It was not long after electricity was discovered that humans started searching for the best ways to store it. Alessandro Volta, for instance, created the first battery all the way back in 1800, using zinc and copper discs. Several decades after, one of the oldest examples of a rechargeable battery—the lead-acid battery—was invented in 1859. In the centuries since, energy storage technology has evolved to include an array of methods as vast as it is varied. Today, the energy storage evolution shows no signs of slowing down and remains a highly dynamic field with headline-worthy innovations arriving almost daily.
Given the intermittency of some renewable energy sources like wind and solar, energy storage has been generating even higher interest and investment as a complementary technology. Many in the industry see storage as the linchpin through which renewables can uproot a centuries’ old model of power generation, transmission, and distribution (i.e., a heavy reliance on centralized fossil fuel power plants). With energy storage, even if the wind isn’t blowing or the sun isn’t shining, renewable energy can be available for the grid.
Batteries and other energy storage methods are used around the world as a solution not only to mitigate the issue of intermittent renewable energy generation but also for some of the world’s most complex energy challenges. Large and small, energy storage devices provide owners and operators the means to store energy for use at any time of the day, unlocking potentially significant value streams depending on the application.
Given the sensitivity of much of the world’s electrical infrastructure to the increasingly severe effects of global climate change, energy storage devices also offer a great deal of resiliency to homes, buildings, and communities that will benefit from the safety and security that comes with having a reliable source of backup power. So, what are the various ways that energy can be stored, and how exactly do all of these methods work?
MECHANICAL STORAGE METHODS
PUMPED HYDROELECTRIC STORAGE
Pumped hydroelectric storage (PHS) has been in use since the 1920s and is the most common form of grid-scale energy storage. Today, PHS makes up roughly 96% of global energy storage capacity.
There are 43 PHS projects operating in the United States, generating a collective 22.9 gigawatts (GW) of power. Most of those projects were built in the 1970s, and there has not been a new pumped hydro plant opened since 2012. Pumped hydroelectric storage offers huge energy storage capacity, but it also requires a large area and specific terrain to be feasible.
Pumped hydro has a relatively simple methodology. During off-peak hours electric pumps pull water from a lower retention pool to a higher retention pool. This stores electricity in the form of potential energy in the water of the elevated reservoir. When there is energy demand, the water is allowed to flow back down to the lower retention pond, turning hydroelectric turbines on the way which generates electricity.
The power generation is just like what would happen at a typical hydroelectric dam. Some PHS systems, called open-loop systems, are built on rivers or dams that are generating electricity from continually moving water. These systems also have an upper reservoir that water will be pumped to when electricity rates are cheapest, usually at night.
By using cheap energy at night, a pumped hydroelectric storage system ensures that water will be available to generate power during the most expensive parts of the day. PHS systems are net consumers of energy, meaning they use more electricity than they create, but are a viable way to save money for significant energy users.
COMPRESSED AIR ENERGY STORAGE
Another mechanical storage method, compressed air energy storage (CAES), has been providing cities with on-demand energy since the 1870s. Cheap or excess electricity is used to pump air into an underground cavern or container at high pressures. When energy is needed, the air is released, heated, and expanded to drive turbines that produce electricity.
The ideal setting for CAES systems is actually salt caverns, which offer flexibility without pressure loss and don’t react negatively with the stored oxygen. Aquifers and depleted natural gas fields are also being investigated as alternative compressed air storage sites.
There is only one utility-scale CAES plant currently operating in the United States. The McIntosh Power Plant in Alabama stores compressed air in a salt mine located a half-mile directly below the plant and releases it to an on-site natural gas unit to produce energy. Most natural gas plants spend about half their energy compressing air to use in the generation process. By having ample storage available, the McIntosh Plant can use only the cheapest energy to compress air that is available for later use.
Most storage systems are designed to provide steady energy for hours at a time. Still, other storage technologies are used only to supply short bursts of electricity to help with power management. This service is the primary purpose of flywheels, also called flywheel energy storage (FESS).
Flywheels are spinning mechanical devices that store electricity as kinetic energy. They consist of a mass that is rotated faster and faster by a motor when energy is being stored. This process typically occurs in a vacuum to minimize energy loss due to drag. When energy is needed, the spinning mass acts similar to a turbine, losing speed to create electricity.
Traditionally flywheels use a steel mass that rotates on bearings. Newer constructions have been achieving higher revolutions per minute by using a carbon fiber mass and magnetic levitation.
As mentioned, the benefit of flywheels is their ability to instantaneously send energy to balance out fluctuations in the electricity supply. They are also sometimes used as an interim backup power source to ensure there is no loss of electricity when backup generators are needed. Flywheels also saw short-lived but fascinating use as an alternative bus propulsion system.
THERMAL STORAGE METHODS
THERMAL ENERGY STORAGE
This method stores energy in the form of heat or cold. Excess or cheap electricity is used to heat or cool some material, usually water, but air, rocks, or salt may also be used. That material is then stored in heavily insulated tanks or natural caverns deep in the ground. When electricity is needed, the thermal energy of the material is captured and transformed back into electricity, often via steam turbines.
For example, many modern solar thermal plants use their excess energy to heat molten salt. At night the molten salt is then used to generate steam that runs turbines, transforming the heat back into electricity. However, just like the thermos keeping coffee warm, thermal storage works best in the short term, as insulation is not perfect, and heat will be lost to the atmosphere.
A newer thermal energy storage method is showing better long term effectiveness. Liquid Air Energy Storage (LAES) uses excess energy to cool down air until it liquefies. The liquid air is then stored in insulated tanks. When energy is needed, the liquid air is vaporized back to a gaseous state that then spins turbines to generate electricity.
Another common use of thermal energy storage is improving the efficiency of large scale heating and cooling operations. During the summer, naturally occurring hot air and water can be stored underground to be later used during the winter. Additionally, facilities can produce ice during “off-peak” night hours that is then used to lower the energy demand for air conditioning systems during the day.
CHEMICAL STORAGE METHODS
Chemical storage is perhaps the most well-known form of energy storage today. From your TV remote and laptop to multi-megawatt grid ancillary systems, batteries that store energy chemically can provide power in a moment’s notice.
The two most common types of solid-state batteries are lead-acid and lithium-ion. Lead-acid was the first-ever rechargeable battery invented and remains popular as a low-cost energy storage solution. However, lead-acid’s low energy density makes it less applicable to large grid-scale applications.
Lithium-ion batteries are newer to the scene, with commercial applications starting in the early 1990s. Lithium-ion got its start in consumer products like laptops and cellphones but is now also used for residential and grid-tied systems. In fact, lithium-ion makes up the majority of the 862 megawatts of battery storage currently installed in the United States.
Energy storage systems for solar are frequently composed of lithium-ion batteries because they are better at fully discharging and recharging frequently and have much longer lifespans than their lead-acid counterparts. Independent Power uses both lithium-ion and lead-acid batteries for installations as certain conditions can favor one or the other. Flow Batteries
Like solid-state batteries, flow batteries can also serve as solar storage systems to hold excess energy during the day and discharge it at night. The difference between flow batteries and solid-state batteries is the internal makeup. Flow batteries use reduction and oxidation reactions to store energy in liquid electrolyte solutions, whereas solid-state batteries store it in the actual material that comprises the electrodes.
Additional research into flow batteries will help to bring down the cost and increase the scale at which these batteries can be produced, as they currently make up only a small percentage of the battery market. Flow batteries can be more flexible than solid-state batteries in meeting specific power and energy requirements, so they see some large scale application. An energy storage project in the Hubei Province in China, for instance, is using flow batteries on a massive scale and will eventually offer up to 10 megawatts of power capacity.
The energy storage industry is seen by many to be the biggest key to expanding renewable energy generation across the globe, and many different types of storage methods will play a role in the transition. Batteries will undoubtedly play a significant role, especially at smaller residential and commercial scales. If you are curious about how solar plus storage can work for your home or business, please reach out to one of our experienced professionals for more information or start your clean energy journey by obtaining a free quote!