A. M. Nilsson1* and A. Roos1

1 Uppsala University, Department of Engineering Sciences, The Angstrom Laboratory, Uppsala, Sweden
* Corresponding Author, Annica. Nilsson@Angstrom. uu. se

Abstract

With modern society facing the task of reducing energy consumption in all areas of life, modern windows provide an enormous potential to reduce energy consumption for the heating, cooling and lighting of buildings. For future buildings and for the retrofitting of older buildings the window is more and more becoming an integral part of the building’s energy system. In this paper we present the optical properties for a selection of different window coatings and discuss their impact on the performance of the window. Special emphasis is put on switchable glazing. Optimum performance for switchable glazing is often a trade off between minimum energy consumption for cooling heating and lighting. This can sometimes be in conflict with occupancy preferences. As an example we show how different control strategies for electrochromic windows can influence the energy balance of the window, and that small variations in the control algorithm can lead to improvements. The results were obtained by using the WinSel window simulation tool.

Keywords: Switchable glazing, control strategies, energy balance calculations

1. Introduction

As a reaction to climate change and depletion of energy resources the EU has declared energy efficiency as one of the top priority issues of the political agenda. In the “Action plan for energy efficiency” the European Commission has recognised that the largest cost effective savings potential lies in the residential and commercial building sectors. Thus, the demanding goal recently set by the member states is to save 20% (compared to the level of 2005) of the overall energy consumption in the building sector by the year 2020. Today buildings account for about 40% of the European energy consumption and in 2002 the European Commission published an EU Directive on the energy performance of buildings. Moreover, all member states must introduce an energy performance certification system for buildings, which should be implemented already by the end of 2008. The directive emphasizes the importance of energy efficiency in buildings and requires that new production and major refurbishments meet minimum standard for energy performance. It is important that the standard includes renovations of the current building stock in addition to new construction. The majority of buildings will still predate the standards, but substituting old building components with more energy efficient ones can still provide significant energy savings.

The window is perhaps the single building component with the highest impact on the energy performance, and window technology has undergone major improvements over the past 20 years, transferring the window from an energy-drain to a possible resource in the energy supply system. The development can mainly be attributed to the improvements in large area glass coating technology, making it possible to design low emissivity (low-e) glazing with different solar transmittance. Low emissivity is necessary for obtaining a low thermal transmittance (U-value), i. e. a low heat transfer through the window. This is, together with a high solar transmittance, especially

important in heating dominated climates. To avoid overheating and reduce the energy needed for air conditioning on the other hand, the solar transmittance should be minimized for cooling dominated climates. [1]

The most recent advancement in window coating technology is electrochromic coatings [2,3]. These switch between a bleached, highly transparent, state and a dark, absorbing, state, when a low electric voltage is applied across the switchable coating. Previous simulation studies have shown that, compared to static low-e coatings, significant energy savings can be obtained with variable transmittance glazing [4-6]. When designing the electrochromic control strategy it is important that both energy and daylighting issues are considered in order to avoid glare and reduce electric lighting use, while energy consumption for both heating and cooling is minimized.

In this study, we have used the WinSel window energy balance simulation tool to estimate possible energy saving potentials when an electrochromic window is controlled with four different strategies for energy-efficiency and daylighting [7,8]. The simulations were performed for three locations, Stockholm, Brussels, and Rome, with two static solar control coatings as references. The objective has not been to develop the “best” control strategy, but to illustrate the complexity of the problem and to give some insight into what possibilities these coatings offer. The objective was also to show that useful results can be obtained using a simple window energy balance simulation tool without having to perform detailed building simulations.

2. Background