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Low Delta T (∆T)is Far Too Expensive
for District Cooling
Eric M. MoeIDEA Annual Conference
Scottsdale, ArizonaJune 2007
Common(and Expensive)
Myths
Energy – Capacity – Complexity - Comfort
Myth: Without decoupling buildings or indirect connections, existing (low ∆T) cooling coils are incompatible with the new (high ∆T) plant.
chilled water plant41/57F design
(5.0/13.9C)
1000 toncustomer
45/55F coils(7.2/12.8C)
1500 toncustomer
44/56F coils(6.7/13.3C)
41F(5.0C)
16F (8.9C) ∆T
10F (5.6C) ∆T12F (6.7C) ∆T
Reality: Colder water and better control will deliver greater than design ∆T at peak and part load
Energy Labs Coil 5WC-0 806-54x160-A36/6C
0
10
20
30
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60
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80
0 50 100 150 200 250
Flow Rate (gpm)
Coo
ling
Load
(ton
s)
41F (7.2C) EWT45F (5.0C) EWT
Colder CHWST to Coil Increases ∆T∆T Rises Above Design at Part Load
10.0F (5.6C) ∆T
16.2F (9.0C) ∆T
19.8F (11.0C) ∆T
Avoid the Expense: Design with cold water and better control to achieve high ∆T at cooling coils
• Design the chilled water plant and distribution for high ∆T despite low ∆T cooling coils in buildings
• Simplify customer interconnections– Direct connect if possible, HEX if required– Maintain the supply water temperature to coils– Avoid return water temperature control
• Rely on (high quality) pressure independent control– Achieve high ∆T performance across coils – Eliminate balancing, even as a system expands
Myth: System performance (including ∆T) can be optimized at the building interface alone
T
T
LAT
T
T
LATVFD (w/ 2-way & balancing valves)
Building level return watertemperature control
Decoupling (blending)
HEX (indirect connection)
Flow limiter (balancing)
Reality: Low ∆T at coils commonly leads to rising supply water temperature which adversely affects performance for the utility and its customer
32
42
52
62
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82
Two Weeks in July 2007
Tem
pera
ture
(deg
F)
0
6
11
17
22
28
Tem
pera
ture
(deg
C)
Wet Bulb Outside Air Temperature
Supply Water Temperature To Building
Supply Water Temperature To Coils
Avoid the Expense: Achieve high ∆T at coils to reduce total energy use, retain customers, simplify systems, and get paid
• For the Chilled Water Utility– Re-capture lost latent cooling revenue
• Eliminate low ∆T at the loads• Maintain low chilled water supply temperature to coils
– Acquire, satisfy, and retain customers• More comfort, greater efficiency, less equipment, lower costs
• For the Connected Customer– Minimize complexity
• Direct connections, HEX if required, no balancing– Reduce pump and fan energy consumption
• Higher ∆T in building, maintain low supply air temps• Remove pumps if not required
Myth: District cooling utilities can’t control what customer’s choose to do within their buildings.
• Lowest first cost design• Insufficient maintenance, dirty coils• Bypasses, 3-way valves, C/S pumps• Bad pump, pipe, and valve sizing practice• Minimal engineering, oversized equipment• Poor chilled water flow control• Low leaving air temperature
Reality: District cooling utilities may develop rate structures that influence customer design and performance
$/ton-hr ∆T (°F) ∆T (°C) gpm/ton$0.22 ≤13 ≤7.2 1.85$0.21 14 7.8 1.71$0.20 15 8.3 1.60$0.19 16 8.9 1.50 chilled water plant design$0.18 17 9.4 1.41$0.17 18 10.0 1.33$0.16 19 10.6 1.26$0.15 ≥20 ≥11.1 1.20
It may take a carrot to add a stick to change existing long term contracts!
Example: ~ 25,000 ton commercial plant with ongoing (expensive) low ∆T issues
• New plant designed for 10°F (5.6°C) ∆T - coils have 15°F (8.3°C) ∆T capability with 40°F (4.4°C) supply
• Direct customer connections in original design, no decoupled buildings or heat exchangers
• Additional chiller added after startup due to low ∆T performance in buildings
• Utility now has a rate structure that penalizes customers with poor ∆T performance
• Some customers are adding heat exchangers to try to deal with low ∆T
• Rising supply water temperature is creating comfort issues in customer buildings
What to Do: Explore common low ∆T issues relative to coil, distribution, and plant capability
• Low return temperature (to the plant)• High supply temperature (to cooling coils)• Where does the excess water go?
– Overflow running chillers– Operate additional chillers– Blend return water with supply– Quickly deplete TES capacity
What to Do: Use ARI certified software to fully understand coil capability at peak and part Load
Energy Labs 5WC-0 806-36x160-A14/10C
0%
20%
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120%
0% 20% 40% 60% 80% 100%
Coil Design Flow (%)
Coo
ling
Load
(%)
Design Conditions (16.0F, 8.9C ∆T)
Actual Peak Load (21.6F, 12.0C ∆T)
explore what happens withchanges to the entering waterand leaving air temperatureconditions
What to Do: Assess the economic benefit of correcting low peak and part load ∆T
Example: 12,000 Ton (Growing) Level 1 Trauma Center“10°F (5.6°) ∆T Coils with a 16°F (8.9°C) ∆T Plant”
– Retrofit project– Pressure independent control (DeltaPValves), no new coils– Reduced gpm/ton by over 60% raising part load ∆T from 7°F (3.9°C)– Removed building pumps and bridges– Increased peak load ∆T from 12 to 16°F (6.7 to 8.9°C) – 7,082,381 kWh annual savings (equivalent lbs CO2 reduced)– 53,631 kW reduction at peak (campus has CHP and reverse metering)– Increased available system capacity by ~ 3000 tons– Improved system reliability and comfort control– Eliminated waterside balancing requirements
$1,260,000 investment ($105/ton)$708,238 annual savings (plant energy alone)
1.78 years simple payback
What to Do: To drive good design, create chilled water contracts that vary with ∆T performance
Example: 4,500 Ton District Cooling (Airport) Customer“Penalties in contract for less than 18°F (10°C) ∆T”
– New construction project– Pressure independent control (DeltaPValves) at coils– 38/56°F (3.3/13.3°C) chilled water plant design– Thermal storage (ice) is fully utilized to minimize peak load– Cold water maintained all the way to cooling coils– Customer achieves 20-24°F (11.1-13.3°C) ∆T at all loads– Distribution managed with a single secondary pump in central plant– No excess pumping, piping, control, balancing, or heat transfer
equipment in the terminal buildings
