A photovoltaic system, also known as a PVA system or solar generator, is a solar power system in which part of the solar radiation is converted into electrical energy by means of solar cells. The typical direct type of energy conversion is called photovoltaics. In contrast, other solar power plants (e.g. solar thermal power plants) work via the intermediate steps of thermal energy and mechanical energy. Each module is an independent installation. Several solar modules are considered to be a single installation under certain circumstances for the sole purpose of determining the installed capacity. PV systems are also subject to the Waste Electrical and Electronic Equipment Directive (WEEE), which means that manufacturers have to take them back at the end of their life.
The nominal output of conventional solar systems ranges from the low single-digit kW range, as is common for residential rooftop systems, to a few MW for commercial rooftop systems, while ground-mounted solar systems are usually located at least in the MW range.
Depending on the size and type of system, individual solar modules are connected in series to form so-called strings. The solar modules, the smallest building blocks of a solar system to be distinguished in technical applications, consist of a series connection of solar cells, which are hermetically sealed and no longer accessible for repair. In the case of crystalline solar cells, the individual cells are first manufactured individually and then connected by metal foils. In thin-film cells, the production of the compounds is integrated into the processes for forming the cells.
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By connecting solar cells in series, with a voltage of only about 0.5 V, and then solar modules, the voltage adds up. Whereas 20 years ago the maximum system voltage was limited to 110 V in order to meet safety regulations, today’s solar modules tested according to protection class II make a system voltage of 1000 V possible. A further increase to 1500 V DC, the limit value of the low voltage definition according to VDE0100, is being worked on.
Even a square, crystalline solar cell with an edge length of 156 mm (6+”) can be loaded with several amperes at the operating point at maximum solar radiation. When several individual cells are connected in series, the current is determined by the solar cell with the lowest value. If necessary, several of these strings of the same voltage and characteristics are connected in parallel; this adds up the currents of the individual strings. Parallel connection of individual modules is particularly common in stand-alone operation.
The solar modules are usually attached to a substructure, which ideally aligns the modules in such a way as to ensure the highest possible or as constant energy yield as possible over the year. The substructure can also be tracked by the sun (astronomical, sensory) in order to achieve a higher energy yield.
If it is a grid-connected system, the direct current generated in the solar module(s) is converted into alternating current with the help of a solar inverter and fed into the power grid. Apart from the conversion losses, which reduce efficiency, the feed-in of smaller plants is usually complete and unrelated to the actual electricity demand, as long as the grid has sufficient voltage quality at the feed-in point.
Grid-connected solar systems normally do not provide the power grid with balancing power, as the aim is to generate a maximum of renewable energy.
Inverters can also deliver or absorb reactive power into the grid if required, which has also been required by the medium-voltage directive since mid-2010 for higher-performance systems that feed into the grid at the medium-voltage level.
In off-grid systems, the energy is temporarily stored unless it is used directly. Storage in energy storage devices, usually lead-acid batteries, requires the use of a charge controller. In order to be able to use the stored energy for conventional electrical appliances, it is converted into alternating current with the help of an off-grid inverter. For grid-connected systems with storage systems, there is now a trend towards lithium batteries. The smaller dimensions with the same capacity are advantageous, but the higher purchase price is currently still having a disadvantageous effect.
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