IMBSIR 79-1737

Testing of Pebble- Bed and

Phase-Change Thermal Energy

Storage Devices According to

ASHRAE Standard 94-77

Dennis E. Jones

James E. Hill

Building Thermal and Service Systems Division

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

May 1979

Sponsored by

The Department of Energy

1 2th and Pennsylvania Ave., NW

Washington, D.C. 20461

mTb-$r‘

NBSIR 79-1737

• * < TESTING OF PEBBLE-BED AND

PHASE-CHANGE THERMAL ENERGY

STORAGE DEVICES ACCORDING TO

ASHRAE STANDARD 94-77

Dennis E. Jones

James E. Hill

Building Thermal and Service Systems Division

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

May 1979

Prepared for: Research and Development Branch for Solar Heating and Cooling

Office of Conservation and Solar Applications

Department of Energy

Washington, D.C. 20545

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

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CONTENTS

Pas.e

ABSTRACT iv

1. INTRODUCTION 1

2. ASHRAE STANDARD 94-77 2

3. NBS TEST FACILITY 7

4. EXPERIMENTAL PROCEDURE 18

5. PHASE-CHANGE DEVICE TEST RESULTS 19

6. PEBBLE-BED TEST RESULTS 32

7. CONCLUSIONS AND RECOMMENDATIONS 38

8. REFERENCES 40

iii
TESTING OF PEBBLE-BED AND PHASE-CHANGE THERMAL ENERGY STORAGE

DEVICES ACCORDING TO ASHRAE STANDARD 94-77

by

Dennis E. Jones

James E. Hill

ABSTRACT

The American Society of Heating, Refrigerating, and Air Conditioning

Engineers (ASHRAE) has recently adopted ASHRAE Standard 94-77 - Methods

of Testing Thermal Storage Devices Based on Thermal Performance.

Experiments have been completed at the National Bureau of Standards in

which a 7 m^ (250 ft^) pebble-bed and a similarly-sized 264 MJ (250,000

Btu) phase-change unit utilizing sodium sulfate decahydrate, both using

air as the transfer fluid, were tested in accordance with this Standard.

A description of the test procedure, test apparatus, and detailed test

results is given. Some problems were encountered in using the Standard

for these kinds of thermal energy storage devices, and modifications to

the Standard are recommended based on these experiments.

KEY WORDS: ASHRAE Standard 94-77; Glauber's salt; latent heat storage;

pebble-bed; phase-change unit; solar energy storage; thermal

storage device.

iv
TESTING OF PEBBLE-BED AND PHASE-CHANGE THERMAL STORAGE

DEVICES ACCORDING TO ASHRAE STANDARD 94-77

by

Dennis E. Jones

James E. Hill

1 . INTRODUCTION

Beginning in early 1974, the staff of the National Bureau of Standards

(NBS) began the development of thermal test procedures for the two primary non-conventional components of solar heating and cooling systems

solar collectors and thermal energy storage devices. Recommended

procedures for testing both components were published in late 1974 and

early 1975 [1, 2] in a format consistent with the standards of the

American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE). The procedures were later published with supplementary

information explaining the rationale behind each procedure [3, 4, 5].

In July of 1975, ASHRAE formed Standards Project Committee 94-P to

develop a standard for the testing of thermal energy storage devices.

In June, 1976, a draft of a standard was published and submitted to the

Society for review. It was based substantially on the previously published NBS recommended procedure [2], The ASHRAE draft was modified

according to the review comments received and then forwarded to the

ASHRAE Standards Committee for approval. In February, 1977, the ASHRAE

Board of Directors approved and authorized publication of ASHRAE Standard 94-77 - Methods of Testing Thermal Storage Devices Based on Thermal

Performance [6],

The purpose of this report is to describe the results of an experimental

study conducted at NBS during 1978 in which a 7 m^ (250 ft^) pebble-bed

and a 264 MJ (250,000 Btu) phase-change unit utilizing sodium sulfate

decahydrate, both using air as the transfer fluid, were tested in accordance with Standard 94-77. The study demonstrates the applicability of

the Standard to both sensible-heat and latent-heat storage devices. A

similar experiment was previously completed on a 1.9 nr (500 gal) water

tank. [7,8] .

It should be noted that a similar and parallel process has occurred with

the NBS-recommended test procedure for solar collectors [1], ASHRAE

Standard 93-77 - Methods of Testing to Determine the Thermal Performance

of Solar Collectors was adopted and published in February, 1977 [9].

NBS has conducted a series of experiments demonstrating the use of this

standard for typical commercially available water-heating and air-heating

collectors [10,11].

1
2. ASHRAE STANDARD 94-77

The tests outlined in ASHRAE Standard 94-77 consist of the following:

1. one test to determine a heat loss factor for the thermal

energy storage device,

2. two tests to determine the response characteristics (charge

capacity) of the device to a step increase (above the initial

temperature) in the entering transfer fluid temperature, and

3. two tests to determine the response characteristics (discharge

capacity) of the device to a step decrease (below the initial

temperature) in the entering transfer fluid temperature.

Heat Loss Test

The heat loss test consists of passing the transfer fluid through the

storage device with an inlet temperature of 25°C (45°F) above the

ambient air temperature. After steady-state conditions are reached,

measurements are made of the average temperature difference between the

inlet and outlet transfer fluid and the ambient air temperature over a one-hour period (see Figure 1). Steady-state conditions are achieved by

circulating the transfer fluid through the storage device until the

inlet and outlet transfer fluid temperatures vary by less than + 0.5°C

(+ 0.9°F) during a one-hour period.

The heat loss factor is defined in the Standard by:

L = heat loss factor, J/(s*°C ) (Btu/(h*°F))

w, = mass flow rate of the transfer fluid for the heat loss test,

kg/s (lb/h)

c

t f = specific heat of the transfer fluid, J/(kg*°C) (Btu/ (lb* °F)

)

t. = temperature of the transfer fluid entering the storage

device, °C (°F)

t = temperature of the transfer fluid leaving the storage

, . Ox-1 / O T-. \

The mass flow rate of the transfer fluid to be used for the heat loss

test is determined by:

L

)dx

( 1 )

where

device, °C (°F)

tscl ( 2 ) c tf (14400s) (25°C)

2
3

•i-l 3U

d)

4-1COCU

0) X os&oz

CD

ISCO

>-o 5 3

CO

CD

<4-1 o30

•r4 4Jn

•r4 U3>

4J

0) CO U <u 3 4-4

4-1

CC) CO 3 CO

0)

O

Q- t-H E I <1) 4-)

4-1 Ctf 1 Cl) 0> X E

•r4 0)

4->

X

QO

0CE <1— z Z H

CO s < o LLJ 2 f— h— Q QC wuu 0 O QQ <

ee _! 5s —1o 0) LU

Q_

LU

3>aLU

a 5 o cc CO< u3

00 CD i i ' oc 59 1— •H QC<

1—<

UJ

—i

a- 5 <o

3:o

h—CO .=

«—

aLU ;E

II II II cu _o C_J

00c

•H

Vj3

T3 3
where

TSCl

= theoretical storage capacity of the thermal energy storage

device for the heat loss test, J (Btu).

The theoretical storage capacity for the heat loss test, TSC^, is the

amount of energy that could be stored in the device if all the device

components would undergo an increase in temperature of 25 °C (45 °F),

starting with an initial temperature equal to t a

, the ambient air temperature. It is further specified that it be calculated as the summation

of the product of mass and specific heat of the various components com- prising the device (storage medium, container, insulation, etc.) multiplied by the temperature increase of 25 °C (45 °F).

Charge and Discharge Tests

The concept of test fill time is introduced and defined in the Standard

as

T f = TSC (3) wtf c tf

where

tf = test fill time, s

TSC = theoretical storage capacity of the thermal energy storage

device for the temperature increase At, J (Btu)

w

t £ = mass flow rate of the transfer fluid, kg/s (lb/h)

At = temperature step of the inlet transfer fluid from an initial

temperature, t^, to a final temperature, t^ + At, °C (°F).

A sensible heat storage device with no heat loss to the ambient and with

perfect stratification of the storage medium would be completely charged

or discharged in the time defined by equation (3). Since such an ideal

storage device does not exist, the fill time defined above is found to

be less than the time required to completely charge or discharge actual

storage devices. In the original analysis that led to the proposed test

procedure, several alternate ways of specifying the test period and/or

flow rate to be used were considered [1,3]. It was concluded that thermal energy storage devices of widely differing designs could be equitably compared if each were tested for the same period of time defined by

equation (3). In other words, the flow rates for the different devices are adjusted so that the amount of energy entering or leaving the device

(denominator of equation (3)) per unit thermal energy storage capacity

(numerator of equation (3)) is the same for each device.

The Standard specifies that the charge and discharge tests be conducted

for specific test fill times and step changes in the inlet temperature

of the transfer fluid. The recommended temperature step change values

4
for devices which use both air and liquid were based on consideration of

the way in which they are currently used in typical solar heating and

cooling systems. The fill times were chosen based on typical ratios of

solar collector size to storage size and flow rates required for optimum

collector performance [5]. However, as a result of the experiments described in this report, it has been found that the air flow rates recom- mended by the Standard are much too high.

The recommended values specified in the Standard for devices using air

as the transfer fluid are:

T c>

Td “ 2 h

it = 35°C (63°F)

and

T c T d “ 4 h

it = 35°C (63°F)

where the subscripts c and d denote the charge tests and discharge tests,

respectively. With the test fill times designated, the mass flow rates are determined by the use of equation (3). Therefore, the mass flow

rates for the transient tests are determined by:

t c At (4)

c tf

wd = TSC

t c At

d tf

(5)

In cases where the storage device was designed for a particular flow

rate, the Standard allows use of this design flow rate.

Figure 2 represents the charge and discharge cycles of a thermal energy

storage device undergoing the transient response tests in accordance

with the Standard. The initial temperature of the storage medium is

chosen based on the intended operating range of the device. The flow is

adjusted to the value, wc

, defined by equation (4), and the device is

brought to a uniform initial temperature. The temperature of the entering transfer fluid is then raised in a step-like manner from the initial

temperature, t i# to a final temperature, + At, and measurements necessary for computing the charge capacity are made over the charge test

fill time, t c

. The storage medium is then allowed to reach steady-state

conditions at the temperature t^ + At. Once steady-state conditions are obtained, a discharge test is performed by decreasing the entering transfer fluid temperature in a step-like manner from t^ + At to a value of

t^. Measurements necessary in computing the discharge capacity are then

recorded over the discharge test fill time, t d

. The measurements
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required for the charge and discharge test computations are the temperature difference between the entering and leaving transfer fluid over the

test fill time, the transfer fluid flow rate, and the ambient air temperature.

The charge and discharge capacities are computed as:

C c wc

c tf LT c ( ti +

t t in - out

2

t a)

( 6 )

and t = x

c

d “ wd

c tf ( C OUt

T = 0

- t in>dT (7)

where

C

c = charge capacity of the thermal energy storage device,

J (Btu)

Co = discharge capacity of the thermal energy storage device,

J (Btu).

It should be noted that the charge and discharge capacities are a function of the specific heat of the transfer fluid and hence a function of

the transfer fluid used.

3. NBS TEST FACILITY

The tests conducted and described in this paper were performed on a 7 m

(264 ft^) pebble-bed and a 264 MJ (250,000 Btu) phase-change unit at the

NBS site in Gaithersburg, Maryland.

Phase-Change Unit

The phase-change device consisted of 726 plastic trays containing a Glauber Salt-water mixture (sodium sulfate decahydrate) . The phasechange temperature for this material is 32°C (89°F). Tray dimensions

and manufacturer’s specifications are shown in Figure 3. The trays were arranged in an array 26 trays high, 4 trays wide, and 7 trays in the

flow direction. The array tested was short two trays. The tray array

was contained within a 1.3 cm (1/2 in) plywood cell arrangement which

was insulated on the outside with 15 cm (6 in) of foamed insulation.

Air was the heat transfer fluid used with this device. Figure 4 shows

a schematic of the test specimen.

7
SPECIFICATIONS

EUTECTIC SALT: CONTAINER

o Sodium Sulfate Decahydrate

NAo S0. - 10Ho 0)

2 4 2

o Latent heat of fusion

112 MJ/m3 at 32°C

3 o Density-1458 kg/m

o Specific heat: kJ/(kg*K)

NAo S0. - 10 Ho 0 (solid) 1.92

2 4 2 Saturated at 32°C 3.26

Solution of NAo S0. 3.51

2 4

Water 4.18

o High density polyethylene

o Density-0.96 gms/cc

o Products of combustion-CO^ + H^O

o Flash point-340°C

o Coefficient of linear expansion-

.0013 m/m °C

o Thermal conductivity-0.53 W/(m*K)

o Specific heat-0.458 cal/gm

o Nominal wall thickness-0.05 cm

o Sonic sealing

o Empty weights-250 grams each

o Full weight-1724 grams each

o Volume per tray-1019 cc

Figure 3. Phase-change thermal energy storage tray and eutectic

material specifications

FLOW

8
PHASE-CHANGE THERMAL ENERGY STORAGE UNIT

TOP VIEW

THERMOCOUPLE

LOCATIONS

Figure 4. Schematic of the phase-change thermal energy storage

test unit

9
The nominal thermal energy storage capacity of the unit was 264 MJ

(250,000 Btu) although the actual capacity would vary according to temperature swing and the exact number of trays. A theoretical energy

storage capacity of 414 MJ (392,000 Btu) was calculated using the manufacturer's specifications and a temperature swing of 35°C (63°F) chosen

in accordance with Standard 94-77 and used during the test. The thermal

energy storage capacity is a combination of the latent-heat and sensibleheat capacity of the storage material. Table 1 summarizes the thermal

energy storage contribution of the various components. The theoretical

latent contribution was approximately 65 percent of the total capacity.

Pebble-Bed

The pebble-bed was fabricated at NBS from plans shown in Figure 5.

The pebble-bed container was constructed using 1.27 cm (0.5 in) plywood

and 4 x 10 cm (2 x 4 in) studs with 8 cm (3 in) glass fiber batts between

the studs. Building sealant and duct tape were used on all joints. The

upper air flow plenum was formed by leaving a 20 cm (8 in) space between

the rocks and the top of the container. The lower air flow plenum was formed using blocks as shown covered with a heavy metal screen. The

pebble-bed was composed of 3.8 to 5 cm (1.5 to 2 in) washed river gravel

from a local source. After construction, it was necessary to seal all

external seams with an industrial epoxy tape sealant in order to stop

the air leakage. The internal dimensions of the pebble bed were 2 m by

2 m (6.5 ft by 6.5 ft) in horizontal cross-section and 1.8 m (6 ft) in

height. The pebble bed is shown in Figure 6 as it was tested. The flow

direction was from top to bottom during all tests.

The thermal energy storage capacity of the pebble-bed was 9.80 MJ/°C

(5160 Btu/°F) computed from the bed dimensions and assuming a rock density of 1538 kg/m (96 Ib/ft ) and a specific heat of 0.88 (kJ/kg*°C)

(0.21 Btu/(°F*lb)) for the rock. Using the 35°C (63°C) temperature

swing, the theoretical energy storage capacity was 343 MJ (325,000 Btu).

The latent energy storage effects due to moisture exchange with the air

stream, to be described later in this report, were not taken into account

in calculating the theoretical storage capacity.

Test Loop

The test loop is shown schematically in Figure 7. Room air first

entered a proportionally-controlled 25 kW electric duct heater, where

it was heated to the desired test temperature. The conditioned air traveled from the heater through a length of flexible ducting to the upstream

measurement section, the thermal energy storage device (TESD), and the

downstream measurement section. The measurement sections contained

pressure taps for pressure measurements across the TESD, sensors for

wet-and dry-bulb temperature measurements, and a thermopile across the

two sections for temperature difference measurements. Another section

of flexible ducting was connected from the downstream measurement sec- tion to a blower-nozzle arrangement which was used to measure air flow.

Air was exhausted from the blower to outside the test cell to avoid

over-heating problems.

10
TABLE

1.

THEORETICAL

STORAGE

CAPACITY

OF

THE

PHASE-CHANGE

THERMAL

ENERGY

STORAGE

DEVICE

?o rH 4J

CO 4) •H

4-1 tn 60 i—1

CO

m o CO o i o O 60 cO •H • • • • • i • H 3 3 uo m CM m CM i o

cu o CO cO CM i o

U-l 3 4.) a. rH O W CO CO o

S'?

0)60

cO 6o

Vj 4-1 O •H

4-1 1—1

Sn os so OS co LO i CM

CO bi kJ

cO3 • CO •

r—H • m • o • rH i • CO

cO

H r-' CO CM CO rH 1 60 a CO rH

m

U cO <U CJ cwo

•H CJ * CO CM so CM CM

MH 4-1 0 00 O'. in Mf

m UO

•H cO • • • • • • • O (D M 60 I—1

rH CO CO CM CM

0) 33

m

Q- CM co

4) f N 60 Ss CJ CM iH CM rH rH r-H 3 O 0 CO CM CO CM CM CM 3

rJ s-/ 333

4J

CO 4= 3 60 CJ CM CM SO SO vO

0) •H o CO CO

m

m

m j CL, 33 s ' s

<u H3

0)

p-l >s >S

CO 6£ cO os UO co OS

CO 3 J«S 3 CM rH

O CO H H • • • •

CO i—i s

03 •H

•H 3

iH cr O •H COA A

4-1 4J o CO 3

CM <U <U CJ

33 33 33 o O CM

i—1

0) 4) CO

4-1 I—1

iH 3 s 1 a X X O (H,. <u o •H •H •H 3 'O’ •H CO CO o 4-1 60 o o CO 3 3 •H 3

a, CO 3 4> 4) 4-) rH 0 Pt-i CO CO CO TJ 3 O CM 3 O 3 3 o cO i—1 O 3 z CL, D* H K

11
12

Figure

5.

Schematic

of

the

pebble-bed

thermal

energy

storage

test

unit
13

Figure

6.

Photograph

of

the

instrumented

pebble

bed
£3

14

Schematic

of

the

NBS

thermal

energy

storage

test

loop.
The measurement sections approximated typical duct size ratios and were

constructed in accordance with the requirements of Standard 94-77 with

respect to pressure taps and placement of sensors as shown in Figure 7.

Thermocouples were used for dry-bulb temperature measurements and a wet-bulb thermocouple (Figure 8) was used for wet-bulb temperature

measurements [12], A six-junction thermopile was used for temperature

difference measurements. Thermocouples and the thermopile were all

laboratory fabricated from premium 24-gage type-T thermocouple wire.

Both duct sections were heavily insulated with 15 cm (6 in) of glass

fiber insulation. Flow mixers were not used.

Several methods were employed for measuring air stream humidity during

the various tests. The first technique was to place psychrometers in

both the inlet and exit air flow ducts. The psychrometers consisted of

wet-and dry-bulb temperature sensing type-T thermocouples and were con- structed and applied as specified in ASHRAE Standard 41.1-74 [12].

After the tests were completed on the pebble-bed and the data reduced,

it was found that the inlet duct psychrometer indicated a sudden

increase in absolute humidity ratio when the dry-bulb temperature was suddenly increased by the electric heater. This indicates that the

inlet psychrometer was not reading properly since moisture was not being

added to the air stream. Noting this effect from the data, the test

duct was disconnected and inspected and the inlet wet-bulb sensing

element was found to be resting on a rubber stopper. Apparently the

supporting thermocouple wire had slipped. It was also noted that the

outlet wet-bulb sensor was dirty, probably due to dust from the pebble

bed. As a result, the wet-bulb readings for the pebble bed tests were

not reliable.

An error sensitivity analysis was conducted for the psychrometer in

order to gain insight into the problem of the psychrometric measurements

for these tests. For a properly wetted wet-bulb temperature sensing

element, errors in specific humidity were calculated to be in the range

of 5 to 10 percent at low relative humidities due to radiation heat

exchange between duct walls and the wet-bulb element. Radiation effects

can be compensated for fairly accurately; however, they are generally

ignored. Further analysis indicated that a 0.5°C (1°F) measurement

error in the inlet wet-bulb temperature at conditions present during the

tests would result in a 7% error in inlet specific humidity and a 6% error in the energy exchange across the thermal energy storage device.

A second type of sensor used on the outlet of the pebble-bed was a com- mercial relative humidity transducer which depends on the absorbic equilibrium properties of lithium chloride for its response. During the

first charge test on the pebble-bed, the relative humidity transducer

gradually drifted to a reading of 0% relative humidity and remained

there for the rest of the tests. The drift was probably due to fouling

of the element caused by dust from the bed.

A third type of sensor employed was a "dew-probe" which measures the dew

point temperature of the air. In this device, a mirrored surface is

cooled until the surface starts to fog. A photoelectric sensor is used

15
mercury-in-gtass

thermometer —

OE

as m®E

Jl

as

as 5 e

s16

mercury-in-gtass

tfiermometer

psychrometer

ftermocoupta

psychrometer
to maintain the surface temperature at the dew-point and a thermocouple

indicates temperature. This device is considered more reliable than the

transducer even though it can be contaminated by foreign substances in

the air. It was not available for the pebble-bed tests.

The flow measuring apparatus was a blower nozzle arrangement built in

accordance with Standard 94-77. Flexible magnetic mounting strips were

fixed to the nozzles and worked in combination with the air pressure to

hold the nozzle in place and form a tight seal. Five interchangeable

nozzle sizes were available: 5, 6.5, 7.5, 10, and 12.5 cm (2, 2.5, 3,

4, and 5 in) throat diameters, which provided a range of flow rates from

0.03 to 0.41 wr/s (70 to 950 ft^/min). The pressure difference across

the nozzle was determined using a 0-1.2 kPa (0-5 in 1^0) inclined manometer in parallel with an electronic pressure transducer. The pressure

transducer was an elastic diaphragm type. The nozzle discharge gauge

pressure was determined using a 0-2.5 kPa (0-10 in H2 O) vertical manometer. A precision barometer was used to measure barometric pressure.

The blower was a centrifugal industrial model with a 3 phase, 220-Volt,

3-horsepower motor capable of delivering 0.47 m

J /s (1000 ft^/min) at

standard conditions and at a static head of 2.5 kPa (10 in H2 O). In a typical solar heating system installed in a house, a half-horsepower or smaller motor is generally sufficient since air flow nozzles are not

used. The major pressure loss experienced in the tests was 0.25 to 0.75

kPa (1 to 3 in H2 O) pressure drop across the nozzle. The amount of air

delivered by the blower was controlled by a bayonet-type damper on the

blower outlet (coarse control) and a stovepipe-type damper on the inlet

(fine control). The blower assembly was mounted on rubber vibration

isolators and duct connections to the blower were made using flexible

rubber connections which resulted in nearly complete vibration isolation

of the blower.

Data from the various sensors consisted of either millivolt signals

(thermopile and pressure transducer) or thermocouple signals, which were

fed to a commercial data aquisition system (DAS). The DAS was capable

of handling 20 millivolt channels and 40 thermocouple channels. The scan rate was variable, although all tests were done at a 2-minute scan

rate. An internal electronic reference junction was used for thermocouples. Data were collected and stored on a reel-to-reel magnetic tape

drive and data reduction was accomplished on the large-scale digital

computer at NBS.

The remainder of the instrumentation for these tests consisted of four

thermocouples for ambient temperature measurement and thermocouples

located within the thermal energy storage units. The pebble-bed was

instrumented with 13 thermocouples placed along a line through the

center in the direction of flow. The phase change unit was similarly

instrumented with seven thermocouples for air temperature through the

centerline, seven thermocouples for tray temperature through the centerline, and other thermocouples scattered throughout the unit.

17
Prior to any testing, the thermal energy storage units were subjected to

a smoke test to determine if air leaks were present. The smoke test

consisted of pressurizing the units to about 0.25 kPa (1 in. ^0), introducing smoke from a smoke bomb into the unit, and visually checking for

leaks. This method was found to detect very small leaks.

4. EXPERIMENTAL PROCEDURE

Heat Loss Test

The heat loss test was carried out in accordance with Standard 94-77

except for using a temperature difference of 35°C (63°F) rather than

25°C (45°F) as specified. The reason for this departure was that it was more convenient to do the heat loss test at the end of a charge test

which was done at 35°C. It was felt this would not compromise the test

results in any way. All test cycles described in this report consisted

of a charge test followed by a heat loss test and finally followed by a discharge test.

For the heat loss test, the inlet fluid temperature was set at 35°C

(63°F) above ambient temperature and the outlet fluid temperature was allowed to reach a steady- state condition. The difference between inlet

and outlet fluid temperatures was averaged over a several hour period.

The heat loss coefficient, L, was then calculated according to the equation (1). A major problem with the heat loss test was found to be the

stability and measurement of the low temperature difference across the

unit. Typical ranges of temperature difference were 1.5 to 3°C (3 to

6°F). The fluid inlet temperature controller was required to maintain a steady temperature only within +1°C (±2°F) which made it very difficult

to accurately measure the 1.5 to 3°C temperature difference. In order

to overcome these problems, the heat loss rate was averaged over several

hours rather than the one-hour test period as specified.

Transient Charge Tests

All transient tests were performed in accordance with the Standard

except that the recommended flow rates were not used. Both thermal

energy storage units were designed for specific flow rates and these

specified flow rates were used in the tests. In general, the flow rates

specified in the Standard are much higher than experienced in most solar

systems using air as the transfer fluid. The Standard specifies fill

times of 2 and 4 hours, whereas in a typical system the flow rate is such

that the fill time is approximately 6 hours. A summary of all transient

tests on both units is presented in Tables 2 and 3, respectively.

Transient charge tests were performed by first circulating room temperature air through the device until the inlet and exit temperatures were within 0.5°C (1.0°F). The flow rate was then adjusted to the desired

value and the air heaters were then turned on starting the test. The

required test fluid inlet temperature was generally attained within 5 minutes, meeting the requirements of the Standard that 90% of the step

change be achieved within 2% of the test fill time. The charge test

18
continued until the exit fluid temperature reached steady-state conditions at which time the heat loss test was started. For all tests on the pebble-bed, the flow direction was top to bottom. Flow rates for

all tests in both units were approximately 0.42 and 0.21 m~Vs (900 and

450 ft^/min).

Transient Discharge Tests

The transient discharge tests were performed following the heat loss

tests. The flow rate was not adjusted prior to starting the discharge

tests since adjusting flow rate could have caused problems with the

stability of the inlet temperature. Thus the discharge test was started

by simply cutting off the heaters and introducing unconditioned room temperature air.

An important exception to the ASHRAE test requirements for this test program was that the phase-change device was not cycled through the phasechange temperature 30 times prior to testing. Tests performed at NBS

were primarily to evaluate the testing techniques and not to evaluate

the particular unit.

5. PHASE-CHANGE DEVICE TEST RESULTS

Test on the phase-change unit were run at the NBS facility in July and

August, 1978. Five complete charge - heat loss - discharge test cycles

were completed, three at the flow rate recommended by the manufacturer

and two at one-half the recommended flow rate. Air stream pressure drop across the device was 0.21 kPa (0.85 in. ^0) at the recommended flow

rate.

Heat Loss Test

A heat loss test was included in all five test cycles and resulted in an overall heat loss coefficient of 85.5 kJ/(h*°C)(45 Btu/(h*°F)) for both

test flow rates. An estimation of the heat loss factor utilizing basic

heat transfer theory, the material properties and dimensions of the

phase-change container, and assuming no air leakage, yielded an overall

heat loss coefficient of 17 kJ/(h*°C)(9 Btu/(h*°F)). Heat loss during

the transient tests generally was on the order of 5 to 10 percent of the

energy transferred during a test fill time period.

A major heat loss problem in these types of thermal energy storage

devices is air leakage to or from the unit. The storage container which was fabricated at the NBS site contained sealant which dried out and

cracked shortly after installation. The unit had to be resealed and

taped. Metal packing bands were necessary to hold the unit together.

The dry wall recommended by the manufacturer was not used. Smoke tests

verified the unit was sealed although there was some smoke diffusion

through the foamed insulation.

19
Charge and Discharge Tests

The results from the charge and discharge tests for the five test cycles

of the phase-change unit are shown in Table 2. The charge capacity was found to be approximately 220 MJ (2.1 x 10^ Btu) using the fill time as defined in Standard 94-77 and described earlier in this report. A more

meaningful measure of performance in the authors' opinion is to divide

the charge capacity by the theoretical storage capacity which yields an average value of 0.56. This is in comparison to values of between 0.80

and 0.90 for a similarly-sized water tank tested at NBS [7,8].

It is proposed that the ratio of the charge or discharge capacity to the

theoretical storage capacity be denoted as the devices' performance factors (PF). Using such performance factors rather than the charge capacity (C

c

) and discharge capacity (C^) presently used in ASHRAE Standard

94-77 has several advantages

1. The performance factor is a dimensionless number which is independent of the physical size of the unit.

2. Use of the performance factor tends to normalize the test

results in more consistent values. Variation between tests of

temperature and flow rate do not affect the value of the performance factors nearly as much as they do the charge and discharge

capacities presently used [7,8].

3. The performance factor relates the performance of the unit to

the theoretical maximum performance of any completely sensibleheat storage device.

The discharge capacity test results were very scattered. Transient test

results for a phase-change device are in general highly dependent on the

initial temperature, the phase-change temperature, and the temperature

step function. This effect can most readily be seen in the discharge

tests where the difference between the input temperature and the phasechange temperature averaged only about 6°C (11°F). Referring to Table 2,

it is evident that for the discharge tests, the closer the input temperature was to the phase-change temperature, 32 °C (89 °F), the smaller the

discharge capacity. The tests at NBS were affected by doing the tests

in a summer in a poorly-conditioned space which resulted in lack of

control over the charge test initial temperature and discharge test

input temperature. Strict requirements should be included in the Standard on the value of the temperatures to be used for phase-change units.

The discharge capacity averaged approximately 130 MJ (1.2 x 10^ Btu) and

the corresponding average value of the performance factor for the discharge tests was 0.33.

The low values for discharge capacity obtained indicate a possible problem with this particular phase-change unit for space heating. During

discharge, once the sensible heat above the transition temperature (32°C)

has been extracted, the rate of energy extraction is limited by the

20
TABLE

2.

PHASE-CHANGE

THERMAL

ENERGY

STORAGE

DEVICE

TEST

RESULTS*

O)o

32OH

T3 s^

<U CD

Cd T3 JZ

rs a) rs bo I M

st CO as sz

cdo

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cj

O l-i 1 3 o 1 M eel 4J 1 00

0) s o 1 UO M3

(X i-i cd ! • • O Cm 1 CM 1l-i 0)

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>N

<u 4J 00 -H /—\

Cd o ! st

m 1 Is cd £ 0 • 1 O v£> 1 4-1 cd 'O 00 1 C/3 c_> no CM 1

0100 fn

Is 4-1 cd •H /~n J= O 1 pH as o Cd £ 1 • •

CO &.s^ 1 rs CO

•H cd 1 no rs Q CJ 1 CM CM

0)00u 4-1 CO •H

.3 o 13 1 CM no o cd £ 1 • •

CO

a 1 rs

m

•H cd 1 CM

m

O cj 1 CM r-H

01 Oo oo 4-1

CT3 •H

1-1 o s CM rH sO o cd £ •

4-1 O-s^ »"H no pH

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cd »M

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l-i

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rH 3 00 CM

w CM CM rH pH 00 00

O

Cm pel Jrl rs ai rs 00 rs 00 as as 00 00 ai S-/ rH to rH r-H rH rH 3

T3 hJ o

a)

W Is •H rH 3 i < no cO i pH o CM CM o Is CM rH E i PH • • I • • • • • ai •H •H •H ,3 i 00 CO i r-H o CM r—H

CO

03 pH

H i CM i oo CM OO CM

m CO

O pz ai £ 4Jo o 3

rs rH a> i 0-i no

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Ph

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ai 11 rH | o CJ Q CJ Q CJ Q CJ a CJ Q

rH r-H CM CM oo oo St st

m

m *

cd T3

21
difference between the entering fluid temperature and the transition

temperatureo It may be difficult to heat a space with a source temperature of 32°C (89°F) 0 It would be useful to designers if the test results

from Standard 94-77 tests included a plot of delivery heat rate as a function of time.

ASHRAE Standard 94-77 suggests a plot be provided showing the time variation in transfer fluid temperature for each test. Figure 9 shows such a plot for those tests involving the ASHRAE-recommended test fill times

and temperature step changes. The abscissa represents time from x=0 to

t=t

c j and the ordinate is a non-dimensional quantity:

t in - c out (8

)

At

In equation (8), the initial step change in transfer fluid inlet temperatures, At, is a constant. However, the difference between the transfer

fluid outlet and inlet temperatures decreases with time yielding the curves of Figure 9. For a sensible-heat thermal energy storage device

with perfect stratification and no heat loss to the ambient, the temperature of the inlet transfer fluid, t^n

, and the outlet transfer fluid,

t 0ut 9 would remain constant for the entire test fill time period (time

for one entire volume change), at which time they would become equal.

A curve of test results for such a device would be represented by a rectangle in Figure 9.

The concept of using a non-dimensional plot showing the storage capabilities of thermal energy storage devices relative to those of an ideal

device has been proposed [5]. Such a non-dimensional plot is shown in

Figure 10 for ideal thermal energy storage units. The ordinate is the

nondimens ional quantity as before:

tom t - out

At

The abscissa represents a non-dimensional time defined by: wtf c tf At (9)

TSC

where t = time, s

An ideal, perfectly-insulated, completely-stratified, sensible-heat thermal storage device is represented by the dashed rectangle in Figure 10.

A value of non-dimensional time equal to 1.0 corresponds to the time

required to fully-charge such an ideal device (test fill time). The area under the curve for this ideal device is 1.0.

If a perfectly-insulated, completely-mixed, sensible-heat thermal energy

storage device was possible, the resulting curve would be the exponential

solid curve in Figure 10. Again the total area under the curve out to an

22
.s -C S)

05 05 -SC

-SC JC CM

CO

LT5 05 CO

II II cs

CD

II § s 5 § o U o O

C_3 O ocq

O

CVJ

OCO

Ol LC5 e\j o o

*\i CM CO CM CO II II n

ii «-« <

<1 <i <1 o C S

j=

05

r-i to

r**« CO CO o

II II ii II u CO -a a o • O

CD

CD

LU

CO

CD

cvi

--

O « O #

CNI

"I1 E

ID 1 V

IDl

SS31N0ISN3WIQ ‘ ' UIDj . U|j

•l—1 01—

>4-4 a)GO

5-1 tO <U d

>4-1

O

CO 4-1 C CO

to

5-i >>

4-) GO

5-1

d 0) •H d

a)

do i—l •H tO

4-4 0

to 5-4 •H a)

S-I -d

to 4J >

a;

<u 60 e d

•H to

4-4 -do

>4-4 l O a)

co

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r—1Cl

o.

0) T3 .d

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u

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<5* <u Cu Cl 4-) X 0 CO

c/o <15 <D C 4-1 4-)

<15 S-i d

GO

•H

23
Sdeai

sensibleheat

storage

unit

V) J

e©

'v>

\ V

ssajuoisuaiuip ' tnoj . m

24

Figure

10„

Theoretical

plots

of

ideal

thermal

energy

storage

units
abscissa value of 00 is 1.0. However, if the area under the curve were calculated from dimensionless time of 0 to 1.0, the value would be 0.632,

which corresponds to the ratio of energy stored by the ideal completelymixed sensible-heat device over the test fill time compared to the ideal

stratified device. This is precisely the performance factor described

above and as a result, such a dimensionless plot would be an ideal way

to show graphically the meaning of the performance factor.

The foregoing analysis was based on the performance of sensible-heat

thermal energy storage devices. Devices which use latent-heat for

energy storage behave differently. A perfectly-insulated, completelystratified, ideal latent-heat device with a phase-change temperature, t m , would behave as shown in Figure 10. Note that the ideal device would

not be fully-charged at a dimensionless time of 1.0. Also note that the area under the curve between a dimensionless time of 0 and 1.0 depends

on the relative values of the phase-change, initial inlet, and step

change in inlet temperature. Any latent-heat thermal energy storage

device will also store some energy in the form of sensible heat. The curve for an ideal device with both latent and sensible-heat contributions is difficult to visualize; however, the fill time for such an

ideal device can be calculated as explained below.

The theoretical storage capacity (TSC) of the combined latent-sensible

device is:

TSC = MCp At + M Hl (10)

where M = mass of storage material, kg

Cp = sensible specific heat of the material, J/(kg*°C) (Btu/lb*°F))

= latent heat of the material at the phase-change temperature,

tm , J/kg (Btu/lb)

The fill time components can be calculated from the following expression:

T _ TSC l F x ~ w

tf c tf Atx

( 11 )

For the sensible-heat portion, Atx

is the difference between the inlet

fluid temperature, t in , and initial storage temperature, t i# For the

latent-heat portion, Atx

is the difference between the inlet fluid temperature, t. n , and the phase-change temperature, tm

. Consequently, for

the combined latent-sensible device:

25
t f = ( 12 ) Mc

P O^in - tj)

+

M HL w

tf c tf (tin " fc i> wtf c tf ( fc in " tm>

or mhl

~ tl}

.

Ty = M°p (tjn

tj) + (tin ~ tm )

( 13 ) w

tf c tf (tin ~ tf)

Note that if the phase-change temperature were exactly half way between

the initial temperature, t^ 9 and the input temperature, t^n , equation

(13) would reduce to:

tf = Mc

p + 2 MHl (i4) w

tf c tf

Also note that if the sensible-heat contribution were small and could be

assumed to be zero:

xp

=

2 MHL (15)

W

tf C tf

In other words, the fill time would be exactly twice that called for in

the present version of Standard 94-77.

It is recommended that the fill time for latent-heat storage devices be

calculated using equation (12) or (13) (denoted hereafter in this report

as modified fill time) instead of equation (3) as is presently required

in Standard 94-77. In this way, latent-heat devices will be evaluated

more equitably with sensible-heat devices.

The data from the transient tests on the phase-change device were re- analyzed using the modified fill time and the results are also shown in

Table 2. The modified charge capacity (the charge capacity calculated

using the modified fill time) was calculated to average 230 MJ

(2.2 x 10-* Btu) compared to 220 MJ (2.1 x 10^ Btu) using the fill time

of equation (3). The performance factor for the charge tests averaged

0.59 (compared to 0.56). Because the input temperature was so close to

the phase-change temperature during the discharge tests, the modified

fill time for most of the tests was on the order of several days. The

tests were not run that long and as a result, the discharge capacities

and corresponding performance factors could not be calculated using

the modifed fill times.

The test results for the transient tests were plotted in accordance with

the completely dimensionless plot described above. Figure 11 is such a plot for charge tests at two flow rates using the Standard 94-77 fill

time. A second plot using the modified fill time is shown in Figure 12.

A plot of the inlet and outlet transfer fluid temperatures as a function

of time is shown in Figure 13 for the charge test where t c = 7.2 h. It

26
PHASE-CHANGE

THERMAL

ENERGY

STORAGE

UNIT

CHARGE

TEST

8) JL cn COLO

r* 55

ii ns

S

ci

o

oto

C\J LO CN CM

cn II

ii <1

<1 JZ j£ o

to

r**

ii II o w

o

o

°

O

o

D o . o

o

o

o

o

o

o

o

o

o

o

o

CM CO

GO

GO

4-0 o

5

G_3

GO

CO

%

o

°

O

o

o

°

o

CM

o

CM CM

SS31N0ISN3WI0

‘ \ V

jnoj . utj

27

Figure

11.

Plot

of

dimensionless

temperature

versus

dimensionless

time

for

charge

tests

on

the

phase-change

thermal

energy

storage

test

unit

at

two

flow

rates,

using

the

ASHRAE

Standard

94-77

fill

time
PHASE-CHANGE

THERMAL

ENERGY

STORAGE

UNIT

CHARGE

TEST

CO

en

cs

LO

Itg

o'

o

csi

ng

oo«a

td

ess

^ <3

00

II II63 Ko

o

o

o

o

°

c%s

o

°

8

O

o

o

<2>

o

n S>

co

SM

GO CO

O

SS31NOISN3IAIIQ

' inoj _ u\

x

CM

28

WtfCt<AtT

,

DIMENSIONLESS

TSC

Figure

12.,

Plot

of

dimensionless

temperature

versus

dimensionless

time

for

charge

tests

on

the

phase-change

thermal

energy

storage

test

unit

at

two

flow

rates,

using

the

modified

fill

time
PHASE-CHANGE

THERMAL

ENERGY

STORAGE

UNIT

CHARGE

TEST

29

TIME,

h

Figure

13.

Plot

of

inlet

and

outlet

transfer

fluid

temperatures

during

charge

test

on

the

phase-change

thermal

energy

storage

test

unit
distinctly shows the temperature step increase in the inlet transfer

fluid and the outlet fluid temperature variation with time.

Internal Behavior

The thermocouples placed in the airstream and pressed into stacking

slots yielded temperature readings which agreed very closely at each

tray location. This indicates a high heat transfer rate from the fluid

to the tray. The thermocouples in the stacking slot were not immersed

in the phase-change material (PCM). Figure 14 is a plot of air temperatures at seven tray locations along the flow path length. The plot shows

a high degree of stratification in temperature in the direction of flow.

In the length of 0.3 m (1 ft), the temperature changed as much as 8°C

(15 °F) along the same flow channel. The fluid exit temperatures, however, were not consistent with the stratification observed in the trays

as can be seen in Figure 14. This leads one to conclude that a significant portion of the flow was somehow by-passing the trays. It is possiible that a large part of the flow passed down the spaces between the

top trays and the tops of the eight plywood cells where there were no tray support legs to restrict flow. Fourteen thermocouples in the other seven flow cells all exhibited the same temperature for the corresponding

location in the flow cell shown in Figure 14, indicating no flow imbal- ance between the eight flow cells.

Thermal Performance Degradation

One possible problem area with phase-change devices is the degradation

of performance due to either separation of phase-change material or super-cooling due to lack of nucleation sites. Hence, 30 cycle preconditioning is required by Standard 94-77 prior to performing the performance

tests. As mentioned previously, the approach used in these tests was to use no pre-conditioning. The degradation that did occur over the five

cycles can be seen however for the charge test results in Table 2.

The degradation of the unit can be checked in two ways: (1) by observing

the changes in the charge and discharge capacities determined in accord- ance with the Standard, or (2) by observing the changes in the estimated

total thermal capacity of the unit which was also determined experimentally from the measurements made and is given in Table 2.

Problems were experienced during test cycles (1) and (3) with the proportional temperature controller used to control the inlet temperature to

the device so that the charge and discharge capacities indicated from

these tests are not valid. Test cycles (4) and (5) produced similar

results for charge capacities.

The second technique of estimating the total thermal energy storage

capacity of the unit after each cycle was done by charging the unit

much longer than the test fill time and until the exit temperature was constant. The measured amount of energy charged into the device less an allowance for heat loss was then assumed equal to the total thermal

capacity. An unsteady inlet temperature (which occurred during cycles

30
PHASE-CHANGE

THERMAL

ENERGY

STORAGE

UNIT

CO

<Dd

3u

cO 60 d d

<u <u cl c

e a)

0)

4-1 i—

I

a} d s

•H d

d 0) -d

i-H XJ

cO3 a) d 60

ai c

•u to d e

ITS

CO

CNJ

0) o a> a 4-1 e d

a) <U

4-J 60 60

1 c d

0) •H 14 6 d o

•H 3 4-J H T3 w

<ud

3

60

•H

P>H

C_3

3o 3dlUVH3dW3JL Olltli

31

MEASUREMENT

LOCATION
(1) and (3)) did not affect the results since a numerical integration of

the difference in temperature between inlet and exit (see equation (6)) was done. On the first charge test, the total thermal capacity of the

unit was calculated to be 90% of the theoretical capacity. On cycle (2),

the total capacity dropped to 70%. On cycles (4) and (5), the capacity

was approximately 60% of the theoretical capacity.

6. PEBBLE-BED TEST RESULTS

Pebble-bed tests were completed at the NBS test facility in August and

September, 1978. Seven complete charge - heat loss - discharge test

cycles were run, four at the flow rate recommended by the manufacturer

and three at one-half the recommended flow rate. Air stream pressure

drop at the recommended flow rate was 0.068 kPa (0.27 in. IL^O).

Heat Loss Test

A heat loss test was included in all seven test cycles and resulted in an overall heat loss coefficient of 95 kJ/(h*°C)(50 Btu/h*°F) for both

flow rates. An estimation of the heat loss factor utilizing basic heat

transfer theory, the material properties and dimensions of the pebblebed containers, and assuming no air leakage, yielded an approximate over- all heat loss coefficient of 36 kJ/(h*°C)(19 Btu/h°°F)). Heat loss during the transient tests generally was on the order of 3 to 8 percent of

the energy transferred during a test fill time period.

Charge and Discharge Tests

The results for the charge and discharge tests for all seven test cycles

of the pebble-bed are shown in Table 3. The charge capacity determined

ranged from 258 to 320 MJ (2.44 x 10^ to 3.03 x 10^ Btu) and the

discharge capacity from 247 to 293 MJ (2.34 x 10^ to 2.78 x 10^ Btu).

Charge and discharge capacities are highly dependent on test temperatures

as has been indicated previously.

The performance factors for the charge tests as defined in the previous

Section ranged from 0.79 to 0.87 and from 0.80 to 0.86 for the discharge

tests. In general, the performance factor for the charge tests was higher at higher flow rates due to less heat loss over the shorter fill

time. Moisture exchange with the air stream was not taken into account

in the calculation of charge or discharge capacity.

A plot of charge and discharge test results at both flow rates as required by the Standard is shown in Figure 15.

The non-dimensional plot of the same two charge tests as on Figure 15

is shown in Figure 16. Note that the results for the two different flow

rates fall nearly on the same curve, indicating that the thermal perfor- mance of the pebble-bed is not greatly affected by flow rate. Similar

plots of all charge tests followed the same curve.

32
TABLE

3.

PEBBLEBED

TEST

RESULTS* 3y33 fa s o

fa 4J o o 3 nO NO o 1 i—H

m ON 3 Is

fa 3 00 00 00 00 1 00 oo r-. oo oo 00

fa fa • • 0 • • * • • • • 3 o o o o o o o o o o

fa

>4

cu 4-1 fa 00 -H

co CO

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i—1

CJ

Q CJ

Q CJa CJ

a CJ QO OO

Q

y

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m no no rs rs

CJ

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0)3ouu3

co

0)

4-1 0c3

331I *

33
o

o

o

o

i inoj . ujj

ssaiuojsuauijp

Figure

15.

ASHRAE-recommended

plot

of

time

variation

in

transfer

fluid

temperature

for

the

pebble-bed

thermal

energy

storage

test
Pebble

bed

O

_ to

O

<3 O

6

_

O CM

r

<?

<p

Hs

_ es

' 5

\%

%ia

K,

'p<

b.

x

cn J£

CM

IIs

ooOS

toCO

II

LT5

IIo

K*

05

JC

soC5

eo

ii§

ooCM

(Sn

ii

iiu

K*

O <3

co

-I

__i ^

CM

CM CM

l V

ssaguofsuauijp ' in0j

35

TSC

Figure

16.

Plot

of

dimensionless

temperature

versus

dimensionless

time

for

charge

tests

on

the

pebble-bed

thermal

energy

storage

test

unit

at

two

flow

rates
Internal Behavior

The thirteen thermocouples spaced evenly along the flow path within the

pebble-bed were used to produce the plot in Figure 17 for a typical

charge test. The pebble-bed showed a large degree of stratification as evidenced by the shape of the curve in the first hours of the test.

Exiting bed temperatures did not vary significantly from the exiting

duct temperatures throughout the tests indicating that the flow was uniform across the cross-section of the pebble-bed.

Moisture Exchange Effects

Pebble-beds are known to exchange moisture with the air stream which has

a significant effect on the performance of the bed [14,15]. Attempts

were made during the tests to quantify this process, but problems with

the measurement and control of air stream humidity as previously indicated prevented any quantitative analysis.

In general, moisture is released from the pebble bed during charging and

absorbed during discharging [14,15]. The mechanism of moisture exchange

is a sorption process in which moisture is exchanged between the air and

the fine pores of an absorbent. During the process, there is an energy

exchange equal to the latent heat of vaporization of the water. The

quantity of water finally held by a given sorbent when equilibrium has

been reached is dependent only on the relative humidity of the contacting

air-water vapor mixture [13]. This ratio is largely independent of the

temperature of operation and of the vapor pressure at which the water

exists in the air mixture.

Although the exchange of moisture in the rock bed was not well characterized by these tests, the bed should behave in general as follows:

1. Initially the bed will be at thermal and moisture equilibrium

with the cool pre-charging air stream assuming entering temperature and humidity are held constant.

2. Charging, which is accomplished with air of high temperature

and low relative humidity, will raise the temperature of the

rocks and extract moisture from them. Energy is stored in both

the sensible temperature increase of the rocks and in the

"dryness" of the bed.

3. Discharging, which is accomplished with air of low temperature

and high relative humidity, will lower the temperature of the

rocks and deposit moisture in them. Energy removed from the

bed is contributed by the sensible heat of the rocks and the

heat of vaporization as the water changes from vapor to liquid.

As an illustration of the storage of heat through "dryness", imagine an

ideal massless desiccant which is at an equilibrium condition of 21 °C

(70°F) and 10% relative humidity. If air at 21 °C (70°F) and 50% RH with

a corresponding enthalpy of 58.7 kJ/kg (25.2 Btu/lb) is introduced into

36
Tc

=5

4. h

Pebble

bed

A

t

=35

9.

°C

w

=

1782

kg/h

OS

CQ

E

B

e

"J3s

OSE

as§

«3

as

00

•r-1 &-

as o

0o 'ajniejaduiai pm|j

37

Plot

of

inlet

and

outlet

transfer

fluid

temperatures

during

a

charge

test

on

the

pebble-bed

thermal

energy

storage

test

unit
the unit, the desiccant absorbs 0.0023 kg (0.005 lb) of water per kg(lb)

of air in bringing the air from 50% RH to the 10% RH equilibrium condition. A total of 12.3 kJ/kg (5.3 Btu/lb) of heat will be released from

the condensed water. Air exiting from this massless unit will then have

an enthalpy of 71.1 kJ/kg (30.5 Btu/lb) and a temperature of 40°C

(104°F) o

The above example illustrates the large effect moisture transfer could

have on thermal energy storage capacity. In fact, use of desiccants as thermal energy storage media has been suggested [14,15], A desiccant

thermal energy storage device would have the advantage of a high energy

density at relatively low temperatures.

It is apparent from analysis of the phenomena of moisture exchange with

porous beds that the present Standard should include requirements to cover this situation. Inlet humidity conditions for tests must be estab

lished in order to rate various thermal energy storage devices equitably

In Section 8.2 of the Standard it is suggested that an energy balance

approach be used for rating a thermal energy storage device when there

is a net moisture exchange between the air stream and the device. In

contrast, Close suggests a rating based on sensible heat exchange only

when the thermal energy storage device is used for heating purposes [14]

Close’s recommendation seems to be the most feasible for pebble-beds

since the moisture exhange process is one that will occur in a random

fashion and primarily based on the system design and geographical location of the system.

In order for the moisture exchange process to be controlled properly in

the test, Standard 94-77 requires modification. Section 8.2 states that

the inlet and exit dew-point temperatures shall be equal and remain constant which will be impossible for some devices. Three alternate

approaches for establishing the inlet humidity conditions are:

1. Use a closed test loop and a drier to remove all significant

quantities of moisture from the air and the bed prior to starting the test.

2. Use a closed test loop, but initially have the bed at equilibrium at a specified condition prior to a transient test. This

approach would simulate a closed loop with a solar collector

such as exists in a solar heating system.

3. Use an open test loop with the inlet humidity conditions con- trolled to a specified condition.

7. CONCLUSIONS AND RECOMMENDATIONS

A total of five tests on a phase-change thermal energy storage unit and seven tests on a pebble-bed were completed in accordance with ASHRAE

Standard 94-77. It was found that the heat loss characteristics and the

amount of energy charged into or discharged from the device could be

38 '
quantified through the use of the Standard. However, based on this

study, the following recommendations are made:

1. Flow rate requirements in ASHRAE Standard 94-77 for thermal

energy storage devices utilizing air as the transfer fluid

should be changed to reflect flow rates normally seen in

installed solar systems. The flow rate is currently specified

in terms of the test fill time. This may have to be changed if

recommendation 3, below is adopted.

2. The temperature difference for the heat loss test should be

changed from 25°C to 35°C so that the heat loss test can be

conducted conveniently in conjunction with the transient charge

and discharge tests.

3. For thermal energy storage devices in which energy storage capacity is temperature dependent (i.e. a phase-change device), the

relationship between the phase-change temperature, initial temperature, and input temperature should be specified so that the

devices can be consistently tested and rated.

4. For phase-change thermal energy storage devices, the fill time

used in the computation of the charge and discharge capacity

should be calculated using equation (13) or (14) in this report.

5. For latent-heat energy storage devices, delivery heat rate and

fluid exit temperature should be included in the test results

required to be reported by the Standard.

6. Thermal energy storage devices which exchange moisture with the

air flow stream (e.g. a pebble-bed) should be tested using

specified inlet air conditions.

7. The charge and discharge capacities of a pebble-bed should be

based on sensible heat exchange only.

8. The dimensionless plots described in this report should be used

in the analysis of test results. The utilization of such plots

should provide a suitable means for comparing thermal performance of different thermal energy storage devices.

9. The performance factor described in this report provides a relative rating factor independent of the size of the unit and

should be used in rating thermal energy storage devices.

10.

Even though the test results should allow a relative ranking of

different thermal energy storage devices, the relationship

between test results and real-world system performance needs to

be established for typical devices.

39
8 . REFERENCES

1. Hill, J.E., and T. Kusuda, "Method of Testing for Rating Solar Collectors Based on Thermal Performance," NBS Report NBSIR 74-635,

December, 1974,

2. Kelly, G„E,, and J.E. Hill, "Method of Testing for Rating Thermal

Storage Devices Based on Thermal Performance," NBS Report NBSIR

74-634, April, 1975.

3. Hill, J.E., Streed, E.R., Kelly, G.E., Geist, J.C., and T. Kusuda,

"Development of Proposed Standards for Testing Solar Collectors and

Thermal Storage Devices," NBS Technical Note 899 , February, 1976.

4. Hill, J.E., and E.R. Streed, "A Method of Testing for Rating Solar

Collectors Based on Thermal Performance," Solar Energy , Vol. 18,

pp 421-429, 1976.

5. Hill, J.E., Kelly, G.E., and B.A. Peavy , "A Method of Testing for

Rating Thermal Storage Devices Based on Thermal Performance," Solar

Energy , Vol. 19, pp. 721-732, 1977.

6. ASHRAE Standard 94-77 —Methods of Testing Thermal Storage Devices

Based on Thermal Performance, ASHRAE, 345 E. 47th St., N.Y., N.Y.

10017, 1977.

7. Hunt, B.J., Richtmyer, T.E., and J.E. Hill, "An Evaluation of

ASHRAE Standard 94-77 for Testing Water Tanks for Thermal Storage",

NBS Report NBSIR 78-1548, November, 1978.

8. Hunt, B.J., Richtmyer, T.E., and J.E. Hill, "Testing of Water Tanks

for Thermal Storage According to ASHRAE Standard 94-77," ASHRAE

Transactions , Vol. 85, Part I, 1979.

9. ASHRAE Standard 93-77 —Methods of Testing to Determine the Thermal

Performance of Solar Collectors, ASHRAE, 345 E. 47th St., N.Y.,

N.Y. 10017, 1977.

10. Hill, J.E., Jenkins, J.P., and D.E. Jones, "Testing of Solar Collectors According to ASHRAE Standard 93-77," ASHRAE Transactions , Vol. 84, Part II, 1978.

11. Hill, J.E., Jenkins, J.P., and D.E. Jones, "Experimental Verification of a Test Procedure for Solar Collectors," NBS Building Science

Series 117 , December, 1978.

12. ASHRAE Standard 41.1-74 Standard Measurements Guide: Section on Temperature Measurements, ASHRAE, 345 E. 47th St., N.Y., N.Y. 10017,

1974.

13. ASHRAE Handbook of Fundamentals , ASHRAE, 345 E. 47th St., N.Y.,

N.Y. 10017, 1977.

40
14. Close, D.J., and T.L. Pryor, "The Behavior of Absorbent Energy

Storage Beds," Solar Energy , Vol. 18, pp. 287-292, 1976.

15. Close, D.J., and R. V. Dunkle, "Use of Adsorbent Beds for Energy

Storage in Drying of Heating Systems," Solar Energy, Vol. 19,

pp. 233-238, 1977.

41
iS-i 14A (REV. 9-76)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 79-1737

i 4. TITLE AND SUBTITLE

Testing of Pebble-Bed and Phase-Change Thermal Energy

Storage Devices According to ASHRAE Standard 94-77

5. Publication Date May 1979

'

7. AUTHOR(S)

Dennis E. Jones

8. Performing Organ. Report No. James E. Hill

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. Projact/Ta3K/WofkJ

K .J42>4'41?

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, state, ZIP)

R & D Branch, Solar Heating and Cooling

Off of Asst. Sec. for Conservation & Solar Appl.

DoE, Washington, D.C. 20545

13. Type of Report & Period Covered Final

M> Ag&rtcy Cede

15. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less factual summary of moat significant information. If document includes a significant bibliography or

literature survey, mention it here.) The American Society of Heating, Refrigerating, and Air Conditioning Engineers

(ASHRAE) has recently adopted ASHRAE Standard 94-77 - Methods of Testing Thermal

Storage Devices Based on Thermal Performance. Experiments have been completed at

the National Bureau of Standards in which a 7 m3 (250 ft3) pebble-bed and a similarly-sized 264 MJ (250,000 Btu) phase-change unit utilizing sodium sulfate

decahydrate, both using air as the transfer fluid, were tested in accordance with

this Standard. A description of the test procedure, test apparatus, and detailed

test results is given. Some problems were encountered in using the Standard for

these kinds of thermal energy storage devices, and modifications to the Standard are recommended based on these experiments.

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

separated by semicolons)

ASHRAE Standard 94-77; Glauber's salt; latent heat storage; pebble bed; phasechange unit; solar energy storage; thermal energy storage device.

18. AVAILABILITY fy) Unlimited

[

i For Official Distribution. Do Not Release to NTIS

[ Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC " 20402, SD Stock No. SN003-003-

|y Order From National Technical Information Service (NTIS), Springfield, VA. 22161

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