Building integrated concentrating solar systems - Solar Energy Sciences and Engineering Applications - page 558

Table 17.2.1 Classification of solar concentrators based on the concentration ratios and their
applications.

Concentration
ratio (X) Traking requirements, type of system Application

1-2 Stationary, CPC Heating, cooling, building integration

2-10 Quasi-stationary, CPC and Parabolic trough Power generation, heating and cooling

10-100 1-axis tracking, Parabolic trough Power generation

100-10000 2-axis tracking, Parabolic dish, Fresnel lens Power generation, CPV

10000-100000 2-axis, solar tower, solar furnace Power generation, materials

assessment, laser

17.2.2.1 Non-imaging optics

Non-imaging optics provide effective and efficient collection, concentration, transport
and distribution of energy in applications where image forming is unnecessary (Welford
and Winston, 1982). In imaging optics an image is formed at the exit aperture or on
a screen, whereas for non-imaging optics no image of the object is formed (Winston
1980). In an “ideal'' non-imaging concentrator the first concentrator aperture is radi-
ated uniformly from a Lambertian source. The absorber then receives a uniform flux
(Leutz, 1999b). The Sun approximates to a Lambertian source, although its brightness
is not uniform and its wavelength-dependent brightness changes significantly across
its disc. Practical non-imaging concentrators are designed with one or two pairs of
acceptance half-angles that accept light (for example, diffuse insolation) incident at
angles other than the almost paraxial rays of the Sun. Concentrated solar fluxes are
thus non-uniform (Winston and Hinterberger, 1995) and flux densities at the absorber
in a non-imaging solar concentrator are influenced by solar disc size and solar spectral
irradiance (i.e. colour dispersion) (Leutz et al., 2000) and by the proportion of diffuse
insolation, particularly at low concentrations (Rabl, 1985).
For non-imaging optical systems, the edge-ray principle (Winston, 1974) states
that extreme rays entering a concentrating system through an entrance aperture must
be extreme rays when leaving this system through another aperture (i.e. receiver or
absorber) for maximal optical concentration.
Non-imaging systems can be made either by using a refracting lens or by using
reflective mirrors (Boes and Luque, 1992). Fresnel lenses may offer flexibility in non-
imaging optical design. For photovoltaics, uniformity of solar flux maintains electrical
efficiencies by minimizing electrical energy losses (Leutz, 1999b). Non-imaging Fresnel
lenses allow uniformity of flux at the photovoltaic material to be achieved as manufac-
turing errors at the back and front faces of Fresnel lenses are partially self-correcting.
In contrast, an angular error in the plane of a mirror leads to twice this error in the
reflected beam.

17.2.2.2 Non-imaging optics: examples
a) Compound Parabolic Concentrator (CPC)
Developed originally for the detection of Cherenkov radiation in particle physics exper-
iments (Hinterberger and Winston, 1966), a CPC for solar energy applications consists