ENERGY INSIGHTS OPTIMISING HYDRONIC SYSTEMS FOR ENERGY SAVINGS Tom Pak
May 9th 2012
ENGINEERING ADVANTAGE
World's energy consumption 40% of the world's energy consumption is used in buildings*
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50% of this is in HVAC systems alone*
(*) Sources: European Commission EPBD (point 6, pp1) & US Department of Energy’s “Buildings Energy Data Book” 2
ENGINEERING ADVANTAGE
Energy savings on HVAC in buildings
Building structure
HVAC installation
Human factor
(insulation, double glazing, …)
• Use of new technologies • System approach of hydronic design • Shorter pay-back times
• Avoid interferences with the HVAC system • Educate tenants and maintenance team • Never-ending task
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• Best way to save energy • Larger energy savings • Long pay-back times
Building modifications require adaptation or modernization of the HVAC installation to take into account new heat gains/losses 3
When modifying a HVAC installation one must take into account the capabilities of people using the installation ENGINEERING ADVANTAGE
Energy savings via hydronic optimization Optimising a building's HVAC system can reduce its energy consumption by 30% :
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‒ By avoiding the deterioration of production unit efficiencies, ‒ By optimizing the energy efficiency of the hydronic distribution, ‒ By guaranteeing a stable and accurate room temperature.
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ENGINEERING ADVANTAGE
IMPROVING PRODUCTION UNIT EFFICIENCIES
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Chillers Coefficient of Performance (COP) is used to indicate chiller efficiency: Condenser
COP =
Pevaporator Pcompressor
≈ 2.5K 4K6
Tr
Ts Evaporator
Chilled water
Heat transfer (and thus COP) is good when Log Mean Temperature Difference between water and refrigerant is kept high
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‒ Evaporator refrigerant temperature remains constant ‒ Supply water temperature Ts is usually kept constant ‒ Thus return water temperature Tr must be kept "high" to keep LMTD high
Tr
Ts Refrigerant saturated suction temp.
Keeping a high Tr (thus a high ∆T = Ts-Tr) provides higher COP at partial load
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ENGINEERING ADVANTAGE
Effect of a decrease of the return water temp. on COP
COP
Example : Chiller: 200 tons (703 kW) Water condenser temperatures: 29,5°/35°C Supply temperature of chilled water Ts : 7°C 5,2 5 5
4,8 4,6 4,6
4,4 4,4
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4,2 4 10,5
11
11,5
12
12,5
13
Return temp. chilled water Tr [°C]
15%
A reduction of return temperature of chilled water can lead to a 15% drop of the COP 25
ENGINEERING ADVANTAGE
Variable flow – proportional control The ∆T through a terminal unit increases when the flow reduces.
2-way circuit (variable flow) qp
Thus the return water temperature increases when the flow reduces. Cooling
H C STAD
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Return temp. Tr
All benefits for chiller COP.
26,0 24,0 22,0 20,0 18,0 16,0 14,0 12,0 10,0 8,0 6,0
Temperature regime: Ts/Tr/Ti = 7/12/24°C
Variable flow circuit
0%
20%
40%
60%
80%
100%
Flow through terminal unit 26
ENGINEERING ADVANTAGE
Constant flow system – proportional control The ∆T through a terminal unit increases when the flow reduces.
3-way diverting circuit (constant flow) qp qs
H STAD-B
qb
But the flow through the terminal unit is reduced by bypassing an increasing fraction of the primary flow (at temperature Ts).
C
STAD-P
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Return temp. Tr
Constant flow
Thus the return water temperature decreases when the flow reduces !!!
Variable flow
Cooling
26,0 24,0 22,0 20,0 18,0 16,0 14,0 12,0 10,0 8,0 6,0
Temperature regime: Ts/Tr/Ti = 7/12/24°C
Variable flow circuit Constant flow circuit 0%
20%
40%
60%
80%
100%
Flow through terminal unit 27
ENGINEERING ADVANTAGE
Variable flow system with 2-way on-off control When some CV are closed: – there is less total flow and Dp in piping – and thus more available Dp everywhere in the system – open valves receive a flow that is higher than design flow
At partial load in the system, if a valve is open: q >= qdesign
> qd open
0 qd
open shut
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0 qd open shut
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open
> qd
open
> qd
>qd
open
ENGINEERING ADVANTAGE
On-off control – Flow increase at partial load
Total system flow
Manually balanced variable flow on-off control system 100 identical units; Pump head 150 kPa; Terminal unit 20 kPa; On-off CV 5 kPa 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%
Temperature regime: Ts/Tr/Ti = 7/12/24°C
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0%
20%
40%
60%
80%
100%
System load
At 50% load, the total flow in system reaches 73% of the total design flow. This is a 46% increase w.r.t. the required flow (50%) at 50% load. Seasonal flow increase lead to an estimated increased pumping energy consumption equal to +3% of total plant energy consumption 29
3%
ENGINEERING ADVANTAGE
On-off control – Emission at partial load At flow that is near design flow, emitted power does not increase much with the flow
Emission
120%
100%
80%
60%
40%
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Room t°
Control signal switches on/off when room temperature deviates much beyond the thermostat differential
0% 0%
20%
40%
60%
80%
100%
120%
140%
160%
Flow
At partial load in the system, if a valve is open:
Design set-point 24°C
(Troom - Tset-point) >…=…< Time
30
20%
P ≈ Pdesign
q ⋅ ∆T P= k
q ≥ qdesign
∆T ≤ ∆Tdesign
ENGINEERING ADVANTAGE
Return water temp. degradation
Return temp. Tr
Manually balanced variable flow on-off control system 100 identical units; Pump head 150 kPa; Terminal unit 20 kPa; On-off CV 5 kPa 14
Temperature regime: Ts/Tr/Ti = 7/12/24°C
12 10 8 6 4 2 0
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0%
20%
40%
60%
80%
100%
System load
Below 50% of the load, which represents typically 70% of the cooling season, the return water temperature is degraded by 1.5 to 2°C. This will result approximately in a 3 to 4% increase in seasonal chiller energy consumption 31
3%
ENGINEERING ADVANTAGE
Return water temp. – proportional vs on-off control Cooling Temperature regime: Ts/Tr/Ti = 7/12/24°C 26,0 24,0 22,0 20,0 18,0 16,0 14,0 12,0 10,0 8,0 6,0
On-off control 26,0 24,0 22,0 20,0 18,0 16,0 14,0 12,0 10,0 8,0 6,0
Return temp. Tr
Return temp. Tr
Proportional control
2-way proportional 3-way proportional 0%
20%
40%
60%
80%
100%
3-way on-off, MBV 0%
20%
40%
60%
80%
100%
System load
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Flow through terminal unit
2-way on-off, MBV
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ENGINEERING ADVANTAGE
Case Study - Local University campus buildings
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Renovation of 2 University buildings with a total of 9840 m2 Installed cooling capacity: – Building 1 : 1452 tons refrig. – Building 2 : 1730 tons refrig. Work performed during summer 2010 Results compared for Oct.-Nov. 2009 vs 2010 DP controllers paired with manual balancing valves installed
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ENGINEERING ADVANTAGE
Local University campus building 1 – chiller saving
Power input [kW]
Chiller Power Input vs. Cooling Load
Variable secondary flow with differential pressure bypass
2009 2010
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Dp controllers at on-off control FCU groups and PAU/AHUs and re-balanced
Cooling load [kW]
Annualized 22% chiller energy saving
22% 34
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ENGINEERING ADVANTAGE
Local University campus building 2 – chiller saving
Power input [kW]
Chiller Power Input vs. Cooling Load
Variable flow primarysecondary system
2009
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2010
Cooling load [kW]
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Addition of Dp controllers at FCU groups zones and pressure independent control valves for PAU/AHU and re-balanced Annualized 16.5% chiller energy saving
16%
ENGINEERING ADVANTAGE
Case study - business hotel at Sheung Wan > First LEED and BEAM Plus Platinum certifications of new-built hotel > 38 floors, 274 rooms > Chiller capacity 667 kW x 2 nos > Variable primary flow design > Hydronic balancing design
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> Chiller plant balancing > Dp control at each floor and modulating terminals > Balancing and control valve at FCUs
ENGINEERING ADVANTAGE
Hydronic balancing in variable primary flow
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4 Balancing & control valve for proportional balancing
3 DP stabilization against large Dp fluctuations in part load 1 Balanced chiller plant
2 Proper “fail-safe” bypass valve sizing and characteristic ENGINEERING ADVANTAGE
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Computerized hydronic system design
TA Select 4 hydronic system calculation
ENGINEERING ADVANTAGE
Computerized hydronic system design
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> Pipe sizing per BEC recommendations > Balancing valve sizing and pre-settings for easy and efficient commissioning > Minimum control valve authority checks (suggest > 0.25) to ensure indoor comfort with stable temperature control > Correct Index circuit identification to help determining optimal VFD sensor location and set-point > Pump head optimization: > Initial : 350 kPa > Select 4 : 236 kPa (-33%) > Final set : 250 kPa (-29%)
ENGINEERING ADVANTAGE
Computerized hydronic system design
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> Bypass valve sizing calculation
ENGINEERING ADVANTAGE
Tips for reliable and energy efficient VPF system > Make use of computerized software for hydraulic design calculations > Chiller plant must be balanced to avoid uneven distribution of flow > Proper bypass valve sizing and select with good rangeabilty, linear characteristic (or equal percentage characterisitic at 0.25 authority) valve > Use of high precision flow meter and local controller to monitor total return flow as fail-safe measure against chiller minimum flow > Use of DP controllers
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> to stablize Dp fluctuations to avoid overflow and low dT problems > to maintain good control valve authority necessary for stable temperature control > as flow/DP/DT/energy measuring station for energy audit purpose > Make system sustainable and adaptive to future changes
> Locate VSP sensor at index circuit and adjust set-point to the DP of the most demanding stablized circuit – potentially up to 40% pump energy saving > Make use of computerized balancing method and instrument to produce balancing report without manual record of measurements and verfications
ENGINEERING ADVANTAGE
Conclusion Important energy savings can be achieved by taking care of: ‒ Maintaining cold/warm return water temperature to condensing boilers/chillers (~15%), ‒ Having an adequate pressurisation avoiding scaling/fouling in boilers/evaporators (~5-10%).
Minimising pumping costs requires having (up to 40% on 7-17% in cooling): ‒ A good pressure maintenance to avoid corrosion leading to aging and fouling, ‒ An optimized hydronic distribution design (Dp control), ‒ A systematic methodology of balancing and commissioning.
Stable and accurate room t° control gives (~5-20%) energy savings by: Copyright © TA Hydronics SA. All rights reserved.
‒ Avoiding oversizing in on-off systems, ‒ Using state-of-the-art thermostatic radiator valves, ‒ Sizing of modulating control valves for good minimum authority.
The approach "He who can do more can do less" is to be avoided 92
ENGINEERING ADVANTAGE
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ENGINEERING ADVANTAGE
