Utilisation of Passive Solar Energy - Renewable Energy: Technology, Economics and Environment - page 108

Good transparent covers are characterised by high g -values and low U -values.
The single glass panes and insulating glazing used in the past offered very high g -
values on the one hand, but also very high, and disadvantageous, U -values on the
other. Noble gas fillings between the two glass panes, characterised by low ther-
mal conductivity, low specific thermal capacity and high viscosity, help further
reduce the thermal transmission by convection between the two glass panes. Fur-
thermore, optimum adjustment of the pane spacing ensures the lowest possible U -
values. Table 3.1 illustrates examples of optimum spacing and lists the physical
characteristics of some of the most common filling gases.
So-called low ε-coatings help reduce thermal losses due to radiation exchanges
in between the two panes. These coatings reduce the emission coefficient ε , for
long wave radiation from originally 0.84 to 0.04. For short wave radiation these
coatings are highly transparent. Low ε-coated double and triple glazing with noble
gas fillings and panes with infrared reflecting coating ensure both: low U-value s
(thermal transmittance coefficient) and high g -values (energy transmittance fac-
tor).

Table 3.1 Optimum spacing between panes and thermodynamic properties of some win-
dow filling gases at 10 °C /3-1/
Filling
gas

Optimum spac-
ing between
panes in mm

Thermal
conductivity
in W/(m K)

Density
in kg/m 3

Dynamic
viscosity
in Pas

Specific ther-
mal capacity
in J/(kg K)

2.53 10 -2
1.648 10 -2
0.9 10 -2
1.275 10 -2

1.75 10 -5
2.164 10 -5
2.34 10 -5
1.459 10 -5

Air
Argon
Krypton
SF 6

15.5
14.7
9.5
4.6

1.23
1.699
3.56
6.36

1,007
519
345
614

By developing glazing with a high energy transmittance factor g, and transpar-
ent thermal insulation material (TI), transparent covers that offer both a high en-
ergy transmittance and good thermal insulation properties, are obtained. Table 3.2
illustrates some examples of g and U -values of some typical glazing types and a
selection of transparent insulation systems. To indicate the energy transmittance
also the diffuse g -value is considered.
Table 3.3 shows the equivalent U -values corresponding to different glazing
types. By selecting state-of-the-art south facing double-glazing with thermal insu-
lation, heat losses can be nearly compensated; triple glazing can achieve energy
gains. The heat gain of a high-class north facing triple glazing can even exceed its
heat transmission.