BOLD signal change data analysis. Problems with correlations analyses.

Diverse causes behind frequency fluctuations in power grids
The use of renewables like the sun and wind can cause fluctuations in power grids. But what impact do these fluctuations have on security of supply? To answer this question, scientists from Jülich and Göttingen worked together with colleagues in London and Tokyo to analyse different types of fluctuations in several power grids in Europe, Japan, and the USA – and came to surprising conclusions. Their study was published today in the peer-reviewed journal Nature Energy.
Our power grid works at a frequency of 50 hertz – usually generated by turbines, for example in hydro- or coal power plants, which rotate at a speed of 50 revolutions per second. "When a consumer uses more electrical energy from the power grid, the grid frequency drops slightly before an increased energy feed-in re-establishes the original frequency," explains Benjamin Schäfer from the Max Planck Institute for Dynamics and Self-Organization (MPIDS) in Göttingen and lead author of the study. "Deviations from the nominal value of 50 hertz must be kept to a minimum, as otherwise sensitive electrical devices could be damaged."
Renewable energy generation also causes grid frequency fluctuations because the wind does not always blow at the same speed and clouds constantly alter the feed-in from photovoltaic systems. A frequent suggestion for integrating renewable energy generators into the power grid involves breaking the grid down into small autonomous cells known as microgrids. This would allow a community with a combined heat and power unit and its own wind and photovoltaic generators, for example, to operate its energy systems in an autonomous manner.
But what impact would this division into small cells and additional renewable generators have on the power grid? To answer this question, scientists from Forschungszentrum Jülich and MPIDS analysed the grid frequency fluctuations in power grids in different regions of the world – and using mathematical models, they predicted potential vulnerabilities and their causes.
Two surprises in one analysis
Firstly, they collated measurements from Europe, Japan, and the USA. Then, they systematically analysed the data and were surprised on two accounts. "The first surprise was that the grid showed particularly strong fluctuations every 15 minutes," says Dirk Witthaut from Jülich’s Institute of Energy and Climate Research und the Institute for Theoretical Physics of the University of Cologne. "This is the exact time frame during which generators on the European electricity market agree on a new distribution for the electricity generated – this alters how much electricity is fed into the grid, and where. In Europe at least, power trading therefore plays a key role in balancing grid frequency fluctuations."
The second surprise was that statistical grid fluctuations around the nominal value of 50 hertz do not follow the expected Gaussian distribution, which is a symmetrical distribution around an expected value. Instead, more extreme fluctuations are probable. Using mathematical models, the scientists calculated the expected fluctuations depending on the grid size and estimated the degree to which the fluctuations depended on renewables.
Power trading as a key factor
A comparison of the investigated regions showed that a large proportion of renewables did indeed lead to greater grid fluctuations. "For example, the share of wind and solar generation in the United Kingdom is much higher than in the USA, leading to greater fluctuations in grid frequency," explains Dirk Witthaut. For an increased share of renewables, the scientists therefore recommend increased investment in an intelligent adjustment of generator and consumer according to the grid frequency – known as primary control and demand control.
One of the most interesting findings of the study, however, is that grid frequency fluctuation caused by power trading appeared to be more significant than fluctuation caused by renewable feed-in.
The scientists also discovered that small power grids show larger fluctuations. "Our study indicates that dividing large and thus very slow grids – such as the synchronous grid of Continental Europe – into microgrids will cause larger frequency fluctuations," says Benjamin Schäfer. "Technically, microgrids are therefore only an option if today's very stringent frequency standards were to be relaxed."
Publication: "Non-Gaussian Power Grid Frequency Fluctuations Characterized by Lévy-stable Laws and Superstatistics" by Benjamin Schäfer, Christian Beck, Kazuyuki Aihara, Dirk Witthaut, Marc Timme, Nature Energy, DOI: 10.1038/s41560-017-0058-z
More information
Institute of Energy and Climate Research, Systems Analysis and Technology Evaluation (IEK-STE), Forschungszentrum Jülich
Contact
Dr. Dirk Witthaut
Institute of Energy and Climae Research, Systems Analysis and technology Evaluation (IEK-STE)
Phone.: +49 2461 61-3397
E-Mail: d.witthaut@fz-juelich.de

Silicon-air battery achieves running time of over 1,000 hours for the first time
Silicon-air batteries are viewed as a promising and cost-effective alternative to current energy storage technology. However, they have thus far only achieved relatively short running times. Jülich researchers have now discovered why.
In theory, silicon-air batteries have a much higher energy density and are also smaller and lighter than current lithium-ion batteries. They are also environmentally friendly and insensitive to external influences. Their most important advantage, however, is their material. Silicon is the second most abundant element in the Earth's crust after oxygen: it is cheap and its reserves are practically inexhaustible.
However, the silicon-air battery does still have a few crucial blemishes: for example, the flow of current stops after a relatively short period of time. Only assumptions have been made thus far as to why this is the case: does a protective layer form spontaneously on the silicon anode? Is the electrolyte at all suitable? Is there a problem with the air electrode? Attempts to rectify this problem by improving the components have proven to be less than successful. The best result was achieved through the use of a special, high-quality electrolyte based on an ionic liquid. This helped increase the battery’s running time to several hundred hours, but contradicted the fundamental idea of the battery: to provide a cost-effective alternative to lithium-ion batteries.
Scientists at Jülich’s Institute of Energy and Climate Research (IEK) suspect another cause for the short running time: the consumption of the electrolyte. As part of the AlSiBat project funded by Germany’s Federal Ministry of Education and Research, the researchers developed a pump system in which the electrolyte fluid – potassium hydroxide dissolved in water – was refilled from time to time. “If the silicon anode remains in contact with the electrolyte, the battery will continue running,” explains Hermann Tempel from the IEK’s Fundamental Electrochemistry. The battery is thus able to achieve a running time of over 1,100 hours, or almost 46 days, he adds. “Until the silicon is fully used up. The battery can subsequently be recharged by exchanging the anode, in other words mechanically.”
The scientists are now looking for a way to keep the battery running without having to refill the electrolyte. “We need to stop the battery from self-discharging,” explains Hermann Tempel, noting how this leads to the electrolyte fluid being used up. Additives in the electrolyte could help here, he says. “The battery is not yet perfect, but we now know what we have to work on.”
Test set-up for the silicon battery: the battery itself is only the size of a button cell and is located in the hollow cylinder in the middle of the acrylic glass casing. The thin channels that pass through the housing control the supply and outlet of the electrolyte fluid.
Copyright: Forschungszentrum Jülich
Learn more:
Institute of Energy and Climate Research – Fundamental Electrochemistry
Interview with the inventor of the silicon-air battery, Prof. Yair Ein-Eli from Technion - Israel Institute of Technology
Contacts:
Prof. Rüdiger-A. Eichel
Director at the Institute of Energy and Climate Research
Fundamental Electrochemistry (IEK-9)
Tel: +49 2461 61-4644
Email: r.eichel@fz-juelich.de
Dr. Hermann Tempel
Institute of Energy and Climate Research
Fundamental Electrochemistry (IEK-9)
Tel: +49 2461 61-96570
Email: h.tempel@fz-juelich.de