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Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 130-133
Calcium Chloride Hexahydrate: A Phase-Changing Material for Energy Storage Hans Fellchenfeld' and Sara Sarlg Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, 9 1904 Jerusalem, Israel
Salt hydrates, in particular sodium sulfate decahydrate and calcium chloride hexahydrate, are promising materials for the storage of solar heat. Their tendency to degenerate to lower hydrates can be overcome by the addition of nucleators and thickening agents. Such systems have performed for over 1000 cycles of cooling and heating under laboratory conditions without any signs of deterioration. They are becoming increasingly available to the general public. Some applications are passive and active climate control for residences and hothouses. solar assisted heat pumps, and heat recovery.
Background Energy storage subsystems are essential parts of devices for direct use of solar radiation and for energy conservation by heat recovery from low-temperature sources. As energy storage in the form of sensible heat necessitates large volumes, it is preferable to use the latent heat capability of phase change. The technological problems of liquidvapor transitions render such systems too cumbersome for practical purposes. On the other hand, materials with solid-liquid phase change at appropriate temperatures have been extensively investigated for the development of storage systems. The most common system of this type is composed of ice and water with a melting point below the ambient and has been in use for millenia. Ice is cut from lakes during winter and stored underground in cellars for the hot days of summer. This is an example of long term storage to level out seasonal variations. Most applications nowadays try to attenuate diurnal changes. Phase-Changing Materials Though the lists of candidate materials compiled by leading research groups include many attractive organic materials, very few of them (if any) have passed the barrier of technological and economical criteria. Since the cheapest inorganic substance, water, has too low a melting point for positive heat storage, it is natural to look at salt hydrates. These compounds are more expensive and have lower heats of melting, but they retain many of the attractive features of water. In fact, most prototypes and advertised commercial products utilize inorganic salt hydrates. It is interesting to note that the first formulation of a salt hydrate, sodium sulfate decahydrate, with its nucleating agent, borax, was advocated for heat storage already in 1952 by Maria Telkes. However, many years of tenacious research were needed to perfect a system six to eight times more compact than a tank of water of equivalent thermal content. Storage systems of this kind are particularly attractive because they can be incorporated not only into newly constructed buildings, but also into a vast number of existing houses. The technological criteria for phase-changing materials (PCM) to be used in storage systems are obvious: suitable temperature of phase transition, congruent melting and solidification, nontoxicity, limited corrosivity and dilatibility. The economic criterion, availability at low cost, restricts the number of desirable salt hydrates to only a few. The economically favored candidates do not fulfill all the technological requirements. 0196-4321/85/1224-0130$01.50/0
Difficulties in the Use of Salt Hydrates Overcooling is very common with materials of high solubility. It can be counteracted by rough surfaces, the introduction of seed crystals, violent motion, or nucleating agents. The latter approach has been the most fruitful in PCM application. A specific nucleator should have a structure similar to that of the crystal it is to nucleate. Stratification on repeated melting and crystallization is a vexing problem common to all pure salt hydrates. The cause of this phenomenon is easily understood. Salts which crystallize as hydrates also possess modifications of lower hydrates and/or of the anhydrous salt. Unfortunately, the liquid with the composition of the desired salt hydrate is, in general, not in equilibrium with the same solid salt hydrate at its melting point but rather with a lower solid hydrate. This incongruent melting leads to more and more severe stratification when equilibrium conditions prevail during crystallization. The undesired lower hydrate will fall to the bottom of the container and will not convert to the higher hydrate even if conditions change and the higher hydrate becomes thermodynamically the stable modification. On remelting, the top layer will now be poorer and the bottom richer in salt than the original melt. At the next cycle of cooling, the bottom layer will solidify a t a lower temperature and a still lower hydrate or the anhydride may crystallize out. Though diffusion will attenuate this phenomenon, the bottom layer will, after many cycles, approach the composition of the eutecticum salt/water and the top will become depleted of salt. Experimentally this stratification shows up as a decrease in the heat absorbed on melting and released on crystallization, and in the spreading of the enthalpy peak over a wide range of temperature. The suggested remedies include: addition of another salt to make the mixture congruent, addition of thickening agents in order to avoid the movement of crystals toward the bottom, addition of such nucleating agents which favor the crystallization of the desired hydrate only, and thorough mixing. Formation of large crystals is caused by a slow rate of crystallization when temperature changes are only small. This tendency is undesirable mainly for three reasons: the rate of remelting is lowered, large crystals may damage the containing wall, and slow crystallization tends to exclude the additives. Calcium Chloride Hexahydrate Calcium chloride hexahydrate with its nucleating agent, strontium chloride hexahydrate, fulfils most of the economic and technological requirements. It is available from 0 1985 American
Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985 131
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Figure 1. Apparatus for stability test.
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Figure 2. Thermal cycle of stability test for calcium chloride hexahydrate.
the Dead Sea and some other salt lakes and as a byproduct from chemical industry. The potential use of calcium chloride at competitive prices in technological devices prompted fundamental studies of this compound. Carlsson et al. (1979) reexamined the phase diagram of CaCl2/H2O and the influence of small quantities of added SrClz thereon in order to define compositions and temperature ranges which would ensure the avoidance of the formation of calcium chloride tetrahydrate. Stability Tests. As the stability of a storage system is the most important criterion of PCM performance, the influence of thermal cycling on the heat of fusion was studied. The experimental setup simulated the conditions to which the storage salt was to be subjected. The sample blend consisted of calcium chloride hexahydrate with an admixture of the nucleating agent, strontium chloride hexahydrate, and silica gel as the thickening agent. The sample (ca. 2 g) was enclosed in a hermetically sealed glass tube. It was alternatively heated and cooled over a temperature range from 18 to 38 "C or occasionally in the range from 15 to 45 "C. The simple experimental system which was placed in a cold room is shown in Figure 1. The thermistor formed one arm of a Wheatstone bridge, the unbalance of which was fed to the analog input of a minicomputer (IBM Series/l). The computer converted the voltage to a temperature reading at time intervals of 1min. When the set maximum temperature was reached, the computer switched the heating circuit off. The printout tabulated temperature and time. A plot of temperature vs. time was obtained from the analog output of the computer on an XY recorder and is shown in Figure 2. Thus the whole system (water bath and sample) first cooled down spontaneously in the cold room and was then heated to the set upper temperature. The rate of heating depended on the balance between the water content of the bath and the energy input. It was adjusted for a 30-min melting period. The rate of cooling before and after crystallization was dependent on the quantity of water and the temperature of the cold room. The time required for crystallization was determined by the rates of crystal
growth and of heat transfer through the solid phase. Care was taken to ensure total melting before the start of the cooling, and total solidification before the start of heating. The average duration of a whole melting-solidification cycle was 3-4 h. The influence of the addition of 1to 5 % KC1, NaC1, or LiCl on the temperature of melting and crystallization and on the stability of the system was also tested. A typical curve of a thermal cycle is shown in Figure 2. The rate of natural cooling (part A of the curve) was 0.33 "C/min. In the absence of any thermal event, the cooling would presumably continue along branch A'. The crystallization process released thermal energy which kept the sample between 28.7 and 27.0 "C for about 48 min even though the water bath continued to cool down. After the completion of crystallization, cooling of the solid sample continued along curve C'. On reaching the lower set temperature, the computer-monitored heating started, as represented by branch H. The rate was about 0.5 "C/min. At 28 "C, melting started, absorbing most of the energy and allowing only a slight increase in the temperature of the sample. After completion of melting, the temperature rose much faster (branch H") than the extrapolstion from the initial heating rate (brach H') would suggest. This was evidently due to heat transfer being faster through the melt than through the solid phase. The undercooling of ca. 1.8" at about 28 "C is characteristic of the tested system. The shape of the crystallization curves remained remarkably constant during six months through 1000 cycles. In fact, curves with equal undercooling were identical; the 1000th cycle could be substituted for the 12th or 200th cycle. The reproducibility of the thermal characteristics as well as the visual observation of the recurrent melting and solidification of the whole sample indicated that the latent heats did not change. However, this particular experimental arrangement, though suitable for stability determination, was not acceptable for latent heat measurements because of uncontrolled heat dissipation in the cold room. Measurements of the Latent Heat of Melting. Cantor (1978) checked the thermal behavior of sodium sdfate decahydrate with a differential scanning calorimeter (DSC). His measurements showed a decrease in heat of fusion with thermal cycling and also considerable undercooling. The addition of the nucleating agent reduced the undercooling but did not eliminate it entirely. In Figure 3 the thermograms of crystallization and of melting of calcium chloride hexahydrate determined by us with a Mettler DSC are shown. It will be noted that the liquid undercooled to -16.8 "C before crystallization, and melting started at around 0 "C. Integration of the area under the curves showed that on crystallization, instead of the expected 209 J/g, only 86 J / g was released and on melting 71 J/g was absorbed. It is clear that a considerable amount of lower hydrates had also crystallized out on cooling. Considering the successful application of sodium sulfate decahydrate and calcium chloride hexahydrate in prototypes of storage devices and experimental houses, the failure to establish its stability in DSC measurements made the method of measurement suspect. This point was brought home by Abhat and Malatidis (1981). They confirmed that experiments carried out in apparatus remote from practical latent heat storage designs, especially with respect to hermetic closing, yielded nonrelevant results of undercooling and thermal stability. In DSC measurementsthe rate of temperature change is forced and too fast, and the sample is too small, to correlate well with conditions encountered in practice. Furthermore, the
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985 TEMPERATURE
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Heat f l o w exothermal - - 50 000 mW
TEMPERATURE
Heat f l o w exothermal 2 0000 mW
O C
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Figure 3. Differential scanning calorimetry: calcium chloride hexahydrate without additives: (A) crystallization; (B) melting. 7
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r
r
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i I
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25
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"C A- Thermocouple B-Stirrer C-Sample ' container D-Dewar W-Water
Figure 5. Net heat input as a function of temperature of melting of calcium chloride hexahydrate: (A) before deterioration; (B) after deterioration. 36 -
Figure 4. Experimental setup for the determination of the latent heat of melting of calcium chloride hexahydrate.
temperature sensor is outside the sample container and is therefore influenced by the different heat transfer coefficients of solid and liquid. The extremely small samples used in DSC measurements (about 10 mg) may also have adverse effects because of the nonhomogeneity of the salt hydrate/nucleating agents blend. To obtain more reliable data, we carried out latent heat determinations with large samples (typically 200 g) of calcium chloride hexahydrate. The sample container with the solid sample was submerged in a weighed quantity of water within a Dewar. The water was heated by passing a measured stabilized electric current through a resistance coil. The water had to be well mixed to ensure fast heat transfer. As magnetic stirrers introduced, by induction, errors in the measurement of the thermocouples, the sample vessel was built in an annular form and a mechanical stirrer was passed through the axial channel. This design also decreased the path which heat had to travel in order to reach the working fluid, and thereby improved the time constant of heat transfer without appreciably reducing the mass of the sample. The design of the apparatus is shown in Figure 4. The thermocouples were connected to the analog input board of the minicomputer which calculated the difference between the electric energy input and that needed to raise the temperature of the water (+ water equivalent) in the Dewar. This value H was plotted against the average temperature T of the sample. The straight parts of the curve, before the occurrence and after the completion of melting, were both extrapolated to 29.3 "C. The heat of
t-
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Wt. %CaCI2
Figure 6. Phase diagram of calcium chloride/water.
melting was calculated by dividing the difference of H values at this temperature by the weight of the sample. This method gave consistent results with different heating currents. The extrapolation from the straight lines before and after melting corrected for errors due to uncontrolled heat losses. The time constants of the immersed sensors were many orders of magnitude faster than the rate of change of temperature. One run took typically 4-6 h. In Figure 5, curve A shows melting of calcium chloride hexahydrate before, and curve B after deterioration occurred. Some of our results are summarized in Table I. In these experiments strontium chloride hexahydrate crystals were present, but no thickening agent. It will be noted that the maximum heat of melting was obtained with a CaC1$H20 ratio of 1:6 exactly. The values of the heat of melting were a little higher than those published in the literature. Possibly the latent heat determinations were carried out on samples with diverse thermal histories. Additives. The high heat of melting of calcium chloride hexahydrate is obtained only in the first few cycles of melting and solidification. After several cycles the efficiency decreased due to the formation of calcium chloride tetrahydrate and the ensuing stratification discussed above. This undesirable process could be avoided by the addition
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985 133 Table I. Heat of Melting of Calcium Chloride Hexahydrate run heat of melting, ratio no. J / a of samde CaCl,:H,O 190 1:6.1 101 185 L6.1 102 188 1:6.1 103 182 1:6.1 104 222 111 1:6.0 112 213 1:6.0 113 212 1:6.0 114 205 1:6.0 115 210 1:6.0 121 194 1:5.7 122 189 1:5.7 180 1:5.7 123
of a suitable nucleating agent. This can be easily understood by reference to the phase diagram calcium chloride hexahydrate/water (Figure 6). The vertical lime at 50.66% represents the composition of the hexahydrate. The area above the hexahydrate and tetrahydrate curves represents the region in which only solution exists. It is therefore apparent that on cooling a solution with the composition of the hexahydrate to about 31.5 "C, the tetrahydrate may crystallize out. However, unless seed crystals are present, this will rarely happen, and the liquid is easily undercooled to below 30 "C, to a region in which the hexahydrate is the stable form. The purpose in adding specific nucleators is to ensure that in the region of stable hexahydrate, the hexahydrate will in fact crystallize out and not an unstable modification like the tetrahydrate. The suggested nucleators for calcium chloride hexahydrate are strontium and barium salts which form in the environment sparingly soluble hydrates with dimensions similar to those of calcium chloride hexahydrate. The thickening agent is added in order to increase the viscosity of the solution. This slows the rate of growth of existing crystals and makes for a larger number of small crystals. Furthermore, it keeps the crystals localized so that even if by mischance the undesired tetrahydrate were formed, it would, on heating, remelt at approximately its original position and not cause stratification. Rate of Temperature Change. Nucleating agents and even seed crystals of calcium chloride hexahydrate, which prevented degeneration at conditions of gradual heating and cooling, were not able to do so when the material was subjected to severe thermal shock. The almost instantaneous degeneration was demonstrated when the material was melted outside the Dewar and then introduced into the Dewar containing ice-cold water (in order to measure the heat of crystallization). A lower hydrate crystallized out. We suggest that in this case Ostwald's rule, requiring the unstable modification to crystallize out first, was followed. In the other extreme case, after crystallization between 29 and 27 "C took 70 h or more, a significant decrease in the heat of melting was encountered. We suggest that
under these conditions large, pure crystals were formed which counteracted, by exclusion, the effect of additives. In storage devices coupled with solar energy systems very slow cooling may sometimes occur. It is therefore fortunate that a degenerate blend of calcium chloride-water-nucleating agent can be regenerated by remelting and mixing, and thus be restored to its original heat storage capacity. Ratio of Calcium Chloride/Water. Inspection of the phase diagram (Figure 6) may suggest that stratification can be avoided by the addition of water to calcium chloride hexahydrate until the composition of the peritectic point is reached where the tetrahydrate curve cuts the hexahydrate curve at 6.12 mol of H,O/mol of CaC12 However, when crystals form in a stagnant solution, they fall to the bottom. Thus the composition of the bottom layer will tend toward the molar ratio 6:1, irrespective of the overall composition. Since the overall ratio is 6.12:1, the solution at the top becomes even more dilute. It is therefore best to chose a ratio as near as possible to 6:l.
Thermal Energy Storage Materials It must be obvious by now that pure salt hydrates are useless as heat storage materials. Indeed, for development and commercial use, blends are offered in packages which are carefully formulated and engineered to avoid stratification. Most of them are based on Glauber salt or on calcium chloride hexahydrate. Their phase-change temperatures are very similar. They can be adjusted downward for a few degrees by the addition of other salts. If another range of temperatures is desired, a different salt hydrate must be used. We had some success with calcium bromide hexahydrate, which melts around 38 "C but which is expensive. Applications of these materials can be found in climate control both for residences and for greenhouses in conjunction with solar heaters and heat pumps. It is hoped that the use of such systems will become widespread and contribute to the conservation of fossil fuel. Registry No. CaC1,.6H20, 7774-34-7; SrC12.6Hz0,10025-70-4; KC1, 7447-40-7; NaC1, 7647-14-5; LiC1, 7447-41-8.
Literature Cited Abhat, A.; Malatidis, N. A. Conference on New Energy Conservation, Technologies and their Commercialization, IEA Berlin, 1981. Cantor, S . Thermochim. Acta 1878, 26, 39. Carlsson, B.; Stymme, H.; Wettermark, G. Sol. Energy 1878, 23, 343. Feilchenfeld, H.; Fuchs, J.; Kahana, F.; Sarig, S. Isr. J. Chem. 1882, 22, 269. Feilchenfeld, H.; Fuchs, J.; Sarig S. Sol. Energy 1884, in press. Haber, 8. International Congress on Energy for Small and Medium Sized Countries, Tel-Aviv, Israel, 1984. Marano, R. T. Thermochim. Acta 1878, 26, 27. Sarig, S. Israel Patent 59103 (pending), 1980. Telkes, M. I n d . Eng. Chem. 1952, 4 4 , 1308. Telkes, M. IECEC Meeting, Unlversity of Delaware, 1975. Williams, R. H. "Towards a Solar Civilization"; MIT Press: Cambridge, MA, 1978.
Received for review July 31, 1984 Accepted October 12, 1984
