Page 46 - RSE - Results of the Apollon Project

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New intelligent Concentrating Modules

(iCM)

(RSE, TECNALIA)

State of the art

Accurate and reliable tracking is an important issue to maintain a high CPV system output power. Very often we
do not realize that improvement on module effciency can be useless if sun tracking is inaccurate and not reliable.
This topic was not addressed in the Work programme “Concentrating photovoltaics: cells, optics, modules”, but it has
been deemed important by the APOLLON Consortium with a view to geting economically-attractive concentrating
photovoltaics; therefore new solutions have been researched under APOLLON. Inaccurate sun tracking can occur
for several reasons.

p Tracking strategy: so far open loop methods and close loop methods or a mixture of both have been developed
for tracking the sun. The frst ones are based on astronomical calculations of the sun position. Owing to the
refractive local-dependent nature of the atmosphere, these methods do not allow reaching the precision
requested in high CPV systems and are usually utilised along with the second ones. The close loop methods
are based on:
w tracking accuracy sensors, usually PV devices or position sensitive devices (PSD)
w Modules Power Feedback tracking.
p Electronic control is the frst cause of tracking inaccuracy.
p Flexure and torsion of the frame structure where modules are loaded.
p In felds module installation: this can be inaccurate.
p Thermal dilatation of the materials: undesired misalignment can arise under the numerous temperature
cycles undergone by the CPV system.
p Mechanical deformation: wind effects.

The overall tracking inaccuracy value given by the sum of above mentioned factors must be lower than the optical
acceptance angle of the CPV system, otherwise MJ cells will not be properly illuminated and a strong current (power)
mismatch factor will arise, penalising the power output of the CPV system. Of course the solutions for obtaining an
accurate and reliable tracking should not be expensive and material/energy consuming, otherwise this may impose an
unaffordable penalty on the competitiveness of the CPV technology and a strong environmental impact.
the Problem of tracking Accuracy Sensors Cost (tAS)

One of the most advanced tracking strategy makes use of closed-loop sun tracking control based on TAS that
can measure the misalignment between the normal to the module plane and the direction of the sunlight rays that
hit the module surface. In this case, the accuracy of the solar tracker depends on the accuracy of the TAS. Among
the TASs, the Position Sensitive Sensors (PSD) and CCD sensors exhibit the best characteristics in term of resolution
and precision. In the case of the PSD, the spatial resolution is related to the smaller variation of current that can be
measured by an electronic card that acquires the 4 anodic currents, while in the case of the CCD sensors it depends
on the number of pixels available in the quadrant.

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