Page 23 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

14 Fundamentals of Ocean Renewable Energy

systems, and George Westinghouse backing the rival AC current. DC current
had the advantage that it transmitted electrical power at the same voltage as
that used by the lamps and motors of the customers. However, this required
the use of large costly distribution wires, and DC current is only suitable when
transmission distances are small (e.g. a service radius of up to a kilometre).
Although the press at the time, with the help of Thomas Edison, instilled fear in
the public relating to the dangers of high-voltage AC current, it had several
advantages over DC systems. AC could be transmitted over long distances
using relatively small wires at high voltage, then the voltage reduced using a
transformer to make the electricity suitable for public consumption. In addition,
AC power stations could be larger and more efficient than their DC counterparts,
and the distribution network was less costly. AC won the war of currents.
Early grid systems were inefficient and fragmented. However, in 1926, the
UK National Grid was formed as a result of the Electricity Supply Act, which
standardized electricity supply, transmitting AC current at 132 kV and 50 Hz.
5
Today, most countries transmit electricity at a frequency of 50 Hz, but the
transmission voltage varies within and between grids (although 132 kV is still
commonly used). However, the main thing that has changed is the scale. Modern
grid systems are many thousands of kilometres in length: the UK grid has a
length of around 25,000 km, and in China the grid is over 500,000 km long
(although this is formed of six separate grids). These complex systems connect
the electricity that is produced by a large number of diverse electricity generators
to consumers.
Most large-scale power generators are thermal power stations, where heat
energy is converted into electrical power. In general, this is achieved by
producing steam, which spins a steam turbine and drives an electrical generator.
The greatest variation between thermal power stations is the fuel source. As
mentioned in Section 1.1, coal is the most common source of fuel for thermal
power stations around the world, followed by natural gas, then nuclear (Fig. 1.5).
Note that in the case of natural gas, power stations generally use a combined
cycle, where a gas turbine is combined with a steam turbine. Other power
stations that are connected to electrical grids include hydroelectric, solar, wind,
and of course, marine.
It is desirable for the electricity that is fed into grid systems to have certain
characteristics. In particular, the electricity that a power station produces should
ideally be:

1. predictable
2. reliable
3. dispatchable

5. Example exceptions are the United States, Canada, Brazil, Mexico, and Saudi Arabia, where the
grid frequency is 60 Hz.