Photovoltaics is on the rise in the USA. This article explains how a photovoltaic system works and what components it is made up of.
How does photovoltaics work?
A photovoltaic system converts sunlight into electrical energy. Solar modules consisting of solar cells are used for this purpose. The sunlight sets the electrons in the solar cells in motion and generates direct current. This is converted into alternating current by the inverter and made available to the household.
The way a photovoltaic system works is somewhat more complex than described above. A PV system consists of several components that are essential for efficient operation.
Which components are needed to produce electricity?
A PV system consists of just a few components. The most important ones are
*Solar meter and feed-in meter
Alternatively, an electricity storage unit and energy manager are also used. The individual components are described in more detail below.
Solar modules and solar cells
Solar modules contain solar cells that convert sunlight into electrical energy. The solar cells are connected in series so that their voltage adds up. As a rule, a solar module consists of 60 or 72 cells or 120 to 144 half cells. They achieve an output of 300 to 400 Wp, although solar modules with over 600 Wp are now available on the market.
Solar cells are mainly made of silicon. Silicon is a semiconductor material with photovoltaic properties. When sunlight hits the solar cell, it excites the electrons. Their movement generates electricity. Two differently doped layers are needed for the solar cell to generate electricity:
1、The top layer is called the n-doped layer. It contains silicon and phosphorus. Silicon has four bound electrons, while phosphorus has five electrons. This additional electron is free in the layer;
2、The lower silicon layer is p-doped with boron. Boron has one electron less than silicon, creating a hole;
3、The free electrons from the n-doped silicon-phosphorus layer migrate into the p-doped layer and fill the holes. This forms a boundary layer of boron atoms with four electrons. These atoms become stationary as they no longer have any holes;
4、Electron migration creates electric poles. When electrons migrate, the upper layer becomes positively charged and the lower layer negatively charged. Sunlight releases electrons from boron atoms in solar cells. The electrons are attracted to the positive pole and migrate to the upper layer. This process takes place in all solar cells that are exposed to sunlight.
The excited electrons are discharged from the upper solar cell layer. This takes place via an electrical conductor, usually a metal grid on the back of the solar module. When the sun shines, more and more electrons are pushed through the metal contacts and conducted through the solar cables.
There is a metal contact on the underside of the solar module that is connected to the solar cables. The electrons flow through the cable and reappear in the bottom layer. By remaining in constant motion, they generate an electrical voltage.
Solar cables connect the modules of a solar system. They are weatherproof and UV-resistant and transport the electricity between the PV modules. There are various ways to connect or switch these cables. This has an effect on the current voltage, the current strength and the overall output:
1、When connected in series, the solar modules are connected in series. The positive cable is connected to the negative cable. The voltage of all modules adds up, while the amperage remains the same. At the end, the first and last module each have a cable that is connected to the inverter. This is the most common type of circuit and the one with the fewest cables.
2、In parallel connection, negative cables are connected to negative cables and positive cables to positive cables. This increases the current while the module voltage remains the same. In the end, there are still two cables connected to the inverter. The advantage is that the shading of one module has no influence on the current yield of the others. The disadvantage is that more cables have to be laid and the installation is more complex.
The solar meter measures the total electricity generated by the photovoltaic system. This is crucial for determining the yield generated and the cost-effectiveness of the PV system. The solar meter is installed on the direct current side, i.e. before the inverter.
A solar power inverter makes it possible to use the solar power generated in the household. Solar power is direct current, while households and the public grid use alternating current:
1、Direct current flows constantly in one direction, from negative to positive. The strength of the current remains constant over time;
2、With alternating current, the current flow regularly changes direction. The frequency, measured in Hertz (Hz), indicates how often this change takes place per second. In Europe, electricity grids operate at 50 Hz, which means the direction changes 50 times per second.
PV inverters use sophisticated circuits to generate a sinusoidal wave for electronic devices. The switches open and close power lines quickly, thus changing the direction of the current. In order to achieve a uniform sine wave, the switching frequency is divided into smaller segments with different current intensities.
To monitor and optimize PV systems, modern inverters contain MPPT (Maximum Power Point Tracking). They influence the electrical current and voltage in order to operate the solar system close to its maximum power point.
Due to high electricity prices, it is nowadays worth storing surplus electricity instead of feeding it into the grid. An electricity storage system is integrated into the PV system for this purpose. This makes it possible to use the self-generated solar power outside of the generation times. This in turn increases self-consumption and the profitability of the system.
An electricity storage system consists of a positive electrode (anode), a negative electrode (cathode) and an electrolyte as a conductive liquid. The electrolyte surrounds the two electrodes. If the solar system generates excess electricity, the electrons move through the electrolyte from the cathode to the anode. The anode is fully charged with electrons. At the anode, the electrons react and form atoms. In this way, excess electricity is stored in the form of chemical energy.
During discharge, the atoms move back to the cathode. There they are converted back into electrons. The electrons are available as electric current and are fed into the household circuit.
To make an energy storage system particularly worthwhile, combine it with an energy management system.
Energy management system
The task of an energy manager for PV systems is to increase self-consumption of solar power in the household and reduce electricity costs. The energy management system identifies and utilizes potential energy savings. It records and analyzes energy flows and sources, develops ideas for improvement, evaluates cost-effectiveness and implements them. Most energy management systems are controlled via an app or software.
Consumption meters and feed-in meters
If you connect a PV system to the public grid, you need a consumption meter and a feed-in meter:
1、The feed-in meter measures the electricity fed into the grid;
2、The consumption meter measures the electricity consumed in the household.
The consumption meter is usually already present. The feed-in meter is only installed when the PV system is commissioned, as soon as you register the system with the grid operator and it is approved by them. Nowadays, bi-directional meters are usually installed, which combine consumption and feed-in meters.