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Renewable systems and energy storages for hybrid systems 151

in number of cycles is a critical factor, especially for batteries because the efficiency and op-
erability of batteries reduce with age. Many properties determine the life of battery systems
including, operating temperature, state of charge, electricity current levels, cycle depth and
frequency, among others.
7. Depth of discharge (DoD): This is also more critical for battery storage systems. Most bat-
teries suffer gradual degradation when subject to prolonged partial or improper charging/
discharging. DoD gives the amount of capacity discharged with respect to its total capacity.
This has a direct impact on the electrolyte degradation and cycle life of the battery. A battery
with a lower DoD will be capable of delivering higher cycle life.
8. Self-discharge: This refers to the amount of charge lost during inactivity. This is also more
prevalent in batteries. Self-discharges affect the shelf-life of batteries and is directly affected
by factors such as operating temperatures and operating environment.
9. Operating temperatures: Many storage systems like compressed air and superconducting
magnetic storage require specific operating temperatures to operate. Storage systems are
also closely affected by the temperatures prevalent in the operating environment because
heat is directly related to energy and exergy efficiencies.

5 Types of ESS

With the knowledge of the above characteristics of ESS let us explore the various
types of ESS participating in the power grid applications (Fig. 8.1) [5].

1. Pumped Hydro Storage (PHS): PHS is a mature and widely implemented form of energy
storage for power/grid applications. Many water reservoirs have come to be utilized for PHS
projects with the aim of saving energy in times of energy availability by pumping water to

Figure 8.1 Types of energy storage systems [5].
Source: © 2016 A.H. Fathima, K. Palanisamy, Energy storage systems for energy
management of renewables in distributed generation systems, in: L. Mihet-Popa (Ed.), Energy
Management of Distributed Generation Systems, InTech (2016), under CC BY 3.0 license.
Available from: http://dx.doi.org/10.5772/62766.