How Long Can Solar Power Be Stored?
As the world transitions to renewable energy sources like solar power, energy storage becomes increasingly important. Solar power can only be generated when the sun is shining, so storing surplus solar energy allows it to be used at night or on cloudy days. Energy storage also helps stabilize the electrical grid by releasing stored solar power during periods of peak demand. Without storage, solar power’s growth would be constrained by its intermittent availability.
There are several methods for storing solar power, including batteries, pumped hydro storage, compressed air storage, hydrogen production, and thermal storage. The length of time solar power can be stored depends on the storage technology. Key factors include the storage capacity, efficiency, and discharge rate. For example, lithium-ion batteries provide short-term storage of a few hours to days, while pumped hydro facilities can store solar energy for many months.
This article will provide an overview of the different storage methods and examine the capabilities and limitations of existing solar energy storage technologies.
Solar Panels and Direct Use
Solar panels, also known as photovoltaic panels, produce direct current (DC) electricity from sunlight. The photovoltaic cells in the panels convert the energy from photons of light directly into electricity through the photovoltaic effect.
The main limitation of solar panels is that they only produce electricity when the sun is shining. The electricity must be used right away or converted to be stored for later use. Solar panels do not have any built-in storage capacity on their own.
Using the electricity as it is produced is called direct use. This could involve powering appliances, lights, machinery or other electrical loads that are running while the sun is out. Direct use avoids any losses from storing and retrieving the electricity.
However, since solar energy generation follows a day-night cycle, while electricity demand often occurs at all hours, the electricity often cannot be used directly when it is produced. Some form of energy storage is required for solar power to be a primary energy source.
Batteries
Batteries are one of the most common ways to store solar energy for later use. Some key battery technologies used for solar storage include lithium-ion, lead-acid, sodium-sulfur, and flow batteries. The chemistry and materials used in these batteries affects how much energy they can store and for how long.
Lead-acid batteries are the most mature battery technology, but have limitations in storage duration. They can typically discharge over 10-20 hours. Lithium-ion batteries are able to store power for slightly longer, with discharge durations of 6-10 hours.
For even longer energy storage, sodium-sulfur and flow batteries are better suited. Sodium-sulfur batteries use molten electrodes to achieve discharge times of 6-12 hours. Flow batteries pump electrolyte fluids through storage tanks and can discharge over 10-20 hours.
Emerging battery chemistries using alternative materials are enabling longer discharge durations. For example, zinc-hybrid cathode and iron-flow batteries can store and discharge energy over days and even weeks versus just hours.
As battery technologies continue advancing, longer solar energy storage durations are becoming feasible. This better enables power supply to continue overnight and during extended cloudy periods.
Pumped Hydro Storage
Pumped hydro facilities can store energy from solar power by using excess electricity to pump water from a lower reservoir to an upper reservoir. When electricity is needed, the water can be released from the upper reservoir through a turbine to generate electricity (Hydropower.org). This allows solar energy captured during the day to be stored for use at night or during peak demand times.
While pumped hydro requires specific geographic conditions, it currently provides over 90% of energy storage capacity worldwide. Existing facilities have stored energy for periods over 10 hours, while new concepts could extend duration to weeks or months (Blakers, 2021). For example, Gordon Butte Pumped Storage in Montana will store over 35 hours of peak energy output. However, suitable sites with reservoirs and elevation changes are geographically limited.
Compressed Air
Compressed air energy storage (CAES) is a system that compresses ambient air and stores it in an airtight underground cavern or structure. The compressed air is later heated and expanded through a turbine to generate electricity when power demand is high (Renewable Energy World, 2022). Compressed air exists naturally in underground geologic formations and can be used to store energy. This is a capable tool for long-duration energy storage, with many systems holding charge for up to 15 years (PV Magazine, 2023).
The primary advantage of CAES is its ability to store large amounts of energy with minimal losses over long durations, making it suitable for supplying power when renewables like solar are not generating electricity. Unlike batteries which degrade over time, the underground caverns used for CAES can retain the stored energy consistently for many years. This provides a reliable and cost-effective storage solution to supplement renewables and balance the grid (Renewable Energy World, 2022).
Thermal Storage
Thermal storage involves using thermal mass to store excess heat collected by solar thermal collectors. This allows the stored heat to be used at a later time, including overnight or through cloudy weather. Thermal storage extends the usability of solar thermal systems.
One common approach is to use molten salts. Solar heat is used to melt salts, which can retain high temperatures for extended durations when insulated properly. The molten salts are then used as needed to generate steam to drive turbines. This allows solar thermal plants to provide power consistently. Other materials like graphite and concrete can also store thermal energy.
Most thermal storage systems for solar power provide anywhere from 5-15 hours of storage. However, some new designs aim for longer durations. For example, one system using concrete storage aims to store thermal energy for up to a week. Overall, thermal storage allows solar heat to remain usable from hours to days after it is initially collected.[1]
Hydrogen Fuel
Solar energy can be used to produce hydrogen fuel through a process called electrolysis, which uses electricity to split water into hydrogen and oxygen. The hydrogen can then be stored and used as a fuel source when needed. Hydrogen is attractive as an extremely long-term storage method for solar energy. According to the Fuel Cell & Hydrogen Energy Association, “Hydrogen allows vast quantities of clean energy to be stored for long durations for use in peak demand and seasonal energy balancing.”
Hydrogen has a very high energy density by weight, so it can store a lot of energy in a relatively small space. It can be stored in pressurized tanks or geological formations like underground caverns. According to an article from Power Magazine, “hydrogen storage caverns and tanks can retain fuel for months or years with little to no energy losses.” This makes hydrogen capable of providing seasonal storage across an entire winter when solar production is lower.
The downside is that converting solar energy to hydrogen and back is a relatively inefficient process. According to research from IEEE Spectrum, round-trip efficiency rates for hydrogen storage systems are around 20-30%. So a significant amount of the original solar energy is lost. However, the tremendously long storage capabilities may make up for the efficiency challenges in some applications.
Capacity Needs
As more renewable energy like solar and wind come online, there is an increasing need for energy storage capacity to meet clean energy goals and ensure grid reliability. Current projections estimate that the world will need over 1 terawatt-hour (TWh) of energy storage capacity by 2030, compared to just 0.5 TWh today.1 In the United States, approximately 31 gigawatts (GW) of new energy storage will be required by 2025 to support the transition to renewable energy.2
Current battery storage installations are far below what is needed to enable high renewable energy penetration in the coming decades. For example, the entire battery storage capacity installed in the U.S. by the end of 2020 was just over 1 GW.2 Meeting renewable energy and decarbonization goals will require a massive scale up of grid energy storage in the next 5-10 years. This presents technical and economic challenges, but new battery chemistries and storage technologies may help accelerate deployment. Overall, a 5-10x increase in storage capacity is likely needed within this decade.
New Technology
There are several promising new technologies in development for longer duration energy storage for solar power and other renewable sources like wind:
Liquid Air Energy Storage (LAES) – This system uses electricity to cool air until it becomes a liquid, which is then stored in insulated tanks. When electricity is needed, the liquid air is pumped to low pressure and expands back to a gas, running a turbine to generate electricity. LAES can potentially store energy for weeks. Challenges include improving efficiency and reducing costs.
Underground Compressed Air – Air is compressed and pumped into underground geologic formations like old mines or saline aquifers. When electricity is needed, the air is released to power a turbine. This can provide storage for months. Making the process more efficient and finding suitable underground sites are key challenges.
Molten Salt Storage – Molten salt is used to store heat collected from solar thermal plants. This allows thermal energy to be stored for hours or even days. Bringing down costs and scaling up capacity are areas needing improvement.
Flow Batteries – These batteries use liquid electrolytes pumped through a reactor to store chemical potential energy. They can be scaled more easily and discharged over longer periods than lithium-ion batteries. Improving membrane and electrolyte chemistry to increase efficiency and lower costs is a focus.
While great progress has been made, longer duration energy storage technologies still need refinement to increase efficiency, lower costs, and scale up to grid-level capacity. But they show promise for enabling much higher levels of renewable energy in the future.
Conclusion
In summary, current battery storage solutions can provide up to 8 hours of energy storage for solar power. However, longer duration storage of 10-100 hours is needed to fully transition to renewable energy and retire baseload power plants. Promising long duration solutions currently being developed include pumped hydro storage, compressed air storage, thermal storage, hydrogen storage, flow batteries, and alternative chemical battery technologies.
While costs are still high for many emerging long duration storage technologies, continued research, development, and deployment incentives can help improve performance and drive down costs over time. Key areas of focus include developing new battery chemistries optimized for long discharge cycles, integrating thermal storage into concentrated solar plants, leveraging underground geological formations for pumped hydro and compressed air storage, and producing ‘green’ hydrogen from solar power for long-term energy storage.
With sufficient investment and innovation, long duration solar storage can become economical at scale within the next decade. This will enable solar power to reliably meet electricity demand around the clock and across seasons, which is essential for transitioning to a 100% renewable energy future powered by the sun.
