NBSIR 74-634

Method of Testing for Rating Tliermal

Storage Devices Based on Thermal

Performance

George E. Kelly

James E. Hill

Thermal Engineering Section

Center for Building Technology

National Bureau of Standards

Washington, D. C. 20234

Hill, J. E.,, Kelly, G. E., Peavy, B. A.,

A method of testing for rating thermal

storage devices based on thermal perfoi'mance.

May 1975

Solar Energy 19, 7?l-732 (l977). , Interim Report i 462

7i- L 3 y

Prepared for Energy Research and Development Administration

Solar Energy Division

1800 G street Washington. D C. 20550

NBSIR 74-634

METHOD OF TESTING FOR RATING THERMAL

STORAGE DEVICES BASED ON THERMAL

PERFORMANCE

George E. Kelly

James E. Hill

Thermal Engineering Section

Center for Building Technology

National Bureau of Standards

Washington. D. C. 20234

May 1975

Interim Report

Prepared for Energy Research and Development Administration

Solar Energy Division

1800 G Street Washington, D. C. 20550

U S, DEPARTMENT OF COMMERCE. Rogers C.B. Morton. Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

Method of Testing for Rating Thermal Storage

Devices Based on Thermal Performance

George E. Kelly and James E. Hill

Themial Engineering Section

Center for Building Technology

National Bureau of Standards

Washington, D.C. 20234

ABSTRACT

A study has been made at the National Bureau of Standards of the different

techniques that could be used for testing thermal storage devices and

rating them on the basis of thermal performance. This document outlines

a proposed standard test procedure based on that study. It is written

in the format of a standard of the American Society of Heating, Refrigerating, and Air Conditioning Engineers and gpecifies the recommended apparatus, instrumentation, and test procedure.

Key Words: Standard test; thermal storage^

solar energy, standard, tliermal

thermal performance,

test

i
TABLE OF CONTENTS

PAGE

SECTION 1. PURPOSE 1

SECTION 2 . SCOPE 1

SECTION 3. DEFINITIONS i

SECTION 4. CLASSIFICATIONS . . 3

SECTION 5. REQUIREMENTS ^;

SECTION 6. INSTRL'MENTATION 5

SECTION 7. APPARATUS AND METHOD OF TESTING . . . 7

SECTION 8. TEST PROCEDURE AND CALCULATIONS • • 11

SECTION 9. DATA TO BE RECORDED AND TEST REPORT 22

SECTION 10. NOMENCLATURE 23

SECTION 11. lIEFERENCES 25

TABLES 27

FIGURES 30

ii
SECTION 1. PURPOSE

1.1 The purpose of this standard is to provide a standard procedure for

determining the thermal performance of thermal energy storage

devices that are used in systems to provide the thermal requirements

for heating, cooling, and the generation of domestic hot water in

buildings

SECTION 2. SCOPE

2.1 This standard applies to sensible-heat and latent-heat type thermal

energy storage systems. In addition, it is limited to those

storage devices in which a fluid enters the device through a single inlet and leaves the device through a single outlet. Storage

devices having more than one inlet and/or outlet may be tested

according to this standard, but each flow configuration involving

a single inlet and a single outlet must be tested separately. This

standard is not applicable to those configurations in which there

is simultaneous flow into the storage device through more than

one inlet or simultaneous flow out of the storage device through

more than one outlet. The fluid can be either a gas or liquid

but not a mixture of the two.

2.2 This standard does not address factors relating to cost or consideration of requirements for interfacing with a specific heating

and cooling system. Consequently, the test results do not provide

all the information required for a complete evaluation of the thermal energy storage system.

2.3 The test procedure and equipment outlined in this standard are most easily adaptable to thermal energy storage system having

capacities on the order of 10^ J (10^ Btu) or less.

SECTION 3. DEFINITIONS

3.1 AMBIENT AIR

Ambient air is the air in the space surrounding the thermal energy

storage system.

3.2 CYCLING

Cycling of a latent -heat type storage device is a process in

which the temperature of the system is raised and lowered in a cyclic manner and the phase of the storage medium is changed twice

in each temperature cycle.

1
' 3.3 EFFECTIVE CAPACITY FOR HEAT REMOVAL

The effective capacity for heat removal is the amount of heat

that can be removed from the storage system during a period of

time equal to the fill time and for a specific set of values for

t^, the initial temperature of the storage system; At, the

temperature difference between t^ and the temperature of the

entering fluid; and m, the mass flow rate of the fluid through

the storage system. .

3.4 EFFECTIVE CAPACITY FOR HEAT STORAGE

The effective capacity for heat storage is the amount of heat

that can be stored in the storage system during a period of

time equal to the fill time and for a specific set of parameters

t£. At and m.

3.5 FILL TIME

The fill time is the duration of a single transient test as specified in Section 8 in which energy is either added or extracted

from the storage system.

3.6 RATE OF HEAT LOSS I.

The heat loss rate is the rate that heat is lost from the

storage system per degree temperature difference between the stor-

, age medium temperature and the average ambient air temperature.

3.7 STORAGE CAPACITY

The storage capacity of a thermal energy storage system is

defined as the heat that can be stored in a system undergoing a At increase in temperature from its initial value t^.

3.8 PERFORMANCE COEFFICIENT FOR HEAT REMOVAL

The performance coefficient for heat removal is the ratio of the

effective capacity for heat removal to the amount of heat that

could be removed from an equal volume of water in an ideal

water tank under the same conditions.

2
3.9 PERFORMANCE COEFFICIENT FOR HEAT STORAGE

The performance coefficient for heat storage is the ratio of the

effective capacity for heat storage to the amount of heat that

could be stored in an equal volume of water in an ideal

water tank under the same conditions.

3.10 STANDARD AIR

3 3 Standard air is air weighing 1.2 kg/m (0.075 Ib/fC ), and is

equivalent in density to dry air at a temperature of 21.1°C (70°F)

and a barometric pressure of 1.01 X 10^ N/m^ (29.92 in. of Hg.).

3.11 STANDARD BAROMETRIC PRESSURE

1.01 X 105 N/m2 (29.92 in. of Hg.).

3.12 STORAGE MEDIUM

The storage medium is the material in the storage system in

which the major portion of the energy is stored.

3.13 STORAGE SYSTEM

The storage system is defined as the container (s) plus all contents

of the container(s) used for storing thermal energy in a system.

The transfer fluid and other accessories such as heat exchangers

within the thermal storage container(s) are considered as part

of the storage system.

3. 14 SPECIFIC HEAT

The specific heat of a substance is the quantity of energy necessary to produce a unit change in temperature of a unit mass.

3.15 TRANSFER FLUID

The transfer fluid is the fluid that carries energy in and out

of the storage system.

SECTION 4. CLASSIFICATIONS

4.1 In this standard thermal energy storage systems are classified

according to the method they use to store energy and the type of

transfer fluid they employ.

3
4.1.1 Sensible heat storage systems are those in which the heat

absorbed by or removed from the system results in an in- crease or decrease in the temperature of the storage medium and there is no change of phase of any portion of the

* storage medium. Typical systems employ pressurized water,

unpressurized water, rock, brick or concrete as the storage medium.

Latent -heat storage systems are those involving a change

of phase of the storage medium. In this type of system,

most of the heat added to or removed from the system goes

into changing the enthalpy of the storage medium during a change of phase process. Some heat is also stored as sen- sible heat, since charging and discharging of the storage

device usually involves a finite change in the temperature

of the system.

4.1.2 A storage system will use either a liquid or a gas as the transfer fluid. The most common liquids are water or a water-ethylane glycol solution. The most common gas is air.

SECTION 5. REQUIREMENTS

This standard covers only those thermal energy storage systems , that can be treated as black boxes and that do not alter the

phase or composition of the transfer fluid passing through them.

Latenc-heat type storage systems evaluated under this standard

shall have been cycled (see definition of cycling) through their

cliange of phase at least 30 times prior to being tested.

The transfer fluid used in evaluating the performance of a thermal

energy storage system shall have a known specific heat that varies

by less than +0.5% over the temperature range encountered during

a test.

The room where the testing of the storage system is performed

shall have its temperature controlled to the extent that the

average ambient air temperature t , determined by the average of

the four temperatures measured as^specified in Section 8.9, varies

between extremes by less than + 1.0°C (+ 1.8°F) during a test.

4
SECTION 6. INSTRUMENTATION

6.1 TEMPERATURE MEASUREMENTS

6.1.1 Temperature measurements shall be made in accordance with

ASHRAE Standard 41-66, Part 1 [1].

6.1.2 Temperature Difference Measurements Across the Thermal

Storage System . The temperature difference of the transfer

fluid across the thermal storage system shall be measured

with

a. Thermopile (air or liquid as the transfer fluid)

b. Calibrated resistance thermometers connected in

two arms of a bridge circuit (only when a liquid

is the transfer fluid)

6.1.3 The accuracy and precision of the instruments and their

associated readout devices shall be within the limits as follows

Instrument Accuracy^ Instrument Precision

Temperature + 0.5°C (+ 0.9°?) + 0 . 2°C (+ 0.4°^)

Temperature + 0 . 1°C (+ 0.2°F) + 0 . l^C (+ 0.2°F)

Difference

6.1.4 In no case shall the smallest scale division of the

instrument or instrument system exceed 2 1/2 times the

specified precision. For example, if the specified

precision is + 0.1°C (+ 0.2°F), the smallest scale

division shall not exceed 0.25°C (0.5°F).

6.1.5 The instruments shall be configured and used in

accordance with Section 7. of this standard.

6.1.6 When thermopiles are used, they shall be constructed in

accordance with ANSI Standard C96. 1-1964 (R 1969) [2].

The ability of the instrument to indicate the true value of the measured

quantity.

Closeness of agreement among repeated measurements of the same physical

quantity.

5
6.2 LIQUID FLOW MEASUREMENTS

6.2.1 The accuracy of the meter including a calibration, if

furnished, shall be equal to or better than + 1.0% of

the measured value.

6 . 3 RECORDERS AND INTEGRATORS

; 6.3.1 Strip chart recorders used shall have an accuracy equal

to or better than + 0.5% of the temperature difference

and/or voltage measured for each test with the exception

of the heat loss rate test. In the test to determine

the heat loss rate, the accuracy of the strip chart

recorder shall be equal to or better than + 2.0%.

6.3.2 Electronic integrators used shall have an accuracy equal

to or better than + 1.0% of the measured value for each

test with the exception of the heat loss rate test. In

the test to determine the heat loss rate, the accuracy

of the electronic integrator shall be equal to or better

than + 4.0%.

6.4 AIR FLOW MEASUREMENTS

When air is used as the transfer fluid, air flow rate shall be

determined as described in Section 7.

6.5 PRESSURE MEASUREMENTS

6.5.1 Nozzle Throat Pressure . The pressure measurement at the

nozzle throat shall be made with instruments that shall

permit measurements of pressure to within +2.0% absolute

and whose smallest scale division shall not exceed 2 1/2

times the specified accuracy [3].

6.5.2 Air Flow Measurements . The static pressure across the

nozzle and the velocity pressure at the nozzle throat

shall be measured with manometers that have been

calibrated and are accurate to within + 1.0% of the

reading. The area of the nozzle shall be determined by

measuring its diameter to an accuracy of + 0.20% in four

places approximately 45 degrees apart around the nozzle

in each of two planes through the nozzle throat, one

at the outlet and the other in the straight section near

the radius [3].

6
6.5.3 Pressure Drop Across the Thermal Storage System . The static

pressure drop across the thermal storage system shall be

measured with a manometer having an accuracy of 2.49 N/m^

(0.01 in. of water).

6 . 6 T IME AND MASS MEASUREMENTS

Time measurements and mass measurements shall be made to an accuracy of + 0.20% [3].

SECTION 7. APPARATUS AND METHOD OF TESTING

7.1 AIR AS THE TRANSFER FLUID

The relative position of the thermal energy storage system, the

temperature measuring instrumentation, the air flow measuring

apparatus and the reconditioning apparatus is shown in Figure Bl^.

7.1.1 Test Ducts . The air inlet test duct, between the air flow

measuring apparatus and the thermal energy storage system,

shall have the same cross-sectional dimensions as the inlet

to the storage device. The air outlet test duct, between

the thermal energy storage system and the reconditioning

apparatus, shall have the same cross-sectional dimensions

as the outlet of the storage device.

7.1.2 Temperature Measurement Across the Storage System . A thermopile shall be used to measure the difference between the

inlet air temperature and outlet air temperature of the

thermal energy storage system. It shall be constructed

from calibrated type T thermocouple wire all taken from a single spool of wire. No extension wires are to be used

in either its fabrication or installation. The wire

diameter must be no larger than 0.51 mm (24 AWG) and the

thermopile shall be fabricate^ as shown in Figure B2. There

shall be a minimum of six junctions in the air inlet test

duct aad six junctions in the air outlet test duct. These

junctions shall be located at the center of equal cross- sectional areas.

The recommended apparatus consists of a closed loop configuration,

open loop configuration is an acceptable alternative provided that

test conditions specified herein can be satisfied.

7

An

the
During all tests, the variation in temperature across the

air inlet and air outlet test ducts shall be less than +

0.5°C (+ 0.9°F) at the location of the thermopile junctions.

The variation shall be checked prior to testing utilizing

instrumentation and procedures outlined in reference [l].

If the variation exceeds the limits above, mixing devices

shall be installed to achieve this degree of temperature

uniformity. Reference [4] discusses the positioning and

performance of several types of air mixers.

The measuring junctions of the thermopile should be located as near as possible to the inlet and outlet of the thermal energy

storage system. The air inlet and air outlet ducts shall be insulated in such a manner that the calculated heat loss from

these ducts to the ambient air would not result in a temperature change for any test of more than O.OSOC (0.09OF) between

the temperature measuring locations and the storage system.

7.1.3 Dry and Wet Bulb Temperature Measurements . Thermocouples

or other devices giving a continuous reading shall be used

to measure the wet and dry bulb temperature at the locations

in the air inlet and air outlet ducts shown in Figure Bl.

ASHRAE Standard 41-66, Part I [1 ] shall be followed in raaking these measurements.

7.1.4 Duct Pressure Measurement s. The static pressure drop across

the thermal energy storage system shall be measured using a manometer as shown in Figures Bl and B3. Each side of the

manometer shall be connected to four pressure taps that are con- nected to an external manifold on the air inlet and air outlet ducts.

The pressure taps should consist of 6.4 mm (1/4 in.) nipples soldered to the duct and centered over 1 mm (0.040 in.) diameter

holes. The edges of these holes on the inside surfaces of the

ducts should be free of burrs and other surface irregularities [3].

7.1.5 Air Flow Measuring Apparatus . The air flow shall be mea-

' sured with the nozzle apparatus discussed in Section 7 of

ASHRAE Standard 37-69 [3j . As shown in Figure B4 , this apparatus consists basically of a receiving chamber, a discharge chamber and an air flow measuring nozzle. The distance from the center of the nozzle to the side walls shall

not be less than 1 1/2 times the nozzle throat diameter, and

diffusers shall be installed in the receiving chamber at

least 1 1/2 nozzle throat diameters upstream of the nozzle

and 2 1/2 nozzle throat diameters downstream of the nozzle.

The apparatus should be designed so that the nozzle can be

easily changed and the nozzle used on each test shall be

selected so that the throat velocity is between 15 m/s

(2960 fpm) and 35 m/s (6900 f pm) . Details on nozzle con- struction and discharge coefficients that may be used are contained in Section 7.3 of ASHRAE Standard 37-69 [3]

8
An exhaust fan capable of providing the desired flow rates

through the thermal energy storage system shall be installed

in the end wall of the discharge chamber. The dry and wet

bulb temperature of the air entering the nozzle shall be

measured in accordance with ASHRAE Standard 41-66, Part I

[ l] . The velocity of the air passing through the nozzle

shall be determined by either measuring the velocity head

by means of a commercially available pitot tube or by measuring the static pressure drop across the nozzle with a manometer. If the latter method is used, one end of the

manometer shall be connected to a static pressure tap located flush with the inner wall of the receiving chamber

and the other end to a static pressure tap located flush

with the inner wall of the discharge chamber, or preferably, several taps in each chamber should be connected through

a manifold to a single manometer. A means shall also be

provided for measuring the absolute pressure of the air in

the nozzle throat.

7.1.6 Air Leakage . Air leakage through the air flow measuring

apparatus, air inlet test duct, the thermal energy storage

system and the air outlet test duct shall not exceed + 1.0%

of the measured air flow.

7.1.7 Air Reconditioning Apparatus . The reconditioning apparatus

shall control the dry bulb temperature of the air

entering the storage system to within +1.0*^ (+ 1.8°F) of

the desired test values at all times during the tests. Its

heating and cooling capacity shall be selected so that dry

bulb temperature of the air entering the reconditioning

apparatus may be raised or lowered by an amount equal to

the largest required in Section 8.

7.2 LIQUID AS THE TRANSFER FLUID

The test setup for thermal energy storage systems employing liquid

as a transfer fluid is shown in Figure B5^.

7.2.1 Temperature Measurement Across the Storage System. The

temperature difference between the transfer fluid entering

and leaving the storage system shall be measured using

either two calibrated resistance thermometers connected

in two arms of a bridge or a thermopile made from

calibrated, type T thermocouple wire all taken from a single spool of wire. The thermopile shall contain any even number of junctions constructed according to the

recommendations in ANSI Standard C96. 1-1964 (R 1969) [2].

a The recommended apparatus consists of a closed loop configuration. An

open loop configuration is an acceptable alternative provided that the

test conditions specified herein can be satisfied.

9
Each resistance thermometer or each end of the thermopile

is to be inserted into a well [6J located as shown in

Figure B5. To insure good thermal contact, the wells shal]

be tilled with light oil. The wells should be located just

downstream of a right angle bend to insure proper mixing [1].

To minimize temperature measurement error, the wells should

be located as close as possible to the inlet or outlet of

the storage device. In addition, the piping shall be insulated in such a manner that the calculated heat loss from

this piping to the ambient air would not cause a temperature

change for any test of more than 0.05'^C (0.09^F) between

each well and the storage system.

7.2.2 Additional Temperature Measurements. The temperature of the

trlnifer fluid at the two locations cited above shall also

be measured by inserting appropriate sensors into the wells,

ASHRAE Standard 41-66, Part I [1] shall be followed in

making these measurements.

7.2.3 Pressure Drop Across the Storage System. The pressure drop across the thermal energy storage system shall be measured

using static pressure tap holes and a manometer. The edges

of the holes on the inside surfaces of the pipe should be

free of burrs and should be as small as practicable but not

exceeding 1.6 mm (1/16 in.) diameter [5]. The thickness of

the pipe wall should be 2 1/2 times the hole diame.ter [5].

7.2.4 Liquid Transfer Fluid Reconditioning Apparatus . The recon- ditioning apparatus shall control the temperature of the

transfer fluid entering the storage system to within + 1.0°C

(+ 1.8°F) of the desired test values at all times during the

tests. Its heating and cooling capacity shall be selected

so that the temperature of the liquid entering the recon- ditioning apparatus may be raised or lowered by an amount ' equal to the largest required in Section 8.

7.2.5 Additional Equipment . A pressure gauge, a pump and a means

of adjusting the flow rate of the transfer fluid shall be

provided at the relative locations shown in Figure B5 . In

addition, a pressure relief valve and an expansion tank

should be installed to allow the transfer fluid to expand

and contract freely in the apparatus^.

Figure B5 should not be interpreted to mean that the relief valve and

expansion tank necessarily be located below the thermal energy storage

unit

10
SECTION 8. TEST PROCEDURE AND CALCULATIONS

8.1 GENERAL

The method that has been most commonly employed in testing of

water storage tanks in Japan [11, 12, 13] is to cause the transfer

fluid entering the storage device to undergo a step change

in temperature and to measure the temperature of the

transfer fluid leaving the storage unit. By integrating the

difference in temperature between the inlet and outlet over the

testing period and multiplying the result by the transfer

fluids' mass flow rate and specific heat, one can determine the

amount of heat added or removed during this time period^. This

energy balance is shown conceptually in Figure B6 where the area under the curve represents the energy absorbed during the

time period shown. If the time period chosen for the test were

some characteristic time depending upon the size of the storage

device chosen, the heat storage capability of different devices

could be compared. This will be illustrated by citing typical

results taken from reference [14].

Yang and Lee [14] performed an analysis to determine the nature

of the transient heat transfer between a heat storage unit and

a circulating or transfer fluid due to variations in the inlet

temperature of the transfer fluid. The configurations chosen

for analysis are shown in Figures B7, B8, and B9. Figure B7

shows a specific-heat type storage device in which a liquid

storage medium is heated or cooled by a fluid passing through

thin tubes inside the container. Figure B8 shows a pebble-bed

type unit in which the transfer fluid comes in direct contact

with the storage medium. Figure B9 shows as in Figure B7, a heat-exchanger type storage device except in this case, the

transfer fluid is circulated around tubes which encapsulate a latent-heat type material such as a salt hydrate.

The basic one-dimensional transient equations governing the

temperature distribution of both the transfer fluid and storage

medium are presented and solved using the Laplace transformation

technique. Yang and Lee point out that the boundary conditions

most appropriate to simulate a real storage device would be some arbitrary variation of inlet fluid temperature with time.

a This is strictly true only if the losses from the outside of the

storage unit are negligible. Otherwise, the losses must be accounted

for in the energy balance.

11
However^ since It ±8 Impractical to calculate the ayatem response

for every possible Inlet variation and since the storage system

Is described by linear equations, Its dynamic characteristics

may be conveniently Investigated by using a step Input or a sinusoidal input. Solutions are given in [14] for both the step

Input and sinusoidal input for the configurations of Figures

B7 and B8 but only for the step input for the latent-heat type

device.

Typical results are shown In Figures BIO and Bll for a water tank,

in which water is also circulated through tne heat exchanger as the transfer fluid and the input is a step function. Figure

BIO shows the temperature distribution of the transfer fluid as

a function of position down the tube and time. Figure Bll shows

the same thing for the storage medium or water in the tank. These

results were for the case of negligible resistance to heat transfer at the interface between the tubes and the storage meJium.

One should note that it has been possible to present the results

in dimensionless form. The temperature difference between

the fluid and the initial value is divided by the difference

between the inlet value and the initial value (step function)

to get t'''. The space dimension is divided by the total path

length through the storage device to get x * and time has been

made dimensionless by dividing it by the time required for a fluid particle to travel through the system of length 2., A/u

where u is the flow velocity. The process of normalizing

the results has been possible due to the fact that the system

behavior is described by linear equations. In reality, the

response of storage devices will only approximate a linear

behavior. Consequently the testing procedure Included herein

has been written In such a way as to determine the response of

the system to different step Inputs and for both heat

storage and heat removal processes even though the results of

the linear theory would indicate this is unnecessary.

To demonstrate how different storage devices can be compared

based on their response to a step function input, the results

of reference [14] have been used to plot the curves in Figure

B12. The curves are plots of dimensionless temperature difference

12
between the inlet and outlet of a storage tank configured as in

Figure B7. Both curves are for the same flow rate of water

(transfer fluid) through the storage device. The only difference

between the two is that on one hand there is a finite resistance

to heat transfer on the outside of the pipes (h^ = 56.7 W/ (m^ . ^K)

typical of natural convection in the tank and on the other, there

is negligible resistance (h^ = «>) . The area under the curve is

proportional to the amount of energy transferred into or away

from the storage unit. As can be seen, the device with

the smallest resistance to heat transfer is clearly the more

effective one for absorbing or releasing the energy.

Up until this point, emphasis has been placed on discussing

the comparison of storage devices based on their response to a step increase in inlet fluid temperature. Other possibilities

exist.

A second method that could be employed would be to subject the

transfer fluid entering the storage unit to a constant influx of

heat, Q. This would result in raising the temperature of the

entering transfer fluid (assuming the specific heat of the

transfer fluid is constant) by a fixed number of degrees above

the outlet temperature. By measuring the time dependent outlet

temperature one could obtain information chat would be useful

in designing collector-storage systems. While this method

simulates more closely the real interaction between a collector

and a storage device, it has the disadvantage that one cannot

measure the energy storage and removal capability of the unit.

This is due to the fact that if one measured the heat absorbed by

the storage unit over a period of time, it would just be equal to

Q X the test period or the amount of energy added to the system.

Thus the only way of comparing different storage devices would be

to compare plots of outlet temperature versus time for different

values of Q choosen so as to take into account the different sizes

of the storage units being compared. The storage device with

the lowest average outlet temperature would probably be considered

best because this would tend to maximize the efficiency of a collector.

13
A tlilrd method would be to use a time varying Q and to measure the

outlet temperature as a function of time during the testing period.

This would allow one to simulate the output of a collector over one

or more days and to determine the response of the storage device.

If the time dependence of Q resulted In an oscillating inlet

temperature, one would also be able to look at the degree of

stratification attained in the storage unit. This method has the same disadvantage as the second method in that it would be very

difficult to compare the performance of different storage devices.

In addition, one has the problem of deciding on what is the typical

cycle for Q; not an easy task when one considers that the output

of the collector depends not only on the weather but on the

particular storage unit employed. The major advantage of this

method would be that by Inserting an array of thermocouples in

the storage medium, the experimenter could measure the temperature

stratification in the unit.

Stratification, which in water tanks results from different temperature water seeking its own temperature ( or density) level, is a desirable characteristic for operation in a solar heating and

cooling system since the inlet fluid temperature varies up and

down with time. If large stratification results when the inlet

temperature is either constant or a monotonic (increasing or decreasing) function of time, it is probably a result of short

circuiting flow (ie. dead space in a water tank). This short

circuiting of the flow could result in higher fluid temperatures

to the collector and thus decreased efficiency, which could easily

off-set the advantages of stratification.

To test the transient response of the storage unit, the first

method of measuring the response of outlet temperature to a step

change in inlet temperature, was chosen as a basis for the

test. This method was selected because:

a) it permits the determination of effective storage

capacity and thus allows an easy comparison of

different types of storage units,

b) it appears to be the most fundamental approach

since linear theory shows that the outlet temperature

response to a constant or variable heat flux Q can be predicted if one knows how the outlet temperature

changes with a step change in inlet temperature, and

c) it is felt that the relative performance of storage

devices using this method will be the same if either

of the other procedures were used.

14
8. 2 STORAGE CAPACITY, FILL TIME, EFFECTIVE CAPACITY FOR HEAT

STORAGE, EFFECTIVE CAPACITY FOR HEAT REMOVAL

The storage capacity SC (t-j^, At) of a thermal energy storage

system is defined as the energy that can be stored in a system

undergoing a At increase in temperature from its initial value

t^. The value SC (t-j^, At) shall be calculated by adding up the

amount of heat absorbed by the individual components in the

storage system (storage medium, tank, heat exchanger, insulation,

etc.) in raising their temperature from t^^ to (t-^ + At). If

there is an uncertainty in the heat storage ability of the

storage medium, a representative sample shall be tested in a calorimeter to determine the amount of heat required to raise

its temperature from tj^ to (t^ + At),

The concept of fill time as introduced in this test procedure

is used for the purpose of specifying the time of the transient

tests. Recall that in the analysis of Yang and Lee [14], it was possible to present the response characteristics of the thermal

storage unit in terms of a dimensionless time where real time

is divided by some characteristic time 1/xi, the effective path

length of the transfer fluid divided by the effective transfer

fluid velocity. This allows a convenient way of comparing

storage units of the same basic design but of different sizes.

However, if one is going to compare, for example, a water storage

tank where the transfer fluid and the storage medium are the same with a storage tank and embedded heat exchanger where the volume of

water-- in the heat exchanger (transfer fluid) at any instant is

only 1/10 of the volume of thfe water in the storage tank (storage

medium) , then comparing the response of the two over the same

time period I In would be unfair. The device with the embedded

heat exchanger would appear to have something on the order of

10% of the storage capacity of the other device when compared

over the same number of time periods £/u. Consequently, a different time scale is introduced herein that will allow

storage units of entirely different designs to be

compared on an equitable basis.

If a storage unit has a specified thermal capacity or storage

capacity SC (t^, At), and the transfer fluid of specific heat

c^£(tj^) is flowing through the device at a constant flow rate m and has an inlet temperature At, above the initial temperature of the storage device, then the fill time is defined by

T . SC(ti, At)

F m ctf(ti) At

In the testing procedure, all storage units are tested for

the same fill time and then the thermal responses compared.

15
In other words, if two different sensible-heat devices

had the same ultimate storage capacity and were being tested

over the same At but one used water (water tank) and the other

air (pebble-bed type) as the transfer fluid, and the flow rates were such that the fluid dwell times (determined by llu) were

identical, it would be unfair to compare the response of the two

units over the same real time period. One would be able to

charge the water tank with considerably more energy over the same

time period (approximately by the ratio of ((m Cj-f )^^^.gj.) / ( (m ^tt^ g,±t^

The recommendation here would be to test the two units tor the same fill time as defined by equation (1). Consequently, the

flow rates would be adjusted so that ( (m c^-^r) )/((m Ct-c) . )

> v^ater axr =1. In other words, the flow rates for the cirrerent devices are adjusted so that the amount of energy entering the device (or

leaving for an energy removal test) per unit thermal storage

capacity is identical for the two devices.^

The fill time t^, and the flow rate of the transfer fluid m are related in an inverse manner according to equation (1) . One

possibility for specifying the test conditions of the transient

tests would be to specify the At to be used, allow the experimenter

to select an m, and then, depending upon the properties of the

transfer fluid (c^f) and storage capacity of the unit, conduct

*the tests for some fraction or integral of the fill time. However, the

experimenter who was concerned about the optimum performance of his

unit would no doubt select a flow rate where the relationship between

the energy transfer rate and pressure drop (or power required to push the

fluid through the device) was an optimum. In actual installations,

the flow rate through and At across the storage device is controlled

by the flow rate through and At across the collector and/or ,' building heating and cooling system. The flow rate is usually

proportional to the collector size and in turn the storage capacity

is usually proportional to the collector size. Consequently, the

ratio of SC/m is constant within certain limits which means

according to equation (1), the fill time is constant within

certain limits. As a result, in the test procedure, two different

fill times are specified that are felt to be typical of installed

systems, the At's are specified according to whether a liquid or air is the transfer fluid^, and the flow rate to be used is

aOne should recognize that for an ideal water tank where there is

entirely piston-type flow, the above definition of fill time is identical

with the fluid dwell time.

b

Air heating collectors impose a much higher At on the storage device

than do liquid heating collectors.

16
calculated according to equation (1).

The effective capacity for heat storage EC ^^±y ^F^ defined

as the net amount of heat flowing into a storage system during

the time period (t^, Tq + t^) when the entering transfer fluid,

that is initially at temperature tj^, undergoes at At step increase

in temperature at time t^. It may be calculated using the equation:

^ + -T o F

^^hs ^^'i' ''f^ ^ ^ \f <'^i> J

St (t) dT

TO

where 6t (x) = [t-j^^^ (x) ~ t (x)] is the difference between the

inlet temperature tj^^ and tEe outlet temperature t at time x, t^ is the average ambient air temperature, and m has the value

specified in Section 8.6.

The effective capacity for heat removal EC^^ (t^ + At, - At, x^)

is defined as the net amount of heat removeS from a storage system

during the time period (x^, x^ + x^) when the entering transfer

fluid, which is initially at temperature (t^ + At), undergoes a At step

decrease in temperature at time x^. It may be determined using

the formula:

T ' + T o 'f

EC. (t . + At

hr 1

' At, Tp) A ctf (t.) J6t (t) dT (3)

8.3 GENERAL TEST REQUIREMENTS

All of the tests require that the temperature of the storage medium,

prior to the start of data taking, be uniform at the desired temperature and that there exist steady flow of the transfer fluid

through the storage system during a test. To achieve the former,

the transfer fluid shall be circulated through the testing

apparatus until steady state conditions are achieved and the

inlet and outlet temperatures vary by less than + 0.5°C (+ 0.9°F)

during a one hour period. The initial temperature is then defined

to be the arithmetic average of the inlet and outlet temperatures.

Steady flow of the transfer fluid shall be considered achieved

if the flow rate varies by less than + 1.0% during a test.

17
For all tests, with the exception of the one to determine the heat

loss rate, the temperature of the storage medium shall remain

within the normal operating range of the storage medium. For

latent heat-type storage systems this means that the storage

medium shall undergo a change of phase as the temperature of the

storage medium is both increased and decreased by At in the manner

discussed in Section 8.1.

During tests involving air as the transfer fluid, the dry bulb

temperature shall remain above the dew point temperature, and the

inlet dew point temperature shall equal the outlet dew point temperature and both shall remain constant. The latter shall be con- sidered accomplished if the inlet and outlet dew point temperatures

vary by less than + 0.5^C (+ 0.9^F) during a test.

18
8 .4 TESTS TO BE Pli:RF0RT4ED

The first test to be performed on a thermal energy storage system

is the determination of the heat loss rate, L. The test method

for doing this is discussed in Section 8.5. After this is completed, additional tests are required to evaluate EC^^ (t^, /At, Tp)

and ECj^j. (tj^ + At , - At, Tp) for a specific tj^ and a given pair of

parameters Tp and At. The set of parameters Tp and At are to be

chosen depending on the type of thermal energy storage system, so that all of the following combinations of the variables Tp and At are tested:

Tp = 2 hr, 4 hr and At = 16 (2 8.8°?), 8°C (14.4°F) for

thermal energy storage systems using a liquid transfer fluid,

Tp = 2 hr, 4 hr and At - 50Oc (90Of), 2 8°C (50.4°F) for

thermal energy storage systems using air as the

transfer fluid.

When a phase-change type thermal storage system is being tested that

has been designed to be "charged" or "discharged" over a specific

time period, this time period shall be used as the fill time for

testing in lieu of the above specification.

The value of t^ to be used shall be chosen based on the intended

application for the thermal energy storage system. The performance

of these tests is discussed in Section 8.6.

8 . 5 METHOD FOR DETERMINING THE HEAT LOSS RATE

The flow rate of the transfer fluid shall be adjusted to the value

(see equation (1)):

SC (t^, 25"C)

"

(c^. (t^ + 250C)) (1 hr) (250C)

and the temperature of the transfer fluid entering the storage system shall be raised 25 9:: (45^) above the average ambient air temperature t^. After the storage system has reached a uniform, steady

state temperature, the average value of 6t shall be determined over

a one hour period. This average, that will be called 6t, is to be

obtained by integrating §t over this time period and then dividing

by the time period. The rate of heat loss L from the storage system shall then be determined from

L = m c,. (t + 2^^C) 6t

tr a2^C

(5)

19
PERFORMANCE OF A TEST INVOLVING A At STEP CHANGE IN TEMPERATURE

AND A Tp FILL TIME

The test described in this section is to be performed on both sensible and latent heat -type storage systems. However, because the

performance of a latent heat-type storage system is usually affected by its immediate past temperature history, the test herein described shall be performed twice on this kind of storage system.

After the test has been completed once, the storage system shall

be allowed to reach a uniform temperature and the test is then to

be repeated. Only data collected on this second test shall be re- duced and reported as discussed in this section and in Section 9.

After the storage medium has reached a uniform initial temperature

t^, the flow rate shall be adjusted to the value:

' sc (t , z^)

ih = '—\ (6)

The temperature of the transfer fluid entering the storage system

shall be increased in a step-like manner to the new value t^ + At

at some time . During the time period (t^, t^,), the difference

between the temperature of the transfer fluid entering and leaving the storage system, 6t (t ) = [t^^ (t) - t^^^. (t)] , shall either

be recorded on a strip chart recorder or integrated over time using an electronic integrator. If a strip chart recorder is used,

6t (t) shall be manually integrated over time after the test is

completed. The integrated value

J

6t (t) dT

plus a knowledge of m and c^j (tj ) will allow determination of the

effective storage capacity ECha (ti, At, tj') by means of equation

(2).

The last phase of the test Involves a step-like decrease In the

temperature of the transfer fluid entering the reconditioning

apparatus. The temperature of the entering transfer fluid shall

!>c maintained at t^ + At until the temperature of the transfer

fluid leaving the storage system is no longer changing with time.

Following this, the temperature of the entering transfer fluid

shall be reduced from (t^ + At) to t^. Assuming this step change

takes place at t^, the quantity

t' + T_, o F

J

6t (t) dT

20
is to be evaluated over the time period (t ' , t' + t„) either by

direct electronic integration or by recording fit (t; on a strip

chart recorder and then manually integrating. Equation (3) is

then to be used to find the effective heat removal capacity ECj^^

(t^ + At, - At, Tp). , ^

8.7 AN INDEPENDENT CHECK ON THE INTEGRAL OF fit

As an independent check, the inlet temperature, t^^, an^ the outlet temperature, t^^^. , of the transfer fluid shall be recorded on strip chart recorders. The quantities

T + T_, T ' + T

' o F o F

I fit (t) dx and T fit (t ) dr shall be calculated using these

T t'

o o

recordings and compared with the identical quantities that are obtained by using the primary method which measures fit (t ) =

\-^±-n (t)

- t^^^ (t)] directly. In order for a test to be valid, the values

obtained on the check must be within + 10% of the results obtained

using the primary method.

8.8 AIR FLOW RATE CALCULATIONS

The air flow rate through the nozzle is calculated by the following equations

Q . = 1.41 C A (P v' )°'^ (7a) mi n V n

V ' = 10.1 X 10^ V /P (1 + W ) (7b) n n n n

The air flow rate of standard air is then:

Q = Q^,/(1.2 v' ) (7c)

s mi n

8.9 MEASUREMENT OF AMBIENT AIR TEMPERATURE

The ambient air temperature t^ shall be the arithmetic average

temperature of the test area, determined by four calibrated,

temperature sensors. ASHRAE Standard 41-66, Part I [1] shall

be followed in making these measurements. The sensors shall lie

in a horizontal plane approximately at the vertical midpoint of

the storage system and shall be approximately 0.6 m (23.6 in.)

from the sides of the storage system.

21
8.10 ESTIMATION OF POWER REQUIREMENTS

In order to ©stlmate Ehe power required to move the

transfer fluid through the thermal energy storage syetem, the

following equation shall be used:

P = m AP / p (8)

SECTION 9. DATA TO BE RECORDED AND TEST REPORT

9.1 TEST DATA

Table Bl lists the measurements that are to be made during the var- ious tests.

9.2 TEST REPORT

Table B2 specifies the data to be reported in testing a thermal energy storage system. The performance coefficient for

heat storage, PC^^^ , is defined in Table B2 by the formula:

EC (t At, T )/v : PC. = (9)

where c ^ and p„ ^ are the specific heat and density, respectively,

of water at the temperature t^ + At/2. The performance coefficient

for heat removal is siuilarly defined by:

EC. (t + At, - At, T )/V

PC^ =

" — (10)

^H^O Ph^O

The performance coefficients for heat storage and heat removal, as defined above, compare the performance of a thermal energy storage system with the theoretical performance of an ideal water tank

of equal volume and having perfect piston-type flow and zero heat loss

22
Both PC^g and Pt^j^j. ^re to be calculated for each of the eight test

that determine the effective heat storage and removal capacities

for a specific t^. In addition, a single plot is to be provided

showing the time variation in outlet temperature of the transfer

fluid for these eight tests. The quantity

t (t) out

At

shall be plotted as the ordinate, while the abscissa shall show

the time periods (t , t + t„) and (t ' , t' + T„).

^ o o F o o F

SECTION 10. NOMENCLATURE

"2'

EC. (t. , At, T^) hs 1 F

^Sr ^^1 ' ^*

^F^

SC (t^, At)

m

PC

hr

PC

hs

n

area of nozzle, m

nozzle coefficient of discharge

specific heat of water, J/(kg« OC)

specific heat of transfer fluid at t-,

J/(kg.^C)

nozzle throat diameter, m

effective capacity for heat storage, J

effective capacity for heat removal, J

storage capacity, J

heat loss rate, W/^c

mass flow rate of transfer medium, kg/s

performance coefficient for heat removal

performance coefficient for heat storage

2 absolute pressure at nozzle throat, N/m

estimated power required to move the

transfer fluid through the thermal

energy storage system, W

velocity pressure or static pressure

difference across nozzle, N/m^

3 measured air flow, m /s
3 standard air flow, m /

initial temperature of storage system, °C

temperature of transfer fluid at inlet,

°C

average ambient air temperature , °C

temperature of transfer fluid at outlet,

oc

specific volume of air at dry and wet

bulb temperature conditions existing at

the nozzle but at standard barometric

pressure, m^/kg dry air

specific volume of air at the nozzle,

rcr/kg dry air

3 volume of the storage system, m

total mass of storage system, kg

humidity ratio at nozzle, kg H20/kg dry

air

pressure drop across storage system,

N/m2

step change in temperature ,°C

inlet temperature, t^^^, minus outlet tem*

perature , t ^ ^ out

average of f)t (t ) during test for L,°C

density of the transfer fluid, kg/m

density of water, kg/rn^

time f

fill time, s

initial time

time at which temperature of transfer

fluid is decreased from tj^ + At to

t

1

24
SECTION 11. REFERENCES

1. "Standard Measurements Guide: Section on Temperature Measurements",

ASHRAE Standard 41-66, Part 1 , American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 345 East 47th Street,

New York, N.Y. 10017, January, 1966.

2. "American Standard for Temperature Measurement Thermocouples C96.1-

1964" (R 1969), American National Standards Institute, 1969.

3. "Methods of Testing for Rating Unitary" Air Conditioning and Heat Pump

Equipment", ASHRAE Standard 37-69 , American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 345 East 47th Street,

New York, N.Y. 10017, April, 1969.

4. Faison, T.K. , Davis, J.C., and P.R. Achenbach, "Performance of

Louvered Devices as Air Mixers, NBS Building Science Series 27 , March, 1970. (Available from the Superintendent of Documents,

U.S. Government Printing Office, Washington, D.C. 20402 - order

by SD Catalog No. C 13.29/2:27, $0.30.)

5. "Instruments and Apparatus, Part 2, Pressure Measurement", Supplement

to the ASME Power Test Codes, American Society of Mechanical Engineers,

345 East 47th Street, New York, N.Y. 10017, July, 1964.

6. "Instruments and Apparatus, Part 3, Temperature Measurement", Supplement to the ASME Power Test Codes, American Society of Mechanical Engineers, 345 East 47th Street, New York, N.Y. 10017, March, 1961.

7. Davis, J.C., "Radiation Errors in Air Ducts Under Isothermal Conditions Using Thermocouples, Thermistors, and a Resistance Thermometer", NBS Building Science Series 26 , November, 1969. (Available

from the Superintendent of Documents, U.S. Government Printing Office,

Washington, D.C. 20402 - order by SD Catalog No. C 13.29/2:26, $0.25.)

8. Davis, J.C., Faison, T.K. , and P.R. Achenbach, "Errors in Temperature

Measurement of Moving Air Under Isothermal Conditions Using Thermocouples, Thermistors, and Thermometers, ASHRAE Transactions , Vol.

72, Part 1, 1966.

9. "Fluid Meters, Their Theory and Application", 5th edition, American

Society of Mechanical Engineers, 345 East 47th Street, New York,

N.Y. 10017, 1959.

10. "Instruments and Apparatus, Part 5, Measurement of Quantity of Materials, Chapter 4, Flow Measurement", Supplement to the ASME Power

Test Codes, American Society of Mechanical Engineers, 345 East 47th

Street, New York, N.Y. 10017, February, 1959.

25
11. Nakajima, Y., "Studies on the Weighting Functions of Thermal Storage

Water Tanks, Part I", Transactions of the Japanese Architectural Society , Vol. 199, pp. 37-47, September, 1972 (Japanese).

12. Nakajima, Y., "Studies on the Weighting Functions of Thermal Storage

Water Tanks, Part II", Transactions of the Japanese Architectural Society , Vol. 200, pp. 75-83, October, 1972 (Japanese).

13. Matsudaira, H. , and Y. Sakakura, "Analysis on the Thermal Capacity

of Heat Storage Water Tanks, Part I", Journal of the Society of Heating , " Air-Condltlonlng, and Sanitary Engineers of Japan , Vol. 46, No. 4,

pp. 11-17, April, 1972 (Japanese).

14. Yang, W.J., and CP. Lee, "Dynamic Response of Solar Heat Storage

Systems", ASME paper 74-WA/HT-22, presented at the ASME Winter

Annual Meeting, New York, 1974.

26
Table Bl Test Data to be Recorded

Item

U I

•r-l 0) <d S

(30 u

C Qj

•r-t M-l > M

-I c O CO > S-1 C H

M

CO jr

CO 3

<U CO -r-) H c«

Date

Observer

Equipment Name Plate Data

5t (t ) Across Storage System

Inlet Temperature, t in

Outlet Temperature, t out

Inlet Wet Bulb Temperature

Outlet Wet Bulb Temperature

Times

Liquid Flow Rate

Barometric Pressure

Gauge Pressure at Inlet

Gauge Pressure at Nozzle Throat

Nozzle Throat Diameter

Velocity Pressure at Nozzle Throat or Static

Pressure Difference Across Nozzle

Dry Bulb Temperature at Nozzle Throat

Wet Bulb Temperature at Nozzle Throat

Pressure Drop Across Storage System

Ambient Air Temperature

Initial Temperature t^

Step Change At

27

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X
Table B2 Data to be Reported

General Information

Manufacturer .......

Mode 1 #

Serial #

Storage Medium . .

Transfer Fluid .......

Weight of Storage System, W

Volume of Storage System > V t • •

Normal Operating Temperature Range

Minimum Transfer Fluid Flow Rate

Maximum Transfer Fluid Flow Rate .

Maximum Operating Pressure

Flow Configuration Tested (picture)

Heat Loss Rate Test

t ' a SC (t , 25°C)

Si T(t)

m

^tf <^n>

^ c^^ (t.^) 6t ^ "

t. - t in a

28
Table B2 (continued)

Transient Tests

t a

t

1

At

m

P

^tf •

•

SC (t^, At)

EC, (t. + At, - At, T„) hr 1 F

^H2 0 PH2O

EC (t + At, - At, T )/v

pQ = —ti£ = —

USCOMM-NBS-DC

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NBS-114A (REV. 7-71)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. I'Um.K A TION OK Ki;POK l' NO. 2. (iov't Acci'ssion No. NBSIR 74-634

3 0 1^ (u* ( 1 p 1 1" n t s A t' c (' s s i ofi N I >

,

4. I'n i.i', ,ANn suHi i ri.i'

Method of Testing for Rating Thermal Storage Devices Based on Thermal Performance

5 1^11 K 1 1 1" 1 f 1 ( \n 0:if*' May 1975

6. Performing Organization (.ode

7. AUTHOIUS)

George E» Kelly and James Hill

8. Performing Organ. Report No.

9. PKKKORMlNCi OIUiANl/ATlON NAMI' AND ADDRMSS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

1ft 1 13r,<i<.i-F /"r'Hil- /WnrL 1 Im 1 1 M/i u. 1 rt>)ttt/ la.sK/worK iJiiii i\o. 4624417

11. (.ontract/Cirunt No.

AG493

12. Sponsoring Orgnnization Name and Clomplctc Address (Street, City, State, ZIP)

Energy Research and Development Administration

1800 G Street

Washington, D. C. 20550

13. Type of Report & Period

(-ovcred

Interim Report

14. Sponsoring Agency (lode

15. SUPPl.KMENTARY NOTHS

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.)

A Study has been made at the National Bureau of Standards of the different techniques that

could be used for testing thermal storage devices and rating them on the basis of ther-|

mal performance. This document outlines a proposed standard test procedure based on th^t

study. It is written in the format of a standard of the American Society of Heating,

Refrigerating, and Air Conditioning Engineers and specifies the recommended apparatus,

instrumentation, and test procedure.

i

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)

Standard test; thermal storage, thermal performance, solar energy, standard, thermal

test

18. AVAILABILITY Unlimited 19. SECURITY CLASS

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21. NO. OF PAGES

Q Fef Official Distribution. Do Not Release to NTIS

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20. SECURITY CLASS

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22. Price

[Jx] Order From National Teciinical Information Service (NTIS)

Springfield, Virginia 22151 UNCLASSIFIED 3.75

USCOMM-OC 29042-P74