Environmental Engineering Reference
In-Depth Information
3.2.1 Definitions
Terms. The translucence of walls is often described by the terms opaque, trans-
parent, and translucent, as well as by solar aperture surface.
The opaque building envelope is not permeated by light and includes, for in-
stance, brick walls, or a roof covered with tiles.
Transparent and translucent parts (e.g. windows) of the building are permeable
by solar radiation. In general, the word transparent means clear, whereas translu-
cent parts of the building can not be seen through. In terms of solar energy utilisa-
tion, the word transparent is also used to describe external parts of the building
that can be seen through but that are not clear, in order to express their permeabil-
ity not only by visible light but also by other components of the solar spectrum.
The term solar aperture surface refers to the translucent envelope surface that is
suitable for solar energy utilisation.
Key figures. Following are some definitions of the main key figures used in terms
of passive solar energy utilisation.
Transmission coefficient. The transmission coefficient τ e indicates the share of
global radiation incident on the irradiated structural element, which is transmitted
through the glazing into the building as short wave radiation. It also considers the
invisible wavelengths of solar radiation. If the transmission coefficient refers to
the vertical radiation incidence, it is designated as τ e * .
Secondary heat flow. The secondary heat flow factor q i indicates the share of
global radiation G g which is absorbed by a structural element and re-radiated into
the building in the form of long wave radiation and convection (see /3-1/). Trans-
parent elements (glazing) also warm up a little by the absorbed incident radiation
and thus also present a secondary heat flow.
Energy transmittance factor (g-value). In addition to the energy supplied by radia-
tion transmission (i.e. in addition to transmission coefficient τ e ), the g- value g or
energy transmittance factor also includes the secondary heat flow q i . It has been
defined for a vertical radiation incidence and the same temperatures on both sides
of the structural element /3-2/. For transparent building parts (glazing), it consists
of the transmission coefficient τ e * , assuming a vertical radiation incidence, and the
secondary heat flow q i (see Equation (3.1)). q in represents the heat flow added to
the structural element. The secondary heat flow q i is calculated by means of this
added heat flow and the global radiation G g .
g
*
+
q
with
q
=
q
/
G
(3.1)
e
i
i
in
g
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