Lecture Objectives:

• Model HVAC Systems– HW3 Asignemnet

• Learn about eQUEST software– How to conduct parametric analysis of building envelope

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?

HW3System simulation

Simplified model (use in your HW3):

• Use the results from HW2 and calculate the sensible cooling requirement for 24 hours for ten identical rooms like the one from HW2.

• If infiltration/ventilation provides 1 ACH calculate the latent load from infiltration 24 hours for ten identical rooms like the one from HW2.

• Calculate the total cooling load for 24 hours for ten identical rooms like the one from HW2.

• Use this as Q cooling () for HW3

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 ()

Modeling of Chiller

CAPFT

Chiller model acronyms:

Available capacity as function of evaporator and condenser temperature

EIRFT

Full load efficiency as function of condenser and evaporator temperature

EIRFPLR

Efficiency as function of percentage of load

PLR

Part load:

EIRFPLEIRFTCAPFTPP NOMINAL

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

Part Load Ratio

Energy Input Ratio as Function of Part Load Ratio

Energy Input Ratio as Function of Temperature

CAPacity as Function of Temperature

HW3Chiller 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

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

Building-System-Plant

Plant(boilerand/orChiller)

Building

HVAC System(AHU and distribution systems)

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

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 _)(

eQUEST software