Building integrated concentrating solar systems - Solar Energy Sciences and Engineering Applications - page 566

Figure 17.2.8 a) Ring Array Concentrator (RAC), and; b) Slat Array Concentrator (SAC) (Vasylyev,
2002; 2005). (Picture source: Chemisana, 2011).
A new concept in the field of Fresnel reflection systems is the so called Non-imaging
Reflective Lens (NIRL) concentrator, of which there are two types: the axially symmet-
ric Ring Array Concentrator (RAC) and the linearly symmetric Slat Array Concentrator
(SAC) (Vasylyev, 2005). These operate by using mirrors to direct and concentrate light
onto a receiver behind the optical element, thus emulating a lens (Figure 17.2.8). The
high concentration RAC requires two-axis tracking, whereas the medium concentra-
tion SAC can be employed with either one- or two-axis tracking (Vasylyev, 2004).
This type of concentrator combines the high optical efficiency achievable by mirrors
with the flexibility of design which is characteristic of lenses. The principle drawback
of these systems is that solar tracking is achieved by movement of the whole system,
incurring the aforementioned restrictions with regard to architectural integration.
The University of Lleida is currently developing concentration technology which
uses reflection, in a similar way to the systems developed by Chemisana and Rosell
(2009), but with a design which prioritizes architectural integrability. The system con-
sists of a linear Fresnel reflector which focuses radiation in a manner analogous to a
lens. The receiver remains static and solar tracking is achieved by a simple and effec-
tive way by rotation of the individual mirrors. Thus, overall movement is minimized,
facilitating incorporation into buildings and offering different possibilities for suiting
the varied requirements of specific installations (Figure 17.2.9). High and medium
concentration reflective systems are summarized in Table 17.2.2.
iii) Linear Fresnel lenses
Firstly, before commenting on the different properties and characteristics of Fresnel
lenses when applied to BICPV, two systems must be mentioned. Although of low
architectural integrability, as per the systems described previously, these are the first
references of this kind of linear concentrator.
These products are both formed by arched Fresnel lenses situated on a solar tracker.
The first, designed by Entech Solar (USA) (O'Neill et al., 1990), uses a two-axis tracker
and a PV or PVT receiver. The second, designed by SEA Corp. (later Photovoltaics