Low temperature latent heat thermal energy storage: Heat storage materials

SolarEne,T¢~,Vol 10,No.4.pp 313-332.t983 PrintedinGreatBrilain

110384192X/83/04031~-2050300/0 ~ 1983PergamonPres~lhl

LOW TEMPERATURE LATENT HEAT THERMAL ENERGY STORAGE: HEAT STORAGE MATERIALS A. ABHAT Institut for Kernenergetik und Energiesysteme (IKE), University of Stuttgart, Stuttgart, FRO

(Receiced 6 November 1981; accepted 18 May 1982)

Abstract--Heat-of-fusion storage materials for low temperature latent heat storage in the temperature range 0-120°C are reviewed. Organic and inorganic heat storage materials classified as paraffins, fatty acids, inorganic salt hydrates and eutectic compounds are considered. The melting and freezing behaviour of the various substances is investigated using the techniques of Thermal Analysis and DifferentialScanning Calorimetry. The importance of thermal cycling tests for establishingthe long-term stability of the storage materials is discussed. Finally, some data pertaining to the corrosion compatibilityof heat-of-fusion substances with conventional materials of construction is presented. 1. INTRODUCTION Efficient and economical heat storage is the key to the effective and widespread utilization of solar energy for low temperature thermal applications. Amongst the various heat storage techniques of interest, latent heat storage is particularly attractive due to its ability to provide a high energy storage density and its characteristics to store heat at a constant temperature corresponding to the phase transition temperature of the heat storage substance. The term "Latent Heat Storage", as we generally understand it today, applies to the storage of heat as the latent heat of fusion in suitable substances that undergo melting and freezing at a desired temperature level. Consequently it is also often called the "Heat-of-Fusion" storage. Typical heat-of-fusion storage substances wellknown to all of us are ice, paraffin or Glauber Salt. The term "latent heat storage" may also be applied to include the heat stored in substances, such as Diaminopentaerythritol, wherein heat is stored as the heat of crystallization, as the substance is transformed from one solid phase to another. The stored heat is recovered in a likewise manner as the original solid phase is regained. Excluded in the present definition of "latent heat storage" is, however, the heat stored in materials that undergo a liquid-to-vapor phase transition, e.g. water-tosteam. Although the latter phase transitions are associated with a latent heat of phase transition that is almost an order-of-magnitude higher than that for solid-to-liquid or solid-to-solid phase change, the practical problems of storing a gaseous phase and the necessity of pressurized containers for this purpose rule out their potential utility. The review article relates to the discussion of heat-offusion storage, a technique which is also of the greatest current practical value. A temperature range of 0-120°C

to cover a variety of low temperature applications, such as domestic hot water production, direct or heat-pump assisted space heating, green house heating, solar cooling, etc. is considered. It should, however, be emphasized here, that although heat storage in solid-solid phase transitions is much less understood today, it does hold out future promise.

fThe following abbreviations are used in the text: LTES = latent heat thermal energy storage. PCM = phase change (heat-of-fusion) storage material.

Desired material properties A large number of organic and inorganic substances are known to melt with a high heat of fusion in any

2. BASIC REQUIREMENTSOF A LATENT HEAT THERMAL ENERGY STORET

Any latent heat thermal energy storage system must possess at least the following three components: (1) a heat storage substance~ that undergoes a solid-to-liquid phase transition within the desired operating temperature range and wherein the bulk of the heat added is stored as the latent heat of fusion; (2) a containment for the storage substance; (3) a heat exchanging surface for transferring heat from the heat source to the heat storage substance and from the latter to the heat sink, e.g. from the solar collector to the heat storage substance to the load loop. The development of a latent heat thermal energy storage (LTES) system hence involves the understanding of two essentially diverse subjects: heat storage materials and heat exchangers, The flowchart in Fig. 1 provides an overview of the different stages that may be involved in the development of an L T E S system and of the specialized problems that need to be tackled. The present paper is confined to the discussion of heat-of-fusion storage materials. The measurement techniques are outlined and the short and long-term behaviour of various heat storage substances is presented. Finally, some information pertaining to the compatibility of the storage substances with construction materials is provided. 3. HEAT-OF-FUSION STORAGEMATERIALS

SEVok30,No.4--8

3 13

314

A. ABHAT LATENT HEAT THERMAL ENERGY STORAGE SYSTEMS

f

I

I

MATERIALINVESTIGATION q CHOICE OF MATERIALS IN THE APPROPRIATE .... TEMPERATURE RANGE

_

HEATEXCHANGERDEVELOPMENT I

]

CHOICE OF HEAT EXCHANGER CONCEPT(S)

1 I

HEAT STORAGE MATERIALS

FPROPERTY DATA

I

-

-

MATERIALS?

I

THERMAL ANALYSIS, PARAMETRIC INVESTIGATION

1

EXPERIMENTAL 7 INVESTIGATION]

LABORATORY MODELS J

FREEZING CHARACTERISTICS

1

INTEGRATION WITH HEATING/ COOLING SYSTEMS

PILOT UNITS

~Z~

LONGLIFE ~l CONSIDERATIONS

I SIMULATION I n~,~S~ LJCOMPARISONS vKtu~, i FORMANCEOFJ HEAT STORE I AND HEATING/l I COOLING l L SYSTEM

[CO~;~I~I~ITYI -

-

FIELDTESTS

L

yr.-l/ J

FINAL COST ANALYS S _J

~

COMMERCIAL PRODUCTI ON

Fig. 1. Flowchart providingan overviewof the various steps in the developmentof a latent heat storage system. required temperature range, e.g. 0-120°C. However, for their employment as heat storage materials in LTES systems, these phase change materials must exhibit certain desirable thermodynamic, kinetic and chemical properties. Moreover, economic considerations of cost and large-scale availability of the materials must be considered. The various criteria that govern the selection of phase change heat storage materials are summarized in Table I.

Candidate heat storage materials It is quite apparent that no single material can fully satisfy the long list of criteria listed in Table 1. Tradeoffs are hence made in the selection of candidate heat storage materials. Within the operating temperature range of 0-120°C, candidate PCMs are grouped into the families of organic and inorganic compounds and their eutectica, as seen in Fig. 2. Sub-families of organic compounds include paraffin and non-paraffin organics. Figures 3(a, b), respectively show the latent heat of fusion per unit mass and per unit volume of some heat storage materials of interest in the temperature range 0-120°C[1, 19, 23, 29]. Of particular interest is Fig. 3(b), which provides information on the compactness of an LTES system for a given amount of energy storage. Figure 3(b) shows that the organic compounds have a volumetric latent heat storage capacity in the range 125200 kJ/dm ', whereas that of the salt hydrates is almost twice as much, between 250-400 kJ/dm 3. Notwithstanding the disadvantages concerning volume requirements, the organic substances serve as important heat storage materials due to the several desir-

able properties they possess in comparison with inorganic compounds. Some of these advantages include their ability to melt congruently, their self-nucleating properties, and their compatibility with conventional materials of construction[3]. We shall now examine some of the important heat storage material groups in some detail. Para~ns. Paraffins, as we normally understand them, are substances having a waxy consistency at room temperature. Chemically speaking, paraffin waxes consist primarily of straight-chain hydrocarbons with only a small amount of branching, such as 2-methyl groups, near the end of the chain. Paraffins contain in them one major component called alkanes, characterized by C,H2,+z; the n-alkane content in paraffin waxes usually exceeds 75 per cent and may reach lOOper cent[13]. Depending on the chain length of the alkane in the paraffin, paraffins may be even-chained (n-paraffin) or odd-chained (iso-paraffin). Pure paraffins contain only alkanes in them, for example the well-known paraffin octadecane, C~,H38. The melting point of the alkanes increases with the increasing number of carbon atoms; alkanes containing 14--40 C-atoms possess melting points between 6 and 80°C and are generally termed as paraffins. Commercial waxes, on the other hand, may have a range of about 8-15 carbon-numbers [13]. In their solid phase, paraffins are generally found in two allotropic modifications that differ in their physical properties and the crystal structure. The primary modification existing at higher temperature, i.e. slightly above the melting point of the substance, is soft and

Low temperature latent heat thermal energy storage: heat storage materials Table 1. Desired properties of phase change heat storage materials [3] A.

THERMODYNAMIC CRITERIA THE PHASE CHANGE MATERIAL SHOULD POSSESS O A MELTING POINT IN THE DESIRED OPERATING TEMPERATURE RANGE O HIGH LATENT HEAT OF FUSION PER UNIT MASS, SO THAT A LESSER AMOUNT OF MATERIAL STORES A GIVEN AMOUNT OF ENERGY O HIGH DENSITY, SO THAT A SMALLER CONTAINER VOLUME HOLDS THE MATERIAL O HIGH SPECIFIC HEAT TO PROVIDE FOR ADDITIONAL SIGNIFICANT SENSIBLE HEAT STORAGE EFFECTS O HIGH THERMAL CONDUCTIVITY~ SO THAT THE TEMPERATURE GRADIENTS REQUIRED FOR CHARGING AND DISCHARGING THE STORAGE MATERIAL ARE SMALL O CONGRUENT MELTING: THE MATERIAL SHOULD MELT COMPLETELY ~O THAT THE LIQUID AND SOLID PHASES ARE IDENTICAL IN COMPOSITION. OTHERWISE~ THE DIFFERENCE IN DENSITIES BETWEEN SOLID AND LIQUID CAUSE SEGREGATION RESULTING IN CHANGES IN THE CHEMICAL COMPOSITION OF THE MATERIAL /20/ O SMALL VOLUME CHANGES DURING PHASE TRANSITION, SO THAT A SIMPLE CONTAINMENT AND HEAT EXCHANGER GEOMETRY CAN BE USED.

B,

KINETIC CRITERIA THE PHASE CHANGE MATERIAL SHOULD EXHIBIT I LITTLE OR NO SUPERCOOLING DURING FREEZING, THE MELT SHOULD CRYSTALLIZE AT ITS THERMODYNAMIC FREEZING POINT. THIS IS ACHIEVED THROUGH A HIGH RATE OF NUCLEATION AND GROWTH RATE OF THE CRYSTALS. AT TIMES, THE SUPERCOOLING MAY BE SUPPRESSED BY INTRODUCING NUCLEATING AGENT OR A "COLD FINGER" IN THE STORAGE MATERIAL /20/.

C.

CHEMICAL CRITERIA THE PHASE CHANGE MATERIAL SHOULD SHOW I CHEMICAL STABILITY I NO CHEMICAL DECOMPOSITION, SO THAT A HIGH LTES SYSTEM LIFE IS ASSURED I NON-CORROSIVENESS TO CONSTRUCTION MATERIALS e THE MATERIAL SHOULD BE NON-POISONOUS, NON-FLAMMABLE AND NON-EXPLOSIVE

D.

ECONOMIC CRITERIA THE PHASE CHANGE MATERIAL SHOULD BE I AVAILABLE IN LARGE QUANTITIES I

INEXPENSIVE

Paraffins 1 Compounds

Non-parafhn 1 Orgamcs ~]

OrgonlCS Eutectic~"

)

I,~orgo~n,cs

compounds I

Eutectlc~:

}

Fig. 2. Families of phase change heat storage materials[3].

315

A. ABHAT

316 450

l

A,

i

400 -~

350

o ~E

300

~

250

25 • 29 22

37

36

19 27 28 7

• 24

200

u_ "6

150

~

100

3. PARAFFIN 5838 4, PARAFFIN 6035 5. PARAFFIN6403 6. PARAFFIN6499 7, OCTADECANE

Paraffins Non-Paraffin Organics • Inorganic Compounds [] Eutectic Compounds

L21

c .o u'l

PARAFFINS

1, PARAFFIN 5913 2, PARAFFIN 6106

23•

33A ,&013 35 19

r, N

• 26

A A

9

31•

20

~6 A18

39•

4

A • 10 30

8

5

A

E 41

Ig A

~2

17

•32

•

38

T

B.

ORGANICCOMPOUNDS

8, 9. 1U. 11, 12. 13. 14. 15. 16. 17. 18.

CAPRILIC ACID POLYGLYCOLE 600 CAPRICACID LAURICACID MYRISTIC ACID POLYGLYCOLE 6000 LOXIOL G 32 PAI241TICACID STEARICACID BIPHENYL

PROPIONAMIDE 19. NAPTHALENE I

0

I

I

10

20

30

40

50

60

70

90

80

I

100

110

120

Melting Point [°C]

(a)

C.

f

'

371

'

'

25

% 27~

400 22

3

19 Paraffins Non-Paraffin Organics • Inorganic Compounds [] Eutectic Compounds

34

28

350 24

E

33

300' b21 30 23 c

c

._(2 LL

26

200

3s% 41 & 32 />,11

719

150

A 17

39

~2o

E3

LO

15

18

19 12 ? 8

® "1-

31

•

250

A

1

19

t0

20. ACETAMIDE IHQRGAN1CCOMPOUNDS

21. H20 22. LICLO3.3H20

582 k J/din3

450 E "E3

I

14

23. 24. 25. 26, 27, 28. 29. 30. 31, 32. 33, 34, 35. 36. 37. 38, 39.

NAOH.31/2H20 KF.4H20 LINo3.3H20 CACL2,6H20 NA2SOh.IOH20 NA2CO3,1OH20 NA2HPO4.12H20 ZN(NO3)2.6H20 CA,BR2,6H20 KF.2H20 NA2S203.5H20 NI(NO3)2.6H20 NACH3COO.3H20 NAOH.H20 BA(OH)2.8H20 MG(NOs)2.6H20 MGCL2.6H20

p,

EUTEGTIC COMPOUNDS

100 50 -

3

I

0

10

L

20

30 (b)

a

40

50

I

60

I

[

70

80 Melting

90

100

Point

[ °C]

110

120

40. 21,1 WT,% PROPIONAM1DE + 74.9 % PALMITIC ACID 41. 53 MOL-%MG(NO3)2.6H20 + 47 MOL-%MGCL2.6H20 42. 27 MOL-% LINO3 + 68 MOL-% NH4NO3 + 5 MOL-%NH4CL

Fig. 3. Latent heat of fusion per unit mass and per unit volume of selected phase change heat storage materials undergoing a phase transition within the temperature range 0-120°C.

plastic, the individual crystals are needle-shaped. The secondary modification existing at lower temperature, i.e. below the melting point, is hard and brittle, the crystals are disc-shaped. The transition from one crystal form to another is reversible [28]. Paraffins qualify as heat-of-fusion storage materials due to their availability in a large temperature range and their reasonably high heat of fusion. Furthermore, they are known to freeze without supercooling [4]. Due to cost considerations, however, only technical grade paraffins may be used as PCMs in latent heat stores. Table 2 lists some technical grade materials, which are essentially

paraffin mixtures and are not completely refined of oil. Manufacturers' data on their physical properties as well as cost data are also included in Table 2. Non-para#in organics. Although a number of nonparaffin organics have been included in Figs. 3(a)--(b), we shall restrict our discussion to the group of materials called "Fatty acids". Other non-paraffin organics may, however, be assumed to have properties similar to those of the fatty acids. Fatty acids. Fatty acids are organic compounds characterized by CH3(CH2h,COOH with heat of fusion values comparable to that of paraffins. Some fatty acids

Low temperature latent heat thermal energy storage: heat storage materials

317

Table 2. Physical properties and cost data of some paraffins PARAFFIN

DISTRIBUTION OIL OF ~ENT C-ATOMS

~E~I~

% _ 1)

CL5 - C16 C]_3 - C24

20

C18 C16 - C28

0CTADECANE 6106 3)

KJ/KG

°C

C14

5913 3)

HEAT OF FUSION

~I~/~

DENSITY SPECIFIC AT HEAT 20 *C 70 *C AT 1(]0 *C KJ/DM 3 KG/~ 3 KJ/KG.K

THERMAL COST COIk~)UCT I Vl TV (1979) (SOLIDPHASE)

W/M.K

REF.

I]M/KG

4.5

165

1,20 2)

20

8

153

0,50 2)

20

0.50

4

22 - 24

189

144

0.900 0.760

2.1

0.21

0

28

244

189

0.814 0.774

2.16

0,15

5

42

44

189

145

0,910 0.765

2.1

0.21

45 - 48

210

165

0.817 0,786

2.5

50 60

189 189

145 150

i0.91P 0.769 0.gL~O 0,795

2.1 2.1

0.21

0,60

4

62 - 6 4 66 - 68

189 189

150 157

0.915 0,790 0.930 0.830

2.1 2.1

0,21 0.21

1,00 0,80

4 4

P].16 4) 5838 3)

C20 - C33

< 0.5

6035 3)

C22 - C45

4

6403 6499 3)

C23 - CLE5 C2_].- C50

< 0,5 3

48 58

-

-

-

150,0

4

0.70 0,44 2) 1,00

0.21

i) - IMPLIES DATA NOT AVAILABLE

3) MANUFACTURERS OF TECHNICALGRADEPARAFFINS5913, 6106, 5838, 6035,

2) COSTESTIMATESARE FOR 1974 (REF. 2(])

4) MANUFACTURERS OF PARAFFINP116: SUN OIL COMPANY,USA

4 20 4

6403 AA~)6499: 'TER HELLPARAFFIN, HI~'~JRG, FRG

Table 3. Physical properties and cost data of some fatty acids MATERIAL

MELTING POINT/RANGE *C

CAPRILIC ACID

16,5

CAPRIC ACID

31.5

HEAT OF FUSION

DENSITY

KJ/KG

KG/DM3

149

KJ/DM3

128

1.033(10 °C)

S P E C I F I C THERMAL COST HEAT CONDUCTIVITY (1979) KJ/KG K W/M.K DM/KG -

1)

0.148(20 °C)

4

0.149(40

4

0.862(8O °C)

153

136

0.886(40 *C)

42 - 44

178

155

0.870(50 *C)

1,6

MYRISTIC ACID

54

187

158

0,844(80

1,6(s) 2) 2.7(L)

PALMITIC ACID

63

187

159

0.847(80 °C)

STEARIC ACID

70

203

191

0.941(40 *C) 2,35(125°C)

LAURIC ACID

REF,

*C)

°C)

0.147(50 *C)

2,50

4

2.50

4

0.165(70 *C)

2,30

4

0,172(70 *C)

2,O0

4

1) -IMPLIES DATA NOT AVAILABLE 2) S - SOLID; L - LIQUID

of interest to low temperature LTES applications are listed in Table 3. Fatty acids are known to possess a reproducible melting and freezing behaviour and freeze with little or no supercooling[4, 19]. They hence qualify as good PCMs. Their major drawback, however, is their cost which is 2-2.5 times greater than that of paraffins. Salt-hydrates. Salt hydrates, characterized by M.nH20, where M is an inorganic compound, form an important classs of heat storage substances due to their high volumetric latent storage density (see Fig. 3(b)). In fact, their use as PCMs has been propagated as early as 1947124]. Table 4 provides a list of some salt hydrates melting in the temperature range 0-120°C along with their thermophysical properties and cost data. The major problem in using salt hydrates as PCMs is that most of them melt incongruently, i.e. they melt to a saturated aqueous phase and a solid phase which is generally a lower hydrate of the same salt. Due to density differences, the solid phase settles out and collects at the bottom of the container, a phenomenon

called decomposition. Unless special measures are taken, Ihis phenomenon is irreversible, i.e. during freezing, the solid phase does not combine with the saturated solution to form the original salt hydrate. Another important problem with salt hydrates is their poor nucleating properties resulting in supercooling of the liquid salt hydrate prior to freezing. Suitable measures must be adopted to eliminate supercooling or reducing it to a minimum. Typical methods suggested in the literature for this purpose are: (1) addition of nucleating agents that have a crystal structure similar to that of the parent substance [25]: (2) using a "cold finger" in the PCM[20]; (3) promoting heterogeneous nucleation by using (rough) metallic heat exchanging surfaces in contact with the salt hydrate[4]. We shall examine the influence of these techniques on the melting and freezing characteristics of salt hydrates in greater detail at a later stage. The selection of a salt hydrate as a PCM can be eased by an understanding of the binary phase diagrams, MH20, for which the reader is referred to Ref. [30]. To illustrate the use of the phase diagrams, two examples

318

A. ABHAT Table 4. Physical properties and cost data of some salt h rdrates MATERIAL

MELTING POINT

*C

HEAT-OF-FUSION KJ/KG

KJ/DM3

SPECIFIC HEAT

DENSITY KG/DM3

THERMAL CONDUCTIVITY

KJ/KGK

W/M.K

2.09 (S)2) 4.18 (L) 2)

2.2 (S) 2) 0.6 (20"C)

2.39 (L)

0

333

306

0.917 (0 *C) 0,998 (20"C

KF,4H20

18.5

231

336

1,455 (18"C) 1,447 (20"C)

CACL2,6H20

29.7

171

256

1.710 (25"C) 1,496 (L)

1,45 (s)

NA2S04,10H20

32,4

254

377

1.485 (S)

1,93 (S)

0,544

NA2HP04,12H20 35,0

281

405

1.520 (s) 1.442 (L)

ZN(N03)2.6H20

36,4

147

304

NA2S203.5H20

48.0

201

BA(0H)2.8H20

78,0 116,0

H20 1)

MGCL2,6H20 1) 2)

DATA FOR

H20 IS

1.84 (s)

COST

(1979)

REF,

DM/KG

.3)

23

_4)

23 0.36

4

0,10

4

1,70 (s) 1,95 (L)

0,514 (32%) 0,95 0,476 (49"C)

16

2.065 (14"C)

1.34 (s) 2.26 (L)

2.40

4

322

1.73 (S) 1.67 (L)

1.46 (S) 2.39 (L)

0.30

4

267

581

2.180 (S)

1,17 (S)

1.75

4

165

239

1.57 (20"C) 1,442(78"C)

1.72 (S) 2.82 (L)

0,20

23

INCLUDED FOR THE SAKE OF COMPARISONS

S - SOLID; L - LIQUID

3) NEGLIGIBLE

4)

-

IMPLIES DATA NOT AVAILABLE

80

L

~,m

c_) o e~

2O

D

- -

"1-

% z

-20

z

-40 0

20

80

t,0 60 Weight % Zn(NO3)2

Fig. 4. Phase diagram for Zn(NO0:-H20 system[15]. have been included here: (a) Zn(NO3)-H20 system; (b) Na2SO4-H20 system. (a) Zn(NO3)2-H20 system. Figure 4 shows the phase diagram for Zn(NO3)2-H20 between the temperature limits from - 4 0 to +80°C[15]. If a liquid mixture of Zn(NO~)2 and H20 is cooled below 80°C, 5 different hydrated salts of Zn(NO3)2, as shown in Fig. 4, crystallize out of solution. Of these, Zn(NO0.9H,O has an incongruent melting point, whereas the remaining 4 hydrates have a congruent melting point. The 6-hydrate, with a melting point of 36.4°C, additionally has a high latent heat of fusion and is hence well-suited as a heat storage material. (b) Na2SO4-H20 system. The second example we wish to discuss here is the well-known storage material sodium sulphate-10-hydrate or Glauber salt. This material has been extensively investigated so far for use in LTES systems. Figure 5 shows a partial phase

50 4O 36' 32.4o 30

Liquid

T

.~

....... -r

', ii

7

20

t

Liquid ~.

Fp

B E

~

5

/ ..........

/-,so,.,°.2°i

-

N°2SO~ ]

.......

i

100W/o H20 Weight Percent No2SO~ Fig. 5. Partial phase diagram for Na:SO4-H20 system[9].

Low temperature latent heat thermal energy storage: heat storage materials diagram of the sodium sulphate-water system/9]. Na2SO4" 10H20 decomposes peritectically on heating to 32.4°C to yield anhydrous sodium sulphate and a saturated solution of Na2SOn in water. The solid crystals containing 44 per cent anhydrous Na2SO4 and 56 per cent water by weight change to a mixture of 15 per cent anhydrous Na2SO4 and 85 per cent saturated solution of Na2SO4 in water (10H20). The 15 per cent anhydrous Na2SO4 remains insoluble and settles down as a white bottom sediment. It is not possible to dissolve this material by the usual expedient of increasing the temperature, because the highest solubility of Na2SO4 in water occurs at the melting point with solubility decreasing at higher temperatures. Due to the large density difference between the saturated solution (1350kg/m 3) and the anhydrous solid Na2SO4 (2550kg/m3), gravity induced separation results. Consequently melting is partly incongruent due to segregation/26]. Now, if the mixture of the saturated solution and anhydrous salt is cooled below 32.4°C, sodium sulphate can be absorbed in the solution only as rapidly as water can diffuse through solid sodium sulphate decahydrate to the anhydrous sodium sulphate particles on which the decahydrate particles form. Since peritectic solidification reactions are characteristically much slower than congruent solidification or eutectic solidification, and because the rate limiting process here is solid state diffusion, even stirring cannot significantly affect the rate of absorption of Na2SO4, which therefore remains settled at the bottom of the container/9]. If the phase diagram for the Na2SO4-H20 system is now carefully followed, it can be seen that the solid state diffusion as a reaction step can be eliminated by using excess water in the system. A recommended

319

composition/9] is 30 per cent Na2S04 and 70 per cent H,.O by weight, corresponding to 68.2 per cent Na2SO4 • 10H20 and 31.8 per cent excess water. At 15°C (point 2), this mixture comprises 58.1 per cent Na2SO4.10H~0 and 41.9 per cent solution of composition 10.6 per cent Na2SO4 and 89.4 per cent H20 by weight. When the temperature of the mixture is raised beyond 15°C, the solubility of the decahydrate increases with the increase in temperature up to 32.4°C. At the liquidus line, the decahydrate completely goes into solution and no segregation hence takes place. Now if the solution is cooled from 36°C (point 5 in Fig. 5), decahydrate crystals begin to separate as the solution reaches the liquidus line (at 30°C). With further reduction in temperature, the system enters into a two-phase region, Na2SO4.10H20 and liquid. The amount of each constituent can be easily calculated at any temperature by using the lever rule. The stored thermal energy in the system is released when the Na2SO4-10H:O crystals separate from the solution/9].

Eutectics of organic and inorganic compounds. Eutectics of organic or inorganic compounds qualify as latent heat storage materials as they possess a fixed melting/freezing point. Some candidate materials having acceptable values of heat-of-fusion are listed in Table 5. It should be mentioned here that the search for eutectics is rather recent, and only limited data on thermophysical properties and cost are presently available. Other compounds. A material group that deserves mention for use in LTES systems is the clathrate hydrate. True clathrate hydrates are continuous solid water structures containing closed cavities within which are guest molecules that do not interact strongly with water. These guest molecules act to stabilize the "ice"

Table 5. Physical properties and cost data of some organic and inorganiceutectic compounds MATERIAL (WEIGHT ~ OF COMPOUND IN BRACKETS)

MELTING POINT °C

KJ/KG

4

234

NA2S04 (31%) NACL (13 %) KCL (16 %) H20 (40 %) CACL2 NACL KCL H20

(48 %) (4.3 %) (0.4 %) (47.3 %)

HEAT-OF-FUSION KG/DM 3

1)

COST

KJ/KG K

DM/KG

_

20

CA(N03)2.4H20 (67 %) MG(N03)2.6H20 (33 %)

30

PROPIONAMIDE (25.1%) PALMITIC ACID (74.9 %)

50

192

MG(N03)2,6H20 (53 %)2) MGCL2.6H20 (47 %)

59.1

144

232

MG(N03)2.6H20 (53 %)2) AL(N03)2.9H20 (47 %)

61

148

249

81.6

111

205

136

REFERENCE

16

_1)

26.8

LIN03 (27 %)2) NH4N03 (68 %) NH4CL (5 %)

SPECIFIC HEAT

0,324)

228 1,96 (S) 2.40 (L) 1.34 (s) 3) 3.16 (L) 3)

7, 19 0.445)

-

1.07 (S) 3) 2.20 (L) 3)

20

3.405)

29

20 29

1) - IMPLIESDATA NOT AVAILABLE

4) COSTDATA IS FOR 1974

2)

PROPORTIONSARE IN MOL

5) COSTDATA IS FOR 1977 (JAPANESE MARKET)

3)

S - SOLID; L - LIQUID

320

A. ABHAT

Table 6. Details of measurement techniques employedfor the determination of melting and freezing behaviour of phase change heat storage materials[6] MEASURB'~ IECI-INIQUE

.~P/~qATI~ DESIGN+

PERKIN-EU"ER DIFFERENTIAL SC.Pd~I NG CAI.O- CALORIMETER, MERRY (I~C) MODELDSC-2

ANALYSIS (TA)

TEST TI.A~S OR C.VJ~S AMPULES FIAT Ca_ASS CONTAINERS LAE~TORY MODELS OF PASSIVELY OPERATING LATENT HEAT STORAGE SYSTEM IESle4S:

~F.ASLIRB~TS MADE

DATA EVALL~TED

1-10 MG

VARIATID~I OF THE 11-lERMAL ENERGY WlTH IME S.aMPLETO ENABLE IT TO ~ HEATINGOR COOLINGAT A ~ T A N T PREDETERMINEDRATE, "n-IE OUTPUTOF THE MEASUREMBqTS IS THE ENERGY-TIME I)IAGRAM~ ALSOCALLE)

1) FORMOF D-I£ B ~ I C 0F E M 0 ~ MIC PEAKS ON THE 2) MELTINGPOINT/RANGE 3) FRE.ZIN6 eOmT/RAI~E 4) DEGREEOF ~ I I ~ 5) HEATSOF FUSIONAND SOL[DIFICATION 6) SPECIFICHEAT AS A FUNCTIONOF I~TLIE

i0 G

VARIATION OF "I~MPERATIJRE AT SELECII~D LOCATIOI~WITHIN THE HEAT STORAGESLIBST/~ICERESIJLT[NG FR(~ ENERGYINPUT/OUTPOT TO T~E S~c'tJE, THE OUTPUT OF THE MEASUREMENTS IS THE TE'PERATUR£TIME DIAGR~

1) SHAPE OF THE ~ ' n J R E - T I M E

QUANTITYOF HEAT STORAGE SUIBST/~CE IN THE TESTS (ORDER OF MAGNITUDE)

IOOG

1-10 KS

2) 3) 4) 5) 6)

DI~ MELTING~ I:I~.EZII~ PLKr-r_ttS, HOMOGENEITYOF "WE GIRIGINAI. SI.IBST~NI~FOU.D~IK~MELTING, ETC, MELTINGP011~/R,~(~ FREEZINGPOIN T / R ~ DEGI~_EOF S I . ~ I N 6 CONGI~IBW/INCONQ~UENTMELTING I~ITION OF THE ORIGINAL SIJIBST,qqC,I-,, ~M.TION OF NEW Pf-I~.S, SE(g~C~,TIONEFFECTS

1) E i X L ~ FINNED HEAT PIPE HEAT EXolmccm/2/ 2) FINNEDANNULL6 MEAT EXELEMENT

/5/

DETAILS OF TEST APPARATUS DESIGN ARE PROVIDED IN APPENDIX I

structure. There are also other structures in which the guest molecules participate in the water lattice directly, these substances are known as semi-clathrates and include hydrates of amines and tetra alkylammonium salts [20]. Examples of clathrate hydrates are[20]: SO2.6H20 (mpt), 7°C; AHF+ =247kJ/kg), C2H40" (6.9)H20 (rap, ll.l°C). Examples of semi-clathrate hydrate are [20]: (CH3)3N.(10~/4H20) (mp, 5.9°C, AHF=239kJ/kg), Bu4NOH.32H20 (mp, 30.2°C). Like salt hydrates, clathrate hydrates may melt congruently or incongruently. They also tend to supercool and hence suitable nucleating techniques must be found to promote crystallization. 4. MELTING AND FREEZING CHARACTERISTICS

By melting and freezing characteristics of phase change heat storage materials, we understand the behaviour exhibited by these materials during heating and cooling, viz. melting and freezing ranges, congruency of melting, nucleation characteristics, supercooling of the melt and stability to thermal cycling. A comprehensive knowledge of the melting and freezing behaviour of heat-of-fusion storage substances, and particularly its reproducibility as a consequence of repeated melting and freezing of the substances, is hence essential for the assurance of the long-term performance of a latent heat store.

fmp =

melting point. AH~- = heat-of-fusion.

Measurement techniques Primarily two measurement techniques are employed for the determination of the melting and freezing characteristics: (1) Differential Scanning Calorimetry (DSC), and (2) Thermal Analysis (TA). Table 6 provides details of the two measurement techniques and a listing of data that may be evaluated. The two techniques distinguish themselves in terms of the type of measurements made, the quantity of the sample used in the tests and the speed with which results can be obtained. For example, the DSC provides quick and reliable results in the form of Energy-Time diagrams (Thermograms) using very small quantities of the sample (ca. 1-10mg)). Evaluation of the thermograms yields rather precise values of the phase transition temperatures during melting and freezing of the sample, the heats of fusion and solidification and the specific heat variation as a function of temperature. The DSC is, however, a severe test for substances that supercool, since the supercooling tendencies are maximized due to the small quantity of the samples and the poor nucleation conditions in the DSC testing pans. Consequently, for materials like salt hydrates, the DSC fails to provide meaningful information on the degree of supercooling and on the freezing point of the substance[7, 19]. The Thermal Analysis technique, on the other hand, involves the determination of Temperature-Time (T-t) diagrams, or the Heating and Cooling curves, recorded during the melting and freezing of the sample. The TA-technique uses about 10 g to a few kilograms of the sample, depending on the apparatus used, and is hence slower. With proper care, the rate of heating and cooling

Low temperature latent heat thermal energy storage: heat storage materials

A. Heating curves

B.

70

o

,Fr

0

60~

20

=

-'

,

?,

40

,

,

, ~

,

,

,

3o

t00

.

.

t I minl

.

.

~'

~----____~,.,.

20

I?o, " . . . aO

N

50

Paraffin 61o5

Tin

Cooling curves

6o

(a)

.-~0

120

.

0

140

.

(b)

_J.--g/

321

eo

Louric

~o Cn J

,

20

'

40

60

80

100 12D t (mini

140

'

_ 5o ~_ ~ 4°rT,

Acid

It~

It

I

30 I

"i'o'

2Oo

11min]

't4o

20

, 20

,

i

i

40

i 60

i

J ~

~

i 100

J

60

,

i-

,-s o ~ ~

i~ 140

,

,

i

,

r ~

Na2HPO4. V 4o 12 H20 30 2O

10 O

i 120

rim=hi

(c) ~ ]o

I 0

t.O

80

120

160

2(10

240

280

i 2

0

t.

6

10

tlm~l

~lh]

12

Fig. 6. Typical heating and cooling curves for representative heat storage materials belonging to the material families of (a) commercial paraffins, (b) fatty acids,and (c) inorganic salt hydrates [3]. can be well controlled and relatively accurate results can be gained. If the tests are undertaken in glass containers, they suffer from the disadvantage that metallic surfaces that generally form the heat exchanging surface within a latent heat store, or other means that are employed to improve heat transfer in a latent heat store, are absent. Nucleating conditions different to those in actual LTES systems hence exist. Experiments done in scaled-sown test models are the most accurate as conditions similar to those in large-scale LTES systems are simulated within them. These tests are also the slowest in comparison to those in glass apparatuses due to the large quantities of phase change materials they require (1-10 kg). Thermal cycling of the sample to obtain data on the reproducibility of the melting and freezing behaviour of the substance may be undertaken using both measurement techniques. Any variations observed in the form of the endothermic or exothermic peaks on the DSC-thermograms, or alterations in the values of the thermophysical properties of the thermally cycled heat storage materials, serve as a measure of the change in their melting and freezing characteristics. In the TAmeasurements, on the other hand, changes in the form of the temperature-time diagrams or in the heat transfer rate into or out of the sample provide information on the influence of thermal cycling. We shall now discuss some results for the melting and freezing characteristics of PCMs belonging to three materials groups: paraffins, fatty acids and salt hydrates.

Paraffins (a) TA-measurements. Figure 6(a) shows typical heating and cooling curves obtained during TA-measurements with a technical grade n-paraffin, paraffin 6106. As

seen in Fig. 6(a), the paraffin exhibits two freezing ranges: a narrow freezing range characterized by QN (Region II) and a larger freezing range characterized by NP (Region III). The two freezing ranges respectively signify a liquid-to-solid transition and a solid-to-solid transition[3]. A part of the total latent heat of fusion is stored in the substance during each of these transitions. Not all commercial waxes (all of these are essentially paraffin mixtures) display a freezing behaviour as shown in Fig. 6(a). For the sake of comparisons, the cooling curves obtained during slow cooling of three different paraffins are presented in Fig. 7 [17]. The paraffins included in the diagram are seen to vary extensively in their freezing interval. While the n-paraffin 6106 exhibits a rather fiat freezing plateau during the time period of the experiments, the iso-paraffins 6403 and 6035 undergo freezing in a temperature range. In fact, the freezing interval of paraffin 6035 is extremely large, rendering it completely unfit for use in an LTES system. 90

,

,

,

,

,

1-80~ It \ ~ 70I', ~.._ ® 60 = 50

-Paraffin 61.03 - - - Paraffin 6035 - - - - P a r a f f i n 6106

~

\ \ ~

~o " ~

"" "" ~--~~

13° 2o

4

2i

;

~

,i

~i

Time [h]

Fig. 7. Comparison of cooling curves for three different commercial paraffins[17].

322

A. ABHAT

Table 7. Melting and freezing range of Paraffin 6106 obtained from Thermal Analysis(TA) measurements[3]

]

MANUFACTURER'S RECOMMENDED

42

-

44 °C

PARAFFIN 6&O3 HEATING 2 CYCLE RUN NO 1L2 HEATING RATE 5KtMIN

FREEZING RANGE

MEASURED FREEZING RANGE

38 - 43 *C

MEASURED MELTING RANGE

36 - 43 "C

Another important observation with paraffin is the large difference between the experimentally measured freezing range and the manufacturer's data, as seen through th6 example of Paraffin 6106 in Table 7. This result is of particular importance to the design and operation of a LTES system, which calls for an exact knowledge of the phase transition temperature range of the heat storage material. Further quantification of these results can be carried out using the DSC-technique. (b) DSC-measurements. Typical thermograms for nand iso-paraffins are presented through the example of Paraffin 5838 and Paraffin 6403 in Figs. 8 and 9, respectively[7]. The n-paraffin exhibits a sharp peak around 46°C and secondary peaks at temperatures between 12 and 46°C, whereas the iso-paraffin exhibits one peak which is, however, smeared over a large temperature range of about 43 K. Evaluation of the peaks shows that both paraffins undergo solid-liquid and solidsolid phase transitions. The D$C technique additionally provides a quantitative measurement of the energy content associated with each of the phase transitions. Some results from the DSC-measurements of the phase transition temperatures and enthalpies of commercial paraffins are presented in Table 8. For the sake of comparisons, results from measurements with 99 per I

12 09

I~--

]

I

I

PARAFFIN 5838 HEATING 1CYCLE RUN NO 73 HEATING RATE 25 K/MIN

06 03 0 tz:

300

I

I

t

2

&

305

310

6 J 315

8 TIME MIN I I 320 TEMPERATURE [K]

RAFFIN OLING

5838 3 CYCLE

-08

LINNOGRA;8E 25 K/MIN (.9 -12

6

33o~

320

1_ 310

a 9

TIME 15 J_ [ MINI TEMPERATURE { K ]

Fig. 8. Typical DSC-thermogramsfor n-paraffinsshown through the example of Paraffin 5838[7].

/

10

G2~

b

280

L

L

290

300

I 8 I 320

310

1'0 I~N]12 "~141 iTIWE j 330 3~0 350 TEMPERATURE [K} ]

025 X

I

N

G

PARAFFIN 6~03 COOLING 2 CYCLE RUN NO 1~3 RATE 5K/MIN

ua

-175

3/,0

I

I

J 330

~ 320

I 310

I 300

ITINE [MIN! 290 280 TEMPERATURE[KI

Fig. 9. Typical DSC-thermogramsfor iso-paratfinsshown through the example of Paraffin6403[7]. cent pure octadecane, a research grade paraffin, are also included in the Table (from [51). For all commercial paraffins tested, the deviation between the manufacturer's data and the measured values is noteworthy. All paraffins exhibited two phase transitions. Whereas the n-paraffins experienced the primary phase transition (solid-liquid) in a narrow temperature range of about 2 K, the corresponding temperature range for iso-paraffins was large (-> 14 K). The research grade n-octadecane, on the other hand, exhibited only one solid-liquid phase transition in a narrow temperature range of 1.5 K. Evaluation of the thermograms furthermore showed that while the total phase transition enthalpy of the commercial paraffins agreed closely with the manufacturer's data, the enthalpy associated with the solid-solid phase transition was rather significant (ca. 30 to 50 per cent of the total). The above results are of particular importance for the choice of paraffins for low temperature applications, wherein the temperature excursions of the store are generally limited to 10-15 K about the melting point. Only n-paraffins may hence be selected. Although the oil content of the paraffins does not play any significant role, the amount of heat stored in a a narrow temperature range is dependent on the phase transition enthalpy of the primary solid-liquid transition, which should be measured for the paraffin in question.

Low temperature latent heat thermal energy storage: heat storage materials

323

Table 8. Phase transition temperatures and associated phase transition enthalpies for paraffins, obtained from Differential Scanning Calorimetry (DSC) measurements[5] PARAFFIN

PW~UFACTURER'S DATA

MEASUREDVALUES

OIL- FREEZING HEATOF ~ "Ig,ANSTRUC- CON'- RANGE FUSION SITIONTEMRERANGE TURE TENT

TYPE

C~IN

~ITION

Fr

6106

/%/

/'C/

/KJ/KC-,/

I'C/

N

5

189 189 189 189

19 - 44

N

0.5

6035 6A133

tSO tSO

4 0,5

LQ - 44 48 - 50 58 - 60 62 - 64

6499

iso

3

66 - 68

189

23 - 66.6 -6 - 71.6

28 - 29

246

27

OCTADECm~3)

1) AHI ~

AH2 REPRESENTTHE ~ I E S

2) PARAFFIN~

122 -8

- 48.3 -

er1

all.l_ 1)

I'C/

IKJIKG/

40.7 - 44

06.2 - o,8.3

64.4

39

-

64,5

51,7 - 66,6 2)

- 28.5

27 -

28,5

TEMPERATURE~

pT2

aH2 i)

/'C/

/KJ/V.C,/

129.8 134.4 ~5.7 129.8

19- ~.7 12.7 - 46.2 -8 - 39

2)

2)

TRANSITION

TOTAl.

PROPORT IONS

IKJI~cV

AH %

AH

49.2

179

72.5 27.5

63.0

i~.4 168.9

68.0 32.0 50.7 49.3

189

682 31.3

83.2

23 - 51,7 59.2 2)

230

ASSOCIATE/)WITH THE SOLID-LIQUID ~

EXHIBITED A PEAKsPREADOVERA ~

~E

SOLID-SOLID PHASE TRANSITION

SOLII)-LIQUID

lt6

2)

230

100

2)

SOtlD-SOLIDPHASETRANSITIONS RESPECTIVELY

OF ABOUT78 K, SO THATTHE ~

PHASETRANSITIONS

COULDNOT BE DISTINGUISHEDFROMEACHOTHER 3) N-OC-TAI~EC~/~E(99 ~ PURE) WAS EMPLOYEDAS A REFERENCEPARAFFINDURINGTHE INVESTIGATIONS

z, LAURIC ACID HEATING 2 CYCLE RUN NO 15 HEATING RATE 2

3

2 1

0

"

312

~-~

:

I

3

TIME( MIN]

I

I

I

31&

316

31B

LAURIC

I

C

-'1-

L,.

Ro..o

] CYCLE

I

1,

I

il

/

/

I/ I/ 15

IO

)

TEMPERATUREI K ]

317

31~

20 31--15

2S 31"J&

TIME [MINI I. T E M P E R ~

K"'~ |

Fig. 10. DSC-thermograms of lauric acid, representative of fatty acids [6]. Fatty acids (a) TA-measurements.

The melting and freezing characteristics of fatty acids are presented through the example of lauric acid in Fig. 6(b), which contains the heating and cooling curves for this material [3]. The freezing plateaus are long and flat and no supercooling is

evident, though in some cases, a small degree of supercooling (-0.5 K) has been measured[4]. This behaviour is representative of all fatty acids as well as of polyethylene glycols with melting points between 15 and 70°C[19]. (b) DSC-measurements. Fatty acids possess good melting and freezing characteristics, as may be seen in Fig. 10 from the sharp peaks traced on the DSC-thermograms during the heating and cooling of lauric acid. Results from the DSC-measurements with two fatty acids [6] are summarized in Table 9. The difference between the melting and freezing points given in the Table is a measure of the supercooling of the substances. As mentioned earlier, yet smaller amount of supercooling (ca. 0.5 K) were observed in TA-tests undertaken using somewhat larger quantities of the substance. The small amount of supercooling during the freezing of fatty acids does not hamper their potential use as heat storage substances. Although fatty acids are good heat storage substances, they are somewhat too expensive for large-scale heat storage applications. They are, however, far cheaper than research grade paraffins and are recommended for the function tests of new "passively-operating" heat-offusion storage system designs. Recent discussions with manufacturers indicate that the cost of some fatty acids can be brought down by about 50 per cent as a result of large scale production. For LTES systems operating within a narrow temperature range, the cost of heat storage material per kilojoule of stored energy may then compare fairly well for fatty acids and commercial paraffins.

Table 9. DSC-measurements with fatty acids [6] HEAT STORAGE SUBSTANCE

NUMBER OF THERMAL CYCLES PERFORMED

MEASURED VALUES MELTING POINT 1%1

LAURIC ACID PALIMITIC ACID

FREEZING POINT 1%1

3

43,5 -'I"0,05

39,9

5

61,2 +- 0.07

59.9 _+ 0.13

*

0.05

HEAT OF FUSION /KJ/KG/

HEAT OF SOLIDIFICATION

/KJ/KGI 2.5

169.3

-'!" 2 . 0

168,8

+

196.1

+ 2.0

197.0

-'! 3 . 0

324

A. ABHAT

\ 50"

Crystals

N°2C0310H20

No2SQ 10H20 2O Time [hi

Fig. l l. Freezing (cooling) curves for various inorganic salt hydrates[26].

Salt hydrates (a) TA-measurements. Typical heating and cooling curves for salt hydrates are shown through the example of Na:HPO4 • 12H20 in Fig. 6(c). Melting of the material takes place at a constant temperature, whereas freezing is associated with appreciable supercooling[3]. Freezing curves for several other salts obtained from tests in glass capsules[26] are presented in Fig. 11. For all these materials, the melt does not freeze at its thermodynamic freezing point, but is supercooled by several degrees below the freezing point. The supercooled liquid hence exists in a highly metastable state. Formation of or introduction of a single crystal nucleus into the melt causes a spontaneous crystallization of the whole melt. In the case of salt hydrates, the geometry of the test

apparatus has been found to have a significant influence on the melting and freezing behaviour of the material[6]. Table 10 compares results from TA-measurements with the salt hydrates carried out in three different experimental apparatuses. Poor nucleation conditions in the glass test tube apparatus (Apparatus A)--which is also the most commonly suggested apparatus in the literature-cause substantial supercooling of the salt hydrates. In fact, the degree of supercooling observed during the cooling of Na2S203"5H20 was so large ( > 40 K) that no freezing of the substance occurred within the temperature range used in the tests. As a consequence, the tests were terminated after merely 2 thermal cycles. The amount of supercooling could be reduced by using glass containers (Apparatus B) containing a shallow bed of the salt hydrate. The larger glass surface aided in somewhat improving the nucleation conditions in the melt and the shallow bed assisted in eliminating segregation effects in the molten material. Both Apparatuses A and B are, however, far remote from practical latent heat store designs. Experiments to investigate the melting and freezing characteristics of salt hydrates were hence undertaken in laboratory models (Apparatus CI, C2) of two latent heat stores that have been recommended for large scale applications [2, 5]. The heat storage concepts selected employ large metallic heat exchanger surfaces (fins) for heat transfer into the poorly conducting heat-of-fusion storage substance, and are suitable for "passively-operating" latent heat stores. Results contained in Table 10 now indicate a very favourable freezing behaviour of the salt hydrates, characterized by their small degree of supercooling. This improvement, in comparison to the values measured using Apparatus A and B, results from the presence of the aluminum heat exchanger surfaces contacting the salt hydrate, which strongly promote heterogenous nucleation in the molten salt[4]. (b) DSC-measurements. The calorimetric measurements with salt hydrates[7] are illustrated here through the example of the incongruently melting calcium chloride

Table 10. TA-measurementswith inorganicsalt hydrates[6] SALT HYDRATES

LITERATURE VALUE OF FREEZING POINT

TEST + APPARATUS USED

NO, OF THERMAL CYCLES PERFORMED

M E A S U R E DVALUES (AVERAGES) DEGREE OF SUPERCOOLING

I°CI CACL2,6H20

29,7

/K/ A C1

10 5O 100

25 5 i

A C2

10 20

4

A

2 40 20

40 ii 5

B

ZN(N03)2.6H20 NA2S203.5H20

36,4 48.0

B

c2

+ DETAILS OF TEST APPARATUS DESIGN ARE PROVIDED IN APPENDIX A

- GLASS TEST TUBES

B

- GLASS CONTAINERS

C1 - LABORATORY MODEL OF THE FINNED HEAT PIPE HEAT EXCHANGER DESIGN / 2 / C2 - LABORATORY MODEL OF THE FINNED ANNULUS HEAT EXCHANGER ELEMENT / 5 /

8

Low temperature latent heat thermal energy storage: heat storage materials 6-hydrate. Experimentation with this substance was undertaken in two operation modes: Mode 1--tests in hermetically sealed pans, and Mode 2--tests in nonhermetically sealed or "open" pans, wherein a hole was punched in the hermetically sealed pans. Figures 12 and 13, respectively present typical thermograms obtained during the heating and cooling of CaCI2.6H20 in hermetically sealed and open pans[6, 21]. Tests in the hermetically sealed pans (Mode 1) delivered thermograms with a single endothermic or exothermic peak which was reproducible in form even after thermal cycling up to 11 cycles. Operation in open pans (Mode 2), on the other hand, yielded thermograms with a single endothermic peak during the first melting

325

process but with two or more peaks during subsequent cooling and heating processes. The salt hydrate CaCI2.6H20 thus undergoes decomposition during tests in the open pans. By the fifth thermal cycle, a clear secondary peak is observed at 45°C, signifying the partial decomposition of CaCI2.6H20 into its lower form CaCI2.4H_,O. Results pertaining to the melting point and the heat of fusion of four salt hydrates tested [6] are summarized in Table 11. The values of the thermophysical properties of the substances as obtained from measurements with various samples tested in hermetically sealed pans (Mode 1) are seen to fall in a small range and the average values agree fairly well with literature values. On the other 0 ~

2,5

uq 2,0 I

RUN NO : 183

RUN NO.

COOLING : /,,.CYCLE

COOLING : t,.CYCLE

RATE : tr o

2.5 KIMIN

RATE "

.!

:

184.

2.5

K/NIN

1,5 0 2

=:

1.0

/_

ud 6 <[

0.5

B

0 0

I

290

I

3O0

I

3~0 TEMPERATURE

i

2

250

2/*0

[K]

230 TEMPERATURE [K]

!

20 o3 u

z o

1.5

RUN NO.: 197

RUN NO. : 198

H E A T I N G : 11 CYCLE

COOLING : 11. CYCLE

RATE :

RATE

2.5 K / M I N

2.5 K / M I N

I..&. r,,"

0

1.0 w z w

0.5

0

t

290

I

300

I

1

310 TEMPERATURE [K]

260

250

2t,0 TEMPERATURE [ K ]

Fig. 12. DSC-thermogramsfor calciumchloride 6-hydrate, tested in hermeticallysealed pans[6].

326

A. ABHAT

I•

r

1

1

T

r

q

i

I.,I(~ AT I ~ I.CYCLE RUN NO. 106 HEATINO RATE 1.2~" KIMIN

|

COOLING I.CYCLE RUN NO 10g

CI8

I

COOLING RATE 1.25 KI HIN

0.t.

I

2

3

t,

i

i

5

8

i • - TIME IMIN] 2~78 TEMPERATt.h~.IK] 258

f.__---~1.21

"0.6 COOLING 3 CYCLE RUN NO 113 COOLING RATE 2.5K! MIN

HEATING 5 CYCLE RUN NO. 116 HEATING RATE 5.0K/MIN

m~08

-2.2

-3£I

}

0 I

~5 TIME[ MIN]

L

290

t 2

e,

30(]

310

J

T kME [ I~IN]

TEMPERATURE [K]

Fig. 13. DSC-thermogramsfor calcium chloride 6-hydrate, tested in non-hermetically sealed (or "open") pans [6].

Table 11. DSC-measurementswith inorganic salt hydrates [6] ,SALT HYI~TE

LITERATUREVALUE OF

OPERATION

MF.LTINGI.r~kT-OFPOINT FUSION

ME/k,~IRE]) VALUES

NO. OF ,SAI'I~ES

TOTAl. NO. THERM~. CYCLES

~

OF VALLF.S - I

MELTING

HEAT-OF-

POINT

FI.~1ON

I

NE~T--~-

MELTING POINT

FIEION

(PRIMARY

I TR~SITION)

/'C/

c~.~0

I

i.~

/~,,~

/'C/

i

/KJ/Kr.,/

I J

152

29.7

ZNt%b.r~2o ~.4 N,~S,~.~H,~ ~.o

201

N,(%>2.F~H20~.7

188

NOTES; +

I

/KJ/KG/

Z~7

136 - 175

4 2

a,8.5-49.0

190 -

13

53.5-57.0

145 - 160 "t

I

29

1 2

1 1

i I

1

3

i OPERATIO~I M01DE: ,].-TESTS IN HB~4ETICAU_y ~ 2-1"IE~ IN OPEN PANS

") ,~CAI'IF.RTOO ~

~,5-~.0 36.0-57,0

5

1

+)

,

]6,5

20O +)

1

55,2

+)

I

P~WWS

TO QBI"AIN MEANINGFUL AVERAGES

+) NO EVALUATION ~ I ~ ,

AVB~6E OF 8 ~

CYCLES

hand, results from measurements in open pans show a large scatter as a consequence of the decomposition of the salt hydrates. In practical systems, the use of salt hydrates is hence recommended only in hermetically sealed or encapsulated heat-of-fusion storage system designs. It is furthermore recommended to use the salt hydrates in an atmosphere free of air[6, 7]. Modified salt hydrates. The discussion in the preceding section has been restricted to the investigation of the behavioural characteristics of "pure" salt hydrates i.e. in which no foreign substances are intentionally added to prevent segregation and/or to eliminate supercooling of the substance prior to crystallization. There have been several attempts in the literature to "modify" salt

hydrates to obtain a reproducible melting and freezing behaviour. Although only limited success has been reported with the methods employed, we shall discuss them here briefly for the sake of completeness. (a) Suspension media. Addition of suspension media or thickening agents to the salt hydrate to prevent separation of the solid and liquid phases has been recommended [l l, 27]. The use of a thickener also assists in suspending the nucleating agents within the heat storage medium bulk, which otherwise tend to collect at the container bottom due to density differences. Thickening agents, however, displace a part of the salt hydrate in the heat store, thus reducing the volumetric heat storage capacity of the heat store. Furthermore, they work towards a lowering of the melting point of the heat

Low temperature latent heat thermal energy storage: heat storage materials

327

Table 12. Some suspension media for use with inorganic salt hydrates HEAT OF FUSION MATERIALS

~USPENSlON MEDIA

CACL2,GH20

HYDROXY ETHYL CELLULOSE

REFERENCE

11

CA(NO3)2,4H20

POLYACRYLIC ACID

11

NACO3.1OH20

POLYETHYLENE OXIDE

11

NA2HPOh.12H20

STARCH

11

NA2S203.SH20

wOOD PULP

11

NA2SO4.10H20

CLAY (BENTONITE, ATTAPULGITE)

11, 26

Table 13. Some nucleatingagents for use with inorganic salt hydrates HEAT STORAGE SUBSTANCE (PCM)

NUCLEATINGAGENT (SEEDCRYSTAL)

DEGREE OF SUPERCOOLING

WITHOUT SEED CRYSTAL

K LICLO3.3H20

KCLOq, NA2SIF6, K2SIF6, BASIF6

2 - 8 1)

KF.4H20

PUMICE STONE

15 - 24

CACL2.6H20

BACO3, SRCO3, BAF2, SRF2

10 - lq

NA2SO4.10H20

BORAX

ZN(NO3)2.BH20

ZNO, ZN(OH)2

KF.2H20

AL203

REFERENCE

WITH SEED CRYSTAL

K 0.5

23

9.5

23

3 - 5 1)

Ii,

14

2 - 5

8, 23, 26

23

2 - 7

i - 6

4, 19

25 - 35

5 - 8

23

i ) MINIMUM AND MAXIMUM VALUES OBSERVED

storage substance. Table 12 lists some suspension media recommended for different salt hydrates. (b) Nucleating agents. A nucleating agent is a material having a crystal structure similar in lattice spacing to that of the heat storage substance. They serve as nuclei for the PCM crystals to grow on them during freezing of the PCM and are also termed as "seed-crystals". For good results, measures should be taken whereby the nucleating agents are homogeneously dispersed within the bulk PCM and/or they are in contact with the heat transfer surface. Table 13 provides a list of some of the commonly recommended nucleating agents for use with salt hydrates and the reduction in the degree of supercooling attained with their use. (c) Extra water principle. Extra water may be added to a salt hydrate to allow dissolution of the anhydrous salt in the water at the melting point of the salt hydrate, so that the heat storage medium becomes a saturated salt solution at the melting point. During cooling, the solubility of the salt in water for temperatures below the melting point decreases with decreasing temperatures resulting in crystallization of the salt hydrate. Thus at temperatures below the melting point, the storage medium consists of a salt hydrate solid phase and solution. Upon repeated heating and cooling, the solid phases of the salt hydrate then completely go into solution or crystallization out of solution takes place. Soft stirring of the medium may, however, be necessary to overcome the density differences between the salt solution and the solid phases/23/. The stirring effect has been achieved in some practical systems by employing the direct contact heat exchange principle, wherein cold oil sprayed through fine pores directly in the molten phase of the heat storage substances simultaneously cools down the liquid and creates a mixing effect/12/.

Using a mixture of 68.2 wt per cent Na2SO4 • 10H20 and 31.8wt per cent H20, Biswas[9] reports good results in comparison to pure Glauber salt. Nucleation of the decahydrate occurred readily, even without the addition of borax. Furbo[14] reports some preliminary results with several different salt hydrates and finds them attractive in use in comparison to sensible heat storage in water. The major disadvantage of the extra water principle is, however, the loss in volumetric heat storage capacity in relation to that of the pure salt hydrate. For example, with Biswas' composition of Na2SO4.10H20 and n20, a mass about 50 per cent larger and a volume 70 per cent larger than for an ideally efficient system based on pure Na2SO4.10H20 would be required to store the same amount of heat [9]. (d) Other techniques. Another technique to avoid phase separation in incongruent melting salt hydrates is that suggested by Carlsson et al.[lO], who propose chemical modification of an incongruent heat-of-fusion system to make it behave as a congruent system. Through the addition of 2 wt. per cent SrCI2 • 6H20, the melting point of the resultant salt hydrate was altered so that the melting point maximum for CaCI2.6H20 coincided with the point where equilibrium between CaCI2.6H20 CaCIE.4H20 and the solution exists. Upon melting of the modified salt hydrate, the peritectic point was thus bypassed and the formation of the lower salt hydrate (CaCI2.4H20 in this case) avoided. 5. THERMALCYCLING One of the most severe tests that phase change heat storage materials must undergo is thermal cycling involving repeated melting and freezing of the heat storage materials. For example, for a 20-yr life of a one-day heat

328

A. ABHAT

50 To

S fo

--

T

o

~ 2 0 0 --~-rJo ~

~

~

,

,

I

Sample A. Not Thickened Sample B, Thickened Ambient Air Temperature

I,

I:'

,s

Cycle No

,._..-

~

~200

201 100 Hours ~

200

1000 I

1

i

1

2

3

._L. - ~-

Cooling

Heating

I

Time [ h]

/.

Fig. 14. Temperatureprofilesmeasured during thermal cyclingtests with two samples of Glauber salt plus 3 wt. per cent Borax. Sample B uses in addition 8 per cent thickener as suspensionmedia for the salt hydrate and nucleating agent[27]. store, the phase change material experiences one meltingfreezing cycle dailp or a total of 7365 cycles during the system life. The influence of thermal cycling on the phase change material characteristics must be measured experimentally. Laboratory measurements comprising at least 10002000 cycles are recommended, particularly with the inorganic salt hydrates, to establish the long-term thermal stability of the PCM. The measurements should include the temporal variation of temperature within the PCM and the temporal variation of the heat transfer rate during the charging and discharging of the storage substance. The thermal energy stored in the PCM during each cycle should be computed and the variation of the stored energy with the number of thermal cycles should be studied. A large gap exists today in the area of thermal cycling tests. Limited thermal cycling (120 cycles) carried out with paraffins and lauric acid exhibited no degradation of the materials[4]. Short-term thermal cycling (up to 90 cycles of Na2HPO4 • 12H20 showed that the material melts congruently when it is not contaminated with the heptahydrate, i.e. when the formation of the heptahydrate has been prevented by appropriate nucleation[27]. Test results describing the temperature variation in Na2SO4' 10H20 over 1000 heating-cooling cycles are available[27]. Two material samples of Na2SO4.10H20 were used in these tests--one sample comprising the phase change material and 3 per cent Borax by weight as nucleating agent, and the second sample comprising the same constituents as above plus 8 per cent thickener that formed a thixotropic gel. The results of these thermal cycling tests are presented in Fig. 14. The material with thickener showed no degradation in the temperature

profile following cycling. However, the material without the thickener contained approximately 30-35 per cent liquid at the end of the cooling cycle. A sediment layer formed which remained throughout the testing period. As a consequence, the non-thickened material attained a higher temperature during its heating cycle, as seen in Fig. 14. The heat stored in Glauber salt as a function of thermal cycling was measured using standard calorimetric techniques[22]. Two different samples consisting of Na2SO4" 10H20 with 3 per cent Borax and thickened material comprising 88wt. per cent Glauber salt+ 2.64wt per cent Borax+9.36 wt per cent attapulgite clay were included in the tests. The samples weighed 150 g each and were cycled within a temperature range 27-38°C. Figure 15 shows the measured thermal energy per gram of mixture (AH) as a function of thermal

Glaubers SaLI+ 3 % Borax •¢/•TheoreticaJ-Pure

6O

251 0 J

5O

A.

4O

AH kJ/kg

t

30

1255

20 10

l[ ;!;

½ • = Pure Glaubet's Sa# + 3 % Borax • • Glouioers Sort + Borax ~"Aftoputqtte

t Clay

o 2b 6b ,6o ,,o ,8o 21o 2~o 36o 3~o ~ ,.&o No. of Cycles

Fig. 15. Thermal energy stored in unthickened and thickened mixtures of Glauber salt and Borax as a function of the number of thermal cycles[22].

Low temperaturelatent heat thermal energy storage: heat storage materials cycling for the two materials. The results in Fig. 15 reveal that the thermal capacity of the pure salt declines quickly from an initial value of 238 kJ/kg to 63 kJ/kg after 40 cycles. The thermal capacity of the thickened salt also declines--albeit slowly--to 125kJ[kg in the 140th cycle[22]. Both the pure Glauber salt and the thickened mixture are thus unfit for long-term use as latent heat storage materials, despite the fact that the results in Fig. 14 for the temperature profiles showed the thickened material to possess a reproducible behaviour upon thermal cycling. 6. COMPATIBILITY WITH MATERIALS OF CONSTRUCTION

Knowledge regarding the compatibility of phase change heat storage materials with conventional materials of construction is of particular importance for the assurance of the life of an LTES system. Only limited compatibility data is, however, presently available and further work in this direction is recommended. The choice of the operating conditions and evaluation techniques plays an important role in the corrosion investigations. In most tests, the samples of the construction materials are immersed in the liquid phase (melt) of the heat storage material contained in air tight bottles, held at a constant temperature, uusally 5-20 K above the melting point of the corresponding PCM. Several samples are used, one to a bottle, and they are removed from the bottles after predetermined time intervals, cleaned and analysed. More realistic operating conditions, however, require that the PCM be thermally cycled during the corrosion tests. Gravimetric analysis, optical and scanning electron microscopy techniques, and chemical anlysis of the corrosion products are the conventional evaluation methods employed. Gravimetric tests prior to and following the

329

corrosion tests after predetermined contact times provide the sample mass loss, from which the reduction in sample thickness with time (for uniform surface corrosion) and the corrosion rate may be computed. In general, according to accepted norms, a metal experiencing a thickness reduction of <-0.1 mm/a is termed corrosion resistant. For cases where the corrosion rate attains a linear value, an estimate of the lifetime (e.g. 20-yr life) of a metal in contact with a PCM is possible. Microscopy techniques, on the other hand, provide information on the type of corrosion, for example, pitting corrosion, cracks, etc. The results from various corrosion studies undertaken with phase change materials for latent heat storage applications are presented in Table 14[17, 18, 23, 29]. The organic materials are seen to be compatible with the metals tested. With the salts, however, one needs to be careful as preferential compatibility may exist. Stainless steel is the only metal that was found compatible with all phase change materials tested. Mild steel is a fairly good material being corrosion resistant to most PCMs in Table 14, with the exception of Zn(NO3)2.6H_,O and the Mg(NO3)2-MgCI2 eutectic. The corrosion investigation with mild steel joined by the tin solder and several inorganic salt hydrates and their eutectics[23] showed that the mass loss is rather low and decreases with time to negligible values. Copper was found compatible with all heat storage materials tested, except sodium thiosulphate 5-hydrate. When immersed in the melt of Na2S~O3.SH20, copper exhibited rapid corrosion and a black layer of CuS was seen to form within just 10 days of contact. The mass loss of the sample after 300 days of contact was found to be 8.17g and the thickness reduction 610#m. A photograph of the corroded sample taken after 50 days contact is presented in Fig. 16[17].

Table 14. Corrosion investigations with selected phase change heat storage materials HEAT STORAGE MATERIAL

c)

PHASE TRANSITION TEMPERATURE OF P(24 (°C)

TEMPERATURE LEVEL OF MELT DORING TESTS (APPROX.) (°C)

R£F.

MATERIAL OF CONSTRUCTION STAINLESS MILD STEEL STEEL 1.4301 1.0330

TIN PLATED COPPER AL.qg,5 MILDSTEEL

ALMG3

i, LAURIC ACID

44

65

+

+

0

+

+

+

17, 18

2. WAX ESTER, LDXIQL G 32

58

80

+

0

o

+

0

+

17

0

23

3. LICLD3.3H20

8.1

20

0

0

+

O

0

4. CACL2,~%0

29.7

50

+

+

O

+

-

5. NA2S04.10H20

32.4

50

+

0

0

+

O

+

O

+

18

55

+

36.4

55

+

48.0

70

+

+

0

58.0

60

0

0

+

116,0

140

0

0

+

60

0

0

+

70

+ A)

_ B)

O

9, ~ . 3 H 2 0

59.1

+

+

17, 18

0

0

0

25

0

0

0

23

O

O

0

23

0

O

20

- ~t~CL2.~ 2 0 (4?MOL%) NOTATIQ[~L: + Q~RR0610~ RESISTANT - UNSUITABLE O r"ETAL-PCMPAIR NOT INVESTIC~TED

SE Vol. 30, No.

8

O

35.0

7. Z~(N%)2,e~20 8. N~203.5~0 10, MG(N03)2,Q'i20

+

1.7, 18

6. NA2HM34.12H20

+

18

A) TESTS WITH STAINLESS STEEL AISI-403

B) TESTS WITH ] ~ ~

STEEL

330

A. ABHAT

Fig. 16. Surface of Cu99.9 sample after 50 days contact with the melt of sodium thiosulphate 5-hydrate[17]. Table 14 reveals that aluminum (A199.5) and aluminum alloy AIMg3 are incompatible with the salt hydrates investigated, with the exception of Na2S203.5H20. With Na2HPO4.12H20, samples of both metals experienced surface thickness reduction as well as surface attack, with a white layer of aluminum hydroxide AI(OH)3covering the metal surface after a short period of contact[17]. Figure 17 shows results for the variation of the surface thickness reduction and the mass loss (corrosion) rate with time for A199.5 and A1Mg~ samples immersed in molten Na2HPO4.12H20. Initially the rate of corrosion is fairly high, but drops with time and attains an asymptotic value after about 100 days of contact. Scanning electron microscopy investigations indicated pitting corrosion with trans and intercrystalline cracks on the A199.5 surface after 80 days of contact, as seen in the photograph in Fig. 18. The aluminum alloy AIMg3,on the other hand, exhibited fewer cracks and tended more towards shallow pit formation[17]. From these and other results, it may be concluded that mild steel and stainless steel are essentially compatible with most heat storage materials, whereas aluminum and copper or their alloys are only preferentially compatible. Furthermore, common plastics are stated to be generally corrosion resistant to most inorganic salt hydrates and their eutectic compounds[23]. 4

v

AS

c o

-o

AI 9 9 . 5

---O----

--'-t~-.-

AI M g 3

~

-.4~-.--

n

E

30

w

-o

~ 20

2 ...i

l

i

i

i

oi

•

2o

~

'E--

~

"~--"-'-t

'

•~

,~o

.

~.

L

i ,6o

'

0 200

Time [ d ]

Fig. 17. Temporal variation of the surface thickness reduction and corrosionrate of A199.5and AIMg3samples in contact with sodium hydrogenphosphate 12-hydrate[17].

Fig. 18. Scanning electron microscope photograph of AIMg3 sample surface showing pitting corrosion after 80 days contact with the melt of sodium hydrogenphosphate 12-hydrate[17]. 7. S U M M A R Y

Latent heat storage in the temperature range 0-120°C is of interest for a variety of low temperature applications, such as space heating, domestic hot water production, heat-pump assisted space heating, green house heating, solar cooling, etc. The development of dependable heat storage systems for these and other applications requires an understanding of two essentially diverse subjects: heat-of-fusion storage materials and heat exchangers. The present paper reviews the state-ofthe-art of heat storage materials. Inexpensive commercial paraffins, fatty acids, inorganic salt hydrates, and eutectica of organic and inorganic compounds are the important materials families, to which candidate heat-of-fusion storage materials with phase transition temperatures within the temperature range 0-120°C belong. While the inorganic salt hydrates are generally preferred due to their higher volumetric heat-of-fusion in comparison to the organics, they usually suffer from the disadvantages of supercooling and decomposition upon melting. The knowledge of the melting and freezing characteristics of the heat storage materials, their ability to undergo thermal cycling and their compatiblity with construction materials is essential for assessing the short and long-term performance of a latent heat store. Using two different measurement techniques--differential scanning calorimetry and thermal analysis--the melting and freezing behaviour of the heat storage materials is determined. Results from measurements undertaken with representative organic and inorganic substances are discussed. Commercial paraffins are characterized by two phase transitions--a solid-liquid and a solid-solid phase transition--which may be spread over a large temperature range depending on the paraffin concerned, nParaffins are preferred in comparison to their iso-counterparts, as the desired solid4o-liquid phase transition is generally restricted to a narrow temperature range. Fatty acids are organic materials with excellent melting and freezing characteristics and may have a good future scope, if their cost can be brought down. Inorganic salt hydrates on the other hand, must be carefully screened

Low temperature latent heat thermal energy storage: heat storage materials for congruent, "semi-congruent", or incongruent melting substances with the aid of phase diagrams. Incongruent melting salt hydrate may be "modified" to overcome decomposition by adding suspension media, or extra water, or other substances that shift the peritectic point. The use of salt hydrates in hermetically sealed containers is recommended. Moreover, the employment of metallic surfaces to promote heterogeneous nucleation in a salt hydrate is seen to reduce the supercooling of most salt hydrates to a considerable extent. Finally, some results from thermal cycling tests and corrosion investigations are presented to illustrate their importance in the appropriate choice of materials from the standpoint of the life of a latent heat store.

Acknowledgements--The article forms part of a lecture given by the author at the Ispra course "Thermal Energy Storage" organized by the Commission of European Communities (CEC) in June 1981. The full lecture is included in the course proceedings published by D. Reidel Publishing Co., Holland on behalf of CEC. REFERENCES

1. A. Abhat, S. AbouI-Enein and G. Neure, Latent heat storage for application to solar energy systems in dwellings. (In German). Verein Deutscher Ingenieure, VDI-Berichte Nr. 288, pp. 97-104 (1977). 2. A. Abhat, Performance studies of a finned heat pipe latent heat thermal energy storage system. In SUN, Mankind's Future Source of Energy (Edited by F. de Winter and M. Cox), Vol. I. pp. 541-546. Pergamon Press, New York (1978). 3. A. Abhat, Short term thermal energy storage. Revue Physique Applique 15, 477-501 (1980). 4. A. Abhat et al., Development of a modular heat exchanger with an integrated latent heat store (in German). Report No. BMFT FB-T 81-050, Ger/nan Ministry for Science and Technology, Bonn, FRG (1981). 5. A. Abhat, S. AbouI-Enein and N. A. Malatidis, Heat-offusion storage systems for solar heating applications. In Thermal Storage of Solar Energy (Edited by C. den Ouden), pp. 151-171. Martinus Nijhoff Publishers, The Hague, Holland (1981). 6. A. Abhat and N. A. Malatidis, Determination of properties of heat-of-fusion storage materials for low temperature applications. In New Energy Conservation Technologies and Their Commercialization (Edited by J. P. Millhone and E. H. Willis), Vol. 1, pp. 847-856. Springer-Verlag, Berlin, FRG (1981). 7. A. Abhat, S. Aboul-Enein and N. A. Malatidis, Latent heat thermal energy storage--Determination of properties of storage media and development of a new heat transfer system (in German). Report No. BMFT-FB-T 82-016, German Ministry for Science and Technology, Bonn, FRG (1982). 8. S. Aboul-Enein, Investigation of a latent heat store for solar space heating systems (in German). Diplomarbeit, Report No. 5 D-0, IKE, University of Stuttgart, Stuttgart FkG (1977). 9. D. R. Biswas, Thermal energy storage using sodium sulphate decahydrate and water. Solar Energy 19, 99-100 (1977).

331

10. B. Carlsson, H. Stymne and G. Wettermark, An incongruent heat-of-fusion system--CaCI2.6H20--made congruent through the modification of the chemical composition of the system. Solar Energy 23, 343-350 (1979). 11. D. Chahroudi, Suspension media for heat storage materials. Proc. Workshop on Solar Energy Subsystems for the Heating and Cooling of Buildings. Charlottensville, Virginia, U.S.A., 56--59 (1975). 12. D. D. Edie et al., Latent heat storage using direct contact heat transfer. In Sun H (Edited by K. W. Boer and B. H. Glenn), pp. 640-644. Pergamon Press, New York (1979). 13. Encyclopedia of Polymer Science and Technoh~gy, Vol. 14, pp. 768-769. Wiley, New York (1971). 14. S. Furbo, Heat storage with an incongruently melting salt hydrate as storage medium. In Thermal Storage of Solar Energy (Edited by C. den Ouden), pp. 135-146. Nijhoff, The Hague (1981). 15. Gmelin's Handbuch der anorganischen Chemie, Verlag Chemie GmbH, Berlin, FRG, Various Editions (1927-1966t. 16. D. V. Hale, M. J. Hoover and M. J. O'Neill, Phase Change Materials Handbook. NASA Contractor Report NASA CR61363, NASA Marshall Space Flight Centre, Alabama I1971). 17. D. Heine and A. Abhat, Investigation of physical and chemical properties of phase change materials for space heating/cooling applications. In SUN: Mankind's Future Source of Energy, (Edited by F. de Winter and M. Cox), Vol. 1, pp. 500-506. Pergamon Press, New York (1978). 18. D. Heine, The chemical compatibility of construction materials with latent heat storage materials. Proc. Int. Conf. on Energy Storage, pp. 185-192. Brighton (1981). 19. G. A. Lane and D. N. Glew, Heat-of-fusion systems for solar energy storage. Proc. Workshop on Solar Energy Storage Subsystems for the Heating and Cooling of Buildings, pp. 43-55. Charlottensville, Virginia (1975). 20. H. G. Lorsch, K. W. Kaufmann and J. C Denton, Thermal energy storage for solar heating and off-peak airconditioning. Energy Conversion 15, 1-8 (1975). 21. N. A. Malatidis and A. Abhat, Investigation of the thermophysical behaviour of calcium chloride 6-hydrate for use as heat storage material in latent heat stores (in German). Forschung Ingenieur-Wesen 48, 1-26 (1982). 22. S. Marks, An investigation of Glauber's salt with respect to thermal cycling. Solar Energy 25, 255-258 (1980). 23. J. Schr6der, R. and D systems for thermal energy storage in the temperature range from -25°C to 150°C. Proc. Seminar New Ways to Save Energy, 495-504. Reidel, Dordrecht, (1980). 24. M. Telkes, Solar house heating--A problem of heat storage. Heating and Ventilating 44, 68-75 (1947). 25. M. Telkes, Nucleation of supersaturated inorganic salt solutions. Industrial and Engineering Chemistry 44, 1308-10 (1952). 26. M. Telkes, Solar energy storage. ASHRAE J. 16, 38 (1974). 27. M. Telkes, Thermal storage for solar heating and cooling. Proc. Workshop on Solar Energy Storage Subsystems for the Heating and Cooling of Buildings, pp 17-23. Charlottesville, Virginia, U.S.A. (1975). 28. J. Teubel, W. Schneider and R. Schmiegel, Erd61paraffine, VFB Deutscher Verlag fiir Grundstoff, Leipzig, GDR (1%5). 29. N. Yoneda and S. Takanashi, Eutectic mixtures for solar heat storage. Solar Energy 21, 61-63 (1978). 30. M. Zief and W. R. Wilcox, Fractional Solidification, Vol. 1. Marcel Dekker, New York (1967).

A. ABHAT

332

APPENDIX

I TEMPERATURE RECORDER

CONiROL

I

TEST TUBE THERHOCOUPLE

THERMOSTAT /

HEAT STORAGE MATERIAL

A. Glass test tube apparatus[4] I~r moco~q~e II

/,

/

II

II

__1 __ .~1

/r

~ --

Seal

g'~'~/

~ed,~

910

B. Glass container apparatus[4] THERMOCOUPLES TE2 TE3 TEe

TEl

TE5

STORAGE CHAMBER ,

}

" ~ C H A N G E R

ALUMINIUM FINS

~

P

I

P

E

HEAT STORAGECONTAINER MATERIAL

C1. Laboratory model of the finned heat pipe heat exchanger[2]

CALORIMETER

THERMOCOUPLE WIRING C2. Laboratory model of the finned annulus heat exchanger element [5]