Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 428

Figure 14.3.7 Wind velocity field around five different parabolic troughs.

hubs. The advantages of this configuration are higher rigidity via interlinking and no
cutting or welding during construction. Lastly, the ENEA design is based on a torque
tube with precise reflector supported by arms.
The structure has to be fixed to the ground with foundations which add signif-
icantly to the overall cost of solar fields. To reduce these associated costs, research
activity is also being done on advanced foundations, an example of which is

steel-reinforced, drilled pier foundations.

14.3.6 Parabolic trough performance
This section tries to give an overview of parabolic trough performances during a typi-
cal year. As mentioned above, the parabolic trough transfers solar radiation to a fluid
flowing in the HCE: this conversion is affected by optical and thermal losses. Optical
losses depend mainly on K ( θ ) contribution, and secondarily on end-losses and shad-
owing. In order to give an idea of the impact of K ( θ ) on overall performance, the
optical efficiency ratio (OR) is introduced. This is calculated as the ratio between off-
design optical efficiency and nominal optical efficiency. The hourly OR calculated for
a parabolic trough-based solar plant located in the United States at a latitude of 34 ◦
with N-S axis tracking is shown in Figure 14.3.8.
It can be noted that during the winter, even at midday, optical efficiency is about
half the nominal optical efficiency. This is due to the above-mentioned high incidence
angle consequence of low solar altitude. For example, the cosine effect, which is only
one part of the optical penalty, is equal to 0.6 (during winter, solar altitude at midday
is about 40 ◦ -50 ◦ ). On the contrary, during summer, optical efficiency at noon is close
to nominal conditions because the incidence angle is about 10 ◦ .