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The large-scale battery system can supply the equivalent of up to 10,000 households with electricity for one hour.
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Energy storage device in electricity grids
Projektinfo 12/2017

Five different battery types add up to a capacity of 5.7 MWh and a total output of 5.8 MW. The battery storage system is therefore pre-qualified for the provision of primary balancing power.
© E.ON Energy Research Center (E.ON ERC), Aachen
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Modular battery storage system supplies balancing energy

How can large-scale battery storage systems help stabilise electricity grids and at the same time be profitable? This question is being investigated by scientists at RWTH Aachen University in collaboration with industrial partners using the large-scale M5BAT battery storage system. Using a modular and flexibly scalable batteryinverter concept they are testing the interaction of different accumulator technologies and are developing business models for decentralised storage systems in real grid operation. The initial results are now available following a year in measurement operation.

In order for electricity grids to remain stable, the consumption and production must always be in harmony. This is posing increasing technical challenges as a result of the intermittent power generated by wind and solar energy and the shutdown of conventional power plants. Electricity grid operators therefore require additional possibilities to respond flexibly to imbalances. One option is provided by battery storage systems that can compensate for short-term fluctuations with response times in the millisecond range. Depending on their size and design, these provide balancing power at different grid levels and help to stabilise the voltage and frequency, such as for providing reactive power compensation. Unlike, for example, pumped storage systems, battery storage systems are not bound by any geographic or geological prerequisites and can be assembled in a comparatively short time and integrated into the grid where they are needed.

The currently largest battery system, which is used among other things for research purposes, is located in a former office building on the premises of RWTH Aachen University. The “modular multi-megawatt multi-technology medium-voltage battery storage system”, or M5BAT for short, produces 5.8 MW with a storage capacity of 5.6 MWh.

The output and capacity of the system roughly correspond to the electricity consumed by 10,000 households in about an hour. A special feature of the research storage system is its hybrid design and, based on this, its intelligent operating concept: It uses three different lithium-ion technologies for short-term power storage and output. These are supplemented with two lead-acid battery types for short to medium discharging and charging times.


Battery technologies complement one another

The researchers want to test the complex interaction of different cell chemistries in order to develop cost-effective and flexible storage systems that can provide all the important system services in the grid. At the inauguration of the system, Professor Dirk Uwe Sauer from the Institute for Power Generation and Storage Systems (PGS) at RWTH Aachen University explained: “More than 25,000 battery cells in six strings with different lithium-ion battery technologies and four different lead battery strings will be closely and individually monitored from day one. This will enable us to gain valuable information about the ageing, reliability and durability. With an intelligent battery management system, we also want to demonstrate how the overall operation can be optimised using a hybrid system with different technologies.” Lithium-ion and lead-acidbatteries offer different characteristics, whose strengths are being combined and utilised in the project. “The modular design of the battery storage system enables us to control and operate each of the ten strings. In addition, this system structure facilitates the exchange and thus the testing of further battery technologies. Moreover, we can exchange battery strings in our system and thus test further battery technologies,” says Jeanette Münderlein from ISEA Aachen, who is responsible for the project.

In addition to the technical issues, the cooperation partners from science, industry and the power sector are also exploring economic aspects. The extent to which the storage systems can be operated in a grid-supportive manner is still hotly debated, which is why there are only limited marketing possibilities for refinancing. It is therefore intended to determine the extent to which battery storage systems can be economical in accordance with the technology and marketing strategy as part of the project. This shall be done by marketing the storage system on the electricity exchange. The basic prerequisite is its integration into the medium-voltage network, which shall be achieved via the neighbouringsubstation. This will enable fully autonomous second, minute or hourly balancing power to be provided. The thermal behaviour of the batteries is also being analysed to optimise the air-conditioning of the energy storage system and to additionally reduce the energy consumption.

Dynamic – lithium-ion batteries

Lithium-ion batteries generally have a high efficiency and a long cyclic life compared with lead-acid batteries. Their good charging acceptance makes them suitable or load profiles with large dynamics. The cell types have different characteristics and operating characteristics depending on the electrode material and the electrolyte. The scientists are investigating two high-energy accumulators and one high-performance accumulator in different battery strings:

Four parallel strings of the battery storage system consist of lithium manganese oxide cells (LMO). This technology is characterised by high energy density and offers price advantages relative to other lithium-ion battery technologies. Despite the lack of inherent safety, there are reliable systems on the market as the technology is increasingly being used in the automotive sector.

Lithium iron phosphate (LFP) cells are used in other battery strings. The cobalt-free accumulators are intrinsically very safe and can supply higher currents than LMO cells for short-term loads. The highest currents during charging and discharging are tolerated by lithium- titanate (LTO) cells. These also surpass other kinds of lithium cells in terms of reliability and efficiency. Their very long cyclic life is also hardly affected by deep discharges. Their lower cell voltage, which results in a lower energy density, and in particular their high production price are disadvantageous.

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