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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
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https://www.flickr.com/photos/usask/sets/72157636130538285 3
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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
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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)
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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%
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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%
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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
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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.
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http://vanee.edenenergy.com/hw-vanee-airexchanger.php
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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.
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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
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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?
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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
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Experimental Approach: Test Facility
15
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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
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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)
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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
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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
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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• …
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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%
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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
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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.
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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)
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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
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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
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