ASHRAE Journal - August 2024 - 42

TECHNICAL FEATURE
(approximately 7,600 hours per year in Sacramento).
This allowed the data center to operate independently
of the central chilled water plant whenever conditions
were appropriate for economization. This eliminated the
need for all of the equipment (pumps, cooling towers,
chillers) in the chilled water plant to run during most
nights and weekends. It comprised 53% of HVAC savings
for this step.
* Replaced four existing water-cooled CRAH units
with two air-cooled variable-speed CRACs, sized to
meet the data center cooling load and facilitate the
airside economization detailed above. The operation
of the new CRAC units is based on the return air
temperature within the data center. Temperature
sensors were placed in the cold aisle and then
connected to the CRACs and fans. If the return air
temperature nears the maximum setpoint, the CRACs
decrease the supply air temperature for the data
center. The outdoor air temperature (OAT) determines
whether the CRACs use economization or direct
expansion (DX) cooling to provide the conditioned
supply air. Although air-cooled CRACs are typically less
efficient than water-cooled CRAHs, the new equipment
allowed the central plant to be fully decoupled from
the data center. The measure also improved cooling
resiliency and comprised 22% of HVAC savings for
this step.
Step 3: Redesign Infrastructure to Improve Resilience
(7% of Total Energy Savings)
To enhance the data center's resilience, the project
implemented critical measures such as upgrading the
building's power systems, installing energy storage
(with 2N redundancy) and backup generation and
increasing redundancy of critical components (i.e.,
2N redundant power distribution units (PDUs) and
power distribution, 2N redundant cooling). The team
did the following:
* Installed dual lithium-ion battery energy storage
systems with a backup natural gas-powered standby
generator that together can provide sufficient backup
power to critical infrastructure through an extended
electrical grid outage.
* Replaced the existing UPS with two parallel UPSs
right-sized to the new data center to add redundancy
and increase the efficiency of the UPS system.
* Added A- and B-side PDU power supplies to
42
ASHRAE JOURNAL ashrae.org A U G UST 2024
introduce redundancy of the power distribution.
* Installed two CRACs to add redundant cooling.
These power infrastructure measures reduced energy
waste and reduced vulnerability to system failure. In
addition, the equipment was sized to allow for future
growth of the data center, and the configuration would
allow for using the ASHRAE data center supply air
setpoints of up to 81°F (27°C).
Results
The project yielded impressive results across several
key performance indicators:
Overall System Efficiency. The data center's yearly
kW/ton decreased from 3.66 (0.96 COP) to 0.82
(4.29 COP). The kW/ton represents the ratio of energy
consumption by the cooling equipment (i.e., chillers,
cooling towers, pumps and CRAHs or CRACs) to the
rate of heat removal from the system. The data center's
power usage effectiveness (PUE) dropped from 3.78
to 1.42. PUE represents the ratio of total data center
energy consumption compared to the total energy
consumption of IT equipment. The pre-project PUE was
calculated based on one year of data from the building
management system (BMS) for the chiller plant, CRAHs,
fans and UPS.
To attribute baseline loads to the data center, the
chiller plant load-including chiller, cooling tower
and pump power-was analyzed for a year. Due
to the proportion of loads, during unoccupied or
economization hours the data center accounted for
substantially all building loads. The post-project PUE
was calculated based on BMS data for the central
plant and logged data for the new CRACs. Post-project
loads attributable to the data center were isolated
from total building load and used to calculate the
PUE. Future upgrades to further enhance PUE could
include increasing IT loads in the data center (i.e.,
using some of the spare capacity that was designed
into the data center retrofit), reducing the size of
the CRACs and UPSs, reducing redundancy from 2N
to 1N, reducing the buffer between the data center's
temperature setpoints and the maximum allowable
ASHRAE setpoints to increase economization hours
and implementing technologies such as indirect
evaporative cooling.
IT and UPS Loads. As a result of the virtualization,
consolidation and migration of IT equipment/loads,
http://www.ashrae.org
ASHRAE Journal - August 2024

Table of Contents for the Digital Edition of ASHRAE Journal - August 2024
