Balancing Renewables with Li-ion Energy Storage

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Li-ion energy storage systems are being deployed around the world to balance output from wind and solar, and the first grid-scale projects are demonstrating the technology’s impressive potential. 

The inherently unpredictable and variable nature of wind power can present significant integration challenges when increasing the penetration of wind turbines within already highly stressed medium-voltage (MV) distribution grids. Energy storage systems (ESSs) based on lithium-ion (Li-ion) battery technology are now starting to play an increasingly important role in helping control the output of wind farms as well as providing ancillary services to the grid. Furthermore, for wind plant installations remote from a strong grid connection point, energy storage can provide a cost-effective, fast-track alternative to grid reinforcement.

According to the U.S. Department of Energy’s Global Energy Storage Database, there are 102 Li-ion battery installations worldwide in operation or in development as of March 2014, with an estimated combined storage of more than 175 MWh.

The technology behind Li-ion battery systems has been developing fast in recent years, and the chemistry’s high energy density (up to 385 Wh/liter and 180 Wh/kilogram) means that significant levels of storage capacity can be packed into a relatively compact and lightweight footprint. Li-ion ESSs are now able to store energy at the megawatt scale, and integrated containerized systems can be connected in parallel to deliver multiple megawatt-hour storage capacity.

One advantage of Li-ion ESSs is that voltage, power, and storage capacity can be precisely tailored to the requirements of specific installations because these industrial-scale batteries are built up from “strings” of individual cylindrical cells, such as Saft’s VL cells, each of which has a storage capacity of just a few amp-hours and a nominal voltage of 3.6 V. These strings are combined in series and parallel into modules, racks, cabinets, and ultimately containers.

Operators of wind turbines and wind farms are keen to make the most of their generation capacity so that they can sell electricity to the grid at times of peak demand and avoid curtailment. Grid operators, on the other hand, are eager to ensure that wind farms supply high-quality power that meets the demands of their grid codes and avoids high ramp rates caused by rapid changes in wind conditions.

To stabilize output from wind farms, an ESS must handle significant daily energy flows, high power output, and very dynamic charge/discharge behavior at variable depth of discharge. The Intensium Max concept was developed to offer the ideal combination of energy and power output, with the ability to deliver high performance in demanding cycling conditions over a lifespan exceeding 10 years.

Taking a containerized approach to an ESS offers multiple advantages. It means not only that a single technical solution can address multiple applications but also that a single battery system can enable grid operators to network multiple value streams. For the wind power plant, an ESS can facilitate grid integration, smooth intermittent generation, reduce ramp rates, and shape power outputs. For the MV grid, an ESS can help manage power flows to reduce feeder congestion during demand and generation peaks, provide local dynamic voltage support, and enable black start and islanding.

Battery Management

When engineering a Li-ion ESS, a vital consideration is ensuring that a battery management system is in place to control the charge and discharge of each battery module and monitor its state of charge and health. Both thermal and electrical management are important to ensure a homogenous state of charge and operating temperature under dynamic operating conditions. This delivers safety and reliability throughout a long service life. The value in such a fully integrated “plug and play” solution is that operators receive a containerized product including a management system that can be monitored either locally through a human machine interface or from a remote control room.

While Saft has delivered Li-ion ESS solutions for a number of installations, including integrating solar photovoltaic (PV) and other on-grid applications, three projects highlight the benefits of the technology and how it can be applied to suit different circumstances.

Overcoming Intermittency in Canada

Since taking delivery of two containerized ESS units in 2013, a remote community in Canada is smoothing out the fluctuations in power from its wind turbines. Cowessess First Nation (CFN) is now benefiting from a $5.5 million project delivered with the Saskatchewan Research Council (SRC) that comprises an 800-kW wind turbine installed with a 1-MWh Li-ion ESS that overcomes the issue of intermittency of wind power on the prairies.

For this project, Saft supplied two Intensium Max 20E systems, which have a total capacity of 1 MWh (Figure 1). The ESS limits the ramp rate to 10% per minute of the turbine’s rated output and provides up to 400 kWh of peak-shaving capability. CFN has reduced the volatility of its wind power by 70%, an important consideration for remote communities.

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1. Smooth output. The 1-MWh Saft Intensium Max Li-ion energy storage systems inside the two containers at the base of this wind turbine reduce the turbine’s ramp rate and smooth out fluctuations in its output. Courtesy: Saft

Funded by a government agency, the SRC delivered the project in part to demonstrate the benefits of energy storage to other First Nations communities that are looking to overcome intermittency. It worked in close cooperation with CFN to consult on the best location for the new installation and within Canada’s well-developed framework for developing wind farms to minimize the impact on wildlife and make the most of natural wind resources.

Maximizing Wind Potential in France

A project in the Aube region of France has brought together several partners to test innovative equipment and management tools for electricity distribution grids in rural environments. As part of the VENTEEA project sponsored by ADEME (the French Environment and Energy Management Agency), consortium leader ERDF is installing an ESS supplied by Saft that combines more than 1 MWh of storage capacity and 2 MW of power. The system will serve a 12-MW wind power plant and demonstrate multiple services, including smoothing of output as well as grid voltage and frequency management.

The location of the ESS on the grid between two MV feeders (a dedicated feeder and a mixed feeder) was chosen for its high testing potential. The intention is to test the various services provided by the battery in order to improve the overall system performance for both the wind power generating company and the distribution grid operator. It is anticipated that the Li-ion ESS will increase the grid’s integration capacity for renewable energy, contribute to stabilizing the grid, and drive an overall increase in energy efficiency.

Predictability and Manageability

Pellworm Island, off the North Sea coast of Germany, represents a vision of the renewable energy mix of the future—the island’s current share of renewables already corresponds to the country’s national target for 2050. Its annual energy production of around 21 GWh from wind turbines, PV power plants, and biogas plants is around three times the annual consumer load of 7 GWh. There is also a remarkably high level of night storage heaters and heat pumps.

Yet, even with this large excess of local production, the island’s community of 1,200 people still relies on its connection with the mainland grid, via two 20-kV subsea cables, for balancing local surpluses and importing energy at peak periods when demand exceeds supply.

An E.ON-led pilot project, running from 2012 to 2015, is now implementing a smart grid for Pellworm, based on a combination of intelligent control technology, flexible load management, and central energy storage. One of the main aims of the project is to increase the island’s self-consumption of its own renewable energy generation and transmit less energy to the mainland.

Different storage technologies are being implemented on Pellworm to cover the range of needs to store and deliver energy over time scales ranging from “minutes-to-hours” to “hours-to-days.” A combination of commercially available storage systems within an overarching hybrid storage system concept has enabled a reduction in investment costs. Retrofitting thermal loads with the potential for flexibility also reduces the system cost for the integration of renewable energy sources.

The “hours to days” storage is provided by a 200-kW, 1.6-MWh Vanadium redox-flow battery. Load flexibility in terms of “hours” storage is provided by a combination of night storage heaters and heat pumps, with an average energy capacity per household of 135 kWh and an average power of 17 kW.

The “minutes to hours” storage is provided by a Saft Intensium Max 20 Li-ion battery system providing 560 kWh of energy storage and 1 MW power (Figure 2). The system will smooth intermittent generation and reduce ramp rates as well as help to manage power flows within the MV grids, making wind and solar energy a predictable and manageable contribution to the Pellworm energy mix.

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2. Multi-talented storage. The Saft Intensium Max system on the right is part of a hybrid energy storage system (ESS) being developed on Pellworm Island in Germany—where total annual production from renewables is three times local consumption—as a test of how ESSs can support very deep renewable integration. Courtesy: Saft

Speaking about the project, Dr. Klaus Peter Röttgen, head of E.ON Innovation Center Energy Storage, said, “E.ON regards energy storage as a key innovation that will help improve the grid performance and especially the integration of renewable energy, and Li-ion batteries are one of the most interesting and important technologies in this sector. SmartRegion Pellworm Island is therefore a crucial lighthouse project that will enable us to evaluate how Saft’s Li-ion technology can operate in a real-world situation to help make smart grids even smarter.” ■

Michael Lippert is marketing and business development manager for Saft.