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Global Optimisation of Chiller Sequencing and Load Balancing Using Shuffled Complex Evolution
IAIN STEWARTA (MENG)LU AYEA (FAIRAH, FAIE, PHD)
TIM PETERSONB (PHD)
A Renewable Energy and Energy Efficiency Group, Department of Infrastructure Engineering,The University of Melbourne, Vic 3010, AustraliaB Environmental Hydrology and Water Resources Group, Department of Infrastructure Engineering, The University of Melbourne, Vic 3010, Australia
Evolutionary Chiller Optimisation (ECO)
• The why?
• A brief history of building controls
• Chillers and VSDs
• The how? Improving system efficiency
• Aims- Optimising equipment loading
• Model- Structure, inputs & outputs
• Results- Modelled savings
Why?
• Energy costs
• Carbon emissions
• Peak demand reduction
40%
35%
15%
20%
Typical Commercial Building End Use Energy
HVAC
Lighting
Equipment
Other
92PJ
22MtCO2
A Brief History of Building Controls
Pneumatic controls….
Direct Digital Control (DDC)
A Brief History of Building Controls
Building Management System (BMS)
A Brief History of Building Controls
Reciprocating vs Centrifugal Chillers
Chiller COP vs Loading
0
2
4
6
8
10
12
0% 25% 50% 75% 100%
Ch
iller
CO
P
Chiller Load %
New centrifugal variable speed chiller
Old reciprocating chiller
100%300%
Variable Speed Drives (VSDs)
Variable Speed Drives (VSDs)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Ener
gy C
on
sum
pti
on
Pump Speed
Pump Speed vs Energy Consumption
<50%
10% 80%
Technological improvements
• Capabilities of Building Management Systems
• Improved chiller part load efficiency
• Affordability of Variable Speed Drives
Current Control Technique
• Return water temperature staging
Switch-on thresholds Switch-off thresholds
From 1 chiller to 2 chillers: Trtn > 11.94 °C From 2 chillers to 1 chiller: Trtn < 9.28 °C
From 2 chillers to 3 chillers: Trtn > 11.85 °C From 3 chillers to 2 chillers: Trtn < 10.10 °C
From 3 chillers to 4 chillers: Trtn > 11.81 °C From 4 chillers to 3 chillers: Trtn < 10.51 °C
Trtn = Temperature of return chilled water from the building
The Opportunity
• Maximize cooling system efficiency [COP]
– Non linear efficiency curves
– Multiple pieces of equipment
– Inherently difficult for humans
Aims
• Accurately simulate waterside plant equipment
• Optimise equipment loading for cooling loads
– Maximise plant COP for all cooling loads
”All models are wrong but some are useful” – George Box
ECO
1. Simulate waterside plant– Energy inputs
• Flow rates• Mechanical work• Efficiencies• Physical limits
– Refrigeration delivered2. Optimise equipment staging and loading using genetic algorithm for a range of cooling loads
Modelling assumptions
• Chiller turn down ratio = 10:1
• Condenser and chilled water pumps operate at minimum flow rate when chillers are at minimum output (eg. 50% pump speed when chiller is at 10% capacity)
• Pumps speed increases linearly with chiller energy
• Maximum temperature difference across chillers = 7°C
• Fans excluded from simulation
Plant layout
Plant Simulation
Inputs Simulation Output
Simulation of chilled water plant (Chillers &
Pumps)*Fans excluded
Cooling Load [kWr]
Low load chiller energy [kW]
High load chiller 1 energy [kW]
High load chiller 2 energy [kW]
Input energy [kWe]
System COP
Plant simulation
Ǘ𝑄𝑟 = Ǘ𝑚cw𝑐𝑤(𝑇𝑖 − 𝑇𝑜)
• where, Ǘ𝑚cw is the mass flow rate of the chilled water [kg s-1]
• 𝑐𝑤is the specific heat of chilled water [kJ kg-1K-1]
• 𝑇𝑖 and 𝑇𝑜 are the inlet and outlet chilled water temperatures [°C] across the chiller.
Regression analysis
Ǘ𝑄𝑟 = aP + bP2
where, Ǘ𝑄𝑟 is the refriegeration energy [kWr], P is the chiller input power [kWe], and a & b are scalars to be
calculated in the regression analysis
y = -0.0185x2 + 8.8461xR² = 0.9224
0
200
400
600
800
1000
1200
1400
0 50 100 150 200
Co
olin
g ca
pac
ity
[kW
r]
Input Energy [kWe]
Regression of Chiller [kWr vs kWe]
Results of chiller regression analysis
Chiller #Input Power
[kWe]Nominal
Capacity [kWr] a b R2
1 59 289 8.7957 -0.0699 0.9236
2 178 988 8.8461 -0.0185 0.9224
Modelling Structure
InputSimulation & Optimisation
Outputs
Simulation of chilled water plant (Chillers &
Pumps)*Fans excluded
Cooling Load [kWr]
Low load chiller energy [kW]
High load chiller 1 energy [kW]
High load chiller 2 energy [kW]Shuffled complex
evolution algorithmObjective function:
∑ Plant equipment energy
Shuffled Complex Evolution
• Deterministic & probabilistic
• Competitive evolution
• Clustering
• Steepest decent
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 500 1000 1500 2000 2500 3000 3500
Ch
ille
r In
pu
t P
ow
er
[kW
r]
Cooling Load [kWr]
Chiller Input Power [%] vs Cooling Load [kWr]
Low LoadChiller
High LoadChiller 1
High LoadChiller 2
5.9
5.95
6
6.05
6.1
6.15
6.2
6.25
6.3
6.35
0 500 1000 1500 2000 2500 3000 3500
Syst
em
CO
P
Cooling Load [kWr]
System COP vs Cooling Load [kWr]
5.9
5.95
6
6.05
6.1
6.15
6.2
6.25
6.3
6.35
0 500 1000 1500 2000 2500 3000 3500
Syst
em
CO
P
Cooling Load [kWr]
System COP vs Cooling Load [kWr]
Response surface, COP vs Chiller Loading [%]
0
20
40
60
80
100
120
140
0 500 1000 1500 2000 2500 3000 3500
Ene
rgy
Savi
ngs
[kW
e]
Cooling Load [kWr]
Energy Savings [kWe] vs Cooling Load [kWr], aggregated into bins of 100kWr
0
20
40
60
80
100
120
140
0 500 1000 1500 2000 2500 3000 3500
Ene
rgy
savi
ngs
[kW
e]
Cooling Load [kWr]
Energy savings [kWe] vs Cooling load [kWr]
148,269
18,683
166,952
21,579
11,594
33,173
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Chillers Pumps Total
Modelled Energy Savings, 2016 [kWh]
Modelled energy consumption 2016 Energy savings from observed consumption
Results
Chiller 1 Chiller 2 Chiller 3 Pumps Total
2016 actual [kWh] 95,254 71,480 3,114 30,277 200,125
Modelled control strategy [kWh] 82,014 50,877 15,378 18,683 166,952
Energy savings [kWh] 13,240 20,603 (12,264) 11,594 33,173
% Savings 13.9% 28.8% -393.8% 38.3% 16.6%
40%
35%
15%
20%
Typical Commercial Building End Use Electricity
HVAC
Lighting
Equipment
Other
92PJ
22MtCO2
Savings
15PJ
4MtCO2
15-25% Peak
demand reduction
Next steps…
• Test ECO in real world
• Represent chillers and fans using neural networks to increase optimisation opportunities (adaptive control)
• Incorporate ambient conditions and fan/condenser pump speed modulation (dynamic control)