Lecture Objectives:
• Cooling towers and modeling • Project 1
• Thermal storage systems
Cooling Tower Performance Curve
Most important variable is wet bulb temperature
TCTS = f( WBToutdoor air , TCTR , cooling tower properties)
or for a specific cooling tower type
TCTS = f( WBToutdoor air , R)
from chillerOutdoor WBT
TCTS
R
Temperature difference:
R= TCTR -TCTS
TCTR
to chiller
WBT
TCTS
Cooling Tower Model Model which predict tower-leaving water temperature (TCTS) for arbitrary entering water temperature (TCTR) and outdoor air wet bulb temperature (WBT)
Temperature difference:
R= TCTR -TCTS
22444
2444
2444 ][][ RWBTiWBThgRWBTfWBTedWBTcWBTbaTCTS
Model:
For HW 3b:
You will need to find coefficient a4, b4, c4, d4, e4, f4, g4, h4, and i4 based on the graph from the previous slide and two variable function fitting procedure
Combining Chiller and Cooling Tower Models
EIRFPLEIRFTCPFTPP NOMINAL
3 equations from previous slide
Function of TCTS
22444
2444
2444 ][][ RWBTiWBThgRWBTfWBTedWBTcWBTbaTCTS
Add your equation for TCTS
→ 4 equation with 4 unknowns (you will need to calculate R based on water flow in the cooling tower loop)
Merging Two Models
Finally: Find P() or
The only fixed variable is TCWS = 5C (38F) and Pnominal and Qnominal for a chiller (defined in nominal operation condition: TCST and TCSW); Based on Q() and WBT you can find P() and COP().
Temperature difference:
R= TCTR -TCTS
22444
2444
2444 ][][ RWBTiWBThgRWBTfWBTedWBTcWBTbaTCTS
Model:
Link between the chiller and tower models is the Q released on the condenser: Q condenser = Qcooling + Pcompressor - First law of Thermodynamics
Q condenser = (mcp)water form tower (TCTR-TCTS) m cooling tower is given - property of a tower
TCTR= TCTS - Q condenser / (mcp)water
)()()(
PQCOP
Two variable function fitting(example for a variable sped pump)
Function fitting for a chillerq = f (condensing and evaporating T)
70 2 4 6 8 10
0
50
100
150
200 25 C35 C45 C
q[kW]
Tevaporator [C]
Merging Two Models
Finally: Find P() or
The only fixed variable is TCWS = 5C (38F) and Pnominal and Qnominal for a chiller (defined in nominal operation condition: TCST and TCSW); Based on Q() and WBT you can find P() and COP().
Temperature difference:
R= TCTR -TCTS
22444
2444
2444 ][][ RWBTiWBThgRWBTfWBTedWBTcWBTbaTCTS
Model:
Link between the chiller and tower models is the Q released on the condenser: Q condenser = Qcooling + Pcompressor ) - First law of Thermodynamics
Q condenser = (mcp)water form tower (TCTR-TCTS) m cooling tower is given - property of a tower
TCTR= TCTS - Q condenser / (mcp)water
)()()(
PQCOP
Low Order Building Modeling
Measured dataor Detailed modeling
Find Q() = f (DBT)
For Austin’s Office Building
Number of hours
Hours in a year
kW
Model: (Area = 125,000sf)
0 10 20 30 400
200
400
600
800
1000
Coo
ling
wat
er d
eman
d [k
W]
Outdoor temeprature [C]
Model
=0 when building is off
Used for component capacity analysis
For project 1 you will need Q()for each hour
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 900
4
8
12
16
20
Q=-0.45 +0.0448*t
Q=--27.48+0.5152*t
Q [t
on]
t [F]
Yearly based analysis: You will need Q() for one week in July Use simple molded below and the Syracuse TMY2 weather file posted in the course handout section
Example of CHP + Cooling
We need a thermal storage somewhere in this system !
Thermal storage
• Store heat• Many issues to consider (∆T, pressure, losses,…. )
• Store cooling energy• Chilled water
• For cooling condenser • For use in AHU (cooling coils)
• Ice storage • Compact but…
• Other materials (PCMs) that change phase the temperature we need in cooling coils
• Many advantages, but disadvantages too!
On-Peak and Off-Peak Periods
This profile depends on the type of building(s) !
Chilled water tankUse of stored cooling energy
Store Use
Which one is better ?
Depends on what you want to achieve:-Peak electric power reduction-Capacity reduction-…..
Downsizing the Chiller
• Lower utility costsLower on-peak electrical consumption(kWh)Lower on-peak electrical demand (kW)
• Smaller equipment sizeSmaller chillerSmaller electrical service (A)
• Reduced installed costMay qualify for utility rebates or other incentives