Basics of the photovoltaic thermal module - Solar Energy Sciences and Engineering Applications - page 159

building integrated with a PV system (retrofitted on a tilted roof) in Rome receiving
an annual solar insolation of about 1530 kWh/m 2 /year. The study reported that EPBT
was reduced from 3.3 years (standalone system) to 2.8 years by integrating the PV
modules to the building. Despite these improvements, commercial application of PV/T
air collectors is still marginal, but it is expected to be wider in the near future with
many building facades and inclined roofs expected to be covered with photovoltaics.
PV facades are already well established and are closely identical to PV/T facades.
Hence, by replacing expensive facade cladding materials by PV facades, it is expected
that the costs on a module level will be low compared to all other applications. How-
ever, on a system level the situation may be different; since PV facades are often
unglazed, the temperature levels that can be reached are limited, and the costs of the
additional infrastructure required may outweigh the benefits of the use of this heat, so
it is essential to come up with alternative low-cost system designs. Yet another issue
is that these systems are not yet standardized. However, due to the current strong link
between this type PV/T systems and the existing building projects, efforts are made
to formulate codes based on an architectural point of view and PV manufacturing
constraints (Butera et al., 2005).
5.2.4 PV/T concentrator
The combination of solar radiation concentration devices with PV modules appear
to now be a viable method to reduce system cost, replacing the expensive cells with
a cheaper solar radiation concentrating system. By concentrating, a (large) part of
the expensive PV area is replaced by a less expensive mirror area, which is a way to
reduce the payback time. This argument serves as the main driving force behind PV
concentrators. Concentrating photovoltaics present higher efficiency than the typical
ones, but this can be achieved only when the PV module temperature is maintained
as low as possible (Othman et al., 2005). The concentrating solar systems use reflec-
tive and refractive optical devices and are characterized by their concentration ratio
(CR). Concentrating systems with CR > 2.5 must use a system to track the sun, while
for systems with CR < 2.5, stationary concentrating devices can be used (Winston,
1974). The distribution of the solar radiation on the absorber surface (PV module)
and the increase in its temperature are two problems that affect the electrical output.
The uniform distribution of the concentrated solar radiation on the PV surface and
the suitable cooling mode together can contribute to an effective system operation and
the achievement of high electrical output. PV/T absorbers can be combined with low,
medium or high concentration devices, but so far, only low CR PV/T systems have
been mainly developed so far.
Reflectors of low concentration, either of flat type (Sharan et al., 1985; Al Baali,
1986; Garg et al., 1991) or of Compound Parabolic Concentrator (CPC) type (Othman
et al., 2005; Garg and Adhikari, 1998; Garg and Adhikari, 1999; Garg and Adhikari,
2000) have been suggested. Tripanagnostopoulos et al., (2002) suggested a diffuse
reflector to increase both electrical and thermal output of PV/T systems. Garg et al.
(1991) presented a simulation study of the single-pass PV/T air heater with plane
reflector. They further extended their work on a hybrid PV/T collector with integrated
CPC troughs (Garg and Adhikari, 1998; Garg and Adhikari, 2000). Both the studies
confirmed that the total efficiency of a PV/T collector with a reflector was marginally