Roundtrip efficiency

Roundtrip efficiency is a key performance metric for an energy storage system (ESS) that evaluates the energy losses incurred during a complete charging and discharging cycle.^[1] 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.^[1]

The efficiency can be expressed as a percentage using the formula:

${\displaystyle {{\text{Roundtrip Efficiency}}(\%)}={\frac {\text{Energy Out}}{\text{Energy In}}}\times 100}$

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.^[1]

Factors affecting efficiency

The round-trip efficiency of a storage system accounts for losses from multiple sources. These can include:^[1]

• Conversion inefficiencies
• Heat dissipation

For the green ammonia and green ammonia the main factors are:

• water electrolysis voltage required for production of hydrogen (the energy required for ammonia synthesis is relatively small). The hydrogen production energy linearly depends on the required voltage (that in turn depends on the catalyst used in anode and cathode);^[2]
• efficiency of the power plant that burns the fuel (combined cycle gas turbine provides the highest efficiency of 64% assumed for high-end estimates).^[2]

Achieving high efficiency requires careful selection of energy storage technologies, optimization of system components, and the use of advanced control strategies to minimize energy losses.^[1]

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.^[3]

Comparison of Round-trip Efficiency for Energy Storage Methods^[4]

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%
TES Thermal	~98%	~97% – 99%
Pumped hydro	~75%	~65% – 82%
Green hydrogen	~40%[5]	28 – 52%[6]
Green ammonia		23 – 42%[6]