A Modeling Study to Determine the
Effectiveness of an
Energy Recovery Ventilator (ERV)
by
Roy Pastor
An Engineering Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
MASTER OF ENGINEERING
Major Subject: MECHANICAL ENGINEERING
Approved:
_________________________________________
Norberto Lemcoff, Primary Project Adviser
_________________________________________
Ernesto Gutierrez-Miravete, Co-Adviser
Rensselaer Polytechnic Institute
Hartford, Connecticut
December, 2010
(For Graduation January 2011)
i
© Copyright 2010
by
Roy Pastor
All Rights Reserved
ii
CONTENTS
LIST OF TABLES ............................................................................................................. v
LIST OF FIGURES .......................................................................................................... vi
ACKNOWLEDGMENT ................................................................................................ viii
NOMENCLATURE ......................................................................................................... ix
ABSTRACT ...................................................................................................................... x
1. INTRODUCTION ....................................................................................................... 1
1.1
Background ........................................................................................................ 1
1.2
Previous Work.................................................................................................... 3
1.3
Problem Description........................................................................................... 4
2. METHODOLOGY ...................................................................................................... 6
2.1
Physical Model ................................................................................................... 6
2.2
Mathematical Model .......................................................................................... 6
2.2.1
Fluid Dynamics ...................................................................................... 6
2.2.2
Heat Transfer .......................................................................................... 7
2.2.3
Mass Transfer ......................................................................................... 7
2.2.4
Inlet Conditions ...................................................................................... 8
2.2.5
Boundary Conditions ............................................................................. 8
2.2.6
Heat and Mass Transfer Effectiveness ................................................. 10
3. FINITE ELEMENT MODEL .................................................................................... 11
3.1
ERV Dimensions and Parameters .................................................................... 11
3.2
Fluid Dynamics ................................................................................................ 12
3.3
Heat Transfer .................................................................................................... 13
3.4
Mass Transfer ................................................................................................... 13
3.5
Meshing ............................................................................................................ 13
4. RESULTS .................................................................................................................. 14
4.1
Problem Scenarios ............................................................................................ 14
iii
4.2
4.3
4.4
4.5
ERV Effectiveness with Equal Supply and Exhaust Flow............................... 14
4.2.1
Summer Conditions.............................................................................. 14
4.2.2
Winter Conditions ................................................................................ 20
ERV Effectiveness with Varying Exhaust Flow .............................................. 23
4.3.1
Summer Conditions.............................................................................. 23
4.3.2
Winter Conditions ................................................................................ 27
ERV Effectiveness as the Exhaust Height is Reduced..................................... 29
4.4.1
Summer Conditions.............................................................................. 29
4.4.2
Winter Conditions ................................................................................ 31
ERV Effectiveness as the Diffusion through the Membrane is Varied ........... 33
4.5.1
Summer Conditions.............................................................................. 33
4.5.2
Winter Conditions ................................................................................ 34
5. CONCLUSION.......................................................................................................... 37
6. REFERENCES .......................................................................................................... 39
7. APPENDIX A ............................................................................................................ 40
7.1
Sensible and Latent Effectiveness Calculation ................................................ 40
iv
LIST OF TABLES
Table 1. ERV Basic Dimensions ..................................................................................... 11
Table 2. Supply Conditions and Properties for Summer and Winter Seasons ............... 11
Table 3. Exhaust Conditions and Properties for Summer and Winter Seasons .............. 12
Table 4. Membrane Properties and Parameters .............................................................. 12
Table 5. Number of Elements ......................................................................................... 13
Table 6. Sensible and Latent Effectiveness with Equal Velocities (Summer) ............... 14
Table 7. Sensible and Latent Effectiveness with Equal Velocities (Winter) .................. 20
Table 8. Sensible and Latent Effectiveness at Varying Exhaust Flow (Summer) .......... 23
Table 9. Sensible and Latent Effectiveness at Varying Exhaust Flow (Winter) ............ 27
Table 10. Sensible and Latent Effectiveness with Varying Height (Summer) .............. 29
Table 11. Sensible and Latent Effectiveness with Varying Height (Winter) ................. 31
Table 12. Sensible and Latent Effectiveness with Varying Diffusion (Summer) .......... 33
Table 13. Sensible and Latent Effectiveness with Varying Diffusion (Winter) ............. 34
Table 14. Boundary Integration for Countercurrent Flow (Summer)............................. 40
v
LIST OF FIGURES
Figure 1. Comparison of HVAC Systems with and without ERV ................................... 2
Figure 2. Schematic of a Cross-Flow Membrane ERV [2] .............................................. 3
Figure 3. Schematic of a Quasi-Counter Flow Membrane ERV [4] ................................ 4
Figure 4. Schematic of a Countercurrent Flow Membrane ERV ..................................... 5
Figure 5. Schematic of a Concurrent Flow Membrane ERV ............................................ 5
Figure 6. Pictorial Description of the Mathematical Model ............................................. 8
Figure 7. Summer Sensible Effectiveness for ERVs ...................................................... 15
Figure 8. Summer Latent Effectiveness for ERVs ......................................................... 15
Figure 9. Countercurrent Flow Temperature Profile at Varying Channel Location ....... 16
Figure 10. Concurrent Flow Temperature Profile at Varying Channel Location ........... 17
Figure 11. Countercurrent Flow Concentration Profile at Varying Channel Location .. 18
Figure 12. Concurrent Flow Concentration Profile at Varying Channel Location ......... 19
Figure 13. Winter Sensible Effectiveness for ERVs ...................................................... 21
Figure 14. Winter Latent Effectiveness for ERVs .......................................................... 21
Figure 15. ERV’s Temperature Profile at x = 1.25 m .................................................... 22
Figure 16. ERV’s Concentration Profile at x = 1.25 m .................................................. 22
Figure 17. Temperature Profile at Varying Exhaust Flows (Countercurrent) ................ 24
Figure 18. Temperature Profile at Varying Exhaust Flows (Concurrent) ...................... 24
Figure 19. Concentration Profile at Varying Exhaust Flows (Countercurrent) .............. 25
Figure 20. Concentration Profile at Varying Exhaust Flows (Concurrent) .................... 25
Figure 21. Summer Sensible Effectiveness for ERVs with Varying Exhaust Flow ....... 26
Figure 22. Summer Latent Effectiveness for ERVs with Varying Exhaust Flow .......... 26
Figure 23. Winter Sensible Effectiveness for ERVs with Varying Exhaust Flow ......... 28
Figure 24. Winter Latent Effectiveness for ERVs with Varying Exhaust Flow ............ 28
Figure 25. Summer Sensible Effectiveness with Varying Height .................................. 29
Figure 26. Summer Latent Effectiveness with Varying Height ..................................... 30
Figure 27. Countercurrent Temperature Profiles with Varying Height .......................... 30
Figure 28. Concurrent Temperature Profiles with Varying Height ................................ 31
Figure 29. Winter Sensible Effectiveness with Varying Height ..................................... 32
Figure 30. Winter Latent Effectiveness with Varying Height ........................................ 32
vi
Figure 31. Latent Effectiveness with Varying Diffusion (Summer) .............................. 34
Figure 32. Latent Effectiveness with Varying Diffusion (Winter) ................................. 35
Figure 33. Winter Concentration Profiles for Varying Diffusion (Countercurrent) ....... 36
Figure 34. Winter Concentration Profiles for Varying Diffusion (Concurrent) ............. 36
vii
ACKNOWLEDGMENT
I want to thank my wife for supporting me in my life and during my time in college. I
also want to thank Professors Norberto Lemcoff and Ernesto Gutierrez-Miravete for
their assistance and guidance in completing my project. Lastly, I want to thank Electric
Boat and Rensselaer Polytechnic Institute for providing me the opportunity to obtain my
Master’s Degree in Mechanical Engineering.
viii
NOMENCLATURE
c
water concentration (kg/m3)
cp
specific heat, J / kg K
d
channel height or membrane spacing, m
D
diffusivity, m2/s
F
volume force field, N
h
convective heat transfer coefficient, m/s
k
thermal conductivity, W/m K
L
channel length
n
normal vector
N
mass flux, mol/m2 s
q
heat flux, W/m2
Q
heat source, W
R
reaction rate, mol/m3 s
t
tangential vector
T
temperature, C or K
U
overall heat transfer coefficient, W / m2 K
u
velocity field, m/s
x,y,z
coordinates
Greek Symbols
ts
time scaling coefficient

density, kg/m3

dynamic viscosity, kg/m s
Subscripts
ave
average
e
exhaust
i
inlet
L
latent
o
outlet
s
supply
S
sensible
ix
ABSTRACT
The purpose of this project is to evaluate the effectiveness of an energy recovery
ventilator (ERV) during the summer and winter seasons using finite element modeling
methods. An energy recovery ventilator is a new concept in ventilation systems that
allows heat and mass transfer between two airstreams separated by a membrane. Two
configurations were investigated in this study: the countercurrent and the concurrent
flows. The effects of varying the following parameters were examined: flows through
the supply and exhaust ducts, flows through the exhaust duct only, height of the exhaust
channel, and diffusion coefficient through the membrane. The results showed that the
countercurrent flow configuration is more effective than the concurrent flow
configuration. For equal supply and exhaust channel flows the effectiveness of the ERV
increases as the velocity decreases. The total effectiveness varied between 0.51 and 0.61
and between 0.43 and 0.47, for the countercurrent and concurrent configurations,
respectively. It was also observed that, for a fixed supply flow, the effectiveness
increases as the exhaust flow decreases. However, the temperature and concentration
change along the supply channel is lower. Therefore, this will not be an ideal solution
for reducing the energy consumption of ventilation systems. The effectiveness of the
ERV also increases as the height of the exhaust channel decreases. Results also show
that the larger concentration and temperature changes occurred in the exhaust channel.
On the other hand, for the same volumetric flowrate, a higher velocity in the exhaust
channel has a positive effect on both the effectiveness and temperature/concentration
changes. Lastly, as the diffusion rate through the membrane increases, the effectiveness
of the ERV also increases. A better membrane will help reduce the equipment size.
x
1. INTRODUCTION
1.1 Background
In recent years, the need to conserve energy is receiving more attention. Therefore, there
is a push in many engineering systems to use less energy, while maintaining the same
functions and exceeding the performance required by earlier systems. This is the case
for heating, ventilating, and air conditioning (HVAC) systems, that are required to
provide comfort and quality air for occupants in buildings or offices, within reasonable
installation, operation, and maintenance costs.
A traditional HVAC system will typically consists of coils, fans, heaters, ducts, and
filters. The purpose of the coil is to reduce the air temperature and control the humidity
of the incoming air, which is vital for dehumidification. The fan is the driving force to
allow the conditioned air to flow through the ducts in buildings or offices. A heater is
used to control the thermal comfort in the space, and heat the cold air that exits the
cooling coil. The ducts are used to distribute the conditioned air to various locations in a
building. To provide quality air, a filter is used to prevent airborne bacteria, dust, and/or
odors that may exist in the outside air to be distributed in the conditioned spaces.
Using a traditional HVAC system for buildings that require high volume of outside air
for heating and cooling will require more powerful ventilation systems to meet the
buildings demands. This can be accomplished by using larger coils, fans, and/or heaters.
Superior ventilation systems will increase operating and equipment costs. Therefore, a
larger system is not a viable solution for conserving energy and meeting system needs.
In fact, one of the major costs for ventilation systems is the dehumidification of the
incoming air from the outside environment. This is because the outside air must first
past through a cooling coil where it is cooled below the saturation temperature of the air
to allow condensation. The cold air must then be reheated, since the conditioned space
requires a higher temperature to meet the proper thermal comfort (21-24°C, 30-60%
Relative humidity [1]). As a result, limiting the usage of cooling coils and heaters not
only reduces the energy cost but also the maintenance cost for an HVAC system.
As an example of the cost savings that may be achieved through the use of an ERV, a
typical HVAC system will be compared to one with an ERV installed. The conditions
1
that will be used in this analysis are outside air at 35°C with 49% relative humidity
(RH), while the condition in the space is 24°C with 50% RH. For an HVAC system
with 20% fresh air and 80% recirculated air, the mixed condition is at 26.2°C and 50%
RH.
In order to dehumidify the mixed air, the air will be cooled to a saturation
temperature of 13°C. Based on the psychometric chart (Figure 1), the enthalpy of the air
must be reduced by approximately 18 J/gm of dry air to reach the coil condition. The
cooled air will then be heated by increasing the enthalpy of the air by 15 J/gm of dry air.
When an ERV is used, the mixed condition of the air will be 24.6°C with 50% RH.
Therefore, the reduction of enthalpy required to reach the coil condition is approximately
13 J/gm of dry air, while the heating of the air will be the same. It can be seen that
significant savings can be achieved by using an ERV.
Figure 1. Comparison of HVAC Systems with and without ERV
2
1.2 Previous Work
To reduce the energy consumption of ventilation systems, research in areas such as airto-air energy recovery ventilator (ERV) or enthalpy exchanger is being carried out. The
ERV allows reducing energy consumption because it takes advantage of the conditioned
air that is normally exhausted out of the buildings, to either heat or cool (sensible heat)
and humidify or dehumidify (latent heat) the incoming outside air. Therefore, this
allows the ERV to be used during all the seasons. The heat and moisture transfer is
possible because the water vapor-permeable membrane or plate, located between the
conditioned and supplied air, allows the heat and moisture to pass through the
membrane. The cost of the ventilation system will be reduced, because an ERV does not
have the complexity generally found in rotary dehumidifiers or cooling coils. The
simplified design of the ERV also reduces the maintenance cost, because it does not
have any moving parts that can wear over time and only routine cleaning is required.
Therefore the ERV allows the ventilation system demands to grow, while maintaining
air quality required by buildings and offices as mandated by state and local codes based
on ASHRAE standards, but it does not increase the energy consumption of the
ventilation system.
The most common ERV design found in the market is the cross flow design, due to its
simplified design, and the ease of duct sealing required for ERV systems. A depiction of
a cross flow ERV design is shown in Figure 2. Due to the popularity of cross flow ERV
systems, Zhang et al. [2] analyzed the heat and mass transfer in an ERV through the use
of numerical analysis and conducting a test of a commercial product in a test lab. Min et
al. [3] analyzed the performance of ERVs by changing the membrane spacing and the
thickness of the ventilator through numerical computation.
Figure 2. Schematic of a Cross-Flow Membrane ERV [2]
3
Another type of ERV that has been studied is a quasi-counter flow design. Zhang [4]
conducted a study of an ERV with a quasi-counter flow design, because a countercurrent
flow membrane ERV has a much higher effectiveness than a cross flow design. To
overcome the problem of a countercurrent flow design, Zhang designed an ERV that
combines cross and countercurrent flow in one system. A schematic is shown in Figure
3.
Figure 3. Schematic of a Quasi-Counter Flow Membrane ERV [4]
1.3 Problem Description
Based on previous research of ERV systems, it was determined that countercurrent and
concurrent flows ERV have not been evaluated significantly. Therefore, in this paper
the effectiveness of countercurrent and concurrent flows will be evaluated and compared
to each other. This paper will not focus on the complexity of creating a countercurrent
or concurrent flow membrane ERV or the cost required to build it. It is assumed that
implementation of countercurrent and concurrent flows ERV will be feasible. In the
countercurrent flow membrane ERV, the exhaust and supply air flow in opposite
direction, as shown in Figure 4.
4
d
Exhaust Air

Porous Membrane
d
Supply Air
L
Figure 4. Schematic of a Countercurrent Flow Membrane ERV
In a concurrent flow membrane ERV, the exhaust and supply air flow in the same
direction, as shown in Figure 5:
d
Exhaust Air
Porous Membrane
d

Supply Air
L
Figure 5. Schematic of a Concurrent Flow Membrane ERV
One study that is conducted in this project is to evaluate the impact of ERV’s
performance by varying the air velocity through the supply and exhaust channels. In
addition, the ERV is evaluated by varying the exhaust channel air velocity only. It is
known that in most ventilation systems some of the air from conditioned spaces is
discharged directly out to the environment, as is the case of kitchen and bathroom
exhausts.
Also, the effect of reducing the height of the exhaust channel, while
maintaining constant air velocity through the ERV is analyzed. Lastly, the ERV is
evaluated by varying the diffusion coefficient through the membrane.
The studies
mentioned above will be carried out for both summer and winter conditions. To model
the ERV’s performance, both countercurrent and concurrent flow will be analyzed
through the use of COMSOL, a commercial software package.
5
2. METHODOLOGY
2.1 Physical Model
Typical membrane-based ERV with countercurrent and concurrent flows are shown in
Figures 4 and 5, respectively. The ERV design that will be analyzed is a core that
contains alternate layers of membranes to separate and seal the exhaust and supply
airstreams passages. As described above, a countercurrent flow ERV is designed so that
the exhaust and supply airstreams flow in opposite direction, while a concurrent flow
ERV is designed so that the exhaust and supply airstreams flow in the same direction.
As the exhaust and supply air flow through the ERV, the airstreams will exchange heat
and moisture through the membrane.
Since the ERV has a symmetric design, the
domain that will be evaluated will contain the membrane and only half of the channel
volume of the supply and exhaust airstreams, as shown in Figures 4 and 5.
2.2 Mathematical Model
Based on the physical model described above, several assumptions will be made to assist
in the modeling of the countercurrent and concurrent flow ERVs:

Heat and mass transfer processes are in steady state

The physical properties of the air are constant

In the membrane, only heat conduction and water diffusion exists

Heat conductivity and water diffusivity in the membrane are constant
The guideline used for the mathematical model was obtained from the modeling guide
documentation provided with the COMSOL software [5].
2.2.1
Fluid Dynamics
The governing fluid dynamics equations for the ERV are the momentum transport
equations and the equation of continuity for incompressible fluids:



u
T
   u   u      u    u  p  F


t
 u  0
(1)
(2)
where  is the density,  is the dynamic viscosity, u is the velocity field, p is the
pressure, t is the time, and F is the volume force field. In equation (1), for steady state
6
problems the first term of the equation is zero. In addition, assuming that the ERV flow
is laminar and free of any force field, then equation (1) will simplify to:
2u    u   u  p  0
2.2.2
(3)
Heat Transfer
The governing heat transfer equation (conduction and convection) for the ERV is shown
below:
 ts  c p
T
    k T   Q   c pu  T
t
(4)
where cp is the heat capacity, k is the thermal conductivity, T is the temperature, ts is the
time scaling coefficient, and Q is the heat source. For steady state problems, the first
term of equation of equation (4) is zero. Also, assuming no heat source in equation (4),
the heat transfer equation simplifies to:
  kT    c pu  T
2.2.3
(5)
Mass Transfer
The governing mass transfer equation (diffusion and convection) for the ERV is shown
below:
 ts
c
     Dc  cu   R
t
(6)
where c is the concentration, D is the diffusion coefficient, and R is the reaction rate.
Additionally, for steady state problems the first term of equation (6) is zero. Since the
reaction rate is also zero, then equation (6) simplifies to:
   Dc  cu   0
(7)
A pictorial description of the equations described above is shown in Figure 6.
7
Heat Convection/
Conduction
Water Convection/
Diffusion
Exhaust Air
Heat Conduction
Heat Convection/
Conduction
Porous Membrane
Supply Air
Water Diffusion
Water Convection/
Diffusion
Figure 6. Pictorial Description of the Mathematical Model
2.2.4
Inlet Conditions
The inlet conditions for the countercurrent flow ERV based on the assumptions and the
equations described for the heat and mass transfer are the following:
Supply Air:
us
x 0
 usi
Ts
x 0
 Tsi
cs
x 0
 csi
(8)
Exhaust Air:
ue
xL
 u ei
Te
xL
 Tei
ce
x L
 cei
(9)
where subscripts e, i, and s corresponds to the exhaust, inlet, and supply, respectively.
For concurrent flow, the same boundary conditions are used, except that for the exhaust
flow the boundary is located at x = 0, in lieu of x = L.
2.2.5
Boundary Conditions
2.2.5.1 Fluid Dynamics
For the fluid dynamics, it will be assumed that the no slip condition exists at the
membrane surface. For a no slip condition, the velocity of the fluid at the wall will be
8
zero. At the system boundary of the ERV (the symmetry plane, at the center of the
supply and exhaust channels) the following conditions exist:
un  0
(10)
t    pI  u  n  0
(11)
which indicates no penetration and vanishing shear stresses. In equations (10) and (11),
n indicates the normal vector, while t is the tangential vector.
At the outlet of the ERV it is assumed that Dirichlet condition exists for the pressure and
the viscous stress is small.
p  p0
(12)
u  n  0
(13)
2.2.5.2 Heat Transfer
Continuity of the heat flux is assumed at the membrane interfaces.
n   q1  q2   0
(14)
where q is the heat flux.
At the symmetry plane, at the center of the supply and exhaust channels, it is assumed to
have symmetry, that is
n   kT   0
(15)
At the outlet of the ERV, the boundary condition is a convective flux, where equation
(15) is also valid.
2.2.5.3 Mass Transfer
It is also assumed that there is continuity of the mass flux at the membrane interfaces.
Therefore, the boundary condition is
n   N1  N2   0
(16)
where N is the mass flux.
At the symmetry plane, at the center of the supply and exhaust channels, it is assumed to
have symmetry, and the boundary condition is
n    Dc  cu   0
(17)
9
For the outlet of the ERV the boundary condition is convective flux, namely
n    Dc   0
2.2.6
(18)
Heat and Mass Transfer Effectiveness
The heat transfer effectiveness of the ERV is a way to measure its ability to transfer
sensible and latent heat. In order to calculate the sensible heat transfer effectiveness, the
change in sensible heat transfer of the supply and exhaust flow will be divided by twice
the maximum sensible heat transfer possible for this system. The sensible heat transfer
effectiveness is shown below:
S 
 s c psus d s Tsi  Tso    ec peued e Teo  Tei 
2   c pud min Tsi  Tei 
(19)
where o in the subscript is the outlet.
For the latent heat transfer effectiveness a similar approach to that described for the
sensible heat transfer effectiveness will be used, except that the latent heat transfer is
used in lieu of the sensible heat transfer. The equation for the latent effectiveness is
shown below:
L 
 sus d s  csi  cso    eue d e  ceo  cei 
2   ud  min  csi  cei 
(20)
10
3. FINITE ELEMENT MODEL
3.1 ERV Dimensions and Parameters
Based on the mathematical model described above, the COMSOL finite element
software will be used to model the ERV and analyze its capacity to transfer sensible and
latent heat. The ERV basic dimensions were taken from Reference [3] and are shown in
Table 1.
Table 1. ERV Basic Dimensions
Length (mm)
Height (mm)
Membrane Height (mm)
250
2
0.1
The inlet conditions for the supply and exhaust streams for both the summer and winter
seasons were obtained from Reference [6]. References [7] and [8] were used to evaluate
the system properties required to solve the differential equations. This data found is
shown in Tables 2 and 3 for the supply and exhaust streams, respectively.
Table 2. Supply Conditions and Properties for Summer and Winter Seasons
Inlet Dry Bulb Temperature (C)
Inlet Dry Bulb Temperature (K)
Inlet Wet Bulb Temperature (C)
Inlet Dry Bulb Temperature (K)
Relative Humidity (%)
Pressure (mbar)
Density (kg/m^3)
Dynamic Viscosity (kg/m*s)
Thermal Conductivity (W/m*K)
Diffusion (m^2/s)
Concentration Air (mol/m^3)
Concentration Water (mol/m^3)
11
Summer
Winter
35.000
1.700
308.150
274.850
26.000
0.600
299.150
273.750
49.340
82.020
56.280
6.910
1.145
1.284
1.895E-05 1.738E-05
0.026
0.024
2.680E-05 2.120E-05
39.550
44.342
1.085
0.248
Table 3. Exhaust Conditions and Properties for Summer and Winter Seasons
Exhaust Dry Bulb Temperature (C)
Exhaust Dry Bulb Temperature (K)
Exhaust Wet Bulb Temperature (C)
Exhaust Wet Bulb Temperature (K)
Relative Humidity (%)
Pressure (mbar)
Density (kg/m^3)
Dynamic Viscosity (kg/m*s)
Thermal Conductivity (W/m*K)
Diffusion (m^2/s)
Concentration Air (mol/m^3)
Concentration Water (mol/m^3)
Summer
Winter
24.000
21.000
297.150
294.150
17.000
14.000
290.150
287.150
49.590
45.866
29.850
24.877
1.188
1.200
1.844E-05 1.830E-05
0.025
0.025
2.484E-05 2.436E-05
41.014
41.432
0.600
0.467
The membrane properties were determined at the average inlet temperatures of the
supply and exhaust streams. Reference [7] was used to determine the other parameters
of the membrane. The diffusion and the thermal conductivity through the membrane
were taken from Reference [4].
Table 4. Membrane Properties and Parameters
Inlet Dry Bulb Temperature (C)
Inlet Dry Bulb Temperature (K)
Inlet Wet Bulb Temperature (C)
Inlet Dry Bulb Temperature (K)
Density (kg/m^3)
Thermal Conductivity (W/m*K)
Diffusion (m^2/s) Membrane
Concentration (mol/m^3)
Diffusion (m^2/s) Air to H20
Summer
Winter
29.500
11.350
302.650
284.500
21.500
7.300
294.650
280.450
1.160
1.240
0.130
0.130
8.000E-06 8.000E-06
40.269
42.838
2.680E-05 2.272E-05
3.2 Fluid Dynamics
The initial stage of the finite elements modeling is to solve the flow through the ERV for
both countercurrent and concurrent flows. The fluid dynamics model that was selected
is the incompressible Navier-Stokes, steady state model in COMSOL. It was assumed
that the wall of the membrane has a no slip condition, and the symmetry plane at the
center of the channels of the supply and exhaust flows are the system boundaries. The
velocity profiles obtained in this stage were used as input to model the heat and mass
transfer in COMSOL.
12
3.3 Heat Transfer
For the heat transfer of the ERV, the conduction and convection, steady state model in
COMSOL was used. In this multiphysics model, COMSOL will solve the heat transfer
in the ERV. It was assumed that the inner boundaries satisfy the continuity condition,
and the symmetry plane at the center of the supply and exhaust channels is defined as
thermal insulation. It will also be assumed that there is no velocity flow through the
membrane. The results from the heat transfer analysis will be used to calculate the
sensible effectiveness of the ERV.
3.4 Mass Transfer
In the final stage of the analysis, the convection and diffusion, steady-state model will be
selected in COMSOL. This model will be used to determine the ability of the ERV to
humidify or dehumidify the air, by providing the following assumed parameters:
diffusion constant for the membrane, and air and vapor concentration in the air for both
supply and exhaust streams. In this model, the inner boundaries will also satisfy the
continuity condition, and the symmetry plane at the center of the supply and exhaust
channels is defined as insulation/symmetry.
3.5 Meshing
To mesh the model, the mapped mesh parameter is used. This provides more flexibility
and the user has better control in preventing the meshing of the model from exceeding
the computer’s memory that can be used with COMSOL. In order to solve the ERV in
COMSOL, quadrilateral meshes were used, and were divided into equal spaces as
defined in Table 5.
Table 5. Number of Elements
d

L
10
10
200
Based on the meshing described, the ERV will have 6000 elements in a quadrilateral
meshed model.
13
4. RESULTS
4.1 Problem Scenarios
In order to evaluate the effectiveness of the ERV, various scenarios will be evaluated.
The first scenario is to evaluate the ERV with velocities through both channels of the
ERV between 1.0 and 1.5 m/s. The ERV will then be evaluated when the supply
velocity is held at 1.5 m/s, while the velocity through the exhaust is varied from 1.0 to
1.5 m/s. The ERV will also be evaluated as the height of the exhaust channel is reduced
to 1.33 x 10-3 m, while the velocity through the channels is set at 1.5 m/s. The final
analysis is to vary the diffusion coefficient through the membrane from 8 x 10-6 to 8 x
10-8 m2/s. The ERV is evaluated for both summer and winter conditions, and for each
season the countercurrent and concurrent flow configuration is analyzed.
4.2 ERV Effectiveness with Equal Supply and Exhaust Flow
4.2.1
Summer Conditions
Through the use of COMSOL and the methods described in section 3, the sensible and
latent effectiveness of the ERV were evaluated for the summer conditions using the data
from Tables 1-4. The effectiveness of the ERV was calculated using equations (19) and
(20). A summary of the results for the different supply and exhaust velocities for the
summer condition is shown in Table 6.
Table 6. Sensible and Latent Effectiveness with Equal Velocities (Summer)
Velocity
(m/s)
1
1.25
1.5
Countercurrent Concurrent Countercurrent Concurrent
S
S
L
L
0.605
0.474
0.609
0.478
0.553
0.451
0.555
0.456
0.509
0.427
0.511
0.433
It can be seen that for both countercurrent and concurrent flow the latent effectiveness of
the ERV is nearly equal to the sensible effectiveness. This is because the diffusion
coefficient in the membrane is relatively high and the control lies in the fluid phases.
The difference between the sensible and latent effectiveness of the ERV varied between
0.002 and 0.006. The effect of changing the diffusion coefficient through the membrane
is evaluated later in the report. The results also show that as the velocity increases, the
14
sensible and latent effectiveness of the ERV decreases. For countercurrent flow, the
sensible effectiveness decreases by 0.096 as the velocities through the channel increase,
while for the concurrent flow the sensible effectiveness decreases by 0.047. The latent
effectiveness of the ERV for countercurrent flow decreases by 0.098, while the
concurrent flow decreases by 0.045.
This occurs because at slower velocities the
residual time increases and a greater heat and mass transfer through the membrane of the
ERV are possible.
Plots for the sensible and latent effectiveness for the countercurrent and concurrent flow
are shown in Figure 7 and 8, respectively.
Sensible Effectiveness
0.620
0.600
0.580
0.560
e
0.540
Countercurrent
0.520
Concurrent
0.500
0.480
0.460
0.440
0.420
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
Speed (m/s)
Figure 7. Summer Sensible Effectiveness for ERVs
Latent Effectiveness
0.620
0.600
0.580
0.560
e
0.540
Countercurrent
0.520
Concurrent
0.500
0.480
0.460
0.440
0.420
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
Speed (m/s)
Figure 8. Summer Latent Effectiveness for ERVs
It can be seen that for both sensible and latent effectiveness the countercurrent flow ERV
is more effective than the concurrent flow configuration, which is expected for the
15
configuration evaluated. The countercurrent flow is more effective because the average
driving force is higher than for the concurrent flow. In the concurrent configuration,
although the temperature and/or concentration differences at the inlet are quite high, they
decrease rapidly and are very small at the channels exit. Therefore, for the same size
ERV the countercurrent flow will have a greater temperature or concentration variation
as the air flows from inlet to outlet, when compared to the concurrent flow. The sensible
effectiveness of the countercurrent flow was 0.082-0.131 better than the concurrent flow
as the channel velocities decreased. For the latent effectiveness, the countercurrent flow
was 0.078-0.138 more effective than the concurrent flow as the channel velocities
decreased.
The full details of the calculation of the sensible and latent effectiveness may be found in
Appendix A of the report.
To better understand the results for the sensible and latent effectiveness of the ERV, the
temperature and concentration profiles for the ERV are plotted. The results obtained for
a channel flow of 1.25 m/s at various axial positions (x = 0, 0.125, and 0.250 m), is
plotted as a function of the vertical distance. The plot of the temperature profile is
shown in Figures 9 and 10 for countercurrent and concurrent flow, respectively.
Temperature vs y for Summer Countercurrent Flow
310
308
306
304
T (K)
x= 0
x = 0.125
x = 0.25
302
300
298
296
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0.0045
y (m)
Figure 9. Countercurrent Flow Temperature Profile at Varying Channel Location
At the inlet of the supply channel the temperature is uniform and equal to 308.15 K.
However, the temperature gradually decreases through the membrane, and a larger
temperature variation can be seen throughout the exhaust channel.
16
The average
temperature at the outlet of the exhaust channel is approximately 303 K. The average
temperature was calculated using COMSOL boundary integration as described in
Appendix A. At the channel axial midpoint (x = 0.125 m) the temperature gradually
decreases from the bottom of the supply channel to the top of the exhaust channel. The
maximum temperature in the supply channel is approximately 306 K, while the
minimum in the exhaust channel is approximately 299 K at the top of the ERV. At the
end of the ERV (x = 0.250 m) a similar behavior to that at x = 0 m can be seen. The
temperature at the exhaust inlet is uniform and equal to 297.15 K, while the average
outlet temperature of the supply channel is 302 K. It can be seen that the maximum
temperature change occurs across the channel for the supply flow at x = 0.250 m, while
for the exhaust flow it occurs at x = 0 m. Therefore, for the countercurrent flow the
temperature changes by approximately 6 K from the inlet to the outlet of the ERV, for
both supply and exhaust flows.
Temperature vs y for Summer Concurrent Flow
310
308
306
x= 0
T (K)
304
x = 0.125
x = 0.25
302
300
298
296
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0.0045
y (m)
Figure 10. Concurrent Flow Temperature Profile at Varying Channel Location
For the concurrent flow the temperature of the supply and exhaust streams at the inlet of
the ERV (x = 0 m) is uniform. However, there is a large temperature variation across the
membrane. The supply and exhaust inlet temperatures for the concurrent flow are
identical to the countercurrent flow, namely 308.15 K and 297.15 K, respectively. At
the channel axial midpoint, the behavior is the same as that described for the
countercurrent flow. However, the temperature variation is not as steep. The maximum
temperature in the supply channel is approximately 305 K and it decreases to 300 K at
17
the symmetry of the exhaust channel. At the outlet of the ERV, the supply and exhaust
outlet temperatures are almost equal.
The supply average outlet temperature is
approximately 303 K, while the exhaust average outlet temperature is approximately 302
K. Therefore, for the concurrent flow the temperature changes from inlet to outlet of the
ERV by approximately 5 K, for both supply and exhaust flows.
The results for the countercurrent and concurrent flows as shown in Figures 8 and 9,
proves that the countercurrent flow is more effective than a concurrent flow ERV. The
concurrent flow results in a 5 K temperature change from inlet to outlet, while an
equivalent size countercurrent flow ERV has a temperature change of 6 K. In addition,
if the length of the ERV was increased, the countercurrent flow outlet temperature still
has a potential to change by 5 K, while the concurrent flow may change by only 0.5 K.
Therefore, the countercurrent flow ERV configuration is more effective to be used in
HVAC system to reduce the cost and energy consumption, over the concurrent flow
configuration.
A similar plot was also created for the concentration profile along the channel as
described above for the temperature profiles. The countercurrent and concurrent flow
cases are shown in Figures 11 and 12, respectively.
Concentration vs y for Summer Countercurrent Flow
1.2
1.1
c (mol/m^3)
1
0.9
x= 0
x = 0.125
x = 0.25
0.8
0.7
0.6
0.5
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0.0045
y (m)
Figure 11. Countercurrent Flow Concentration Profile at Varying Channel
Location
For the countercurrent flow at x = 0 m, the supply inlet concentration is 1.085 mol/m3,
while the exhaust outlet average concentration is approximately 0.865 mol/m3 (Figure
18
11). At the channel axial midpoint, the maximum concentration in the supply channel is
approximately 1.0 mol/m3, while the value at the symmetry plane of the exhaust channel
is approximately 0.7 mol/m3.
At the end of the ERV, the supply average outlet
concentration is approximately 0.821 mol/m3, while the exhaust inlet concentration is
0.6 mol/m3. Therefore, for the supply flow the concentration decreases by 0.264 mol/m3
from inlet to outlet, while the exhaust flow shows an increase of concentration of 0.265
mol/m3.
Concentration vs y for Summer Concurrent Flow
1.2
1.1
c (mol/m^3)
1
0.9
x= 0
x = 0.125
x = 0.25
0.8
0.7
0.6
0.5
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0.0045
y (m)
Figure 12. Concurrent Flow Concentration Profile at Varying Channel Location
At the membrane inlet for the concurrent flow configuration there is a large variation of
concentration across the membrane, similar to that observed for the temperature (Figure
12). At the outlet of the ERV, the supply average concentration is approximately 0.868
mol/m3, and the exhaust average concentration is approximately 0.817 mol/m3.
Therefore, for both supply and exhaust flows it showed a variation of about 0.217
mol/m3 from inlet to outlet of the ERV.
It can be seen that the results for the concentration profiles for both countercurrent and
concurrent flow are similar to that for the temperature profiles.
Therefore, the
countercurrent flow configuration is also more effective in mass transfer than the
concurrent flow configuration.
19
4.2.2
Winter Conditions
The ERV performance at the winter conditions was evaluated through the same
approach as that used for the summer conditions. A summary of the results for the
winter condition is shown in Table 7.
Table 7. Sensible and Latent Effectiveness with Equal Velocities (Winter)
Velocity
(m/s)
1
1.25
1.5
Countercurrent
S
0.590
0.536
0.493
Concurrent
S
0.472
0.445
0.419
Countercurrent
L
0.592
0.537
0.493
Concurrent
L
0.475
0.449
0.423
The results for the winter conditions shows that the latent effectiveness of the ERV for
both countercurrent and concurrent configurations have a similar behavior as that
described for the summer conditions. As the velocity increases, the sensible and latent
effectiveness of the ERV decreases. It can be seen that as the velocity increases for the
countercurrent flow, the sensible and latent effectiveness of the ERV decreases by 0.098
and 0.099, respectively. However, for the concurrent flow, both the sensible and latent
effectiveness decrease by 0.052.
It is also observed that the sensible and latent
effectiveness are slightly lower than that for the winter conditions. The countercurrent
flow sensible effectiveness for the winter condition is 0.016 lower than for the summer
conditions, while the latent effectiveness 0.018 lower. For the concurrent flow winter
conditions, the sensible effectiveness at the highest velocity is 0.008 lower than that for
the summer conditions, while the latent effectiveness is 0.010 lower. This variation is
due to the operating conditions assumed for the supply and exhaust flows during the
winter conditions.
The sensible and latent effectiveness for the countercurrent and concurrent flow are
plotted in Figures 13 and 14, respectively.
20
Sensible Effectiveness
0.600
0.580
0.560

0.540
Countercurrent
0.520
0.500
Concurrent
0.480
0.460
0.440
0.420
0.400
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
Speed (m/s)
Figure 13. Winter Sensible Effectiveness for ERVs

Latent Effectiveness
0.600
0.580
0.560
0.540
0.520
0.500
Countercurrent
Concurrent
0.480
0.460
0.440
0.420
0.400
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
Speed (m/s)
Figure 14. Winter Latent Effectiveness for ERVs
The plot shows that for both sensible and latent effectiveness the countercurrent flow
ERV is more effective than the concurrent flow configuration. This behavior is similar
to that observed for the summer conditions. The sensible effectiveness of countercurrent
flow is between 0.068 and 0.111 higher than the concurrent flow. However, for the
latent effectiveness the countercurrent flow is between 0.066 and 0.109 higher than the
concurrent flow.
The countercurrent and concurrent flow ERV temperature profiles at the channel axial
midpoint, as a function of the vertical distance, and at a speed of 1.25 m/s, for both
summer conditions are shown in Figure 15. It can be observed that, for countercurrent
flow for both summer and winter conditions, the maximum temperature difference
across the channel is slightly greater than for the concurrent flow. This indicates that the
21
heat transfer driving force for the countercurrent flow is greater, which leads to a higher
sensible effectiveness.
Temperature vs y at Midpoint of Channel
310
305
300
Countercurrent Flow Summer
Concurrent Flow Summer
Countercurrent Flow Winter
Concurrent Flow Winter
T (K)
295
290
285
280
275
0
0.001
0.002
0.003
0.004
0.005
y (m)
Figure 15. ERV’s Temperature Profile at x = 1.25 m
The corresponding concentration profiles are plotted in Figure 16. The same behavior as
that for the temperature profiles is observed. This also indicates that the countercurrent
flow is more effective than the concurrent flow configuration.
Concentration vs y at Midpoint of Channel
1.1
1
0.9
T (K)
0.8
Countercurrent Flow Summer
Concurrent Flow Summer
Countercurrent Flow Winter
Concurrent Flow Winter
0.7
0.6
0.5
0.4
0.3
0.2
0
0.001
0.002
0.003
0.004
0.005
y (m)
Figure 16. ERV’s Concentration Profile at x = 1.25 m
22
4.3 ERV Effectiveness with Varying Exhaust Flow
4.3.1
Summer Conditions
Using the same approach as described in section 4.2, the ERV was analyzed for a
constant supply flow of 1.5 m/s, while the exhaust flow was varied from 1 to 1.5 m/s. A
summary of the results for the summer conditions is shown in Table 8.
Table 8. Sensible and Latent Effectiveness at Varying Exhaust Flow (Summer)
Velocity Supply Velocity Exhaust Countercurrent Concurrent Countercurrent Concurrent
(m/s)
(m/s)
S
S
L
L
1.5
1
0.649
0.531
0.648
0.532
1.5
1.25
0.560
0.464
0.561
0.468
1.5
1.5
0.509
0.427
0.511
0.433
The trend is similar to that observed when the supply and exhaust flows were equal
(Table 6). The countercurrent flow effectiveness is higher than the concurrent flow, and
the sensible and latent effectiveness are almost identical. However, as the exhaust flow
decreases the sensible and latent effectiveness increase. Exhausts flow slower than the
supply is more effective than an ERV with equal channel flows, because the temperature
and concentration variation is much higher, which results in a higher heat and mass
transfer.
Analyzing the results, it can be seen that for the countercurrent flow configuration, the
sensible and latent effectiveness increase by 0.140 and 0.137, respectively, as the
velocity of the exhaust flow decreases. For the concurrent flow, the sensible and latent
effectiveness increases by 0.104 and 0.099, respectively, as the velocity decreases.
The temperature profiles for both countercurrent and concurrent configuration at x =
0.125 m and at varying channel flows are shown in Figures 17 and 18, respectively.
23
Temperature vs y for Countercurrent Flow at Varying Exhaust Speed
307
306
305
304
303
T (K)
Exhaust 1.5
Exhaust 1.25
Exhaust 1
302
301
300
299
298
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0.0045
y (m)
Figure 17. Temperature Profile at Varying Exhaust Flows (Countercurrent)
Figure 18. Temperature Profile at Varying Exhaust Flows (Concurrent)
Figures 17 and 18 show that the temperature variations in the supply channel are similar
to those in the different exhaust flows. For the countercurrent flow, the variation of the
maximum supply inlet temperature is approximately 0.4 K as the velocity changes, while
for the concurrent flow, the variation of supply temperature is approximately 0.2 K. The
variation of the maximum temperature at the exhaust channel outlet is approximately 1.0
K as the velocity changes, for both countercurrent and concurrent flow configuration.
Since, a slower exhaust flow shows a greater temperature variation, the sensible
effectiveness will be higher than channels with equal flow.
24
However, a further review of the temperature profiles shows that the increase of sensible
and latent effectiveness is due to the larger temperature variation of the exhaust flow. A
higher temperature variation of the exhaust flow will not reduce the energy
consumptions of the HVAC system, because the exhaust stream will be discharged to the
outside environment. Therefore, to reduce the energy consumption of HVAC systems,
the temperature and concentration variations at the supply airflow must increase. In this
way, the temperature and moisture concentration of the supply airflow will be reduced,
and the size of the cooling coil may also be reduced.
The concentration profiles are plotted for both countercurrent and concurrent flow in
Figures 19 and 20, respectively.
Figure 19. Concentration Profile at Varying Exhaust Flows (Countercurrent)
Figure 20. Concentration Profile at Varying Exhaust Flows (Concurrent)
25
We observe the same outcome as that described for the temperature profiles, where the
countercurrent flow is more effective than the concurrent flow.
The concentration
variation for the countercurrent flow, as the exhaust flow decreases, is approximately
0.01 mol/m3, while for the concurrent flow it is slightly lower than 0.01 mol/m3. It can
also be seen that the concentration gradient is much higher for the exhaust channel as
compared to the supply channel. The concentration variation for both countercurrent
flow and concurrent flow in the supply channel is approximately 0.025 mol/m3. As
described for the temperature profile, the higher sensible and latent effectiveness of the
ERV will not reduce the energy consumption of the HVAC systems, because the exhaust
air is discharged to the outside environment.
The sensible and latent effectiveness for both countercurrent and concurrent flows are
plotted in Figures 21 and 22.
Sensible Effectiveness
0.700
e
0.650
0.600
Countercurrent
0.550
Concurrent
0.500
0.450
0.400
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
Speed (m/s)
Figure 21. Summer Sensible Effectiveness for ERVs with Varying Exhaust Flow
Latent Effectiveness
0.700
0.650
e
0.600
Countercurrent
0.550
Concurrent
0.500
0.450
0.400
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
Speed (m/s)
Figure 22. Summer Latent Effectiveness for ERVs with Varying Exhaust Flow
26
For the velocity range considered, the sensible effectiveness for the countercurrent flow
is between 0.082 and 0.118 more effective than the concurrent flow. However, for the
latent effectiveness, the countercurrent flow is between 0.078 and 0.116 more effective
than the concurrent flow.
4.3.2
Winter Conditions
The winter conditions of the ERV were also evaluated for a constant supply flow of 1.5
m/s while the exhaust flow was varied from 1 to 1.5 m/s. A summary of the results for
the winter condition is shown in Table 9.
Table 9. Sensible and Latent Effectiveness at Varying Exhaust Flow (Winter)
Velocity Supply Velocity Exhaust Countercurrent Concurrent Countercurrent Concurrent
(m/s)
(m/s)
S
S
L
L
1.5
1
0.648
0.539
0.654
0.545
1.5
1.25
0.561
0.472
0.563
0.477
1.5
1.5
0.493
0.419
0.493
0.423
The results for the winter condition are the same as for the summer conditions. Due to
the high diffusion rate through the membrane, the sensible and latent effectiveness for
both countercurrent and concurrent flow are almost equal. The sensible and latent
effectiveness for both countercurrent and concurrent flows differ between 0.000 and
0.006. In addition, for the countercurrent flow the sensible effectiveness decreases by
0.155 as the exhaust velocity increases, while the latent effectiveness decreases by
0.161. However, for the concurrent flow the sensible effectiveness decreases by 0.120
with increasing velocity, while the latent effectiveness decreases by 0.122.
Both the sensible and latent effectiveness of the countercurrent flow are higher than the
concurrent flow as shown in Figures 23 and 24, respectively. The results show that as
the velocity through the exhaust flow decreases, the effectiveness increases when
compared to a system with equal supply and exhaust channel flow.
27
Sensible Effectiveness
0.680

0.630
0.580
Countercurrent
0.530
Concurrent
0.480
0.430
0.380
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
Speed (m/s)
Figure 23. Winter Sensible Effectiveness for ERVs with Varying Exhaust Flow
Latent Effectiveness
0.680
0.630

0.580
Countercurrent
0.530
Concurrent
0.480
0.430
0.380
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
Speed (m/s)
Figure 24. Winter Latent Effectiveness for ERVs with Varying Exhaust Flow
From the plot for of sensible effectiveness, it can be seen that the countercurrent flow is
between 0.074 and 0.109 more efficient than the concurrent flow. On the other hand, the
countercurrent flow shows a latent effectiveness between 0.070 and 0.109 higher than
the concurrent flow.
However, just as described for the summer conditions, the increasing effectiveness is
deceiving, because it does not reduce the energy consumption of the HVAC system.
Therefore, the temperature and concentration profiles must also be evaluated because the
calculation of the sensible and latent effectiveness does not provide the right conclusion
on the overall performance of the ERV. Since the same conclusion can be drawn for the
winter conditions, the temperature and concentration profiles will not be plotted.
28
4.4 ERV Effectiveness as the Exhaust Height is Reduced
4.4.1
Summer Conditions
In this analysis, the summer conditions are evaluated as the channel velocities are held at
1.5 m/s, while the height of the exhaust channel is reduced to 1.33 x 10-3 m. A summary
of the results for the summer condition is shown Table 10.
Table 10. Sensible and Latent Effectiveness with Varying Height (Summer)
d
(m)
1.33E-03
2.00E-03
Velocity Countercurrent Concurrent Countercurrent Concurrent
(m/s)
S
S
L
L
1.5
0.697
0.549
0.693
0.551
1.5
0.509
0.427
0.511
0.433
The results show that, as the height of the exhaust channel is reduced, the sensible and
latent effectiveness for the ERV increase. The sensible and latent effectiveness for the
countercurrent flow can be seen to increase by 0.188 and 0.182, respectively. On the
other hand, the sensible and latent effectiveness of the concurrent flow increase by 0.122
and 0.118, respectively.
The sensible and latent effectiveness for both countercurrent and concurrent flow, are
plotted in Figures 25 and 26, respectively.
Summer Sensible Effectiveness with Varying Height
0.800
0.700
e
0.600
0.500
Countercurrent
0.400
Concurrent
0.300
0.200
0.100
0.000
0.00E+00
5.00E-04
1.00E-03
1.50E-03
2.00E-03
2.50E-03
Height (m)
Figure 25. Summer Sensible Effectiveness with Varying Height
It can be seen that as the height of the exhaust channel is reduced, the sensible
effectiveness is between 0.082 and 0.148 higher than the concurrent flow.
29
Summer Latent Effectiveness with Varying Height
0.800
0.700
0.600
e
0.500
Countercurrent
0.400
Concurrent
0.300
0.200
0.100
0.000
0.00E+00
5.00E-04
1.00E-03
1.50E-03
Height (m)
2.00E-03
2.50E-03
Figure 26. Summer Latent Effectiveness with Varying Height
The plot for the latent effectiveness shows that the countercurrent flow is between 0.078
and 0.141 more efficient than the concurrent flow.
However, as it was determined earlier in section 4.3, the effectiveness of the ERV does
not provide an accurate indication regarding the reduction of the HVAC systems energy
consumption. The temperature profiles of the ERV with varying heights are plotted for
both countercurrent and concurrent flow at the channel axial midpoint as a function of
the vertical distances in Figures 27 and 28, respectively.
Temperature vs y at Varying Height for Countercurrent
307
306
305
T (K)
304
303
Height 2e-3 (Countercurrent)
Height 1.33e-3 (Countercurrent)
302
301
300
299
298
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0.0045
y (m)
Figure 27. Countercurrent Temperature Profiles with Varying Height
30
Temperature vs y at Varying Height for Concurrent
306
305
304
T (K)
303
Height 2e-3 (Concurrent)
Height 1.33e-3 (Concurrent)
302
301
300
299
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0.0045
y (m)
Figure 28. Concurrent Temperature Profiles with Varying Height
The temperature profiles for the countercurrent and concurrent flow show that the
effectiveness increases, due to the larger temperature variations of the exhaust flow. It
can be seen that, for the countercurrent flow, the exhaust average outlet temperature for
the reduced channel height is approximately 302 K, while the average outlet temperature
for equal supply and exhaust channel height is about 300 K. For the concurrent flow, the
average outlet temperature for the reduced and non reduced channel height is almost
equal to the countercurrent flow. Therefore, by reducing the height of the ERV the
reduction of energy consumption for the HVAC systems will not be realized. The
supply channel height should be reduced, in lieu of the exhaust channel.
The concentration profile for the ERV will not be plotted, since a similar profile will be
observed as shown for the temperature profiles.
4.4.2
Winter Conditions
A summary of the results for the summer conditions is shown in Table 11.
Table 11. Sensible and Latent Effectiveness with Varying Height (Winter)
d
(m)
1.33E-03
2.00E-03
Velocity Countercurrent Concurrent Countercurrent Concurrent
(m/s)
S
S
L
L
1.5
0.703
0.566
0.697
0.566
1.5
0.493
0.419
0.493
0.423
31
The result shows the same phenomenon for the winter conditions, as was observed for
the summer conditions. It can be seen that the sensible and latent effectiveness are
almost equal, the difference being in the range 0.000-0.006. For the countercurrent flow,
the sensible effectiveness increases by 0.210 when the exhaust channel height is
reduced, while the latent effectiveness increases by 0.204. The concurrent flow sensible
effectiveness can be seen to increase by 0.147 as the exhaust channel height is reduced,
while the latent effectiveness increases by 0.143.
The countercurrent and concurrent sensible and latent effectiveness are plotted in
Figures 29 and 30, respectively.
Winter Sensible Effectiveness with Varying Height
0.800
0.700
0.600
e
0.500
Countercurrent
0.400
Concurrent
0.300
0.200
0.100
0.000
0.00E+00
5.00E-04
1.00E-03
1.50E-03
2.00E-03
2.50E-03
Height (m)
Figure 29. Winter Sensible Effectiveness with Varying Height
Winter Latent Effectiveness with Varying Height
0.800
0.700
e
0.600
0.500
Countercurrent
0.400
0.300
Concurrent
0.200
0.100
0.000
0.00E+00
5.00E-04
1.00E-03
1.50E-03
2.00E-03
2.50E-03
Height (m)
Figure 30. Winter Latent Effectiveness with Varying Height
From the data for the sensible effectiveness at the different exhaust channel heights, it
follows that the countercurrent flow effectiveness is between 0.074 and 0.137 higher
32
than for concurrent flow. For the latent effectiveness, the countercurrent flow is between
0.070 and 0.131 more effective than the concurrent flow.
4.5 ERV Effectiveness as the Diffusion through the Membrane is
Varied
4.5.1
Summer Conditions
A final analysis is to study the effect of the membrane diffusion coefficient on the
effectiveness of the ERV. A summary of the results for the summer condition is shown
in Table 12.
Table 12. Sensible and Latent Effectiveness with Varying Diffusion (Summer)
Velocity
(m/s)
1.25
1.25
1.25
Diffusion Coefficient Countercurrent Concurrent Countercurrent Concurrent
(m^2/s)
S
S
L
L
8.00E-08
0.553
0.451
0.072
0.072
8.00E-07
0.553
0.451
0.345
0.324
8.00E-06
0.553
0.451
0.555
0.456
It can be seen that the variation of the diffusion coefficient of the membrane does not
affect the sensible effectiveness of the ERV. However, as expected there is a large effect
on the latent effectiveness. The difference between the sensible and latent effectiveness
for the countercurrent flow, varies between by - 0.002 and 0.481 as the diffusion
coefficient decreases.
However, for the concurrent flow the sensible and latent
effectiveness difference varies between - 0.005 and 0.379 as the diffusion coefficient
decreases. It can also be seen that the as the diffusion increases, the latent effectiveness
of the countercurrent flow increases by 0.483, while for the concurrent flow it increases
by 0.384.
Since the sensible effectiveness of the ERV does not change as the diffusion coefficient
is varied, the comparison of the countercurrent and concurrent flow will only be plotted
for the latent effectiveness (Figure 31).
33
Summer Latent Effectiveness for Varying Diffusion Coefficient
0.600
0.500

0.400
Countercurrent
Concurrent
0.300
0.200
0.100
0.000
0.00E+00 1.00E-06 2.00E-06 3.00E-06 4.00E-06 5.00E-06 6.00E-06 7.00E-06 8.00E-06 9.00E-06
Diffusion Coefficient (m^2/s)
Figure 31. Latent Effectiveness with Varying Diffusion (Summer)
It can be observed, that the countercurrent flow configuration has a higher latent
effectiveness than the concurrent flow. The countercurrent flow effectiveness is up to
0.099 higher than the concurrent flow, as the diffusion coefficient of the membrane
increases. The change of the latent effectiveness is also parabolic. Therefore, for a low
diffusion coefficient the countercurrent and concurrent flow effectiveness are equal.
However, as the diffusion increases the countercurrent flow ERV outperforms the
concurrent flow ERV.
4.5.2
Winter Conditions
The summary of the results for the winter conditions is shown in Table 13.
Table 13. Sensible and Latent Effectiveness with Varying Diffusion (Winter)
Velocity
(m/s)
1.25
1.25
1.25
Diffusion Coefficient Countercurrent Cocurrent Countercurrent Cocurrent
(m^2/s)
S
S
L
L
8.00E-08
0.536
0.445
0.073
0.072
8.00E-07
0.536
0.445
0.340
0.320
8.00E-06
0.536
0.445
0.537
0.449
The results show that the sensible effectiveness for winter conditions do not change.
The difference between the sensible and latent effectiveness for the countercurrent flow
ranges from -0.001 to 0.463 as the diffusion coefficient decreases, while for the
concurrent flow it ranges from -0.004 to 0.373. In addition, the latent effectiveness for
34
the countercurrent flow increases by 0.464 as the diffusion increases, while for the
concurrent flow it increases by 0.377.
A comparison of the latent effectiveness for both the countercurrent and concurrent flow
is shown in Figure 32.
Winter Latent Effectiveness for Varying Diffusion Coefficient
0.600
0.500
0.400
e
Countercurrent
0.300
Concurrent
0.200
0.100
0.000
0.00E+00 1.00E-06 2.00E-06 3.00E-06 4.00E-06 5.00E-06 6.00E-06 7.00E-06 8.00E-06 9.00E-06
Diffusion Coefficient (m^2/s)
Figure 32. Latent Effectiveness with Varying Diffusion (Winter)
The plot of the latent effectiveness shows that as the diffusion increases the effectiveness
also increases. It can also be observed that as the diffusion increases, the countercurrent
flow latent effectiveness is up to 0.088 higher than the concurrent flow.
To better understand the effect of varying the diffusion coefficient of the membrane, the
concentration profiles of the countercurrent and concurrent flows are plotted in Figures
33 and 34, respectively.
35
Concentration vs y for Countercurrent Flow (Winter)
0.5
0.45
c (mol/m^3)
0.4
8.00E-06
8.00E-07
8.00E-08
0.35
0.3
0.25
0.2
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0.0045
y (m)
Figure 33. Winter Concentration Profiles for Varying Diffusion (Countercurrent)
Concentration vs y for Concurrent Flow (Winter)
0.5
0.45
c (mol/m^3)
0.4
8.00E-06
8.00E-07
8.00E-08
0.35
0.3
0.25
0.2
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0.0045
y (m)
Figure 34. Winter Concentration Profiles for Varying Diffusion (Concurrent)
The results show that the concentration variation for the lowest diffusion coefficient is
almost zero at the supply and exhaust channel. However, a large concentration variation
exists across the membrane. For the midrange diffusion coefficient, a concentration
variation is much lower than for the highest diffusion coefficient. The variation of
concentration for the supply and exhaust channels, as the diffusion coefficient varies, is
approximately 0.025 mol/m3 for both countercurrent and concurrent flow.
The
concentration profiles also shows that the variation for the countercurrent flow is much
higher than for the concurrent flow.
36
5. CONCLUSION
Based on the results, it was determined that the countercurrent flow ERV is more
effective than a concurrent flow ERV.
This is due to the much higher overall
temperature and/or concentration variation as the air travels from the inlet to the outlet of
the ERV. For the ERV that was modeled in this report, the countercurrent flow also has
the potential to greatly improve its effectiveness if the size of the ERV increases.
However, for the concurrent flow the increase of the sensible and latent effectiveness
will not be achievable, because at the ERV outlet the supply and exhaust channels
streams are almost in equilibrium.
In the analysis of equal channel flows, it can be seen that the ERV effectiveness
increases as the flow through the channel decreases. The effectiveness of the ERV
improved as the velocity decreased, because at a slower speed the air has more time to
transfer its heat and moisture from the supply to the exhaust channel or vice versa.
Therefore, the temperature and concentration variation is much higher. So in an ideal
system, the velocity through an ERV should be reduced as much as possible to reduce
the energy consumption and operating cost of an HVAC system. However, to process
the same air flowrate a very larger ERV system would be required.
When the supply and exhaust flows are not equal, the effectiveness of the ERV increases
as the difference between the flows in the channels increases. However, the increase in
the effectiveness is misleading because the greatest temperature and concentration
variations occurred in the exhaust channel. A higher variation in the exhaust flow results
in a smaller temperature and concentration variation in the supply channel. As a result,
the incoming outside air in the summer conditions will have a higher temperature and
moisture content, which leads to a higher energy consumption to meet the requirements
of the conditioned space. Therefore, while evaluating the performance of the ERV, it is
also important to analyze the temperature and concentration profiles. The decrease of
channel velocity would have been beneficial, if the supply channel had a lower velocity,
in lieu of the exhaust channel. The same disadvantages mentioned above regarding the
unit size would apply in this case.
37
It was also observed that for a constant velocity, as the height of the exhaust channel
decreases, the sensible and latent effectiveness also increase. However, the increase of
the ERV effectiveness was also realized on the exhaust channel. On the other hand, if
we compare the effectiveness of the two cases with the same volumetric flowrate in the
exhaust channel, that is reduced height with high velocity and standard height with
reduced velocity, we observe a higher efficiency for the former due to a higher transfer
rate at the air/membrane interface.
In the final analysis, the variation of the diffusion coefficient was found not to affect the
sensible effectiveness of the ERV.
However, the latent effectiveness of the ERV
increases concurrently with the higher diffusion coefficient through the membrane.
Therefore, using the proper membrane between the channels is important in order to
reduce or increase the relative humidity of the incoming outside air.
In all of the analysis of the ERV, it showed that the summer and winter conditions have
similar performance characteristics. However, for the summer conditions the goal is to
reduce the incoming air temperature and relative humidity, while the opposite is true for
the winter conditions.
It is suggested that for further analysis, the ERV should be compared with a cross flow
configuration. This requires a three dimensional analysis in COMSOL. The results
would allow to determine whether the cross flow ERV is more effective than the
countercurrent or concurrent flow ERV. The cross flow model may also be used to
compare previously analyzed models, such as in the research done by Zhang et al. [2]
and Min et al. [3].
38
6. REFERENCES
[1] “Thermal Comfort”, Wikipedia, September 26, 2010
http://en.wikipedia.org/wiki/Thermal_comfort, Web. November 11, 2010
[2] Zhang, L.Z.; Jiang, Y., Heat and mass transfer in a membrane-based energy recovery
ventilator, Elsevier Ltd., Journal of Membrane Science (1999) 29-38
[3] Min, Jingchun; Su, Ming, Performance analysis of a membrane-based energy
recovery ventilator: Effects of membrane spacing and thickness on the ventilator
performance, Elsevier Ltd., Applied Thermal Engineering 30 (2010) 991-997
[4] Zhang, Li-Zhi, Heat and mass transfer in quasi-counter flow membrane-based total
heat exchanger, Elsevier Ltd., International Journal of Heat and Mass Transfer 53
(2010) 5478-548
[5] COMSOL Multiphysics Modeling Guide, COMSOL Version 3.5a, COMSOL AB,
1998-2008
[6] ANSI/AHRI Standard 1060 2005 Standard for Performance Rating of Air-to-Air
Exchangers for Energy Recovery Ventilation, Air Conditioning, Heating, and
Refrigeration Institute (AHRI), 2005
[7] Cengel, Yunus, Heat and Mass Transfer A Practical Approach, Third Edition,
McGraw-Hill Companies, New York, 2007, Pages 782 and 860
[8] “Psychometric Calculations”, Sugar Engineers,
http://www.sugartech.co.za/psychro/index.php, Web. November 16, 2010
39
7. APPENDIX A
7.1 Sensible and Latent Effectiveness Calculation
In order to calculate the sensible and latent effectiveness, the boundary integration
feature of COMSOL was used to aid in the calculation of the average temperature and
concentration at the supply and exhaust, inlet and outlet boundaries.
The average
temperatures and concentrations were calculated using the following equations,
respectively.
d
Tave 
 T  u  dy
0
(21)
d
 u  dy
0
d
cave 
 c  u  dy
0
(22)
d
 u  dy
0
Using the boundary integration for COMSOL, the countercurrent flow summer
conditions with equal channel flows are summarized in Table 14.
Table 14. Boundary Integration for Countercurrent Flow (Summer)
Countercurrent (T*u) Countercurrent (u) Countercurrent (Tave)
m^2*K/s
m^2/s
K
Supply Inlet
Supply Outlet
Exhaust Inlet
Exhaust Outlet
0.916746
0.900770
0.884021
0.900771
0.002975
0.002977
0.002975
0.002977
The channel velocity for this data was 1.5 m/s.
308.150
302.576
297.150
302.577
The sensible effectiveness was
calculated using equations (19) and (21). A similar approach will be used for the latent
effectiveness except the concentration average will be calculated using equation (22) and
the equation for the latent effectiveness is equation (20).
All the calculations for the sensible and latent effectiveness for the various cases, may be
found in the excel file ERV Properties and Calculations.xls submitted along with this
report.
40