Designing with Wind
The New York Chapter of U. S Green Building Council -- now re-named the Urban Green Council -- held another monthly installment meeting in their High performance Building Salon Series last week. The use of wind -- or more accurately -- natural ventilation for buildings was discussed in detail by two speakers.
The first speaker, Matt Herman, from Buro-Happold Engineers gave a broad overview on the current thoughts and directions that natural ventilation is now taking.
Many people think of natural ventilation as just simply being able to open a window, skylight or door, and allow for the prevailing winds to enter the building. Natural ventilation has appeal to the public as they perceive it to be "cool" and they like it. In addition, it is often felt to be a major cost savings for the building. In dealing with a high performance building, it is far more involved. Many factors influence when and if natural ventilation makes sense for a building. These include first, the existence of prevailing wind and its direction in relation to the orientation of the building, the building shape, and mass. In addition, the building envelope and facade features also play an important role, as does relative humidity.
The ASHRAE Codes deals with both ventilation (62.1) and Thermal Comfort (55.1) and clearly spell out parameters that must be followed by all buildings. These are based on the idea of Human Comfort, which in general terms, is defined as a temperature of 70 degrees F., and a relative humidity of 50%. As we know, there are more times than not, that the prevailing outdoor conditions DO NOT meet this criteria.
A system of measure known as Psychometric Charts is used here, and can often be extended by use of the provision of shading. Cross ventilation is a very critical factor here, in that we need air movement for any comfort or ventilation level to obtained. A U, E or H shaped building will often work well here, when placed in the direction of prevailing winds will capture this free flowing air.
As mentioned in the intro above, other factors such as solid mass can be tricky, as often the buildings are thin enveloped.
The Day-Night influence can play a role as well here, too. In this area, the presence of a solid mass containing floor slab (or other mass in the building) can retain the night cooling for use during the daytime occupancy period.
There is a need to know design tools in order to optimize natural ventilation. Mixed mode - steel primary structure with concrete slab structures and light holes to allow for air flow within a ceiling are often key to the successful application, as this will help to avoid heat build-up.
Dynamic Thermal Modeling is needed here. This is similar to energy modeling (E-Quest, DOE-2, Transys, etc.) but has an added component to it. This is the change in state, namely pressure. In the process, a Nodal Network is created for energy and mass, and then is communicated into the operation of the building. Airflow and pressure displacement are linked to thermal comfort in a mixed mode system that includes facade and mechanical system. Software used here to analyze the system includes Transys combined with Comis, as well as Energy Plus. Once Dynamic simulation is done, then a process known as Thermal Fluid Dynamics is performed. This will show the heat flow process for the building and give a good idea as to the level of thermal comfort that can be expected.
One possible solution here can involve Air Speed Variation - Negative Pressure Zones that will lead to a balance of pressure.
Weather data is of utmost importance here, in order to do the modeling This includes prevailing wind direction, speed, temperature, and relative humidity, as well as percentage of sunlight. Indirect Solar Gain can be a BIG factor, and often occurs in hot humid areas such as Singapore, but New York City has its share of days when this is critical. This phenonomen is based on the fact that humidity (really the presence of particultate matter in the air) will disperse the solar radiation to other areas of the building facade outside of the direct rays of the sun. This factor will result in the ASHRAE 55.1 limits being exceeded by a large amount. Good candidates for natural ventilation are often academic facilities such as public grade and high schools that close during the hot humid periods, but are in use during the shoulder seasons of spring and fall.
Building Stack Effect is another key factor in the application of natural ventilation. This is akin to the chimney effect, and the thus,the higher the building is, the greater the airflow from the bottom floors to the top areas will be.
Building Shading and Daylight Harvesting are also involved in the design process, as is the facade design. The facade is thought to have an effect here for six meters. Internal and external sensors are used, but often have a long payback period. It is IMPERATIVE for a Sequence of Operation to be established. This means taking advantage of natural ventilation when weather and humidity conditions allow, and shutting off the ventilation when conditions are adverse. This must be programed into the Environmental Simulation of the building.
In summary, Matt Herman feels that the six foot facade factor, zone of comfort, zoning, and solar shading /daylight harvesting are key to successful use of natural ventilation.
The second speaker, Shanta Tucker, from Atelier Ten provided us with several specific applications of natural ventilation. She emphasized the need to incorporate building shape, including slanting features, into the design process. The potential for natural ventilation as well as the total number of hours needs to be determined. For example, a cool climate, such as the United Kingdom and other parts of Northern Europe and Scandinavia will often succeed very well with natural ventilation. In the U. K., she mentioned Kew Gardens, specifically the Alpine House.
Shanta went on to cover an application in a hot climate, in which thermal mass did NOT make sense. This was the Bird Island House. Here, shading of the building envelope was of far more importance to the success of natural ventilation.
She covered a third application close to NYC in Connecticut, the Kroon Building, in which an attempt at creating a near carbon neutral building was done. In this case, high thermal mass and high R-values of insulation where used in conjunction with the natural ventilation.
It is also of importance to take into account the internal loads. This includes lighting, appliances, such as copy machines and other equipment, as well as occupancy loads. A good commissioning team is VERY important, as it must be determined that the building is performing as it was designed to do; if not corrective action must be taken.
Shanta also mentioned the factor of good and bad days for natural ventilation, and the need for a system of alerts -- e-mails to close windows and vents, or automatic sensors to do this. She also said that wind turbines can be difficult to incorporate.
Rules of thumb here include:
I brought up the question of condensation build up in the building and its envelope, and the need for sensors to monitor for this. Mold can be a real problem here.
One final thought that I would like to add here is the potential possibility of synergies in the area of Optimization of Energy Credit in the LEED process. We have the ability to combine the natural ventilation source with either pre-heating or pre-cooling of the air to comply with not only the ASHRAE 62.1 requirements for ventilation of the building BUT to gain additional points in the Optimize Energy Performance - Credit 1.
This can easily be done by bringing the incoming needed fresh air into the building by a process that I described in an earlier blog posting. Instead of utilizing powered air handling equipment, the natural force of the wind will draw the air in and out of the building for a zero carbon cost. In this process, the air is either pre-heated or cooled by drawing it into a series of pipes that are buried at least seven feet below ground surface level. At this level, the ground temperature in our area is about 55 degrees F. year-round. Thus, in the winter months, the air will be pre-heated, reducing heating loads. In the summer months, it will be pre-cooled, reducing cooling loads, while also potentially de-humidified as well. In the case of de-humidification, as I described in my earlier entry, there is process available in which the piping is fitted with a condensate drainage system, and is embedded with silver particles, which naturally inhibit mold growth. When combined with heat recovery systems, such as enthalpy wheels, the efficiency is further increased.
Thus, we not only improve on the Optimize Energy Credit area, but improve on the Indoor Environmental Quality credit areas of Increased Ventilation and Thermal Comfort, as well. While it is realized that natural ventilation cannot be used on a daily basis, the days that it can be used will certainly reduce the energy loads, by not needing to run air handling equipment to move the air into and out of the building.
The first speaker, Matt Herman, from Buro-Happold Engineers gave a broad overview on the current thoughts and directions that natural ventilation is now taking.
Many people think of natural ventilation as just simply being able to open a window, skylight or door, and allow for the prevailing winds to enter the building. Natural ventilation has appeal to the public as they perceive it to be "cool" and they like it. In addition, it is often felt to be a major cost savings for the building. In dealing with a high performance building, it is far more involved. Many factors influence when and if natural ventilation makes sense for a building. These include first, the existence of prevailing wind and its direction in relation to the orientation of the building, the building shape, and mass. In addition, the building envelope and facade features also play an important role, as does relative humidity.
The ASHRAE Codes deals with both ventilation (62.1) and Thermal Comfort (55.1) and clearly spell out parameters that must be followed by all buildings. These are based on the idea of Human Comfort, which in general terms, is defined as a temperature of 70 degrees F., and a relative humidity of 50%. As we know, there are more times than not, that the prevailing outdoor conditions DO NOT meet this criteria.
A system of measure known as Psychometric Charts is used here, and can often be extended by use of the provision of shading. Cross ventilation is a very critical factor here, in that we need air movement for any comfort or ventilation level to obtained. A U, E or H shaped building will often work well here, when placed in the direction of prevailing winds will capture this free flowing air.
As mentioned in the intro above, other factors such as solid mass can be tricky, as often the buildings are thin enveloped.
The Day-Night influence can play a role as well here, too. In this area, the presence of a solid mass containing floor slab (or other mass in the building) can retain the night cooling for use during the daytime occupancy period.
There is a need to know design tools in order to optimize natural ventilation. Mixed mode - steel primary structure with concrete slab structures and light holes to allow for air flow within a ceiling are often key to the successful application, as this will help to avoid heat build-up.
Dynamic Thermal Modeling is needed here. This is similar to energy modeling (E-Quest, DOE-2, Transys, etc.) but has an added component to it. This is the change in state, namely pressure. In the process, a Nodal Network is created for energy and mass, and then is communicated into the operation of the building. Airflow and pressure displacement are linked to thermal comfort in a mixed mode system that includes facade and mechanical system. Software used here to analyze the system includes Transys combined with Comis, as well as Energy Plus. Once Dynamic simulation is done, then a process known as Thermal Fluid Dynamics is performed. This will show the heat flow process for the building and give a good idea as to the level of thermal comfort that can be expected.
One possible solution here can involve Air Speed Variation - Negative Pressure Zones that will lead to a balance of pressure.
Weather data is of utmost importance here, in order to do the modeling This includes prevailing wind direction, speed, temperature, and relative humidity, as well as percentage of sunlight. Indirect Solar Gain can be a BIG factor, and often occurs in hot humid areas such as Singapore, but New York City has its share of days when this is critical. This phenonomen is based on the fact that humidity (really the presence of particultate matter in the air) will disperse the solar radiation to other areas of the building facade outside of the direct rays of the sun. This factor will result in the ASHRAE 55.1 limits being exceeded by a large amount. Good candidates for natural ventilation are often academic facilities such as public grade and high schools that close during the hot humid periods, but are in use during the shoulder seasons of spring and fall.
Building Stack Effect is another key factor in the application of natural ventilation. This is akin to the chimney effect, and the thus,the higher the building is, the greater the airflow from the bottom floors to the top areas will be.
Building Shading and Daylight Harvesting are also involved in the design process, as is the facade design. The facade is thought to have an effect here for six meters. Internal and external sensors are used, but often have a long payback period. It is IMPERATIVE for a Sequence of Operation to be established. This means taking advantage of natural ventilation when weather and humidity conditions allow, and shutting off the ventilation when conditions are adverse. This must be programed into the Environmental Simulation of the building.
In summary, Matt Herman feels that the six foot facade factor, zone of comfort, zoning, and solar shading /daylight harvesting are key to successful use of natural ventilation.
The second speaker, Shanta Tucker, from Atelier Ten provided us with several specific applications of natural ventilation. She emphasized the need to incorporate building shape, including slanting features, into the design process. The potential for natural ventilation as well as the total number of hours needs to be determined. For example, a cool climate, such as the United Kingdom and other parts of Northern Europe and Scandinavia will often succeed very well with natural ventilation. In the U. K., she mentioned Kew Gardens, specifically the Alpine House.
Shanta went on to cover an application in a hot climate, in which thermal mass did NOT make sense. This was the Bird Island House. Here, shading of the building envelope was of far more importance to the success of natural ventilation.
She covered a third application close to NYC in Connecticut, the Kroon Building, in which an attempt at creating a near carbon neutral building was done. In this case, high thermal mass and high R-values of insulation where used in conjunction with the natural ventilation.
It is also of importance to take into account the internal loads. This includes lighting, appliances, such as copy machines and other equipment, as well as occupancy loads. A good commissioning team is VERY important, as it must be determined that the building is performing as it was designed to do; if not corrective action must be taken.
Shanta also mentioned the factor of good and bad days for natural ventilation, and the need for a system of alerts -- e-mails to close windows and vents, or automatic sensors to do this. She also said that wind turbines can be difficult to incorporate.
Rules of thumb here include:
- Cross Ventilation -- must be 5 times the roof hight.
- Opening height equals the external size.
- Single-sided ventilation is VERY limited in its effectiveness.
- Stack Ventilation.
- Passive Ventilation.
- Return on Investment (ROI) works AGAINST natural ventilation -- people must be committed to the need for it, and want to do it for its inherent environmental benefits.
I brought up the question of condensation build up in the building and its envelope, and the need for sensors to monitor for this. Mold can be a real problem here.
One final thought that I would like to add here is the potential possibility of synergies in the area of Optimization of Energy Credit in the LEED process. We have the ability to combine the natural ventilation source with either pre-heating or pre-cooling of the air to comply with not only the ASHRAE 62.1 requirements for ventilation of the building BUT to gain additional points in the Optimize Energy Performance - Credit 1.
This can easily be done by bringing the incoming needed fresh air into the building by a process that I described in an earlier blog posting. Instead of utilizing powered air handling equipment, the natural force of the wind will draw the air in and out of the building for a zero carbon cost. In this process, the air is either pre-heated or cooled by drawing it into a series of pipes that are buried at least seven feet below ground surface level. At this level, the ground temperature in our area is about 55 degrees F. year-round. Thus, in the winter months, the air will be pre-heated, reducing heating loads. In the summer months, it will be pre-cooled, reducing cooling loads, while also potentially de-humidified as well. In the case of de-humidification, as I described in my earlier entry, there is process available in which the piping is fitted with a condensate drainage system, and is embedded with silver particles, which naturally inhibit mold growth. When combined with heat recovery systems, such as enthalpy wheels, the efficiency is further increased.
Thus, we not only improve on the Optimize Energy Credit area, but improve on the Indoor Environmental Quality credit areas of Increased Ventilation and Thermal Comfort, as well. While it is realized that natural ventilation cannot be used on a daily basis, the days that it can be used will certainly reduce the energy loads, by not needing to run air handling equipment to move the air into and out of the building.
Posted by Alan L. Englander at 5/18/2009 12:35 PM 
Categories: Direct Solar Gain, Building Mass, Building Stack Effect, Cross Ventilation, Nodal Netwirk, Diffuse or Indirect Solar Gain, Natural Ventilation, Relative Humidity, Comfort Range, Building Facade, Prevailing Wind Direction, Dynamic Thermal Modeling
Categories: Direct Solar Gain, Building Mass, Building Stack Effect, Cross Ventilation, Nodal Netwirk, Diffuse or Indirect Solar Gain, Natural Ventilation, Relative Humidity, Comfort Range, Building Facade, Prevailing Wind Direction, Dynamic Thermal Modeling
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