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Energy Storage Systems


Batteries, power-to-gas, and power-to-heat systems provide a critical function in the energy transition: they balance out fluctuations in the electrical grid and thus enable the continued expansion into renewable energy and eMobility. The technology is currently being tested on a broad front.

The increasing proportion of fluctuating power generators is leading to problems. The capacity of rapidly-controllable power plants no longer suffices to balance out the variable production levels from wind turbines and solar arrays. The first effects have already been felt in northern Germany: because turbines sometimes produce more power than the electrical grid can hold, they must be capped. Due to this necessity of supply management, 3.5 billion kilowatt hours were lost in 2016 – that is enough to supply a large German city for a year.

“For this reason, we need storage devices, in addition to new networks, for temporary balancing of fluctuating energy supplies,” says Urban Windelen from the Federal Association of Energy Storage (BVES). Previously, grid operators did not have a compelling reason to use them, because they could cover the necessary flexibility in cheaper ways. Examples include flexibly deployable fossil fuel power plants, or electrical trading with neighboring states. However, multiple analyses have shown that these measures will no longer suffice in the future. The proportion of renewable power in the electrical mixture is currently around 40% in Germany. According to predictions, at 60% the electrical system will no longer maintain its stability without additional storage devices.

The merging market in eMobility will also be dependent on storage devices. “If all Germans were to drive eVehicles, then we would need 15 to 20% more electricity than today, just for personal cars. That is around 100 terawatt hours,” according to Peter Birkner, Honorary Professor at the University of Wuppertal. The infrastructure will be particularly stressed, primarily from 5 to 8 pm, when the expected peak load due to the eVehicles coincides with the current peak loads. Storage units could alleviate this problem, because they would provide additional output at this time.

“Multi-Use” as the Model for the Future

Solar charging stations would provide a concrete solution. Increasing numbers of companies are investing in solar arrays linked to lithium-ion batteries, which allow them to potentially cover a large proportion of their energy consumption with their own solar power. These batteries could also supply charging stations so that fewer eVehicles would depend on the public electrical grid. Discount chain Aldi-Süd has already incorporated the concept into some of its stores. The advantages include: the company saves on energy costs by producing its own supply; the charging stations attract additional customers; the grid is also protected. Windelen points out that “Multi-use applications of this type produce large benefits.”

However, the energy transition needs more than simply batteries; long-term storage devices, like power-to-gas systems are also in demand. Whenever wind farms produce too much electricity, the excess power is converted to hydrogen using electrolysis. The gas is stored in tanks and then used when demand for electricity increases again. It can also be converted to methane and supplied to the existing natural gas network that supplies heat, power plants, and natural gas stations. Power-to-gas thus has a doubled-value: it reduces the electrical load on the grid and also links the heating and eMobility sectors, both of which have drawn the short straw in the energy transition.

To ensure that storage is available when needed, industry and research are testing batteries and power-to-gas in numerous projects. This includes a pilot project run by TenneT, a transmission network operator, and Sonnen, a storage device manufacturer: a battery pool of 6000 residential energy storage units is used to prevent network bottlenecks in a targeted manner. “Instead of throwing away green energy in the north, we store it. Instead of spinning up large power plants in the south, we call up solar energy from the storage devices,” explains Philipp Schröder, Head of Sales and Marketing at Sonnen.

A specific software technology, the blockchain, ensures that the storage devices are networked and the intelligent loading management system adjusts them individually to the actual situation in the TenneT network. All transactions for the project are stored on distributed computers at the participating companies, instead of on one central server. This offers the advantage that the actuators function better with one another, and thus detailed processes can also be implemented – an important aspect in promoting the decentralized energy sector.

Relief at All Network Levels

While TenneT and Sonnen are concentrating on the transmission network, scientists at the Bavarian Centre for Applied Energy Research (ZAE Bayern), are demonstrating how storage devices can help to equip local networks for the increase in solar electricity (see page 26). They have set up a photovoltaic array in Arzberg in Bavaria with batteries and electrolysis, and then connected this test field to the local network in the Schlottenhof district of Arzberg. This area also has a number of PV arrays with storage devices. The networks in solar-supplied regions normally reach their limits quickly. This is not the case in Arzberg, because less energy is supplied to the network during midday due to the intelligent control of the storage devices, which prevents dangerous peak loads. “The potential for the future provision of solar electricity in a targeted fashion is enormous,” states ZAE Project Leader Philipp Luchscheider.

There are additional methods for using photovoltaics in ways that protect networks. The building technology company, leitec, has installed an ice energy store in their building, which they use in combination with heat pumps to heat and cool their building (see page 34). The ice energy store includes an underground cement tank that holds up to 400,000 liters of water. In the winter, heat pumps withdraw energy to ensure that the ice energy store freezes from the inside out. In the summer, the resulting ice mass is used to supply the collectors in the offices with cold water to lower building temperatures. The electricity necessary to operate the heat pump is supplied primarily by a PV array on leitec’s roof – no burden is placed on the network.

WAGO technology can help implement storage management. Telecontrollers organize the communication between individual components at the field and control levels. The collect all data via digital and analog signals using MODBUS for example, translate them into the required communication protocol, like IEC 60870-5-101/-104 or IEC 61850, and transmit them via a data line. The reverse is also true, the controllers can access the storage device and all associated systems from the control center. The data flow is thereby protected from unauthorized access by encrypting the data using TLS 1.2, and by employing specifically secured connections, like IPSec or OpenVPN according to the BDEW White Paper. In addition, WAGO helps to link storage device projects into larger, flexibly controllable aggregates, so-called virtual power plants. The electrical, gas, and heat supply is becoming more complex due to the number of decentralized energy producers. This can lead to a confusing network with different interfaces from different manufacturers. With WAGO components, you can conquer communication problems, because their controller fulfills the demands of the VHPready industrial standard (Virtual Heat and Power). It functions like a translator so that control centers and systems understand one another. The technology for integrating storage devices into smart grids is already available.

Text: Heiko Tautor, WAGO