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Abstract
This article evaluates the impact of effective sky temperatures on building radiation exchange under clear, cloudy, and dusty conditions for extremely hot and dry climates. In part, a dusty sky temperature model has been introduced as a function of atmospheric aerosol optical depth. The sky radiative exchange was evaluated using a one-dimensional transient heat transfer model with numerical calculations performed using the fully implicit finite-difference method. The newly available ASHRAE 2013 clear sky model was evaluated and implemented to calculate the hourly incident solar radiation for a horizontal roof under the extremely hot–dry climate conditions of Riyadh, Saudi Arabia. Results showed that in clear sky conditions, sky longwave radiation contributes to a reduction of the total heat gain. A daily mean clear sky cooling around 2645 and 2385 W-hr/m2 was estimated for July and January, respectively. In contrast, cloud and dust covers increase effective sky temperature and diminish the role of sky radiative cooling. Depending on severity, the mean contributed sky cooling heat exchange was found to range between 436 and 1636 W-hr/m2 for dust storm and scattered cloudy sky conditions, respectively. Similarly, the ASHRAE 2013 clear sky model and the sky temperature models were shown for four other extremely hot–dry global sites.
Nomenclature
| AOD | = | aerosol optical depth |
| ab | = | beam air mass exponents |
| ad | = | diffuse air mass exponents |
| c1, …, N | = | roof layers thermal capacities (J/kgK) |
| Eb | = | beam normal irradiance (W/m2) |
| Ed | = | diffuse horizontal irradiance (W/m2) |
| E0 | = | solar constant (W/m2) |
| Fss | = | view factor with respect to sky |
| fcloud | = | cloud sky fraction |
| Kt | = | clearness index |
| k1, …, N | = | roof layers thermal conductivities (W/mK) |
| L1, …, N | = | roof layers thicknesses (m) |
| m | = | air mass |
| N | = | roof multiple layers |
| Pv | = | vapor pressure (millibars) |
| qconv | = | outside roof heat convection (W/m2) |
| qi | = | combined internal heat transfer (W/m2) |
| qsky | = | sky longwave radiation (W/m2) |
| qsolar | = | absorbed solar radiation (W/m2) |
| Tamb | = | ambient air temperature (°C) |
| Tdp | = | dew point temperature (K) |
| Tsky | = | effective sky temperature (°C) |
| Tx = L | = | exterior surface temperature (°C) |
Greek
| τb | = | beam pseudo-optical depth |
| τd | = | diffuse pseudo-optical depth |
| ρ1, …, N | = | roof layers densities (kg/m3) |
| ϵ | = | exterior surface emissivity |
| ϵcloud | = | cloudy sky emissivity |
| ϵdust | = | dusty sky emissivity |
| ϵsky | = | sky emissivity |
| ϵsky-clear | = | clear sky emissivity |
| β | = | solar altitude angle |
| σ | = | Stefan–Boltzmann constant (W /m2K4) |