scispace - formally typeset
Search or ask a question
Journal ArticleDOI

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

01 Aug 2015-Solar Energy (Pergamon)-Vol. 118, pp 28-40
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 (3 min read)

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.

Did you find this useful? Give us your feedback

Citations
More filters
Journal ArticleDOI
TL;DR: In this paper, the effect of dust on electrical and thermal behavior of photovoltaic panels is investigated, which leads to different panel behaviors in the thermal, optical and electrical aspects.

122 citations

Journal ArticleDOI
TL;DR: In this paper, a series-connected power MOSFET is used to subtract part of the reverse voltage from the shaded solar cell, thereby acting as a voltage divider.

105 citations

Journal ArticleDOI
14 May 2020-Energies
TL;DR: A comprehensive review and highlights recent advances, ongoing research, and prospects, as reported in the literature, on bypass diode application on photovoltaic modules, as well as the state of the art in the field.
Abstract: Solar photovoltaic (PV) energy has shown significant expansion on the installed capacity over the last years. Most of its power systems are installed on rooftops, integrated into buildings. Considering the fast development of PV plants, it has becoming even more critical to understand the performance and reliability of such systems. One of the most common problems faced in PV plants occurs when solar cells receive non-uniform irradiance or partially shaded. The consequences of shading generally are prevented by bypass diodes. A significant number of studies and technical reports have been published as of today, based on extensive experience from research and field feedbacks. However, such material has not been cataloged or analyzed from a perspective of the technological evolution of bypass diodes devices. This paper presents a comprehensive review and highlights recent advances, ongoing research, and prospects, as reported in the literature, on bypass diode application on photovoltaic modules. First, it outlines the shading effect and hotspot problem on PV modules. Following, it explains bypass diodes’ working principle, as well as discusses how such devices can impact power output and PV modules’ reliability. Then, it gives a thorough review of recently published research, as well as the state of the art in the field. In conclusion, it makes a discussion on the overview and challenges to bypass diode as a mitigation technique.

80 citations

Journal ArticleDOI
TL;DR: In this article, the authors proposed two different techniques for advanced inspection mapping of PV plants; aerial triangulation and terrestrial georeferencing, which were tested in two grid-connected PV systems, of a total installed power of 70.2 KWp.

78 citations

Journal ArticleDOI
TL;DR: In this article, the design and development of a hot spot mitigation technique using a simple, low-cost and reliable hot spot activation technique is proposed using a FLIR i5 thermal imaging camera.

73 citations

References
More filters
Journal ArticleDOI
TL;DR: The findings contained in this paper will prove to be of considerable interest to maintenance personnel at large-scale PV plants and also to those responsible for setting module quality standards and specifications, and even the PV module manufacturers themselves.
Abstract: A number of findings have shown that the test procedures currently available to determine the reliability and durability of photovoltaic (PV) modules are insufficient to detect certain problems. To improve these procedures, ongoing research into the actual performance of the modules in the field is required. However, scientific literature contains but few references to field studies of defective modules. This article studies two different localized heating phenomena affecting the PV modules of two large-scale PV plants in Spain. The first problem relates to weak solder joints whilst the second is due to microcracks on the module cells. For both cases, the cause is identified, and consideration is given with regard to the effect on performance, the potential deterioration over time, and a way to detect the problems identified. The findings contained in this paper will prove to be of considerable interest to maintenance personnel at large-scale PV plants and also to those responsible for setting module quality standards and specifications, and even the PV module manufacturers themselves.

41 citations

Proceedings ArticleDOI
18 Nov 2009

37 citations

01 Jan 2009

36 citations


"Experimental observations on hot-sp..." refers background in this paper

  • ...Second quadrant / - V characteristics of 7 individual cells of a same PV module (Alonso-García and Ruiz, 2006)....

    [...]

  • ...It can be observed that power dissipation at a hot-spot can vary nearly an order of magnitude depending on the defective cell (Alonso-García and Ruiz, 2006; Alonso-García et al., 2003; Herrmann et al., 1997,1998; Muñoz et al., 2011)....

    [...]

  • ...from one cell to another, even within the same PV module (Alonso-García and Ruiz, 2006), the hot-spot temperature also depends on the particular defective cell....

    [...]

Journal ArticleDOI
TL;DR: In this article, a thermo-mechanical cohesive zone model is proposed to predict the evolution of micro-cracks under the action of mechanical and thermal loads in polycrystalline silicon.

26 citations

01 Jan 2005
TL;DR: In this paper, the micro cracks are increased the breakage risk over the whole value chain from the wafer to the finished module, and the actual need to use larger and thinner crystalline silicon wafers increases the risk of yield losses.
Abstract: The mechanical strength of multi- and monocrystalline silicon wafers and cells is strongly dependent on the length and the position of micro cracks in the silicon wafer material. Micro cracks are increasing the breakage risk over the whole value chain from the wafer to the finished module, because the wafer or cell is exposed to tensile stress during handling and processing. The actual need to use larger and thinner crystalline silicon wafers increases the risk of yield losses. Besides the breakage risk in the production, the cells have to withstand the tensile stresses under outdoor operation in the finished modules. Here the tensile stress is induced by temperature changes and mechanical loads from wind and snow. Currently, commercial PV modules have 25 years warranty and their reliability over this time period is simply considered as proven, when a module type was successfully qualified according to IEC 61215. In industrial practice, there is currently no parameter for the mechanical strength of cells and wafers available, which guarantees the reliability of an individual module. Wafers of different thickness were laminated into modules, which were then exposed to mechanical load tests similar to IEC 61215. The micro crack propagation due to the load test was monitored by scattered light on the micro crack planes in the near infrared range. Finally, the power drops resulting from mechanical load tests were compared for standard modules using cells with and without micro cracks.

24 citations