energy performance of buildings group heating, ventilating, & air-conditioning: diagnostics...

Energy Performance of Buildings Group

Heating, Ventilating, & Air-Conditioning: Diagnostics & Controls to ImproveAir-Handling System Performance

Craig Wray, P.Eng.

Indoor Environment DepartmentLawrence Berkeley National Laboratory

Tel: 510-486-4021 Email: [email protected] http://epb.lbl.gov

American Physical Society Short CoursePhysics of Sustainable Energy: Using Energy Efficiently and Producing It Renewably

UC Berkeley, 1 March 2008

AcknowledgmentsAcknowledgments

• Assistant Secretary for Energy Efficiency and Renewable Energy, Office of the Building Technologies Program, U.S. Department of Energy

• California Energy Commission PIER Program

• Max Sherman, Iain Walker, Darryl Dickerhoff (LBNL)

• Cliff Federspiel (Federspiel Controls)


OverviewOverview

• Background– Opportunities for improvement

• Duct Leakage Diagnosis– Measuring leakage flows using the DeltaQ test

• Duct Pressure Diagnosis & Control– Demand-based reset with DDC/non-DDC controls

• Ventilation Control– Intermittent ventilation and efficacy


Opportunities for ImprovementOpportunities for Improvement• Duct Leakage and Operating Pressure

– Thousands of field assembled joints– System pressures not uniform or constant; impossible

to know location of each leak and pressure difference across each leak

– Unnecessarily closed dampers restrict flow– Large energy savings possible from sealing ducts and

optimizing duct static pressures

• Ventilation– Standards specify constant ventilation rates– Energy intensive process; sometimes can reduce IAQ– Intermittent ventilation more appropriate in some cases


OverviewOverview






Why Use DeltaQ Duct Leakage Test?Why Use DeltaQ Duct Leakage Test?

• Fast and easy– No register covering (less damage potential)– Coincidentally measures envelope leakage– Uses familiar equipment (blower door)– Self-diagnostic for uncertainty– Can be automated

• Accurate– Leaks to outside under operating conditions

• BUT…– Need a computer– Need to operate central blower


PressurizingAirflow

DeltaQ Airflows and PressuresDeltaQ Airflows and Pressures


AirHandler

Blower DoorDeltaQ = -80100

Air Handler OFF

10 Pa

030

10 Pa10

10 Pa

6010 Pa

20

Air Handler ON

2040 Pa

100060 -40 Pa

60

BuildingEnvelope

SupplyReturn

DeltaQ Test DataDeltaQ Test Data• Green = blower on• Red = blower off• Difference = DeltaQ


DeltaQ ModelDeltaQ Model

DeltaQ(P)=Qon(P)-Qoff(P)

Qon(P)=Qenv(P) + Cs(P+Ps)ns + Cr(P-Pr)nr

P = Envelope added pressurePs = Supply PressurePr = Return Pressure

Cs=Supply leak coefficientCr=Return leak coefficientQs=Supply leak flowQr=Return leak flow

Q=C(P)n

Qoff(P)=Qenv(P) + Cs(Pns) + Cr(Pnr)

DeltaQ(P)=Cs((P+Ps)ns-Pns) + Cr((P-Pr)nr-Pnr)

DeltaQ(P)=Qs((1+P/Ps)ns-(P/Ps)ns) - Qr((1-P/Pr)nr+(P/Pr)nr)


Pressure Scanning Error SurfacePressure Scanning Error Surface


Ps Pr

Solution at lowest error

For each supply and returnpressure pair, the least squareserror is calculated by comparingthe estimated ΔQ to the measured ΔQ

Error

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-5 -4 -3 -2 -1 0 1 2 3 4 5

House Pressure/Leak Pressure

Pre

ssu

re C

orr

ecti

on

/ H

ou

se P

ress

ure

10% 20% 30% 40%

Leakage Fraction

Duct Flow Resistance CorrectionDuct Flow Resistance Correction

,1/

, ,

,

, ,

2

,

1 1 11r son

r s

nonr s r s

ah

r s

a s sr rh

Q PP

Q

P

Q P

Q

P

Flow through leakDifference between flow throughair handler and flow through ducts


OverviewOverview






Variable-Air-VolumeVariable-Air-VolumeSystem SchematicSystem Schematic


Return Fan

Supply FanCooling Coil

MixingBox

Air Filter

MinimumOA

EconomizerOA

ReliefAir

Ce

iling

Plen

um

Zone P1

Zone P2

Zone P3

Zone P4

Zone C1

ReturnAir

SupplyAir

ReheatBoxes

DownstreamLeakage

UpstreamLeakage

10.9 C19.9 C

22.7 C

22.4 C

11.7 C

23.5 C

23.2 C

21.9 C

23 C

24.9 C

11.7 C

11.7 C

11.7 C

11.7 C

11.7 C

19.9 C

0 kg/s

5.79 kg/s

1.04 kg/s

5.79 kg/s

5.79 kg/s

MassFlow

Temp.

0.64 kg/s0.71 kg/s

0.87 kg/s

0.96 kg/s

1 kg/s

1.11 kg/s

0.97 kg/s

1.08 kg/s

1.1 kg/s

1.22 kg/s

0.51 kg/s

0.71 kg/s

5.79 kg/s

-82800 W

0 W

0 W

0 W

0 W

0 W 6350 W

Coil Load

Load

4500 W

1620 W

Power

22.7 C

19.9 C

4.75 kg/s

7630 W

9970 W

10270 W

11100 W

14650 W

83% flow

79% flow

79% flow

91% flow

80% flow

71% flow

Lighting Load

Legend

VAV Duct Leakage Case: 10% Upstream / 10% Downstream (7 am, July 29)

(1 1/ )n

system system

Q P QW

VAV System ControlVAV System Control

C

F

FMD

supply fan

terminal

pressuresensor

flowsensor

variable-speeddrive

controller


Duct Static Pressure Reset IssuesDuct Static Pressure Reset Issues• DDC systems with reset capability already exist, but

suffer from:– Inaccurate, open-loop position measurement

– Failures at terminal boxes

– Limited bandwidth and limited programming capabilities

• Many systems have pneumatic terminal controls

• Using total supply airflow signal from airflow station expands reset applicability

• Aggregation of terminal box flows makes control more robust to single terminal failure


Diagnostic PrincipleDiagnostic Principle• Terminal flows are regulated by thermostat,

independent of duct static pressure

• Test Procedure– Start at high pressure

– Incrementally lower pressure

– Record flow signal at each step

• Complicating Issues– Flow stabilizes slowly

– Zone temperatures can change

– Noisy measurements

– Ducts leak (pressure-dependent)


Diagnostic: Dual-Model EstimationDiagnostic: Dual-Model Estimation• Model components

1. Constant component2. Time-varying component3. Leakage flow4. Starved behavior

• “In-Control”:

• “Starved”:

• At critical pressure, both models predict same flow; solve for transition using least squares fit

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 50 100 150 200 250 300 350

supply duct static pressure, Pa

sup

ply

du

ct f

low

, m3 /s

in-control

critical pressurestarved


0N

c t pQ Q C T C P

1 20 1 2

0

1N N N NtS p

C TQ C P C P C P C P

Q

11000

11500

12000

12500

13000

13500

14000

14500

15000

0.2 0.3 0.4 0.5 0.6 0.7 0.8

supply duct static pressure, in. w.c.

su

pp

ly d

uc

t a

irfl

ow

, CF

M

starved

in control

critical pressure

operatingpressure

Haas School of BusinessHaas School of Business


UCOPUCOP

0

20000

40000

60000

80000

100000

120000

140000

0 0.2 0.4 0.6 0.8 1 1.2static pressure, in. w.c.

sup

ply

air

flo

w, C

FM

starved

in-control

operatingpressure

criticalpressure


County of AlamedaCounty of Alameda

0

2000

4000

6000

8000

10000

12000

14000

0 0.2 0.4 0.6 0.8 1 1.2 1.4

static pressure, in. w.c.

sup

ply

flo

w, C

FM

starved

criticalpressure

in-control

operatingpressure


OverviewOverview






Intermittent Ventilation:Intermittent Ventilation:When Steady Won’t Always DoWhen Steady Won’t Always Do

• Ventilation (for acceptable IAQ) should not always be constant

• May be periods of the day when outdoor air (OA) quality is poor and one wishes to reduce amount of OA entering building

• Economizer operation can over-ventilate a space from IAQ point of view; energy savings can be achieved by reducing ventilation rates at other times to account for over-ventilation

• Demand charges or utility peak loads may make it advantageous to reduce ventilation for certain periods of the day

• Some HVAC equipment may make cyclic ventilationmore attractive than steady-state ventilation

– Example: residential or small commercial systems that coupleventilation to heating and cooling system operation


What’s The Problem?What’s The Problem?• Constant target ventilation (Aeq)

• Intermittent ventilation with cycle time (Tcycle),over-ventilation (Ahigh) for fractional time fhigh, andunder-ventilation (Alow) for fractional time flow

• Equivalency = same dose for constant contaminant source– Sherman & Wilson (1986); Std 136

• Means to demonstrate equivalency not obvious:– Designers want flexibility to use intermittent ventilation, but also

want to follow standards & guidelines

– Average not always same as constantEnergy Performance of Buildings Group

Efficacy is LinkEfficacy is Link• Provide calculation method to assess equivalency

– Find the temporal ventilation effectiveness (“efficacy”)of a given pattern of ventilation

• Definition:

• Typical Use:

(1 )eq

low low low high

A

f A f A

/

(1 )eq low low

highlow

A f AA

f


Hyperbolic Cotangent?Hyperbolic Cotangent?

• Nominal Turnover:

• Fraction of time under-ventilated: flow

• Recursive equation numerical solution• Use efficacy for design

2

2

1 coth( / )

1low

low

f

f

( )

2eq low cycleA A T


Air Change Rates & Turn-Over TimesAir Change Rates & Turn-Over Timesach (1/h)

Turn-Over Time (h) DESCRIPTION

0.15 6.67 Infiltration rate of new homes

0.25 4.00 Infiltration rate of commercial buildings

0.3 3.33Ventilation requirement of almost empty commercial buildings[from Std 62.1-2004]

0.5 2.00Office space requirement [from Std 62.1-2004];also large home [from Std 62.2-2004]

0.7 1.43 Ventilation requirement for small homes

1.0 1.00 Infiltration rate of older homes

2.0 0.50 Conference room requirement [from Std 62.1-2004]

4.0 0.25 High density space (e.g., theater lobby)


Efficacy TrendsEfficacy Trends


Efficacy for Different Under-Ventilation Fractions

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.000 1.000 2.000 3.000 4.000 5.000

Nominal Turn-over, N, (air changes)

Ve

nti

lati

on

Eff

ec

tiv

en

es

s (

-)

25%

50%

75%

N<<1N>>1

flow

Notch Ventilation at Various Air Change Rates

1

2

3

4

5

6

0 2 4 6 8Under-Ventilation Time (h)

Ve

nti

lati

on

Mu

ltip

liie

r(-)

0.5 ACH

1.0 ACH

0.5 ACH wi/ infiltration

1.0 ACH w/ infiltration



90% Efficacy at Various Air Change Rates

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 4 8 12 16 20 24

Cycle Time (h)

Un

der

-ven

tila

tio

n F

ract

ion

(-)

0.350.5124

ε > 90% for region to left and below each curve


Maximum Under-Ventilation

0

2

4

6

8

10

12

0 4 8 12 16 20 24

Cycle Time (h)

Un

der

Ve

nti

lati

on

Tim

e (

h)

No max

0.35

0.5

1

2

4

Low-Density Spaces

High-Density Spaces


Capacity Required for 0.35 ach

0

0.5

1

1.5

0 1 2 3 4 5 6 7 8

Desired Under-Ventilation Time (h)

On

-cy

cle

Ve

nti

lati

on

(A

CH

)

4 hr cycle time 8 hr cycle time12 hr cycle time 24 hr cycle time

Questions?Questions?






energy performance of buildings group heating, ventilating, & air-conditioning: diagnostics...

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