Solar panels are advancing rapidly with greater efficiency and lower prices resulting in a huge increase in demand. However despite the massive investment in solar technology, solar panel construction hasn’t changed much over the years. Most solar panels still use a series of silicon crystalline cells sandwiched between a front glass plate and a rear polymer plastic back-sheet supported within an aluminium frame.
Once installed on a rooftop, solar modules are subjected to severe conditions over the course of the expected 25+ year life. Extreme variations in temperature, humidity, wind and UV radiation can put enormous stress and strain on a solar module. Fortunately most panels are well engineered to withstand the weather extremes but even though some panels can still fail in several ways, including water ingress, cell micro-fractures and potential induced degradation or PID, this is why it is vital solar panels are manufactured using only the highest quality components.
The 6 Main components of a solar panel
· Extruded Aluminium frame
· Tempered Glass - 3 to 4mm thick
· Silicon PV cells with busbars
· Encapsulation - EVA film layers
· Polymer rear backsheet
· Junction box - diodes and connectors
Several panel manufacturers are ‘vertically integrated’ which means the one company supplies and manufactures all the main components including the silicon ingots and wafers used to make the solar PV cells. However many panel manufacturers assemble solar panels using externally sourced parts including cells, polymer back sheet and encapsulation EVA material. These manufacturers can be more selective about which components they chose but they do not always have control over the quality of the products so they should be sure they use the best suppliers available.
Silicon photovoltaic cells or PV cells convert sunlight directly into DC electrical energy. The performance of the solar panel is determined by the cell type and characteristics of the silicon used, with the two main types being monocrystalline and polycrystalline silicon. Additionally the base of the cell can be built using different additives to create either a positive p-type silicon or negative n-type silicon. However there are several different cell configurations available which offer different levels of efficiency and performance. See more information in the complete solar PV technology review.
Most residential solar panels contain 60 cells linked together via busbars in series to generate a voltage between 30-40 volts depending on the type of cell used. Larger solar panels used for commercial systems contain up to 72 cells and in turn operate at a higher voltage. The busbars are electrical contacts which interconnect the cells and allow the current to flow in a circuit.
The front glass sheet protects the PV cells from the weather and impact from hail or airborne debris. The glass is typically high strength tempered glass which is 3.0 to 4.0mm thick and is designed resist mechanical loads and extreme temperature changes. The IEC minimum standard impact test requires solar panels to withstand an impact of hail stones of 1 inch (25 mm) diameter traveling up to 60 mph (27 m/s). In the event of an accident or severe impact tempered glass is also much safer than standard glass as it shatters into tiny fragments rather than sharp jagged sections.
To improve efficiency and performance high transmissive glass is used by most manufacturers which has a very low iron content and an anti-reflective coating on the rear side to reduce losses and improve light transmission.
The aluminium frame plays a critical role by both protecting the edge of the laminate section housing the cells and providing a solid structure to mount the solar panel in position. The extruded aluminium sections are designed to be extremely lightweight, stiff and able to withstand extreme stress and loading from high wind and external forces.
The aluminium frame can be silver or anodised black and depending on the panel manufacturer the corner sections can either be screwed, pressed or clamped together providing different levels of strength and stiffness.
EVA stands for ‘ethylene vinyl acetate’ which is a specially designed polymer highly transparent (plastic) layer used to encapsulate the cells and hold them in position during manufacture. The EVA material is extremely durable and tolerant of extreme temperature and humidity, so it plays an important part in the long term performance by preventing moisture and dirt ingress.
The lamination either side of the PV cells provides some shock absorption and helps protect the cells and interconnecting wires from vibrations and sudden impact from hail stones and other objects. A high quality EVA film with a high degree of what is known as ‘cross-linking’ can be the difference between a long life or a panel failure due to water ingress. During manufacture the cells are first encapsulated with the EVA before being limited within the glass and back sheet.
The backsheet is the rear most layer of common solar panels which as acts as a moisture barrier and final external skin to provide both mechanical protection and electrical insulation. The backsheet material is made of various polymers or plastics including PP, PET and PVF which offer different levels of protection, thermal stability and long term UV resistance. The backsheet layer is typically white in colour but is also available as clear or black depending on the manufacturer and module.
‘Tedlar’ PVF material from Dupont is known as one the leading high performance back sheets for PV module manufacturing.
The junction box is a small weather proof enclosure located near the top on the rear side of the panel. It is needed to securely attach the cables required to interconnect the panels. The junction box is important as it is the central point where all the cells sets interconnect and must be protected from moisture and dirt.
The junction box also houses the bypass diodes which are needed to prevent back current which occurs when some cells are shaded or dirty. Diodes only allow current to flow in one direction and a typical 60 cell panel has 3 rows of 20 PV cells and in turn there are 3 bypass diodes, one for preventing reverse current to each of the 3 sets of cells. Unfortunately bypass diodes can fail over time and may need to be replaced, so the cover of the junction box is usually able to be removed for servicing, although many modern solar panels now use more advanced long lasting diodes and non-serviceable junction boxes.
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