Energy Efficient Buildings
Energy Efficient Cooling Plants
Chiller Plant Control
A chiller plant typically includes chillers, chilled water pumps, cooling tower pumps, and cooling
towers with fans. A chiller plant with constant-speed primary chilled water pumps and variable
speed secondary chilled water pumps is shown below.
Cooling Tower Fan
Chilled Water Supply
AHU 1
Chiller 1
VFD
Secondary
Chilled Water
Pumps
Chiller 2
Condenser
(Cooling Tower)
Pumps
AHU 2
AHU 3
dP
Chilled Water Return
Primary
Chilled Water
Pumps
Typical temperature design guidelines for chiller plants include:
Tevaporator = 10 F
Tcondenser = 10 F
Tchilledwatersupply = 42 F
Tcoolingtowerreturn = 80 F
The total energy use of the chiller plant includes the energy use by the chiller, pumps and
cooling tower fans. Historically, as energy efficiency became more important, attention was
originally focused on improving chiller efficiency since the chiller was the single biggest energy
user in the chiller plant. As a consequence, chiller energy use declined over the last thirty years
from over 0.75 kW/ton to less than 0.50 kW/ton today.
As chiller efficiency improved, the energy use by supporting fans and pumps became a larger
fraction of chiller plant energy use. In addition, designers and operators became increasingly
aware of interaction effects between the components, and the potential to drive energy use
even lower by optimizing the system rather than components. This chapter discusses chiller
plant control to reduce total system energy use. Specifically, it considers how the design
guidelines shown above could modified to enhance energy efficiency.
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Source data: Trane, 2000, “Chilled Water System Design and Operation”, CTV-SLB005-EN.
Reduce Cooling Tower Fan Energy Use
Cooling tower fan energy use can be reduced by reducing the friction loss as air flows through
the cooling tower and by reducing the flow rate of air. Friction losses are much smaller in
induced flow-cooling towers than forced-flow cooling towers. In addition, improved packing
designs increase evaporative contact area while reducing friction losses. Low-friction cooling
towers can be identified by comparing rated fan motor power per rated cooling capacity.
Cooling tower fan energy use can also be reduced by better air flow control. The temperature
of the water leaving a cooling tower is typically controlled by varying the air flow rate through
the cooling tower. In older cooling towers, air flow rate was varied by cycling the cooling tower
fan on and off. A more energy efficient method of control is to vary the fan speed since friction
pressure drop is lower at lower air flows; the fan affinity law of fluid work varying with the cube
of flow applies to cooling towers as well duct systems. Two speed cooling tower fan motors
approximate this type of control. Today, full variable speed control is achieved by controlling
fan speed with a variable-frequency drive.
The table and graph below show simulated cooling tower fan electricity use for a constantspeed on/off and variable-speed 10-hp fan motor running continually in Dayton, OH with a 10 F
temperature drop and an 80 F condenser supply temperature. The constant-speed fan is on
47% of the year, while the variable speed fan runs at 37% of full speed. During hot, humid
weather, the fraction energy savings from variable-speed cooling tower fan control are less
2
since the cooling tower fan must operate at close to full load. However, during cool, dry
conditions, the fraction energy savings from variable-speed cooling tower fan control are
significant. Overall, fan energy was reduced by about 64%.
Month
======
1
2
3
4
5
6
7
8
9
10
11
12
======
Year
CSfon
VSffs
Ecsf(kWh) Evsf(kWh) Esav(kWh) FracSav
====== ====== ====== ====== ======
======
0.26
0.17
1,467
69
1,398
0.95
0.27
0.18
1,368
71
1,297
0.95
0.33
0.23
1,862
153
1,709
0.92
0.38
0.26
2,061
223
1,838
0.89
0.49
0.36
2,745
572
2,173
0.79
0.69
0.58
3,740
1,794
1,945
0.52
0.87
0.8
4,856
3,658
1,198
0.25
0.8
0.71
4,473
2,860
1,614
0.36
0.63
0.51
3,405
1,510
1,895
0.56
0.39
0.27
2,151
226
1,925
0.89
0.33
0.22
1,793
147
1,646
0.92
0.29
0.19
1,621
99
1,523
0.94
====== ====== ====== ====== ======
======
0.48
0.37
31,543
11,381
20,162
0.64
Reduce Cooling Tower Water Flow Rate
After cooling tower fan energy use is addressed, energy savings from reducing cooling tower
water flow rate can be explored. Cooling towers are typically designed to operate with a fixed
flow rate of water. A typical flow rate for cooling towers is 3 gpm per rated ton of chiller
capacity. However, reducing the water flow rate reduces pumping costs and improves cooling
tower effectiveness. Moreover, if system is originally designed for less flow, smaller pipes and
pumps can reduce first costs. Consider the following example.
Example
3
A cooling tower is originally designed and operated with 5 gpm of water per ton with a 10 F
temperature gain through the condenser. The required elevation head is 10 ft H20 and the
friction head is 20 ft H2O. The pump is 70% efficient and the pump motor is 90% efficient. The
water flow rate is then reduced to 3 gpm per ton. If the wet-bulb temperature of the air is 60 F,
determine a) the water temperature leaving the cooling tower at 5 gpm, b) the water
temperature leaving the cooling tower at 3 gpm, c) the pumping power at 5 gpm per ton, d) the
pumping power at 3 gpm per ton, and e) the fraction reduction in pumping power.
a) From the cooling tower performance chart for a tower operated at 5 gpm/ton, the
temperature of water leaving a cooling tower is about 80 F when the temperature range is 10 F
and the wet-bulb temperature of the air is 60 F.
b) If the tower flow rate were reduced to 3 gpm/ton, the new temperature range can be found
from an energy balance on the condenser.
Qcond = V1 p cp Tcond1 = V2 p cp Tcond2
Tcond2 = (V1 / V2) Tcond1 = (5 gpm/ton / 3 gpm/ton) 10 F = 16.7 F
From the cooling tower performance chart for a tower operated at 3 gpm/ton, the temperature
of water leaving a cooling tower when the temperature range is 16.7 F and the wet-bulb
temperature of the air is 60 F is about 78 F.
Thus, the temperature of water leaving the cooling tower declines with the lower flow rate. As
long as the temperature of water to the condenser is greater than the minimum temperature
required by the chiller, reducing the temperature of water to the condenser improves the
efficiency of the chiller.
c) The initial pump head is:
h1 = helev + hfric = 10 ft H2O + 20 ft H2O = 30 ft H2O
The electrical power supplied to the pump motor at 5 gpm/ton is:
P1 = V1 h1 / [ 3,960 (gpm-ftH20/hp) Epump Emotor ]
P1 = 5 gpm/ton 30 ftH20 / [3,960 (gpm-ftH20/hp) 0.70 0.90] x 0.746 kW/hp
P1 = 0.0449 kW/ton
d) Reducing the flow rate to 3 gpm/ton reduces the pump work to overcome friction according
to the pump affinity laws. The fluid work to overcome friction at 5 gpm/ton was:
Wf1 = V1 hf1 / 3,960 (gpm-ftH20/hp)
Wf1 = 5 gpm/ton 20 ftH20 / 3,960 (gpm-ftH20/hp) = 0.0253 hp/ton
4
According to the fan affinity law, the fluid work to overcome friction at 3 gpm/ton is:
Wf2 = Wf1 (V2 / V1)3 = 0.0253 hp/ton (3 gpm/ton / 5 gpm/ton)3 = 0.00546 hp/ton
The fluid work to overcome the elevation head is:
We2 = V2 he / 3,960 (gpm-ftH20/hp) = 3 gpm/ton 10 ftH20 / 3,960 (gpm-ftH20/hp) = 0.00758
hp/ton
Assuming the efficiencies of the pump and motor remain the same, the total electrical power to
the pump motor at 3 gpm/ton is:
P2 = (Wf2 + We2) / (Epump Emotor)
P2 = (0.00546 hp/ton + 0.00758 hp/ton) / (0.70 x 0.90) x 0.746 kW/hp = 0.0154 kW/ton
e) Thus, reducing the flow rate from 5 gpm/ton to 3 gpm/ton reduced the electrical power to
the pump motor by
(0.0449 kW/ton - 0.0154 kW/ton) / 0.0449 kW/ton = 66%
The result in the example above indicates the savings potential from reducing the flow rate of
condenser water through the cooling tower. In practice, this can sometimes be achieved by
measuring temperature difference of water across the condenser, and reducing flow if the
temperature difference is consistently small.
Condenser Water Temperature Control
Chiller efficiency improves with lower condenser water temperature. However, cooling tower
fan energy use increases to deliver lower condenser water temperature. This suggests an
optimum condenser water temperature may exist which would minimize total cooling tower
fan plus chiller energy use.
The affect of condenser water temperature on total cooling tower fan plus chiller electricity use
can be modeled by solving a system of equations that includes cooling tower and chiller
performance. Input values must be known for:
Qevap (tons) = actually cooling load
Qcap (tons) = cooling capacity of chiller
Twb (F) = ambient wet-bulb temperture
Tcsp (F) = set point temperature of cold water leaving cooling tower
DWF (gpm/ton) = design water flow rate to cooling tower
5
To solve the system using successive substitution, start by assuming a temperature range Tr,
across the cooling tower and then solving the following set of equations. The chiller fraction
loaded, FL, is:
1) FL = Qevap / Qcap
The minimum water temperature delivered by the cooling tower, Tc, is given by:
2) Tc = a + b Twb + c Tr + d Twb2 + e Tr2 + f Tr Twb
However, cooling tower fans cycle on and off to maintain the water leaving the cooling tower at
a set temperature Tcsp. Thus, to incorporate cooling tower control, Equation 2 must be
followed by the following algorithm. From an energy balance, the fraction of time the cooling
tower fan runs, Fon, and the actual entering and leaving cooling tower water temperatures Th
and Tc are:
3)
If Tc >= Tcsp then
‘fan runs continuously
Fon = 1
Tc = Tc
Th = Tc + Tr
Else if Tc < Tcsp then
‘fan cycles on and off to maintain Tcsp
Th = Tcsp + Tr
Fon = (Tcsp – Th) / (Tc – Th)
Tc = Tcsp
End if
Compressor input power per ton of evaporator cooling, KWPT, is given by:
4) KWPT = a + b FL + c FL2 + d Tc + e Tc2 + f Tc FL
Then, from an energy balance on the chiller, compressor input power, Wcomp, heat rejected by
the condenser, Qcond, volume flow rate of water through the condenser, Vw, and
temperature rise across the condenser, Tr, are given by :
5) Wcomp = KWPT Qevap
6) Qcond = Wcomp + Qevap
7) Vw = DWF Qcap
8) Tr = Qcond / (Vw pw cpw)
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where pw is the density of water and cpw is the specific heat of water. The value for Tr can
then be substituted back into the start of the algorithm and the algorithm repeated until Tr
converges. After convergence, cooling tower fan power, Wctf, can be calculated as:
Wctf = Fon RHP FML / Emotor x 0.746 kW/hp
Where RHP is cooling tower fan rated horsepower, FML is fraction motor loaded, Emotor is the
efficiency of the motor. The total power of the cooling tower fan and compressor is:
Wtot = Wctf + Wcomp
Example
Consider a 500-ton chiller operated at 300 tons with a 30-hp cooling tower fan and design
water flow rate of 3 gpm/ton. The fan motor is 90% efficient. The outdoor air wet-bulb
temperature is 60 F. Calculate total cooling tower fan plus compressor electrical power for
cooling tower water set point temperatures of 80 F, 70 F and 60 F.
Use of the algorithm shown above produces the following results. The minimum total cooling
tower fan plus compressor electrical power (192 kW) occurred at a cooling tower water set
point temperature of 70 F, which is 10 F greater than the outdoor air wet bulb temperature.
This suggests that total cooling tower fan plus compressor electrical power might be minimized
over an entire year by resetting the cooling tower water set point temperature according to
outdoor air wet-bulb temperature.
When this algorithm is incorporated into an hour-by-hour simulation program, the affect of
condenser water temperature set point on total cooling tower fan plus chiller electricity use can
be tested. For example, the following results shown below are for a 500-ton chiller operated at
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300 tons with a 30-hp cooling tower fan and design water flow rate of 3 gpm/ton in Dayton,
OH, and a cooling tower water set point temperature of 60 F.
The results below show total cooling tower fan energy use for both a constant speed fan Ecsf
and variable speed fan Evsf, compressor energy use, Ec, and total energy use for various cooling
tower set-point temperatures. These results indicate that chiller plus cooling tower fan energy
use can be reduced from a design condenser water temperature of 80 F by setting the cooling
water temperature equal to the minimum condenser temperature recommended by the chiller
manufacturer. Chiller plus cooling tower fan energy use can then be further reduced by varying
cooling water temperature with outdoor air wet-bulb temperature.
Tcsp
(F)
80
70
60
50
Twb + 10 F
Ecsf
(kWh/yr)
52,296
93,217
134,651
173,341
1,498,886
Evsf
(kWh/yr)
5,541
48,493
99,355
151,437
94,166
Ec
(kWh/yr)
1,739,450
1,542,125
1,406,825
1,328,278
1,350,674
Ecsf + Ec
(kWh/yr)
1,791,746
1,635,342
1,541,476
1,501,619
1,500,559
Evsf + Ec
(kWh/yr)
1,744,991
1,590,618
1,506,180
1,479,715
1,444,840
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These results are similar to an analysis by Trane, which showed that varying condenser water
temperature with outdoor air wet-bulb temperature resulted in lower total energy costs than
the design condenser water temperature or the minimum condenser water temperature that
the chiller could accept.
Source data: Trane, 2000, “Chilled Water System Design and Operation”, CTV-SLB005-EN.
All Variable Flow Cooling Plants
Traditionally, cooling plant pumps, fans and chillers were driven with constant speed motors.
Load following capability was provided by bypass, mixing, staging and on/off control. Today,
load following capability can be provided by varying the flow of the pumps, fans and chillers
with variable frequency drives. Variable flow control can significantly reduce energy use over
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traditional constant flow systems. The figure below shows an all variable-speed cooling plant.
Variable-speed cooling plants have been documented to use as little as 0.5 kW/ton at high
loads and 0.3 kW/ton at low loads (Erpelding, Ben, 2008, “Monitoring Chiller Plant
Performance”, ASHRAE Journal, April, pp. 48-52.)
Cooling Tower Fan
VFD
Chilled Water Supply
AHU 1
AHU 2
AHU 3
Chiller 1
dP
VFD
VFD
Chiller 2
VFD
Condenser
(Cooling Tower)
Pumps
Bypass
Valve
Flow
Meter
Chilled Water Return
VFD
Primary
Chilled Water
Pumps
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