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