The successful integration of energy storage with wind-power production holds great possibilities for the industry. Storing wind energy helps even the difference between the electricity supply and demand, and creates additional revenue streams for wind-plant owners.

Dan Girard • Director, Business Development • Renewable Energy and Energy Storage, S&C Electric Company • www.sandc.com

One of the barriers holding back broader adoption of wind energy is the impact it will have on electric power grids. As a variable resource, wind energy acts differently than the conventional generation sources managed by most grid operators. Wind energy production is also subject to rapid, somewhat unpredictable fluctuations in output, which can impact the stability of power grids, creating control problems for grid operators and reliability issues for customers. As a result, many electric utilities and transmission system operators have developed stringent grid-interconnection requirements to mitigate the impact that wind energy resources can have on the power grid. If a wind farm does not meet interconnection requirements, it faces mandated production curtailments, limiting plant productivity and profitability.

Natural gas is often cited as a complementary resource that, when paired with wind energy, can ensure a predictable and stable power supply. However, natural gas generation is a carbon-emitting energy source. It is also prone to significant cost fluctuations as the price of natural gas rises and falls due to shifts in global demand as well as supply availability. Concerns about future regulations on hydraulic fracturing technology (or “fracking”), for instance, have raised questions about the future price and availability of natural gas. For these reasons, there is continued interest in other approaches to address the variability issues associated with wind energy.

An emerging technology, however, holds great potential to accelerate the adoption of wind energy resources, without the drawbacks of natural gas. Energy storage can help address the intermittency issues of wind energy and make wind generation perform more like conventional generating plants. In the process, energy storage can also help wind-plant operators create new revenue streams and maximize plant profitability.

Making wind plants act like conventional generating plants

The main priority of grid operators is to maintain a stable, reliable power system. Conventional power plants provide a number of functions beyond power generation to ensure power grid stability, such as contingency reserves to maintain generation and load balance. Frequency regulation is another important ancillary service of fossil fuel-fired power plants—one that is crucial to maintain a stable power grid. With energy storage, wind energy plants can offer similar capabilities to grid operators, and can thus duplicate the functionality typically available from conventional power plants. By providing these additional functions, wind energy plants can foster much greater utility acceptance of wind energy resources.

First and foremost, energy storage helps utilities balance electricity supply and demand. It stores excess wind energy generated when demand is low and can be easily dispatched when needed. There is no time lag waiting for stored electricity to come online, as there can be in starting up conventional generating plants to meet higher than anticipated demand. In this respect, energy storage makes wind a more flexible energy source, letting grid operators react and adjust quickly.

Energy storage also makes it practical to reliably predict and schedule wind plant output, much as it’s possible to schedule output from conventional power plants. Plant operators typically need to schedule outputs in advance, about 14 to 36 hours. Energy storage can also provide wind leveling, which assures that actual plant output matches scheduled output. If actual wind output is below what was scheduled, the energy storage makes up the difference with energy stored when demand was low. In this way, energy storage enhances the ability to predict plant production. The batteries can also store excess wind generation when actual output exceeds what was scheduled, so the electricity can be sold at a later time.

Ramp-rate control, usually provided by conventional power plants, is another important consideration for wind plants. Energy storage can counter increases and decreases in wind plant output, and give power system operators more time to react to output changes. This smoothing function improves overall grid efficiency and power reliability.

Finally, energy storage can provide frequency regulation. Frequency regulation is an ancillary service typically provided by conventional generating plants, and it’s a necessary function to protect the reliability and stability of power grids. Grid operators must continually balance electricity supply and demand in real time to maintain the frequency of the power grid. In addition, a battery-based energy storage system operates more efficiently than a traditional power plant when it comes to frequency regulation. Traditional generators balance grids by speeding up or down, a process that consumes energy and results in greater carbon emissions.

Addressing grid interconnection requirements

Many transmission system operators have implemented strict grid interconnection requirements to ensure that fluctuations in wind energy generation do not impact power grid reliability. If these interconnection requirements are not met, wind plants may face production curtailments, and in some cases, they may not be able to sell any electricity generated at the plant.

Reactive compensation solutions that provide rapid response, such as dynamic static compensators (DSTATCOMs), have proven effective at mitigating some of the impact on power systems. DSTATCOMs provide VAR support, but real power is needed to level electricity generated by wind energy plants.

Energy storage, a source of real power, provides another approach to meet grid interconnection requirements. It provides the real power along with a fast response needed to firm voltage levels and effectively fill gaps created by large voltage swings and fluctuations. It offers the potential to provide a more complete solution to these grid interconnection requirements while also delivering additional capability.

Increasing plant profitability

Energy storage can do more. For example, it gives wind-plant owners a new tool to maximize plant revenues and plant profitability. It also gives wind plants the capability to meet demand at any time of day—particularly during peak demand when power prices are typically at their highest. Energy storage lets plants store energy generated off-peak so it can be sold when demand is high, regardless of whether the wind is blowing.

Energy storage also provides wind plants the ability to sell ancillary services such as frequency regulation to transmission system operators, and, thus, opens a potential new revenue stream. By providing such services, along with functions to help protect power grid reliability, energy storage can offer a solid return on investment for plant owners. On a larger level, this helps make wind a more attractive option for generation. Flexibility and profitability are crucial for boosting investment in clean energy resources.

Real-world proof

Wind-energy storage is a new technology, but it is showing great promise in real-world renewable energy applications. The most prominent example is Xcel Energy’s Wind-to-Battery project initiated in 2009 and based in Luverne, Minn., at the 11.5-MW MinWind Energy LLC wind plant. This project was intended to help address questions about what storage can do to integrate renewable energy generation in a real-world context. The project used a 1.0 MW (7.2-MWh) sodium-sulfur, or NaS, battery-based system connected to the grid via an S&C PureWave storage management system, which managed battery charging and discharging.

The project has demonstrated that energy storage can be effective in delivering a wide variety of functions, including time-shifting energy, wind smoothing, and dispatched wind-leveling. Batteries were also evaluated for frequency regulation capabilities.

Despite wind variability, the project demonstrated that it needs a relatively small amount of power and energy to better integrate a wind plant with the power grid. For instance, roughly 15 to 20% of a wind plant’s nameplate power rating and just 2 to 3 hours of battery storage makes the wind plant look like a traditional dispatchable resource.

The Wind Energy Institute of Canada also recently initiated a project to evaluate the benefits of energy storage when used with wind energy. They are installing a 1 MW (2 MWh) energy storage system at their Wind R&D Park on Prince Edward Island, featuring sodium nickel chloride batteries connected to the power system by S&C’s PureWave SMS. The project will demonstrate the ability of energy storage to store excess energy in times of low demand and to supply it for use in times of higher demand, along with other functions. The institute anticipates that this demonstration of wind-energy generation with storage will facilitate the integration of more renewable energy into the generation mix.

Energy storage applications in microgrids are also growing. Chevron Energy Solutions recently completed a microgrid system serving the Santa Rita Jail in Dublin, Calif. The microgrid serves the 113-acre correctional facility and combines wind, solar, and fuel cell energy sources, along with emergency diesel backup, to generate electricity for the facility. An energy-storage system plays an important role in the microgrid. When the facility is off the local utility grid, the energy stored balances the microgrid’s load with the total power generated from on-site sources by automatically and instantly storing energy generated that is in excess of load requirements. The energy-storage system also supplies electricity when demand exceeds the output of onsite generation sources. WPE