Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 419

Figure 14.3.1 Schematic of conversions and losses in a generic concentrating system.

Optical efficiency accounts for the difference between solar radiation available
at the reflector and the amount of radiation effectively transferred to the receiver, while
thermal efficiency accounts for the thermal losses to the ambient mainly as radiative
losses. The resulting solar field efficiency can be written as:

· Q FLUID
G

· Q FLUID
· Q REC

· Q REC
G

η SF =

A =

×

A = η opt × η th

(14.3.3)

· ·

where · Q REC is the heat concentrated on the receiver, η

opt is the optical efficiency and

η

th is the thermal efficiency.
Solar field efficiency strongly depends on the operating conditions: ambient tem-
perature, wind speed, sun position, solar radiation and material properties, etc. In
particular, optical efficiency varies significantly with the position of the sun, while
thermal efficiency is dependent on ambient temperature and solar radiation.
Optical efficiency is usually defined at the design conditions (nominal optical effi-
ciency) and then corrected during different operating conditions to take account of the
variation in material properties according to the incidence angle of radiation.
The overall optical efficiency depends on several contributions, which are usually
split to outline their differences: nominal optical efficiency, geometrical losses and the
K(

θ ) as expressed in equation 3.4.

η opt = η opt − peak · η geom · K ( θ ) (14.3.4)

where η geom is the geometrical efficiency,

which includes shadowing, tail-end losses and blocking, and K ( θ ) is the correction
of the efficiency for the incidence angle. K ( θ ) takes into account the variation in

η opt − peak is the nominal optical efficiency,