Neutral daylight illumination with variable transmission glass: Theory and validation

Abstract

The dynamic control of daylight is seen as key for the effective exploitation of natural illumination in buildings. Traditional control solutions are invariably used in a sub-optimal manner: blinds/shades are left down for long periods and lights are left switched on. A glazing with a transmissivity that varies continuously between clear and dark extremes, and which can be controlled automatically, could be much more effective in providing a 'well-tempered' daylit environment that meets occupants needs. Amongst the different types of variable transmission glazing that have undergone extensive research and development in the last few decades, those based on electrochromism appear to have the best performance characteristics and the greatest market potential. Electrochromic (EC) glazing generally exhibits a shift in spectral transmission as the glass darkens, e.g. causing it to appear blue as it tints. Occupants, however, are believed to prefer a neutral spectrum of daylight illumination without any pronounced hue. In this paper, the authors show that it is possible to maintain a neutral spectrum of illumination with electrochromic (EC) glazing under normal operation provided that just a small proportion of the EC glazing is kept in the clear state. A theoretical model to predict the daylight spectrum resulting from any arbitrary combination of clear and tinted glazing is described. Predictions from the model are compared with measurements of the daylight spectra in an office with EC glazing under various states of tint. The predicted spectra show excellent agreement with the measurements. The model is applicable to any combination of clear and/or tinted glass panels irrespective of the glazing type(s). The paper concludes with a discussion of design considerations for the effective deployment of EC glazing.

Optimum Glazing Configurations for Visual Performance in Algerian Classrooms under Mediterranean Climate

“…The ratio of minimum to maximum illumination must be greater than 0.5 glazing for daylighting and glazing for views [39,40]. For the use of bilateral lighting, reflective and tinted glasses are applied in the top part of windows in order to moderate the penetration of high angle sunlight [41] and to prevent the direct sunlight that causes glare, especially in summer. The clear glass is also applied in the lower part to maximise the view.…”

Evaluation of a Mixed Method Approach for Studying User Interaction with Novel Building Control Technology

“…In-built delays ("dead-bands") are included to avoid control system switching during periods of varying illuminance (e.g., fast-moving clouds). The lowest glazing zones are generally left untinted, as glare rarely occurs due to daylight through those zones; keeping some panes clear maintains the natural feel of the room [29]. When a user presses a wall switch for a specific zone, the system overrides the "Auto" mode, tinting the zone to the user-selected tint level.…”

“…A detailed overview of the case study setup and glazing characteristics is provided in [29]; the main points are summarised below.…”

Daylighting buildings: Standards and the needs of the designer

“…The control was configured so that, unless overridden, the lower zone would always remain clear to ensure a neutral spectrum of daylight illumination in the space. 46 Based on early experience of the refurbished Architecture Building, it does seem hopeful that many of the major drawbacks of over-glazed buildings could be avoided altogether by substituting standard clear double glazing with zoned EC glass. The daylighting design (that is, performance) of the space would therefore be largely achieved after construction through the programming of the control system.…”