Energy storage techniques
Energy storage techniques involve converting energy into a different form for later use, playing a crucial role in balancing supply and demand, especially with the rise of variable renewable energy sources. This process is essential as it allows for energy to be temporarily stored when production exceeds consumption and released when needed, mitigating the challenges posed by the unpredictable nature of renewables. There are various types of energy storage systems, categorized by the energy carrier used, storage duration, and efficiency. Common methods include pumped storage hydropower, which converts excess electricity into gravitational potential energy by moving water to elevated reservoirs, and compressed air energy storage, which uses compressed air in underground caverns. Additionally, batteries, including both primary and rechargeable types, serve as versatile energy storage solutions by converting chemical energy into electrical energy. Other techniques encompass flywheels for kinetic energy storage and thermal storage methods, such as sensible and latent heat systems, which store heat energy for later use. As the energy landscape evolves, innovative storage solutions, including thermal and hydrogen-based systems, are gaining attention, highlighting the importance of energy storage in a sustainable future.
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Energy storage techniques
Summary: Energy storage involves converting energy into a different form for use at a later time and is becoming much more relevant as less-predictable renewable electricity capacity is installed.
The term energy storage refers to the temporary storage of energy for use at a later time, usually because the supply and demand for a particular energy carrier do not coincide. There are various types of energy storage according to the type of energy carrier, the time for which they can store, the speed at which they can be loaded and unloaded, as well as various other nontechnical, economic, and environmental characteristics.
Nature’s own stores include different types of energy that are converted, either naturally or through human intervention, into other types of energy. The best-known example is the sun, which continuously converts massive amounts of chemical potential energy into electromagnetic radiation in the form of light and heat, and is the source of most energy on Earth—only tidal and geothermal energy derive at least partly from other sources. Another example is fossil fuels such as oil, gas, and coal, which are widely combusted by humans to extract the chemical energy stored in them and convert it to another useful form, such as heat or power. Nuclear power can also be added to this list, as it is based on utilizing the potential energy within unstable atoms, which is released as ionizing radiation when these atoms radioactively decay. Finally, there is a great deal of gravitational potential energy stored in the water cycle on Earth; as water flows downhill, this potential energy is converted to kinetic energy, which can be used to generate electricity, as in the case of hydropower.
The versatility of electricity, which is manifested through its many applications, is contrasted with the disadvantage that this energy carrier cannot be stored, at least not in its original form. Hence, electricity generation systems have traditionally evolved to generate precisely as much electricity as is currently needed: in other words, they are led by demand. Power systems based on a large number of centralized power stations generating electricity for transmission across national and/or international networks have therefore evolved to enable rapid adjustment of the electricity generated to the load at any given time.
If the difference between supply and demand in the electricity network at any one time exceeds very small tolerances, the network can become unstable due to the fact that the frequency of the network has to be adjusted to account for the discrepancy. In most European countries, most electrical appliances are designed to operate at a frequency of 50 hertz, such that only small deviations away from this value may result in damage to these appliances, as well as to the network.
The large growth in electricity generation from renewable resources over the past few decades, such as in Germany and Spain, has meant that the whole electricity system is no longer as controllable as it once was. This problem is expected to worsen in the future through the continued development of renewable energy and the growth in electric vehicles, which frequently need charging for extended periods. Although there are models for short-term wind forecasting, for example, the supply of electricity from renewable energy cannot be precisely predicted, especially over large time frames. Apart from measures to turn off power plants such as wind turbines, renewable electricity supply cannot easily be controlled. This means that there is a renewed interest in storage devices for electricity in order to balance supply and demand and thus account for the fact that, on some days, large amounts of electricity are generated when they are not needed (i.e., supply exceeds demand), and vice versa.
Available devices for storage of electricity generally convert it into potential energy. The most ubiquitous electricity storage devices used for these purposes, such as network balancing, are pumped storage hydroelectric plants. In these plants, excess electricity is used to pump water up from a basin into a reservoir (i.e., convert it into gravitational potential energy) until a time when electricity is needed again and the water flows out of the reservoirs to turn turbines and generate electricity.
The overall conversion efficiency is typically 80 percent, depending upon the turbine and type of plant. A similar principle is applied to pressurized air storage devices, which exploit the thermodynamic properties of gases to compress air, which is then later decompressed through a turbine. This technology is known as compressed air energy storage (CAES), and can be used in conjunction with gas turbine plants in order to improve the overall generation efficiency. The principle of this technology, which relies on underground caverns to store the compressed air, means that the location of any power plant wishing to exploit this concept is crucial.
Another form of device for storing electrical energy is the battery, which converts stored chemical energy into electricity, and vice versa. Primary batteries can only be used once, whereas reusable or rechargeable batteries can be recharged and thus used in both directions. There are a wide variety of rechargeable battery types, which vary greatly in terms of their capacity, charging/discharging time, and deterioration over time (i.e., number of cycles).
Yet another type of energy storage device is a flywheel, which is a large rotating mass designed to store (rotational) kinetic energy. When the flywheel is accelerated up to a certain speed, its large mass means that it has a large momentum (actually moment of inertia), so provided there is little friction on the shaft upon which the wheel is mounted, the flywheel will store this kinetic energy as it continues to rotate. Due to its large mass, the flywheel tends to resist changes in its rotational velocity; hence, they are used in applications where the demand for energy is continuous, but the supply is discontinuous—such as in a vehicle, to temporarily store the rotational energy from the drive shaft when the clutch is disengaged and the vehicle comes to a stop. They are also used for applications where a rapid discharge of the stored energy is required, such as in wind-up toys.
Finally, another type of energy store is that for storing heat energy. There are various types of thermal storage devices, including: sensible heat storage, whereby the store actually heats up, as in the example of an insulated hot water tank; latent heat storage, which exploits a change in phase of a material, such as when salt is employed as a storage medium for solar thermal power plants; and finally thermo-chemical heat storage, whereby heat is stored in a chemical (endothermic) reaction, which can later be reversed (an exothermic reaction) to release the heat. In general, the latter approach is not constrained to heat storage, but is also applied in fuel cells, for example, in order to generate and store electricity, typically in the form of hydrogen. In fact, the vision of a future energy system based largely on hydrogen as a chemical energy store is shared by many scientists.
Bibliography
Baxter, R. Energy Storage: A Non-Technical Guide. Tulsa, OK: PenWell Corporation, 2006.
“Electricity Explained: Energy Storage for Electricity Generation.” US Energy Information Administration (EIA), 28 Aug. 2023, www.eia.gov/energyexplained/electricity/energy-storage-for-electricity-generation.php. Accessed 1 Aug. 2024.
“Energy Storage.” International Renewable Energy Agency, 2022, www.irena.org/Energy-Transition/Technology/Energy-Storagewww.energy.gov/eere/analysis/energy-efficiency-vs-energy-intensity. Accessed 1 Aug. 2024.
Huggins, R. A. Energy Storage. Heidelberg, Germany: Springer, 2010.
Roth, Kurt, and James Brodrick. “Seasonal Energy Storage.“ ASHRAE Journal, January 2009.