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