The use of natural ventilation in European commercial office buildings has received much attention in the last decade. Natural ventilation is thought of as a low energy cooling strategy which can provide year round comfort, with flexible user control, at a low capital and maintenance cost. A key consideration in adopting natural ventilation is climate. Architects and engineers in northern European countries may be more willing to embrace natural ventilation because of the cold to moderate climate. Yet, we must be aware that climate is not the primary barrier to the use of natural ventilation. The main barrier may be a lack of design tools and simple understanding of the principles of natural ventilation. A further discussion of natural ventilation is not possible without first defining exactly what it is. There are two related concepts: the use of natural ventilation to control indoor air quality and the use of natural ventilation to control temperature during the summer.
Controlling indoor air quality appears to be more of a concern during the wintertime when interior spaces must be heated to provide a certain level of thermal comfort. In a building without mechanical ventilation or air-conditioning, IAQ is a concern because most windows in the building may be closed, preventing the exchange of fresh air with stale air. Care must be exercised in the operation of windows; opening a window for a long period of time when the outside temperature is much less than the comfort level temperature may cause more energy to be consumed than desired. Controlling airflow rates becomes the ultimate consideration.
Using natural ventilation to prevent overheating within a building presents a very different challenge to maintaining acceptable IAQ standards. In this case, natural ventilation is used especially at night when outdoor air is typically cooler than daytime air. This applies to a cold to moderate European climate and not necessarily to the climate of a city such as Washington DC, which experiences very hot and humid summer days. For summertime cooling, important considerations are internal heat loads and external solar gains, as well as building characteristics, such as thermal mass and insulation level, and the overall building floor and site layout. Controlling airflow rates is not as much a concern. The higher the airflow availability, the greater the cooling effect. As long as occupants are comfortable, rigorous control of airflow is not necessary.
Natural ventilation is permitted in Europe not only because of the more temperate climate, but also because of a culture that favors generous ventilation to provide air freshness, as well as a strong connection with the outdoor environment. With these preferences comes a wider tolerance for temperature fluctuations. These factors contrast starkly with the US culture of preferring a constant temperature environment at a fairly low set point of around 68 °F during the summer. While there is increasing environmental awareness in the United States, an understanding of sustainability among government officials is limited. In contrast, in the United Kingdom, "green" building practices are considered progressive, socially responsible, and correct. Because European economies have not enjoyed the same amount of vigor and success as the United States economy, every construction project that is funded is highly visible to the public and must demonstrate long-term sustainability. In the UK, the Building Research Establishment (BRE) has created a rating system (Environmental Assessment Method) that gives an indication of the environmental-friendliness of a building. This system is an effective way to encourage green building. In a sense, it fosters positive competition among architects and engineers to create the "best" building. A building that is considered "green" becomes a reflection of not only the designers, but also those that work in the building. In addition to an organization such as the BRE, there are many sources of leadership pushing the use of natural ventilation. They include colleges and universities, large electric power companies, and small electronic corporations. Pre-dominant architects that have been knighted, such as Sir Norman Foster, play very important roles. Unlike in the United States, natural ventilation is a very central and key concept used. To not consider natural ventilation verges on being considered irresponsible behavior.
Environmental leadership is not as well defined in the United States. While the government proposes environmental legislation, very little of it promotes passive ventilation systems. The Environmental Protection Agency features some programs, such as the ENERGYSTAR rating system, that may sound like the BRE assessment program, but focus on different green building aspects. Highly efficient windows and chillers appear to be the central focus, rather than looking at "whole building design." On another note, the US Department of Energy (DOE) appears to be significantly more aware of the need for sustainability. The DOE's Center of Excellence for Sustainable Development, as well as the Energy Efficiency and Renewable Energy Network (EREN), encourages green buildings that are designed using an integrated approach. These buildings should be designed to promote conservation of energy resources, use renewable energy, conserve water, consider environmental impacts and waste minimization; create a comfortable and healthy working environment; reduce operation and maintenance costs; and address issues such as access to public transportation and related infrastructure. A green building is ideally designed with its entire life cycle in mind, rather than say, the first ten years, for example.
The need to consider sustainability has never been more critical than right now. Currently there are more than 76 million residential buildings and nearly 5 million commercial buildings in the United States. These buildings together consume one-third of all the energy consumed in the US, and two-thirds of all electricity. By the year 2010, another 38 million buildings may be constructed. From a more critical standpoint, buildings are a major source of pollution. They account for 49 percent of sulfur dioxide emissions, 25 percent of nitrous oxide emissions, and 10 percent of particulate emissions. To cap this all, buildings product 35 percent of the carbon dioxide emissions in the United States, the chief cause of global warming.
The use of natural ventilation is not a new concept by any means. Before air-conditioning was invented, civilizations had to use their own innovative spirit and creativity to maintain thermal comfort. In the 20th century, building designers and architects were aware of many of the principles of natural ventilation that will be described. Yet, because of an abundance of cheap energy in the United States, air conditioning was readily embraced by designers. In particular, architects found new "freedom" with the use of air conditioning. We can look at the buildings that exist today and notice that facades feature a lot of windows. Air-conditioning allowed architects to use massive amounts of glass in their designs; in the process, buildings became more and more reliant on active cooling.
The concept of cross ventilation is fairly simple. Let's consider a ten story rectangular building. When wind hits one side of a building (windward side), the air will speed up in order to flow around the building to the opposite (leeward side). This creates a positive pressure on the windward side and a negative pressure on the leeward side. If windows are open in the building on both the windward and leeward side, air will be forced through the building. The process is similar to how lift is generated by an airplane wing. The difference is that the building is very blunt compared with an airfoil; and is situated vertically instead of horizontally like the airfoil. As long as the outside temperature is lower than the indoor temperature, cross ventilation can be very effective in cooling down the interior spaces of a building. How well cross ventilation works is dependent on many factors including: building location with respect to pre-dominant wind directions, interior layout of offices, corridors, furniture, and other potential airflow obstructions, sizing of windows and other openings.
Figure 1-The above diagram is provided to illustrate how airflow passes through a room by cross ventilation. Cold air comes in from the left side and absorbs heat from the indoor environment. It then exits through windows on the opposite side of the building. The use of automated windows is gaining popularity in European countries to provide optimal airflow control. They are not typically used in the United States.
Stack Ventilation
The concept of stack ventilation is also fairly simple. Think of a chimney with a fire burning below. The air heated above the fire rises through the chimney and out the top. In this case, air is rising at a fairly high velocity. The same principle can be used to ventilate a space. As air increases in temperature, its density drops. This causes it to rise past air that is at a lower temperature. A stack will concentrate this effect. Typically, the longer the stack, the more airflow you can get. The main benefit of using stack ventilation is that wind is not required.
Figure 2-The above diagram illustrates an airflow path driven by the stack effect. Hot air rising out the roof of the building creates a vacuum in the interior space; this vacuum is used to bring cooler air from the outside into the building. In this particular scenario, the vacuum is enhanced by wind. Note that wind is not necessary for stack ventilation and that this method can be incorporated into a building in many more ways than having just a skylight.
Night Cooling / Thermal Mass
Typically, outdoor temperatures are lower at night than during the day. The concept of night cooling rests on this very fact. Cooler night air is brought into a building space to "flush" out warm, stale air that accumulates during the day. While the concept of night cooling is fairly straightforward, its implementation rests on many considerations. Exposing a building to the outside at night can be a security risk. Also, if a building is cooled down too much at night, workers may be uncomfortable the following morning. In that case, heaters may be turned on, needlessly using energy.
Thermal mass can be incorporated into a building structure to absorb heat during the daytime hours. The key to the success of many naturally ventilated buildings appears to lie in the use of thermal mass. Typically this mass is incorporated into ceilings and walls in the form of concrete or brick. At night, cooler outside air is brought in to bring the temperature of the thermal mass back down to pre-occupancy levels. While the outdoor temperature may fluctuate by ten to fifteen degrees C, the interior of a building with a significant amount of exposed thermal mass may experience only temperature fluctuates of two to three degrees.
Designing a naturally ventilated building is a greater challenge for building engineers than designing an air-conditioned one. In a sealed building, what goes on outside the building is not so important to an air-conditoning designer. The designer is able to assume uniform temperature distribution within a particular zone. If the temperature rises above a certain setpoint, the air conditioning is turned on and cool air is brought into the space. A natural ventilation designer cannot make any such assumptions. Mother nature becomes the driving force. We all know from experience that weather patterns change continuously. A designer must consider fluctuating temperature, humidity, wind speed, and wind direction andd thus the interior layout of the building becomes crucial. The physical location and orientation of the building becomes crucial also. Outdoor pollution and noise are yet more tings that must be factored into the design. With so many variables to consider, one can see the amount of effort required in designing an effective naturally ventilated building. The challenge is not insurmountable by any means though. It is our own responsibility to take charge of our planet and embrace design strategies that can reduce global energy consumption and pollution.