Today, in well-insulated buildings, ventilation and cooling may account for about 50% of the energy requirements and a well controlled and energy efficient ventilation system is prerequisite for low energy consumption for the building.

The ventilation system for Albertslund Library is based upon natural driven forces caused by the temperature difference and the differential pressure between indoor and outdoor. The natural ventilation system is fan assisted for assuring a comfortable indoor thermal climate on sunny days with rain and high gusting winds.

The success of hybrid ventilation system depend on utilizing the benefits of natural ventilation and that the design of the system is integrated, from the start and in subsequent design stages. This is the case for Albertslund library where a close teamwork with integrated energy design as the main topic was initiated from the very beginning.

1.3 Design

The design of the new library is characterized by big open areas and a high roomheight, which gives a good possibility for use the building for fresh air distribution and gaining good air quality. Renovation of the facade and installation of skylights makes the building highly suitable for natural ventilation by using the facades as fresh air intakes and remove the exit air through the skylights. Thereby, the building will act as a ventilation duct for distributing the fresh air inside the library. The fresh air is then distributed by displacement.

Fresh air intakes are situated along the facade between the facade windows. A convector is integrated in the intake for preheating of the fresh air, as seen in figure

11. The fresh air intakes are controlled with a damper in each intake which all are connected and controlled by the BMS, see figure 3.

The exit air is ventilated to the outside through openable windows in the skylights, see figure 11. The openable high performance windows are regulated by wind direction, indoor temperatures, indoor CO2 level and rain by the BMS. In both sides of each skylight hidden panel convectors are installed for avoiding cold draughts and to contribute to space heating of the library.

The size of the openable area in the skylights are dimensioned after a demand of rate of air change of 3 h"1 (24 000 m3) found from the internal heat and CO2 load. The program ContamW 2.0 has been used for dimensioning the size of the openable area in the intake and exit. Is has been found that the openable area needed in the intake is minimum 7 m2 and that 20 mi2 of openable area is needed in the skylights for exist. Here the openable area has been downwind prioritized.

1.4 Heating season

During the heating season the air change is kept at a minimum by regulating the damper in the fresh air intake and thereby the amount of fresh air after the CO2 level during opening hours. The natural driven forces will keep the CO2 at a satisfying level during the heating season. A convector integrated in the fresh air intake assures that the temperature of the incoming fresh air minimum is 18 °C so

cold draught is avoided. Fan assistance is used when the openable windows are shut because of high winds or rain. Furthermore, openable high performance windows are used in the facade with user control for removal of excess heat during summer.

1.5 Summer

Outside the heating season the natural ventilation system is controlled as during the heating season. However the dampers in the fresh air intakes and the openable windows are regulated by the indoor temperature during opening hours. When needed, the natural ventilation system will also be active during the night with a lower setpoint for indoor temperature then during the opening hours thus cooling the library. Preheating of the ventilation air is shut off outside heating season.

During very hot and sunny days where the natural ventilation cannot keep the indoor temperature at a satisfying level, the fans which are installed in the gable of every 2nd. skylight, will kick in.