Long-term storage for green electricity: the current technologies

What are long-term storage systems for green electricity?

Carbon dioxide (CO₂) is a gas that occurs naturally in the air. However, it is also produced by the combustion of fossil fuels, which are widely used in industrialized nations for production processes, transport and other purposes. The problem is that CO₂ is a “greenhouse gas”, meaning that it contributes to global warming and is a major factor in climate change.

So-called renewable energies (REs) from solar and wind power plants are emission-free, i.e. they do not generate any carbon dioxide. So we need to replace natural gas and petroleum with REs as primary energy sources as soon as possible. This applies particularly to electricity generation, but also to heating and cooling, as well as in the transport sector (alternative drive concepts).

But renewable energies rely solely on particular weather conditions. Otherwise, photovoltaic and wind power plants don’t produce electricity. On the other hand, under the right conditions they are able to produce a lot - sometimes more than is needed.

This excess energy could be fed into the power grid when the sky is overcast or there is no wind. But this means there needs to be a way to store it on a large scale so it can be available on demand. Long-duration energy storage (LDES), also known as seasonal storage, can fulfil this function. Research into these systems is currently underway.
 

The advantages and disadvantages of long-term storage systems

Once LDES technologies are fully developed, wind and solar energy will be guaranteed for a longer period of time, regardless of weather and grid conditions: for example, at night when no solar power is available. This means they could contribute to the decarbonisation of fossil industrial plants, because they can cover their electricity needs free from CO₂ around the clock, with renewable energies. An overview of the application of long-term storage:

• Increasing the share of renewable energies in the energy mix.
• Ensuring resilience to unreliable grids (such as isolated or off-grid locations) over long periods of time.
• Enabling power purchase agreements (PPAs) - individually negotiated, long-term electricity supply contracts for renewable electricity around the clock.
• LDES capacities can, in part, dispense with the need for construction of additional power plants or transmission and distribution infrastructure.
• However, long-term energy storage also has some disadvantages and challenges:
• One of the biggest drawbacks of using an energy storage system is the cost. While the prices for solar modules have fallen significantly in recent years, energy storage systems are still relatively expensive. However, they are expected to become cheaper as technology improves and demand increases.
• Energy storage systems require regular maintenance, including monitoring the state of charge. This can be difficult for companies which may not have the time or expertise to perform maintenance work.
• Energy storage systems have a limited lifespan, typically between 5 and 15 years depending on the technology used. For example, when batteries reach the end of their life, they need to be replaced: this can be costly and time-consuming.
• Part of the electricity has to be used for the storage process itself.
   

The long-term storage technologies with potential

For companies to become climate-neutral on a large scale, the capacity of long-term storage must increase significantly. These are some of the technologies showing potential:

• Pump hydropower: Electricity is used to pump water into a reservoir. When water is released from the reservoir, it flows down through a turbine to generate electricity.
• Compressed air: Using electricity, air can be highly compressed and is then stored in underground caves. When power demand is high, the compressed air is released to generate electricity via an expansion turbine generator.
• Flywheels: Electricity is used to accelerate a flywheel (a type of rotor), which conserves the energy as rotational kinetic energy. When energy is needed, the rotational force of the flywheel is used to drive a generator. Some flywheels have magnetic bearings, operate in a vacuum to reduce drag, and can reach speeds of up to 60,000 revolutions per minute.
• Batteries: Similar to conventional rechargeable batteries, very large batteries can store power until it is needed. These systems may use lithium-ion, lead-acid, lithium-iron or other battery technologies.
• Thermal energy storage: Electricity can be used to generate thermal energy, which is then stored. For example, electricity can be used to produce chilled water or ice during times of low demand and later used for cooling during times of peak electricity consumption.
• Power-to-X: This refers to the conversion of electricity into another energy source. One method is hydrogen storage. Here, hydrogen is produced from water and the solar power which is generated. This is then stored until the electricity is needed again.

In addition to these technologies, new technologies are currently being developed, such as flow batteries, supercapacitors and superconducting magnetic energy storage devices.