Page 169 - Hybrid-Renewable Energy Systems in Microgrids

Renewable systems and energy storages for hybrid systems 153

5. Superconducting Magnet Energy Storage (SMEC): It consists of superconductive coil, pow-
er conditioning system, refrigerator, and vacuum where energy is stored in the magnetic
field created by DC current circulating through a superconducting coil. The advantages of
the SC are
• Rapid response
• Instantaneous power availability
• Used in small-scale storage systems.
These systems are still in a developing stage and are hoped to develop into a most sought
after ESS for power grid applications.
6. Battery Energy Storage (BESS): Batteries store energy in electrochemical form in the elec-
trolyte. They have a set of low-voltage/power battery cells connected in parallel and series
to achieve a desired electrical characteristic. Each cell is made up of a liquid, paste, or solid
electrolyte together with anode and cathode, which undergo chemical reactions. The reac-
tion is reversible and let the battery deliver the absorbed energy for discharging. Available
in a wide range of characteristics, they can be adopted for diverse applications from backup
reserves to electric vehicles. The construction of battery is composed of positive and nega-
tive electrodes, which react electrolyte producing a circulating current. The rate and type
of these chemical reactions is deterministic of the energy absorbed or discharged from the
battery. Larger battery systems are formed by interconnecting individual battery cells in the
form of arrays. Battery cells are connected in series to ramp up the desired voltage. Every
battery has an energy rating which gives the energy capacity the battery can hold depending
on mass/volume of electrolyte. The power rating of the battery, however, will depend on the
contact area/ reaction surface area between the electrolyte and electrodes [11,12].
Some of the advantages are
• Rapid response time
• Small self-discharge loss
• Round-trip efficiency is high
• High power and energy densities
• Mature technology
• Ease of commercial availability
Many types of batteries are available with varied characteristics and can be used for
a variety of applications from power grid to electric vehicles.
1. Lead acid: A flooded battery has a positive electrode made of lead dioxide (PbO 2 ) and the
other electrode is a simple lead sponge, both immersed in a solution of sulfuric acid acting
as the electrolyte. These types of batteries have maintenance issues with the liquid electro-
lyte and electrode corrosion. Recent approaches have replaced the liquid electrolytes with
gel-based and valve regulated batteries. Improvement in efficiency can be achieved by
using advanced materials. Advantages include easy installation, low costs, longer reten-
tion of stored charge, and high efficiencies. Disadvantages encountered are maintenance
issues, lower specific power, premature failure if operated with partial discharging and
hazardous.
2. Nickel-based batteries: The cell is made of nickel electrodes and cadmium electrolyte. Re-
cent approaches have replaced cadmium with other materials such as metal hydrides and
zinc oxides due to rising environmental concerns. Advantages include fast discharging
cycles, low cost per cycle, long life, suitable for renewable applications. Disadvantages
encountered are costs, toxicity of cadmium, memory effect, highly exothermic (esp. NiMH),
and continued maintenance. In November 2003, the European Commission drew up a pro-
posal for new directives including recycling targets of 75% for this type of battery.