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UNDERSTANDING CHILLER EFFICIENCY: A chiller is a machine that removes removes heat from a liquid via a vapor-compression 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 la rger 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 building’s 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 be tween 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 7°C instead of 5°C 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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Figure 1‐ Round Tube Plate Fin Condenser Coil
Figure 2 ‐ Microchannel Condenser Coil
Figure 3 ‐ Tube Cross Section
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 h ours are at design con diti ons! 1200
95 1067 952
1000 871 s r u o H l a u n n A
923 949 861
90 873
800
85 633 510
600 400
E C W T 80 ( ° F )
602
75
313
200 81 4
70
107 14
0
65 115- 110- 105- 100- 95- 90- 85- 80- 75- 70- 65- 60- 55- 50- 45119 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 xaxis 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 ho urs 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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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 Chi llers:
IPLV ratings for water cooled chillers can be calculated using the following equation: Load %
ECWT °F
Energy Efficiency
Operating Time %
100
85
EER
1
75
75
EER
42
50
65
EER
45
25
65
EER
12
1
2
3
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 44ºF 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 65°F 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
75% (525 Ton)
72.5
EER
42
50% (350 Ton)
65
EER
45
25% (175 Ton)
65
EER
12
1
2
3
4
NPLV = EER X 1% + EER X 42% + EER X 45% + EER X 12% 1
2
3
4
IPLV / NPLV for A ir Cooled Chil lers:
IPLV ratings for Air cooled chillers can be calculated using the following equation: Load %
Am bi ent Ai r Tem per atu re °F
Energy Efficiency
Operating Time %
100
95
EER
1
75
80
EER
42
50
65
EER
45
25
55
EER
12
1
2
3
4
IPLV = EER X 1% + EER X 42% + EER X 45% + EER X 12% 1
2
3
kW/TR or COP can also be used instead of EER for IPLV Calculations
4
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IPLV calculations are based on 44ºF 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 122°F) or the evaporator leaving water temperature is different from 44ºF 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 115°F will have NPLV calculated as follows
Load %
Am bi ent Ai r Tem per atu re °F
Energy Efficiency
Operating Time %
100% (350 Ton)
115
EER
1
75% (262.5 Ton)
80
EER
42
50% (175 Ton)
65
EER
45
25% (87.5 Ton)
55
EER
12
1
2
3
4
NPLV = EER X 1% + EER X 42% + EER X 45% + EER X 12% 1
2
3
4
An nu al Energ y Cost Analy si s (Fo r a Singl e Chi ll er) HVAC system is the largest consumer of electricity in commercial build ings. 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 An nu al Ener gy Cos t:
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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e p y T r o t a r o p a v E
R O S S E R P M O C W E R C S D S V D R A D N A T S
r o t a r o p a v E m l i F g n i l l a F d i r b y H s s a P 2
r o t a r o p a v E m l i F g n i l l a F d i r b y H s s a P 3
e p y T n a F r e s n e d n o C
Round Tube Plate Fin Condenser Coil
Cooling Capacity @ 115 Full Load °F; EWT/LWT = Efficiency EER 54/44 °F
Microchannel Condenser Coil
Part Load Efficiency NPLV EER
AEC (SR)
AEC (SR)/ Ton
Cooling Capacity Part Load Full Load @ 115°F; Efficiency NPLV EWT/LWT = Efficiency EER EER 54/44 °F
AEC (SR)
AEC (SR)/ Ton
Low Speed Fans
309.3
7.128
14.59
413,607
1,337
321.1
7.639
14.93
419,608
1307
Low Speed Fans with Variable Speed Drive
309.3
7.128
16.36
368,859
1,193
321.1
7.639
17.27
362,753
1130
High Airflow Fans
327
7.219
12.78
499,206
1,527
335.7
7.567
13.87
472,213
1407
High Airflow Fans with Variable Speed Drive
327
7.219
16.35
390,205
1,193
335.7
7.567
17.23
380,128
1132
Low Speed Fans
317.1
7.241
14.85
416,613
1,314
329.5
7.765
15.18
423,494
1285
Low Speed Fans with Variable Speed Drive
317.1
7.241
16.62
372,245
1,174
329.5
7.765
17.55
366,304
1112
High Airflow Fans
335.6
7.345
13.04
502,120
1,496
344.6
7.701
14.12
476,150
1382
High Airflow Fans with Variable
335.6
7.345
16.61
394,199
1,175
344.6
7.701
17.50
384,185
1115
Speed Drive
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e p y T r o t a r o p a v E
R O S S E R P M O C W E R C S D S V V L P N D E Z I M I T P O
r o t a r o p a v E m l i F g n i l l a F d i r b y H s s a P 2
r o t a r o p a v E m l i F g n i l l a F d i r b y H s s a P 3
e p y T n a F r e s n e d n o C
Round Tube Plate Fin Condenser Coil
Cooling Capacity @ 115 Full Load °F; EWT/LWT = Efficiency EER 54/44 °F
Microchannel Condenser Coil
Part Load Efficiency NPLV EER
AEC (SR)
AEC (SR)/ Ton
Cooling Capacity Part Load Full Load @ 115°F; Efficiency NPLV EWT/LWT = Efficiency EER EER 54/44 °F
AEC (SR)
AEC (SR)/ Ton
Low Speed Fans
309.3
7.128
15.53
388,572
1,256
321.1
7.639
15.77
397,257
1237
Low Speed Fans with Variable Speed Drive
309.3
7.128
17.55
343,848
1,112
321.1
7.639
18.44
339,737
1058
High Airflow Fans
327
7.219
13.51
472,232
1,444
335.7
7.567
14.55
450,144
1341
High Airflow Fans with Variable Speed Drive
327
7.219
17.54
363,732
1,112
335.7
7.567
18.36
356,732
1063
Low Speed Fans
317.1
7.241
15.86
390,082
1,230
329.5
7.765
16.09
399,542
1213
Low Speed Fans with Variable Speed Drive
317.1
7.241
17.89
345,819
1,091
329.5
7.765
18.79
342,131
1038
High Airflow Fans
335.6
7.346
13.82
473,781
1,412
344.6
7.701
14.85
452,743
1314
High Airflow Fans with Variable
335.6
7.346
17.88
366,200
1,091
344.6
7.701
18.73
358,956
1042
Speed Drive
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
An nu al Energ y Cost Analys is (For a Multi pl e 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 a nalysis reports. For more details on how YORKcalc™ software can help you quickly and easily answer challenging questions on chiller pla nt energy consumption, call your local Johnson Controls office.
