Page 168 - Hybrid-Renewable Energy Systems in Microgrids

152 Hybrid-Renewable Energy Systems in Microgrids

higher altitudes and then generating energy from it in the reverse process. Advantages of
pumped hydro systems are:
• Largest power and energy rating
• Long lifetime
• High efficiency
• Very small discharge losses.
The major hurdle in this type of storage is the geographical dependence of the site and long
gestation periods. Many ideas are also being proposed and developed for PHS.
2. Compressed Air Energy Storage (CAES): As the name suggests the ESS stores energy in the
form of compressed air in natural underground caverns or constructed air chambers. They
are two types of CAES: Diabetic and Adiabetic. The heat released while compressing the air
is the main reason for loss of efficiency in the ESS and the system is thus Diabetic in nature.
If the heat can be preserved and reused for decompression, then the system is Adiabetic with
much higher efficiencies. Electrical compressors are used to compress air and store it in
either an underground structure. Two CAES units are in operation in Huntorf, Germany and
MacIntosh in Alabama, USA. Advantages of CAES are:
• Largest power and energy rating
• Long lifetime
• Very small discharge losses.
The major hurdle in this is same as in PHS, geographical dependence, large-scale invest-
ments, and mostly suitable for only grid level applications.
3. Flywheel Energy Storage (FES): Flywheels are rotating masses, which store energy in thef
form of kinetic energy. The rotational energy is stored in an accelerated rotor, which is con-
structed as a massive rotating cylinder. During the charging process, the rotor is accelerated
to a very high speed, which can reach from 20,000 to over 50,000 rpm. The energy is stored
in the flywheel by keeping the rotating body at a constant speed. For discharging, the rotor
decelerates and drives the machine as a generator. The advantages of the FESS are
• Excellent cycle stability
• Long life
• Little maintenance
• Cost
• High power density
• High efficiency
The major drawback of the method is that the FES is unsuitable for applications demanding
prolonged energy storage as they lack the ability to hold on to the charge because of break-
ing losses.
4. Supercapacitors/Ultracapcaitors (SC/UC): Large capacitors with electrolyte solution placed
between two solid conductors form the SC. Compared to conventional capacitors, it has much
larger capacitance and energy density and can store/discharge energy in large quantities with-
in a short time up to >100 times its volumetric capacity. The advantages of the SC are
• Unlimited cycle stability
• Extremely high power density
• Fast charging and discharging
• High reliability
• No maintenance
• Long lifetime
• Operation over a wide temperature range.
These advantages make them an attractive option for power quality and electric vehicle
applications. The major drawback of this ESS is similar to flywheels as it also suffers from
self-discharges.