REFERENCE

STAND & TECH

AlllDb D3ibm

NIST

PUBLICATIONS NBSIR 78-1548

An Evaluation of ASHRAE

Standard 94-77 for Testing

Water Tanks for Thermal

Storage

B. J. Hunt

T. E. Richtmyer

J. E. Hill

Building Thermal and Service Systems Division

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

October 1978

i

wo

Sponsored by

The Department of Energy

Research & Development Branch for Solar Heating and Cooling

Office of the Assistant Secretary for Conservation and

Solar Applications

Washington, D.C. 20545

NBSIR 78-1548

AN EVALUATION OF ASHRAE

STANDARD 94-77 FOR TESTING

WATER TANKS FOR THERMAL

STORAGE

B. J. Hunt

T. E. Richtmyer

J. E. Hill

Building Thermal and Service Systems Division

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

October 1 978

Sponsored by

The Department of Energy

Research & Development Branch for Solar Heating and Cooling

Office of the Assistant Secretary for Conservation and

Solar Applications

Washington, D.C. 20545

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

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director
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IV- ?*-
CONTENTS

Page

Abstract Iv

1. Introduction 1

2. ASHRAE Standard 94-77 Test Procedures 2

3. NBS Test Facility 8

4. Experimental Procedure 13

5. Test Results and Analysis 29

6. Discussion of Results 38

7. Conclusion and Recommendations 46

8. References 47

9. Appendix A. Time-Temperature Relationships for the Transfer

Fluid during the Transient Storage Tests... 49

10. Appendix B. Dimensionless Time-Temperature Relationships

for the Transfer Fluid during The Transient

Storage Tests 59

iii
TABLES

Table 1. Transient test parameters used for the charge and discharge

tests.

Table 2. Results of the transient tests performed on the 1.9 m

(500 gal) water tank.

'i Table 3. Rating coefficients for the 1.9 m (500 gal) water tank.

Table 4, Times for the achievement of the step-change in entering

transfer fluid temperature during the transient tests,

3 Table 5. Mass flow rates per tank cross sectional area for the 1,9 m

(500 gal) test tank and the test models of [13]

.

vi
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 components of solar heating and cooling systems, solar collectors

and thermal storage devices. Recommended procedures for testing these

two components were published in December 1974 and May 1975, respectively,

[1, 2] in a format consistent with standard test procedures 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 storage devices. The NBSrecommend procedure [2] was submitted to the Committee as a working

draft for their consideration. In June 1976, a draft of the Standard

was published and submitted to the Society for review. It should

be noted that the preparation of this proposed standard by the

Project Committee was performed on an unusually accelerated schedule

relative to the pace at which ASHRAE standards are normally written.

This was partially a result of 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 1977 in which a 1.9 m^ (500

gal) water thermal storage tank was tested in accordance with Standard

94-77. The study demonstrates the applicability of the Standard

to this type of sensible-heat storage device for liquid systems.

Similar experiments are presently being conducted at NBS on residentialsize pebble-bed and phase-change storage devices for air systems.

The results of these experiments are expected to be completed and

published before the end of 1978,

It should be noted that a similar and parallel standards development

process occurred utilizing 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 [7], NBS has conducted a series of experiments demonstrating the use of this Standard for testing typical commercially

available water-heating and air-heating collectors [8],

1
2. ASHRAE STANDARD 94-77 TEST PROCEDURES

The method outlined in ASHRAE Standard 94-77 for testing thermal storage

devices consists of the following:

1. one test to determine the heat-loss f actor, L, of the thermal

storage device (Heat Loss Test),

2. two tests to determine the charge capacity, C , of the device

as the result of a step Increase in the entering transfer

fluid temperature (Charge Test), and

3. two tests to determine the discharge capacity, Cjj,of the

device as the result of a step decrease in the entering

transfer fluid temperature (Discharge Test).

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 obtained,

measurements are made of the average temperature difference between the

inlet and outlet transfer fluid and are recorded 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 ^tf

(3600s) (25“C)

where,

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

Wj^ = mass flow rate of the transfer fluid

for the heat loss test, kg/s (Ib/h)

tj^^ = temperature of the transfer fluid entering

the storage device, "C (®F)

tout ” temperature of the transfer fluid leaving

the storage device, °C (®F)

Cj.£ = specific heat of the transfer fluid,

J/(kg*“C) (Btu/(lb;“F)).

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

test is determined by:

wl = TSCl

(14400s) (25“C)

( 2 )

where

,

TSCj^ = theoretical storage capacity of the thermal

storage device for the heat loss test, based

on a At = 25®C (45“F), J(Btu).

The theoretical storage capacity for the heat loss test is the amount of

energy capable of being stored in the device if all the device components

undergo an increase in temperature of 25“C (45*F) above an initial temperature equal to t^, the ambient air temperature. It is further specified that it be calculated as the summation of the products of mass and

specific heat of the various components comprising the thermal storage

device (storage medium, tank, insulation, etc.) multiplied by the temperature step from the initial temperature, t^^, to a final temperature, t^

+ 25"C (t^ + 45®F).

Charge and Discharge Tests :

These tests deal with the determination of the response characteristics

of the thermal storage device to a temperature step increase or decrease

in the inlet transfer fluid (charge and discharge tests, respectively).

This quantifies the energy storage capabilities of the device.

In the Standard, the concept of test fill time is Introduced and defined

as

:

T

f

TSC

"tf 'tf

(3)

where.

= test fill time, s

= duration of a single transient test in which

energy is either added or extracted from the

storage device,

Wj.£ = mass flow rate of the transfer fluid , kg/s (Ib/h)

4
TSC = theoretical storage capacity of the thermal

storage device for the transient tests based

on a At = 15“C (27°F), J (Btu)

At = temperature step of the inlet transfer fluid

from an initial temperature, t^, to a final

temperature, t^ ± At, ®C (®F).

A water tank 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.

ASHRAE Standard 94-77 specifies that the charge and discharge tests be

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

transfer fluid temperature. The recommended temperature step change

values 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 [5]. The fill times were chosen based on typical

ratios of solar collector size to storage size and flow rates required

for optimum collector performance.

The recommended values specified in the Standard for devices using liquid

as the transfer fluid are:

With the test fill times designated, the mass flow rates are simply

determined by use of equation (3). Therefore, the mass flow rates for

the transient tests are defined as:

T^,T^ = 7200s (2h)

At = 15“C (27'’F)

and

T^,T^ = 14400s (4h)

At = 15°C (27“F)

w, TSC

c (4)

( 5 )

5
where,

= mass flow rate of the transfer fluid for the

charge test, kg/s (Ib/h)

w^ = mass flow rate of the transfer fluid for the

discharge test, kg/s (Ib/h)

= test fill time for the charge test, h

= test fill time for the discharge test, h.

Figure 2* represents the charge and discharge cycles of a thermal 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. After the device

is brought to a uniform initial temperature (steady-state conditions),

the flow is adjusted to the value, w^, defined by equation (4), The

temperature of the entering transfer fluid is then raised in a step-like

manner from the initial temperature, t^^, to a final temperature, t^^ + At,

and measurements necessary for computing the charge capacity are made

over the charge test fill time, The measurements required are the

temperature difference between the transfer fluid inlet and outlet over

the test fill time, the transfer fluid flow rate, and the ambient

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

conditions at the temperature t^^ + At. This is represented by the asymptotic

approach of the outlet temperature curve to being parallel to the inlet

temperature curve shown in Figure 2. Once this steady-state condition

is 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 tj^. Measurements necessary in computing the discharge capacity are then recorded over the discharge test fill time, x^. It should be noted

that whenever the charge and discharge tests are performed in series

as described above, the values of the test fill times (t^,, Xj) are equal as well as the temperature step increase and decrease CAt). Also,

the flow rate established for the charge test (w^) is maintained the same for the discharge test (w^^).

The charge and discharge capacities are defined in the Standard respectively as:

= w. tf

t +

in

2

( 6 )

* Figure 2 is an ideal representation. Refer to the temperature vs,

time plots in Appendix A for a representation of what occurs in an actual application.

6
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= discharge capacity of the thermal storage device,

J (Btu).

Observe that in the computations for the charge capacity, the quantity

of heat loss occurring over the test period, t^, is accounted for in the

right hand portion of equation (6), This term does not appear in equation (7) since energy is being extracted from the device over the test

time, T^. Also observe that the charge capacity is a function of the

specific heat of the transfer fluid. This makes the rating of the device

dependent on the fluid used.

3. NBS TEST FACILITY

The tests conducted and described in this report were performed on a 1.9

m"^ (500 gal) water tank which is built into a complete solar heating

and cooling system at NBS.

NBS has been experimentally investigating the performance of whole build'

ings and their heating systems since 1940, when its first environmental

chamber was completed. In the early 1970' s, a prefabricated townhouse

with a floor area of 110 m^ (1200 ft^) was purchased on the open market

and Installed in a large 2100 m^ (70,000 ft^) environmental chamber.

Environment-controlled tests were performed and the results were used

to verify an NBS-developed computer program for estimating the heating

and cooling loads of buildings [9, 10],

During 1974, the house was relocated onto the NBS grounds and a solar

heating and cooling system was retrofitted to it. The system consists

of 45 m^ (485 ft^) of double-glazed, flat-plate water-heating solar

collectors, 5.7 m^ (1500 gal) of water storage, a 10,000 W (3 ton)

lithium bromide absorption air cooling unit and a 17,6 kW electric

boiler. The plumbing is quite complex to allow for the versatility

needed in performing various controlled tests (see Figures 3 and 4),

The house was also fitted with a highly versatile data acquistion system

(DAS). The house is capable of being either manually controlled or operated completely by the DAS, The DAS consists of;

8
SOLAR ARRAY

Figure 3. Plumbing schematic of the NBS solar townhouse facility.

9
10
1. a six-digit integrating digital voltmeter,

2. a scanner permitting the voltmeter to read any of 400

channels of data,

3. a magnetic tape system to record measured values,

4. four data source input channels that allow the input of

binary coded decimal information,

5. a relay register with 48 separate contacts that enable

the system to control external measuring devices as well as pumps, motors, valves, etc., that comprise the

solar system, and

5. a mini-computer which serves as the control for the

total system.

The entire house and the solar system is fitted with various temperature

and physical property sensors, all of which are wired to the DAS.

O As shown in Figures 3 and 4, the solar system consists of 3.8 m^ (1000

gal) and 1.9 m^ (500 gal) water storage tanks interconnected with the

solar collectors via an elaborate plumbing system. These tanks were fabricated of ASME flange-quality steel, are unlined, and are available

on the open market. They were manufactured in a manner enabling the

transfer fluid to enter or exit the containers at any given 0.61 m

(2 ft) elevations by means of hand-operated valves (see Figures 5 and

6). The tanks are covered with 10.2 cm (4 in) of glass fiber insulation,

fitted with type-T thermocouple temperature sensors at 0.36 m (1 ft)

elevations within the tanks, and are exposed to the ambient in an equipment room. The storage medium within the containers is water with a 40

percent concentration of ethylene-glycol. Insignificant quantities of

corrosion inhibitors are also present.

The ASHRAE Standard 94-77 test procedure was performed on the 1.9 m^

(500 gal) tank only. The 3.8 m^ (1000 gal) tank was used as a reservoir

to supply the necessary transfer fluid at the required temperature to

the smaller tank. Type-T thermocouple temperature sensors present at

the inlet and the outlet of the tanks were used in recording the transfer

fluid temperatures at the respective locations throughout the tests.

Six different positive-displacement flow meters were used in monitoring

the transfer fluid mass flow rates. These meters were equipped with

pulse generators producing electronic outputs which were received and displayed by light-emitting diode pulse counters. Each counter was wired

to the minicomputer thus enabling the operator to* automatically read and

record accumulated flow data at predetermined intervals.

11
Figure 5.

O Isometric representation of the 1.9 m (500 gal) water tank

in the NBS solar townhouse facility.

12

ii
Figure 6. Schematic representation of the 1.9 (500 gal) water tank

in the NBS solar townhouse facility.

13
Prior to all testing, the various type-T thermocouples used for the

temperature difference measurements were calibrated. The calibration

was done in two steps. In the first step, the sheathed thermocouple

probes were removed for the test facility and calibrated by the NBS

Temperature Measurements and Standards Division in a constant- temperature both against a secondary standard (platinum resistance

thermometer). After the probes were replaced in the storage tank

test loop, a second calibration check was made to Insure that for

matched pairs of the thermocouples, the uncertainty in temperature

difference measurement would be within + 0.1“C (+ 0.2“F) as required

by Standard 94-77. An apparatus consisting of solid dowels submerged

in a highly viscous oil within a glass thermos was used. The various

thermocouples were placed into the oil and the temperature of the

copper-oil medium raised by means of an electrical resistance heater.

Various temperature levels encompassing the Intended test range were obtained and the DAS was used to print-out the temperatures

sensed per thermocouple. It was found that all temperature difference

discrepancies were within the + 0.1“C (+ 0.2*F) uncertainty range.

Therefore, no data corrections were applied.

The DAS was used to calculate the necessary data and store it on magnetic

tape. An associated line printer was used to obtain that part of the

data required for test calculations. A thermocouple indicator was also

wired to various thermocouple temperature sensors throughout the system.

By operating a manual switch, any of the pertinent temperature sensors

could be observed. This thermocouple indicator proved to be most con- ducive to monitoring the desired temperature of the inlet transfer fluid.

A two-channel strip chart recorder was also wired into the DAS to monitor the transfer fluid inlet and outlet temperatures. The recorder

generated an excellent graphical representation of the inlet and outlet

fluid temperatures. These plots are representative of what was occurlng

during the tests.

14
4. EXPERIMENTAL PROCEDURE

Heat Loss Test :

The heat loss test was carried out in accordance with the Standard. The

DAS was set up to perform a scan every 15 seconds of the various sensors

involved. Those sensors vital to the heat loss computations consisted

of the transfer fluid inlet and outlet temperatures, the storage medium

temperature, the ambient air temperature, and the fluid flow rate.

The transfer fluid was circulated through the thermal storage device

(SI) and through the auxiliary boiler (ON) (see Figure 7). The mass flow

rate was adjusted and set to a value determined by equation (2). This was done by adjusting variable proportional valves VP 2 and VP6. The

boiler was used to raise the inlet transfer fluid temperature to a value

of 25‘*C (45“F) above the average air ambient temperature, t^. The ambient

temperature sensors were located at one-foot Increments from the floor

to ceiling within the equipment room. The DAS printed out a complete listing of these temperatures and also the average ambient temperature. Once

steady-state conditions were reached and maintained for a one-hour period,

all data were recorded.

For comparative purposes, a stagnant heat loss test was performed on the

thermal storage device. This test was not designated or recommended in

the Standard. The temperature of the storage medium was raised in a similar manner with the auxiliary boiler to a value of 65°C (149°F).

The fluid was circulated at the flow rate defined by equation (2) until

steady-state conditions at a uniform 65“C (149®F) were obtained. Once

this state was reached, the inlet and outlet portals to the thermal storage device were sealed off.

The device was allowed to cool off over a period of 48 hours while ambient

air and storage medium temperatures were being recorded. Figure 8 depicts

this temperature history. The figure clearly shows a repetitive rise

and fall in the ambient air temperature during the day and night hours,

respectively. The data were utilized for calculating the amount of heat

loss per hour and a heat loss factor was calculated and compared to

that value determined in accordance with the Standard.

Transient Charge Tests

For the transient tests performed, the recommended values for step temperature change. At, and fill times, t^, were used. Additional tests

were also conducted for other values of the test parameters and a com- plete list for all charge and discharge tests is given in Table 1.

The transfer fluid was circulated through the storage device (1.9 m^

(500 gal water tank)) until steady-state conditions were established.

The value of the initial temperature, t^, was chosen based on the

known application of the device in accordance with the instructions in

the Standard. Figure 9 depicts the range chosen for these tests.

15
Figure 7, Charging the small storage tank with the auxiliary boiler.

16
#

AVERAGE

STORAGE

MEDIUM

TEMPERATURE

O

AVERAGE

AMBIENT

AIR

TEMPERATURE

17

Figure

8,

Time

-Temperature

variation

of

the

storage

medium

and

ambient

air

during

the

stagnant

heat

loss

test.
Table 1, Transient test parameters used for

the charge and discharge tests.

Fill Time Mass Flow Rate Temperatures

Charge

Test

Discharge

Test

Charge

Test

Discharge

Test

Step

Change

Initial

TEST

DESCRIPTION

Tj,(h) Td(h) Wj.(kg/ s) Wjj(kg/s) At(*C) ti(*C)

Charge* 2.0 — 0.259 — 15.3 43

Discharge* — 2.0 — 0.259 13.4 53

Discharge** — 2.0 — 0.251 14.7 67

Charge* 4.0 — 0.130 — 15.4 41

Discharge* — 4.0 — 0.129 14.4 57

Charge 1.0 — 0.531 — 15.3 38

Discharge — 1.0 — 0.528 16.8 49

Charge 0.55 — 0.951 — 13.8 38

Di scharge — 0.55 — 0.951 12.6 69

* Recommended ASHRAE Standard 94-77 values,

** Cool inlet transfer fluid introduced into top of the water tank.

18
INITIAL

TEMPERATURE

RANGE

M

DEVICE UPPER LIMIT

tj HIGH

••

50

At=15*C

A t=15“C

tj LOW

30

DEVICE LOWER LIMIT

Figure 9, Test temperatures used for the charge and discharge test.

19
It can be seen that the device has a designated upper limit of

80°C (176°F) and a lower limit of 30“C (76“F), It has been found that

the storage medium temperature is usually within the range of 50°C to

60°C (122°F to 140°F) during typical system operation. Therefore, this

range, in conjunction with the recommended 15°C (27 “F) temperature step

was utilized in determining the limits as shown in the Figure (40°C and

70°C (104“F and 158®F)), The higher limit was used as the initial temperature in the discharge test and the lower limit used as the initial

temperature in the charge tests. These values were only approximate; the

exact values are given in Table 1,

It is noted that, in reference to Figure 2, a single charge and discharge

test are shown to be performed in series with each other. During this

test program, the transient tests were performed in a different manner

due to the way the system was configured. Two options were available to

achieve and maintain the required transfer fluid temperature step

increase. One method was to follow the plumbing loop shown in Figure 7 and use the auxiliary boiler to obtain the necessary step increase in

the inlet temperature. The second option was to utilize the flow pattern

shown in Figure 10 and pump fluid at t^^ + At from the 3,8 m^ (1000 gal)

storage device to the 1.9 m^ (500 gal) device.

The former method using the auxiliary boiler was not successful in maintaining the required temperature step increase, t^ + At. A variable

alternating current transformer was wired to one of the five electrical

resistance elements in the boiler. This configuration was not sensitive

enough to maintain a constant temperature step. The inlet transfer

fluid temperature fluctuated above and below the desired t^ + At

due to the time lag of the heating element. Once the operator detected

a drop in the inlet temperature from the intended tj^ + At, an increase

in current to the heating element usually resulted in an overshoot

of the desired temperature step, t^ + At.

The second method was found to be quite successful and was therefore

fully utilized for all transient tests performed. The procedure con- sisted of raising the temperature of the storage medium in the 3.8 m"^ (1000 gal) storage device to a value above the desired temperature step

tj^ + At, This was generally accomplished by using the solar collector

panels and circulating the transfer fluid as shown in Figure 11 during

the daylight hours. The auxiliary boiler was utilized when solar energy

was not avaialble. In order to maximize the utility of the system, the

large storage tank was heated to a value above the initial temperature

of the small tank, usually slightly more than t^ + 2At. This allowed

two charge tests to be performed in series and saved both experimental

time and auxiliary energy.

Once the large storage tank medium was heated to a desired value, both

storage devices were isolated as depicted by Figure 12 and their respective transfer fluids circulated until steady-state conditions were established. This was necessary for the small tank, in accordance with the

Standard, and also for the large tank, since any temperature stratification in the large tank would disrupt the desired step increase once the

transfer of fluid was initiated from the large tank to the smaller tank,

20
Figure 10. Charging (Discharging) the small storage tank with higher

(lower) temperature fluid from the large tank (boiler not

in use).

21
Figure 11. Charging the large storage tank with the solar collector

array augmented by the boiler.

22
Figure 12, Obtaining steady-state conditions in both storage tanks prior

to the start of any transient test by circulating the fluid

in a closed loop.

23
While steady-state conditions were being established in the small tank,

equation (4) was used to determine the mass flow rate of the transfer

fluid. The setting of this rate was accomplished by controlling variable

proportional valve VP2 (see Figure 12). While steady-state conditions

were being established in the large tank, flow meter FM3 was monitored

and the flow rate was regulated by proportional valve VP3. Since the reserve medium of the large tank was at a temperature above the desired

step increase, a specific flow rate had to be established such that when

mixed with the fluid from the smaller tank (see Figure 10), the desired

temperature step to the small tank could be established and maintained.

Figure 13 Indicates the mixing process Involved in detennlning the

known mass flow rate through FM3. The initial temperatures of both storage devices were known quantities and established for each test. The

final temperature of the inlet transfer fluid was a known quantity equal

to the initial temperature of the small tank plus the desired temperature

step increase. The mass flow rate of the transfer fluid pumped through

the small storage device was also known and determined by equation (4).

Referring to Figure 13, the following equation for the desired mass flow

rate through FM3 can be derived:

where

W, = W,

At

^^il “ ^2^ ( 8 )

Wj^ = exiting mass flow rate from the large tank,

(through FM3) kg/s (Ib/h)

“f = "c.d

= entering mass flow rate of the transfer fluid

into the small tank, kg/s (Ib/h)

t^j^ = initial temperature of the large tank storage

medium, “C (®F)

t ^2 = initial temperature of the small tank storage

medium, “C (°F).

The transient charge test was begun by transferring the large tank storage medium at it^ elevated temperature to the small tank (see Figure 10).

For all charge tests performed, the inlet transfer^ fluid was introduced

into the top of the small storage tank and the exiting transfer fluid

was removed from the bottom.* This represented the configuration in

* The schematic of Figure 10 may appear to misrepresent the proper

entrance and exit locations. Refer to Figure 5 for true portal

configuration.

24
1

W2

S2

SI (ffl tj,

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MIXIM6

STAGE

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L J S.- - 1

REINTROOUCTION

j l,@W,

51 = 1.9m^ (SOO GALLON) TANK

52 = 3.Sm’ (1000 GALLON) TANK

1| 2 ^ INITIAL STORAGE MEDKIM TEMTEAATUNE SMALL TANK. °C (*F)

ti. - INITIAL STORAGE MEDIUM TEMfERATURE LARGE TANK. °C (°F)

It = FMAL TRANSFER FLUID TEMfERATURE. °C (°F)

W ^ RESPECTME MASS FLOW RATES. k|/t (bM/h)

Figure 13. Schematic representation of the required mixing process for

the conduct of the transient tests.

25
which the device would normally function within any solar system and was

thus tested in this way. Once the transfer was initiated, a thermocouple

indicator and strip-chart recorder were used to monitor the temperaturetime history of the charge cycle as depicted by the plots in Appendix A.

A print-out from the DAS in real time was also used during the tests,

while all pertinent data were stored on the magnetic tape for future

retrieval and reduction.

While the transient charge test was being executed, strict observation

of the outlet transfer fluid temperature profile was necessary. The

only change during the test execution was the regulation of the flow

rate, , through FM3, As the test proceeded, the storage medium of the

small tank stratified with the higher temperature strata at the tank top

where the hotter transfer fluid was being introduced. However, when

enough time elapsed to allow the higher temperature strata to permeate

to the lower elevations of the small tank and begin exiting, the amount

of transferred energy from the large tank was reduced; otherwise, the

inlet transfer fluid temperature to the small tank would rise above the

temperature step increase required for the test.

Therefore, the flow rate through FM3 was progressively reduced as the

test proceeded by manual control of VP3.

Transient Discharge Tests

The procedure for the performance of the transient discharge tests was identical to that utilized for the charge tests in that the large tank

was used as a reservoir of low temperature fluid and the flow arrangement

indicated in Figure 10 was used in performing the tests. The temperature

of the large tank storage medium was lowered to a value enabling a series

of discharge tests to be performed, thus maximizing the utility of the

system as was discussed previously. This usually led to lowering the

storage medium to a value of at least tj^ - 2 At, where t^^ is the initial

temperature of the small tank storage medium.

The lowering of the large tank medium temperature was accomplished by

circulating the fluid as shown in Figure 14 through the liquid-to-air

heat exchanger within the house and through the solar array during the

night hours. It was found that by allowing this process to take place

during the entire night hours, the large tank storage medium would be at

a state of adequately low temperature to allow for a series of discharge

tests to be conducted the next day. It is also noted that this condition was facilitated by performing the charge tests in series during the

day and allowing the cool-down to take place at night, thus yielding a large tank reserve of low temperature fluid and a small tank of high

temperature fluid the following day.

As with the charge tests, steady-state conditions and the setting of the

proper flow rates were established during the period when the two tanks were isolated as depicted in Figure 12, Equation (8) was utilized in

establishing the proper flow rate through FM3. Once the flow rates and

26
Figure 14. Dicharging the large storage tank during night hours via

the liquid-to-air heat exchanger in the test house and the

solar collector array.

27
proper temperature were established, the transfer of fluid in the same mixing mode as for the charge tests was initiated for the discharge tests

(see Figure 10),

For all discharge tests except one, the transfer fluid was introduced to

the cool bottom strata of the small tank and exited from the top warmer

strata. Once again, this represented a typical operating mode of the

thermal storage device being tested. One discharge test was performed

by introducing the colder inlet transfer fluid into the top of the small

tank. This mode of operation minimizes stratification tendencies and causes a more thoroughly- mixed water tank.

As was experienced with the charge tests, the flow rate through FM3

had to be reduced as the test proceeded. The outlet transfer fluid

temperature would eventually become lower so that the amount of

lower temperature fluid being introduced from the large tank had

to be reduced. Once again, this was accomplished by the operator

monitoring the inlet and outlet transfer fluid temperatures via

the available equipment.

28
5. TEST RESULTS AND ANALYSIS

Data Acquisition System

The DAS was programmed to perform a scan of all the temperature sensors every 15 seconds and record the temperatures on magnetic tape. Since

the data made available via the 15 second scan were not always necessary

for the data reduction process, the DAS was programmed to perform a printing of data blocks during the retrieval process at an Interval

representative of the test being executed. That is, for tests involving

long charge and discharge periods, the print-out interval would be

greater than for tests involving shorter ^ periods. In case any one

of the sensors Involved indicated a sudden rise or fall in temperature

to the nearest tenth of a degree, the DAS was programmed to give an immediate print-out of that sensor and the temperature indicated, irrespective of the print-out period utilized in the retrieval process.

For the three types of tests performed (heat loss, charge and discharge),

all were performed to determine quantities defined by equations (1), (6),

and (7), Each of these equations has an integral term of the general

form

:

If a plot were made of the difference in transfer fluid inlet and outlet

temperatures versus time over the test period, d* resulting curve would be as shown in Figure 15. The integral of equation (9) represents

the area under such a curve. Throughout the test period, the DAS

recorded the difference in the inlet and outlet transfer fluid temperature for each scan performed and summed these differences yielding the

value of the integral. The DAS was also utilized in calculating the

instantaneous and average value of the following heat loss term which

appears in the right hand portion of equation (6):

T

(9)

o

^in - *-out

( 10 ) 2

Heat Loss Test

The heat the heat loss factor defined by the ASHRAE Standard in equation

(1) is:
500

6ALL0M

STORAGE

TANK

CHARGE

TEST

T,.

=2

h

;

At=

CM

ililtliili im illll MII m

mtt »!»*>.M!M!M!iAMAM M.M > M* ** u i > nm if. MU1 1 1

1

mn I H III 1 1 1

1

> M mMM I m>t » j ?Y;Y;Y;Y;YYM;Mwy^

^ 1 1 n i i 1

1

1 1 1

1

1 WmmMm i t i i

.

i 1 1 1 1 mmm mmmi « ti hiYiYw

ftWiViY!¥jYiM-iiMAMAMAM H1

1

n1

1

»M1 1 1 1

1

Htt mt > iYiY»iYi

’

i wjwMwww ;t

ilitjiiliii t

.|i|i m It M 1

1

> MMM» w«

MMi

UMi m AMiMiMAMAMAM 1 1 n m n M MMt i i wtwYtYi

j^X^jXf;;|^;;;; ;j i;i;ij sfSAi

i:i; i : i^ AM:MAMAMAMAMA«

«

M M 1 1 mt »

I

SM iii mw |i|iiiiiiiiijiiiiiiiiijXMAMAAMAMAMJ » mi ii ii i< i lYm»nt mi i H tYtYw' iYi' WiMtMwiMiMwM!^

IAi I II M

M

1

1

MII

I

MI MI n I H I Mi¥»Ym

Y

sVi

i

m.> 1 1 mi u 1 1 1 1 1 1 m m

MMM1 1 1 1 ^twMlW¥lM!MlMlM!Ml^MAMAMAM.MAMy.MAMAMAM.^ ** m i m1 1 h i mi lYi YiiViY

l|YwiiWiM?YMifcJ!MlWi>>A»jMAMAMAMAMAMAM.* »’»»Y»^^^^

^ M*! miiiiij;iii

i|||iji|i

jii|ji ^i^ffiy

iy

iiiXi>Xi>XM

T

g

'ww?w??ww.w.'iw'y*w**cc

»

MI m» 1C w'i'!rw'ww?www'?ww'W»

i»x»:

<jA» MAM.MAMAM » mmhi MiitiYiYiYwi

i^iMiMii.MiMiMiMiMiM^AMAMAM.* » ii 1 1 1 1 1 1 ii ii mmi i

o

ui o

o

3.' (*H !‘j)

30

Figure

15.

Inlet

and

outlet

transfer

fluid

temperature

difference

during

a

2-hour

transient

charge

test.
L

WL Ctf

(3600s) (25“C)

T= 3600

T= 0

The specific heat of the transfer fluid was determined by sampling the

storage medium with a hydrometer and establishing that there was a 40%

ethylene-glycol concentration. Referring to the ASHRAE Handbook of Fundamentals [12], ethylene-glycol water solutions with a 40% concentration

have a specific heat of 3,600 J/(kg*®K) (0.85 Btu/(lb*®F) ). By using

this value with the DAS-computed value of the integral and the flow

rate, w^, the heat loss factor, L, defined by equation (1) was found to

be

:

L = 16.16 J/(s-‘*C) (30.61 Btu/(h-"F)).

As was mentioned previously, a stagnant heat loss test was also performed. Reference to Figure 7 depicts the temperature history profile

of the storage medium and the ambient over the 48-hour test period.

Using this data, the heat loss factor was calculated by:

L = = Q At

At avg

^stg ^tf ^^stg

( 11 )

where

,

Q

= rate of heat loss from storage medium, J/s (Btu/h)

Atavg =* average temperature difference between the storage

medium and the ambient over the test period, “C (®F)

At = time period, h

Mstg “ raass of the storage medium, 1890 kg (4165 lb),

Atgj. = change in temperature of the storage medium over the

test period, ®C (“F)

The value was found to be:

L = 16.04 J/(s.‘*C) (30.38 Btu/(h-®F)).

31
The two heat loss factors were in excellent agreement. The value used

in the charge capacity calculations was that one determined in

accordance with the Standard,

L = 16.16 J/(s-“C) (30.61 Btu/(h*°F)).

It should be noted here that the calculation of the heat loss factor

utilizing basic heat transfer theory and the material properties and

dimensions of the thermal storage device and its insulation yielded a theoretical heat loss factor of:

L = 5.67 J/(s*®C) (10.75 Btu/ (h'^F) )

.

The reason for the discrepancy between this value and the higher values

determined experimentally is still being investigated.

Transient Charge Tests

The primary purpose of ASHRAE Standard 94-77 is for the determination of

the "effective"* charge and discharge capacities of the thermal storage

device being tested. The effective charge capacity has been defined by

equation (6) to be:

T =T,

^c f

“ ^tf 1 (bin ” tout)^T ~ (ti + —

-- -- - - t^) ( 6 )

T=0

Referring to equation (6), the known quantities consist of the following:

1. the heat loss factor, L, determined above

2. the mass flow rate, w^, calculated by equation (4)

3. the charge test fill time, x^, specified by ASHRAE Standard

94-77 or calculated by equation (3) when the mass flow rate is

initially known, and

4. the specific heat of the transfer fluid, c^.^.

* So labelled effective since they represent only that portion of the

capacities utilized over the time duration x^.

32
The first of the unknown quantities in equation (6) deals with the integral in the left-hand portion, identical to equation (9) and calculated

by the DAS. The second of the unknown quantities in equation (6) deals

with the right-hand portion, the heat loss over the charge period, t^.

The major part of this term was calculated by the DAS as described

previously.

Table 2 lists the various transient charge tests, the quantities of heat

loss, and the "effective" charge capacities. It should be noted that,

the quantity of heat loss was small compared to the quantity of energy

stored in the device over the testing period.

Transient Discharge Test

The discharge capacity has been defined by equation (7) to be;

The same procedure for computing the effective charge capacity was applied for the computation of the effective discharge capacity. Table 2

also gives a complete listing of the results of these tests.

Data Analysis

ASHRAE Standard 94-77 suggests a plot be provided showing the time variation in transfer fluid temperatures for each test. Figure 16 shows such

a plot for those tests involving the ASHRAE-recommended test fill times

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

T=T^ and the ordinate is a non-dimensional quantity defined by:

In equation (12), it is noted that the initial step change in transfer

fluid inlet temperature. At, is a constant. However, the difference

between the transfer fluid outlet and inlet temperatures decreases with

time yielding the asymptotic curves of Figure 16, For a thermal storage

device with perfect stratification and no heat loss to the ambient, the

temperature of the inlet transfer fluid, t^ and the outlet transfer

fluid, t£ g, would remain constant for the entire test period (time for

one entire tank change), at which time they would become equal. A curve

of test results for such a device would be represented by a rectangle.

(7)

( 12 ) At

33
Table 2. Results of the transient tests performed

on the 1,9 (500 gal) water tank.

TEST

DESCRIPTION

Charge-2

Discharge-2

Discharge-2*

Cc Q

9.29 0.16

7.63

5.72

Charge-4

Discharge-4

9.24 0.21

8.26

Charge-1

Discharge-1

8.91 0.07

9.49

Charge-1/2

Di scharge-1/2

8.03 0.06

7.11

All units. Joules x 10^

*Cool inlet transfer fluid introduced into top of the water tank.

Numerical identification following test description designates the

test period involved, ^d*

C^ designates the "effective" charge capacity with heat loss

accounted for,

C^ designates the "effective" discharge capacity.

Q designates the quantity of heat loss over the charge period.

34
500

GALLON

STORAGE

TANK

40%

GLYCOL

SOLUTION

•

Tj

=

4.0h,At

=

14.4‘’C,

130ml/s

(2.05

GPM)

O

Td

=

2.0h,At

=

13.4X,

261ml/i

(4.12

GPM)

Tc

=

2.0h,At

=

15.3X,

261ml/s

(4.12

6PM)

Tr

=4.0h,At

=15.4‘’C,

131ml/i

(2.07

GPM)

O

O

O

O

o

o

o

cH-a

o

<P

o#

SSJNMNSNIM ‘ tV| TTT

t

35

Figure

16.

ASHRAE-re

commended

plot

of

time

variation

in

transfer

fluid

temperatures

.
Other plots thought to be of value and not specified by ASHRAE Standard

94-77 are included in Appendix A. A plot of the inlet and outlet transfer fluid temperatures against time was prepared for each transient test.

These plots are representative of those generated by the strip chart

recorder used during the conduct of the tests. They distinctly show the

temperature step increase or decrease in the inlet transfer fluid and

the constant outlet temperature maintained up to some point late in the

test period.

The concept of using a non-dimensional plot showing the thermal

storage capabilities of storage devices relative to those of an ideal

device has been proposed [5], Such non-dimensional plots for each transient test performed on the 1.9 m"^ (500 gal) storage device are found

in Appendix B. The ordinate represents the difference between the inlet

and outlet transfer fluid temperatures normalized by the step change in

temperature. At, (identical concept utilized in Figure 16), The abscissa

represents a non-dimensional time defined by:

^tf ^tf At T TSC

(13)

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

the test fill time or fluid dwell time of a perfectly-insulated, com- pletely-stratified water tank. It is noted that the area under the curve on such a plot represents the amount of energy capable of being

stored relative to the ideal device. Therefore, the area corresponding

to the energy storage capability of the ideal device is 1,0 and all

real devices have an area representatively smaller than 1,0. Table 3

lists those transient tests performed on the thermal storage tank and

their respective energy storage capabilities or performance coefficients

based on such a non-dimensional analysis.

36
Table 3. Rating coefficients for the 1.9

(500 gal) water tank

TEST

DESCRIPTION PERFORMANCE COEFFICIENT

%

Charge-2 88

Discharge-2 84

Discharge-2* 55

Charge-4 88

Discharge-4 84

Charge-1 85

Discharge-1 79

Charge-1/

2

86

Discharge-1/2 85

* Cool inlet transfer fluid

water tank.

introduced into top of the

37
6. DISCUSSION OF RESULTS

Temperature Step Change

Of all the transient tests performed, four were specifically called for

in ASHRAE Standard 94-77. The remaining transient tests performed were

at higher flow rates representative of conditions that did exist at one time or another during the operation of the solar heating and cooling

system. For all transient tests performed, a 15“C (27*F) temperature

step was initially attempted. As is seen in Table 1, this temperature

step was not consistently obtained. This is attributed to:

1. The process of raising the temperature of the 3.8 m^ (1000

gal) water tank and obtaining steady-state conditions

was quite tedious (especially at the lower flow rates).

As was mentioned earlier, charge and discharge tests were performed in series by raising or lowering the large tank

medium at least by twice the amount of the temperature

step required. Once either of the two types of tests was

performed, steady-state conditions had to be reestablished

in both tanks.

The smaller tank proved to be less of an encumbrance in the

establishment of steady-state conditions. However, as Figure 10 shows, the transfer fluid was allowed to reenter

the large tank, enabling the mixing process to take place

within the closed loop. Since the large tank was twice

the volume of the analler tank, after one test run was completed, the large tank storage medium was at non-steadystate conditions. Therefore, it was circulated as shown

in Figure 12 to reestablish a uniform storage medium temperature once again. This would usually result in a final

uniform storage medium temperature only approximately equal

to the desired 15®C (27®F) temperature step. This resulting storage medium temperature was accepted and a second

test run was performed,

2. There was difficulty in regulating the flow rates. The

problem was traced to mechanical backlash within the proportional valves. This resulted in an improper mixing of the

storage medium of the larger tank with the test tank medium,

3. Cold or hot slugs within the loops themselves were credited for some of the initial step change error. Once the

exchange was initialized, a considerable length of 3,2 cm (1-1/4 in) piping had to be flushed before the large tank

reservoir fluid reached the small test tank. That fluid

flushed was at a temperature different from that of the

reservoir fluid.

38
The Standard states that the time to reach 90% of the transfer fluid

step change shall not exceed 2% of the test duration, For the

transient response tests performed, this criterion was not met. Table 4

shows the actual times and temperature steps achieved. Reference to the

graphical plots of temperature versus time reveals the lag involved in

reaching the desired temperature step.

The best results for minimizing the time required to reach the desired

step change in temperature resulted in precirculating the transfer fluid

in the lines to avoid hot or cold slugs. Once the transfer from the

large tank to the small tank was initialized, no attempt was made to

obtain a precise 15°C (27 ®F) temperature step when the transfer did not

yield such a step.

Non-Dimensional Analysis

The purpose of the ASHRAE Standard 94-77 test procedure is to provide a standard method for determining the thermal performance of thermal energy

storage devices. This procedure was applied to a 1.9 m'^ (500 gal) water

tank and the "effective" charge and discharge capacities were determined.

However, the meaning of such values tends to become blurred when they are utilized in any attempt to compare thermal storage devices. It was for this reason that the non-dimensional analysis was proposed [5].

The utility of such a non-dimensional analysis becomes apparent when one refers to Table 2. This table reveals a variety of "effective" charge

and discharge capacities. For example, the 2-hour charge test performed

resulted in a C^, equal to 9.29 x 10^ J where the 2-hour discharge test

resulted in a equal to 7.63 x 10' J. The reason for such a discrepancy is revealed by referring to Table 1. This table shows that there

existed a 2“C difference between the temperature step change achieved

per test.

Reference to Table 3 reveals that the charge and discharge tests in the

above-cited example display nearly the same performance coefficients

(88% and 84%). A further viewing of this table reveals that all the

transient tests exhibited nearly the same performance coefficients

except the reverse mode Discharge-2 Test (this particular test was performed to deliberately eliminate any possibility of stratification

and consequentially the storage capability was much lower). Therefore,

it is seen that by utilizing the "effective" charge and discharge capacities obtained via the ASHRAE Standard in a non-dimensional analysis,

a meaningful overall rating coefficient can be obtained. It should also

be pointed out that since the dimensionless time used in the abscissa of

the non-dimensional plots includes the transfer fluid specific heat,

the performance coefficient should also be independent of the transfer

fluid. Since only one fluid was used in this study, this could be not

verified.

39
Table 4. Times for the achievement of the stepchange in entering transfer fluid

temperature during the transient tests.

TEST

DESCRIPTION

t *C

(.90)

•c

t ( .02)x

min

Actual Time

Utilized

min

Charge-2 15.3 13.8 120 2.4 3.6

Discharge-2 13.4 12.1 120 2.4 4.8

Discharge-2* 14.7 13.2 120 2.4 4.5

Charge-4 15.4 13.9 240 4.8 7.0**

Discharge-4 14.4 12.9 240 4.8 3.6

Charge-1 15.3 13.8 60 1.2 1.8

Discharge-1 16.8 15.1 60 1.2 6.0**

Charge-1/

2

13.8 12.4 33 0.7 2.2

Discharge-1/

2

12.6 11.3 33 0.7 1.4

* Cool inlet transfer fluid introduced into top of the water tank.

** Unexplained cold or hot slug present at test Initiation.

40
Miscellaneous

Upon completing the tests and analysis of data described above, it was decided to reanalyze the results obtained in accordance with a recent

paper dealing with the stratification of hot water tanks [13], In this

paper, geometric and dynamic parameter effects on thermal stratification

were studied. This study involved plexiglass cylindrical test models,

10.4 cm (4.1 in) and 24.2 cm (9.54 in) in diameter, utilizing water as the transfer fluid. The geometric configuration of these models (inlet

and outlet port locations, proportions, etc.) were similar to the 1.9

m"^ (500 gal) storage tank used in the present study.

The test procedure utilized in [13] was identical to the discharge tests

described in this paper. That is, the cold transfer fluid (water) was introduced into the bottom of the model tanks and the hot fluid removed

from the top elevation. Determination of the inlet and outlet transfer

fluid temperatures during the tests was performed using conventional

instrumentation.

A unique concept of extraction efficiency, n, was introduced in [13].

It is defined as:

n= (^ t*)/V

where

,

^ = volume flow rate, m^/s (gal/min)

t* = time elapsed until ^f,e ^i _ 0.90, s ^f,i “ H

3

V = volume of tank, m (gal).

Several factors thought to influence the extraction efficiency were studied in reference [13]. Those of major concern included the inlet port

location, inlet geometry, mass flow rate, tank length to diameter ratio,

and temperature variation of inlet and outlet transfer fluid. The individual effects of these factors on the extraction efficiency were presented in the paper. Then the effects of all factors were incorporated

into one correlation curve. This was accomplished by utilizing three

dimensionless groups; Reynolds number, Re^, (based on the portal diameter), Grashof number, Gr^, (based on the tank diameter), and the ratio

L/D. Figure 17 is the correlation of extraction efficiency taken from

[13] with actual data taken in the NBS tests.

The cluster of data points to the left on Figure 17 represent those tests

performed on the models and shown in [13]. A least-squares fit of all

data taken in both tanks (96 cases) yielded this correlation. The curve

is mathematically represented as:

41
001

in

in

CO

C9

in

in

o

T3

>%O

c

IV

u

COu

4-1 X

OJ

"c

CO

QJu

oo

(VV43

00

CN

A/(.)A) = » A3N3l3<Jd3

A 2
n = 1 - [exp - 0.067 (L/D)®*^®] (14)

The data obtained from the transient charge and discharge test performed

in the present stvidy were reanalyzed in accordance with the procedure presented in reference [13]. The correlation of extraction efficiency for

each test (8 cases) is plotted in Figure 17 for comparison. The tank

efficiencies range in the present study from 75 to 85 percent.

The results of the NBS tests show a pronounced spread along the abscissa

relative to the test model results of [13]. By individually analyzing

the three dimensionless terms comprising the abscissa, the following

determinations were made:

1.

The Reynolds number is defined as:

Red 4 V

irvd

where

,

d = tank inlet diameter, m (ft)

2 2

V = kinematic viscosity, m /s (ft /s)

For the NBS tests, higher volume flow rates were used.

However, the inlet diameter and fluid viscosity were also larger

resulting in approximately the same Re5molds number values for

the present tests and those reported in [13].

2. The ratio of tank length to tank diameter, L/D, utilized for the

test models was similar to the values used in the present study.

3, The Grashof number is defined as:

Grjj = [p^gB(At)D^]/y

where

,

p = transfer fluid density, kg/m^ (Ib/ft^)

2 2 g = gravitational force, m/s (ft/s )

B = coefficient of thermal expansion 1/®C (1/®F)

y = absolute viscosity, kg/(m*s) (lb/(s*ft)).

All values comprising the Grashof number are similar in quantity for both the test models and the NBS tank except the

tank diameter, D. The test model values were on the order

of 0.24 m (0.80 ft) and 0.10 m (0.34 ft) in contrast to

the 0.91 m (3 ft) value of the NBS tank used in this study.

43
It is noted that this value is cubed in the numerator of

the Grashof number.

The much larger tank and hence much larger Grashof number is considered

to be the major reason for the discrepancy between the correlation of

[13] and the data of the present study as Indicated in Figure 17, However, Lav an and Thompson do point out in reference [13] that deviations

from their correlation should be expected for commercially available

tanks made of steel. As noted, steel has a thermal conductivity two

orders of magnitude larger than plexiglass and the Increased conductive

heat flow through the walls could be expected to adversely affect the

stratification and thus the efficiency.

Table 5 compares the mass flow rates per cross sectional area utilized

for the NBS tests and the model tests of [13], Due to the large diameter

of the tank tested in this study, the flow rates were insufficient to

generate values as large as those listed in the table for the test models.

44
Table 5. Mass flow rate per tank cros sectional area for the 1.9 m

(500 gal) test tank and the

test models of [13].

Tank Diameter, D,

NBS I IT* I IT*

1.91m (3 ft) 0.24m (0.80 ft) 0.10m (0.34 ft)

1.70 (145) 0.70 (60) 3.52 (300)

3.42 (291) 1.88 (160) 9.39 (800)

6.96 (593) 14.67 (1250) 73.39 (6250)

12.54 (1068) 17.61 (1500) 88.07 (7500)

All units, kg/(m^*s) x 10“^ (lb/(ft^*h))

* Illinois Institute of Technology model water tank studies [13]

45
7. CONCLUSIONS AND RECOMMENDATIONS

A total of nine tests structured after ASHRAE Standard 94-77 were used O to test a 1,9 m (500 gal) water tank thermal storage device. A description of these tests, the data reduction procedure, and the test results

have been presented in this paper. It was found that the heat loss

characteristics and the amount of energy charged into or discharged from

the device could be quantified through the utilization of the Standard,

However, based on this study, the following recommendations are made:

1, Leniency should be allowed with respect to the specified

test requirements for a 15® C (27 ®F) temperature step change.

It was shown that adequate results were obtained without a precise 15®C step provided the results were presented in

non-dimensionless terms,

2, The dimensionless plots described in this paper should be

used in the analysis of the results. The utilization of

such plots should provide a substantial means for comparing

the thermal performance of different storage devices.

3, An incorporation of the stagnant heat loss test as performed

in this study should be allowed as an alternate technique

in the Standard,

46
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. 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.

8. 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.

9. Peavy, B.A. , Burch, D.M. , Powell, F. J. , and C.M. Hunt, "Comparison

of Measured and Computer-Predicted Thermal Performance of a Four

Bedroom Wood-Frame Townhouse," NBS Building Science Series 57,

April, 1975.

10. Burch, D.M., Peavy, B.A. , and F.J. Powell, "Comparison between Measured and Computer-Predicted Hourly Heating and Cooling Energy

Requirements for an Instrumented Wood Framed Townhouse," ASHRAE

Transactions , Vol. 81, Part II, 1975.

11. Hill, J.E. , and T.E. Richtmyer, "Retrofitting a Residence for Solar

Heating and Cooling - The Design and Construction of the System,"

NBS Technical Note 892, November, 1975.

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

,

N.Y. 10017, pp. 275-284, 1972.

13. Lavin, Z,, and J, Thompson, "Experimental Study of Thermally Stratified Hot Water Tanks," Solar Energy, Vol. 19, pp. 519-524, 1977,

48
9 . APPENDIX A

Time-Temperature Relationships for the Transfer Fluid

during the Transient Storage Tests

49
TEMPERATURE

VS.

TIME

STORAGE

TANK

CHARGE

TEST

Tc*

4hi

At*l5.4*C-,

I3lml/t

(2.07GPM1

MOm

to

<0

lOM

lOQ

lO

00

csi

cvi

cvi Ncsi o

csi

<0

CSI

00

c>

COdds

50

r.h
TEMPERATURE

VS.

TIME

STORAGE

TANK

CHARGE

TEST

Tc»2h

261.6

ml/s

(4.12

GPM)
TEMPERATURE

VS.

TIME

Q.

CD

00

tn

UJsso E

lO

rom

CSJ

00d

<0od3

52
o

«0d

<Pod

0.2
54

TEMPERATURE

VS.

TIME

STORAGE

TANK

DISCHARGE

TEST

T.

2

h.

, At

»

I4.7*C;

250

ml/t

(3.98)

56

r,h
TEMPERATURE

VS.

TIME

STORAGE

TANK

DISCHARGE

TEST

T.

*lh,At«l6.8*Ci530ml/t

18.38GPM)

57

T*h
Io

T,h
10. APPENDIX B

Dimensionless Time-Temperature Relationships for the

Transfer Fluid during the Transient Storage Tests

59
500

GALLON

STORAGE

TANK

CHARGE

TEST

SS3nN0ISN3Wia

60

DIMENSIONLESS
500

GALLON

STORAGE

TANK

CHARGE

TEST

04

Xw

- !*» 61

SS31NOISN3NIO

.

OtMENSIONLESS
500GALLDN

STORAGE

TANK

CHARGE

TEST

62

Sa^LL,

DIMENSIONLESS
500

GALLON

STORAGE

TANK

CHARGE

TEST CVJ

GO

d

1-

-I o =

Qcn

d

CJ

O

CM 00

d

CO

o o

IV

CM

d

SS3"IN0ISN3INia *

63

DIMENSIONLESS
500

GALLON

STORAGE

TANK

DISCHARGE

TEST

CNJ o <D <0 ^ CM — — c> o do

SSDNOISNBWIO

64

DIMENSIONLESS

T.S.C
w

SS91NOISN3WIG ' T|^ .1^

<

u

E

o

i/3

65

DtMENSIOMLESS
500

GALLON

STORAGE

TANK

DISCHARGE

TEST

t*s

at

TOP

TANK

ai

O

CO

66

DIMENSIOMiESS
500GALLDN

STORAGE

TANK

DISCHARGE

TEST CM

COd

C£> d

I-z

E

CJ> (O

d

CMo

CM 00

d

COd

SS31NOISN3Wia‘

o W

CMd

67

DIMENSIONLESS
500GALUON

STORAGE

TANK

DISCHARGE

TEST CM

<30 d

Io

«> t^

Q

h6

CMO

CM <30 <0 d d o

SS3“IN0ISN3INia ‘

CMd

68

DIMENSIONLESS
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 78-1548

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

AN EVALUATION OF ASHRAE STANDARD 94-77 FOR TESTING

WATER TANKS FOR THERMAL STORAGE

5. Publication Date October 1978

6. Performing Organization Code

7. AUTHOR(S) Bernard J. Hunt, Thos. E. Richtmyer, James E. Hill 8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 742 4417

11. Contract/Gtant No.

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

Department of Energy

R&D Branch for Solar Heating and Cooling

Office of Asst. Secy, for Conservation & Solar Applications

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES Washington, D.C. 20545

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

bibliography or literature survey, mention it here.) The National Bureau of Standards proposed a standard test procedure for rating

thermal storage devices, in mid-1975. Early in 1977, the American Society of

Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) adopted ASHRAE

Standard 94-77 Method of Testing Thermal Storage Devices Based on Thermal

Performance, which is based substantially on the NBS procedure. In order to

evaluate the Standard, NBS has conducted an experiment in which a 1.9 m^ (500 gal)

water tank, built as part of a residential solar heating and cooling system, was

tested in accordance with this Standard. A description of the test apparatus,

test procedure, and detailed test results are given. It was found that there were no major problems encountered in using the Standard for this kind of

thermal storage device. In addition, suggestions are made for minor modifications

in the Standard.

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 Standards; evaluation of test procedure; solar

heating components; standard test procedure; thermal storage tests; water tank

thermal storage.

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1

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