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