Session 3, Paper 2, CIBSE ASHRAE Technical Symposium, Dublin, Ireland, 3-4 April 2014
The dynamic control of daylight has been called the “Holy Grail” of the fenestration industry. Electrochromic (EC) glass is believed to be the leading contender in the race to manufacture a glazing technology that will achieve the accolade set down by Steve Selkowitz in 1998. With recent investment in the scaling-up of production capacity, EC glass is now set become a mainstream glazing product. As EC glass darkens (‘tints’) the peak in the spectral transmittance curve shifts to the blue. Whilst control of the luminous and thermal environment is highly desirable, occupants are believed to prefer daylight illumination that is perceived as neutral rather than tinted. Thus the question regarding the neutrality of the illumination spectrum is an important one that needs to be addressed. In this paper the authors show that it is possible to maintain an effectively neutral spectrum of daylight illumination in a space with EC glass in normal operation, provided that a relatively small proportion of the glass is left in the clear state. A theoretical formulation giving the overall spectral transmittance curves for any arbitrary combination of clear and tinted EC glazing in varying proportions is outlined. Applying the theoretical model it should be possible to configure and/or control an actual EC glass installation so that neutral daylight illumination results. The theoretical model is tested using measurements of the daylight spectra in an office space with EC glazing for various combinations of clear and tinted glass.