ASHRAE Journal - December 2020 - 32

TECHNICAL FEATURE

substation would be needed to
support the added mechanical
cooling equipment. However,
under the same codes, the team
successfully used archived
OMNI power meter data to
show that a new substation was
unnecessary since the most
recent year of monitoring data
showed it never exceeded 60%
of the total individual equipment specified peak power ratings.3 This decision saved about
$2 million-a great example
of how advanced data analytics enabled managerial capital
budget decisions involving collaboration with the lab's facilities division.

FIGURE 3 Psychrometric chart showing Oakland weather and cooling strategies.

Annual Psychrometric Chart of Oakland, Calif.
(Relative Humidity Lines are Stepped by 10%, Wet-Bulb Lines by 10°F)

WB = 70

WB = 60

WB = 50
Direct Evaporative Cooling
WB = 40
Mixing

Outdoor Air

Return Air

ASHRAE Recommended
30

ASHRAE Allowable (A1)
65
70
75
80
85
90
Dry-Bulb Temperature (°F)

100

105

Need for Ongoing Commissioning
While compressor-free cooling system design provides large energy savings, it also brings challenges with
indoor environmental control. This was another operational driver for a highly detailed OMNI instrumentation capability. NERSC operators can pull three " levers "
to meet the varying cooling load: airside economizer,§
waterside economizer,# and direct evaporative cooling
inside the AHUs. Sophisticated control sequences are
needed to meet LBNL's energy efficiency goals, which
require operator attention. Therefore, an ongoing commissioning (OCx) process for cooling system troubleshooting and optimization is being used. To facilitate
this process, a commissioning consultant was hired in
2016. They conducted an assessment revealing a potential of three gigawatt-hours of annual energy savings.4
To help understand control sequences, various outdoor air conditions are illustrated on the psychrometric chart in Figure 3 with annual hourly local weather
data points. (The psychrometric chart for Oakland is
used to approximate Berkeley weather.) The actual
local weather in recent years has seen hotter conditions
more frequently than shown in the chart. The chart

shows the " recommended " and " allowable " ranges of IT
equipment intake air conditions for the " A1 " class data
centers outlined in the ASHRAE thermal guidelines.8
This most recent guideline had relaxed the low end of
the humidity range from 42°F (5.5°C) dew point (DP)
to 16°F (-9°C) DP for all data center classes compared to
the previous 2011 version9 that was used during Wang
Hall design (which houses NERSC). In response to the
new guideline, the NERSC team implemented control
sequence changes eliminating humidification when
outdoor air is cold and dry (below 42°F [5.5°C] DP),
which saved water by operating the direct evaporative
coolers less.
Outdoor air is cooler and with higher relative humidity
than the " recommended " range most of the year (green
area in the chart). Therefore, mixing outdoor air with
return air to achieve desirable supply air conditions is
a primary strategy. In addition, cooling tower water is
used in AHU cooling coils when outdoor air is warmer
than 74°F (23°C).II For the limited hours when the outdoor air is even warmer (about 80°F [27°C] or higher)
but less humid, evaporative coolers are used to bring the

§Using

cool outdoor air as a low-energy cooling source to avoid or reduce direct-expansion based cooling for energy savings.

#Using

cooling tower generated water as a low-energy cooling source.

IIThere

is a temperature rise of about 5°F to 6°F (2.8°C to 3.3°C) between the AHU supply air and rack intake air.

ASHRAE JOURNAL

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D ECEM BER 2020

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ASHRAE Journal - December 2020

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