Summary


Passive Ventilation (Natural Ventilation) is a method of air circulation reliant on harnessing air and wind
pressure differences to distribute fresh air through a structure. Ventilation systems are a key part of a building's internal structure because they ensure that internal air remains comfortable to building users and does not become stagnant or polluted. In most modern day buildings, active ventilation systems such as air conditioners are used to circulate this air. Unlike traditional air conditioners and other mechanical air circulation devices, true passive systems use no electricity to operate. Instead, wind, air pressures, and other heat transfer principles are used to move air around a structure.

Passive ventilation can be incorporated into proposed building designs or the principles can be used to update existing structures. Ultimately, the goal of passive ventilation is to keep a structure at a comfortable temperature in a cheaper and more environmentally friendly manner. Prior to modern times, namely before the widespread use of electricity, passive cooling techniques were the primary method used to circulate air through structures. Today, many consumers are choosing to return to these practices to reduce their energy footprint. Many modern passive ventilation designs can be incorporated in buildings without completely compromising the control and comfort mechanical air movers provide.

History


Before the invention of electricity and air conditioners, passive ventilation was the primary way to circulate air throughout a building. As early as structures existed, passive ventilation techniques have been used. Historical systems were used to distribute both hot and cool air though a structure.
Although passive ventilation was used throughout the ancient world, societies developed different methods based on their individual climate, natural resources, and ingenuity. In societies from early Native Americans to Vikings to Jordon period (3rd-1st Century BCE) Japanese people, many structures were build such that significant portions of the building were dug down into the earth. These structures relied on the thermal properties of the earth and soil, specifically its heat absorption, to keep the building at lowest extreme temperatures than the outdoor air. The earth surrounding the structure's walls acted as a heat sink to absorb extra heat from the building or increase the indoor temperature by through diffusion of heat into the structure. This technique of passive temperature control does not rely so much on air circulation but more on heat diffusion; however, internal air circulation is necessary to distribute conditioned air throughout the indoor structure [1] .
In addition to buildings dug in the earth, some ancient buildings were also raised off the ground. Throughout ancient India, buildings were raised up on stilts. This building style created an arid patio area below the building that could be used for outdoor recreation and help people keep out of the overwhelming sun. This type of ventilation is primarily a cooling mechanism and therefore most effective in tropical regions[2] .
badgir-kaveh-farrokh-yazd.jpeg
Traditional Middle Eastern buildings were designed with badgirs, translated as wind catchers. These structures extended from the buildings several feet above the roof to capture wind and funnel it through the rest of the structure. This structure brought fresh air into the building and also allowed for a good amount of air flow to cool off the structure. Many badgirs still dot the skyline of the Middle East including the historic city of Yazd, Iran. The wind catchers shown in the photo to the right are from modern day Yazd. The mechanisms used by badgirs will be further described in Wind Ventilation, a subsection of the Modern Passive Ventilation Methods section. In addition to using wind catchers, many historic Middle Eastern structures also employed Solar Chimneys. Instead of allowing wind to flow into these and down into the building, solar chimneys are intended to use pressure differentials to pull air up into the chimney and funnel warm air up through the chimney. The mechanics behind these structure will be further detailed in the Stack Ventilation section[3] .
Parthenon Ventilation.jpeg
In ancient Rome, passive ventilation methods were used to keep breezes moving throughout a building to creating the illusion of cooler temperatures. Hot air would leave through a vent in the celling and then cool air would come in through open doorways and windows at the ground floor of the building. In other words, fresh air would be blown in while the stagnant polluted air would be pushed out the outlet. This method of ventilation is shown to the left in the depiction of air flow in the Pantheon in Rome, Italy[4] .



Modern Passive Ventilation Methods


The modern methods of passive cooling do not differ in principle but rather in design from ancient methods. Wind and pressure differentials are still employed to move air throughout a structure, but some improvements have been made to all for the necessary air flow in less noticeable ways. For example, instead of building a structure that is completely open with no doors or window pains, strategically places fans can be used to direct air flow. Although a design like this would not be 100% passive ventilation, the use of just fans significantly reduces energy consumption compared to air conditioning systems and uses unconditioned air flow[5] . Additionally, some currently used passive ventilation methods do still rely on open doors and windows to allow for air flow.

Modern passive ventilation methods are generally both sustainable and economical; however, the biggest draw back is often a lack of precise indoor climate control that users of traditional HVAC systems are accustom to. Additionally, some general limitation exist across all ventilation methods. Although passive ventilation can be used in a wide range of climates and conditions, the system works best when building are less than 45 ft in wide perpendicular to the main wind streams, have open floor plans, are located in low humidity regions and exposed to moderate climates moderate climate (0-90°F)[6] .

Wind Ventilation

Wind ventilation is most common method for passive ventilation. This method requires little technology or additional structures and is essentially what many homeowners occasionally do already, opening the windows. In any house, opening the windows creates wind ventilation but structures intended to be primarily wind ventilated are designed to maximize this ventilation and create air flow throughout the structure, not only in rooms with windows facing the prevailing wind direction. In structures specifically designed for passive ventilation, windows and doors are strategically located so that when the breeze enters the structure is makes a path around the whole room before leaving. Typically structures reliant on this kind of ventilation will account for the geographic location of the structure and the wind speed in that area to determine room sizes, building dimensions, and open surface area (often windows) needed to allow adequate air flow. Additionally, to maximize the ventilation provided by this method, the inlet and outlet's for the air should be positioned such that their is the maximum pressure differential between the two openings. In other words, to get the best air flow, the vertical distance between the inflow and outflow locations should be maximized.[7] .
Venting.jpg
Additional factors to consider are changing the type and size of inlets and outlets and laying out a path with limited obstacles for air to flow through. The main ventilation windows should be located on the wall perpendicular to the direction of prevailing winds of the area. The direction of the prevailing wind is location dependent and can be found using the national weather services databases. This ventilation technique can become incredibly effective if the expected air flow carefully considered and the structure laid out accordingly. An entire house can be wind ventilated by carefully planning air flow inlets and outlets. Larger structures can also wind ventilated but the farther the room gets from the building side exposed to the prevailing winds, the less air flow; therefore, building depth is a limiting factor for wind ventilation. Proper use of this technique can be seen in the image to the right[8] .
In addition to traditional window ventilated structures, badgirs or wind catchers use similar principles to operate. In the case of wind catchers, the structure that is catching the wind extends high into the sky above obstructions to pick up wind and then funnel this air flow through the structure. They are an effective method of using wind ventilation in areas where obstructions block wind from reaching the sides of the building or where wind speeds are not sufficient at the ground floor to ventilate the structures. However wind catchers do require additional elements, namely the stacks themselves to be built.
Advantages
Disadvantages
- Easy to build without major modifications to traditional building plans
- Hard to use as the only ventilation method (it is not always windy)
- Can be used in any climate part of the year
- Not suitable for many climates year round
- Great for residential buildings
- Hard to effectively incorporate in large buildings
- Relatively cheap, do not require significantly altered building designs
- Requires open floor plan with few obstacles in wind's path


Stack Ventilation (Buoyancy)

Stack ventilation, also called buoyancy ventilation, relies on fluid density principles to operate. Since hot air is less dense than cooler air, in a room with various temperature air, the hottest air rises[9] . Stack ventilation makes use of this temperature differential and funnels the warmer air that has risen through an opening at the highest point of the building and out of the structure[10] . When this warm air leaves, cold air is drawn in to replace it. In comparison to wind ventilation, this technique can work in buildings without significant wind speeds.
US MINT Passive ventilation.jpg

An example of stack ventilation can be seen in the illustration of the United States Mint Building in San Francisco, California. As shown to the left, cooler air is drawn in from outside through the sides of the building and pulled upwards towards the stacks when it warms. Finally, the warmed air moves out of the building through vents mounted strategically in the roof structure.
Structures like solar chimneys belong to this classification of stack ventilation.



Advantages
Disadvantages
- Not reliant on wind
- Requires additional structures (stacks)
- Relatively cheep, only stacks have to be added
- Requires open floor plan to promote air flow
- Can cool larger buildings
- Climate dependent to be effective, if the air coming into the building is at an uncomfortable temperature then the ventilation method brings in uncomfortably warm air

Night Purge Ventilation

Night purge ventilation involves ventilation a structure at night when the outside air is coolest. This type of ventilation is really a subset of another kind such as wind ventilation or stack ventilation but with a couple key distinctions. As the name suggests, the only ventilation that happens is during the night. This method is primarily for climates where the most comfortable outdoor temperatures are at night but during the day temperatures climb, making ventilation using this warmer temperature air uncomfortable. During the daytime, windows, vents, and doors are all kept shut. At night, vents and windows are opened allowing the cooler air in and the warmer, stagnant indoor air out. This air is cooler than the daytime air so, when the vents windows are closed again during the day, the indoor air is cooler than the outdoor air.

This method is highly dictated by the climate. The daytime and night time temperatures must be reliably different so that during the day the outdoor temperature is too hot to be comfortable but during the night time the outdoor temperature is comfortable. Additionally, this method introduces some security concerns because as the inhabitants of the building prepare for sleep or gone for the day, the windows or vent apparatus is opened allowing in cooler night airflow. Since the windows and vents are open when no one is awake, security is a concern[11] .

Advantages
Disadvantages
- Can effectively ventilate structures in climates that are too hot to comfortably live in during the day time
- Limited climate to use: must be uncomfortable temperature during the day but comfortable temperature at night

- Requires vents/windows to be open when residents are asleep or building occupants are likely gone

Fan/Duct Ventilation

Many previous ventilation methods are effective in moderate climates and can keep structures ventilated at a reasonable temperature most of the year, but no 100% passive system can provide total comfort and control everyday. Therefore, hybrids of passive and active ventilation methods have been developed. Fan and duct ventilation does use energy to power the mechanisms, but this energy is significantly less than a normal heating, ventilation, and air conditioning (HVAC) system uses.
fan ventilation.jpg
Although systems do vary from structure to structure, fan/duct ventilation systems generally only use small fans running at low energy consumption (often consuming less energy then a handful of light bulbs) to move air into a structure[12] . This method resembles traditional HVAC systems in that it requires duct work with simple input and outputs to connect inflowing air to each area of the structure. The system works by drawing in through the low pressure created by the spinning fan[13] . This air then circulates through the house to ventilate the entire structure.
An additional feature of this type of ventilation is the possibility of installing a heat recovery system as well. The system can be set up so that as the warm air leaves the house and cold air enters, the two pass by one another and the cool inflowing air is heated by the outflowing air. This system can operate during cooler times of the year and allow for the structure to use significantly less energy to heat the building. In some systems 90% of heat can be transferred to the intake. An example of this system can be seen in the image to the right[14] .
Advantages
Disadvantages
- More temperature control
- Requires additional structures (duct work and fans)
- Able to be used for a wider range of climates
- Not completely passive, requires input energy to power the fans
- Can cool larger buildings
- More infrastructure required increases the cost in comparison to other passive ventilation methods

Implementation


Calculations

In order to properly design a system, the designer must first understand the required air flow that a building needs. This requirement then can be compared to the ventilation and air flow that potential systems can provide. These parameters are important because they control how fast polluted indoor air can be replaced by fresh exterior air. A simple formula outlined below can be used to calculate the amount of air flow a potential system can create [15] . This value can compared to standards such as those published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). Specifically, standard ASHRAE 62.2 details required air flow for rooms with different intended poses as well as general guiding principles [16] .

Qwind= K*A*V
Qwind = Normal Volumetric Flow Rate (M^3/h)
K= Coefficient of Effectiveness
A= Opening Area (smaller opening)
V= Outdoor (uninterrupted) wind speed

Standard wind speed for various areas can be found here Standard Wind Change Rates[17] and inputed for the outdoor windspeed in the air flow equation. This equation is the most basic of the calculations involved in proper passive ventilation designs . Other considerations including modeling air flow, evaluation obstacles of effective air flow, and building orientation must also be completed. In buildings needing as much passive ventilation as possible, additional factors such as window angle and vent lotions will also be extensively evaluated.


Sustainability

These methods of ventilation are incredibly sustainable in modern construction and are easy to implement in many environments. As many of these methods are limited to temperate climates, passive ventilation is not for every structure in every place but can be implemented and sustained in many areas. The energy consumption on 100% passive ventilation designs is her and even when fans are introduced, the electrical requirement is small and could be provided by a small solar cell or other renewable energy source [18] . Passive ventilation requires no materials that are not traditional building materials so does especially not put a constraint on nonrenewable resources either.

Construction Application


To Implement these ventilation methods into construction, the designer needs to be aware of intended structure's surrounding environment and the locations typical wind movements. From proper knowledge of these variables, the designer can select the most suitable ventilation technique and plan out the structure accordingly. For the most part, passive ventilation systems are easier to install than HVAC systems because they some only involve proper positioning of vents and windows. Therefore less work on the construction and maintenance sides are required[19] . The primary maintenance required is fan replacement and potential mold removal. In some highly humid climates mold can grow within the venting of these systems. This problem can be mitigated prior to construction and monitoring systems as well as proper coatings and preventative maintenance can be included to prevent mold growth. For systems that do use fans, some additional maintenance is necessary but this work is comparable to that required for HVAC systems and often less extensive because of the limited amount of fans.

Financial Considerations


Passive ventilation is generally both a sustainable and economical decision. Logically, passive ventilation systems significantly reduce household electricity. Pure passive ventilation systems as well as hybrid systems reduce or eliminate energy used to power HVAC systems, which varies from 40-60% of building energy usage depending on climate [20] . In addition to this long term reduction in energy usage, capital costs for passive ventilation systems tend to range for 10-15% lower than building designs with air-conditioning systems [21] . Although both financial and sustainable considerations point to converting traditional HVAC systems to passive ventilation methods, in addition to not being viable in all climates and locations, occupants or passive ventilation buildings must be alright with the system occasionally failing to provide a comfortable environment. If a building wants to both have the control associated with HVAC systems and the sustainability and energy savings associated with passive systems, both can be installed but this double installation requires additional capital investment that may never be recouped.

Case Studies


Enschede Tax Office Extension

Location: Enschede, Netherlands
external image EnschedeTaxOfficeExtension.jpg[22]
Key features:

  • Naturally vented
  • Self regulation constant flow fresh air vents installed
  • Night cooling ventilation
  • In the daylight, light shelves are used
  • Externally shaded
  • Low internal heat gains

Biggest hurdles:
  • Initial commissioning of passive systems
  • Maximizing cooling potential in hot summer months [23]

The Lanchester Library

Location: Coventry University, England
external image 102206-004-9417EF84.jpg[24]
Key features:

  • Naturally ventilated with almost no air conditioning use
  • Uses light wells extensively
  • Integrated building management system
  • Maintains student comfort level inside
  • Night cooling
  • Ventilation exhaust stacks along the building perimeter
  • Daylight and solar shading

Biggest hurdles:
  • Maintaining comfortable indoor temperatures
  • Design choices - Architects versus College's wants
  • Some noise propagation throughout the building
  • Initial cost - 19 million euros [25] [26]

GSW Headquarters

Location: Berlin, Germany
external image gsw-headquarters.jpg?w=500[27]
Key Features:

  • Renovated 1950s building
  • Stack Ventilation
  • Embedded ducts to distribute fresh air and collect stagnant air
  • Cross Ventilation
  • Automatic and manually operated windows
  • Energy savings of 30-40% compared to pre-renovation energy usage
  • Naturally ventilated for up to 70% of the year

Biggest Hurdles:
  • Renovating a 1950 building
  • Building height - taller than most passive ventilation buildings
  • Reworking the building layout to accommodate cross ventilation [28] [29]

Recent Research


Across the globe, passive ventilation technologies continue to be researched and design methods improved. A brief list of some key areas of research is included below. Please follow the links for additional information.

1. Heat recovery technology - The importance of heat recovery related to passive ventilation is a topic of increasing interest in the passive ventilation discussion. Systems that combine passive ventilation with passive heating of incoming air currently show great promise in decreasing heating costs as well as supporting integration of passive ventilation systems into a wide variety of structures. This paper is an overview of many heat recovery devices currently used across the industry.
http://eds.a.ebscohost.com.research-db.letu.edu/eds/command/detail?sid=2742a40e-e48f-4372-889d-2ae67fb712c5%40sessionmgr4004&vid=3&hid=4110

2. Design methods for evaluation the potential of naturally ventilated buildings - Passive ventilation is a generally desirable sustainable design feature because it requires little additional initial investment (if any at all) and can generally improve energy costs if the climate and local conditions are right. This article looks at the potential for buildings in the early stages of design for natural ventilation. In other words, the study attempts to model possible natural ventilation advantages prior to progressing building plans too far along.
http://eds.b.ebscohost.com.research-db.letu.edu/eds/command/detail?sid=c0a15a09-def8-4860-8d16-86f541922743%40sessionmgr120&vid=4&hid=108

3. Design of natural ventilation in high-rise buildings - Passive ventilation has been primarily used in small story, residential construction but shows promise in large scale and high-rise construction. This study looks at effectively optimizing natural ventilation in high-rise buildings through 3D computer modeling.
http://ac.els-cdn.com/S0378778814005180/1-s2.0-S0378778814005180-main.pdf?_tid=982d1c76-9977-11e5-8653-00000aacb35e&acdnat=1449117643_58557aa9e2fea326efd55a282615631f

4. Using natural ventilation in various climates - One of the factors that cannot be controlled about passive ventilation is the climate of proposed buildings. Adapting passive ventilation techniques for different climates is a current challenge in the industry. The following papers look at natural ventilation in different climates.
Mexico - http://ac.els-cdn.com/S0306261914005303/1-s2.0-S0306261914005303-main.pdf?_tid=0df7ff34-9978-11e5-ac01-00000aab0f02&acdnat=1449117841_2c5d6b5714a45acc237cc965d79e67be
Denmark - http://ac.els-cdn.com/S0306261913008283/1-s2.0-S0306261913008283-main.pdf?_tid=7a295f04-9978-11e5-87f5-00000aacb360&acdnat=1449118023_2ad82830e5660e16a4ac51ee764b49b9
Greece, Germany, and Australia - http://ac.els-cdn.com/S0360132313002862/1-s2.0-S0360132313002862-main.pdf?_tid=b32de4a0-9978-11e5-b597-00000aab0f27&acdnat=1449118118_eaf8783003373e7f76703350deadc152


Videos


Below are videos further explaining the concept of passive ventilation. Refer to the text below the video for additional information on the video content.


Fan-drive air ventilation systems - this video looks at fan/duct ventilation systems and heat recovery[30] .


Wind and ecosystem ventilation systems - this video explains different passive ventilation and cooling techniques used in various climates[31] .


Passive design strategies for heating, cooling, and ventilation - this video shows how passive heating, cooling, and ventilation can all be combined together to reduce energy consumption [32] .


Further Information


For further information and specific inquiries, the following list includes credible experts in various passive ventilation technologies.

American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) https://www.ashrae.org/home
This group publishes relevant standards and requirements for indoor air quality. Passive ventilation designs generally must meet ASHRAE's specifications

CoolVent http://coolvent.mit.edu
This website offers free downloadable software intended to simulate natural ventilation in buildings. In addition to the software, the cite explains natural ventilation basics as well.

Journal of Architectural Engineering http://ascelibrary.org/journal/jaeied
Passive Ventilation really includes a lot of different civil engineering research topics and is included in many journals. Although not the only journal to include passive ventilation research, the journal does look into design and architectural aspects of passive ventilation.

Whole Building Design Guide https://www.wbdg.org/resources/naturalventilation.php
In addition to providing helpful information about natural ventilation, this website also puts passive ventilation into the larger context of sustainable building design.

Your Home http://www.yourhome.gov.au/passive-design/passive-cooling
This guide is published by the Australian government and recommends some steps that homeowners wishing to adopt passive ventilation techniques can easily take to retrofit their homes.



[33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44]



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