Page 170 - Design of Solar Thermal Power Plants

3.3 THERMAL PERFORMANCE OF PARABOLIC TROUGH COLLECTOR 155

According to the testing requirements of concentrating solar collectors,
this standard has mainly considered the respective massive technical
problems, which include:
Influences of tracking/driving system and surface precision of reflector
on thermal performance of collectors; Appropriate selection of stan-
dardized factors for incident solar radiation; Research on quasi steady
state test conditions for solar collectors with high concentration ratios; As
the high temperature synthetic oil applied in testing is lack of sufficiently
precise specific thermal power parameters, the specific thermal power of
synthetic oil shall be determined through calorimetry; Testing and anal-
ysis on vertical incidence and angular incidence on aperture of receiver of
collector; In the case that heat-transfer fluid within the collector does not
flow, sunshine may cause damage to the collector; therefore, requirements
on pretreatment of solar collector shall be cancelled; For large-scale solar
collectors, most of the solar simulators may introduce interferences and
uncertainty; therefore, it is specified to perform the test outdoors under
the clear weather.
This published standard shall be applied in axial or biaxial concen-
trating solar collectors; influences of solar scattering radiation are negli-
gible, only influences of direct radiation, as well as determination of
optical responses of collectors toward different solar incidence angles and
thermal performance of vertically incident solar radiation under different
operating temperatures shall be considered. Methods in this standard are
requested to achieve quasi steady state conditions, measure certain
environmental parameters, and determine the product of inlet and outlet
temperature difference of heat-transfer fluid within the collector and
thermal capacity of heat-transfer fluid. The test method has provided
experiment and calculation procedures in order to determine such
parameters as response time, incidence angle correction factor, range of
near-vertical incidence angle, and the thermal gain rate corresponding to
the near-vertical incidence angle.
According to the definition, response time refers to the time required
for the temperature increase of heat-transfer fluid within a specified col-
lector after the step change of solar radiation, which has determined the
time necessary for achieving quasi steady state conditions. Thermal per-
formance of a collector corresponding to a random incidence angle is
obtained through the calculation of the incident angular modifier (IAM)
and thermal performance of collector under near-vertical incidence.
Measurement of IAM is carried out when the collector heat loss is at the
minimum level; therefore, inlet temperature of heat-transfer fluid during
the measurement is equivalent to or close to the ambient air temperature.
In case that the tested collector is mounted on a biaxial tracking test
platform, the thermal performance test is able to achieve the condition of
vertical incidence of solar radiation through the aperture of receiver of