Transparent facades in low energy office buildings Numerical simulations and experimental studies

Abstract

Windows are a key component in the building envelope. They are often, thought of as energy drains and something associated with excessive energy demands in a building. However, in order to assess the energy performance of a window, several factors must be addressed. The most important issues to consider are energy losses due to heat transmission through windows, energy gains from solar radiation as well as transmitted visible light and the influence on artificial lighting demands. Factors like thermal and visual comfort in buildings are additional factors that need to be assessed and addressed. Existing work that has been carried out within this field of topic lacks in addressing two major factors that are important in the context of this thesis: Buildings situated in cold climates. Buildings with highly insulated envelopes. Thus, the need for further research with these aspects in mind emerges and performance assessments of the glazed elements by themselves as well as in combination with shading systems have been carried out. Focus has been on both the thermal and optical performance of systems, and systems performance assessments for whole buildings. The objective of this thesis has been to investigate the performance of windows and various window solutions in the context of low- or zero-energy buildings situated in a cold climate. Both state-of-the-art solutions as well as more theoretical studies of what today’s and tomorrow’s windows can look like are studied. Experimental research has been used in combination with numerical simulations to assess and characterize the performance of windows and solar shading devices. The component-level performance characteristics have been used as input to the analysis on the whole building scale for selected case studies. Likewise, the case study results have been used as a baseline for design criteria for components. One of the aims for the future is to develop solutions for the transparent components of the building to take advantage of highly insulating multi-pane glazing, thus minimizing heat losses. At the same time, the potential energy and lighting gains from solar radiation should be harvested and utilized for heating and lighting. The case studies showed that the thermal properties of the glazing units play a vital role when trying to reduce energy demands in office buildings. Based on this, a choice was made to investigate the possibilities of improving the thermal transmittance values for glazing units. A review of currently available technologies providing low thermal transmittance values was also carried out. One of the main results from the simulation work is that cooling demands are becoming a dominating factor in office buildings with well-insulated envelopes, even in what is commonly considered to be a heating-dominated cold climate like Oslo in Norway. Hence, it is important that the design of the window and glazed façades used in such buildings takes into account not only thermal properties, but also optical properties related to solar insolation. Low window U-values combined with an SHGC close to 0.4 were found to be the optimum for the sample office building situated in a cold (Oslo) climate. This ensures an optimal balance where as much as possible of the useful solar gains are harvested while, at the same time, the solar gains that lead to cooling demands are kept at a minimum. These findings support the need for a more holistic assessment of both thermal and optical properties of windows. However, the Norwegian building regulations only focus on the thermal transmittance of windows and not the solar gains and visible transmittance in an explicit way. Future regulations should be clearer in addressing these aspects when both thermal and visual conditions are considered. The debate related to the introduction of the passive house concept in Norway has been coloured by a certain disregard for windows. It has been a common perception that window areas should be minimized in order to reduce the energy demand of passive houses. However, modern windows can perform well in low- or zero-energy buildings. Windows with 4-pane IGUs will be equal to or even better than highly insulated opaque walls (i.e. equal to passive house standard insulation level) with respect to the total heating and cooling demands in the sample office building. This shows that it could be possible to move away from passive houses with small window areas by using state-ofthe- art windows, thus expanding the flexibility in the architecture, design and layout of future low- or zero-energy buildings. The choice of shading control strategy can have significant impacts on the energy demand of offices. Depending on strategy, the energy demand can either increase or decrease compared to an unshaded office cubicle. The potential for reduction of energy demands was found to be as large as 9 % if the right shading strategy is chosen. Furthermore, it was found that the improper use of shading systems will lead to an increase in the total energy demand. This increase in energy demand can be as high as 10 %. Hence, it can be concluded that the wrong use of shading systems will lead to an increase in the total energy demand. This is caused by the fact that the wrong shading strategy will block more of the beneficial solar gains than the unwanted solar gains leading to cooling demands. In addition, glare problems must be addressed and reduced to an acceptable level. Thus, it becomes obvious that modern buildings and the demands of its users make shading devices necessary in order to maintain visual and thermal comfort and also to reduce cooling demands during certain periods of the year. The introduction of controllable solar shading systems is therefore vital to reducing the energy demands; however, such shading devices should not be used without careful planning. Improving the energy performance of windows should not be seen as an exercise in adding more and more layers to the insulated glazing units (IGUs), even though a building’s total energy demand reductions can be as high as 20 % if double-pane glazings are replaced with four-pane glazing units. Likewise, if a triple-pane unit is interchanged with a four-pane glazing unit, total energy demand reduction was found to be as high as 7 %. A major argument against multi-pane IGUs (with four or more layers) is that the weight will increase and make transport, handling and mounting of windows impractical or impossible in addition to extra loads on the load-bearing structure of the frame and surrounding structure. Also, visible solar transmittance will be reduced and the inhabitants’ visual perception of the IGUs will likely be impaired. It was found that the only practical way of reducing the thermal transmittance of IGUs without adding additional glazing layers is to reduce gas thermal conductivity. This could be achieved by reducing the gas pressure in the cavities and thus moving towards vacuum glazing. Another alternative to improve thermal properties is by adding glass layers while at the same time trying to keep the weight of the units low. Using thinner glass layers is a possible solution to this. However, for the layers to be effective, it is vital that the beneficial surface properties from the traditionally low-emissivity coated glass panes are kept. This leads to challenges for extremely thin glass layers, with thicknesses as low as 0.1 mm. As a supplement to the theoretical studies which were performed, measurements were carried out for two selected technologies. The possibilities of using in-between pane shading devices to reduce the thermal transmittance of the glazing units when deploying the shading slats were investigated. The effects of operating the shading devices with various slat angles and blind positions were studied. A reduction of U-values when deploying the shading devices were found to be in the magnitude of 1 to 3 %, making it marginal from a global perspective. Taking into account the thermal bridging effects formed by the shading devices themselves, there are even fewer benefits to this system if the aim is to reduce the thermal transmittance value of the windows. Any beneficial effects expected to be achieved by using integrated venetian blinds as an additional layer in the IGU were found to be counteracted by the thermal bridging of the shading hardware. Hence, shading devices with properties like the ones measured (with aluminium slats) should not be considered as an effective system for reducing the U-values of windows. However, several actions could be taken to improve the efficiency of the shading devices. As a second system with a novel glazing system incorporating phase change materials were also studied. For this kind of product, one is moving away from the traditional notion of windows, as the product is no longer transparent and only a translucent appearance is maintained, but it nonetheless shows some interesting properties which should be taken into account. The utilization of thermal inertia in direct coupling with incident solar radiation is a relatively new concept, but the aim is still to reduce energy demands in the buildings in which it is installed. Another strategy for reduction of energy demands and improvement of comfort in buildings is through the use of thermal mass. A study was carried out where the thermal mass is coupled with a transparent façade element. The element encompasses a layer of phase change material together with a solar shading device in a four-layer glazing unit. The results showed that the latent heat storage capacity of the PCM layer was utilized during a climatic load similar to that of a Norwegian summer day. Beyond this, further studies need to be carried out in order to understand and describe the entire effects of such a system. However, these studies have given results that indicate that the methods used for characterization of the transparent façade element are relevant and that the results form a good base for further studies of such technologies. This work has shown that transparent facades in future low-energy, nearly-zero or zeroenergy office buildings can have good energy performance provided they are well planned with respect to glazing technology, constructions and materials and solar shading solutions.