''11105 140107

NBS

publications

NBSIR 81-2422

Factors Affecting the Service

Lives of Phase Change Storage

Systems

December 1 981

Issued February 1982

Prepared For: U.S. Department of Energy

Office of Solar Heat Technologies

^[•'onservation and Renewable Energy

Washington, DC 20585

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NATIONAL BUREAU

OF STANDARDS

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NBSIR 81-2422

FACTORS AFFECTING THE SERVICE

LIVES OF PHASE CHANGE STORAGE

SYSTEMS

DEC 9 1982

do~V~cxclc< -c

3e. too

* % I

I

John B. Ings

Paul Wencil Brown

December 1 981

Issued February 1982

Prepared For: U.S. Department of Energy

Office of Solar Heat Technologies

Conservation and Renewable Energy

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

Phase change storage systems currently in use or in advanced stages of

development are identified. Possible modes of degradation which may affect

service lives are considered. Specifically, the effects of crystal growth,

crystal segregation, supercooling, corrosion and thermal decomposition are discussed. The generic basis for the development of performance tests for

inorganic phase change materials is described.

Key Words: crystal growth; encapsulants ; failure mechanisms; nucleating agents;

phase change storage; service life prediction.
«n

vo

TABLE OF CONTENTS

Page

1. Introduction I 2. Identification of PCM's and Encapsulants . 2 3. Degradation of PCM's . 5

3.1 Degradation of Inorganic Salt Hydrate PCM's 5

3.1.1 Crystal Segregation - Life of Thickening Agents ......... 5 3.1.2 Supercooling - Life of Nucleating Agents 5 3.1.3 Crystal Growth - Life of Salt Hydrates 6

3.2 Degradation of Organic PCM's 7

4. Degradation of Encapsulating Materials .............................. 8

. Development of Performance Test for Phase Change Systems 10

. References 14

iv
1 . INTRODUCTION

An efficient method of storing solar energy is its conversion into the latent

heat of fusion of a material which undergoes a reversible phase change in an appropriate temperature range. When such a material undergoes a phase change

on heating, energy, as latent heat, is absorbed by the phase change material

(PCM). Only after the phase change is complete and all the latent heat

absorbed, will the temperature of the PCM again increase. When the PCM is

allowed to cool, the reverse takes place with heat being released as sensible

heat as the temperature drops and as latent heat during the phase change.

PCMs used in solar applications are typically either organic materials, such as paraffin wax, where the phase change is the melting and solidification of the

wax, or inorganic salts, where the phase change usually involves dehydration.

An example of the latter is sodium sulfate decahydrate (Na 9 S0^ ‘IOH9 O) which

is commonly called Glauber's salt. When Glauber's salt is*"heated from room

temperature about 0.4 cal/gram-°C is absorbed. At 32°C a phase change occurs

resulting in the formation of a saturated aqueous solution of sodium sulfate

and a crystalline Na2

S04 precipitate. The latent heat absorbed in this phase

change is about 60 cal/gram.

The advantages to using PCMs as energy storage media are twofold. First PCM

storage systems are compact when compared with sensible heat storage units such

as rock bins or water tanks. For example, about eight times more volume is

required to store a given amount of heat using a water tank than is required

for a sodium sulfate decahydrate PCM [12]. Similarly, the relative volume

advantage over rock bed storage is about 17 to I [12]. A parallel advantage

exists with respect to relative weight of the structured bins [ 12 ] and this

allows much greater flexibility in choosing a location for the storage unit.

The second advantage is that thermal stratification does not occur in phase

change storage systems, because temperatures remain nearly constant throughout

the charge-discharge cycle. If the melting point is chosen so that the storage

unit provides heat at a temperature slightly above the minimum temperature

required by the system, then the output from the solar collector need be only a few degrees warmer than the minimum temperature regardless of whether this

storage unit is charged or discharged. By contrast, a sensible heat storage

system typically operates at 22 to 33 °C above its minimum operating temperature

when it is fully charged.

1
7_ • IDENTIFICATION OF PCM'S AND ENCAPSULANTS

A review of Che literature identified the PCM systems currently in use or with

potential for widespread use and identified problems associated with the PCM's

and their containment systems. A summary of the PCM systems currently in use or with future potential are given in tables 1 and 2. The status of development of the PCM storage systems, listed in these tables is divided into three

groups: commercially available; advanced research/demonstration stage; and

exploratory research stage. 3ased on this literature review the most promising

systems appear to be paraffin wax encapsulated in steel or plastic, and salt

hydrate encapsulated in plastic or in bulk form (tanks). Hydrated salts are not contained in steel due to corrosion problems. The types of PCM system are divided into two groups, organic and inorganic, and further divided into four

types of encapsulation or containment: macroencapsulation in metal or plastic,

microencapsulation, and bulk storage in tanks. Macroencapsulation is the

containment of the paraffin wax or salt hydrate, such as Na^SO^'lOH^O , in conveniently sized, easily handled containers. The containers”may be long plastic

tubes, ’’gallon" cans, plastic trays, foil pouches, sausage chubs or a variety of

other packaging forms. Microencapsulation refers to packaging schemes in which

small granules or pellets of PCM, about 1/8 inch in diameter, are packaged in

individual envelopes. An example is the use of form-stable, high density polyethylene pellets approximately 1/8 inch in diameter as a PCM. The polyethylene

is lightly crosslinked so that when it undergoes the phase change from a crystalline to an amorphous state the pellets retain their shape and form. The

advantage of using small sized pellets is that the heat exchange between the

PCM and the transfer fluid is improved. Bulk storage refers to the storage of

PCM’s in large tanks, equipped with heat exchangers.

2
commercially

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Table

2.

Typical

Combinations

of

Encapsulating

and

Organic

Phase

Change

Materials

Presently

Available

or

Being

Considered

for

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3. DEGRADATION OF PCM'

3

3.1 degradation OF INORGANIC SALT HYDRATE PCM'S

3.1.1 Crystal Segregation - Life of Thickening Agents

Typical salt hydrate PCMs include CaC^^f^O, Na 2

S 203 Na 2

S 04 « 10 H20 and

Na 2 HP 04 * 12 H20 . CaCl 2

* 6 H20 and ^28303 « 5 H20 melt congruently. Na 2 HP 04 » 12 H20

melts semicongruently when the heptahydrate ('^HPC^'IIbO) is present [ 2 , 12 ] and congruently if it is not present. Na2

SC> 4

* 10 H20 melts incongruently . A

congruently melting salt is one which is completely soluble in its water of

hydration at its transition temperature. When a salt is not completely soluble

in its water of hydration, the undissolved salt can precipitate and settle to

the bottom of its container where it cannot be easily rehydrated. If, however,

the precipitated material becomes fully soluble in its water of hydration at

a temperature a few degrees above its melting point, the material is said to

melt semicongruently. If the precipitated material does not become fully soluble as the temperature is raised, the material is said to melt incongruently.

Resistance to rehydration which results ‘ from incongruent melting is a major problem with salt hydrate PCM’s. The problem is the most serious for NagSC^lOl^O

and the least for CaCl^’bt^O. This is because, while CaCl 2

* 6 H20 melts semicongruently in a technical sense, its behavior is very similar to that of a congruently melting salt [3].

The crystal segregation due to incongruent melting of the salt hydrates can be

controlled by thickening agents. Thickening agents are additives used to fix

the salt in a gel to eliminate settling. The effectiveness of the thickening

agents listed in table 3 have been examined experimentally with a wide range

of results [12, 13, 14]. The least expensive organic thickening agents, such

as wood pulp, paper pulp, and starch [12, 13] appear to work well in short-term

tests, but are believed to slowly decompose by bacterial or enzymatic action.

The use of formaldehyde or other preservatives to prevent the decomposition

has been suggested [14] but their effectiveness has not been verified experimentally. Other organic thickeners such as polyvinyl alcohol, hydroxethyl cellulose, and methyl cellulose also appear to work well in short duration tests but

their cost is probably prohibitive [13]. Inorganic substances such as clays

that can form thixotropic gels have also been used as thickening agents [ 12 , 15]

.

When thixotropic materials are stirred or vibrated vigorously, they exhibit a relatively low viscosity which facilitates the initial container filling

operation, but when left undisturbed a gel structure forms which hinders

subsequent settling.

The thickening agents that appear to hold the most potential for commercial

applications are inorganic thixotropic gels and stabilized wood pulp, starch

and certain proprietary elastomeric materials. Test methodologies to evaluate

the performance of thickening agents are needed.

3.1.2 Supercooling - Life of Nucleating Agents

Supercooling is prevalent in salt hydrate PCM’s. As a salt hydrate PCM cools,

its temperature can drop below its normal melting point without a change in

5
Table 3. Potential Thickening Agent

Organic Inorganic

wood shavings clays (montmorillonite type,

attapulgite)

saw dust diatomaceous earth

paper pulp

methyl cellulose

hydroxyethyl cellulose

poly (vinyl alcohol)

poly (acrylic acid)

starch

silica gel

proprietary gels
phase and without the release of latent heat. The temperature will continue

to drop until crystals of solid PGM nucleate. At that point rapid crystal

growth occurs accompanied by an increase in temperature. The problem associated with supercooling is that the system may cool below the point at which

useful heat can be extracted. A method to nucleate PCM crystallization is

usually required to prevent supercooling. The method commercially used is to

add a nucleating agent to the encapsulated PCM to provide surfaces on which

the salt hydrates will crystallize. Effective nucleating agents are crystalline materials whose interatomic lattice spacings are within 15 percent of

that of the salt to be crystallized [ 16 ] . In addition, the nucleating agent

must freeze at a temperature above the freezing point of the salt hydrate.

Nucleating agents have been found for the commonly used salt hydrates; however

some of these agents are proprietary [ 12 , 14 ]. A typical example of a salt

hydrate/nucleating agent/ thickening agent system is Na9S04 *01120 containing

three to five weight percent Na2B40y0H20 (borax) as a nucleating agent [ 12 ] and silica gel as a thickening agent. At the freezing point of Na2S04

*

10^0

borax crystals are present and provide nuclei on which the Na2S04*10H20 crystals may grow. Because the density of borax is slightly greater than that of

the saturated Na2S04 solution, the borax nuclei will settle unless a thickening

agent such as silica gel is present.

The life of a nucleating agent may be limited by a variety of factors. For

example, when silica gel was used as a thickener, it was observed to react

with borax and inhibit its nucleating capacity. Thus, both the settling of a nucleating agent and its reduction in nucleating capacity could limit the useful life of a nucleating agent and the thermal storage system. Test methods

are necessary to evaluate the effectiveness of nucleation agents as well as the

compatibility of the nucleating agent, thickening agent and phase change

material

.

3 . 1.3 Crystal Growth - Life of Salt Hydrates

The rate of growth of crystals of a hydrating salt will affect the rate of heat

transfer and hence system efficiency. The crystal growth rate has been determined to be 0.055 cm/hr for Na2S04*10H20 for temperatures near the transition

temperature [ 16 ]. Cyclic testing of Na2S04*10H20 using both wood pulp and

attapulgite clay as thickeners has shown that large crystals of Na2S04 have

formed after about 200-300 cycles [ 13 , 15 ]. Because the large crystals resist

rehydration, a reduction in the system efficiency of as much as 40 percent

results [ 15 ]

.

It is known that impurities can alter the crystal growth rate and the crystal

habit [ 17 ]. In general, impurities decrease crystal growth rates. In light

of this, the service life of salt hydrate, and more specifically, changes in

crystal growth rate and growth habit with thermal cycling in thickened systems

must be ascertained.

7
3.2 DEGRADATION OF ORGANIC PCM'S

Organic materials may be slightly less desirable as PCM's than inorganic salt

hydrates due to lower heats of fusion, higher cost and the danger associated

with the storage of large quantities of flammable substances. They do, however, melt congruently and generally do not supercool. This allows repeated

thermal cycling with no apparent loss in thermal performance. The most promising organic systems appear to be paraffin waxes encapsulated in steel or plastic. The major problems with paraffin waxes are the relatively high degree of

shrinkage upon solidifying and the low thermal conductivity of the solid waxes.

Waxes undergo 5 to 15 percent shrinkage upon solidifying. When used in bulk,

this shrinkage could result in void formation which would affect heat transfer

within the system.

Little information regarding the long-term chemical stability of organic PCM’s

is available. Potential problems include the generation of volatile degradation products leading to bloating of the container, the generation of moisture

and the generation of organic acids which would be aggressive towards the encapsulation materials. Test methods to assess these potential problems need to be

developed

.

8
4. DEGRADATION OF ENCAPSULATING MATERIALS

Metals with the exception of stainless steel have not, in general, been chosen

as a containment material for commercially available systems or advanced

research demonstration projects due to corrosion. Rather, salt hydrates are typically macroencapsulated in plastic materials and many bulk storage systems

utilize plastic tanks and plumbing. Thermal storage systems using paraffin

encapsulated in steel are not expected to develop corrosion problems. Paraffin,

in general, is quite inert and corrosion may be a problem only if impurities

or trace amounts of moisture are present. Waxes, however, are not compatible

with polyethylene and polypropylene since they can cause stress cracking in

these materials.

When plastics or plastic-foil laminated films are used to contain the a salt

hydrate PCM, film puncture by salt hydrate crystals may be a problem [18]. The

relatively slow rehydration of the salt, as would be expected during in-service

cycling of the salt hydrate storage system, may produce preferential grain

growth capable of penetrating the thin plastic film encapsulation. This phenomenon may not be evident from rapid cycling tests.

The potential problem of water migration through the plastic container walls

has been cited in the literature [3, 12]. Water lost through the plastic container walls or container closures can not conveniently be replaced, and would

in time reduce the system efficiency since insufficent water would be available

to completely rehydrate the PCM salts. However, values of moisture loss rates

have been reported to be as low as 0.25 percent water per year [19] (through

high density polyethylene). Other data on the rate of moisture loss through

plastic is also available [20]. It is possible that rapid cyclic testing typical of much of the present solar energy research efforts may not reveal effects

due to water loss or the settling of the PCM salts, both of which would occur

in the time frame of real application [3].

9
5. DEVELOPMENT OF PERFORMANCE TESTS FOR PHASE CHANGE SYSTEMS

3ased on the PCM and encapsulation materials currently in use, it is apparent

that the technology is tending towards the utilization of salt hydrate PCM's.

Typically, these systems are incongruently or semicongruently melting, inorganic

salt hydrates in which the hydrated phase is heterogeneously nucleated on cooling below the transition temperature. Clay thickened, borax nucleated, Glaubers

salt ( NagSO^ lOHgO) encapsulated in plastic is one example of a system of this

type. It is also apparent that there is a need for methods of test by which the

durability of phase change systems of this type may be assessed. Current voluntary consensus standards methods may be applied to various physical property

measurements. However, tests by which the long-term performance of phase change

systems may be evaluated, both in terms of material degradation and in terms of

loss of energy storage capacity, are less well developed.

Two major obstacles to the development of performance tests can be identified.

First, the diversity of possible materials and system designs that must be taken

into consideration require that test methods be sufficiently flexible to be

relevant to unanticipated materials and system designs. Second, performance

tests generally require both acceleration and simulation. Acceleration can

usually be attained. However, simulation can be difficult if failure mechanisms are unknown or poorly understood. In spite of this, test parameters can

be identified and constraints hypothesized based on the assumption that a performance test must involve thermal cycling. Figure 1 shows a simplified

generalized"'cycle for a salt hydrate phase change system where:

T?vjax is the maximum temperature which occurs in the cycle,

Tfj is the transition temperature on heating,

Tq is the transition temperature on cooling

TmIN i s minimum temperature which occurs in the cycle.

a and a 'are the rates at which the system is heated from Tkijjj to Tq and

from Ty to T^^, respectively.

3 and $' are the corresponding cooling rates.

tfl and t£ are the times spent at T^ and Tq, respectively.

t\jAX and t^iij are the times spent at Tj^^x and ^MIN*

It is possible to qualitatively consider the response of a phase change

material in terms of the above parameters based on the cycle shown in figure 1

.

Tn^ax must be above the transition temperature on heating, Ty, and Tj.^xn musC be

below the transition temperature on cooling, T^;. The latter may be the case initially, but may not obtain after N cycles. One failure mechanism involves

the depression of Tq due to the persistence of crystals of anhydrous material

at increasingly lower temperatures resulting from preferential crystal growth

or loss in effectiveness of nucleating agents.

I 10
3MfLLVH3dWH

Figure

1.

A

generalised

time-temperature

cycle

for

a

salt

hydrate

phase

change

system
Therefore, the variation in Tq, with the number of cycles provides a diagnostic

tool for assessing one mode of failure. In addition, time spent above Tq must

be greater than the time required for complete melting, i.e.,

C H +

‘MAX

+

T

"> * ( Tw. v ~ T, gr

) ^ b complete melting, anc*

similarly, the time spent below must be greater than the time required for

complete freezing, i.e.,

C C +

C MIN + ( TH

* TMIN^

( T

C " TMIN^

> t complete freezing*

if the maximum amount of heat is to be stored and delivered. These criteria

have material dependencies in that t^ and t^ may increase with an increasing

number of cycles. This is because t^ and ty depend on crystal size distributions which will be affected by the heating and cooling rates, and by the

maximum and minimum temperatures and their durations.

The total cycle time may be defined as

'cycle “ C MAX + C MIN + C H + C C + ( Ti - T

H

i MIN

) + ( t

max

" t h

) - ( t

max " T

C. _ Tr T

a

;

( c \

This, in turn, allows a definition of a minimum cycle time:

'T* rp *p rp . , _ .. r , /H ~ l MINx , /MAX " i Hv /MAX “ tv , C cycle, min “ C H

“ c C + ( ) + C . ) " ( ) (

otMAX a MAX MAX

T

C TMINn

^MAX

where the maximum values of the heating and cooling rates are determined by the

thermal conductivities of the system components and the system geometry.

The response of the phase change material will vary according to the shape of

the thermal cycle imposed. The average crystal size will show a dependence on

a’ and 8- If these rates are low the system spends relatively more time near

the transition temperatures. Under this circumstance crystal nucleation is

less favorable but the growth rates of those crystals which do form will be

relatively higher. This results in the formation of fewer but larger cyrstals.

Alternatively, if heating and cooling rates are high, a larger number of

smaller crystals will form.

The maximum temperature will affect the size distribution and population of

anhydrous crystals in two ways. The maximum cycle temperature will determine

the relative amount of anhydrous material present through solubility considerations. In addition, Tmax in combination with the total time spent above the

transition temperatures will affect the size distribution of both the anhydrous

material and the nucleating agent. Coarsening of larger crystals and dissolution of smaller ones will occur with time at a temperature dependent rate.

12
This phenomenon, called Ostwald ripening, occurs because only particles of one size can be in equilibrium with the solution at a given concentration. Over

time this may result in a decrease in the total number of nucleation sites on which the hydrated salt may crystallize. Further, because this phenomenon

also results in an increase in the average size of the anhydrous crystals, the

time required for complete transformation to the hydrated salt will increase.

If this time exceeds

^MIN + C C

TMIN x _ ( T

C ~ TMIN

) a 3

the fraction of energy stored which is delivered to the environment will

decrease

.

Because optimal test methods should meet the criteria of acceleration and

simulation, it becomes possible to identify and model failure mechanisms. Once

this is accomplished and the relationships between the system performance and

the test parameters have been developed, service life prediction may be achieved.

13
References

1. Haver, C., Remilland, R. , "Passive Solar Collector Wall Incorporating

Phase Change" in Proc. Second National passive Solar Conference, Vol . 2 , University of Pennsylvania, Philadelphia, PA, 3/16—13/1978 -

2. Baylin, F., "Low Temperature Thermal Energy Storage: A 3tate-of-the-Art

Survey", SERI/RR-54-164 , Solar Energy Research Institute, Golden, Co.

3. Cole, R. , Nield, K . , Rohde, R., Wolosewicz, R. , "Design and Installation

Manual for Thermal Energy Storage", Report No. ANL-79-15, 1979, Argonne

National Laboratory, Argonne, IL.

4. Murray, 0., "Phase-Change Storage in a Brewery Solar System", in Solar

Energy Storage Options, Vol. 1 , (C0NF-790328-P1 , NTIS).

5. Murray, 0., "Solar Pas turization of Beer: A Pilot Application", Master

Brewers Assoc, of Amer. Tech. Quart., Vol. 16, No. 1, 8-11, (1978).

6. Botham, R. , Jenkins, G. , Ball, G., "Form-Stable Crystalline Polymer

Pellets for Thermal ENergy Storage: Phase 1", Report No. ORO/5159-lO,

Monsanto Research Corp., Dayton, OH, VA.

7. Mehalick, E., Tweedie, A., "Two Component Thermal Storage Materials Study,

Phase 2", Report No. COO/2845-77/1, General Electric Co., Philadelphia,

PA.

8. University of Delaware, Institute of Energy Conversion, Private

Communication with Dr. Stephan Marks, (1980).

9. Kedl, R. , "Advanced Energy Storage Concepts with Building Materials,"

in Proc. Solar Energy Storage Options, Vol. 1 , (C0NF-790328-P1 NTIS).

10. International Thermal Instruments Co., An Immisicible Fluid, Phase Change,

Heat Storage Device , Del Mar, CA, (1979).

11. Herrick, C., "Progress Report on the Rolling Cylinder Heat Storage,"

Proc. Solar Energy Storage Options, Vol. 1 , (C0NF-790328-P1 , NTIS).

12. Telkes, M. , "Thermal Storage for Solar Heating and Cooling," in

Proceedings, Workshop on Solar Energy Storage Subsystems for the Heating

and Cooling of Buildings , NSF-RA-N-7 5-041 , p. 17-23, (1975).

13. Chahroudi, D., "Suspension Media for Heat Storage Materials," in

Proceedings, Workshop on Solar Energy Storage Subsystems for the Heating

and Cooling of Buildings , NSF-RA-N-7 5-04 1 , p. 56-59, (1975).

14. Telkes, M., "Solar Energy Storage," ASHRAE Journal, # 9 , p. 38,

(1974).

14
15. Marks, S., "An Investigation of the Thermal Energy Storage Capacity of

Glauber's Salt with Respect to Thermal Cycling,” Solar Engineering, 25 , p. 255-258, (1980).

16. Telkes, M. , "Nucleation of Saturated Inorganic Salt Solutions," Ind.

Eng. Chem. , Vol . 44, pp. 1308-1310, (1952).

17. Davey, R. , "The Control of Crystal Habit” in Industrial Crystallization

78 , North-Holland Pub. Co., Amsterdam, (1979).

18. Lane, G. , Glew, D., Clark, E., Rossow, H. , Quiqly, S., Drake, S. and

Best, J., '’Heat^of -Fusion Systems for Solar Energy Storage," in Proceedings, Workshop on Energy Storage Subsystems for the Heating and Cooling

of Buildings, NSF-RA-N-7 5-041 , p. 55, (1975).

19. Telkes, M. , "Heat of Fusion Systems for Solar Heating and Cooling," Solar

Engineering, Vol. 22, p. 22, (1978).

20. Roff, W. and Scott, J., Fibers, Films, Plastics, and Rubber, 3utterworths

Pub. Co., (1971).

15

NSS-1 14A REv. 2-80

U.S. 3EPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 81-2422

2. Performing Organ. Report No.

4. 1 1 1 Lh AND SUBTITLE

3. Publication Date February 1982

Factors Affecting the Service Lives of Phase Change Storage Systems

5. AUTHOR(S) __ John B. Ings and Paul Wencil Brown

s. PERFORMING ORGANIZATION (If joint or other than NBS. see Instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City . Store, ZIP ) U.S. Department of Energy

Office of Solar Applications for Buildings

Conservation and Renewable Resources

Washington. DC 20585

10. SUPPLEMENTARY NOTES

" ’ " “

Document describes a computer program; SF-I8S. FlPS Software Summary, is attached. U- o' TMS1 ’-'fom.cU ,f — <, Phase change storage systems currently in use or which are in the advanced

stages of development are identified. Various possible modes of degradation which may affect service lives are considered. Specifically, the effects of crystal growth, crystal segregation, supercooling, corrosion and thermal

decomposition are discussed. The generic basis for the development of

performance tests for inorganic phase change materials is described.

12. KEY_WORDS (Six twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons) crystal growth; encapsulants; failure mechanisms; nucleating agent; phase change storage; service life prediction.

13. AVAILABILITY

‘ “ —

y; Unlimited For Official Distribution. Do Not Release to NTIS

1—' 20402

Fr° m SuDerintendent of Documents, U.S. Government Printing Office, Washington, D.C.

~Xj Order From National Technical Information Service (NTIS), Soringfield, VA. 22161

14. NO. OF

PRINTED PAGES

19

15. Price

S6 . 00