Natural Ventilation and Daylighting

Natural Ventilation and Daylighting

In the quest to build energy efficient buildings, natural ventilation and daylighting go hand in hand. To begin with, both techniques are highly dependent on the placement and orientation of windows as well as the building shape and orientation. In addition, the use of both natural ventilation and daylighting turns a building into a dynamic system which varies with the natural cycles of the external environment. In this way, they allow the occupants of a building to become more connected with their natural surroundings.

Occupant Comfort and Control

One of the downsides of natural ventilation is that it is rarely possible to ensure complete occupant comfort without mechanical means of environmental control 100% of the time. It is generally considered acceptable if the number of “uncomfortable” days for a building is limited to 3 to 5% of the total number of days of occupancy during the year. This can be compensated for by building a maximum amount of user control into the system.

One of the most direct ways to ensure user control is to use windows that can be manually opened by the occupants. As windows are generally a main source of natural ventilation as well as daylighting, they are crucial to the creation of a comfortable environment. For example, windows can be designed to rotate about a horizontal axis such that when they are opened, they not only allow fresh air to enter, they also act as a light-shelf that reflects direct sunlight up onto the ceiling. In a different scenario, a window could be opened slightly in cool weather so as to allow fresh air to enter while solar radiation through the glazing warms up the occupant. As windows play such an important role in both natural ventilation and daylighting, their placement and selection requires careful planning.

The Importance of Designing for Daylighting and Natural Ventilation from the Start

In order to ensure a proper implementation of daylighting and natural ventilation, it is essential that these be incorporated into the design process in the earliest stages of the design process. This is because both methods depend heavily on such things as the building envelope and orientation. Implementation becomes more and more difficult and costly if they are added at a later time during the design process. Ideally, daylighting and natural ventilation should be built into the building starting from the very beginning of the design process.

Building examples

In order to provide a better understanding of natural ventilation and daylighting, we examine two buildings: the Reichstag and the Düsseldorfer Stadttor. The Reichstag is the New German Parliament in Berlin, which was renovated by Norman Foster and Associates. The Düsseldorfer Stadttor was built as a portal for the city of Düsseldorf, also in Germany – it was designed by Petzinka, Pink und Partner. Both buildings were designed with the idea in mind of minimizing CO2 emissions by using renewable energy sources. Within this vision, natural ventilation and daylighting play an important role without sacrificing the architectural quality of the buildings.

Natural Ventilation

Natural ventilation is used to deliver fresh air to a building without the use of mechanical fans or air-conditioning units. The motion of air promotes evaporation, thus improving comfort under high temperatures. If combined judiciously with other energy-efficient design techniques, it can also be used to replace mechanical air-cooling units. Thus, naturally ventilated structures can have significantly lower rates of energy consumption – up to 10-30% of the total energy consumption [Whole Building Design Guide]. Natural ventilation can be divided into two types of effects, buoyancy and wind. Buoyancy is commonly referred to as stack effect.

Stack Effect

Stack effect is related to the variation in density which occurs when air is heated. Because hot air has a lower density than cold air, it tends to rise while the cold air falls. This leads to large-scale motions of air. Stack effect is also affected by humidity: dry air has a higher density than air which is humid. Consequently, the stack effect helps to reduce the humidity in the air, thus further improving occupant comfort [Whole Building Design Guide]. The stack effect is put into use in the dome of the renovated Reichstag, which was designed by Norman Foster.

Wind

Natural ventilation can also be driven by wind. Wind causes a pressure difference: on the windward side of a building, the pressure is higher, while on the leeward side the pressure is lower. This pressure differential causes the air to be drawn through the building, while the amount of ventilation depends on how porous the envelope is. In order to properly design a building to take advantage of wind effects, it is necessary to have a good understanding of the seasonal wind direction and speed on the building site. This information can be summarized from wind-rose diagrams – these can be obtained from the National Oceanographic and Atmospheric Administration (NOAA).

Daylighting

There was a time when daylighting was the only source of light in buildings, and it absolutely had to be considered in building design. With the progress of technology and the relatively inexpensive cost of electricity, daylighting could be easily replaced with artificial lights and architects and designers began to forget about it. As the quality and brightness of artificial light improved, windows became smaller reducing the amount of natural light entering the interior of our buildings. Eventually people realized that the quality of artificial light was inferior in many ways and that designs which considered daylighting had several benefits besides superior light quality.

Daylighting should and must be considered from the start of any design, as it has to work in combination with artificial lights. Daylight varies and cannot be moved, although with intelligent design it can be effectively transmitted. Artificial light does not vary, though it is potentially monotonous, but it can be placed where required. The easiest way to combine the two is to provide switches which are easily accessed by occupants of a building so that they may easily turn lights on or off as required. Leaving it entirely up to occupants may not be effective, so automation can be used to control lights based on lighting levels. Systems divided into zones, which control small numbers of lights depending on local requirements, are particularly effective as lights further from windows often need to be turned on sooner in the afternoon.

Generally there are two major types of required lighting: ambient lighting and task lighting. In designing with daylight, as with artificial light, both types of lighting must be considered. Ambient lighting is the more general lighting of a space whereas task lighting is more focused and usually brighter, intended to be used for a specific task. Direct sunlight is too bright to be used for either type of lighting, and may cause overheating. However, sunlight bounced off surfaces and diffused is very useful. Light shelves bounce the sunlight onto the ceiling and clerestories can be used to bounce sunlight off a vertical wall. Window louvres can be used to bounce light as well as shade from the sun and direct incoming air. Windows are not only for lighting, but also for a view and for fresh air and ventilation—this is the point of interaction between daylighting and natural ventilation, and one can see how they can work very well together.

Reichstag: The German Parliament Building

Berlin, Germany, 1992-1999

Architect: Foster and Partners

After the fall of the Berlin wall and the re-unification of Germany, the decision was made to move the German parliament to Berlin from Bonn. At the same time, the decision was made to house parliament in the historic Reichstag, its former home. Major renovations were required before it could serve in this purpose, as it had been mutilated in previous renovations since WWII. The dome had been removed in the mid-1950s and the interiors had been covered over in plasterboard in the 1960s. One of the four major issues in the reconstruction of the Reichstag was a commitment to sustainable, renewable, environmentally friendly architecture, which was financially and morally supported by the European Commission.

The commitment to an environmentally friendly agenda was an important one. Prior to the renovations, the existing mechanical services produced an incredible 7000 tonnes of carbon dioxide per year. With the renovations complete, the building’s carbon dioxide output was reduced by 94% to only 440 tonnes per year. These reductions were achieved using several different methods. The new service system is powered by refined vegetable oils, which release far less carbon dioxide, and excess heating and cooling is stored in underground water for use at a later date. These services are essentially invisible to the average observer; the other energy saving measures in the building are not only visible, but act as a focal point of the new Reichstag. The lighting and natural ventilation strategies used in the new building are clearly identified by the glass cupola that takes the place of the original dome.

The cupola or ‘lantern’ is an architectural feature in itself, the way it lights up at night as a beacon of light, and has become a landmark in Berlin. Most importantly, it is completely functional as the central component in both the daylighting and ventilation strategies of the building. The cupola measures 40 metres across and is 23.5 metres high

and is fully accessible to the public who can reach an observation deck on the top via two

helical ramps which wind their way around the outside of the dome. The most significant

feature in the cupola is located at its centre, and that is the ‘light sculptor’. The light sculptor is a huge cone weighing 300 tonnes and measures 16 metres across at the top and

2.5 metres across at its base. The cone is covered with 360 reflective glass mirrors, which reflect daylight from outside down into the parliamentary chamber below, significantly reducing the amount of artificial light required. To prevent solar overheating and glare to the occupants of the chamber, the light sculptor is protected by a solar shading shield, which follows the sun through the day. The movement of the shield is automatically controlled and is powered by 100 panels of photovoltaic cells found on the roof of the building, which adapt to the strength and angle of the sun. The shield is an important feature as it allows daylight, but not direct sunlight, to access the chamber. At night, the light sculptor works in reverse, reflecting lights from below into the cupola, lighting it like a lantern.

Besides providing light to the centre of the building, the cupola and light sculptor perform a major role in the ventilation system of the chamber. The natural rising action of warm air, augmented by fans, is used to exhaust stale air and bring in fresh air. Warm air exits though the cone and the top of the cupola, but not before heat exchangers capture and recycle energy from it. The same photovoltaic panels used to power the movable sun shield power the fans in the exhaust and ventilation system. As stale air is exhausted, fresh exterior air from the west portico is drawn in from below, and slowly enters the chamber through grilles in the floor. The gentle rising and spreading out of the air in the chamber is important in that it is more comfortable for the occupants by reducing draughts and noise.

The rest of the building is not forgotten in the overall ventilation and daylighting strategy. Perimeter windows are both manually and automatically controlled, allowing occupants to open or close them as they desire, but also allowing building operators to control them at night to maximize nighttime cooling. The windows are double layered, both for security and to contain a solar shading device. The exterior layer of each window is a protective laminated glass pane with ventilation joints, while the interior layer is a thermally separated glazing system.[1] These windows allow up to five times the air volume in a room to be exchanged every hour. The interesting feature of the windows is the way they showcase the direct interaction between daylighting and solar shading, and natural ventilation. The void between the two layers of the window serves in both functions, and the placement of solar shading between the panes eliminates the need for exterior sunshades, which would likely clash with the historical details of the building.

The energy reductions in the Reichstag are so significant that the building is able to serve as a power plant for the surrounding buildings and the natural ventilation and daylighting systems have been seamlessly integrated to perform to their maximum potential. The Reichstag clearly demonstrates how the two strategies go hand in hand in the design of a building. The glass cupola may be a beautiful source of daylight but would be a technical nightmare were it not used to exhaust the hot air it traps. The light sculptor is a piece of art, but if not shaded would annoyingly cast direct sunlight into the eyes of those below, and perhaps even lead to overheating. Most significant, however, is the way in which all the different energy saving features are wonderfully integrated into a beautiful architectural composition which is modern and yet respective of the great historical value of the building.

Das Düsseldorfer Stadttor

Architect: Petzinka, Pink und Partner

Client: D.b.R. Düsseldorfer Stadttor mbH

Location: Düsseldorf

Design - completion: 1987-1998

Net floor area: 30,119 m2

Net Volume: 103,547 m3