ASHRAE Journal - February 2024 - 38

TECHNICAL FEATURE
TABLE 2 A comparison of cold-standby and hot-standby cooling.
HSP-BASED COOLING
Redundancy
Normal Mode
Fully Redundant
Equipment
Cooling System
Connectivity Is Enabled
Emergency Mode Active Before Failure
Continuous Cooling
Zero Cooling
Interruption
Cooling Energy
Operating Cost
CSP-BASED COOLING
Partially Redundant Equipment
No Cooling System Connectivity
Active After Failure
Minimum Cooling Interruption
Constant Speed/Frequency Drives: CSP < HSP
Variable Speed/Frequency Drives: CSP > HSP
Constant Speed/Frequency Drives: CSP < HSP
Variable Speed/Frequency Drives: CSP > HSP
loads and energy-saving. In other words, variable
speed chillers, inverter-controlled fans and pumps
with electronically commutated (EC) motors are now
widely used. When each component is controlled by
an inverter, the system can operate at a relatively low
load factor, which maximizes energy savings. This is a
key concept in enhancing the energy efficiency of HSPbased
cooling systems, in contrast to CSP-based cooling
systems (Table 2).
On the other hand, in terms of reliability, cooling
systems that use CSP can incorporate thermal storage
FIGURE 2 Example of data center cooling system with inverter technology.
Inverter
Technology
Three-Phase
AC Power
A
VSD: Variable Speed Drive
VFD: Variable Frequency Drive
AC to DC
DC to AC
C
B
Motor
Air
qCT
Cooling
Tower
w/Fan
Makeup
Water
qP(cw)
Pump
Transformer
Switchgear
IT
Power Substation
Equipment
Air
Room
Air
Chiller
MV
qRM
Chilled
Water
Loop
Pump
UPS
qCR
CRAC/H
w/Fan
FCV
qP(ch)
Secondary
Pump
Tank
Pump
qP(ch)
Primary
Pump
Evaporator
qCH
HX
Economizer
Compressors
Condenser
FCV
equipment, completely eliminating downtime. Even
without thermal storage, equipment turned off for CSP
can be brought back online within 10 to 60 seconds,
restoring normal operation within minutes with
minimal disruption. Although CSP may not be the most
efficient means, the system's reliability can be similar to
that of HSP.
HSP-Based Cooling Strategy
Figure 2 depicts the concept of a centralized cooling
system in a typical data center. HSP-based cooling
systems, which incorporate inverter technology to
maximize low (partial) load efficiency, include variable
speed chillers, pumps (for chilled water and condensing
water circulation) and fans (such as computer room
air conditioning/air-handling [CRAC/H] units and
cooling towers). A variable-speed/variable frequency
drive (VSD/VFD) is a device that regulates the speed of a
motor, converting a constant input power frequency to
a required output frequency. This frequency regulation
enables a wide range of adjustments and results in
a significant reduction in power consumption in
controlled equipment.7 The partial load efficiency of
each component is presented below.
Air
Condenser Water Loop
Data Center Cooling System
MV
Pump
qp(cw)
Condenser
Water Pump
38
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