ENERGY BY NATURE
8
DIRECT EXCHANGE - THE DOUBLE-GLAZED PANEL
The direct exchange technology, combined with the double-glazing is
the only solar thermal
technology that ensures high yields even in winter
.
The glass’ thermal transmittance and its solar factor are the key factors influencing the efficiency of a
solar thermal system.
Single glass has 90% solar factor which lets almost all the available solar radiation in but has a thermal
transmittance U of 5.7 W/m2 K.
A last generation low-emissivity double-glazed panel has a lower solar factor (76% versus 90%) than a
single glass, but a better thermal transmittance: 1.7 W/m2 K.
This means that a double-glazing unit holds more than 340% of energy within the system both during
daytime absorption and night dispersion.
To better understand the effectiveness of the double-glazing unit we will analyse the efficiency
percentage formula.
η = (Ec - Qp)/Io
where
Ec : total radiation captured
Io : incident radiation,
Qp : losses due to conconradiation.
If we regard incident and captured radiation as independent from the product’s manufacturing quality
and if we consider radiation and conduction losses as negligible (the latter may be effectively curtailed
via an excellent insulation), the discriminating element are losses by convection i.e. those caused by the
convective motions that occur in the air space between the radiator and the glass.
The smaller these losses, the higher the efficiency. The air in contact with the receiving surface of the
radiator heats up and tends to rise to the top where it releases much of his energy into the glass, and the
heat then follows its natural path towards the colder environment, i.e. outside.
This phenomenon is more evident in cold climates, characterized by lower temperatures for longer times
of the year. The best way to counter this convective flow towards the surrounding outside environment
in colder climates is using RADIASOL D models with double-glazed units.
Conversely, in warm climate zones where night temperatures never drop below 10°C single glass models
can be effectively used.
air volume
temperature controlled resistor
SINGLE GLASS U value 5,7W/ m
2
K
DOUBLE-GLAZED U value 1,7W/ m
2
K
air volume
gas-filled double-glazing unit
temperature controlled resistor
