November 17, 2023

Energy Storage Systems: Understanding the Duration and Limitations of Energy Storage Capacity

8 Min. Read

Integrating more renewable energy and balancing the grid requires utilities, businesses, and even homeowners to embrace energy storage systems. Excess energy can be captured and stored when the production of renewables is high or demand is low. When demand rises, the sun isn’t shining, or the wind isn’t blowing, that stored power can be deployed.

While the concept of banking excess electricity for use when needed sounds simple, energy storage can be complicated but it is critical to creating a more flexible and reliable grid system. This article explores the types of energy storage systems, their efficacy and utilization at different durations, and other practical considerations in relying on battery technology.

The Temporal Spectrum of Energy Storage

Renewable energy for residential homes, primarily wind and solar power, accounted for 81% of new capacity added globally in 2021. The worldwide push to replace power generated using fossil fuels is growing exponentially, with renewables projected to comprise 95% of power capacity growth through 2026. Some forecast that 80% of U.S. electricity will come from renewable sources by 2050. That transition escalates demand for energy storage technologies that will bank excess power from renewables and both short-discharge it when needed on a short-term and longer-term basis.

Instantaneous vs. Short-Term Storage

True resiliency will ultimately require long-term energy storage solutions. While short-duration energy storage (SDES) systems can discharge energy for up to 10 hours, long-duration energy storage (LDES) systems are capable of discharging energy for 10 hours or longer at their rated power output. Both are needed to balance renewable resources and usage requirements hourly, weekly, or during peak demand seasons and enable the phase-out of traditional sources of electricity.

SDES instruments are becoming more widely used on local levels as property owners add battery walls and other storage mechanisms to manage their energy use, integrate renewables, and use smart panels to schedule device usage. Battery arrays are the short-duration stabilization technology of choice for many because they can be installed quickly, respond when discharging is needed, and have high round-trip efficiency to ensure maximum output. Here are some options:

• Lithium-ion systems dominate the small-scale battery energy storage systems (BESS) market, aided by their price reductions, established supply chain, and scalability. Lithium-ion is just one of the battery storage options in use today.
• Lead-acid options are widely used for rechargeable batteries and are commonly employed in conjunction with solar power installations. Lead-acid battery storage can be scaled to accommodate needs from residential to utility-scale deployment, however lithium-ion is more powerful and requires less space than lead-acid batteries, making it a more ideal energy storage option for residential settings than lead-acid.
• A vanadium redox flow battery (VRFB) uses chemical energy from two chemical components dissolved in electrolyte fluid flowing through the rechargeable central unit from two exterior tanks. These flow battery systems can store and release large volumes of energy with durations ranging from hours to days but are also scalable for multi-day durations. VRFB systems are a sustainable solution for long-term energy storage and facilitating grid stability, but this is not yet as viable of a solution for residential energy storage.

Long-Term Energy Storage

LDES systems are needed to help realize the potential of renewable power generation throughout the country. Some, including scalable SDES systems like flow batteries, are deployed in places, but more cost-effective viable options are needed. Here are some LDES options:

• One LDES system utilized successfully for decades is pumped storage hydropower systems. However, pumped hydropower requires flowing water and reservoirs built at different elevations. These requirements limit wider adoption.
• Thermal storage systems use heating and cooling in thermal energy mediums, such as aluminum alloys and molten salts. The mediums can be stored for several days before being pumped into a generator to run a turbine and generate electricity.
• Some solutions are not practical for widespread use. For example, researchers found the storage of hydrogen in underground salt caverns has a 120-hour (5-day) duration, but there are only two viable caverns in the U.S. Other hydrogen-based storage solutions are being tested.

Other types of LDES systems expected to be adopted for use include compressed air energy storage and liquid air energy storage. The adoption of these technologies has the same constraints as hydropower, thermal storage, and hydrogen-based options in terms of location suitability challenges and cost constraints. Considerable effort and funding are being deployed to develop new or more cost-effective LDES technologies. By some estimates, the need for LDES in 2040 will be 400 times the present-day level.

Factors Influencing Storage Duration

Like a common household battery, an energy storage system battery has a “duration” of time that it can sustain its power output at maximum use. The capacity of the battery is the total amount of energy it holds and can discharge. An SDES with a duration of 4-6 hours in a home may be used to keep the lights on or the refrigerator cold during an outage. On a broader scale, utility-sized SDES systems may be used to replace wind power on a day with no wind.

Battery Technologies

Different battery chemicals affect the energy storage duration achieved. Lithium-ion storage systems currently dominate the space, reportedly comprising approximately 90% of storage capacity in use in the U.S. The use of other battery technologies is growing as the industry works to mitigate concerns about the limited lithium supply and meet growing demand.

The two other battery technologies being widely utilized are lead and VRFB, but there are factors to consider when selecting the most appropriate battery chemistry for the energy storage need. Both technologies are mature, with lead batteries originating in the 19th century and VRFB technology being developed by NASA over 50 years ago.

Lead batteries are the most sustainable, being composed primarily of recycled materials. They reportedly have a recycle rate of 99%. Lead works best for shorter durations and shallow depths of discharge. Additional battery cells can be linked to increased duration, but they are not designed to be LDES options. They can last decades, depending on usage and maintenance. A lithium battery is only useful for 10–15 years.

VRFBs are ideal for short- or long-duration energy output with very low degradation of components. The flow tanks can easily be expanded to increase duration and allow utility-scale deployments. They last far longer than the other options, with a 20- to 30-year lifecycle being common.

Temperature and Environment

One factor affecting the lifetime of a battery energy storage system is temperature. Batteries in a hot atmosphere (over 90 degrees F) may overheat, which shortens the lifetime of the battery. Conversely, very cold temperatures also shorten the lifetime because the battery has to work harder and operate at a higher voltage to charge successfully.

Lead and lithium are the most sensitive to higher temperatures. Their ideal operating temperature is between 68°F and 90°F, with degraded performance above this range. VRFBs will tolerate higher temperatures, well over 100°F.

Practical Constraints and Innovations

Energy storage systems are designed to be used intermittently along with renewable energy or grid sources. They are not backup generators to be deployed solely during outages. However, they do have constraints to consider, including cyclic life and degradation of effectiveness.

Degradation and “Cycle Life”

All battery-based energy storage systems have a “cyclic life,” or the number of charging and discharging cycles, depending on how much of the battery’s capacity is normally used. The depth of discharge (DoD) indicates the percentage of the battery that was discharged versus its overall capacity. Overcharging or keeping it plugged when fully charged will drain the battery more than if the battery is nearly drained before charging. For example, a battery may have 15,000 cycles at a 10% DoD but only last 3,000 charging cycles at an 80% DoD.

Effective Load Carrying Capability (ELCC) measures the electric production ability when the grid is likely to encounter shortfalls and is a consideration of wind and solar renewable power or energy storage. The ELCC of energy storage is higher than that of renewables since the stored power can be dispatched at any time but is limited by its duration. If the grid has a very high load for eight hours and the storage only has a 6-hour duration, the storage system cannot be at full capacity for eight hours. So, its ELCC and its contribution will only be a fraction of its rated power capacity.

An energy storage system capable of serving long durations could be used for short durations, too. Recharging after a short usage period could ultimately affect the number of full cycles before performance declines.

Likewise, keeping a longer-duration system at a full charge may not make sense. There must be a balance between establishing capacity and scaling when needed. For example, if temperatures are rising and the use of air conditioning is expected to impact grid load, maximizing stored power to reduce the load makes sense. Conversely, with mild temperatures and reduced energy needs, it may not make as much sense to store excess energy.

Advancements in Storage Solutions

Cost constraints are huge challenges for developing new energy storage options. There are emerging technologies being explored that could improve and extend energy storage duration, but long-duration innovations must be tested over long periods while incurring punitive debt financing.

While VRFBs are easily scalable, their high upfront capital costs have also been a deterrent in spite of available energy storage government incentives. In addition, supply chain and material issues have hampered the large-scale adoption of lithium-ion and VRFB energy storage systems. To combat material issues, some flow battery manufacturers are developing integrated vertical business models and entering the raw material supply chain. In the interim, utilities, businesses, and homeowners are capitalizing on the benefits of government incentive programs to boost storage capabilities and embrace renewables now.

While supply chain issues still exist and there are shortages of critical minerals, the growth of SDES is expected to continue. Longer-term energy storage systems that have longer durations are being explored when shorter-term options, such as VRFBs, can be expanded to boost durations.

Demand for energy storage systems is increasing as renewable energy sources come online. While large-scale systems are costly, government incentives make adopting the technology viable, and small residential-scale battery banks in garages or utility-wide storage fields are more affordable.

Determining the right energy storage system for your property and needs isn’t easy. Qmerit is a national leader in deploying energy storage systems, solar arrays, EV chargers, and other tools to allow you to maximize your use of renewable energy and reduce your utility bills. Consult with our network of experienced, highly trained installers to find the best solution to meet your energy storage needs now and in the foreseeable future. Contact Qmerit today to learn more.