PAÑAMARCA

The integration of the double skin façade systems in offi ce buildings is crucial for thermal performance and energy use during the occupation phase. Stec & Paasen (2003) presented a paper in which they describe different HVAC strategies for different double skin façade types. Accord-ing to the authors, the integration procedure of double skin façades in the building should include (a) defi ning the functions of the double skin façade in the building, (b) selecting the type of the double skin façade, its components, materials and dimensions of the façade that fulfi l the requirements, (c) optimizing the design of the HVAC system to couple it with the double skin façade, and (d) selecting the control strategy to supervise the whole system.

The authors briefl y introduce the concept of different cavity depths and describe its infl uence on the air temperatures inside the cavity. According to them, the dimensions of the façade together with the openings deter-mine the fl ow through the façade; narrower cavities result in higher fl ow resistance and smaller fl ow through the cavity and a higher increase in air temperature in the cavity. The authors conclude that (a) in the cold period it is more suitable to use narrow cavities to limit the fl ow and increase the cavity temperature and (b) in the hot period the double skin façade should work as a screen for the heat gains from radiation and conduction.

It is diffi cult to claim in general whether the narrow or deep cavities will perform better because in one case the cavity temperature and in the other case the temperature of the blinds will be higher.

Examples concerning the infl uence of different depths on the properties of the cavity are shown in “Second Skin Façade Simulation with Simulink Code” by Di Maio and van Paassen in (2000). In “Modelling the Air Infi l-trations in the Second Skin Façade” in (2001) the same authors conclude that “narrow cavities are more useful, because they can deliver a higher and hotter air fl ow compared to the air fl ow delivered by wide ones”.

3.2.2.1 Contribution of double skin façades to the HVAC strategy

As Stec et al. (2003) describe, an HVAC system can be used in the follow-ing three ways in a double skin façade offi ce buildfollow-ing:

• full HVAC system (the double façade is not a part of the HVAC) which can result in high energy use. The user can select whenever he/she prefers mechanically controlled conditions inside or natural ventilation with the use of the double skin façade).

• limited HVAC system (the double façade contributes partly to the HVAC system or plays the major role in creating the right indoor climate). In this way the double façade can play the role of:

o pre-heater for the ventilation air o ventilation duct

o pre-cooler (mostly for night cooling)

• no HVAC. The double façade fulfi ls all the requirements of an HVAC system. This is the ideal case that can lead to low energy use.

During the heating periods the outdoor air can be inserted from the lower part of the façade and be preheated in the cavity (Figure 3.3). The exterior openings control the air fl ow and thus the temperatures. Then, through the central ventilation system the air can enter the building at a proper temperature. During the summer, the air can be extracted through the openings from the upper part of the façade. This strategy is usually applied to multi storey high double skin façades.

AHU

Winter Summer

Figure 3.3 Double skin façade as a central direct pre-heater of the supply air.

During the whole year, the double skin façade cavity can work only as an exhaust duct without the possibility of heat recovery for the HVAC system (Figure 3.4). It can be applied both during winter and summer to the same extent. The main aim of this confi guration is to improve the insulation properties in the winter and to reduce the solar radiation heat gains during the summer. There are no limitations to individual control of window opening.

AHU

Winter/Summer

Figure 3.4 Double skin façade as an exhaust duct.

It is also possible to use the double skin façade as an individual supply of the preheated air (Figure 3.5). This strategy can be applied in both the multi-storey and box window types. An exhaust ventilation system im-proves the fl ow from the cavity to the room and to the exhaust duct. Extra conditioning of air is needed in every room by means of VRV system or radiators. This solution is not applicable for the summer conditions, since the air temperature inside the cavity is higher than the thermal comfort levels. Also in this case there are no limitations to individual control of window opening.

Box window Multi-storey

Figure 3.5 Double skin façade as an individual supply of the preheated air.

Finally, the double skin façade cavity can be used as a central exhaust duct for the ventilation system (Figure 3.6). The air enters through the lower part of the cavity and from each room. The supply ventilation system stimulates the fl ow through the room to the cavity. Heat can be recovered by means of heat pump or heat regenerator at the top of the cavity. Because the air in the cavity is not fresh air, the windows cannot be operable.

AHU Winter

Figure 3.6 Double skin façade as a central exhaust duct for the ventilation system.

As Stec et al. (2003) describe, generally supply façades couple better with the winter systems in which their preheating properties can be used. The exhaust façade is more effi cient in cooling the cavity in the summer.

Problems arise when one façade needs to couple both of the periods, in which case the construction must be adjusted for summer and winter conditions.

3.2.2.2 Examples of coupling double skin façades and HVAC

In 2001, van Paassen and Stec wrote a paper “Controlled Double Façades and HVAC” that deals with the preheating aspects of double skin façades.

The authors claim that for the winter period the most signifi cant parameter should be the heat recovery effi ciency. The main aim of the paper was to show the usability of the cavity air for ventilation purposes. According to the authors, it is possible to defi ne by simulation how the heat recovery ef-fi ciency depends on the outside conditions, the dimension of the cavity, the area of inlet and outlet for outside air and the height of the building.

For the simulations, the authors chose the following four double skin façade types:

1. Double skin façade with controlled airfl ow through the cavities (Figure 3.7). The façade is a multi-storey façade with no opening junctions that allow the air to be extracted out. There is only one inlet for the ventilation airfl ow at the bottom of the façade. It is controlled by an air damper such that the air supply to the cavity

is just enough for ventilating all the rooms above. The controlled trickle ventilator delivers the desired airfl ow to each room (80 m³/h)

Figure 3.7 Coupling DSF and HVAC; Controlled air fl ow in the cavity.

2. There are no open junctions on each fl oor, no controlled airfl ow in the cavity and open dampers in this system (Figure 3.8). Ad-ditionally, the upper part of the façade is open allowing the air to be extracted.

Figure 3.8 Coupling DSF and HVAC; Uncontrolled air fl ow in the cavity.

3. There are open junctions between the outside and the cavity on each fl oor, which cause heat exchange between air inside the cav-ity and outside air. The main airfl ow is the same as in the second system (Figure 3.9). The authors claim that this should be the best system for summer time when cooling is required, but due to the open junctions preheating of the cavity air will be much lower than in the other systems with closed junctions.

Figure 3.9 Coupling DSF and HVAC; Open junctions in each fl oor.

4. There are open junctions on each level, but the storeys are separated from each other (Figure 3.10). Consequently each storey creates its own system. The authors claim that in practice this can be the most convenient system since the same module can be used on each storey and the problems due to large temperature gradients at different levels in the cavity can be avoided (on each storey there is more or less the same temperature in the cavity).

Figure 3.10 Coupling DSF and HVAC; Each storey is separated.

The conclusions drawn by the authors were that:

• The dimensions of the cavity, (height and width) have the great-est infl uence on the heat and fl ow performance in the double skin façade and hence they are the most important parameters in design-ing the double skin façade.

• High-rise buildings with very narrow cavities may not ensure the airfl ow in the cavity needed for ventilation purposes.

• In general, a double façade with airtight junctions and proper air-fl ow control in the cavity is an interesting pre-heater for ventilation air. In a four storey building with cavity width of 0.2 m an overall heat recovery effi ciency of 40% can be obtained. According to the authors this effi ciency can be increased to 72% if the ventilation fl ow inside the cavity is properly controlled. A disadvantage is the vertical temperature gradient inside the double façade. It gives lower comfort or higher cooling capacities at higher fl oors.

• Splitting the cavities of high rise buildings into separate parts by combining for example four storeys with their own inlets and outlets can be essential. If this is done for each fl oor the effi ciency can drop to 35%.

• In order to use the double façade for night cooling and for heat recovery, controlled dampers in the open junctions are needed.

During cooling periods they should be fully open.

3.2.2.3 Control strategy

A crucial point when integrating double skin façade systems in buildings is to defi ne a control strategy that allows the use of solar gains during the heating period and provides acceptable thermal comfort conditions dur-ing the whole year. In the case of cavities with all year round mechanical ventilation, there is a high risk of overheating the offi ces during the sum-mer months, when the design of the double skin façade is not coupled properly with the strategy of the HVAC system. According to Stec et al.

(2003) this system allows the outside conditions to infl uence the indoor climate. As the authors describe, an effi cient control system can manage rapidly changing outside conditions. Successful application can only be achieved when the contributions of all the devices can be synchronized by an integral control system.

According to the authors, the control system of the building should take into consideration the following principles:

• the occupants should be able to infl uence everything, even if their intervention wastes energy.

• energy saving can be achieved when the control system takes maxi-mum advantage of the outside conditions before it switches over to the air conditioning system.

• all control systems must be focused on realization of the required comfort with the lowest energy use.

• during the unoccupied period the control system is focused only on the energy savings, while during the occupied period it must also be focused on comfort.

According to the authors, the main tasks that the control system has to fulfi l are to: (a) keep the right level of temperature inside the building, (b) supply suffi cient amount of ventilation air to the building and (c) ensure the right amount of light inside the building.

3.2.3 Energy performance of buildings with