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UNDERSTANDING CHILLER EFFICIENCY:
A chiller is a machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This liquid can then
be circulated through a heat exchanger to cool air or equipment as required. As a necessary byproduct, refrigeration cycle creates
waste heat that must be exhausted to ambient or, for greater efficiency, recovered for heating purposes
The chiller efficiency depends on the energy consumed and the cooling delivered. Absorption chillers are rated in fuel consumption
per ton cooling. Electric motor driven chillers are rated in kilowatts per ton cooling.
Below are a few simple formulas for converting between various units of energy efficiency for electric motor driven chillers
KW/ton = 12 / EER
KW/ton = 12 / (COP x 3.412)
COP = EER / 3.412
COP = 12 / (KW/ton) / 3.412
EER = 12 / KW/ton
EER = COP x 3.412
If a chillers efficiency is rated at 1 KW/ton,
COP = 3.5
EER = 12
Cooling Load in - kW/ton
The term kW/ton is commonly used for larger commercial and industrial air-conditioning, heat pump and refrigeration systems.
The term is defined as the ratio of energy consumption in kW to the rate of heat removal in tons at the rated condition. The lower
the kW/ton the more efficient the system.
KW/ton = Pc / Er
Where
Pc = energy consumption (kW); Er = heat removed (ton)
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Coefficient of Performance - COP
The Coefficient of Performance - COP - is the basic parameter used to report efficiency of refrigerant based systems.
The Coefficient of Performance - COP - is the ratio between useful energy acquired and energy applied and can be expressed as
COP = Eu / Ea
Where
COP = coefficient of performance
Eu = useful energy acquired (btu in imperial units or Watts in SI Units)
Ea = energy applied (btu in imperial units or Watts in SI Units)
COP can be used to define both cooling efficiencies and heating efficiencies (for heat pumps)
Cooling - COP is defined as the ratio of heat removal to energy input to the equipment
Heating - COP is defined as the ratio of heat delivered to energy input to the equipment
COP can be used to define the efficiency at single standard or non-standard rated conditions, or as a weighted average of seasonal
conditions. The term may or may not include the energy consumption of auxiliary systems such as indoor or outdoor fans, chilled
water pumps, or cooling tower systems.
higher COP - more efficient system
COP is dimensionless because the input power and output power are measured in the same units. COP is an instantaneous
measurement i.e. both the energy acquired and energy applied have to be measured at any specific given point in time (Either full
load condition or any partial load condition). Most air conditioning equipment manufacturers provide COP values at full load
conditions and it does not reflect how the equipment performs at part load conditions.
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Energy Efficiency Ratio - EER
The Energy Efficiency Ratio - EER - is a term generally used to define cooling efficiencies of unitary air-conditioning and heat pump
systems.
The efficiency is determined at a single rated condition specified by an appropriate equipment standard and is defined as the ratio of
net cooling capacity - or heat removed in Btu/h - to the total input rate of electric power applied - in Watts. The units
of EER are Btu/Wh.
EER = Ec / Pa (3)
Where
EER = energy efficient ratio (Btu/Wh)
Ec = net cooling capacity (Btu/h)
Pa = applied electrical power (Watts)
This efficiency term typically includes the energy requirement of auxiliary systems such as the indoor and outdoor fans.
higher EER - more efficient system
Similar to COP, EER is an instantaneous measurement taken at a particular point in time and does not reflect how the equipment
performs across entire range of its capacity modulation.
Factors affecting Chiller Efficiency:
In order to understand chiller efficiency, we must understand that the purpose of a chiller is to remove heat from any buildings chiller
water circuit and to reject it to the ambient by using either an air cooled condenser (For Air Cooled Chillers) or a combination of water
cooled condenser / cooling tower (For Water Cooled Chillers). In both cases, most of the power applied to the chillers is for the
compressor which will pump the refrigerant between the evaporator and the condenser. The compressor takes up most of the power
consumption for the chiller as it lifts the refrigerant from a low temperature / low pressure state in the evaporator to a high
temperature / high pressure state in the condenser.
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In order to have a chiller which runs efficiently, the lift between refrigerant temperature in the evaporator and condenser must be
minimized. This can be done by selecting leaving chilled water at a relatively higher temperature (i.e. use of 7C instead of 5C
leaving chilled water temperature will reduce the amount of lift required for the compressor and help improve the efficiency of the
chiller). Design of Evaporator and Condenser can also have significant impact on the overall efficiency of the chiller. For example use
of Microchannel Condenser coil for air cooled chillers can improve the efficiency by around 4% for the same size chiller as compared
to traditional round tube plate fin coils. Using 3 pass evaporator instead of 2 pass can also improve the efficiency of the chiller. Larger
heat exchangers yield higher full load efficiency .Hybrid Falling film evaporators are more efficient than the traditional DX and
Flooded type evaporators and can increase the overall efficiency of the chiller by approximately 5%.
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Figure1RoundTubePlateFinCondenserCoil Figure2MicrochannelCondenserCoil Figure3 TubeCrossSection
Condensing refrigerant temperature depends on the ambient conditions and cannot be controlled. However, condensing
temperatures reduce during off-design conditions (When the ambient temperature is lower than the design condition). Studies
conducted by AHRI show that 99% of the time, the chiller encounters ambient conditions lower than the design condition. In such
instances, use of compressor having Variable Speed Drive can help achieve higher part load efficiencies. Using Variable Speed
Condenser fans can also help in achieving better part load efficiencies. Other operational factors which can affect efficiency include
condenser and evaporator fouling.
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General Weather Pattern in the Middle East
Less than 1% of chiller run hours are at design conditions!
1200 95
1067
1000 952 923 949 90
871 861 873
Annual Hours
800 85
ECWT (F)
633 602
600 510 80
400 75
313
200 70
81 107
4 14
0 65
115- 110- 105- 100- 95- 90- 85- 80- 75- 70- 65- 60- 55- 50- 45-
119 114 109 104 99 94 89 84 79 74 69 64 59 54 49
Dry-bulb Temperature Bins (F)
This graph shows the average weather data for Dubai. We can assume the other cities in the Middle East would be similar. On the x-
axis are the temperature bins. On the y-axis is the number of hours which the chiller has to run at these temperatures
Appearing on the right y-axis is the entering condenser water temperature. In general, the ECWT rises as the dry-bulb temperature
rises (Applicable for Water Cooled Chillers).The chart indicates that most of the operating hours occur at off-design conditions; when
the actual dry bulb (in case of air cooled chillers) or wet bulb (in case of water cooled chillers) is lesser than the design condition.
According to the above chart, Chillers run 99% of the time on part load (off-design) conditions i.e. reduced compressor lift and/or
reduced internal load of the building; hence more consideration must be given to part load efficiency while choosing your chillers
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Understanding Full Load and Part Load (IPLV / NPLV) Efficiencies
The two most common energy efficiency ratings given to chillers are full load and part Load (Integrated Part Load Value - IPLV or
Non Standard Part Load Values - NPLV). Both full load and part load efficiencies can be measured in kW/ton, EER (Btu/Wh) or COP.
Full Load Efficiency
Indicating the efficiency of the chiller at its peak load condition, full-load efficiency is the ratio of the cooling capacity to the total power
input at peak load (when the chiller is running to satisfy the maximum cooling demand of the building)
Full Load Efficiency
Predicts performance at a single operation point
Doesnt anticipate how equipment will respond during off-design conditions
Equipment with excellent full-load characteristics may have less than satisfactory part-load characteristics
Studies conducted by AHRI show that chillers run at full load only 1% of the time.
Part Load Efficiency (IPLV / NPLV)
When designing any chiller plant, part load efficiency must be taken into consideration since 99% of the operating hours for any
chiller are on part load conditions. Part Load means not only reduced tons of cooling required, but also reduced lift (difference
between evaporator and condenser temperatures which the compressor must overcome)
The Integrated Part Load Value (IPLV) is a performance characteristic developed by the Air-Conditioning, Heating and Refrigeration
Institute (AHRI). It is most commonly used to describe the performance of a chiller capable of capacity modulation. Unlike full load
efficiency, which describes the efficiency at full load conditions only, the IPLV is derived from the equipment efficiency while
operating at various capacities. Since a chiller does not always run at 100% capacity, the full load EER, COP or kW/TR is not an
ideal representation of the typical equipment performance. The IPLV / NPLV is a very important value to consider since it can affect
energy usage and operating costs throughout the lifetime of the equipment. Energy codes such as ASHRAE Standard 90.1 specifies
minimum values for the Chiller full load and part load efficiencies
The Integrated Part Load Value (IPLV) rating is targeted to a very specific situation: when the projects design conditions are equal to
the ARI standard conditions. For departures from standard AHRI conditions, the efficiency number is known as the Non-standard
Part-Load Value (NPLV). IPLV is a specialized subset of NPLV. The AHRI recognizes that an NPLV rating cant predict exactly what
the absolute chiller efficiency would be in an actual installation. NPLV does, however, provide a meaningful way of comparing the
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relative efficiency of different chiller models. The actual efficiency may differ from the NPLV by a few percent, but each chiller model
will differ by a similar amount
IPLV / NPLV for Water Cooled Chillers:
IPLV ratings for water cooled chillers can be calculated using the following equation:
Load % ECWT F Energy Efficiency Operating Time %
100 85 EER 1
1
75 75 EER 42
2
50 65 EER 45
3
25 65 EER 12
4
IPLV = EER X 1% + EER X 42% + EER X 45% + EER X 12%
1 2 3 4
kW/TR or COP can also be used instead of EER for IPLV Calculations
IPLV calculations are based on 44F evaporator LWT with a flow rate of 2.4 gpm/ton. Condenser EWT
is 85 F with 3 gpm/ton (as per AHRI 550/590 Standard)
If a chiller is designed to operate at different conditions, including lower/higher evaporator leaving water temperature or different
evaporator flow rates; different condenser EWT or condenser flow rates; the efficiency is called a NPLV (non-standard part load
value). In case of NPLV, the part load entering condenser water temperature should vary linearly from the selected Condenser EWT
at 100% load to 65 F at 50% load, and fixed at 65F for 50% to 0% load.
For example a 700 Ton Centrifugal Chiller with Entering / Leaving Chilled Water Temperature = 56 / 44 F and Entering / Leaving
Condenser Water Temperature = 80 / 90 F will have its NPLV Calculated as follows
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Load % ECWT F Energy Efficiency Operating Time %
100% (700 Ton) 80 EER 1
1
75% (525 Ton) 72.5 EER 42
2
50% (350 Ton) 65 EER 45
3
25% (175 Ton) 65 EER 12
4
NPLV = EER X 1% + EER X 42% + EER X 45% + EER X 12%
1 2 3 4
IPLV / NPLV for Air Cooled Chillers:
IPLV ratings for Air cooled chillers can be calculated using the following equation:
Load % Ambient Air Temperature F Energy Efficiency Operating Time %
100 95 EER 1
1
75 80 EER 42
2
50 65 EER 45
3
25 55 EER 12
4
IPLV = EER X 1% + EER X 42% + EER X 45% + EER X 12%
1 2 3 4
kW/TR or COP can also be used instead of EER for IPLV Calculations
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IPLV calculations are based on 44F evaporator LWT with a flow rate of 2.4 gpm/ton. Condenser EAT is
95 F (as per AHRI 550/590 Standard)
In many cases, equipment is rated for higher ambient air temperature (designed for 115, 118 or 122F) or the evaporator leaving
water temperature is different from 44F or the evaporator flow rate is different from 2.4 gpm/ton. For NPLV Calculations, EER1 will
be selected at the rated ambient condition. For example, an air cooled screw chiller offering 350 Ton Capacity at an ambient
temperature of 115F will have NPLV calculated as follows
Load % Ambient Air Temperature F Energy Efficiency Operating Time %
100% (350 Ton) 115 EER 1
1
75% (262.5 Ton) 80 EER 42
2
50% (175 Ton) 65 EER 45
3
25% (87.5 Ton) 55 EER 12
4
NPLV = EER X 1% + EER X 42% + EER X 45% + EER X 12%
1 2 3 4
Annual Energy Cost Analysis (For a Single Chiller)
HVAC system is the largest consumer of electricity in commercial buildings. Energy efficiency constantly ranks near the top among
project requirements because it has a direct impact on the bottom line in the long term. Lower operational costs are a necessity
regardless of institution or business type. Money saved on operational costs can be diverted to more productive uses.
Annual energy cost of operating a chiller can be estimated using the following formula:
Annual Energy Cost = Real world efficiency NPLV x Energy rate (SR/kWHr) x Average chiller load x Operating hours
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Calculation based on:
Operating hours: 8760 Hours (Annual)
Chiller net capacity: 324.8 Tons
Energy rate: 0.32 SR/ kWHr
Ambient Temp: 115 F
Chilled Water Temp: 54 / 44 F
Average building load profile as defined by AHRI as follows
= 0.01 (100% load) +0.42 (75% load) +0.45(50% load) +0.12 (25% load)
= 0.01(1) + 0.42(0.75) + 0.45(0.5) + 0.12(0.25)
= 0.58
Average chiller load = 0.58 (324.8 TR) = 188.4 TR
Annual Energy Cost:
Chiller 1: NPLV = 14.6 EER (kW/TR = 0.822), Energy cost SR 434,117 per year
Chiller 2: NPLV = 17.8 EER (kW/TR = 0.674), Energy cost SR 355,955 per year
HIGHER EFFICIENCY CHILLER CAN SAVE SR 78,162 EVERY YEAR OF OPERATION!!!
Impact of Chiller Component Selection on Efficiency
As briefed earlier, components used in a chiller can significantly impact its efficiency. Selection of Condenser, Evaporator,
Compressor type and Condenser Fan type can greatly change the IPLV/NPLV of the chiller and affect the annual energy
consumption and cost. In order to understand the extent of this impact on efficiency, we will be take an example of a 350 Ton
Nominal Air Cooled Screw Chiller and see how its efficiency varies by using different components as listed below.
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RoundTubePlateFinCondenserCoil MicrochannelCondenserCoil
CondenserFanType
EvaporatorType
Cooling CoolingCapacity
PartLoad PartLoad
Capacity@115 FullLoad AEC(SR)/ @115F; FullLoad
EfficiencyNPLV AEC(SR) EfficiencyNPLV AEC(SR) AEC(SR)/Ton
F;EWT/LWT= EfficiencyEER Ton EWT/LWT= EfficiencyEER
EER EER
54/44F 54/44F
LowSpeedFans 309.3 7.128 14.59 413,607 1,337 321.1 7.639 14.93 419,608 1307
2PassHybridFallingFilmEvaporator
LowSpeedFans
withVariable 309.3 7.128 16.36 368,859 1,193 321.1 7.639 17.27 362,753 1130
SpeedDrive
HighAirflowFans 327 7.219 12.78 499,206 1,527 335.7 7.567 13.87 472,213 1407
STANDARDVSDSCREWCOMPRESSOR
HighAirflowFans
withVariable 327 7.219 16.35 390,205 1,193 335.7 7.567 17.23 380,128 1132
SpeedDrive
LowSpeedFans 317.1 7.241 14.85 416,613 1,314 329.5 7.765 15.18 423,494 1285
3PassHybridFallingFilmEvaporator
LowSpeedFans
withVariable 317.1 7.241 16.62 372,245 1,174 329.5 7.765 17.55 366,304 1112
SpeedDrive
HighAirflowFans 335.6 7.345 13.04 502,120 1,496 344.6 7.701 14.12 476,150 1382
HighAirflowFans
withVariable 335.6 7.345 16.61 394,199 1,175 344.6 7.701 17.50 384,185 1115
SpeedDrive
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RoundTubePlateFinCondenserCoil MicrochannelCondenserCoil
CondenserFanType
EvaporatorType
Cooling CoolingCapacity
PartLoad PartLoad
Capacity@115 FullLoad AEC(SR)/ @115F; FullLoad
EfficiencyNPLV AEC(SR) EfficiencyNPLV AEC(SR) AEC(SR)/Ton
F;EWT/LWT= EfficiencyEER Ton EWT/LWT= EfficiencyEER
EER EER
54/44F 54/44F
LowSpeedFans 309.3 7.128 15.53 388,572 1,256 321.1 7.639 15.77 397,257 1237
2PassHybridFallingFilmEvaporator
LowSpeedFans
withVariable 309.3 7.128 17.55 343,848 1,112 321.1 7.639 18.44 339,737 1058
SpeedDrive
OPTIMIZEDNPLVVSDSCREWCOMPRESSOR
HighAirflowFans 327 7.219 13.51 472,232 1,444 335.7 7.567 14.55 450,144 1341
HighAirflowFans
withVariable 327 7.219 17.54 363,732 1,112 335.7 7.567 18.36 356,732 1063
SpeedDrive
LowSpeedFans 317.1 7.241 15.86 390,082 1,230 329.5 7.765 16.09 399,542 1213
3PassHybridFallingFilmEvaporator
LowSpeedFans
withVariable 317.1 7.241 17.89 345,819 1,091 329.5 7.765 18.79 342,131 1038
SpeedDrive
HighAirflowFans 335.6 7.346 13.82 473,781 1,412 344.6 7.701 14.85 452,743 1314
HighAirflowFans
withVariable 335.6 7.346 17.88 366,200 1,091 344.6 7.701 18.73 358,956 1042
SpeedDrive
Annual Energy Cost (AEC) = Real world efficiency NPLV (kW/TR) x Energy rate (SR/kWHr) x Average chiller load (TR) x Operating hours (Hr)
= (12/EER) x 0.32 SR/kWHr x (0.58 x Cooling Capacity @ 115 F) x 8760 Hrs
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If we have a detailed look at the tables above, we can see that the 350 TR chiller can cost us as low as SR 339,737 / year and as
high as SR 499,206 / year for its operation. So while designing and executing projects, it is very important to select the right
components and the highest NPLV to achieve highest annual energy cost savings
Annual Energy Cost Analysis (For a Multiple Chiller Plant)
In a single-chiller plant, the chiller sees the full range of building cooling loads: from 100% design load down to 10%, when the chiller
shuts off. In multiple-chiller systems, on the other hand, chillers cycle off as the building-cooling load gets lower, and the load on the
remaining chillers increases. The result is that the individual chillers see higher loads, on average.
Calculating Annual Energy Cost for a multiple chiller plant requires more sophisticated calculations and simulation. Johnson Controls
offers its YORKcalc Chiller plant energy estimation software which can perform chiller plant analysis based on real-world operating
conditions; includes weather data for around 300 cities across the globe and can be a very useful tool in determining annual
operating cost and/or to compare different chiller plants. YORKcalc also considers all pumps/towers in its energy cost analysis with
the ability to generate multiple analysis reports.
For more details on how YORKcalc software can help you quickly and easily answer challenging questions on chiller plant energy
consumption, call your local Johnson Controls office.
