Comfort Study of a Passive Office Building in Wallonia Cody Coeckelenbergh1, Piotr Wierusz-Kowalski2

1 3E nv/sa, www.3E.eu Rue du Canal 61, B-1000 Brussels, Belgium Phone: +32 2 217 58 68 Fax: +32 2 219 79 89 2 MK Engineering Avenue Molière 116, B-1190 Forest, Belgium Phone: +32 2 340 65 00 Fax: +32 2 340 65 01 ABSTRACT

In the context of the construction of the first building in the new « environmental campus » in Marche-en-Famenne developed by the company InvestSud, a first study was realised and finalised the 17th of April 2007 respecting the criteria of a « low-energy » conception. Following the latest developments in low-energy building standards and the developer’s ambition to be at the cutting edge of modern building technology, the project was adapted to meet “passive” standards. Complementary studies by means of dynamic computer simulations were carried out in order to evaluate and control the building’s summer comfort parameters.

The thermal treatment strategies of the ventilation system were defined to guarantee interior comfort restrictions while minimising the use of active systems. These studies have shown that reducing heat gains and evacuating them through passive measures is sufficient to provide a comfortable interior environment. 1 Thermal modelisation of the building 1.1 Presentation of multizone building model The building was modelised in several thermal zones (office north, office south, reception …) as illustrated in Figure 1. These zones were selected based on their orientation, floor and function combining rooms with similar thermal behaviours.

Figure 1 : Thermal and geometrical model of the building

1.2 Hypotheses In order to simulate the thermal behaviour of the building as accurately as possible, parameters concerning the heating, ventilation, infiltration and internal

gains have been introduced for each zone. The hypotheses taken in the construction of this model are briefly described below.

1.2.1 Meteorological data The simulations have been realised using a standard weather reference year for

Saint-Hubert. This reference year is based on a selection of 12 real months, which together represent a representative year. It is recommended to use this reference year to evaluate different concepts as much from an economical perspective as an energetic perspective. 1.2.2 Shadows The simulations take into consideration shadows cast by awnings, exterior solar protections, trees, etc … 1.2.3 User profiles The occupation profiles of the offices and meeting rooms used in the model were defined with the future occupant to correspond as closely as possible to reality.

1.2.4 Heating and cooling The offices, meeting rooms and cafeteria are heated at 20°C during period of

occupation. A detailed comfort study allows to evaluate the risk of overheating and to put into evidence measures necessary to eliminate those risks. 1.2.5 Internal gains For electronic equipment, we can consider that the entirety of power consumed by the equipment is dissipated in the ambient air as heat. All equipment representing internal gains (IT equipment, lighting, …) were included the model.

1.2.6 Ventilation and infiltration The model considers infiltration by all openings (doors, windows, …) while considering wind speed and direction. The air change rate was set at η50 < 0,6 h-1. For the ventilation of the offices, a two way mechanical ventilation unit with high heat recuperation has been considered. The model also considers motorised window openings (0,30 m² per window and 12 m² for the atrium) in order to accurately simulate natural ventilation strategies.

1.2.7 Architectural modeling All the compositions of the walls, floors, roof and glazing were integrated into the building physics model. 2 Ventilation strategy 2.1 Winter period During the winter period, the air is dry and cold. The building will benefit from solar gains through the opening of external shutters. The double-flux ventilation unit with heat recuperation by accumulation (wheel type) is equipped with a hydroscopic film which allows the recuperation of latent heat from the ambient air and avoids a high use of energy for humidity control. A very small heating system is installed in proximity of the ventilation group. This will heat the air coming from the heat recuperation system if necessary. A series of temperature sensors monitor ambient air temperature and a control system adapts the supply air temperature to meet comfort conditions. In order to ensure a high quality of individual control, additional heating coils controlled by thermostat have been installed in each office.

Figure 2 : Winter mode

2.2 Mid-season In mid-season, external climate conditions approach comfort conditions required by occupants. The mechanical ventilation and heating units are stopped and external air is supplied directly to the offices through natural means. The upper part of the

glazing is motorised and controlled by a central regulation. The management of internal conditions is determined through the control of two parameters: 1. solar gains - external shutters permit heat gains that complement internal gains; 2. external air supply - opening of windows and a control of extracted air.

Figure 3 : Mid-season mode

External gains are managed individually per zone, by the opening or closing of the shutters in function of interior and exterior conditions. Air is extracted at the top of the central atrium. The rate of extracted air, and therefore the air change rate in the building, is controlled from the atrium depending on the number of opened windows. Low-energy mechanical extraction fans guarantee a certain air change rate in case the natural flow is insufficient.

2.3 Summer period During the summertime and in periods of high temperatures, external air temperature can exceed internal comfort requirements. It is therefore important to minimize internal and external heat gains. Several strategies have been put in place to realize this: 1. Reduce gains by conduction and infiltration. 2. Protect the building from solar gains with outside shutters. 3. Minimize internal gains by: the centralisation and local evacuation of zones with

high thermal loads such as “print shops” and “data centre”; dimmers on the lighting system based on the natural light present in rooms; presence detectors.

4. Stall and delay temperature evolution in the building. In order to benefit from the thermal mass of the building, and therefore contend with rapid changes of

internal conditions due to an assimilation of heat gains, the ceiling is not completely covered with false ceilings and the mass is in continuous contact with ambient air.

5. Evacuate surplus heat loads [see night-cooling]. These principles are applied in the building and priority is given to passive and low tech solutions. Under these conditions, the heat recuperation system works the other way around by cooling down the supply air with fresher extracted air. This has a high potential of maintaining a comfortable indoor environment.

Figure 4 : Summer mode

2.4 Night-cooling During hot summer days, there is a noticeable decrease in external air temperature (at least 8°C) between nocturnal and diurnal temperature maxima. Night-cooling consists of cooling down buildings at night by means of external air. The process is referred to as a “nocturnal discharge” because heat stored in the building’s thermal mass is evacuated at night. For a sufficient evacuation of calories by night cooling, it is essential to have a significantly higher air change rate at night than that provided for hygienic

purposes : about 4 [vol/h] instead of 1 [vol/h]. The upper portion of the window frames are automatically opened to create an intensive air flow throughout the building. In order to guarantee the necessary air change rate, low-energy fans are used to assist the naturally occurring stack effect.

Figure 5 : Night-cooling mode

The next day, this thermal mass is then able to absorb a significant amount of heat and will assist in avoiding overheating problems.

3 Configuration definition In order to find the technical-economic optimal in terms of comfort and related

investment, several building design and meteorological scenarios are evaluated. 3.1 Reference weather year External mobile shutters, rather than fixed solar shading systems, are used for the south facade. These systems provide better protection from solar gains. For the east and west facades, exterior mobile shutters are also considered. No solar shading device was planned for the north facade. 3.2 Constraining weather file In order to address the specific problems of overheating in the summertime, available data from Uccle in 2003 are used. In addition to this file being in a city (where temperatures are generally higher), 2003 was a year with very strong heat waves in Western Europe and is very constraining in this study. 3.3 Optimisation 1: Solar protection on four facades Following the overheating problems identified for north facing offices, an optimisation study of exterior shutters on all four facades is carried out. 3.4 Optimisation 2: Top-cooling In order to additionally reduce overheating problems, the supply air of the mechanical ventilation unit is cooled to 18°C when the external temperature reaches 24°C. In this case daytime natural ventilation is stopped, the motorised openings are

automatically closed and the ventilation unit is turned back on. A small cooling unit assures the cooling and dehumidification of the supply air. In this scenario, no exterior shutters are installed on the north facade. 4 Synthesis of results The results of the calculations are presented through the number of hours exceeding certain threshold temperatures. It is generally recommended not to exceed 100 hours of occupation above 25°C and 20 hours above 28°C.

The following figure illustrates the average amount of hours with overheating problems as well as the conditions in the rooms with the most problems:

0

50

100

150

200

250

>25°C >26°C >27°C >28°C >29°C >30°CRoom temperature

Tim

e [h

ours

]

Average - Reference weatheryear

Average - Constrainingweather file

Average - Optimisation 1 :Solar protection on fourfacadesAverage - Optimisation 2 :Top-cooling

Maximum - Reference weatheryear

Maximum - Constrainingweather file

Maximum - Optimisation 1 :Solar protection on fourfacadesMaximum - Optimisation 2 :Top-cooling

Figure 6: Number of hours overheating

As clearly demonstrated in Figure 6, the type of weather file has a considerable influence on comfort conditions in the building.

• The reference situation with Saint Hubert’s meteorological data poses no problems: no overheating is observed and requires no mechanical cooling.

• If an extreme weather file (Uccle 2003) is used, the building is subject to a large number of hours with overheating problems.

All the north facing offices (without exterior shutters) largely exceed the number of hours limit to meet comfort conditions with an average of about 230 hours above 25°C and 39 hours above 28°. South facing offices are protected, but still exceed comfort conditions. In the optimisation 1 configuration, the situation reflects an improvement in

comfort conditions by an even temperature distribution. • On average, comfort conditions are nearly reached, however some offices

still present overheating problems which are difficult to accept. The optimisation 2 configuration meets comfort conditions above 28°C for South

offices. • The North facing offices are still frequently subject to temperatures above

25°C. This can be explained by the lack of exterior shutters on this facade. • Cooling needs are 5 MWh/yr with extreme weather conditions.

• Peak power (supply air at 18°C for external air at 30°C) is 16 kW and estimated number of hours working is 400 hours/yr.

5 Analysis and recommendation

In extreme conditions, passive measures are insufficient to guarantee comfort requirements. However, taking into consideration the extreme meteorological conditions considered in this study, with a probable occurrence in Marche-en-Famenne of almost none, the results must be evaluated carefully. These results

should be seen as a test of the building in a very constraining scenario and show that it is possible to reach comfort conditions if certain measures to reduce heat gains are put in place and a passive cooling strategy is implemented. To avoid all risk of overheating problems, a combination of solutions is required, installing exterior shutters on all facades and implementing a top-cooling strategy. Concretely, the options which have been retained in the construction of this building are: 1. Exterior shutters on all four facades,

2. Low-energy extraction fans to guarantee a certain air change rate for night cooling.

3. Due to the low probability of meeting the same weather conditions on site as the Uccle 2003 extremes and the constraining modelling hypotheses, no cooling unit was installed. The ventilation unit is however equipped with a free slot permitting the installation of a cooling unit if necessary.

The building will be operational early September. A monitoring phase will then begin and the regulation parameters will be optimised. Passive House Platform will be kept informed of the evolution of this project.