Simplified Calculation Method for the Energetic Assessment of Glass Double Skin Façades

Transcription

1 Simplified Calculation Method for the Energetic Assessment of Glass Double Skin Façades Abridged Version of Final Report (English) Research Program Future Building Project Duration September 01, 2006 until September 30, 2008 Reference Number By order of Federal Office for Building and Regional Planning (BBR) Responsible Engineers Ingo Heusler, Fraunhofer Institute for Building Physics, Holzkirchen Herbert Sinnesbichler, Fraunhofer Institute for Building Physics, Holzkirchen

2 The Fraunhofer Institute for Building Physics developed a new assessment method for glass double skin façades which can be integrated in the previous calculation specification DIN V The façade gap (air gap) of the glass double skin façades (DSF) has the effect of a thermal buffer zone between indoor and outdoor climate. Dynamic and non-stationary processes take place within this buffer zone, which influence the energy balance of the investigated building. So far, precise calculations of this energy performance were only possible by extensive, dynamic simulation programs allowing the representation of complex building physical processes in the façade gap. Since such calculations are very time-consuming, they are usually not applied in the early stage of planning. For verification according to the energy saving regulation (EnEV) it is, however, necessary to make statements on energy performance already in an early stage of the planning of a building. It must be checked whether the requirements of maximum allowable energy demand are fulfilled or whether optimisations are necessary. In the context of the research program Assessment Method DSF a simplified calculation model was developed to allow the energetic assessment of the façade technology of double skin façade by minor effort. Outdoor measurements of the two different double skin façades were carried out at the test facility for energetic and indoor climatic investigations (VERU, see Fig. 1) allowing the representation of the building physical potentials and differences of the two types of double skin façades chosen as extreme cases as well as the verification of non-stationary simulation models. Fig. 1: exterior view of the test facility for energetic and indoor climatic investigations (VERU). By means of simulation calculations it was possible to develop characteristics of the different types of glass double skin façades to determine the air change rate in the façade gap (air gap) of the double skin façade. These characteristics replace the fixed air change rate of 10 h -1 of the previous model defined according to DIN V

3 The new calculation model The new calculation model can be used already in the early stage of planning on the basis of a few significant parameters of a glass double skin façade. In case of a multitude of parameters the new calculation model would be so comprehensive that the advantage in comparison to the non-stationary calculation in terms of time would be eliminated. Therefore it was specified that only parameters are included which have a significant effect on energy demand. Four fundamental system characteristics were identified, which decisively influence the air change rate of a naturally ventilated double skin façade. The following parameters are regarded for the new calculation model: Type of external glazing Depth of the façade gap Opening surface of the ventilation openings Flow coefficient of the ventilation openings First of all, the glass double skin façade is differentiated in the external layer concerning the type of glazing. Single and double glazing is commonly used in practice. Then the depth of the façade gap must be indicated, which is usually between 0.5 m and 1.5 m for double skin façade constructions. For each combination of external glazing and depth of the façade gap an assessment of the air change rate in the double skin façade can be made by means of the characteristics indicated in the following, if the size and type of the ventilation openings is known. In the process, the aerodynamically active opening surface of the external glass façade A GDF,aero must be determined. It can be determined from the smaller (=representative) surface of the supply and exhaust ventilation openings (A GDF ) and the flow coefficient c v,gdf of the opening. A GDF,aero A c equation 1 GDF v,gdf The determined characteristics establish a linear cohesion of air change rate n ue and the aerodynamically active opening surface of various types of glass double skin façades. n ue z A equation 2 GDF GDF,aero n ue air change rate between the unheated or unchilled building zone (adjacent to the thermally conditioned building zone) and outside in h -1 z GDF distance-dependent air change coefficient (dependent on the distance of the panes of the glass double skin façade) A aerodynamically active opening surface of the external glass façade in m 2 /lfdm GDF,aero The air change rate n ue determined in this way is then used to calculate the thermal transmittance of ventilation according to DIN V

4 External single glazing Fig. 2 gives an example of a characteristic to determine the air change rate n ue of a glass double skin façade with single glazing and a distance between the panes of 0.7m. The respective distance-dependent air change coefficient z GDF is listed in Table 1. Glass double skin façade with external single glazing and distance of the panes 0.7 m Air change rate n ue [h -1 ] ,000 0,025 0,050 0,075 0,100 Representative aerodynamically active opening surface A GDF,aero in m²/m Fig. 2: Table 1: characteristic to determine the air change rate n ue in the façade gap of a double skin façade with external single glazing and a distance between the panes of 0.7 m. distance-dependent air change coefficient z GDF of a glass double skin façade with external single glazing. external single distance of the panes [m] glazing z GDF [-] External double glazing Fig. 3 gives an example of a characteristic to determine the air change rate n ue of a glass double skin façade with double glazing and a distance between the panes of 0.7 m. The respective distance-dependent air change coefficient z GDF is listed in Table 2.

5 Glass double skin façade with external double glazing and distance of the panes 0.7 m Air change rate n ue [h -1 ] ,000 0,025 0,050 0,075 0,100 Representative aerodynamically active opening surface A GDF,aero in m²/m Fig. 3: Table 2: characteristic to determine the air change rate n ue in the façade gap of a double skin façade with external double glazing and a distance of the panes of 0.7 m. distance-dependent air change coefficient z GDF of a glass double skin façade with external double glazing. external double distance between the panes [m] glazing z GDF [-] Exemplification by means of test conditions Two specific office rooms of the same size but with a different glass double skin façade and sunscreen were set up on the outdoor test site of the Fraunhofer Institute for Building Physics in Holzkirchen (see Fig. 4). Both rooms were equipped with the same HVAC- (heating, ventilation and air conditioning) and lighting system. The objective was to generate two identical office rooms concerning orientation, geometry and equipment, but furnished with glass double skin façades which represent extreme cases concerning the thermal properties. The basic model for the simulation calculations is one of the two climatic test rooms. The gross opening surface of the supply and exhaust ventilation openings of the climatic test rooms was approx. 1 m². With a façade width of approx. 3.7 m the representative surface of the opening (A GDF ) can be determined as 0.27 m²/lfdm. This is equivalent to almost 7 % in relation to the total façade surface of approx m² (glazing and ventilation openings). The flow coefficient c v,gdf of the protective grating in combination with the open butterfly valve was approx resulting in an aerodynamically active opening surface of approx m²/lfdm according to equation 1. By means of the characteristic in Fig. 2 for a glass double skin façade with external single glazing and a distance of the panes of 0.7 m or the appropriate distance-dependent air change coefficient z GDF according to Table 1 an air change rate n ue of 1500 x = 35 h -1 can be derived according to equation 2.

6 Fig. 4: exterior view of the two office rooms with glass double skin façade in the test facility VERU. Application instructions The new calculation model is a tool to assess the energy demand of buildings with glass double skin façades within the framework of the energy saving regulation and the related calculation specification DIN V in a simple and during the planning process manageable way according to the monthly balance method. In this respect it is necessary to restrict the number of boundary conditions and parameters significant for glass double skin façades. Due to this approach only certain types of glass double skin façades can be assessed, among them naturally ventilated glass double skin façades which are separated in respective floors. Ventilation of adjacent rooms by means of the double skin façade is not designed in the calculation method at present. External glazing can consist of single or double-pane glazing, the depth of the façade gap can be up to 1.5 m. Any further conditions are related to the ventilation openings of the glass double skin façade. If the double skin façade system to be investigated is different from the above-mentioned specifications, it must be assessed by means of non-stationary building simulation or by measurements as before.