Lecture Objectives:

• Finish wit introduction of HVAC Systems

• Introduce major ES software

Integration of HVAC and building physics models

BuildingHeating/Cooling

SystemPlant

BuildingHeating/Cooling

SystemPlant

Load System Plant model

Integrated models

Qbuiolding Q

including

Ventilation

and

Dehumidification

Example of System Models:Schematic of simple air handling unit (AHU)

rmSfans

cooler heater

mS

QC QH

wO wS

TR

room TR

Qroom_sensibel

(1-r)mS mS

wM

wR

Qroom_latent

TSTO

wR

TM

Tf,inTf,out

m - mass flow rate [kg/s], T – temperature [C], w [kgmoist/kgdry air], r - recirculation rate [-], Q energy/time [W]

Mixing box

Energy and mass balance equations for Air handling unit model – steady state case

SRpSsensibleroom TTcmQ _

mS is the supply air mass flow rate

cp - specific capacity for air,

TR is the room temperature,

TS is the supply air temperature.

changephaseSRSlatentroom iwwmQ __ wR and wS are room and supply humidity ratio

changephasei _ - energy for phase change of water into vapor

The energy balance for the room is given as:

The air-humidity balance for room is given as:

The energy balance for the mixing box is:

ROM TrTrT )1(‘r’ is the re-circulated air portion, TO is the outdoor air temperature, TM is the temperature of the air after the mixing box.

The air-humidity balance for the mixing box is:

ROM wrwrw )1(wO is the outdoor air humidity ratio and

wM is the humidity ratio after the mixing box

)( MSpSHeating TTcmQ

The energy balance for the heating coil is given as:

The energy balance for the cooling coil is given as:

changephaseMSSMSpSCooling iwwmTTcmQ _)(

Non-air system Radiant panel heat transfer model

Room (zone 1)

Radiant Panelc onv ecti

onTsurface

Tsurounding

Tzone_air rad iat ion

Qrad_pan

radiant panel layer (water tube)

air supplysystem

m ,T = const.s s

Qzone

Tw_out Tw_in

Non-air system Radiant panel heat transfer model

)()( __sup_sup airroomairplyairplypair TTmcQ

panradQ _

airpanradzone QQQ _

)()( ,,_ airpanelpanelconvisurfacepanelpaneliradiationconvradiationpanrad TTAhTTAhQQQ

)( ___ inwoutwpwpanrad TTmcQ

The total cooling/heating load in the room

The energy extracted/added by air system

The energy extracted/added by the radiant panel:

The radiant panel energy is:

The energy extracted/added by the radiant panel is the sum of the radiative and convective parts:

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

Example of Plant Models:Chiller

P electric () = COP () x Q cooling coil ()

Chiller model: COP= f(TOA , Qcooling , chiller properties)

OACWSOAOACWSCWS TTfTeTdTcTbaCAPTF 12

112

111

CAPFTQ

QPLR

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

)(

)()(

P

QCOP

Available capacity as function of evaporator and condenser temperature

Energy Simulation (ES) Programs

Structure of ES programs

SolverInterface for input data

Graphical User Interface (GUI)

Interface for result presentation

Preprocessor Engine

Preprocessor

ASCIfile

ASCIfile

Modeling steps

• Define the domain

• Analyze the most important phenomena and define the most important elements

• Discretize the elements and define the connection

• Write energy and mass balance equations

• Solve the equations

• Present the result

ES program

Preprocessor

Solver

Postprocessor

Characteristic parameters

• Conduction (and accumulation) solution method – finite dif (explicit, implicit), response functions

• Time steps • Meteorological data• Radiation and convection models (extern. &

intern.) • Windows and shading• Infiltration models• Conduction to the ground• HVAC and control models

ES programs

• Large variety • http://www.eere.energy.gov/buildings/tools_directory

• DOE2• eQUEST (DOE2)• BLAST • ESPr• TRNSYS• EnergyPlus (DOE2 & BLAST)

eQUEST (DOE2)US Department of Energy & California utility customers

• eQUEST - interface for the DOE-2 solver• DOE-2 - one of the most widely used ES program - recognized as the industry standard • eQUEST very user friendly interface • Good for life-cycle cost and parametric analyses

• Not very large capabilities for modeling of different HVAC systems

• Many simplified models • Certain limitations related to research application - no capabilities for detailed modeling

ESPrUniversity of Strathclyde - Glasgow, Scotland, UK

• Detailed models – Research program • Use finite difference method for conduction• Simulate actual physical systems • Enable integrated performance assessments

Includes daylight utilization, natural ventilation, airflow modeling CFD, various HVAC and control models

• Detail model – require highly educated users• Primarily for use with UNIX operating systems

TRNSYSSolar Energy Lab - University of Wisconsin

• Modular system approach • One of the most flexible tools available • A library of components • Various building models including HVAC • Specialized for renewable energy and emerging

technologies

• User must provide detailed information about the building and systems

• Not free

EnergyPlusU S Department of Energy

• Newest generation building energy simulation program ( BLAST + DOE-2)

• Accurate and detailed• Complex modeling capabilities• Large variety of HVAC models• Some integration wit the airflow programs Zonal models and CFD

• Detail model – require highly educated users • Very modest interface• Third party interface – very costly