Page 357 - HVAC Pump Handbook

Rishel_CH13.qxd 20/4/06 6:40 PM Page 354

Pumping Open Thermal Storage Tanks

354 Pumps for Open HVAC Cooling Systems

A number of studies of the energy consumed by chillers have been
conducted by various companies to determine the value of energy
storage in reducing on-peak demand charges and overall efficient
operation of chillers. The data shown in Table 13.1 are typical of the
findings of some of these studies. This information is for chillers pro-
viding cooling of the chilled water without energy storage.
The data in Table 13.1 demonstrate that the programming of
chillers needs to eliminate the operation of chillers at light loads. One
of the most important advantages of these cool storage systems is the
“flywheel effect” they can have on chiller operation. The proper design
of energy storage systems therefore should provide for chiller opera-
tion at optimal efficiency as well as to ensure adequate cooling during
the peak demand charge period. The actual programming of the
charging of the tanks should not be just to recharge the tanks as fast
as possible but to use the storage and the chillers during light system
loads to achieve optimal overall energy consumption.
For example, in Table 13.1, most of the hours of operation were
from 15 to 50 percent design load on the chiller, where the kilowatts
per ton varied from 1.32 to 0.79. With chilled water storage, the

TABLE 13.1 Chiller Performance
(Mean kw/ton per 5% Increment of Chiller Load)

Percent Number of Kilowatts
chiller load seasonal hours per ton
10–15 108 1.53
15–20 542 1.32
20–25 389 1.15
25–30 303 1.03
30–35 311 0.94
35–40 209 0.87
40–45 225 0.83
45–50 262 0.79
50–55 230 0.76
55–60 137 0.74
60–65 211 0.72
65–70 128 0.70
70–75 120 0.70
75–80 122 0.70
80–85 104 0.70
85–90 32 0.67
90–95 11 0.61
95–100 4 0.59
NOTE: The values for chillers shown in this table
are for chillers manufactured around 1994. Recent
values (circa 2005) have indicated improved chiller
performance; actual data should be secured for any
chiller under consideration.

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