The Pros and Cons of Energy Storage Systems

As we begin to move away from fossil fuels, it is important to recognize that those fuels provided two functions in one. They were both energy sources and energy storage systems. In the new world of renewables, those two functions will have to be provided separately.

The ability to store energy at times when the supply exceeds demands will be a key to the effective utilization of renewable energy. Because many renewable sources (e.g. wind, solar, tidal) are intermittent in nature, storage is useful, both for the times it is available, and not needed, as well as those times it is needed, but not available.

The basic idea is to keep harvesting the energy resource at a steady rate, regardless of the demand. Usually, this results in the most efficient operation. If the demand is less than full capacity, the excess is diverted into some kind of storage medium. Storage could take the form of charged electric batteries, compressed air, mechanical springs or rotating flywheels, pumped water, heat, ice, electrolytic production of hydrogen, or numerous other methods.

This DOE database contains 409 projects from 34 countries. The total represents 121,89GW of energy storage. The aim is to track all energy storage projects worldwide within the next year. [ed note: numbers updated as of October 2013]

Probably the first major modern application of energy storage was pumped storage hydro. In this scheme, excess energy that is available during off-peak hours is used to pump water back up the hill to an upper reservoir, where it can be added to the regular flow during periods of peak demand. Pump storage is one of the more efficient methods of energy storage (around 75 percent) though it has the drawback of not being instantaneously available.

Hydrogen storage utilizes excess electric power to produce hydrogen and oxygen from water by means of electrolysis. Energy can be retrieved by running them back through a fuel cell, or by direct combustion of the hydrogen-oxygen combination to power a gas turbine or other type of engine.

Energy can also be stored in compressed air, in which a motor-compressor is used to pump air into a tank, which is then reversed when the energy is needed, turning the motor into a generator, and the compressor into a turbine. Recent advances, such as the new regenerative device from Lightsail Energy, manages and utilizes the heat generated by the process, which has improved “round-trip” efficiencies into the 70 percent range.

Flywheel energy storage puts excess energy into a heavy spinning rotor, which, due to its large inertia, maintains a very constant speed. These are often used in a vacuum enclosure which eliminates air resistance, resulting in higher efficiency. Systems like this can be used to maintain frequency regulation in conventional fossil fuel plants, eliminating the need for additional fossil “peaker” plants, which are usually among the dirtiest, to perform this function.

Thermal energy storage involves storing the energy in a storage medium at a temperature that will be useful a later time. This is often stored as hot water, or heated rocks or gravel, molten salts or concrete slabs in solar heating applications. Some companies, like Calmac, use off peak energy at night to create ice, which is then stored and used to provide air conditioning in large buildings during the summer. Not only is the electricity cheaper at night, but it often contains a higher mix of wind power, making it cleaner as well. Thermal energy storage is currently second only to pumped storage in U.S. capacity. And is expected to keep going given the growth in solar energy.

Finally, perhaps the holy grail of energy storage is the quest for the perfect battery. Batteries are best because not only are they clean and extremely efficient, but they can provide the stored energy instantaneously. The development of new, very large, highly efficient batteries, suitable for utility scale storage, has become very big business. AES Energy Storage, for example, has, at this writing, some 76 MW of storage in operation or construction, with 500 MW more in development. EOS is another such company, just bringing their first products to the market this year. Their website claims the following elements in their value proposition:

  • Electricity off-peak/peak time shifting
  • Frequency regulation, spinning reserve and other ancillary services
  • Capacity payments (reliability incentive)
  • Load firming for renewables
  • Transmission and distribution capital expenditure deferred

Like many facets of the emerging new energy economy, the biggest payoff comes as the result of the synergy of putting these elements together in a deliberately and carefully designed system: distributed renewable power generation, flexible baseload infrastructure, smart grid, demand response and rapid response energy storage. When combined properly, these elements will perform together like discrete components in an integrated circuit whose primary function is to reliably provide the cleanest and most efficient form of power possible, round the clock, day after day.

But there is one more component in this equation that we dare not leave out, for to do so would be to miss a great opportunity. That would be electric vehicles. EVs could eventually form, as a collective entity, the largest energy storage system of all. At night, when the windmills are turning and the EVs are parked in their garages, they could be using the EV batteries to gobble up all that carbon-free, clean output, while offsetting what had once been the exclusive domain of gasoline. During the day, these same cars and trucks could be connected to smart charging stations that can facilitate the give and take that will become the hallmark of communities that are interconnected by smart networks.

Examples of this vision in action have already been demonstrated in Japan’s Smart Cities program, Ford and Whirlpool’s MyEnergi Lifestyle collaboration as well as a multitude of efforts under the banner of vehicle-to-grid or V2G. A recent report found that these types of “machine to machine” synergies could cut as much as 9 billion tons of CO2 by 2020.

Energy Storage


  • Facilitates effective utilization of intermittent renewable sources
  • Can be combined into smart integrated energy system
  • Reduces need for increased peak generation capacity
  • Enhances grid reliability
  • Performance and cost are continually improving
  • Allows renewable and fossil source to integrate


  • Energy lost in “round trip” inefficiencies
  • Additional cost and complexity
  • Additional infrastructure and space requirements


[Image credit: Radio Rover: Flickr Creative Commons]

RP Siegel, PE, is an inventor, consultant and author. He co-wrote the eco-thriller Vapor Trails, the first in a series covering the human side of various sustainability issues including energy, food, and water in an exciting and entertaining format. Now available on Kindle.

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RP Siegel

RP Siegel, author and inventor, shines a powerful light on numerous environmental and technological topics. His work has appeared in Triple Pundit, GreenBiz, Justmeans, CSRWire, Sustainable Brands, PolicyInnovations, Social Earth, 3BL Media, ThomasNet, Huffington Post, Strategy+Business, Mechanical Engineering, and among others . He is the co-author, with Roger Saillant, of Vapor Trails, an adventure novel that shows climate change from a human perspective. RP is a professional engineer - a prolific inventor with 52 patents and President of Rain Mountain LLC a an independent product development group. RP recently returned from Abu Dhabi where he traveled as the winner of the 2015 Sustainability Week blogging competition.Contact:

One response

  1. Great article RP! A few updates on the DOE International Energy Storage Database:
    – 409 energy storage projects (as of 10/4/2013)
    – 121,89 GW of energy storage in 34 countries
    – 50+ energy storage technologies represented including battery, compressed
    air, flywheel, gravel energy storage, hydrogen energy storage, pumped
    hydro, superconducting magnetic energy storage, and thermal storage.
    – 18 federal and state policies addressing grid connected energy storage

    There’s a one-click download button of all of the information on this free public resource to make graphs and charts. It’s a great resource for industry.

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