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Pictures of the Future
The Magazine for Research and Innovation
 

Smart Grids and Energy Storage

Bottled Sunlight

Siemens and energy provider Enel are testing lithium-ion storage units to store surplus photovoltaic electricity in Italy.

The use of energy from renewable sources requires special attention to grid stability. Siestorage, an innovative energy storage system from Siemens, offers a potential solution — as is being demonstrated in a pilot project Siemens is conducting with energy provider Enel in Italy.

Italy is blessed with plenty of sunshine, so it’s not surprising that the country’s photovoltaic (PV) sector is booming. The grid operated by energy supplier Enel, for instance, includes PV facilities with a power output of over 11,000 megawatts (MW), most of which are connected to the medium-voltage distribution network.

But there’s a dark side to all this: When the midday sun is shining, solar cells produce a large amount of electricity that is then fed into the grid, where it needs to find consumers. However, if clouds appear, power output will drop suddenly. In general, the more fluctuating energy sources, such as sun and wind power, are connected to the grid, the more difficult it is to ensure grid stability. Supply and demand have to be balanced at all times. If they are not, the resulting fluctuations in voltage and frequency can disrupt or even destroy electronic equipment.

Needed: Better Energy Storage Systems

In view of this, it is clear that energy storage systems will become increasingly important in the future. Storage units take in surplus electricity that is not needed at a given time and then feed it back into the grid when demand rises. For decades now, efficient pumped-storage electrical power stations have been used for long-term storage needs. “Unfortunately, there aren’t enough suitable locations to build them in,” says Uwe Fuchs, Sales Manager for Advanced Power Systems and Storage at Siemens. “We therefore need to develop alternatives that can stabilize our power grids.”

According to Deutsche Bank, the German market for electrical storage devices is expected to at least double between 2012 and 2025. An associated investment of roughly 30 billion euros will be required in Germany alone over the next 20 years. By 2040 at the latest, some 40 terawatt-hours (TWh) of electricity will have to be stored on a regular basis, in some cases over a period of several months. The 40 TWh figure is one thousand times higher than the storage capacity of today’s pumped-storage facilities in Germany. By comparison, total German power plant output in 2012 was approximately 618 TWh.

Hydrogen, Methane, or Compressed Air?

Various technologies for dealing with this issue are available. For example, hydrogen storage devices can take in surplus power from wind farms. These devices use electrolysis to produce energy-rich hydrogen gas from water. The hydrogen can be temporarily stored in underground caverns that are already used to hold natural gas. If demand for power rises, the energy-rich hydrogen gas can drive turbines that then supply electricity to the grid. Alternatively, the hydrogen can be converted to methane through a reaction with carbon dioxide; after that the methane can be fed into the natural gas grid. Alternatively, the methane can be used as a base material in the chemical industry or as fuel in fuel cell vehicles.

Energy can also be stored as compressed air. This approach involves pumping air into hollow chambers such as salt domes and then compressing it to a pressure of up to 100 bar. The compressed air is later used to drive a gas turbine, In other words, combustion still requires a fossil fuel such as natural gas, but the combustion air no longer needs to be compressed.

Finally, there are the energy storage systems that everyone knows: batteries. Lithium-ion cells are currently the best batteries for stabilizing distribution grids because they combine high storage capacity with high charge and discharge rates. If load volatility should occur in the grid, such batteries can take in or dispense power within milliseconds, thus balancing out fluctuations in voltage and frequency. Unlike pumped-storage units, batteries only have to make their power available for a few minutes — for example, if temporary cloud cover reduces output at PV facilities connected to a subgrid.

Batteries: Helping to Stabilize the Grid

Siemens began developing energy storage devices several years ago. Siemens Energy Storage (Siestorage) is a modular system that links high-performance lithium-ion batteries with power electronics for connection to the electricity grid. “The system enables us to stabilize both low-voltage grids of 400 volts and distribution grids with ten to 30 kilovolts,” says Fuchs. “The batteries and the control electronics are slide-in units housed in cabinets that can easily be integrated into climate-controlled containers.”

One such container was placed in the city of Isernia in the Molise region of Italy in February 2012. The area is home to a large number of PV facilities connected to the Enel distribution grid. “Our system has the first lithium-ion storage device in Europe; it serves as a powerful energy storage device for the distribution grid,” Fuchs explains. “Its control electronics continually measure the network voltage and frequency. Siestorage absorbs or dispenses energy in line with the situation.” The system in Italy has a storage capacity of 500 kW-hours and can store and release power in the MW output range.

That’s enough to keep the rural subgrid stable, even with the fluctuating output of solar cells. “We’ve been very satisfied with the tests to date,” said Paola Petroni, Director of Network Technologies at Enel’s Network and Infrastructure division, in the most recent Siemens Sustainability Report. Enel, Italy’s largest energy supplier, has over 32 million customers and operates and maintains more than one million kilometers of power lines. “Our Siemens product can handle fluctuations in electricity production as well as the alternating loads caused by several electric vehicle recharging stations,” said Petroni. This ability is mainly due to specialized converters. A great deal of expertise from SIPLINK products (Siemens Power Link) flowed into these devices. SIPLINK products have been used for around ten years as grid couplings at industrial facilities, as well as in conjunction with land-based electrical power outlets that provide electricity to large ships. “The software ensures that Siestorage converters can react flexibly to changes in the grid, especially in the event of frequency fluctuations, which involve extremely rapid processes that occur within seconds,” says Fuchs.

Unique Black Start Capability

When a grid’s instability reaches critical levels, energy suppliers may cut off entire sub grids from the network. Restarting the grid after such a shutdown is called a black start. During an acceptance test in Italy in 2012, Siestorage was able to restart a rural sub grid with its connected PV facilities within just milliseconds after the connection was cut. From that point on, converters in the battery storage unit maintained a frequency of 50 Hertz and ensured stable voltage of 20 kilovolts in isolated operations. “Siestorage’s black start capability is truly a unique feature,” says Fuchs.

Siestorage will also play a key role in a project to be launched in the fall of 2014. Siemens will team up with steel manufacturer Arcelor Mittal in Eisenhüttenstadt, Germany, and the local energy supplier, VEO, to build a backup system for power outages. Here, a Siestorage device will use an electric motor to start the gas turbine in the steel mill’s own gas power plant in order to make sure the factory can continue operating with its separate grid if there’s a blackout. “This scenario is also relevant to the energy transition,” Fuchs explains. “In the future we will need to have many flexible combined-cycle plants that can be black-started quickly if the grid fails — and this is exactly what Siestorage can do. In short, it can be used as an alternative to conventional diesel engines.”

Siestorage is being continually refined. “Our converters and the batteries we’re now using enable us to absorb or dispense a maximum output of two MW using a storage device with a capacity of one megawatt-hour that is housed in a standard 40-foot container,” Fuchs explains. “The use of much higher-performance batteries might also be an interesting option for special applications — for example, the regeneration of braking energy from container cranes or tire test rigs.” Such applications involve handling very large amounts of energy in a short period of time. But not all of this energy can be put into Siestorage yet, due to the system’s short recharging cycle. Experts at Siemens are now working to develop an increasing number of combinations that will allow customers to maximize their devices’ output and their storage capacity. This will ensure that customers’ grids remain as stable and efficient as possible.

Christian Buck