Integrated energy-storage systems

Renewable energy: Uninterrupted availability of power takes on added importance with increased use of renewable energy to generate electricity. Energy storage could particularly benefit wind and solar power, which are subject to supply intermittence—due to dynamic weather changes and diurnal interruptions, respectively. New and existing storage methods offer potential solutions.

By Frank J. Bartos, PE October 27, 2012

Large-scale energy storage has been applied to balance regional power needs via pumped-hydro storage and, in a more limited way, with compressed-air energy-storage (CAES) systems. These methods use traditional energy sources to implement storage and take advantage of local geographical or geological features.

Inherent variability poses severe limits on wide market penetration of wind and solar power. Energy storage becomes crucial. Regulations may also require reserve generating equipment to be on standby, for example, in case of insufficient wind at a certain time or area. Such a costly provision defeats energy efficiency.

A forward-looking perspective envisions large-scale energy-storage systems balancing supply fluctuations on the electricity grid. Newer above-ground CAES and expanded pumped-hydro storage are among possible solutions. Thermal storage may be an answer for concentrated solar power plants. Battery storage is also moving forward. Demonstration projects in various technology sectors are taking place.                  

Energy storage (ES) would enable solar power plants to operate after sundown; or allow wind farms to run at full capacity more often with little concern for periodic curtailment to avoid excess electricity generation. All proposed large-scale ES systems are costly and face challenges to gain public/political support. Some methods require further application experience or development, but winners will emerge in time. Part of that future picture is a viable grid and transmission system to distribute electricity to where it is needed.

Wind, water, gravity

In simplest terms, pumped-hydro storage (PHS) embodies two large water reservoirs at different elevations. Reversible hydroelectric turbines pump water to the top reservoir during non-peak electricity usage; then, during high demand, water released to the bottom reservoir flows through the turbines producing electricity via a connected generator. PHS system efficiency can exceed 75%, according to the National Renewable Energy Laboratory.                 

PHS is the most widely used storage method associated with electricity generation. Numerous plants operate worldwide, but few directly integrate wind or solar energy storage. This is forecast to change in the future. Wind (or solar) power could pump water directly to the top reservoir to increase storage capacity or reduce normal turbine power consumption—depending on when excess renewable energy is available. Given the topology requirements and costs involved, few new PHS plants are being built. However, existing facilities are being enlarged or upgraded.

E.ON AG, one of the largest global power and gas companies, intends to substantially increase capacity of its Waldeck pumped-storage hydroelectric station on Lake Eder in central Germany. A new 300-megawatt (MW) plant is to be built next to the existing Waldeck 2 facility. Start of construction awaits a “final investment decision,” but all required external permits are on hand, according to Alexander Ihl, spokesperson for E.ON. Construction is expected to take four years.

The company mentions plans for another pumped-storage plant in southeast Bavaria with an Austrian partner. Project planning is underway for the 300 MW facility (named project Riedl), though construction is not expected to start before 2014.

“In general, pumped-storage power plants can be used to store excess wind power, which would apply to Waldeck; and Riedl will also be storing energy from renewables,” Ihl said. (A different ES technology initiative from E.ON is covered in Ref. 1, online.)

One prime example of a large PHS facility is the Ludington pumped-storage plant (LPSP) located just south of Ludington, Mich. The plant can generate up to 1,872 MW of electricity from a reservoir measuring 2.5 miles long, 1 mile wide, and 110 ft deep (4 km x 1.6 km x 34 m), situated 363 ft above the eastern shore of Lake Michigan.

Six reversible turbines, each rated at 312 MW, run in reverse during low electricity demand to fill the upper reservoir typically each night, noted Dennis McKee, area manager for governmental and public affairs at Consumers Energy—operator and 51% owner of the Ludington pumped-storage plant. Detroit Edison is a 49% joint owner. Water is released during peak demand periods, driving the turbines in the opposite direction to generate electricity.

Daily generating and pumping needs vary at Ludington. One unit or all six may be employed, depending on customer demand and off-peak power prices.

“We can bring one 312 MW unit online in about three minutes; our record to bring on all six—some simultaneously—is 11 minutes,” McKee said. While not directly connected to wind energy input, LPSP can use any energy source available on the grid for its pumping mode. Wind power could be stored in the future, depending on the quantity of excess energy available. Today, “the plant serves a larger beneficial role to balance regional energy supply and demand, with unmatched capability to provide a large amount of power rapidly,” McKee noted.


Compressed-air energy storage is based on using daily off-peak electricity to compress air into caves, abandoned mines, or other large underground spaces. To produce peak-demand electricity, compressed air is mixed with natural gas and burned in a modified gas turbine-generator unit. Only two CAES plants are known to be in commercial operation—at Huntorf, Germany (290 MW, built in 1978) and at McIntosh, Ala. (110 MW, built in 1991). Any energy source connected to the grid can be used to initially “charge” air into the storage chamber—including wind energy.

CAES operation faces an interesting trade-off. Because of high-pressure air input (1,100 psig; 76.9 bar), much less natural gas is needed to produce the same electric output as a conventional gas turbine plant. Yet the basic CAES process requires additional gas for reheat before the expansion part of the thermal cycle to make up for heat lost during air compression. System efficiency is in the 42% to 50% range for existing plants.

Various process refinements and research and development (R&D) are ongoing to make CAES plants more efficient, such as storing the heat of compression and reusing it to heat the compressed air before expansion, thus avoiding use of extra natural gas. Newer CAES plants are in planning and permit acquisition. Developments also focus on above-ground CAES to reduce costs and simplify installation.

Smaller U.S. companies, with industry and government backing, are advancing CAES technology. One such firm, SustainX, has developed and trademarked an “isothermal” CAES technology (ICAES) that seeks to keep stored air temperatures close to the ambient during compression and expansion. This minimizes energy losses but requires highly efficient heat exchange throughput the process. Isothermal expansion produces electricity without burning natural gas, according to SustainX. Modular above-ground ICAES plants designed around large tanks, process piping, and standard mechanical systems could ease integration of renewable energy.

ICAES developments are progressing from small demonstration plants. Currently, SustainX is building a system with up to 2 MW output that is expected to be “operational in the first half of 2013.” (See more at Ref. 2, online.)

Thermal energy storage

Two so-called “concentrating solar power” technologies—parabolic trough and power tower—could particularly benefit from energy storage because they use a final thermodynamic cycle to produce steam for driving a conventional turbine generator. However, they apply different methods to concentrate and collect solar energy.

Parabolic trough uses parallel rows of parabolic reflectors to focus sunlight on an absorber tube located along each trough’s focal line, generating heat in a fluid (typically oil) pumped through a circuit of such absorber tubes. Heat exchangers transfer heat energy to the power block where it’s turned into steam.

Solar power tower (SPT) deploys a large array of computer-controlled mirrors (heliostats) to concentrate sunlight on a solar receiver steam generator built atop a tall tower structure. Heated from the outside, the receiver is an advanced design boiler with thermodynamic piping and controls, where thermal energy turns water into superheated steam.

Energy storage involves using some energy produced during sunlight to further heat a storage fluid that flows to a reservoir (or tanks). Various reservoir media have been tried and demonstrated, including molten salt, a material that industry experts consider ready, safe, and economically viable for commercial usage. Heat of molten salt is used later to turn water into steam, which is piped to the turbine generator for off-sun power generation. Full exploitation of energy storage could make solar power dispatchable.

A number of parabolic trough and SPT plants operate worldwide, but the former has had more operating experience. Only a few solar plants presently include thermal storage, but thermal power tower offers greater storage effectiveness. Less storage volume is needed because of higher operating temperatures (1,050 F; 566 C) versus those for parabolic trough (about 750 F). 

BrightSource Energy Inc.—one of the companies active in this arena—has developed a molten salt ES system, trademarked SolarPlus, that’s available for future solar power plants.

“Molten salt storage is a well-understood technology used in the solar thermal industry,” said Israel Kroizer, executive VP of engineering and R&D at BrightSource. “Our primary advantage is in our solar thermal technology’s ability to reach higher temperature and pressure levels, which allows our plants to run more efficiently with or without storage capacity.” See further thermal storage developments at Ref. 3, online.

Meanwhile, the world’s largest SPT plant has reached the halfway mark of construction in California’s Mojave Desert as of early August 2012, according to NRG Energy (developer) and BrightSource Energy, thermal power tower technology supplier. Google is an investor in the project. The three-unit, 370 MW (net) Ivanpah Solar Electric Generating System (SEGS) is reportedly on track for completion, with unit 1 to supply power to Pacific Gas & Electric starting in mid-2013. However, energy storage is not part of the Ivanpah plant.                                                                                          

Battery storage is an ES sector comprised of multiple technologies in various stages of development. Demonstration projects and commercial installations are moving from modest size toward the 50 MW mark. Battery storage faces a number of issues like power density, material-intensive manufacturing, physical size, and more. Some of these represent work in progress. Future technology breakthroughs can also be expected. Many companies are active in this sector, among them AES Corp. and Xtreme Power. Notable developments are discussed in Ref. 4, online.

Markets, limitations

Investment in energy-storage (ES) technologies is growing worldwide. Storage capacity is expected to accelerate over the next decade, with projects accounting for more than $122 billion in the 2011-2021 time period, according to Pike Research, a consulting firm in global clean technology market analysis. Pumped hydro storage and “advanced flow batteries” are seen as technologies contributing to the largest part of total revenues, followed by other battery types and CAES. Pike Research sees the top two ES applications in wind power integration and load leveling/peak shifting, at 50% and 31% of the total market, respectively.

A similar robust growth forecast for the ES market was noted by the Electricity Storage Association (ESA) in February 2012, based on a report from the Copper Development Association. Over the next five years, 2-4 GW of energy storage could be developed—depending on financial incentives, the report stated—along with cost reductions as ES technology demonstration projects reach commercialization.

"The next five years will be critical and provide enormous opportunity to move storage technologies to full commercialization," said Brad Roberts, ESA executive director.

Pike Research noted several factors that presently limit growth of ES, including:

  • Inflexible electricity market structures
  • High capital costs
  • Disconnection between asset owners and parties benefiting from such projects
  • Grid instability—inherent and also exacerbated by integration of new, renewable energy. 

However, the company added that energy-storage technology benefits “will start to overcome those barriers in the next few years.”

Growing use of renewable energy faces challenges to be met from technology, current economic factors, and societal acceptance. Wider availability of energy storage systems would allow wind and solar energy to benefit from better power market pricing and take an appropriate share of electricity generation within our mix of power sources.

-Frank J. Bartos, PE, is a Control Engineering contributing content specialist. Reach him at Edited by Mark T. Hoske, content manager CFE Media, Control Engineering, Plant Engineering, and Consulting-Specifying Engineer,

Key concepts

-Integrated energy-storage systems help solar and wind energy deliver power when needed, rather than when generated

-Forms of storage include pumped-hydro, compressed-air, battery, and thermal systems

-Viable controls for the grid and transmission system are needed to distribute electricity to where it is needed 

Consider this

Electricity on demand: Solar and wind renewable energy often require infrastructure. Storage and control systems help electricity “flow” in the correct direction, when and where needed.                 

Go Online  

Worth reading: “5 things you need to know about energy storage”

Each of the four online articles below has more coverage.