It is of course very cool, such a row of solar panels on your roof. You have your own local energy factory. But how exactly are solar panels put together? And how do they produce electricity? Our in-house physics teacher and Project Manager Adam explains. This time: layers and coatings.
You may not know it yet, but a solar panel consists of different layers. And all these layers have their own properties and functions. This week Adam explains exactly which layers there are in a solar panel, and what they are good for. Including: how do the different coatings that are applied to solar panels work? And: what are those stripes on a solar panel good for?
FRAME OF THE SOLAR PANEL
The frame keeps all layers of the solar panel firmly together. Furthermore, the frame makes it easier to attach the solar panels to a roof.
GLASS PLATE OF THE SOLAR PANEL
The top of a solar panel consists of safety glass. This plate protects the solar cells not only against weather influences such as hail and snow, but also against high temperatures. Adam: ‘Glass conducts heat relatively well. You want a solar cell to be able to dissipate its heat quickly, because it produces less power when it is warm. Glass is very suitable for this. Metal would be even better, but of course no sunlight can pass through it. ‘
ANTI-MIRROR COATING: COPIED FROM GLASSES
An anti-reflective coating has been applied to the plate, which ensures that as much sunlight as possible reaches the solar cells. You can see whether a surface has a high reflectivity by how much a surface reflects. ‘Much of the technology used on solar panels is copied from glasses. If you didn’t have an anti-reflection coating on your glasses, you could see your own eye partly in your glasses. ‘ Since solar cells work on sunlight, you want the layers above the cells to reflect as little as possible.
‘This is a more technical story: how much a material reflects depends on the difference in refractive index between two materials. The refractive index of air and glass are very different from each other. By choosing a coating with a refractive index in between that of glass and air, you reduce the difference, and thus ensure that less light is reflected.
‘ Such an anti-reflective layer can be applied in various places in the solar panel, both on the glass and on the solar cells themselves. By applying it in several places you increase the amount of light that reaches the solar cells. Two materials are most often used for this coating on solar panels: silicon nitride and titanium dioxide. The thickness of the layer depends on the type of light you want to transmit, but you should think of nanometers. Of course you cannot see the layer with the naked eye! ‘ With solar panels it is important that the coating allows visible light to pass through.
WATER? RATHER NOT
The anti-reflective coating is also water and dirt repellent. When it rains, almost all dirt washes off the panels. This means that solar panels only need to be cleaned once every two years. A material that repels liquid is called hydrophobic. ‘The coating on solar panels gives the panel a very specific roughness. If a layer is too smooth, the entire water drop will settle on it. While if you have the right roughness, the drop will stay on it nicely so that the drop can slide off the coating more easily.
You look at the surface tension of the drop: the dots have to be at the correct distance. ‘ Far enough to make as little contact as possible, but also not so far that the drop falls apart and becomes a puddle. ‘This works the same as the water-repellent spray you can spray on your shoes. The spray contains very small particles that create the right roughness so that water droplets do not adhere to your shoes but roll off. ‘
The effect of different material roughnesses on water droplets. (Tuteja et al., 2008)
Directly above and below the solar cells is a plastic layer that ensures that the solar cells remain dry. These layers are laminated together so that they are well waterproof. They also function as a kind of cushion to protect the solar cells from shocks.
FINGERS AND BUS BARS
Thin silver-colored lines run on each solar cell. These lines transport the direct current that is generated in the solar cells to the point of attachment with the rest of the system. ‘This is really just the electrical circuit. The silver lines ensure that electron transport can take place. Electrons are actually very simple particles: when they see a conductor, they prefer to move along that conductor.
‘ The two larger lines are called the bus bars, while the fingers run to the sides of the solar cells from the bus bars. Silver is currently most commonly used in solar panels. ‘But a lot of research is being done into whether less rare materials can be used. Or, for example, transparent materials, because now the fingers and bus bars still provide some shade on the solar cells. ‘
Wider white lines are also visible on some panels. This is the back plate you see. There must be some space between the solar cells so that they can expand when they get hot. If the back plate is black, these stripes are less visible. Many people prefer this, but the black back plate absorbs more solar heat, making the panel slightly less efficient.
SOLAR CELLS: THIS IS WHERE THE MAGIC HAPPENS!
The solar cells are the stars of the solar panel, because they convert light into electricity. Adam will explain exactly how this works next time.
This plastic or glass plate provides a neat and sturdy finish to the back of the solar panel. It protects the solar cells against high temperatures and moisture and insulates the panels, which improves the performance and safety of the panels.