There are two main objectives:
1) Conduct detailed analysis and experimentation of topics as a follow-on to the previous Concerted Action on PV arrays and prepare additional design practices and guidelines for PV systems;
2) Develop reliable low-cost Si-based solar irradiance sensors and field calibration methods for Si thermopile sensors.
2000 commercially produced solar cells have been studied. Dispersions in series and stunt resistances were low. The dispersion in dark current was lower than that in photocurrent. The main criterion to use would seem to be current, using both short circuit current and the current at a voltage near the operating voltage.
A code has been developed to analyse the behaviour of solar cell modules in a partially shaded array, and to predict the position of hot spots. Comparison with experiment confirmed the accuracy of the code, subject to accurate input data on reverse bias characteristics of the modules and on shunt resistance.
It has been found that bypass diodes do not act as simple switches. When a diode is in operation, the current is shared between the diode and the strings, and hot spots can develop even if the block is reverse biased on the diode voltage alone. Two kinds of hot spot can be generated: shaded cells in reverse bias, and nonshaded cells when string current is forced into them. So called equilibrium lines proved to be a waste of cabling.
The work consist of the following four tasks:
1) Array Design Analysis / Test Method Evaluation;
2) Outdoor Module and Array Test Methods;
3) Study of Array Types and Effect of Orientation on Power Output;
4) Solar Irradiance Sensors and Calibration.
As a result of the action present uncertainties in many topics of PV array design and testing methods can be reduced. More standardized designs will increase reliability and make PV systems less expensive. The design guidelines develop for the most important points will be useful to anyone in design of small or large PV plants.
Outdoor module tests executed in laboratories located in different climatic areas on selected representative modules of present commercial production will allow a better understanding of their thermal behaviour, associated electrical parameters and a direct comparison of some state-of-art modules. The knowledge of a good relationship between air ambient temperature and module temperature will yield a much better test and IV extrapolation techniques and models.
The comparison of the energy collection among selected tracking systems and fixed tilt arrays performed for at least one year in very different climatic areas of Europe will also supply further elements for basic decision on array design and economic comparison. A much better mathematical model should result as well as energy prediction methods for use in economic analysis.
The results of the proposed campaign will ensure a clear decision upon the type and design of future radiation sensors for the monitoring of photovoltaic plants. The results will also allow for a sound scientific base to judge questions like degradation, calibration, and durability of sensors. The results should eliminate many uncertainties about Si vs. thermophile sensor differences, how to do an effective calibration at the PV site without removing the sensors, and allow better array performance prediction.
GU14 8PA Farnborough