP
The heatexchanger pressure drop in the twophase flow is much greater than that for singlephase flow alone for various reasons among which are the irreversible work done by the gas on the fluid and the fact that the presence of the second fluid reduces the cross sectional area of flow for the first fluid. The twophase pressure drop per unit length during the boiling process may be expressed as
~:(x)W/ALl, d5,
(1)
0
where @, is a parameter which is a function of a dimensionless variable x such that x=
~~A~/A~l,/~~P/A~l,1°~5;
(2)
[AP/AL], and [AP/AL], are the pressure drops per unit length that would exist if the liquid and gaseous phases, respectively, are assumed to flow alone. Assuming smooth pipes for a Reynolds number range of 10,000i NRC2 120,000 yields x = h/Pgl”~’ k?ghl”~5.
(3)
Accordingly, [AP/AL]r, = k(ti)‘,‘j @:(x)(1  #.8 d&
(4)
0
where k = O.l84/$*/2g,D’.* A’.‘.
(5)
The pressure drop was evaluated by a stepwise solution employing the LockhartMartinelli correlation45 for isothermal, twophase flow in pipes, assuming an average fullydeveloped liquidphase flow velocity of 1.22 m/s and an inside tube diameter of 16.6mm. The results are presented in Fig. 4. _s:MEAN HEAT TRANSFER COEFFICIENTS
In the solar boiler, it is important to evaluate the heattransfer coefficients that occur during convective boiling and/or in the bubbleexpression regime. The stirring action of a bubble occurs mainly because of movement of the bubble boundary during its growth while it is still attached to the heating surface. Experience with singlephase flow indicates that heattransfer coefficients are related to fluid properties operating at the same conditions. Hence, dependence on viscosity, thermal conductivity and specific heat of the two phases is expected. From bubble dynamics and boiling correlations, it follows that density, latent heat of vaporization and surface tension are also significant. A large number of correlations for evaluating the heattransfer coefficients of boiling fluids has been proposed. However, most of these are not reliable beyond the range of the data on which they were based, except for a correlatiotP which is limited to vertical flow applications and a more recently developed technique.47 The latter appears to be considerable superior to other available predictive techniques in the range of applications. The twophase heat transfer coefficient may be expressed as h7P = @I, where h, is calculated from the DittusBoelter Reynolds numbers of the two phases; also,
(6)
equation for all values of the superficial
Nu, = 0.023[p,[ VD(1  x)/j~,]~.~Pr:~
(7)
and 1+5 is a correlation parameter, which depends on the convection number, boiling number and Froude number. These dimensionless groups are, respectively, defined as follows: co = [l/(x  1)]0~8(pJp,)0.5, ffiY
Vol. 5. No. 7E
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EZZAT WALI
636
6.0
1.0 0
200
100 Saturation
temperature,
‘C
Fig. 4. Twophase pressure drop per unit length of candidate fluids in smooth pipes for 10000I 12oooO.
Bo = h
(9)
Vhf,,
Fr = V’lg D.
w
Assuming an average, fullydeveloped liquidphase velocity of 1.22 m/s and an inside tube diameter of 16.6 mm, the mean heattransfer coefficients were calculated iteratively for the candidate fluids. The results are shown in Fig. 5. 6. MAXIMUM HEAT FLUX
Boilers for organic heattransfer fluids are generally designed to operate at a low heat flux. The allowable heat flux is usually limited by the maximum film temperature. In order to achieve efficient heat transfer and operation, it is important to know the maximum heat flux attainable with nucleate boiling. This value can be predicted with reasonable accuracy because it does not depend appreciably on bubble dynamics, surface conditions and nucleation phenomena. Figure 6 shows the variation of the maximum heat flux of the candidate fluids with saturation temperature employing Zuber’s correlation,” viz. [q/Al,,, = (d24)h,&%ggC(~,  PJP(
1 + ~el~do.5.
(11)
Increasing the saturation temperature increases the vapor density without appreciably affecting the other terms in the above equation and consequently the maximum heat flux increases. It should be noted (see Fig. 6) that each fluid has a specific :*lturation temperature at
Working fluids for solar Rankine cycle cooling systems
637
r
so
0.0 0
\’ R113
%
2 i.I? =
7.0
6.0
I
E ; t 6 t
5.0
‘0 r 6 E :: 2m
4.0
3.0
E I2 .o
I .o
0.0 0
50
100
150
Saturation
temperature
200
250
, ‘C
Fig. 5. Twophase mean heat transfer coefficients of candidate fluids for a 16.6mm inside tube diameter and a veclocity of 1.22m/s for the average flow of the fullydeveloped liquidphase system.
which the maximum heat flux reaches a peak value. At this point, the latent heat of vaporization begins to dominate the increase of the vapor density if the saturation temperature is increased. 7. CONCLUSIONS
AND RECOMMENDATIONS
Selection of the appropiate fluid depends on the design of the heat exchanger. It is necessary to strike a balance between the gain in heattransfer rates and the accompanying increase in pumping requirements. Of the candidate fluids, Rl 14 and FC75 have, respectively, the lowest and highest pumping requirements (Fig. 4) and heat transfer rates (Fig. 5). The limited experimental data available on the efficiency of the Rankine cycle show that the performance of Rl 13 is superior to that of R11 which, in turn, is superior to that of R114. The fluid R114 has a low boiling point at atmospheric pressure (3.lW.J).On the other hand, Rl 13 and Rl 1 have high values (47.6”and 23&C, respectively). Furthermore, the heattransfer rate for Rl 14 is low relative to that for Rl 13 and Rl 1 (Fig. 5). Therefore, Rl 14 is excluded as a potential candidate. In addition to having poor pumping and maximum permissible heatflux characteristics (Figs. 4 and 6) FC75 has a low vapor pressure in the condensing temperature range. This will cause a large difference across the condenser surface walls and lead to the requirement for a very large condenser volume, thereby increasing the leakage rate to the system. Static thermal stability tests, which may be considered as acceptable screening approach for evaluating the approximate temperature limitations, clearly indicate that steel should not be recommended for use in systems containing Rl l/lubricant heated to 121°Cand Rl 13/lubricant heated to 149°C. The results of the combined fluid/copper and fluid/copper/lubricant combinations absolutely show that the stability of R113 is superior to that of R11 up to 149°C.
EZ2.A.T
638
WALI
8.0
7.0
6.0 , 
l
I 
1
3.c 1.
2.f )
1, 0
I ___ zuu
I 100 Soturatlon
temperature,
I __A
JvUb
,
‘C
Fig. 6. Maximum heat flux of candidate fluids.
The preceding conclusions suggest that Rl 13, followed by FC88, are the leading candidate fluids for safe operation of the Rankinecycle for the solar cooling of buildings. However, dynamic experimental investigation should be conducted to evaluate the thermal stability of these two fluids in the presence and absence of lubricants in copper, steel and alloy conduits. Test loops should be operated at temperatures, pressure,A and flow rates similar to recently proposed Rankinecycle solar cooling system. AcknowledgementsThis work was supported by the University of Petroleum and Minerals, Dhahran, Saudi Arabia. The author is grateful for technical support by the Energy and Environment Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California. REFERENCES
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Working fluids for solar Rankine cycle cooling systems
639
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