What is PID?

The acronym PID refers to Potential Induced Degradation. Initially, this phenomenon was associated to the back contact cell technology, the corrosion of thin film panels and the band silicon manufacturer processes. However, today PID has also been related to standard silicon technology. Thus, PID may affect  most of the PV facilities installed modules, manufactured with this common and extensively-used technology.

Potential Induced Degradation originates a loss of power caused by the negative potential between the solar cells and the earth. This negative potential is responsible for an accumulation of sodium ions proceeding from the glass into the solar cells. Hence, this negative potential damages the p-n junction and, consequently, jeopardizes the cell performance.

The conditions that have mostly been identified as causes for the appearance of PID are high relative humidity, temperature and leakage currents as well as the use of a high negative potential between the cells and the earth in absolute terms. It has been showed that the effect of PID in an operational facility can generate a loss of power at module level of up to a 50% of the module performance if no actions are undertaken to prevent or reduce it.

How to detect PID?

The Potential Induced Degradation, apart from the monitoring information, can be detected by analyzing the IU-curves, Infrared and Electroluminescence at module level.

How to mitigate PID?

Once PID is confirmed at site, two different solutions have been found that may be taken into consideration to halt the degradation of the modules. They are: at the one hand the grounding of the negative pole of the inverter (commonly known as hard grounding), and on the other hand the inducing of a potential shift during the night. The goal of both options is to increase the potential up to positive values in order to avoid the PID occurrence.

What about “PID-Free” certificates and ISO certified methodology?

In the last couple of years, several institutes and certification companies have run tests to modules, aiming to figure out their degradation due to PID in relation to a specific conditions’ setting. In fact, currently lacks an ISO-certified methodology to evaluate objectively the effect of PID in accordance to every single manufacturer or module.


Termination of the EPC warranty

The termination of the EPC warranty is an important milestone within the life cycle of any PV project, being the moment when the contractual relationship between the EPC contractor and the project company ends. Given the importance of the final acceptance of the plants, it is very common to find relevant topics for both parties that must be settled during this step, such as withheld amounts and liquidated damages. With the aim of deeming the compliance of the terms agreed as per the contracts, the companies often agree to follow up a procedure which is usually detailed in the agreement itself. Nevertheless, and having into account that there is not always an independent party in charge of those tests, disagreements among the players are frequent at this time.

Common tests to be performed at the termination of the warranty to assure the contractor has fulfilled the terms as per the EPC contract.

In order to provide an independent and accurate analysis of a given solar facility’s performance, PVdiagnosis recommends the following scope to assure the compliance of the EPC contract terms:

  • Historical analysis of production and irradiance for the period of application of the EPC warranties.

  • Thermographic analysis of panels, boxes and main equipment.

  • Quality inspection of the main equipment on the site.

  • Inverter performance analysis for assessment of anomalies and supplier characteristics verification.

  • Panel measurements by independent lab, management, analysis and comparison of results for acceptance tests and guarantee claiming processes.

  • Study and assessment of numerous weather stations for comparison and correlation with own PVdiagnosis calibrated weather station.

  • Assessment of facility performance (by means of 72-hour test) for acceptance tests and value verification.

  • Production of panel analysis reports focused on technical anomalies and incidents affecting claim processes.

Why involve a technical third party to perform an independent analysis

By involving a third party to perform an independent analysis of the facility, the project company may clearly understand how well its asset is performing. Additionally, an independent analysis facilitates the appraisal of the facility by third parties depending on the project company’s approaches, for instance: present the current status of the asset to potential buyers –disinvest– and / or to finance entities –start a project finance or keep calm the ongoing one–. Since normally no other claims are possible once the deal on EPC warranty is closed, PVdiagnosis strongly recommends to carry out a detailed review of the facility to avoid further technical issues that may be aggravated in the medium term placing the project’s business model at risk.

Examples of some issues detected by PVdiagnosis that are susceptible to be checked at the termination of the EPC warranty.

  1. PID: Potential Induced Degradation originates a loss of power in the modules caused by the negative potential between the solar cells and the earth. For further information, you may visit: http://pvdiagnosis.com/index.php/en/noticias.html

  2. Discrepancies between the figures declared by the EPC contractor and the real ones. There are several points to be reviewed in depth when monitoring data is analyzed such as: FM events, curtailments, degradation of the modules, snow, etc. in order to comply with the EPC warranty in terms of PR and Availability.

  3. Calibration of Weather Stations. It is recommended to calibrate weather stations by an independent party to avoid deviations within the irradiance data collected.

  4. Low performance and malfunctions of inverters.

What about the snail tracks on solar PV modules?

The term snail track is used in the photovoltaic industry to refer to the apparition of discolored lines (resembling the marks left by a worm or snail) crossing the solar cell. This defect has been observed in crystalline silicon modules, both poly and monocrystalline, from several manufacturers after some time of outdoor exposure.

The aspect of these lines varies from one module to another affecting differently the cell area. In some cases, the tracks zigzag along the cell, leaving some cell fragments isolated while in some other cases they cross the cell parallel to the busbar. Moreover, this kind of discoloration has also been observed at the cell edge and along the busbar, being called in these cases as “framing” and “finger prints”, respectively.

Origin of the snail tracks

The presence of snail tracks has been traditionally related to the existence of cracks or microcracks in the modules. However, the cracks due to the modules’ sealing cannot be considered as the cause of the snail tracks, but only as a prerequisite for its apparition. Although the origin process of snail tracks is not yet well characterized, it is believed that the cracks induce a path for the moisture to get to the front side of the cell where it contacts with the cell's fingers leading to an electrochemical reaction between those fingers and some chemical elements present in the EVA film.

Monitoring of the snail tracks

Although the existence of snail tracks in a module is an indicator for the presence of an electrochemical degradation process as well as for the existence of cracks in the EVA, it has not been proved that they are a direct cause of power loss. Manufacturers usually do not accept warranty claims related to the snail tracks themselves, but only in those cases where the module performance is affected.

Once the modules with snail tracks are found in a photovoltaic facility, it is recommended to monitor its evolution and test whether the performance of the modules is among the guaranteed features.

Analyzing the issue

Aiming to recognize modules affected by snail tracks, it is advisable to firstly carry out a visual inspection of the facility. Once the modules are identified over the field, several tests can be performed to point out anomalies based upon their expected behavior such as electroluminescence tests – to detect inactive areas in the module–, IR images – to observe hot spots caused by heterogeneous currents in the module–  or IV curves at the module level – to check the performance parameters–.

How could PVdiagnosis help you on dealing with the Snail track issue?

PVdiagnosis offers an independent and specialized service designed for anyone interested in assessing the operational condition of a given solar PV facility or portfolio. The main goal of PVdiagnosis is to optimize energy production, verify durability conditions and ensure technical and financial feasibility by making sure that equipment and facility contractual guarantees are complied.

PVdiagnosis has the professional team and the specialized technical equipment required to perform the aforementioned on-site tests. Additionally, should further analysis be needed, PVdiagnosis can also manage the shipment of a sample of modules to a specialized laboratory and its subsequent analysis and report of the results.

The goal of all these tests are (i) to verify how the snail tracks are affecting the facility performance and (ii) give the project company the valuable support in the event of raising a formal claim against the module manufacturer.

Additionally, since PVdiagnosis is an independent and a renowned technical advisor with a track record of services in excess of 500MW, we can boast a close relationship with many of the main module manufacturers which is particularly useful in some cases.

Measuring the Perfomance of PV Plants

 The performance of a PV plant is a term used to refer to how well a plant is operating. Although the concept “performance” is easy to understand, the complex operational dynamics of PV plants make it one of the most discussed topics between operators and owners, especially when the operation and maintenance contract includes bonus or penalties according to the plant performance.

Monitoring: Equipment used

The most standard method for calculating the efficiency of the plant is the calculation of the Performance Ratio. This parameter relates the real energy produced and the energy that could have been produced according to the power installed and the solar resource available. In most of the cases, the production data can be directly obtained by the energy meters that usually have low failure rate and high accuracy. However, for monitoring the radiation reaching the surface of the modules, it is necessary to install sensors able to measure this value.

The pyranometers are the standard instrument for measuring the global amount of solar radiation reaching a surface in a 180º range. Its operating principle is based on the measurement of the difference of temperature between a dark surface and a clear one using a thermopile. The ISO 9060 defines three different categories of pyranometers depending on its characteristics and accuracy:

  • Secondary standard: High quality pyranometer with high expected levels of accuracy around 1%-2%.

  • First class: Good quality pyranometer, with accuracy levels around 5%

  • Second class: Moderate quality pyranometer with 10% accuracy

Besides the pyranometers, the calibrated solar reference cells are also commonly used in the photovoltaic market. They are constructed similarly to the silicon modules that could be found installed in a photovoltaic plant and have the same operating principle, i.e. they convert the photons reaching their surface into an electrical current. The main difference between reference cells and pyranometers is their spectral response. While pyranometers have flat spectral response (same for all wavelengths), the reference cells have different response depending on the wavelength. 

PV performance measurement_1 PV performance measurement_2

The need for recalibration

The radiation sensors, both pyranometers and reference cells, have to be calibrated before they are used due to the importance of having accurate values of radiation in the PV generator plane as they are essential to determine the operating efficiency of the facility. In this concern, calibration allows traceability of the sensor to the certified labs.

Commercially available sensors usually came with a calibration certificate for a period of validity of one or two years and manufacturers recommend recalibrating them every two years in order to maintain their accuracy.

However, the calibrations are made in controlled laboratory conditions that are usually far from the real conditions to which a sensor is exposed in a plant. As a complement to recalibration, PVdiagnosis performs an on-field test of the sensors installed in a facility. The study aims to verify the radiation sensors present in the facility by contrasting their registered measurements with the values obtained by the calibrated sensors of PVdiagnosis. Additionally, it is done an assessment on the condition of the equipment and on the location where they are installed, regarding their exposure to elements which could affect the accuracy of the measurements, such as shade or dirt.

As a result of the test, the parameters of linear correlation between both data series are calculated in order to determine the accuracy level of the sensors in the plant and obtain the equations to correct their responses.

Summer Campaign - Campagna estiva

PVdiagnosis offers an independent and specialized service designed for anyone interested in assessing the operational condition of a given solar PV facility or portfolio. The main goal of PVdiagnosis is to optimize energy production, to verify durability conditions and to ensure technical and financial feasibility by making sure that equipment and facility contractual guarantees are complied with.

PVdiagnosis collects wide technical expertise auditing PV plants, independently assessing the technical features of the projects during their acquisition as well as elaborating their technical lifetime reports.

The main services offered by PVdiagnosis are:

  • Thermographic analysis of panels, string boxes and main equipment.
  • Inverter performance analysis of anomalies and verification of the supplier features.
  • Panel measurements performed by an independent laboratory. Management, analysis and comparison of the results.
  • Study and assessment of the on-site weather stations. Comparison and correlation with our own calibrated weather station.
  • Calibration of sensors according to the O&M contract.
  • Assessment of the facility’s performance (by means of 72-hour test)
  • Production of panel analysis reports focused on technical anomalies and incidents affecting claim processes.
  • Measurement and assessment of I/V Curves of panels and series, aimed at characterizing and analyzing them qualitatively.
  • Quality inspection.

Besides the above, PVdiagnosis is a company certified by TÜV according to the standards of the ISO 9001/2008 and has an extensive experience in excess of 900MW.

Campagna estiva di PVdiagnosis: conosci il tuo impianto fotovoltaico

PVdiagnosis offre un servizio indipendente e specialistico, progettato per chiunque sia interessato a valutare la condizione operativa di un determinato portfolio o specifico impianto solare fotovoltaico. L'obiettivo principale di PVdiagnosis è quello di ottimizzare la produzione di energia, verificare le condizioni di durata e di garantire la fattibilità tecnica e finanziaria facendo in modo che le attrezzature e le garanzie contrattuali degli impianti vengano rispettate.

PVdiagnosis dispone di una vasta esperienza nella conduzione di verifiche indipendentivalutazione tecnica degli impianti durante i processi di compravendita, e di analisi di vita utile dei progetti.

Tra i principali servizi offerti da PVdiagnosis, risaltano le seguenti:

  • Analisi termografica di pannelli, quadri-cabine e attrezzature principali.
  • Analisi delle prestazioni Inverter per la valutazione delle anomalie e verifica le caratteristiche dei fornitori.
  • Misurazione dei pannelli attraverso un laboratorio indipendente, gestione, analisi e confronto dei risultati di prove di collaudo e processi di garanzia.
  • Studio e valutazione delle stazioni meteorologiche per il confronto e la correlazione con le stazioni meteo calibrate dalla stessa PV Diagnosis.
  • Calibrazione annuale dei sensori d’accordo ai vincoli contrattuali O&M
  • Valutazione delle performance di impianto (mediante test di 72 ore) per le prove di collaudo e verifica dei valori.
  • Produzione di report di analisi del modulo fv, focalizzata su anomalie tecniche e gli incidenti che interessano i processi di reclamo.
  • Misurazione e valutazione di I / V Curve di moduli e stringhe, volti a caratterizzarle e analizzarle qualitativamente.
  • Controllo qualità.

PVdiagnosis offre una grande visione e una profonda conoscenza del comportamento operativo di impianti solari fotovoltaici, essendo in grado di fornire un servizio completo con la più alta qualità, su misura in base alle esigenze di ogni ciente. Oltre a 900 MW di impianti valutati avalano la nostra esperienza; la certificazione ISO 9001:2008 garantisce l’ottima qualita dei nostri servizi.