Building integrated concentrating solar systems - Solar Energy Sciences and Engineering Applications - page 564

An important problem associated with linear CPVs is overheating produced by the
high density of light flux received by the cells, the majority of which is transformed
into heat. The high concentration systems mentioned above use a passive cooling
system, facilitated by the reduced dimensions of the cell which allow the use of a
fin-based heat sink. Contrarily, in the case of linear concentration systems, passive
cooling is complicated due to the larger surface area of the solar cells. This results in
less cost-effective dissipators than in the case of insulated cells (Edenburn, 1980). For
solar receivers which receive linearly concentrated light, the most adapted means of
cooling is by active dissipation using liquids such as heat conducting fluids (Florschuetz,
1975). A new group of solar generators has appeared which take advantage of the
evacuated heat stored in the thermal fluid as a bi-product. These are known as hybrid
or co-generation Photovoltaic Thermal Concentrators (CPVT).
Medium concentration systems present a wide range of possible building integra-
tion configurations. The principle designs, grouped by their integration characteristics,
are described below.

i) Parabolic trough concentrators
The installation of parabolic trough concentrators in buildings is similar to that of high
concentration systems; they are generally placed on flat roofs and are ideally hidden
from view. Solar tracking is achieved by rotation of the entire concentrator/receiver
ensemble about a single axis. The majority of devices which use parabolic concentra-
tors are thermal generators (Weiss and Rommel, 2008). Exponents of parabolic CPV
systems are: the Combined Heat and Power Solar System (CHAPS) developed at the
Australian National University, with a concentration factor of 37X, which employs a
photovoltaic/thermal (PVT) module (Coventry, 2005) and the Euclides system designed
at the Polytechnic University of Madrid, with a concentration factor of 38.2X, in
which the cells are passively refrigerated using fins (Luque et al., 1997; Antón and
Sala, 2007). In 2009, Niedermever patented a new concentrating system for PVT
generation (Niedermeyer, 2008).

ii) Linear Fresnel reflectors
With parabolic troughs, daily solar tracking is achieved by moving the entire concen-
trator/receiver ensemble. However, within this range of concentrations good versatility
is offered by systems which work using Fresnel reflection, some of which are worthy
of note (some of the systems described below are included owing to their importance
as concentrating technologies, despite being thermal collectors):

1 Concentrators with 2-axis trackers in which tracking is achieved by movement of

the entire system, such as the BiFres system developed at the University of Lleida
(equipped with a PVT receiver), whose integration in buildings would be restricted
to flat (horizontal) roofs (Rosell et al., 2005) (Figure 17.2.7).

2 Static concentrators in which solar tracking is achieved by movement of the

receiver. This option offers greater scope for integration in buildings as it can
easily be installed on either flat or inclined roofs. Installation on façades, however,
presents certain problems: the mirrors prevent light from passing into the building,
and the mobile receiver, which protrudes outward from the building, creates strain