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for a more flexible use than typical roller blinds but makes the modelling of their optical
behaviour much more complex. Therefore, it is recommended to use detailed models
that can accurately simulate the optical properties of this type of shading device and help
its optimization early in the design process.
In last years a great effort is being made to develop models for venetian blinds
characterization (Barnaby et al. 2009). Although the first attempt to modeling venetian
blind was probably made by Parmelee and Aubele (1952) based on the 2-D ray tracing
technique, until recently, its detailed simulation and integration in building energy
simulation software was often neglected. Besides the ray tracing technique, which is also
used by Yahoda and Wright (2005) and Talmatamar et al. (1995), the net radiation
method is also adopted in several models: the WIS model (van Dijk and Goulding 1996,
Rosenfeld et al. 2001), which is currently incorporated in ISO 15099 (2003), and models
of Rheault and Bilgen (1989), Yahoda and Wright (2004) and EnergyPlus (2005) (based
on the Simmler et al. (1996) work).
This paper presents an experimental and numerical study on the solar-optical properties
of venetian blinds. The experiments were carried out in situ at an outdoor light coloured
venetian blind with horizontal movable slats. A method for determining solar optical
properties – transmittance, reflectance and absorptance – of venetian blinds based on
the net radiation method is also described. A comparison between computed blind solar
transmittance and in situ experimental results is presented. The model can be used for
different sun profile angles, venetian blind geometries, slat properties and rotation
angles. Therefore, it is suitable for comparing different venetian blinds solutions and
optimizing blind control strategies. A proper blind control strategy should minimize HVAC
and artificial lighting energy consumption while maximizing daylight and view to the
outdoor. Hence, a balance should be found between daylighting/views and glare/solar
gains. A venetian blind control strategy, to avoid direct solar radiation but enhance
daylight, is also implemented.
Experimental campaign
An in-situ experimental campaign was conducted at an east oriented fenestration of an
office room from the Civil Engineering and Architecture Building, at Instituto Superior
Técnico, in Lisbon (38º7’N, 9º1’W), Portugal.
The glazing/shading system consists of an indoor single clear glass of 6mm and an
outdoor light coloured venetian blind with horizontal movable slats. The venetian blind
used in these experiments has white horizontal aluminium slats with 0.08m of width,
0.068m of slat spacing and 0.40 of solar reflectance.
The fenestration system was equipped with three pyranometers – one Kipp&Zonen
CMP3 and two LI-COR LI-200 (Fig.1a, b and c). All data was collected by a Campbell
CR10X datalogger (Fig. 1d) at a 1 minute period and registered as 10 minutes averages.
Figure 1 shows the experimental setup and the sensors used and Figure 2 the location
of the measurement equipments. To minimize the reflective effects of the indoor glazing
and the windowsill, their surfaces were covered with a black opaque fabric, as shown in
Figure 1.
