Environmental Engineering Reference
In-Depth Information
Low-iron glass
Transparent insulation
Selective coating
Reflector
Heat insulation
Stainless steel tank
Figure 3.5
Cross-section through an Integral Collector Storage System
Disadvantages of the ICS systems are the high weight and large
dimensions. This makes installation more difficult in many cases. Furthermore,
the system efficiency is usually lower than that of an optimal system with
forced circulation. These are some of the reasons why the integral system type
has not reached a high market penetration to date.
Flat-plate collectors
The most common collectors for solar domestic water heating systems in many
countries today are flat-plate collectors. These mostly consist of three
components:
•
transparent cover
•
collector housing
•
absorber.
An absorber is inside the flat-plate collector housing. This absorber converts
sunlight to heat and transfers it to water in tubes, which passes through the
system. The collector housing is highly insulated on the back and sides to keep
heat losses to a minimum; however, there are still some collector heat losses,
which mainly depend on the temperature difference between the absorber and
ambient air. These losses are subdivided into convection and radiation losses.
Movements of the air cause the convection losses.
A pane of glass covers the collector and avoids most of the convection
losses. Furthermore, it reduces heat radiation from the absorber to the
environment in the same way as a greenhouse. However, the glass also reflects
a small part of the sunlight that can no longer reach the absorber. Figures 3.6
and 3.7 show the mechanism and energy flow in flat-plate collectors.
The
front glass cover
reflects and absorbs a small part of the solar radiant
power
Φ
e
as shown in Figure 3.8; however, the majority of the solar radiation
passes through the glass. The reflectance
ρ
, absorptance
α
and transmittance
τ
