Energy Storage

 Energy Storage

 

The future of energy now lies almost exclusively in the progress to be made in increasing energy storage. Obviously, following such a desirable development, Renewable Energy Sources - e.g. solar energy via photovoltaic systems, etc. - will become advantageous to exploit.

There are many different ways of storing energy, each with its own pros and cons. Tab. 1 below focuses on technologies that can currently provide large storage capacities (at least 20 MW).

 

Large Capacity Energy Storage Technologies (LCEST)

 

	Max Power Rating (MW)	Discharge time	Max cycles or lifetime	Energy density (watt-hour per liter)	Efficiency
Pumped hydro	3,000	4h – 16h	30 – 60 years	0.2 – 2	70 – 85%
Compressed air	1,000	2h – 30h	20 – 40 years	2 – 6	40 – 70%
Molten salt (thermal)	150	hours	30 years	70 – 210	80 – 90%
Li-ion battery	100	1 min – 8h	1,000 – 10,000	200 – 400	85 – 95%
Lead-acid battery	100	1 min – 8h	6 – 40 years	50 – 80	80 – 90%
Flow battery	100	hours	12,000 – 14,000	20 – 70	60 – 85%
Hydrogen	100	mins – week	5 – 30 years	600 (at 200bar)	25 – 45%
Flywheel	20	secs - mins	20,000 – 100,000	20 – 80	70 – 95%

 

But let's look briefly at what each technology presented in Tab. 1 is:

  ·         Pumped storage hydropower

Pumped-storage hydro facilities (PSHF) are large-scale energy storage facilities that use gravity to generate electricity. Water is pumped to higher altitudes for storage during periods of low energy costs and periods of high renewable energy production. When electricity is needed, the water is released back into the lower reservoir, generating power through turbines.

 ·         Compressed air energy storage (CAES)

With compressed air storage, air is pumped into an underground hole, most likely a salt cave, during off-peak hours when electricity is cheaper. When power is needed, the air from the underground cave is released back into the facility, where it is heated and the resulting expansion triggers an electricity generator.

 ·         Thermal (including molten salt)

Thermal energy storage facilities use temperature to store energy. When energy needs to be stored, rocks, salts, water or other materials are heated and kept in insulated environments. When energy needs to be produced, thermal energy is released by pumping cold water over the hot rocks, salts or hot water to produce steam, which drives turbines.

 ·         Lithium-ion batteries

Lithium-ion batteries are by far the most popular battery storage option today and control more than 90% of the global grid battery storage market. Compared to other battery options, lithium-ion batteries have a high energy density and are lightweight. New innovations, such as replacing graphite with silicon to increase battery power capacity, seek to make lithium-ion batteries even more competitive for longer-term storage.

 ·         Lead-acid batteries

Lead-acid batteries were among the first battery technologies used in energy storage. However, they are not popular for grid storage due to their low energy density and short cycle and calendar life. They were commonly used for electric cars, but have recently been largely replaced by longer-life lithium-ion batteries.

·         Flow batteries

Flow batteries are an alternative to lithium-ion batteries. They have a relatively low energy density and a long life cycle, which makes them suitable for providing continuous power.

 ·         Solid state batteries

Solid-state batteries have multiple advantages over lithium-ion batteries for large-scale grid storage. Solid-state batteries contain solid electrolytes which have a higher energy density and are much less prone to fires than liquid electrolytes, such as those found in lithium-ion batteries. The smaller volume and higher safety make solid-state batteries suitable for large-scale grid applications.

 ·         Hydrogen

Hydrogen fuel cells, which produce electricity by combining hydrogen and oxygen, have attractive features: they are reliable and quiet (no moving parts), have a small footprint and high energy density, and are emission-free (when running on pure hydrogen, the only by-product is water). The process can also be reversed, which makes it useful for energy storage: electrolysis of water produces oxygen and hydrogen. Fuel cell plants can therefore produce hydrogen when electricity is cheap, and later use this hydrogen to generate electricity when needed (in most cases, the hydrogen is produced at one site and used at another).

 ·         Flywheels

Flywheels are not suitable for long-term energy storage, but are very effective for equalization and load-shifting applications. Flywheels are known for their long life cycle, high energy density, low maintenance costs and fast response speeds.