Improved Window Technologies for Energy Efficient Buildings

The principal objective of this project is to develop new window technologies for improved control of energy flows through windows in energy efficient buildings. Both solar and thermal radiation are considered. Several improvement possibilities have been investigated, some of which are mentioned below. Windows of Today and Tomorrow One study deals with the best windows and glass systems of today and examines future development possibilities. Vacuum windows, smart windows, windows with thin foils, aerogel windows and self-cleaning windows are presented. Another article has presented different solutions and improvement opportunities for the spacer used to separate the glass panes. Several potential ways of improving the spacers are identified, including development of new materials with improved heat insulation properties and extending the heat flow length by smarter spacer designs. In another study it was found that the spacer lifetime depends on the reflection properties of facade materials. Higher reflection gives a longer service life. Insulating Window Frames An study show how highly insulating window frames can be designed. A number of numerical calculations and parameter variations were carried out. Development of new and improved insulation materials for use in the sash and frame is one way to improve the thermal performance. Another study looks at the calculation methods used to calculate the thermal characteristics of the window frames, and proposes improved methods. For thermally well insulated window frames, small inaccuracies in calculation methods could have a major impact on the results. Aerogel Windows Project partner Lian Trevarefabrikk has made windows with aerogel granulate. Two types of windows have been created, a system with a 30 mm thick layer of aerogel and a system with 60 mm aerogel. Measurements were carried out to see if the theoretical and experimental results match, and also to see if heat loss by convection can occur in such systems. The study shows that convection does not reduce thermal insulation capability of the windows. Further, ageing experiments show that the thermal conductivity of aerogel can increase by 10 % if it is exposed to moisture. In another study, we examine if aerogel windows can provide more energy efficient buildings. The results show that facades with aerogel in parts of the facade can achieve a lower energy demand than double pane windows for hot climates such as e.g. Tokyo and Singapore. In colder climates, as in Oslo, triple glazing better. Window-Wall Heat Transfer We have studied how a window's position in the wall will affect the heat loss through the window and the wall. Walls with vacuum insulation and mineral wool were considered. The window location has a major impact on the amount of heat that is lost through the wall. Heat loss varies with the type of wall and the window's distance from the outside wall. It is more beneficial to place the window approximately in the middle of the wall than against the inside or outside, with respect to heat loss. With regard to moisture, however, it is more beneficial to place the window towards the outside. Improvement of Existing Windows In historic/listed buildings, it is often beneficial to improve the existing windows instead of replacing them. Different systems have been investigated: e.g. adding foil with reflective coating on glass surface, adding insulating shading systems between the panes (in box window), and replacing (single) glass with better multiple-pane glazings using thin glass layers or foils. The last option often offers challenges with regard to weight, because systems with more glasses typically are heavier than single glass, and since the original frame/sub-frame structure is not designed for this (both in terms of weight and width/size). We have therefore been working on reducing the weight of the glazing systems, by looking at alternative glass materials. Impact of Façade Properties on Building Energy Use The impact of various façade designs and thermal properties (e.g. wall U-value, window U-value, wall reflectivity, and window solar heat gain coefficient - SHGC) of an office building in Tokyo has been analyzed. Office buildings with different heights and width/depth ratios have been investigated. A reduction of the window SHGC resulted in the largest reduction in energy use, followed by a reduction of the window's U-value and an increase in the facade solar reflectivity. Low-Emissivity Coatings Materials with a low emissivity can be used to reduce the energy transport from both opaque and transparent parts of a building. Materials with a low emissivity radiate less heat than materials with a high emissivity. The emissivity can also affect the amount of daylight and solar radiation throughput in the windows. Different materials and products that can reduce the energy demand of buildings are presented. The aging properties of some coatings are examined.

Saving energy and carbon emissions are currently a top priority for buildings and constructions. With up to about 60 % of the total energy loss through the building envelope coming from its windows, improved fenestration products have a huge potential to provide large energy savings. The main objective of this project is to develop new window technologies for improved control of energy flows through windows in energy efficient buildings (new and existing buildings). Both solar and thermal radiation are c onsidered. The project will be organized in two different Work Packages (WP): Improved Thermal Insulation (WP1) and Dynamic Properties (WP2). In the first work package new and improved frame and spacer technologies will be in the focus. In WP2 research wi ll be carried out on windows with dynamic properties (controllable U-value and solar heat gain coefficient). All activities will focus on promotion of solutions and methods that can reduce the energy use in buildings. The project will be performed with cl ose co-operation between Norwegian University of Science and Technology (NTNU), SINTEF Building and Infrastructure, Lian Trevarefabrikk AS, and Lawrence Berkeley National Laboratory (LBNL).