Fünf Kilometer nordöstlich von Cottbus in Brandenburg. 220 Millionen Tonnen Braunkohle wurden hier bis zum Jahr 2015 aus dem Boden geholt, um sie im nahe gelegenen Kraftwerk Jänschwalde zu verbrennen. Doch heute ist der ehemalige Tagebau kaum noch wiederzuerkennen. Das riesige Loch in der Erde wird seit einigen Jahren mit Wasser geflutet. Und nicht nur das: Auf dem künstlichen See entsteht die größte schwimmende Photovoltaikanlage Deutschlands.
Damit steht das Projekt in der Lausitz für den Umbau der deutschen Stromversorgung hin zu einem erneuerbaren System. Es soll der Wirtschaft den Weg bereiten in eine Welt, in der kein Kohlendioxid (CO2) mehr ausgestoßen wird: Bis 2030 peilt die Bundesregierung 80 Prozent Ökostrom an, fünf Jahre später soll der Stromsektor „überwiegend dekarbonisiert“ sein. Klimaneutral produzierter Strom soll dann dafür sorgen, dass auch alle anderen Sektoren auf Treibhausgase verzichten können.
Der grüne Strom steht am Anfang aller Versorgungsketten, auch in der Industrie, in Gebäuden und im Verkehr. Diese beruhen heutzutage noch vielfach auf fossilen Energien. Aber wie soll das gehen: ohne Kernkraft, Kohle, Öl und Erdgas auskommen?
Skepsis gegenüber erneuerbaren Energien
Tatsächlich gibt es bislang nur wenige Länder auf der Welt, die ihren Strombedarf vollständig aus Erneuerbaren decken: Albanien, Island, Norwegen, Bhutan, Nepal und Paraguay sind darunter. Sie alle haben eins gemeinsam: Sie verfügen über die geographischen Voraussetzungen, ihren Strom nahezu vollständig aus Wasserkraft zu gewinnen. In Island kommt Strom aus Geothermie hinzu, also Erdwärme. Hierzulande hingegen ist das Potential für die Stromerzeugung aus Wasserkraft und Geothermie begrenzt – Solarparks und Windräder bieten sich deutlich mehr an.
Die Skepsis den erneuerbaren Energien gegenüber ist so alt wie die Energiewende selbst. Zu Beginn des Jahrtausends hatten viele Kritiker kaum mehr als ein paar Prozent Ökostrom im System für möglich gehalten. Die schwankende Stromproduktion der Erneuerbaren sei nicht grundlastfähig, hieß es, als die rot-grüne Bundesregierung im Jahr 2000 mit dem Erneuerbare-Energien-Gesetz (EEG) dafür sorgte, dass grüner Strom vorrangig abgenommen wird, und garantierte Vergütungssätze einführte. Irgendwann würde ein Stromsystem mit einem hohen Anteil Ökostrom zusammenbrechen, weil es nicht kontinuierlich Strom liefern könne.
58 percent of gross electricity consumption in Germany is now covered by renewable energies, and the trend is still rising. A figure that many people would not have dared to dream of a few years ago. The electricity system has still not collapsed. Nevertheless, a few engineers continue to express concern that renewable energies alone could “never keep a power grid functioning for more than 24 hours” and that they are “not controllable”. What is the truth behind these concerns, and what could a vision for the German electricity system of the future look like?
Sufficient space available
Kathrin Goldammer is optimistic. “From an engineering perspective, we can say: Yes, 100 percent renewable energy is possible in Germany,” says the director of the Reiner Lemoine Institute (RLI) in Berlin. She is researching precisely this goal. “All the necessary technologies – above all wind power, photovoltaics, storage – are available.” There is also enough suitable space for wind turbines and solar parks to fully cover Germany’s electricity needs, says the scientist, pointing to a calculator from her institute. This applies even if electricity demand will increase dramatically in the coming years. “A system in which we electrify the transport sector, heat and German industrial processes or equip them with green molecules is also feasible.” Various energy system studies assume that gross electricity consumption could double to around 900 to 1100 terawatt hours by 2045.
There are greater concerns about security of supply. In the traditional energy world, a distinction was made between three types of power plants: base load, medium load and peak load. Lignite plants and nuclear power plants ran virtually around the clock (base load). Hard coal power plants were responsible for medium load, while gas power plants could only achieve a few hundred full load hours a year: they only stepped in when they were needed.
The big joker flexibility
The new energy world no longer knows this – no base load, medium load or peak load, only renewable energies and flexibility. Nuclear power plants are already completely shut down, but brown coal and hard coal power plants are also increasingly being pushed out of the market by cheaper solar and wind power. They can generate electricity competitively in fewer and fewer hours a year – their full load hours are decreasing. Researchers assume that these power plants will no longer be able to operate at all in a few years due to rising prices in the European emissions trading system.
Flexibility remains – the big wildcard that should complement electricity production from renewables and ensure that security of supply is maintained around the clock. Solar systems, which are more heavily used in summer, and higher wind power generation in winter complement each other well seasonally. However, electricity consumption is traditionally higher in winter than in summer because people heat more and turn on more lights. Gas-fired power plants should therefore generate electricity flexibly mainly in winter. In the best case scenario, these will eventually be operated with green hydrogen instead of natural gas.
A scenario that the consulting institutes Prognos and Consentec have designed for the think tank Agora Energiewende shows what a climate-neutral electricity system could look like in 2035. In it, renewable energies contribute the overwhelming share of 89 percent to net electricity generation, while hydrogen power plants cover the rest with 7 percent. Since Germany exports more electricity abroad than it receives from there, the share of renewables in electricity consumption as a net exporter is mathematically more than 100 percent. Because there are not yet that many gas-fired power plants, the federal government is planning to put 12.5 gigawatts of power out to tender. A capacity mechanism is intended to ensure that there are enough flexible electricity producers on the market in the medium term.
In the scenario mentioned, there are two different types of controllable power plants in 2035: One part – around 20 gigawatts – runs an average of 3,000 hours a year. This corresponds roughly to the capacity utilization of many gas-fired power plants today. A second part of power plants, on the other hand, only runs for a very few hours a year – they are, so to speak, insurance against rare extreme weather events. While the first group of power plants burns hydrogen, the second group could also use ammonia, which is easier to store and transport than hydrogen.
Demand must keep up
Now comes the big paradigm shift: In a system with a very high proportion of renewable energies, electricity demand must also become more flexible in order to ensure a constant balance between supply and demand. This means that households and companies must use electricity as soon as it is generated.
This applies primarily to large consumers. Electric cars, for example, can be charged flexibly in a climate-neutral system, some even charge in two directions and thus act as electricity storage (vehicle-to-grid). Many heat pumps can also behave flexibly. In hours when a lot of green electricity is generated, heating rods or electrode boilers can provide local heat, district heating and heat for industry (power-to-heat). Electrolyzers also ideally produce green hydrogen in the hours when the electricity cannot be used for other purposes. In summer this is mainly during the day, in autumn and winter especially in phases with strong winds.
Large and small battery storage systems and, to a lesser extent, pumped storage power plants play a key role in making electricity demand more flexible. In summary, the more flexible storage systems and demand are, the fewer gas-fired power plants will be needed and the closer we get to the vision of an electricity system that is 100 percent based on renewable energy.
To ensure that electric cars, heat pumps and home storage systems can actually behave flexibly, they are controlled using smart electricity meters. To do this, network operators need to know much more about the status of their networks and consumption units than they do today. Intelligent network control also reduces the need for investment. Nevertheless, the electricity system of the future will require power lines that are almost 50 percent longer than today – around 50,000 kilometers in the Agora scenario. Ideally, today’s network will then be optimized and strengthened. This could mean, for example, converting existing power lines from 220 to 380 kilovolts so that less electricity is “lost” along the way. Another way to strengthen power lines is to use high-temperature conductor cables.
Power grid must remain stable
In addition, the exchange of electricity with neighboring European countries plays a crucial role in the climate-neutral electricity system of the future. In addition to controllable power plants, flexible demand and electricity storage, other countries can also provide flexibility. In Agora’s scenario, Germany will once again become a net exporter of electricity from 2027 onwards, also due to new submarine cables to Norway and England.
And what about the concerns that an electricity system with a very high proportion of renewable energy is “not controllable”? In fact, the grid operators must not only ensure that there are enough cables of sufficient quality, but also that daily operations run smoothly. The operators must ensure that electricity really always arrives when it is needed.
Conventional power plants have actually provided some of the services necessary for grid stability. The more of them go offline, the more urgently these services need to be procured elsewhere. How is that supposed to work? Firstly: the instantaneous reserve. This refers to the rotating masses of the synchronous generators in conventional power plants. At the moment they almost automatically keep the frequency in the power grid stable, which must always be at 50 hertz. This applies even if demand changes abruptly or a power plant fails, and until the control power is activated after a few seconds.
What’s missing?
The instantaneous reserve gives the power grid valuable seconds in critical cases. In the future, renewable energies will also be able to participate in the instantaneous reserve. One possible way to play this role is to use wind turbines in which turbines also rotate – but these must be equipped with special frequency converters. The time until the control power is activated could also be shortened.
Secondly, the so-called reactive power is essential to keep the power grid stable. More precisely, the voltage that is necessary to transport electricity over long distances. Reactive power is also traditionally provided by large conventional power plants and their synchronous generators. Technical alternatives in a power system dominated by renewables can be grid operating equipment such as rotating phase shifters, which are already used in the transmission grid. The grid operator Tennet, for example, has just installed such a phase shifter weighing 1,000 tons in a substation in Hesse.
The technical prerequisites can therefore be created for an electricity system with a very high proportion of renewable energy. What is still missing? For Berlin engineer Goldammer, more needs to be done. “What we urgently need to continue to work on is the rapid expansion of the networks and generation plants,” says the researcher. “In addition, the right incentives are lacking to integrate battery storage and other flexibilities in such a way that they serve the system.” Then Germany could actually one day be the first country in the world to cover its electricity needs predominantly from solar power and wind power. Germany’s largest floating photovoltaic system in Brandenburg is also helping with this. It is possible.
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