NBS

PUBLICATIONS

NATL INST. OF STAND & TECH

A111Q7 310111

NBSIR 84-2920

Thermal Performance Testing of

Passive Solar Components in the

NBS Calorimeter

M. E. McCabe

C. E. Hancock

M. Van Migom

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

August 1984

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U.S. Department of Energy

Office of Solar Heat Technologies

assive and Hybrid Solar Energy Division

Washington, DC 20585

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NBSIR 84-2920

THERMAL PERFORMANCE TESTING OF

PASSIVE SOLAR COMPONENTS IN THE

NBS CALORIMETER

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M. E. McCabe

C. E. Hancock

M. Van Migom

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

August 1984

Prepared for U.S. Department of Energy

Office of Solar Heat Technologies

Passive and Hybrid Solar Energy Division

Washington, DC 20585

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director
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ABSTRACT

Studies of the thermal performance of passive solar buildings have indicated a need for precise measurement of solar heat gain and thermal heat loss or gain

for modular passive/hybrid solar components. A description of the design,

calibration, and initial operational results for a new calorimetric test

facility designed to perform these measurements is presented in this report.

It is anticipated that this test facility will provide a substantial improvement in the measuring techniques for passive and hybrid solar components over

the field test cells currently in use.

Thermal performance data were taken for four passive solar test articles

during the winter of 1982-1983, including two windows and two collector-storage

walls. The U-Values and Shading Coefficients for the two windows were measured

and correlated with outdoor environmental variables. Operational problems

with the test facility are described and future improvements are suggested. The

application of the test results for evaluating standard laboratory test

procedures for windows is discussed. A draft procedure for measuring and

reporting U-Values for windows based on laboratory testing is presented.

iii
TABLE OF CONTENTS

Page

ABSTRACT iii

LIST OF TABLES v LIST OF FIGURES vi

1. INTRODUCTION 1 1.1 BACKGROUND 1 1.2 PASSIVE SOLAR COMPONENTS: CLASSIFICATION AND EXISTING TEST

METHODS 1 1.3 TEST METHODS FOR DIRECT GAIN FENESTRATION 2

1.4 TEST METHODS FOR PASSIVE SOLAR SYSTEMS 4

1.5 NEED FOR CALORIMETRIC TEST FACILITY 4

2. DESCRIPTION OF NBS PASSIVE SOLAR CALORIMETER 6

2.1 METERING CHAMBER ASSEMBLY 6

2.2 FLUID CONDITIONING SUBSYSTEMS 10

2.3 BUILDING-TEST ARTICLE INTERFACES 10

2.4 DATA ACQUISITION SYSTEM 11

3. PERFORMANCE EVALUATION PROCEDURES 23

4. TEST FACILITY CHECKOUT AND CALIBRATION 26

4.1 ANALYSIS OF ERROR 26

4.2 METERING CHAMBER CALIBRATION 27

4.3 AIR VELOCITY AND TEMPERATURE DISTRIBUTION 29

4.4 AIR INFILTRATION 30

4.5 TEMPERATURE TRACKING TESTS 31

5. DESCRIPTION OF TEST ARTICLES 32

5.1 TRIPLE-GLAZED WINDOW 32

5.2 SINGLE-GLAZED WINDOW WITH INSULATING SHUTTERS 34

5.3 COLLECTOR- STORAGE WATER WALL 34

5.4 COLLECTOR- STORAGE PCM WALL „ 37

6. TEST RESULTS . . 39

6.1 TRIPLE-GLAZED WINDOW 39

6.2 SINGLE-GLAZED WINDOW WITH INSULATING SHUTTERS 44

6.3 COLLECTOR- STORAGE WATER WALL 45

6.4 COLLECTOR- STORAGE PCM WALL 45

6.5 CORRELATION NEEDS FOR COLLECTOR-STORAGE WALLS 48

7. EVALUATION OF INITIAL OPERATION OF NBS PASSIVE SOLAR CALORIMETER ... 50

8. USE OF THE TEST RESULTS FOR EVALUATING EXISTING TEST PROCEDURES

FOR WINDOWS 52

9. NOMENCLATURE 54

10. REFERENCES 56

1 1 . ACKNOWLEDGMENT 58

12. APPENDIX A - PROCEDURE FOR MEASURING AND REPORTING U-VALUE FOR

FENESTRATION SYSTEMS 59

IV
LIST OF TABLES

Page

Table 1. Test Results for Triple-glazed Window 42

Table 2. Test Results for Single-glazed Window with Insulating

Shutters 46

Table A-l. Proposed Test Conditions for Sealed Glazing Unit and

Fenestration System 62

v
LIST OF FIGURES

Page

Figure 1. Photograph of NBS/DoE passive solar test building 8

Figure 2. Floor plan of passive solar test building 8

Figure 3. Schematic drawing of metering chamber 9

Figure 4. Photograph of metering chamber with solar absorber panel

partially removed 12

Figure 5. Photograph of metering chamber in horizontal position 12

Figure 6. Schematic drawing of solar absorber panel 13

Figure 7. Photograph of solar absorber panel 13

Figure 8. Solar absorber panel cooling water circuit 15

Figure 9. Photograph of photovoltaic array used for the emergency

freeze protection system 15

Figure 10. Air-conditioning unit cooling water circuit 16

Figure 11. Photograph of fluid conditioning systems 16

Figure 12. Vertical aperture - test article interface 17

Figure 13. Photograph of vertical aperture interface with standard

double-glazed window 18

Figure 14. Skylight test article - horizontal aperture

interface 18

Figure 15. Photograph of test fixture installed in horizontal aperture . 19

Figure 16. Photograph of skylight test article installed in horizontal

aperture 19

Figure 17. Schematic drawing of data acquisition system 20

Figure 18. Photograph of normal incidence pyrheliometer installed in

tracking mount 21

Figure 19. Photograph of vertical aperture with pyranometer installed to

right 21

Figure 20. Photograph of three-axis wind sensor installation 22

vi
LIST OF FIGURES

Page

Figure 21. Photograph of instrumentation, control and data

acquisition rack 22

Figure 22. Schematic drawing of performance evaluation procedures 24

Figure 23. Metering chamber energy balance 24

Figure 24. Schematic drawing of guard heater panels 28

Figure 25. Air velocity distribution in metering chamber 28

Figure 26. Schematic drawing of triple-glazed window and test frame .... 33

Figure 27. Photograph of triple-glazed window test article 33

Figure 28. Schematic drawing of single-glazed window with insulating

shutters 35

Figure 29. Photograph of single-glazed window with insulating shutters . 35

Figure 30. Photograph of collector-storage water wall installed in

test frame 36

Figure 31. Drawing of collector-storage water wall installed in test

building 36

Figure 32. Photograph of collector-storage PCM wall 38

Figure 33. Environmental conditions for triple-glazed window 40

Figure 34. Net heat transfer rate for triple-glazed window 40

Figure 35. Correlations for triple-glazed window 43

Figure 36. Environmental conditions - collector-storage water wall 47

Figure 37. Net heat transfer rate and temperature - collector-storage

water wall 47

Figure 38. Environmental conditions - collector-storage PCM wall 49

Figure 39. Net heat transfer rate and AT - collector-storage PCM wall .. 49

Figure A-l. Hypothetical performance for sealed glazing unit 62

vii

1. INTRODUCTION

1 . 1 BACKGROUND

Passive solar buildings are structures designed to use radiant solar energy for

space heating and may take advantage of available environmental heat sinks such

as the outside air, the ground and the sky, for space cooling. Frequently,

elements of a passive solar building's thermal envelope such as walls, windows,

roofs, and floor slabs are used to transfer heat between the inside occupied

spaces and the outside environment. Since the intensity of solar radiation and

the external thermal environments vary with time, designers of passive solar

buildings require information on the dynamic thermal behavior of the various

building envelope components in order to integrate them into successful building designs.

The thermal behavior of building elements under both steady-state and transient

conditions can be predicted using numerical heat-transfer simulation models,

provided the thermophysical properties of the materials are known and the

thermal boundary conditions at each interface can be determined. In practice

however, nonhomogeneities in the materials used in building construction and

complexities in calculating thermal boundary conditions often result in significant uncertainties in performance predictions based on thermal modeling.

These considerations have led to recent efforts to experimentally determine the

thermal characteristics of materials, components, systems, and entire buildings

with considerable emphasis placed on measurement of entire buildings. This

report describes a new experimental facility designed to measure thermal

characteristics at the component level. The testing of full-scale passive

solar components in this facility is anticipated to lead to a better understanding of the basic heat transfer mechanisms involved in a passive solar

building and thereby contribute to an improved technology base.

1.2 PASSIVE SOLAR COMPONENTS: CLASSIFICATION AND EXISTING TEST METHODS

In previous work [1]*, a survey of passive solar products resulted in an interim

classification consisting of ten different generic passive solar components for

purposes of thermal testing. That report defined a passive solar component as being: (1) capable of mass production economies; (2) fabricated in modular

incremental sizes to permit ready integration into buildings; and (3) designed

to utilize solar energy in conjunction with natural thermal processes such as conduction, free convection, radiation and evaporation to reduce fossil energy

consumed in buildings for purposes of space heating, space cooling, and/or

domestic water heating.

Direct Gain Fenestration (DGF), such as windows, glazed doors, and skylights, and

Collector-Storage Wall (CSW) modules were identified as being most advanced in

terms of commercial availability and acceptance by the design community for use

* Numbers in brackets identify the references listed in Section 10

1
in passive solar buildings, therefore higher priority was assigned to experimental

evaluation of those components.

In the previous report, a survey of available thermal test procedures was performed to assess the applicability of existing test methods for passive

solar components. Test procedures that are useful for the DGF classification

were identified, and recommendations were made for evaluation of these

laboratory-based procedures by comparison with field-based testing under controlled interior conditions. Recommendations were also made for the development

of new test procedures for component classifications for which test methods

were not available. A program to develop test procedures for CSW modules was also presented.

1.3 TEST METHODS FOR DIRECT GAIN FENESTRATION

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers

(ASHRAE) has used a simple design procedure for predicting the instantaneous rate

of solar and thermal energy transfer through building fenestration systems [2-5] . That procedure, while orginally intended for determining summer design-day heat

gains through fenestration systems to permit sizing of cooling systems, is also

being used to characterize DGF systems used in passive solar heating applications.

However the thermal performance of fenestration systems is known to exhibit

substantial variation in response to climatic conditions such as ambient temperature, air velocity, and solar irradiance. Therefore it is possible that

thermal analysis of buildings containing substantial glazed area, when based

on the single-point, design-day thermal characteristics of the installed

fenestration, might result in substantial error in the calculated energy

consumption or an incorrect choice between alternative fenestration systems.

1.3.1 Heat Transfer Considerations

The transfer of heat through a solar irradiated window into a building space can

be considered to consist of the following three processes:

1. Direct transmission of radiant solar energy,

2. Convective and radiative transfer of radiant solar energy absorbed by

the glazing,

3. Convective and radiative transfer of heat due to outdoor-to-indoor

temperature difference.

In the ASHRAE procedure, the two solar-driven processes are combined into a single

term, and the net rate of heat transfer into the conditioned space per unit area of glazing is given by:

qNET = F lx + U(T0 -Ti) ( 1 )

2
where

F is the solar heat gain coefficient,

I T

is the solar irradiance, w/m^ (Btu/hr f

t

z

)

,

U is the overall coefficient of heat transfer, w/m2°C (Btu/hr ft2°F),

Tc-Ti is the outside-to-inside temperature difference, °C (°F).

It is common practice to reference the solar heat gain coefficient F, for a particular glazing or shading product to a reference glazing Fref> consisting

of a single sheet of double-strength clear glass. The ratio F/Fref is

called Shading Coefficient. Manufacturers of glazing products characterize

their products by providing Shading Coefficient data at a selected value of

solar irradiance, ambient temperature, and external heat transfer coefficient

corresponding to the ASHRAE summer design condition. Fenestration U-Value

data are presented in the ASHRAE handbook [5], for both winter and summer

design conditions.

1.3.2 Fenestration Test Methodology

The simplified design procedure of ASHRAE has led to a thermal test methodology

for fenestration systems in which Shading Coefficient is determined independently of U-Value. Shading Coefficient is often measured in outdoor calorimeters under conditions of natural convection and with zero outside-to-inside

temperature difference. U-Value is measured in laboratory hot boxes under

forced convection conditions and with no solar irradiation. The test methodology assumes that the net heat-transfer rate per unit area of fenestration

can be determined by adding the contribution due to solar irradiance (Shading

Coefficient x Solar Heat Gain Factor)** and the contribution due to temperature

difference (U x AT).

This ASHRAE design procedure appears to provide reasonably accurate results for

simple fenestration systems, which have low solar energy absorbing components

such as single or double-glazed, clear windows. However, its application for

characterizing the more complex fenestrations proposed for use in passive

solar applications is unknown [6]. Multiple-glazed fenestrations having

spectrally-selective , low-emittance coatings, which reduce radiative heat

loss without substantially reducing solar gain, are now commercially available.

New optical switching materials, which modulate solar gain in response to

temperature or solar intensity, are also being developed [7]. The development

of these new high-performance fenestration systems suggests that the existing

test methodology and building fenestration characterization should be carefully

reviewed to determine their adequacy for passive solar applications.

** Tabulated values of Solar Heat Gain Factor for reference glazing are provide! in

Ref. 5.

3
1.4 TEST METHODS FOR PASSIVE SOLAR SYSTEMS

Test methods for passive solar systems and components have evolved differently

from the test methods for building fenestration systems. Most often, testing

was performed on passive solar systems installed in small outdoor test rooms or cells, with naturally occurring climatic conditions and with widely varying test room air and surface temperatures [8].

Although the results of those passive solar testing programs have yielded

significant information on the operating principles and the relative thermal

efficiencies of various passive solar systems and components, the use of such

test rooms with uncontrolled air and surface temperatures may not be useful

for standard testing of modular passive solar components. In the test rooms,

the air and surface temperatures are often uncontrolled , and therefore are sensitive to the thermal characteristics of the particular room, such as its

insulation thickness, surface area, thermal mass, and air leakage rate. Uncontrolled air and surface temperatures in test rooms tend to modify temperaturedependent heat-transfer mechanisms, such as free convection and infrared

radiative transfer. This results in measured thermal performance that is

strongly facility-dependent, a highly undesirable characteristic for a standard

test procedure.

1.5 NEED FOR CALORIMETRIC TEST FACILITY

In consideration of the shortcomings of the existing test methods for both

fenestration and passive solar systems, the need for a new calorimetric test

facility was established. The test facility would provide controlled interior

surface and air temperatures, thereby simulating an ideal room, and would

accurately measure the rate of heat transfer between the the passive solar

component and the indoor environment.

The potential use of the test facility is twofold: (1) to provide a comparison

between outdoor test data from the calorimeter and data from indoor test

facilities which may use combinations of simulated solar radiation, ambient

temperature, and wind velocity; (2) to measure the thermal performance of

passive solar components under realistic field conditions and attempt to quantify

the contributions from various heat transfer mechanisms. The former use would support the development or validation of new and existing standard test

methods and the latter use would support research activities for the passive

solar program sponsored by the U.S. Department of Energy. For example, natural

convection in enclosures is being investigated using both modeling and experimental techniques based on idealized boundary conditions in which vertical

surfaces are isothermal. One of the design features of the test facility is

to measure surface heat transfer rate and resolve it into convective and radiative components. Data of this type are useful to obtain a better understanding

of the thermal behavior of passive solar components and the interaction with

the buiding space in which they are installed. More specific details on the

conceptual design and performance specifications for the test facility are given in references 9 and 10.

4
The remainder of this report describes the design, construction, calibration,

instrumentation, and performance evaluation procedures for the NBS passive solar

calorimeter. The initial winter season of operation is described and results

are presented for four passive solar test articles. Operating problems

encountered during the initial test period and recommended improvements in the

facility are described. The use of the field test results for evaluating

existing laboratory procedures for measuring U-Value of windows is discussed.

An outline of a draft procedure for measuring and reporting U-Value of

fenestration systems such as windows, patio doors, and skylights is presented

in the appendix.

5
2.

DESCRIPTION OF NBS PASSIVE SOLAR CALORIMETER

The passive solar calorimeter is installed in one of the test rooms in the

NBS/DOE Passive Solar Test Building, which is located at the NBS Annex immediately south of the main NBS campus in Gaithersburg, MD. The test building was constructed in 1981 to obtain thermal performance data for full-scale passive

solar systems in buildings. A photograph of the exterior of the test building

is shown in figure 1. A floor plan of the building showing the interior layout

of the building experiments and the calorimeter is provided in figure 2.

Detailed information on the passive solar features and instrumentation installed

in the Passive Solar Test Building are given in reference 11.

An advantage to locating the solar calorimeter in the Passive Solar Test

Building is the opportunity for the simultaneous thermal testing of two

identical passive solar components: one in the idealized room environment

provided by the calorimeter and the other in the more realistic, full-scale

room environment provided by one of the test cells. This will allow qualitative assessment of the affect of the calorimeter environment on component

thermal performance.

The passive solar calorimeter test facility consists of the following major

assemblies and subsystems, each of which is described in the following

sections

1. metering chamber assembly,

2. fluid conditioning subsystems,

3. building-test article interface, and

4. data acquisition system.

2.1 METERING CHAMBER ASSEMBLY

The function of the metering chamber assembly is to completely enclose the

interior of a test article with controlled air and surface temperatures and

thereby simulate an interior space in a building. An energy balance

establishes the net rate of heat transfer between the test article and

the metering chamber.

The metering chamber assembly, shown schematically in figure 3, includes a five-sided metering chamber constructed of rigid polyurethane foam insulation

150 mm (6 inches) thick everywhere except 100 mm (4 inches) on the bottom.

The metering chamber can be mounted either vertically against the south wall

or horizontally against the ceiling of the calorimeter room. It is externally

supported by a structural frame of welded aluminum angles which has no penetrations through the insulation. The metering chamber assembly is mounted on a welded steel dolly with casters to permit movement of the assembly in the

calorimeter room. A pair of telescoping-tube trunnion supports are attached

to the dolly to permit rotation and the raising and lowering of the metering

chamber to the ceiling test position using a hand-operated winch.

6
Figure 4 is a photograph showing the air handling equipment installed in the

metering chamber with the solar absorber panel removed for clarity. Figure 5 is a photograph of the metering chamber assembly installed in the horizontal

test position for a component installed in the building roof. In the vertical

position, the metering chamber is bolted against the building interface and

sealed with closed-cell foam gaskets to minimize infiltration air exchange

with the test room. In the horizontal position, the metering chamber is

clamped and sealed against the ceiling of the test room using telescoping

steel pipe columns located at each corner.

The metering chamber surface temperature and air temperature are individually

controlled by the solar absorber panel conditioning unit and the air-conditioning

unit, respectively. To assist in developing the conceptual thermal design of

the metering chamber and to aid in sizing the absorber panel and air-conditioning

units, a 65-node thermal network of the metering chamber assembly was prepared.

The thermal model consisted of an finite-difference representation of the

metering chamber, the absorber panel and coolant tubes, and a single-glazed

window representing a "worst-case" test article. The general purpose thermal

analyzer program SINDA [12], was used to solve the heat transfer equations.

The results of the design study are presented in reference 9.

The average surface temperature is maintained in the calorimeter by forced

circulation of water through the solar absorber panel. The panel, shown schematically in figure 6, is a soldered assembly of copper sheets and tubes painted

with a black paint having measured solar absorptance of 0.95 and normal infrared

emittance of 0.88. Figure 7 is a photograph of the absorber panel assembly

prior to painting. The solar absorber panel fits within the metering chamber

and is positioned sufficiently close to the test article to absorb almost all

the solar energy transmitted by DGF type test articles and to provide a temperature

controlled, nearly isothermal, radiative heat sink (or heat source) for the

test article. Temperature control is achieved by sensing the average surface

temperature of the panel with a grid of six thermocouples wired so that

these junctions are in a parallel circuit. The averaging thermocouple grid

provides a control signal to a solid-state temperature controller which

regulates the duty cycle of a water heater located in the absorber panel

inlet pipe.

A uniform air temperature is maintained in the calorimeter by circulation of

air at low velocity between the test article interior surface and the absorber

panel. Temperature control is achieved in the air-conditioning unit by first

removing heat and then adding heat to the circulating air using the water-cooled

heat exchanger and an electric heater, respectively. The cooling water flow

rate and inlet temperature to the heat exchanger are manually set to provide a constant rate of energy removal from the airstream based on anticipated ambient

conditions. A pair of duct heaters located downstream of the cooling heat

exchanger are used to increase the air temperature to maintain the setpoint

value. The average air temperature in the metering chamber is sensed by a grid

of six type T (copper-constantan) thermocouples wired in parallel. The averaging

thermocouple grid provides a control signal to a solid-state temperature controller

which controls the duty cycle of the air heater. Control of air velocity and

direction in the metering chamber is provided by a solid-state motor controller,

7
Figure 1. Photograph of NBS/DoE passive solar test building

15.75m (51 6

TEST ARTICLE COMPONENT

CALORIMETRY Figure 2. Floor plan of passive solar test building

8
Figure 3

.

Schematic drawing of metering chamber

9
which regulates the speed and direction of a belt-driven, axial-flow fan. The

fan circulates air over the test article in the downward direction in the winter

and upward direction in the summer in accordance with the motion that would

result from free convection

flow.

2.2 FLUID CONDITIONING SUBSYSTEMS

The function of the fluid conditioning subsystems is to provide temperature

control for the absorber panel and for the air cooling coil in the metering

chamber. Distilled water is used as the heat transfer medium in both fluid

conditioning subsystems.

Figure 8 shows a schematic drawing of the solar absorber panel water circuit,

which contains the primary circulating pump, emergency pump, heat rejection

pump, 1955, (52 gal.) storage tank, 2500W (8532 Btu/h) immersion heater,

solid-state temperature controller, and an air-cooled water chiller. Thermal

energy from the absorber panel is transferred to the storage tank from which it

is rejected to ambient air by the 2600 W (9000 Btu/h) water chiller. Provision

for emergency circulation of storage tank water by a battery-operated, low

capacity pump during power outage was made to prevent freezing of the absorber

panel during conditions of subfreezing ambient temperature. Sufficient

thermal energy is contained in the storage tank to maintain the temperature

of the absorber panel above freezing for at least 24 hours. The emergency

coolant pump storage battery is kept on a trickle charge using a small photovoltaic array shown in figure 9.

A schematic drawing of the air-conditioning unit is shown in figure 10. This

unit consists of a circulating pump, a 1145, (30 gal.) storage tank, a 500W

(1700 Btu/h) immersion heater, and a 700 W (2500 Btu/h) water chiller. The

thermal energy absorbed in the water/air heat exchanger in the metering chamber

is rejected to the calorimeter room through the water chiller. Control of the

calorimeter room temperature is provided by the building heating and cooling

system to a nominal _+ 1°C (+ 2°F) of the room set point. Figure 11 is a photograph showing both the fluid conditioning subsystems installed in the test

facility.

2.3 BUILDING-TEST ARTICLE INTERFACES

The purpose of the building interface is to provide structural support for

the test article and thermal isolation from the building. Flanking loss, the

undesired heat flow between the metering chamber and outdoors, is minimized

by carefully sealing joints to prevent air leakage and by reducing heat conduction by providing the building interface with a substantial thickness of rigid

insulation between outdoor and indoor surfaces.

Each passive solar component to be tested is assembled in a suitable

test frame and installed in either the vertical south-facing aperture, or the

horizontal aperture of the NBS Passive Solar Test Building. Details of the

passive solar components and test frames are provided in section 5. Figures

10
12a, b, and c, shows the vertical building interface construction details

at the head, sill and vertical sections, respectively.

Figure 13 is a photograph of the vertical aperture of the building interface

with a double-glazed window provided for nonglazed components. Modular passive

solar components provided with a self-contained glazing system are accommodated

by removing the double-glazed window and sealing the test article's glazing

system against the steel plates of the building interface.

Test articles substantially smaller in size than the nominal 1.26 by 2.09 m

(49-1/2 by 82-1/2 inches) test article can be accommodated in the test facility

by installing blanking heater panels in the unoccupied portions of the aperture.

The electric strip heaters in the blanking panels are temperature controlled to

offset heat loss and thereby maintain nearly adiabatic boundary conditions at

their interior surfaces. Details of the blanking panels are provided in

section 4.

Figure 14 shows the installation of a test article in the horizontal aperture

of the building interface. In this case, the nominal 1.22 by 1.54 m

(48 by 72 inches) horizontal aperture is fitted with a special, insulated test

fixture to accommodate a 0.79 by 0.79 m (30-1/4 by 30-1/4 inches) test skylight.

Figure 15 is a photograph of the test fixture and figure 16 shows the

skylight installed in the horizontal aperture.

2.4 DATA ACQUISITION SYSTEM

The data acquisition system consists of the measurement sensors, signal

conditioning circuits, display meters, and data scanning and storage devices

shown schematically in figure 17. Measured variables include temperature

(absolute and differential), solar irradiance (total and direct normal), flow

rate, velocity, and electric power. The primary measurement sensors used in

the test facility are thermocouples, differential thermopiles, pyranometers

,

a pyrheliometer , turbine flow meters, two types of air velocity sensors, and

watt transducers.

Temperature is measured with 24 gauge, type-T (copper-constantan) thermocouples. Sixteen surface-mounted thermocouples are attached to the absorber

panel and eight thermocouples are installed in the air space between the absorber

panel and the test article to monitor the operation of the metering chamber and

the temperature control systems. Thermocouples also measure the temperature at

various locations on the metering chamber walls and the test article, in the two

fluid conditioning subsystems, and of the outdoor and room environments.

The temperature change of the water in each fluid conditioning subsystem within

the metering chamber thermal envelope is measured with a differential thermopile.

Each thermopile consists of 30 pairs of type-T thermocouple junctions located at

the inlet and outlet pipes at the exterior of the metering chamber. Turbulence

inducing pipe fittings are located immediately upstream of each thermopile to

insure well-mixed flow.

11
Figure 4. Photograph of metering chamber with solar absorber panel

partially removed

Figure 5. Photograph of metering chamber in horizontal position

12
32 mm [1'/« in) O.D.

Figure 6. Schematic drawing of solar absorber panel

Figure 7. Photograph of solar absorber panel

13
Turbine flowmeters are used in each of the water conditioning subsystems to measure flow rate. These sensors develop a signal that is proportional to

the volume of fluid flow. A f requency-to-voltage converter makes this signal

compatible with the data logger.

A pyrheliometer and two pyranometers are used to measure the direct normal

and the total solar irradiance, respectively. A photograph of the pyrheliometer

installed in a sun tracking mount is shown in figure 18. One of the pyranometers

is installed horizontally at the roof aperture and the other is installed

vertically at the wall aperture. The pyranometers measure the total amount

of solar irradiation including both beam and diffusely reflected radiation

from the sky, ground, and building. Figure 19 is a photograph of the vertical

pyranometer installed next to the aperture at approximately mid-height.

The electrical power and energy supplied to the metering chamber are measured

with watt transducers and an electronic integrating module, respectively. The

watt transducers consist of Hall Effect devices that provide an output analog

signal of instantaneous real power. The analog power signal is integrated to pro

vide an analog signal proportional to the number of watt-hours of energy.

Transducers measure all the electrical power and energy input to the metering

chamber, including the air heaters, the fan motor and speed control, and the

test article (if it requires operating energy).

The outside wind speed and direction are sensed with three, mutuallyperpendicular wind driven propellers located approximately 3 m (10 ft.) in front

of the vertical aperture. Each propeller drives a generator which produces a voltage signal proportional to the component of wind velocity in the direction

in which the propeller is facing. Figure 20 is a photograph of the wind sensor

installation.

The data acquisition system initially installed in the test facility used an analog data logger to scan and print the temperature and millivolt sensor readings and a nine track tape recorder to store data on magnetic tape. The data

logger has the capability of reading 40 type-T thermocouple channels for temperature measurement and 20 channels for other analog voltage measurements, including solar radiation, outdoor wind velocity, metering chamber air velocity, water

flow rate, and electric power and energy.

Visual display of up to 30 channels of temperature data is available with a digital panel meter having a resolution of 0.1°C (0.2°F). Other analog sensor displays such as pressure, temperature, and flowrate, are available using pressure gauges, liquid-in-glass thermometers, and rotameters, respectively. These sensors are primarily used to monitor operation of the test facility. Figure

21 is a photograph of the instrumentation cabinet for the data acquisition system

visual displays and control equipment in the passive solar calorimeter.

14
Figure

ni —{x n

1417

Figure 8. Solar absorber panel cooling water circuit

15
CONDITIONING UNIT CHILLED WATER CIRCUIT

Figure 10. Air-conditioning unit cooling water circuit

Figure 11. Photograph of fluid conditioning systems

16
(a) HEAD SECTION

lb] SILL SECTION

|C] VERTIClf SECTION

mtqwr of arrows dunk*

Figure 12. Vertical aperture - test article Interface

17
Figure 13. Photograph of vertical aperture interface with standard double

glazed window

ACRYLIC SKYLIGHT DOMES |12 mm| (1/8 In) CLEAR /WHITE (TEST ARTICLE A) CLEAR/CLEAR (TEST ARTICLE B|

Figure 14. Skylight test article horizontal aperture interface

18
Figure 15. Photograph of test fixture installed in horizontal aperture

Figure 16. Photograph of skylight test article installed in horizontal

aperture

19
Figure 17. Schematic drawing of data acquisition system

20
Figure 18. Photograph of normal incidence pyrheliometer installed in

tracking mount

Cr

Figure 19. Photograph of vertical aperture with pyranometer installed to

right

21
Figure 20. Photograph of thre-axis wind sensor installation

Figure 21. Photograph of instrumentation, control and data acquisition rack

22
3. PERFORMANCE EVALUATION PROCEDURES

Thermal performance evaluation for each test article was performed using the

minicomputer located in the Center for Building Technology in NBS. The

performance evaluation procedures are shown schematically in figure 22.

Sensor data, scanned by the data logger, are recorded on magnetic tape and

printed on paper tape. Approximately one minute is required to scan the 60

data channels. The scan rate is four scans per hour. Electronic modules were

used to continuously scan and integrate the voltage signals from the differential thermopiles, turbine flow meters, watt transducers, pyranometers and

wind sensors, thereby improving the accuracy of these measurements over that

possible with the relatively slow scan rates available with the data logger.

A data file containing the raw scan data is created in the CBT minicomputer

from the data stored on the paper or magnetic tape. The scan data are converted to engineering units using measured calibration constants, which are stored

in a separate data file. The scan data in engineering units are then used to

perform a heat-balance calculation, and a listing is made of all computed and

measured energy flows. The scan data heat-balance information is then integrated to provide hourly average values, which are archived. Further evaluation

of the hourly test results is performed using a statistical analysis program

and a graphics plotting routine.

Figure 23 is a schematic drawing showing the energy transfer across the

boundaries of a control volume located within the metering chamber. It is

assumed that: (1) there is negligible air infiltration exchange between either

the outside or the room air and the metering chamber air; (2) there are no latent heat transfer processes due to evaporation or condensation of moisture

at cold surfaces; and (3) there is no energy storage within the metering chamber

other than possibly within the test article. The latter assumption is justified

by controlling the mean absorber panel temperature, T^BS> an(l the mean

chamber air temperature, T^ir to constant values. Based on these assumptions,

the net instantaneous rate of thermal energy transfer between the test article

and metering chamber, QneT> as given by:

Qnet = Qabs + Qcc + Qls - Qhc * Qfan ( 2 )

where: *

(3)

* Nomenclature defined in section 9

23
SCAN DATA

Figure 22. Schematic drawing of performance evaluation procedures

PASSIVE SOLAR

TEST ARTICLE

Figure 23. Metering chamber energy balance

24
Qcc " ^CC C

P

( TOUT,C " tin,c> dt > and

Qur = ~ fr e

2

a dt • HC AtJ{-Z)K HC

( 4 )

( 5 )

QFAN is the electrical energy input to the fan motor and it is assumed

that all electrical energy supplied to the heating coils and for operating

the fan and test articles (if provided) is converted into thermal energy.

Q ls is the heat transfer rate between the metering chamber and the surrounding

room due to conduction through the chamber walls and piping and convection

through the instrumentation wiring openings. Q^g is measured in the calibration

tests, during which the test article is replaced by an adiabatic guard heater

panel

.

QnET consists of both convective and radiative components. The convective

component is imposed on the metering chamber air stream adjacent to the test

article and the radiative component is imposed on the absorber panel. For test

articles with windows, the convective and radiative components may have opposite

senses, i.e., solar radiative heat gain during the daytime being positive, while

convective heat gain is negative (heat loss). The individual heat-balance

relations for the absorber panel and metering chamber air suggest that if the

mean temperatures of both the absorber panel and the adjacent air stream are

equal, so that there is no net convective heat exchange between each other,

then the absorber panel heat gain is due entirely to radiative heat transfer

from the test article. This allows the test article's net heat-transfer rate,

QNET * to be resolved into its convective and radiative components.

One of the design objective for the calorimeter was to resolve the net heat

transfer rate into convective and radiative components. This proved to be

difficult to achieve and will be further discussed in the next section.

25
4. TEST FACILITY CHECKOUT AND CALIBRATION

Upon completion of construction of the test facility in November 1982, a series

of tests was performed to verify to operation according to specifications.

These experiments were designed to test the ability of the mechanical and

control systems and the data acquisition system to operate properly as planned.

Individual instruments were calibrated, accuracies were determined, and an estimate of overall uncertainty in the measured results was made. Calibration

of the metering chamber itself was necessary so that the small amount of

heat loss through the chamber walls could be determined and accounted for in

the overall energy balance. This was accomplished by placing guard heater

panels in the vertical aperture to create an adiabatic wall and then operating

the fluid conditioning equipment. The tests designed to check the normal

operating conditions helped to establish adequate control procedures but also

revealed some shortcomings of the calorimeter’s capabilities. Improvements

in the data acquisition and control systems were implemented as a result of

the tests and the performance was found to be adequate. Testing for collecting

useful data started in January 1983.

4.1 ANALYSIS OF ERRORS

The magnitude of the errors that might be expected in the test results are discussed in this section. The analysis of error is important since a high

level of overall accuracy is desired and several special measurement problems

exist. The measurement of small temperature differences that are a consequence

of the attempt to separately measure the radiative and the convective components

of heat flux were found to present the most difficult measurement problem. The

significance of the errors in the test results was established and the

principal sources of these errors identified. Specific improvements to the

operation and measurement systems also discussed.

The individual measurements which must be made to determine the test article

net heat transfer rate are defined by equation 2. In that equation, and

Qfan are measurements of electrical energy which can readily be made with

accuracies of better than 0.5 percent. Q^g is not measured directly but is

determined from the metering chamber heat balance measured during calibration.

The uncertainty in that heat balance component is the most significant error

in the overall energy balance, however, the magnitude of Q^g is usually

small compared to other terms in the equation and thus is not generally a large contributor to the total error. The determination of Q^g and its

impact on the overall energy balance is discussed further in section 4.2.

The greatest potential error occurs from the heat balance components, Q/yjs

and Qcc> which are the changes in enthalpy of the fluid conditioning systems.

Each component requires the measurement of a mass flow rate and a temperature

difference (AT). Of the two, uncertainty in the measurement of the absorber

panel fluid AT, generally constitutes the largest portion of error in the

energy balance. This is due primarily to the conflicting goals of maintaining

26
an isothermal absorber panel (both spatially and temporally) and of maintaining

a measurable AT. To meet the isothermal requirement requires a large flow

rate and hence, the fluid AT should be near zero. However, to achieve better

accuracy in the measurement of temperature difference, a large AT should be

maintained. For the results presented in this report, the mass flow rate of

the absorber circulating fluid was held constant and the absorber AT allowed

to change in response to changing environmental conditions. Since the magnitude

of Qabs varies over a range of about 20 to 1 , the measurement of absorber AT

at night has a relatively low accuracy and isothermal conditions are not closely

maintained during periods of peak daytime solar radiation.

For the existing measurement system, the 95 percent uncertainty interval

for temperature difference of measurements is estimated to be 0.02°C (0.04°F).

This error in the AT measurements results in a 15 watt uncertainty interval

for Qabs anc* a 5 watt uncertainty interval for Q^. These errors are much

larger than all other measurement system errors combined and so must receive

the greatest attention for future improvements. A control system that provides

for variable flow rates depending upon the magnitude of QabS a re l a tively

simple modification that should produce significantly improved measurement

capability.

4.2 METERING CHAMBER CALIBRATION

One terra in the energy balance equation Q^g , which accounts for heat lost

through the insulated walls of the metering chamber is given by:

Qls = < ua)mc ( tair “ trm)

The overall heat-transfer coefficient, (UA)j^c> of the metering chamber was determined experimentally by placing guard heater panels in the test article

aperture to prevent heat loss and by maintaining the guard heater surface temperature equal to the metering chamber air and absorber panel temperature.

When the conditioning and measurement systems were operated and a value of

QnET = 0 established by virtue of the guard heaters, Q^g was obtained by the

energy balance, and (UA)^jc could be calculated.

The guard heater panels are shown schematically in figure 24. They consist

of three sections which completely fill the vertical aperture for the metering

chamber calibration. The guard heater panels can also be used separately as individual adiabatic blanking panels to accommodate different size test

articles in the vertical aperture. Each guard heater panel consists of an aluminum plate sandwiched between two pieces of rigid polyurethane foam insulation, with 102 mm (4 in.) insulation on the interior side of the plate and 51 mm (2 in.) on the exterior. Resistance-foil strip heaters mounted on the

plates are controlled by solid-state temperature controllers to maintain a constant average surface temperature equal to the setpoint temperature of the

metering chamber. This design constrains the heat transfer rate through the

blanking panels to less than 0.45 (1.54 Btu/h) watts when the guard heater plate

temperature is within 1.0°C (1.8°F) of the metering chamber air temperature.

27
Panel B Guard Heater

25x51 (1x2)

Wood Insert

Note: Dimensions shown

are in millimeters

(inches)

Interior Surface 1 6 ( 1/4) Tempered

Hardboard (typ.) 102(4) Urethane

Foam Insulation

5 (3/ 1 6) Aluminum Plate

with Strip Heaters 51(2) Urethane

Foam Insulation

SECTION A-A

13(1/2) Plywood

'Exterior Surface White Paint

F-icnire 24. Schematic drawing of guard heater panels

(a) UPPER SECTION

Absorber Panel Surface-^

1 + 0.27 + 0.37 + 0.30 + 0.37 0.43 +

305

mm

(12 in) + 0.37 + 0.40 + 0.27 + 0.34 0.37 +

1 + 0.12 + 0.15 + 0.15 + 0.12 0.12 + —1220 mm (48 in) —

Test Article Surface

(b) MID SECTION

+ 0 37 + 0.40 + 0.37 + 0.1 2 0.06 +

+ 0. 18 + 0.18 + 0.30 + 0.2 1 0.12 +

+ 0 06 + 0.09 + 0.12 + 0.06 0.03 +

Note: Ve locity measured at locations denoted by + , given

in meter /sec. To c onvert to ft/ sec, mult ply by 197

Figure 25. Distribution of air velocity (m/s) in the metering chamber

28
The calibration tests were performed by operating all fluid conditioning equipment

and instrumentation as they would be operated under normal testing conditions.

The systems were allowed to run for several hours to reach equilibrium conditions after which the data was recorded for a period of about 8 hours . This test was repeated several times and from these data an average (UA)mc was calculated.

Tests were also conducted without operation of the absorber panel in an attempt

to reduce overall uncertainty.

The best estimate of the average (UA)mc obtained through the tests was 2.4 W/°C

(4.5 Btu/h°F) . This agrees reasonably well with the UA calculated from material

properties of about 2.0 W/°C (3.8 Btu/h°F). It should be noted that there are other possible heat-loss mechanisms that do not depend only on the difference

in temperature between the room and the metering chamber interior and are therefore not included in this term. These include air infiltration and

conduction to the outside environment either through the test article and its

support frame or through or around the metering chamber itself. Another potential

source of measurement error is due to exterior surfaces of the chamber being

exposed to different ambient temperatures. Measurements indicated the bottom

of the chamber to be about 6°C (3°F) colder and its top to be about 4°C (2°F) warmer than the average (mid-height) room temperature.

4.3 AIR VELOCITY AND TEMPERATURE DISTRIBUTION

The velocity and temperature distributions of circulating air in the metering

chamber were measured to determine their compliance with design criteria. These

criteria specified uniform velocity and temperature across the aperture and a maximum velocity of less than .45 m/sec. (1.5 fps) to minimize disturbance of

natural convection at the surface of the test article. Development of these

criteria is discussed in reference 9. Honeycomb flow straighteners were

placed in the inlet section of the air duct to obtain a uniform air velocity

distribution and a speed controller on the fan motor was adjusted to provide

the desired velocity. The measurement was accomplished by closing the aperture

of the calorimeter with a plywood panel and inserting a hot wire anenometer

probe through small holes drilled in the panel. Readings were taken at nine

points across the width, four points along the vertical dimension, and at

three different depths for a total of 108 velocity measurements.

Figure 25 presents the most important results which show a small variation

from side-to-side in a given horizontal plane but considerable variation from

front to rear, with the higher velocities being near the absorber panel and

monotonically decreasing toward the aperture. The velocity distribution in a vertical plane implies that there is some mixing from front to back in the

lower portions. The average air velocity appears to satisfy the design

criterion, especially the velocities nearest the test article which are sufficiently low so there is little chance for forced convection heat transfer. It is

also clear however, that at some locations, velocities greater than the design

value may induce some localized turbulence. Performing the same traverse

pattern with a thermocouple revealed that there were no temperature differences

greater than 0.1°C (0.2°F) anywhere within the air flow stream.

29
Some potential operational and measurement problems with the fan were discovered

during checkout. The system is designed so that the electric motor which operates the belt-driven fan is not located within the air flow stream. Hence when

trying to discriminate between radiation and convection in the measurements, it

is not clear whether the fan energy should be associated with the air or absorber

(i.e., convection or radiation) portion of the energy balance. The recommended

solution to this problem is discussed in section 7

.

4.4 AIR INFILTRATION

Although air infiltration is not explicitly measured in the calorimeter, the

assumption was made that due to careful construction, it would be an insignificant

component of the energy balance. To test the assumption, a pressurization

test was performed on the metering chamber and the air leakage rate was determined

as a function of pressure difference. This was accomplished by sealing the

calorimeter aperture and installing a small blower to pressurize the chamber.

The flow rate through the blower and the induced pressure difference across

the walls of the chamber were measured at several blower speeds, providing a set of points to be fit to the equation:

V = C(AP) n (7)

where:

V = volume flow rate of air, l/s (CFM)

AP = pressure difference, Pa (in H2 O)

C,N are parameters to be determined.

Parameters estimated from the data yield values of C = 0.131 and N = 0.93

in SI units and C = 5.1 and N = 0.93 in English Units.

This equation provides a quantitative measure of the air leakage characteristics

of the metering chamber not associated with a test article. At a rating point

of 4.0 Pascals (0.016 in. H2 O) , the computed leakage would be about 0.7 air changes

per hour. However, in practice the driving forces for infiltration through

the metering chamber (wind and temperature difference) are very small because

of its location within a controlled environment.

The more important consideration is to determine the air leakage characteristics

of the test articles. If there is air leakage through the test article from

the outside, this heat flow will show up in the energy balance and obscure the

identification of other heat transfer mechanisms. Each test article is likely

to have different air leakage characteristics and the interface with the calorimeter aperture may also leak. It is possible to perform a pressurization test

for each test article as installed in the aperture or to use continuous tracer

gas monitoring, but these tests would still not provide a quantitative measure

of heat flow due to infiltration because it would not be known whether air

coming into the calorimeter is from the surrounding room or from the outside.

It is therefore desirable to minimize air leakage of all test articles to

eliminate this consideration from the energy balance.

30
4.5 TEMPERATURE TRACKING TESTS

Several separate control devices must be independently adjusted to provide the

desired temperature control. The solid-state temperature controllers which

control the heaters in both the air and absorber loops each have three parameters

that determine their behavior; setpoint, bandwidth, and cycle-time. The two

chillers are turned on and off by thermostats with adjustable setpoints and

bandwidths. The response characteristics of the system are also sensitive

to the liquid and air flow rates in the conditioning loops. A series of tests

was performed to select values of control variables that would provide adequate

control and to qualitatively determine the sensitivity of the system to each

variable

.

These tests primarily consisted of operating the entire calorimeter system in

its normal mode and observing the temperature history of the air and absorber

on a strip-chart recorder. As individual adjustments were made on the controllers, their effect could be observed, the objective being to eliminate any

steady-state oscillation of the temperatures and to achieve equality of T^ir

and T^bs to within the accuracy of the thermocouples. Another part of this

test consisted of subjecting the system to step changes in radiation energy

input by alternately opening and closing the aperture. These tests were

intended to measure the response time of the system.

The tests demonstrated that it is difficult without a supervisory computer

controller to coordinate the operation of several independent controllers to

provide effective control over a wide range of inputs. The controllers are single-input and single-output devices, however, the variables to be controlled

are functions of multiple-independent inputs. Control settings that provide

correct operating of the calorimeter under a particular set of environmental

conditions (e.g., day) may leave the system without adequate control under

other conditions (e.g., night). It was found that control settings must be

carefully chosen and periodically revised to minimize deviation from the control

objectives

.

31
5. DESCRIPTION OF TEST ARTICLES

Four test articles were tested in the calorimeter between January and March

1983, including two direct gain fenestrations and two collector-storage wall

modules. The facility is designed so that test articles can easily be changed

within a single day. A typical test lasts from one to three weeks. As previously

discussed, guarded heater panels of various sizes allow the dimensions of the

test aperture to be changed to accommodate components that are not sized to

fill the entire height of the aperture. A previously calibrated test frame is

used with components of nominal 0.91 m by 1.22 m (36 by 48 in.) dimensions.

The frame consists of a 0.15 m (6 in.) thick extruded polystyrene insulation

layer sandwiched between 6 mm (1/4 in.) plywood interior and exterior surfaces.

Any heat flux through the frame is subtracted from the measured value of OneT

by use of the heat-transfer coefficient determined by calibration.

5.1 TRIPLE-GLAZED WINDOW

A triple-glazed window was the first component to be tested. This window was mounted in the test frame described above and consisted of top and bottom

double-glazed exterior sections, with interior single-glazed sections. The

white-enamel aluminum frame was designed with a thermal break to limit

conduction losses and provision was made for sealing against air leaks at all

movable interfaces. The window is of the single hung type with the top sash

fixed and the bottom moveable. An insect screen provided for the bottom half

of the window was not in place during the test. A schematic drawing showing

a vertical section through the window and test frame is shown in figure 26. A

photograph of this test article appears in figure 27.

Because this test article is smaller than the aperture, a guard heater panel was installed blanking out the lower section of the aperture. The test frame was attached to the steel plate at the outside surface of the aperture with wood

screws. A tight fit was achieved between the aperture and the test article by

placing precut rigid foam insulation in the larger gaps and insulating foam

strips in the smaller gaps to minimize both air leakage and extraneous

conduction losses.

Thermocouples made from type-T, 30 gauge wire were mounted at the center of each

of the six glass sections of the window to measure surface temperatures for

future comparison with computer model predictions. To maximize surface contact

with the glass and to minimize the area exposed to solar radiation, the thermocouple lead wires were separated and formed into a 2.4cm (1 in.) diameter

loop. The junction and loop was bonded to the glass with a thin layer of

clear epoxy adhesive. However, despite these precautions the thermocouples

are considered to provide reliable surface temperature measurements only at

night, with daytime readings being less accurate due to the heating effect

caused by solar radiation absorption at the junction and lead wires. A

photovoltaic-cell type pyranometer was also mounted immediately behind and

parallel to the interior glass to directly measure transmitted solar radiation.

Testing of the triple-glazed window began on January 15 and continued through

January 25, 1983. Test results from six of the days are presented in section 6.1.

32
(EXTERIOR)

Figure 26. Schematic drawing of triple-glazed window and test frame

Figure 27. Photograph of triple-glazed window test article

33
Some difficulties were encountered in operating the absorber panel chiller

when ambient temperatures dropped below 0°C, thus only a limited set of data

are available when all system components were operating properly.

5.2 SINGLE-GLAZED WINDOW WITH INSULATING SHUTTERS

The second item tested was a fixed single-glazed window with interior bifold

insulating wood shutters. Construction of the shutters is shown schematically

in figure 28. A photograph of the component mounted in the standard test

frame is shown in figure 29. The shutters are normally closed at night and

during periods of low solar radiation and opened during the daytime to admit

solar energy into a building. Air leakage through the shutters is minimized

by foam weatherstripping between moveable pieces. The fixed single-glazing

was caulked and sealed to the test frame and therefore considered to have

essentially no air leakage. Daytime and nighttime tests for this test article

with shutters open are of value for comparison between measured data and published

ASHRAE data for single-glazing. Tests with shutters closed were interesting

only at night or during very cloudy daytime periods as normal operation would

dictate that they would be open during the day. The test article was installed

in the aperture in a manner similar to that of the triple-glazed window,

using the same guard heater panel and method of edge sealing. One thermocouple

was installed at the center of the glass surface on the test article. The

solar cell pyranoraeter was used to measure transmitted solar radiation when

the shutters were open.

The tests began on January 26 and ended on February 6 producing seven days of

useful data, although only three days were with closed shutters. Test results

are presented in section 6.2. Problems with operation of the chiller persisted

and thus limited the data considered to be useful for determining shading

coefficient. These data were considered adequate for characterization of the

insulating shutters and data collected for the single-glazing without shutters

was considered useful for evaluating the capabilities of the system and instrumentation.

5.3 COLLECTOR-STORAGE WATER WALL

A test article utilizing water as a thermal storage medium was the first of two

collector-storage walls to be tested. Figure 30 shows a photograph of the test

article. This component, referred to as a water wall, consisted of opaque black

plastic tanks, 1.14 (45 in.) high, 1.18 (46 1/2 in) wide and 0.18 (7in.) deep.

Each tank will hold approximately 200 £(53 gal.) of water and is mounted

behind the standard double glazed window. An assembly with two of these tanks

installed in the calorimeter aperture is illustrated in figure 31. In order

to accommodate both tank modules, the upper tank was modified by cutting off the

top 0.30M (12 in.) to allow it to fit within the vertical aperture and the opening

was sealed with a layer of polystyrene insulation to prevent heat loss and water

evaporation

.

Thermocouples were installed on the front and rear surfaces of the water tanks

and at three levels in the water inside each tank. Since the water wall is

opaque, no measurements with the solar cell pyranoraeter were made. Also, the

34
VERTICAL SECTION - INSULATING BIFOLD SHUTTER

Figure 28

Figure 29. Photograph of single-glazed window with insulating shutters

IT"

Insulating test frame —

V

1

Fixed

glassExterior

side

ff

1

; ' Wood mounting frame J | Polyurethane foam strip

Polyisocyanurate insulation with aluminum foil faces

Air gap

3mm(1/8) birch plywood

(both sides)

127(5) (1-5/16)

Hinge

Schematic drawing of single-glazed window with insulating shutters

35
Figure 30. Photograph of collector-storage water wall installed in test frame

Figure 31 Drawing of collector-storage water wall installed in test building

36
absorber panel circulating loop was not used in this test to improve the accuracy

of the Qnex measurement. Qjjet was measured exclusively by the air circulation

loop. The direction of flow of conditioned air was from top to bottom at the

surface of the test article, opposing the predominant direction of free convection.

This situation has the potential for biasing test results, however no quantitative

estimates are available as to the magnitude of this effect.

Testing of the water wall began on February 22 and continued through March 6.

Data from five typical days for this period are presented in section 6.3.

Local wind speed and direction measurements were not available due to damage

to the anemometers.

3.4 COLLECTOR-STORAGE PCM WALL

The collector-storage wall utilizing phase change materials (PCM) as the

storage medium was installed in the aperture behind the standard double-glazed

window. This component consists of five translucent pods, each containing 12

kg (291bs) of calcium chloride hexahydrate. The pods are made from a sealed

fiberglass reinforced polymer and are .41 (16 in.) high, 1.22 (48 in.) wide,

and have a maximum thickness of 0.05 (2 in.). The PCM melts at 27.2°C (81°F)

and has a latent heat of fusion of 190 kj/kg (82 Btu/lb) Figure 32 is a photograph of this component installed in the aperture.

Because the PCM pods were sealed units, special instrumentation was not

installed within the storage medium. Thermocouples attached to the interior

and exterior surfaces of the pods comprised the only direct measurements on

this component. Although the test article has a solar transmittance varying

between 0.15 and 0.25 depending on the melt-state of the PCM, this component

of the energy balance was not measured either with a pyranometer behind the

module or with the calorimeter absorber panel. The small amount of transmitted

radiation was absorbed at the absorber panel and this heat was transferred to

the circulating air. As was the case with the water wall, the forced flow of

circulating air was in the opposite direction of the predominant free convection

flow from the test article.

The testing began on March 8 and ended on March 22. Test results are presented

in section 6.4. Data acquisition problems were encountered later in the

test leaving only five days of useful data.

37
(a) individual module showing solid phase (darker region) and

liquid phase (lighter region)

(b) installation in test building

Figure 32. Photographs of collector-storage PCM wall
6. TEST RESULTS

A large range of detail and sophistication in the data is potentially available

from the calorimeter test results. Different levels of detail would be useful

to different users. Among the possible uses of the calorimeter test results

would be the characterization of component thermal performance, evaluation of

test procedures, computer model validation and basic heat transfer studies.

Only the first utilization is addressed in this report.

At a primary level, a test article can be treated as a "black box” with no special instrumentation applied directly. The net rate of heat transfer,

QNET and the environmental variables are measured with the standard facility

instrumentation. Heat-transfer studies and model validation, however, demand

more specific instrumentation that may be different for each type of component

and for each research objective. The most important results may be the heattransfer parameters which can be used for rating components or performing design

analyses. This implies that the data be reduced to one or two parameters that

will be most useful. Alternatively, those who are developing models to compare

analytical predictions with measured data may be more interested in detailed

data covering a longer period of time. Collector-storage wall components are relatively insensitive to short-term changes in the outdoor environment so to

obtain more meaningful results, the performance testing is carried out over a period of many days. On the other hand, the response of direct gain fenestrations such as windows is essentially instantaneous and, therefore a long

continuous period of measurements is not required.

A general framework for presenting the calorimeter results is as follows:

• environmental data are presented in plots versus time,

• net heat flux data are presented in plots versus time,

• thermal performance parameters are estimated and summaries of data are made

,

• correlations between parameters and environmental variables are made,

where suitable correlations exist.

6.1 TRIPLE-GLAZED WINDOW

The environmental conditions during part of the triple-glazed window test are plotted as a function of time in figure 33 for a three day period in January

1983. Figure 33a shows total solar radiation incident on the vertical southfacing surface varied from mostly overcast conditions on January 17 to nearly

cloudless on January 18. Figure 33b shows the outside ambient temperatures

ranged from -10°C to 2°C (14°F to 36°F) representative of typical winter weather.

Figure 33c shows wind speeds averaged over a 15 minute scan period ranged from

near zero to 3.6 m/s (8.1 mph).

Figure 34 shows the measured net heat transfer rate of the triple-glazed window

closely follows the vertical solar irradiance during the daytime, with maximum

daytime gain of 500W (1700 Btu/h) and maximum nighttime heat loss of 100W

(341 Btu/h).

39
Jan. 16 Jan .17

Noon

W

E 1000

(/) H

I-<

£

Oo

</> \

E

500

5

0 1

WIND SPEED

1 1 1 1 1 1 1 1 1 1 1 20 40

TIME (hrs)

60

Figure 33. Environmental conditions for triple-glazed window

Jan. 16 Jan. 17 Jan. 18

Noon Noon Noon

500

w 400

l-h

< 300

7- 200

UJz

o 100

o -100

0 10 20 30 40 50 60 70 80

TIME (hrs)

Figure 34. Net heat transfer rate for triple-glazed window

40

MPH

°F

BTU/hr

ft
Integrated daily average of the test results are presented in table 1 for

six sampling periods consisting of one complete day and five incomplete days.

Rows I and 2 present the average ambient temperature and wind speed, respectively, for each sampling period. Rows 3 and 4 present the cummulative radiant

solar energy incident on the window and the total heat gain, Qjjet> respectively.

Rows 5 and 6, respectively, resolve the value of QjjeT i nt° a daytime and a nightime component.

The parameters U (overall heat transfer coefficient) and F (dimensionless

solar heat gain coefficient), which are presented in rows 7 to 9 of table 1, are the most important results for a direct gain fenestration. These results

are obtained by expressing equation (1) in terms of the total opaque heat

transfer area A of the window, including the frame and glazing, as:

QNET = (FIT + UAT)A (8)

At a given instant of time, U and F cannot be determined simultaneously

because there are two unknowns and only one equation. However, by using re- gression analysis and a large sample of data, it is possible to determine

U and F based on measured values of OneT, It and AT. At night when I-j = 0.0, U can be independently determined, or during the day if AT = 0, F can

be directly determined.

The data are treated in three groups for the regression analyses. The "night

only" data group provides a good estimate of U but gives no information

about F. The "night and day" group usually estimates U approximately

equal to U from the "night only" group and provides an estimate of F as well.

This group, which uses all available data, is believed to provide the best

estimate of average U and F values. For comparison, the "day only" group

usually provides a significantly lower estimate of U while F is essentially

the same as obtained from the day and night group. No explanation of why the

"day only" U-Values are significantly lower then the "night only" U-Values is

apparant. Theoretical analysis of heat transfer in a solar irradiated window

suggests that the convective and radiative heat transfer coefficients between

each of the glazing layers should increase between night and day, due to

absorption of radiant solar energy in each glazing layer and the resulting

increased surface and air temperatures. The increased heat transfer coefficients

should result in a slight increase in U-Value instead of the substantial decrease,

as observed in the test results.

Figure 35(a) shows a plot of net heat transfer rate as a function of vertical

solar irradiance. It is interesting to note the intersection of the regression

line with the QneT = 0 line, which indicates a threshold radiation level

for useful heat collection at the particular ambient temperature. The high

degree of correlation of OneT with I-p in figure 35(a) as indicated by the lack

of scatter of data points suggests that during the daytime, the dominant heat

transfer mechanism in the window is by direct radiant transmission and that

heat loss to the outside ambient is of much lower significance.

Figure 35(b) shows a plot of nightime U-Values versus measured wind speed, indicating that U-Values tend to increase with increasing wind speed. Correlation

41
Table 1 . Test Results for Triple-Glazed Window

DATE

Jan

16/17

Jan

17/18

Jan

18/19

Jan

19

Jan

23/24

Jan

24/25

Time Period when

date were Available 0915-0600 0615-0600 0616-0245 1600-2300 2015-0600 0615-0130

Average Ambient

Temperature, °C

-3.1 -4.0 -6.7 -5.0 1.5 3.2

Average Wind Speed,

m/s

1.21 0.94 1.93 0.37 0.59 0.83

Total Incident Solar

Radiation, kj

15172. 8839. 22168. 994. — 5399.

QNET Total, kj 1779 . -2706. 3749. -1503. -2027. -1705.

Qnet Day»

^5824. 2077. 7503. 142. — 383.

QNET Night, kj -4045. -4783. -3754. -1645. -2027. -2088.

U Night Only, w/m2 c 2.60 2.73 2.68 2.41 2.21 2.24

Night and day

U(w/m2 °F) 2.53 2.44 2.65 2.20 2.06 2.02

F 0.52 0.49 0.49 0.50 — 0.39

Day only

U(w/m2 °C) 1.69 1.63 2.69 1.49 — 1.62

F 0.49 0.44 0.49 0.50 — 0.36

Correlation Coeff., QNET

vs. I T

0.99 0.98 0.99 0.99 — 0.89

Correlation Coeff., U vs. Wind Speed

0.16 0.48 -0.39 0.05 -0.11 0.39

Correlation Coeff., U vs. Temperature

0.23 -0.26 -0.19 0.00 0.40 0.00

42
U-VALUE,

watts/m

(Btu/hr ft 2 ) 100 200 300

s:\

3

4-1 03

(a) Net heat flux vs total solar irradiance

1.0

MILES/hr

2.0 3.0 4.0 5.0

0.8

u.o

0.6

0.4

(b) U-Value vs wind velocity

Figure 35» Correlations for triple-glazed window

43

Btu/hr

-ft
coefficients between Q^ET and U and the environmental conditions are presented

in table 1. These coefficients show that Q^et i s highly correlated with I^,

as might be expected, and a weak dependence of U-Value on wind speed and ambient

temperature is noted.

The calorimeter test results contrast with previous laboratory test results

[15] for the same test article. In the laboratory tests, the lower sash at

the window was covered by an insect screen, which was not present during the

calorimeter tests. The laboratory test results showed a U-Value of approximately 1.68 w/m2 °C (.30 Btu/hr/f

t

2 ° F) at zero windspeed and -8°C (18°F)

ambient temperature as compared to a U-Value of 2.3 w/m2 °C (.41 Btu/hr/f

t

2 ° F)

determined by extrapolation of the field test data to zero windspeed. Most

likely the insect screen which covered approximately one-half the window area,

contributed to the lower U-Value measured in the laboratory. Another possible

cause of the difference between the field and laboratory test results is radiation loss to the cold night sky, which is absent in the laboratory test.

Another comparison can be made based on winter design U-Value in the ASHRAE

Handbook [5]. ASHRAE U-Value data varies between 1.90 and 2.50 w/m 2 °C, (0.33

and 0.44 Btu/hrf

t

2 ° F) , based on adjustment factors for a metal frame with a thermal break. Additional adjustment of these data to convert from design

wind speeds of 6.7 m/s (15 mph) to still air conditions provides an estimated

range of U-Value between 1.7 and 2.1 w/m2 °C, (0.30 and 0.37 Btu/hrft 2 °F) which

is approximately 9 to 26 percent lower than the calorimeter measurements.

6.2 SINGLE-GLAZED WINDOW WITH INSULATING SHUTTERS

Table 2 summarizes daily performance for five days of data available for the

single-glazed window with insulating shutters. Data for Jan. 27, and Feb. 4

to 5 are with the shutters open and data for Feb. 1 and 2 are with the shutters

closed. The first row of the table shows hours during which useable data are available. These data are arranged in the format previously described for the

Triple-Glazed window. Note that complete 24 hour periods are not available

for this test article so that daily total values are not directly comparable.

The most useful estimates of U-Value for these data are shown for the

"Night Only" case. These estimates range from 3.92 to 5.42 w/m2 °C (0.69 to

0.95 Btu/hrft 2 °F) for single-glazing with shutters open and 0.80 to 0.99

w/m2 °C (0.14 to 0.17 Btu/hrft 2 °F) with shutters closed. These can be compared

with ASHRAE U-Values for single glass ranging from 6.2 w/m2 °C (1.09 Btu/hrft 2 °F)

for winter design conditions with 6.7 m/s (15mph) wind to 4.9 w/m 2 °C (0.86

Btu/hrft 2 °F) for still air. Previous laboratory measurements of this component

with closed shutters indicated a U-Value of 0.90 w/m2 °C (0.16 Btu/hrft 2 °F)

[15].

The highest measured solar heat gain coefficient F for single glass was 0.59

which is significantly lower than the ASHRAE value of 0.87. The size of

this sample of data, however, is relatively small and no definitive conclusions

are drawn. The correlation coefficients presented in the last three rows of

the table show similar trends to those for the triple-glazed window.

44
Qnet is highly correlated with irradiance and again there is no general re- lationship between U-Value and wind speed. There is some negative correlation

between ambient temperature and U-Value when no insulation is used.

6.3 COLL ECTOR- STORAGE WATER WALL

Figures 36 and 37, respectively, show plots of climatic conditions and water

wall thermal performance for a four day sequence in early March of 1983.

As shown in figure 36(a), the first day was completely overcast, followed by

two very bright days and then by a partially overcast day. Figure 37(a) shows

the net rate of heat transfer between the water wall and the metering chamber.

Delivery of stored solar energy during the night is noted and there is an approximate five hour time lag between peak solar irradiance and peak heat

transfer rate. Figure 37(b) shows the temperature distribution on the outer

and inner surfaces and in the water at mid-height of the upper module. The

lower module displayed similar temperature distributions. Vertical temperature

(not shown) was also measured in both modules. The maximum vertical temperature

difference in the lower module was approximately 6°C (11° F) in contrast to a maximum difference of 3°C (5.4°F) measured in the upper module. The difference

in vertical temperature distribution between lower and upper modules was apparantly due to the foreshortened upper module used in the test.

6.4 COLLECTOR-STORAGE PCM WALL

Test data for the collector-storage PCM wall are summarized in figures 38 and 39.

Solar irradiance at the vertical aperture, and ambient temperature are plotted

in figures 38(a) and 38(b), respectively, for a three day period in mid-March

1983. Ambient air temperature varied between 1 and 13°C (34 and 55° F) and sky

conditions varied from mostly clear on day one to overcast on day three. It was observed that the interior surface temperature of the test-article never reached the PCM melting point of 27.2°C, (81°F) indicating that the PCM did

not completely melt.

Figure 39(a) shows a plot of Qnet and figure 39(b) shows plot of the temperature

difference, Ty^LL -^AIR» during the measurement period. Delivery of stored

energy at night is observed, however, almost no time-lag existed between the

time of peak solar irradiance and peak heat transfer rate. That suggests that

direct transmission of solar energy is the dominant heat transfer mechanism for

the translucent PCM module, and this test article has less effective thermal

storage than the water wall. It was also observed that there are some periods of

time when OnET i s negative and temperature difference is positive. This suggests

the possibility of significant thermal losses through the aluminum support

frame provided by the manufacturer.

Unseasonably warm ambient temperatures and difficulties with the data

acquisition system resulted in the termination of data gathering for the field

test. However, the test article remained in place in the calorimeter and was visually observed for three additional months. One observation of significance

was that leakage of liquid (probably a solution of CaCl2 in water) occurred at

the fill ports of several of the PCM modules. During the entire three month

45
Table 2. Test Results for Single-Glazed Window with Insulating

Shutters

Date Jan 27 Feb 1/2 Feb 2/3 Feb 4/5 Feb 5

Time Period Data were Available 1245-2345 1845-0545 0615-0545 1545-0600 0615-2245

Average Ambient Temperature, (°C) 2.7 2.9 11.1 -4.0 -1.5

Average Wind Speed, (m/s) 0.66 0.75 2.66 2.66 0.51

Total Incident Solar Radiation, (kJ) 7333. — — 394. 21334.

QNET Total, (kJ) 11. -919. -714. -6424. 4124.

Qnet Day, (kJ) 2711. 955. 6897.

QNET Night, (kJ) -2700. -919. -714. -7379. -2773.

U Night Only (w/m2 °C) 4.55 0.80 0.99 5.42 3.92

Day and Night

U (w/m2 °C) 4.28 0.80 1.00 5.50 3.76

F 0.59 0.26 0.56

Day Only

U (w/m2 °C) 2.00 — 1.16 5.14 3.03

F 0.50 0.26 0.52

Correlation Coefficient, Qnet vs. I t 0.92 0.38 0.88

Correlation Coefficient, U vs. Wind Speed

-0.59 0.42 0.07 -0.09 0.94

Correlation Coefficient, U vs. Ambient Temperature

0 H 1 -0.09 o

r*“4 01 -0.46 -0.63

46
Figure

Figure 37.

1000

800

co 600

>-<

5 400

200

0

1 —I 1

a) VERTICAL

- SOLAR J IRRADIANCE

11

1— > _L_ JJL

300

- 200 ~

100 5

DAY NUMBER

36. Environmental conditions - collector-storage water wall

DAY NUMBER

Net heat transfer rate and temperatures - collector-storage

water wall

47
observation period, neither complete melting nor complete solidification were

ever observed. The PCM material separated into solid and liquid phases which

never recombined.

6.5 CORRELATION NEEDS FOR COLLECTOR- STORAGE WALLS

In contrast to the non-energy storing test articles such as the Triple-Glazed

Window, standard methods of correlation for collector-storage walls do not yet

exist, therefore, generalization of the test results presented here for the

water wall and for the PCM wall are not possible. One of the potential uses

for thermal performance data for energy storage devices such as those tested

in the NBS calorimeter is to provide a correlation data base. Techniques using

thermal response coefficients to normalize collector-storage wall test data have

been described [13]. Frequency response techniques [14], have also been suggested as a means of generalizing dynamic test data for passive solar buildings,

and have been proposed for validation at the component level. Based on carefully controlled test conditions and accurate heat transfer data, it is believed

that the validity or limitations for both the thermal response coefficient and

the frequency-response concepts can be established. Future testing of collectorstorage walls is planned for the NBS calorimeter in order to provide such a data base.

48
Figure 38. Environmental conditions - collector storage PCM wall

1000

50

0

co

8

4

0

O

-4

1 2 3

DAY NUMBER

Figure 39. Net heat transfer rate and AT - collector-storage PCM wall

49
7. EVALUATION OF INITIAL OPERATION OF NBS PASSIVE SOLAR CALORIMETER

Operation of the calorimeter during the first three months of 1983 demonstrated

the ability of this test facility to perform calorimetric measurements on various types of passive solar components. The facility was generally able to

maintain controlled interior conditions while test articles were exposed to

the actual exterior environment. Measurements made with the calorimeter can

provide insight into basic heat transfer mechanisms as well as detailed information on the performance of specific components. However, several operational

and measurement problems were noted during these tests, which could significantly affect the accuracy of the results. The nature of some of these problems

is discussed and possible solutions are recommended in the following paragraphs.

One of the design features of the facility is the capability to measure net

heat flux and to resolve this into radiant and convective components. To

accomplish this, average absorber temperature and average air temperature

inside the metering chamber must be equal within very small tolerances. The

present control system relies upon having the air and absorber temperature

control signals operate on two separate controllers, which does not tend to

enforce the equality of air and absorber temperatures. Steady-state temperature difference bewtween values for T^ir and T^g were observed to vary

between 0.0 and 1.0°C as environmental conditions slowly changed, due to the

different response characteristics of the air and absorber panel fluid

conditioning loops. This temperature control problem limits the capability

to resolve radiative and convective components of heat flux, however, it does

not appear to have any significant impact on overall measurement accuracy.

The recommended improvement is to measure temperature difference between the

air and absorber panel with a differential thermopile and to control the air

and absorber conditioning loops to minimize this temperature difference. It

is anticipated that this can be accomplished by using a micro-computer and by

making some component changes in the fluid conditioning loops.

Another problem in distinguishing between radiative and convective fluxes

arises if any terms of the energy balance equation can not be associated

exclusively with either the air stream or absorber panel. In the present

system, the fan is powered by belt-driven motor that is located in the space

behind the absorber panel, which is not in the conditioned air flow stream.

Because the motor is not cooled directly by air from the circulating loop, an unknown fraction of the fan energy is probably radiatively transferred to the

absorber panel. This does not affect the overall energy balance of the calorimeter, but it constitutes an uncertainty for the resolution of radiation and

convection. The recommended solution to this problem is to change the fan to

a direct-driven fan, thereby eliminating any possible heat transfer to the

absorber panel.

It has not yet been determined by experiment whether the forced circulation of

air causes any disturbance in the natural convection heat transfer from test

articles. In testing fenestration systems having high overall thermal resistance, the temperature difference between the interior surface and adjacent

air is small. This suggests that convective heat flows might easily be disturbed by fan forced air. However, in that case, the film resistance is a

50
small part of the total resistance and thus the measurement error incurred is

small. The more important situation arises in testing thermal storage walls

which rely upon free convection to transfer heat to the interior. In that

case, the calorimeter's forced flow of air may cause significant change, especially if the natural convective flow reverses direction over a day and the

forced flow does not. It is recommended that air flow velocities be set as low as possible while still retaining adequate control and that provision be

made for automatically reversing forced flow direction when the free convection

flow changes.

It was apparent from the error analysis of section 4.1 that the measurement of

Qabs was t ^e greatest single source of error. QabS varies over an extremely

wide range from more than 600W(2050 Btu/h) of heat rejection on a typical

winter day to less than 30W(100 Btu/h) of heat addition on a typical winter

night for the same test article. The specific problem is that the mass flow

rate of water through the absorber is constant, producing AT’s of 1 to 2°C

(2 to 4°F) during daytime radiation but less than 0.1°C (0.2°F) at night. One

obvious solution for this problem is to control the flow rate to a range which

provides a reasonably measurable AT. This might be accomplished by using

a multiple-speed pump or a throttling valve. It is anticipated that flow rate

changes would be infrequent, perhaps changing only twice daily. Another

possible solution would be to install electrical heaters on the absorber panel

rear surface. The test facility would be operated so that the conditioning

loop controls the absorber panel during the day with the heaters shut off and

the heaters control the panel temperature during the night, with the conditioning loop shut off.

51
8. USE OF THE TEST RESULTS FOR EVALUATING STANDARD TEST PROCEDURES FOR WINDOWS

One of the original objectives proposed for the NBS Passive Solar Calorimeter

was in the development of outdoor test procedures for measuring thermal performance of passive solar components. A description of existing laboratory test

procedures for thermal performance measurements of windows was presented in a previous report [1], and recommendations were made for comparison between

laboratory and field measurements. The NBS calorimeter was proposed as the

test facility to perform the field measurements and this use was a primary

factor motivating construction. Laboratory measurements of U-value had

previously been made for the two fenestration systems (Triple-Glazed Window

and Single-Glazed Window with Insulating Shutters) described in this report.

The laboratory test results for the Triple-Glazed Window, which displayed

considerable sensitivity to wind velocity and wind direction, and the SingleGlazed Window with Insulating Shutters, are presented in reference 15. The

original plan was to compare the laboratory data with the field test data from

the calorimeter, and to recommend one of two alternative laboratory testing

methods as being preferred. The choice would be based on the laboratory test

results that best correlated with the test results from the NBS Calorimeter.

After completion of the analysis of the field test results for the TripleGlazed Window, it was concluded that there are insufficient data to make such a determination. First, it was determined that the test article had a slightly

different configuration between the laboratory tests and the field test.

During both of the laboratory tests, the lower window sash was fitted with an

insect screen, which inadvertantly, was not installed during the field test.

The presence of an insect screen on the exterior of a window has an unknown,

but possibly significant, effect on both the air flow rate and convective

heat transfer coefficient at the glass surface, as well as on the radiative

heat transfer between the outer glazing surface and the environment. (Personal

observations of frost patterns on double-glazed windows with partial insect

screen covers often show significant frost buildup on the unscreened portion

with litle or no frost on the adjacent screened section.) A second problem

was that wind velocity measured during the field test never approached the

6.7 m/s (15 mph) conditions imposed during the laboratory tests. Most often

the measured wind velocities were below 3 m/s (5 mph). At those low wind

conditions, difference between the two laboratory test results was not significant, therefore the field test results are inconclusive for making a selection

between alternative laboratory tests.

A third problem in using the calorimeter test results for windows as a basis

for comparison with laboratory test results is the inability to control

climatic conditions, such as wind, cloud cover, ambient temperature and solar

radiation. This is very evident in the observed scatter of the data. Balcomb

[16], reported a large degree of scatter in measuring nighttime U-Value for a double-glazed window installed in an outdoor test cell having a collector-storage

wall. In addition to wind, he concluded that there are other effects which

lead to variations in the glazing U-Value, such as changes in the heat transfer

mechanisms between the storage wall and glazing. It is evident that data

must be taken over much longer time periods then originally planned, possibly

52
an entire month, to have sufficient variation in the climatic conditions to

enable meaningful attempts at correlating U-Value with climate variables.

In subsequent work, it became apparent that at least three other test methods

are reported as useful for measuring U-Values for windows. Since those test

methods and/or test facilities were not considered in the original evaluation

of laboratory methods and there is no statistical significance to the limited

number of samples tested, it appears inappropriate to draw conclusions and to

make any recommendations at the current time.

Based on discussions with researchers from several universities and government laboratories, fenestration manufacturers, testing laboratory operators,

and information obtained by participation in standards committees, the proper

method of simulating wind in laboratory testing of windows appears to be unresolved. Furthermore, since most fenestration systems display a significant

variation in U-Value with exterior surface coefficient, the appropriateness

of testing windows at a single specified wind velocity of 6.7 m/s (15 mph) is

open to question. In addition, the variation in fenestration U-Value due to

variations in ambient temperature that occur between summer and winter might

also be significant, especially with regard to predicted energy performance

for passive solar buildings with large glazing areas.

It appears that a testing procedure which produces U-Value data appropriate

both for seasonal energy calculations and for performance ratings would be of

greater value than that available from the current practice, which provides

data for a single, winter-design condition. It is obvious that substantial

research will have to be undertaken to resolve such issues as the effects of

wind velocity and direction, sky and ground temperature, and solar irradiance

on the field behavior of fenestration systems. It is proposed, however that a procedure for performance testing of fenestration systems be prepared which

address at least some of the needs of the potential users of these data. With

these limited objectives in mind, an outline of a standard procedure for measuring and reporting U-Values of fenestration systems has been prepared and

presented in appendix A.

53
9 . NOMENCLATURE

Cp specific heat, kj/kg °C (Btu/lb°F)

e electromotive potential, volts

F solar heat gain coefficient, dimensionless

Fref reference glazing solar heat gain coefficient, dimensionless

I T total solar irradiance normal to fenestration, W/m^ (Btu/hrft^)

M mass flow rate, kg/sec (lb/hr)

P pressure, pascals (in. H2 O)

Qabs rate of heat removal in absorber panel (Eq. 3), W(Btu/hr)

Qcc rate of heat removal by air heat exchanger (Eq. 4), W(Btu/hr)

Qfan fan power, W(Btu/hr)

Ohc rate of heat added by electrical heaters (Eq. 5), W(Btu/hr)

Ols rate of heat transfer between metering chamber and room air, W(Btu/hr)

QnFp net rate of heat transfer through window or test article, W(Btu/hr)

O O q N ET heat transfer rate per unit area (Eq. 1), W/m (Btu/hrft )

R electrical resistance, ohms

T temperature, °C (°F)

t time, sec

U overall heat transfer coefficient, W/m2°C ( Btu/hrf t^ 0 F)

(UA)mc overall conductance of metering chamber, W/°C (Btu/hr°F)

V velocity, m/ s (mph)

A difference

SUBSCRIPTS

ABS absorber panel

AIR metering chamber air

54
cc cooling coil

HC heating coil

RM room air

i inside air

o outside air

55
10 . REFERENCES

1. M. McCabe, W. Ducas , M. J. Orloski, and K. N. Decorte. "Passive/Hybrid

Solar Components - An Approach to Standard Thermal Test Methods," NBSIR

81-2300, July 1981.

2. D. J. Vild, "Solar Heat Gain Factors and Shading Coefficient" ASHRAE

Journal , 1964.

3. E. A. Farber et al., "Theoretical Analysis of Solar Heat Gain Through

Insulating Glass with Inside Shading," ASHRAE Journal , 1964.

4. C. W. Pennington et al. , "Experimental Analysis of Solar Heat Gain Through

Insulating Glass with Indoor Shading," ASHRAE Journal , 1964.

5. ASHRAE Handbook — 1981 Fundamentals , (Atlanta: ASHRAE, 1981).

6. Private communication with John Yellott, President, John Yellott

Engineering Associates, Inc.

7. S. Selkowitz, C. Lampert, and M. Rubin, "Advanced Optical and Thermal

Technologies for Aperture Control," Proceedings of the Passive and Hybrid

Solar Energy Update , Washington, D.C., September 1983.

8. F. Moore, "Passive Solar Test Modules," Passive Solar Journal , Vol. 1,

No. 2, Spring 1982.

9. M. McCabe, S. T. Bushby, and W. Ducas, "Conceptual Design and Performance

Specification for the Passive Solar Component Calorimeter," letter report

from NBS to DoE, March 1982.

10. M. McCabe, S. Robinson, and J. LeCourt, "Calorimeter Test Facility for

Measuring Thermal Performance of Passive/Hybrid Solar Components,"

Proceedings of ASHRAE/PoE Conference of Thermal Performance of the Exterior

Envelope of Buildings II , Las Vegas, Nevada, December 1982, pp. 678-686.

11. K. W. Lindler, "National Bureau of Standards Passive Solar Test Building

Handbook," NBS-GCR 82-398, August 1982.

12. "SINDA - Systems Improved Numercial Differencing Analyzer," available from

COSMIC, Uiversity of Georgia, Athens, 30602.

13. W. Kennish, M. Ahmed, M. McCabe, annd M. McKinstry, "Determination of Thermal

Performance Characteristics of Modular Passive Solar Storage Walls,"

Proceedings of the 5th National Passive Solar Conference , Vol. 5.2, October

1980.

14. Subbarao, K. , "Determining Passive Building Dynamics from Performance Data"

AIChE 38th Annual Heat Transfer Conference, Seattle, Washington, July 1983.

56
15. Ducas, W., McCabe M. , Cholvibul, R. , and Wormser, P., "U-Value Measurements

for Windows and Movable Insulations from Hot Box Tests in Two Commercial

Laboratories," draft paper submitted to ASHRAE.

16. Balcomb, J. D., "Dynamic Measurement of Nighttime Heat Loss Coefficients

Through Trombe Wall Glazing Systems," Proceedings of The Sixth National Passive

Solar Conference, Vol. 6, September 1981.

57
11. ACKNOWLEDGMENT

The authors wish to thank all persons who contributed to the design, construction,

and operation of the test facility. In particular, continued financial and

management support from the Passive and Hybrid Solar Energy Division, Office

of Solar Heat Technologies, U.S. Department of Energy, is acknowledged. The

original conceptual design of the facility was the result of discussions with

John Jenkins. Steve Bushby provided the refined engineering calculations using a thermal model. The final design and construction drawings drew heavily upon the

engineering and architectural services provided by the Plant Division of NBS,

in particular, Jim Hyatt and Ken DeCorte. The test facility was constructed

by engineering technicians in the Center for Building Technology, in particular,

Donn Eberts, Frank Rankin, and Walt Ellis, with substantial assistance from

the undergraduate students, Allan Swim, Jenifer Cooley, Michael O’ Boyle, and

Susan Reiley. William Ducas assisted in the design of the instrumentation and

selection of sensors. Graduate students Susan Robinson and John LeCourt assisted

in development of operational procedures and in the initial operation of the

facility

.

John Yellott, Professor Emeritus at Arizona State University and designer,

owner, and operator of a similar calorimetric test facility, provided a very

useful review of the conceptual design and proposed instrumention, calibration,

and operating procedures. In addition, John Yellott, William Brown of the

National Research Council Canada, and Joseph Klems of Lawrence Berkeley

Laboratory, each provided outside peer review and substantial commentary on

the draft manuscript of this report, for which we are indebted. Finally,

substantial technical review and management support from within NBS came from

Kent Reed, Robert Jones, and Robert Dikkers.

58
Appendix A

Outline of Draft Standard Procedure for Measuring and Reporting

U-Value for Fenestration Systems

Background

Calculation of the rate of heat transfer through fenestration systems, such

as windows, patio doors, and skylights commonly performed in building energy

analysis, is often based on data given in the ASHRAE Handbook of Fundamentals

for U-Value and Shading Coefficient. Those data are based on two sets of

extreme conditions which never occur simultaneously. Therefore, their application for the design of residential and commercial buildings with large areas

of fenestration, presents the possibility of substantial error in energy performance prediction, and poor selection between alternative fenestration systems.

A fenestration system usually consists of a glazing unit installed in an opaque

frame. In the absence of solar radiation, the transfer of heat through a fenestration system consists of components of energy convected and radiated

through the glazing unit, conducted through the frame, and transferred by

infiltration at joints and seals. Since convective and radiative heat transfer

across enclosed air spaces are temperature-dependent processes, variation in

the heat transfer rate through the glazing component can occur due to variations

in ambient temperature. In addition, significant variation in the heat transfer

rate can occur due to wind-induced convection at the exterior surface. Analysis

of conductive heat flow through framing members is often difficult because of

complex framing geometry, the presence of dissimilar materials with unknown

values of contact resistance, and the possibility of thermal bridges at the

interface with the glazing unit. Predictive methods for analyzing thermal

behavior of fenestration systems would be limited to a few relatively simple

frame configurations which would probably be inadequate for the majority of

available systems.

The preferred alternative to establishing thermal performance ratings of

fenestration systems is measurement of U-Value in laboratory test facilities in

which outdoor conditions are simulated. Although a number of different laboratory

procedures for testing of building thermal envelope components are available,

no consensus currently exists as to which procedure is most suitable for

measuring the U-Value of fenestration systems. Comparison between two existing

test procedures conducted in two commercial testing laboratories, revealed

substantial differences between measured U-Value for a triple-glazed window.

These differences were attributed to the different techniques used by each

laboratory to simulate wind.

It is evident that a standard procedure is required for measuring and report imthe U-Value of fenestration systems, which includes the sensitivity to variations

in the ambient air temperature and the convective heat transfer coefficient .it the exterior surface. The following outline describes a proposed standard

procedure that will provide useful thermal performance data for many types >:

59
fenestration systems. It is anticipated, however, that a number of research

tasks will be required to validate or modify the proposed standard, prior to

its acceptance by consensus standards writing organizations.

Objective: To define a standard procedure for measurement of U-Value for

fenestration systems

Scope

The procedure is limited to fenestration systems including windows, patio doors,

and skylights in which the glazing component consists of either; (1) a singleglazing layer, or (2) a multiple-layer, factory-sealed glazing unit (SGU),

either of which must have one surface exposed to outdoor ambient conditions

and the other surface exposed to indoor conditions.

Approach:

U-Value measurements shall be performed on individual SGU’s and on fenestration

systems containing a SGU installed in a frame.

Apparatus

Test data shall be obtained on test articles at steady-state conditions in

calibrated hot boxes such as described in ASTM C976 or in guarded hot boxes

such as described in ASTM C236. A heat flow meter ( HFM) shall be used to measure both the exterior and interior surface heat transfer coefficients at

the specified test conditions.

Test Conditions:

A range of outdoor conditions shall be simulated in the outdoor chamber of the

hot box, varying between summer and winter temperatures and between forced and

free convection on the exterior surface. Solar radiation, air infiltration, or moisture transfer will not be included. Indoor test conditions in the metering

chamber shall be maintained constant at 21.1 + 1.1°C (70°F + 2°F), with natural

convection conditions maintained at the interior surface of the test article.

Relative humidity shall be measured and controlled to a value such that no moisture condensation occurs on any surface in the metering chamber.

The SGU and the HFM are installed in the normal orientation, with vertical

orientation for windows and doors, sloped or horizontal orientation for skylights. The proposed exterior test conditions for the SGU, the HFM and the

fenestration assembly are shown in Table A-l. Ifypothetical test results for

the SGU at the proposed test conditions are plotted in Figure A-l. It is

assumed that statistical analysis of the measured data for each SGU can be

used to produce a repeatable functional relationship between the overall

conductance-area product (UA), ambient temperature, and exterior surface heat

transfer coefficient.

60
The overall UA of a fenestration system shall be measured utilizing a representative SGU installed in a standard size frame assembly at the test

condition specified in Table A-l. All joints and moving surfaces shall be

sealed with tape at the interior to exclude possible air infiltration.

Removable insect screens or sunshading devices that are not an integral part

of the SGU shall be removed prior to testing. The contribution of the frame

conductance to the overall UA of the system shall be determined by subtracting

that of the SGU from that of the system. Heat transfer through the frame is

assumed to be essentially conductive and therefore not sensitive to variation

in exterior convection and ambient temperature.

Heat Flow Meter ( HFM)

The HFM consists of a sealed and laminated, three-layer assembly having its

outer and inner layers identical in material, thickness, and surface treatment

to those of the SGU. The middle layer consists of a 6.3 mm (1/4 inch) layer of

aged polystyrene or polyurethane insulation. The size and shape of the HFM

shall be similar to that of the exterior surface of the SGU. The thermal

conductance of the HFM or a representative sample thereof shall be measured in

a guarded hot plate apparatus similar to that of ASTM C177, at the same surface

temperatures as those used in the measurement of exterior surface coefficient.

Arrays of 25 calibrated thermocouples are installed on opposite sides of each

surface of the HFM at locations corresponding to approximately equal areas, and

the average surface-to-surface temperature difference is measured during each

calibration test. Similar arrays of 25 calibrated thermocouples are installed

in the free stream on both the exterior and interior side of the HFM and the

average surface-to-air temperature differences are measured. These data are used to determine the conductive heat flow rate and the average exterior surface

and interior surface heat transfer coefficient at each test condition specified

in Table A-l.

Performance Rating:

The thermal performance of a fenestration system is the sum of the glazing

heat flow component and frame heat flow component as follows:

UAsys( to> ho) = UApR + UAgGU (T0 , ho)

It is anticipated that thermal performance measured in tests on a standard size

fenestration system can be scaled for systems having smaller or larger dimensions

than those tested. Therefore, these results will be assumed applicable to

other fenestration systems containing an identical SGU and for similar framing

and sash components, provided the overall projected area of the system is not

less than 50 percent or greater than 150 percent of that of the actual conf igur it i m

tested

.

61
SGU

OVERALL

HEAT

TRANSFER

COEFFICIENT-AREA

UA

Table A-l. Proposed Test Conditions for Sealed Glazing Unit and Fenestration

System

« 5

Test No.

Test

Article

Text

(°C) (°F)

Exterior

Convection

Coefficient

1 A SGU 32 (90) Free

IB SGU -8 (18) Free

2 A HFM 7 (45) Free

2B SGU 7 (45) Free

3 A HFM 7 (45) Forced low speed

0.5±0. 1 5 w/m 2 °C

3B SGU 7 (45) (2.8±0.8)Btu/hrft 2oF

4 A HFM 7 (45) Forced high speed

1.0+0. 2 w/m 2 • °C

4B SGU 7 (45) (5.7± 1.1)Btu/hrft 2 °F

5 Fenestration

system

7 (45) Free

Figure A-l. Hypothetical performance of sealed glazing unit

62
NBS-114A IREV. 2 -60 ) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

SHEET (See instructions)

REPORT NO.

NBSIR 84-2920 October 1984

4. TITLE AND SUBTITLE

THERMAL PERFORMANCE TESTING OF PASSIVE SOLAR COMPONENTS IN THE NBS CALORIMETER

5. AUTHOR(S)

Michael E. McCabe, Charles E. Hancock, Marc Van Migom

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

U.S. Dept, of Energy

Division of Passive and Hybrid Solar Energy

Forrestal Building

Washington, D.C. 20585

10. SUPPLEMENTARY NOTES

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

11.

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

bi bl iography or literature survey, mention it here)

Studies of the thermal performance of passive solar buildings have indicated a need

for precise measurement of solar heat gain and thermal heat loss or gain for modular

passive/hybrid solar components in the outdoor environment. A description of the

design, calibration, and initial operational results for a new calorimetric test

facility designed to perform these measurements is presented in this report. The

test facility is located at the National Bureau of Standards in Gaithersburg, MD,

and it is anticipated that it will provide a substantial improvement in the measuring

techniques for passive and hybrid solar components over the field test cells currentl>

in use. Thermal performance data were taken for four passive solar test articles

during the winter of 1982-1983, including two windows and two collector storage walls.

Test results are correlated as U-values and Shading Coefficients for the two windows.

Operational problems with the test facility are described and improvements suggested.

The application of the test results for evaluation of standard laboratory test

procedures for windows are discussed. A draft procedure for measuring and reporting

U-Values for windows based on laboratory testing is presented.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by • Calorimeter; fenestration test methods; insulating shutters; passive solar; phasichange storage wall; shading coefficient; solar energy; U-value; water stor.i,-. v.ill

13. AVAILABILITY

'

[}£] Unlimited

QH For Official Distribution. Do Not Release to NTIS

PH Order From Superintendent of Documents, U.S. Government Printing Office, Washington. D.C

~ 20402.

[X~] Order From National Technical Information Service (NTIS), Springfield, VA. 2216 I

14. NO. OF

PRINTED PAGES

70

15. r $10.00

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