The world is facing a climate crisis that requires urgent action to reduce greenhouse gas emissions and limit global warming. The power sector, which accounts for one-third of total carbon emissions, has a central role to play in decarbonizing the economy. However, the transition to renewable energy sources, such as wind and solar, poses new challenges for the stability and reliability of the power grid, as these sources are variable and intermittent.
One solution to this problem is long-duration energy storage (LDES), which can store energy for prolonged periods and release it when needed. LDES can provide system flexibility, which is the ability to absorb and manage fluctuations in demand and supply, and to integrate different forms of energy, such as power, heat, hydrogen, and others. LDES can also help decarbonize industrial heating services, which are responsible for another 20% of global carbon emissions, by supplying zero-carbon electricity and heat.
What is long-duration energy storage?
LDES is a group of technologies that can store energy for hours, days, or even weeks, and can scale up economically to meet the needs of the power system. LDES technologies include mechanical, thermal, electrochemical, and chemical storage, each with different characteristics and applications. Some examples of LDES technologies are:
- Pumped hydro storage (PHS): This is the most mature and widely used LDES technology, which uses excess electricity to pump water from a lower reservoir to a higher one, and then releases it through a turbine to generate electricity when needed. PHS can provide large-scale storage capacity and fast response, but it is limited by geographical and environmental constraints.
- Compressed air energy storage (CAES): This technology compresses air using excess electricity and stores it in underground caverns or tanks, and then expands it through a turbine to generate electricity when needed. CAES can provide medium to large-scale storage capacity and moderate response, but it requires natural gas or other fuels to heat the air during expansion, unless it uses adiabatic or isothermal processes that store the heat as well.
- Liquid air energy storage (LAES): This technology liquefies air using excess electricity and stores it in insulated tanks, and then vaporizes it through a turbine to generate electricity when needed. LAES can provide medium to large-scale storage capacity and moderate response, and it can also capture waste heat or cold from other sources to improve efficiency and performance.
- Hydrogen energy storage (HES): This technology uses excess electricity to split water into hydrogen and oxygen through electrolysis, and then stores the hydrogen in tanks or pipelines, and then either uses it as a fuel for various applications, such as transport, industry, or heating, or converts it back to electricity through fuel cells or turbines when needed. HES can provide large-scale storage capacity and long-term storage, but it requires high capital and operational costs, and faces technical and regulatory barriers.
- Thermochemical energy storage (TCES): This technology uses excess electricity to heat a metal oxide material up to high temperatures, triggering a chemical reaction that releases oxygen and stores heat in the form of chemical energy, and then reverses the reaction by introducing air, which consumes oxygen and releases heat, which can be used for industrial processes or power generation. TCES can provide medium to large-scale storage capacity and long-term storage, and it can use abundant and low-cost materials, such as iron oxide or manganese oxide.
- Flow batteries: These are electrochemical devices that store energy in liquid electrolytes, which are pumped through a cell stack to produce electricity when needed. Flow batteries can provide small to medium-scale storage capacity and long-term storage, and they can vary their power and energy independently by adjusting the size of the cell stack and the volume of the electrolytes. Flow batteries can use different chemistries, such as vanadium, zinc-bromine, or organic compounds.
The table below summarizes the main features and applications of some LDES technologies.
| Technology | Power range | Energy range | Discharge duration | Response time | Round-trip efficiency | Levelized cost of storage | Main applications |
|---|---|---|---|---|---|---|---|
| PHS | 100 MW – 1 GW | 1 – 10 GWh | 4 – 12 hours | Seconds | 70 – 85% | 100 – 200 $/MWh | Grid balancing, frequency regulation, peak shaving, black start |
| CAES | 10 – 100 MW | 100 – 500 MWh | 4 – 12 hours | Minutes | 40 – 60% | 150 – 200 $/MWh | Grid balancing, peak shaving, load following, renewable integration |
| LAES | 10 – 100 MW | 100 – 500 MWh | 4 – 12 hours | Minutes | 50 – 70% | 150 – 200 $/MWh | Grid balancing, peak shaving, load following, renewable integration, waste heat/cold utilization |
| HES | 1 – 100 MW | 10 – 1000 MWh | 12 – 168 hours | Minutes | 30 – 50% | 200 – 300 $/MWh | Grid balancing, peak shaving, load following, renewable integration, sector coupling |
| TCES | 10 – 100 MW | 100 – 500 MWh | 12 – 168 hours | Minutes | 60 – 80% | 100 – 150 $/MWh | Grid balancing, peak shaving, load following, renewable integration, industrial heat electrification |
| Flow batteries | 1 – 10 MW | 10 – 100 MWh | 4 – 12 hours | Seconds | 70 – 85% | 150 – 250 $/MWh | Grid balancing, peak shaving, load following, renewable integration, microgrids |
How can long-duration energy storage enable zero-carbon power and heat?
LDES can enable zero-carbon power and heat by providing system flexibility and sector coupling, which are essential for integrating high shares of renewable energy sources and decarbonizing hard-to-abate sectors. Some of the benefits of LDES are:
- Balancing supply and demand: LDES can store excess renewable energy when supply exceeds demand, and release it when demand exceeds supply, thus smoothing out the variability and intermittency of wind and solar power. LDES can also provide ancillary services, such as frequency and voltage regulation, to maintain the stability and quality of the power grid.
- Shifting peak load: LDES can reduce the need for peaking plants, which are usually fossil-fuel-based and expensive to operate, by storing energy during off-peak periods and releasing it during peak periods, thus lowering the peak demand and the electricity price. LDES can also defer or avoid costly grid upgrades by relieving congestion and increasing the utilization of existing transmission and distribution lines.
- Enhancing resilience: LDES can improve the reliability and security of the power system by providing backup power and black start capability in case of grid outages or emergencies. LDES can also support the development of microgrids and distributed energy resources, which can operate independently or in parallel with the main grid, and provide local power and heat to communities, especially in remote or rural areas.
- Electrifying industrial heat: LDES can help decarbonize industrial heating services, which account for about 20% of global carbon emissions, by supplying zero-carbon electricity and heat. LDES can use renewable electricity to heat a storage medium, such as metal oxides, molten salts, or phase change materials, and then deliver the heat to industrial processes, such as cement, steel, or chemicals production, at temperatures ranging from 100°C to 1500°C. LDES can also use waste heat from industrial processes to improve the efficiency and performance of the storage system.
- Coupling different sectors: LDES can enable the integration and optimization of different energy sectors, such as power, heat, hydrogen, and others, by storing and converting energy across different forms and vectors. For example, LDES can use renewable electricity to produce hydrogen, which can be used as a fuel for transport, industry, or heating, or converted back to electricity when needed. LDES can also use renewable electricity to produce heat, which can be used for district heating, cooling, or hot water, or converted back to electricity when needed.
What are the challenges and opportunities for long-duration energy storage?
LDES is a promising solution for enabling zero-carbon power and heat, but it also faces several challenges and barriers, such as:
- Technical maturity and innovation: LDES technologies are at different stages of development and deployment, and some of them are still in the early stages of demonstration and commercialization. LDES technologies require further research and innovation to improve their performance, efficiency, durability, and safety, and to reduce their costs, environmental impacts, and resource requirements.
- Market design and regulation: LDES technologies are not adequately recognized and rewarded for the value and services they provide to the power system and the society. LDES technologies face unfair competition from subsidized fossil fuels and other conventional technologies, and they lack clear and consistent policies and regulations that support their development and deployment. LDES technologies need a level playing field and a stable and predictable market environment that reflects their true costs and benefits, and that enables their participation and integration in the energy markets.
- Social acceptance and awareness: LDES technologies are often unfamiliar and misunderstood by the public and the stakeholders, and they may face opposition or resistance due to perceived or real impacts on the environment, the landscape, the communities, and the interests of different groups. LDES technologies need to engage and communicate with the public and the stakeholders, and to address their concerns and expectations, and to demonstrate their value and potential for enabling zero-carbon power and heat.
Despite these challenges, LDES technologies also have significant opportunities and potential, such as:
- Cost reduction and competitiveness: LDES technologies are expected to experience significant cost reductions and performance improvements in the coming years, as a result of technological innovation, economies of scale, learning effects, and market competition. LDES technologies can become more competitive and attractive for investors and customers, and can capture a larger share of the energy storage market, which is projected to grow exponentially in the future.
- Diversification and complementarity: LDES technologies can offer a diverse and complementary portfolio of solutions that can meet different needs and applications of the power system and the society. LDES technologies can also synergize and cooperate with other energy technologies, such as short-duration storage, demand response, smart grids, and others, to optimize the overall system performance and efficiency, and to enhance the resilience and security of the energy supply.
- Innovation and leadership: LDES technologies can foster innovation and leadership in the energy sector, by creating new business models, services, and products, and by opening new markets and opportunities. LDES technologies can also contribute to the advancement of science and technology, and to the creation of jobs and value, and to the achievement of the sustainable development goals.
