In the US, about 59% of electricity was generated from fossil fuels in 2024. Of that fossil fuel-generated electricity, approximately 73.3% came from natural gas, while the remainder came from other sources like coal and petroleum. With the US energy grid generating energy around the clock, this becomes a larger problem of not only greenhouse gas emissions, but also wasteful use of energy sources. On top of the greenhouse gas emissions and the generation of electricity that may not always be in use, using fossil fuels requires combustion of fuel, which yields less than 50% net electricity generation and creates heat and chemical waste (greenhouse gases). From this, the question arises: why can’t the energy just be saved for later? In fact, it can and should be to avoid wasting electricity. To reduce greenhouse gas emissions and meet net zero goals, the power grid must replace fossil fuel power plants with cleaner energy systems that include large-scale energy storage. This will enable a more reliable and sustainable grid.
Current Energy Storage Technologies
There are several energy storage technologies that have been deployed to date. Some of the oldest utility-scale technologies include pumped hydroelectric storage (PHS) and compressed air energy storage (CAES). PHS involves pumping water from a lower water reservoir to a higher one, utilizing potential energy to generate electricity. CAES works in a similar manner by compressing air into specialized storage tanks, where the air can be accessed during peak hours when electricity is in high demand, though this method typically burns natural gases in the process. With the advent of nuclear power plants, molten salts have also emerged as a way to store thermal energy from nuclear reactors as well. In recent years, however, battery storage systems have garnered the most attention. Battery storage systems can be implemented in a variety of locations, from large utility-scale plants to more rural microgrids, making it an integral part of the effort to integrate larger amounts of energy storage into the power grid infrastructure. Many are familiar with the lithium-ion battery, which is used in our smartphones, laptops, and even electric vehicles and energy grid storage systems. The state-of-the-art technology boasts high energy densities and long cycle life. However, the best of these types of batteries use other critical materials such as cobalt and nickel in addition to lithium (i.e. NMC811 Li-ion batteries). This makes these technologies vulnerable to supply chain disruptions and also raises concerns about child labor and unsafe working conditions, like those seen in the Democratic Republic of Congo. For this reason, though lithium batteries are considered the “cream of the crop” today, alternative technologies need to be considered.
Alternative Battery Technologies
One of the most promising candidates for grid energy storage is the redox flow battery. They are batteries that utilize tanks that contain liquid electrolyte, an anolyte and catholyte, which flow through carbon current collectors and are separated by a membrane. This setup enables the battery to undergo its redox reaction, which is what allows the battery to store and release energy. Though these batteries have relatively lower energy and power density than lithium-ion batteries by the cell, increasing the volume of each tank, stringing multiple cells together, or changing the architecture of the current collectors and membranes can improve these properties. The decoupling of both energy density and power makes these batteries attractive options for grid energy storage compared to lithium-ion batteries, where the energy density and power cannot be changed independently of one another. These batteries also have a much longer cycle life than other types of batteries, where the membranes can be replaced in the case of battery degradation. Redox flow batteries also have multiple safety features that lithium-ion batteries don’t have. Some include the ability to prevent short-circuits in the case of cell rupturing as well as non-flammable electrolytes (in most cases). The current collectors are also made out of carbon, which is typically much cheaper than the aluminum or copper used in lithium and sodium-ion battery current collectors (Sandia National Laboratory 2024). The most widely commercialized type of flow battery is the all-vanadium battery, which utilizes vanadium as the primary component in both the catholyte and the anolyte. While this system is relatively simple, vanadium is also a critical material like lithium, making it vulnerable to supply chain disruptions as well as costly. While research is being done on cheaper types of electrolytes (i.e. all-iron electrolytes), the vanadium redox flow battery still has a higher voltage. Advances in this technology require more insight into the electrolyte composition to enable higher voltage redox flow batteries.
Schematic of a redox flow battery. The green chamber holds the catholyte, and the blue chamber holds the anolyte. The tan blocks through which each electrolyte is flowing are current collectors. The clear panel is the separator. The red is the load through which current is flowing. (Sandia National Laboratory)
Sodium batteries, both sodium-ion and sodium metal, can be thought of as chemical cousins to the lithium-ion/metal battery. They have a similar function to their lithium analogues with a slight penalty to their energy density due to the size of the sodium ion being larger than that of the lithium ion. However, they are good candidates for grid energy storage applications due to their comparable energy density to lithium-ion batteries while using more abundant battery materials, with sodium metal batteries having similar energy densities (250 W l-1) to higher-end lithium-ion batteries (260 W l-1). One of the more well-known sodium metal battery technologies that has been commercially deployed is the sodium-sulfur battery, which uses molten sodium and sulfur as electrodes. This battery can be seen as a medium between the long cycle life and cost-effectiveness of the redox flow battery and the energy and power density of the lithium-ion battery, though improvements need to be made to the design to enable higher-power applications, such as better electrolyte performance. Another sodium metal battery used for grid energy storage is the Zero Emissions Batteries Research Activity (ZEBRA) battery, which uses a mixture of sodium chloride and nickel, depositing sodium on the anode and nickel chloride on the cathode. This battery removes the concern of using molten sodium metal, which is very volatile in atmospheric conditions and can spontaneously combust. However, the use of nickel still raises concern, as it is relatively low in abundance, and its mining has negative environmental impacts.
[Left] Schematic of a sodium-sulfur battery (Andreas Poullikkas). [Right] Schematic of a ZEBRA sodium-nickel chloride battery. (Westbrook, GlobalSpec)
Integration into the Grid
Energy storage systems take advantage of grid usage patterns by storing energy when demand and costs are low and releasing energy when they are high, operating on a “save-it-for-later” principle. This makes these technologies complement renewable energy generation, where the technologies have often been criticized for their intermittent nature. It also reduces operational costs by eliminating the need for 24 hour facility operation and increases access to energy by providing emergency reserves of power in the case of a direct power outage. However, their standardization across the various energy grids in the United States poses a significant challenge, such as regulations with different state governments and integration into existing power grid infrastructure. In Chicago, there have been delays in energy storage technology advancement due to policies not falling in line with other developments, such as provisions for growing data center development. There is also the issue of cost, including grid integration, repurposing of power plants, and technology infrastructure, which could hinder the progress of implementing these systems. Despite these challenges, the US is predicted to keep growing its energy storage capacity, with California and Texas leading the charge. With the cuts to tax credits relating to renewable energy and energy storage, such as the Clean Electricity production tax credit (PTC) and investment tax credit (ITC), it is uncertain that this trend will hold. The solutions to these problems lie within the strategic placement of these energy storage systems. One way of reducing the cost of this transition to a cleaner energy grid with more energy storage capacity lies in utilizing existing storage technologies. For instance, Illinois could still meet its 2025 renewable energy goals by deploying short-term storage on retiring fossil fuel plants, according to a 2024 NRDC article. This solution could also be applied to other states that are aiming to increase their energy storage capacities, potentially accelerating the rate at which states adopt cleaner energy grids. However, energy storage systems should also be built with existing renewable energy resources to accelerate the transition and improve grid energy storage.
Added battery capacity to the U.S. energy grid (From Cleanview 2024).
As of 2023, California and Texas have the largest shares of installed battery capacity in the United States (From the EIA 2023).
What Happens After?
The growth of energy storage systems has the potential to create a new energy and manufacturing market in the US that prioritizes efficiency and circularity, though it will not be easy to replace our current market. While batteries are a pretty universal energy storage technology, geographical locations also need to be considered since proximity to large water reservoirs and varying altitudes could make PHS storage more ideal. States that already use nuclear power could also make use of molten salt thermal energy storage. In places where batteries are the best option, the use of sodium or redox flow batteries should be prioritized in order to create a more robust infrastructure for energy storage. By operating on a “save it for later” principle, the US could reduce the thermal and chemical waste produced by its energy systems and potentially set a precedent for other industries, creating an economy that produces less waste. Beyond the economic sector, could this way of operation influence the broader economic culture of countries such as the US? The constant generation of electricity could be juxtaposed with American culture, one which has been associated with hyper-consumption. Could the innovation of energy storage trigger a domino effect, shifting values from always consuming to only necessary consumption? Or could the opposite happen, where energy becomes cheaper to use and people use more? Though it’s hard to say if the two are related, there’s no denying that constant generation and consumption are two defining characteristics of both the US economy and energy grid. It’s possible that a shift to a “save it for later” principle may alter the way the US approaches energy policy and research.
Reader Question
How will cuts to clean energy tax credits impact grid energy storage innovation and investment on the state level?