The points to consider when choosing great solar panels
Like any manufactured product, not all solar panels are equal. Some use the absolute best materials and equipment to manufacture them and others do not. Some conduct extensive research and development into long term performance and have deep understanding of how the materials will behave and others do not. However, it’s not easy to tell the differences simply by looking at a solar panel, so what can go wrong and how can you tell them apart?
Solar panels are made of a composite of chemically treated glass, chemically treated silicon cells, various plastics, aluminium, sealants and interconnecting wiring. Solar panels spend their entire life in the blazing sun and freezing rain, and incredibly harsh environment for any material, let alone a composition of different ones bonded together. Under such conditions, materials can – and do – chemically change over time reacting with each other and creating new chemicals and compounds as the age. Their elasticity also changes over their life affecting the stresses and tensions they place on each other.
Solar manufacturers with a strong understanding of materials science who invest heavily in research and development are able to understand and predict how these effects are likely to occur. Some, who have been manufacturing solar panels for many years also, have the benefit of hindsight having seen what can go wrong in the real world too.
Typically, manufacturers who understand these issues well will have accreditation to ISO standards for manufacturing and will have advanced quality assurance processes and controls described on their promotional material. True quality assurance is about understanding what causes variation and controlling it, not just checking the final result and hoping to catch mistakes.
Here are some of the commonly seen issues that can occur when materials science is not well understood.
Delamination occurs when the bond between the plastics (on the back) and the glass (on the front) separate. This is catastrophic for a solar panel because it allows air and moisture to creep inside which will cause corrosion and imminent failure. Delamination will occur if:
- Inferior plastics are used or
- If the thermal properties of the plastics are poorly understood so it doesn’t melt to exactly the right point or
- The plastics or the glass or not perfectly clean and compatible or
- The laminating machine is inferior and poorly regulated for pressure and temperature
When you are assessing a solar panel, look very closely for bubbles, creases or imperfections on the plastic rear surface as an indication of the workmanship in lamination. Ask your supplier if they know who supplies the plastics and look for evidence of quality brands.
Sealants are used to seal the junction box on the rear of the solar module to prevent moisture getting into connections and sometimes to bond the laminates solar panel into its aluminium frame. If poorly matched, some sealants can react with the plastics causing premature degradation and although non-flammable sealants are available, cheaper, flammable alternatives are available and are sometimes used to save money.
Frame sealants need to flexible to allow for some thermal expansion and contraction between the glass laminate and the aluminium frame.
When you are assessing a solar panel, look very closely for evidence that the sealants are applied precisely and carefully as a sign of good workmanship. Excess sealant indicates poor control over the dosage and could mean that too much is in some places, and too little in others. Ask your supplier if they know who supplies the sealants and if they are non-flammable.
Silicon solar cells are made of very thin wafers, usually around 0.20mm thick. Although they have some ability to flex, they can and do suffer from stress induced cracks which are so small they are almost impossible to see with the naked eye. These are called micro cracks and can lead to loss of power and potentially failure over time.
Micro cracks can be caused by poor handling of the solar cells during assembly (usually through non automated systems), poorly maintained equipment or transport damage from vibration or shock if improperly packaged. It is not possible to see micro cracks but good manufacturers use Infra-Red scanning cameras which can detect and isolate micro cracked cells during processing and use the feedback to tune their machines better.
When you are assessing a solar panel, look for quality packaging and careful transport by your installer as a sign of good workmanship. Ask your supplier if they know whether the supplier uses Infra-red scanning or other techniques to minimise and control micro cracking.
Yellowing and browning
Similar to the snail trail issue, yellowing or browning is an indication of a poor understanding of materials science and is also typically caused by an uncontrolled chemical reaction between materials causing the (usually) white plastics to turn yellow or brown.
It has also been known to occur on the surface of the cell, in a chemical reaction between the chemicals on the surface of the silicon cell and the chemicals used to treat the glass. Yellowing and browning are almost always something that occurs in the first few years of exposure to sunlight. Fortunately, (historically) performance does not appear to significantly affected due to yellowing or browning however, it is a sign that something uncontrolled is happening.
When you are assessing a solar panel, look very closely and make sure there is absolutely no evidence of yellowing or browning as a sign of good workmanship. Ask your supplier what the manufacturer understands about the issue and how they have addressed the material science to avoid its occurrence.
Inclusions and alignment
When a solar panel is being assembled and prepared for lamination it is crucial that no dirt, blobs of solder, dust, hair or other foreign particles are inadvertently included or they can cause bubbles in the laminate or air gaps which can lead to premature failure. A 100% perfectly clean environment is essential in manufacturing.
The alignment of solar cells is also an important indicator of workmanship in a solar panel. Poorly aligned solar cells are indicative of poor process control and can introduce stress into the interconnections or the potential for current leakage between cells.
When you are assessing a solar panel, look very closely and make sure there are absolutely no inclusions accidentally laminated under the glass and that the solar cells are perfectly aligned as a sign of good workmanship. Ask your supplier whether the manufacturer uses automated or manual assembly methods when they assemble their solar panels.
LID and PID
Potential Induced Degradation (PID) is a relatively recently discovered phenomenon which can occur in solar panels although a concise understanding of what causes it is still missing. Recent tests results presented at a solar conference in 2012 showed that solar panels which are susceptible can lose as much as 30% of their rated power through this effect.
What is known is that sodium enrichment occurs between the chemicals used on the surface of the solar cell and the glass in certain circumstances, causing a leakage of current between the cells and the solar panel frame. The intensity of the degradation depends on the module type, the ambient environmental conditions and the position of the solar panel in the array string. A high system voltage (eg 600V +) and high ambient humidity can lead to a moisture film on the solar panel surface. It is assumed that a drift of positive ions occurs from the glass towards the solar cell in the opposite direction, which is believed to cause degradation in the solar cell.
Light Induced Degradation (LID) is another phenomenon that occurs in solar panels, although it is better understood. All solar cells degrade when they are first exposed to sunlight to some degree, varying depending on both the technology type and some of the chemical characteristics which scientists are still learning about. It is generally accepted that the effect is most pronounced if boron oxygen defects are more prevalent within the silicon structure, although it can occur on the surface layers too. The result is simply described as an increase in the density of the material which makes the movement of electrons more difficult, which results in less power flow.
Thin Film solar panels for example, exhibit relatively higher levels of initial degradation (in the first 30 days) but manufacturers understand this well and over-rate their power to take into account the losses they know will occur.
However, in Monocrystalline and Multcrystalline solar panels, the effect is more subtle (in the order of around 0.5% power loss) and not all manufacturers take it into account or understand it as well. The effect is more pronounced in Monocrystalline panels although Multcrystalline panels with the can also suffer from the effect if the oxygen content of the silicon is particularly low. Some manufactures understand LID well although no standard tests or standards are in place.
When you are assessing a solar panel, ask your supplier whether they have undertaken any PID or LID testing and what if any test results can be provided to demonstrate their knowledge of the issue and what they have done to minimise its effects.
Solar cells within solar panels are interconnected using a flat, conductive ribbon in series and parallel connection. Most use a tin coated copper ribbon although the number of connections, width and thickness varies by manufacturer.
An issue increasingly well understood by quality manufacturers is how these interconnectors are connected (soldered) to the silicon solar cell and how the connection method maximises conductivity without placing undue stress on the solar cell. Poor interconnections can cause micro cracking in the solar cells through thermal expansion and contraction stress or the interconnector can break. In the worst case, a poor solder connection which opens and closes with thermal movement can cause an arc and potentially burn out or combust the plastic backing materials.
For example, some manufacturers solder the entire length of the interconnector, and some only solder every few millimetres, theoretically reducing stress caused by thermal expansion and contraction. Some use thinner interconnectors which can theoretically stretch more, avoiding stress.
When you are assessing a solar panel, observe the neatness, alignment, size and quantity of the interconnectors. Ask your supplier what type of interconnector and soldering method is used to demonstrate their knowledge of the issue and what they have done to minimise its effects.
The vast majority of solar panels are fitted with aluminium frames which provide mounting attachment points and protection for the edges of the glass laminate. Importantly, the expansion and contraction ratio of aluminium and glass are different and hence the materials used to join them together are important to avoid stress build up.
Most manufacturers use butyl rubber or flexible double sided tapes which allow some movement but ensure the glass is retained within the frame. Obviously, the long term adhesion strength and UV stability of these materials is crucial and cost saving measures through the use of inferior materials have demonstrated rapid failure of the bond.
The frame itself can come in various types from double walled and highly rigid, to quite thin walled and relatively flexible. Both serve the same purpose when attached securely to quality mounting systems, although logically a more rigid frame offers less chance of distortion and potentially coming loose over time. In double walled frames, it is important that the cavity between the walls is vented so that ice cannot build up inside and split the frame in cold climates.
Some frames are screwed together and some are joined using push fit mitre joints. When manufactured correctly using tight tolerances they are equally effective.
When you are assessing a solar panel, look very closely and observe the type and integrity of the materials used to bond the frame to the glass laminate. Ask your supplier about the materials used for bonding and if using push fit mitre joints what they do to ensure that tolerances are held in specification.
A seemingly subtle but important feature in the construction of solar panels is the junction box. Attached to the rear of the solar panel, it serves several important functions. Firstly, it seals the rear of the solar panel where the interconnectors exit. As such, it is vital that it is well sealed so that moisture cannot enter the panel. Some are permanently sealed, others can be opened up but either way quality moulding’s and sealing faces are important. Most are made of plastic and some are cast aluminium.
Secondly, it houses connection points for the cables which exit to allow you to connect each solar panel together. These connections need to handle the extremes of weather over decades and handle quite high currents so quality connections are important. It also houses small electronic devices called diodes which protect the solar cells in the case some are partially shaded. Some diodes exhibit higher losses and create higher temperatures than others, leading to potential stress.
Lastly, the majority are filled or partially filled with a sealant to protect the entry to the solar panel and sometimes filled to the brim to fully encapsulate the diodes, connections and cables. The quality and flammability of this sealant is an important factor too.
When you are assessing a solar panel, look very closely and observe the type and integrity of the junction box to ensure it fits well, is perfectly sealed and if operable, that it seals up perfectly when re-closed. Ask your supplier about whether the materials are flammable and look for quality workmanship.