www.usask.ca

Frosting in HRVs/ERVs and its impact on energy recoveryCarey Simonson, Professor and Mohammad Rafati, PhD studentDepartment of Mechanical Engineering, University of SaskatchewanSaskatoon, SK, CANADA

2016 NWT and Nunavut Construction Association Residential Construction Workshop: HRVs and Ventilation Issues in the NorthYellowknife, NWTFebruary 18, 2016

https://www.flickr.com/photos/usask/sets/72157636130538285 3

Air-to-Air Energy Exchanger Test Lab

Test conditions• -40°C to +40°C• 20% to 90% RH• 400 cfm (200 L/s)

200 cfm (50%)

5 cfm (1%)

U of S frosting fouling

Heat and MoistureExchange (Energy Exchange)

Heat ExchangeOnly

AdjacentDucting

Non-AdjacentDucting

Overview of air-to-air energy exchangers

Heat Exchange – Adjacent Duct

Energy Exchange – Non-adjacent Duct

Heat Exchange – Non-adjacent Duct

Energy Exchange – Adjacent Duct

Heat Wheel Flat PlateExchanger

(aluminum orsolid plastic)

Heat Pipe

GlycolRun-AroundSystem

Energy (Enthalpy)Wheel

Flat PlateExchanger

(paper or membrane) Twin-Tower Enthalpy Recovery Loop

2004 ASHRAE Handbook—HVAC systems and equipment handbook. © American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Pump

Supply Exchanger

Exhaust Exchanger

liquid flow

airflow

Run-around,

Membrane Energy

Exchanger (RAMEE)

0

1

2

3

4

5

6

-20 -15 -10 -5 0 5 10 15 20

W (g

/kg)

T (°C)

50% RH

75% RH100% RH

25% RH

Why do exchangers frost?• Less condensation and frosting

when exchanger has:– Lower effectiveness (ε)– Moisture transfer

20°C40% RH

0°C75% RH

ε=50%ε=75%

0

1

2

3

4

5

6

-20 -15 -10 -5 0 5 10 15 20

W (g

/kg)

T (°C)

50% RH

75% RH

100% RH

25% RH

Energy exchanger frosting• Frost (rule-of-thumb)

– when line joining indoor and outdoor states crosses saturation line

• Reduce/avoid frosting– Decrease ε (bypass, speed control,…)– Preheat outdoor air– Increase moisture transfer?

Preheat outdoor air

20°C40% RH

-20°C, 75% RHε=75%

NSERC Smart Net-zero Energy Buildings strategic Research Network (SNEBRN) • 2011-2016• 29 Professors• 15 Universities• 5 M$ from NSERC + 2 M$ from NRCan, industry, utilities• VISION:

– The vision of SNEBRN is to perform the research that will facilitate widespread adoption in key regions of Canada, by 2030, of optimized NZEB energy design and operation concepts suited to Canadian climatic conditions and construction practices

8

www.usask.ca

Other projects NSERC Engage project

• Frosting of air-to-air membrane energy exchangers• Jan-June 2013• 25 k$ from NSERC, in-kind from dPoint Technologies

Mahmood, G. and Simonson, C.J., 2012. Frosting conditions for an energy wheel in laboratory simulated extreme cold weather, Proceedings of the 7thInternational Cold Climate HVAC Conference, Calgary, AB, November 12-14, 2012, pp. 223-232.

http://vanee.edenenergy.com/hw-vanee-airexchanger.php

11

Frosting in exchangers may : Decrease effectiveness Reduce air flow Increase fans’ power consumption Deflect the plate or membrane

Imag

e C

ourte

sy o

f JL

Hock

man

Cons

ultin

g In

c.

12M. Rafati Nasr, M. Fauchoux, R. W. Besant, and C. J. Simonson, “A review of frosting in air-to-air energy exchangers,” Renew. Sustain. Energy Rev., vol. 30, pp. 538–554, Feb. 2014.

0

10

20

30

40

50

60

2 1 2 2

Num

ber

of p

ublis

hed

wor

k

year

Published papers Review papers

Distribution of the published work from 1980-2013 on frosting in heat/energy

exchangers.

020406080

100120140

Heat Pumpsor

Refrigrationsystems

Plate HeatExchanger

EnergyWheels

Other typesof air-to-airexchangers

NotSpecifiedN

umbe

r of

Pub

lishe

d W

ork

Distribution of the published work from 1980-2013 based on the type of heat/energy

exchanger.

Membrane Energy

Exchanger

Literature ReviewTotal: 201

Project Overview

13

Objectives

FROSTING LIMIT: EXPERIMENTAL APPROACH

FRO

STIN

G LI

MIT

: THE

ORE

TICA

L M

ODE

L

FRO

STIN

G-DE

FRO

STIN

G AN

D RE

QU

IRED

DE

FRO

STIN

G TI

ME

ENERGY IMPACT OF FROSTING

Develop a test facility to investigate

frosting in exchangers

Quantify the frosting limit operating

conditions through experiments

Quantify the energy impact of frosting-defrosting cycles in

energy recovery

Develop a theoretical model

to predict the frosting limit

1 24 3

• HRV/ERV operating conditions without frosting?

• Suitable frost protection strategies?

• Energy impact of frosting?

Experimental Approach: Methodology

14

Methods to detect frosting

• Visual method (endoscope)

• Pressure drop (∆P)

• Effectiveness (ε)•

Frosting limit

• Heat Exchanger• Energy Exchanger• Repeatability test

Operating conditions

• 𝑇𝑇𝑆𝑆𝑆𝑆 = 0℃ 𝑡𝑡𝑡𝑡 − 30℃(controlled)

• 𝑇𝑇𝐸𝐸𝑆𝑆 = 22℃(controlled)

• 𝑅𝑅𝑅𝑅𝐸𝐸𝑆𝑆 = 10% 𝑡𝑡𝑡𝑡 50%(Controlled)

• 𝑅𝑅𝑅𝑅𝑆𝑆𝑆𝑆 = 35% 𝑡𝑡𝑡𝑡 50%(uncontrolled)

• 𝑄𝑄 = 40 𝑐𝑐𝑐𝑐𝑐𝑐 (18 𝐿𝐿/𝑠𝑠)• Test duration: ~3 ℎ𝑟𝑟

for the main frosting test

Experimental Approach: Test Facility

15

Experimental Approach: Test Facility

16

Pito

t-tu

be

Endoscope

Experimental Approach: Visual Method

17

Before the test

7min into the test 82 min 194 min

Right after the test (194 min)

Condition: 𝑇𝑇𝑆𝑆𝑆𝑆 = −20℃𝑅𝑅𝑅𝑅𝐸𝐸𝑆𝑆 = 20%

Heat Exchanger

Experimental Approach: Visual Method

18

2 min into the test 180 min45 min

Before the test Right after the test (180 min)

Energy Exchanger Condition: 𝑇𝑇𝑆𝑆𝑆𝑆 = −20℃𝑅𝑅𝑅𝑅𝐸𝐸𝑆𝑆 = 20%

0

40

80

120

160

0 60 120 180

Δp (P

a)Time (min)

19

Conditions: 𝑇𝑇𝑆𝑆𝑆𝑆 = −20℃𝑅𝑅𝑅𝑅𝐸𝐸𝑆𝑆 = 20%

Heat Exchanger

Supply

Experimental Approach: ∆P Method

0

40

80

120

160

0 60 120 180

Δp(P

a)

Time (min)

Energy Exchanger

Supply

Increase in ∆P→ Frosting due to partial blockage

No change in ∆p in supply → No frost in the supply side

Energy exchanger has lower ∆p change

Experimental Approach: ε Method

20

Conditions:𝑇𝑇𝑆𝑆𝑆𝑆 = −20℃𝑅𝑅𝑅𝑅𝐸𝐸𝑆𝑆 = 20%

Reduction in effectiveness due to frosting,

More effectiveness reduction in the heat exchanger,

Relatively low rate of change in the effectiveness

Small change in the latent effectiveness

30

40

50

60

70

0 60 120 180

Sens

ible

effe

ctiv

enes

s, ε

s(%

)

Time (min)

Energy Exchanger

10

20

30

40

50

0 60 120 180

Late

nt e

ffect

iven

ess ,

εl (%

)

Time (min)

Experimental Approach: Methods comparison

21

Method Advantages Disadvantages

Visual

-Easy to use

-Quickly detects frosting

-Low uncertainty

-Inexpensive

-Does not show the amount of frost inside the

exchanger

-Not practical in frost protection systems

because it is qualitative method

-Frequent maintenance required

𝜀𝜀

-Shows the effect of frosting

on the energy recovery

-Quantitative method

-Not reliable in practical applications due to

high uncertainty

-Complex in terms of installation of different

sensors and measurement

∆𝑝𝑝

-Easy to use

-Relatively quick response to

the presence of frost

-Quantitative method

-Relatively expensive

-Frequent calibration is needed

-Sensitive to flow rate

Experimental Approach: Frosting limit

22

0%

10%

20%

30%

40%

50%

60%

-35 -30 -25 -20 -15 -10 -5 0

RHEx

haus

t

Tsupply(°C)

no frostfrost

Heat Exchanger

0%

10%

20%

30%

40%

50%

-35 -30 -25 -20 -15 -10 -5 0

RHEx

haus

t

TSupply(°C)

Frosting limit

Energy Exchanger experiences frosting at almost 5 °C lower Supply temperature or 5%RH higher Exhaust relative humidity

Frosting region

23

Objectives

FROSTING LIMIT: EXPERIMENTAL APPROACH

FRO

STIN

G LI

MIT

: THE

ORE

TICA

L M

ODE

L

FRO

STIN

G-DE

FRO

STIN

G AN

D RE

QU

IRED

DE

FRO

STIN

G TI

ME

ENERGY IMPACT OF FROSTING

Develop a test facility to investigate

frosting in exchangers

Quantify the frosting limit operating

conditions through experiments

Quantify the energy impact of frosting-

defrosting modes in energy recovery

Develop a theoretical model

to predict the frosting limit

1 24 3

Theoretical model: Frosting limit

24

Methodology• Heat & Mass transfer equations

Frosting conditions

• Subzero surface temperature• Condensation on the cold surface

Assumptions

• Frosting initially forms in the first exhaust air channel • The supply air temperature passing over the first channel

is constant• Negligible condensation heat release• …

Theoretical model: Exchanger properties

25

Exchangers specificationParameter Value Reference

Number of channels for each flow, n 31 Lab measurement

Half duct height, a 1.52 ± 0.07𝑐𝑐𝑐𝑐 Lab measurement

Half duct width, b 3.61 ± 0.07 𝑐𝑐𝑐𝑐 Lab measurement

Apex angle, 𝛼𝛼 49° ± 2° Lab measurement

Hydrodynamic diameter, 𝐷𝐷ℎ 2.63 ± 0.16𝑐𝑐𝑐𝑐 Lab measurement

Exchanger width 𝑋𝑋, height 𝑅𝑅, depth 𝐿𝐿 165 ± 2 𝑐𝑐𝑐𝑐 Lab measurement

Membrane thickness, 𝛿𝛿 100 ± 0.02 𝜇𝜇𝑐𝑐 Lab measurement

Membrane water vapor diffusivity, 𝐷𝐷𝑤𝑤𝑝𝑝 2.0 × 10−6 ± 3 × 10−7 𝑐𝑐2/𝑠𝑠 Lab measurement

Thermal conductivity of membrane, 𝑘𝑘𝑝𝑝 0.44 𝑊𝑊/(𝑐𝑐 𝐾𝐾) (L. Zhang, 2008)

Thermal conductivity of fin, 𝑘𝑘𝑐𝑐 237 𝑊𝑊/(𝑐𝑐 𝐾𝐾) (L. Zhang, 2008)

Thermal conductivity of air, 𝑘𝑘𝑎𝑎 0.0258 𝑊𝑊/(𝑐𝑐 𝐾𝐾) (ASHRAE, 2009)

Density of air, 𝜌𝜌𝑎𝑎 1.12 𝑘𝑘𝑘𝑘/𝑐𝑐3 (ASHRAE, 2009)

Water vapor diffusivity in the air, 𝐷𝐷𝑣𝑣𝑎𝑎 2.82 × 10−5 𝑐𝑐2/𝑠𝑠 (L. Zhang, 2008)

Kinetic viscosity of air, 𝜈𝜈𝑎𝑎 15.52 × 10−6 𝑐𝑐2/𝑠𝑠 (ASHRAE, 2009)

Volumetric air flow rates, 𝑄𝑄 20.8 ± 0.5 𝐿𝐿/𝑠𝑠 Lab measurement

Face velocity, 𝑢𝑢𝑎𝑎 1.5 + 0.1 𝑐𝑐/𝑠𝑠 Lab measurement

Reynolds Number, Re 228+22 Lab measurement

Exchanger type 𝜀𝜀𝑠𝑠 𝜀𝜀𝑙𝑙

Theoretical Experimental Theoretical Experimental HRV 58% ± 3% 58% ± 2% − − ERV 58% ± 3% 58% ± 2% 32% ± 3% 32% ± 4%

Theoretical model: validation

26

Energy Exchanger

Heat Exchanger

Theoretical model: Parametric Study

27

56% 66% 71% 74%

0

32%

47%

56%

62%

66%

68%

0.0

1.0

2.0

3.0

0.5 1.5 2.5 3.5 4.5 5.5

εs(%)

ε l(%

)

NTU

l

NTUs

-10-20-30

RHEI=30%

TSI(°C)

Frosting Region

No Frosting Region

NTUℎ =𝑈𝑈𝑈𝑈 𝑠𝑠�̇�𝑐 𝑐𝑐𝑝𝑝

NTU𝑙𝑙 =𝑈𝑈𝑈𝑈 𝑙𝑙�̇�𝑐

𝜀𝜀 ⁄𝑠𝑠 𝑙𝑙 = 1 − 𝑒𝑒𝑒𝑒𝑝𝑝𝑒𝑒𝑒𝑒𝑝𝑝 −𝑁𝑁𝑇𝑇𝑈𝑈 ⁄s 𝑙𝑙

0.78 − 1𝑁𝑁𝑇𝑇𝑈𝑈 ⁄𝑠𝑠 𝑙𝑙

−0.22

28

Objectives

FROSTING LIMIT: EXPERIMENTAL APPROACH

FRO

STIN

G LI

MIT

: THE

ORE

TICA

L M

ODE

L

FRO

STIN

G-DE

FRO

STIN

G AN

D RE

QU

IRED

DE

FRO

STIN

G TI

ME

ENERGY IMPACT OF FROSTING

Develop a test facility to investigate

frosting in exchangers

Quantify the frosting limit operating

conditions through experiments

Quantify the energy impact of frosting-

defrosting modes in energy recovery

Develop a theoretical model

to predict the frosting limit

1 24 3

Defrosting of exchangers

29

Bypass the supply air

Auxiliary Exchanger

Using PCM Partial blockage of the supply air

Defrosting of exchangers

30M. Rafati Nasr, M. Fauchoux, R. W. Besant, and C. J. Simonson, “A review of frosting in air-to-air energy exchangers,” Renew. Sustain. Energy Rev., vol. 30, pp. 538–554, Feb. 2014.

y = 0.697x

y = 0.946x

0

40

80

120

160

200

0 60 120 180

Fros

t wei

ght (

gr)

Time (min)

Energy Exchanger

y = 2.217x

y = 2.710x

0

40

80

120

160

200

0 60 120 180

Fros

t mas

s (g)

Time (min)

HX_-20C_30%RH

HX_-25C_30%RH

Heat Exchanger

Frost accumulation rate

31

frost accumulation rate is almost constant

frost accumulation rate in the heat exchanger is 3 times of the energy exchanger

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0.0 1.0 2.0 3.0 4.0

Nor

mal

ized

fros

t (w

ater

) mas

s,

mf*

Normalized defrosting time, t*df

HX_-25C_30%RH

HX_-25C_20%RH

Heat Exchanger

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0.0 1.0 2.0 3.0 4.0

Nor

mal

ized

fros

t (w

ater

vap

or)

wei

ght

Normalized defrosting time, t*df

EX_-25C_30%RH

EX_-25C_20%RH

Energy Exchanger

Frost removal rate

32

Normalized defrosting time

𝑡𝑡𝑑𝑑𝑑𝑑∗ =Defrosting time

Frosting duration

Defrosting time is from when the defrosting phase starts

frost removal rate decreases with time

energy exchanger has a higher frost removal rate and much shorter defrosting time

The trend in the heat exchanger is independent of the amount of frost accumulated

Defrosting time requirement

𝐷𝐷𝑇𝑇𝑅𝑅 =Defrosting duration

Frosting duration 33

Required defrosting time ratio

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-30 -25 -20 -15 -10

Defr

ostin

g tim

e ra

tio, D

TR

Supply Temperature (°C)

Heat Exchanger

𝑅𝑅𝑅𝑅𝐸𝐸𝑆𝑆 = 20%

𝑅𝑅𝑅𝑅𝐸𝐸𝑆𝑆 = 30% DTR can be very different

depending on the operating conditions and exchanger type

The methodology can be applied for commercial HRVs/ERVs

0.0

0.5

1.0

1.5

-30 -25 -20 -15 -10

Defr

ostin

g tim

e ra

tio, D

TR

Supply Temperature (°C)

Energy Exchanger

34

Objectives

FROSTING LIMIT: EXPERIMENTAL APPROACH

FRO

STIN

G LI

MIT

: THE

ORE

TICA

L M

ODE

L

FRO

STIN

G-DE

FRO

STIN

G AN

D RE

QU

IRED

DE

FRO

STIN

G TI

ME

ENERGY IMPACT OF FROSTING

Develop a test facility to investigate

frosting in exchangers

Quantify the frosting limit operating

conditions through experiments

Quantify the energy impact of frosting-

defrosting modes in energy recovery

Develop a theoretical model

to predict the frosting limit

1 24 3

Energy impact of frosting-calculation method

35

tot

faverage t

t )(0 ⋅= εε

ttot

time

Defrosting

tf tf tf

tdf tdf

ε (%)

tdf

0ε

Defrosting

Energy impact of frosting-calculation method

36

Total sensible heat required

Recovered sensible heat by exchanger

Energy impact of frosting: calculation method

37

Total energy required

Total energy recovered by exchanger

Energy impact of frosting

38

Energy loss due to frosting depends on:1) Climate (higher in cold climate)2) Exchanger (lower with ERV)3) Frost control method (lower with frost prevention)

% loss in energy recovery due to frosting

66%58%

53%

26%

12% 8%

0%

20%

40%

60%

80%

100%

Saskatoon Anchorage Chicago

HRV

ERVFrost-defrost cycle

(defrost by bypassing the supply air)

33%

21%

10%15%

7% 4%0%

20%

40%

60%

80%

100%

Saskatoon Anchorage Chicago

HRV

ERV

Avoiding frost(Preheating supply air)

39

Objectives

FROSTING LIMIT: EXPERIMENTAL APPROACH

FRO

STIN

G LI

MIT

: THE

ORE

TICA

L M

ODE

L

FRO

STIN

G-DE

FRO

STIN

G AN

D RE

QU

IRED

DE

FRO

STIN

G TI

ME

ENERGY IMPACT OF FROSTING

Develop a test facility to investigate

frosting in exchangers

Quantify the frosting limit operating

conditions through experiments

Quantify the energy impact of frosting-

defrosting modes in energy recovery

Develop a theoretical model

to predict the frosting limit

1 24 3

www.usask.ca

Future research Energy analyses

• Other climates• Other commercial HRVs/ERVs with actual frost-control

schemes• “Real” indoor humidity conditions

Verify model and lab data with field data Frost-free exchangers

• Design• Control

41

CollaborationMovement towards low/zero energy buildings

● Test facilities● Simulation tools● Expertise● HQP (students)

InnovationSolution

Industry University

Acknowledgement

42

Acknowledgement

43

• University of Saskatchewan

• Smart Net-Zero Energy Research Network (SNEBRN)

• The Natural Sciences and Engineering Research Council of Canada (NSERC)

• dPoint Technologies Inc.

• Norwegian University of Science and Technology

• NWT & Nunavut Construction Association

Frosting in HRVs/ERVs and its impact on energy recoverySlide Number 2Slide Number 3Air-to-Air Energy Exchanger Test LabOverview of air-to-air energy exchangers Why do exchangers frost?Energy exchanger frostingNSERC Smart Net-zero Energy Buildings strategic Research Network (SNEBRN) Other projectsSlide Number 10Slide Number 11Literature ReviewProject OverviewExperimental Approach: MethodologyExperimental Approach: Test FacilityExperimental Approach: Test FacilityExperimental Approach: Visual MethodExperimental Approach: Visual MethodExperimental Approach: ∆P MethodExperimental Approach: ε MethodExperimental Approach: Methods comparisonExperimental Approach: Frosting limitSlide Number 23Theoretical model: Frosting limitTheoretical model: Exchanger propertiesTheoretical model: validationTheoretical model: Parametric StudySlide Number 28Defrosting of exchangersDefrosting of exchangersFrost accumulation rateFrost removal rateDefrosting time requirementSlide Number 34Energy impact of frosting-calculation methodEnergy impact of frosting-calculation methodEnergy impact of frosting: calculation methodEnergy impact of frostingSlide Number 39Future researchSlide Number 41AcknowledgementAcknowledgementThank you for your attention