I n d . Eng. Chem. Res. 1988,27, 679-684 Mustafaev, R. A. Znzh. Fiz. Zh. 1973,24,663. Nederbragt, G. W.; Boelhouwer, J. W. M. Physica 1947, 13, 305. Neufeld, P.D.; Janzen, A. R.; Aziz, R. A. J . Chem. Phys. 1972,57, 1100. Ogiwara, K.; Aral, Y.; Saito, S. Znd. Eng. Chem. Fundam. 1980,19, 295. Parckinson, W. J. Ph.D. Dissertation, University of Southern California, Los Angeles, 1974. Passut, C. A.; Danner, R. P. Znd. Eng. Chem. Process Des. Dev. 1972, 11, 543. Rappenecher, K. Z. Phys. Chem. 1910,72,695. Rastorguev, Yu. L.; Keramidi, A. S. Fluid Mech.-Sou. Res. 1974,3, 156. Rastorguev, Yu. L.;Pugach, V. V. Zzu. Vyssh. Ucheb. Zaved., Neft Gar. 1970,13,69. Rastorguev, Yu. L.; Bogatov, G. F.; Grigor’ev, B. A. Khim. Tekhnol. Topl. Masel 1974,9,54. Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids; McGraw-Hill: New York, 1977. Roder, H. M.; Nieto de Castro, C. A. J. Chem. Eng. Datu 1982,27, 12. Rowley, R. L. Chem. Eng. Sci. 1982,37,897. Rowlinson, J. S.; Watson, I. D. Chem. Eng. Sci. 1969,24, 1565. Saji, Y.; Kobayashi, R. Cryogenics 1964,4, 139. Sakiadis, B. C.;Coates, J. AZChE J. 1957,3,121. Silgadro, R. B.; Storroy, J. A. J. SOC.Chem. India 1950,69(8),261. Smith, W. J. S.; Durbin, L. 0.;Kobayashi, R. J . Chem. Eng. Data 1960,5,316. Starling, K. E. Ph.D. Dissertation, Illinois Institute of Technology, Chicago, 1962. Starling, K. E. Fluid Thermodynamic Properties for Light Petroleum Systems; Gulf Houston, TX, 1973.
679
Steele, W. A.; Hanley, H. J. M., Eds. Transport Phenomena in Fluids; Marcel Dekker: New York, 1969;p 209. Stephan, K.; Lucas, K. Viscosity of Dense Fluids; Plenum: New York and London, 1979. Tanaka, Y.;Kubota, H.; Makita, T.; Okazaki, H. J . Chem. Eng. Jpn. 1977,10(2),83. Tham, M. J.; Gubbins, K. E. Ind. Eng. Chem. Fundam. 1969,8,791. Tham, M. J.; Gubbins, K. E. Znd. Eng. Chem. Fundam. 1970,9,63. Thornton, E. Proc. R. Phys. SOC.(London) 1960,76, 104 Touloukian, Y. S.;Liley, P. E.; Saxena, S. C. Thermophysical Properties of Matter; IFI/Pleum: New York-Washington, 1975; VOl. 3. Trautz, M.; Sorg, K. G.Ann. Phys. 1931,10,8190. Trevoy, D. J.; Drickamer, H. G. J. Chem. Phys. 1949,17, 582. Tufeu, R.;Le Neindre, B. Int. J. Thermophys. 1980,1,375. Twu, C.H. Fluid Phase Equilib. 1983,11, 65. Uhlir, A., Jr. J. Chem. Phys. 1952,20, 463. Van Itterbeek, V. A,; Van Paemel, 0. Physica 1941,8(1),133. Vargaftik, N. B. Tables on the Thermophysical Properties of Liquids and Gases; Wiley: New York, 1975. Venart, J. E. S. “Advances in Thermophysical Properties at Extreme Temperature and Pressures”. 3rd ASME Symp. on Thermophys. Properties, ASME, New York, 1965;p 237. Vitagliano, V.; Zagari, A.; Sartorio, R. J. Chem. Eng. Data 1973,18, 370. Yusibova, A. D.; Agaev, N. A. Gazou. Prom. 1969,14(6),46. Ziebland, H.; Needham, D. P. Proceedings of the 4th Symposium on Thermophysical Properties, ASME, New York, 1968;p 296. Zwanzig, R. Ann. Rev. Phys. Chem. 1965,16,67.
Received for review May 18, 1987 Accepted October 2, 1987
Thermal Energy Storage by Agitated Capsules of Phase Change Material. 1. Pilot Scale Experiments Zeki Z. SoZen,+John R. Grace,* and Kenneth L. Pinder Department of Chemical Engineering, University of British Columbia, Vancouver, B.C., Canada V 6 T 1 W5
Segregation due to incongruent melting has severely limited application of Glauber’s salt as a phase change energy storage material. A 96% Glauber’s salt/4% borax mixture was encapsulated in 25-mm-diameter hollow spheres and agitated in a liquid fluidized bed of diameter 0.34 m. The heat storage efficiency was improved by increasing the superficial water velocity and by decreasing the cooling rate. Heating rate had little effect. Fluidization provided enhanced heat transfer to or from the storage medium and resulted in a steady-state heat storage efficiency of about 60% after repeated heating and cooling cycles. The heat storage efficiency decreased to 38% of the theoretical capacity in only seven cycles under fixed bed conditions, but most of the original capacity was recovered within three cycles when these capsules were refluidized. In space heating and domestic hot water systems, it is desirable to store energy as heat (thermal energy) since transformation of energy from one form to another results in a loss of available energy. The simplest way of storing thermal energy is as sensible heat where a material absorbs energy by increasing its temperature without undergoing any change of phase. However, thermal energy storage by phase change energy (latent heat) of a suitable material has the advantage of higher energy density (giving considerably smaller volumes) and relatively isothermal behavior (Telkes, 1975). It is possible in principle to use any reversible change with high heat of absorption or release, but solid-liquid phase changes are generally preferred for space heating and domestic hot water needs. A number of studies (Lane et al., 1975, 1976; Heine and Abhat, 1978; Telkes, 1980; Fellchenfeld and Sarig, 1985) +Presentaddress: Yasar Holding Company, Sehit Fethi Bey Caddesi No. 120,Alsancak 35210, Izmir, Turkey.
0888-5885/88/2627-0679$01.50/0
have been performed to find the most promising phase change materials (PCM)for thermal energy storage (TES). Glauber’s salt (Na2S04.10H20)is one of the most extensively studied phase change materials for solar energy storage because of its low price, suitable phase change temperature, high latent heat, and availability as a suitable nucleating agent (borax) (Telkes, 1952). The major problem associated with Glauber’s salt is segregation due to incongruent melting (Hodgins and Hoffman, 1955). About 15 wt % of the mass of Glauber’s salt forms insoluble anhydrous Na2S04crystals in a saturated Na2S04 solution upon melting. The anhydrous Na2S04crystals precipitate to the bottom because their density of 2680 kg/m3 is considerably larger than that of the saturated solution (Telkes, 1980). During cooling, anhydrous Na2S04 crystals at the top of the precipitate re-form Glauber’s salt crystals, blocking diffusion of water to the inner part of the precipitate and preventing the remaining anhydrous Na2S04crystals from rehydrating. The result is a loss in $3 1988 American Chemical Society
680 Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988
thermal storage efficiency from cycle to cycle. Hodgins and Hoffman (1955) showed that crystallization started upon the surface of a rotating heat-transfer coil inside a vessel containing Glauber’s salk movement of the surface with respect to the saturated solution did nothing to remove the crystal mantle, nor to prevent segregation of Na2S04crystals at the bottom of the tank. Internal mixing of the tank using air or an immiscible fluid was investigated by Chadwick and Sherwood (1981). Fouda et al. (1984) studied the operating characteristics of a heat storage system with 68.2% by weight Glauber’s salt/31.8% by weight H 2 0 mixture in direct contact with varsol as an immiscible heat transfer fluid. Although thermal efficiencies were generally above 85% and often above 90%, the theoretical heat storage capacity was only about 60% of that of a eutectic Glauber’s salt mixture between 20 and 40 OC (Sozen, 1985). There have been a number of claims of stable Glauber’s salt systems over extensive numbers of freezing/melting cycles using encapsulation in discrete cells and/or thickeners (Telkes, 1975,1976; Chahroudi, 1975; Marks, 1980; Page and Swayne, 1981). However, such claims are not necessarily reliable (Marks, 1980; Page and Swayne, 1981). Encapsulation in discrete cells and the use of thickeners seek to prevent gross separation between phases, thus ensuring that all solids including nucleators which participate in the rehydration process are held in “uniform” suspension at all times. During repeated thermal cycling, the heat storage efficiency decreases because of cell rupture by sharp crystals and due to poor nucleation. Herrick and Golibersuch (1977) and Herrick and Zarnoch (1979) used a rotating drum to prevent segregation with good results. However, rotating drums provide a very low surface area per unit storage volume requiring higher heat-transfer fluid temperatures during the heating period and a consequent loss of collection efficiency (Duffie and Beckman, 1980) or a bigger pump or blower, resulting in higher operating costs. Chen and Nelson (1983) pelletized Glauber’s salt in 12-mm-diameter tablets with the addition of 4% fumed silica, 2% clay, and 0.5% calcium stearate by weight to improve compacting, mold release, and coating material adhesion. Water permeation of the encapsulating material was a problem. The coating, 4% asphalt and 16% latex by weight, suffered 4.3% weight loss in air at 40 OC over a period of 69 days. Encapsulation also added substant i d y to the cost. Wood et al. (1981) believe that automatic encapsulation processes, such as cold encapsulation using ambient-temperature-activated polymers, offer potential for bulk encapsulation at reduced cost.
Experimental Heat Storage System Desirable characteristics of a phase change heat storage system include (1)low cost of phase change material, (2) complete phase change with 100% latent heat release, (3) reproducible melting and freezing cycles, (4) high rate of heat transfer between storage material and heat-transfer medium, (5) small temperature difference between storage and heat-transfer media, (6) efficient use of storage volume, (7) low capital cost, (8) low cost of operation and easy maintenance, and (9) safety and lack of toxicity, odor, etc. Encapsulated Glauber’s salt offers most of these characteristics when sufficient external mixing is supplied. Encapsulation increases the heat-transfer area per unit storage volume enormously: A storage unit containing 0.2 m3of Glauber’s salt in 25-mm spherical capsules has about 30 times more heat-transfer area than a cylindrical rotating drum with a length-to-diameter ratio of 2 of the type used by Herrick and Zarnoch (1979). Since the interfacial area
Steon
ieGter
Figure 1. Schematic diagram of experimental equipment. T’s denote thermocouples.
between the anhydrous Na2S04precipitate and the saturated solution is also much larger in capsules compared to a bulk Glauber’s salt storage unit, encapsulation helps to alleviate segregation and should give improved heat recovery. Mechanical mixing of the capsules and their contents is also simpler and more efficient than mixing the entire storage volume. Fluidized beds cause the particles within them to undergo rotation, collision, and vibration. Fluidization should therefore reduce segregation within Glauber’s salt capsules. At the same time, the high surface area of the capsules should give enhanced heat transfer between the storage and heat-transfer media leading to rapid energy recovery or delivery and higher collection efficiencies (Duffie and Beckman, 1980). Hollow polypropylene spheres of 25-mm outside diameter produced by Euro-Matic Company of Denmark were used to encapsulate the phase change material. A fluid mixture consisting of 96% by weight Glauber’s salt and 4% borax (both analytical grade) was injected by using an automatic syringe into 5800 spheres through 2.1-mm-dim e t e r holes, one drilled in each sphere. A 5% by volume air space was left in each sphere to increase the mixing efficiency and to allow for volume changes which accompany freezing and melting. The holes were than sealed by heat on a hot plate. The average capsule density was 1340 kg/m3 with a standard deviation of 17.3 kg/m3 based on a randomly chosen sample of 70 capsules. The polypropylene wall material contributed, on average, 13.8% of the capsule weight and 17.6% of the volume. The capsules were translucent, allowing observation of their contents during experiments. In addition to these 5800 capsules, mixtures of 25%, 15%, and 5% excess sodium sulfate and 10% excess water by weight were each injected into 25 capsules to study the effects of excess water and excess sodium sulfate. To reduce capsule segregation, the air spaces of these 100 capsules were adjusted so that the total capsule mass was the same as that of the vast majority of capsules containing stoichiometric proportions of sodium sulfate and water. The permeability of the wall material was checked over a period of 1year for capsules stored in both air and water. No weight change was observed in either case. A schematic diagram of the experimental system used for thermal cycling of the capsules is given in Figure 1. The main component was a cylindrical PVC fluidization column (equipped with three view ports), with an inside diameter of 0.34 m and a height of 1.37 m from the dis-
Ind. Eng. Chem. Res., Vol. 27, No. 4,1988 681 tributor to the top restraining screen. For most cases the bed height a t minimum fluidization was 0.85 m. The distributor was designed to produce a pressure drop of 125 mmHzO under minimum fluidization conditions and to provide uniform distribution of water into the bed. A closed water recirculation system allowed the superficial water velocity in the bed to be varied without changing the flow rate of heating or cooling water from the constant temperature and head tank. Water was recirculated by a 3.7-kW centrifugal pump. The column and water recirculation system were completely wrapped with 75-mmthick Fiberglas insulation. Water from a temperaturecontrolled 0.5-m3 constant head storage vessel was used for cooling and heating. The water flow rate was measured by a Cole-Palmer paddle-wheel sensor with accumulator. Heating and cooling periods of the column were changed by adjusting the inlet flow rate and temperature. Temperatures were measured at the points shown in Figure 1by copper-constantan thermocouples and monitored by Omega 410 Series digital temperature indicators with 0.1 K sensitivity. Outputs of the inlet and outlet thermocouples were connected through Omega-CJ electronic ice-point compensators to a Servagor Model 220 recorder. The curves were digitized and integrated for the calculation of total heat input to, or output from, the system as well as for the calculation of instantaneous heat-transfer rates. Thermocouples were calibrated against a digital high sensitivity quartz thermometer. The experimental system was designed and calibrated as a calorimeter to measure the heat released or gained by the capsules. Extreme care was taken during calibration to determine the rate of heat loss from the system, the heat input from the pump and the sensible heat capacity of all components inside the energy balance boundary so that the error in the energy storage capacity evaluations for the capsules was less than f0.1%. Full details are given elsewhere (Sozen, 1985). The system was charged with 5656 standard capsules (the balance being kept aside for other experiments described in part 2 of this series) and 100 nonstandard capsules, described above. The total weight of the phase change material inside the 5756 capsules was 51.2 kg. Thermal cycling of the capsules was performed between 20 and 40 "C, and all the evaluations in this study are based on this temperature interval. Experiments were performed under fixed bed conditions in the s p e column with the same capsules simply by decreasing the superficial water velocity in the column below the minimum fluidization velocity. Theoretical Considerations The theoretical heat storage capacity of the contents of the capsules includes the latent heat of phase change for Glauber's salt at 32.4 "C and the sensible heat in solid and liquid phases below and above this temperature: Qth
= zmkCpkl(40 - 32.4) k
+
mGsAGs
+ CmkCpk~(32.4- 20) k
(1)
For components other than Glauber's salt, specific heat capacities at 30 "C were used. The specific heat capacities, total weights of the materials in the 5756 capsules, and energy stored in each component appear in Table I. The total theoretical heat storage capacity for the contents of the capsules is seen to be 14683 kJ, with 84% of this total due to the latent heat of phase change. Calculations were performed to estimate the source and the magnitude of resistances to transfer to or from the heat
Table I. Thermal Properties, Total Component Masses, and Theoretical Energy Stored by Different Materials inside the 5756 Fluidized Capsules" material Glauber's salt
borax (NazB407. 10HzO) extra NazS04 extra water air total
Cpk,
mass. ke kJ/(kpK) 3.307liq 49.045
251.03
1.758 sol. 1.613
N.A.
0.967 4.178 0.996
N.A. N.A. N.A.
2.048 0.100 0.011 0.002 51.21
kJ/ke
heat stored. kJ 1233 (8.4%) 12312 (83.9%)
1.069 (7.3%) 66 (0.4%) 2 (0.01%) 1 (0.006%) 0.04 (0%) 14683 (100%)
'Temperature range, 20-40 OC; thermal properties are from Perry and Chilton (1973),Telkes (1975),and Weast (1976).
storage medium. The unknown mixing and crystallization behavior inside the capsules adds some complexity to these calculations. The external resistance to heat transfer, estimated by considering convective transfer to a single sphere (Zahavi, 1971), is only 0.25 K/W at a superficial velocity of 0.11 m/s. The resistance to heat transfer across the plastic capsule wall is more than an order of magnitude higher at 4.02 K/W. Resistance to heat transfer from inside the capsule depends on the degree of crystallization; based on a simplified symmetrical model which also assumes good mixing of any liquid present in the capsules (Sozen, 1985),the internal resistance was calculated as 0, 1.29, 3.21,6.96, and 22.7 K/W for 0, 25%, 50%)75%, and 100% by volume crystallization of Glauber's salt inside the capsule, respectively. The resistance due to the capsule wall is clearly important, especially at low degrees of crystallization. For real systems, heat transfer between the PCM and water, already high as shown below, could clearly be improved further by using a thinner capsule wall or an encapsulating material of higher thermal conductivity. Experimental Results and Discussion Fluidized Bed Results. The capsules initially fluidized at about the calculated (Grace, 1982) minimum fluidization velocity (58.8 mm/s). Fluidization was uniform throughout the bed volume, with no observable channelling or nonuniformities. It was not possible to observe any capsule segregation for U/Umf< 2.1. For higher velocities, some capsules adhered to the top restraining screen when all 5756 capsules were present in the bed. While there were some gentle inter-particle collisions, back-and-forth rocking, and rotation around vertical axes, full rotation of capsules around a horizontal axis did not occur because of the density gradient inside the capsules. Instead, the air bubble stayed at the top, and there was some observable precipitate at the bottom of the capsules. The amount of precipitate did not appear to increase from cycle to cycle during freezing-melting cycles under fluidized bed conditions in contrast to an observed increase in the precipitate layer under fixed bed conditions. Water inlet and outlet temperatures and heat-transfer rates between the contents of the capsules and the cooling medium (water) are shown in Figure 2 for a typical cooling run. The sharp changes at about 21 min are due to nucleation after some subcooling. The continuous decrease in the heat-transfer rate which follows is caused by an increase in the internal resistance due to solidification coupled with a decrease in the temperature driving force. The average and maximum volumetric heat-transfer rates,
682 Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988
"I
10
20
30 40 TIME (minuter)
so
so
I
I
90 110 130 SUPERFICIAL VELOCITY (mm/s)
70
Figure 2. Inlet and outlet temperatures and capsule-to-water heat-transfer rate for a typical fluidized bed cooling run.
150
Figure 4. Percent theoretical heat recovery from the capsules aa a function of superficial fluidizing velocity. Temperature range, 40-20 "C; number of capsules, 5756 and 4328 for diamonds and triangles, respectively.
-I-
\\
1
lo 10
20
40 TIME ( m i n u t e s )
30
50
60
Figure 3. Inlet and outlet temperatures and water-to-capsule heat-transfer rate for a typical fluidized bed heating run.
based on the inside volume of the capsules, are 60 and 181 kW/m3, respectively. Temperatures and heat-transfer rates for a typical heating run are plotted in Figure 3. If the temperature change due to heat input from the pump was subtracted from the output curve, the two temperature curves would not cross. The early increase in heabtransfer rate is caused by the initial increase in the temperature difference between the water and PCM. The subsequent nearly constant rate suggests that melting is proceeding. Once melting begins, salt crystals maintain a nearly constant temperature, while the outlet water temperature rises slowly due to the heat release. As the crystallized PCM melts, the path for heat transfer becomes greater inside the capsule, compensating for the increase in heat transfer which would be expected from the increased temperature difference. The subsequent decrease in heat-transfer rate after about 40 min is due to the decrease in the temperature difference between the water and PCM after melting is complete. The average and maximum volumetric heat-transfer rates are 63 and 91 kW/m3, respectively. The average rates during the cooling and heating runs are such that the energy can be discharged or charged in about 65 min with inlet and outlet temperatures as in Figures 2 and 3. High heat-transfer rates are an important advantage of the fluidized storage system, allowing solar collectors to work more efficiently and permitting rapid energy charging or discharge. The heat storage efficiency of the capsules in the fluidized bed during the first heating run after filling the capsules was 90.9% of the theoretical heat storage capacity. The efficiency decreased over the first three heating and cooling cycles to about 60% of that theoretically possible and then remained almost unchanged over the next 93
0
io
do 90 COOLING PERIOD (minuter)
120
1 0
Figure 5. Percent theoretical heat gain by capsules as a function of the duration of cooling run. Temperature range, 20-40 "C; U = 96 mm/s; number of capsules, 5756.
cycles under fluidization conditions (before any fixed bed tests). It was found that the normal heat release efficiency was always 4-5% less than the heat gain efficiency. Crystallization between the end of a cooling run and the beginning of a heating run and temperature gradients inside the capsules are believed to account for this apparent difference. The superficial velocity in the column and the times of heating and cooling were the independent variables for the experiments under fluidized bed conditions. Since the storage capacity of the first cycle under new conditions may be affected by the conditions of the previous cycle, experimental heat storage capacities reported in this study were evaluated from the second cycle for each set of conditions. The performance of the system was found to be improved by increasing the superficial water velocity as shown in Figure 4. The slight decrease in efficiency above U = 123 mm/s for all 5756 capsules present is believed to be due to immobilized capsules adhering to the top restraining screen. To allow more space for bed expansion, about 25% of the capsules were extracted, leaving 4328 regular capsules in the bed. The superficial velocity was then varied between 130 and 153 mm/s. As shown in Figure 4 by triangular symbols, the efficiencies then increased, but for U > 145 mm/s, the expanded bed again filled the available space, causing another decrease in efficiency. The increase in efficiency (prior to capsule immobilization) is more gradual at higher velocities, probably due to a reduction in inter-capsule collisions (and hence
Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988 683
r
I
1,
I 50 75 100 HEATING PERIOD (minutes)
25
125
Figure 6. Percent theoretical heat recovery from the capsules as a function of the duration of heating run. Temperature range, 40-20 "C; U = 96 mm/s; number of capsules, 5756.
I
- -1
Fluidized
Bed Conditions
-Fixed
1
3
1
'
6 9 12 NUMBER OF CYCLES
I
Bed d Conditions
15
Figure 7. Decline and recovery of heat storage capacity resulting from melting/ freezing cycles under fixed bed and refluidization conditions. U = 52 and 123 mm/s, respectively; number of capsules, 5656.
in intra-capsule mixing) resulting from increased bed expansion. The density gradient inside the capsules appears to have prevented full capsule rotation, even at the highest superficial velocities investigated. Hence, complete suspension of the anhydrous sodium sulfate crystals in the solution wm never obtained under fluidized bed conditions. A significant improvement in heat release resulted from increasing the cooling period as shown in Figure 5. A longer cooling period allows more time for diffusion of water into the anhydrous sodium sulfate precipitate. The degree of subcooling, crystal size, and loss of efficiency due to microencapsulation of anhydrous sodium sulfate crystals (see part 2) are also expected to be influenced by the cooling period. Results of varying the heating period between 36 and 121 min appear in Figure 6. In this case, there was little or no change in the heat recovery efficiency. Heating rate is expected to have no effect on the subsequent crystallization from congruent melting mixtures. Fixed Bed and Refluidization Results. The capsules showed a considerable decrease in their heat recovery during the first seven cycles under fixed bed conditions as indicated by the circles in Figure 7. After seven cycles the heat recovery efficiency of the capsules in the fixed bed between 40 and 20 "C had dropped from 57% to 38% of the theoretical heat storage capacity, and the performance continued to decrease with further cycling. Although the loss of heat storage capacity during thermal cycling under fixed bed conditions is a well-known disadvantage of Glauber's salt and many studies have concentrated on solving this problem as discussed above, there are few
studies which report the loss quantitatively as a function of the number of thermal cycles. Hodgins and Hoffman (1955) reported that the heat storage capacity of their Glauber's salt system (a shallow tray) fell to as low as 16.5% of the theoretical capacity. For a pure Glauber's salt system, about 16% of the theoretical heat storage capacity between 20 and 40 "C is due to the sensible heat of the material, the remaining 84% being due to the latent heat of phase change. The Hodgins and Hoffman result indicates that Glauber's salt in a fixed tank probably undergoes little or no phase change after repeated thermal cycles. In our case, the contact area per unit volume between the anhydrous sodium sulfate precipitate and the sodium sulfate solution is larger than that in a bulk-filled tank because of encapsulation. Therefore, it is reasonable to expect somewhat higher heat storage capacities even under fixed bed conditions. This is an advantage of encapsulation, but the recorded efficiencies are still too low for economical operation. When the capsules which had been cycled under fixed bed conditions were refluidized at a superficial velocity of 123 mm/s, about 97.5% of the initial heat storage capacity available before the fixed bed cycles was recovered within three cycles as shown by the triangles in Figure 7. This rapid recovery contrasts with fixed bed and thickened Glauber's salt systems in which the salt has to be replaced (usually together with its container) after a number of cycles due to the unrecoverable loss of heat storage capacity. This result also shows the possibility of intermittent fluidization for heat storage applications where continuous fluidization is not economically viable. The ability to recover lost heat storage capacity also makes the use of encapsulated Glauber's salt more practical since capsules do not have to be maintained above 32.4 "C when transported, the system can be turned off when not needed, and special precautions for power breakdowns, malfunctioning of the pump, etc., are not required.
Conclusion A novel system is described in which Glauber's salt is encapsulated in 25-mm-diameter hollow spheres and agitated when fluidized by the heat transfer medium water. This arrangement led to improvements over fixed bed thermal energy storage systems. In addition to providing a high rate of heat transfer to or from the heat storage medium, the fluidized capsules showed reduced loss of efficiency due to cycling, maintaining nearly 60% efficiency over 96 cycles. Under fixed bed conditions, the heat storage capacity of the same capsules dropped rapidly to about 38%. Almost all of the fluidized bed heat storage capacity was recovered within three cycles when these capsules were refluidized after fixed bed cycling. The performance of the system was improved by increasing the superficial water velocity and by decreasing the cooling rate. The heating period had little influence on the performance. While fluidization of encapsulated Glauber's salt has been shown to improve the feasibility of phase change materials for thermal energy storage, further improvements (see part 2) are clearly possible since the experimental storage never exceeded about 60% of the theoretical capacity after repeated cycling. Acknowledgment Financial assistance from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
I n d . Eng. Chem. Res. 1988, 27, 684-691
684
Nomenclature C, = specific heat capacity, kJ/(kgK) C,k, = specific heat capacity of component “k” inside the capsules in the liquid phase, kJ/(kgK) Cpk8= specific heat capacity of component “k” inside the capsules in the solid phase, kJ/(kgK) mGS= mass of Glauber’s salt undergoing phase change inside the capsules, kg mk = mass of component “k” inside the capsules, kg Qth = theoretical heat storage capacity of the contents of the capsules in the column, k J U = superficial velocity of water in the column U,, = minimum fluidization velocity Greek Symbol XGS = latent heat of phase change for Glauber’s salt, kJ/kg
Registry No. Glauber’s salt, 7727-73-3; borax, 1303-96-4; polypropylene, 9003-07-0.
Literature Cited Chadwick, D. G.; Sherwood, K. H. Report UWRL/P-81/05, 1981; NTIS, Washington, D.C. Chahroudi, D. In Proceedings Workshop on Solar Energy Storage Subsystems for Heating and Cooling of Buildings; Wiley: New York, 1975; p 56. Chen, J.; Nelson, R. Report ORNL/TM-8543, 1983; NTIS, Washington, D.C. Duffie. J. A.: Beckman. W. A. Solar Engineering - of , Thermal Processes; Wiley: New York, 1980. Fellchenfeld, H.; Swig, S. Znd. Eng. Chem. Prod. Res. Deu. 1985,24, 130. Fouda, A. E.; Despaul, J. G.; Taylor, J. B.; Capes, C. E. Sol. Energy 1984, 32, 57. Grace, J. R. In Handbook of Multiphase Systems; Hetsroni, G., Ed.; Hemisphere: Washington, D.C. 1982; Chapter 8.1.
Heine, D.; Abhat, A. In Proceedings of International Solar Energy Society Congress; New Delhi, India, 1978; Vol. I, p 500. Herrick, C. S.; Golibersuch, D. C. Information Series Report 77CRD006, 1977; General Electric Company, Schenectady, NY. Herrick, C. S.; Zarnoch, K. P. Technical Information Series, Report No. 79CRD249, 1979; General Electric Company, Schenectady, NY. Hodgins, J. W.; Hoffman, T. W. Can. J. Technol. 1955, 33, 293. Lane, G. A.; Best, J. S.; Clarke, E. C.; Glew, D. N. Report NSF/ RANN/RE/C-906/FR/76/1, 1976; NTIS, Washington, D.C. Lane, G. A,; Glew, D. N.; Clarke, E. C.; Rossow, H. E. In Proceedings of Workshop on Solar Energy Storage Subsystems for Heating and Cooling of Buildings; Wiley: New York, 1975; p 43. Marks, S. B. In Proceedings of 25th Intersociety Energy Conuersion Engineering Conference; Wiley: New York 1980; Vol. 1, p 259. Page, J. K. R.; Swayne, R. E. H. In Proceedings of International Conference on Energy Storage; Wiley: New York, 1981; Vol. 1, 165. Perry, R. H.; Chilton, C. H. Chemical Engineer’s Handbook, 5th ed.; McGraw-Hill: New York, 1973. Sozen, Z. Z. Ph.D. Dissertation, University of British Columbia, Vancouver, 1985. Telkes, M. Znd. Eng. Chem. 1952,44, 1308. Telkes, M. In Proceedings of Workshop on Solar Energy Storage Subsystems for Heating and Cooling of Buildings; Wiley: New York, 1975; p 17. Telkes, M. U S . Patent 3 986 969, 1976. Telkes, M. In Solar Materials Science; Murr, L. E., Ed.; Academic: New York, 1980; Chapters 11 and 12. Weast, R. C. CRC Handbook of Chemistry and Physics; CRC: Cleveland, 1976. Wood, R. J.; Gladwell, S. D.; O’Callaghan, P. W.; Probert, S. D. In Proceedings of International Conference on Energy Storage; Wiley: New York, 1981; Vol. I, pp. 145-158. Zahavi, E. Int. J . Heat Mass Transfer 1971, 44, 835.
Received for review April 2, 1986 Revised manuscript received October 6 , 1987 Accepted October 21, 1987
Thermal Energy Storage by Agitated Capsules of Phase Change Material. 2. Causes of Efficiency Loss Zeki Z. Sozen,? John R. Grace,* and Kenneth L. Pinder Department of Chemical Engineering, University of British Columbia, Vancouver, B.C., Canada V 6 T 1 W 5
In an effort to further improve the heat storage efficiency and to determine which factors contribute most to losses in efficiency, capsules were tested in a rotating tube and in a rotating drum a t different speeds. Full capsule rotation around a horizontal axis improved the storage efficiency over the efficiencies achieved in part 1. Impacts resulting from motion in the drum gave small additional improvements in the efficiencies over those realized in the rotating tube, with a maximum storage efficiency of 83% for a stoichiometric Glauber’s salt mixture with 4% borax. At high rotation speeds, centrifugal forces had a negative effect. Heat storage efficiencies increased with addition of excess Na2S04and also with excess H 2 0 compared to eutectic salt mixtures. On a storage per unit volume basis, 4743% by weight Na2S04was found to be the optimum concentration, the corresponding optimum on a per unit mass basis being 44-47% by weight Na2S04. Segregation was found to account for less than half of the loss in heat storage capacity, the remainder being caused by subcooling and microencapsulation. Strong agitation and the presence of borax reduced the thermal efficiency loss due to subcooling. Part 1 showed that Glauber’s salt, encapsulated in 25mm-diameter hollow spheres and agitated by water in a 0.34-m-diameter liquid fluidized bed, is a promising thermal energy storage system. The heat-transfer area, increased enormously by encapsulation, led to an average heat-transfer delivery or recovery rate of about 60 kW/m3 (based on the inside volume of the capsules) between 20 and 40 “C. Fluidization also provided uniform gentle Present address: Yasar Holding Company, Sehit Fethi Bey, Caddesi No. 120, Alsancak 35210, Izmir, Turkey. 0888-5885/88/2627-0684$01.50/0
agitation of the capsules which led to much less loss in heat storage efficiency than under fixed bed conditions. On the other hand, the heat storage capacity of the phase change material (96 wt% Glauber’s salt and 4 wt % borax) in the fluidized bed system was about 60% of the theoretical value, leaving a considerable margin for further improvement. density gradients inside the capsules prevented full capsule rotation around horizontal axes through their centers, even at high superficial velocities. Therefore, complete suspension of the anhydrous sodium sulfate crystals was never obtained under fluidized bed conditions 0 1988 American Chemical Society
