ASHRAE Journal - November 2022 - 60

(wet) cooling by location for Denver, Phoenix and Los
Angeles. Power generation methods and their water use
vary across the western U.S. and the country, and those
differences by state are factored into the analysis for the
three locations studied. Data are specific to the locations
cited.
In this study, the water resource
requirement for the generated power
is expressed in gallons/kWh for the
three locations analyzed. Data from
NREL, technical report TP-550-33905,
defined the regional differences in
water use, as shown in Table 3.7
Energy. This study looked at the
Regarding such choices, Chiddix stated, " Municipalities
requiring data centers to use less energy and water onsite
may not have considered the full implications of
these requirements. " The results of such requirements
may actually result in use of higher amounts of both
energy and water.
Additional operating methods for
TABLE 3 Power plant water consumption.
POWER PLANT WATER
CONSUMPTION,
gallon/kWh
COLORADO
ARIZONA
CALIFORNIA
power requirement for a 1,500 kW cooling load using
a standard efficiency water-cooled chiller, including
chiller pumps and cooling tower, vs. an air-cooled
chiller system at full load. The facility cooling systems'
energy use for wet vs. dry is calculated as Wet Chiller
Energy plus Tower Energy vs. Air-Cooled Chiller Energy
for a given climate and is listed in Table 4.
Water. Water consumption at the power plant equals
energy use of the system multiplied by power plant
water consumption per unit energy. For the evaporative
system, " on-site " water consumption is then added back
to get the total water requirement. Results are shown in
Tables 5 and 6.
Overall, it is not surprising that energy use for the
evaporative system is substantially lower than for the
air-cooled system. The energy reduction is 65% to 66%
for each of the three cases. It is surprising that the total
water use, when power generation is
considered, is higher for the air-cooled
system than for the evaporative system
in all cases. The reduction for evaporative
cooling is 23% to 59% in varying
climates in the western U.S. Even in
the cooler Colorado climate, air-cooled
water consumption exceeds evaporative
system consumption.
Water Cooled
Air Cooled
DIFFERENCE
ENERGY REDUCTION FOR
WATER COOLED
Owners of data centers must weigh
multiple cooling options and consider
comprehensive operating costs as
key factors in design choices. Watercooled
chiller, air-cooled chiller, direct
evaporative, adiabatic and other types
of cooling systems are all in play.
60
Water Cooled
Air Cooled
DIFFERENCE
WATER USE REDUCTION FOR
WATER COOLED
ASHRAE JOURNAL ashrae.o rg N O V E M B E R 2022
1.20
7.85
4.64
evaporative cooling technologies that
create energy savings are 1) " free cooling "
in sufficiently cool climates (with
enough cool/cold hours to overcome
the added cost of a plate and frame
heat exchanger and any other additional
equipment) and 2) " variable
flow, " which involves reducing water flow or fan speed
during cool/cold periods.
When considering energy use and generated CO2,
these studies demonstrate why evaporative cooling is the
more sustainable system for meeting U.S. building cooling
needs.
The Impact of Cooling System Choices
Driving energy efficiency in building cooling and
refrigeration can create a significant reduction in U.S.
CO2 emissions and can also have a positive impact on
CHILLER TYPE
Air Cooled
Water Cooled
TABLE 4 Full-load chiller power.
DENVER (kW/ton)
1.250
0.431
Source: Chiddix and Zion (Reference 8)
TABLE 5 Cooling system energy use for 1,500 kW data center.
CHILLER TYPE
DENVER
ANNUAL ENERGY (kWh)
1,610,748
4,663,470
3,052,722
65.5%
TABLE 6 Cooling system water use for 1,500 kW data center.
CHILLER TYPE
DENVER
ANNUAL WATER (gallon/yr)
3,593,000
4,645,000
1,052,000
22.6%
PHOENIX
ANNUAL ENERGY (kWh)
1,726,603
4,999,089
3,272,486
65.5%
LOS ANGELES
ANNUAL ENERGY (kWh)
1,592,062
4,663,470
3,071,408
65.9%
PHOENIX (kW/ton)
1.340
0.462
LOS ANGELES (kW/ton)
1.250
0.426
PHOENIX
LOS ANGELES
ANNUAL WATER (gallon/yr)
14,844,000
36,182,000
21,338,000
59.0%
ANNUAL WATER (gallon/yr)
7,732,000
16,640,000
8,908,000
53.5%
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