Steps towards a vision - a multifunctional glass facade
Since the presentation of Mike Davies' idea of the polyvalent façade , the vision of a multifunctional building skin has produced a number of technological developments taking first steps in this direction. Switchable façades with electrochromic or gasochromic glazing have been investigated for many years. Research has mainly concentrated on material and device development . Large-area chromogenic devices are in the discussion for building applications. Recently a large European project has treated the architectural and technical integration of switchable devices into buildings as well as durability and lifecycle aspects . Glazing companies still balk at the risk of a market introduction of glazings which can change its colour dynamically and thus change characteristic properties like total solar energy transmittance and visual transmittance. In this situation it might seem hazardous to try and incorporate even more functions into a glazing unit than the mentioned technologies. In spite of these considerations, it has been the goal of this research to develop a glass façade system that actively controls not only visual light transmission and the solar energy transmission but may even incorporate further processes in the building envelope The extraction and provision of heat within the transparent façade, i.e. at the interface between the artificial interior and natural exterior environments, offers the possibility to completely control the energy flows and comfort conditions at the façade. For this purpose, a four-layer glazing element with two fluid-filled cavities and one gas-filled cavity between the panes was designed (see Figure 1).The outer liquid layer in cavity I reduces the solar irradiation by absorption even in the clear state. Excess solar heat gains are kept outside. During periods of very high solar radiation, the clear fluid in the outer cavity I is replaced by a coloured one thus increasing absorption. In cavity III the liquid layer holds the surface temperature constant at room temperature - in summer it can be cooled down whereas in winter heat can be added to the room. A heat pump could be used for providing the temperature levels needed for these purposes. These large-area, transparent heating and cooling elements show uniform temperatures over their entire surface, which leads to an increased interior comfort range since convection is reduced due to large areas with uniform temperatures. The gas-filled chamber II in the middle, along with the Low-E coating of the third glass pane, further reduces energy transmission in winter (heat losses) and during the summer (solar gains). Shading, heating and cooling therefore become an integral part of the glazed transparent building envelope which is adaptable to the requirements of the different seasons (Figure 2) Of course this vision is not without risk. It has to be transformed from an idea to a reliable technological system before it can be used by designers and engineers to produce completely different buildings.