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SOLAR PANELS
How is the production cycle of a solar panel?
Photovoltaic modules are considered doubly green:
1. they produce green-renewable energy
2. They can be recycled
3. At the end of its life, each panel has the possibility of being 95% recycled although nowadays the costs are higher than manufacturing them again especially in the process of separating the solar cells from the encapsulant. The PV Cycle organization is in charge of promoting panel recycling in the industry through research for process improvements.
4. It takes approximately 2 to 2.5 years for the module to produce the same energy that was used to manufacture it (TRE).
How is a solar panel manufactured?
As shown in the picture, in the first phase we have chemical silicon with 99.9% purity.
This silicon melts into a mono or polycrystalline ingot depending on the speed of the crystallization process. Polycrystalline ingots are the result of a faster and more uncontrolled crystallization process than monocrystalline ingots. They have a high diffuse light yield but the overall performance of the polycrystalline panel is lower than that of the monocrystalline. Monocrystalline ingots can be either P-type or N-type depending on the doping applied to the silicon.
Once the ingot is obtained, it is cut into wafers. There are different wafer sizes, but the smaller the number of wafers, the lower the voltage drop.
This wafer is then subjected to chemical surface treatments in order to increase its performance:
Anti-reflective (AR) treatments
- Additional coatings such as PERC technology so that the cell absorbs ultraviolet light.
- Split cells so that if one part of the cell is damaged (Hot cell) it does not impair the function of the rest.
- Slim finger
Once the cells are finished, the panel construction is started. Generally, 60 or 72 cells are used per panel. If the cells are split cells, 120 or 144 split cells are used.
Finally, the complete system is made, with all the panels necessary for the installation, as well as the support material and wiring.
What is the cost of the production process of a solar system?
Obtaining the silicon, its smelting process and subsequent crystallization to obtain the ingot constitutes 20% of the total cost of the formation chain.
The chemical processes applied for the creation of the cells and the assembly of the panel contribute to 30% of the total cost.
The remaining 50% of the cost is the integration of the system, the assembly of the panels on the support structures, their connection and their correct positioning.
Therefore, the competitive advantage lies in this last step, since the creation of the solar panel already has a competitive cost.
What types of cells are there?
There are different types of cells depending on the shape and treatments applied. Three types of cells can be classified:
- Front Contact Polycrystalline: These cells are differentiated by straight corners and cell efficiencies between 14-16%. If PERC (Passive Emitter Rear Cell), Split cell or Slim fingers treatment is applied they can reach efficiencies of 18-20%.
- Front Contact Monocrystalline: These cells are differentiated by straight corners and cell efficiencies between 15-17%. If PERC (Passive Emitter Rear Cell), Split cell, Slim fingers treatment is applied, efficiencies of 19-23% can be reached.
- Back Contact Monocrystalline: These cells are differentiated by straight corners and no front fingers, they have cell efficiencies between 15-17%. If PERC (Passive Emitter Rear Cell), Split cell, Slim fingers treatment is applied, they can reach efficiencies of 21-24%.
What does the efficiency of a solar cell depend on?
The efficiency of a solar cell and ultimately the photovoltaic panel depends primarily on 3 factors:
- Spectral response - ability to capture the widest range of wavelengths of solar radiation. For example, PERC (Passive Emitter Rear Cell) technology is able to capture the blue areas of the spectrum.
- Ability to reduce heat losses in the current flow through the cells. For example, reduction of the fingers cross-section.
- Reduction of electron recombination, e.g. with copper plated rear cell or reduction of cell size (split cell) or silicon texturing.
What are the differences between flat silicon and textured silicon?
If flat silicon is used for the modules, the incident light (I), hits the surface with reflection R. R varies from 0 to 1. The light reflected by the surface (RI) is reflected at the same angle as the incident light.
On the contrary, on a rough surface, the reflected light (RI) can impact the silicon surface again, reducing the reflection R2I.
What are the main innovations in cell/panel manufacturing in the last 2 years?
In recent years, various technologies have been applied to solar cells in order to increase the performance of the solar panel. The most prominent are:
Split cell: The 72-cell panels become 144 cells because they are split in two. This new configuration means that the current flowing through each individual cell is halved, thus reducing heat losses in each cell. This allows the entire panel to operate at a lower temperature which increases the performance of the panel. This new configuration increases the efficiency of the panel by about 1%.
- Multibus bar: The simplest front contact panels/cells have 2 bus-bars (conductors) per cell that are in charge of collecting the electrons to conduct them to the panel output. Currently, each cell has 4 or more bus-bars which reduces the recombination of electrons since the distance the electrons travel is shorter. This technology aids photon collection and reduces the series resistance of the panel. It also increases the efficiency of the panel by 1.5%-2%.
- PERC technology (Passive Emitter Rear Cell): It consists of a passivation/coating on the back side of the cell that allows capturing the photons of the blue spectrum of the solar radiation. The PERC acts as a mirror and is able to introduce back into the cell those photons that would pass through the silicon. This is why many older panels are blue because it is the only color not captured by the cell that is visible to the naked eye. PERC technology increases efficiency by 1 to 3%.
What is the structure of a solar panel like?
Silicon solar panels, regardless of the cell technology used, have the same construction scheme as shown below:
Glass or crystal: Structural element of the panel through which photons pass into the cell array. The crystals for solar panels are:
A. Tempered glass to obtain greater mechanical resistance, especially to impact and bending. They are low in Fe (iron) to avoid heat loss.
B. They are treated with anti-reflective coating (AR coating) to reduce the reflection coefficient and increase the refraction of photons. The glass treatment increases the efficiency of the panel by 3-5%.
Cell array: The cells are interconnected with each other through the welding of bus bars. The material used is a light alloy typically consisting of silver or lead with tin (Sn60Pb40 or Sn96Ag4). It is a delicate process and excessive pressure on the contacts or excessive heat input can cause cracks.
EVA: Acronym for "Ethyl vinyl acetate" is a thermoplastic polymer with a high degree of transparency, high flexibility and good mechanical properties for embedding photovoltaic cells. EVA is the main factor in solar panel manufacturing that affects PID (Power Induced Degradation) due to the loss of electrical insulation over time.
TEDLAR: This is the common name for polyvinyl fluoride (PVF). It is an adhesive resin that offers an optimum balance of durability, wear and water resistance as well as high fire resistance.
RTV: Acronym for "Room Temperature Vulcanization" silicon. It is a polymeric adhesive widely used in the industry to fix the junction box to the TEDLAR. The use of this material lies in its high degree of adhesive compatibility with PVF and its low cost.
J-box: The junction box is the interface between the PV cell system and the electrical system. It is the interface between the PV cell system and the electrical system, from which the electrical cables run to connect the consecutive modules in a series or string. The box has a certain number of diodes (depending on the morphology of the module) that serve as protection and energy optimization elements.
How do bifacial solar panels differ?
It is important to emphasize that bifacial panels do NOT have twice as many cells as monofacial panels. They are the same panel with two fundamental differences:
- The backsheet or Tedlar is replaced by a layer of transparent Tedlar or other glass (glass-glass solar panel).
- The cells are connected on both sides. Tin buses circulate on both the front and the back of the solar panel in order to collect the electrons generated on both sides.
What is the difference between monocrystalline and polycrystalline cells?
Monocrystalline cells perform better in direct radiation. Polycrystalline cells work better in diffuse radiation conditions.
Why do solar panels of different power ratings have the same data sheet?
Because it is the same panel with different powers. Each cell is classified according to the power by flash test. They are grouped by categories to give different power families.