Energy storage is commonly classified into five categories: chemical, thermal, mechanical, electrical, and electrochemical (Figure 1). The first four categories refer to the form in which energy is stored. Electrochemical is a separate category that is used to classify the wide range of battery technologies and refers to the type of reaction they are based on. Within each of those categories, there is a range of concepts that utilize the respective energy forms or reactions to store energy. Almost all energy storage technologies are based on these concepts.
Figure 1 – The five categories of energy storage with respective energy storage concepts. Coloured shapes show categories. Bullet points name concepts that utilize respective energy forms/reactions. Chemical, Thermal, Mechanical, and Electrical refer to the form of energy that is stored. Electrochemical refers to the type of reaction battery technologies use to convert electrical into chemical energy and vice versa.
Technologies within the chemical category store energy in the form of chemical bonds. These chemical bonds are multi-purpose energy carriers and can be converted to electricity or used in other energy sectors, such as transport, heating, or as feedstock in industry. They can occur naturally as fossil fuels or be produced synthetically with electricity. The concepts refer to the chemical that is produced. The production of hydrogen gas from electricity through water electrolysis is the key enabler of all electricity storage concepts that convert the electrical input energy into chemical energy as can be seen in Figure 2. Hydrogen can be processed to ammonia by addition of nitrogen, hydrocarbons by addition of carbon dioxide (e.g. methane, kerosene) or alcohols, also by addition of carbon dioxide (e.g. methanol, ethanol). These chemical compounds have higher volumetric energy density than hydrogen. The amount of chemical energy stored is given by the heating value of the respective chemical compounds.
Figure 2 – Schematic of electricity storage through conversion of electricity into chemical energy. Amount of energy stored is given by the heating value of the respective chemical compounds.
Technologies within the thermal category store energy as heat. Electricity may be used to generate the heat that is stored. The heat can be removed later and used as heat directly or re-converted to electricity. There are three concepts to store thermal energy: sensible heat, latent heat, and thermochemical heat. Sensible heat is the thermal energy associated with heating or cooling of a material without changing its physical state. Latent heat is the energy associated with the phase change of a material between the solid, liquid, and gaseous state. Thermochemical heat is associated with a reversible chemical reaction or sorption process that releases or consumes large amounts of thermal energy. Figure 3 depicts the concepts and shows the formulas to quantify the amount of thermal energy stored.
Figure 3 – Three concepts of thermal energy storage: a) sensible heat, b) latent heat, and c) thermochemical heat, including the respective formulas to calculate the amount of thermal energy stored.
Mechanical energy is a collective term for gravitational, elastic, and motion energy. The respective concepts utilising these forms of energy storage are gravitation, compression, and linear or rotational motion. Figure 4 depicts them graphically and displays the formulas to quantify the mechanical energy stored. Pumps, turbines, and electric motors/generators are the machines that convert electrical into mechanical energy and vice versa for the respective electricity storage technologies.
Figure 4 – Schematic of the different mechanical energy storage concepts, a) gravitation, b) compression, c) linear motion and d) rotational motion, including the formulas to calculate the respective amount of mechanical energy stored.
Electric energy can also be stored directly using the concepts of capacitance or inductance. Capacitance separates positive and negative charges on two conductive plates. The energy is stored in the electric field between them. Inductance uses the magnetic field generated by an electric current flowing through a superconducting coil to keep it flowing until needed. Figure 5 shows sketches of both concepts and the respective formulas to calculate the amount of electrical energy stored.
Figure 5 – Schematic of electrical energy storage through capacitance (a) and inductance (b), including respective formulas to quantify the amount of electrical energy stored.
The final category classifies battery technologies based on the electrochemical reactions that take place within them. Energy is stored as the electrochemical potential between two materials that could react to form a new one. The net chemical energy of forming the new material (Gibbs free energy) is balanced by the electrostatic energy between the two separated materials. The dominant concepts are sealed and flow batteries. In sealed batteries, electrodes constitute the active material separated by an ion-conducting electrolyte. All components are in a confined battery cell. In flow batteries, the active material is not the electrode itself but two liquid electrolytes that can be circulated and stored outside of the system. Figure 6 shows both concepts with the examples of lithium-iodine (sealed) and vanadium flow batteries.
In panel a), lithium (Li) and iodine (I2) electrodes are separated by an electrolyte. Both materials naturally want to react to form lithium-iodine (LiI). The net chemical energy of that reaction is directly converted into electrical energy during discharge in the form of electrons that travel through the closed connector from the lithium anode (i.e. releasing electrons during discharge) to the iodine cathode (i.e. consuming electrons during discharge). This enables iodine ions to move to the lithium anode through the electrolyte where lithium-iodine is formed.
In panel b), a similar electrochemical process takes place with ions of vanadium pentoxide. The difference is that the vanadium pentoxide electrolyte is the active material. It is liquid and can be pumped to the electrodes for the electrochemical reactions to take place, thus the system is not confined. Electrolyte tanks and electrode cell can be scaled fully independently.
Figure 6 – Schematic of primary lithium-iodine battery cell (a) and secondary vanadium redox-flow battery (b), including respective formulas to quantify the amount of electrical energy stored.
Electrochemical technologies are further classified as primary or secondary batteries. In primary batteries, the electrochemical reaction cannot be reversed, so they are not rechargeable. In secondary batteries, applying an external potential that is higher than the battery’s potential reverses the electron flow. This restores the battery’s initial electrochemical potential, enabling it to be fully recharged.