10th July 2018
Identifying reliable raw materialfor any product is a crucial to ensure that the product lives for its lifetime. With the competitive market condition along with requirement of being cost competent selecting correct raw material to strive a perfect balance between commercial and technical terms in the product is a challenge. The scenario in case of solar PV module is no different. A solar module, other than solar cell which actually generates power output also houses encapsulant(s), backsheet, solar glass(es), (anodized aluminum) frame and junction box (Figure 1). With the plethora of suppliers of each raw material (both locally and abroad) claiming their product's superiority (both technically and commercially) over others,ensuring solar module producing power for 25 years (above a fixed percentage) could be a perplexing task. While each raw material used in solar module has a specific function, we believe that these 3 material i.e. backsheet, encapsulant glass are of prime importance and any effect on these could significantly affect the performance of entire power plant. Additionally they may significantly affect the LCOE of the power plant which may (sometimes) render the intended economics from the plant. This blog aims to educate its readers on the importance and effects of the three raw material on solar module.
Figure 1: BoM of a standard solar 72 cell module (Source: Google images)
A backsheet in the solar module primarily performs three important functions namely DC isolation, UV/temperature fluctuation/moisture resistant and internal reflection. Anyone with knowledge on history of system voltage would know that it has been upgraded from 600V to (currently in practice) 1000V to the new shift towards 1500V. This means that the operating voltage of the module is high and any one coming in direct contact with it could get a hazardous shock. The backsheet forms a DC insulation layer between the solar cell and the outer world. A backsheet is also impervious to UV rays and moisture. It also acts as barrier to the outer temperature changes to the solar cells, all of which could downgrade the performance of solar module. The third important function of a backsheet is to reflect the light internally. Any light which passes through the cell is not essentially utilized for solar power generation. In order to give such (photons) light another chance to generate electricity, they must be reflected back towards the cell. This function is undertaken by the inner most surface of backsheet. Apart from these a backsheet is also expected to adhere to the encapsulant, hold the entire cell assembly in place and so on. With such functions selection of correct backsheet is a must.
The Figure 2 below some of the common yet challenging problems faced by the backsheet. Starting with top left and going clockwise, the first issue is bubbles and hotspot in backsheet. A backsheet is expected to perform optimally over a certain range of temperature. However such temperature ranges may not be feasible for backsheet, thanks to its low quality raw material. A solar module may operate at different temperatures which after a certain point of time heat up the backsheet and lead to formation of bubbles (as shown). This would after a prolonged exposure cause a hole in backsheet which would lead to further degradation of module rendering it useless. Next concern is the yellowness of the backsheet. Primarily caused due to UV exposure, this indicates that the backsheet is not compatible to the local climatic conditions. Additionally the technology of backsheet and its manufacturing process may also be an issue. The yellowness index (a measure of how much the material has yellowed) of these backsheet was as high as 8 to 10 (<5 is acceptable limit till then end of lifetime). Such yellowness also means that the module is at risk of getting exposed by UV and may stop generating the desired power output. The effects of moisture on the backsheet cannot be ignored. As seen in figure below, the moisture (along with abrasive particles sometimes) may peel of (or even sometimes erode) the backsheet. It may also generate uneven stress in the layers of backsheet which causes cracking the outer layer of the backsheet. This leaves the solar cells vulnerable to outside atmosphere and in the long run, the effects of moisture on solar cell (like corrosion, hotspot, PID) are fairly visible.
Figure 2: Various types of degradation backsheet.
The defects are (starting from top left and moving clockwise) hotspot and bubbles, yellowing of backsheet, erosion of backsheet especially between cell to cell gap & cracks in backsheet (Source: Du Pont)
The encapsulant in the solar module has three important role namely transmittance of light, holding the cell assembly and adhering to glass & backsheet. Firstly the encapsulant is required to transmit all the light falling on it. This is to ensure that the cell assembly below gets adequate light to generate power output. Secondly the encapsulant holds the cell assembly together. This is to ensure that the solar cells do not touch each other and get short circuited. To ensure this two properties namely gel content (for elastomers) and shrinkage comes into play. Gel content of any encapsulant ensures that they have adequate intermolecular strength which could hold the module together. Encapsulant generally have a tendency to shrink at 150+ °C laminating temperatures. Such shrinkages need to be in limit to ensure that there are no misalignments of strings and/or cell short circuiting. Thirdly it is also important there they adhere adequately to the glass on front side and backsheet at the back side of solar module. Additionally to this, an encapsulant is also required to be stable at elevated temperatures and high UV exposure.
There are lots of ways by which a solar encapsulant could possibly degrade. Out of which only few of them are actually visible (as shown in Figure 3) and with (almost) all such degradation being irreversible, it is important to understand and limit such degradation in power plant. Delamination of encapsulant after certain period of time (as seen from figure below) is one of the concerns for power plant. One of the prime causes of such delamination is moisture ingression. Such ingression may be when the modules are in operation in field or at the module manufacturing facility (where the local climatic condition along with interaction with encapsulant). The delamination would primarily hinder the entry of sunlight leading the module to perform lower than other modules. The moisture ingressions could further lead to corrosion and hotspot formation. The next visible effect on the encapsulant is its yellowing. Similar tobacksheet above, this yellowing is caused by UV exposure however it is a serious concern here. The yellowing may be directly proportional to power loss experienced by solar module. The encapsulant may further turn green or brown leading to total failure of solar module.
Figure 3: Various degradations -Encapsulant delamination (on left) and Encapsulant yellowing (on right) (Source: Google images)
The next most important raw material in solar PV module is a solar glass. The glass in the PV modules has the following main functions namely enable transmission while minimizing reflection, mechanical strength & rigidity andcompositional stability. The glass is the first surface that the light interacts with. Thus it becomes extremely important that the glass transmits the light to maximum level while lowering the reflection off its surface. In its natural form, the glass reflects 4 to 10% of incident light on it which may lead to notable loss of power output. The currently available glass are hence coated over the front surface with an anti-reflective coating (ARC) which ensures that such reflection is minimized to as low as 1% (in many cases).The next function of the glass is to provide mechanical strength & rigidity to the solar module. As the module is made to last for 25 years, it is critical that they areprotectedfrom external weather and shocks internals. The glass in use are generally tempered which ensures that the glass has adequate strength for such purpose. Also as they are exposed to all the types of radiations in 25 years, the glass is expected to mostly reflect such radiations which may significantly affect the performance of solar module. For such reason the glass it is important that the required component is added/removed as each of them have specific role to play are also expected to be stable for its lifetime.
While glasses are optimized, there are still a few concerns which sometimes leads to irreversible consequences in PV module. The ARC coating has to be of a specified thickness in order to achieve maximized transmission. It is seen that the ARC coating of some glasses weather out (partially or fully) after few years. The stability of such coating has still been an issue. The degradation of ARC coating while attributed to local climatic conditions, is also significantly affected if proper cleaning cycle is not carried out (as articulated by solar manufacturer/ EPC contracted). Such degradation however significantly reduces the power output of module. The non-tempered glass is brittle in nature. This means that non-tempered would break easily and in sharp edges. Additionally they also have less strength compared to tempered glass. This makes tempering process most important, however sometimes improper tampering and/or slightest defect in raw material could lead to potential decrease in strength of glass. The solar module which may continuously be incident with small stones (carried by wind), hail stones (in few countries) may break the glass (as shown in figure below). While the solar glass comes are said to have low iron content, the blue-green tint could be still seen in glasses from few manufacturers. This signifies that the glass still has adequate iron content which reduces the transmission of the entire spectrum of visible light. This leads to lower power production from solar module. Additionally increased iron contentmay also lead to PID rendering the module useless (as we explained you in our previous blog "What's and why of PID!").
Figure 4: Few variations available for solar glass (left) & shattered solar glass (right) (Source: Google images)
As evident from above, the selection of correct raw material is critical and any change/ deviation from standard BoM must be extensively tested and fully verified. This is to ensure that the module would live up to its claimed lifetime without any hassle. Additionally it is also important to select correct BoM based on end requirements of the customer. One must also consider the local climatic conditions where the module must be installed at and follow all instructions provided by module manufacturer/ EPC provider to ensure that the degradation of power plant is well within the limits.
We at Waaree Energies ensure that all the modules are manufactured as per the standardized BoM. Additionally sample modules from each lot are tested extensively to ensure that they meet more than the desired standards. All our suppliers are verified and audited fromtime to time to ensure that they supply us only premium grade material. An expertise like such enables end customer to be assured that their plant would be up and running for 25+ years ensuring more than desired return.
Let us all pledge to make solar energy the primary source of energy in the near future.
RAHE ROSHAN HAMARA NATION