Passivhaus Conference: High-performance Windows
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POSTER PRESENTATION
High-Performance Windows: Implications for Thermal Comfort in Summer and Winter L D Danny Harvey, Dept. of Geography, University of Toronto, [email protected] Mark Siddall, Dewjo’c Architects, UK, email: [email protected]
1
Window performance and elimination of perimeter heating units
Windows are normally a source of significant heat loss, prompting the installation of heating units at the base of the windows so as to maintain a sufficiently warm inner-glazing surface temperature to prevent cold downdrafts and condensation or undue radiant asymmetry. However, if the heat loss coefficient (U-value) of a window is sufficiently small, the window temperature will be warm enough to avoid all three conditions without using perimeter heating units. The maximum permitted window U-value will be smaller the colder the coldest expected conditions. Figure 1 shows the window U-value that produces inner glazing temperatures of 16°C, or 17°C, as a function of the outdoor temperature, assuming a 20°C indoor air temperature and no incident solar radiation (the results are based on detailed heat flow calculations). Inasmuch as the three conditions listed above will be satisfied at these temperatures, these curves can be interpreted is the maximum permitted U-value, below which perimeter heating units can be eliminated, as a function of the coldest anticipated outdoor temperature (the design temperature) 3.0 Maximum Permitted U-Value (W/m/K)
Indoor Air Temperature = 20oC
2.5
2.0
Effective radiant temperature at o Inner glazing T = 16 C inner surface at16° C
1.5
Effective radiant temperature Inner glazing T = 17oat C inner surface at17°C Perimeter heating required
1.0
Perimeter heating not required
0.5
0.0 -40
-30
-20
-10
0
10
Outdoor Design Temperature (oC)
Figure 1. Whole window U-value below which perimeter heating is not needed as a function of the minimum expected winter outdoor temperature and of the minimum permitted temperature of the innermost glazing surface. NOTE: Whole window U-value, calculated in accordance with BS EN 10077, but also refined to include Ψinst. (psi value of installation thermal bridge) in order to accord with PassivHaus Standard.
Nuremberg 2008
1
POSTER PRESENTATION
2
Implications for overheating and glazing surface temperatures
Low window U-values can be obtained through the use of multiple glazings, lowconductivity gas fills, and low-e coatings. The latter tend to have both higher reflectivity and higher absorptivity in parts of the solar spectrum than uncoated glass, and so will tend to reduce solar heat gain (which offsets some of the energy savings arising from reduced infrared heat loss) and/or lead to very high glazing temperatures. Here, we examine innerglazing surface temperature and solar heat gain during extreme conditions for unshaded triple-glazed windows. Our results are based on detailed radiative transfer calculations at 122 different wavelengths, using spectral and angle-of-incidence dependent optical properties for glass with two different low-e coatings (referred to as T291 and T606, having solar transmittances of 0.291 and 0.606, respectively) and for uncoated glass (T907), Figure 2 shows the inner glazing temperature, U-value, and g-value (direct + indirect solar heat gain as a fraction of solar irradiance) for an unshaded west-facing window at 5 PM on 21 June, 50°N latitude, clear skies, and 40°C outdoor air temperature, and an unshaded south-facing window in winter at noon with -16°C outdoor temperature. Also given are the winter U-values and g-values..
60
T291T907 T606T907
50 40 30 20 10 0
60
o
50 N, South window, Winter noon, Clear Skies, -16oC ambient, 2 841 W/m solar irradiance T291T907 T606T907
50 40 30 20 10 0
1.6
0.9 T291T907 T606T907
1.4
T291T907 T606T907
0.8 0.7
1.2
0.6
1.0
g-value
Winter U-value (W/m2/K)
70
o
70
80
50oN, West window, Summer 5 PM, Clear Skies, 40oC ambient, 2 810 W/m solar irradiance
Innermost glazing-surface temperature ( C)
Innermost glazing-surface temperature (oC)
80
0.8
0.5 0.4
0.6 0.3 0.4
0.2
0.2
0.1
0.0
0.0 None 1
2
3
4
Surfaces with low-e coating
5
65 2&
None 1
2
3
4
5
65 2&
Surfaces with low-e coating
Figure 2. Inner glazing surface temperature under extreme summer and winter conditions for triple glazed windows with low-e coatings (T291 or T606) on the glazing surfaces indicated on x axis label, and for uncoated glass (T907) for the other glazing layers.
2
Nuremberg 2008
High-Performance Windows: Implications for Thermal Comfort in Summer and Winter L D Danny Harvey, Dept. of Geography, University of Toronto, [email protected] Mark Siddall, Dewjo’c Architects, UK, email: [email protected]
1
Window performance and elimination of perimeter heating units
Windows are normally a source of significant heat loss, prompting the installation of heating units at the base of the windows so as to maintain a sufficiently warm inner-glazing surface temperature to prevent cold downdrafts and condensation or undue radiant asymmetry. However, if the heat loss coefficient (U-value) of a window is sufficiently small, the window temperature will be warm enough to avoid all three conditions without using perimeter heating units. The maximum permitted window U-value will be smaller the colder the coldest expected conditions. Figure 1 shows the window U-value that produces inner glazing temperatures of 16°C, or 17°C, as a function of the outdoor temperature, assuming a 20°C indoor air temperature and no incident solar radiation (the results are based on detailed heat flow calculations). Inasmuch as the three conditions listed above will be satisfied at these temperatures, these curves can be interpreted is the maximum permitted U-value, below which perimeter heating units can be eliminated, as a function of the coldest anticipated outdoor temperature (the design temperature) 3.0 Maximum Permitted U-Value (W/m/K)
Indoor Air Temperature = 20oC
2.5
2.0
Effective radiant temperature at o Inner glazing T = 16 C inner surface at16° C
1.5
Effective radiant temperature Inner glazing T = 17oat C inner surface at17°C Perimeter heating required
1.0
Perimeter heating not required
0.5
0.0 -40
-30
-20
-10
0
10
Outdoor Design Temperature (oC)
Figure 1. Whole window U-value below which perimeter heating is not needed as a function of the minimum expected winter outdoor temperature and of the minimum permitted temperature of the innermost glazing surface. NOTE: Whole window U-value, calculated in accordance with BS EN 10077, but also refined to include Ψinst. (psi value of installation thermal bridge) in order to accord with PassivHaus Standard.
Nuremberg 2008
1
POSTER PRESENTATION
2
Implications for overheating and glazing surface temperatures
Low window U-values can be obtained through the use of multiple glazings, lowconductivity gas fills, and low-e coatings. The latter tend to have both higher reflectivity and higher absorptivity in parts of the solar spectrum than uncoated glass, and so will tend to reduce solar heat gain (which offsets some of the energy savings arising from reduced infrared heat loss) and/or lead to very high glazing temperatures. Here, we examine innerglazing surface temperature and solar heat gain during extreme conditions for unshaded triple-glazed windows. Our results are based on detailed radiative transfer calculations at 122 different wavelengths, using spectral and angle-of-incidence dependent optical properties for glass with two different low-e coatings (referred to as T291 and T606, having solar transmittances of 0.291 and 0.606, respectively) and for uncoated glass (T907), Figure 2 shows the inner glazing temperature, U-value, and g-value (direct + indirect solar heat gain as a fraction of solar irradiance) for an unshaded west-facing window at 5 PM on 21 June, 50°N latitude, clear skies, and 40°C outdoor air temperature, and an unshaded south-facing window in winter at noon with -16°C outdoor temperature. Also given are the winter U-values and g-values..
60
T291T907 T606T907
50 40 30 20 10 0
60
o
50 N, South window, Winter noon, Clear Skies, -16oC ambient, 2 841 W/m solar irradiance T291T907 T606T907
50 40 30 20 10 0
1.6
0.9 T291T907 T606T907
1.4
T291T907 T606T907
0.8 0.7
1.2
0.6
1.0
g-value
Winter U-value (W/m2/K)
70
o
70
80
50oN, West window, Summer 5 PM, Clear Skies, 40oC ambient, 2 810 W/m solar irradiance
Innermost glazing-surface temperature ( C)
Innermost glazing-surface temperature (oC)
80
0.8
0.5 0.4
0.6 0.3 0.4
0.2
0.2
0.1
0.0
0.0 None 1
2
3
4
Surfaces with low-e coating
5
65 2&
None 1
2
3
4
5
65 2&
Surfaces with low-e coating
Figure 2. Inner glazing surface temperature under extreme summer and winter conditions for triple glazed windows with low-e coatings (T291 or T606) on the glazing surfaces indicated on x axis label, and for uncoated glass (T907) for the other glazing layers.
2
Nuremberg 2008
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