The definitive guide to how solar panels produce electricity
The main element of a solar electric (photovoltaic) system is the solar (photovoltaic, PV) cell.
Hereinafter the terms ‘solar’ and ‘photovoltaic’ are used interchangeably.
We would like once again to remind that term ‘solar’ could be somewhat confusing because it’s also used for solar-thermal collectors used in solar water heating systems.
In this article ‘solar’ is only used with the same meaning as ‘photovoltaic’. Photovoltaic systems operate by solar light rather than by solar heat.
Video about the basics of solar panels. Discover more about:
- what solar panel is,
- existing main types of solar panels,
- the most efficient type of solar panel,
- what affects the energy output of a solar module.
A photovoltaic cell can generate DC voltage when exposed to sunlight. DC voltage can cause DC current to flow in a closed circuit:
One solar cell produces DC voltage of 0.5 volts.
A group of PV cells forms a PV module.
In a PV module the cells can be connected in series and/or in parallel, and encapsulated in an environmentally protective laminate.
PV module is the basic building block of PV solar systems. Modules can be connected together to get the configuration producing the desired power output.
For example, 36 solar cells connected together in a PV module generate a voltage that is enough to charge batteries and run small motors.
PV panel is a group of PV modules connected together. Y
You can discover more about the different types of solar panels here.
A group of PV solar panels builds up PV array.
PV array is the complete PV unit generating electricity.
For this reason, PV array being a part of a whole PV system can also be called ‘PV generator’ or ‘solar generator’.
PV array is a configuration of several solar panels assembled and used to collect sunlight. A PV array consists of racks and mounting hardware positioned to receive maximum sun exposure throughout the day.
Photovoltaic arrays can be mounted on a roof or on the ground. The size of a PV array depends on the total amount of energy that needs to be generated.
A PV array can be built to provide power to one house, or to an entire office complex.
Here is a summary of the basic PV units, starting from the smallest one:
PV Cell -> PV Module -> PV Panel -> PV Array -> PV system
Photovoltaic panels are made of semiconductor material.
The difference between conductors, insulators, and semiconductors:
- Conductors – contain freely moving electrons
- Insulators – do not contain freely moving electrons
- Semiconductors – can have freely moving electrons only upon certain conditions.
Semiconductors are placed between conductors and insulators.
This means that depending on the environment, semiconductors can act either like a conductor or like an insulator.
The crucial condition that prompts the PV module’s semiconductor material to behave as a conductor is the sunlight. Sunlight provides the electrons with energy so that electric current starts flowing.
A typical representative of a semiconductor is silicon (Si). Silicon is widely used in solar panels production since it can be easily produced from sand.
Generally, silicon behaves like an insulator – it contains no freely moving electrons.
In its natural state silicon is of no use for producing PV generating units – if there are no free electrons available, no current is able to flow.
When does a semiconductor behave like a conductor?
To get freely moving electrons typical for conductors, silicon is doped with boron and phosphorous.
A solar cell consists of a P-type and N-type of silicon combined together.
The N-type silicon forms the side of the cell that faces the sun, while the P-type silicon is the side facing away from the sun.
At the boundary of between a P-type and N-type semiconductor, a PN junction is formed.
When a PN junction is exposed to sunlight, electric current starts flowing as a result of the photovoltaic effect.
Let’s shed more light on how the solar cell produces electricity.
The solar cell has anti-reflection surface helping in absorbing the maximum of solar radiation falling onto it.
Also, the cell is formed by a silicon P-N junction consisting of two separate layers of semiconductor connected together- one P-type and one N-type.
The N-type layer has an excess of electrons.
Electrons have a negative charge.
The P-type layer consists of an excess of holes with a positive charge.
The direction of the movement of the electrons and holes is from the areas of their highest concentration to the lowest one.
That is, electrons strive to move to the P-type layer.
In contrast, holes try to move to the N-type layer.
Electrons create a current flow while moving from N-type to P-type layer.
However, when reaching the P-type layer electrons combine with the free holes there and cancel each other.
Similarly, holes cancel each other with the free electron after moving to N region.
As a result, a depletion layer at the border area of P-N junction is being formed.
It comprises a small positively charged zone within the N-type layer and negatively charged zone within the P-type layer.
Both zones are located at the border area between the two layers.
These zones contain atoms with excess either of electrons/ negatively charged zone/ or holes/positively charged zone/.
As a result, the internal zones form energy barrier at the border area which stops from moving free electrons from the N-type layer to the P-type layer and the holes from the P-type layer to the N-type layer.
Eventually, the system reaches an equilibrium state characterized by no possible further movement of any holes or electrons.
That is because only a small amount of electrons and holes have enough energy to do so initially.
This movement may only continue if an external energy source is being applied. Such a source is solar radiation.
When solar radiation reaches P-N junction, it energizes electrons and increases their charge potential.
This increased potential allows them to overcome the energy barrier at the borderline and the current starts to flow via the load. It continues until the delivered sun radiation energy is high enough to sustain the flow.
If no load is connected, the current can’t flow even at the presence of sunlight.
A solar module consists of solar cells connected together and encapsulated against various climatic conditions.
The goal of connecting solar cells together is achieving higher voltage, current, and power output. The maximum number of cells in a solar module is limited by the module’s size and weight.
Solar cells can be connected either in series or in parallel. Cells connected in series increase voltage, while cells connected in parallel increase current.
Here is pictured the most common structure of a standard solar module:
To enable connecting to other modules by proper wiring, a junction box is mounted on the back of every PV module. Certainly, PV modules should be mounted in a way that no water penetration into the junction boxes is avoided.
In a solar module, the solar cells are usually connected in series in order to provide the higher voltage.
PV modules should be light and small enough to be installed on roofs, sometimes upon adverse conditions.
To get even higher voltage, solar modules are connected in series. A group of solar modules connected in series is called ‘string’:
Efficiency is an important parameter of PV modules. Module efficiency shows what part of the solar energy fallen onto the module’s surface is converted into electrical power.
Every PV module has its rated power or peak power, denoted in kWp.
The peak power of the module, however, is not the real power the module can generate.
The real power output of the module is always less than the rated power, due to the following factors:
- Manufacturer power tolerance
- Dirt and dust
- Cable losses
- Inverter efficiency
Manufacturer power tolerance is the percentage within which the manufacturers guarantee that the real power output will be the same as the rated power output. Such percentage is never 100, the typical value is 95, since PV modules operate in an environment different than Standard Test Conditions.
Dirt and dust cause losses when accumulated on the surface of a photovoltaic module. Dirt and dust particles could block the sunlight and thus reduce the power output. The content of dirt and dust in the air may vary with location and is usually the highest in the urban environment. Certainly, in regions with heavy rainfall dirt losses tend to be zero.
Temperature is one of the most important factors to be considered when designing a PV system. Temperature influences all the three main electrical parameters of a PV module – voltage, current, and power. When the weather gets warmer, the output voltage goes down and vice versa – when the weather gets colder, the voltage goes up. Things are different with power – when the temperature goes up, output power increases too.
Cable losses are inevitable in any PV system, especially when cables are long, which should be avoided if possible. A fairly acceptable value of cable losses is between 3% and 5%.
Inverter efficiency denotes what part of the input DC power is converted into AC power. The percentage is never 100, but values of inverter efficiency between 90% and 95% are widely assumed in practice.
Shading must be avoided since even small shadows could severely reduce the performance of a PV module. A PV module consists of cells, and when gets shaded, any cell turns into a heat-dissipating resistor boosting dramatically the temperature of the PV module. This results not only in the unexpected reduction of the output voltage but also in shortening the life cycle of cells and modules. When mounted on the roof, PV modules could easily underperform due to shading caused by trees, chimneys and other roof protrusions that can hard to eliminate.
How do solar panels work: Video by Richard Komp from TED- Ed: