Energy Storage

Energy storage captures energy and stores it for later use. Reacting to peak electrical demands and the unpredictability of wind and solar generators, energy storage balances supply and demand, while operating on renewable energy rather than fossil fuels or nuclear power.
WorldContributor (Own work), CC BY-SA 4.0, Wikipedia Commons

History of Energy Storage 

The lithium battery was the first commercially successful endeavor in energy storage. Introduced to the market in the 1970s, lithium batteries underwent fifteen years of research and development, culminating into the first lithium-ion battery prototype.

Lithium-ion batteries are a popular rechargeable battery type for portable electronics and used in most home electronics today. However, an underwhelming battery life and predisposition to thermal runaway has researchers working to improve their durability, safety, and charging capabilities.

Recent advancements in energy storage have produced six uniquely innovative technological systems, each capable of moderating wind and photovoltaic (PV) energy consumption.

When connected to wind and/or PV generators, energy storage technology is designed to support energy demands, alleviate environmental impact and offer cost-effective alternatives to traditional practices. 

How Energy Storage Works 

Energy storage involves a device that stores energy, or an accumulator, and the conversion of that energy from difficult forms to storable forms. Currently, energy storage is dominated by pumped hydro. Advancements in the industry have produced various innovative technologies capable of inducing a global shift into the epoch of sustainable energy.

In the video below, by Energy Storage North America, policymakers, utility executives and energy storage pioneers come together to discuss how energy storage is changing the face of the electric grid.

Energy storage technologies have reinvented processes and increased the likelihood of generating clean, renewable energy to support utility energy demands. The diverse approaches being deployed worldwide are defined by six main categories.

(The following information is courtesy of the Energy Storage Association.)

1. Solid State Batteries 

Fundamentally, a battery is a device containing one or more electrochemical cells that convert stored chemical energy into electrical energy. Each cell contains a cathode (i.e. positive terminal) and an anode (i.e. negative terminal). Electrolytes facilitate the flow of ions between the electrodes and terminals, which allows currents to flow out of the battery to perform work.

Advances in technology and materials have produced more reliable and economical technologies, including:

Electrochemical Capacitors (ECs)

These “ultra-capacitors”, or “electric double-layer” capacitors, store a charge physically rather than chemically, which makes the process fast, reversible, and infinitely repeatable. The double layer equates to an unlimited charge-discharge cycle life and low life-cycle costs. ECs are viable options for grid and frequency regulation, bulk energy storage, hybrid transportation, medical devices, and even portable electronics. 

Image credit: O. Usher (UCL MAPS), CC BY 3.0, Wikipedia Commons. Lithium-ion coin cells

Lithium Ion (LI-ION) Batteries

Li-ion technology has flexible applications ranging from power buffering in hybrids to providing energy to electric-only vehicles. Its energy storage applications include energy-type batteries of a few kilowatt-hours in rooftop photovoltaic arrays to multi-megawatt batteries for grid ancillary services.

Nickel-Cadmium (Ni-Cd) Batteries

Ni-Cd batteries have found limited success in energy storage due to their suboptimal energy density, yet remain relevant by boasting manageability, longevity, and reliability.

Sodium Sulfur (NaS) Batteries

NaS batteries are primarily manufactured for stationary application. Since they operate at high temperatures (typically anywhere between 300 to 350 degrees Celsius), NaS batteries are not suited for intermittent operation, despite their impressively efficient use of energy.

2. Flow Batteries (or Cells) 

Similar to a fuel cell and a conventional battery, a flow battery consists of two chemical components dissolved in electrolyte liquids contained within the system and separated by a membrane. Rechargeability, the result of replacing the electrolyte liquid and recovering spent material for re-energization, is a notable advantage of flow batteries. A basic distinction between conventional batteries and flow cells is how the energy is stored. The former’s is stored in the electrode material, while the latter’s is stored in the electrolyte liquid.

Different classes of flow cells have since been developed:

Redox Flow Batteries (RFB)

The separation of power and energy is a key distinction of RFBs because the energy is stored in the volume of electrolytes, while the power capability is determined by the size of the stack of electrochemical cells. This allows for the design flexibility to tailor power capability and energy storage for any given application. RFBs work with energy storage applications with power ratings from 10’s of kW to 10’s of MW and storage durations of two to ten hours. Examples of RFBs include the iron-chromium (ICB) flow battery, the Vanadium Redox (VRB) Battery, and the zinc-bromine (ZNBR) battery, which is actually a hybrid RBF. 

3. Flywheels 

A flywheel is a rotating mechanical device with a spinning, motor-driven mass in its center. It drives a device similar to a turbine with spinning force to produce electricity. As the rate of rotation slows, the flywheel can be recharged by increasing its rotational speed once again. Modern flywheels are made of carbon fiber materials, stored in vacuums to reduce drag, and employ magnetic bearings, enabling rotation speeds of up to 60,000 RPM.

Companies like Velkess have redefined the potential of flywheels by introducing a flexible system and radical cost reduction. The company’s video demonstration elaborates on the physical properties contributing to the functionality of the Velkess Flywheel.  

Flywheel technology can gradually harness energy from intermittent energy sources and deliver a continuous supply of reliable power to the grid. Able to respond instantly to grid signals, they can also regulate frequency and improve electrical quality. Learn more about flywheel energy storage systems

4. Compressed Air Energy Storage (CAES) 

Compressed Air Energy Storage (CAES) plants are similar to pumped-hydro power plants, but instead of pumping water from a lower pond to an upper pond during periods of excess power, a CAES plant compresses and stores ambient air in an underground cavern. The pressurized air is heated and expanded in an expansion turbine, driving a generator to produce power.

The diabatic CAES Method separates the air combustion and turbine process, which means the CAES turbine can generate 3 times the output for the same natural gas input. Air compression uses lower cost excess energy during off peak periods or excess wind energy unable to meet the daily demand cycle. The adiabatic method achieves up to 70 percent higher efficiency when the heat of compression is recovered and used to reheat the compressed air during turbine operations.

CAES plants can be applied in areas like: 

  • Price arbitrage 
  • Balanacing energy (supply & demand) 
  • Higher utilization and greater integration of renewable energy 
  • Ancillary services 
  • Stabilizing conventional generation 
  • Provision of black-start services 

Advanced-adiabatic compressed air energy storage (AA-CAES) is an evolution of the traditional CAES, which incorporates more efficient treatment methods of  heat during compression. Instead of dumping heat into the atmosphere, AA-CAES removes the heat and stores it separately, re-injecting it at the expansion stage.

Isothermal compressed air energy storage (CAES) is emerging as a new technology engineered to overcome limitations from traditional (diabatic or adiabatic) CAES. Isothermal CAES technologies compress and expand gas near-isothermally over a wide pressure range, improving round-trip efficiency and lower capital costs. 

5. Thermal 

Image Credit: Mathieudu68 - Own work, CC BY-SA 3.0, Wikipedia Commons; Panneaux Solaires

Thermal energy storage technologies temporarily reserve energy produced in the form of heat or cold air for future use. The optimal design of thermoses and coolers translates into more effective energy storage throughout the day.

Modern solar thermal power plants produce their energy during sunlit hours. Thermal storage holds onto excess energy produced during peak sunlight. A facility can also use ‘off-peak’ (or night) electricity rates to cool a building and lower daytime energy demand. Thermal energy storage systems include:

Pumped Heat Electrical Storage (PHES)

The following elements are required in PHES: 

  • Two low-cost steel tanks filled with mineral particulate 
  • A means of efficiently compressing and expanding gas 

To store energy, the electrical energy pumps heat from a ‘cold store’ to a ‘hot store’ (similar to how a refrigerator operates). To recover the energy, the heat pump is reversed to become a heat engine, which removes heat from the hot store. Waste heat is then delivered to the cold store and produces mechanical work.

Hydrogen Energy Storage

Electrolysis converts electricity into hydrogen, which can be stored and re-electrified in fuel cells with efficiencies of up to 50 percent. Despite limited round trip efficiency, a much higher storage capacity than that of batteries or pumped hydro and CAES has sparked interest in hydrogen energy storage. Electric hydrogen can produce synthetic liquid fuels from biomass and boost the efficiency of biomass utilization.

Liquid Air Energy Storage (LAES)

Also referred to as Cryogenic Energy Storage, LAES uses electricity to cool air until it liquefies, stores the liquid air in a tank, reverts the liquid air into a gaseous state and applies that gas towards rotating turbines and electricity generation. The system offers large-scale, long-duration energy storage and uses industrial waste heat/cold from applications such as thermal generation plants and steel mills to improve system efficiency. 

6. Pumped Hydro-Power (PHS) 

Image Credit: 河川一等兵 (Own work), CC BY-SA 4.0, Wikipedia Commons; Setoishi Dam

For the last century, the most common type of energy storage around the world has been pumped hydroelectric facilities employing utility-scale gravity storage technology. A hydroelectric dam creates electricity by surging water through a turbine and storing it through a series of pumps, which elevates water into an upper reservoir acting as an on-demand energy source.

Traditionally, PHS involves an upper and lower reservoir, but subsurface pumped hydroelectric storage potentially utilizes abandoned mines, caverns and man-made storage reservoirs. Site availability and reduced environmental impact are two appealing advantages of this alternative.

Recent project proposals are classified as closed loop or open loop, depending on their connection with rivers or other flowing water bodies. Closed-loop pumped storage projects have two reservoirs that are essentially isolated from a free flowing water source. Virtually all existing pumped storage projects in the U.S. are open loop systems, which use free flowing water for the upper or lower reservoir. 

Why Energy Storage is Necessary 

Consumer culture in the United States sustains rapid urbanization by inflating both the housing and automobile industries at a rate tantamount to near ecological collapse. Energy demands surge as urban populations swell, with 40 percent of U.S. energy being consumed by residential and commercial buildings. Generating enough energy to satisfy electrical supply and demand translates into an egregious exploitation of fossil fuels. In fact, it is estimated that fossil fuels are used to generate 67 percent of electricity. 

Consequently, the U.S. has managed to burn nearly a quarter of existing fossil fuels, despite making up only five percent of the global population. Excessive extraction of nonrenewable resources degrades the environment through global warming, air and land pollution, radioactive elements, and greenhouse gases. Reports now link 31 percent of greenhouse gas emissions to energy practices dependent upon fossil fuels. 

Image credit: U.S. EP, 2016; U.S. Greenhouse Gas Emissions by Gas, 1990–2014

After decades of non-renewable energy practices, public interest has shifted towards investing in alternative energy systems as a potential solution to the current ecological crisis. Harnessing renewable energy (e.g. wind, PV, water) is a progressive step toward balancing consumer behavior and environmental impact. Wind and solar power are both relatively economical options—hovering at about two to four cents per kilowatt hour (kwh)—yet their unpredictable natures are forcing researchers to consider energy storage procurement goals for utilities. 

Wind and PV Energy Are Unpredictable 

The electrical demands of residential and commercial buildings fluctuate throughout the day, experiencing peak demand or peak load at approximately the same hour every day (usually around 5:30 at night). It is possible for the peak demand to exceed the maximum supply of electricity able to be generated, resulting in power outages and load shedding.

The ability to reliably meet demand is considered the capacity value of a utility generator. While most fossil fuel dependent systems operate at a normal capacity, renewable energy is subject to natural variability. Wind and hydro power depend on a steady supply; solar power is limited by the finite hours of sunlight each day. This unpredictability hinders the capacity value of renewable energy, rendering it less reliable than non-renewable energy.

Maintaining high capacity values (and eliminating fossil fuel dependency) requires the application of standard capacity value tools to maintain a reliable grid via wind and solar contributions. Various approaches can be implemented including demand response and energy storage. 

Is Energy Storage Economical? 

A global transition to renewable energy boosts the global market value of utility-scale energy storage systems. The energy storage market continues to grow on a global-scale that expands across the United States, China, Japan, and India. Market analysis has forecasted a USD 3.32 billion worth of India’s renewable energy storage systems market for 2022.

Business models and associated financing instruments are merging to invest capital in utility-scale Li-ion battery storage projects. Flow batteries have emerged as the best contenders for long-term, cost-effective energy storage applications. Withstanding rigorous application, utility-scale advanced battery application can expect tremendous growth in North America and Western Europe. Global revenue for advanced batteries will experience tremendous growth—today’s $231.9 million revenue will rise to $3.6 billion by 2025.

Energy storage system price points are dropping, which coincides with a global demand for upgraded electrical infrastructure. Integrating new, clean resources into a system that stabilizes electric supply and demand will generate sustainable communities. If the cost of storage systems continues to match those of wind and solar power, boosting storage capacity will be more economical than generating capacity. 

Storage would allow cheap, renewable energy to overcome intermittence and achieve flexibility. After widespread adoption, it would also stimulate technological improvement and significantly reduce global greenhouse gas emissions, while guaranteeing savings for end-users.

Benefits of Energy Storage

Electric Energy Time-Shift

Sometimes referred to as “buy low-sell high” or price arbitrage, electric energy time-shift is taking advantage of the cost of generating electricity when the price of that electricity is relatively low. Generally, electricity is cheaper at night than during the day, and energy arbitrage would theoretically empower customers with the ability to purchase cheap power and consume it whenever price and demand have peaked. 

Electric Supply Capacity

The benefit of reducing the need to install generation capacity is avoiding the cost associated with building and owning that equipment.

Ancillary Services

Key ancillary services include:

  • Reserves or backup power 
  • Hourly reconciliation of electricity supply and demand to stabilize the grid 
  • "Voltage support" that is required to maintain the grid's voltage

The input and output of most renewable energy storage systems support more responsive and flexible ancillary services. Reduction in cost relative to generation-based ancillary services is linked to a reduced need for generation capacity, generation operations costs, fuel, maintenance and wear.

Transmission and Distribution (T&D) Systems

A renewable storage system connected to the grid extends the life cycle of T&D equipment by alleviating some of the peak demand that typically burdens older, heavily-loaded equipment.

End-User Costs

Electricity bill management involves either a reduced cost for electric energy or reduced financial losses for electric energy, along with reduced electric demand. The reduction in financial losses from poor power quality and outages is another benefit.

Renewables Integration (RE)

Storage supports the effective and reliable integration of renewable energy electricity generation. Benefits include output variability, which can be either short-term or long-term. The happy marriage of storage and variable RE results in constant power output, especially during times of peak demand. 

Disadvantages of Energy Storage

Lack of policy support and government funding threatens the viability of emerging technologies and innovations. Conducting research and developing prototypes are costly, and even companies backed by private investments are debilitated by a price curve or external financial pressures.

Velkess, the company responsible for developing flexible flywheel technology, has announced an indefinite hold on Velkess development. A letter addressing customers and backers blames plunging oil prices for the company’s failure, stating that such a price drop “has undermined the economic case for clean energy.”

Competing against a mostly-nonrenewable advanced energy industry worth 1.3 trillion USD is especially challenging for humble start-ups and independent developers. Without government funding or regulation, the future of energy storage remains undefined. 

Energy Storage and Green Jobs

Within a decade, the energy storage systems market will undergo dramatic growth and wide-scale utilization. Advanced battery technologies, like lithium-ion flow batteries, are leading the wave, and there are numerous technologies in the design-process.

Vendors encourage diversity and competition, which affords aspiring industry professionals countless opportunities to innovate in exciting ways. Solutions vary across projects and regions, representing a complex industry that will evolve rapidly, as partnerships form to help companies leverage their strengths.

Global presence is expected to grow exponentially, and that translates into a boost in green employment opportunities. Contributing to the green economy starts with obtaining accreditation from LEED or WELL, two building rating systems that support energy storage programs and aim to minimize environmental impact, while enhancing personal health and wellness. 

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More on Energy Storage

As part of the U.S. Department of Energy, the Advanced Research Projects Agency has compiled a comprehensive, routinely-updated list of energy storage related projects and other technical categories. 


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