1.
Electricity generation
Electricity generation is the process of generating electric energy
from other forms of energy.
The fundamental principles of electricity generation were discovered
during the 1820s and early 1830s by the British scientist Michael
Faraday. His basic method is still used today: electricity is
generated by the movement of a loop of wire, or disc of copper
between the poles of a magnet.
For electric utilities, it is the first process in the delivery of
electricity to consumers. The other processes, electricity
transmission, distribution, and electrical power storage and
recovery using pumped storage methods are normally carried out by
the electric power industry.
Electricity is most often generated at a power station by
electromechanical generators, primarily driven by heat engines
fueled by chemical combustion or nuclear fission but also by other
means such as the kinetic energy of flowing water and wind. There
are many other technologies that can be and are used to generate
electricity such as solar photovoltaics and geothermal power.
Methods of generating electricity
There are seven fundamental methods
of directly transforming other forms of energy into electrical
energy:
•
Static electricity, from the physical separation and
transport of charge (examples: triboelectric effect and lightning)
•
Electromagnetic induction, where an electrical generator,
dynamo or alternator transforms kinetic energy (energy of motion)
into electricity
•
Electrochemistry, the direct transformation of chemical
energy into electricity, as in a battery, fuel cell or nerve impulse
•
Photoelectric effect, the transformation of light into
electrical energy, as in solar cells
•
Thermoelectric effect, direct conversion of temperature
differences to electricity, as in thermocouples, thermopiles, and
Thermionic converters.
•
Piezoelectric effect, from the mechanical strain of
electrically anisotropic molecules or crystals
•
Nuclear transformation, the creation and acceleration of
charged particles (examples: betavoltaics or alpha particle
emission)
Static electricity was the first form discovered and
investigated, and the electrostatic generator is still used even in
modern devices such as the Van de Graaff generator and MHD
generators. Electrons are mechanically separated and transported to
increase their electric potential.
Almost all commercial electrical generation is done
using electromagnetic induction, in which mechanical energy forces
an electrical generator to rotate. There are many different methods
of developing the mechanical energy, including heat engines, hydro,
wind and tidal power.
The direct conversion of nuclear energy to
electricity by beta decay is used only on a small scale. In a
full-size nuclear power plant, the heat of a nuclear reaction is
used to run a heat engine. This drives a generator, which converts
mechanical energy into electricity by magnetic induction.
Most electric generation is driven by heat engines.
The combustion of fossil fuels supplies most of the heat to these
engines, with a significant fraction from nuclear fission and some
from renewable sources. The modern steam turbine (invented by Sir
Charles Parsons in 1884) currently generates about 80 percent of the
electric power in the world using a variety of heat sources.
All turbines are driven by a fluid acting as an
intermediate energy carrier. Many of the heat engines just mentioned
are turbines. Other types of turbines can be driven by wind or
falling water.
Sources
include:
-
Steam - Water is boiled by:
-
Nuclear fission,
-
The burning of fossil fuels (coal, natural gas, or
petroleum). In hot gas (gas turbine), turbines are driven
directly by gases produced by the combustion of natural gas
or oil. Combined cycle gas turbine plants are driven by both
steam and natural gas. They generate power by burning
natural gas in a gas turbine and use residual heat to
generate additional electricity from steam. These plants
offer efficiencies of up to 60%.
-
Renewables. The steam generated by:
-
Biomass
-
The sun as the heat source: solar
parabolic troughs and solar power towers concentrate
sunlight to heat a heat transfer fluid, which is then
used to produce steam.
-
Geothermal power. Either steam under
pressure emerges from the ground and drives a turbine or
hot water evaporates a low boiling liquid to create
vapour to drive a turbine.
-
Ocean thermal energy conversion (OTEC ):
uses the small difference between cooler deep and warmer
surface ocean waters to run a heat engine usually a
turbine.
-
Other renewable sources:
-
Water (hydroelectric) - Turbine blades are acted upon by
flowing water, produced by hydroelectric dams or tidal
forces.
-
Wind - Most wind turbines generate electricity from
naturally occurring wind. Solar updraft towers use wind that
is artificially produced inside the chimney by heating it
with sunlight, and are more properly seen as forms of solar
thermal energy.
Reciprocating engines
Small electricity generators are often powered by
reciprocating engines burning diesel, biogas or natural gas. Diesel
engines are often used for back up generation, usually at low
voltages. However most large power grids also use diesel generators,
originally provided as emergency back up for a specific facility
such as a hospital, to feed power into the grid during certain
circumstances. Biogas is often combusted where it is produced, such
as a landfill or wastewater treatment plant, with a reciprocating
engine or a microturbine, which is a small gas turbine.
Photovoltaic panels
Unlike the solar heat concentrators mentioned above,
photovoltaic panels convert sunlight directly to electricity.
Although sunlight is free and abundant, solar electricity is still
usually more expensive to produce than large-scale mechanically
generated power due to the cost of the panels. Low-efficiency
silicon solar cells have been decreasing in cost and multijunction
cells with close to 30% conversion efficiency are now commercially
available. Over 40% efficiency has been demonstrated in experimental
systems. Until recently, photovoltaics were most commonly used in
remote sites where there is no access to a commercial power grid, or
as a supplemental electricity source for individual homes and
businesses. Recent advances in manufacturing efficiency and
photovoltaic technology, combined with subsidies driven by
environmental concerns, have dramatically accelerated the deployment
of solar panels. Installed capacity is growing by 40% per year led
by increases in Germany, Japan, California and New Jersey.
Other generation methods
Various other technologies have been studied and
developed for power generation. Solid-state generation (without
moving parts) is of particular interest in portable applications.
This area is largely dominated by thermoelectric (TE) devices,
though thermionic (TI) and thermophotovoltaic (TPV) systems have
been developed as well. Typically, TE devices are used at lower
temperatures than TI and TPV systems. Piezoelectric devices are used
for power generation from mechanical strain, particularly in power
harvesting. Betavoltaics are another type of solid-state power
generator which produces electricity from radioactive decay.
Fluid-based magnetohydrodynamic (MHD) power generation has been
studied as a method for extracting electrical power from nuclear
reactors and also from more conventional fuel combustion systems.
Osmotic power finally is another possibility at places where salt
and sweet water merges (e.g. deltas, ...)
Electrochemical electricity generation is also
important in portable and mobile applications. Currently, most
electrochemical power comes from closed electrochemical cells
("batteries"), which are arguably utilized more as storage systems
than generation systems, but open electrochemical systems, known as
fuel cells, have been undergoing a great deal of research and
development in the last few years. Fuel cells can be used to extract
power either from natural fuels or from synthesized fuels (mainly
electrolytic hydrogen) and so can be viewed as either generation
systems or storage systems depending on their use.
2. Electric power transmission
Electric power transmission or "high voltage electric
transmission" is the bulk transfer of electrical energy, from
generating power plants to substations located near to population
centers. This is distinct from the local wiring between high voltage
substations and customers, which is typically referred to as
electricity distribution. Transmission lines, when interconnected
with each other, become high voltage transmission networks. These
are typically referred to as "power grids" or just "the grid".
Historically, transmission and distribution lines
were owned by the same company, but over the last decade or so many
countries have liberalized the electricity market in ways that have
led to the separation of the electricity transmission business from
the distribution business.
Transmission lines mostly use three-phase alternating
current (AC), although single phase AC is sometimes used in railway
electrification systems. High-voltage direct-current (HVDC)
technology is used only for very long distances (typically greater
than 400 miles, or 600 km); submarine power cables (typically longer
than 30 miles, or 50 km); or for connecting two AC networks that are
not synchronized.
Electricity is transmitted at high voltages (110 kV
or above) to reduce the energy lost in long distance transmission.
Power is usually transmitted through overhead power lines.
Underground power transmission has a significantly higher cost and
greater operational limitations but is sometimes used in urban areas
or sensitive locations.
A key limitation in the distribution of electricity
is that, with minor exceptions, electrical energy cannot be stored,
and therefore must be generated as needed. A sophisticated system of
control is therefore required to ensure electric generation very
closely matches the demand. If supply and demand are not in balance,
generation plants and transmission equipment can shut down which, in
the worst cases, can lead to a major regional blackout. To reduce
the risk of failures, electric transmission networks are
interconnected into regional, national or continental wide networks
thereby providing multiple redundant alternate routes for power to
flow should (weather or equipment) failures occur. Much analysis is
done by transmission companies to determine the maximum reliable
capacity of each line which is mostly less than its physical or
thermal limit, to ensure spare capacity is available should there be
any such failure in another part of the network.
3. Electric power distribution
Electricity distribution is the final stage in the
delivery (before retail) of electricity to end users. A distribution
system's network carries electricity from the transmission system
and delivers it to consumers. Typically, the network would include
medium-voltage (less than 50 kV) power lines, electrical substations
and pole-mounted transformers, low-voltage (less than 1 kV)
distribution wiring and sometimes electricity meters.
4. Control
To ensure safe and predictable operation the
components of the transmission system are controlled with
generators, switches, circuit breakers and loads. The voltage,
power, frequency, load factor, and reliability capabilities of the
transmission system are designed to provide cost effective
performance for the customers.
Communications
Operators of long transmission lines require reliable
communications for control of the power grid and, often, associated
generation and distribution facilities. Fault-sensing protective
relays at each end of the line must communicate to monitor the flow
of power into and out of the protected line section so that faulted
conductors or equipment can be quickly de-energized and the balance
of the system restored. Protection of the transmission line from
short circuits and other faults is usually so critical that common
carrier telecommunications are insufficiently reliable, and in
remote areas a common carrier may not be available. Communication
systems associated with a transmission project may use:
•
Microwaves
•
Power line communication
•
Optical fibers
Rarely, and for short distances, a utility will use
pilot-wires strung along the transmission line path. Leased circuits
from common carriers are not preferred since availability is not
under control of the electric power transmission organization.
Transmission lines can also be used to carry data:
this is called power-line carrier, or PLC. PLC signals can be easily
received with a radio for the long wave range.
Optical fibers can be included in the stranded
conductors of a transmission line, in the overhead shield wires.
These cables are known as optical ground wire (OPGW). Sometimes a
standalone cable is used, all-dielectric self-supporting (ADSS)
cable, attached to the transmission line cross arms.
Some jurisdictions, such as Minnesota, prohibit
energy transmission companies from selling surplus communication
bandwidth or acting as a telecommunications common carrier. Where
the regulatory structure permits, the utility can sell capacity in
extra dark fibers to a common carrier, providing another revenue
stream.
