Environmental Engineering Reference
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higher compared to the systems without concentrators. Due to the increase in solar
radiation, the average plate as well as solar cell temperatures had shown a sharp rise, as
expected. Hence, the system performance in terms of electrical efficiency was low, due
to the fact that the cell performance is dependent on its temperature. To overcome such
overheating issues, Othman et al., (2005) designed a new double-pass photovoltaic-
thermal air collector with fins to enhance the heat extraction. It was observed that the
cell temperature was reduced by a few degrees which had a positive influence on the
cell efficiency.
A simple low concentrating water-cooled type PV/T collector of the building inte-
grated type investigated by Brogren and Karlsson (2001). It incorporates PV/T string
modules with low cost aluminum foil reflectors with a CR of 4.3 times. With reference
to medium concentration devices, PV/T systems based on linear parabolic reflectors
(Chemisana et al., 2011) or linear Fresnel reflectors (Rosell et al., 2005) have been
investigated. Although concentrators of low or medium CR are interesting devices
to be combined with photovoltaics, 3D Fresnel lens or reflector type concentrators
have been recently developed, aiming at the market of concentrating photovoltaics.
The concept of combined linear Fresnel lenses with PV/T absorbers has also been
attempted (Tripanagnostopoulos et al., 2007). Chemisana et al. (2011) carried out a
study on a photovoltaic-thermal module for Fresnel linear concentrator. An advanced
solar unit was designed to match the needs of building integration and concentrat-
ing photovoltaic/thermal generation. The unit contained three basic components: a
domed linear Fresnel lens as primary concentrator, a compound parabolic reflector as
secondary concentrator and a photovoltaic-thermal module. Models for the electrical
and thermal behavior of the system were developed and validated experimentally and
were found that the predicted results showed a good agreement with experimental
measurements.
Even though the PV efficiencies of concentrated PV/T systems have proven to be
high, the market share for such systems is very minimal, which is mainly due to the
fact that these systems are rather bulky, disqualifying them for many PV applications.
Also, since concentrating devices require tracking either one axis or two axes, it makes
building integration impossible. Furthermore, not all climates are suitable for high ratio
concentration, because it depends on the amount of direct irradiation received. In the
aesthetic point of view, the concentrating systems provide different reflections and opti-
cal effects, which are unusual to the built environment and also they might prevent such
systems from being placed visibly in the facade construction. One of the feasible options
may be to install the concentrator on a horizontal roof (e.g. PV/T systems integrated
with booster reflector in parallel rows). One more additional point worthy to note is,
though the small cell area allows the use of more efficient and expensive PV material,
the combination of glazing and reflectors increases the stagnation temperature which
may in turn lead to degradation of materials. For electrical performance, the unifor-
mity of the irradiance may be compromised, increasing mismatch losses. However, this
drawback might be overcome to certain extent by using diffuse reflectors.
5.3 PV/T MODULE CONCEPTS
In a PV cell, part of the solar spectrum does not contribute to the electricity production.
Photons with energy lower than the band gap do not have enough energy to create
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