Building integrated concentrating solar systems - Solar Energy Sciences and Engineering Applications - page 555

In terms of the aesthetics of the modules, frameless modules look very similar to
window glass and the individual module is hard to recognize. Its smooth surface has
a high aesthetic value. On the other hand, framed modules give a totally different
effect. Firstly, the frames can be heavy and thus determine the total impression of
the surface. Moreover, the highly visible frames divide the surface into modules and
every individual module is very recognizable. As a solution, smaller frames in the same
colour as the cells can be used and are less visible. Also, the soldering between the
cells is a small detail but is important for the image of very visible PV systems. Older
techniques had very visible and not very smooth soldering. However, new techniques
have hidden the soldering better, e.g. by moving it towards the back. In addition,
modules vary significantly in size, while the glazing is available as single and double
(insulating) glass. In general, thin-film modules allow greater freedom to select size
and colour than c-Si modules (Reijenga, 2003).
Regarding the temperature of the PV panels, module efficiency and thus the
amount of electricity produced, decreases as the temperature increases for mono and
polycrystalline silicon cells (amorphous silicon cells present little temperature depen-
dence). In many non-BIPV applications, modules are mounted on free-standing frames
with ambient air on both sides (for cooling of both sides). In contrast, some BIPV appli-
cations install the modules in close contact to building material, such as roofs or wall
insulation, and the lack of circulating air increases the module's temperature. Hence, a
good design criterion for mono or poly silicon applications is to allow as much cooling
as possible by providing air flow behind the module and minimizing the effect of insu-
lation. This is not an important issue for amorphous silicon modules (Fanney et al.,
2001; Reijenga, 2003).
In terms of cooling of PVs and thus increasing their efficiency, hybrid PVT (Pho-
tovoltaic Thermal) panels have been developed. These systems combine a PV panel
with a thermal collector and in this way they produce electricity and heat simultane-
ously. In parallel, they offer other advantages, such as generation of higher electricity
output than a standard PV panel, maximization of the available roof space, etc. PVTs
are classified according to the kind of heat removal fluid: PVT/water or PVT/air and
according to the type of fluid circulation: passive or active. In the literature there are
several studies about these types of systems (Tripanagnostopoulos et al., 2002; Tonui
and Tripanagnostopoulos, 2007; Ibrahim, 2011).
Finally, it should be mentioned that high-efficiency operation requires substantial
changes to the traditional inverter technologies. For example, the use of micro-inverters
can reduce the overall cost of BIPV systems (Ericsson and Rogers, 2009).
From this brief introduction to building integration of solar energy, it can be noted
that research concerning BIPV started earlier than BIST. In fact, as an indicator, all the
IEA Tasks focused on BIPV have now been concluded, while new Tasks regarding
BIST are ongoing. Globally, the characteristics of how to integrate a photovoltaic or a
solar thermal system reflect the same principles. The main difference is centred on the
materials and the auxiliary systems needed, which with regard to building integration
in the case of PV were more beneficial at the outset. At present, research conducted
into new materials and configurations for solar thermal systems seeks to overcome
difficulties for their building integration and make strong advances in this field.
In the next section, the ideas described above are discussed in depth for building
integration of concentrating systems.