By Steven Shackell
Battery technology may date back 2,000 years. Discovered in the 1930s in modern-day Iraq, the Baghdad Battery — also known as the Parthian Battery — consists of a clay jar, a copper cylinder and an iron rod that likely acted as electrodes when paired with an electrolyte solution such as vinegar. While some researchers still debate its use, it could be one of the first examples of an energy storage system.
We’re still working to perfect that technology, racing to create efficient long-term energy storage that ranges from board-level batteries to mega-grid-level hydro storage. This article examines energy storage breakthroughs and modern battery systems across a range of applications.
Board-level energy storage
Small battery energy storage systems
Batteries power most modern portable electronic devices. Lithium ‘coin’ batteries, such as the CR2032 from BeStar Technologies, are the primary energy source in watches, small lights, calculators, garage door openers, car key fobs, pedometers and many more small electronic devices. Small batteries vary widely, with differing form factors. A common size like the CR2032 stores around 230mAh of energy.
Lithium-ion batteries are a staple of small-scale energy storage, accounting for over 34% of market share in small electronics. Their advantages over lead acid, alkaline and nickel-metal hydride battery technologies include higher power density, lighter weight, longer life and limited temperature sensitivity.
Capacitor energy storage
Supercapacitors are a newer realm of energy storage devices, now used in applications that require rapid energy storage and release. Because supercapacitors can store large amounts of energy at relatively low voltages and high capacitance, they have several advantages over battery storage.
Supercapacitors have a much longer lifespan than batteries. A typical lithium-ion battery can cycle 500 to 10,000 times, while supercapacitors can tolerate 100,000 to one million cycles. Supercapacitors, like those in the NEDL series, can be charged nearly one thousand times faster than similar-capacity battery systems. Unfortunately, supercapacitors can lose as much as 20% of their charge per day due to self-discharge, so they are not ideal for long-term energy storage systems.
Grid-level energy storage systems
Storing large amounts of energy (over 1kWh) requires dedicated systems that vary drastically in size and capacity. Here are several examples of grid-level energy storage systems that offer long- and short-term storage at scale.
Residential battery energy storage
Perhaps the most recognizable form of grid-level energy storage systems, residential battery systems can be used as backup energy sources for residential use. Devices like the Tesla Powerwall and LG Chem RESU are commonly paired with solar panel assemblies to collect excess energy for subsequent use.
While lithium-ion battery technology is used in 34% of small electronic devices, it accounts for over 90% of the technology used in large-scale battery energy storage systems, per EESI. Lithium-ion technology is so widely adopted and impactful that the 2019 Nobel Prize in Chemistry was awarded to John B. Goodenough, Stanley Whittingham and Akira Yoshino for their contribution to lithium-ion technology. Lithium-ion battery storage systems can store up to 100MWs of electricity, have a power density of 200-400 Wh/liter and can achieve up to 95% efficiency.
Thermal energy storage
There are several types of thermal energy storage devices, including molten salt, ice storage systems, hot water tanks and aquifer thermal energy storage (ATES) systems, which use temperature (entropy) to store energy. In many cases, excess heat is stored in thermally conductive materials and then retrieved to generate electricity.
For example, molten salt energy storage (MSES) facilities are used in commercial applications for short-term energy storage. In MSES, molten salts are heated to over 1000degF and stored in insulated containers. When energy is needed, cold water is pumped through the molten salt to create steam, which is then passed through turbines to generate electricity. These systems can generally store up to 150MWs, have an energy density of 70-210 Wh/liter and can be up to 90% efficient.
Compressed Air Energy Storage systems
Pressure can also be used to store potential energy. Compressed air storage systems (CAES) use electricity to pump air deep underground into sealed holes that can sustain high pressure. This high-pressure air can then be heated and passed through an air turbine to generate electricity.
One advantage of CAES systems is that they can be used for mid- to long-term energy storage systems. There are only a few CAES systems around the world, but their energy storage capabilities are massive, ranging from 110MW to 315MW while achieving 70% efficiency. Several more CAES systems are planned in the U.S., Canada, China, Australia, Germany and other parts of Europe. Theoretically, smaller-scale CAES systems could be used for residential purposes, but these systems have yet to reach the market.
Pumped-storage hydropower
The highest capacity form of energy storage currently available is pumped-storage hydropower (PSH). These large-scale energy storage plants use gravity to store electricity. PSH systems work by electrically pumping water from a low elevation to a higher elevation where it can be stored. When electricity is needed, the water is released back to the lower elevation pool via turbines, which generate electricity much like how hydroelectric dams work.
Caption: The Kruonis Pumped Storage Plant in Lithuania
Pumped-storage hydropower energy storage systems can vary in size. For example, a new PSH facility in Walpole, Western Australia, can store 1.5MWs of electricity: enough to power 500 homes for two days. Meanwhile, the largest PSH energy storage system on the planet is in Bath County, Virginia, and can generate over 3,000 MWs with a total storage capacity of 24,000MWhs. That’s the stored energy equivalent of 34.7 billion CR2032 lithium-ion batteries.
PSH systems are the largest energy storage systems used in the modern era. However, their energy density is one of the lowest of all storage solutions, ranging from 0.2 to 2 watt-hours per liter (1/200th of a lithium battery). Storing the same amount of energy inside a common lithium battery requires 200 times the total area in a PSH system. Despite their need for square footage, PSH systems still reach efficiencies of 85%.
Modern energy storage systems
There is vast diversity in energy storage technology today. Whether these systems rely on pressure, gravity, chemical potential, thermal potential or capacitance, they all serve the same purpose: stabilizing and supplying power demand at a variety of scales. From small board-level applications like portable electronics to large-scale grid-level systems that enable renewable energy integrations, each of these technologies represents modern solutions for energy storage. While the most common applications are lithium-ion battery energy storage systems, the landscape is evolving in pursuit of more efficient, cost-effective and environmentally conscious solutions.
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