Energy Storage on the Grid

The idea of using energy storage to increase the efficiency of power systems is almost as old as the industry itself. There was a burst of activity in the area of electricity storage in 1880's, especially in Britain that took a significant lead over the United States in this area. The first application of large-scale energy storage in the US occurred in 1929 when pumped hydroelectric power plan was placed in service. Pumped hydro which involves pumping water from lower elevation to a higher elevation and using this to generate electricity at the time of peak demand is still the most widely used storage mechanism for electricity and are used to manage grid frequency and provide reserve generation capacity. Other common forms of bulk storage are -

• Compressed  Air Energy Storage (CAES)
• Batteries
• Sodium Sulphide (NaS)
• Nickel-Cadmium 
• Li-ion (Cobalt Oxide or Phosphate) 
• Vanadium Redox
• Zinc Bromine
• Lead-Acid
• Flywheels
• Superconducting Magnetic Energy 
• Electrochemical Capacitors

The following table shows the breakdown of major bulk storage technologies in the grid presently. 

Storage in the Grid

Storage Type	US (MWs)	Rest of the World (MWs)
Pumped Hydro	22,000	88,000
CAES	110	367
NAS	8	250
NiCad	26	0
Other	10	10

With the advent of Electric Vehicles (EVs), a significant amount of distributed storage is expected to come online in the next few years. Optimal control and management of these EVs offers a significant opportunity to increase the of efficiency of the electric grid.

Storage Model

The key physical attributes that differentiate the various energy storage technologies and therefore determine which applications they are most suited for are -

• Storage Capacity [MWh] - This is the total energy that can be stored in the system. 
• Power Rating [MW] - The maximum power output (or input) capacity. This is the rate of charging (or discharging).
• Efficiency - The ratio of energy discharged by the system to energy input to the system to charge it. Conversion Efficiency refers to the losses experienced when charging the system. Storage Efficiency refers to the 'leakage' losses during the time energy is stored. 
• Reaction Time - The time needed to "turn on" the system and begin charging or discharging or switch between the charging/discharging modes.
• Cost - Cost has many components including fixed cost of installing the battery and the cost to charge or discharge the battery. Energy density is an important factor in determining the cost as it has an influence on the real-estate costs. Note that the cost of discharging the battery may not always be negligible. For example, in a CAES system, natural gas might be needed to run the turbine for converting compressed air into electricity. 

Economic Benefit of Electricity Storage

There are many economic benefits that storage can provide. These include -

1. Load leveling between off-peak and on-peak times
2. Peak Generation
3. Arbitrage
4. Spinning Reserve
5. System Regulation
6. Deferred cost of Transmission and Distribution upgrades
7. Environmental Impacts