Environmental Controls I/IGEnvironmental Controls I/IGEnvironmental Controls I/IGEnvironmental Controls I/IG

Lecture 12Cooling Loads

Cooling Degree HoursEnergy Performance Ratings

Annual Fuel Consumption

Lecture 12Cooling Loads

Cooling Degree HoursEnergy Performance Ratings

Annual Fuel Consumption

Cooling LoadsCooling LoadsComputed for worst case scenario: Late summer afternoon

at outdoor design dry bulb temperature

Include: Insolation from sun Heat gain from people,

lights, and equipment Infiltration in

residential buildings Ventilation in

nonresidential buildings

SR-3

Summer Design ConditionsSummer Design ConditionsDesign Dry Bulb TemperatureMean Daily Range

S: p. 1496, T.B1

Determine Design Equivalent Determine Design Equivalent Temperature Difference Temperature Difference

(DETD)(DETD)Construction type Outdoor design temperatureMean daily range

L: 0-16ºFM: 16-25ºF

H: 25+ºF

S: p. 1613, T.F.5

Determine Envelope U-Determine Envelope U-valuesvalues

Calculate ΣR and then find U for walls and roofs.

Note: this method ignores floors, doors, and window U-values

Determine DCLFDetermine DCLFGlazing TypeDesign TemperatureShadingOrientation

S: p. 1615, T.5.6

Determine Area QuantitiesDetermine Area Quantities

Perform area takeoffs for all building envelope surfaces on each facade:

gross wall areawindow areadoor areanet wall area

4’

Elevation

4’

12’

100’

8’

1200 sf

64 sf

368 sf

768 sf

-

-

InfiltrationInfiltration

S: p. 1617, T.F.7

Ventilation AnalysisVentilation Analysis

Non-residential buildings use ventilation to provide fresh air and to offset infiltration effects.

ASHRAE Standard 62-2001 (S: p. 1598, T.E.25)

Estimates the number of people/1000 sf of usage typePrescribes minimum ventilation/person for usage type

ASHRAE 62-2001ASHRAE 62-2001

Defines space occupancy and ventilation loads

S: p. 1598, T.E.25

Ventilation—Sensible CLFVentilation—Sensible CLF

S: 1617, T.F.7

People — Sensible GainPeople — Sensible Gain

Determine number of people

Activity level

S: p. 1617, T.F.8

LightsLights

Determine wattage of lighting/square foot

ASHRAE 90.1 prescriptive levels Count fixture loads and add together

Note: add 15% for ballasts where applicable (e.g., fluorescent lights)

EquipmentEquipment

Determine operating wattage of equipment/square foot

ASHRAE 90.1 prescriptive levels Count actual loads and add together

Note: include a diversity factor (20-30%) if specific usage patterns are unknown.

EquipmentEquipment

Use manufacturer’s data or other references to obtain heat gain data.

Standby mode Copiers Monitors Printers CPU “energy star”

S: p. 1618, T.F.9

Latent LoadLatent Load

Not calculated separately:

Apply a factor as a percentage of the total sensible cooling load

Dry climates: 20%Moist/Humid climates: 30%

Cooling Load Example Cooling Load Example ProblemProblem

Building: Office BuildingLocation: Salt Lake City

Building: 200’ x 100’ (2 stories, 12’-6” each)

Uwall= 0.054 Btuh/sf-ºF

Uroof= 0.025 Btuh/sf-ºF

Cooling Load Example Cooling Load Example ProblemProblem

Determine Building Envelope Areas (SF)

Building: 200’ x 100’ (2 stories, 12’-6” each)

N E S WGross Wall 5,000 2,500 5,000 2,500Windows 1,000 500 2,000 500Doors 20 20 50 20Net Wall 3,980 1,980 2,950 1,980

Roof/Floor Slab 20,000

Determine Design Equivalent Determine Design Equivalent Temperature Difference Temperature Difference

(DETD)(DETD)Roof Construction type: Light color, vented,

ceilingDesign temperature: 95ºFMean daily range: 32ºF

L: 0-16ºFM: 16-25ºF

H: 25+ºF

DETD=31.0ºF

S: p. 1613, T.F.5

Cooling LoadsCooling LoadsInsert roof values

SR-3

0.025 20,000 31.0 15,500 15,500

Determine Design Equivalent Determine Design Equivalent Temperature Difference Temperature Difference

(DETD)(DETD)Wall Construction type (see given) Design temperature: 95ºFMean daily range: 32ºF

L: 0-16ºFM: 16-25ºF

H: 25+ºF

DETD=11.3ºF

S: p. 1613, T.F.5

Cooling LoadsCooling LoadsInsert roof values

Insert wall values

SR-3

0.025 20,000 31.0 15,500 15,500

N 0.054 3,980 11.3 2.429E 0.054 1,980 11.3 1.208S 0.054 2,950 11.3 1,800W 0.054 1,980 11.3 1,208 6,645

Glazing TypeDesign TemperatureShadingOrientation

Determine Window DCLFDetermine Window DCLF

S: p. 1615, T.F.6

Cooling LoadsCooling LoadsInsert roof values

Insert wall values

Insert glass values

SR-3

0.025 20,000 31.0 15,500 15,500

N 0.054 3,980 11.3 2.429E 0.054 1,980 11.3 1.208S 0.054 2,950 11.3 1,800W 0.054 1,980 11.3 1,208 6,645

N 1,000 14 14,000E 500 35 17,500S 2,000 20 40,000W 500 35 17,500

89,000

ASHRAE 62-2001ASHRAE 62-2001

Defines space occupancy and ventilation loads

S: p. 1598, T.E.25

Ventilation Load — Sensible Ventilation Load — Sensible

40,000 sf x 5people/1,000sf = 200 people

200 people x 17 cfm/person = 3,400 cfm

Ventilation Load — Sensible Ventilation Load — Sensible CLF CLF

Design Temperature: 95ºFCommercial Building: Ventilation

S: p. 1617, T.F.7

Cooling LoadsCooling LoadsInsert roof values

Insert wall values

Insert glass values

Insert outdoor air values

SR-3

0.025 20,000 31.0 15,500 15,500

N 0.054 3,980 11.3 2.429E 0.054 1,980 11.3 1.208S 0.054 2,950 11.3 1,800W 0.054 1,980 11.3 1,208 6,645

N 1,000 14 14,000E 500 35 17,500S 2,000 20 40,000W 500 35 17,500

89,000

N/A N/A N/A

3,400 22.0 74,800 74,800

People — Sensible GainPeople — Sensible Gain

Determine number of people: 280

Activity level: moderately active office work

S: p. 1617, T.F.8

Cooling LoadsCooling LoadsInsert roof values

Insert wall values

Insert glass values

Insert outdoor air values

Insert people values

SR-3

0.025 20,000 31.0 15,500 15,500

N 0.054 3,980 11.3 2.429E 0.054 1,980 11.3 1.208S 0.054 2,950 11.3 1,800W 0.054 1,980 11.3 1,208 6,645

N 1,000 14 14,000E 500 35 17,500S 2,000 20 40,000W 500 35 17,500

89,000

N/A N/A N/A

3,400 22.0 74,800 74,800

200 250 50,000

Lighting LoadLighting Load

Lighting load: 1.5 w/sfEquipment load: 0.5 w/sf

Cooling LoadsCooling LoadsInsert roof values

Insert wall values

Insert glass values

Insert outdoor air values

Insert people values

Insert lighting values

Insert equipment values SR-3

0.025 20,000 31.0 15,500 15,500

N 0.054 3,980 11.3 2.429E 0.054 1,980 11.3 1.208S 0.054 2,950 11.3 1,800W 0.054 1,980 11.3 1,208 6,645

N 1,000 14 14,000E 500 35 17,500S 2,000 20 40,000W 500 35 17,500

89,000

N/A N/A N/A

3,400 22.0 74,800 74,800

200 250 50,000

40,000 1.5 204,780

40,000 0.5 68,260 323,040

Cooling LoadsCooling LoadsSensible Heat Gain: 508985 Btuh

Latent Heat Gain (20%): 101,797 Btuh

Total Heat Gain:610,782Btuhor50.9 Tons

Tons=Q/12,000SR-3

0.025 20,000 31.0 15,500 15,500

N 0.054 3,980 11.3 2.429E 0.054 1,980 11.3 1.208S 0.054 2,950 11.3 1,800W 0.054 1,980 11.3 1,208 6,645

N 1,000 14 14,000E 500 35 17,500S 2,000 20 40,000W 500 35 17,500

89,000

N/A N/A N/A

3,400 22.0 74,800 74,800

200 250 50,000

40,000 1.5 204,780

40,000 0.5 68,260 323,040

508,985101,797610,78250.9

2.5

1.1

14.6

16.783.3

52.8

12.3

100.0

Cooling Degree HoursCooling Degree HoursCooling Degree HoursCooling Degree Hours

Cooling Degree HoursCooling Degree Hours

Relative indicator of warmth

S: p. 1496, T.B.1

Cooling Degree HoursCooling Degree Hours

Balance Point Temperature (BPT): temperature above which cooling is needed

CDH(BPT)= ODBT-BPT

If temperature (ODBT)=91ºF

CDH74 =ODBT-74=91-74=17 cooling degree-hours

Energy Performance RatingsEnergy Performance RatingsEnergy Performance RatingsEnergy Performance Ratings

Performance RatingsPerformance Ratings

COP: coefficient of performanceEER: energy efficiency at full loadSEER: seasonal energy efficiency

ratio

Note:SEER≈COP x 3.413

Annual Fuel ConsumptionAnnual Fuel ConsumptionAnnual Fuel ConsumptionAnnual Fuel Consumption

Annual Fuel Usage (E)Annual Fuel Usage (E)

E= UA x CDH(BPT)

SEER

where:UA: cooling load/ºFCDH(BPT): degree hours for balance pointSEER: seasonal energy efficiency rating

Calculating UACalculating UA

QTotal= UA x ΔT

UA= QTotal/ΔT

From earlier example:QTotal= Btuh ΔT= 95-75=20ºF

UA=610782/20= 30,539 Btuh/ºF

Annual Fuel Usage ExampleAnnual Fuel Usage Example

Compare two systems to determine what is the expected annual electrical usage for an apartment in Salt Lake City if its peak cooling load is 12,000 Btuh?

UA=Q/ΔTUA=12,000/20= 600 Btuh/ºF

Determine SEERDetermine SEER

Obtain SEER from manufacturer’s data or Convert COP to SEER

SEER: 5-15

For this example:SEER1=6.8

SEER2=10.2

Annual Fuel Usage — Annual Fuel Usage — Electricity Electricity

E= UA x CDH74 SEER

E1 =(600)(9,898)/(6.8) =873,353 wh/yr

=873 kwh/yr

If electricity is $0.0735/kwh, thenannual cost = $64

Annual Fuel Usage — Annual Fuel Usage — Electricity Electricity

E= UA x CDH74 SEER

E2 =(600)(9,898)/(10.2)=582,235 wh/yr

=582 kwh/yr

If electricity is $0.0735/kwh, thenannual cost = $43

Simple Payback AnalysisSimple Payback AnalysisSimple Payback AnalysisSimple Payback Analysis

Simple PaybackSimple Payback

Cooling System Cost ComparisonFirst

Cost ($)

System 1 500

System 2 600

Simple PaybackSimple Payback

Cooling System Cost ComparisonFirst Annual Incremental Incremental Simple

Cost Fuel Cost First CostAnnual Savings Payback ($) ($/yr) ($) ($/yr) (yrs)

System 1 500 64 --- --- ---System 2 600 43 100 21 4.8

Payback exceeds 3 years, select system 1Other factors?