ASHRAE Journal - March 2013 - 67

2013 Technology Award Case Studies

Annual Operating Cost

HVAC Power (Supply + Relief Fans) (kW)

shows how the annual cost of the mechanical system described
36
here (green line) would change without the separation of cold
32
After 10/12; After Fan
and warm air provided by the chimney cabinets (orange line).
Optimization – PUE of 1.15 (After
28
Signiﬁcantly, the consistent distance between the green and
Accounting for UPS Power)
24
orange curves shows how the system described here provides
20
energy savings using VAV fan airﬂow reduction when the room
is lightly populated and by refrigeration avoidance when the
16
Before 10/12; Prior to Fan
room is more fully populated (when compared to the stringent
12
Optimization – PUE of 1.22
Washington State energy code baseline). This “assured savings”
(After Accounting
8
characteristic removes the uncertainty around how populated the
for UPS Power)
4
system might actually turn out to be, and helped us make the
0
case for the conservation incentive with our electric utility.
120
140
160
180
200
220
240
260
Different methods may be used to modulate evaporative meCooling Load (Power Out of IT Equipment) (kW)
dia. We considered staged control (either parallel or series banks
Figure 5: TCS actual power observations before and after
of separately wetted media) before determining that the face and
described fan optimization.
bypass method would provide the most steady conditions in the
space and consume the least
Annual Cost of Energy for Servers
amount of water (the staged
$70,000
Open Aisles & Chilled Water CRAH
method can frequently overUnits With No Economizer (Not
shoot the setpoint and put too
$60,000
Permitted by Seattle Energy Code)
much moisture into the room).
Open Aisles & Chilled Water CRAH
$50,000
Units with Water Economizer (PerThe face and bypass dampmitted by Seattle Energy Code)
ers modulate to prop up the
$40,000
Open Aisles & Chilled Water Air
indoor humidity in the winter
Handlers with Air Economizer,
$30,000
and to cool the discharge air
Steam Humidiﬁer (Not Permitted
by Seattle Energy Code)
in the summer. The humidity
$20,000
Open Aisles & Air Handlers with
setpoint is reset seasonally. A
Adiabatic Humidiﬁer, Air Economiz$10,000
low relative humidity avoids
er (Seattle Energy Code Baseline)
structural condensation and
$0
Chimney Cabinets & Air Han0
10 20
30 40
50 60 70
80 90 100
dler with Adiabatic Humidiﬁer, Air
saves water in the wintertime,
Economizer (Used at TCS)
Percent Populated
but a higher relative humidity
is necessary in the summer
Figure 6: Annual calculated cost comparison at various populated percentages (Seattle 400 kW computer room with 77°F 25% to 70% RH cold aisles.) Costs include water, electricity and maintenance.
when the evaporative cooling
of 100% outdoor air occurs.
When no water evaporation is necessary at all, face and
Even if a data center chooses to use a water economizer, the
bypass dampers are opened to save fan power and, after a separation of hot and cold air can reduce the chiller demand and
time delay, the water is drained from the evaporative sump annual refrigeration energy use signiﬁcantly. Additionally, this
until evaporation is required again. The system uses a non- separation and airﬂow modulation prevents “hot spots,” provides
chemical water treatment method and provides conductivity fault-tolerant air distribution in the room; and lightly loaded data
control of blow-down within each supply air handler.
centers can reduce their fan speed and energy use signiﬁcantly.

Conclusion
With a design wet-bulb temperature
below 70°F (21°C), there are lots of
opportunities in the Paciﬁc Northwest
for data centers built like TCS to enjoy a low PUE without any refrigeration. However, anywhere a data center
is constructed, or can be retroﬁt with a
means of separation between the cool
supply air and the warm return air (as
TCS was able to do using chimney
cabinets), there will be energy savings
and a reduction in the need for refrigeration.
March 2013

www.info.hotims.com/44630-6

ASHRAE Journal

67
ASHRAE Journal - March 2013

Table of Contents for the Digital Edition of ASHRAE Journal - March 2013
