Page 254 - Design of Solar Thermal Power Plants

4.2 PRINCIPLES FOR CONCENTRATION FIELD LAYOUT 237

largest extent by sun tracking and concentrating direct solar radiation.
However, the working geometries of heliostats and parabolic trough
concentrators are different from each other. A heliostat reflects and
concentrates solar radiation onto a fixed position that does not change
over time, whereas in a parabolic trough system, the heat-absorbing tube
moves with the concentrator to track direct solar radiation, and thus the
target heat-absorbing tube is moving. The parabolic trough concentrator
applies a single axis to track solar radiation, whereas the heliostat applies
two axes to track solar radiation.
Incidence angles are generated by solar positions at different
time points on the aperture of the parabolic trough solar collector,
resulting in cosine effect, end-loss effect, and so on. All of these may
influence whether the parabolic trough solar collector receives sufficient
energy.
For most parabolic trough solar power plants that use collectors on a
northesouth axis that track Sun movement from east to west and morning
to dusk, the incident sunbeam is never normally from directly above.
Therefore, three major influences of oblique incidence of the sunbeam on
the collector aperture must be considered, including cosine effect,
end-loss effect, and incident angle impacting factor.
Direct normal solar irradiance (DNI) is irrelevant to the tracking
position of a parabolic trough solar collector. The part of DNI perpen-
dicular to the aperture area of the collector is the available normal
irradiance (ANI) for the solar field collector and can be calculated as the
product of DNI and the cosine value of the incident angle, namely
ANI ¼ DNIcosq. In the case where the incident angle is 0, solar radiation
is parallel to the normal of the collector aperture, namely ANI ¼ DNI.
In the case where the incident angle does not equal 0, cosine effect
modification shall be conducted. Thus it can be seen that although DNI
resources in winter are still quite high, available solar resources are
greatly reduced by the influences of cosine effect, and therefore solar
resources in winter are normally much less than those in summer. In the
case of establishing a parabolic trough system in a high-latitude area,
special attention must be paid to the influences of ANI on annual power
generation.
In addition to considering the cosine effect caused by solar incident
angle, a certain geometrical relationship exists between the incident angle
and the reflective concentrator and absorber in the case of nonvertical
incidence of sunbeam onto the reflective concentrator. This may result in a
situation where solar radiation that is reflected and concentrated by the
parabolic trough mirror surface is missed by an absorber end piece when
the absorber end is close to the Sun during the parabolic trough operation,
as shown in Fig. 4.10. L end refers to the length of the parabolic trough