The technologies for various energy storage systems (ESSs) are rapidly evolving, with ongoing research aimed at creating efficient and reliable solutions. These technologies play a crucial role in various industries such as manufacturing, services, renewable energy and portable electronics. Currently, the focus is on enhancing energy storage capacities to ensure reliable and cost-effective power system operations. Key trends in ESS include the shift from conventional lithium-ion (Li-ion) batteries to innovative chemicals offering improved stability, density and longevity. There is also a focus on developing solutions that store renewable energy from large-scale projects. Meanwhile, a transition towards more flexible and portable distributed ESS is being witnessed. ESS technologies encompass a range of methods for capturing, storing and releasing energy to meet demand when needed. REGlobal provides an overview of the key technologies explored in IIT Roorkee’s report titled “Study on Advanced Grid-scale Energy Storage Technologies”…

Pumped storage hydropower

Pumped storage hydropower (PSH) is a well-established grid-scale energy storage technology capable of providing storage capacities at the GWh scale. There are three primary types of PSH plants: fixed speed, variable speed and ternary PSH with hydraulic short circuit. Fixed-speed PSH plants maintain a constant motor/generator rotation synchronised with the grid frequency, representing the most widely used and mature technology globally. In contrast, variable speed PSH utilises power electronic converters to enable motor/generator operation at speeds independent of the grid frequency, offering advantages in optimising pumping efficiency. Ternary pumped storage, a newer technology, incorporates hydraulic short-circuit concepts to facilitate rapid changes in generation or pumping power levels. One of the significant benefits of PSH for the grid lies in its capability to deliver GW level power for extended durations, exceeding six hours. Operators are well-versed in this technology, particularly fixed-speed PSH, facilitating its seamless integration into grid operations. Additional advantages include natural inertia, voltage support, blackstart capabilities and contributions to short-circuit current. In terms of cost, PSH offers competitive per kW and per kWh levellised costs of storage, with estimates of Rs 35,000 per kW and Rs 3.91 per kWh, respectively (excluding pumping power costs). However, the capital cost may vary depending on specific site requirements and conditions.

Compressed air energy storage

Compressed air energy storage (CAES) represents a grid-scale energy storage technology capable of providing long-duration energy storage at the MW to GW level. It operates by compressing air into large natural salt caverns, subsequently utilising this compressed air to generate electricity through specialised gas turbines. However, a significant challenge facing CAES is the scarcity of suitable caverns for storing compressed air. Unfortunately, such caverns are limited, resulting in minimal global development of CAES projects.

Solid gravity energy storage

Solid gravity energy storage (SGES) operates by converting electrical energy into gravitational potential energy and vice versa, utilising electromechanical equipment to vertically move a heavy object within a gravitational field. The use of high density solids enhances their adaptability to different geographical locations, increases energy density, improves cycle efficiency, and offers better economic feasibility. SGES exhibits grid-supportive characteristics similar to PSH and CAES. Despite its potential, SGES technology is still in the research and development phase, with small-scale pilots being initiated globally. In India, Gravitricity has collaborated with NTPC for a pilot project. However, due to its early deployment stage and the absence of large-scale pilots or projects compared to pumped storage projects, many countries lacks specific policies or manufacturing incentives for SGES.

Flywheel energy storage

Flywheel energy storage operates based on the principle of momentum and energy conservation. When a flywheel reaches its rated speed and spins in a vacuum, its momentum remains constant (in ideal conditions). As this energy is converted into electrical energy, the conservation of energy causes kinetic energy to transform into electrical energy. Advanced flywheel energy storage systems offer high energy density and efficiency with minimal losses, making them suitable for short to medium duration applications. These flywheels rotate at high speeds ranging from 10,000 rpm to 100,000 rpm. Currently, grid-scale flywheel energy storage systems are mainly utilised in e-mobility and isolated grids. In larger grids, the availability of inertia from the rotating masses of coal and thermal power plants is expected to suffice for meeting grid requirements.

Batteries

Among the battery technologies available, such as Li-ion batteries, vanadium redox flow batteries, molten-sodium batteries and lead-acid batteries, Li-ion batteries and vanadium redox flow batteries (VRFBs) are considered the most suitable options for grid-scale storage applications due to their cost-effectiveness, performance, cycle life and technological maturity. Both Li-ion and VRF batteries have been deployed in grid-scale battery energy storage system installations, with a few hundred megawatts of capacities. These technologies play a crucial role in balancing the grid by streamlining renewable energy generation, providing grid backup, enhancing resilience, facilitating load shifting and offering ancillary services, ensuring a consistent energy supply. The deployment strategy for battery-integrated grid-scale storage should take into account various factors such as grid requirements, levellised cost analysis, reliability, environmental impact, battery pack design, safety aspects, maintenance and integration with the grid. However, there are challenges related to the scarcity of lithium and cobalt resources in many countries. Although large lithium reserves have been discovered in various regions, the production of other raw materials is limited here. The current cost of Li-ion batteries ranges from Rs 13,600 per kWh to Rs 20,000 per kWh, while VRFBs cost Rs 24,000-Rs 32,000 per kWh. However, projections by organisations like BNEF and NITI Aayog suggest that Li-ion battery costs may decrease to around Rs 5,500 per kWh by 2030. Similarly, the cost of vanadium flow batteries is expected to reduce to approximately Rs 8,700 per kWh by 2030, as per predictions by the IRENA report.

PEM fuel cell and electrolysers

Hydrogen holds the promise for reducing carbon emissions in energy generation when combined with renewable energy sources. Fuel cell technology is well developed and commercially viable. Polymer electrolyte membrane electrolysis technology is in the engineering phase, while alkaline electrolysers are already operational. These technologies can contribute to the grid in two ways: “power-to-power” mode, where hydrogen produced during off-peak hours using renewable energy can be converted back into electricity during peak hours to enhance reliability.

Alternatively, the “power-to-gas” mode can utilise hydrogen generated through water electrolysis using surplus renewable energy for various industries like cement and ammonia production. Factors such as raw material processing, component manufacturing and end-of-life recovery are integral to the supply chain of these technologies. Currently, the capital cost of an electrolyser is approximately Rs 78,375 per kW, with hydrogen priced at Rs 330-Rs 453 per kg. However, with large-scale adoption (capacities exceeding 50 GW), these costs could decrease significantly to around Rs 14,025 per kW, potentially resulting in hydrogen production costs of less than Rs 82.5 per kg.

Thermal energy storage

Research institutions are exploring innovative technologies such as high melting phase change materials and supercritical CO2 power cycles on a laboratory scale to integrate thermal energy storage (TES) for grid-integrated applications. The levellised cost of electricity (LCoE) for CSP plants without TES ranges from Rs 11.20 per kWh to Rs 19.03 per kWh. However, implementing an efficient TES system can significantly reduce the LCoE of CSP plants.

Outlook

The analysis of ESS technologies indicates that certain technologies are better suited for large-scale deployment. Factors such as the availability of competing technologies, high costs and the absence of a suitable market scenario influence this suitability. For instance, while flywheel energy storage can serve traction applications and isolated grids by providing synthetic inertia, larger grids may benefit more from rotating machines like synchronous condensers due to their ability to deliver high power, natural inertia and short circuit strength. However, the costs associated with these alternatives are considerably high, primarily due to the use of composite materials in rotor manufacturing and magnetic bearing systems, which are currently not produced in many countries. Overall, based on the report’s analysis, PSH, CAES and TES are expected to offer more favourable economic prospects.