The amount of solar energy that hits the earth far exceeds the energy needs of the entire human population. That is why photovoltaic systems, which convert solar energy into electricity, are a particularly productive form of regenerative power generation. Like wind energy, however, solar energy is volatile because it depends on solar radiation. For this reason, surplus solar power must be stored temporarily.
Advantages and disadvantages of solar energy
Photovoltaic systems capture sunlight in solar cells on roofs and in ground-mounted systems and converts it directly into electricity. As an infinite source of energy, the sun is available in unlimited quantities and free of charge. The total amount of solar energy reaching the earth's surface is more than five thousand times greater than mankind's energy needs1. The operation of photovoltaic systems is emission-free. Another advantage: photovoltaic systems can be integrated directly into built-up areas - no minimum distances are required. However, the generation of solar electricity is very volatile. It depends on solar radiation and weather conditions. At night it is even zero. Therefore, without intermediate storage, it is not possible to guarantee an energy supply that meets demand. Due to the low energy density of solar radiation, solar power generation requires relatively large areas. In addition to generating electricity, solar energy is used to generate heat and, in sunny regions, to operate solar thermal power plants that use heat and water vapour to generate electricity. The expansion of wind and solar energy can reduce dependence on raw material-producing countries, as no fossil or nuclear fuels need to be imported - or at least only smaller quantities.
Solar energy in Germany
In Germany, the number of companies and private households generating their own electricity from solar energy is increasing. According to the Federal Statistical Office, in March 2022, 2.2 million photovoltaic systems with a total nominal capacity of 58,400 MW were installed on roofs and properties: an increase of 10.1 % compared to the same month of the previous year2. Today, most electricity is generated from photovoltaics in Bavaria, followed by Baden-Württemberg and North Rhine-Westphalia. In 2021, 50.0 billion kilowatt hours of electricity were generated from PV systems in Germany. However, despite the high number of newly built PV systems, this only means an increase of about 1 % in 2021 compared to the previous year due to the low-sun weather3. In the first quarter of 2022, the share of photovoltaics was 6.3 % of total electricity generation in Germany: an increase of 34 % compared to the corresponding period of the previous year4.
Solar energy in Europe and worldwide
165 GW is the total installed PV capacity in the EU5. Germany leads with 59.9 GW of installed capacity, followed by Italy. Sun-rich countries such as Spain or France are lagging behind. According to the industry association SolarPower Europe, Spain is only number 9 in the EU with 384 W of installed capacity per inhabitant. The ranking is led by the Netherlands with 765 W and Germany with 715 W. France is not among the top 10, but in this country, measures are to accelerate the expansion of photovoltaic systems6. More than three GW per year are to be installed by the end of 2025. The target is a capacity of more than 35.6 to 44.5 GW by 20287. Globally, the People's Republic of China is leading the way and the expansion of PV capacities is in full swing there. In May 2022 alone, PV systems with a total capacity of 6.83 GW were newly installed in China. According to the China Renewable Energy Engineering Institute (CREEI), up to 100 GW of new PV capacity is expected to be installed in 20228.
Intermediate storage of surplus solar power
The accelerated expansion of photovoltaic and wind plants is not enough to make the energy transition successful. The storage of solar and wind power in large quantities is one of the many challenges. When too much electricity is generated from wind and solar plants, the surplus electricity must be stored. On days when there is little wind or sun, electricity must be fed into the grid to meet demand. Lithium and lead-acid batteries are used as short-term storage for stabilising the electricity grid, installed in so-called battery parks. They ensure a balance in the electricity grid. If, for example, a gust briefly pushes up the power generation of a wind farm, the batteries store the excess energy. Or they supply electrical energy for a short time if a cloud shadows a photovoltaic system and the output drops.
To compensate for fluctuations in electricity consumption during the course of the day, the surplus electricity (e.g. at night or when the wind is strong) is stored in pumped-storage power plants. In this process, the water is pumped into a reservoir at a higher altitude. When needed (e.g. wind lull or peak consumption), the water is pumped down the mountain. This drives a turbine that generates electricity. Pumped storage power plants are ideal for storing large amounts of energy. For nature conservation reasons, however, their construction is subject to particularly strict regulations.
Future: Decentralised batteries and hydrogen production
In the future, many small decentralised batteries, e.g. batteries in uninterruptible power supply systems, could be controlled intelligently so that they behave like a large energy storage unit. When there is a power surplus, they are charged, and during demand peaks, they feed part of their capacity back into the grid.
The production of hydrogen using surplus wind energy and its storage over a longer period of time is also a solution to bridge the windless periods and the winter months with very low solar irradiation. In this case, the hydrogen is converted into electrical energy when needed.
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