Design and Simulation of a Net Zero Energy Healthy Home

7/27/2019 Design and Simulation of a Net Zero Energy Healthy Home

1/8

2nd

Canadian Solar Buildings Conference

Calgary, June 10 14, 2007

1

DESIGN AND SIMULATION OF A

NET ZERO ENERGY HEALTHY HOME IN MONTREAL

Type of Paper: Refereed

Jos A. Candanedo1, Sevag Pogharian2, Andreas K. Athienitis1, Andr Fry3

1Dept. of Building, Civil and Environmental Eng., Concordia University, Montral, Canada

Tel.: (514)-848-2424, ext. 7080, e-mail:[email protected] Pogharian Design, Montral, Canada

Tel.: (514)-935-5210, Fax: (514)-935-9672, e-mail: [email protected] Inc., Sorel-Tracy, Qubec, Canada

Tel.: (450-780-0608), Fax: (450-780-0062), e-mail: [email protected]

ABSTRACT

In 2006, Canada Mortgage and Housing Corporation

(CMHC) launched the Net Zero Energy Healthy

Housing (NZEHH) competition (recently renamed

EQuilibrium Housing), an initiative to encourage the

design of energy efficient and environmentally

friendly homes. This paper presents the energy

design process ofAlstonvale Net Zero House, one of

the winning designs of this competition. Key features

of the house include a unique passive solar design

with adjustable shutter area, a BIPV/T roof with heat

recovery linked to an air-water heat pump and a large

thermal storage tank connected to a floor heating

system. This paper presents the models and

simulations that were performed to optimise the

proposed design.

INTRODUCTION

The objectives of theEQuilibrium Housing initiative

can be briefly summarised as follows:

1. To develop grid-tied homes with net zero, or nearnet zero, energy consumption over a twelve

month period.

2. To achieve high resource efficiency during itsconstruction and operation.

3. To obtain a high quality indoor environment.4. To reach specified levels of affordability and

marketability.

CMHC formulated a point system attributing

weighting factors to the variables that were

considered for the evaluation of the projects.

Alstonvale Net Zero House, the design solution

discussed in this article, is one of the twelve selected

teams from across Canada to continue to the next

phase: the construction of the house.

Team Montral ZERO, the team that designed

Alstonvale Net Zero House, began work circa August

2006. The team included architects, engineers in

diverse disciplines (civil, mechanical, and electrical),

consultants, water management expert and suppliers.

Most of the team members had previous experience

in the design of energy efficient homes.

The location chosen for the design was the town of

Hudson, Qubec (N4527, W749), a suburb of

Montral. The objective market for this house is a

mid-income Canadian family.

DESIGN PROCESS: INITIAL

APPROACH

Although general guidelines exist for designing an

energy efficient and environmentally friendly house

in a cold climate (good level of insulation, windows

facing south, etc.), an optimal solution requires a

methodical approach, taking numerous design

constraints into consideration.

In a traditional approach to building design all the

professionals involved work sequentially: thearchitect makes a conceptual design, the structural

engineer then performs the structural calculations,

and finally the electrical and mechanical engineer

each make their contributions. However, the

complexity of the design of a net-zero energy home

requires that all the professionals involved

collaborate very closely from the beginning.

One of the requirements of the competition was the

organisation of a design charrette. This word has

been used traditionally by architects to refer to an

intensive brainstorming session in which several

professionals contribute ideas.

Figure 1. Rendering of Alstonvale Net Zero House


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The charrette for the design ofAlstonvale Net Zero

House took place on October 10th

and 11th

2006. The

first architectural sketches and design proposals were

presented during this charrette, and subjected to the

scrutiny of the team members. General consensus

was found on the following recommendations:

1. A high-quality building envelope.2. Implementation of passive heating and cooling

design strategies.

3. Use of technologies taking advantage ofrenewable energies (photovoltaic panels, solar

thermal energy).

4. Energy efficient lighting and appliances.5. Adequate construction materials to guarantee a

healthy indoor environment.

6. Restriction of the dimensions of the house to aminimum size guaranteeing health and comfort of

the occupants.

7. Allowing for future expansions of the house byleaving a section of the interior unfinished.

8. Employing standard construction methods asmuch as possible.

9. Water management strategies to reduce waterwaste.

Other design decisions were not so straightforward.

For instance, several alternatives were discussed

concerning the heating and cooling systems, as well

as the method for distributing the heat in the

building. The general consensus was that building

simulations, also an important requirement in the

competition, were needed to decide on these andother key issues such as the values of several

important parameters (size of the PV system, window

sizes, R-values of the building envelope components,

thermal mass of the building, etc.).

SIMULATIONS

HOT2000 Simulations

CMHC rules required all the contestants to submit

numerical simulations of the performance of the final

design of the house. HOT2000, a program developed

by NRCan as an assessment tool for professionals in

the building industry, was the mandatory tool for two

simulations (http://oee.nrcan.gc.ca). The first one

required that parameters such as the HVAC system,

temperature set-points and hot water consumption be

fixed in order to evaluate the performance of the

building envelope alone. HOT2000 simulations

generate an index called EGH (which stands for

Energuide for Houses, the name of a previous

government incentive program) which is taken as an

indicator of the performance of the building

envelope. The minimum acceptable value of EGH

was 82.

The second simulation was intended to account for

the effect of using renewable energies and alternative

HVAC systems, while keeping adequate levels of

comfort and health for the building occupants, and

guaranteeing that reasonable assumptions of

energy usage were employed. A new index, labelled

EGH*, was calculated using the results of the second

simulation, as follows:

Annual Estimated Energy Consumption* 100 20

Reference Energy ConsumptionEGH

=

(1)

The reference energy consumption, calculated as a

property of a given house, depends on the volume of

the building, the temperature of the tap water, the

number of heating degree-days, and the intended

heating system (i.e., furnace or electric).

The minimum acceptable value of EGH* was 90;

however, the competitions marking system was non-

linear. For instance, EGH* ratings of 90, 95 and 98

correspond to 20%, 45% and 72% of the marks

assigned to the energy performance section. Thiscircumstance made it extremely desirable to reach or

approach EGH* = 100, corresponding to net zero

energy performance.

CMHC also required that the parameters employed

for the second simulation be used to determine the

necessity of a cooling system. No cooling system

was required for cooling loads below 1500 MJ per

year.

The HOT2000 simulations were very useful as a

decision making tool for some critical aspects of the

building envelope. For instance, the size of the

overhangs on the south facing faade had to be

increased in order to obviate the need for an air

conditioning system. It was also found that above a

critical R-value (near R-30) the benefit of increasing

the insulation in the walls was very marginal, and not

economically justifiable because of the cost of

additional insulation in walls.

Custom Simulations

Although the submission of the simulations in

HOT2000 was a competition requirement,

supplementary simulations performed in other

software packages could also be submitted.

HOT2000 cannot represent the performance of new

renewable energy systems (such as BIPV/T) and

passive solar behaviour of the custom is not

accurately represented in this bin method-based

software. Also, HOT2000 can not accurately model

thermal mass and overheating. It was found that

other tools, more suitable for this purpose, were

therefore necessary.

Mathcad 2001, a general purpose mathematical

programming tool, was employed for simulating the

most innovative elements ofAlstonvale Net Zero

House. Mathcad has been used as a tool for building

simulation (Athienitis, 1994; Athienitis, 1999;Tzempelikos, 2005). An advantage of using Mathcad


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is the flexibility that it allows for incorporating new

models.

The forcing functions (i.e., heat source and

temperatures) are based on hourly data of solar

radiation and temperature from a typical

meteorological year file (TMY2) for Montral

(TRNSYS 16). This kind of file includes relatively

extreme conditions, such as very cold or very hot

days, but maintains the yearly average values

corresponding to a given location. Since the time step

used in the calculations was 150 s (2.5 min), linear

interpolation functions were used to estimate

intermediate values between the hourly values

available in the TMY2 file. This procedure generated

a set of approximately 17,300 data points per

variable per month.

The Perez model (Perez et al., 1990) is a useful tool

for calculating solar radiation incident on surfaces

having any given orientation, based on two inputvalues: beam radiation and diffuse horizontal

radiation. The Perez model was used to calculate the

value of solar radiation for the north, south, east and

west walls, and north and south roofs. Sol-air

temperatures were calculated for every wall, based

on the incident solar radiation, the exterior

temperature and the external heat transfer

coefficients.

The area of the windows for every orientation was

introduced as an input to calculate the incoming solar

heat gains.

Figure 2 shows the result of the Mathcad simulation

for the interior temperature, for the first 15 days of

February. Passive solar heat gains account for most

of the reduction of the heating load. Auxiliary

heating is needed on days 35 and 38.

BIPV/T System

Several alternatives were considered as the main

source of heat for the space heating system: a ground

source heat pump, a heat pump linked to a Canadian

tunnel system, and a BIPV/T system, also with a heat

pump.

In the BIPV/T system considered (Figure 3), a cavity

is located underneath the PV panels. At the bottom of

the cavity, there is an absorber plate made of

corrugated metal. When solar radiation allows,

exterior air is drawn upwards, recovering some heat

from the PV panels. A glazed section following the

PV panels is intended to further increase the

temperature of the air in the cavity during the winter.

The hot air exiting the cavity can be used as the heat

source of an air-to-water heat pump. When its

temperature is high enough (typically higher than

40C), it can be passed through an air-to-water heat

exchanger, without resorting to the refrigerationcycle. A large water tank is used as a heat reservoir.

Some benefits of the BIPV/T with glazed air

collector at the top coupled to an air-water heat pump

as compared to a separate geothermal heat pump

were apparent: the cost of digging or drilling is

avoided; advantage is taken of the existing framing

system to support the PV panels; and the cooling

effect due to the circulating air improves theelectrical conversion efficiency of the PV panels.

Obviously, the BIPV/T system can only recover heat

when solar radiation is available. The interaction of

the BIPV/T system with the storage tank, and with

the rest of the house, is critical for assessing the

performance of the system. It was necessary to

determine whether the heat supplied during the

heating season would justify relying on the BIPV/T

system as the primary heating source (the installation

of an auxiliary heating system, employing renewable

fuels, had already been decided).

As previously mentioned, HOT2000 is not the ideal

tool for simulating the capabilities of a BIPV/T

system coupled to a heat pump. The closest module

available in HOT2000 was an air-source heat pump.

Glazed

area

PVpan

els

Airflo

w

Absorbing Plate

Insulation

Figure 3. Conceptual representation of the PV-

thermal system intended for EQuilibrium House #1.

Figure 2. Exterior temperature from TMY2 file

(Toutit) and simulated interior temperature (Tinit),

February 1st to February 15th.

32 34 36 38 40 42 44 4630

20

10

0

10

20

3030

30

Tinit

Toutit

4631 LSTit

day

Absorber Plate


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aspect ratio of the cavity is much larger than unity

(as in this case). The presence of the PV-panel

framing together with the unavoidable surface

imperfections, and the corrugated surface of the

absorbing plate, strongly suggests the use of a

correlation that takes into consideration the surface

friction factor (or equivalent friction factor, sincethe cross section is not uniform) for the

determination of the Nusselt number. For instance, it

is not appropriate to use the common Dittus-Boelter

correlation, which is intended for fully developed

turbulent flow in smooth pipes (Incropera and

DeWitt, 2002; Lienhard and Lienhard, 2006). The

problem is further complicated as the Reynolds

number is in the vicinity of the critical region

between laminar and turbulent flow for most of the

flow rates considered. In the laminar flow regime,

the thermal boundary layer is not fully developed for

most of the cavitys length, and it is reasonable to

expect relatively high heat transfer coefficients.

Depending on the flow regime, two correlations were

used for calculating the heat transfer coefficient in

the cavity (Lienhard and Lienhard, 2006): the

Gnielinski correlation for turbulent regime:

( )

( ) ( )0.5 2/ 3

/ 8 ( 1000) Pr

1 12.7 /8 Pr 1

D

D

f ReNu

f

=

+ (2)

and a correlation for thermally developing, laminar

flow

1/ 3

2/ 30.06683.6570.04

D

GzNu

Gz= + +(3)

In both cases, the hydraulic diameter was used as the

length scale.

The exterior convection heat transfer coefficient on

the surface of the PV panels and glazing is a strong

function of the wind speed. The McAdams

dimensional equation (Duffie and Beckman, 2006),

based on wind speed, was used for these calculations.

Incorporating an additional vertical glazing section

has been suggested as a method for recovering heat

during the winter (Pantic, 2007), when the solaraltitudes are low, while avoiding overheating during

the summer months.

Figure 5 compares the outlet temperature of the

cavity and the exterior temperature, for a particular

configuration with 1.4 m of vertical section for the

first 15 days of February. According to this

simulation, the temperature rise within the cavity can

exceed 50 C.

Figure 6, showing the temperature rise for 3 days in

winter and one day in late spring, illustrates the

effect of the vertical section. During the winter

months, the vertical section keeps warming the air(although the change in slope is hardly noticeable).

However, in June, the presence of the vertical section

reduces the exit temperature.

In general, high temperatures are desirable in order to

have higher COP values when operating the heat

pump, but higher temperatures are usually associated

with low flow rates, and consequently, lower heat

removal from the roof.

For the basic simulations it was assumed that the fan

of the heat pump would drive the flow through the

system, overcoming the pressure drop of its own heatexchanger. The pressure drop across the BIPV/T

system is small due to the low velocities, large cross

sectional area of the cavity and short hydraulic length

of the roof. The design also considered bypassing the

first heat exchanger when the heat pump is running.

A separate variable speed fan is being considered for

controlling the flow rate, with the possibility of also

bypassing the heat pump heat exchanger. The proper

size of the fan is currently under study.

Figure 6. Simulated temperature rise for Julian days

356 (December 22nd

), 4 (January 4th

), 40 (February

10th), and 161 (June 10th).

0 0.93 1.85 2.78 3.7 4.63 5.55 6.48 7.40

10

20

30

40

50

6060

0

T356dn

T4dn

T40dn

T161dn

7.40

coordn

Figure 5. Exterior temperature (TMY2) and

simulated exit temperature of the BIPV/T cavity

from February 1st to February 15th. Roof slope =

45, air speed = 0.5 m/s, cavity height = 2 in, length

of PV section = 5 m, length of glazing section = 1m, length of vertical section = 1.4 m.

32 34 36 38 40 42 44 4630

20

10

0

10

20

30

40

50

60

60

30

Texitit

Toutit

4631 tit

day


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FEATURES OF THE FINAL DESIGN

The final configuration of the mechanical system is

shown in Figure 7. After numerous simulations the

final design ofAlstonvale Net Zero House included:

1. A 5.5 kW photovoltaic system will beinstalled on the roof (45 tilt angle, due

south azimuth). According to a mandatory

RETScreen simulation performed for this

competition, this system should generate

approximately 6745 kWhr annually.

2. A two stage, 3.5 ton heat pump (14 kW)will be used to recover the heat from the

roof and use it to heat the water of a large

storage tank.

3. The temperature of the storage tank for theradiant floor heating system will often be

below the required temperature for domestic

hot water (55 C). For this reason, and toserve as a significant supplementary energy

source, the inclusion of a hydronic solar

collector was deemed appropriate. A 40-gal

tank will be used to store the domestic hot

water. The main storage tank and the small

DHW will be linked through a coil (Figure

7) which will permit, if necessary, heat

transfer both tanks. The use of efficient

faucets and nozzles, and a heat exchanger,

will allow the reduction of the daily use of

domestic hot water to 120 L. The annual

water heating load will be about 9,000 MJ.

4. The annual consumption of the main

electrical appliances has been estimated at

1,435 kWhr (5166 MJ), or 3.93 kWhr per

day. Adding lighting and other loads, the

use of electricity for uses other than

ventilation or heating are about 4,358 kWhr

per year (12130 MJ) or 11.94 kWhr/day.

Moderate use of non-essential loads (stereo,TV, computer, coffee maker, etc.), from a

few minutes to a couple of hours daily, has

been assumed. The house has been designed

for two adults and two children occupying it

50% of the time.

5. In order to improve thermal comfort aradiant floor heating system will be the main

heating method. An additional hydronic

heating coil will provide supplementary

heating to the ventilation air.

6. No air conditioning system will be installed.A solar chimney on the roof of the house,

with an opening to the east, is intended totake advantage of low pressures due to the

winds coming from the west thereby

enhancing the effect of natural convection

to remove hot air from the house.

7. The south facing windows have been set toapproximately 29% of the habitable area

(172 m2), or 43% of the south faade, to

take advantage of solar heat gains. Several

measures are taken to avoid overheating due

to this rather large glazing area: a) a large

thermal mass behind the windows, b)

controllable roller blinds, working under an

Figure 7. Mechanical System of Alstonvale Net Zero House.


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anticipatory control algorithm, and c)

adequately sized overhangs (especially for

summer). The fact that most of the south

facing windows are located in front of a

central hall, also helps to distribute the heat

more evenly in the house.

8. A fireplace or furnace fed by a renewablefuel will provide auxiliary heating.

9. The fresh air intake will pass through a heatrecovery ventilator (HRV).

KEY FINAL RESULTS

The HOT2000 simulations corresponding to the final

design produced the results shown in Table 1.

The space heating consumption corresponds to the

electricity consumption of the heat pump (6,872 MJ)

plus the energy consumed by the fuel of the auxiliary

heating system (2,057 MJ). If ethanol is used as the

fuel, this would correspond to about 90 L per year.

The space heating load is much larger than the

effective energy used to supply it. According to the

HOT2000 simulation, the gross annual heating loadis nearly 89,299 MJ. Passive solar gains through the

south-facing windows reduce this load by providing

54,174 MJ for the space heating load. Internal loads

account for 12,157 MJ. The rest of the heating load

(20,910 MJ) is provided by the heat pump. The

effective COP of the heat pump would then be

20,910/6,872 = 3.04. A sizeable amount of energy,

2,841 MJ per year, is used to drive the ventilation

system.

According to this simulation, domestic water heating

would use approximately 496 MJ of electric energy

per year. The rest of the load is provided by the solarcollector.

The custom simulations provided similar results: the

annual energy consumption is about 25,546 MJ, of

which 9,856 MJ correspond to space heating. The

simulations estimate less electric energy consumption

for space heating, but the use of the auxiliary heating

source (renewable fuel) is larger (more than 5,000

MJ).

Perhaps the most relevant fact from both the custom

and HOT2000 simulations is the realization that they

mutually validate their results of heating loads.

EGH and EGH* ratings

The EGH rating ofAlstonvale Net Zero House was

estimated to be about 85. The EGH* rating obtained

from the HOT2000 simulation was 99.6. Based on

the numbers from the Mathcad simulation, the EGH*

rating would be about 101 (more energy is obtained

from renewable sources than it is consumed annually

by the house).

CONCLUSIONS

This paper presents salient features of the design

process ofAlstonvale Net Zero House. Details of a

supporting simulation in Mathcad are presented and

discussed.

BIPV/T roofs, coupled with a heat pump and a

storage system, are a realistic alternative as a heat

source for Canadian homes.

The construction ofAlstonvale Net Zero House has

been scheduled to begin this year. We are lookingforward to the monitoring of the performance of the

house.

BIPV/T systems have the potential to become an

important technology for residences in the Canadian

climate. Further research is recommended to evaluate

several design alternatives such as the use of multiple

inlets for the air in order to increase the efficiency of

heat recovery and the incorporation of fins of

different shapes. A discussion on the design of solar

air collectors (akin to BIPV/T systems) is presented

by Duffie and Beckman (2006).

The house control system should take into accountflow rate variation (and associated changes in the

exit temperature of the cavity and heat recovered),

heat pump COP, storage tank and indoor air

temperatures. In particular, weather forecast data,

essential for predictive control, would improve the

performance of the system.

ACKNOWLEDGEMENTS

The collaboration of all the members of team

Montreal ZERO is gratefully acknowledged. The

authors would like to express their recognition to

CMHC for its commitment towards energy efficient,green buildings. Financial support of this work was

provided in part by NSERC through the Solar

Buildings Research Network.

NOMENCLATURE

COP Coefficient of Performance of the heat pump

f Darcy-Weisbach friction factor, defined by

this expression:

( )2 / 2

P D

L V

Gz Graetz number ( /RePrD x )

Annual Energy Consumption MJ kWhrSpace Heating 8929 2480.3

Water Heating 496.3 137.9

Ventilation 2841 789.2Lighting/appliances 15688 4357.8

Total 27954.3 7765.1

Table 1. Annual Energy Consumption

(HOT2000 Simulation)


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DNu Local Nusselt number (hD/k)

DNu Average Nusselt number (hD/k)

Pr Prandtl number ( / )

DRe Local Reynolds number ( /VD )

TMY2 Typical meteorological year file (2nd

generation)

REFERENCES

Athienits, A.K.. 1994. Buiding Thermal Analysis,

Mathcad Electronic Book.

Athienitis, A.K. 1999. Thermal Analysis of

Buildings in a Mathematical Programming

Environment and Applications. Building and

Environment, Vol. 34, pp. 401-415.

Duffie, J.A.; Beckman, W.A. 2006. Solar

Engineering of Thermal Processes. Third

Edition. John Wiley & Sons, New Jersey, USA.

Incropera, F.P., DeWitt, D.P. 2002. Fundamentals of

Heat and Mass Transfer. Fifth Edition. John

Wiley & Sons.

Jones, O.C. 1976. An Improvement in the

Calculation of Turbulent Friction in Rectangular

Ducts. ASME Journal of Fluids Engineering,

Vol. 98, pp. 173-181.

Lienhard, J. IV; Lienhard, J. V. 2006. A Heat

Transfer Textbook. Third Edition. Internet

Book. Phlogiston Press.

Pantic, S. 2007. Energy Analysis of Photovoltaic

Thermal System Integrated with Roof and

HVAC System. Masters Thesis. Concordia

University, Montral, Canada.

Perez, R., Ineichen, P., Seals, R. 1990. Modeling

Daylight Availability and Irradiance

Components from Direct and Global Irradiance.

Solar Energy Vol.44, No.5, pp. 271-289.

TRNSYS 16. 2004. Transient Simulation Studio.

Tzempelikos, A. 2005. A Methodology for

Integrated Daylighting and Thermal Analysis ofBuildings. PhD Thesis. Concordia University,

Montral, Canada.

www.builditsolar.com/Projects/SpaceHeating/SolarS

hed/solarshed.htm

http://oee.nrcan.gc.ca

www.solarhouse.com

www.stsscoinc.com

APPENDIX: THERMAL NETWORKS

The mathematical model developed in Mathcad 2001

makes use of the thermal network concept, so named

because of its reliance on analogies between heat

transfer phenomena and electric circuits. Electric

potential and current are analogous to temperaturedifference and heat flux. Similarly to electric

resistors, thermal resistances provide a good model

to calculate the heat flux between points at different

temperatures. Thermal capacitances adequately

describe the heat storage capacity of materials and

the resulting time lag between heat flux and their

temperature change. Despite the simplicity of their

operating principle, thermal networks are a powerful

tool for treating complex heat transfer problems.

Electric circuit analysis techniques can be transposed

and applied for simplifying and solving thermal

networks.

Finite volume numerical analysis methods can also

be used to study the transient response of the thermal

network to forcing functions, such as exterior or

interior heat sources, or temperature variations. The

heat balance principle is used to write the equations.

For the Alstonvale Net Zero House simulations, a

fully explicit scheme was used: the temperatures

corresponding to a future time step (p+1) are

functions only of the temperatures of the current time

step (p). For instance, for the nodes having a thermal

capacitance the energy balance equation in thermal

network form is given by (Athienitis, 1994):

( )11

np p p p p

i i ij j i i

ji

tT Q U T T T

C

+

=

= + +

(4)

Where Ti refers to the temperature of the node, Cito

its capacitance, Qi represents all the incoming heat

sources, Tj refers to the temperature of adjacent

nodes, andUij refers to the conductances between Ti

andTj.

Figure 8 shows a typical thermal sub-network

employed for the construction of the larger model for

the entire building. Node 15 and 16 represent the

state of two planes in the wall. C15 represents thethermal inertia of the wall. Thermal resistances

connect nodes 15 and 16, and node 15 with the

exterior. A current source corresponds to the solar

the solar heat gains incident on the walls interior

surface, while a voltage source stands for the sol-

air temperature on the exterior.

R15_16 1516 R15_o

C15 Teo_northS16

Figure 8. Typical network model for a wall.

Design and Simulation of a Net Zero Energy Healthy Home

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