lecture objectives: learn about chiller modeling water energy storage models
TRANSCRIPT
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
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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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