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WORKING FLUIDS FOR SOLAR, RANKINE-CYCLE COOLING SYSTEMS EZZAT WALES University of Petroleum & Minerals, Dhahran, Saudi Arabia (Received 30 August 1979) Abstract-Data are presented-on 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 R-l 13, followed by fluorinert fluid FC-88, are potential candidate working fluids for this type of application. NOTATION A BO co D Fr g & h
heat-transfer surface area of the tube boiling number convection number inside tube diameter Froude number gravitational acceleration conversion factor heat-transfer 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 fully-developed, liquid-phase average velocity vapor quality
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 two-phase flow I. INTRODUCTION
Several research and development activities are presently in progress for application of the Rankine cycle in operating vapor-compression refrigeration cycIes for the solar cooling of buildings.‘4 Few operational experimental installations have been made for the development of low power level Rankine-cycle 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 Rankine-cycle 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 nozzle-et&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
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 turbine-blade erosion, will increase significantly the turbine disc stresses, especially at high shaft speeds. Moreover, a two-phase 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 Rankine-cycle systems, are the halogenated compounds R-l l,‘*” R-l 13,‘szo R-114~‘~Z2the fluorinated compounds FC-88tLz6 FC-75.” toluene CP-25.‘“” dowtherm A;” thiophene CP-34;” caloria HT-43,34monochlorobenzene ~CB,“T~ and mokoisopropylbiphenyl MIPB.” 2. SAFETY AND EFFICIENCY
The Underwriter’s Laboratories Classification System3’ and the safety test results of DuPont co.39 and the 3-M [email protected]
the hologenated compounds R-l 1, R-l 13, and R-l 14 and the fluorinated compounds FC-88 and FC-75 as nonflammable and their liquids or vapors as non-hazardous 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 Rankine-cycle efficiency when using the resulting five candidate fluids and the ideal Carnot efficiency of water-cooled systems at 30°C and of 4o.c
R-II R-l 13 R-114 FC-75
6arber aDillard Pigmore88arber Kmetics Corp. Bronicki
& 3 36
. 6 .-E .u Z 0 d
” ._ i
l5D inlet temperature,
Theoreticaland experimental Rankine-cycle efficiency variations with expander inlet for a 30°C water-cooled condenser.
Working fluids for solar Rankine cycle cooling systems
z? _ E .o p 5 20.0 0 5 e ‘3 5 a 10.0
Fig. 2. Theoretical and experimental Rankine-cycle efficiency variations’with expander inlet temperature for a 46°C air-cooled condenser.
air-cooled 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 FC-75 because the thermodynamic properties at low vapor pressures are not known. The available experimental data of some Rankine-cycle systems developed by other investigators’* I ‘*20,36are also indicated on the figures, Saturation vapor-pressure curves are shown in the temperature-entropy plane in Fii. 3. It may be seen from this figure that the slope of the saturation vapor curves of R-113, R-114, FC-88, and FC-75 on the temperature-entropy diagram are positive while that for R-11 is approximately infinite. The molecular weights of the candidate fluids FC-75, FC-88, R-113, R-l 14, and R-l 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
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
vapor for flourlnert
_ 150 i?!
0.20 compounds, Cal/g-‘K
Fig. 3. Saturation vapor curves of candidate fluids plotted on a temperature-entropy 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 R-l 14, R-l 13, and R-l 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 R-113 and R-l 1 in copper increased to 0.42 and 0.77%, respectively, while in steel containers the rate increased sharply to 23.0% for R-113 while the R-11 container burst. DuPont fluid/lubricant tests at 121°C also indicate a rate of 0.76% for R-113/topper/lubricant and 4.4% for R113lsteeUlubricant after 1.8 years of tests, and showed a rate of 0.36% for R-ll/copper/lubricant and 16.0% for R-l 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 FC-88/20% Krytox oil in copper and steel capsules at 176.7”C.The static test results at the 3-M CO.~ on FC-88 and FC-75 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
The heat-exchanger pressure drop in the two-phase flow is much greater than that for single-phase 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 two-phase pressure drop per unit length during the boiling process may be expressed as
where @, is a parameter which is a function of a dimensionless variable x such that x=
[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.
Accordingly, [AP/AL]r, = k(ti)‘,‘j @:(x)(1 - #.8 d&
where k = O.l84/$*/2g,D’.* A’.‘.
The pressure drop was evaluated by a step-wise solution employing the Lockhart-Martinelli correlation45 for isothermal, two-phase flow in pipes, assuming an average fully-developed liquid-phase 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 heat-transfer coefficients that occur during convective boiling and/or in the bubble-expression 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 single-phase flow indicates that heat-transfer 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 heat-transfer 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 two-phase heat transfer coefficient may be expressed as h7P = @I, where h, is calculated from the Dittus-Boelter Reynolds numbers of the two phases; also,
equation for all values of the superficial
Nu, = 0.023[p,[ VD(1 - x)/j~,]~.~Pr:~
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. 7-E
Fig. 4. Two-phase pressure drop per unit length of candidate fluids in smooth pipes for 10000I 12oooO.
Bo = h
Fr = V’lg D.
Assuming an average, fully-developed liquid-phase velocity of 1.22 m/s and an inside tube diameter of 16.6 mm, the mean heat-transfer coefficients were calculated iteratively for the candidate fluids. The results are shown in Fig. 5. 6. MAXIMUM HEAT FLUX
Boilers for organic heat-transfer 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.
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
2 i.I? =
E ; t 6 t
‘0 r 6 E :: 2m
E I2 .o
Fig. 5. Two-phase 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 fully-developed liquid-phase 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
Selection of the appropiate fluid depends on the design of the heat exchanger. It is necessary to strike a balance between the gain in heat-transfer rates and the accompanying increase in pumping requirements. Of the candidate fluids, R-l 14 and FC-75 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 R-l 13 is superior to that of R-11 which, in turn, is superior to that of R-114. The fluid R-114 has a low boiling point at atmospheric pressure (3.lW.J).On the other hand, R-l 13 and R-l 1 have high values (47.6”and 23&C, respectively). Furthermore, the heat-transfer rate for R-l 14 is low relative to that for R-l 13 and R-l 1 (Fig. 5). Therefore, R-l 14 is excluded as a potential candidate. In addition to having poor pumping and maximum permissible heat-flux characteristics (Figs. 4 and 6) FC-75 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 R-l l/lubricant heated to 121°Cand R-l 13/lubricant heated to 149°C. The results of the combined fluid/copper and fluid/copper/lubricant combinations absolutely show that the stability of R-113 is superior to that of R-11 up to 149°C.
6.0 , -
I ___ zuu
I 100 Soturatlon
Fig. 6. Maximum heat flux of candidate fluids.
The preceding conclusions suggest that R-l 13, followed by FC-88, are the leading candidate fluids for safe operation of the Rankine-cycle 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 Rankine-cycle solar cooling system. Acknowledgements-This 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
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