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Heat storage capacity of sodium acetate trihydrate during thermal cycling
Solar Energy Vol 33, No. 3/4, pp. 373-375, 1984 Printed in the U.S.A.
0038-092X/84 $3.00 + .00 © 1984 Pergamon Press Ltd.
TECHNICAL NOTE Heat storage capacity of sodium acetate trihydrate during thermal cycling TAKAHIRO W A D A , RYOICHI YAMAMOTO a n d YOSHIHIRO MATSUO
Central Research Laboratory, Matsushita Electric Industrial Co., Ltd., Kadoma, Osaka 571, Japan
(Received 29 July 1983: accepted 28 June 1984) 1.
INTRODUCTION
Sodium acetate trihydrate (CH3CO2Na.3H20) has recently attracted attention as a useful heat storage material because of its large latent heat of fusion (about 264 J g-~) [1, 2]. However, this material melts incongruently and severely supercools [3, 4]. Wada and Yamamoto [5] have shown that the addition of Na4P207" 10HzO as a nucleation catalyst to CH3COzNa. 3H20 has solved the latter problem satisfactorily. The problems caused by incongruent melting of CH3COzNa.3H20 are subtle and difficult to solve. Telkes [6, 7] studied Glauber's salt (NazSO4.10H20) for latent heat storage and found that addition of a thickening agent such as attapulgite clay was effective for homogeneously suspending the Na2SO4 particles in the melt and prevented Na2SO4.10H20 from forming a metastable condition where the undissolved Na2SO4 particles coexisted with newly formed Na2SO4.10H20 crystals and excess solution. Marks [8, 9] measured the heat storage capacity of Na2SO4.10H20 thickened by using attapulgite clay during thermal cycling and showed that the addition of attapulgite clay is useful for solving the problems caused by incongruent melting. In order to investigate the decreasing heat storage capacity of CH3CO2Na.3H20 during thermal cycling, we performed calorimetric measurements on three kinds of samples. Sample a comprised guaranteed grade CH3CO2Na-3H20, sample b comprised technical grade CH3CO2Na.3H~O and sample c comprised technical grade CH3CO2Na.3H20 thickened by using polyvinyl alcohol. All three samples also comprised Na4P2OT" 10H20 as a nucleation catalyst for CH3CO2Na.3H20. Aging tests show that impurities contained in technical grade CH3COzNa-3H20 prevent anhydrous CH3CO2Na particles in the melt from settling on the bottom of the container and so, the heat storage capacity of sample b decreased more slowly than that of sample a. The performance of thickened sample c showed little worsening after 500 thermal cycles.
kept for 60 rain at both 70 and 40°C. At various times the sample was taken out of the water bath and calorimetric measurements were performed on it. Heat storage capacity was measured by the standard calorimetric technique in which the heat evolved by the sample is equated to the heat absorbed by the calorimeter's water. A molten sample whose mass and temperature were known was quickly placed into water whose temperature was recorded as a function of time. Prior to the measurement, the sample was brought to a uniform temperature above its melting point, about 60°C. Calorimetric measurements provide a precise and direct measure of the heat storage capacity of the sample. The total heat stored is the sum of sensible heat and latent heat of fusion. The latent heat storage capacity was computed by subtracting the contribution of the specific heat of CH3COzNa.3H20 (Cp ( c r y s t ) = 1.7 J g i K l, Cp Imelt) = 2.9 J g ~ K -~) from the measured heat storage capacity. It was assumed that only CH3CO2Na'3H20 contributed to the heat storage capacity measured because the heat storage capacity of the nucleation catalyst and the thickener was very small compared to that of CH3CO:Na.3HzO. 3. RESULTS AND DISCUSSION The decrease in latent heat storage capacity of sample a with an increase in the number of thermal cycles is shown in Fig. 1. In this figure, AHi is the latent heat of fusion of CH3CO2Na.3H:O. The latent heat storage capacity of this sample is initially 254 J g i; after 30 cycles it declines to 200 J g '; and after 400 cycles it declines to 1 6 0 J g I. The decrease in latent heat capacity of sample b with an increase in the number of thermal cycles is shown in Fig. 2. The latent heat storage capacity of this sample is initially 259 J g - J : after 30 cycles it declines to 235 J g - ~; and after 400 cycles it declines to 200 J g 1 300
2. EXPERIMENTAL Guaranteed grade CH3CO2Na.3H20, Na4P207"IOH20, polyvinyl alcohol (degree of polymerization, n ~ 500), acetone and liquid paraffin were obtained from Wako Pure Chemical Industries. Technical grade CH3CO2Na.3H~O was obtained from Nippongoseikagaku Co., Ltd. The thickened mixture was prepared by mixing technical grade CH3CO2Na.3H20 with polyvinyl alcohol, acetone, liquid paraffin and water at 70°C for 30 min. Table 1 shows the initial composition of the three kinds of samples. About 80 g of CH3CO~Na.3H20 or thickened mixture and 0.8 g of Na4P2OT.10H20 were placed in a stainless steel vessel, 30 mm inner diameter, 100 mm high and 1 mm wall thickness. This vessel was sealed and put into a water bath. The sample was consecutively heated and cooled at the rate of 5°C min 1 between 70 and 40°C, and
IO 250q
,b, -2
O~
2oo'
150
06
I00
04
0
200
No of
400
600
Cycles
Fig. 1. Decreasing of latent heat storage capacity of sample a with increase in number of cycles.
373
374
TECHNICAL NOTE Table 1. Composition of the samples sample name
sample
component
percent by weight
guaranteed grade CH3CO2Na.3H20
a
99.0
Na4PzO7.1OH20
i.O
............................................................ teehnical grade CH3CO2Na-3H20
sample b
99.0
Na4P2OT-10H20
1.0
............................................................ technical grade CH3CO2Na-3H20
93.0
H20 sample c
3.5
polyvinyl
alcohol
I.O
acetone
0.5
liquid paraffin
i.O
Na4P207.1OH20
i.O
From Figs. 1 and 2, it is clear that the heat storage capacity of sample b decreases much more slowly than that of sample a during thermal cycling. Apparent phase changes present in sample a comprising guaranteed grade CH3CO2Na-3H20 during thermal cycling is shown in Fig. 3. In the molten state, the upper layer is liquid paraffin preventing water evaporation, the intermediate layer is CH3CO2Na solution and the lower layer is the mixture of CH3CO2Na solution and anhydrous CH3CO2Na particles settling on the bottom of the graduated cylinder. In the frozen state, the upper layer is also liquid paraffin, the intermediate layer is unfrozen CH3CO2Na solution and the lower layer is the mixture of CH3COENa'3H20 crystals and anhydrous CH3CO2Na particles. Apparent phase change present in sample b comprising technical grade CH3COENa'3H20 are also shown in Fig. 4. Whichever grade CH3CO2Na.3H20 is used, it is observed from Figs. 3 and 4 that the extent of settling of anhydrous CH3COzNa particles in the molten state and the volume of the unfrozen CH3CO2Na solution in the frozen state increase during thermal cycling, respectively. It is also clear that anhydrous CH3CO2Na particles in the melt of sample b settle more slowly than that of sample
a and accordingly, the volume of the unfrozen CH3CO2Na solution in the frozen state sample b increases more slowly than that of sample a. This result is in good agreement with the calorimetric measurement mentioned above. The differences between sample a and sample b are considered to come from the impurities, NaCl, MgC12, HCO2Na, etc., contained in technical grade CH3CO2Na'3H20. Impurities such as HCO2Na are supposed to be adsorbed onto the surface of anhydrous CH3CO2Na particles in the melt and prevent anhydrous CH3COzNa particles from growing (which is due to: (l) an Ostwald ripening process where fine CH3CO2Na particles dissolve preferentially and several large anhydrous CH3COENa particles grow and (2) from sintering together, that is, anhydrous CH3COzNa particles bonding together to form a solid mass). Such an impurity effect is indicated for Glauber's salt by Marks [8]. He called such impurities 'crystal habit modifiers'.
300 I0 f i rst
25O
20th
lOOth
molten state (at 70°C) o8
200
D
I
<3
06
150
0.4
I00 0
I
--
200
400
600
No. of Cycles Fig. 2. Decreasing of latent heat storage capacity of sample b with increase in number of cycles.
first
20th
iOOth
frozen state (at 40~C) Fig. 3. Apparent phase changes in sample a.
375
Technical Note I
l
r
'_'zzL. IOOth
20th
first
tent heat storage capacity after many cycles. The addition of a thickening agent has greatly retarded the decline in storage capacity of CH3COzNa'3H20, because the thickener prevents anhydrous CH3CO2Na particles from settling on the bottom of the container. The thickener used in this experiment comprises polyvinyl alcohol, acetone and liquid paraffin but the action of these components cannot be well understood.
molten s t a t e ( a t 7 0 ° C
4. CONCLUSIONS Calorimetric measurements have been performed on three kinds of samples. Aging tests show that the heat storage capacity of a sample comprising technical grade CH3CO2Na.3H20 decreases more slowly than that of a sample comprising guaranteed grade CH3CO2Na'3H20 and the performance of a thickened sample shows little worsening during thermal cycling tests.
first
20th
IOOth
Acknowledgement--The authors wish to express their thanks to Dr Ryoichi Kiriyama for his useful discussions and to Drs Tsuneharu Nitta, Eiichi Hirota and Masanari Mikoda for their continuous encouragement.
frozen s t a t e (at 4 0 ° C Fig. 4. Apparent phase changes in sample b. The latent heat capacity of sample c with a number of cycles is shown in Fig. 5. The latent heat capacity of this sample is about 230 J g - ], and this latent heat capacity scarcely decreases at all during thermal cycling. The addition of a thickening agent considerably improves the la-
300
PO 250 'I "o
•
T
O8
2OO I<::]
06
150
I00
[> I I
-
04 200
400
600
No of Cycles
Fig. 5. Latent heat storage capacity of sample c with number of cycles.
REFERENCES I. M. Telkes, Solar Materials Science (Edited by L. E. Murr), Chap. 11. Academic Press, New York (1980). 2. A. Pebler, Dissociation vapor pressure of sodium acetate trihydrate. Thermochim. Acta 13, 109 (1975). 3. F. de Winter, Bottled caloric revisited. Solar Energy 17, 379 (1975). 4. K. Narita and J. Kai, Latent heat storage materials. J. Int. Electr. Engineer, Japan 101, 15 (1981). 5. T. Wada and R. Yamamoto, Studies on salt hydrate for latent heat storage, h crystal nucleation of sodium acetate trihydrate catalyzed by tetrasodium pyrophosphate decahydrate. Ball. Chem. Sac. Japan 55, 3603 (1982). 6. M. Telkes, Nucleation of supersaturated inorganic salt solutions. Ind. Engng Chem. 44, 1308 (1952). 7. M. Telkes, Thixotropic mixture and method of making some. U.S. Pat No. 3,986-969, 19 October (1976). 8. S. Marks, Thermal energy storage using Glauber's salt: improved storage capacity with thermal cycling. Proc. lntersoc. Energy Convers. Eng. Conf. 15th. 1, 259 (1980). 9. S. Marks, An investigation of the thermal energy storage capacity of Glauber's salt with respect to thermal cycling. Solar Energy 25, 255 (1980).
0038-092X/84 $3.00 + .00 © 1984 Pergamon Press Ltd.
TECHNICAL NOTE Heat storage capacity of sodium acetate trihydrate during thermal cycling TAKAHIRO W A D A , RYOICHI YAMAMOTO a n d YOSHIHIRO MATSUO
Central Research Laboratory, Matsushita Electric Industrial Co., Ltd., Kadoma, Osaka 571, Japan
(Received 29 July 1983: accepted 28 June 1984) 1.
INTRODUCTION
Sodium acetate trihydrate (CH3CO2Na.3H20) has recently attracted attention as a useful heat storage material because of its large latent heat of fusion (about 264 J g-~) [1, 2]. However, this material melts incongruently and severely supercools [3, 4]. Wada and Yamamoto [5] have shown that the addition of Na4P207" 10HzO as a nucleation catalyst to CH3COzNa. 3H20 has solved the latter problem satisfactorily. The problems caused by incongruent melting of CH3COzNa.3H20 are subtle and difficult to solve. Telkes [6, 7] studied Glauber's salt (NazSO4.10H20) for latent heat storage and found that addition of a thickening agent such as attapulgite clay was effective for homogeneously suspending the Na2SO4 particles in the melt and prevented Na2SO4.10H20 from forming a metastable condition where the undissolved Na2SO4 particles coexisted with newly formed Na2SO4.10H20 crystals and excess solution. Marks [8, 9] measured the heat storage capacity of Na2SO4.10H20 thickened by using attapulgite clay during thermal cycling and showed that the addition of attapulgite clay is useful for solving the problems caused by incongruent melting. In order to investigate the decreasing heat storage capacity of CH3CO2Na.3H20 during thermal cycling, we performed calorimetric measurements on three kinds of samples. Sample a comprised guaranteed grade CH3CO2Na-3H20, sample b comprised technical grade CH3CO2Na.3H~O and sample c comprised technical grade CH3CO2Na.3H20 thickened by using polyvinyl alcohol. All three samples also comprised Na4P2OT" 10H20 as a nucleation catalyst for CH3CO2Na.3H20. Aging tests show that impurities contained in technical grade CH3COzNa-3H20 prevent anhydrous CH3CO2Na particles in the melt from settling on the bottom of the container and so, the heat storage capacity of sample b decreased more slowly than that of sample a. The performance of thickened sample c showed little worsening after 500 thermal cycles.
kept for 60 rain at both 70 and 40°C. At various times the sample was taken out of the water bath and calorimetric measurements were performed on it. Heat storage capacity was measured by the standard calorimetric technique in which the heat evolved by the sample is equated to the heat absorbed by the calorimeter's water. A molten sample whose mass and temperature were known was quickly placed into water whose temperature was recorded as a function of time. Prior to the measurement, the sample was brought to a uniform temperature above its melting point, about 60°C. Calorimetric measurements provide a precise and direct measure of the heat storage capacity of the sample. The total heat stored is the sum of sensible heat and latent heat of fusion. The latent heat storage capacity was computed by subtracting the contribution of the specific heat of CH3COzNa.3H20 (Cp ( c r y s t ) = 1.7 J g i K l, Cp Imelt) = 2.9 J g ~ K -~) from the measured heat storage capacity. It was assumed that only CH3CO2Na'3H20 contributed to the heat storage capacity measured because the heat storage capacity of the nucleation catalyst and the thickener was very small compared to that of CH3CO:Na.3HzO. 3. RESULTS AND DISCUSSION The decrease in latent heat storage capacity of sample a with an increase in the number of thermal cycles is shown in Fig. 1. In this figure, AHi is the latent heat of fusion of CH3CO2Na.3H:O. The latent heat storage capacity of this sample is initially 254 J g i; after 30 cycles it declines to 200 J g '; and after 400 cycles it declines to 1 6 0 J g I. The decrease in latent heat capacity of sample b with an increase in the number of thermal cycles is shown in Fig. 2. The latent heat storage capacity of this sample is initially 259 J g - J : after 30 cycles it declines to 235 J g - ~; and after 400 cycles it declines to 200 J g 1 300
2. EXPERIMENTAL Guaranteed grade CH3CO2Na.3H20, Na4P207"IOH20, polyvinyl alcohol (degree of polymerization, n ~ 500), acetone and liquid paraffin were obtained from Wako Pure Chemical Industries. Technical grade CH3CO2Na.3H~O was obtained from Nippongoseikagaku Co., Ltd. The thickened mixture was prepared by mixing technical grade CH3CO2Na.3H20 with polyvinyl alcohol, acetone, liquid paraffin and water at 70°C for 30 min. Table 1 shows the initial composition of the three kinds of samples. About 80 g of CH3CO~Na.3H20 or thickened mixture and 0.8 g of Na4P2OT.10H20 were placed in a stainless steel vessel, 30 mm inner diameter, 100 mm high and 1 mm wall thickness. This vessel was sealed and put into a water bath. The sample was consecutively heated and cooled at the rate of 5°C min 1 between 70 and 40°C, and
IO 250q
,b, -2
O~
2oo'
150
06
I00
04
0
200
No of
400
600
Cycles
Fig. 1. Decreasing of latent heat storage capacity of sample a with increase in number of cycles.
373
374
TECHNICAL NOTE Table 1. Composition of the samples sample name
sample
component
percent by weight
guaranteed grade CH3CO2Na.3H20
a
99.0
Na4PzO7.1OH20
i.O
............................................................ teehnical grade CH3CO2Na-3H20
sample b
99.0
Na4P2OT-10H20
1.0
............................................................ technical grade CH3CO2Na-3H20
93.0
H20 sample c
3.5
polyvinyl
alcohol
I.O
acetone
0.5
liquid paraffin
i.O
Na4P207.1OH20
i.O
From Figs. 1 and 2, it is clear that the heat storage capacity of sample b decreases much more slowly than that of sample a during thermal cycling. Apparent phase changes present in sample a comprising guaranteed grade CH3CO2Na-3H20 during thermal cycling is shown in Fig. 3. In the molten state, the upper layer is liquid paraffin preventing water evaporation, the intermediate layer is CH3CO2Na solution and the lower layer is the mixture of CH3CO2Na solution and anhydrous CH3CO2Na particles settling on the bottom of the graduated cylinder. In the frozen state, the upper layer is also liquid paraffin, the intermediate layer is unfrozen CH3CO2Na solution and the lower layer is the mixture of CH3COENa'3H20 crystals and anhydrous CH3CO2Na particles. Apparent phase change present in sample b comprising technical grade CH3COENa'3H20 are also shown in Fig. 4. Whichever grade CH3CO2Na.3H20 is used, it is observed from Figs. 3 and 4 that the extent of settling of anhydrous CH3COzNa particles in the molten state and the volume of the unfrozen CH3CO2Na solution in the frozen state increase during thermal cycling, respectively. It is also clear that anhydrous CH3CO2Na particles in the melt of sample b settle more slowly than that of sample
a and accordingly, the volume of the unfrozen CH3CO2Na solution in the frozen state sample b increases more slowly than that of sample a. This result is in good agreement with the calorimetric measurement mentioned above. The differences between sample a and sample b are considered to come from the impurities, NaCl, MgC12, HCO2Na, etc., contained in technical grade CH3CO2Na'3H20. Impurities such as HCO2Na are supposed to be adsorbed onto the surface of anhydrous CH3CO2Na particles in the melt and prevent anhydrous CH3COzNa particles from growing (which is due to: (l) an Ostwald ripening process where fine CH3CO2Na particles dissolve preferentially and several large anhydrous CH3COENa particles grow and (2) from sintering together, that is, anhydrous CH3COzNa particles bonding together to form a solid mass). Such an impurity effect is indicated for Glauber's salt by Marks [8]. He called such impurities 'crystal habit modifiers'.
300 I0 f i rst
25O
20th
lOOth
molten state (at 70°C) o8
200
D
I
<3
06
150
0.4
I00 0
I
--
200
400
600
No. of Cycles Fig. 2. Decreasing of latent heat storage capacity of sample b with increase in number of cycles.
first
20th
iOOth
frozen state (at 40~C) Fig. 3. Apparent phase changes in sample a.
375
Technical Note I
l
r
'_'zzL. IOOth
20th
first
tent heat storage capacity after many cycles. The addition of a thickening agent has greatly retarded the decline in storage capacity of CH3COzNa'3H20, because the thickener prevents anhydrous CH3CO2Na particles from settling on the bottom of the container. The thickener used in this experiment comprises polyvinyl alcohol, acetone and liquid paraffin but the action of these components cannot be well understood.
molten s t a t e ( a t 7 0 ° C
4. CONCLUSIONS Calorimetric measurements have been performed on three kinds of samples. Aging tests show that the heat storage capacity of a sample comprising technical grade CH3CO2Na.3H20 decreases more slowly than that of a sample comprising guaranteed grade CH3CO2Na'3H20 and the performance of a thickened sample shows little worsening during thermal cycling tests.
first
20th
IOOth
Acknowledgement--The authors wish to express their thanks to Dr Ryoichi Kiriyama for his useful discussions and to Drs Tsuneharu Nitta, Eiichi Hirota and Masanari Mikoda for their continuous encouragement.
frozen s t a t e (at 4 0 ° C Fig. 4. Apparent phase changes in sample b. The latent heat capacity of sample c with a number of cycles is shown in Fig. 5. The latent heat capacity of this sample is about 230 J g - ], and this latent heat capacity scarcely decreases at all during thermal cycling. The addition of a thickening agent considerably improves the la-
300
PO 250 'I "o
•
T
O8
2OO I<::]
06
150
I00
[> I I
-
04 200
400
600
No of Cycles
Fig. 5. Latent heat storage capacity of sample c with number of cycles.
REFERENCES I. M. Telkes, Solar Materials Science (Edited by L. E. Murr), Chap. 11. Academic Press, New York (1980). 2. A. Pebler, Dissociation vapor pressure of sodium acetate trihydrate. Thermochim. Acta 13, 109 (1975). 3. F. de Winter, Bottled caloric revisited. Solar Energy 17, 379 (1975). 4. K. Narita and J. Kai, Latent heat storage materials. J. Int. Electr. Engineer, Japan 101, 15 (1981). 5. T. Wada and R. Yamamoto, Studies on salt hydrate for latent heat storage, h crystal nucleation of sodium acetate trihydrate catalyzed by tetrasodium pyrophosphate decahydrate. Ball. Chem. Sac. Japan 55, 3603 (1982). 6. M. Telkes, Nucleation of supersaturated inorganic salt solutions. Ind. Engng Chem. 44, 1308 (1952). 7. M. Telkes, Thixotropic mixture and method of making some. U.S. Pat No. 3,986-969, 19 October (1976). 8. S. Marks, Thermal energy storage using Glauber's salt: improved storage capacity with thermal cycling. Proc. lntersoc. Energy Convers. Eng. Conf. 15th. 1, 259 (1980). 9. S. Marks, An investigation of the thermal energy storage capacity of Glauber's salt with respect to thermal cycling. Solar Energy 25, 255 (1980).