Page 191 - Design of Solar Thermal Power Plants

176 3. GENERAL DESIGN OF A SOLAR THERMAL POWER PLANT

In the equation, e 0 , e 1 , e 2 , a, b, c and d are seven undetermined
parameters. It is necessary for them to obtain experimental data for
identification by utilizing parabolic trough collector test; G eni is an effec-
tively averaged direct irradiance while considering cosine loss, end sec-
tion loss of tube and the influences of heat-transfer fluid that
passes through the parabolic trough collector. It depends on the measured
solar normal direct irradiance G DN , cosine factor F cos , end loss correction
factor F end , flow time s p of heat-transfer fluid passing from the inlet to the
outlet of a parabolic trough collector, and sampling interval s s of experi-
mental data.
dT fo /ds and dT fi /ds are two first-order derivatives, which need to be
based on the discretization method in the governing equation of nu-
merical heat transfer by utilizing the differential expression of derivatives
deduced through the Taylor expansion method, and handled by applying
the mean difference method, then

dT fo T fo ðn þ 1Þ T fo ðn 1Þ
ðnÞ¼
ds 2Ds
dT fi T fi ðn þ 1Þ T fi ðn 1Þ
ðnÞ¼
ds 2Ds
in which n refers to the quantity of experimental data during the test
(n > 1); Ds refers to the equivalent time interval of two random adjacent
numbers of experimental data.
The first three terms to the right of Eq. (3.72) refer to optical charac-
teristics of parabolic trough collector varying along with the incidence
angle; the fourth and fifth terms refer to the effective thermal capacity of
absorber and heat-transfer fluid of parabolic trough collector; whereas the
last two terms refer to heat losses of parabolic trough collector. They are
mainly determined by the difference between inlet temperature of
heat-transfer fluid within the parabolic trough collector and ambient air
temperature, which also include the dual influences of radiation heat
exchange loss and convection heat exchange loss. In addition, there is a
certain relationship between the dynamic test model and the steady state
test model in ASHRAE 93 standard.
Although Eq. (3.72) is not a linear equation, a linear expression can still
be obtained through the treatment toward the respective quadratic term,
based on which, thermal performance dynamic test model of parabolic
trough solar collector applies MLR on the basis of the least-square
serial methods as the method to identify the seven undetermined
coefficients.
In order to verify the dynamic test model, it shall be applied in
the parabolic trough solar collector in this research. Thus by using the
experimental condition I test data in Section 3.3.3.6, and applying the