Solar radiance measurements for fixed and tracking photovoltaic systems


Professor Nicola Pitrone
Universita degli Studi Catania
Italia
E-mail: npitrone@diees.unict.it

Co-Author

Professor G. Tina
 

Abstract: In order to increase the profitableness of photovoltaic systems (PVS) it is decisive to investigate as deeply as possible the features of the primary energy source (the solar energy). This task can be done during the planning or design phase making the right choices on the technology to adopt, on the deployment of the arrays, on the type of the system (fixed or tracking) and also during the operation as well.
In any case suitable instruments to measure some variables connected with the solar radiation are needed. In particular the following considerations should be taken into account:
Planning of PVS: just one solar radiation sensor, instead of two, would allow us to make some measurements of the components (beam and diffuse) of the solar radiation; this characteristic is crucial: in fact, very often the only radiation data available to predict energy production of a PVS, in a given site, is the global radiation on horizontal surface and also, when the diffuse component is available, it has been calculated by means of simple experimental expressions that calculate the diffuse components as a function of clearness index KT .
On the other hand a long term campaign of measurements of both components could be very useful to evaluate the energy performances of system based on tracking systems. Indeed, if the the diffuse component is small in comparison with the beam one, the system equipped with a tracking system (e.g. concentrating PVSs) is more advantageous as far as the economical aspect is concerned.
Operation of PVS: the correct operation of a PV plant can be attained on the basis of monitoring the primary energy (the solar radiation).
To calculate the efficiency of the PV field the solar radiation that reaches the PV array should be measured.
If the PV panels have a tilt angle different from zero, one or more pyranometers parallel to the arrays have to be installed.
In this case, an instrument that can measure both components of the solar radiation could be very useful. In fact, by means of straight calculations it is possible to calculate the radiation that strikes a surface whatever inclination it has, if the beam and diffuse components are known.
Most PVSs undergo some type of monitoring for at least a few years after their installation. Such monitoring can have several goals:
• Ensure that the system is operating properly.
• Assess the performance of system components, pinpoint faulty devices or devices operating below their nominal performance.
• Permit the calibration of design and simulation tools.
• Reveal improvements to the design and increase the understanding of the designer.
Despite its usefulness, monitoring is often overlooked. In the past few years, the authors have encountered numerous examples of improper monitoring. For example, in one case, some important variables as diffuse inplane irradiance were simply not monitored. In another case, improper connections of some sensors to the data logger led to useless numbers. In a third case, improper calibration of an instrument led to recording useless data collected for several years. In another case, a data file was inadvertently erased several months after it was recorded – and no back up was available. And in countless cases, the data gathered have been properly analyzed after months – or years.
Presenting a review of updated references on this subject and reporting the theoretical basis and qualitative considerations that justify the research on new multipurpose radiation sensors is the purpose of the speakers.
Recent developments will be presented by the authors in a paper.