What is needed as a long-term energy store?

December 16, 2025 •

3 min read

According to the Department of Energy, long-duration energy storage (LDES) is defined as systems capable of delivering electricity for 10 or more hours. A long-term energy store is crucial for managing the variable output of renewable energy sources, ensuring a stable and reliable power supply around the clock.

Quick Summary

This article explores the technologies required for effective long-term energy storage, detailing the methods, components, and challenges associated with storing surplus renewable energy. It covers mechanical, thermal, and chemical approaches that are essential for grid stability and decarbonization.

Key Points

Diverse Technologies: A resilient grid requires a mix of technologies, including mechanical (PHES, CAES), thermal (TES), and chemical (Hydrogen) storage, rather than a single solution.
Grid Stability: Long-term energy stores are essential for balancing the intermittency of renewable energy sources like wind and solar, ensuring a reliable power supply over weeks or months.
Scalable Solutions: Technologies like hydrogen stored in geological caverns and PHES are highly scalable, making them suitable for large-scale, utility-level storage needs.
Efficiency Trade-offs: While some technologies offer higher round-trip efficiency (PHES), others, like hydrogen, prioritize very long-term storage capacity over efficiency.
Market and Policy Frameworks: Successful deployment of long-term storage depends on supportive government policies, clear market signals, and innovative business models to incentivize investment.
Geographical Suitability: Several technologies, especially PHES and CAES, have significant geographical constraints, while others like TES and hydrogen offer more locational flexibility.
Cost Considerations: The cost-effectiveness of long-term storage is improving, but high capital costs for large-scale projects and technology-specific expenses remain challenges that require long-term financial planning.

The Core Challenge: Matching Supply and Demand

The fundamental challenge for a modern energy grid increasingly powered by intermittent renewables like solar and wind is matching electricity supply with demand. While short-duration battery storage is effective for managing daily fluctuations, longer-term and seasonal variations require more robust solutions. Long-term energy storage is needed to save excess energy generated during periods of high production (e.g., sunny summer days) for use during periods of high demand or low production (e.g., cloudy winter nights).

Mechanical Long-Term Energy Storage

Mechanical systems convert electrical energy into potential or kinetic energy. Pumped Hydro Energy Storage (PHES) is the most common utility-scale method. It is efficient but geographically limited. Compressed Air Energy Storage (CAES) compresses air for storage. Liquid Air Energy Storage (LAES) liquefies air, offering more locational flexibility.

Thermal Long-Term Energy Storage

Thermal energy storage (TES) stores energy by heating or cooling a medium. This can involve materials like molten salts, sand, or phase-change materials. Thermo-chemical storage is also being explored.

Chemical Long-Term Energy Storage: Hydrogen

Hydrogen storage is important for seasonal energy balancing. Surplus renewable electricity produces hydrogen via electrolysis, which is stored and converted back to electricity using fuel cells or turbines. While less efficient than batteries, it offers vast storage capacity.

Long-Term Energy Storage Technologies Comparison

A comparison of several technologies can be seen in the table below:


Feature	Pumped Hydro (PHES)	Compressed Air (CAES)	Hydrogen Storage	Thermal (TES)
Energy Capacity	Very high (GWh)	High (GWh)	Very high (GWh+)	High (MWh-GWh)
Storage Duration	Long-term (hours to days)	Long-term (hours to days)	Very long-term (weeks to months)	Long-term (hours to seasonal)
Efficiency	High (70-85%)	Medium (40-70%)	Low (30-50%)	Medium-High (up to 90% for sensible)
Geographical Constraints	High (requires specific terrain)	High (requires underground caverns)	Medium (access to caverns)	Low (flexible location)
Maturity	Very Mature (commercial)	Mature (commercial, with advanced variations developing)	Developing (commercialization increasing)	Mature (commercial)
Cost	High CAPEX, Low OPEX	High CAPEX	High (especially conversion components)	Medium-Low (depending on material)
Primary Use Case	Large-scale grid balancing, peak shaving	Grid stability, load shifting	Seasonal energy balancing, decarbonization of heavy industry	District heating, industrial processes

The Role of Software and Market Design

Smart grid management and market mechanisms are important for optimizing long-term energy storage. Supportive regulations and incentives can help attract investment.

Conclusion: A Diverse Portfolio for a Resilient Grid

A mix of technologies is needed for effective long-term energy storage. PHES is an option where geography permits, while hydrogen and thermal storage can address seasonal and industrial needs. Continued development and policy support are key to scaling these solutions for a resilient grid.

What is needed as a long-term energy store: A Checklist of Essentials

A combination of suitable infrastructure and supporting frameworks are needed for long-term energy storage, including:

Mechanical Systems: Like PHES, requiring specific geography and significant capital. CAES needs stable underground formations.
Hydrogen Systems: Utilizing renewable electrolysis and requiring storage infrastructure.
Thermal Systems: Depending on the storage medium and requiring insulation.
Supportive Frameworks: This includes policy, market mechanisms, advanced R&D, smart grid integration, and lifecycle assessment.

What are the key components of a long-term energy store?

Frequently Asked Questions

Short-term storage, typically lasting minutes to a few hours (like most lithium-ion batteries), is for daily grid balancing. Long-term energy storage, lasting from hours to months or seasons, is needed to address extended periods when renewable energy generation is low.

Long-term storage is crucial for integrating high percentages of intermittent renewable energy (wind and solar) into the grid, as it ensures electricity is available even during prolonged periods of low generation.

Pumped Hydro Energy Storage (PHES) is the most mature and widely deployed technology for large-scale, long-term energy storage, having been in use for decades.

Hydrogen storage involves using excess renewable electricity to split water into hydrogen and oxygen via electrolysis. The hydrogen is then stored and can be converted back to electricity when needed using fuel cells or turbines.

Limitations include high capital costs, specific geographical requirements for some technologies (PHES, CAES), lower round-trip efficiency for others (hydrogen), and market/regulatory barriers for investment.

Thermal energy storage (TES) saves energy by heating or cooling a medium, such as water, sand, or molten salts. This heat can be used for district heating, industrial processes, or converted back to electricity.

Promising emerging technologies include advanced thermal energy storage systems like Polar Night Energy's sand battery, green hydrogen from renewable electrolysis, and gravity-based systems.