Free Cooling: Economizers in

Data Centers

David R. Pickut, P.E.VP Operations Engineering

Equinix, Inc.03/15/08

Agenda

• Define “Free”

• Differentiate and explain air-side and water-side economizers

• Operating temperature/humidity ranges

– Geographic applicability

– Potential operating hours per year

– Potential operating cost savings

• Real-life examples and caveats

Why “Free” Cooling?

• Not 100% free – but a very substantial improvement in operating cost– Air-side economizer

• In mild and cold climates, use outside air when conditions are right – don’t need to run chillers

– Water-side economizer• In cold climates, achieve required heat rejection just by pumping the water thru cooling towers (very close to “free” vs. running chillers)

• “Free” – but not necessarily easy

EPA Report to Congress

• In the final “Server and Data Center Energy Efficiency” report, energy-efficient cooling systems characteristics identified:

– Water-side economizers one “best practices”component in reducing PUE to 1.5 (Table 3-5, Page 53). • PUE (Power Usage Effectiveness) = total data center energy use / IT equipment energy use

– Air-side economizers an “energy-efficiency improvement opportunity” (Table 3-6, Page 54)

Operating Expense Savings

• Example of overall potential, using a data center with 2.5 MW of computer room load as an example– Assume for 2.0 PUE, 40% of energy for cooling, located in the Northeastern US, $0.09 per kWh

– Total electric bill will run approx $3.5M/year

– Potential annual savings:• $250k via air-side economizer

• $225k via water-side economizer

Air-side economizer

– Requires additional heat exchanger, piping, valves, and controls (physical space and cost) vs. basic plant

– Controls complexity

– Tricky to operate in freezing and sub-freezing conditions

– Requires additional OA louvers, return and exhaust duct systems (physical space and cost) vs. conventional cooling

– Controls complexity

– Direct impact on computer room environment – little room for error

Water-side economizer

Features of Free Cooling Systems

Both have components that must be operated and

maintained properly – if not, savings will not be realized

Psychrometric Chart

• Great visual tool for calc’s involving air properties (enthalpy)• Mechanical engineers use this all the time – or computer programs

with the algorithms to produce the same results• Too complex for the purposes of this presentation (but we’ll come

back to it, briefly, a bit later)

Air-side Economizer

100% air recirculation

Economizer: Full volume of cool outside air (OA) or

mixing of OA and return air

If OA is clean, 50ºF & RH

acceptable – bring it in!

Define the Operating Range

• Potential savings a function of acceptable temp/RH range – directly relates to potential operating hours/year

• Temp and RH (in the computer room)– “Classical” computer room standards

– ASHRAE standards

– IT/Network equipment mfgr specifications (and warranty requirements)

– Your experience and needs

ASHRAE Guidelines• Broader operating temp/RH ranges than “classical”

computer room environmental specifications

• From “ASHRAE Thermal Guidelines for Data Processing Environments” 2004:

For a Class 1 Data Center:

Dry-bulb Temp (°F) % RHAllowable Recommended Allowable Recommended 59 to 90 68 to 77 20 to 80 40 to 55

Close to the “classical” range

A bit broader than the “classical” range

MUCH broader ranges

Manufacturer Requirements

• Examples:– IBM BladeCenter S: 50-95ºF, 8-80% RH– HP c3000 blade enclosure: 50-95ºF, 20-80% RH– Dell PowerEdge blade: 50-95ºF, 8-80% RH, plus RH variation not greater than 10% per hour

(For all, temp ratings at sea level; RH values non-condensing conditions.)

• Operation outside manufacturer specifications has warranty implications and may result in equipment damage

Operating Parameters

• Rate of temperature change in the computer room has become an issue for some installations– Equipment-specific; not universal– TIA/EIA Standard 942, Telecommunications Infrastructure for Data Centers: max 9ºF/hr

– ASHRAE: 3.6ºF/hr (for some equipment)– Some IT equipment manufacturers continuously monitoring chip-level temperatures

• Broader range = greater energy savings– But must monitor closely and control effectively

Geographic Considerations

• Air-side economizers– Easy – mild climate areas (such as SF)

– Medium difficulty – climates with mild spring and fall, hot, humid summers and rapidly changing weather conditions (such as Chicago, NY, DC)

– Doesn’t work – year-round hot weather (such as Miami, New Orleans) or LA (near ocean)

– Need clean outside air (or necessary filtration)

Temperature BINs• Weather history for a specific location – hours per year

(by month) in specific DB and WB ranges

• Facilitates calculation of potential operating hours on economizer systems

Air-side Economizer – hours/yr

• Estimated (potential) hours/yr within specific temperature range– Based on weather history – not guaranteed for a specific

year, month

– When air is cool and too dry - have to humidify

Potential Hours per Year

Location

Full economizer

Partial (mixed air) None

Reading, UK 8,100 300 360 DC metro 5,700 600 2,460 Silicon Valley 8,500 260 - Dallas, TX 4,500 450 3,810

Assumed design air requirements in computer space: 68 deg F at min 35% RHDesign air requirements directly affect potential hours per year.

Detailed Operations Analysis

• Need to be realistic – don’t assume everything is ideal

– Sustained ideal weather conditions do not happen often

– Difficult to keep systems at 100% operational status with 100% accuracy

– Operating close to limits = less room for error

Detailed Calculations

Calculated “practical” hours are 51% lower than “ideal” >>

Typical Control Parameters

• Real-life example (Silicon Valley)

• Air-economizer mode when these conditions met:

– OA below 68ºF– AND OA temp less than RA temp

– AND OA enthalpy less than RA enthalpy

• If OA dew point is below 42ºF but not more than 52ºF, run the humidifier

Real-life OperationSample AHU - Silicon Valley

% Economizer

0

25

50

75

100

1-Feb

2-Feb

3-Feb

4-Feb

6-Feb

7-Feb

% Economizer Op

30

50

70

OA Temp

% Economizer

OA Temp

Air-economizer Components• Inherently problematic

– Poor quality and/or poor maintenance will degrade savings quickly

– Controls are complex

• Monitor operations and savings – if savings are not being achieved, find out why!

Cooling via Chiller

Cooling Load

Cooling Tower

Approx 0.9 kW per Ton of cooling(cooling tower fan motor, PLUS energy to chiller)Note: Example temps shown are hot weather.

Evaporator

Condenser

Compressor

Chiller

Energy Input

90°F

95°F

85°F

42°F

50°F

Cooling via Water-side Economizer

Cooling Tower vs. Ambient Temp100% Flow (1,000 GPM/cell)

10 Degree RangeDesign Conditions: 95F EWT / 85F LWT / 78 F WB

30

34

38

42

46

50

54

58

62

66

70

74

78

82

86

90

94

98

102

106

110

114

118

0 10 20 30 40 50 60 70 80

Wet Bulb Temp (F)

Leaving W

ater Temp (F)

Design Point

100% Fan Speed

50% Fan Speed

25% Fan Speed

Tower Design

• Some tower types not suitable for economizer operation – proper selection/specification critical

• Piping, valves and controls can be complex

– 2-speed or variable speed on fans

– Water flow rate control

– Bypass piping and valves for start-up/shut-down

– Reverse fan operation for de-icing

Induced-draft tower – good application Forced-draft tower – not so good

Geographic Considerations

• Water-side economizers

– Easy – climates with cool fall and spring, and cold winter weather (such as Chicago, NY, DC)

• Tower freezing can be a problem in severe cold

– Medium difficulty – climates with moderate weather year-round (such as SF)

– Doesn’t work – hot, humid climates

Potential Operating Hours

• Similar to air-side systems, review BIN weather data for specific geographic area– Focus on WB temperature (cooling towers evaporate water

to lower temps)

– Can design for partial economizer operation to extend potential operating hours

– Depends on LWT parameter:

• Example: in Baltimore, if 43ºF LWT needed, potential is 1,900 hours/yr. If LWT increased to 50ºF, operating hours increase to 2,900 hours/yr.

Freezing Weather

• Many caveats associated with operation in freezing weather

– Excessive ice build-up can damage tower components

– Danger to personnel if/when manual ice removal is needed

– Operational errors can result in loss of cooling

• Make sure design engineers and operating engineers understand the challenge, know what to do

Summary

• These techniques are time-proven; they work

– Have to carefully evaluate conditions specific to the site geographic area

– The computer room is a critical environment – can’t allow energy efficiency goals to compromise uptime or the room environmental operating parameters

• Must maintain systems and monitoreffectively

Useful Links

• EPA final Report to Congress on Server and Data Center Energy Efficiency– http://www.energystar.gov/ia/partners/prod_development/do

wnloads/EPA_Datacenter_Report_Congress_Final1.pdf

• LBNL paper on air-economizers and air quality– http://hightech.lbl.gov/documents/DATA_CENTERS/Econom

izerDemoReportMarch13.pdf