1. Free Cooling Application for Energy Savings at Purdue
Jim Braun
Travis Horton
Bonggil Jeon
Rita Jaramillo
September 2012

2. Overview Problem Statement Background Northwest Chiller Plant Overview
Free Cooling
Plume abatement
Preliminary Findings
Free Cooling Approach
Free Cooling Modeling
Cooling Tower Modeling
Northwest Plant Free Cooling Model
Results
Free Cooling Performance with All CT
Free Cooling Performance with Concrete CT
Economic comparison
Conclusions
Next Steps

3. Problem Statement Cooling is required in the winter months at Purdue
2010
Average
~6,000 Ton
Peak cooling load
for a typical house in Indiana = 3 Ton
Outdoor air temperature less than chilled water supply temperature (~47F)
~$3,000 /day

4. Problem Statement Why do we need cooling during the winter?
150+ Buildings on campus
Fact:
Cooling from October to May is done exclusively at Wade Power Plant.

5. Problem Statement
Wade Power Plant

6. 80% of winter cooling done by Steam Chillers
Problem Statement
Winter Cooling and Heating is provided by Wade Power Plant
80% of winter cooling done by Steam Chillers

7. Northwest Chiller Plant
Problem Statement
Northwest Chiller Plant
Wade Power Plant

8. Northwest Chiller Plant
Problem Statement
Northwest Chiller Plant
Plume Effect
Northwest Chiller Plant only supplements additional cooling during the summer.
Northwest Plant could potentially be utilized for free- cooling during the winter months.
Issues related to cold weather operation of cooling towers:
Plume effect
Water freezing

9. Problem Statement
Is it feasible to operate the Northwest Plant in free cooling mode during the coldest months of the year?
What are the modifications and costs for implementing free cooling technology at the Northwest Plant?
What are the modifications of the cooling towers for cold weather operation?
What are the performance characteristics and economic benefits of free cooling implementation at Northwest Plant?

10. Northwest Chiller Plant Overview
Cooling Towers
Electric Chillers
System Pumps

11. Northwest Chiller Plant Overview
Electric Chillers

12. Northwest Chiller Plant Overview
Concrete Cooling Tower

13. Northwest Chiller Plant Overview
Metal Cooling Towers

14. Northwest Chiller Plant Overview
Chilled Water Flow Schematics

15. Northwest Chiller Plant Overview
Condenser Water Flow Schematics

16. What is Free Cooling?
Technology that utilizes low ambient air to cool the water in a chilled water system.
Cooling without operating the compressor of a centrifugal liquid chiller (ASHRAE).
Favorable Facility Conditions
Cooling demand is require throughout the year
High Internal Heat gains due to equipment, lighting, Solar glazing, people, etc…
3,700 + hours where outdoor temperature is below chilled water supply (47F).
Chiller

17. Plate Frame Heat Exchanger System
Free Cooling Method
Plate Frame Heat Exchanger System
Simple installation
No compromise to chiller operation
No contamination of chilled water
Easy change over
Requires lowest condenser water temperature
Larger footprint for additional equipment
CWS pump sized to handle additional resistance

18. Plume Abatement
Plume prediction using psychrometric chart (ASHRAE 2008)
Plume abatement methods
Heating the tower exhaust with natural gas burners.
Heating the tower exhaust with hot-water or steam coils.
Installing precipitators.
Spraying chemicals at the tower exhaust .
ClearSky and Air2Air (SPX Technologies).
Hybrid wet-dry cooling tower (SPX Technologies)
Air leaving tower 2
Indicates fog will form
Air entering tower 1

19. Preliminary Findings Description Unit 2009 2010 2011
Potential free cooling hours (Twb<45°F) hr
3,862
3,779
3,840
Free cooling potential
ton-hr
22,237,252
19,712,398
18,997,609
Total campus cooling load
93,336,089
90,608,510
93,154,092
Free cooling percentage %
23.8
21.8
20.4
Description
Unit
2009
2010
2011
Potential free cooling hours hr
3,862
3,779
3,840
Free cooling potential
ton-hr
22,237,252
19,712,398
18,997,609
Total campus cooling load
93,336,089
90,608,510
93,154,092
Free cooling percentage %
23.8
21.8
20.4

20. Free Cooling Approach Cooling towers performance modeling
Cold well
T = 47F
T ~ 54.5F
Cooling towers performance modeling
NW Plant FC performance modeling
Heat Exchanger sizing
Economic comparison

21. Cooling Tower Modeling
How much cooling capacity can the cooling towers deliver at different wet bulb temperatures?
Data from performance curves
Mathematical Model in EES
𝑵𝑻𝑼=𝒇 𝑾𝒂𝒕𝒆𝒓 𝒇𝒍𝒐𝒘 𝒓𝒂𝒕𝒆 𝑨𝒊𝒓 𝒇𝒍𝒐𝒘 𝒓𝒂𝒕𝒆
Performance Modeling in TRNSYS

22. Cooling Tower Modeling
Concrete Counter-flow Cooling Tower Model Fitting

23. Cooling Tower Modeling
Metal Cross-flow Cooling Tower Model Fitting

24. Cooling Tower Modeling
Schema of the model built in TRNSYS
Water at 51°F
Ambient air
Relative Humidity 50%
Wet bulb
temperature
varies
Description
Concrete CT
Metal CT
Number of cells 3 9
Inlet water flow rate per cell
6,000 gpm
2,567 gpm
Air flow rate per cell
497,012 cfm
250,100 cfm

25. Cooling Tower Modeling
Maximum Capacity of All Cooling Towers

26. Northwest Plant Free cooling model
Schema of the model built in EES
Data:
CWR Temp
CWR flow rate
Wet bulb temp
e = 85%
P P P P P P6
T = 47F
Maximum wet bulb temperature for free cooling : 38F
Actual data from Wade Power Plant 2009 to 2011
Control towers and pumps to meet campus load up to maximum capacity.

27. Northwest Plant Free cooling model
Cooling tower fans and pumps sequencing
2 sp. fan
VSF
Off Off Off Off Off Off Off Off Off
Off
50%
100% %
0 - 50%
50%
100%
Concrete Tower  5 stages for bringing fan capacity online and offline
Adjust variable-speed fan (VSF) up to 50% to meet target plant cfm.
When VSF reaches 50% speed, set Fan 1 to low speed (50%) Adjust VSF to meet target plant cfm.
When VSF reaches 50% speed, Set Fan 3 to low speed (50%) Adjust VSF to meet target plant cfm.
When VSF reaches 100% speed, Set Fan 1 to high speed (100%) Adjust VSF to meet target plant cfm.
When VSF reaches 100% speed, Set Fan 3 to high speed (100%) Adjust VSF to meet target plant cfm.

28. Northwest Plant Free cooling model
Cooling tower fans and pumps sequencing
2 sp. fan
VSF
Off
100% %
Off Off Off Off Off Off Off Off Off ON ON ON ON ON ON ON ON ON
P P P P P P6
Metal Towers Sequencing  9 stages for bringing fan capacity online and offline
When VSF reaches 100% sp., turn on Fan 4A. Adjust VSF to meet target cfm.
When VSF reaches 100% sp., turn on Fan 4B. Adjust VSF to meet target cfm.
Add additional fans for towers #4, 5, and 6 until meeting target plan cfm.
Pump Sequencing  Match pump control to tower control

29. Northwest Plant Free cooling model
Plume Abatement
Selected method : heating the tower exhaust with natural gas burners.
Air entering tower
Air leaving tower 1 2
Energy for plume abatement  Energy to heat the air exiting each cooling tower cell to a temperature that avoids the plume.
Calculations made for each CT cell each time step (0.5 h) of simulation
Tangent to saturation curve 3
Indicates fog will form

30. Results Free cooling performance with all cooling towers
Free cooling performance with concrete cooling tower only
Economic comparison

31. Free Cooling Performance with All Cooling Towers
Free cooling compared to actual campus load

32. Free Cooling Performance with All Cooling Towers
Average free cooling efficiency
Average electric chiller efficiency = 0.55 kWh/ton

33. Free Cooling Performance with All Cooling Towers
Electricity Savings (relative to 0.55 kWh/ton)

34. Free Cooling Performance with All Cooling Towers
From 6am to 8pm
Energy for Plume Abatement

35. Free Cooling Performance with All Cooling Towers
Annual estimates for free cooling with all cooling towers
Description
2009
2010
2011
Average
Campus Load (Ton-hr)
93,336,089
90,608,510
93,154,092
92,366,230
Free cooling hours (hr)
2,682
2,913
2,705
2,767
Free cooling energy (Ton-hr)
15,129,230
15,196,931
14,574,313
14,966,824
Free cooling fraction
0.162
0.168
0.156
Free cooling efficiency (KW/Ton)
0.125
0.121
0.131
0.126
Electricity Savings (KW-hr)
6,422,614
6,525,651
6,109,390
6,352,552
Total plume hours (hr)
1,614
2,017
1,495
1,708
Energy for plume abatement (Therm)
99,298
125,216
76,128
100,214
Day-time plume hours (hr)
879
1,117
801
932
Energy for day-time plume abatement (Therm)
50,524
63,724
38,552
50,933

36. Free Cooling Performance with All Cooling Towers
Plate and Frame Heat Exchanger Sizing
Side
Fluid
Flow Rate
Inlet
temp.
Outlet temp.
Pressure
drop
Hot
Water
10,000 gpm
55.0 F
45.0 F
15.0 psi
Cold
43.0 F
53.0 F
Cooling towers maximum flow rate : 41,000 gpm  4 heat exchangers are necessary.
Estimated cost of each unit : $223,070
Estimated cost of the system: $2,750,000
Operating in free cooling with only the concrete CT will require 2 heat exchangers  How is free cooling performance affected?

37. Free Cooling Performance with Concrete Cooling Tower Only
Average Annual Free Cooling Estimates ( )
Description
Concrete CT Average
Campus Load (Ton-hr)
92,366,230
Free cooling hours (hr)
2,767
Free cooling energy (Ton-hr)
13,218,050
Free cooling fraction
0.143
Free cooling efficiency (KW/Ton)
0.092
Electricity Savings (KW-hr)
6,049,371
Energy for plume abatement (Therm)
105,752
Energy for day-time plume abatement (Therm)
53,529
Nr. of heat exchangers required 2
Installed cost estimate
$1,375,000
All CT
Average
92,366,230
2,767
14,966,824
0.162
0.126
6,352,552
100,214
50,933 4
$2,750,000

38. Economic Comparison All cooling towers Free Cooling with
Concrete tower
All cooling towers
Alternatives of plume abatement
1. FC without plume abatement
2. Only day-time plume abatement
3. All-day plume abatement
4. Reject day-time hours with plume
5. Reject all-day hours with plume
Economic parameters
Electricity price = $0.05 per kW-h
Natural gas price = 0.65 $/Therm

39. Economic Comparison
Annual Cost Savings
Payback period

40. Conclusions
Free cooling operation with only the concrete tower gets ~13 million Ton-hr of annual free cooling, equivalent to $302,000 electricity savings (95% of the savings obtained with all CTs and only half of the system installed cost).
The selected alternative for plume abatement was natural gas burners. The cost of energy is ~$69,000 for plume abatement all day, and ~$35,000 for day-time only (6am to 8pm).
The Best alternative is free cooling mode using only the concrete cooling tower and day-time plume abatement  ~$268,000 annual cost savings , and 5 years payback period.
During the coldest months (January, February and December) the campus demand can be almost satisfied through free cooling. The savings for the shoulder months (March, October and November) could be increased significantly through the use thermal energy storage.

41. Next Steps… More accurate estimation of installed cost of the system.
Design system to control campus chilled water flow into the system, so maximum flow and cooling capacity are not exceeded.
Consider application of thermal storage.