The Power of Containment - A Multitude of Storage Options for Renewable Energy

April 25, 2017

Executive Summary

The idea of capturing energy for later use is certainly not a new concept. From gathering and storing plants and seeds until needed to fuel our bodies with energy, to capturing wind and solar power to feed our modern-day needs for electricity, this approach is well-established. The United States is continuously looking to expand energy generating technologies, specifically those that offer renewable options. In 2016 the U.S. Energy Information Administration reported that electrical-generating facilities are expected to add more than 26 gigawatts (GW) of utility-scale capacity to the power grid. The majority will be from three sources - solar (9.5 GW), natural gas (8.0 GW) and wind (6.8 GW). If actual energy additions follow this plan, 2016 will be the first year additions to solar-based generation will exceed all other energy generating methods.1

Much of this can be attributed to lower battery production costs, which serves as the primary system for collecting solar energy. As technology enhancements lower production costs, more deployment of renewable energy options become possible.

Renewable Energy Sources & Storage Technologies

In simplest terms “renewable energy resources are naturally replenishing but flow‐limiting. They are virtually inexhaustible in duration but limited in the amount of energy available per unit of time.”2 Capturing this energy potential for later consumption requires technologies that can harness these energy sources for integration into existing electrical power grids. The amount of electricity that can be generated is relatively fixed, but demand fluctuates throughout the day. These fluctuations are normally predictable and the use of renewable energy storage systems can smoothly manage the power supply needs at their greatest peak demands. For example, excess electrical energy generated by solar power during the day can be used throughout the night. Energy storage systems are designed to accumulate energy when production exceeds demand and to make it available when needed.

The energy storage industry continues to evolve as advances in technology have created more options and opportunities. The Energy Storage Association has divided energy storage technologies into six main categories3:

  • Solid state batteries ‐ a range of electrochemical storage solutions that includes advanced chemistry batteries and capacitors.
  • Redox flow batteries ‐ a battery with electrolyte solution that directly stores energy. This type of battery has a longer cycle life and quicker response.
  • Flywheels ‐ mechanical devices that store rotational energy.
  • Compressed air energy storage ‐ the use of compressed air to create a potential energy reserve.
  • Thermal ‐ capturing heat and/or cold into a medium to create energy on‐demand.
  • Pumped hydro‐power ‐ converting energy into potential energy by pumping water into reservoirs at higher elevations.

Capabilities & Limitations

An in‐depth analysis of each of these technologies provides insight into their applications, service capabilities, and limitations.

  • Solid state batteries convert stored energy into electricity through the use of electrochemical cells. Advances in technology and materials have significantly increased the reliability and output of modern battery systems. Recent innovations include electrochemical capacitors that can be charged and discharged simultaneously, instantly providing an almost unlimited operational lifespan. The result is batteries with longer life, higher power and better reliability that can be used in portable electronics’ applications, including medical devices.4

    Other solid state battery types such as lithium ion are already used in consumer products and are now expanding to new markets such as electric vehicles and personal power storage such as Tesla’s PowerWall, which is intended for home backup power and off‐ grid use.

    Nickel‐Cadmium, a legacy battery technology, has been enhanced for better power storage capacity and is now found in new applications such as telecommunications as well as off‐grid renewable energy storage systems.5
  • Sodium sulfur batteries, originally developed back in the 1960s by Ford Motor Company, are now being produced primarily for fixed location applications. This energy storage resource provides stabilizing renewable energy output and ancillary services. Examples include backup power supplies that add resilience to wind energy capacity and peak shaving, a technique that reduces electrical power consumption and offers a way to reduce the amount of energy purchased by U.S. utilities.6
  • Redox (reduction‐oxidation) flow batteries are a rechargeable technology with two chemical components dissolved in liquids that are separated by a membrane within a contained system. Applications include storing excess electrical power and providing load‐balance by releasing power during peak demand periods. These systems provide storage for power generated by renewable sources such as wind and solar. Electric vehicles and cellphone base stations can use this storage system as standalone power systems in areas where grid power is unavailable.7
  • Flywheels technology provides stored electricity in the form of spinning mass. At its most basic level it is simply a wheel on an axel which stores and regulates energy by continuously spinning and is one of humanities most familiar and oldest technologies.8 Excess electrical energy powers a motor that drives and spins the flywheel, which stores mechanical energy. It can be tapped instantaneously when electrical energy is needed. The flywheel drives the motor, which acts as a generator and produces electrical power. In turn, the flywheel’s mechanical energy is converted back to electrical power and can be recharged by using the motor to increase its rotational speed.9

    Today, carbon fiber flywheels systems are used with magnetic bearings which levitate the wheel within a vacuum enclosure, allowing it to spin in a nearly friction‐free environment. Furthermore, this technology is not affected by extreme temperature variations, requires minimal maintenance and has a lifespan measured in decades rather than years. Advances in materials and capabilities have prompted an interest in this renewable energy storage option by large companies and government agencies.

    Utility companies use it for load‐balancing between peak power usage cycles and for the storage of surplus energy during low power demand cycles. NASA has dedicated resources to flywheel systems in the hope of completely replacing batteries in space applications.10 Through the use of a “flywheel farm” concept, a large company or utility service can store megawatts of power that can be integrated into an electrical power‐grid when needed.11
  • Compressed air energy storage (CAES) offers a method of storing and generating massive amounts of power by compressing air to very high pressures, and then storing the compressed air in large underground caverns ‐ typically depleted wells or aquifers. When power is neededand wind turbines or solar plants are operating at reduced output, the compressed air is released and runs through turbines to generate electrical power.

    At present there are only two CAES plants operating in the world – one located in Huntorf, Germany and the other in McIntosh, Alabama. The benefits associated with this energy storage system are similar to other sources for providing energy load‐balance and greater integration of renewable energy, thereby reducing carbon emissions. There are significant drawbacks for this technology, however, as locating underground caverns suitable for storing compressed air and the associated cost for building and operating such plants may outweigh the return on investment.12
  • Thermal energy storage (TES) is based on the concept of collecting and storing heat or cold during non‐peak periods for later use. Excess heat, for example, can be collected from solar collector panels and stored as a heat source for buildings or industrial processes, or winter air can be collected to serve as air conditioning during hot summer days. There are three kinds of TES systems: 13
    • Sensible heat storage based on storing thermal energy by heating or cooling a liquid or solid storage medium through temperature transfer with water. For example, a solar hot water heater works by transferring the solar heat collected during the day into circulating cold water. The water retains the heat and can be used to heat a building at night. 
    • Latent heat storage‐‐or phase change materials (PCM)‐‐is the process of changing a solid material into a liquid state. There are a number of PCMs such as certain fatty acids, inorganics, salt hydrates and others that are used in latent heat storage systems. A simple example is the capture and release of energy as water turns to ice and ice to water.
    • Thermo‐chemical storage (TCS) uses a chemical reaction to store and release thermal energy. This chemical reaction results in the adhesion or adsorption of matter to the surface of another substance and can collect or release heat or cold on demand. However, this energy storage option remains under study and is not used as a commercial option yet.

According to the U.S. Energy Information Administration, thermal energy storage is perhaps the most economical and widely used storage technology.14 This technology, specifically sensible heating storage, is normally connected directly to the heating or cooling system serving a dedicated location such as a commercial building or manufacturing facility. It is integrated into a building’s HVAC system.

  • Pumped hydropower uses the power of gravity to generate electricity by containing water in an elevated reservoir and releasing it to a generator turbine located at a lower elevation. The water’s potential energy is converted to mechanical and electrical energy in the turbine. During low peak periods, power from the grid or other renewable source is used to pump water back into the dam or to another elevated reservoir. In turn, during peak demand the power grid can be supplied with electricity through a controlled release of the water from the dam. Pictured below is the Seneca Pumped Storage Generating Station in Pennsylvania, where they generate electricity from both a dammed river source and a reservoir built above the river dam system.


    According to the Energy Storage Association, some companies are creating gravitational systems that use gravel instead of water, based on the same concept as a reservoir dam.16 When electricity is needed, gravel is released to cascade downward, creating the force needed to activate turbines and generate electricity.

    Pumped storage hydroelectric projects have served as a renewable energy storage source for decades and provide benefits beyond electrical power to the grid. Examples include flood control, irrigation and water‐based recreational activities such as local fishing, boating and swimming.

Drawbacks of these Storage Methods

These various energy storage systems offer important benefits. Primarily, they enable storing power during low demand periods and subsequently releasing and using that power during high demand periods or when renewable power systems are unable to generate power. That said, it takes a strong commitment and financial backing to support the requirements necessary to implement such systems. Furthermore, they are not all “clean” and without their own significant hazards.

  • Batteries
    • Acid ‐ batteries have acidic electrolytes, which are corrosive and can cause injury and property damage. In addition, the acids can be harmful to the environment and must be properly disposed.
    • Flammable gas ‐ batteries emit hydrogen gas which is a flammable, easily ignitable and can cause an explosion if not properly vented.
    • Flammable electrolyte – lithium ion batteries have flammable electrolytes that can cause significant property damage if the electrolyte is vented due to thermal runaway or if the battery is damaged.
    • Electrical shock ‐ if not properly connected, it can create arcs leading to injury and property damage.
  • Flywheel ‐ This system relies on a no‐ or low‐friction environment. If it is not properly designed using materials capable of withstanding the required spinning forces, it may fly apart leading to a catastrophic failure with potentially harmful consequences to people and property.
  • CAES ‐ A key drawback is the dependence on specific geographic locations requiring a large underground reservoir, a structure able to retain compressed air and suitable for the construction of a power plant near or above the underground reservoir site. 17
  • TES ‐ These storage techniques have been utilized for years. Sensible heat energy storage can be attractive due to low cost. However this pricing advantage is lost due to the high temperatures needed to transfer heat to a material and the rate at which the materials deteriorate due to repeated high temperature exposure. PCM can consist of organic and inorganic materials, both of which create a challenge in function as most organic PCMs have low thermal conductivity, large changes in volume during phase change and are flammable. Inorganic materials are corrosive to most metals and have decomposition issues that impact their phase change properties and their effectiveness. 18
  • Pumped hydropower – While very efficient, the economic and environmental impacts are enormous as dams and reservoirs are expensive to build and require changes to landscapes that may affect the ecosystem for miles around.

Conclusion

As researchers continue to develop and enhance viable renewable energy storage options, there needs to be willingness on the part of government, private industry and consumers to recognize and accept both the advantages and disadvantages of these systems. Much of the technology for energy storage systems already exists and is deployed to varying degrees around the world. Through continued study, convincing evidence may emerge reinforcing these energy storage options as better suited to meet our power needs while sustaining a safer environment.

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References

1 (March 1, 2016). “Solar, natural gas, wind make up most 2016 generation additions.” U.S. Energy Information Administration. Accessed December 2016. http://www.eia.gov/todayinenergy/detail.php?id=25172

2 U. S. Energy Information Administration Glossary. Definition of renewable energy resources. Accessed January 2017. http://www.eia.gov/tools/glossary/index.cfm?id=R

3 “Energy Storage Technologies.” Energy Storage Association. Access January 2017. http://energystorage.org/energy‐storage/energy‐storage‐technologies

4 Ibid 3

5 Ibid 3

6 Ibid 3

7 “Flow Battery‐Applications.” Wikipedia. Accessed January 2017. https://en.wikipedia.org/wiki/Flow_battery#Applications

8 Putnam, Christopher, S. (October 12, 2007, Updated 20, February 2016). “The Mechanical Battery.” Accessed January 2017. https://www.damninteresting.com/the‐mechanical‐battery/

9 “How Energy Works.” Union of Concerned Scientists Science for a healthy planet and safety world. Accessed January 2017. http://www.ucsusa.org/clean‐energy/how‐energy‐storage‐works#bf‐toc‐ 4

10 Ibid 8

11 Ibid 9

12 Romm, Joe. (August 31, 2009). “The Holy Grail of clean energy economy is in sight: Affordable storage for wind and solar.” Think Progress. Accessed January 2017. https://thinkprogress.org/the‐holy‐grail-of‐cleanenergy‐economy‐is‐in‐sight‐affordable‐storagefor‐ wind‐and‐solar‐378d8117c286#.cqt6ab1pp   

13 (January 2013). “Thermal Energy Storage ‐ Technical Brief” IRENA. Accessed January 2017, https://www.irena.org/DocumentDownloads/Publicatio/IRENAETSAP%20Tech%20Brief%20E17%20Thermal%20Energy%20Storage.pdf  

14 Ibid 2

15 (June 29, 2012). “Electricity storage: Location, location, location…and cost” U.S. Energy Information Administration. Accessed January 2017. http://www.eia.gov/todayinenergy/detail.php?id=6910

16 Ibid 3

17 “Fact Sheet to accompany the report ‘Pathways for Energy Storage in the UK’”. Low Carbon Futures. Accessed January 2017. http://www.lowcarbonfutures.org/sites/default/files/CAES_final_0.pdf

18 “Thermal Energy Storage.” Energy Storage Sense. Accessed January