Energy Efficient Building Design
College of Architecture
Illinois Institute of Technology, Chicago
INTERNAL HEAT GAINS (IHG)
The sources of internal heat gains (IHG) include:
1. PEOPLE (sensible and latent heat gain)
2. LIGHTS (sensible heat gain only)
3. EQUIPMENT
(a) Receptacles or electrical plug loads (sensible heat gain only)
(b) Processes such as cooking (sensible and latent heat gain)
IHG can be a major component of the total building cooling load. This is particularly true of nonresidential (commercial, institutional and industrial) buildings. IHG for lights can be calculated if
the type and number of lighting fixtures are known. This is also true for electrical equipment.
IHG for people and process loads are approximate since the level of activity varies.
IHG loads for each hour of the year is estimated on the basis of percent of peak design load. Like
the hourly weather data that affects energy loads due to the building envelope, infiltration and
ventilation, internal loads can vary from hour to hour and year to year.
A range of IHG design hour values from low, average and high can estimated on the basis of type
of building. This is the type of information that is available. Such estimates apply to a particular
region, country, economy and society. After the building is designed and built, it can be underused or over-used. The building can be used for purposes other than what it was designed for. In
the case of office buildings, lighting loads have decreased due to more efficient lighting and
equipment loads have increased due to computers and telecommunication equipment.
In the case of under-usage, building’s control system will adjust the cooling system at the expense
of inefficient use of the cooling equipment. In the case of over-usage, the building’s cooling
capacity must be increased. Poor judgment in estimating IHG can result in unsatisfactory
operation. As with building envelope loads, IHG estimating procedures are therefore rigorous and
precise using the best information available for the given type of building.
Latent heat (moisture or water vapor) from people and equipment added to the space is an
instantaneous cooling load. Sensible heat generated by internal heat sources (people, lights and
equipment) is a time-delayed cooling load. As with solar radiation heat entering the space, part of
sensible heat generated by internal sources is first absorbed by the surroundings and then
gradually released into the air increasing it’s temperature. The air temperature is sensed by the
control system (thermostat) which operates the cooling system and equipment. So there is a timedelay in the corrective action also.
To allow for the time delay due to thermal storage, Cooling Load Factors (CLF) were developed
to estimate the heat gains from internal heat emitting sources. CLFs are based on the time (hour)
when the internal source starts to generate heat load and the number of hours it remains in
operation. This information is expressed as hourly internal load profiles (percent of design).
Instructor: Varkie C. Thomas, Ph.D., P.E.
Skidmore, Owings & Merrill LLP
ARCH-551 (Fall-2002) ARCH 552 (Spring-2003)
F14 - 1
Energy Efficient Building Design
College of Architecture
Illinois Institute of Technology, Chicago
PEOPLE (P)
Q-ps
= N-p * Fu * qs * CLF-h (sensible heat gain)
Q-pl
= N-p * Fu * ql (latent heat gain)
Q-ps
Q-pl
N-p
= Sensible Heat Gain (SHG) from people
= Latent Heat Gain (LHG) from people
= Number of people (maximum or design from occupancy criteria for building)
Fu
= Diversity factor or percentage of maximum design for each hour of the day
= 0 when there are no people in the room
= 1 when the maximum design number of people are in the room
0 <= Fd <= 1
qs
= sensible heat gain (SHG) per person for the degree or type of activity in the space
(ASHRAE Table 8.18 ). Examples, 245 btu/hr per person when working in an
office and 580 btu/hr per person performing heavy manual work in a factory.
ql
= latent heat gain (LHG) per person for the degree or type of activity in the space
(ASHRAE Table 8.18 ). Examples, 155 btu/hr per person when working in an
office and 870 btu/hr per person performing heavy manual work in a factory.
CLF-h = Cooling Load Factor (CLF) for given hour. This depends on zone type, hour entering
space, and number of hours after entering into space (ASHRAE Table 8.19).
The sensible heat has to be first absorbed by the surroundings and then released into the air. The
cooling load factor accounts for this time delay. The latent heat is an instantaneous cooling load
so there is no cooling load factor associated with it. The following table gives examples of SHG
and LHG from people. It demonstrates the range of heat gain values due to people. When this
information is combined with design space occupancy density (25 ft2/person for an aerobics class
and 250 ft2/person for an apartment) the heat gain from people becomes very significant.
Level of Activity
Seated at rest
Seated, light work
Moderate office work
Standing, walking slowly
Light bench work
Dancing
Heavy work
Typical
Application
Theater
Office
Office
Retail Sales
Factory
Nightclub
Factory
Heat Gain / Person btuh
SHG (qs)
LHG (ql)
245
105
245
155
250
200
250
250
475
275
545
305
870
580
Figure ??
Instructor: Varkie C. Thomas, Ph.D., P.E.
Skidmore, Owings & Merrill LLP
ARCH-551 (Fall-2002) ARCH 552 (Spring-2003)
F14 - 2
Energy Efficient Building Design
College of Architecture
Illinois Institute of Technology, Chicago
Example
The following example is for the 5 zones of a rectangular building where the solar heat gain (24oN
latitude) for each zone peaked at different hours. Assume that the design number of people enters
the space at 8:00 AM and remains in the space until 6:00 PM (10 hours). In reality the number of
people per hour will vary and this must also be taken into account.
Building : 120'L x 120'W x 20'H
N
W
SHG/Person
250
People /Zone
10
Zone Peak People Cooling Loads
Zone
Pk Hr
Hrs IN
CLF
BTUH
N
17
9
0.92
2,300
Latitude : 24 N
E
9
1
0.62
1,550
Zone Type = C
S
15
7
0.89
2,225
Total Hours in Space = 10
W
17
9
0.92
2,300
Month/Day : July 21
E
o
S
Q(people) = SHG/P * Np * CLF
Building Envelope (Walls and Windows) Cooling Loads
ZN/HR
8
9
10
11
12
13
14
15
16
17
18
N
13,600
14,720
16,560
18,480
19,920
21,600
22,000
22,320
23,120
24,000
21,680
E
43,920
48,320
46,480
39,760
33,120
30,960
28,880
27,040
25,040
22,480
19,040
S
9,920
12,320
14,640
17,440
19,440
21,440
21,920
22,240
21,440
19,680
17,200
W
10,320
12,720
15,040
17,200
19,200
26,320
37,600
48,880
56,560
58,320
46,480
Cooling Load Factors for People
People ZONE Type = C
Hours: 8:00 AM to 6:00 PM (10 Hours Total)
Total
Time of Day
Hours
8
Space
Number of hours after entry into space
0
9
10
11
12
13
14
15
16
17
18
19
0
1
2
3
4
5
6
7
8
9
10
2
0.60
0.68
0.14
0.11
0.09
0.07
0.06
0.05
0.04
0.03
4
0.60
0.68
0.74
0.79
0.23
0.18
0.14
0.12
0.10
0.08
6
0.61
0.69
0.74
0.79
0.83
0.86
0.28
0.22
0.18
0.15
8
0.61
0.69
0.75
0.79
0.83
0.86
0.89
0.91
0.32
0.26
10
0.62
0.70
0.75
0.80
0.83
0.86
0.89
0.91
0.92
0.94
12
0.63
0.71
0.76
0.81
0.84
0.87
0.89
0.91
0.93
0.94
14
0.65
0.72
0.77
0.82
0.85
0.88
0.90
0.92
0.93
0.94
16
0.68
0.74
0.79
0.83
0.86
0.89
0.91
0.92
0.94
0.95
18
0.72
0.78
0.82
0.85
0.88
0.90
0.92
0.93
0.94
0.95
250
People /Zone
Occupancy (People) Cooling Load, Each Zone
Cooling
1,550
1,750
Instructor: Varkie C. Thomas, Ph.D., P.E.
1,875
2,000
2,075
Skidmore, Owings & Merrill LLP
SHG/Person
2,150
2,225
2,275
2,300
10
2,350
ARCH-551 (Fall-2002) ARCH 552 (Spring-2003)
F14 - 3
Energy Efficient Building Design
College of Architecture
Illinois Institute of Technology, Chicago
LIGHTS (L)
Q-l
= (W * 3.412) * Fu * Fs * CLF-h (sensible heat gain)
Q-l
W
= Sensible Heat Gain (SHG) from lights
= Lighting power output in Watts (Btu/hr = W * 3.412)
Fu
= Usage factor or percentage of maximum design for each hour of the day
= 0 when all lights are off
= 1 when the maximum design number lights are on
0 <= Fu <= 1 Example Fu = 0.5 when 50% of lights are on.
Fs
= Service Allowance Factor or Multiplier (accounts for ballast losses in fluorescent lights
and heat returned to return air ceiling plenum in the case of air-light fixtures)
CLF-h = Cooling Load Factor (CLF) for given hour. This depends on zone type, total
hours that lights are on, and number of hours after lights are turned on,
The sensible heat has to be first absorbed by the surroundings and then released into the air. The
cooling load factor accounts for this time delay.
Example
Section through Lights, Ceiling and Space
Plan View of Flourescent Lights
Light ON
Light OFF
Light
Ceiling Plenum
Light
OFF
ON
Fs1 = 1.2 = Ballast Factor
Return Air from Space
Ceiling
Fs2 = 0.8 = Light Heat to Plenum (20% to plenum)
(only 80% of light heat enters space)
Two 48" x 24" flourescent light fixtures at 100 watts / fixture.
Fs = Fs1 * Fs2 = 1.2 * 0.8 = 0.96
Fu = 0.50 (50% of lights are on)
Q-l = W * 3.412 * Fu * Fs * CLF-h = 200 * 3.412 * 0.50 * 0.96 * CLF-h = 327.5 * CLF-h
Instructor: Varkie C. Thomas, Ph.D., P.E.
Skidmore, Owings & Merrill LLP
ARCH-551 (Fall-2002) ARCH 552 (Spring-2003)
F14 - 4
Energy Efficient Building Design
College of Architecture
Illinois Institute of Technology, Chicago
Example
The following example is for the 5 zones of a rectangular building where the solar heat gain (24oN
latitude) for each zone peaked at different hours.
Building : 120'L x 120'W x 20'H
Watts/ft2
1.5
Watts
N
W
E
S
Q(lights) = Watts * 3.412 * CLF
Zone Peak People Cooling Loads
21,000
Zone
Pk Hr Hrs ON
CLF
BTUH
Month/Day : July 21
N
17
9
0.88
18,480
Latitude : 24oN
E
9
1
0.68
14,280
Zone Type = D
S
15
7
0.86
18,060
Total Hours Lights ON = 10
W
17
9
0.88
18,480
Building Envelope (Walls and Windows) Cooling Loads
ZN/HR
8
9
10
11
12
13
14
15
16
17
18
N
13,600
14,720
16,560
18,480
19,920
21,600
22,000
22,320
23,120
24,000
21,680
E
43,920
48,320
46,480
39,760
33,120
30,960
28,880
27,040
25,040
22,480
19,040
S
9,920
12,320
14,640
17,440
19,440
21,440
21,920
22,240
21,440
19,680
17,200
W
10,320
12,720
15,040
17,200
19,200
26,320
37,600
48,880
56,560
58,320
46,480
Cooling Load Factors for Lights
Lights ZONE Type = D
Hours: 8:00 AM to 6:00 PM (10 Hours Total)
Total
Time of Day
Hours
8
Space
Number of hours after entry into space
0
9
10
11
12
13
14
15
16
17
18
19
0
1
2
3
4
5
6
7
8
9
10
8
0.66
0.72
0.76
0.79
0.81
0.83
0.85
0.86
0.25
0.20
10
0.68
0.74
0.77
0.80
0.82
0.84
0.86
0.87
0.88
0.90
12
0.70
0.75
0.79
0.81
0.83
0.85
0.87
0.88
0.89
0.90
14
0.72
0.77
0.81
0.83
0.85
0.86
0.88
0.89
0.90
0.91
16
0.75
0.80
0.83
0.85
0.87
0.88
0.89
0.90
0.91
0.92
Lighting Cooling Load, Each Zone
Cooling
14,280
15,540
Instructor: Varkie C. Thomas, Ph.D., P.E.
16,170
Watts/ft2
16,800
17,220
Skidmore, Owings & Merrill LLP
17,640
18,060
1.5
18,270
Watts /Zone
18,480
18,900
ARCH-551 (Fall-2002) ARCH 552 (Spring-2003)
21,000
0
F14 - 5
Energy Efficient Building Design
College of Architecture
Illinois Institute of Technology, Chicago
EQUIPMENT (E)
Equipment consists of three categories
1. electric resistance sensible load (ex. Toaster)
2. electric inductive sensible load (ex. Motor)
3. sensible and latent loads (ex. Electric or Gas Tea Kettle)
Equipment Sensible Heat Gain
Q-es
= (W * 3.412) * Fu * Fp * CLF-h (sensible heat gain from electric resistance, btu/hr)
Q-es
= 2545 * ( HP / Eff ) * Fu * Fp * CLF-h (sensible heat gain from electric motor, btu/hr)
W
= Equipment output in Watts (Btu/hr = W * 3.412)
Fu
= Usage factor or percentage of maximum design for each hour of the day
= 0 when all equipment are off
= 1 when the maximum design number equipment are on
0 <= Fu <= 1 Example Fu = 0.5 when 50% of equipment are on.
Fp
= Part load operating factor for motor type. Example, a compressor operating at 50%
capacity might still use 80% of electric power.
HP
Eff
= Rated electrical horsepower of equipment motor (Btu/hr = HP * 2545)
= Motor Efficiency
CLF-h = Cooling Load Factor (CLF) for given hour. This depends on zone type, total
hours that lights are on, and number of hours after lights are turned on,
The sensible heat has to be first absorbed by the surroundings and then released into the air. The
cooling load factor accounts for this time delay. The heat generated by a motor is a cooling load
only if the motor is located inside the air-conditioned space or in the ducted supply air stream and
it is not directly exhausted away from the source.
Equipment Latent Heat Gain
Q-el
= Mw * Hfg * Fu * Fp
Q-el
= latent heat gain from equipment (btu/hr)
Mw
= Mass (lbs) of water converted to steam (evaporated or boiled)
Hfg
= Heat (btu/hr) required to convert 1 lb of water to steam = 1075 at standard conditions
The latent heat from equipment such as tea kettles and dish-washers is an instantaneous cooling
load. Cooling Load Factors (CLF) do not apply to latent loads.
Instructor: Varkie C. Thomas, Ph.D., P.E.
Skidmore, Owings & Merrill LLP
ARCH-551 (Fall-2002) ARCH 552 (Spring-2003)
F14 - 6
Energy Efficient Building Design
College of Architecture
Illinois Institute of Technology, Chicago
Usage Factors (Fu)
These are also referred to as operating schedules or profiles
Occupancy Schedule (Profile)
Weekday Hour
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Occupancy (%)
0
0
0
0
0
5
10
20
95
95
95
95
50
95
95
95
95
30
10
5
0
0
0
0
Occupancy Profile
100
80
Hour
60
40
20
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Percent of Design
Lighting Schedule (Profile)
Weekday Hour
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Lighting (%)
5
5
5
5
5
10
35
50
90
90
90
90
90
90
90
90
90
90
50
35
10
5
5
5
Lighting Profile
100
Hour
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Percent of Design
Equipment Schedule (Profile)
Weekday Hour
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Receptacles (%)
0
0
0
0
0
10
20
50
90
90
90
90
50
90
90
90
90
70
50
30
20
5
0
0
Receptacle Loads Profile
100
Hour
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Percent of Design
Instructor: Varkie C. Thomas, Ph.D., P.E.
Skidmore, Owings & Merrill LLP
ARCH-551 (Fall-2002) ARCH 552 (Spring-2003)
F14 - 7
Energy Efficient Building Design
College of Architecture
Illinois Institute of Technology, Chicago
EXAMPLE (Internal Heat Gains)
Building Type: Factory
Dimensions: 600’ Long, 300’ Wide, 30’ High. Area = 18,000 ft2, Volume = 540,000 ft3
Zone Types
Heat Source
Zone Type
Solar
A
People
C
Lights
D
Equipment
C
Occupancy Criteria
Occupancy density = 1 Person per 100 ft2 (100 ft2/P)
1/3 of occupants performing office work (seated). 250 SHG/P 200 LHG/P
2/3 of occupants performing factory work (heavy). 600 SHG/P 900 LHG/P
All occupants enter space at 8:00 AM and leave at 6:00 PM (10 hours total).
Occupancy Profile: 70% at 9:00 AM, 90% at 2:00 PM, 80% at 5:00 PM
Lighting Criteria
2000 fluorescent 48” x 24”light fixtures with two 40 watt lamps per fixture. Lighting
ballast factor = 1.2.
1000 fluorescent 48” x 24”return air light fixtures with two 40 watt lamps per fixture.
30% of light heat returned to ceiling plenum. Lighting ballast factor = 1.2.
300 incandescent 100 watt light fixtures.
Lighting Profile: 90% at 9:00 AM, 100% at 2:00 PM, 90% at 5:00 PM
Equipment Criteria
50000 watts of miscellaneous electrical (plug in receptacle) loads
100
five (5) horsepower (HP) pieces of equipment (drills, etc.)
1500 lbs/hr of steam generated by various processes.
Equipment Profile: 50% at 9:00 AM, 80% at 2:00 PM, 60% at 5:00 PM
Calculate:
The Sensible and Latent heat gains from People, Lights and Equipment at:
(1) 9:00 am (2) 2:00 pm (3) 5:00 pm
=====================================================================
Heat Gain from People
Q-ps
Q-pl
= N-p * qs * Fu * CLF-h
= N-p * ql * Fu
(sensible heat gain)
(latent heat gain)
No. of people = 180,000 ft2 / 100 ft2 / Person = 1,800 people
Office:
No. of people at office work = 1800 * 1/3 = 600 = Np
SHG = 600 people * 250 btuh/person (qs) = 150,000 btuh = ( Np * qs )
LHG = 600 people * 200 btuh/person(ql) = 120,000 btuh = ( Np * ql )
Instructor: Varkie C. Thomas, Ph.D., P.E.
Skidmore, Owings & Merrill LLP
ARCH-551 (Fall-2002) ARCH 552 (Spring-2003)
F14 - 8
Energy Efficient Building Design
Factory:
College of Architecture
Illinois Institute of Technology, Chicago
No. of people at factory work = 1800 * 2/3 = 1200 = Np
SHG = 1200 people * 600 btuh/person (qs) = 720,000 btuh = ( Np * qs )
LHG = 1200 people * 900 btuh/person (ql) = 1,080,000 btuh = (Np * ql)
Total SHG = 150,000 + 720,000 = 870,000
Total LHG = 120,000 + 1,080,000 = 1,092,000
Zone Type = C (given). Total Hours in space (8:00 AM to 6:00 PM) = 10
Fu (fraction of max people)
LHG = 1,092,000 * Fu
CLF (Zone-C)
SHG = 870,000 * Fu * CLF-h
9:00 AM
0.7
764,000
0.62
377,580
2:00 PM
0.9
982,800
0.86
673,380
5:00 PM
0.8
873,600
0.92
640,320
=====================================================================
Heat Gain from Lights
Q-l
= (W * 3.412) * Fu * Fs * CLF-h (sensible heat gain)
Zone Type = C (given). Total Hours that lights are ON (8:00 AM to 6:00 PM) = 10
Fu (fraction of max) and CLF vary with time. Fs is constant for given fixture.
(1) Fluorescent Fixtures (Fu and CLF not considered)
2000 fixtures with two 80 watt lamps. 2000 * 80 * 2 = 320,000 watts. Ballast Factor = 1.2 (Fs)
Sensible Heat Gain = W * Fs * 3.41 = 320,000 * 1.2 * 3.41 = 1,309,440 btu/hr.
(2) Air-Light Fluorescent Fixtures (Fu and CLF not considered)
1000 fixtures with two 40 watt lamps. 1000 * 40 * 2 = 80,000 watts
30% of heat to ceiling plenum, 70% (0.7) to space. Ballast Factor = 1.3. Fs = 1.3 * 0.7 = 0.91.
Sensible Heat Gain = W * Fs * 3.41 = 80,000 * 0.91 * 3.41 = 248,248 btu/hr.
(3) Incandescent Fixtures (Fu and CLF not considered)
300 fixtures at 100 watts each. 300 * 100 = 30,000 watts. Fs = 1.0 for incandescent
Sensible Heat Gain = W * 3.41 = 30,000 * 3.41 = 102,300 btu/hr.
9:00 AM
2:00 PM
5:00 PM
Fu (fraction of max usage)
0.9
1.0
0.9
CLF (Zone-D)
0.68
0.84
0.88
-----------------------------------------------------------------------------------------------(1) Fluorescent
801,377
1,099,930
1,037,076
(2) Air-Light Fluorescent
151,928
208,528
196,612
(3) Incandescent
62,608
85,932
81,022
-----------------------------------------------------------------------------------------------TOTAL
1,015,913
1,394,390
1,314,710
Instructor: Varkie C. Thomas, Ph.D., P.E.
Skidmore, Owings & Merrill LLP
ARCH-551 (Fall-2002) ARCH 552 (Spring-2003)
F14 - 9
Energy Efficient Building Design
College of Architecture
Illinois Institute of Technology, Chicago
Heat Gain from Equipment
Q-es
Q-es
Q-el
= (W * 3.412) * Fu * Fp * CLF-h (sensible heat gain from electric resistance, btu/hr)
= 2545 * ( HP / Eff ) * Fu * Fp * CLF-h (sensible heat gain from electric motor, btu/hr)
= Mw * Hfg * Fu * Fp (latent heat gain from equipment, btu/hr)
Zone Type = C (given). Total Hours equipment is ON (8:00 AM to 6:00 PM) = 10
Fu (fraction of max), Fp (part load efficiency) and CLF vary with time.
(1) Miscellaneous electrical (resistance) loads
50,000 watts. Sensible Heat Gain = 50,000 * 3.41 = 170,500 btu/hr.
(2) Motors (inductive) loads
100 five HP motors. Sensible Heat Gain = 100 * 5 * 2545 = 1,272,500 btu/hr.
(3) Steam
1500 lbs/hr. Latent Heat Gain = 1,500 lbs/hr * 1075 btu/lb = 1,612,500 btu/hr.
9:00 AM
2:00 PM
5:00 PM
Fu (fraction of max usage)
0.5
0.8
0.6
CLF (Zone-C)
0.62
0.86
0.92
-----------------------------------------------------------------------------------------------(1) Miscellaneous (resistance)
52,855
80,705
89,001
(2) Motors (inductive)
394,475
875,480
664,245
-----------------------------------------------------------------------------------------------TOTAL (sensible)
447,330
956,185
753,246
-----------------------------------------------------------------------------------------------(3) Steam (latent)
499,875
1,109,400
841,725
-----------------------------------------------------------------------------------------------Summary (Internal Cooling Loads)
-----------------------------------------------------------------------------------------------Sensible Heat Gain (SHG)
9:00 AM
2:00 PM
5:00 PM
People
377,580
673,380
640,320
Lights
1,015,913
1,394,390
1,314,710
Equipment
447,330
956,185
753,246
-----------------------------------------------------------------------------------------------Total SHG
1,840,823
3,023,955
2,708,276
-----------------------------------------------------------------------------------------------Latent Heat Gain (LHG)
9:00 AM
2:00 PM
5:00 PM
People
764,000
982,800
873,600
Equipment
499,875
1,109,400
841,725
-----------------------------------------------------------------------------------------------Total LHG
1,263,875
2,092,200
1,715,325
Tons = (SHG + LHG) / 12,000
258.7
426.3
368.6
Instructor: Varkie C. Thomas, Ph.D., P.E.
Skidmore, Owings & Merrill LLP
ARCH-551 (Fall-2002) ARCH 552 (Spring-2003) F14
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