Global warming and fossil fuel consumption are two major themes we hear about almost on a daily basis. Renewable energy has come a long way and the future is looking greener with the public interest in them. One problem with such types of energy is that they are very dependent on the environment and hence can be intermittent. For example, a solar panel generates more than enough electricity to power your home during the daytime when the sun is feeding it energy in abundance, but nighttime and cloudy days would provide little to no energy to this setup. Similarly, when there is no wind, wind turbines will not be able to generate electricity.

Many scenarios could be mentioned limiting the usability of these systems yet they are implemented efficiently thanks to Energy Storage Systems.

What are Energy Storage Systems?

Energy storage systems are the batteries in your phone, your computer, your smartwatch and rechargeable LED light. They are in many electrical and electronic device that you are using. They are even as simple as your water tank and vacuum flask.

The future is very dependent on these technologies and trillions of dollars are being invested in research and development to improve them. This development would help your phone and laptop to last longer while disconnected, and your future electric car to drive further.

There are many types of energy storage solutions which range from lithium ion batteries to air energy. There’s no perfect solution and each have their own pros and cons.

Lithium-ion batteries as Energy Storage

NASA started research on Lithium(Li)-ion batteries in the 1960s and since then batteries came a long way. Today, Li-ion batteries are in billions of devices in production and in use. They have one of the highest energy densities (amount of energy in a given space or volume) of any battery technology available today, meaning that you can store a good amount of energy even for small size battery. Today’s Lithium-ion batteries have the ability to store 270+ Wh/kg, nearly five times the energy-density of traditional Lead/Acid batteries and double that of Nickel.

Li-ion battery schematics

As the ions of lithium move from the negative electrode “Cathode” through an electrolyte to the positive electrode “Anode”, they generate electricity. Reversing this movement of ions recharges the battery making it a good rechargeable battery.

The lithium compound is usually the positive electrode in these batteries while the negative electrode could be made of graphite or other compounds being tested and researched.

Battery pack prices used to cost near $1000 per kWh just ten years ago. Today’s prices have gone to just a portion of that and are almost $130 per KWh. They are expected to dip below the $100 mark by 2024.

Quantum-Scape, a startup founded in 2010, have attracted over $2 billion dollars in investments and are developing their own solid-state lithium-metal batteries which will provide the next revolution in battery storage.

Thermal Energy Storage

Thermal energy applications are widely spread and thermal energy is considered one of the most important types of energy paving the way for Thermal Energy Storage systems (TES) into application. From storing heat within liquid or even cold thermal energy, TES systems are very important to conserve thermal energy.

Many applications, whether small or large, can be discussed where even a double insulated water vacuum flask is a type of thermal energy storage tool that helps keep the temperature of the liquid inside conserved for a long period of time.

Going into bigger applications, water tanks that are interconnected with solar panels are a great example of heat thermal storage system that would conserve the heat energy collected from the sun so it is not diffused quickly outside the system.

Applications for cold thermal energy systems (CTES) exist as well and can be seen in air-conditioning systems, be it buildings’ central system or a small cold water-based air cooler where the water is brought down to low temperatures and then heat-convection and radiation would slowly take place with the ambient air to make it cooler.

For a water system that is heated to a difference of over 100 degrees Celsius, a liter of water (almost 1 kg of water) could store about 117 Wh/kg which is not as economical as a lithium ion battery.

Malta.Inc, a startup based in UK, has over $140 million dollars in funding having the government as well as some big names as investors. Below is an overview of their implementation of an energy harvesting, thermal energy storage, and distribution system.

Malta.Inc system

Air Energy Storage

Compressing the air and then getting the energy back while decompressing it is one of the methods to store excess energy for use on demand. Research on air energy storage systems started early in the 1800s and the first large scale application of such systems took place in the 1970s.

In air energy storage systems, when there is an excess of energy, a gas (could be air or any other) is pressurized from ambient pressure of 1 bar (atmospheric pressure) to over 70 bar of storage pressure. Releasing this pressure would allow the regenerating of energy (for example generating electricity through a turbine) for later use.

Air could be the world's next battery - SINTEF

Advanced Compressed Air Energy Storage systems (CAES) claim to have a 265 Wh/kg energy density.

Hydrostor, a Canadian startup claim to be the leading developers of Advanced Compressed Air Energy Storage (A-CAES) projects which is for long duration storage of more than 8 hours and the “battery” can last for more than 50 years.

Flywheel Energy Storage

NASA’s FES System

You might know the flywheel from your car, the large disk that is connected to the rotational shaft of the engine (like the crankshaft) which once starts rotating its inertia will allow it to keep rotating even when the engine stops. It is a very simple application that has been in use for hundreds of years and still finding development and applications.

One newer application is the advanced Flywheel Energy Storage (FES) system used by NASA where the flywheel is put in a vacuum chamber (to reduce energy losses due to air resistance) where the flywheel is rotated to high speeds over 50,000 RPM reaching full-energy-capacity in very short duration.

They are very efficient, sometimes even over 90%, and according to the materials used 400 Wh/kg could be achieved “though most typical systems of this type in use today are at 130 Wh/kg.”

Revterra, the US based startup are providing low-loss flywheel energy storage solutions. They are using carbon fiber composites as well as active magnetic levitation, magnetic bearings and superconductors to store rotational energy with very low losses.

Flywheel Energy Storage System

Rust Battery Energy Storage System

The basics of Rust battery storage system is that when oxidization occurs the material releases electrons which can be used to generate electricity [Discharge]. The reaction can then be reversed by shooting electrons at it which helps restore the battery to its initial stage [Charge]. NASA had taken interest in the concept back in as early as the 1960s.

The huge energy density of these types of batteries of 764 Wh/kg makes them a main topic when it comes to the energy storing systems of the future.

Form Energy, a startup that is backed by the likes of Jeff Bezos and Bill Gates and rounding up over 350 million dollars in funding is promising the development of a battery that lasts a hundred hours.

Why Rust Batteries could be the future.

Gravity Energy Storage

Imagine yourself holding a ball while standing on a cliff; you release the ball from your hand without pushing it, what happens next? The ball will fall towards the ground due to the gravitational forces between the ball and the earth. This is simply an energy conversion where the already stored energy “Potential energy” (from the height) is converted into a kinetic energy apparent in the movement. The potential energy is initially supplied to the system “the ball in this case” while carrying it upon the hill.

This is a very simple concept that is being built upon to store energy where the surplus electrical current supplied to the grid is used to actuate a crane lifting some heavy objects (Charging). Once energy is in demand over the grid, these lifted objects are dropped moving an electricity generator along the process creating electricity (Discharging).

Gravity energy storage systems could reach a 90% efficiency and can have energy density rounding to about 4 Wh/kg which is very low but considering the big weights of the systems make them functional.

Energy Vault is a startup that creates gravity and kinetic-energy-based long-duration energy storage systems promising large scale applications up to multi GWh.


The importance of energy storage systems is the capabilities they give us to distribute the time-controlled supply of energy effectively over the duration of demand. While Lithium-ion batteries are seeing billions in investments, other types could prove to be better suitable for a greener energy-sufficient future.

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