Common problems in PV plant - Part 2

21 November 2017 at Warree

In our previous article "Common problems in PV plant - Part 1" we introduced the rapid growth and expansion of solar market. With the continued pace of expansion, there are problems which are bound to happen. The previous article discussed the common problems which are encountered during planning and installation of the solar power plant. This article shall educate you on the solar PV module specific problems which are evident on field after significant amount of time.

Dark or deep blue colour (of solar modules) is what we think of first when we visualize a solar power plant. However this blue colour does not stay put due to many factors and problems ultimately leading to energy loss in the solar power plant. The common problems in solar module are mentioned here. One of the most common problems that occur in solar module is appearance of discoloured lines at random location like the trail of the snail (hence the name snail trail). The cause of formation of snail trail (Figure 1) is formation of nanoparticles of silver carbonate (Ag2CO3) due to ingression of moisture and carbon dioxide (CO2) through the cracks/micro cracks in the module. Such cracks may be generated during the transport or at manufacturing line. Such nanoparticles are capable of absorbing and scattering light and may cause a yearly degradation of 0.50% per year (value may vary depending on the condition of solar module). While this number is not much, the micro-cracks/cracks may penetrate more causing significant decrease of power at a later stage.

Figure 1: Snail trail in solar module (Source: Google images)

While not visible immediately Potential Induced Degradation (PID) effect and its impact on power output of the solar system is adverse and irreversible (in many cases). PID affects the solar strings which are at maximum negative potential or solar cell which are at the negative terminal of the module. Such negative potential of module causes sodium positive charges (from the glass) to settle on the solar cell causing the generated electrons to recombine at the surface resulting to reduced power output. This effect forms a daisy chain affecting the modules at the negative potential in the string leading to significant drop in power output from the power plant. The easiest method to detect PID is by generating the I-V curve or EL image of solar module (Figure 2). The irregularity in the I-V curve (when compared to standard I-V curve) or the darkening of single side of solar module in EL image shows that the module(s) are PID affected.

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Figure 2: IV curve & EL image of PID affected solar module (Source: Waaree white paper& PVTech)

As we mentioned in our previous article the solar market and hence the suppliers of the raw materials are expected to rise to huge numbers. In order to sustain the constantly reducing price few suppliers compromise with the quality of the product or raw material leading to compromised output from solar power plants. Ethyl Vinyl Acetate (EVA) is amongst the commonly used encapsulant in solar modules. An encapsulant is expected to have high transmittance to ensure the incidence of maximum light on solar cells. Additionally they are also expected to be UV resistant but not all the EVA(s) actually are resistant to UV. Such EVA after a prolonged exposure to UV light splits acetic acid from its molecule (Figure 3). This affects the colour and hence the transmissivity of the EVA leading to reduction in power output from the solar module.

Figure 3: EVA discoloration in solar module (Source: PVTech)

The solar module is made up of layers of glass, EVA, solar cell, EVA and back-sheet with the entire assembly surrounded with Aluminium frame for module's rigidity. Before framing the module, the entire assembly is laminated in a laminator so that it stays put. While it is known that different make of EVA, back-sheet, glass requires to be laminated at different temperature and pressure for altered time duration, the exact number may not be known leading to various problems. The most common of those problems is delamination of solar module (Figure 4). Delamination may be present as spots (like air bubble) which may progress within the module causing rupturing of the entire module. Delamination is an irreversible process and requiring the faulty module to be replaced.

Figure 4: Delamination of solar module

Operation & Maintenance (O&M) of solar power plant is one of the most important aspects for efficient running of power plant. Regular cleaning of solar module is mandatory in O&M as there is regular settlement of dust on the modules. While the cleaning is regular the dust still settles (in some cases where efficient cleaning is not done) (Figure 5) in the empty spaces present between the glass and outer (Aluminium) frame of solar module. This is not immediately evident as the dust builds up in the empty space. However after few years of inefficient cleaning, this is clearly visible. Primarily causing reduction in power output of solar module, such dust also tends to formation of hotspot due to generation of differential power output between outer cells and the cells in middle.

Figure 5: Losses due to soil accumulation at edges & Oxidation of solar cell (Source: Google images)

Also evident from the figure above are few different (generally pale red or brown) coloured cells. This is due to oxidation of solar cells which happens either due to moisture ingression in module (via the micro cracks) on field or due to contamination of cell with moisture at manufacturing line. This leads to reduced power output from solar module and in addition to it also leads to failure of module in longer run needing replacements.

This post was related to problems specific to solar module. We would continue this article in the next post where electrical problems shall be discussed. Keep looking this space for our next article.

Let us all pledge to make solar energy the primary source of energy in the near future.