What is solar energy storage?

Solar energy is one of the most abundant and clean sources of renewable energy on the planet. However, it also has a major drawback: it is intermittent and variable. This means that the amount of solar energy available at any given time depends on factors such as the weather, the time of day, and the season. Therefore, solar energy alone cannot meet the constant and reliable demand for electricity in modern society.

This is where solar energy storage comes in. Solar energy storage is the process of capturing and storing excess solar energy when it is available, and releasing it when it is needed. By doing so, solar energy storage can overcome the limitations of solar energy and make it more compatible with the grid and the consumers.

Types of solar energy storage

Solar energy storage can be classified into four main types, based on the form of energy that is stored: pumped hydro, batteries, thermal, and mechanical. Each type of storage has its own characteristics, advantages, and disadvantages, which determine its suitability for different applications and scenarios.

Pumped hydro

Pumped hydro is the most widely used and mature form of energy storage, accounting for about 96% of the global energy storage capacity. Pumped hydro works by using excess electricity to pump water from a lower reservoir to a higher reservoir, where it is stored as gravitational potential energy. When electricity is needed, the water is released from the higher reservoir and flows through a turbine, which converts the potential energy back to electricity.

Pumped hydro offers several advantages:

1. Abundant Energy and Power Capacity: Pumped hydro is capable of storing and delivering vast amounts of energy and power, ranging from hundreds of megawatts (MW) to several gigawatts (GW), over several hours or even days.

2. High Efficiency: With round-trip efficiencies of 70-85%, pumped hydro ensures minimal energy loss, making it highly efficient in energy storage and conversion processes.

3. Longevity: Pumped hydro facilities boast decades-long lifespans with minimal degradation and maintenance costs.

4. Cost-effectiveness: Particularly in large-scale and long-duration applications, pumped hydro stands out for its low operating and levelized costs.

However, there are also drawbacks to consider:

1. High Initial Investment: Building the necessary infrastructure such as dams, reservoirs, pipes, and turbines requires substantial upfront capital.

2. Limited Site Availability: Finding suitable sites with adequate water resources, elevation differences, sufficient land area, and acceptable environmental and social conditions can be challenging.

3. Extended Construction Time: The construction and commissioning of pumped hydro facilities typically span several years or more, leading to long lead times.

4. Environmental Impact: Pumped hydro projects can have adverse effects on the environment, including water loss, habitat disruption, land use implications, and greenhouse gas emissions.

Some examples of pumped hydro projects that are integrated with solar energy are:

• The Abengoa Solar Complex in Spain, which consists of two solar thermal power plants and a 140 MW pumped hydro plant that can store up to 7.5 hours of energy.
• The Kidston Solar Project in Australia, which is a 250 MW solar photovoltaic (PV) plant and a 250 MW pumped hydro plant that can store up to 8 hours of energy.
• The Helios Power Project in Chile, which is a 600 MW solar PV plant and a 300 MW pumped hydro plant that can store up to 13 hours of energy.

Batteries

Batteries are the most popular and versatile form of energy storage, especially for solar energy. Batteries work by storing electrical energy in the form of chemical energy, which can be converted back to electrical energy when needed. Batteries can be classified into different types, based on the chemistry and configuration of the electrodes and electrolytes, such as lead-acid, lithium-ion, sodium-sulfur, flow, and others.

Advantages

• High power density: Batteries can store and deliver high power, ranging from kilowatts (kW) to megawatts (MW), for short to medium durations, typically minutes to hours.
• High efficiency: Batteries can achieve round-trip efficiencies of 80-95%, meaning that only 5-20% of the energy is lost during the storage and conversion process.
• High flexibility: Batteries can be easily installed and integrated with solar energy systems, either on-grid or off-grid, at various scales and locations, such as residential, commercial, industrial, and utility.
• High functionality: Batteries can provide multiple services and benefits for the grid and the consumers, such as peak shaving, frequency regulation, voltage support, backup power, and demand response.

Drawbacks

• Low energy density: Batteries can store and deliver low energy, ranging from kilowatt-hours (kWh) to megawatt-hours (MWh), for short to medium durations, typically minutes to hours.
• High degradation: Batteries degrade over time and use, losing their capacity and performance, and requiring replacement or recycling.
• High cost: Batteries have high capital and operating costs, especially for large-scale and long-duration applications.
• Environmental impact: Batteries can have negative impacts on the environment, such as resource depletion, pollution, and safety hazards.

Some examples of battery projects that are integrated with solar energy are:

• The Hornsdale Power Reserve in Australia, which is a 150 MW/194 MWh lithium-ion battery system that is co-located with a 315 MW wind farm and provides grid services and arbitrage.
• The Kauai Island Utility Cooperative in Hawaii, which is a 28 MW/100 MWh lithium-ion battery system that is co-located with a 28 MW solar PV plant and provides peak shaving and renewable integration.
• The Banana Solar Park in Queensland, Australia, which is a 5 MW/20 MWh vanadium redox flow battery system that is co-located with a 6 MW solar PV plant and provides grid stability and reliability.

Thermal

Thermal energy storage is the process of storing thermal energy in the form of heat or cold, which can be used for heating or cooling applications, or converted back to electricity when needed. Thermal energy storage can be classified into three main types, based on the temperature and material of the storage medium: sensible, latent, and thermochemical.

Thermal energy storage advantages:

• High energy capacity: Thermal energy storage can store and deliver large amounts of energy, ranging from megawatt-hours (MWh) to gigawatt-hours (GWh), for medium to long durations, typically hours to days.
• Low cost: Thermal energy storage has low capital and operating costs, especially for medium to high temperature applications.
• Low environmental impact: Thermal energy storage can use natural or recycled materials, such as water, rocks, salt, ice, or phase change materials, which have low or zero emissions and minimal waste.

Thermal energy storage drawbacks:

• Low power density: Thermal energy storage can store and deliver low power, ranging from kilowatts (kW) to megawatts (MW), for medium to long durations, typically hours to days.
• Low efficiency: Thermal energy storage can have low round-trip efficiencies, ranging from 40-80%, meaning that 20-60% of the energy is lost during the storage and conversion process, due to heat losses and parasitic loads.
• High complexity: Thermal energy storage can have high technical and operational complexity, requiring sophisticated design, control, and integration with solar energy systems and other components.

Some examples of thermal energy storage projects that are integrated with solar energy are:

• The Andasol Solar Power Station in Spain, which is a 150 MW solar thermal power plant that uses molten salt as the storage medium, and can store up to 7.5 hours of energy.
• The IceBank system by CALMAC, which is a commercial and industrial cooling system that uses ice as the storage medium, and can store up to 6 hours of cooling capacity.
• The Solana Generating Station in Arizona, USA, which is a 280 MW solar thermal power plant that uses molten salt as the storage medium, and can store up to 6 hours of energy.

Mechanical

Mechanical energy storage is the process of storing mechanical energy in the form of kinetic or potential energy, which can be converted back to electricity when needed. Mechanical energy storage can be classified into three main types, based on the mechanism and device of the storage system: flywheels, compressed air, and hydrogen.

Mechanical energy storage advantages:

• High power density: Mechanical energy storage can store and deliver high power, ranging from kilowatts (kW) to megawatts (MW), for short to medium durations, typically seconds to hours.
• High efficiency: Mechanical energy storage can have high round-trip efficiencies, ranging from 80-95%, meaning that only 5-20% of the energy is lost during the storage and conversion process, depending on the type and design of the system.
• Long lifespan: Mechanical energy storage can have long lifespans, ranging from 10 to 20 years or more, with minimal degradation and maintenance costs.

Mechanical energy storage drawbacks:

• Low energy density: Mechanical energy storage can store and deliver low energy, ranging from kilowatt-hours (kWh) to megawatt-hours (MWh), for short to medium durations, typically seconds to hours.
• High cost: Mechanical energy storage can have high capital and operating costs, especially for large-scale and long-duration applications.
• Environmental impact: Mechanical energy storage can have negative impacts on the environment, such as noise, vibration, land use, and greenhouse gas emissions, depending on the type and design of the system.

Some examples of mechanical energy storage projects that are integrated with solar energy are:

• The Beacon Power flywheel system in New York, USA, which is a 20 MW flywheel system that provides frequency regulation and grid stability services.
• The McIntosh CAES Plant in Alabama, USA, which is a 110 MW compressed air energy storage (CAES) system that is co-located with a natural gas power plant and provides peak shaving and load leveling services.
• The Hydrogen Energy Supply Chain project in Australia and Japan, which is a pilot project that uses solar energy to produce hydrogen from brown coal, and transports it to Japan for power generation and industrial use.

Benefits of solar energy storage

Solar energy storage can provide various benefits for different stakeholders and applications, such as homeowners, businesses, utilities, and society.

Fill in the gaps

Solar energy storage can help fill in the gaps of solar energy by providing electricity when solar energy is not available or sufficient. It can store solar energy during the day and release it at night when solar energy drops to zero. Additionally, solar energy storage can store solar energy during sunny days and release it during cloudy or rainy days when solar energy is reduced or interrupted. This can increase the availability and continuity of solar energy, reducing dependence on other energy sources such as fossil fuels.

Increase energy resilience

Solar energy storage can enhance the energy resilience of consumers by providing backup power and energy security. It can store solar energy on-site and use it to power critical loads during grid outages or emergencies. Moreover, solar energy storage enables consumers to operate independently from the grid or in a microgrid mode during grid congestion or instability. This enhances self-reliance and self-sufficiency, protecting against power interruptions and price fluctuations.

Reduce carbon footprint

Solar energy storage contributes to reducing the carbon footprint of the energy system by increasing the utilization and integration of solar energy. It stores and delivers clean and renewable solar energy, displacing the use of fossil fuels, the primary source of greenhouse gas emissions and climate change. Additionally, solar energy storage reduces emissions associated with electricity transmission and distribution by enabling local and distributed generation and consumption of solar energy. This contributes to mitigating global warming and achieving environmental and sustainability goals.

Save money on electric bills

Solar energy storage optimizes the use and value of solar energy, leading to savings on consumers' electric bills. It can store and use solar energy to reduce net electricity consumption from the grid, lowering energy charges. Moreover, solar energy storage can store and sell solar energy to the grid or other consumers, earning revenue from feed-in tariffs, net metering, or demand response programs. Additionally, it can store and buy solar energy at different times, capitalizing on time-of-use pricing, peak shaving, or arbitrage opportunities. This increases the return on investment and shortens the payback period of the solar energy system.

Conclusion

Solar energy storage is a key enabler and enhancer of solar energy, which can overcome its limitations and increase its performance and value. Solar energy storage can provide various benefits for different stakeholders and applications, such as balancing electric loads, filling in the gaps, increasing energy resilience, reducing carbon footprint, and saving money on electric bills. Solar energy storage can also overcome various challenges and barriers, such as technical, economic, regulatory, and social factors, by leveraging various opportunities and potentials, such as new technologies, markets, policies, and behaviors.