4.2 Linear Fresnel Lenses
Firstly, before commenting the different properties and characteristics of Fresnel lenses when
applied to building integrated CS, two systems must be mentioned. Although of low
architectural integrability, as the systems described previously, they are the first references of
this kind of linear concentrators.
These products are both formed by arched Fresnel Lenses located on a solar tracker. The first,
designed by Entech Solar (USA) [16], uses a two axis tracker and a PV or PVT receiver. The
second, designed by SEA Corp. (later Photovoltaics Internacional) [17, 18] uses a one axis
tracker and a PVT receiver. Recently, Entech Solar has announced two new systems;
TermaVoltTM II (PVT) and SolarVoltTM II (PV). Both systems are based on the same
technology but using different receivers. Entech has resized the initial prototypes designed in
the 80s into these two smaller and low-cost devices applicable for both ground and roof-mount
applications.
The ability of linear Fresnel lenses to separate the beam from the diffuse solar radiation makes
them useful for illumination control in the building interior space. The Fresnel lenses are
advantageous because they can combine both the concentrating element and the optically
transparent window. The use of Fresnel lenses as a transparent covering material for lighting
and energy control of internal spaces has recently attracted special attention [19].
In addition to mentioning the general benefits of Fresnel lenses, some comparison should be
made between those which are image forming and those which are anidolic. Image forming
Fresnel lenses for solar applications require high precision tracking. Non-imaging lenses, often
2nd European Conference on Polygeneration – 30th March -1st April 2011 – Tarragona, Spain
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convex and arched in shape and designed for medium concentration, using one axis tracking,
have been devised as highly competitive solar collectors. If the tracking requirements are
minimized, the cost reduction achieved by reduction of the PV cells’ surface area outweighs the
cost of the optical elements [20, 21].
The concept of using a fixed concentrator with a tracking absorber has been mentioned in the
past [22-24]. It is based on a stationary wide angle optical concentrator that, whatever the
location of the sun, transmits the input radiation onto a small moving focal area, which, in turn,
is tracked by the receiver. Following this approach, the University of Lleida has developed a
prototype based on a stationary Fresnel lens which focuses solar radiation onto a PVT receiver
which tracks the moving focal area [24, 25]. The advantages of this type of CS make it
architecturally versatile, allowing integration onto flat or inclined roofs or as lightweight
façades, windows etc. Thus their characteristics
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