Solar energy use in buildings - Solar Energy Sciences and Engineering Applications - page 597

Thermotropic layers are polymer mixtures or hydrogels which are inserted in a
homogeneous mixture between two windowpanes and reduce the light permeability
by up to 75% with rising temperature.
For example, electrochrome thin films made from tungsten oxide can be evapo-
rated on windowpanes with conductive oxide coatings. The two thin film electrodes
are connected by a polymer ion conductor. When an external electrical field is applied
and causes cat-ions (e.g. Li ) from the counter-electrode to accumulate onto the tung-

sten oxide, the transmittance falls dependent on wavelength to 10-20%. A tight seal
at the edges is very important for long-term stability. The first commercially avail-
able glass with a maximum surface of 0.9 m

+

2.0 m achieve a reduction of the total
energy transmission factor from 44% in the bright status to 15% in the dark status
(U-value

×

1.6Wm − 2 K − 1 ). In systems with a lower U-value of 1.1 W m − 2 K − 1 , the
g-value falls from 36% to 12%.

=

18.5 TRANSPARENTTHERMAL INSULATION (TTI)

Since the early 1980s, several thousand square metres of transparent thermally
insulated façade systems have been installed throughout Germany. Compared to con-
ventional thermal insulation in buildings, transparently insulated external walls can
well utilize incoming solar radiation. The energy-saving potential for the application
of such solar systems is high. For example, if a fifth of all existing building façades
in Germany were equipped with transparent insulating systems, approximately 15%
of the heat needed for room heating could be supplied. The technology is particu-
larly interesting for the renovation of old buildings with heavy, very heat-conducting
walls. Transparent insulation can be attached directly onto an external wall and a
transparent plaster is used to protect the material from weather. The potentially high
costs of glass composite structures can be significantly abated by foregoing complex
frame constructions. Transparent thermal insulation can also be applied to provide
daylighting of building interiors. The elements of the thermal insulation can scatter
and direct light and provide for the even illumination of a room. Combined with very
good heat-insulating properties, transparent thermal insulation has the potential for
large-scale application on external building walls.

18.6 OPERATIONAL PRINCIPLE OF TRANSPARENT
THERMAL INSULATION

If short-wave solar radiation hits an external wall, the radiation is absorbed and con-
verted into heat. Although the external surface warms up, most of the heat produced
is transferred to the outside air. Only a small portion of the heat produced reaches the
building interior. If a transparent insulating layer is attached in front of the wall (in the
simplest case, as a windowpane), heat emission to the outside is made more difficult.
In addition to the transmission coefficient for solar radiation and the heat resis-
tance of the transparent thermal insulation, the main parameters influencing the extent
of the useful heat gain are the absorption coefficient, heat conductivity and storage
capability of the adjoining wall. The wall itself produces a time delay in the heat flow