Building integrated concentrating solar systems - Solar Energy Sciences and Engineering Applications - page 574

Figure 17.2.14 Fluorescent technology. The main part of reemitted photons are trapped in the layers
and guided by total internal reflection to the PV cells placed on the edges. Photon
loss occurs because of nontrapped emission or absorption by other dyes. Radiation
frequencies noncaptured by the dyes are transmitted through the layers and can be
captured and collected by a second fluorescent device or other system which take
benefit of them.
trapped and guided to the edges of the concentrator, where solar cells convert it into
electricity. This concept was investigated intensively in the early 1980s (Wittwer et al.,
1981; Seybold and Wagenblast, 1989). After 20 years of progress in the development
of solar cells, fluorescent dyes and new concepts, several groups (Luque et al., 2005;
Van Roosmalen, 2004; Goldschmidt et al., 2006; Richards and Shalav, 2005; Rau
et al., 2005; Goldschmidt et al., 2007; Danos et al., 2006; Debije et al., 2007; Slooff
et al., 2007) are currently reinvestigating the potential of fluorescent concentrators.
In the quantum dot concentrator, the luminescent dye is replaced by quantum dots.
Quantum dots are crystalline semiconductors which degrade less than organic dyes.
Quantum dots can be tuned to the absorption threshold by the choice of dot diam-
eter. Red shift between absorption and luminescence is primarily determined by the
variance of dot sizes, which in turn can be optimized by choice of growth conditions.
Reabsorption can therefore be minimized and high efficiencies and high concentra-
tion ratios achieved (Barnham et al., 2000). Of the systems which use this kind of
technology, the organic dye-based Organic Solar Concentrator (OSC), designed in the
Massachusetts Institute of Technology (MIT) and commercialized by Covalent Solar,
has had the most impact, particularly within the field of building integration. The
primary advantages of this system are that it does not require tracking and that its
geometry is completely planar. It is formed of a stack, the principle layers being the
OSC and the PV cells. This system is aesthetically superior to conventional PV systems;
the colour is tuneable, better views through, transparent metal oxide contacts are not
required and they may be formed with flexible plastics. Owing to their versatility, their
position in buildings varies from atriums and roofs to windows. The concentration for
which the system works with best efficiency, when combined with a variety of PV cell
types, is 3 suns (Currie et al., 2008).