13 Solar radiation

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L.D.D Advanced Building Physics - Solar radiation and Windows 1. SOLAR AIR TEMPERATURE This section develops a concept that is convenient fro analyzing solar gains due to absorption by opaque exterior surfaces of a building. If the surrounding environment can be characterized by a single temperature T0 for radiation and convection, the resulting heat flow per unit area q = Q/A, from the environment to the surface, is given by: q = h0 (T0 - TS) where: h0: sirface heat transfer coefficient (radiation + convection) TS: surface temperature If, in addition, a radiative flux αI is absorbed on the surface, the total heat flow becomes: qtot = h0 (T0 - TS) + αI To simplify load calculations it is preferable to work with an equation of the first form rather then specifying both a temperature and a flux. This can be accomplished if one replaces the outdoor air temperature T0 by an equivalent temperature: T0 + αI/h0 Not only the solar flux but also infrared exchanges can be treated in this manner, in particular those with the sky when the radiation temperature of the sky is below the air temperature. Both effects are included in what is called the sol-air temperature, defined as: In terms of solar air temperature, the total heat transfer at the surface is therefore: qtot = h0 (T0S - TS) L.D.D If the total heat transfer coefficient (including h0) is U, the extrra heat flow is UΔT0S. Keeping only the solar contribution in ΔT0S, we find out that the solar heat gain ΔQsol through an opaque surface is: 2. WINDOWS Windows, from the point of view of energy, are the most important elements of the envelope of a building, providing at once insulation and rolar radiation. 2.1 U values of windows The heat flow from the interior to the exterior passes through three resistances in series:…