ASHRAE Journal - November 2022 - 58

Specifics of Resource Utilization in Cooling
This section details studies comparing the resource
efficiency of air and evaporative (wet) cooling processes.
This is a resource comparison, not a cost-based evaluation.
Other cooling processes are available, but are not
the subject of this article or the case studies cited here.
The method of heat rejection and the predominant
energy production system supplying the electrical power
vary widely from state to state and region to region. The
following case studies are presented to illustrate total
grid resource evaluation methods for energy and water,
with some surprising results.
Case Study: Comfort Cooling
For comfort cooling, a highly relevant
study was published in 2008
by environmental consultant Jerry
Ackerman. The study used data from
Pacific Gas & Electric and the National
Renewable Energy Laboratory (NREL)
to evaluate a typical 400 ton (1407 kW)
cooling system for a building in
California.6 It is not surprising that significant energy
and CO2 reductions exist for the evaporative system
compared to air cooling. But the study also compared
water use, including the water consumption for energy
generation. In this case, total water use for evaporative
cooling was lower than that for air cooling. Findings
included:
* Annual energy consumption for evaporative cooling
is 190 MWh vs. 440 MWh for air cooling, a reduction of
56.8%.
* Greenhouse gas emission, measured in lb of CO2,
for evaporative cooling is 294,500 vs. 682,000 for air
cooling, a parallel reduction of 56.8%, based on average
U.S. plant emissions of 1.55 lb of CO2/kWh generated.
*
Total water use in gallons for evaporative cooling
was 1,530,000 vs. 1,944,000 for air cooling, which represents
a reduction of 21.3%. Site water use for evaporative
cooling included water for makeup, assuming six cycles
of concentration. Power plant water use is average for
California; each kWh produced consumes 4.4 gallons of
water. The study notes that water consumption per kWh
varies by state and region.
Evaporative cooling systems also provide additional
benefits, including lower operating noise, fewer fans,
58
ASHRAE JOURNAL ashrae.o rg N O V E M B E R 2022
TABLE 1 Pounds CO2 emitted per million British thermal units (Btu) for various fuels.
FOSSIL FUELS
Coal (Anthracite)
Coal (Bituminous)
Coal (Lignite)
Coal (Subbituminous)
Diesel Fuel and Heating Oil
Gasoline (Without Ethanol)
Propane
Natural Gas
WATER COOLED
(EVAPORATIVE)
ANNUAL ENERGY CONSUMPTION, MWH
ANNUAL EMISSION,a lb CO2
ANNUAL WATER USE, GALLONS
(ON-SITEb + POWER PLANTc)
190
294,500
1,530,000
lb CO2/MMBtu
228.6
205.7
215.4
214.3
161.3
157.2
139.0
117.0
TABLE 2 Energy, emissions and water use for air-cooled vs. water-cooled (evaporative) systems.
AIR COOLED
440
682,000
1,944,000
REDUCTION
56.8%
56.8%
21.3%
aAverage U.S. energy plant emits 1.55 lb of CO2 for each kWh generated.
bSite water includes water for makeup, assuming six cycles of concentration.
cIn California and many western states, each kWh produced consumes 4.42 gallons of water. On average in the U.S.,
each kWh produced consumes 2 gallons of water.7
smaller site area and use of sustainable recycled plastic
materials.
Data from the study by Ackerman are summarized in
Table 2.
Case Study: Data Center Cooling
In the information infrastructure space, dramatic
increases in data generation, storage and digital processes
have spawned massive data centers with substantial
attendant heat generation. Effective and efficient
cooling of data centers to meet these information
demands represents important resource and environmental
choices. Evaporative cooling has often proven to
be the optimum choice, reducing energy, CO2 emissions
and even water use for these facilities when examined
on a total system resource basis. Evaluation of data center
water use for cooling systems must use this type of
holistic analysis, including water used both on-site and
by electricity generation.
A 2017 study by Tim Chiddix and Brook Zion works
through a resource comparison of air-cooled and watercooled
building systems.8 The study's authors calculated
the energy requirements of these systems and went on
to calculate the amount of water used both on-site and at
the point of electricity generation for air vs. evaporative

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