1. When: Monday 26, 4 pm Where: ECJ Building, Classroom 3.402
Seminar: Modeling and Simulation for Smart, Sustainable, and Resilient Cities
Dr. Wangda Zhuo. Associate Professor
Building Systems Engineering, UC Boulder
When:
Monday 26, 4 pm
Where:
ECJ Building, Classroom 3.402

2. Lecture Objectives: Finish with intro for Project 1:
Chiller - Cooling towers and modeling

3. Cooling Tower Performance Curve
TCTR
Outdoor
WBT
from chiller
TCTS
to chiller
Temperature difference:
R= TCTR -TCTS
TCTS
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)
WBT

4. 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
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

5. Modeling of Water Cooled Chiller
(COP=Qcooling/Pelectric)
Chiller model:
COP= f(TCWS , TCTS , Qcooling , chiller properties)

6. Modeling of Water Cooled Chiller
Chiller model:
Chiller data: QNOMINAL nominal cooling power,
PNOMINAL electric consumption for QNOMINAL
Available capacity as function of evaporator and condenser temperature
Cooling water supply
Cooling tower supply
Full load efficiency as function of condenser and evaporator temperature
Efficiency as function of percentage of load
Part load:
The consumed electric power [KW] under any condition of load
The coefiecnt of performance under any condition
Reading: page 597.

7. Combining Chiller and Cooling Tower Models
Function of TCTS
3 equations from previous slide
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)

8. Merging Two Models Temperature difference: R= TCTR -TCTS 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
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().

9. Low Order Building Modeling
Measured data or
Detailed modeling
Find Q() = f (DBT)

10. For HW3a (variable sped pump efficiency) you will need Q()
Yearly based analysis:
You will need Q() for 365 days x 24 hours
Use simple molded below and the Syracuse, NY TMY weather file posted in the course handout section 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 4 8 12 16
Q= *t
Q= *t
Q [ton]
t [F]
TMY 3 for Syracuse, NY

11. For Austin’s Office Building
Model: (Area = 125,000sf)
Hours in a year kW
Used for component
capacity analysis
Model
=0 when building is off
Reading assignment:
Chapter: 2
Number of hours

12. Modeling of chilled water tank (stratified vs. mixing)
From building
To chiller
Stratification
To building
From chiller
Mixing happens if the supply temperature vary
Mixing model: mcpDT/D = Qin –  Qout

13. Stratification Dr. Jing Song’s PhD results Flow time at 20 minutes
CFD domain
Flow time at 1 minute

14. Stratified model (simplified)
However even if the chiller supply constant T
the return water from building is not constant!
From building
To chiller T1 T2
Building
Building T3 Tn
To building
From chiller
For a constant T supply
it is a very simple model
chiller
chiller
Model details in “Solar Engineering of Thermal Process”

15. Tank model Flow indicator: Flow for each node: Energy balance:
Building
Building
Flow for each node:
Energy balance:
chiller
chiller