Energy Storage

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Hydrogen Energy Storage

Videos from CNBC:

Sustainable Energy YouTube Channel

Why hydrogen is becoming a big deal, CNBC 2017

Why hydrogen is becoming a big deal, part one | Sustainable Energy duration 15:49
Why hydrogen is becoming a big deal, part two | Sustainable Energy duration 10:38

Green Hydrogen is the name given to hydrogen that is produced from renewable energy. Most hydrogen today is generated from industrial, non-renewable energy processes. Details of the pros and cons are discussed at:

Green hydrogen is gaining traction, but still has massive hurdles to overcome - CNBC - December 2020.

Green Hydrogen has positive implications for Hydrogen Powered Transportation as costs reportedly plummet for fuel cells and renewable energy. Among the benefits of hydrogen are:

  • hydrogen is non-toxic
  • dissipates rapidly when spilled
  • can be generated and stored by off-peak solar and wind energy by electrolysis
  • can be stored in salt-domes
  • refuels vehicles in minutes

Government funding for green hydrogen may have recently (as of Dec 2020) got a boost;

Trump Admin. Drops Green Hydrogen Bomb On Fossil Energy — Just For Starters - CleanTecnica - December 2020

From the US Department of Energy, Office of Energy Efficiency and Renewable Energy:

H2@Scale is a concept that explores the potential for wide-scale hydrogen production and utilization in the United States to enable resiliency of the power generation and transmission sectors, while also aligning diverse multibillion dollar domestic industries, domestic competitiveness, and job creation.

One line of R&D to benefit is Concentrated Solar Power + Hydrogen:

Weird Contraption Marries Concentrating Solar Power To Produce Hydrogen, Eventually Clean Technica - February 2018

Green Hydrogen depends on the electrolysis of water in a variety of different electrolyzers:

  • Alkaline Electrolyzers
  • Proton Exchange Membrane (PEM) Electrolyzers
  • Solid Oxide Electrolyzers (SOEC)

as detailed in Electrolyzers 101: What they are, how they work and where they fit in a green economy - Cummins - Nov. 16, 2020

Gravity Energy Storage

A review of the status of gravity storage is hopeful that 2021 will see its value:

Gravity Energy Storage Will Show Its Potential in 2021 - IEEE Spectrum - Jan 5, 2021.

Energy Vault

Energy Vault has created the world’s only cost-effective, utility-scale gravity-based energy storage system that is not dependent on land topography or specific geology underground. When energy is needed a tower of 36 ton blocks is disassembled, recovery energy as they are lowered from a tower. When energy is abundant the tower is rebuilt. Advance computer control makes this possible with round-trip efficiencies of 80 to 90%. See

Energy Vault


The Gravitricity system suspends weights of 500 - 5000 tonnes in a deep shaft by a number of cables, each of which is engaged with a winch capable of lifting its share of the weight. Electrical power is then absorbed or generated by raising or lowering the weight. Unused mine shafts are candidate sites. See;

Gravitricity Ltd. - a company based in Scotland.

ARES Systems

Advanced Rail Energy Storage (ARES) energy storage technology employs a fleet of electric traction drive shuttle-trains, operating on a closed low-friction automated steel rail network to transport a field of heavy masses between two storage yards at different elevations. The facilities are highly scalable in power and energy ranging from a small installation of 100MW with 200MWh of storage capacity up to large 2-3GW regional energy storage system with 16-24GWh energy storage capacity. See;

Advanced Rail Energy Storage LLC. - based in Washington State

Chemical Energy Storage


Extracting clean fuel from sunlight, - Richard Harth, Arizona State University - September 3, 2019

“In new research appearing in the Journal of the American Chemical Society (JACS), the flagship journal of the ACS, lead author Brian Wadsworth, along with colleagues Anna Beiler, Diana Khusnutdinova, Edgar Reyes Cruz, and corresponding author Gary Moore describe technologies that combine light-gathering semiconductors and catalytic materials capable of chemical reactions that produce clean fuel.”

Cryogenic Energy Storage

Liquid Air 'Batteries'

Air turns to liquid when cooled down to -196°C (-320˚F), and can then be stored very efficiently in insulated, low pressure vessels. Exposure to ambient temperatures causes rapid re-gasification and a 700-fold expansion in volume, which is then used to drive a turbine and create electricity without combustion. Such facilities are suitable for large scale storage and can compete with hydrokinetic storage (pumped water).

Highview Power claims round-trip energy efficiencies of up to 70%

Mechanical Energy Storage

Compressed Air Energy Storage

Industrial development of compressed air energy storage (CAES) has made some advances. Since the 1870’s, CAES systems have been deployed to provide effective, on-demand energy for cities and industries. While many smaller applications exist, the first utility-scale CAES system was put in place in the 1970’s with over 290 MW nameplate capacity. Care has to be taken since compressing air heats it up and releasing the pressure cools it down. Thermodynamics have to be managed carefully to the decompress phase doesn't require fuels to heat it up.

Unfortunately, large-scale CAES plants are very energy inefficient. Compressing and decompressing air introduces energy losses, resulting in an electric-to-electric efficiency of only 40-52%, compared to 70-85% for pumped hydropower plants, and 70-90% for chemical batteries. See;

"Ditch the Batteries: Off-Grid Compressed Air Energy Storage" - Low-Tech Magazine - no date.

Flywheel Energy Storage Systems

Flywheel energy storage systems (FESS) use electric energy input which is stored in the form of kinetic energy. Kinetic energy can be described as “energy of motion,” in this case the motion of a spinning mass, called a rotor. The rotor spins in a nearly frictionless enclosure. When short-term backup power is required because utility power fluctuates or is lost, the inertia allows the rotor to continue spinning and the resulting kinetic energy is converted to electricity. See;

"Turn Up the Juice: New Flywheel Raises Hopes for Energy Storage Breakthrough" - Scientific American - Chris Nelder - April 10, 2013

Flow Batteries

Aluminum-air Flow Batteries

Researchers at the School of Energy and Chemical Engineering at Ulsan National Institute of Science and Technology claim to have developed a new type of aluminum-air flow battery for EVs. The new battery outperforms existing lithium-ion batteries in terms of higher energy density, lower cost, longer cycle life, and higher safety. Aluminum-air flow batteries are primary cells, which means they cannot be recharged via conventional means.

"A novel catalyst for high-energy aluminum-air flow batteries" - - Ulsan National Institute of Science and Technology - Oct 15, 2018

Redox Flow Batteries (RFB)

The Korea Advanced Institute Of Science And Technology reports progress with Zinc/Bromine Redox Flow Batteries that have benefits over lithium-ion batteries:

New Technology Improves Next-Generation Aqueous Flow Batteries - SciTechDaily - December 27, 2020

What Are Flow Batteries? - a Review

Robert Cordingley
Originally published in our Winter 2021 Quarterly Newsletter.

Commercial scale redox flow battery storage - Australia

In our research on energy storage we came across a type of battery known as Flow Batteries. If you are like us, you may have thought what on earth are they, how useful are they, and do I need one? So we dug a little deeper.

As renewable energy comes on line in more places and in larger facilities, the demand to level out the daily fluctuations in power demand against daily and longer term varying levels of renewable generation is frequently met by Li-ion battery storage. Other energy storage options are in development and some may be ready for adoption. Storage at one scale will be needed to provide hours of electrical energy overnight, while others will be needed to provide days of electrical energy as weather patterns change.

Batteries of one kind or another offer relative short term storage, needing to be recharged once drained. Li-ion batteries may be able to provide up to 6 hours of storage. Some systems such as hydrogen fuel-cells can provide power for as long as the hydrogen supply lasts. Price performance will play a big part in how designers buildout storage facilities in either community or utility scale renewable energy facilities. Flow batteries look like they will play a part.

By the end of 2018, there were 869 MW of large-scale battery storage in operation in the country, reflecting 1,236 MWh of capacity. More than 90% were lithium-ion systems. (1)

“Earlier [in 2020], Pacific Gas & Electric (PG&E) filed a request with California regulators for approval to move ahead with five lithium-ion projects — amounting to 423 MW/1,692 MWh — scheduled to come online next August, bringing the total amount of storage the utility has under contract to more than 1 GW, representing more than 4 GWh of capacity.” (ibid.)

The Basic Technology

Regular batteries, Li-ion or lead acid batteries don’t have any moving parts. Flow batteries are fundamentally different comprising liquid materials that are pumped around and interact through a membrane. They have very limited degradation compared to Li-ion batteries but relatively low energy density. On the other hand flow batteries have a better 8-12 hour discharge time. Note that the low energy density is not an issue for stationary storage systems, unlike in transportation applications.

Flow batteries circulate a liquid electrolyte through cells as shown in the following figure.

Flow Batteries Schematic.jpg

The capacity of the system is dependent on the capacity of the tanks of electrolytes. The two halves of the cell above are separated by a special membrane, somewhat similar to a fuel-cell. During charging, electrons released at the positive electrode drive a chemical reaction to oxidize the electrolyte on that side. As the electrons are pushed around the circuit during charging to the negative electrode the electrolyte there is chemically reduced. When discharging the battery, the process is reversed. The two types of electrolytes are known as ‘redox’ pairs; they are chemical compounds that can reversibly undergo reduction and oxidation, hence the ‘redox’ name. The choice of redox pairs defines the type of redox flow battery (RFB). Both electrolytes can be based on vanadium chemistry as in the Vanadium Redox Flow Battery (VRFB).

While different chemistries (redox pairs) have different performance characteristics, their cells can be stacked to meet different electrical requirements while all being in the same state of charge.(2) Many configurations are possible with multiple tanks servicing a few cells to a pair of large tanks serving lots of cells. Heat management is easier than with Li-ion batteries with the addition of heat exchangers in the fluid handling systems. No electrolytes are flammable though they might be toxic and or caustic. Standard chemical handling practices can manage these safely with ease.

The technology is now well understood with facilities ranging in size from multi-MW installations bearing similarities with chemical plants to packaged facilities in standard shipping containers. Even hybrid installations with a combination of Li-ion and flow-batteries are being considered.

Storage Applications for Grid Modernization

Similar to other batteries, response times of RFBs in milli-seconds solves the ramp up problems renewables create when the sun goes down or the wind drops and some other generator has to come online. Conventional power plants have a significant ramp up time often on the order of hours to get to maximum power output.

Storage Costs

Situations vary, but for 100% grid conversion to renewables MIT researchers (3) estimate the cost of storage needs to fall to roughly $20/kWh. 2018 costs of Li-ion puts on a price tag of $175/kWh though this may have fallen since then. China has claimed $100/kWh for one of their battery powered bus fleets. Gas ‘peaker’ plants are estimated to deliver $5/kWh but of course rely on fossil fuel and emit CO2. However if renewables only need to meet demand 95% of the time the researcher say the price tag for storage works out at $150/kWh. Vanadium Redox flow batteries (VRFB) are estimated to cost $100/kWh.

With further development this too could fall. Researchers at Warwick University in the UK, in cooperation with colleagues at Imperial College London, say they have found a way to dramatically reduce the cost of redox flow batteries to £20/kWh (about $28/kWh) or less using inexpensive materials like manganese and sulfur which are found in abundance nature. (4)

Prospects for RFB

Forbes reports in their article “Why Vanadium Flow Batteries May Be The Future Of Utility-Scale Energy Storage” (6) of instances of businesses considering VRFB to have the ability to operate off-grid. Every 10-20 years the membrane may need replacing but this is a small cost compared to Li-ion batteries where the entire battery pack needs to be replaced. It also turns out that due to the fire risk, Li-ion batteries have to be spaced out further than VRFB because their chemicals are non-flammable. Meanwhile, vanadium can also be recycled indefinitely much more easily than Lithium which may end up in short supply.


  1. - Oct 2020
  2. - 2020
  3. - Sept 2019
  4. - Jan 2021
  5. - Oct. 2020

Liquid Metal Batteries

For a good overview see this video from the guy at 'Just Have a Think'. Commercial scale demonstration is on the cards and costs look promising, with lots of advantages over Lithium-ion for large scale static storage.

Conventional Battery Technology

Types of Batteries

Conventional battery technology includes a number of chemistries.

  • Lithium-Ion
  • Lithium-Ion Polymer
  • Lithium Iron Phosphate
  • Nickel-Cadmium
  • Lead-acid
  • Silver Oxide
  • Alkaline

From IDTechX, a report on the future of battery technologies is available for purchase (from $5,750):

"Solid-State and Polymer Batteries 2019-2029: Technology, Patents, Forecasts" - undated

"This report covers the solid-state electrolyte industry by giving a 10-year forecast till 2029 in terms of numbers of devices sold, capacity production and market size, predicted to reach over $25B. A special focus is made on winning chemistries, with a full analysis of the 8 inorganic solid electrolytes and of organic polymer electrolytes. This is complemented with a unique IP landscape analysis that identifies what chemistry the main companies are working on, and how R&D in that space has evolved during the last 5 years."

Limits on Lithium

Different scenarios predict that wholesale adoption of electric vehicles might produce a lithium shortage at some time in the future, in 17, 50 or 350 years in the future depending on assumptions:

Is There Enough Lithium to Maintain the Growth of the Lithium-Ion Battery Market? - GTM - June 2015

The 'good news' is that there are alternatives to lithium in battery development, projections of car ownership can fall if we adopt the use as-needed model or have smarter transportation solutions. In the as-needed model, instead of owning a personal vehicle, you will call a car rental company and a driverless car turns up at your location to take you anywhere.

Green Lithium

While somewhat of a misnomer (lithium is a silvery-white metal), 'green lithium' is lithium obtained with minimal impact on the environment. Most lithium today is mined from hard rock or taken from salt lakes, leaving scars on the landscape. The process to refine lithium also generates its own CO2 emissions, perhaps as much as 15 tonnes of CO2 for every tonne of lithium and the refining process itself uses large amounts of water.

Geothermal brine is a hot, concentrated saline solution that has circulated through very hot rocks and become enriched with elements such as lithium, boron and potassium. In other words, the energy-intensive process of extracting lithium from solid rock is powered by naturally occurring geothermal energy. Geothermal waters rich in lithium have been found in hot springs in Cornwall, UK. Extracting lithium from geothermal waters has a tiny environmental footprint compared to traditional processes:

The new 'gold rush' for green lithium - BBC Future Planet - Nov. 2020.

Energy Density of Different Storage Technologies

The higher the energy density of a fuel or storage technology, the more energy may be stored or transported for the same amount of volume. The energy density of a fuel or storage technology per unit mass is called the specific energy of that fuel or storage technology.

The higher the numbers for renewable technologies the easier it becomes to replace fossil fuels in transportation, for example Gasoline has a specific energy of 12,888.9 Watt-hours/kg, while Lithium-Ion batteries are 100.00–243.06 Watt-hours/kg. However, available energy has to be considered. Internal combustion engines have a 20-40% efficiency, while the efficiency of batteries/electric motors are expected to be higher, and depends on other features like braking energy recapture.

Costs of Storage Technologies

In a paper from Joule, the authors estimate that energy storage capacity costs below a roughly $20/kWh target would allow a wind-solar mix to provide cost-competitive baseload electricity in resource-abundant locations such as Texas and Arizona. Relaxing reliability constraints by allowing for a few percent of downtime hours raises storage cost targets considerably, but would require supplemental technologies. Finally, they discuss storage technologies that could reach the estimated cost targets. See:

Storage Requirements and Costs of Shaping Renewable Energy Toward Grid Decarbonization Sept, 2019