NBS BUILDING SCIENCE SERIES 109

Simplified Analysis of Thermal

and Lighting Characteristics

^P0m Windows: Two Case Studies

.U58 {mj

NO . 109*ARTMENT OF COMMERCE • NATIONAL BUREAU OF STANDARDS o. 1978

C.2
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards* was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and

technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to pro- mote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Institute for Computer Sciences and Technology, the Office for Information Programs, and the

Office of Experimental Technology Incentives Program.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consist- ent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific conununity, industry,

and commerce. The Institute consists of the Office of Measurement Services, and the following center and divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Center for Radiation Research — Laboratory Astrophysics^ — Cryogenics' — Electromagnetics' — Time and Frequency".

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational insti- tutions, and Government; provides advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials, the Office of Air

and Water Measurement, and the following divisions:

Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services developing and promoting the use of avail- able technology; cooperates with public and private organizations in developing technological standards, codes, and test methods; and provides technical advice services, and information to Government agencies and the public. The Institute consists of

the following divisions and centers: Standards Application and Analysis — Electronic Technology — Center for Consumer Product Technology: Product

Systems Analysis; Product Engineering — Center for Building Technology: Structures, Materials, and Safety; Building

Environment; Technical Evaluation and Application — Center for Fire Research: Fire Science; Fire Safety Engineering.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition, and effective utilization of automatic data processing equipment; and serves as the principal focus wthin the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer

languages. The Institute consist of the following divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE OF EXPERIMENTAL TECHNOLOGY INCENTIVES PROGRAM seeks to affect public policy and process

to facilitate technological change in the private sector by examining and experimenting with Government policies and practices in order to identify and remove Government-related barriers and to correct inherent market imperfections that impede

the innovation process. THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS; promotes the development of the National Standard Reference Data System and a system of in- formation analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services

to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following organizational units:

Office of Standard Reference Data — Office of Information Activities — Office of Technical Publications — Library —

Office of International Standards — Office of International Relations.

^ Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

" Located at Boulder, Colorado 80302.
WATIONAli BURKAU

OF STANDAEDa

UJiUAKY

NIAK Z I 1978

NBS BUILDING SCIENCE SERIES 109 /y

Simplified Analysis of Thermal

and Lighting Characteristics

of Windows : Two Case Studies

Tamani Kusuda

Belinda Lowenhaupt Collins

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Sponsored by:

Energy Research and Development Administration Washington, D.C. 20545 and

U.S. Department of Housing and Urban Development

Washington, D.C, 20410

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

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistartt Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued February 1978
Library of Congress Catalog Card Number: 77-600071

National Bureau of Standards Building Science Series 109

Nat. Bur. Stand. (U.S.), Bldg. Sci. Ser. 109, 113 pages (Feb. 1978)

CODEN: BSSNBV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1978

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

(Order by SD Catalog No. C13.29/2: 109). Stock No. 003-003-01892-9 Price $2.75

(Add 25 percent additional for other than U.S. mailing).
PREFACE

The work covered in this report has been conducted within the framework

of a National Bureau of Standards (NBS) Interdisciplinary Research

project on the energy-related performance of windows. This effort has

been supported in part by NBS and by the Energy Research and Development

Administration (Mode 2 of Contract E(49-l) 3800), jointly with the

Department of Housing and Urban Development (Contract No. RT 19 3012) as

a portion of the Building Energy Performance Standards Program,

111

J
SI CONVERSION UNITS

The units and conversion factors given in this table are in agreement

with the International System of Units or SI system (Systeme International

d 'Unites). Because the United States is a signatory to the 11th General

Conference on Weights and Measures which defined and gave official

status to the SI system, the following conversion factors are given.

Length

1 in = 0.0254* meter

1 ft = 0.3048* meter

Area

2 -4 2

1 in = 6.4516* x 10 meter

1 ft^ = 0.0929 meter^

Volume

3 -5 3

1 in = 1.638 X 10 meter

1 gal (U.S. liquid) = 3.785 x 10~^ meter^

-3 3

1 liter = 1.000* x 10 meter

Energy

3

1 Btu (International Table) = 1.055 x 10 joule

Power

1 Btu/hr = 0.2930 watt

Temperature

°C = ~ (°F - 32)

9

Illumination

1 ft candle = 10.76 lux

*Exactly

iv
TABLE OF CONTENTS

Page

1.0 Introduction 1 1.1 Purpose 1 1.2 Scope 2

2.0 Relation of Windows to Total Energy Consumption 5 2.1 Relationship to Building Heating/Cooling System 5 2.2 Window Heat Gain/Loss Analysis 6

2.3 Daylight/Artificial Light Trade-Off 6

3.0 Analytical Approach 9

3.1 Computer Model Description 9

3.1.1 Office Module 10

3.1.2 Residential Module 10

3.2 Thermal Calculations 13

3.3 Daylight Calculations 15

4.0 Comparison Conditions for Computer Model 21

4.1 General Requirements 21

4.2 Specific Conditions 24

4.2.1 Mode I: External Loads Only 24

4.2.2 Mode II: External Plus Internal Loads 24

4.2.3 Mode III: Daylight Plus Thermal Loads 25

5.0 Results 27

5.1 Office Module Results 27

5.1.1 Mode I: External Loads Only 27

5.1.2 Mode II: External Plus Internal Loads 33

5.1.3 Mode III: Daylight Plus Thermal Loads 33

5.1.4 Summary of Office Results 38

5.2 Residential Module Results 45

5.2.1 Mode I: External Loads Only 46

5.2.2 Mode II: External Plus Internal Loads 46

5.2.3 Mode III: Daylight Plus Thermal Loads 46

5.2.4 Summary: Operating Costs 51

6.0 General Conclusions 57

7.0 Recommendations for Future Research 61

References 65

Appendix A: HVAC System and Window Design A-1

Appendix B: Daylight Calculation Procedures B-1

V
LIST OF FIGURES

Page

Fig. 1. Design module for a midrise office building 12

in Washington, D.C.

Fig. 2. MODE I CASE 1. Seasonal heating and cooling 29

requirements for an office module in

Washington, D.C. as a function of percentage

of window area with a shading coefficient

of 1.0 (0.9) and a load factor of 1.0.

Fig. 3. MODE I CASE 1. External seasonal heating and 32

cooling requirements for an office module in

Washington, D.C. as a function of percentage

of window area for single and double glazing;

shading coefficient = 0.55: LF = 0.5.

Fig. 4. MODE II CASE 1. Seasonal heating and cooli.ig 34

requirements considering both external and

internal loads upon an office module in

Washington, D.C. for single and double glazing.

Shading coefficient = 0.55: LF = 0.5.

Fig. 5. MODE II CASE 1. Estimated annual operating costs 35

for the seasonal heating and cooling requirements

of an office module in Washington, D.C. for both

single and double glazing.

Fig. 6. MODE III CASE 1. Seasonal heating and cooling 36

requirements for an office module with daylight

utilization in Washington, D.C. as a function

of window area and amount of glazing.

Shading coefficient = 0.55: LF= 0.5.

Fig. 7. MODE III CASE 1. Estimated annual operating 37

costs for an office module in Washington, D.C.

using daylight.

Fig. 8. MODE III CASE 4. Seasonal heating and cooling 39

requirements for an office module in

Washington, D.C. with selective management,

thermostat setback, and daylight. LF = 0.5;

shading coefficient varies with season.

Fig. 9. Comparison of estimated costs for MODE CASE 1, 40

external plus internal loads, with costs for MODE

III CASE 4, daylight, management, and thermostat

setback.
LIST OF FIGURES (continued)

Page

Fig. 10. Comparison of estimated yearly operating costs 43

for a north-facing window in a commercial

module with a wall U value of 0.07 and a storage load factor of 0.5; shading coefficient

=1.0 except when management used.

Fig. 11. Comparison of estimated yearly operating costs 44

for a south-facing window in a commercial module

with a wall U value of 0.07 and storage load

factor of 0.5; shading coefficient = 1 except

when management is used.

Fig. 12. MODE II CASE 1. Seasonal heating and cooling 47

requirements for internal and external loads

for a residential module. Wall U value = 0.07,

shading coefficient = 1.0, LF = 0.1.

Fig. 13. MODE II CASE 3 and 4. Seasonal heating and 48

cooling requirements for a residential module

with management, and management plus setback.

Fig. 14. MODE III CASE 1. Seasonal heating and cooling 49

requirements for a residential module with

daylight

.

Fig. 15. MODE III CASE 4. Seasonal heating and cooling 50

requirements for a residential module with

daylight utilization, window management, and

thermostat setback.

Fig. 16. Comparison of estimated yearly operating costs 52

for a north-facing window in a residential

module with a wall U value of 0.07, load factor

of 0.1, and shading coefficient of 1.0 for

different modes and cases of operation.

Fig. 17. Comparison of estimated yearly operating costs 53

for a south-facing window in a residential

module with a wall U value of 0.07, load

factor of 0.1, and shading coefficient of 1.0.

Fig. 18. Estimated yearly operating costs for a residential 54

module using electric heating and cooling. The

two upper curves represent a bare window (MODE III

CASE 1). The two lower curves represent a window

in which daylight, management, and setback are used.

Wall U value = 0.07, LF = 0.1.

vii
LIST OF TABLES

Page

1. Factors which affect window performance variables. 3

2. Office module design assumptions. 11

3. Residential assumptions. 14

4. Shading devices and coefficients. 16

5. Equations for window calculations. 17

6. Daylight/artificial light trade-off. 20

7. Comparison conditions. 22

8. Comparison of the effects of varying shading 30

coefficients for an office module for MODE I CASE 1 and a storage load factor of 1.0.

9. Comparison of the effects of varying shading 31

coefficients for an office module for MODE I CASE 1 and a storage load factor of 0.5.

10. Comparison of estimated yearly energy costs for a 41

commercial module with two different wall u values

with gas heating and electric cooling. LF = 0.5,

SC = 1.0.

viii
Simplified Analysis of Thermal and Lighting

Characteristics of Windows: Two Case Studies

by

Tamami Kusuda and Belinda Lowenhaupt Collins

Abstract

Results of a simplified analysis for annual heating, cooling, and lighting

requirements associated with windows are presented. The analysis includes

the effects of window size, heat transfer, solar shading, and compass

orientation for typical commercial and residential modules located in a

climate typical of Washington, D.C, Three different modes of operation

with respect to heating" and cooling requirements through windows were

assessed: external loads only; external and internal; and external,

internal, and daylight. In addition, the effects of selective fenestration

heat-transfer management, such as planned emplo37ment of thermal shutters

and shading devices, and off-hour temperature setback were considered.

This analysis assumed that daylight could replace or supplement artificial

light whenever it could supply a specified minimum level of illumination.

The use of daylight was found to offer the greatest potential for reducing

energy costs, particularly when combined with selective fenestration

management

.

Key Words: Daylighting; energy conservation; fenestration design;

solar heat gain; window management.

ix

1 . INTRODUCTION

1.1 PURPOSE

Many previous assessments of the energy performance of windows

have considered only the heat loss and solar heat gain aspects of

windows. Yet, historically, a window's basic functions are more closely

related to lighting the interior, providing natural ventilation,

and serving as a means of visual communication with the outdoors. In

the modern building, however, the lighting and ventilating functions of

windows have largely been replaced by electric lighting and by mechanical

air handling devices, respectively. Total reliance upon these systems

is not necessarily the best way of reducing the energy consumption of

a building, however. The possible energy contributions of both natural

illumination and ventilation should be reassessed, before design

recommendations and energy standards, which require sophisticated

illumination and HVAC (heating, ventilating, and air-conditioning)

systems are developed.

1
In this report, a simplified analysis of the thermal and daylighting

performance of single- and double-glazed windows of various sizes is

used to illustrate ways in which the natural lighting and solar benefits

of windows can partially or fully offset the normal heat conduction disadvantages of windows. Although equally important, the effects of natural

ventilation are not dealt with in this paper. Window area, orientation,

and thermal resistance are evaluated with respect to the seasonal heating

and cooling requirements for a limited case in the Washington, D.C. area.

In addition, the effects of varying the operation of a window are studied

by modeling both the selective management of internal window coverings,

and the substitution of natural light for artificial light (whenever

practicable) . Selected results from these calculations are presented to

illustrate the analytical approach employed, as well as the effects on energy requirements and costs.

1.2 SCOPE

Any comprehensive examination of the energy effects of windows must

consider many factors, some of which are summarized in Table 1. A more

detailed identification of these factors is made in Window Design Strategies

to Conserve Energy .— Only a few of these will be analyzed in this

paper, which will consider thermal loads, both internal and external,

and daylight. The energy consequences of natural ventilation which

should be considered in a comprehensive assessment of windows are not

analyzed here because the losses due to air infiltration and benefits

due to ventilation and cooling through an openable window must be quantified

first. The difficulty of extending this analysis to several building

types, and the lack of detailed data on the extent of air infiltration,

prompted the investigators to restrict the present study to the thermal,

daylight, and management analysis.

2/ An earlier report, Windows and People ,— summarized some of the

psychological factors associated with windows. The life-cycle costs

associated with the computer model presented in th^s report are treated

in an Economic Evaluation of Windows in Buildings .—

* Raised figures indicate literature references at the end of this report.

2
00

60 -H C d

•H ^

(U ^ 14-1 (0 1 O CO

0) rH (U CO iH >,rH

CO CO 0) 4-1 d o u 13 •H t3 iH

OS rH <U H CO ^

C o CO d CJ O o cr CO C •H O CU •H

CO o 4J •H rH bO a

CO •H 4J CO 4J o O nH CJ o 4-» •s.

CO rH 4J C!

tiO iH CCJ 4-» rH a O (U CO rH •H rH •H 1 4-1 4J O iH •H 4J 0) O 4-1 d

CO CO CO cs H-r >^ CD O

<U 0) c CO (1) •H CO 0) O

H! H Q H > PM < O

cvjon<rinvor^oocyiO

rH

d

o

•H

4-» CO 4J

(U00 CO rH

(U ^ o o CO d •H •H •H •H M-l 4-1 4J !-i o CO CO 0)

CO CO U

C 0) d CO CO CO 1

(U

4-1 u d o rH CO O

(U bO > 4-t o •H > C d o o > >-i 4-) o •H (U o o CO 0) o

0) o rH •H Pn 0) 4J d

4J •H d 4J 4-) -d a

CO 4J CO d d bO CO § 4-t d > •H > 0) d d (U u-\ •H CO •H o 4-1 0) 4-1 o •H N u d

iH rH CO >i •H d CU o O U w w CJ

uil cn CO PM o

rH CO pq rH CNJ

(U CO x: B U rH

COd

d

o

00 'H d J

•H a

4-1 d

•H d

LO vo

ood

•HN

CO iH

00

t4HO

COu

CO

4-1 O

CO 4J U

d <u N -H e

•H 5-1 d

CO O 3

0)>

•H

CO

(U ^

CO

CO rH

CO •H rH H CO (U -H

4J M

eg 0) B 4J 00 i

d

•H (U N 0

CO CO rH J-l

00d

d

COo

00d

a. cu

4J -H

o

d

4J

COd

o

o

o

>>

4-1

CO

IS

13O

d

d

o

•H

4-1 COO

O

(U 4-J

CO CD ^ OCOd

CD* CO PL,

CO CD (U (U o o

> >

(U (U -d

00 00 d d

•H -H

CO CO -d

CD CD

CO CO g g

(U (U 4J 4J d X M W

rHCNrO<rmvOr-^OOONOrHCS

2. ElELATION OF WINDOWS TO TOTAL ENERGY CONSUMPTION

2.1 RELATIONSHIP TO BUILDING HEATING/COOLING SYSTEM

Window heat loss or heat gain is not necessarily directly related to the

energy required for heating or cooling. The interrelationship between

window heat gain or heat loss and the total energy consumption of

a given building for heating and cooling is complex for the following

reasons

1) Solar heat coming through a window will be absorbed by the interior

surfaces of the building,

2) Building interior surfaces release a part of the absorbed solar

heat into room air by the process of convection,

3) Simultaneously, room air absorbs heat from lighting fixtures,

occupants, equipment, and infiltration air, to achieve a temperature balance,
4) Also contributing to this heat balance process is the heat loss

or gain through windows and other parts of the building to the

ambient air through conduction and infiltration processes.

5) If the room air temperature becomes higher than the cooling

thermostat set-point, it will be cooled by a cooling coil. If

it becomes lower, for instance during a winter night, than the

set-point of the heating thermostat, it will have to be heated

by the heating coil.

6) Energy required for heating or cooling room air varies

considerably depending upon the heating, ventilating, and

air-conditioning system operation. For those who are unfamiliar with HVAC systems (heating, ventilating, and

air-conditioning) , a brief description of typical systems is

presented in Appendix A.

2.2 WINDOW HEAT GAIN/LOSS ANALYSIS

In order to compute the thermal and lighting loads associated

with windows, the effects of a large number of variables must be

considered simultaneously. One of the most critical factors is window

area. Most current designs for energy conservation in buildings have

called for reductions in window area. This concept, was implemented in

the GSA Norris Cotton Building at Manchester, N. H.— Yet, the thermal

effects of window area need to be assessed in combination with other

variables such as orientation, shading devices, glazing materials, and

thermal resistance. Any consideration of window performance should

involve an assessment of the interactions among these variables for

different window areas. Furthermore, an examination of the energy effects

of windows must contain an evaluation of the potential gains and losses

associated with the use of daylight, as well.

2.3 DAYLIGHT/ARTIFICIAL LIGHT TRADE-OFF

The energy used for the indoor lighting of commercial buildings and

schools constitutes a major portion of the electricity consumed in these

buildings. Maximum use of natural light can reduce electricity consumption for electric lighting. In addition, the use of daylight can also

reduce the cooling loads resulting from electric illumination systems.

For approximately the first 15 ft into a room, daylight can be the primary

source of illumination,— with artificial light needed only on overcast

days, at night, or for local task lighting. Beyond this zone, daylight

may provide some of the ambient illumination, with artificial lighting

providing the task illumination. This second illumination zone extends

from about 15 ft to 40 ft into the room. Artificial lighting must be

relied upon for interior spaces, unless another source of natural light,

such as a skylight, is available.—

6
The utilization of daylight, however, requires larger window areas,

which tend to result in increased winter heat los^. (especially at night)

and summer heat gain (especially during the day) .— The reduction of

electric energy usage for lighting and cooling by using daylight could,

therefore, be partly or completely offset by an increase in energy

consumption for heating and cooling due to the increased conduction heat

loss and solar heat gain. In addition, visual task performance and

comfort might be adversely affected by glare or veiling reflections

produced by daylight or direct sunshine. Although the adverse conduction

heat-loss and solar heat-gain effects can be reduced somewhat by the

proper application of controllable shading devices or insulating shutters,

studies are needed to determine an optimum amount of fenestration. This

optimum would balance the thermal benefits and disadvantages resulting

from the increased use of daylighting.

The net contribution of daylight to the energy load upon a building (or a room) is determined by calculation of the amount of light entering

through a window, the amount of heat loss or gain induced by the same window, and the amount of heat gain due to the artificial lighting.

Calculations of the heat released by occupants and equipment, as well as of conductive heat loss or heat gain, and infiltration heat loss or heat gain, should also be included.

7

3. ANALYTICAL APPROACH

3.1 COMPUTER MODEL DESCRIPTION

A computer model was developed to study a limited number of variables

in a typical situation so that the effects of variations in window

parameters upon both thermal performance and lighting levels could be

easily calculated. The goal was to isolate the thermal and illumination

effects associated with a window and to model these effects in a simplified

building situation. Thus, the theoretical performance attributed to each

variation in a specific window parameter could be identified and compared.

Because of the differences in internally generated loads between

offices and residences, the performance of a window wall in each

building type was simulated in some detail. Comparisons were made

between a wall with a range of glazed areas and a solid wall for each

building type. The office situation is treated first.

9
3.1.1 Office Module

For the office comparisons, a typical two-person office in a Washington, D.C. building was modeled. The thermal performance of the

window wall and of the module was simulated. This analysis does not,

however, attempt to model the complex interactions between the module

and other rooms within a building, or the total building thermal performance,

or the contributions made by a central HVAC system.

The module itself was designed to simulate a two-person office with a width of 12 feet, a depth of 15 feet, and a height of 10 feet. The

office was assumed to have only one external facade and to be located in

the middle floor of a highrise building. Thus, all the non-facade areas were considered adiabatic (permitting no heat transfer) . The module

was designed in accordance with good energy-conservation practice with

insulated walls, moderate light levels, and tightly fitting windows.

The windows were considered to be fixed, to avoid the problems associated

with air leakage and draughts around openable windows. Window size was varied from 0 sq ft to 90 sq ft (0% to 75% of the window wall) . Table 2

presents a complete list of assumptions used in the analysis of the

module while Figure 1 illustrates the design itself.

A fixed thermal load due to human occupancy was set. It was assumed

that 1.8 persons occupied the office; in other words, that it was a oneperson office with periodic visitors or a two-person office in which

each person was present about 90% of the time during the occupied hours.

The lighting was assumed to be fluorescent, as is typical of many offices.

The level of illumination used for the calculation is equivalent to 50 fc,

in accordance with good energy conservation practices as recommended by

GSA. The possible contribution of natural illumination to the overall

level of illumination within the office was ignored initially, because

it was treated in a later analysis.

2 A rather nominal equipment load of 0.5 watt/ft was used to simulate

office equipment such as typewriters and calculators, but not computers.

A low level of air leakage, equivalent to 0.25 Ac/h (air changes per

hour) was also assumed, modeling a room with sealed well-fitting windows

and minimal air infiltration.

3.1.2 Residential Module

A similar analysis, using somewhat different assumptions was performed

for a residence. The residential module was assumed to be a familyroom-kitchen with the internal loads and occupancy times typical of

such rooms. Again, only the windowed wall was considered exposed to

the outdoors, while all the other surfaces of the room were considered

to be adiabatic. The external wall was assumed to be 18' long; the room

depth was 15'; and the height was 8'. Thus the external wall area was

144 sq. ft. Smaller window sizes, such as those typical in conventional

residential construction, were examined, as follows: 0, 8, 12, 20, and

40% of the window wall or 0, 12, 18, 30, and 60 sq. ft.

10
TABLE 2. OFFICE MODULE DESIGN ASSUMPTIONS

Module

2 12' wide window wall area = 120 ft

10' high floor area = 180 ft

15' long (deep) volume = 1800 ft

Module Construction

Wall U value =0.15

Glass thickness = 1/4" plate

Glass strength = double strength

Window operation = none

Frame type = aluminum with thermal break in double-glazed windows

Window Sizes

0% = no window

10% = 12 sq. ft.

25% = 30 sq. ft.

50% = 60 sq. ft.

75% = 90 sq. ft.

Persons

Number = 1.8 (1 per 100 ft^)

Heat load = 260 BTU/hr /person @ 70-80'*F

Occupancy = 8 hr/day, 9 AM - 5 PM: Assumed to be sensible only and

based on figures for sedentary adult

Lighting

Type = fluorescent

Output = 3.25 watts/sq. ft.

130 lumens /sq. ft.

40 lumens /watt

50 fc

Equipment

Office = 0.5 watts/sq. ft.

Air Leakage

Leakage = 0.25 air change/hr.

Assume tightly constructed building with fixed windows.

Adiabatic Model

Heat transfer to adjacent spaces is ignored in this calculation.

11
12
The internal residential loads were assumed to be generally lower than

in the office building. Thus the equipment load was about 0.52 watts/sq

ft; lighting level about 0.65 watts/sq ft., and person occupancy equal

to 0.5. Air leakage was assumed to be 0.5 Ac/hr, a value larger than in

the office situation because well-made but operable windows were modeled.

No assumptions were made about natural ventilation through these windows.

Lighting in houses is typically incandescent, with the hours of operation

at the discretion of the resident. Typical levels of artificial light

used either during the day or at night have not yet been determined.

As a result, a very low level of illumination, about 6.7 fc or 0.65 watts/

sq ft of incandescent light, in terms of daily average value, was assumed

in order to simulate as conservative a situation as possible. It was assumed that this level of artificial illumination prevailed even during

daylight hours, because at least this level would have been necessary

had the room been windowless. See Table 3 for a detailed list of residential

assumptions

.

In both the office and residential models, the comparisons were extended

to include the dollar costs associated with heating, cooling, lighting,

and equipment loads. This involved dividing therms (100,000 Btu)

by the equipment efficiency and multiplying by the fuel cost. In the

residential case two sources of heat were modeled; gas heat, and electric

heat. For both, the cooling, lighting, and equipment were assumed to be

electric.

3.2 THERMAL CALCULATIONS

In the thermal analysis, calculations were made of the heat gain and

loss through each of the five window areas located in a facade of fixed

U value for both office and residential modules. In addition to size,

the following parameters were varied: building orientation, window U

value (single or double glazing), shading coefficient (different glazing

materials, internal and external shading devices), and season.

The calculations were made by using the monthly average value of daily

total radiation data for Washington, D.C. derived from the Liu-Jordan

technique as used in reference 7. Also included in the calculation is a Storage-Load-Factor Concept which approximates the capability of the

building structural mass and internal heat distribution system to make

use of excess daytime heat gain for nighttime heating. This factor is

introduced to account f or the fact that not all of the daytime net heat

gain (difference between the solar heat gain and conduction loss) through

the window is available to reduce the heating requirement during the

night. Since specific values for the load factor have not been established,

several selected values representing extreme and moderate amounts are used for this calculation.

Each calculation of the thermal loads upon a vertical surface was performed

for an average day consisting of average temperatures and solar radiation

for each of the twelve months of the year. The effects of varying

13
TABLE 3. RESIDENTIAL ASSUMPTIONS

I . Building Specifications

Room = 18' wide, 15' long, 8' high

Construction = brick veneer

U value =0.07

Exterior wall = 144 sq. ft.

Window sizes = 0, 12, 18, 30, and 60 sq. ft. or 0%, 8%, 12%, 20%, 40

Construction = wood, double-hung, weatherst ripped

Interior surfaces were assumed to be adiabatic in this calculation.

II. Internal Loads - Daily Average Basis

8/ Equipment Load = 0.52 watts/sq. ft. = 3511 kWh/year—

dishwasher = 363 watts blender = 15

stove = 1175 frying pan = 186

refrigerator = 1137 toaster = 39

disposal = 30 clock = 17

TV-color = 440 stereo = 109

Occupancy

Duration = 0.5 for 8 hours occupancy from 0700 to. 2300

Load = 260 Btu/hr.

Lights

Type = Incandescent

Output = 0.65 watts/sg- ft

Illumination = 6.7 fc—

Air Change

0.5 air change/hour

Shading Coefficient

1.0 (0.9) unless window

managed

Storage Load Factor

0.1

III. Operating Costs

Electric cooling = 3c/kWh

Electric heating = 3c/kWh

Gas heating = 30c/therm

Electric equipment = 3(;:/kWh

^/ Daylight Level

It is recognized that a reference illumination level of 6.7 fc is too

low for many household tasks. This value is chosen, however, to make

the average illumination level consistent ^Ith the daily average lighting

energy consumption figure of 0.65 watts/ft given in Table 3. The

average lighting energy figure is equivalent to using one 100 W bulb

for 10 hours per day and three 100 W bulbs for 6 hours per day, with

no lighting used at night. It is difficult for the type of energy

analysis reported here, which is based upon daily average data, to simulate

instantaneous and/or dynamic illumination requirements. Using a low level

of illumination, such as 6.7 fc, as the reference level for turning off

the artificial illumination may increase the calculated benefit of

daylighting and could make the daylight analysis appear more favorable

than it should. 14
window shading devices were studied by using several different shading

coefficients (from 1.0 to 0.2). By examining the list of options in

Table 4, one may pick an existing shading device that corresponds to

each of these shading coefficients. Equations, definitions, and assumptions

used in the thermal load analysis are given in Table 5.*

The calculations were performed for the average day by balancing the

daily total solar heat gain and daily net total conduction heat loss.

If, for a typical day, the daily total solar heat gain was greater than

the daily total net heat loss, the daily heat balance was termed positive;

otherwise it was negative. The calculations were carried out for each

of the twelve months for double and single-glazed (vertical) windows for

eight compass orientations and over a horizontal surface. The seasonal

total values were obtained by summing the products of the number of days

in the month times the average-day net heat loss (negative values) or net heat gain (positive values) . The heat losses and gains were tallied

separately to give the seasonal heating requirement and cooling requirements,

Two seasons were modeled; heating (October through April) and cooling

(May through September) . Although the calculations do take into account

the fact that a part of the daytime excess heat gain is made available

during winter days to assist the nighttime heating requirement, it was assumed that there was no cooling requirement during the heating season. Likewise, it was assumed that there was no heating requirement during

the cooling season. The assumption that there was no cooling during the

winter or heating during the summer, may have resulted in the use of

slightly less energy to maintain room temperature in both seasons than

would otherwise be the case. 3.3 DAYLIGHT CALCULATIONS

The energy calculation associated with daylight dealt only with the

usefulness of daylight as the primary lighting source in the zone nearest

the window. The artificial lighting was assumed to be dimmable, to

enable it to supplement daylight whenever the latter fell below a certain

minimum value. No assumptions were made about individual use of task

lighting. In addition, the visual performance aspects of daylight, such

as glare, ESI (Equivalent Sphere Illumination), and VCP (Visual Comfort

Probability) were not considered in this analysis. An exact daylight

calculation requires a very comprehensive analysis of light^^nj^rref lection

among all the surfaces in the room as described by DiLaura.

Since the exact daylight calculation is very laborious and timeconsuming, the daylight calculation proc^^uj^ .used in this study is

based upon the daylight factor approach. This concept, which has

been used by European researchers as a simplified method for approximating

available daylight within a room, is described in Appendix B. The

Daylight Factor is defined as the ratio between the illumination on a horizontal plane at a reference point in the room and the illumination

on a horizontal plane under the open sky, both without direct sun beam.

*Steady-state heat transfer relationships are used in this analysis

because one is dealing strictly with the daily average values. Such

simplifications may not be justifiable if one is dealing with the hourly

calculations, however.

15
TABLE 4. SHADING DEVICES AND COEFFICIENTS

Shading

Coefficient •U" Value

Glass Type

Clear single (SG)

Clear double (DG)

Reflective single

Reflective double

1.0

0.9

0.4

0.3

Suimner Winter Sp ./Fall

1.06

0.54

1.13

0.55

1.09

0.54

Devices

Venetian Blinds + SG 0.55

Thermal Drapes + SG 0.2 ~ 0.35

Fabric Shade + SG 0.4

External Sunscreens

+ SG 0.2

Vertical Fins + SG 0.3

Horizontal Overhang

+ SG 0.1 0.2

Drapes and Sunscreen

+ SG 0.1

Vented Heat Absorbing

DG 0.1

Horizontal Overhang

+ Venetian Blinds

+ DG 0.1

16
TABLE 5. EQUATIONS FOR WINDOW CALCULATIONS

facade ''wall "^gULass

q , = U (TOT - t.) A • 24 hrs. ^all wall X wall

q ^ = U ^ (SATG - t.) A ,

• 24 hrs • LF

^glass glass i glass

a IDT ° TOT = TOT + -— X —tt: Average sol-air temperature for walls

no z4

6 / TOT = average outdoor temperature—

a = absorptance of surface for solar radiation

a = 0.9

h = Coefficient of heat transfer by radiation and convection

o at the outdoor surface

h^ = 4 in spring, fall (months 4, 5, 10, 11)

h^ = 3 in summer (months 6, 7, 8, 9)

h^ = 5 in winter (months 12, 1, 2, 3)

IDT = Daily total solar insolation upon a specified surface

per unit area

t^ = indoor temperature thermostat setback

t. = 78°F summer = 84°F summer

1

t. = 72°F winter = 62° winter

X

A_ J = facade area in square ft

facade

^wall ^facade ^glass

A ^ = window or glass area in square feet (here equal to §X3S S

^facade ^ percentage of window wall)

U = Overall heat transfer coefficient for wall wall

17
U , = overall heat transfer coefficient for glass glass

(See specific coefficients on Table 4)

SATG = average sol-air temperature for glass

0.87 X IDT ^ ^ */

SATG = TOT + X S.C.-

24 X U - glass

SATG^ = average sol-air temperature for single glass

SATG^ = average sol-air temperature for double glass

TOT = average outdoor temperature

0.87 = solar transmittance for single glass

S.C. = Shading coefficient

= Solar heat gain through fenestration

Solar heat gain through single-glazed double-strength glass

LF = storage load factor

_ Useful heating obtained from the daytime excess heat gain

Total daytime excess heat gain

^/The equation assumes that there is no heat absorption in the standard

double-strength single-glazed window glass.

18
Daylight Factors were calculated for several areas of glazing, internal

and external reflectances, and cloud cover. The amount of illumination

within the room at 15 ft from the window was then calculated. In the

trade-off analysis, whenever room illumination supplied by daylight fell

below a 50 fc minimum (at the reference point) in the office module or 6,7 fc in the residential module, supplementary artificial light was added to maintain the minimum lighting level,* and the electric power

saved in lighting was calculated. — See Table 6 for assumptions made

in calculating electric power savings.

*In this calculation it is assumed that the artificial illumination level

is automatically and perfectly adjusted to maintain the reference level

without loss of illumination efficiency.

19
TABLE 6. DAYLIGHT/ARTIFICIAL LIGHT TRADE-OFF

° Calculate INDOOR illumination = DALITE

° window size h = 8 ft. high

° wall/floor reflectance RFW =0.4

° ceiling reflectance RCN = 0.6

° exterior obstruction angle a = 0.0

° distance from window = 15 ft.

° height above floor h^ = 3 ft.

° Indoor Natural Illumination = DALITE

DALITE = (SKYLIT * (SO + XIRC) + SUNLIT * ERG) * M * G * B * SHC/100

SKYLIT : sky illumination

SUNLIT : sun illumination

SC : sky component

ERG : external reflection component

XIRG : internally reflected component

° Task Requirement = DALITE + Supplementary Artificial fc

° Electric Power Saved

Office Residential

A = floor area

CU = coefficient of utilization 0.55 0.7

LE = lamp efficacy 40 lumens/

watt

19.4 lumens/

watt

LLF = light loss factor 0.7 0.7

Illumination requirement 50 fc 6.7 fc

Watts* = A * illumination requirement 3.25 w/ft

GU X LE X LLF

0.65 w/ft^

*3.413 Btu/watt

20
4 . COMPARISON CONDITIONS FOR COMPUTER MODEL

4.1 GENERAL REQUIREMENTS

In order to simulate a fairly realistic building situation, three different

modes of operation were studied. The first, Mode I, analyzes only the

external loads and simulates an unoccupied room with no loads from

people, equipment, or lights. The second mode of operation, Mode II,

combines internal loads from people, equipment, and electric lights with

the external load calculations of Mode I. Finally, Mode III computes

the effects of substituting daylighting for artificial light in the

calculations described for Mode II. In all three modes, there is assumed

to be no heating in the cooling season, or cooling in the heating season. Table 7 depicts the various modes of comparison.

For each of the three modes of operation, four different cases, or patterns of use were studied. In case 1, the internal thermostat setting

was maintained at one value all day for each of the twq seasons. In case 2, a provision was made for unoccupied-hour temperrature setback in

21
TABLE 7. COMPARISON CONDITIONS

\ Cases

Modes

1. Constant

Internal

2. Off-Hour

Thermostat

Spthark

3. Management

- Shutters

TA.LLn WnvV-l_LlL- n t" p T1. - Blinds in

Summer

4. Management

and Setback

I. External

Loads *

II. External and

Internal Loads * * *

III. Daylight plus

Thermal * * *

Window Variables

° Window size: 0, 10, 25, 50, 75% for office

0, 8, 12, 20, 40% for residence

° Single and Double Glazing

° Building Orientation: N, NE, E, SE, S, SW, W, NW

° Average Day in Each of the Twelve Months

Assumptions

° Case 1: Constant internal temperatures as specified in

Table 5.

° Case 2: Thermostat Setback during night and unoccupied

hours.

Setback Temperature (t.)

62°F for months li, 1, 2, 3

84°F for months 6, 7, 8, 9

72°F for months 4, 5, 10, 11

Case 3: Management Options

Winter ; Use Thermal Shutters for Months

12, 1, 2, 3 during night hours.

Vary "U" value as follows:

Single Glazing = 1.13 for Day

Single Glazing + Shutters = 0.5 for Night

Double Glazing = 0.55 for Day

Double Glazing + Shutters =0.2 for Night

*Data from these conditions will be reported in detail.

22
Summer : Use white Venetian Blinds during daylight hours for

Months 6, 7, 8, 9

Changes Shading Coefficient during Daylight Hours

SC goes from 1.0 to 0.55 for single glazing

SC goes from 0.9 to 0.5 for double glazing

Blinds set at 45° slat angle

Additional Assumptions

Heating system efficiency = 0.65

Cooling coefficient of performance = 3.0

Energy costs

gas = 30c/therm

electricity = 3c/kWh

Daylight Calculation Data

Window maintenance factor

Window material factor

Window glazing bar factor

Floor reflectance

Wall reflectance

Ceiling reflectance

Window obstruction

angle

1.0

1.0

1.0

0.4

0.4

0.6

0°

23
each of the seasons. (For offices, the unoccupied hours include weekends

and national holidays.) In Case 3, thermal shutters were used during

winter nights, and Venetian blinds during summer days, to manage the

window. In case 4, the window management option was combined with the

unoccupied-hour setback option. A more detailed explanation of the

assumptions for each of the modes and cases is given in Section 4.1.

Each of the four cases was evaluated in each of the three modes so that

a comparison could be made of the relative effectiveness of each mode/case

combination.

In addition to calculating the thermal loads in each of the three modes,

the probable costs of heating, cooling, and lighting for each mode were

estimated to determine the total annual expenditures in dollars for

energy. The estimate was made by calculating the amount of fuel used

for both heating and cooling, as well as the subsequent dollar cost.

The heating system was assumed to be gas, at 300/ therm; the cooling

system was electrical, at 3c/kWh. Non-cooling electrical costs for

equipment and lights were also calculated. Costs aj^ .based upon 1975

figures and are given for comparison purposes only.

4.2 SPECIFIC CONDITIONS

Each mode of operation involved modeling a set of specific conditions

A complete description of the conditions appears below. It is important

to remember that the data are applicable only to a situation in which

each of these conditions is met, before substantive conclusions can be

drawn about the results contained in this report . Further experimental

assessment of actual thermal and lighting loads is needed to verify

these findings.

4.2.1 MODE I: External Loads Only

In Mode I, the calculations given in Table 5 are performed as a reference

condition without internal loads. Although not representative of actual

use, such calculations have occasionally been used to predict the thermal

performance of windows. Modeling is confined to the climatic conditions

of Washington, D.C.; and the additional effects of variation in shading

coefficient and storage load factor are simulated. In Mode I, the

effects of external loads only are studied. The combination of these

loads with internal loads is then analyzed in Mode II Case 1 to compare

their effects upon seasonal heating and cooling requirements.

4.2.2 MODE II: External Plus Internal Loads

Mode I presents an incomplete picture of the thermal performance of

windows. In Mode II, the analysis was extended to include an evaluation

of the balance between external and internal loads. Only in rare instances

is a building operated without internal equipment, lighting, and people

present in its interior. Each of these sources generates heat, which

may decrease the need for supplementary heating during the winter, but

will increase the demand for air-conditioning during the summer. An

24
understanding of the balance between internal and external loads is

essential for determining the contribution of daylight and winter sunshine.

The heat-balance equation involves balancing the transmission loss

against the internal heat generation. Ideally, a condition of zero heating energy consumption would result when winter solar heat gain

through the window and internally generated heat by the lights, equipment,

and occupants could supply the heating requirements, to offset the heat

loss due to conduction and infiltration heat losses (at least during

daylight hours). During the non-heating seasons, cooling energy consumption

would be reduced by minimizing the heat gain, thus lessening the load

upon the air-conditioning system. In Mode II, constant internal temperature,

thermostat setback, window management, and a combination of both management

and setback were simulated to determine the relative effects of each upon

window performance.

4.2.3 MODE III: Daylight Plus Thermal Loads

Mode III combines the daylight calculations described before with an assessment of the internal and external loads from the window. In this

way, the combined effects of both heat and light could be quantified to

determine the extent of energy savings due to the use of natural rather

than artificial light for different combinations of window design and

operation. The contributions of daylight to room energy use were simulated

by assuming that daylight could supplement or replace artificial light,

depending upon the amount of external illumination at the reference point.

All calculations were determined for one point on a horizontal surface

15 ft (4.2 m) from the window and 3 ft (0.9 m) from the floor to model

the edge of the perimeter zone of a larger room. It should be pointed

out that use of this particular reference point represents a "worst-case"

analysis, because the quantity of daylight increases closer to the window.

The trade-off analysis balances heat loads from the windows against

usable daylight. It also calculates the reduction in internal loads

due to the absence of heat from the artificial lighting. Thus, the

analysis computes possible savings in fuel for both lighting and cooling.

25
I
5 . RESULTS

Selected results are presented for each of the three modes of operation.

Results were obtained for seasonal heating and cooling requirements in

therms, and for yearly operating costs in dollars. Yearly operating

costs include the cost of energy for lighting and equipment as well as for heating ^nd cooling. Life cycle costs will be treated in a subsequent

publication.— Results for the office simulation are given first, and

for the residential situation, second.

5.1 OFFICE MODULE RESULTS

5.1.1 MODE I: External Loads Only

For the office module, a number of calculations were performed which

assessed only the external loads imposed upon the windows, with no consideration of the effects of internal loads. The effect of varying

the amount of shading was also assessed, as well as the ability of the

27
building to store any accumulated heat. In all, six runs were made

using three shading coefficients (1.0, .55, .25) and two storage load

factors (1.0 and 0.5).

An examination of the seasonal heating and cooling requirements given in

Figure 2 reveals that, for single glazing, the heating requirement

increases steadily with increasing area for north and east/west orientations,

but decreases for southern orientations. The addition of double glazing

reduces the heating requirement substantially, particularly for north

and east/west orientations. The cooling requirement, however, remains

about the same. Figure 2 also demonstrates that for both glazing types,

the heating requirement for south-facing windows is less than that of a solid wall (0% window), and approaches zero for double-glazed windows

that comprise 25% or more of the window wall.

The data depicted in Figure 2 were obtained with a shading coefficient

of 1.0 (0.9 for double glazing), indicating no shading. Table 8 presents

the effects of varying shading coefficient upon seasonal heating and

cooling requirements. Table 8 demonstrates that lowering the shading

coefficient from 1 to .25 reduces the cooling requirement below the non- shaded condition for all orientations with both single and double glazing.

However, the change in shading coefficient increases the heating requirement

for all orientations. For example, reducing the shading coefficient

from 1 to 0.25 (Table 8), increases the heating requirement for the

largest northern single-glazed window from 101 to 137 therms, and from 10

to 107 therms for a similar south- facing window. Similar, although

smaller, increases occur for double glazing. Clearly, the effect of

shading in winter is detrimental, because it blocks the solar heat gain

which would otherwise reduce the heating requirement. These data suggest

that shading is most effective when used only at times of high solar

heat gain and high air temperature. Since selective shading appears

more effective than year-round use, this approach is subsequently

employed as one of the two management options.

In Table 9, the calculations of heating and cooling requirements are repeated using a storage load factor of 0.5 instead of 1.0 to simulate

the effects of reducing the storage capacity of the building. A higher

load factor represents a building system which is more capable of storing

and utilizing excess daytime solar heat gain for nighttime heating

requirements.

In brief, a comparison of Tables 8 and 9 demonstrates that reducing the

load factor decreases the cooling requirements. This decrease is

most noticeable for the two largest window sizes, on all orientations for

both types of glazing. The heating requirement was increased slightly,

particularly for southern, single-glazed windows. Changing the shading

coefficient preserves the same relative relationships as that obtained

with a load factor of 1.0. In view of the fact that, in most office

buildings, the storage load factor should be less than 1, all subsequent

comparisons were run with a load factor of 0.5. A shading coefficient

of 0.55 was used except where otherwise stated. Figure 3 depicts seasonal

heating and cooling requirements under these circumstances for Mode I. 28
X3

OQ

SIAia3Hl 'SlN3LM3y!nD3H 3l\in000/9NllV3H 1VM0SV3S

c 1 (1) •H r- 0)

0) o CO o Q

bO O C • e •r-l 00c

QJ CO •H O rC to

•r-l CO

y-i 1—1 CO tiO o

x; 0) c 4-1 r-l

•r-l o

o CO0) X)

cn CO c

4-) CO C

o <v 1 r—

1

<D c GO U •H c

•r-l •r-l in cr i+j 0) o u

^1 c

OJ M-l M) C CO t—

1

•i-i 4J

,—1 C U-l o 0) o

o o o ;j

0) o

-o o- •u c o

cC CO o M-l M)C c

•r-l o CO u •r^ O

cC 4-» 1—1 (U o

j:;

un CO

1—1 X)

CO C

c CO CO o

tn tn CO CO

Q) • CO o•

• • o

Q • H

C

<; o o o u

Ml 4J M C c

•r^ w •r-< Q o O to •H

IS :2 M-l

4Jc

(Ue

•r-l cr

0)

C

•H

•U

CO

(U 43

•uC •

0) 10

cn 4J

0) C u cu a H

(U 0) M ^13

crM

c .-I

QJ o a o

o o

-a -u c c

CO (1) CO

cn J-i 0) (u c a

•r-l (1) r-l U

CM

60.

•r-l

29
(1)

C T—l 0 0o c OJu u 1^

•H U-l O o c O to 0

4-1

V-i CJ ao to c u-l lit Ha to « 4Jc o

CO e

60 0) 01 00 c W) •H •r-l to Q) 3

T—

cr o

XI to 0) 4-1 as > a! CO H

U-l

M 0 c

•H

m I—

o u o

<D

u

w

U-l CO

U-l XI <C w c O

to

01 i-i J= 00

4-1 c w •r-t Q U-l 4-1 o O to

HI c ac

u o o

(0 u-l

l-l to

ll c

(0 o

a. to e (0 o 0) u CO

J33OQ

60C

CO o CO

00 uO CN

Ca

00 CO o m

cr

cS o o 00 CO CO CO

bOC

•r-( CN 00 o o .—4 o

<T <t CN

_

CN ON 1 1 CO UO 1 1 1 s o CO

0) 1 1 1 11 1

cr o o <!• CO CO CO CN 1 1 1 1 1

•r-( LO

n3 1 1 1 1 1 1

00 <1" 1

CSI1 1 1 CN1 CN1

CO

O CO

00 UO CN

uC

P! 00 00 1— CN CN

CO

OJ o v£) UO 00

CO CO c

•H

CN 1^ CO

_

<j 0Q

<T CN CO

CN

~? On UO CO . *y ON CN UO 1 1 1 <-H CN ' 1 * O o II ' II ON 1 t 1 1 UC c

CT* ^ u

QJ CO ^ 1 CO • H

Pd 1 1 t 1^

00 OJ n o

*^ LO o CO

u CN

cd 1 1 1 1 00c

*H o CN <t xj 00 rH <J- cd c CM CN CN CO CN CM o 1 1 j= 1 1 1 CO • f-l w u

cfl 4J P3 c x: 4=

CO <U

U

U j-t J-)

U i-J dJ 'r-t D V-t D 13 u u u u O o CO O o CO < O w CO W PL,

u-l 00 r-.

r-~ ON CO

COI OI

00I oIII OoIII

00 • H

4J

CO

CD

0)

J-,

•H tou

OJ

<v

g

0)

4_) OJ u 5>

dJ • H

<-M 60

}_( to

CO • H to

4-} 4JCD toaPQ

•H

4_) CO00 o

CD 0

IS

0)

CO o

CO 4J

OJ

CO T3

0)

u •H

OJ

CI. G.a3

to iJ PQ 0)

LiO XI O1—

4-1 to3 B e u

0) 4J x: to

4-1 0)

0)

4-1 toc

tfl o H H

4-1 to H

30
0

u-l

U o o

u-lo

4Jc u

<u o

•H 4-1 o o

r^ to

U-l

14-1 fl4

0) T3 o to u o

hJ 60c 01

•r-l 60

to

n) o

CO 4-1 CO

(D txO .—1 C •o

XI r-l c

n) to H

to > w o < w

4-1M

O

0)

w

U-l

Q

U-l w

g

(1)

4-1 fo

U-l (1) O 3 c T3 o 0 w s

•r-l 0)

CO

a

a. •rJ B U-l o U-l u O

60c

to I—I o

t—I soo oo

bOc

•H

Ifl

60c c

OJ0

(1)

l-l • l-l 3

D"

01

60cD

D*

0) OS60cD

cr

0)

60c

60c I ooo

-3-I

<»

r-HI3O

C/3

OCN CO I II I

00n r-^ ^ CN CM I I I

00 —I <J- —I CN CN I i I

OS

O I ^I 0 o r~- 00 1 I I

vO -H ITi CM fn IT) I I I

00 '—I •-I cs I I o

II I

CTn oo

inI

u-lI

31
•H •

(U a

rH •HN 60

CO d

o rH •H S 60 +J

CO (U CU <u o j:: •H

4-1 3 4-1 O d

o T3 CU

CO

CI X) dj

n3 C 5-1 CO U CU O (U M • M-4 iH CO bO CO 4-1 W a d

4-1 •H 0 CU d CO B

(U (U B u • M

OJ o CO •H

!-< d

•H d w

CO (U cr <u 0- !-i OJ >-l 0

5-1 CO 60d

bO d •H S o CO r-l •H 0

iH d CO 0

O OJ 0

O d

O •H 4-1 r-) d

Xl o (U X) CD

nj 0) 0) CU 60 ^ u

bO CO CO P- C u CO cu •H d 0 5-1 •u CU m o CO

0) >-l in 0

a- -2 0 B

COC O II CO O

CO d

CO o d

Q) •H CO

CO 4-1 O LTl CO iH d in (U

CO 3 d

C 4-1 d •H

J-i OJ CO II u •n

CO 4-1 •H w CO d iH

<u 0

•H C/D

• CJ 0

•H Q M-l • W 4-1 CO C/7 rv CU 4-1 <3 d 0 d

CJ o CU

4-» B

60 60 CU M d d u

•H •H •H w 13 d Q CO CO cr

CO (U ^3 C/2

SlAiaaHl 'll\l3lAI3aiimBy 9l\in003/9I\lllV3H ivnosvbs

<uu

d

60

•H

Pl4

32
Data involving off-hour temperature setback and window management (cases

2, 3, and 4) are not presented for Mode I, because this mode represents

external loads upon the window, and does not reflect variations in

internal conditions.

5.1.2 MODE II: External Plus Internal Loads

Figure 4 represents a situation in which internal loads are added to the

external load calculations and a constant internal temperature is maintained. When the external thermal loads depicted in Figure 3 are compared

with the combined loads given in Figure 4, it is apparent that the

addition of internal loads has increased the cooling requirement while

decreasing the heating requirement for a single-glazed window. For a double-glazed window, the cooling requirements exceed the heating requirements

for all cases. Data reported in Figure 4 represent a reasonable baseline

against which all subsequent data can be compared for the office module.

Annual operating costs were also calculated. These include heating and

cooling costs as well as electricity costs for lighting and equipment.

Figure 5 shows that dollar costs increase as a function of increasing

single-glazed window area by $22 to $35 for different orientations. This

increase is much less pronounced for double-glazed windows, however,

with an increase of only about $5-$10 per year.

Although the data for cases 2, 3, and 4 are not presented here, the

calculated results indicate that the use of thermostat setback decreases

operating costs by $5-$10 regardless of window size. Selective management

reduces operating costs of the windows somewhat more. A combination of

both thermostat setback and window management is even more effective in

reducing annual operating costs. These effects are treated in greater

detail in the next section.

5.1.3 MODE III: Daylight Plus Thermal Loads

In MODE I a daylight/artificial light trade-off is made along with he

assessment of internal and external loads. Daylight is assumed to

supplement (or replace) artificial light to maintain an overall light

level of 50 fc at the reference station, 15 ft from the window.

Figure 6 depicts the thermal requirements when using daylight in a room with a constant temperature setting. It demonstrates that the use of

daylight appears to increase heating requirements noticeably over the

baseline case (Figure 4), but to decrease cooling requirements. Using"

double glazing cuts the heating requirement in half, but affects the

cooling requirement only slightly. Northern orientations exhibit the

greatest heating requirement and the least cooling requirement.

Compared with the costs for the baseline case, Figure 5, the use of

daylight reduces operating costs substantially. As can be seen in

Figure 7, when daylighting is used, increasing single-glazed window area

lowers costs for all window sizes except the largest. For double glazing,

costs decline up to 50% glazing, but even those for 75% glazing are $20-

33
CD

OQ

o CD O

CO

CD

I

oCO oIT)

ID

I

cc oLT) <

CNI o

00c

U <-t U 3

X O

U

rC 13 CO •U d QJ o CO x;

oor-i a

Q) W U

W O

d "4-1 o

o u o

d

u

d

. . 0) a E

0) cu u u

•r-l 3

cr

cuu

" CO d

O d r-J •U CU o

00 O

d o o

B

cuu

•Hcr th

cu x: T3

to c

00 IS d

•H d

CO CO (U

C/5 CO cu

(U o (U oo

3 T) o x: o

CO G

00 cu Q

d o

4-1 M-4 . CO M-l lO

(U o .

d

CO

CO

COd

o

COCO

cu

d

CO

x)d

CO

COcud

3 " -O in -H

c/:i CO m I—

T3 • o

CO O CO

• o

t] r-< C_) to CO CO w u

<! d d

CU . g

4J 00 OJ d d v-i •i-l -H -i-l N 3 Q XI CO O d r-J S CO 00 )^

SIAIHBHl 'iN3l/\l3yinD3a 9Nn000/9NliV3H 1VN0SV3S o-

<uu

3

00

•r-l

34
QQ

Oa

in

<

o

O

LO

CM

o o o o

CO

o CD

CO

oLO o

CO CN

CO

INGLE CO

1 1 1

1 1 1

1 1 1

GC<

CD

LO oo

CMo

o

oC35 o

CO

o CD

CO o

in

o o

CO

o o CM saviioa 'SisoG SNiivaBdo ivhnnv

tiO 00 c

c •r-l •H N

1—

(

cfl o

o 60 o

cu

r-l c J3o

OCX) c

•r-l 4-1 c

nj CO

(U (U 1—

<

I—

1

CjO c

c •r-l o C/l CO

(tJ 01 U

tfl o

(U u

4J o

o •

<4-l •

tn Q

U

cnoo o

4J 00 ftO c

•t-l •r-l 4J

tfl cn

CO

0) 13 a. • o c cn

•r-l 'U

1—1 CO nj <v O

1—1 O

c

c w

(fl o CO e f—

1

X) 1—1

(U O

4J o

CO •r-l g

•r-l y-i c

•u o

C/5 CO w c (1) to cx

• M-l

r-l o w cn cn CO •u 1—1 c O

o

i M

cn

•r-l C

o- 0) MO re

CU

35
ZDOo

ooo o oCO

<

OO

oID

LD

<

OO

SmiHi 'SlN3IAI3ainD3a 9Nn00G/9NliV3H 1VN0SV3S

•H

4-1 3

•H cr

15 o re

<U •u

1—1 c 00 d d

o •^^ O 6 4-1 e CO ea o

•H CO

M-l d

y-t CO CU o re es c CO )-( ep u o j_( O CO

U-l C3 CO 4J •H r—

d

03 & Q CU u e

c M-l CU

<u o !-i B CO •rH <u fi 3

u o d w

•H •H (U CU 3 4-) a

cr U o

(1) ti 00 u 3 d

d •H W) CO rH

q! CO O

•H to O

r-l CO <u O

o CO d

0 •H 4J o U

i-l

en XI Xl to Q tu CO ^CO CL

&0 d CO

ci o Q 5-1 •1-1 4J CO CO c: • t-1 •H o ^

1—1 as o

I

CO IS II CO c

so in LF nd

CO CO

CU d

Ol 0 in to •H 03 4J d

CO o •H

I—

N

•H II

r-l w 1-1 -o w •i-( u •H <: 4J r—

o d o

CO M 4-1 M rd 00

00 d

•H •H to O CO

vO

CUU

3

00

•I-l Pu

N 4-) CO d

-I QJ 00 e

36
cnZDOo

eg

DC

<

a

CD

2 g

o0"5 O

OO

oCO oLO CO CD

00

UJ

CD

oo

o

LO

CNI

cc<

oo

oCD o CD

CO o oCO oLO oCO oCM sa\nioa isoo 9Niivy3do ivhnnv

(It

CO{J o CO a Q

•1-4 M-f

[

1 1 Q Q

T3

J-( Q CO

t|

1 Q<)

j_( CO w

CO j_l o

r-4

Of) Q

r)

CO

5_(

>-« O

o

CO

—J *

n3 mj

'r-1 13 r—

1

ctJ CO

1 1 03 •i-t w CO

• •

T-H • w Q

<d

CJ co M u M 60 M c

•H QO CO s :2

00

37
$25 less than for the 0% window. Again, south-facing windows cost less

to operate than windows on the other orientations.

The data in Figures 4 and 6 suggest that the heat generated by the

electric lighting can reduce the heating requirement. It is, however,

more expensive and less efficient to heat a room with electric lighting

than with gas heat. In the summer, of course, removal of the heat from

the lights reduces the cooling requirements as well as the demand for

electricity. In summary, the use of daylight appears to offer a marked

potential for reducing overall energy consumption in buildings.

Figure 8 depicts the use of daylight combined with selective window

management and unoccupied-hour temperature setback. When the seasonal

heating and cooling requirements for this case, given in Figure 8, are compared with the case in which only daylight is used. Figure 6, it is

apparent that the use of window management and temperature setback

reduces the heating requirement for single-glazed windows. The cooling

requirement is reduced only slightly. Double glazing cuts the heating

requirement but does increase the cooling requirement over the singleglazed case. When daylight, management, and double glazing are used,

the heating and cooling requirements are less than those for the zero window case for window areas up to 50% glazing on all orientations

except north. There, the heating requirement jumps sharply for areas above 25%.

The reduction in yearly operating costs over the baseline condition is

most dramatic when daylight and management are both used, as can be seen

in the lower portion of Figure 9. Furthermore, costs are reduced below

those for a solid wall by as much as $35 for 50% single glazing on the

south or $26 for a similar north window. They begin to rise again for

75% glazing for all but the south exposures, but are still well below

those for the 0% window. The difference in costs is even more dramatic

when compared with the baseline case in which neither management nor daylight are used. These costs are given in the upper portion of Figure

9.

Table 10 demonstrates the effects of lowering wall U value for the

different modes of operation. Lowering the wall U value (increasing

its thermal resistance) reduces estimated costs slightly. These data

are presented to demonstrate that the model can be extended to different

design conditions.

5.1.4 Summary of Office Results

In Figures 10 and 11 estimated yearly operating costs for a window in an office module are plotted for four different conditions: bare window

(external plus internal loads only); management and setback; daylight;

and daylight, management, and setback. Figures 10 and 11 apply to walls

with a U value of 0.07, a storage load factor of 0.5, and a shading

coefficient of 1.0 (except during summer management conditions). The

comparative effectiveness of the various conditions of operation for

38
5

uj oo ^ ;s lD 00

\

\

LU

m 711

—J \/ 1 / DO

J /\ 1 / ZDOa f II <i e- / ^j:' / pc>-

1 1 1 1 1 1 1 1 1 ^

C3

CNI

3

CC<

o

CDO o

03

o

00

CD

I

CD

CO OIX) C3 OCO

CD

CN

CD

CO

in

I

CSI O

o o 7= o

03

o

00

o oCO CD

1X5 O oCO o o CM »—

SIAIdBHl ' SlN3mainD3a 9l\in003/9NllV3H 1VN0SV3S

• c II 4-1 •H C

OJ

(U e

u

• iH D 4-1 e x: cr W)

•rH u H

•H >^ GO

U-l to C

u-< -o •H 0 4-i CO

(U

CD CU x; u 4-1 0 c

CJ 0)

cfl XI QJ u 4-1 >-i c dJ a

01 cn OJ e

0) 4J cn u CO cn 4-1

•l-l 4-i d

c QJ 0 H e

0) e 1—

1

QJM

<u XI •H

boc 4-1 ^ cr

•H QJ

<-{ CO S-i 0 4J XI 0 C bO

(U XI C

e d •H

CO tH M 0

CO to CO 0

iH CJ

CO 0

c e X> 4J e C

4-i QJ

cfl > Cfl 0) •H QJ

4-1 c U

a QJ CL

rH QJ

CT3 H 0 S-i c 0) 0 CO

cn • rH

cfl c

QJ 4-1 0

•H cn

CO

0)

CO <r CJ XI w Q 4-1 CO CO •H

<: CO 0 C QJ 0 CO M 4-1 OJ •H

1—

1

bO •H i-H M S-4 •H CO X3 w 4= > •H Q CO i-H 0 CO 0

CO

00 •H

39
CD

ZDO

o <

O

CVJ

13

CU

CO

0)

CO

o

o oCDCD

CO oCD OLn oCO o

0)

CO CO C

!-i CO (U e

4-1 C -

•H 4-t

CO txO >H rH

CU >,

CO

to

fi -

(U

4-1 pq

0) <:

<

LU

GC

O

Csl

W H • C/D CD o -H M S M

!-i CU O QJ

<4-l ^-1

CD -U M CD )-i O O 0)O

CD ^ iH

CDO

a

CU u

COB

•H

•H 0) & -C

4-J CD

CU

CD

73

4-t c

SaVllOQ 'SISOG 9NllVa3dO IVnNNV

0)

V-i 01 43 CD

4-1 (U

M-lo >-i a

0) CU o cx

CD 3 ^ •H ^ S-i (U u

CO x: CO a 4-1 ^

e 4J o (U U ^ CD

CU

£J0

40
H

C7N rH CJN LO CN

LO rH CN CO o lO • • • • • • • • • r ON rH ON CO o

CO CX3 LO LO LO CO LO

LO CO LO 00 LO LO LO o • • • • • • • • • CNl -J" ON CO CJN o LO

>>D LO LO CN •<r

/-I •P

•rlt>

i> 1—1

CN) o rH CN <f" <t

O CN! LO CO CN CN CN rH r~~ CD ^* * * * * o CM LO CJN o CTi rH rH CM 3 o 60 LO LO LO CM CO CO C II 60 O I-i •H c S -t-i iH 3. •rH CJ O o iH CM N •vf (DO CO LO vD

iH i) o cjo Cd CO CO CN LO 00 CJN Cd iH CJ o * * * rH • * 60 • •rH W iH •H CNl CO CO CJ CTi o O d ON o O

CJN CO LO LO LO LO •rl CO -Jcd (u N

CU {Ii * 1 1—1

cd 6 cd r t\ 60 1—1 w 1—.

CM .5

CO CTi

O CJ o CO CX5

O bO <u •H LO CO CJN o o iH * CO • • • CD * * * * 60 o o o ON ON ON rH o o o

cd 4-J CO LO LO 1 <)" <f 60 LO LO LO

cd "H

d

i-t

D II CO •rl

O

X CJ CO CJ 1 CTi CM CM cd On 00 CN

O o

CO CO CTv o CO LO 1 rH LO o

CO cd lH <u * * * 4-* * * * * * C! o CO rH rH 00 l-J 00 CN

CO

H <X> ON CO LO LO rd CO LO

O rCl * iH 60

CJ -l-J o d) P •rl

'ri CO

C 1—1

>• !3 II cd CO CX3 d) rH rH 00 >-> <} LO

O

60 PQ CO CM £ LO Cd C7N LO

(-1 CO >-l o * * * dJ • * * * * * <U CL) o 60 o LO 00 rH CO 00

C! 3 4-t t

Cd LO LO LO CO -J- <t

M rH o C3

cd cd cd > iH ro CO LO rH CJN ON CJN

i-t

O rHO 00 CO rH

cd Iz) cd CO * • * • • • • o CM CO o CO CO o vO CJN

i-J • LO LO LO CO CO CO

I—1 O

1) cd II r-l

O CN CN CjN <r 00 CJN cd 3. t—1

1^ o LO CJN

O

O • • * • * * • •H C vO O CN rH rH LO

cr> LO LO LO lO lO •vf

CD

M

14-1 >4-( ro ro CJN 1

rH CJN CJN CO

O 'rl CNO

Q o * * * • * * * * * CM CNl o rH rH CN CO CN o o CO CO LO LO LO LO LO LO LO

CO I3 •H H

}-l cd

Q-i O

CJ cu cd

d) cd> cd 12

5: CO CO CO O W

w

rH n3

C!

Cd •H

:s :s

41
CO o _o LO CNJ LO 1—

CO 1 r\ LO 1 r\ V ri CO V ri V ri • iH o On rH CO CN O rH CN

CNI CO Ln LO LO LO CNI CO CO o 1—

CT* o CN CTn LO o o

00 CO /—

^ Cn| rn , 1 * * * * * * o vO CO CNl rH CO

rH CM cs| lO vO lO LO lO lO rH CNl CN

Ou CO LO CO

f-i CO 1 CvJ t t~~1 CO , [ I 1 r~l

VJ •rl^3 CO LO CO <^ CO V ri LO

CO CO CO lO lO LO LO LO CNj CN CN

p—

ou

i-i Ln o» CO "H CN CN t I~l CO 00 LO csj c^ r~i

OX) rrt CO * _ *

I-i /—\ 1 r-~i Oil rH CN| rH Oi OU CO o>

rl f-l LO lO lO LO lO r* rr\ CO

CO •H N 11 rrt vO rn

CN| CO r~i CO CO

CO

rh •-t lO CO o*\

Cj

O * Co (11 /—

(—

o^

V—

1^ r—

lO lO LO r^, J 1u 1 LO 1 1 C\ ni •—

—

LJ '<r CO O rH LO o r-| Q lo LO

CO uO CO CO LO lO r~1

4-1 * * * 1 1 • * 0^ LO 4-1 rH <r CO rH CN CO 1—

CNl CO lO LO lO lO 4-1 CNI CO CO

c! CO

n\U> •n OU

OU [ J *H

cd Ln LO LO CO 00 <r t—1 CJ^ LO

CS| CO lO CO CO

Co CO CO • iH PQ CNI o o CO CN Q a^ 00

lO ou LO lO lO rH CN CNJ

*> cd

4-1 {-< CO 0^ 00 LO 1—

CNI CO

•r~1 I—

CN CO lO CO LO CO CO lO

rH a\ ro in <r

O CNl <!

CO LO in in lO in CM CM CNJ

CO CO in vO o in CNJ I—

in <r CN 1 * o <f 00 CO

r—\

CNI CO CO CO <f <r o rH rHO CN <! to UO in in in in -d- <r

CO CO CO 1—o> 0^ CO

<r CTN LO <r o rH o rH O CN CO O CN CO CNJ in u-i UO LO in in in in in in LO in

12 :s

w

ww

CO vD CO

00 00 <r

CM in

rH rH rH

CO 00

00 CTi 00

rH rH rH

M CO vO 00 G 00 in o

•H • « N ON

nj CNI CNJ CM

rHO

(U in1

• • • o rH rH O <r Q1 in

in 00

O • • • cd ON

<r <r <1-

4-1 (U to in oo p • • •

C! CvJ 00 o

<U rH B

(UM)

cd CvJ rH CNI \D §S in ON rH

rH CNI

u

MO CO

•H 00 rH in

00 rH rH

(d CNI CO CO

rH

CT\ CO

rH CO CM<r

in CO

ON

d rHO

in in in

42
1

CQ

OQ \

O

\ I W

-Jul

//

//

y

/

/

/

2

C9

U

<:

OQ

I—UJ+

i ^ ^

^ UJ

00 CO < <

o t> o

<

LO CC

<

Oa

LD

CNI

o03 ooo

CD oCO o CD ooo oCM

\

\

\

\

U3

CO

\

\

\

\

\

\

\

\

\

f

I

I

\

f

\

\

\

\

/

\ /

\ /

/

i

<

<00

s s

< a s2 2 —I —

!->->-

UJ < < CO a a

> D O

I

o < Lf> UJ

GC<

Oo

CSI

o oCO o co oLD oCO oCM

60C

CJ

CO T3 0)

14-1 C CO

1 CO 3

4-1 4-1 U o c o OJ c o e

(U CO O CO u C

fo ue ma 1—1 cn CO c

4-1 > OJ

COo

CJ I—

1

00 .-1 a

c CO (U

•l-l s o u X

to CO <u u

(1) J= •—^ a 4J o •H II s o

I—

1

CO u 1—13

in O • e o

T5

OJ 1—1 CM

4J CO O

CO •r-l 5 o U

• p-l u o

CO i o

CO o IM

CM o o X)

CO CO c o

o c 1—1 CO l-l

• I-l u s oo

CO o CO CX 73 M c O

O • 1-1 4-1 CO

<uu

3

OO

$ Nl SiSOa 9NliVa3dO AiyV3A a3iVI/\lliS3

43
o

4-J 1 M-l d) Xi o C3s u (U

<u o cv

CO

CO sR U

j-4 M o d)

^-^ r-l

r-l 1>

1» CO CO

4-> 4-1 CO CU O CO O O

?s 60 4-1 (U C •H •H 1—1

J-l CO II I—(

(U 3 CJ a. CO o i

.—1—1 * CO o

CO •r-l CJ M-l u O

X) iV4

(U o

J-) o 4J

CO o O B CO •H CO M-l

4-) CO

C T3

Q) •H COO

1—

o

(U

C3 W) O •H CO

CO !-i • •H O

bO 4J <u

COC CO CO • 1-1 3 & o TJ o CO C! CO

4-1 CO •H

<u

l-l

44
both single and double glazing for a north-facing window can be readily

seen from Figure 10. The differences between the four conditions indicate

the possibilities for reducing energy consumption through different

operating practices. The use of daylight, whether with or without

management, appears attractive when double glazing is used. Figure 11

demonstrates that orienting the window to the south lowers all costs and

improves the effectiveness of daylighting for the single-glazed case. The calculations reported for the office module for the various modes of

operation and cases of study indicate that the energy balance at the

window can be reversed. If nothing is done to the window, increasing

its area increases both the heating and cooling requirements (except on southern orientations) as well as operating costs. If extra glazing,

window management, off-hour thermostat setback, and daylight are utilized,

increasing window area no longer increases energy costs, and will, in some instances, actually decrease them.

Shading the window during the winter, however, may actually increase the

heating requirement by preventing the use of solar heat gain. Furthermore,

failure to use thermal barriers during winter nights can increase energy

losses and operating costs substantially. Effective use of daylight also

requires that it be substituted for artificial lighting (as long as a minimum level of illumination, such as 50 fc, is maintained). In this

trade-off analysis, no attempt was made to assess the quality of the

lighting for task performance. Rather, the analysis was based solely on the quantity of light and heat generated by the two sources. The thermal analysis described for the office module demonstrates that

the window can perform better than a solid wall, if the total window

system is designed and operated in accordance with the assumptions

presented here. There appears to be a range of window sizes occupying

25 to 50% of the window wall which can minimize yearly operating costs

for an office module located in a climate similar to Washington, D.C.

Additional calculations are needed, however, to extend this analysis to

other window sizes, climatic regions, and geographic locations, as well

as to a total building. In addition, field verification of the predicted

reductions in thermal requirements and operating costs due to the use of

daylight and management is necessary.

5.2 RESIDENTIAL MODULE RESULTS

The analysis of the three modes employed for the office situation,, was also used in a typical residential application. As before, the

analysis focused on window performance, so that heat flow within and

between rooms was not modeled

.

The effects of varying window size, thermal resistance, shading, and

orientation were assessed for the three modes and four cases in the

residential situation. Thus the effects of external loads only (Mode

I), external plus internal loads (Mode II), and external, internal, and

daylight (Mode III) were modeled for the following cases: constant

temperature (Case 1); nighttime setback (Case 2); window management

(Case 3) ; and management plus setback (Case 4)

.

45
Results from the calculation of seasonal heating and cooling requirements for several different types of operation are presented first

Then, summary graphs, depicting estimated yearly operating costs for a north-facing and a south-facing window are given to compare the effects

of the different operations.

5.2.1 MODE I: External Loads Only

As in the office situation, consideration of only the external loads

demonstrates that both the heating and cooling requirements increase

substantially with increasing window area for the residential module.

The increased heating requirement is greatest on northern exposures and

smallest on southern exposures. It is reduced substantially by the

addition of double glazing, however. Because the results are very

similar to those for the office situation, the data for external loads

only are not presented graphically.

5.2.2 MODE II: External Plus Internal Loads

The addition of internal loads to the external loads decreases the

heating requirement slightly, but increases the cooling requirement

substantially. Yet, Figure 12 demonstrates that the heating requirement

still increases rapidly with increasing window area in the single-glazed

case. The upper portion of Figure 13 presents seasonal heating and

cooling requirements for a window with management. The lower portion

presents results for management plus setback. As in the office situation,

window management means that wooden thermal shutters cover the window on winter nights, while white Venetian blinds are used on summer days. The

upper portion of Figure 13 demonstrates that window management decreases

the heating requirement from that needed for the load-only case (Fig. 12)

but increases the cooling requirement , The lower portion of Figure 13

shows that the addition of thermostat setback to window management

reduces the heating and the cooling requirements over those shown in

Figure 12 for all window areas.

5.2.3 MODE III: Daylight Plus Thermal Loads

In Mode III, Case 1 daylight replaces artificial light whenever about 6.7

fc of daylight falls on a horizontal surface 3 ft from the floor and 15

ft from the window. The artificial light is turned off, removing 0.65

watts/sq ft of heat load and the electricity for burning the light.

Brief inspection of Figure 14 reveals that the cooling requirement is

reduced well below that for the bare window (Figure 12), in which no daylight replacement occurred. However, the heating requirement is

substantially higher for both single and double glazing.

Figure 15 represents the case in which the window is used most effectively.

In addition to daylight ing, window management and nighttime temperature

setback are used. The result is to reduce the heating and cooling

requirements compared with the baseline case (Figure 12). Furthermore,

the heating requirement noted for the daylight-only case (Figure 14) is

46
CD

OO

\\\

\\\

\\\

\\\

J

CD

<

o O

CSI

' ^

oo

o o

05

ooo o oCO oLT) oCO

o

3az<

CNI

OO

1—

<

II •

to to C u

^-1 C

0) QJ u

c QJ •H O !-i •H U o :3 O cr

II 0) w QJ u 0 00 C <—t c

01 CO •H E > I—

1

(U o

j-i o

•H o

1-H

II CO XI

•H bOC o

•H QJ CO

I—

I

!—

1

o .

o CO

CJ O 4-J e c n 0) C I—

1

6

CO CO QJ •H 5-j bO 4-1 •H C C D

•H QJ cr

4J QJ

CO •H u

QJ CO

QJ 00c

T—< •l-l CD CO 4-1 C CO O !-i QJ to O

CO

QJ II CO

CO

CO I—

1

• O o

t—

<

I—

1

0) I—

(

I

CO CO CO

CO c

yi C

QJ QJ M 4-1 D- X O

QJ

QJ TD 1—

1

O C

CO O

SIAIHBHi ' SlN3IAI3ainD3a 9Nn003/9NliV3H 1VN0SV3S

QJU

00

•r-l

47
CD<

GC<

Oa

<

oa

O 4-1 M-l ^ C ^ <u wow

C XI t-i (U 4J (X B <u <u

<u w M

•i-i w m

3 3 .-I

CT" >—I O

<U CI, ^

M s

4-1 c <u •H e T)

>—I (U T-l O bO O cfl O

e

w

4Jc

0)

T3G

to t3C - a (u

*J 4J -H C 3

<u <u cr X e 0)

CO n) bO C C C

O to 'i-i W E 4-1 CO to CO 4-1 J3

r-l T3 3

to o

B

CO

1—1

<U CO W .H

CO 4-)

<u M

M .HW

0) <U T) >-i OS CO

on

c

O) • W 4J

0) C

a. 6

<V d) •H3

cr

<uu

w

o

-i

bO W 3

•W C r-^ 0) o

<X O

o o

9)U

3

bO •H

Sl/\ia3Hi 'SiN3lM3yinD3a 9Nn003/9NliV3H 1VN0SV3S

48
m

oo

oo

o Ooo oICD

CO

<

oO

^ 3

a*

03 (1) c

QJ 60

•r-l CO cr x: u II

C

o •CO

tfl 0)S

C D

0)

to a. -H o 3

tio cr c QJ H 4J u s:

CO bO 60

CO -O c

O X

CO O

W

«0 ij II •r-l 5 CO

1—*

QJ O

-I XI 3 B

CO

T3O

c

0) • - 13 CO

r-* iJ

CO C

(Uu

3

bO •H

SIAiaaHi 'SlN3IAI3ainD3a 9Nn003/9NllV3H 1VN0SV3S

49
CD

Oo

\ L oo

CD

CT5 Ooo

• M

CO

4-> to

en w u B

O c Q)

me

CO 0)60 cr" CO Q)

CO

(U 00

>-t

• H • i-i O 4-1 CT" CO

<D c (U

V-i •H

I* oO 4-1 c • •H c (U CO

i-H o CO O •H (U o 4J o CO i

N 0) (1) •1-1 u

1—

rt •H to U o* oO D o (Uu u

4-) X! 1, bO

to bO CO C

0) • i-i •H

JC I—C <—

(U o

r—

CO a o

o u

O J2 4-1 OT 4-1 c

to •H o (U a) > CO CO CO

0) tb re I—

0)

• 3 CO 0) od

4-t u

0) B CO CO

CO 4J r—

CO I—

CO o o CO O

•H B a M 4-) u

c <u CO M 0) 4J

<U • H •H

•K3 CO r-H O (U C3 O

IS CO CO

i-H

(U

SIAia3Hl lN3IAI3ainD3a 9Nn000/9NliV3H 1VN0SV3S

50
cut substantially while the cooling requirement is reduced slightly.

-5.2.4 Summary: Operating Costs

Figure 16 summarizes the estimated yearly costs for six types of operation

for a north-facing window in a residential module for both single and

double glazing. Comparison of the various curves reveals that, if

nothing is done to the window, operating costs increase by about $25 for

single glazing and by $10 for double glazing as window size increases.

Window management reduces this increase, as does a combination of thermostat

setback and management. In fact, with double glazing the detrimental

cost effects of increasing window area are just about removed for management

or management /setback. Figure 16 also demonstrates the beneficial

energy effects of substituting daylight for incandescent light. Costs

are reduced by $15-20 over the case in which setback and management are used — for all single-glazed areas. Similar results hold for double

glazing. Further reduction in costs, in the order of $5-10, are possible

through the use of management and setback. In the most dramatic example,

costs are cut in half for the largest single-glazed window area by the use of daylight, management, and setback. In fact, costs are even about

$25 lower than they are with a solid wall.

Figure 16 also demonstrates that double glazing is most effective in

reducing overall costs when the window is unmanaged — e.g., in the bare

window case, and the daylight only case. This occurs because the management

conditions assume that the thermal resistance of the window is increased on winter nights to about that of a double-glazed window through the use

of tightly-fit ting wooden shutters.

Figure 17 summarizes the overall operating costs obtained for a southfacing window. Simply orienting a window to the south reduces yearly

costs by $2-10 for single-glazed windows for all 6 types of operation.

Costs are also reduced slightly for double-glazed windows. The pattern

of results is very similar to that obtained with north-facing windows

with costs increasing with area for bare windows, and remaining level

for management and setback. Use of daylight causes all costs to remain

lower than those incurred with a solid wall. With the use of daylight,

there is some indication of a window size, of 8-12% for single glazing,

and 20% with double glazing, which minimizes operating costs. When

daylight, management, and setback are all used, estimated costs continue

to decline for double glazing, even for the largest window area.

Figure 18 presents a comparison of costs for two different types of

operation for a residential module with electric heating and cooling.

Costs for the bare window are given by the upper two curves while costs

for a window with daylight, management, and setback are given by the

lower two curves. Because these graphs represent a situation in which

a more expensive fuel is used for heating, costs are higher than those

obtained for Figures 16 and 17 in which a less expensive fuel (gas) was

used. Differences between north and south orientations appear particularly

pronounced for the situation in which daylight, management, and setback

are used.

51
CQ

Oa

o o05 Ooo

CD<

+

IS CD o < < Z CD < t3: LU

CO " CB CD o

2 —< —' —

< < C < < < CQ 2 CO a o a

O < > B O

o

orsi

<

Oa

oCD

CO<

z

LU ^ CD o < < Z 00 < I—

< a S2 S

< Cu < < < 2 (/) Q a a

oCVJ

CC<

o

CO C <U T3 X) 0 CO 0

«d 0 S

1 4-1 C U (U u 0 M

0

c ffe

CO II T3

V-i (1) 0 3 t,M

4-1 I—

m

CO >

j-i 0

CO :=> • 0

0 I—

I— 4-4 bO CO 0

C

•1-1 4-1 4J CO C!

CO CU U J3 • w

Q) a

o- •H •H 0 4-1

4-1 a) <U I—

(

r—

0

M 3 0

CO T30 ao s c

•H

I—• CU CO CO CI

4-> •H 0

4-1 CO i c 4-)

•r-l <u -o CO

4-1 XI C V4

CO •H CO (U

<U CO a

0

I—

(

• 4-1 CO 0 0

C30 C 4-1 CO

CO H 0 cu •H CO ^1 s

CO O

s c O -r-l

(Uu

•H

CO C

4-1 CO

$ Nl SiSOG 9NliVa3dO AiaV3A QBlVIAIIiSB

52
QO

=3Oa

CO

to<

Z +

LU ^

< < Z 03 < t-

+ +

CD<Z

<

Z

^ ^ t— " 55 55 CD S ^ a

€

<

oo

CD

CNI

OO

o

CO

oOO ot

oCO oLDCD CDCO

CD

CSI

CD

cs<

z

<

Z

CD o

+ +

cs<

OQ 2

" CS CS CS

CO S ^ a

€

cc<

oo

ocvi

CXD

00

•H CD O o

CO

1

4-* o

O

CO

1

1 j Cy (J H d)

t-* M-H Cu

C/}

1—' f—

J

O

uu *0

I-i 3

•H f J ^

CD r*U-i to CD M

(-

d) M r \ \J

•rH o 1

1 [

t

[ 1 QJ CL) r\\jCJ CO

(U O Hr\ OD c r*

*rH T3I i-H 'O

Q> CO CO +J -H {-<

CO 4J CO e C

•H CU 4J T3 C

W •H CO

CO

cu r—

1

O • CO o

c

o M-l w O

-Hu !-i CO o OU

e o

o •H CO 3 44

0)

bO

•r-l

CO oC»J

$ Nl iSOO 9NliVa3dO AiaV3A QBiVIAIIiSB

53
CM

OO

<

oa

o

oCNI

CM

CC<

o

— OO

1—

1—1 CO CD CD

r: •H 4-1 4-1 CO C C

(U 0) • CO CO

<u CD 0) 0) I—

!-l CO 3 a. 3

'O Q> cu O !-i cu cB

CO CO CO

cu to > > ni<5 •H u u O U CD C o o

(U 'O CU •H (U cu CO

ra a

0) a

)-l D 1—1 an

CD o o

«\

}-i u +J 4J O C

(U CU CU .c e

CO H H cu

COo • O bOG

(50 -H C

•H O

4-1 O

CD O

<U XI a c

O CObO

.-I c M -H

CO 4-J cu CO >. 0)

<u o

4J -H

CO ^-1 E 4-t •H O

4J (U

CO 1—1 W (U

W) O

CDC II CDE U

O

(U «

CO 4J

CD ,C U

- . CD U C>0 M-f M 1-1 -13 >, CD

CD O

CU i-< XIo ,r; •» ^ tH o

,c • 2^0

XI a II

•i-l (U 5 :3 3

O x-t cu CO

>-i c >

CO -H

CO

cu3

$ Nl S1S03 9NllVy3dO AiaV3A a31Vl/\lliS3

54
When operating costs are considered for a window with management, daylight

utilization, and temperature setback, Figure 18 demonstrates that a single-glazed window results in lower yearly operating costs than an insulated wall system of equal size for all areas except the largest

north-facing window. When double glazing is used, even with electric

heat, windows of all sizes do better than a comparable wall. Costs are reduced most, of course, with a south-facing window and gas heat. Nevertheless, the combined use of daylight, window management, temperature

setback, and double glazing reduces residential energy use and operating

costs substantially.

In summary, the energy balance at the window in a residence is affected

both by fixed design parameters, and by actual operating practices. The

results are similar to those for the office situation, except that the

lowest operating costs are obtained with smaller areas of glazing.

Although questions remain about the ways in which both daylighting and

window management are actually used in homes, these results suggest some ways in which energy resources can be conserved through careful attention

to the design and operation of windows.

55
I,
6. GENERAL CONCLUSIONS

The results from the thermal and lighting calculations demonstrate that

a properly designed and operated window system can reduce overall operating

costs below those for a solid wall for the two types of rooms simulated

in the metropolitan Washington, D.C. area. The reduction is greatest

for managed south-facing, double-glazed windows in which daylight replaces

or supplements artificial light. In all cases, however, reducing operating

costs for windows requires careful consideration of orientation, the balance

between annual heat gains and losses, use of daylight whenever sufficient,

and good management practices including off-hour temperature setback.

The single option that has the most impact is the use of daylight rather

than artificial light. Its use appears to reduce all window operating

costs relative to the baseline conditions, for the assumptions outlined

in this report.

57
The findings from the computer analysis should be treated cautiously.

They are based upon computer simulation, not actual case studies.

Further experimental verification by data collection on window performance

and use in existing buildings is necessary. Furthermore, not all the

parameters that affect window performance in real situations are modeled.

The major omission involves air exchange aroimd and through the window

in the form of natural ventilation and infiltration. In addition, the

effects of the building HVAC system response upon window performance

were not assessed.

Nevertheless, the findings presented in this paper indicate clearly that

annual operating costs can be lowered through the use of window management

and/or daylight. The use of a thermal barrier on winter nights combined

with shading on summer days will cut undesirable heat loss and gain

noticeably, while the combination of these devices with the use of

natural light improves window performance beyond that of a solid wall.

An effective HVAC system which increases the utilization of excess daytime heat gain or storage load factor for the heating season and

decreases it for the cooling season should also improve the overall

performance of windows. Unless window management and daylight are used,

however, windows will increase heating and cooling requirements when

compared to an equivalent wall area. These requirements can be reduced

somewhat through the use of double glazing, fixed shading, and proper

orientation of the window. Nevertheless, the best performance results are obtained when all the options for reduction of heat loss and gain

are utilized and daylight is substituted for electric light, whenever

practicable

,

The results indicate further that there may be a range of window areas which maximizes daylighting and beneficial solar heat gain and minimdzes

undesirable heat gains and losses. This range appears to be centered

between 12% and 20% of the window wall for the residential module and

around 50% of the window wall for the office module, if daylighting,

setback, and management are used according to the design conditions, and

if orientation and double glazing are chosen carefully. The differences

in the range of areas between the two modules indicate the importance of

the assumptions made about the internal loads inside the two modules.

Thus, the external loads, daylight availability, thermal performance of

management devices, orientation, and glazing resistance remain the same

for both situations. In the office situation, however, the internal

loads are much greater due to higher occupancy, higher lighting levels,

and higher equipment loads. It is in the latter situation that daylighting

is particularly energy-effective by reducing loads upon the cooling

system and by reducing the use of electricity for lighting.

This report also makes certain assumptions about patterns of use and

management. These are: 1) off-hour thermostat setback; 2) dimming or turning-off of electric lights whenever sufficient daylight is available

at a given reference point; 3) use of thermal shutters on winter nights;

4) use of Venetian blinds on summer days. Although each of these options can be accomplished manually, there is some question as to the extent to

which people will avail themselves of these energy-saving possibilities

,

It is possible, of course, to circumvent human behavior by using auto58
mation. Thus, the first possibility, thermostat setback, is readily

available as an automatically timed device. The second option can also

be automated through the use of photocell-activated switches and dimmers.

The Building Research,Establishment in Great Britain is currently investigating such devices. — It is more difficult to automate the last two

devices, although automated Venetian blinds have been used infrequently

in England.— On the other hand, it may not be necessary to resort

to automation in all cases. For example, nighttime closing of shutters

or draperies often occurs as a routine matter to preserve privacy or to

prevent cold draughts in the winter. It may be possible to accomplish

some of these options by informing people of the large energy-saving

potential associated with them. Nevertheless, the management options

discussed in this paper do require either informed human behavior or extensive automated controls in order to realize the predicted energy

savings, (Costs of the various management options are not dealt with

here .

)

59

7. RECOMMENDATIONS FOR FUTURE RESEARCH

The limited number of computer calculations summarized in this report

indicate the possibility that, at least in the Washington, D.C. area, a window can conserve rather than waste energy . The conditions under

which energy savings may occur are based upon specific assumptions about

the design and use of windows.

Further research is needed to verify the energy savings predicted in

this paper in both field and laboratory situations. Perhaps the two

most controversial areas dealt with by the model involve the actual

utilization of excess heat gain during the winter, and the exact quantification

of available daylight under different sky conditions, room configurations,

and window designs.

The model also makes some specific assumptions about window management.

Although many theories about human behavior have been advanced to deal

61
with the management of such energy-using building components as lights,

HVAC systems, shading devices, and windows, very little investigation

has been made of typical management behaviors. How do people use lights,

windows, draperies, blinds, and/or shutters? What variables affect the

use of each of these? Answers to questions such as these are essential

for designing effective energy-conservation systems.

Still another area of major concern is the development of valid daylight

prediction techniques under a wide variety of conditions. The calculations

discussed in this paper need to be extended to more complex situations.

The following list summarizes some important items which should be

considered in further daylight research.

a) Hourly variation of available outdoor illumination throughout the

U.S.A. as affected by the sun's altitude angle, cloud cover,

reflected light from the ground and other exterior objects, and

the shadows cast by external objects.

b) Hourly solar heat and light gain through the fenestration as functions of the type, size, location, and shape of the windows

and their shading devices (both external and internal).

c) Hourly indoor illumination by daylight at a selected point and

the minimum additional illumination required by artificial

lighting.

d) Hourly cooling load due to the solar heat gain, lighting,

equipment, occupancy condition, heat from the exterior envelope,

and infiltration.

e) Hourly heat loss through the window as affected by the outdoor

temperature, infiltration, and wind speed and direction.

f) Heating effects of artificial light as compared with the solar

heat gain through the window during the winter.

g) Quantification of the extent of window heat loss and heat gain

and light transmission when controllable shading or shutter

devices are used. By these devices, it would be possible to admit

maximum daylight during the winter day to assist heating and

lighting of the room, and to utilize the options to minimize the

heat loss during the night. The shading device would also be used

to impede excess amounts of solar heat gain. For example,

Rosenfeld and Selkowitz-^' have described a novel scheme

in which direct daylight illumination is beamed by Venetian

blinds from the top of the window to a highly reflective ceiling.

This scheme increases the room illumination without causing

undesirable glare at the working level.

h) The psychological needs of occupants for visual communication

with the outdoors through the window. This need is stronger in

62
some rooms, such as those occupied by recuperating hospital

patients or by factory workers who are engaged in routine and

monotonous work,—

Extensive and systematic studies of the windows should be part of a general program which includes some of the items mentioned above. The

number of experimental studies and observations can be minimized if

selected aspects of fenestration design are studied by computer simulation

techniques to determine the daylight ing effect and heating and cooling

energy consumption on an hourly basis,

NBS is currently developing an hourly daylighting simulation computer

program using U,S, Weather Bureau tapes. This program will be designed to

perform the following calculations pertinent to the daylight analysis

1, Solar energy incident upon the surface of a given orientation

and inclination under clear as well as cloudy sky conditions

can be calculated. The radiation data are determined

separately for the direct and diffuse components,

2 , Solar heat gain through windows is calculated by considering

interior shading devices, exterior overhangs, and- side fins.

3, Heat loss and gain through the exterior envelope are determined

by taking into consideration the comprehensive heat transfer

between the interior as well as thermal mass of the structure

and room air,

4, Heat emitted by the occupant, lighting fixture, and equipment

is included.

With the addition of a daylight computer routine, it is a relatively

simple matter to include in the computer program a scheme for switching

off or reducing the artificial lighting whenever the daylighting level at a given point in the room exceeds a prescribed set value of illumination

for a specified task.

63
ACKNOWLEDGEMENTS

The authors are grateful to the many persons in the Center for Building

Technology who contributed to the preparation of this manuscript. In

particular, we wish to thank John Bean and Bob Chapman for their assistance

with the computer work, as well as Reece Achenbach, Joe Murdock, Don

Quigley, and Art Rubin for their helpful comments and guidance, and

Sharon Rippeon and Susan Stitely for their help with typing.

64
REFERENCES

-1. Hastings, S. R. and Crenshaw, R. , Window Design Strategies to Conserve

Energy . BSS 104, U.S. Department of Commerce, NBS, June 1977.

2. Collins, B. L. , Windows and People: A Literature Survey: Psychological

Reaction to Environments With and Without Windows . BSS 70, U.S.

Department of Commerce, NBS, June 1975.

3. Ruegg, R. T. and Chapman, R, , Economic Evaluation of Windows in

Bui Idings . NBS Report in preparation.

4. Kusuda, T. , Hill, J. E. , Liu, S. T. , Barnett, J. P., and Bean, J. W.

,

Pre-Design Analysis of Energy Conservation Options for a MultiStory Demonstration Office Building , BSS 78, U.S. Dept. of Commerce,

NBS, November 1975.

5. Hopkinson, R. G. , Supplementing daylight in offices. Light and

Lighting , 1961, 54, 296-299.

6. Ho It on, J. K. , Daylighting of Buildings: A Compendium and Study of its

Introduction and Control . NBSIR 76-1098, October 1976.

7. Kusuda, T. and K. Ishii, Hourly Solar Radiation for Vertical and

Horizontal Surfaces for the Average Day in the United States and

Canada, BSS 96, U.S. Dept. of Commerce, NBS, 1977.

8. All Year Energy Savings All Around the House . Pamphlet prepared

by PEPCO, Potomac Electric Power Company, 1900 Pennsylvania Avenue,

N.W. Washington, D.C. 20068.

9. Liu, B. H. and Jordan, R.C., Availability of Solar Energy on Flat Plate

Solar Heat Collectors, ASHRAE Symposium, Bulletin for Low Temperature

Energy Applications of Solar Energy, New York, 1967.

10. DiLaura, D. L. , and Hauser, G. A., On Calculating the Effect of

Daylighting in Internal Spaces , Smith, Hinchman and Grylls Report,

June 197 7.

11. DiLaura, D. L. , LUMEN-II Lighting Analysis System , Smith, Hinchman

and Grylls, June 1977.

12. Lynes, J. A., Principles of Natural Lighting , Applied Science

Publishers Ltd., London, 1968.

65
16. Collins, J. B. and Crisp, V. H. C, "Energy Management and the lES

Code", Current Paper, Building Research Establishment, Department of

the Environment, U.K., 1977.

17. Beckett, H. E. and Godfrey, J. A., Windows - Performance, Design, and

Installation , Van Nostrand Reinhold Company, New York, 1974.

18. Rosenfeld, A. H. and Selkowitz, S., Beam Daylighting, Chapter 5 of

Report of 1975 Berkeley Summer Study on Efficient Use of Energy

in Buildings , LBL 4411, 1976.

19. American Society of Heating, Refrigerating and Air-Conditioning

Engineers, Handbook and Product Directory , 1976 Systems, ASHRAE,

New York, 1976.

20. Meriwether, R. F., Computer Programs for Energy System Analyses,

APEC Journal , July 1971.

21. Kelly, G., Kusuda, T., and Hill, J., Potential for Energy Conservation

in Heating, Ventilating, and Air-Conditioning Equipment for Buildings

in Energy Conservation through Effective Energy Utilization , NBS

Special Publication 403, pp. 163-191, June 1976.

22 . International Recommendations for the Calculation of Natural Daylight , Publication CIE No. 16 (E-3.2), 1970.

23. McNamara, A. C., The potential of location within a room in producing

ESI, Journal of the lES , April 1976, p. 169-176.

66
Appendix A

HVAC SYSTEMS AND WINDOW DESIGN

This section is provided to assist architects and illumination

engineers toward an understanding of typical building HVAC systems,

operation of which significantly affects the energy benefits obtained

from careful window design. An inefficient central HVAC system

could more than offset the energy conservation provided by carefully

designed fenestration. Only a few selected systems are illustrated.

Readers who are interested in more details on this subject should refer

11/

to ASHRAE Systems Handbook.

SINGLE-DUCT CONSTANT AND VARIABLE VOLUME SYSTEMS

Figure A-1 depicts a schematic of a single-duct single-zone system

which could be either constant volume or variable air volume (VAV),

depending upon the air outlet unit in the room's ceiling. In order

to respond to the changing space load, the constant volume system

regulates its heat delivering capacity by changing the supply air

temperature. This is accomplished by regulating the heating coil

temperature during the heating season or cooling coil temperature during

the cooling season. The variable volume or VAV system, on the other

hand, maintains a constant coil temperature during each season, but

regulates the supply air flow rate to meet the changing load. This

can be done by either a damper in the VAV box at the ceiling or by

dumping part of the supply air into the ceiling plenum space. The

air supplied to the space exchanges heat with the air in the space.

A-1
LUco>

MSz

o

N

tu

_JO

H

CO

I O

O

lU

-IO

MOQNIM ^

Figure A-1. Single-Duct System

A-2
ui

H->

CO

I-<UJX

UJ

tc

-I<

z

oc

UI

MOQNIAA

Figure A-2. Terminal Reheat System

A-3
The space air is then returned to the central plant through the return

air duct. Since the occupants of the space that is being conditioned

generate excess carbon dioxide, odor, and smoke, these contaminants

are diluted by mixing them with outside air. This is the reason that

a part of the return air is exhausted and replaced by make-up outdoor

air as indicated in Figure A-1.

On many days during spring and autumn, the space heat loss could

be exactly matched by the heat given off by the lights, equipment, and

occupants; no net heating or cooling is required. The single VAV

duct systems have difficulty in meeting this zero load condition since

the supply airflow cannot be made zero. A minimum of supply air must

be fed into the space regardless of the thermal load in order to satisfy

the space ventilation requirement.

SINGLE DUCT REHEAT SYSTEM

It is not difficult to control the supply air temperature by

regulating the heating or cooling coil temperature if a single duct

system is connected to a single zone such as shown in Figure A-1. In

actual practice however, the system supplies air to more than one space.

Particularly during the intermediate seasons, it frequently happens

that one space calls for heating while others require cooling. It is

impossible to satisfy these different space requirements in the single

duct system described above. A common practice employed is to cool

the central supply air to the lowest likely required temperature and

modulate it up to the desired supply air temperature for the other

spaces by the reheat coil such as shown in Figure A-2.

A-4
It is obvious that the reheating of the air which is once cooled by

refrigeration causes a double expenditure of energy. Obviously, in

this type of system, attempts to reduce solar heat gain by shading

devices results in the increase of energy use rather than energy

saving

.

DUAL DUCT SYSTEM

Another commonly used central all-air system is the dual-duct

system which is illustrated in Figure A-3 . In this system, both heated

air and cooled air are provided in two separate supply ducts to all the

spaces to be air-conditioned. At the ceiling plenum above the space,

mixing boxes are used to yield a desired supply air temperature which

meets the heating or cooling load at a specific instant and at a

specific location. Although the total air flow of the dual duct

system after mixing usually remains constant, it could also be varied

by a VAV box. The dual duct system is definitely not energy conserving

because it requires mixing of heated air with chilled air to obtain the

proper degree of heating or cooling. The office energy requirement of

such a system could be reduced, however, if the heating coil were provided

with hot water from the chilling machine's condenser or reclaimed heat

form the lighting fixtures.

ECONOMIZER CYCLE

When the outdoor temperature and humidity conditions are favorable,

it is possible to shut off the cooling plant and use the cool outdoor

air to cool the interior zone. This operation is called using an

economizer cycle. The dampers indicated in Figures A-1 through A-3 for

A-5
A-6
the outdoor air intake, the return air duct and the exhaust air duct

are controlled to regulate the mixing rate of outdoor and return air

to achieve economizer-cycle operation. The economizer cycle can

obviously be a very energy conserving feature of a HVAC system and

its use should be very much encouraged. Ross F. Meriwether used a

_20/

computer simulation to predict the energy savings by the use of an

economizer cycle with a dual duct and VAV system for an office building

in San Francisco. The results were as follows:

TABLE 1. PREDICTED ANNUAL COOLING ENERGY CONSUMPTION FOR

A SAN FRANCISCO OFFICE BUILDING (TON-HOUR)

No Economizer Cycle Economizer Cycle

Dual Duct 2,040,916 825,393

VAV 1,582,240 745,531

As can be seen from this table, the energy savings resulting from

employing the economizer cycle are significant indeed. The performance

of these HVAC systems actually determines the energy consumption of

a building, sometimes independent of the heat gain and heat loss

21/

through windows

.

For example, total energy demand expressed on the source energy

for various air-side systems with the room heat gain of 10,860 Btu/h

may vary depending upon the system, such as follows:

Constant-volume dual duct system: 24,507 Btu/h

Constant-volume terminal reheat system: 38,215 Btu/h

Variable-volume dual duct system: 10,810 Btu/h

A7
In addition to the air-side system variations, there are numerous

ways of producing heating and cooling effects at the central plants.

For example, there are many ways of attaining heating effects, such

as by electric heating, gas or oil furnace or boiler, heat pump system,

and waste heat utilization. Similarly for cooling, there are gas/oil

absorption chiller, electric centrifugal chiller, electric reciprocating

chiller, gas-engine-driven centrifugal chiller, and gas-engine-driven

reciprocating chiller. Numerous other possibilites exist which would

produce different building energy consumption data even for the same

heating and cooling loads imposed on the heating and cooling coils

in the central air handling system. Also, it is extremely difficult

to determine how much of the daily total heat gain would become the

daily total cooling load (storage load factor). What fraction of the

daily total heat gain in the winter months would assist in reducing

the daily total heat loss also depends upon the heat absorbing characteristics of building thermal mass and building HVAC system.

The important thing to note is that there is no simple way to

express the window heat gain/loss in terms of building energy consumption.

An approach taken in this report is to discuss the effect of window heat

gain in terms of total daily heat gain and heat loss to a given space

irrespective of the complexities of heating and cooling systems.

The resulting analysis, therefore, does not purport to answer all

the questions about windows and energy conservation. It does, however,

seek to quantify and compare some of the energy consequences of variations

in window parameters.

AAppendix B

DAYLIGHT CALCULATION PROCEDUflES

This section describes a procedure for determining the available

indoor illumination, when the outdoor illumination is known, by using

the Daylight Factor concept. The Daylight Factor is defined as the

12,13 / ratio between the illumination on a horizontal plane at a reference

point in the room and the illumination on a horizontal plane under the

open sky, both without direct sun. While the outdoor illumination is

determined by a routine called OUTLIT, the Daylight Factor is determined

as the sum of the sky component (SC), external reflection component (ERC),

and internal reflection component (IRC), with correction factors applied

for the dirtiness of the window and the type of window glazing and framing.

In this procedure, however, the daylighting level is calculated by also

considering the direct sun illumination reflected by external obstructions

when they are exposed to the sun as shown in the routine called DALITE.

In order to determine the sky component (SC), external reflection

component (ERC), and internal reflection component (IRC), several subroutines are developed, such as SCERC, ERC, EOF, SF, and BSF, which are

briefly described as follows:

BCF and BWF : BCF determines a geometric factor which corresponds to

the views of the sky from a specific reference point in a room through a 111

rectangular window. BWF is the calculation of a similar factor

between the reference point and the bright window. The reference point

in this case is a point along the normal to the window through its lower

left-hand corner.

B-1
SF: This is an extension of BCF and BWF to allow the reference

illumination point to be shifted from that along the normal through the

lower left-hand corner of the window to that of any other location.

EOF: This algorithm determines the view factor between the room

reference point and the external obstruction viewed through the window.

The algorithm is so written that the percentage of horizontal

obstruction covering the window area is given for the purpose of describing

the size of the external obstructions.

IRC: In this routine it is assumed that the room is divided into two

parts by a horizontal plane passing through the centroid of the window.

It is assumed in this routine that the external obstruction has the

horizontal edge parallel to and all the way across the window and its

reflection factor is known.

SCERC: This routine simply utilizes the routines SF and EOF to

obtain the sky component (SC) and externally reflected component (ERG)

of the daylight factors.

DALITE: This routine determines the daylight intensity available

at a given point in a given room by utilizing the daylight factors and

outdoor illumination value, as well as other pertinent factors such as

window dirtiness, glazing and frame construction.

OUTLIT: Available outdoor illumination is calculated by this

routine for the typical sky conditions as a function of the sun's altitude

angle, a graphic representation of the equation used is shown in Fig. B-1.

Fig. B-1 also shows a direct beam illumination for cloudless conditions.

Algorithms for all of the routines mentioned are given in the

following sections.

B-2
12,000

DIRECT

a SOLAR ALTITUDE, *» DEGREES

Figure B-1,

B-3
OUTLIT (ALT, ISKY, SKYLIT, SUNLIT)

SKYLIT: Sky illumination, Im/ft

SUNLIT: Direct sun illumination, Im/ft

ALT: Altitude angle, degree

ISKY: Type of sky

1 . mean sky

2. overcast sky

3. cloudless sky

SKYLIT = 53 * ALT, if ISKY = 1

AT T

SKYLIT = 2200 * sin * ^) , if ISKY = 2 ioU

AT T

SKYLIT = 1500 * sin (f=^ * tt), if ISKY = 3 ioU

AT T

SUN = 12847 * EXP (-0.2259/sin * tt)) loU

SUNLIT = 0.5 9, SUN, if ISKY = 1

SUNLIT =0 if ISKY = 2

SUNLIT = SUN if ISKY = 3

This algorithm is developed to represent the outdoor illumination curves given in "Principles of Natural Lighting" by L5mes,

B-4
DALITE (WAZ, SC, SAZ, ERC, XIRC, SKYLIT,. SUNLIT DLFTR, M, G, B)

DALITE: Indoor illumination

SC: Sky component, percent

ERC: Externally reflected component, percent

XIRC: Internally reflected component, percent

SKYLIT: Sky luminance, Im/ft^

SUNLIT: Direct sun luminance, Im/ft^

M: Maintenance factor

Non- industrial area 0.9

Dirty industrial area 0.8

G: Correction factor to be applied for

materials other than clear flat glass

Wired cast windows 0.9

Heavily diffusing glass 0.7

Double glazing 0.85

B: Correction factor for glazing bars

Metal windows 0.8

Wooden windows 0.7

Conventional

DLFTR: Daylight factor %

DLFTR = (SC + ERC + IRC) * M * G * B

6 = (SAZ - WAZ) * Tr/180

IF 5 ^0 , SUNLIT = SKYLIT

DALITE = (SKYLIT * (SC + XIRC) + SUNLIT * (ERC) ) * M * G * B/lOO,

B-5
SCERC (W, H, W]^, H^, D, w, h, f , RX, SC, ERC)

SC: Sky component, percent

ERC

t

RX

Externally reflected component, percent

Transmission coefficient of the window

Reflectance coefficient of the external obstruction

j-^ W

REFERENCE POINTh = vertical

obstruction, ft

External obstruction

viewed from window

from point P

SC = T * SF (W, H, W , H , h, D) * 100

1 1 ERC = RX * r * EOF ( W, H, W , H , h, D) * 100

1 1

B-6
IRC (W, A, p, RFW, RCN, RX, a, D, h)

IRC: Internally reflected component, percent

D: Distance of the reference point from the window

2 W: Window area, ft

2 A: Internal surface area, ft

h: Window obstruction height, ft

p: Average reflectance of the room

RFW: Average reflectance factor of the floor and of the wall area below

the horizontal plane through the window centroid excluding the

window wall

RCN: The same as RFW except that above the horizontal plane through the

window centroid

RX: Reflectance factor of external obstruction

a: Angle of obstruction from center of window (degrees above horizontal)

1. Determine window obstruction constant C as a function of a as follows

a 0 10 20 30 40 50 60 70 80

C 39 35 31 26 20 15 10 7 5

where

h = D * tan (a * TT/ISO)

B-7
SF;

EOF;

SF (W, H, W-^, H^, h, D)

and EOF (W, H, W^, H^, h, D)

Sky factor for a point P in the room

External obstruction factor

REFERENCE POINT

H =

2

HH =

=

H + H

1 + h

+ W

IF (W^ > 0)

SF = BCF (W H D) - BCF (W , H , D) - BCF (W^, HH, D) + BCF (W HH, D)

EOF = BWF (W2, HH, D) - BWF (W-, , HH, D) - BWF (W^, H^ , D) + BWF (W. , H , D)

IF W-j^ < 0

SF = BCF (W^, H^, D) + BCF (W^, H^, D) - BCF (W2, HH, D) - BCF (W^, HH, D)

EOF = BWF (W2, HH, D) + BWF (Wj^, HH, D) - BWF (W2, H-j^, D) - BWF (W^, H-,^, D)

IF SF < 0, SF = 0

B-8
BCF (W, H, D) and BWF (W, H, D)

BCF: Basic geometric factor between the overcast sky and a point in a room

BWF: Basic geometric factor between the window and a point in a room

w

H

REFERENCE POINT

Yet X

Y

A

BCF

BWF

W/D

H/D

/l + Y^

1

-1

[3 (tan"^ X - 1 tan"- X)

A A + 4 (tan'^ f-D

XY

\- [tan"l X - 4 tan-1 2^]

2Tr ' A A

B-9
Table 15-1 DAYLIGHT FACTOR DATA SHEET SAMPLE DATA

Altitude, degree AT T ju

Aziinuth , degree on Solar azimuth , degree C A7 QO

Distance from window ft. u J Window width ft.

W Window he ight f t

un

Q0 Horizontal distance of window WJ. -AD Vertical dist anc e o f window Hi U Room width (along t he window facade

)

KrlW Room length (Normal to the facade) Room height RMH

External obstruct ion % TJT nil u Room distance surface area, ft^ A Calculate * Window area ft^ T»7A WA. Reflection of external obstruction p U.J Reflection of walls and floor s\r w 0u • 7^ 0 . 4 Reflection of ceiling n A 0. 6 Maintenance factor M

Glass convection factor Q

Glazing bar factor 1 Sky ID factor ± olx 1 9 Window obstruction angle ALPHA 0 Daylight factor rvr* T?T'D JJUr Ltx Internal reflection component XIRC

1 Sky luminance SKYLIT Calculate

Sun's luminance SUNLTE Daylight luminance DALITE Transmission coefficient (glass) T 0.85

Sky component SC External reflection component ERC Calculate

*REMARKS ROOM INFORMATION A = 2 (RM(*RMH + RM (*RMW) + ^RM| * - W * Li

157.5?011.25

80

60

0.6 0.8

100

90

TK: 462.01: 3/25/76

B-10
In the following pages sample daylight-factors for the office

module are given as obtained by this program for a point 3 ft above

the floor at 15 ft from a window of 8 ft height located 3 ft above

the floor. As shown in Table B-1, window area was varied from 10,

25, 50 to 75% of the 120 sq. ft. facade. It is assumed in this

sample calculation that a wire cast metal window of transmission

factor 0.85 was used. The reflectance factors for the ceiling and

floor/wall are also varied as shown in the Daylight Factor data

sheet

.

The following parameter values are used for the daylight fact or

calculations

Dark Average Light

Wall/floor reflectance, RFW 0.2 0.4 0.6

Ceiling reflectance, RCN 0.2 0.4 0.6

External obstruction reflectance 0.1 0.4

Figures B-2 through B-5 are obtained from sample calculations to

depict the daylight factors in relation to the percent of window area,

window positions, distance of task, point from the window, and the

external obstructions, respectively.

With the daylight factor increases in near proportion to an

increase in window area, it decreases very rapidly as one moves away

from the window. Both the window height and the lateral position of

the window with respect to the task point have a strong effect on the

daylight factor. Figure B-5 shows that the daylight factors are also

B-11
20 30 40 50 60 70 80 90 100

WINDOW AREA, %

Figure B-2. Daylight factors vs window area % for various wall-ceiling

reflectance values

B-12
Figure B-3. Daylight factors vs window position.

B-13
1 r

0 5 10 15 20 25

DISTANCE FROM WINDOW

Figure B-4. Daylight factors vs distance of task point from the window.

B-14
1 \ \ r WINDOW 50 %

RFW = 0.2

RCN = 0.4 , Rx = 0.4

0 20 40 60 80 100

EXTERNAL OBSTRUCTION ANGLE, °a

Figure B-5. Daylight factors vs external obstruction angles.

B-15
Exact Daylight Calculation Using GLIM

The calculation procedure used in the previous analysis is a simplified version of more exact methods which include the actual simulation

of successive inter-reflection by inside surfaces. The exact calculation

process starts with initial excitation illumination from outside sources,

considers the first "inter-reflection" then the second, and continues the

interaction until the luminous values calculated on all intended surfaces

converge. One of the computer programs that treats this inter-reflection

process is called GLIM "General Lighting Inter-reflection Models" developed by the Applied Research of Cambridge, Ltd. in England. According

to D. Archer, author of GLIM, calculation by the inter-reflection factors

is accurate to within +157o of experimental measurements. This GLIM was

used to check the daylight factor calculated by the simplified NBS daylight routine "DALITE."

Figure B-6 and Table B-2 provide the physical dimensions for the

room and window which were used for the comparison calculation, whereas

Table B-3 is the summary of the 306 GLIM daylight factor calculations.

The sample room used for the comparison faces north and has the following

characteristics

1. Only one heat/light transfer surface which is the north facing

facade.

2. North-facing wall surface area = 12' (3.93"^) wide and 10'

(3.05TM) high.

3. Single-glazed regular windows with no fixed shading. However,

the window location and size will vary as shown in Table B-1.

4. No external shading.

5. Internal shading has two options: (1) being no shading,

and (2) Venetian blinds - open position (light colored).

B-16
Table B-2

NBS Daylight Calculation Data

Window Data

Run

^1

m

^2

m m m m

Area

2

m

Aspect

Ratio %

11 . 23 3.46 0 2.77 . 28 9 .59 .8 0 0 80

2 .23 3.46 . 80 2 .08 .17 7 .19 .6 60

3 . 87 2 . 19 o r\

. 80 2 .19 .06 4.79 1.0 40

. 55 o o o O A T "7 /*\ 1. 70 . 55 4 . 79 .6 40

5 . 23 3 . 46 . 80 1.38 O T

. 87 4 . 79 .4 / 0 40

6 1.42 1.09 o r\

. 80 2 . 19 .06 0 / 02 .40 2.0 20

7 1.19 1.55 .80 1.55 .70 2.40 1.0 20

oo . 23 3 . 4d Q n . oU . by 1 . 56 2 . 40 .2 20

9 1.69 .55 .80 2.19 .06 1.20 4.0 10

10 1.58 .77 .80 1.55 .70 1.20 2.0 10

11 1.42 1.09 .80 1.09 1.16 1.20 1.0 10

12 1.42 1.09 1.20 1.09 .76 1.20 1.0 10

13 1.42 1.09 1.60 1.09 .36 1.20 1.0, 10

14 1.19 1.55 .80 .77 1.48 1.20 .5 10

15 .23 3.46 .80 .35 1.90 1.20 .1 10

16 .23 3.46 1.60 .35 1.10 1.20 .1 10

17 .23 3.46 2.40 .35 .30 1.20 .1 10

Wall width = 3.93 m ' 2 ^1 ^2

Wall height = 3.05 m = + +

% = Glass area/wall area x 100 = • /(3.93 X 3.05)] X 100

Aspect ratio = ZrylX

B-17
ADIABATIC SURFACE

CO

ELEVATION

ADIABATIC SURFACE

ADIABATIC SURFACE

PLAN

4.6m

f

Y2

ADIABATIC SURFACE

0 ILLUMINATION POINT

WINDOW

Zi+Z2+Z3=3.05m

2 Yi+Y2=3.93m

22xY2= 11-98x[window %)

100

WINDOW

CO

CO

Figure B-6

B-18
o Q M H H

<:z WOQ

OS l-l o Qi<O

COo u z Di M <:

Of «po O <*.o o 2 <»c ooOO

a: <^o oo h <i<> o

pa (IP oo

CO

cot

t- I CO 61 Q

B.1-19
Table b-A

Inner Surface

Reflectance

Floor-wall/ce

Window Opening /i for 120 ft^ Facade

iling 75% 50% 25% 10%

.4/. 80

DALITE 6.0 4.0 2 .0.8

GLIM 4 .

o c J . b 1

4 A n 0 . y

.4/.

DALITE 4.4 2.8 1.4 0.6

GLIM 2.8 2.5 1.9 0.7

.2/.

DALITE 4.1 2.7 1.4 0.5

GLIM 2.8 2.4 1.8 0.7

.11

DALITE 3.0 2.0 1.1 0.4

GLIM 2.0 1.8 1.5 0.6

B-20
CO

GC

CD

^ 3

CD

>! 2 <

_ DALITE (NBS) and CIE

1 -

FACADE: 3.93m(L) x 3.05m(H)

ROOM DEPTH: 6.1m

BRIGHT CEILING

MEDIUM WALLS

DARK FLOOR

GLIM

J L

• GLIM

NBS

A CIE

I 20 40 60 80 100

WINDOW % OF FACADE

Figure B-7

B-21
REFLECTANCE

DAYLIGHT FACTORS FLOOR/CEILING/WALL

10.65 .5/.85/.8

3.56 .1/85/4

1.96 .I/.4/2

9.77

3.29

1.85

F/C /W

.5/85/8

.1/85/4

.I/.4/.2

Figure B-8. Effects of Varying Window Size and Room Reflectances Upon

Daylight Factors.

B-22
REFLECTANCE

DAYLIGHT FACTORS FLOOR/CEILING/WALL

7.20 5/85/8

2.66 .1/85/4

1.69 .I/.4/.2

6.31

2.31

L5I

F/C/W

. 5/85/8

. I /.85/.4

.1/4/2

6.34

2.33

1 .53

F/C/W

.5/85/8

.I/.85/.4

.I/.4 /2

gure B-9. Effect of Varying Window Location and Internal Reflectances

Upon Daylight Factor.

B-23
20%

REFLECTANCE

DAYLIGHT FACTORS FLOOR/CEILING/WALL

4.38

L94

L44

.5/85/. 8

.1/85/4

.1/4/2

3.23

LIS

0.77

F/C/W

.5/85/8

.1/85/4

.1/4/2

3.42

0.88

0.48

F/C/W

.5/85/8

.1/85/4

.I/.4/2

Figure B-10. Varying Window Location.

B-24
REFLECTANCE

10% DAYLIGHT FACTORS FLOOR/CEILING/WALL

2.68

0.97

0.62

.5/B5/.8

.I/.85/.4

.I/.4/.2

107*

2.21

0.80

0.56

F/C/W

.5/85/8

.I/.85/.4

.I/.4/.2

1.71

0.45

0.24

F/C/W

.5/ 85/. 8

.1/85/4

.1/4/2

Figure B-11. Varying Window Location.

B-25
strongly affected by the exterior obstruction angle when the reflectance

of the obstruction is high.

6. Reflectance of interior surfaces:

Table B-3 clearly shows that the daylight factors (at 15 feet from the

window) are strongly affected by the reflectance of the room surfaces,

with the same window percentage and with the same inner surface reflectances. The daylight factor could vary as much as a factor of 2 depending

upon the shape and location of the window in relation to given room

dimensions. Table B-4 and Figure B-7 compare the daylight factors

computed by the NBS routine DALITE against those determined by GLIM for

several window sizes and inner surfaces reflectance combinations. Except

for the largest window area (75% of the window wall), the agreements

between these two calculations are reasonably good.

Figure B-7 also indicates the daylight factors calculated by the

CIE (International Commission on Illumination) procedure described in

their publication No. 16 (E3.2), 1970.

Figures B-8 through B-11 are daylight factors as determined by the

GLIM program, shown together with the relative size, shape and location

of windows. The purpose of these figures is to demonstrate the effectiveness of the window design in terms of natural lighting.

It is clear, for example, from Figure B-8 through B-11 that the

location and shape of the window have a greater effect upon the

ceiling 0.85, 0.4

0.8, 0.4, 0.2

0.1, 0.3, 0.5

wa 1

floor

B-26
daylight factor for a small window area than for a larger area. The

daylight factors for 40% window area, shown in Figure B-9, are relatively unaffected by the location and shape of the window while

the opposite is true for a 10% window area (Fig. B-11).

EQUIVALENT SPHERE ILLUMINATION

The daylight calculation presented in the previous pages deals

only with the total illumination available on the horizontal plane

at 15 ft from the window over a task 3' above the floor. These daylight

factors do not indicate the quality of the illumination, however. The

quality of the illumination is usually affected by contrast, brightness

of the task, size of the task, time of viewing, reflected glare, and

veiling reflection and position of the viewer. The only meaningful overal

index which incorporates all or part of these parameters is the Equivalent

Sphere Illumination (ESI). The calculation of the ESI involves describing

the visual performance potential of a real environment of known actual

illumination in terms of the illumination under the reference conditions

of a photometric sphere providing equivalent performance potential.

23/

According to McNamara, ESI can be calculated by an equation such as

illustrated in Figure B-12.

Since GLIM is able to determine the illumination from various sources

in the room, with a slight modification, it would be possible to calculate

ESI by providing required task reflectance and contrast in sphere, and

view angle for the given task. This would add some assessment of the

quality of illumination provided by daylight to the thermal analysis

presented here.

B-27
Ce CRF=^

C

Cg= Effective contrast

C = Equivalent contrast

ESI=yO-'Lr(CRF)-6.54

p = the reflectance of the task In the sphere

Lr = luminance of task in the real world

CRF= Contrast rendition factor

ESI=/)-' Co

\6.54/M \-5.54

Cq= task contrast In sphere

Ej = foot candles from source i

/Sgj = background luminance factor for 9 =

6^ and^=^j

fij = the reflectance of the task in the real world

M = number of sources

Figure B-1

B-28
EXPERIMENTAL VALIDATION OF DAYLIGHT CALCULATIONS

In order to validate the daylighting calculation procedure described

in the previous sections, daylight intensity over a horizontal surface in

one of the north-facing office modules at the National Bureau of Standards

was measured and compared against the calculated values. The measurement

was made by a calibrated recording photometer placed at 15' from the

window on the surface 30" above the floor, on March 4-8, 1977, which

included clear sky, partially clear sky and completely overcast sky

conditions as shown in Figure B-13. In the calculation, it was

assumed that the reflectance of the wall, ceiling, and floor was 0.6,

0.7, and 0.2, respectively. The calculated daylight illumination under

the clear sky condition agrees fairly well with the observed value except

at the low sun's altitude angles. More studies of this nature are needed

to improve the daylight calculation methodology, particularly in terms of

its effect on the total building energy consumption.

It is suggested that a comprehensive research program be carried out

to obtain the following information:

1. Available outdoor daylight and skylight data under clear,

partially clear, and overcast sky conditions in relation to

the solar insolation data

2. Simultaneous measurement of indoor and outdoor illumination

to cover the following variables:

a. Window dimensions

b. Window shading

c. Window orientation

B-29
d. Sun's altitude

e. Interior surface reflectance

f. Exterior obstructions

Precise measurement of indoor surface reflectances and outdoor

obstruction reflectances.

Energy consumption measurement of daylight utilization system

vs. conventional system for heating and cooling of the office

as well as residential buildings

B-30
B-31
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPOR 1 NO. NBS ESS 109

2, Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Simplified Analysis of Thermal and Lighting Characteristics

of Windows: Two Case Studies

5. Publication Date February ly/o

6. Performing Organization Code

7. AUTHOR(S)

Tamani Kusuda and Belinda Lowenhaupt Collins

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

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

Energy Research and Development Administration

Washington, D.C. 20545; and

U.S. Department of Housing and Urban Development

Washington, D.C. 20410

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 77-600071

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

bibliography or literature survey, mention it here.)

Results of a simplified analysis for annual heating, cooling, and lighting

requirements associated with windows are presented. The analysis includes

the effects of window size, heat transfer, solar shading, and compass

orientation for typical commercial and residential modules located in a climate typical of Washington, D.C. Three different modes of operation

with respect to heating and cooling requirements through windows were

assessed: external loads only; external and internal; and external,

internal, and daylight. In addition, the effects of selective fenestration

heat-transfer management, such as planned employment of thermal shutters

and shading devices, and off-hour temperature setback were considered.

This analysis assumed that daylight could replace or supplement artificial

light whenever it could supply a specified minimum level of illumination.

The use of daylight was found to offer the greatest potential for reducing

energy costs, particularly when combined with selective fenestration

management

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

)

Daylighting; energy conservation; fenestration design; solar heat gain;

window management.

18. AVAILABILITY Unlimited

I

' For Official Distribution. Do Not Release to NTIS

IXX! Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C13 .29/2:109

I I Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

113

22. Price$2.75

USCOMM.DC 29042-P74
IIKistelleat

Guideliook

A typical plant can save about 20 percent of its fuel—just by installing waste heat recovery equip- ment. But with so much equipment on the market,

how do you decide what's right for you?

Find the answers to your problems in the Waste

Heat Management Guidebook, a new handbook

from the Commerce Department's National Bureau

of Standards and the Federal Energy Administration.

The Waste Heat Management Guidebook is designed to help you, the cost-conscious engineer or manager, learn how to capture and recycle heat

that is normally lost to the environment during in- dustrial and commercial processes.

The heart of the guidebook is 14 case studies of companies that have recently installed waste heat

recovery systems and profited. One of these applications may be right for you,- but even if it doesn't

fit exactly, you'll find helpful approaches to solving

many waste heat recovery problems.

In addition to case studies, the guidebook contains

information on:

• sources and uses of waste heat • determining waste heat requirements

• economics of waste heat recovery • commercial options in waste heat recovery

equipment

• instrumentation • engineering data for waste heat recovery • assistance for designing and installing waste

heat systems

To order your copy of the lA^as^e Heat Management

Guidebook, send $2.75 per copy (check or money

order) to Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

A discount of 25 percent is given on orders of 100

copies or more mailed to one address.

The Waste Heat Management Guidebook is part of the EPIC industrial energy management program

aimed at helping industry and commerce adjust to the increased cost and shortage of energy.

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

FEDERAL ENERGY ADMINISTRATION/Energy Conservation and Environment

*U.S. GOVERNMENT PRINTING OFFICE I 1978 0-261-238/13
There's

anew

look

to...

. . . the monthly

magazine of the National Bureau of Standards.

Still featured are special articles of general interest on

current topics such as consumer product safety and building

technology, in addition, new sections are designed to . . . PROVIDE

SCIENTISTS with illustrated discussions

of recent technical developments and

work in progress . . . INFORM INDUSTRIAL

MANAGERS of technology transfer activities in Federal and private labs. . . DESCRIBE TO MANUFACTURERS advances in the field of voluntary and

mandatory standards. The new DIMENSIONS/NBS also

carries complete listings of upcoming conferences to be

held at NBS and reports on all the latest NBS publications,

with information on how to order. Finally, each issue carries

a page of News Briefs, aimed at keeping scientist and consumalike up to date on major developments at the Nation's physical sciences and measurement laboratory.

(please delach here) SUBSCRIPTION ORDER FORM

Enter my Subscription To DIMENSIONS/NBS at $12.50. Add $3.15 for foreign mailing. No additional postage is required for mailing within the United States or its possessions. Domestic remittances should be made either by postal money order, express money order, or check. Foreign remittances should be made either by international money order, draft on an American bank, or by UNESCO

coupons. Send Subscription to:

Remittance Enclosed (Make checks payable

to Superintendent of Documents)

n Charge to my Deposit Account No.

NAME-FIRST, LAST

I I I I I I I M I I I I I I I I I I I I I I COMPANY NAME OR ADDITIONAL ADDRESS LINE

STREET ADDRESS

I I I M I I I I I I I I I I I I I I I I I I

I I I I I I I I I I I I I I I I

MAIL ORDER FORM TO:

Superintendent of Documents

Government Printing Office Washington, D.C. 20402

PLEASE PRINT
Announcement of New Publications

of the

National Bureau of Standards

Superintendent of Documents,

Government Printing Oflke,

Washington, D. C. 20402

Dear Sir:

Please add my name to the announcement list of new publications as issued by the National Bureau of Standards.

Name

Company

Address

City State Zip Code

(Notification Key N519)

NBS TECHNICAL PUBLICATIONS

PERIODICALS

JOURNAL OF RESEARCH—The Journal of Research

of the National Bureau of Standards reports NBS research and development in those disciplines of the physical and

engineering sciences in which the Bureau is active. These

include physics, chemistry, engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement methodology, and

the basic technology underlying standardization. Also in- cluded from time to time are survey articles on topics closely

related to the Bureau's technical and scientific programs. As

a special service to subscribers each issue contains complete

citations to all recent NBS publications in NBS and non- NBS media. Issued six times a year. Annual subscription: domestic $17.00; foreign $21.25. Single copy, $3.00 domestic; $3.75 foreign. Note: The Journal was formerly published in two sections: Section A "Physics and Chemistry" and Section B "Mathematical Sciences." DIMENSIONS/NBS

This monthly magazine is published to inform scientists, engineers, businessmen, industry, teachers, students, and consumers of the latest advances in science and technology, with primary emphasis on the work at NBS. The magazine

highlights and reviews such issues as energy research, fire protection, building technology, metric conversion, pollution

abatement, health and safety, and consumer product per- formance. In addition, it reports the results of Bureau pro- grams in measurement standards and techniques, properties of matter and materials, engineering standards and services, instrumentation, and automatic data processing.

Annual subscription: Domestic, $12.50; Foreign $15.65.

NONPERIODICALS

Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with interested industries, professional organizations, and regulatory bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other special

publications appropriate to this grouping such as wall charts, pocket cards, and bibliographies.

Applied Mathematics Scries—Mathematical tables, manuals, and studies of special interest to physicists, engineers, chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of

materials, compiled from the world's literature and critically evaluated. Developed under a world-wide program co- ordinated by NBS. Program under authority of National Standard Data Act (Public Law 90-396).

NOTE: At present the principal publication outlet for these data is the Journal of Physical and Chemical Reference Data (JPCRD) published quarterly for NBS by the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions, reprints, and supplements

available from ACS, 1155 Sixteenth St. N.W., Wash., D.C.

20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research

results, test methods, and performance criteria related to the

structural and environmental functions and the durability and safety characteristics of building elements and systems. Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often

serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures"

published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The purpose

of the standards is to establish nationally recognized require- ments for products, and to provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS administers this program as a supple- ment to the activities of the private sector standardizing

organizations. Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest to the consumer. Easily understandable language and

illustrations provide useful background knowledge for shopping in today's technological marketplace. Order above NBS publications from: Superintendent of Documents, Government Printing Office, Washington, D.C.

20402. Order following NBS publications—NBSIR's and FIPS from

the National Technical Information Services, Springfield, Va. 22161.

Federal Information Processing Standards Publications (TIPS PUB)—Publications in this series collectively consti- tute the Federal Information Processing Standards Register. Register serves as the official source of information in the Federal Government regarding standards issued by NBS

pursuant to the Federal Property and Administrative Services Act of 1949 as amended, Public Law 89-306 (79 Stat

1127), and as implemented by Executive Order 11717

(38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of

interim or final reports on work performed by NBS for outside sponsors (both government and non-government).

In general, initial distribution is handled by the sponsor;

public distribution is by the National Technical Information

Services (Springfield, Va. 22161) in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-aivareness and literature-survey bibli- ographies are issued periodically by the Bureau: Cryogenic Data Center Current Awareness Service. A litera- ture survey issued biweekly. Annual subscription: Domestic, $25.00; Foreign, $30.00.

Liquified Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00.

Superconducting Devices and Materials. A literature survey

issued quarterly. Annual subscription: $30.00. Send subscription orders and remittances for the preceding bibliographic

services to National Bureau of Standards, Cryogenic Data

Center (275.02) Boulder, Colorado 80302.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington. D.C. 20234

OFFICIAL BUSINESS

Penalty for Private Use, $300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE

COM-21S

U.S.MAIL

SPECIAL FOURTH-CLASS RATE

BOOK