AUGUST 21 - 23, 2018 • CLEVELAND, OHIO
Making a Splash: Targeting Water Saving 

Measures for the Highest Impact

NREL ESIF Data Center Water Use Reductions

Otto Van Geet, PE – NREL

NREL/PR-7A40-72044
NREL | 3

NREL Data Center

Showcase Facility

• ESIF 182,000 ft.2 research facility

• 10,000 ft.2 data center

• 10-MW at full buildout

• LEED Platinum Facility, PUE ≤ 1.06

• NO mechanical cooling (eliminates 

expensive and inefficient chillers)

Data Center Features

• Direct, component-level liquid cooling, 

24oC (75oF) cooling water supply

• 35-40oC (95-104oF) return water (waste 

heat) is captured and used to heat offices 

and lab space

• Pumps more efficient than fans

• High-voltage, 480-VAC power distribution 

directly to high power density 60- to 80-

kW compute racks

Compared to a Typical Data Center

• Lower CapEx—costs less to build

• Lower OpEx—efficiencies save

Utilize the bytes and the BTUs! Integrated “Chips-to-Bricks” 

Approach
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Metrics

PUE = “Facility energy” + “IT energy”

“IT energy”

ERE = “Facility energy” + “IT energy” – “Reuse energy”

“IT energy”
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Metrics

WUE = “Annual Site Water Usage”

“IT energy”

the units of WUE are liters/kWh

WUESOURCE =

“Annual Site Water Usage” + “Annual Source Energy Water Usage”

“IT energy”

WUESOURCE = 

“Annual Site Water Usage”

“IT energy” + [EWIF × PUE] 

where EWIF is energy water intensity factor
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Air- and Water-Cooled System Options

Air-Cooled System

• Design day is based on DRY BULB 

temperature

• Consumes no water

(no evaporative cooling)

• Large footprint/requires very large 

airflow rates

Water-Cooled System

• Design day is based on the lower WET BULB 

temperature

• Evaporative cooling process uses water to improve 

cooling efficiency

o 80% LESS AIRFLOW  lower fan energy

o Lower cost and smaller footprint.

• Colder heat rejection temperatures improve 

system efficiency

However, water-cooled 

systems depend on a 

reliable, continuous 

source of low-cost water.
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Traditional Wet Cooling System

95°F (35.0°C)

Process Loop

Heat In

Moist Heat 

Out

Wet Loop

Sized for Design Day

Thermal Duty

75°F (23.9°C)

95°F (35.0°C) 75°F (23.9°C)

Condenser Water Pump
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Basic Hybrid System Concept

95°F (35.0°C)

Process Loop

Heat In

Moist Heat 

Out

Wet Loop

Sized for Design Day

Thermal Duty

75°F (23.9°C)

75°F (23.9°C)

Condenser Water Pump

Process Loop

Dry Loop

Sized for Water Savings

85°F (29.4°C)

Dry Heat 

Out

“Wet” when it’s hot, “dry” when it’s not.

Dry Sensible Cooler

95°F (35.0°C)
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Improved WUE—Thermosyphon
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Applications

Favorable Application Characteristics

• Year-round heat rejection load (24/7, 365 days is best)

• Higher loop temperatures relative to average ambient temperatures

• High water and wastewater rates or actual water restrictions

• Owner’s desire to mitigate risk of future lack of continuous water availability 

(water resiliency)

• Owner’s desire to reduce water footprint to meet water conservation targets

Any application using an open cooling tower is a potential 

application for a hybrid cooling system, but certain 

characteristics will increase the potential for success.
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Sample Data: Typical Loads and Heat Sinks
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Data Center Metrics

PUE = 1.034 

ERE = 0.929 

WUE = 0.7 liters/kWh 

First year of TSC operation (9/1/2016–8/31/2017) 

Hourly average IT Load

= 888 kW 

WUESOURCE = 5.4 liters/kWh

WUESOURCE = 4.9 liters/kWh if energy from 

720 kW PV (10.5%) is included

using EWIF 4.542 liters/kWh for Colorado

Annual Heat Rejection

(with only cooling towers, WUE = 1.42 liters/kWh) 
Otto Van Geet, PE

Principal Engineer, NREL

Otto.vangeet@nrel.gov
NREL | 14

Notice

This research was performed using computational resources sponsored by the 

U.S. Department of Energy's (DOE) Office of Energy Efficiency and Renewable 

Energy and located at the National Renewable Energy Laboratory under 

Contract No. DE-AC36-08GO28308. Funding provided by the Federal Energy 

Management Program. The views expressed in the presentation do not 

necessarily represent the views of the DOE or the U.S. Government. The U.S. 

Government retains and the publisher, by accepting the presentation for 

publication, acknowledges that the U.S. Government retains a nonexclusive, 

paid-up, irrevocable, worldwide license to publish or reproduce the published 

form of this work, or allow others to do so, for U.S. Government purposes.