Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 429

Figure 14.3.8 Annual maps of optical efficiency ratio (OR) for PT technology (Giostri et al., 2013).

For the United States location at latitude north 34 ◦ , the yearly optical efficiency is
53% (meaning that optical losses account for almost half of the solar radiation that is
lost), while the nominal optical efficiency was about 75%.
In addition to optical losses, parabolic troughs are affected by thermal losses. As a
general consideration, the solar field is operated such that inlet and outlet temperatures
are constant; hence, absolute heat losses are constant. Focusing on thermal efficiency,
which was defined in Equation 14.3.3 as the heat transferred to the fluid divided by
the solar radiation impinging on the receiver, it can be seen also as:

· Q FLUID
· Q REC

· Q REC − · Q HT LOS

· Q REC

· Q HT LOS
· Q REC

· Q HT LOS
G

η th = = = 1 − = 1 − (14.3.7)

· A · η

opt

where Q HT LOS is the heat losses [W], G is the solar normal radiation [W/m 2 ], A is the
collector area [m 2 ] and η opt is the optical efficiency as defined in Equation 14.3.5.

Keeping in mind that absolute heat losses are constant, depending only on absorber
size and temperature, it can be noted that thermal efficiency drops when G · η opt

is lower than nominal conditions. Hence, during spring, fall and mostly in winter,
thermal efficiency is significantly lower than in summer. Thermal losses depend also on
ambient temperature since they are proportional to the temperature difference between
the receiver and the ambient (Equation 14.2.3). However, this is a second order effect,
because the temperature variation in typical solar plant sites is about 30-40 ◦ C, while
the average temperature difference between the receiver and the ambient is in the range
of 300 ◦ C.

· A