ASHRAE Journal - May 2024 - 18

COLUMN DATA CENTERS
TABLE 4 Summary of results comparing various ACC operating strategies. See Table 3 to be reminded of the scenario description and chiller operating description.
DETAILS
SCENARIO
CHW SUPPLY (°F)
1
(Baseline)
2-A
2-B
3-A
3-B
68°F
68°F
68°F if OA >65°F
60°F if OA <65°F
68°F
68°F
AVG. SERVER INLET TEMP
MAX SERVER INLET TEMP
MIN SERVER INLET TEMP
NET
X-FACTOR
Avg., Max & Min: 76°F 1.22
Avg., Max & Min: 76°F 1.22
Avg. 73°F
Max 76°F
Min 68°F
Avg. 71°F
Max 80°F
Min 64.4°F
Avg. 69°F
Max 80°F
Min 59°F
1.15
1.09
1.05
ANNUAL COOLING
PLANT ENERGY
USE AT 100% ITE
(kWh)
1,879,867
1,447,401
1,606,858
880,847
880,847
the outdoor air enters the economizer coil first and
then the condenser coil. The cold aisle temperature is
constant and the same as in Scenario 1.
Scenario 2-B: ACC with integrated economizer
(chiller on year-round, but at reduced or no load as a
function of outdoor air dry-bulb temperature, CHWS
reset ability). This scenario is similar to Scenario 2-A
except that the chilled water supply setpoint resets to
60°F (16°C) when ambient temperature is below 64.4°F
(18°C). The reason for modeling this alternative scenario
is to see the dual impacts that this control change
has both on x-factor (server reliability) and energy
consumption.
Scenario 3-A: ACC and airside free cooling (FC):
(chiller on if OA >80.6°F [27°C], CHWS: 68°F [20°C],
64.4°F [18°C] minimum cold aisle temperature
during FC operation). This scenario combines the
baseline chiller examined in Scenario 1 with direct
airside FC. Airside FC allows the data hall to bypass
mechanical cooling and instead use ambient air as a
coolant when outdoor air temperatures are below the
top end of the ASHRAE Recommended Range (80.6°F
[27°C] dry-bulb).
Scenario 3-B: ACC and airside FC: (chiller on
if OA >80.6°F [27°C], CHWS: 68°F [20°C], 59.0°F
[15°C] minimum cold aisle temperature during FC
operation). This scenario is similar to Scenario 3-A
except that it is assumed that ambient air mixes with
hot return air to maintain server inlet temperatures at
a minimum of 59.0°F [15°C] instead of 64.4°F [18°C].
During cooler ambient conditions, the expectation is
18
ASHRAE JOURNAL ashrae.org MAY 2024
ANNUAL CRAH
ENERGY USE
AT 100% ITE
(kWh)
606,331
606,331
524,671
547,170
547,170
TOTAL
ANNUAL
ENERGY
USE (kWh)
2,486,198
2,053,732
2,131,529
1,428,017
1,428,017
CRAH
REDUNDANCY
TYPOLOGY
NEEDED
N + 2
N + 2
N + 2
N + 4
N + 4
90.4-2016
METHOD
CALCULATED
MLC
0.26
0.21
0.22
0.15
0.15
COMPLIES
WITH
90.42016?
Yes
Yes
Yes
Yes
No
COMPLIES
WITH
2021
IECC?
No
No
No
Yes
No
that this option will lower x-factor (i.e., improve server
reliability) relative to Scenario 3-A, but have little impact
on MLC since the drop in the minimum temperature
doesn't involve more chiller operation, but rather only
a change in the mixing ratio of outdoor and return
airstreams.
In all cases, CRAH energy was tracked in addition to
chiller energy. The modeling assumes that CRAH airflow
is equal to server airflow. Since server airflow typically
increases once server inlet temperatures exceed 68°F
(20°C), and all these options assume some operational
time with cold aisle temperatures above 68°F (20°C) and
as high as 80.6°F (27°C], the impact of CRAH energy on
MLC cannot be ignored.
Results
Energy consumption metrics, including annual
cooling plant energy use and CRAH energy use, varied
across different operational scenarios, as did server
reliability (x-factor) and ASHRAE Standard 90.4-2016 vs.
IECC 2021 compliance.
As expected, scenarios incorporating economization
strategies exhibited lower energy consumption than
Scenario 1 (Baseline), which relied solely on mechanical
cooling. Table 4 offers a summary comparison of the
various ACC operating scenarios. A more detailed
explanation of the results is provided below.
Energy Use
Scenario 1 (Baseline). In this scenario, the ACC
operates with no economization with a chilled water
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