Page 178 - Design of Solar Thermal Power Plants

3.3 THERMAL PERFORMANCE OF PARABOLIC TROUGH COLLECTOR 163

applicable for flat-type solar collectors instead of parabolic trough solar
collectors. k qd (q) refers to the collector’s IAM; G* refers to the hemi-
spherical irradiance of aperture of receiver surface of collector; c 1 , c 2 , c 3 , c 4 ,
c 5 are regression coefficients; T m refers to the mean temperature of fluid
within the collector; E L refers to the heat loss; s is the StefaneBoltzmann
4
2
constant, s ¼ 5.67 10 8 W/(m K ); s refers to time; u refers to ambient
wind speed. Therefore, for parabolic trough solar collector under higher
working temperatures and more complex optical effects, there has been
no literature to demonstrate that this function expression remains to be
fully convenient and effective.
By using the mathematical tool of multiple linear regression (MLR), the
two IAMs, k qb (q) and k qd (q) in the quasi dynamic test model are able to be
obtained together with other parameters of the model on a simultaneous
basis, instead of requiring an independent IAM testing process like the
steady state test method mentioned above.
Heat loss in the quasi dynamic test model is expressed by a function
that contains a quadratic polynomial, and it depends on the difference
between the inlet and outlet mean temperature T m parabolic trough
collector heat-transfer fluid and the ambient air temperature T a .
Furthermore, derivatives containing inlet and outlet mean temperature
of heat-transfer fluid of parabolic trough collectors serve as the effective
thermal capacities of collectors, in which dT m =ds can be obtained by
calculating the difference between the current moment T m and previous
moment T m and dividing it by the sampling interval of T fo and T fi .
Although the quasi dynamic test method allows the collector thermal
performance test to last continuously for several hours together with solar
irradiance fluctuation and solar position variation, it still needs to satisfy
certain specified allowable deviation of measurement parameters, which
are shown in Table 3.5. It is a remarkable fact that the testing system re-
quires to strictly control the inlet and outlet temperature of heat-transfer
fluid of parabolic trough collectors and the mass flow of heat-transfer
fluid passing through parabolic trough collectors. However, for a large-
scale parabolic trough solar thermal collection system, it is difficult to
satisfy these test conditions based on its own control equipment.
Based on the inlet temperature of collector and the combination of
natural environmental conditions that contain cloudy and clear days,
testing sequences recommended by the quasi dynamic test method can be
summarized into four types of testing days, and one of them is subject
to the condition of partial cloud. The quasi dynamic test method still
requires testing inlet temperatures of heat-transfer fluid of at least four
evenly separated collectors within the working temperature range of
parabolic trough collectors. Furthermore, collector thermal performance
quasi dynamic test method requires each testing sequence to last for
at least 3 h with an overall testing time of about five testing days.