ASHRAE Journal - November 2019 - 25

TECHNICAL FEATURE

FIGURE 2 Average ocean water temperature by depth for most tropical locations.

FIGURE 1 Typical lake stratification and thermoclines in North America with summer gradients.

0
Summer
Temperature (°C)
5 10 15 20

Thermocline

Hypolimnion
Mud

1,000

Oxygen
Temperature

10
15

4°

Increasing Temperature (°C)
8°
12°
16°

20°

24°

500

2 4 6 8 10 12 14 16
Oxygen (ppm)

IMAGE COURTESY PEARSON.COM

Depth (m)

Epilimnion

0°

1,500
Increasing
Depth
(m) 2,000

Thermocline

Temperature by Depth in
Tropical Location

2,500

Cooling Systems Comparisons
Conventional Cooling Systems
A conventional cooling plant that does not take
advantage of natural cooling reservoirs is composed of
several pieces of expensive and sometimes sensitive
equipment (Figure 3). The chilled water removes the

IMAGE COURTESY OF MARINEBIO.ORG

3,000

Rivers have long been used to support power plant and
manufacturing operations. The water temperature of
rivers tends to vary the most through different seasons
and they are also more closely regulated upstream,
downstream, and across borders.3
While the surface of lakes varies with the weather and
seasons, they have a more constant temperature below
(Figure 1). The temperature becomes more steady with
depth, as the deep water within a lake is less affected by
weather, storms, and other events.
Deep ocean water, like deep lake water, has a very
stable temperature year-round, and can be used for
direct cooling (Figure 2). Even in hot, tropical locations
deep ocean water can be reliable to achieve 59°F (15°C)
at 1,640 ft (500 m) below the surface and about 41°F
(5°C) at about 3,280 ft (1000 m).
While the temperature of surface ocean water can
ﬂuctuate seasonally, it can still be a valuable source of
cooling. In hot regions such as the Middle East, surface
seawater as high as 90°F (32°C) is used for condenser
cooling with conventional systems. This is because even
the higher temperature water is more efﬁcient than air
cooling and using potable water for evaporative cooling
is not allowed due to water use restrictions.

3,500
4,000
4,500

heat from the source load, then uses a chiller to move
the heat from chilled water system to the condenser
water system. A cooling tower then rejects the heat from
the condenser system to the atmosphere. Each step has
energy losses and each water system requires pumps to
move the water.
These systems require specialized care and at times
complex operational and maintenance procedures.
Based on the type of cooling refrigerants, there are
hazards related to the personnel and the environment
as well as standards and regulations on the minimum
equipment performance, the type of acceptable
refrigerant per country, and other safety considerations
for having and operating the equipment. Most existing
systems also rely on evaporative cooling as the method
to reject heat to the atmosphere, and the chemical
and water use of this method have come under more
scrutiny in recent years.
Also included in the capital and operational
investment for typical data centers are redundant
components (chillers, cooling towers, pumps, etc.)
to prevent loss of cooling during maintenance or
equipment failures. Since there are many operational
N O V E M B E R 2 0 19

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ASHRAE Journal - November 2019

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