ASHRAE Journal - January 2022 - 29

COLUMN DATA CENTERS
at various conferences and data
center-specifi c forums. Certain
exceptions exist within the standard
for computer rooms (see paragraph
6.5.1, #11 and #12).
Section 6.6, Alternate Compliance
Path, paragraph 6.6.1 Computer
Room Systems, indicates that HVAC
systems serving the heating, cooling
or ventilating needs of a computer
room shall comply with Sections 6.1,
6.4, 6.6.1.1 or 6.6.1.2, 6.7 and 6.8.
Section 6.6.1.1 states that the
PUE1 (an energy consumption metric
defi ned in the standard) shall be less
than or equal to the values listed in
Table 6.6.1.
In turn, Section 6.6.1.2 states that
the PUE0 (an energy demand metric
defi ned in the standard) shall be less
than or equal to the values listed in
Table 6.6.1, at both 100% and 50% of
the design IT equipment load.
For reference, Standard 90.1-2016
Table 6.6.1 is listed here as Table 1. A
defi nition for PUE1 appears in the
standard.
Standard 90.1-2019
For Standard 90.1-2019, Table
6.6.1 has been eliminated, and the
compliance path under Section 6.6,
Alternative Compliance Path, is simply
a reference to Standard 90.4.4
Standard 90.4-2016
The mechanical system compliance
path for Standard 90.4-2016
consisted of achieving both design
and energy targets for a calculated
entity called the mechanical load
component (MLC). There was both
a design MLC, which needed to be
satisfi ed at 100% and 50% of information
technology equipment
(ITE) load (per Table 6.2.1.1) and an
annualized MLC, which also had
to be met at 100% and 50% of ITE
load (per Table 6.2.1.2). In addition,
a limitation existed on electrical
infrastructure losses, as defi ned by
a metric called the design electrical
loss component (ELC). Section 11 of
the standard allows for trade-offs
between the MLC and the ELC to
achieve overall compliance with the
standard.
Standard 90.4-2019
The design MLC is removed from
the 2019 version of the standard,
and there is only an annualized
energy MLC. In recognition that
data centers operate with a range of
ITE loads, which also vary over time
(typically but not always increasing),
the MLC annual energy calculation
is performed at 25%, 50%, 75% and
100% of the ITE design loads to make
sure the system is designed to be
effi cient over a range of possible ITE
operation. Requirements pertaining
to the calculation are listed in
the new standard, with both binned
and TMY3 weather data modeling
approaches acceptable.
It is diffi cult to make a direct
comparison between the 2016 and
2019 MLC, partly because the 2016
standard had one set of values for all
sizes of data centers, while the 2019
standard differentiates between
small and large data centers, with a
threshold of 300 kW. For small data
centers, the 2019 MLC values are
between 14% and 31% lower than
the 2016 values, while for large data
centers the 2019 values are between
28% and 60% lower, depending on
climate zone (Table 2).
The ELC looks at electrical losses,
but instead of looking at annual
energy consumption, it is more
of a design loss calculation, with
TABLE 1 ASHRAE Standard 90.1-2016.
Maximum PUE (Table 6.6.1 from Standard
90.1-2016).
TOPIC
ASHRAE CLIMATE ZONE
0A
0B
1A
1B
2A
3A
4A
5A
6A
2B
3B
4B
5B
6B
3C
4C
5C
7
8
MAXIMUM POWER USAGE
EFFECTIVENESS (PUE1)
90.1-2016
1.64
1.62
1.61
1.53
1.49
1.41
1.36
1.36
1.34
1.45
1.42
1.38
1.33
1.33
1.39
1.38
1.36
1.32
1.30
part-load energy effi ciency taken
into consideration by calculating
the ELC at both 100% and 50% of
ITE load. The electrical losses are
divided into three segments: incoming
electrical service, uninterruptible
power supply (UPS) and ITE
distribution. For the purposes of
calculating the ELC, the threshold
between a small data center and
a large one is 100 kW, rather than
300 kW for the MLC calculations.
For small data centers, the 2019
ELC is between 11% and 19% lower
than the 2016 ELC. For large data
centers, the ELC is between 8% and
18% lower than in 2016. The adjustment
primarily refl ects continued
improvements in UPS effi ciency
(Table 3).
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