Fluctuating renewable energies

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Fluctuating renewable energies are renewable energies , such as solar energy or wind energy , whose supply fluctuates, i.e. is subject to fluctuations .

Comparison of the individual regenerative producers

Biomass and geothermal energy can be regulated. When generating electricity from solar energy, the output can be reduced, but not increased; in addition, solar energy is more variable than wind energy, but it is also more predictable. Wind turbines can be throttled as required, and in modern variable-speed systems with full converters, short-term, i.e. H. power increase possible for a few seconds. The energy required for this comes from the stored kinetic energy of the rotor and drive train, the speed of which decreases. This means that a sufficient number of wind turbines with a corresponding central system control can contribute to the frequency stability of the power grid within seconds, both with overfrequency and underfrequency .

The generation of energy through biomass consists of a two-part process, the generation and utilization of the biomass. A combined power plant of the Institute for Solar Energy Supply Technology is operated 100 percent by renewable energies, with wind turbines and solar systems being supported by water storage and biomass, if necessary.

Typical wind power forecast with forecast data (green) and actual power (blue)

Wind energy is the worst predictable of all variable energy sources. Network operators rely on a daily forecast of wind power and weather in order to be able to decide which energy sources should be used the next day and which should not. In the case of wind power, the accuracy of the predictions is an average uncorrected error of 8.8% in Germany over a period of two years.

In wave power plants , the kinetic energy of waves generated by wind is converted into electrical energy. The strength of the waves correlates with the wind power, but is less variable due to the mass of the water. Wind energy is proportional to the third power of the wind speed, and the wave energy is proportional to the square of the wave height.

Feed-in of a solar system on three different days

Solar energy is easier to estimate than wind energy, but it is also more variable because it is only available during the day. It is influenced during the day by the degree of cloudiness. Wind power arises from the different degrees of warming of the earth's surface and can make about 1% of solar energy available.

Tidal energy is best predictable compared to any other variable renewable energy source. Twice a day there is a tide change, which is similarly strong every day. It is believed that 20% of the UK's energy needs could be met by tidal energy, however there are only 20 locations in the world where it would be useful to use tidal energy.

Dealing with the variability

As soon as a system is mainly fed by non-controllable energy sources, reacting to fluctuations must become one of the main tasks of a network operator . Instead of switching available sources on and off, the focus must now be on the correct distribution and storage of energy.

Electrical energy can be temporarily stored in pumped storage power plants in the form of potential energy. Processes such as “ power-to-gas ” are being tested on a small scale, with methods for water electrolysis being tested, among other things . One example of a prototype system is the hybrid power plant from the company Enertrag , which went into operation in 2011.

The International Energy Agency notes that too much attention is being paid to the problem of the variability of renewable energies. This obstacle only applies to certain forms of renewable energy, such as wind energy and photovoltaics, and is also dependent on other factors, such as the market penetration of the respective form of energy or the availability of compensation options. The combination of solar energy and wind energy is advisable here, the former is more productive in summer, the latter in winter. In addition, the basic consumption can be covered by geothermal energy or biomass. Thus, the variability of renewable energies is rarely a barrier to the spread of the same. However, if the market penetration is high, additional costs can arise due to more complex fallback systems.

literature

• Martin Kaltschmitt , Wolfgang Streicher, Andreas Wiese (eds.): Renewable energies. System technology, economy, environmental aspects . Springer Vieweg, Berlin / Heidelberg 2013, ISBN 978-3-642-03248-6 .
• Martin Kaltschmitt / Wolfgang Streicher (eds.), Renewable Energies in Austria. Basics, system technology, environmental aspects, cost analyzes, potentials, use , Wiesbaden 2009, ISBN 978-3-8348-0839-4 .
• Volker Quaschning : Renewable energies and climate protection. 3. Edition. Hanser, Munich 2013, ISBN 978-3-446-43809-5 .
• Volker Quaschning: Regenerative Energy Systems. 8th edition. Hanser, Munich 2013, ISBN 978-3-446-43526-1 .
• Viktor Wesselak , Thomas Schabbach , Thomas Link, Joachim Fischer, Regenerative Energietechnik , 2nd expanded and completely revised edition, Berlin / Heidelberg 2013, ISBN 978-3-642-24165-9 .

Individual evidence

1. ↑ Anca D. Hansen, Müfit Altin, Ioannis D. Margaris, Florin Iov, Germán C. Tarnowski, Analysis of the short-term overproduction capability of variable speed wind turbines . Renewable Energy 68 (2014) 326-336, 326f doi : 10.1016 / j.renene.2014.02.012 .
2. ^ The Combined Power Plant: the first stage in providing 100% power from renewable energy . SolarServer. January 2008. Retrieved June 14, 2014.
3. ↑ On-line Monitoring and Prediction of Wind Power  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.  
4. ^ Wind and Waves
5. ↑ Comparing the Variability of Wind Speed ​​and Wave Height Data ( Memento of the original from June 17, 2012 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.  
6. ↑ Wind Turbines: Converting Wind Energy Into Electricity ( Memento of the original from May 15, 2012 in the web archive archive.today ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  
7. ^ Tidal power
8. ^ Contribution of Renewables to Energy Security, page 5. International Energy Agency . Last accessed on August 1, 2014.