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Battery Storage
A battery storage system comprises the battery, dc/ac converter, charger,
transformer, ac switchgear and a building to house these components.
Battery energy storage systems have several advantages in addition to their
load levelling capability. Because battery systems can be added to in small
increments, they offer a means of matching load growth. Battery storage systems
also have dynamic source benefits because they provide spinning reserve, area
frequency and voltage control, and increased system reliability. Because of
their small sizes and because battery storage systems are environmentally compatible
in virtually any area, they can be located near the loads, with a consequential
reduction in system losses.
A disadvantage of battery storage systems is the high initial cost. Also,
batteries using existing technologies require replacement every 8 to 10 years.
Currently, the only battery available for large energy storage applications
is the lead-acid battery, which uses lead electrodes and a sulphuric acid electrolyte.
Advanced batteries such as the sodium-sulphur and the zinc-bromine battery
are being developed for this application.
Typical round trip (ac to ac) efficiencies are around 72%, made up of battery
round trip (dc to ac) efficiencies of about 78% and power conditioning system
efficiencies of about 94%.
Examples of large battery
installations in operation are:
17
MW, 14.4 MWh in Germany; 21 MW, 14 MWh in
Puerto Rico; and 10 MW, 40 MWh in USA.
Note: A 17 MW, 14.4 MWh system would be able to produce 17 MW of instantaneous
electrical power and provide a total of 14.4 MWh of electrical energy before
requiring a recharge.
Compressed Air Energy Storage
Compressed Air Energy Storage (CAES) is a technology in which energy is stored
in the form of compressed air in an underground cavern. Air is compressed during
off-peak periods, stored in a cavern, and then used on demand during peak periods
to generate power with a turbo-generator system.
A typical CAES unit consists of five basic components:
1. Compressor train (compressor,
inter-coolers and after-cooler);
2. Motor Generator;
3. Turbine expander train
(including expanders and combustors);
4. Recuperator; and
5. Underground cavern
Electricity from the grid powers an electric motor, which drives an air compressor.
The heat generated by the compression process is extracted by inter-stage
cooling and after cooling and stored. Most of the electric energy from the
grid is
therefore stored as the pressure potential energy of the compressed air in
the cavern, with the small amount extracted by the compressor coolers is
stored as heat energy.
When air is extracted from the cavern, it is first preheated in the recuperator.
The recuperator reuses the energy extracted by the compressor coolers. The
heated air is then mixed with small quantities of oil or gas, which is burned
in the combustor. The hot gas from the combustor is expanded in the turbine
to generate electricity.
The combustor and turbine components are identical to those used in a conventional
gas turbine. However, instead of having to utilise some of its output to
compress its air needed for combustion, all the power of the turbine can
be used to
generate electricity (its combustion air has already been compressed and
stored). Less fuel is therefore required to generate the same quantity of
electricity,
resulting in a high thermal efficiency of the energy recovery stage. However,
the overall cycle efficiency would be the ratio of the electrical energy
generated to the total energy input (electrical energy from the grid + fuel
energy).
An important performance parameter for a CAES system is the charging ratio,
which is defined as the ratio of the electrical energy required to charge
the system versus the electrical energy generated during discharge (the number
of kWh input in charging to produce 1 kWh output). A low charging ratio
results
in low off-peak electrical energy requirements during the charging cycle.
Fast start-up is an advantage of CAES. A CAES plant can provide a start-up
time of about 9 minutes for an emergency start, and about 12 minutes under
normal conditions. By comparison, conventional combustion turbine peaking plants
typically require 20 to 30 minutes for a normal start-up.
A significant contributor to the cost of a CAES system is the construction
of the underground cavern. Three types of geological formations used for
compressed air storage are salt dome, aquifer and rock caverns. Two of these
are illustrated
below.
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In addition to the geological formation classifications, there
are two classes of cavern design concepts, constant volume (also called un-compensated)
and constant pressure (also called compensated). In a constant volume cavern,
the air pressure is allowed to drop as air is withdrawn from storage. In a
constant pressure cavern, water from a surface reservoir displaces the compressed
air to maintain a constant pressure in the cavern.
The first commercial scale CAES plant in the world is the 290MW Huntorf, Germany,
plant operated by Nordwest Deutsche Kraftwerke (NDK) since 1978. The Huntorf
plant runs on a daily cycle in which it charges the air storage for 8 hours
and provides generation for 2 hours. The plant has reported high availability
of 86% and a starting reliability of 98%. The Huntorf plant has a salt cavern.
The Alabama Electric Co-operative, Inc, in McIntosh, Alabama built the second
commercial scale CAES plant. This plant has the maximum existing CAES cavern
capacity of around 1.8 million cubic metres. It began operation in 1991 and
provides 110 MW of power generation. The cavern for the McIntosh plant was
mined from a salt dome by dissolving salt with fresh water. The cavern which
is 70m in diameter, 305m tall and 460m below grade, supplies compressed air
supporting generation of 100MW for 26 hours. The CAES plant has a full load
nett plant heat rate of 4819 kJ/kWh (74.7 % thermal efficiency) with a charging
ratio of 1.3.
In addition to the NDK and the McIntosh CAES facilities, a 35MW CAES unit
is under construction in Japan. Israel also has a 100MW CAES unit under construction,
which uses an aquifer cavern for storage.
The Regenerative Fuel Cell Energy Storage System
There are several methods to used chemical energy as the form of energy storage.
One of the most commercially advanced of these is the regenerative fuel cell
technology.
The regenerative fuel cell, (sometimes known as redox flow cell technology)
converts electrical energy into chemical potential energy by 'charging' two
liquid electrolyte solutions. This chemical energy is converted back to electrical
energy on discharge.
Regenerative fuel cell systems store or release electrical energy by means
of a reversible electrochemical reaction between two salt solutions (the electrolytes).
The reaction occurs within an electrochemical cell. The cell has two compartments,
one for each electrolyte, physically separated by an ion-exchange membrane.
In contrast to most types of battery system, the electrolytes flow into and
out of the cell through separate manifolds and are transformed electrochemically
inside the cell.
A commercial
application of this system is the Regenesys™ system. This system
has a high speed of response, supplies real and reactive power and is therefore
suited to many different applications on a power system. The first Regenesys™ system
is expected to be operational in 2002 at Little Barford. It will be used in
conjunction with an adjacent combined cycle gas turbine power station to meet
power requirements. The plant is designed to store 120 MWh of energy and discharge
it at a nominal power rating of 15 MW.
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Electricity Generation
The electricity production process involves, in simple terms, the conversion
of energy from a (primary) energy source to electrical energy.
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There are many
sources of energy that may be used and many types of energy conversion
processes. It is important to distinguish between the primary energy
source and the energy conversion processes because some primary energy sources
can be used in several types of energy conversion processes. Conversely,
some energy conversion processes can be used to convert several different
sources
of primary energy.
Energy Conversion Processes
This section will look in particular at the energy conversion processes. These
processes can be grouped in several ways, but the following grouping is
used here:
Conversion of Rotational Energy in a rotating generator;
A rotating generator is the most common means of generating electricity. The
various methods used to develop the rotational energy are discussed.
Electricity Generation By Conversion of Rotational Energy
Turbines - Steam Turbines ,
Hot Gas Turbines ,
Water Turbines ,
Wind Reciprocating
Engines
This section provides brief discussions on how rotational energy can be
produced, with emphasis on turbines & reciprocating engines.
The Generator
Before turbines are discussed, it is pertinent to give some mention to the
item of equipment fundamental to the conversion of rotating energy into electrical
energy and is the final link in the energy conversion process which commenced
with the energy source - the generator.
The major generator components are the
stator, rotor and frame.
The stator, as the name implies, is the stationary portion
of a generator and consists of a core and windings. The stator winding provides
the generator
output voltage and current and which is connected to the electric power system.
The
rotor of the generator is connected to the turbine, either directly or through
a gearbox. It carries the rotating electric field into which direct
current is introduced to produce the electromagnetic field and which is used
to convert mechanical energy to electrical energy in the stator. The amount
of direct current required is produced by an excitation system.
The generator
frame supports the weight of the stator and rotor and acts as a containment vessel
for the coolant gas, which is usually hydrogen for large
machines.
Rotational Energy
Rotational energy is the kinetic energy possessed by a spinning shaft. The
shaft is made to spin by fluid energy imparted to components attached to
it. In the case of a turbine, the components are blades which are driven
by a fluid which may be air, water, gas or steam. In the case of a reciprocating
engine, the components are pistons and connecting rods driven by internal
combustion forces.
Turbines
The main component of any turbine is the rotor. This is mechanically connected
to the rotor of the generator which produces the electrical power output
from the generator stator. All turbine rotors may be considered to be generically
similar in that they all consist of a shaft with blades attached. The actual
detailed design of the rotor is, however, quite different depending upon
the properties of the fluid which drives it.
The turbine rotors for steam, hot
gas, water and wind turbines are very different with respect to size, blade
shape and materials. For example, the rotor of
a steam turbine has many blades and is much smaller in diameter than the rotor
of a wind turbine which may only have three blades made from a quite different
material. The operating duty is quite different also and depends upon the ease
of starting and stopping the turbine, the time involved in reaching full load
and the life consumed each start.
Reciprocating Engines
Reciprocating engines and their use in electricity generation.
Conversion of Chemical Energy in a Fuel Cell or Battery;
A battery converts chemical energy into electrical energy through an electrochemical
process involving stored materials. Fuel Cells are devices that convert
a fuel to electricity also by electrochemical means.
Conversion of Electromagnetic Radiation (Solar Energy) in a Photo
Voltaic cell (which produces an electrical potential when exposed
to light) or by heating a working fluid in an electricity generating
cycle;
Conversion of Kinetic Energy by the MagnetoHydroDynamic (MHD) process in
which the flow of a conducting plasma through a static magnetic field produces
an
electrical current.
Reference
Web site :
http://www.energy.qld.gov.au/electricity/infosite/index.htm
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