Download Design Feasibility Analysis * Stack Ventilation

Design Feasibility Analysis – Stack Ventilation
Straw Bale Eco Center
Architecture 273: Environmental Systems 1
July 11, 2011
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Austin Hesler
Julie Musial
Jeannie Sego
Introduction
Project Context
The past two decades have seen a major paradigm shift, in terms of design and architecture. Traditional
viewpoints of materialistic enterprises, such as having the latest and greatest houses, and cars with no
consideration of environmental impact have given way to a new idealism involving movement back to
nature. Simply conserving or preserving environments is not enough. Land pollution, air pollution, and
water pollution from the building arts has eroded and degraded the environment to the verge of a
breaking point. Therefore, it is the duty of designers and architects to promote social change through
adaption of design intent and processes to enhance and regenerate elements of ecosystems lost in
“traditional” building contexts.
The Straw-Bale Eco Center educational facility, located on Cooper Farm Field Station in Muncie, Indiana
promotes sustainable design by emphasizing the balance between built environment and natural
ecosystems. This EPA p3 award-winning project is composed of two distinct phases. Phase 1, already
complete, focused on the construction of a main educational building and greenhouse in addition to
integrating the “water-wastewater-energy-building-landscape-system”1 components of the regenerative
design. Phase 2, now underway, focuses on final implementation of the integrated systems started
during Phase 1 as well as the construction of a secondary demonstration/efficiency apartment space.
“Water-wastewater-energy-building-landscape-system” refers to the four distinct regenerative systems
utilized throughout the Eco Center site to promote symbiosis of natural and artificial elements. The
“energy system” features both active and passive technologies. This feasibility study will outline a
proposed plan for passive cooling system in the form of stack ventilation. The proposed stack ventilation
system addresses indoor air quality (IAQ), air flow, and diminishes the need for mechanical intervention,
preserving the client’s goal of net-zero energy and carbon-neutral site.
Background
Similar to any design project, the Straw-Bale Eco Center began with the identification of the team
players. It was a collaboration of students from multiple disciplines including Architecture, Landscape
Architecture, Interior Design, Pre-Journalism, Political Science, and Natural Resources & Environmental
Management. Two faculty members also cooperated on the project. After the building permits were
received and established, a work schedule was formed and the project was designed in two weeks.
Throughout the design, it was crucial to de-emphasize the significance of any single component, in order
to better understand sustainable strategies as a collective and connected set of decisions.
1
Gray, Timothy. Straw-Bale Eco Center. Ball State University, 2008. Web. 19 May 2011. http://ecocenter.iweb.bsu.edu/index.htm
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Once the construction of Phase 1 was complete, the group focused on community outreach initiatives to
promote the Eco Center. They also participated in part of the campus tours for the 2007 “Greening of
the Campus” Conference at Ball State University. In addition to the endeavors of the Straw Bale Eco
project, the group also began tours of the facility for Delaware County K-12 students as part of the
“Christy Woods Nature Tour”.
The passive system to be examined in this document is a naturally occurring phenomenon which has
been utilized successfully in a wide range of buildings for centuries. Examples of stack vent systems can
be seen in many forms of vernacular architecture, including schools, churches, and homes 2. Historically,
stack ventilation was used as a main source of thermal comfort in interior spaces due to lack of
advanced cooling systems Today, a recurrence in “traditional” cooling methods arises as a result of
renewed interest in environmental phenomena as well as awareness toward carbon-neutral and netzero energy passive systems.
Focus – Proposal
This proposal’s focus is to outline a potential design solution for a stack ventilation cooling system for
the Straw Bale Eco Center’s new educational and residential space. The design will intend to achieve a
net-zero energy and carbon neutral building while providing thermal comfort using minimal energy. The
implementation of stacks will attempt to achieve three-main purpose/intents including: maintenance of
indoor air quality, removal of heat, and providing air movement. The proposal will outline a singular
system alternative with individual component permutations, providing multiple options for the client.
The analysis of the system alternative will address effectiveness with respect to issues such as relative
humidity, adaptive comfort potential, dual-building function, and sustained comfort.
Deliverables
Upon completion of the ten-week feasibility study, a final report will be presented. The five part study
will outline the proposed stack ventilation solution to passive cooling in a fully comprehensive system
explanation, statement of building and user needs, estimation of site resources, overall solution
implementation feasibility, and project team recommendations. The final report will include the cooling
systems’ impact upon the net-zero energy as well as the carbon-neutral goals of the Straw-Bale Eco
Center.
System Explanation
2
Baker, Nick. Research Associate. Royal Institute for British Architects. University of Cambridge, Martin Centre. 2011.
http://www.architecture.com/SustainabilityHub/Designstrategies/Air/1-2-1-2-Naturalventilation-stackventilation.aspx
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Stack ventilation, a type of natural ventilation, is designed to exhaust warm air out of a building while
pulling cool air into the building. Stack ventilation systems rely on air buoyancy, a function of
temperature differences. When analyzing a stack system it is important to review main components: air
temperature, pressure, and density.
Air Temperature, Pressure, and Density
Stack ventilation is a naturally occurring phenomenon
produced by the stratification of air pressure,
temperature, and density, also known as the thermal
buoyancy effect3. In essence, air rises as it becomes
warmer and less dense. The warmer air “buoys”, hence
the name, to the top of an interior space while the
cooler, denser air sinks to the bottom.
Illustrated in Fig. 1, colder, denser air, indicated by the
blue arrows, flows into the system and sinks to the
lower part of the structure. As it warms, less dense air,
indicated by the red arrows, rises to the top or the
structure. General air flow patterns for stack
ventilation will be addressed in the following Systems
Considerations section.
Figure 1. Stack Ventilation Diagram
The air temperature, pressure, and density within a stack system must work with certain parameters in
order to be effective. For instance, the outdoor air temperature must be cooler than the desired interior
temperature to function properly. This leaves a small opportunity for functionality of a stack ventilation
system in the summer months.
System Considerations
There are three types of stack vent systems to be considered. Option 1 creates a stack out of the
occupied space. Option 2 is to create a stack above the occupied space, as in an atrium. Option 3 is to
remove the stack from the occupied space altogether. Each of the three options has its benefits and
drawbacks. For the first option, if the stack is in the occupied space, the occupants are directly in the
path of the flow created by the stack vent, which can increase thermal comfort. Since occupants are
placed directly in the system, the thermal comfort range is quite small because the air temperature of
this space needs to be raised in order for the system to function. In the second option, the occupants
are still within the stack space but it removes them directly from the system. Since hot air is less dense,
this air will be higher than the level of the occupants, mitigating the effects of raising the temperature.
The last option that removes the stack from the occupied space provides the most benefit with the least
amount of drawbacks because an increased temperature difference can be created to enhance the
effects of the thermal buoyancy without effecting the thermal comfort of the occupants.
3
Baker, Nick. Research Associate. Royal Institute for British Architects. University of Cambridge, Martin Centre. 2011.
http://www.architecture.com/SustainabilityHub/Designstrategies/Air/1-2-1-2-Naturalventilation-stackventilation.aspx
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Upon the determination of the feasibility of the stack ventilation system, careful considerations of the
physical features of the final design need to be analyzed. First of all, the building needs one or more
stacks to actually evacuate the air from the building. The more floors in the building, the more vents
may be needed for the ventilation to work properly. Next, a point of access will need to be configured
to let in the cooler air. The location of the inlet and outlet sizing are critical to the system performance.
The location, quantity, and size can affect the building’s security, façade appearance, and the quality of
incoming air. The location should not be placed in a spot on the building that may allow the vent to suck
in polluted air, such as a designated smoking area. Possibly the most important factor is the difference in
air temperature inside to the air temperature outside. For the system to work, the outside air must be
cooler than the inside air. The exterior finishes and landscaping can be utilized in order to lower the
incoming air temperature. To conclude, the cooler air needs to enter the building through a point of
access that is low on the vertical surface and has to exit through a stack or vent that is high on the
vertical surface.
System Capacity
There are a couple of ways to increase the efficiency of the stack ventilation system. One way is to
increase the height of the stack. Figure 2 displays the correlation of stack area and height with cooling
capacity. In essence, the correlation implies a stack
height that is twice the height of the building will
provide more cooling capacity. Another way the
system can increase capacity is to utilize more of the
building to draw in air. Using the floor, for example,
can create an indirect access for air. In Mechanical
and Electrical Equipment for Buildings, the Olivier
Theatre at the Bedales School utilizes a crawl space
underneath the theater to cool the space4. Another
way eliminates the possibility of a net-zero building.
This method utilizes a fan that can be run to cool the
space or used to warm the space. For a fan to control
the temperature of a space, the direction of the fan
can be manipulated in order to control how cool and
warm air interacts in a space. Even though this may
Figure 1. Relationship between Stack Area and Cooling Potential
eliminate the goal of net-zero, it does assist in keeping
a more steady temperature than solely relying on
natural ventilation.
Statement of Needs
Site/Building Requirements
To determine the feasibility of the stack
ventilation system, the required cooling load of
the space needs to be estimated. The 1997
ASHRAE Fundamentals handbook gives the
following relationship: Q = 60 * Cd * A * sqrt(2 * g
* (Hn - Hb) * ((Ti - To) / Ti))5. Specific square
footages were not given for the educational and
residential space within the building, therefore
the total space is assumed to be educational for
Grondzik, Kwok, Stein, and Reynolds, “Stack Effect”, Mechanical and Electrical Equipment for Buildings, edition New Jersey: John Wiley and Sons Inc., 2010.
130.
5
See Appendix A, page 8, for variable definitions
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Figure 3. ASHRAE Thermal Comfort Chart
the purposes of the calculation. The cooling load factor to be used is 1.5(CFM/SF) 6, for educational
spaces (not to be averaged with the cooling load factor of 1 for residential spaces). To calculate the
required cooling load, the total square footage of 1,500 SF is multiplied by the cooling load factor of 1.5
CFM/SF. The estimated cooling load for the building is 2,250 cfm. The stack ventilation system must
provide this amount of cfm to be feasible.
User Requirements
Although thermal comfort is a condition of mind, design tools can be utilized to “predict” the thermal
comfort of the occupants of the space, which also need to be considered in this analysis. In the ASHRAE
Thermal Comfort Chart, the 0.5 clo (clothing value) should be used since the system feasibility is being
analyzed for the summer months. According to the chart, users will be comfortable at an operable
temperature between 77ᵒ – 82ᵒF with a relative humidity between 10% and 60%.
Estimation of Resources
An estimation of resources must be analyzed in order to document all site aspects that can contribute to
a stack ventilation system. Upon the analysis of these factors, recommendations can be made to
implement the system. Contributing factors potentially making this system possible include site
information, relative humidity, summer dry bulb temperature and solar radiation. See the Site Resources
in Appendix B.
For stack ventilation to function, the outdoor air temperature must be cooler than the indoor air
temperature. According to United States Department of Energy, Energy Efficiency and Renewable
Energy7, the charts below illustrate the dry bulb temperature, extreme temperatures, and relative
humidity for the summer months.
Month
Maximum
Minimum
Daily Average
June
93.2
48.2
68.18
July
96.8
48.92
73.22
August
91.4
51.8
73.22
September
93.2
42.8
68.5
Table 1.1 Monthly Statistics for Dry Bulb Temperature (in degrees Fahrenheit)
Cubic feet per minute/Square feet
7
United States Department of Energy, Energy Efficiency and Renewable Energy, Weather Data, Region 4, USA.
http://apps1.eere.energy.gov/buildings/energyplus/weatherdata/4_north_and_central_america_wmo_region_4/1_usa/USA_IN_Delaware.Count
y-Johnson.Field.725336_TMY3.stat
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Number of Days
June
July
August
September
Max >= 89.6
2
4
3
5
Max <= 32
Min <= 32
Min <= 0
Table 1.2 Monthly Statistics for Extreme Temperatures (in degrees Fahrenheit),
Month
June
July
August
September
Maximum
Minimum
Daily Average
100
18
68
100
38
74
100
31
78
100
27
69
Table 1.3 Monthly Statistics for Relative Humidity (%)
Based upon the information in Table 1.1-1.3, the outdoor air temperatures for summer months are
relatively high, creating smaller opportunities for utilizing the stack ventilation system. In addition, to a
small temperature range, relative humidity plays a large role in occupant thermal comfort. Table 1.3
indicates humidity ranges from 68% to 78%. With relative humidity at such a high level, the positive
effects of the proposed stack system are mitigated with regards to thermal comfort. No occupant will be
thermally comfortable with such a large percentage of humidity, no matter the air flow rate provided by
a system.
While air flow rates are potentially mitigated by high levels of humidity, they may be supplemented by
solar radiation. The proposed site contains trees bordering the western edge of the property, meaning
more than 75% of the site is open to solar radiation. The radiation available on the site during the
summer months may be used to increase the effects of the stack system. The solar radiation may be
used to heat the air within the stack, increasing the thermal buoyancy of the system.
Implementation Feasibility
Feasibility of a stack system in this design would be appropriate from two viewpoints: temperature
difference and the cfm provided via the stack ventilation system. When implementing this design one
must recognize the fact there must be at least a three degree difference between the outdoor air
temperature and the indoor air temperature to maintain proper air flow rates. While it might be difficult
to attain this temperature differential during the summer months, utilizing solar radiation to increase
the temperature difference between indoor and outdoor air temperature will alleviate this potential
problem. The system needs no outside forces to act in order to make it work. Air stratification which, in
essence, “powers” the system happens naturally all year round including windless summer days.
The recommended cooling load factor for an educational space is 1.5 cfm/SF, meaning the total amount
of cfm required for the Straw Bale Eco Center’s proposed space needs to be at least 2,250 cfm. It may be
assumed that maintaining the cfm flow rate within these parameters will help keep thermal comfort for
occupants within the 77ᵒ – 82ᵒF degree range recommended by ASHRAE.
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Recommendations
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Based upon the two aforementioned viewpoints, it is feasible to integrate a stack ventilation system in
the Phase 2 building design. While stack ventilation systems are generally weaker passive cooling
systems and are challenging to execute, they can be used to augment existing systems or may be
enhanced to perform at a higher operating capacity with a few general recommendations.
The overall recommendation of the project team is to implement a stack system for the Straw-Bale Eco
Center’s new construction project. The proposed stack system should utilize either “atrium style” stack
system (Option 2) or a separated stack system (Option 3) mentioned in the System Explanation section.
Either of these two options keeps occupants from being directly involved in the stack, essentially
allowing for greater temperature difference within the stack while minimally affecting thermal comfort.
Of the two options expressed, a separated stack system (Option 3) might be the most effective option in
relation to the scale of the project. In terms of scale appropriateness this option would match existing
buildings on the site and not create a towering space which would overshadow the other structures. In
addition, a stack system separated from the occupied space will decrease any unwanted change in
thermal comfort while increasing the effects of thermal buoyancy. However, instead of being
architecturally integrated to the occupied space, the system will be separated and will serve only one
function of cooling operations.
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Option 2 would architecturally integrate the system into the occupied interior space, but would elevate
it above the plane at which occupation occurs. The elevation of the system would allow for greater
temperature differentials to enhance buoyancy with minimal change to the thermal comfort in the
occupied space. Such changes in the vertical height could cause the scale of this building to be
disproportionate in height to the other buildings on the site. As mentioned in the System Capacity
section, a new stack would need to be twice as tall as the space it is attempting to cool. For example, a
ten foot tall structure would need a twenty foot tall stack in order to create desired cfm rates. In using
the ASHRAE equation for estimating the air flow rate of a stack ventilation system, the resulting cfm
should be higher than or equal to 2,250. The recommended square footage of the opening of the vent
should be 20 to achieve the desired cfm.
References
ASHRAE Handbook of Fundamentals. American Society of Heating, Refrigerating, and Air-Conditioning Engineers
(ASHRAE). Atlanta, GA: 2001
Baker, Nick. Royal Institute for British Architects. University of Cambridge, Martin Centre. 2011.
http://www.architecture.com/SustainabilityHub/Designstrategies/Air/1-2-1-2-Naturalventilationstackventilation.aspx
Gray, Timothy. Straw-Bale Eco Center. Ball State University, 2008. Web. 19 May 2011.
< http://ecocenter.iweb.bsu.edu/index.htm>
Grondzik, Walter T., Alison G. Kwok, Benjamin Stein, John S. Reynolds. Mechanical and Electrical Equipment for
Buildings. 11th ed. New Jersey: John Wiley and Sons Inc., 2010. 130-131.
Konzo, Seichi. The Quiet Indoor Revolution Illinois: University of Illinois, 1992, 130-131.
National Institute of Building Sciences. “Whole Building Design Guide”. Natural Ventilation resource
http://www.wbdg.org/resources/naturalventilation.php
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United States Department of Energy, Energy Efficiency and Renewable Energy, Weather Data, Region 4, USA.
http://apps1.eere.energy.gov/buildings/energyplus/weatherdata/4_north_and_central_america_wmo_region_4/
1_usa/USA_IN_Delaware.County-Johnson.Field.725336_TMY3.stat
Appendices
Appendix A
The 1997 ASHRAE Fundamentals handbook gives the following relationship:
Q = 60 * Cd * A * sqrt(2 * g * (Hn - Hb) * ((Ti - To) / Ti))
where
Q = flow rate in cfm,
Cd = 0.65 (for unobstructed openings),
A = opening area, square feet,
Ti = indoor temp (Rankine),
To = outdoor temp (Rankine),
Hn = height of "neutral pressure point" (for simple systems, assume
1/2 way between top and bottom openings).
Hb = height of bottom opening
g = gravity.
Appendix B
Site Resources
The site is located at 5842 Bethel Ave., Muncie, Indiana. Most of the site is either old farm land or heavily
wooded. The heavily wooded areas are a mix of both deciduous and evergreen trees. The only structures on the
site are a windmill, the Phase One building, a storage building, and a ranch style house. Figure 1 is a site plan
showing all the items listed above. The site is surrounded by a wooded area to the north, a residential
neighborhood and church to the west, a busy road to the south with First Merchants Corporation across the road,
and a smaller wooded area to the east. Figure 1 also shows objects for potential summer shading, potential
winter wind block, potential summer wind block, and potential noise sources.
Figure 2 displays good, bad, and neutral areas on the site for cross ventilation passive cooling. The good areas are
places on the site that have no objects blocking any wind flow during the summer but has objects blocking wind
flow during the winter. The neutral areas are places on the site that have small amounts of objects blocking wind
and/or have noise from high travel routes during all seasons. Whereas the bad areas are places that have large
amounts of wind blocking objects during the summer.
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Figure 4 displays good, bad, and neutral areas on the site for indirect gain (Trombe wall) passive heating. The
good areas are places on the site that have objects for summer shading but not as much for winter shading. The
neutral areas are places on the site that have objects for summer morning shading but no afternoon shading.
Whereas the bad areas are places that have no objects for shading at all. Figures 5-8 are actual pictures of the
site.
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Figure 3 displays good, bad, and neutral areas on the site for stack ventilation passive cooling. The good areas are
places on the site that have no shading or wind blocking objects. The neutral areas are places on the site that
have some wind blocking objects but no shading objects. Whereas the bad areas are places on the site that have
large amounts of shading and wind blocking objects.
Figure 9 displays a three-dimensional plan for sun orientation in
regards to a potential spot for Phase Two building. Each yellow
circle and arch depicts a pathway in which the sun would move
at different times during the year. The potential spot can be
seen in Figure 5 and the building depicted can be seen in Figure
7.
Figure 2
Site Plan
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Stack Ventilation Passive Cooling
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Figure 3