Energy storage
Energy storage refers to the methods and technologies used to store energy for later use, playing a crucial role in various applications, such as utility load leveling, electric vehicles, and solar energy systems. It encompasses a range of storage durations, from fractions of a second to several years, depending on the energy source and requirements. For example, nuclear fuel is stored for a year, while coal or gas can be kept for days, and solar systems may require hourly or daily storage solutions. Key factors in energy storage include the amount of energy stored per unit weight or volume, as well as the speed of charging and discharging processes.
Common forms of energy storage include mechanical, thermal, chemical, electrical, and magnetic energy. Electrochemical cells, like lead-acid and lithium-ion batteries, are widely used for storing electricity and are vital for applications in consumer electronics and electric vehicles. Pumped hydro storage leverages gravitational potential energy by moving water between reservoirs to generate electricity when needed. Additionally, solar heat storage involves capturing and storing thermal energy through sensible and latent heat, using materials such as water and phase change materials (PCMs). This technology has significant applications in space heating and power generation, demonstrating the diverse and evolving landscape of energy storage solutions.
Subject Terms
Energy storage
When more energy is available than is needed at a given time, the excess energy can be stored for later use in a number of ways, including electrochemical cells, pumped storage, and solar heat storage.
Background
Energy storage is important for utility load leveling, electrical vehicles, solar energy systems, uninterrupted power supply, and energy systems at remote locations. Two important parameters to consider when discussing energy storage are the duration of storage and the amount of energy stored per unit weight or volume. Duration of energy storage may vary from a fraction of one second to many years. In a nuclear power plant, nuclear fuel is stored within a reactor for a year. Coal piles, gas and oil storage tanks, and pumped hydro (hydroelectric power) are maintained by power utilities for several days’ use, depending on the need. Similarly, for a solar energy system, energy storage may be required on an hourly, daily, or weekly basis. The amounts of energy stored per unit weight (specific energy) and per unit volume (energy density) are critical in determining the size of a storage system.
Other factors of importance in the design of a storage system include the time rates at which energy can be stored (charging) or removed (discharging) and the number of useful cycles of charging and discharging. Depending on the nature of available energy, it can be stored as mechanical, thermal, chemical, electrical, or magnetic energy. Electrical energy can be either stored as chemical energy in batteries, called electrochemical cells, or stored as mechanical energy by pumping water from a lower elevation to a higher elevation (pumped hydro). Electrical energy can also be converted to thermal energy and then stored as thermal energy.
Electrochemical Cells
An electrochemical cell consists of an anode, a cathode, and an electrolyte. When a cell is connected to a load, electrons flow from the anode to the cathode. In this operation, (loss of electrons) takes place at the anode, and reduction (gain of electrons) occurs at the cathode. The cell chemistry of the well-known lead-acid battery is as follows: The anode is lead (Pb), the cathode is lead (PbO2), and the electrolyte is sulfuric acid (H2SO4). The overall cell reaction is as follows:
The forward reaction represents the change during discharge when the cell is connected to a load, and the backward reaction represents the change that occurs when electric energy is stored. The theoretical voltage and capacity of a cell are functions of the anode and cathode materials. The theoretical voltage can be calculated from the standard electrode potentials of the materials. The capacity of a cell is expressed as the total quantity of electricity involved in the electrochemical reaction and is defined in terms of coulombs or ampere-hours. Theoretically, one gram-equivalent weight of a material will deliver 96,487 coulombs or 26.8 ampere-hours. A battery consists of one or more cells connected in series, parallel, or both depending on the desired output voltage and capacity.
Electrochemical energy storage is more commonly known as battery storage. Batteries are classified as primary and secondary batteries. Only secondary batteries are rechargeable and are therefore suitable for energy storage applications. Lead-acid and nickel-cadmium are well-known rechargeable batteries and are the most commonly used. Lead-acid batteries have been used for more than a century and are still the most popular batteries. An example is the automobile battery. Large numbers of electrochemical cells have been identified that can be used for storing electricity; a few of these are nickel-cadmium, nickel metal hydride, and lithium-iron sulfide. Electric storage in batteries has shown great potential in applications such as cell phones, laptop computers, tools, and electric vehicles and as a means of storing electricity for load-leveling purposes in power plants. The growing interest in electric vehicles is driving innovation in the battery industry. The automobile industry is looking for a better-performing, lower-cost battery. The current favorite, the lithium-ion battery, has led to research into lithium-air and lithium-sulfur batteries, which have much higher energy capacity and lower weight.
Advances in battery technology allowed for the popularization of electric vehicles throughout the 2010s and 2020s. Though gasoline-powered vehicles remained the dominant type of automobile worldwide, the increased practicality of vehicles powered by modern lithium-ion batteries encouraged many consumers to switch to electric. In the third quarter of 2024, electric vehicles made up roughly 8.9 percent of vehicle sales.
Pumped Storage
Another means of storing electricity is to pump water from a lower (which can be a lake or a river) to a higher reservoir. The potential energy stored in water by virtue of its elevation can be used later to generate electricity when needed by using hydraulic turbines. The motors that drive the pumps are reversible and act as electrical generators when water falls from the upper reservoir to drive the turbines. The main components of a pumped storage plant are the upper reservoir, waterway passage, power house, and lower reservoir.
Advantages of pumped hydro units include simple operation, high reliability, low maintenance, long life, quick start from a standstill, and economic generation of peaking electrical energy. Several such systems are in operation in the United States. Power-generating capacities of these systems vary between 5 megawatts and 2,000 megawatts or higher. The overall efficiencies of these power plants vary between 65 and 90 percent (these figures include the efficiencies of pumps, hydraulic turbines, and generators, and losses from the upper reservoir). In spite of the technical and economic viability of pumped hydro, the requirement of a specific type of topography and some environmental concerns limit its application. To overcome these problems, underground pumped hydro storage can be used. In this case a large cavern or an can be used as the lower reservoir.
Solar Heat Storage
When converted into heat, solar energy can be stored in the form of sensible heat and latent heat. Sensible heat is stored in a material by raising its temperature. The amount of sensible heat stored in a material is equal to the product of the mass, specific heat, and the temperature rise of the material. The most common sensible heat storage materials include water, propylene glycol, rocks, and molten salts. Water has the highest specific heat value. The higher the temperature rise, the greater the amount of heat stored. However, the highest temperature is limited by the properties of the material.
Thermal energy can also be stored as latent heat in a material when it changes phase, as from solid to liquid or liquid to vapor. Some materials also change phase from solid to vapor directly or from one solid phase to another. The amount of latent heat stored in a material is equal to the product of the mass of the material and its latent heat. Because materials change phase at a constant temperature, latent heat is stored and retrieved at a fixed temperature known as the transition temperature. Some common phase change materials (PCMs) used for heat storage are paraffin waxes, Glauber’s salt (sodium sulfate decahydrate), calcium chloride hexahydrate, sodium acetate trihydrate, and cross-linked high-density polyethylene.
Solar heat storage has major applications in space heating, crop drying, cooking, electric power generation, and industrial process heat. Heat storage in water is the most economical and well-developed technology. Hot water is stored in tanks made of glass- or stone-lined steel, fiberglass, reinforced polymer (plastic), concrete with plastic liner, and wood. The storage tanks may be located above or below ground. In North America and China, aquifers have been used for long-term storage of hot water. Molten nitrate salt (50 percent sodium nitrate, 50 percent potassium nitrate), also known as Draw salt, which has a melting point of 222° Celsius, has been used as a storage material for a solar thermal power system in an experiment in Albuquerque, New Mexico. This was the first commercial demonstration of generating power from storage. Solar Two, a 10-megawatt solar thermal power demonstration project in Barstow, California, also was designed to use this molten salt to store solar energy. It led to the development of Solar Tres Power Tower near Seville, Spain.
PCMs encapsulated in tubes, trays, rods, panels, balls, canisters, and tiles have been used for solar space-heating applications. The most common PCMs used are hydrated salts of sodium sulfate, sodium thiosulfate, sodium acetate, barium hydroxide, magnesium chloride, and magnesium nitrate. For building space-heating applications, PCM can be encapsulated in the building components themselves. They can be incorporated in the ceiling, wall, or floor of the building. For example, paraffin wax mixtures have been used for heat storage in wallboards.
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