How does a photovoltaic system work?

What are the components of a photovoltaic system?

The main components of a photovoltaic system are:

• Photovoltaic modules: a photovoltaic system captures the energy radiated by the sun through the use of special components called “photovoltaic modules”, which in turn consist of individual silicon cells, i.e. the basic electrical components that convert solar radiation into electrical energy. Several modules connected together in series form a string. Strings connected together in parallel form a solar generator.

• Module support structures: these are the structures that support the modules by fixing them to the roof and, in the case of flat roofs, also provide for their orientation, optimising their exposure by tilting them towards the sun's rays.

• Field switchboard: the field switchboard manages the different strings and protects them from overloads.

• Inverter: this is the electronic device that converts the electrical energy produced by the modules (so-called direct current) into the same type of electrical energy used by residential appliances (household appliances etc.) or industrial appliances (so-called alternating current). For increased installation safety, the inverters incorporate protection devices that cause them to shut down in the event of a blackout or grid disturbance.

• Wiring: connecting cables for photovoltaic panels carrying direct current are subject to intense stresses. Specific solar panel cables with good UV and weather resistance must therefore be used.

• Protection systems: these represent the set of systems (e.g., breakers, residual current devices, limiters, arresters) that protect the installation from adverse events such as overloads, short circuits and lightning, or from direct contacts.

• Metering systems: grid-connected photovoltaic systems have a new meter installed by the local distributor, which measures production; this meter is added to the one already present in the house and will therefore, instead of just energy consumption, the exchange of energy between the house and the grid will also be measured.

Additional components that help to improve the efficiency of the system, which are becoming increasingly popular, are:

• Monitoring system: this is a system that allows you to control your installation remotely, monitoring production and self-consumption trends and checking the status of the inverter.

• Energy Storage: a storage system that allows the efficiency of the installation to be improved by storing the energy produced during the day for use when the system is not producing. This maximises self-consumption of the energy produced without changing consumption habits.

What factors influence the efficiency of a photovoltaic system?

No photovoltaic system is able to convert all the energy it receives from the sun into electricity, but only a part. The efficiency of photovoltaic panels is as high as the percentage of solar radiation energy they are able to transform into electrical energy. High-efficiency photovoltaic panels currently on the market are able to convert between 20 and 23% of the energy they receive under standard conditions, i.e. about one-fifth. However, systems with efficiencies of over 30% are being tested. The conversion capacity of the panels is the main factor influencing the efficiency of a photovoltaic system. However, there are other factors that intervene in the different stages of energy production.

• Service life: the efficiency of panels tends to decline over time, by 0.5% every year, on average. Therefore, at the end of its life (after 20-25 years of operation), a photovoltaic panel will have a significantly lower yield than when it was commissioned.

• Environmental situations such as temperature and dust affect the efficiency of a photovoltaic module. In particular, the higher the ambient temperature, the lower the efficiency. The presence of dust or other volatile materials on the cells negatively impacts efficiency.

• Shading is an inherent inefficiency factor, whether it is a transient phenomenon (e.g. due to weather conditions) or a structural problem (e.g. trees casting their shadow on a group of panels).

• Inverter efficiency: The process of converting electrical current from direct to alternating through inverters results in an energy loss of 3-4%.

• Unevenness and small losses along the distribution network are possible. Although identical to one another in terms of make, model and capacity, each panel on the market may exhibit slightly different characteristics. Cables and connectors can also adversely affect the overall efficiency of the system, but usually these changes are not very noticeable.

• Module orientation: this plays a crucial role in defining the power output of the system and the type of modules to be installed. The optimal solution is to have a south-facing roof, corresponding to the position of the sun at midday solar time. By adding a system of “solar trackers” it is also possible to change orientation and tilt depending on the time of year.

How is the efficiency of a photovoltaic system calculated?

All the factors that contribute to defining photovoltaic performance can be summarised through an index, the System Derate Factor, which is the efficiency reduction coefficient for a photovoltaic system. Some simulations, such as those performed by the PV Watts Calculator of the US Department of Energy's National Laboratory, estimate a standard efficiency value of 86%. However, this is an indicative figure because, as we have seen, some of the factors influencing photovoltaic panel performance depend on unpredictable environmental situations.

The efficiency of a photovoltaic module (i.e., the amount of solar energy it is able to convert into electrical energy) is measured in the laboratory under standard conditions, known as STC, which include an operating temperature of 25º C and solar radiation of 1000 W/m2. The Overall Efficiency of a Photovoltaic System is then calculated using the following formula: Overall PV System Efficiency = (PV Module Efficiency) x (System Derate Factor). For photovoltaic performance %, however, Performance % = (Power / Surface Area / 1000) * 100.