Natural ventilation

Natural ventilation is driven by the pressure differences that occur naturally over a building. When wind moves air over and around a building, it increases pressures on the windward side and lowers pressures on the leeward side. When indoor and outdoor temperatures are different, the warmer air becomes buoyant relative to the colder air resulting in an upward movement of the warmer air. The pressure difference that drives natural ventilation is a result of the combined effects of wind speed and direction and the temperature difference between the indoors and outdoors. Natural ventilation can be facilitated and controlled through strategically placed and shaped openings in a building envelope [22,23].

Figure 2.1 gives four examples of such openings.

The first of the four examples is a stack. In the context of natural ventilation, a stack serves as an example of an opening located at a height above a ventilated room. Due to differences in the density of warm and cold air, warm air is buoyant relative to cold air. When indoor air is warmer than outdoor air, the natural buoyancy of the indoor air can drive it up through a stack. The process driving the ventilation is known as the stack effect. In order for a stack to be efficient, it needs openings at the room level. Stacks are an efficient form of natural ventilation and air flow rates can be controlled by changing the height difference between inlets and outlets and their cross-sectional areas.

If a building design does not allow for the use of the stack effect, it is possible to ventilate at the room level by controlling openingsin the building façade. Figure 2.1 gives three examples of such openings. Here vents are examples of simple openings in the façade. As with a stack, it is possible to control the air flow rates by adjusting the cross-sectional area of a vent by shutters, dampers or similar mechanisms.

Trickle vents are similar to ordinary vents in that they are simple openings to the exterior. However, they are smaller than ordinary vents and allow only small flow rates of air to pass through them at any given time. Where trickle vents do not allow for the same level of control as larger vents, they can be used to establish a base ventilation rate without occupants having to continuously adjust the cross-sectional area. Trickle vents are usually placed in window frames.

Of the three examples given in Figure 2.1, windows can supply the largest air volumes. Ventilation can be controlled by adjusting the size of the cross-sectional area by either opening or closing the window.

Disregarding stack, natural ventilation at the room level can be achieved either by using openings in a single wall or by using openings in two opposing walls. If a room is ventilated by openings in a single wall, the form of natural ventilation is called single-sided. If a room is ventilated by openings in two opposing walls that both face the exterior, the form of natural ventilation is called cross ventilation. Of the two forms, cross ventilation is the most efficient. When wind passes over or round a building, it creates pressure differences across the building; the windward side has a higher pressure than the leeward. Cross ventilation makes use of this pressure difference to drive air flows across a room or building.

2.1.1.1 Pros and cons

A well designed natural ventilation system is passive and efficient in supplying un-conditioned exterior air. Being passive, a natural ventilation system has no need for maintenance and uses no energy to drive ventilation.

Since the driving potential is dependent on the exterior conditions, natural ventilation can be difficult to control [22]. The supply air is un-conditioned and

this can at times affect thermal comfort (TC) and indoor air quality (IAQ). In periods with cold outdoor temperatures, occupants may experience low indoor temperatures and draught. Denmark is not a big country and geographical variations in the climate are small. The Danish coast (which constitutes the majority of the land mass) has a temperate oceanic climate while the inland has a warm-summer humid continental climate (with Köppen climate classifications Cfb and Dfb, respectively). For 28 % of the full year and 90 % of summer, outdoor temperatures allow outdoor air to be used unconditioned [24,25]. With outdoor temperatures averaging a little above 1 °C, Denmark also has mild winters [26]. Consequently, Denmark has had a long tradition for natural ventilation in homes. This tradition continued up until 2010 when the Danish building regulations were updated to include requirements for preheated supply air and a minimum heat recovery (HR) rate on exhaust air of 80 % for multi-family housing [27]. With driving pressures being low and unstable, it is not possible to implement filters or heat exchangers into a natural ventilation system [28]. Therefore, if exterior air is polluted, supply air volumes will bring this pollution indoors [29–31]. However, as the outdoor air is relatively clean, this is generally not a problem in Denmark [32].

It is possible to improve control by adding a control system and fitting actuators to shutters, dampers and windows. However, the system will then no longer be passive or entirely maintenance-free. Arguably, a system fitted with active controls and actuators should be classified as a hybrid system. Still, this may be worthwhile as hybrid systems have been shown to be able to combine the benefits of natural and mechanical ventilation resulting in lower energy demands [22,24].

A natural ventilation system is a part of the fundamental design of a building. To make use of the stack effect, there must be a stack. To make use of cross ventilation, air flowing from one side of a building to another must be unobstructed. Densely occupied rooms – such as meeting rooms and class rooms – and rooms with a large production of heat – such as server rooms – constitute particularly difficult cases. Therefore, it is not always possible to implement a new, efficient natural ventilation system in connection with an

energy renovation; the design of the building that is to be renovated may simply not allow it.

2.1.1.2 Supply air windows

A common supply air window design will take outdoor air in trough a valve at the bottom. From here it will lead the air up through a cavity between window panes before directing it into the conditioned interior via a valve at the top of the window. For any temperature difference across a window, energy moves from high to low temperatures. In a supply air window, heat is entrained in the supply air as it passes up through the ventilated cavity between window panes. If not entrained in the supply air, this heat would otherwise have been be lost to the exterior. This way, supply air windows can both lower the heat loss through a window and preheat supply air. Figure 2.2 shows a sketch outlining the principles governing heat exchange in and around a supply air window.

Figure 2.2: Sketch showing the principles of the supply air window

Published literature (project reports and peer-reviewed journal articles) contains several studies of supply air windows [33–39]. The studies cover many different window designs and mathematical models – both white and grey box – have been created to examine the effects of design variations [40–45]. Consensus seems to be that the supply air window has significant advantages when compared to classical natural ventilation where outdoor air is delivered through fresh air valves. When compared to conventional natural ventilation, studies estimate that for ACHs between 0.4 h^-1 and 0.64 h^-1 supply air windows can help reduce energy demand for ventilation by 11-24 % [33,34,38].

There are two mechanisms that facilitate preheating of the supply air in supply air windows. One, and by far the most important, makes use of the heat loss inherent to all window designs. Heat driven by a temperature difference over the window is entrained in the supply air stream. In this way a well-designed supply air window can have a very low effective U-value (the effective U-value being defined as heat leaving the system by the outermost pane normalised by area and the temperature difference between the interior and exterior). Increasing supply airflows have the effect of lowering the effective U-value.

The second way to preheat supply air is by solar energy. Incident solar irradiation that is not reflected from or transmitted directly through the window, is absorbed in the panes. The window panes can release the absorbed energy to the supply air by convection. Increasing the supply airflow will increase the solar heat gain coefficient (SHGC) and the g-value (here defined as the total solar heat gain divided by the incident solar radiation) as the fraction of energy that is transferred to the supply air grows while the fraction reemitted to the exterior diminishes.

While using solar energy to preheat supply air is efficient, its usefulness is subject to availability. A window’s orientation, shadows cast by nearby objects and structures and the amount of incident solar radiation in occupied hours – especially during dark northern winters – are all factors that can have a negative impact on the usefulness of solar energy as a way to preheat the supply air.