Utilisation of Passive Solar Energy - Renewable Energy: Technology, Economics and Environment - page 106

Diffuse energy transmittance factor (diffuse g-value) . Depending on time and
season, solar radiation strikes the transparent building elements from very differ-
ent angles. On average, solar incidence on transparent surfaces is thus not vertical.
Furthermore, moderate climate is characterised by a high share of diffuse radia-
tion, amounting to about 60 % of the total incident solar radiation and presenting
an average incidence angle of about 60°. The diffuse g -value g diffuse considers the
decreased energy transmittance factor or g -value in case of vertical incidence,
amounting to about 10 % /3-3/. When compared to the conventional g -value g , the
diffuse g -value g diffuse , allows for more realistic results.

Thermal transmittance coefficient (U-value). The U -value or thermal transmit-
tance coefficient is a measure of the heat that is transmitted from the front side of
a façade to the inside, assuming an area of 1 m 2 and a temperature difference of 1
K. It consists of heat transfer from air on one side of the element, thermal conduc-
tivity within the structural element, and thermal transmission from the other side
of element to the air. In case of double-glazing, heat is transmitted by long wave
radiation and convection between the two glass panes. For windows, we distin-
guish the U G -value, which solely refers to the glazing, and the U W -value, which
also considers heat losses of the window frame, and thus refers to the entire win-
dow.

Equivalent thermal transmittance coefficient (equivalent U-value). The equivalent
U -value is a measure which describes the difference between the specific thermal
losses of a structural element and its specific heat gain by solar radiation. Like the
U -and g -values, it also depends on the radiation incidence on the transparent sur-
face and the dynamic behaviour of the building located behind. For its determina-
tion only thermal gains during the heating season must be taken into account, as
overheating of rooms due to the solar radiation on glazed surfaces is not desirable.
A negative equivalent U -value indicates that the heat gained by a transparent sur-
face exceeds its thermal transmission. The Equation (3.2) allows for approximate
estimation of the equivalent U -value U eq by means of the U W -value, referring to
the entire window (including frame), the g -value (energy transmittance factor) and
a correction factor S W for window orientation. The correction factor S W varies
between 0.95 for north facing, 1.65 for east and west facing, and 2.4 for south
facing orientation.

U = U − S g

(3.2)

eq W W

Transmission losses. As illustrated in Fig. 3.1 thermal losses of a building Q . l con-

sist of ventilation and infiltration losses Q . V and transmission losses Q . T . Transmis-
sion losses are calculated by means of the U -values of the respective surfaces (i.e.
of surfaces A n ) and the temperature difference between internal room temperature
θ i and the corresponding external temperature θ e of all structural elements of a