Low Delta T (∆T) is Far Too Expensive for District Cooling Eric M. Moe IDEA Annual Conference Scottsdale, Arizona June 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. 12F (6.7C) ∆T 10F (5.6C) ∆T
1000 ton customer 45/55F coils (7.2/12.8C)
16F (8.9C) ∆T chilled water plant 41/57F design (5.0/13.9C) 41F(5.0C)
1500 ton customer 44/56F coils (6.7/13.3C)
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 80 Cooling Load (tons)
70 10.0F (5.6C) ∆T
60 50
41F (7.2C) EWT
16.2F (9.0C) ∆T
40
45F (5.0C) EWT
19.8F (11.0C) ∆T
30 20
Colder CHWST to Coil Increases ∆T ∆T Rises Above Design at Part Load
10 0 0
50
100
150
Flow Rate (gpm)
200
250
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 LAT
LAT
VFD (w/ 2-way & balancing valves) Building level return water temperature control Decoupling (blending)
T
HEX (indirect connection) T
Flow limiter (balancing)
T T
Reality: Low ∆T at coils commonly leads to rising supply water temperature which adversely affects performance for the utility and its customer 82
28
72
22
62
17
52
42
Supply Water Temperature To Coils
Supply Water Temperature To Building
32
11
6
0 Two Weeks in July 2007
Temperature (deg C)
Temperature (deg F)
Wet Bulb Outside Air Temperature
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 $0.22 $0.21 $0.20 $0.19 $0.18 $0.17 $0.16 $0.15
∆T (°F) ≤13 14 15 16 17 18 19 ≥20
∆T (°C) ≤7.2 7.8 8.3 8.9 9.4 10.0 10.6 ≥11.1
gpm/ton 1.85 1.71 1.60 1.50 chilled water plant design 1.41 1.33 1.26 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 Design Conditions (16.0F, 8.9C ∆T)
120%
Actual Peak Load (21.6F, 12.0C ∆T)
Cooling Load (%)
100% 80% 60%
explore what happens with changes to the entering water and leaving air temperature conditions
40% 20% 0% 0%
20%
40%
60%
Coil Design Flow (%)
80%
100%
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
Questions?
Eric Moe Flow Control Industries
[email protected] Office: 425-483-1297 Cell: 206-890-3266
