NATL INST. OF STAND & TECH

AlllDt D317Db

REFERENCE

NIST

PUBLICATIONS

NBSIR 77-1237

Transpiration Heat Transfer in

Thermal Energy Storage

Devices

B. A. Peavy and W. E. Dressier

Thermal Engineering Section

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

May 1 977

at

loo U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 11A121

TRANSPIRATION HEAT TRANSFER IN

THERMAL ENERGY STORAGE

DEVICES

B. A. Peavy and W. E. Dressier

Thermal Engineering Section

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

May 1 977

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Herman, Under Secretary

Jorden J. Beruch, Assistartt Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Annbler. Acting Director

TRANSPIRATION HEAT TRANSFER

IN THERMAL ENERGY

STORAGE DEVICES

ty

B. A. Peavy and W. E. Dressier

ABSTRACT

The storage of thermal energy at a suitable temperature level from

sources such as solar energy or vaste heat processes can make that

energy available for space heating at a later time period. This report

is concerned with sensible heat transfer that takes place in a thermal

storage device composed of a porous material with a fluid (water)

transpiring through it. Experiments were performed on a prototype

thermal storage device and the results were compared to numerical values

computed from an analytical model. The comparison is shown to be very

good. Further experimentation is considered necessary in order to more

fully examine the effects of stratified and mixing-type flows, heat

losses, conduction heat flow in the heat storage container, and unsteady

initial conditions.

Keywords: Porous media; thermal energy storage; transpiration heat

transfer.

i

«
I

TRANSPIRATION HEAT TRANSFER

IN THERMAL ENERGY STORAGE DEVICES

by

B. A. Peavy and W. E. Dressier

Contents

Page

Introduction ^ .^ - .

Test Apparatus -

- ;

' -^^ . 2

Testing Procedure ^ '

. ^ ^

Analytical Model '

^ - ^

Discussion of Results ' -

*- .10

Acknowledgments ' ^ ' - -j^^

List of Figures ""'

•

' ^

• 25
TRMSPIRATION KEAT TRMSFER

IN THERMAL ENERGY

STORAGE DEVICES

by

B. A. Peavy and W. E. Dressier

INTRODUCTION

With the growing emphasis on the utilization of solar energy for

space heating in buildings, focus is drawn to the requirements for

storage of the energy at a suitable temperature level for use at time

periods when solar energy is not available, such as at night or on

cloudy days when the heat is most often needed. Consideration is also

being given to long-term storage of thermal energy whereby energy is

collected during the summer months for utilization during the winter

season.

Generally, thermal storage may be classified in two ways; namely,

sensible heat : and latent heat storage (change of phase of a material).

This report is concerned with the sensible heat changes that take place

in a porous material with a fluid (water) transpiring through it. An

example of such a storage device is a bed of sand with water seeping

through it. In the daytime, the water flows through a solar collector

array where it absorbs the thermal energy and transfers it into the bed

of sand through which the water passes. At night, the water passes

through the storage bed where it is heated and then flows through one or

more heat exchangers where it transfers the energy to the building for

domestic water heating or space heating. The water returns to the sand

bed to pick up additional heat and the process is repeated.

-1-
There has been relatively little work done on the testing of sensible heat storage devices during typical tremsient operating condition

and there is a need for the development of methods for determining the

effectiveness of these devices.*. This paper deals with the development

of an analytical model for predicting the performance of sensible thermal energy storage units and the comparison of calculated results to

actual experimental results. With the use of the model, the prediction

of the thennaJ. performance of transpiration energy storage lonits should

permit a better understanding of the phenomena involved.

TEST APPARATUS

The general testing approach used was to subject a porous bed of

sand with water flowing through it at a uniform temperature (the bed and

fluid were initially at steady conditions) to a sudden change in entering fluid temperature. The transient temperature distribution was then

determined until steady conditions were again approached. This is the

techniq.ue recently adopted by the heating and air conditioning industry

for testing thermal storage devices.*.

The experimental apparatus, as shown schematicaJ-ly in figure 1, consisted of a U-in. (10.2 cm) inside-diameter plastic cylinder with a 1/Uin. (0.6U cm) wall. The first 2k in. (6l cm) of the cylinder were fitted

every 2 in. (5.1 cm) with thermocouples which were located at the center

of the cylinder. The cylinder was wrapped with a 1-in. (2.5 cm) thicknes

of rigid glass-fiber insulation for the first three tests described in

the next section. An additional 1 in. (2.5 cm) was added for the fourth

* Method of Testing Thermal Storage Devices Based on Thermal Performance

ASHRAE Standard 9^-77, February, 1977.

-2-
and final test. Provision was made for introducing either

hot or cold water from the building supply at the top of the cylinder

by appropriate valving. This technique was used for the first three

tests. Additional steps were taken for the fourth test to control the

temperatiare of the entering fluid to within +0.5F (+0.3C). The porous

3 3 matrix material, Ottawa sand, had a density of 109 lbs/ft (1TU6 kg/m )

and occupied 66 percent of the voliame of the cylinder.

TESTING PROCEDURE

To begin the experiment, a first test was run by passing hot water

through an empty cylinder at approximately 117F {h^ .2C) at a flow rate

-7 3 of 0 .h gal/h (U.2 x 10 m /s). The hot water was then suddenly stopped

and cold water at T^F (23. 3C) was introduced. The temperature distributions were recorded over intervals of time. A graph of the results is

shown in figure 2.

The next step in the testing procedure was to fill the cylinder to

a height of 2k in. (6l cm), with a 20- to 30-mesh silica sand from Ottawa,

Illinois. Two more tests were then run by saturating the sand matrix

with hot water at flow rates of 0.29 (3.0 x lO"''' ) and 0.55 (5.8 x 10~^)

gail/h, (m /s) respectively. The hot water was suddenly stopped and cold

water was introduced at the same respective flow rates and temperature

distributions once again recorded at intervals of time. Graphical representations of these two tests are shown in figures 3 and respectively.

Upon close examination of the results of the last two tests, questions arose concerning the lack of a constant initial matrix temperature,

the failure to get as highly stratified conditions as had been expected

-3-
due to the buoyant effects of placing cold vater on top of hot water,

the effects of radial heat losses on the system, and the possible

conductance of energy along the cylinder walls providing a by-passing

of the porous matrix. Therefore, a number of changes in the system

were initiated. First, constant temperature hot water and chilled

water sources were used to sufficiently reduce temperature fluctuations

during the experiments. Secondly, it was decided to first cool the

matrix and then introduce hot water to prevent hot and cold water mixing

due to buoyancy. A second inch (2.5 cm) of glass fiber insulation was

added to the exterior of the cylinder to reduce radial heat transfer.

Initial and final matrix temperatures were chosen to be above and below

room air temperatures, respectively , to minimize radial heat transfer

due to temperature differences between cylinder and ambient air. Finally,

thermocouples were added to the inside and outside of the cylinder wall

to check for possible lateral conductance of heat past the porous matrix

material. (No evidence was found in the subsequent experiment to support such concern.

)

A final test was made. The total matrix of silica sand and water

was initially cooled to a temperature of 5^F (12. 2C) with a flow rate

-7 3

of 0.3 gal/h (3.2 X 10 m /s. The chilled water was suddenly stopped

and hot water at 106F (Ul.lC) was introduced, while recording temperatures

at time intervaJ-s until a steady-state condition was reached. The graph

of temperature versus time is shown in figure 5.

ANALYTICAL MODEL

The following assumptions were made in formulating the analytical

model

:
1. Thermal, physical, and transport properties are constant.

2. Heat conduction in the porous matrix and fluid flow through

the interstices are one-dimensional in a direction normal to the

bounding surfaces.

3. The temperature of the fluid and solid are assumed to be equal

in any matrix element normal to the fluid flow, and the matrix thermal

properties are treated as effective values. The thermal conductivity

includes heat transfer through both the fluid and solid components of

the matrix. In a similar manner, the volxjmetric specific heat of a

flooded (saturated) matrix is a sum of proportional values of both

components

.

h. The specific heat of the flowing fluid is a definable property

of the fluid as distinct from that of the matrix materials, which form

a fixed volume.

With these assumptions, a heat balance was performed on the solid

material of an elemental slice of the porous matrix, figure 6. For the

treinsient one-dimensional transpiration heat-transfer problem of a

fluid through a porous matrix, the following governing partial differential equation was derived:

2

3 V

2 - 2B = 0 (1)

9x a 3t

m

where v temperature potential

X ratio of the distance in the solid to the length, %

length of fluid flow path through storage unit

am

thermal diffusivity of the storage unit

-5-
t = time

23 = the product of the mass flux of the fluid, G, the fluid

specific heat, c, and the length, divided by the thermal

conductivity, "the storage unit. Gel

\

m

The general solution of (l) is given hy:

V = A + B e^^"" + e^''"^ ^ oo 1 [A cos 6 x+B sin6 x] e (2) n nnn '

2

where y = at/i , and constants A and B satisfy initial and boiindary conditions. Tvo of the three required boundeiry conditions are (l) an initial temperature potential, v = 0 in the region 0 <_ x _< 1 , and (2) for

time greater than zero, the heat flux is zero at x = 1. The following

paragraphs describe alternate solutions of equation (l) depending on the

third boundary condition chosen.

Solution No. 1

For a time greater than zero, the temperature at the matrix entry

face, X = 0, is assumed to be suddenly raised and maintained at a value

V = V. The solution to equation (l) then becomes:

^

2 ~ n

^

o „2 6 sin 6 X e

(3)

6+3+3 2 2 n

where 6 are positive roots of n

3 sin 6 + 6 cos 6 = 0. (^)

Solution No. 2

For times greater than zero, the fluid-entry temperature at x = 0

can more accurately be assumed to be

V = V (1 - e"*^) (5)

-6-
where *f is an independent damping factor to be determined experimentally

for a given transpiration configuration. The solution to equation

(l) then becomes:

" n

^

^ , „ „2 6 sin 6 X e ,rs - = 1 - F(x)e + 2e I ^ 2

2

V - [(6^ + 6 )/ a- 1](6 ^ + + B) n n

for:

= Bsinh d) (1 - x) + 4) cosh (}) (1 - x) g2 ^ ^ Bsinh 4) + 4) cosh 4)

F(x) = Bsin 4) (1 - x) + 4) cos 4> (1 - jO for 3^ < a Bsin 4> +4) cos 4)

= g (1 - ^) 1 for 6^ = a

6 + 1

and a = 4^^ = - a| -

As a result of the experimental physical constraints, equation (5)

more closely approximates the actual fluid-entry temperature in a transpirational heat storage device than does the assumption of a step change

in the fluid temperature.

Solution No. 3

Additional experimental constraints most often require the inclusion

of heat loss or gain radially to the surrounding ambient air temperature.

Consequently the more general governing partial differential equation

alloving for two-dimensional heat transfer within the matrix is given by:

-7-
2 2 2

9r r dr dx m

No radial fluid flow is assumed and the following boundary conditions

are assumed:. ... . ..

at X = 0

' V = V[l - e""^^} (8)

at X = 1 dv/dx =0 (9)

at r = a -X dv/dr = h(v - u) , -a ^ = h(v - u) (10)

m

where U is the temperature potential of the s\irrounding ambient and

h = aH/X , dimensionless , and a is the inside radius of the storage

m

unit. The solution to equation (7) using the foregoing boundary conditions is given by:

^ = 2 he^"" (a, + A„ + A_ + aJ (11)

V 1 2 3

4-*

. , ^ 3: sinh (l-x) + Q cosh (l-x]

n o ^'n

r z , ZN , r ^ B sinh Q. + Q cosh Q, ly„ + h JJ iy„J ^. . n n n

„ , 6 sin 6 x 1 [i> r/al

^2 V / ^ 2 . „2

*3 ' " ^ (y 2 + h^) J (y )

-8-
2 2 2

2 Y

A, = 2e

2 rj (y r/a)e ^ V ^

n, n sin 6 x e -p Y \ o._ n \ m n m m

4 / r„ 2 ^ ,2^ , r„ ^ / o 2^2

Ta and where:

y are roots of yJ, (y) - hJ (y) = 0

6 are roots of 3 sin 6+6 cos 6=0

n

2 2 2 2 2 ^ = (B^ + 6 ) a^/r

n n

2 2

7 =A_±_? _U 22 2 iin,n ^ 2 _^ 2„2, 2 _^ .2,. , V ^n

6 +yic/a+p )/a - 1

m n

2 2

3 sinh d) (1-x) + 6 cosh d) (1-x) „ y ^

T, / N ^n n ^n o2 . n

F (x) = for 3 + —2 ^ *^ 3 sin (}) + cosh d) a ^n ^n n

2 2

3 sin 4) (1-x) + ({) cos 4) (1-x) „ y £

F^(x) = for 3 + < a 3sin d) + d) cos 6 a

n n n

, (,) = e(i-x) ^ 1 for 3^ + V- = -

3 + 1 a2

2 2

2 1^2 ^ ^n

^

I a and Jq(«), J (a), I J«) and I (a) are Bessel functions.

-9-
For computing niunerical solutions of equations (3), (6) or (ll),

particular care must be taken for large values of the dimensionless

parameter , 3 . At x = 1 , the fluid exit from the thermal energy storage

device, the exponential factor can "become very large, and the summation

term must be correspondingly very small such that v/V lies between zero

and unity. It is therefore suggested that computer calculations be

performed in the double-precision mode to avoid excessive loss of significance in the summation of terms. For 6 greater than 30, it may become

necessary to employ some type of extended precision routines to avoid

2 spurious results, depending upon the magnitude of 3-'B Y'

DISCUSSIOM OF RESULTS

Fig\ire 2 shows the temperature history of the experimental unit

filled only with water initially held at a high temperature and where

cold water was introduced at the top of the unit. Due to the reduced

buoyancy of the cold water almost perfect mixing existed as is evident

from the figure. At a given time, the temperature at the exit-face

(bottom) is not much different from that at the fluid entry-face (top).

The reverse test of suddenly introducing hot water to the cylinder

initially filled with cold water was not performed; however, intuitively

one would expect stratification of the warmer layers near the top whereby

the temperature at the fluid exit-face would remain at its initial temperature for a definite time period. At the flow rate of O.U gal/h {k.2 x 10

m /s), it wo\ild be expected that the water temperature leaving the unit

would be the same as the entering water temperature at the end of 3.2 hour

Figure 2 shows this to be true at the end of 1.6 hours and is an indicatio

of the heat loss that occurred from the unit to the ambient.

-10-
In order to compare the results of the analytical model to the

experimental results, it is necessary to determine the thermal properties

of the thermal storage device. The solid portion of the matrix material

is a natural silica sand from Ottawa, Illinois, with a standard mesh size

of 20 to 30. The measured diy density was 109 Ihs/ft (17^6 kg/m ) and

the specific heat was estimated to be O.I85 Btu/lb-F (775 J/kg-C) at

about 85F (29c) from various literature soiirces. The volumetric porosity

was 3h percent. When saturated with water, the water was l6.2 percent

by weigJit. From these data, the density and specific heat for the

thermal storage device, including container, were about ihS.f lb/ft

(2350 kg/m^) and 0.32 Btu/(lb-F) (13^+0 J/kg-C)) respectively. From Woodside and Messmer,* the thermal conductivity of the storage device for a

porosity of 3^ percent is given as 1.9^ Btu/(h'ft'F) (3.36 W/m-C)), and

the thermal diffus ivity .0lil3 ft^/h (I.07 x lO"''' m^/s). For the last

three tests with water flow rates of 0.29, 0.55 and 0.30 (3.0 x 10 ,

5.8 X

10"''', and 3.2 X lO"''' m^/s) the values of 3 = GcJl/2A were li+.31,

m

27.13, and li+.80, respectively.

For the Biot number, h = aH/Xm, values were determined from the

relationship:

aH

+

1

Woodside and Messmer, "Thermal Conductivity of Porous Media,

Unconsolidated Solids," Journal of Applied Physics , (32),

1961, p. 1688.

-11-
where

= thermal conductivity of container, 0.083 Btu/(h'ft'F)

(l.h W/(m-C)

= thermal conductivity of insulation, 0.021 Btu/(h*ft*F)

(.36 W/(m-C)

a = inside radius of container, 2.0 in (5.1 cm)

r^ = outside radius of container, 2.3 in (5.7 cm)

r^ = outside radius of insulation, 3 . 3 in or h.3 in (8.3 or 10.8 cm)

p

= coefficient of heat transfer at r^, 1.08 Btu/(h-ft 'F)

For tests 2 and 3, h = .0233 ( with 1 in. (2.5 cm) of insulation) and

for test h, h = 0.015 (with 2 in. (5.1 cm) of insulation). For tests 2,

3 and h the damping factors, were determined experimentally to be

3.^8, 5.59 and 12, ^respectively

.

The temperature potential, v, is defined by the relationship:

V T - T. X _ X 1

' V T - T.

e 1

where is the temperature at a position x, is the initial temperature, and is the water temperature entering the storage device, such

that ,

V— = 1 e"'^^

V

For tests 2 and 3, the entering water temperatvire was essentially the

same as the air temperature surrounding the storage device; therefore,

U of equation (lO) is unity. For test U,

-12-
^

= (78.9 - 5^.0)/(l06 - 5M = M9.

Given the above information, numerical results for the analytical

model, equation (ll) , are shown in figures 7 and 8 and compared with the

experimental results for the temperature potential, v, at the exit of

the storage device. Except for the initial temperature rise at the

fluid exit for tests 2 and 3, the agreement between the analytical

model and the experimental res\ilts can be considered very good.

The prototype storage unit used for the tests described herein was

small and presented problems that are to be corrected in future testing.

A new storage unit is presently being constructed which will be larger

and contain additional instrumentation necessary for accurately determining heat loss from the unit. However, the data obtained does allow

some general conclusions to be made concerning the effectiveness of

^1

thermal energy storage in a sand bed. Figure 9 shows a plot of 1 - —

for the results of test U as a function of a dimensionless time defined

by:

mC(T - T. ) t

e 1 SC

where

m = mass flow rate of the fluid

SC = the storage capacity of the thermal storage unit

which has its temperature increased from to •

In addition, fig-ore 9 also includes calculated results for two water

tanks of the same volume having the same flow rate through them as did

-13-
the saxid bed of test h. In one case, the water tank is assumed to be

completely stratified and in the second, completely mixed. In both

cases, zero heat loss was assumed from the tanks.

The area under each curve of figuj-e 9 is directly proportional to

the amo\int of energy that was stored in the device. In order to compare

the performance of the three units, it is convenient to choose the time

period 0 to 1 (dimensionless time). This is consistent with the procedure currently being considered by ASHRAH for testing and rating

storage devices that was mentioned previously. Based upon the cuarves in

figure 9, a perfectly mixed water tank is only 63 percent as effective

in storing thermal energy during this trsjisient period as the completely

stratified tank (which of course is never obtained in actual practice).

However, the sand bed remains rather highly stratified at the chosen

flow rate and as a result is within 88 percent as effective as the ideal

case.

CONCLUSION

This paper is a preliminary report to ongoing work in the development of methods for determining the effectiveness of thermal storage

devices, and it must be recognized that certain conclusions must be

reserved until the completion of the ongoing work.

Certainly, heat loss from a thermal storage device can be considered a serious matter, especially when heat loss occurs at a time

when there is no need for heating and the heat is unavailable when it is

needed. Figure 8 gives an indication of the heat loss upon addition of

heat to a device whereby the exit temperature rise attained only about 91

percent of the entry temperature rise. A heated storage unit allowed to

Ik
stand for several hours would lose considerable heat under these conditions.

The analytical model, using thermal properties from the literature,

gives very good agreement to actual experimental results on a thermal

storage device employing the transpiration process. Solution number 3,

although a seemingly cumbersome set of equations, gives numerical solutions

in a very fast manner when programmed for the computer and accounts for

radial and actual heat flows as well as fluid flow. With this model it

is possible to predict the thermal performance of transpiration energy

storage units.

One test was performed using water only (without sand) in the

storage unit, and the results are given in figure 2. This test involves

the introduction of cold water on top of an initially uniform hot water.

There was an almost perfect mixing due to the higher density of the cold

water seeking a lower level and thereby a mixing with the hot water.

Perfect mixing is not a desirable feature for thermal storage units in

that the exit temperature should be maintained as close as possible to

the initial temperature at the beginning of heating or cooling cycle.

For the case cited above, the introduction of cold water at the bottom

of the storage unit would promote stratification, but involves additional

piping and controls to give stratification for both the heating and

cooling cycles of the thermal storage unit.

Figure 3 shows the introduction of cold water on top of a heated

sand-water combination where the exit water temperature is maintained at

a higher level for a considerably longer period of time. The use of a

sand bed may preclude the necessity of extra piping and controls and the

15
reversal of fluid flow that is necessary for the heating and cooling

cycles of water thermal storage devices.

, • . ACKNOWLEDGMENT

The authors wish to express appreciation to Mr. Walter Ellis for

installation of the apparatus euid instrumentation and the day-to-day

operation of the experiments.

16
LIST OF FIGURES

1. Schematic of Prototype Themal Storage Unit.

2. Temperature History of Thermal Storage Unit Initially Filled with

Water at IITF. At Time Equal Zero, Water at T^F and a Flow Rate

of 0.i+ gal/h was Introduced at Top of Unit.

3. Temperature History of Thermal Storage Unit Filled with a SilicaSand-Water Mixture Initially at 116F. At Time EQual Zero Water

at T^F and a Flow Rate of 0.29 gal/h was Introduced.

k. Temperatiare History for Thermal Storage Unit of Figure 3, except

Flow Rate was 0.55 gal/h.

5. Temperature History of Thermal Storage Unit Initially at 5^F. At

Time Equal Zero Water at 106f and a Flow Rate of 0.3 gal/h was Introduced.

6. Transpiration Heat Flow Through Elemental Slice of Thermal Storage

Unit.

7. History of Temperature Potential for Tests 2 and 3 Compared to that

for Analytical Model (equation ll).

8. History of Temperature Potential for Test h Compared to that for

Analytical Model (equation ll)

.

9. Comparison of Temperature Potential for Test h Compared with those

of Ideal Water Tanks.

-17-

HOT

WATER

COLD

WATER

1" = 2.5k cm

THERMOCOUPLES

itCM

CSI

CI

M

M

M

CM

WATER

OUTLET

U4

Figure 1. Schematic of Prototype Thermal Storage Unit.
u

<

- - M

o o o

o

o

0>

o

00

o

(Jo) aiiniVli3dM31

Figure 2. Temperature History of Thermal Storage Unit

Initially Filled With Water at 117F. At Time

Equal Zero, Water at 74F and A Flow Rate of

0.4 gal/h was Introduced at Top of Unit.
(Jo) aHnivaadMi3i

Figure 3. Temperature History of Thermal Storage Unit Filled

With a Silica Sand Water Mixture Initially at 116F.

At Time Equal Zero ^Water at 7AF and a Flow Rate of

0.29 gal/h was Introduced.
<0

<

<

<

H

O

To

in

in

II u

H

<

O

E

(n

<

< + O

cn

o TERV

9/5)

X

LAO

E

z X

II O

-3- o Z 0

LA <

\ CM u

II a>

II M

u. o

J L

- - in

k.3

O

- - w

o o o

o

o

0>

o

09

o

(Jo) 3liniVliadM31

Figure 4. Temperature History for Thermal Storage Unit of

Figure 3, Except Flow Rate was 0.55 gal/h.
\

E

vD

CMCO O

+

X

-—

LA\

u*v O

<r» E

O

X

II -d- o LA

o x:\ CM

II roO)

II

Ll_ O

o e o o o o o

«4 O «» «B r« <• M

(Jo) 3MniVII3dM31

Figure 5. Temperature History of Thermal Storage Unit

Initially at 54F. At Time Equal Zero^ Water

at 106F and A Flow Rate of 0.3 gal/h was

Introduced.
Figure 6. Transpiration Heat Flow Through Elemental Slice

of Thermal Storage Unit.
Figure 7. History of Temperature Potential For Tests 2 and 3 Compared

To That For Analytical Model (equation 11).
Figure 8. History of Temperature Potential For Test A Compared To

, -, ; , That For Analytical Model (equation 11).
Figure 9- CoTToarison of Temperature Potential for Test

h Compared

with those of Ideal Water Tanks.
NBS-^14A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 77-1237

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLF

Transpiration Heat Transfer In Thermal Energy Storage

Devices

5. Publication Date May 1977

6. Performing Organization Code

7. AUTHOR(S)

B. A. Peavy and ¥. E. Dressier

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. U62i+lii6

11. Contract /Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

NBS

13. Type of Report & Period

Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.)

The storage of thermal energy at a suitable temperatiire level from sources such

as solar energy or waste heat processes can make that energy available for space

heating at a later time period. This report is concerned with sensible heat

transfer that takes place in a thennal storage device composed of a porous material

with a fluid (water) transpiring through it. Experiments were performed on a prototype thermal storage device and the resul"bs were compared to numerical values

computed from an analytical model. The comparison can be considered very good.

Further experimentation is considered necessary to define test parameters such as steady initial condition, steady input water temperatures, piston and mixing -type

flows, effects of heat losses, and conduction heat flow in the heat storage container

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only die first letter of the first key word unless a proper name; separated by semicolons

)

Porous media; thermal energy storage; transpiration heat transfer.

18. AVAILABILITY ^^Unlimited

I ! For Official Distribution. Do Not Release to NTIS

[3J Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C15

Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

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(THIS REPORT)

UNCL ASSIFIED

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21. NO. OF PAGES

31

22. Price

$4.00

USCOMM-DC 29042-P74

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