Page 1

Peng Xu, Philip Haves, James Braun, MaryAnn Piette

January 23, 2004Sponsored by the California Energy Commission and

the California Institute for Energy Efficiency

A Case Study of Precooling with Zone

Temperature Reset in a Commercial Office

Building

Page 2

Outline

• Aim

• Test site description

• Precooling and demand shedding strategies

• Test results

• Utility analysis

• Conclusions and future work

Page 3

Aim

Demonstrate the potential for reducing peak-period electrical demand in a moderate-weight commercial building using building structural mass.

Page 4

Executive Summary

• Precooling and zone temperature reset can shift up to 100% of cooling power from on- to off-peak in a moderate-weight commercial buildings

• Electricity peak demand reduced by as much as 2.3 W/ft2

• Potential for cost savings when combined with Critical Peak Pricing

Page 5

Case Study - Introduction

• Started in August 2003

• Purpose - preliminary study to assess potential and need for further work– Potential demand reduction/load shifting in

moderate-weight buildings– Effectiveness of precooling and zone

temperature reset – Thermal comfort

Page 6

Test Site Description• Medium-sized government building

– Santa Rosa, CA– 80,000 ft2 (40,000 ft2 office + 40,000 ft2 courthouse)– 3 stories

• Relatively low mass building structure– 6 in. concrete floor, 4 in. concrete wall, medium

furniture density, standard commercial carpet

• High window-to-wall ratio – Floor to ceiling glazing on south and north façade

• Typical internal loads • Number of occupants

– ~100 (office side)

Page 7

South Facade

Page 8

North Facade

Page 9

Windows

• Single glazing• Tinted glass• Internal blinds

Page 10

Cooling Plant

• West wing: three 75-ton, 30-year old McQuay air-cooled chillers

• East wing: two 60-ton, 10-year old Carrier air-cooled chillers

• Constant-speed water pumps, one for each chiller

• Two stage compressors

Page 11

HVAC Secondary Systems

Five VAV air handling units (three single duct & two dual duct systems):

• Variable-speed supply and return fans

• 50-60 VAV zones • DDC control (Alerton)• No global rest of zone

temperature

Page 12

Building Operation

• 5 am - 8 am - start up

• 8 am - 5 pm - occupancy

• 2-3 hot/cold calls per month

• No major faults, some modest problems: – One undersized cooling coil– Some air balance problems– Lack of reheat coils in single-duct systems

causes temp control problems

Page 13

Precooling & Zonal Reset Strategies

66

68

70

72

74

76

78

80

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

Tem

p (

oF

)

current precooling+zonal reset

extended precooling + zonal reset

unoccupied hours occupied hours unoccupied hours

floating

floating

floatingfloating

floating

zonal reset

precooling

Page 14

Monitoring

• Existing:– Whole building power meters– Chiller power meters– Weather station– HVAC performance data from EMCS

• ~500 data points• 15 minute intervals

• Added:– AHU fans power meters– Operative temperature sensors

Page 15

Operative Temperature & Comfort

Convectiveheat exchange

Radiant heat exchange

RoomRegular temperature sensor

globe

Operative temperature sensor

screen

Page 16

Peak OA Temperature vs. Peak Demand

Whole building power vs Peak OA temp

0

50

100

150

200

250

300

350

400

450

40 50 60 70 80 90 100 110

Outside air peak temperature

wh

ole

bu

ildin

g p

ow

er

(kW

)

-0.2

0.8

1.8

2.8

3.8

4.8

5.8

wh

ole

bu

ildin

g p

ow

er

(kW

/sf)

weekend/holidays

working days

CPP hours

Page 17

Classification of Weather Conditions

40

60

80

100

120

10/1 10/6 10/11 10/16 10/21 10/26 10/31

Ou

tsid

e ai

r te

mp

(oF

)

Hot daysWarm daysCool days

Page 18

Comparison of Baseline & Test Conditions

40

60

80

100

120

10/7 10/8 10/9 10/10 10/11

Warm daybaseline

Warm dayprecooling

Cool daybaseline

Cool dayprecooling

10/2310/22

Page 19

Tests Performed

Limited Precooling + zonal reset

Extended Precooling + zonal reset

Cool days 3Warm days 5 1Hot days 2

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Cool Days – Limited Precooling

Limited precooling (Cool days)

00.5

11.5

22.5

33.5

4

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (hour)

Wh

ole

bu

ildin

g p

ow

er

W/s

qft

Limited precooling 1 Limited precooling 2 baseline

1 W/sf shed

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Warm Day – Limited Precooling

Limited precooling (Warm days)

0

0.5

1

1.5

2

2.5

3

3.5

4

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (hour)

Wh

ole

bu

ildin

g p

ow

er

W/S

qft

baseline Limted precooling

1.4 W/sf shed

Page 22

Extended & Limited Precooling

Limited and Extended Precooling (warm days)

00.5

11.5

22.5

33.5

4

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (hour)

Wh

ole

bu

ildin

g p

ow

er W

/sq

ft

baseline Limited precooling Extended precooling

1.4 W/sf shed

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Hot days – Extended Precooling

Extended Precooling (Hot days )

0

1

2

3

4

5

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (hour)

Wh

ole

Bu

ildin

g p

ow

er

W/s

qft

Extended precooling 1 Extended precooling 2 baseline

2.3 W/sf shed

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Chiller Power: Limited Precooling

Chiller power shedding

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (hour)

Ch

ille

r p

ow

er

(W/f

t)

precooling+DL_10_8 baseline_10_6

0.7 W/sfshed

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Results – Fan Power

response of one supply fan

0

1

2

3

4

5

6

7

8

9

10

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (hour)

fan

po

wer

(kW

)

fan power

1.5 kW shed

precooling zonal reset

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Results – Zone Temp

zone air temperatures

68

69

70

71

72

73

74

75

76

4:00 8:00 12:00 16:00 20:00 0:00

zon

e t

em

pe

ratu

re (

oF

)

Precooling_Temp_operative Precooling_Temp_DDCPrecooling_Average zone temp baseline_temp

Page 27

Critical Peak Pricing (1 of 3)

• Operating time– CPP high price: 3:00pm –6pm– CPP moderate price: 12:00pm –3:00pm

• CPP days (12 days: 5/1 – 10/31)– Average of maximum outside air

temperatures in San Francisco, San Jose, Sacramento and Fresno > 98 oF

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Critical Peak Pricing (2 of 3)

• Current tariff: A-10S

• Non CPP Day Rate– On-peak credit, $0.0559/kWh, (12 pm- 6 pm)– Part-peak credit, $0.0050/kWh, (8:30 am-12 pm)

• CPP Day Rate– Moderate price, $0.24/kWh (12 pm – 3 pm)– High price, $0.82/kWh, (3 pm-6 pm)

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Critical Peak Pricing (3 of 3)

$-

$2,000.00

$4,000.00

$6,000.00

$8,000.00

$10,000.00

$12,000.00

$14,000.00

$16,000.00

$18,000.00

non-CPP dayssavings

CPP days extracost withprecooling

CPP days extracost w/o

precooling

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Market Potential – Cooling in Office Buildings

• State-wide peak demand ~50GW• Commercial cooling - largest contributor: 15%, ~7.5 GW

Cooling Electricity Breakdown*

Large office (>30,000 sf)

Small office (<30,000 sf)

RetailsCollegeRestraurant

School

Health

Hotel

Others

*CEC 1994 electricity consumption

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Conclusions• Precooling & zone temp reset can shed up to 80 –100% of

the cooling peak load, without comfort complaints, even in relatively high outside temperature conditions (90 oF)

• Benefits of nocturnal precooling are unclear• Economic savings can be made if combined with CPP or

similar DR programs Extended Precooling + DL Hot days

0

1

2

3

4

5

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (hour)

Wh

ole

Bu

ild

ing

po

wer

W/s

qft

Extended precooling 1 baseline

2.3 W/sf shed