23.04.20 Can electricity be both renewable and reliable? Hans-Joachim Ziegler • 5 min.

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Summary

In the decades ahead, Europe’s electricity grids will need to undergo a fundamental transformation. Why? So that they can remain reliable and stable despite a bigger much proportion of intermittent green electricity in the energy mix.

The electricity systems of most European countries are very reliable. Germany, for example, had an average of just 13 minutes of service interruption per customer in 2016. Average outage duration was similarly brief in Switzerland, Luxembourg, Denmark, and Italy. 

Going forward, less electricity will be generated in large power stations and more in wind and solar farms as well as small, embedded gas-fired cogeneration units. Wind and solar output depends on the weather and the time of day. It therefore fluctuates continually, creating challenges for grid operators. Moreover, electricity no longer flows in just one direction, namely from power stations to consumers. Increasingly, companies’ cogeneration units and individuals’ solar panels are feeding some of their output back into the grid. 

These changes are significant enough. In the foreseeable future, however, several European countries—including Germany, Sweden, and the United Kingdom—could be able meet their electricity needs without fossil fuels. The question is: will their grids remain reliable and stable under this all-green scenario. The answer is somewhat complicated.

Spinnin’ wheel, got to go ’round 

The turbines at fossil and nuclear power stations turn at 50 revolutions a minute, giving electricity a frequency of 50 Hertz. These turbines are huge and heavy. They therefore tend to want to continue spinning at 50 Hertz even when the voltage in the grid goes up or down owing to sudden changes in production or consumption. This tendency, called grid inertia, helps minimize and slow the effects of voltage fluctuations and thus helps keep the power system stable and reliable. Frequency can also be regulated by increasing or reducing the steam flow to a plant’s turbine (primary control) or by bringing power plants on- or offline (secondary control).

Renewables, by contrast, provide no grid inertia. Solar panels of course don’t spin at all, and wind turbines spin in response to wind speed, not the frequency of the grid. As the proportion of renewables in the electricity mix increases, the amount of available grid inertia will decrease. As a result, changes in production or consumption will affect the grid faster and more dramatically. To maintain stability going forward, the power system will have to adapt in at least three ways. 

Power lines

The virtues of virtual 

Europe may have fewer large power stations in its future, but it can still have them virtually. Virtual power plants (VPPs) consist of hundreds and sometimes thousands of small and medium-sized generating units scattered across a power system. These units are controlled remotely and, together, can provide secondary control energy to eliminate frequency fluctuations as if they constituted a single, larger power plant. Some VPPs have more than 1 gigawatt of capacity. Going forward, adding more VPP capacity will help Europe’s power system retain some of the virtues of the big conventional power stations that are gradually disappearing. 

Get smart  

Most power lines could carry more energy than they currently do. Tapping this surplus capacity would enable them to accept and transport more green electricity. The answer is to make power lines smarter by equipping them with sensors that measure their temperature, that of the surrounding air, and wind speed. Grid control centers use these real-time data to maximize capacity use without overheating the lines. The great thing is: the same stiff breeze that turns wind turbines also cools the power lines that transport their output, thereby increasing line capacity precisely when it’s needed: a true wind-wind situation. In many countries, substations are getting smarter as well, enabling them to manage some local voltage fluctuations automatically and also to transmit real-time data to control centers. Going forward, these and other smart-grid technologies will need to be deployed on a much larger scale. 

Store more 

Today, electricity production precisely matches consumption almost 24/7. At times, storage systems—primarily pumped-storage hydroelectric stations—come online to meet peak load. But as the power system becomes greener and thus more dynamic, much more storage capacity will be needed. The International Energy Agency predicts that the world must increase its storage capacity from 177 GW in 2017 to 266 GW by 2030. More storage will not only make the grid more flexible and reliable. It will also make it possible to harness more renewable energy, for example by storing surplus wind power produced at night when energy consumption is low.

Cheap and efficient storage tomorrow will require R&D investments today. Grid-scale battery systems are the most prevalent form of new storage. Other promising technologies include power-to-gas systems and compressed air storage. In the former, surplus renewable electricity is used to run equipment that transforms water into green hydrogen or methane. In the latter, it’s used to compress air underground or in a cavern; when electricity is needed, the air is heated and released to drive a turbine.

Swift but secure  

Europe wants to decarbonize as swiftly as possible. Still, many European countries will continue to need fossil-fueled power plants in the years ahead to help stabilize their grids. Gas-fired plants are ideal for this. They’re much less carbon intensive than coal-fired plants and can be ramped up and down in a few minutes to balance out the fluctuating feed-in from renewables. 

The EU wants to be a pacesetter in green energy. The changes described above will help ensure that its entire power system’s stability and reliability remain world-class as well.

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