How Is Solar Energy Stored? Energy Storage and Solar Panels

Solar panels harness the free and renewable energy produced by the sun to generate electricity. While they have many advantages, they face a significant drawback: they're unable to produce electricity without sunlight. Consequently, energy production is reduced and reliability suffers at night or during long periods of poor weather.

Solar storage systems offer a solution to this issue. These systems are connected to solar panels and allow them to store surplus solar energy for future use.

Different storage systems offer advantages in different scenarios.

• Short-term, quick-release storage is needed to maintain a stable energy output through short term fluctuations that can occur such as passing clouds.
• Long-term, high-capacity storage can provide energy during long weather events such as storms.

Why Should We Store Solar Energy?

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The maximum amount of solar energy we can generate (our solar capacity) is increasing globally. The UK’s solar capacity increased from 2.68 GW in 2013 to 15.45 GW in December 2023. As technology continues to advance and our solar panels become more efficient, this figure is likely to continue increasing.

Storing solar energy enables continuous and stable access to electricity, even when sunlight is unavailable. This helps to reduce our dependency on non-renewable energy sources, lowers energy expenses, and provides us with more efficient energy grids.

Storing excess solar energy provides many benefits:

• Using solar energy at night – The biggest drawback of solar panels is that they only generate electricity when they are exposed to sunlight. Without a storage system, they are unable to provide electricity at night, so many residential solar panel systems need to be equipped with battery storage.
• Balancing electricity loads – Without storage, electricity generated from solar energy would need to be used instantly. This is not an efficient use of solar panel systems. By using long-term storage systems, excess solar energy generated when demand is low can be banked for peak-demand. For example, long summer days can generate lots of excess energy that may be stored for stormy weather where cloud coverage limits energy production.
• Stable solar generation – Short-term storage of solar energy helps to maintain a consistent output of electricity during brief disruptions such as passing clouds or maintenance. Energy can be released quickly to provide a back-up, improving the reliability of solar panel systems.
• Reducing carbon footprint – Storing generated solar energy allows us to maximise the amount of renewable energy we use. This helps to reduce the demand for energy generated by fossil fuels, and therefore cuts back on our carbon emissions.

Methods of Storing Solar Energy

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The main methods of solar energy storage can be broken down into three categories: battery storage, thermal storage, and mechanical storage. In each case, solar energy is converted into a different form of energy which can easily be released when needed.

Battery Storage

Most residential solar panel setups use electrochemical storage in the form of batteries. Batteries provide an easily accessible energy supply and don’t require masses of space to install.

Batteries store energy via a chemical reaction. Generated solar energy gets pumped into a battery which forces charged particles called ions= to move from a negative electrode to a positive electrode.

At the same time, electrons move from the negative electrode to the positive electrode via an external circuit. The electrons at the positive terminal represent solar energy that has been converted to chemical potential energy. The electrons stay there until the energy is needed, and the battery is considered ‘charged’.

When the solar energy is needed, the battery is discharged. Electrons flow back from the positive terminal to the negative terminal, generating a current of useable electricity.

Lithium-ion batteries are the most popular choice for residential solar panel setups thanks to their long cycle life and high energy density.

Thermal Storage

In thermal energy storage, energy is stored in a fluid or solid material as heat energy. Examples of these include heating and cooling buildings, industrial processes, and power generation. TES is commonly used in concentrating solar power (CSP) plants, where sunlight is focused onto a receiver to heat the storage fluid. TES on a large scale can be achieved using borehole thermal energy storage (BTES).

Most CSP plants use thermal storage in one of two ways: a two-tank direct system or a two-tank indirect system.

In the direct system, solar energy is stored in the fluid that is used to collect it. This fluid is stored in two tanks, one at a low temperature and one at a high temperature.

Solar energy is used to heat up the fluid in the low temperature tank, which then flows into the insulated high temperature tank for storage. When energy is needed, the fluid in the high temperature tank flows through a heat exchanger where steam is generated. This steam drives a turbine and in turn generates electricity. The fluid then returns to the low temperature tank ready to be reheated.

In the indirect system, different fluids are used for heat-transfer and storage. The principle is the same as for the direct system, but an extra heat exchanger is used to transfer energy between the storage fluid and heat-transfer fluid.

Thermal energy storage (TES) systems can be useful at various scales, from small residential applications to large industrial and utility-scale projects. At the residential level, TES can provide heating and cooling for homes, using systems like hot water tanks or phase-change materials in walls. Commercial buildings can use TES to manage heating, ventilation, and air conditioning (HVAC) demands, reducing peak electricity usage and costs. There is also potential create solar communities using TES technology for seasonal energy storage and release. One example of this is the Drake Landing Solar Community, where 100% of space heating was provided to 52 homes using solar power and BTES during 2015-2016.

Mechanical Storage

Large amounts of solar energy produced by solar farms can be stored using mechanical storage. Mechanical storage uses the potential energy of an object to generate electricity. Example of these systems include flywheels, compressed air, or pumped hydro storage. When the stored energy is needed, these systems convert the mechanical energy back into electricity.

Pumped-storage hydropower (PSH) is an example of mechanical storage that uses water. Generated solar energy is used to pump water uphill into a reservoir during periods where energy demand is low and surplus is high. Once the water is in the reservoir it has potential energy.

When the energy is needed, the water is allowed to flow back downhill. It is directed through a turbine which turns and generates useable electricity.

PSH is not the first choice for storing solar power – implementation requires a specific and difficult to achieve landscape. As solar power becomes more popular and its energy production rises, we might see mechanical storage systems being used to manage this increase effectively, especially in larger communities or solar panel networks.

Further Reading

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