Energy is the capacity to do work. It is a quantitative property that can be transferred, but not created or destroyed, a principle known as the law of conservation of energy. Energy storage involves capturing energy for later use, bridging gaps between supply and demand. This capability is fundamental for both living organisms, enabling continuous biological processes, and technological advancements, supporting reliable power systems and integrating intermittent energy sources.
Biological Energy Reserves
Living organisms store energy primarily within specific molecules to power cellular activities. Adenosine triphosphate (ATP) functions as the immediate energy currency for cells, constantly produced and consumed to fuel processes from muscle contraction to nerve impulses.
For short-term energy reserves, the body converts excess glucose into glycogen. Glycogen is a complex carbohydrate, a branched chain of glucose units, primarily stored in the liver and skeletal muscles. Liver glycogen helps maintain stable blood glucose levels for the entire body, especially for organs like the brain during fasting. Muscle glycogen is used directly by muscle cells to meet their own energy demands, particularly during physical activity. Glycogen offers a readily accessible energy source, though it is less energy-dense than fats.
Fats, also known as lipids, serve as the body’s primary long-term and most concentrated energy storage. They store more energy per gram compared to carbohydrates or proteins, making them efficient for prolonged energy needs. Adipose tissue, or body fat, is where these triglycerides are stored. The energy in these biological molecules is held within their chemical bonds, released through metabolic processes like cellular respiration to synthesize ATP.
Chemical Energy in Fuels and Batteries
Chemical energy is stored in various forms through the bonds between atoms and molecules. Fossil fuels, such as coal, oil, and natural gas, are concentrated forms of chemical energy. They originated from ancient organic matter, buried and subjected to immense heat and pressure over millions of years. This energy, captured from the sun through photosynthesis, is released as heat when burned, converting into electricity or used for transportation.
Food also stores energy in its chemical bonds. Macronutrients like carbohydrates, proteins, and fats contain potential energy within their molecular structures. When consumed, the body breaks down these molecules, releasing stored chemical energy to power bodily functions or be stored in biological reserves.
Batteries are devices that store chemical energy and convert it into electrical energy through electrochemical reactions. Inside a battery, two different materials, electrodes, are separated by an electrolyte. During discharge, chemical reactions occur at the electrodes, causing electrons to flow through an external circuit, generating an electric current. For rechargeable batteries, this chemical process is reversible; applying an external electrical current forces the chemical reactions to proceed in reverse, storing energy back within the battery’s chemical bonds.
Physical and Mechanical Storage Methods
Energy can also be stored through physical or mechanical means, without relying on chemical bonds. Pumped-hydro storage stores energy as gravitational potential energy. Water is pumped from a lower to an upper reservoir during surplus electricity, like low demand or high renewable generation. When needed, stored water is released downhill through turbines, generating electricity. This method accounts for a large percentage of utility-scale energy storage in the United States.
Compressed air energy storage (CAES) stores energy by compressing air. Excess electricity powers compressors to force air into large underground caverns or tanks. When energy is required, the compressed air is released and expanded, often heated, to drive a turbine and generate electricity. This technology helps balance energy supply and demand, especially for intermittent renewable sources.
Flywheels store energy as kinetic energy in rotational motion. An electric motor accelerates a heavy rotor to very high speeds. When energy is needed, the flywheel’s rotational energy converts back into electricity as the motor acts as a generator. These systems are known for rapid response times and efficiency in short-term energy delivery.
Thermal energy storage involves storing heat or cold in various materials. This is achieved by changing a material’s temperature (sensible heat storage) or utilizing energy absorbed or released during a phase change, such as melting or freezing (latent heat storage).