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multiple layers of polymer film with a layer of pure silver to provide for high specular
reflectance (Skytrough Brochure); and (iii) polished aluminium (Alanod).
14.3.10 Linear Fresnel performance
This section describes the performance of a solar field using LFR technology and com-
pares it to PT systems. As before, performance will be described from both optical
and thermal perspectives. With regard to thermal analysis, it is much more difficult
to provide a general indication since LFR technology involves more than one receiver
design. Moreover, the receivers are strongly affected by the concentration ratio as
well as by the absorber tube operating temperature, which also varies with manufac-
turer. As a general consideration, Fresnel collectors usually have a higher concentration
ratio than PT, hence the potential for thermal loss reduction (see Equation 14.2.3). As
regards receiver configuration, the single absorber tube case is similar to PT technology
and will have the highest thermal losses due to the radiative contribution. The sec-
ondary receiver and cavity configuration limit radiative losses thanks to the insulation
layer. Since further general considerations are not possible, thermal efficiency of the
Novatech collector will be presented. Experimental results (Novatech Biosol. Techni-
cal data - NOVA 1) show that heat losses for the Fresnel collector at design conditions
are in the range of 15-30 W/m
2
, with a thermal efficiency of 95% (whereas parabolic
trough efficiency was in the range of 90%). However, these losses were calculated at
lower temperature than the corresponding PT case.
Moving to optical efficiency, this is defined as the collected radiation divided by
the DNI multiplied by the total mirror surface
8
(see Equation 14.3.9).
·
Q
REC
G
·
Q
REC
G
η
SF
=
A
total
=
(14.3.9)
·
·
n
·
A
mir
where the total reflecting area (
A
total
) is equal to the number of mirrors (
n
) times the
single mirror reflecting area (
A
mir
).
Assuming this efficiency (in the literature another definition of optical efficiency
is proposed), LFR has a lower optical efficiency than PT even at design conditions or
where the incidence angle is equal to 0
◦
. A schematic explaining the above-mentioned
concept is shown in Figure 14.3.12. This is because even with solar incidence angle
equal to 0
◦
, the primary mirror must be inclined to centre the solar radiation on the
absorber tube (the angle is equal to
θ
which corresponds to the angle between the
solar and the reflected radiation). Hence, optical efficiency is penalized because of the
cosine effect on the mirror's area (
A
·
cos(
θ
i
)). This effect is more significant for mirrors
further from the receiver.
This effect was not present in PT because the parabolic shape makes the mirrors
parallel to the solar radiation at any given point. In order to increase the optical
efficiency in LFR, the average incidence angle should be minimized; minimization
can be performed in two different ways: (i) by increasing the height of the receiver;
8
There is discussion on this definition, since in parabolic trough systems the aperture area is
considered and not the reflective area as in Fresnel. A recent study showed that a different
definition has limited impact on final results (Giostri et al., 2013).
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