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Lecture Objectives:
• Differences in Conduction Calculation in Various Energy Simulation Programs
• Modeling of HVAC Systems
Methods for conduction calculation
• Finite difference or finite volume method
• Weighting factor method
• Response factor method or Response function method
Methods for conduction calculation
• Finite difference or finite volume methods
– Used in your HW assignments– Energy simulation program
• ESPr• Some models in TRANSYS • Some models in EnergyPlus
Conduction with finite difference (volume) method
Example room
F
C
L R
3
3
33
A air node
Ei
System with the finite difference (volume) method for conduction calculation
Weighting factor methods
T external airT internal airQ solar
Orientation andWall (or element) structure
database
Heat flux on internal surface
q
The simplest method
Q HVAC
qBuilding
System when we know the fluxes thought building walls
We need to find other way to calculate fluxes
Response function methods
Used in eQUEST program
Response function methods
NOTATION: θ(x,t)=T(x,)
Ts
0
T
-L / 2 L /2
h
h
h
T o
T
h omogenous wa ll
L = 0.2 mk = 0 . 5 W/ m Kc = 9 20 J/kgK
= 120 0 k g/mp
2
Laplace transformation
Laplace transform is given by
Where p is a complex number whose real part is positive and large enough to cause the integral to converge.
Laplace transformation table
Principles of Response function methods
The basic strategy is to predetermine the response of a system to some unit excitation relating to the boundary conditions anticipated in reality.
Reference:
JA ClarkeBook form the Reference List
Modeling of HVAC systems
• Review – Psychrometrics– Air-conditioning in Air Handling Units (AHU)– Refrigeration cycles
• Building-System-Plant connection
Psychrometrics – review
Air-conditioning in Air Handling Unit (AHU)
Compressorand Condenser
Roof top AHU
Gas/Electric Heater
to building
Fan
air from building
fresh air
Evaporator
filtermixing
hotwatercool
water
Return fan
Supply fan
flow control dampers
AHU
Fresh air
AHU schematic
Outdoor air To room
Exhaust From room
Processes in AHU presented in Psychrometric in psychrometric
OA Case forSummer in Austin
IA
MA
SA
Refrigeration Cycle
T outdoor air
T cooled water
Cooling energy (evaporator)
Released energy (condenser)
- What is COP?- How the outdoor air temperature affects chiller performance?
Building-System-Plant
Plant(boilerand/orChiller)
Building
HVAC System(AHU and distribution systems)
Integration of HVAC and building physics models
Building Heating/Cooling System Plant
Building Heating/Cooling System Plant
Load System Plant model
Integrated models
Qbuiolding Q
including
Ventilation
and
Dehumidification
HW3System simulation
Simplified model (use ii in your HW3a):
• Use the results from HW2 and calculate the sensible cooling requirement for 24 hours for ten identical rooms like the one from HW2b.
• If infiltration/ventilation provides 1 ACH calculate the latent load from infiltration 24 hours for ten identical rooms like the one from HW2b.
• Calculate the total cooling load for 24 hours for ten identical rooms like the one from HW2b.
• Use this as Q cooling () for HW3b
Note: This method:- assumes perfect process in AHU
to control RH sometimes we need to heat and cool at the same time- neglects fan power- dos not consider system properties and control Variable Air Volume or Constant Air Volume
TOA
water
Building users (cooling coil in AHU)
TCWR=11oCTCWS=5oC
Evaporation at 1oC
T Condensation = TOA+ ΔT
What is COP for this air cooled chiller ?
COP is changing with the change of TOA
Plant Models:Chiller
P electric () = COP () x Q cooling coil ()
HW3Chiller model: COP= f(TOA , Qcooling , chiller properties)
OACWSOAOACWSCWS TTfTeTdTcTbaCAPTF 12
112
111
CAPFTQQPLR
NOMINAL
)(
Chiller data: QNOMINAL nominal cooling power, PNOMINAL electric consumption for QNOMINAL
Cooling water supply Outdoor air
OACWSOAOACWSCWS TTfTeTdTcTbaEIRFT 22
222
222
Full load efficiency as function of condenser and evaporator temperature
PLRcPLRbaEIRFPLR 333
Efficiency as function of percentage of load
Percentage of load:
The coefficient of performance under any condition:
EIRFPLEIRFTCAPFTPP NOMINAL
The consumed electric power [KW] under any condition
)()()(
PQCOP
Available capacity as function of evaporator and condenser temperature