building surface radiation properties and heat flux
TRANSCRIPT
Building Surface Radiation Properties and Heat Flux
Ali Joudi1, Harald Svedung2
ABSTRACT
1 Högskolan Dalarna, SERC, SE-79188 Falun, Sweden (E-mail: [email protected])2 SSAB EMEA, SE-78184 Borlänge, Sweden (E-mail: [email protected])
The choice of building materials for new and retrofitted buildings is critical for the building energy performance. The major part of the energy used by a building during its lifetime is used for maintaining a suitable interior thermal climate under varying exterior conditions. This poster addresses the sensitivity of the momentary thermal energy flux through a building envelope, as calculated from a steady-state model, primarily based on surface thermal emissivity (both interior and exterior) and total solar reflectance of exterior surface; a simple two-node steady state model of the heat transfer between the exterior and interior roof surfaces and their surroundings to make the sensitivity analysis in order to point at suitable refinements to dynamic or time-integrated methods of calculating yearly energy use in buildings.
The aim is to show that the explicit treatment of interior irradiative heat transfer between surfaces can be crucial wherever there is a significant radiation asymmetry, e.g., where there are significant temperature differences between surfaces. Detrimental to the effect of interior emissivity would be the convective heat transfer phenomena that coexist variably with the irradiative heat flux. The results indicate potential energy saving by the smart choice of optical properties of interior and exterior surfaces.
This simple steady-state evaluation of a 2-node model indicates that, depending on the level of insulation, a significant reduction of the heat flux though sandwich panels is possible by adjusting the surface optical properties.
In cases where there is a significant radiation asymmetry in the interior of a building, the effect of reduced thermal emissivity on the interior surfaces are more pronounced.
RESULTS
MODEL DESCRIPTION
Incident Solar Irradiance (G)
TSky
RRad,out
TAmbient
RConv,out
Rcond.
Exterior Surface Temperature (Tso)
TInterior Radiation
RRad,in
TAir,in
RConv,in
Interior Surface Temperature (Tsi)
ASSUMPTION / SIMPLIFICATION
1. Steady-state conditions2. One-dimensional conduction, with constant optical properties3. Sky radiation temperature for clear sky is assumed 20 deg C
below the ambient air temperature4. Exterior convective coefficient is dependent to wind speed
adjacent to the surface. Interior convective coefficient is dependent to the direction of the heat flow.
FURTHER LITERATUREPer Erik Nilsson, Achieving the desired indoor climate, Energy efficiency aspect of system
design, ISBN 91-44-03235-8 (2003) p 322Svedung et al., Highly reflective coatings for interior and exterior steel cladding in energy
efficient buildings, submitted to Energy and Buildings (2010)Suehrcke et al., Effect of roof solar reflectance on the building heat gain in a hot climate,
Energy and Buildings 40 (2008) 2224-2235Shan K Wang, Handbook of Air Conditioning and Refrigeration (2001)McGraw-HillASHRAE Handbook of Fundamentals (2005)Dhirendra et al., Effect of atmospheric emissivity on clear sky temperature, Atmospheric
Environment, Volume 29, No. 16, Elsevier Science Ltd (1995)M.Pérez-Garcia, Simplified modeling of the nocturnal clear sky atmospheric radiation for
environmental applications, Ecological Modeling 180 (2004) 395-406Duffie and Beckman, Solar Engineering of thermal processes, 3rd edition (2006)SS-EN ISO 6946:2007 (E) Annex A (2007)
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0 200 400 600 800 1000
Hea
t Flu
x (W
/m2 )
incident solar irradiance (W/m2)
Heat Flux vs. solar irradiance
Total Solar Reflectance = 0.3 IR-Emittance (Exterior) = 0.9 IR-Emittance (Interior) = 0.4
Total Solar Reflectance = 0.1 IR-Emittance (Exterior) = 0.9 IR-Emittance (Interior) = 0.4
Total Solar Reflectance = 0.1 IR-Emittance (Exterior) = 0.9 IR-Emittance (Interior) = 0.9
25°C ambient temp., Clear sky, wind speed of 4 m/s80 cm PIR Sandwich Panel, Tair, in=Tinterior radiation=20°C
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40 60 80 100 120 140
Hea
t Flu
x (W
/m2 )
insulation thickness (mm)
Heat Flux vs. Insulation thickness
Total Solar Reflectance = 0.1 IR-Emittance (Exterior) = 0.9 IR-Emittance (Interior) = 0.9
Total Solar Reflectance = 0.1 IR-Emittance (Exterior) = 0.9 IR-Emittance (Interior) = 0.4
Total Solar Reflectance = 0.3 IR-Emittance (Exterior) = 0.9 IR-Emittance (Interior) = 0.4
25°C ambient temp., Clear sky, wind speed of 4 m/s, PIR insulation800 W/m2 incident solar irradiance, Tair, in=Tinterior radiation=20°C
CONCLUSION
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Hea
t Flu
x (W
/m2 )
interior IR-Emissivity
Heat Flux vs. interior IR-Emissivity
interior radiation temp. Equal to inside air temp.
interior radiation temp. 3 degrees lower than inside air temp.
interior radiation temp. 3 degrees higher than inside air temp.
25°C ambient temp., Clear sky, wind speed of 4 m/s80 cm PIR Sandwich Panel with exterior TSR of 0.5 and IR-Emissivity of 0.9800 W/m2 incident solar irradiance, Tair, in=20°C