6 RESULTS – SUMMARY OF SCIENTIFIC PAPERS
6.6 Paper 5: Thermal performance of in-between shading systems in multilayer
numerical simulations
This journal paper included hot-box measurements of thermal transmittance values (U- values) performed for three insulated glazing units with integrated in-between pane shading systems. The shading devices are venetian-type blinds with horizontal aluminium slats. The windows with double- and triple-pane glazing units have motorised blinds. The shading device is placed in the exterior cavity of the triple-pane glazing. The window with four-pane glazing has a manually operated blind placed in an external coupled cavity. The measurements were compared to numerical simulations using the WINDOW and THERM simulation tools developed at the Lawrence Berkeley National Laboratories.
The uncertainties associated with the hot-box measurements were assessed in accordance with the procedure described in ISO 12567-1:2010 [79]. The total
uncertainty propagation of the measured U-YDOXHV ǻPU
w/Uw, were derived using the
root-mean-square method (RMS). The uncertainty in the sample heat flow was based on the heat balance equation for the metering chamber. The uncertainties stated in this work are given with a coverage factor of two standard deviations and the corresponding 95 % confidence interval.
The aim of the study was to assess the effect of operating the blinds on the U-value of the windows. The U-values as a function of various slat angles and blind positions, as shown in Figure 10, were studied. In the paper, both centre-of-glazing U-values and whole window U-values were measured.
1 2 3 4
Figure 10. Shading slat angles and position configurations illustration,numbers indicating; 1 the blinds fully retracted, 2 the blinds down and the slats in an open KRUL]RQWDOĮ = SRVLWLRQEOLQGVGRZQDQGWKHVODWVLQĮ = 45° position (for the 4- pane window), 4 blinds down aQGWKHVODWVLQDFORVHG YHUWLFDOĮ = 90°) position.
The measured mean U-values for the windows with closed blinds with horizontal slats were unaffected for the window with the 2-pane IGU. The U-value of the 3-pane IGU window was increased by 3 % compared to retracted (open) blinds, whereas the U-value of the window with the 4-pane glazing unit was increased by 1 %.
The measured mean U-values for the windows when closing the blinds with vertical slats were reduced by approximately 3 % for the windows with the 2- and 4-pane glazing unit compared to retracted (open) blinds. The mean U-value of the window with the 3-pane glazing unit was reduced by 1 %.
Shading systems like this are considered by some to be an effective system for reducing the U-values of the glazing units when they are closed. Based on the measurements carried out in this work, it can be concluded that shading devices with properties like the ones measured should not be considered as an effective system for reducing the U- values of windows. The measured and calculated values are shown in Figure 11. The figure shows the U-value on the vertical axis for the 10 different measurement configurations, as described in Table 3, on the horizontal axis.
Table 3. Measurement series description.
Test series ID Test series description
1 2-pane glazing with retracted (open) blinds
2 2-pane glazing with deployed horizontal blinds (closed blinds with horizontal slats)
3 2-pane glazing with deployed vertical blinds (closed blinds with vertical slats)
4 3-pane glazing with retracted (open) blinds
5 3-pane glazing with deployed horizontal blinds (closed blinds with horizontal slats)
6 3-pane glazing with deployed vertical blinds (closed blinds with vertical slats)
7 4-pane glazing with retracted (open) blinds
8 4-pane glazing with deployed horizontal blinds (closed blinds with horizontal slats)
9 4-pane glazing with blinds in 45° position (closed blinds with slats in 45° angle)
10 4-pane glazing with deployed vertical blinds (closed blinds with vertical slats)
Similar trends for the measured values were found in the calculated values. With the exception of the window with the 2-pane IGU, minor reductions in the U-values of the windows were found when closing the blinds.
found for the window with a 3-pane IGU. The U-value of the window with the 4-pane glazing unit is reduced by only 2 %, from 0.654 to 0.647 W/(m²K).
The numerically calculated values were in general found to be lower than the measured values. The reasons for this can be many. Firstly, the actual low-e coatings can be of a poorer quality than what the declared values are stated as. Secondly, even though the pressure difference across the windows (i.e. pressure difference between the warm and cold chamber of the hot box) were found to be close to zero at the start and end of the measurement periods, some air leakages could have occurred during the measurement periods. This will contribute to a higher heat flow from the warm to the cold side. This increase in heat flow contributes to a higher U-value of the window. Thermal bridging effects, other than the ones discussed in relation to the shading devices, could also be a contributing factor for the modelled values being lower than the measured U-values.
Figure 11. Comparison of measured and calculated mean U-values of whole windows, Uwindow.
In order to achieve more effective shading devices, several factors should be explored: x Redesign motors in order to minimize protruding aluminium from the cold to the
warm side.
x Reduce slat thermal conductivity in order to reduce thermal bridging effects. x Improve surface properties of slats (e.g. reduce emissivity) in order to reduce
radiative heat loss from the warm to the cold side.
x Improve airtightness of shading layer by reducing openings between slats when in the closed position and by making tight connections towards the edges of the cavity.