Experimental observations on hot-spots and derived acceptance/rejection criteria

TL;DR: The IES–UPM observations on 200 affected photovoltaic modules are presented, as well as electroluminescence, peak power rating and operating voltage tests have been carried out, and hot-spots temperature gradients larger than 20 °C are proposed as rejecting conditions for routine inspections under contractual frameworks.

About: This article is published in Solar Energy.The article was published on 2015-08-01 and is currently open access. It has received 68 citations till now. The article focuses on the topics: Nominal power (photovoltaic) & Power rating.

Summary

2. Fundamentals of hot-spots

• The by-pass diode assures V ^ 0, thus limiting the negative biasing and the power dissipation in this cell.
• Obviously, the maximum hot-spot temperature is then attained when the group is short-circuited or, which is nearly the same, when the bypass-diode is ON.
• In other words, hot-spot temperature mainly depends on the operating voltage and incident irradiance (which modulates I c ), on the defect gravity (which determines 7SC,D) and on the second quadrant I-V characteristic of the defective cell (which modulates V-D).

Voltage (V)

• The great dispersion in the second quadrant behavior is notorious.
• Associated voltage excursions in the defective module are much larger than that corresponding to the non-defective ones.
• This observation was made at the Cáceres PV plant, at a system with one-axis azimuthal tracking affected by clouds, what explains the evolution of the incident irradiance.
• The latter are slightly hotter than the former.

4.2. Infrared inspection

• Where * stands for the Standard Test Conditions (STC).
• Up to now, there has not been a widely accepted correlation for considering this effect on the heating of PV modules (IEC, 2014) .
• Nevertheless, the authors think that there is a certain advantage of assuming that the hot-spot temperature is proportional to the incident irradiance.

4.3. Electroluminescence

• The objective of this test is to analyze the correlation between the portion of isolated area of a cell affected by micro-cracks and the magnitude of hot-spots.
• Each module was polarized in the fourth quadrant at 25% of the STC rated short circuit current.
• The authors have followed the crack type classification proposed by Kontges et al. (2011) , dividing the affected cells into C-type (those exhibiting only background noise for the inactive cell part) and B-type (those exhibiting a reduced intensity but higher than the background noise).

4.4. Electrical inspection: power rating

• The individual I-V curves of the affected PV modules were obtained with a commercial I-V tracer (Tritec Tri-ka) and extrapolated to STC in accordance with the IEC-60891 (procedure 1), using the current and voltage temperature coefficients given by the manufacturer.
• The incident irradiance and the operating cell temperature were measured by means of a previously calibrated module of the same technology, used as reference.
• The operating voltage of the PV module, when working within the PV array, was measured by simply inserting "T" connectors into the module output wires.
• The voltage losses as regards the non-defective modules can be understood directly as power losses, as the current is common for all the modules connected in series.

5.1. Visual inspection

• Fig. 10 shows examples of visible defects, where micro-cracks cause a current drift and a corresponding heat that leads to the burning of the metallization fingers and to bubbles at the rear of the modules.
• The authors found observable defects in only a 19% of the concerned PV modules, which is a too weak correlation for considering visual defects as a basis for dealing with hot-spots.

5.2. Infrared inspection

• Fig. 12 shows the combined result of these effects.
• Each point in the graph describes the observed T* HS at two different moments.
• Fig. 12a shows the evolution at the Cáceres PV plant between July 2012 (average ambient temperature, June have been considered on this occasion.
• Here, the average AT* HS has decreased by 22%, in an example of seasonal effects overcoming the degradation over time.

5.3. Electroluminescence

• Module temperature and can thus be much larger during the day (when hot-spots are observed) than during the night (when EL images are obtained).
• Whichever the case, EL images, despite being a very useful tool for quality control during the PV manufacturing processes, are not appealing for dealing with hot-spots in the field.
• Along similar lines, other authors have observed that the correlation between the number of cell cracks in a PV module and the power loss is very noisy (IEA, 2014).

5.4. Electrical inspections: power rating and operating voltage

• First, the standard peak power is not a good indicator of the energy production capacity of defective modules, so that it must be disregarded for dealing with hot-spots.
• Second, the correlation between A.T* HS and A.V* HS and thus, power losses during operation, is positive, but the large dispersion does not allow the correlation at individual levels to be applied.
• Apart from that, Fig. 16 shows the relationship between the normalized hot-spot temperature and the operating voltage loss for a more complete ensemble of the 113 PV modules of the three different manufacturers (78 from the Cáceres PV plant and 35 from the Cuenca PV plant).
• It can be observed that the behavior is not the same for every manufacturer (neither in the correlation slope nor in the spread around it).
• Whichever the case, this behavior spread is not relevant here.

6. Discussion

• As regards energy losses, it seems logical to just extend the application of usual warranties to defective modules.
• Hence, it is proposed to reject any module exhibiting hot-spots whose corresponding voltage losses (in relation to a non-defective module being part of the same string), within the PV system in normal operation, exceeds the allowable peak power losses fixed at standard warranties.
• This is also applicable to PV modules with defective by-pass diodes, regardless the derived hot-spot temperature.

7. Proposal

• Finally defective (type II error when establishing a null hypothesis that considers the module as defective), the method will have correctly classified the suspicious elements in more than the 80%> of the cases.
• Other authors have also reached similar conclusions (Buerhop etal., 2011b) .
• Finally, it is worth mentioning that this procedure and these acceptance/rejection criteria have already been applied by the IES-UPM when mediating in conflicts between module manufacturers and EPC over hot-spots problems during the last years.

8. Conclusion

• There is still not a widely accepted reference on how to face this problem within commercial frameworks.
• This paper has reviewed hot-spot related phenomena, paying particular attention to the fact that hot-spot appearance entails also operation voltage losses at the concerned PV module.
• Then, supported by experimental observations on 200 PV modules exhibiting hot-spots, the paper proposes a practical in-field approach to accomplish IR imaging inspection and straightforward acceptance and rejection criteria that address both the lifetime and the operational efficiency of the modules.
• Then, normalized hot-spot temperatures fewer than 10 °C are considered as not problematic, as they are caused by convective heat transfer and slight solar cells mismatch.
• On the other hand, those over 20 °C should imply a module rejection, as they entail significant probability of hot-spot absolute temperature exceed 85 °C.