Roundtrip efficiency
Roundtrip efficiency is a key performance metric for an energy storage system that evaluates the energy losses incurred during a complete charging and discharging cycle. It is defined as the ratio of the energy output from the system during discharge to the energy input supplied during charging. A higher round-trip efficiency indicates lower energy losses and maximizes the usable energy stored in the system, which improves overall performance and reduces operational costs.
The efficiency can be expressed as a percentage using the formula:
Maximizing round-trip efficiency is considered essential for the economic viability and sustainability of energy storage systems, particularly for applications in grid stability, renewable energy integration, and peak demand management.
Factors affecting efficiency
The round-trip efficiency of a storage system accounts for losses from multiple sources. These can include:- Conversion inefficiencies
- Heat dissipation
- water electrolysis voltage required for production of hydrogen. The hydrogen production energy linearly depends on the required voltage ;
- efficiency of the power plant that burns the fuel.
Comparison of storage methods
Different energy storage technologies exhibit a wide range of round-trip efficiencies. The technology is often selected based on its intended application, such as providing power quality and distributed power or serving as bulk energy storage.| Storage Technology | Median Efficiency | Efficiency Range |
| Lead-acid battery | ~75% | ~68% – 82% |
| Li-ion battery | ~85% | ~75% – 95% |
| Sodium–sulfur battery | ~65% | ~62% – 70% |
| Flywheel | ~93% | ~90% – 95% |
| Superconductive | ~90% | ~85% – 95% |
| Compressed air | ~52% | ~42% – 72% |
| Thermal energy storage | ~98% | ~97% – 99% |
| Pumped hydro | ~75% | ~65% – 82% |
| Green hydrogen | ~40% | 28 – 52% |
| Green ammonia | 23 – 42% |