Milan Janak, Slovak Technical University, Bratislava, Slovakia
In the recent years energy efficiency and the passive solar design are being seriously considered in the building design . Wee can see more and more passive solar application such as e.g. conservatories - sun spaces. These glazed spaces provide with thermal buffer and also an excellent extension of the building. However it is non - trivial task to design them for an optimal performance. In Slovakian climatic conditions (cold winters and warm summers) they must perform well around whole year. An usual design practice attempts to maximise solar heat gains and does not consider the consequences for the summer conditions.
Construction company asked for advice about summer performance of the conservatory in single family house in Stupava near Bratislava. There was indication for a summer overheating in the conservatory as originally designed and unfortunatelly already erected (see figure 1 and figure 2).
Figure 1 South view of the conservatory showing large highly transparent roof surfaces exposed to the solar irradiance.
Figure 2 West view of the conservatory showing large highly transparent west wall surfaces exposed to the solar irradiance.
As can be seen from figures 1 and figure 2 , the conservatory has extensive glazing facing directly south and west with almost no shading. These highly transparent double glazing units (normal direct transmittance of 0.73) in combination with limited natural ventilation. All this is indicative of serious summer overheating problems. After an initial consultation with the client it was agreed that an energy simulation will be used to predict the level of internal air temperatures of the conservatory in summer conditions. If necessary, the influence of the different solar control devices would also be evaluated.
The computer model of the conservatory and attached house was created based on geometrical and contructional data provided by the client. A site inspection was carried out to acquire missing data. A climate Test Reference Year (TRY) was based on hourly measurements at Bratislava-Koliba station. Figure 3 illustrates ambient air temperature and direct normal as well as diffuse solar irradiance for a typical hot day in July used in this simulation study.
As can be seen from figure 3 operative temperatures as high as almost 40 oC could be expected. The computer simulation provides evidence that the original conservatory design would very likely cause serious summer overheating and thus occupants thermal discomfort in the conservatory as well as in the adjoining rooms. Because of the fact that the conservatory was almost finished only the additional solar control devices could be used. We have carry out computer simulations to investigate influence of the following solar control devices:
Figure 3 Predicted conservatory operative temperature during a hot day in July. The results are shown for as-is design with virtually no shading and highly transparent double glazing units.
It was determined that none of the solar control devices studied was able to decrease operative temperature in the conservatory under 26 oC - The formally recognised maximum summer thermal comfort temperature for occupied interiors. Therefore, the goal became to decrease internal air temperatures to under the ambient temperature, or in other words, the conservatory environment should not be hotter than that of the exterior air temperature.
The best performance has been predicted for the solar control by application of additional reflective foil on the south roof and west wall glazings of the conservatory. Also the other solar control devices as the external movable roof shading device and internal blinds on the western wall or reflective foil on the roof and internal blinds on the western the wall have shown relative good performance with a maximal operative temperatures of 31.9 oC and 32.6 oC (see figure 4).
Taking into consideration winter conservatory performance, the use of a movable solar control device was suggested to the client. A movable solar control device can be completely closed in hot summer days and completely opened during the heating season to trap useful solar radiation.
Figure 4 Predicted conservatory operative temperature during hot day in July. The solar control reflective foil is applied to the conservatory roof and white reflective blinds are applied to the glazing of the west conservatory wall.
In exposed glazed sun spaces, such as the one described in this case, good performance usually occurs if the interior climate does not get warmer than ambient temperature. The use of the add-on solar control devices was necessary in this case because a particular design had already been built. A much better conservatory design could have been possible, as well as additional costs avoided, if the building performance simulation had been used early in the planning phase to predict environmental conditions. Obviously, at the planing stage a building exists only as idea or sketch and modifications are relatively easy and inexpensive to implement.