Decontamination of water by combined solar advanced oxidation processes and biotreatment - Solar Energy Sciences and Engineering Applications - page 379

significantly more susceptible attack by hydroxyl radicals. Standard glass is not sat-
isfactory because it absorbs part of the UV radiation that reaches it, due to its iron
content. Low-iron borosilicate glass, which has good transmissive properties in the
solar range with a cut-off at about 285 nm (Blanco et al., 2000), would seem to be the
most adequate. Therefore, although both fluoropolymers and glass are valid photore-
actor materials, if a large field with a considerable number of photoreactors is being
designed, there will be a high system pressure drop. So in such cases, fluoropolymer
tubes are not the best choice of material, and borosilicate glass is a better solution.
The original solar photoreactor designs (Dillert et al., 1999) for photochemical
applications were based on line-focus parabolic-trough concentrators (PTCs). In part,
this was a logical extension of the historical emphasis on trough units for solar thermal
applications. Furthermore, PTC technology was relatively mature and existing hard-
ware could be easily modified for photochemical processes. The main disadvantages
are that these collectors (i) use only direct radiation (ii) are expensive (iii) have low effi-
ciencies as they concentrate sunlight therefore increasing temperature and promoting
iron precipitation, and (iv) a high iron concentration is needed to absorb concentrated
sunlight. On the other hand, one-sun (non-concentrating) collectors have no moving
parts or solar tracking devices. They do not concentrate radiation, so efficiency is not
reduced by factors associated with concentration and solar tracking. As there is no con-
centrating system (with its inherent reflectivity), the efficiency is higher than for PTCs,
and they are able to utilize the diffuse as well as the direct portion of the solar radiation.
An extensive effort in the design of small non-tracking collectors has resulted in the test-
ing of several different non-concentrating solar reactors (Blanco et al., 2007). Although
one-sun collector designs possess important advantages, the design of a robust one-sun
photoreactor is no simple matter, due to the need for weather-resistant and chemically
inert ultraviolet-transmitting reactors. In addition, non-concentrating systems require
significantly more photoreactor area than concentrating photoreactors and, as a con-
sequence, full-scale systems must be designed to withstand the operating pressures for
fluid circulation.
Design of a solar collector for a photo-Fenton reactor is subject to some major opti-
mization constraints: (1) collection of maximum solar UV-Vis radiation, (2) working
temperatures below 50 o C, (3) efficiency at low iron concentrations, (4) its construction
must be economical, and finally (5) the system pressure drop must be low. Tubular
photoreactors therefore have a decisive advantage in the inherent structural efficiency
of tubing for flowing water. Tubing is also available in a large variety of materials and
sizes and is a natural choice for a pressurized fluid system. A particular type of low
concentration collector called the Compound Parabolic Concentrator (CPC) is used
in thermal applications. This combination of parabolic concentrators and static flat
systems is also an attractive option for solar photochemical applications (Ajona and
Vidal, 2000). CPCs are static collectors with an ideal reflective surface according to
non-imaging optics that can be designed for any given reactor shape. The entire circum-
ference of the receiver is illuminated, rather than just the front, as in conventional flat
plates. The ideal optics of these concentrating devices thus combines both the advan-
tages of the PTC and static systems (Colina-Márquez et al., 2010). The concentration
factor (R C ) of a two dimensional CPC collector is given by Equation 12.2.8.

1
sin θ a =

A
2 π r

R C , CPC = (12.2.8)