Lecture Objectives:

• Learn about • Chiller modeling• Water energy storage models

Modeling of Water Cooled Chiller

(COP=Qcooling/Pelectric)

Chiller model:

COP= f(TCWS , TCTS , Qcooling , chiller properties)

Modeling of Water Cooled Chiller

CTSCWSCTSCTSCWSCWS TTfTeTdTcTbaCPATF 12

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Chiller model:

Cooling water supply Cooling tower supply

Available capacity as function of evaporator and condenser temperature

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Full load efficiency as function of condenser and evaporator temperature

PLRcPLRbaEIRFPLR 333

Efficiency as function of percentage of load

CAPFTQ

QPLR

NOMINAL

)(Part load:

The coefiecnt of performance under any condition

EIRFPLEIRFTCPFTPP NOMINAL )(

)()(

P

QCOP

Chiller data: QNOMINAL nominal cooling power, PNOMINAL electric consumption for QNOMINAL

The consumed electric power [KW] under any condition of load

Reading: http://apps1.eere.energy.gov/buildings/energyplus/pdfs/engineeringreference.pdf page 597.

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

)(

)()(

P

QCOP

Low Order Building Modeling

Measured dataor Detailed modeling

Find Q() = f (DBT)

For HW3a (variable sped pump efficiency) you will need Q()

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 365 days x 24 hoursUse simple molded below and the Syracuse, NY TMY weather file posted in the course handout section

TMY 3 for Syracuse, NY http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.html

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

Reading assignment: http://www.taylor-engineering.com/downloads/cooltools/EDR_DesignGuidelines_CoolToolsChilledWater.pdfChapter: 2

Modeling of chilled water tank(stratified vs. mixing)

From chiller

To chiller

To building

From building

Mixing model: mcpDT/D = Qin – Qout

Mixing happens if the supply temperature vary

Stratification

Stratification

Inlet Height, hi

Uniform Flow: q(t), Tc(t)

Inlet Width, l

Diffuser Radius, RD

Tank floor: No Slip, Adiabatic

Outlet Pressure Boundary

Main Supply Pipe Wall: No Slip, Adiabatic

Gravity, g

Tank Radius, RW

Tank Wall: No Slip, Adiabatic

Tank Centerline: Symmetry Axis

Initial conditions: Quiescent, Th(Z)

Z

R

Diffuser Pipe

CFD domain

Dr. Jing Song’s PhD results

Flow time at 1 minute

Flow time at 20 minutes

Stratified model(simplified)

From chiller

To chiller

To building

From building

T1

T2

T3

Tn

For a constant T supply it is a very simple model

However even if the chiller supply constant Tthe return water from building is not constant!

Building

Building

chiller

chiller

Model details in “Solar Engineering of Thermal Process”

Tank model

Building

Building

chiller

chiller

Flow indicator:

Energy balance:

Flow for each node:

HW 4

Model a Chiller coupled with the Cooling Tower (from HW3) and plumbing and pump System from HW3 for the building in Syracuse (from HW3)

Chiller model coefficients:

………

Provided in the Handout section of the course website

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