Page 24 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

Introduction Chapter | 1 15

Predictable
The electricity that is generated by thermal power stations is predictable,
because the electricity output can be controlled by altering the fuel supply. This
makes the electricity controllable, but for coal and nuclear (generally known
as ‘base load’ power plants), this controllability cannot be managed over short-
time periods—it will likely take a minimum of several hours to adjust the power
output of a large thermal power station. Therefore, coal and nuclear power
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stations are not dispatchable. By contrast, many renewable power stations are
not predictable, notably wind and solar. These sources of energy depend on
environmental conditions, for example, wind speed or the amount of sunlight
striking a solar panel, and so are intermittent. An exception is tidal power (both
tidal stream and tidal range), which, because tides are governed by astronomical
forces, is extremely predictable (Chapter 3). Note that at relatively low levels of
renewable energy penetration, errors in forecasts are likely to remain within
the parameters that the system is designed to cope with [9]. However, for
higher levels of wind and solar penetration, these forecast errors could become
significant.

Reliable
It is important for an electrical grid system to have a secure level of base load
power—insufficient electricity to meet peak demand, but sufficient to ‘keep the
lights on’. Currently, such continuous base load tends to be supplied by coal
and nuclear power plants. Wind and solar power are weather-dependant, and
hydroelectric can be influenced by droughts; therefore, these forms of renewable
energy are not reliable. Some form of storage would improve the reliability of
renewable energy sources, and tidal lagoons (Chapter 3) present one opportunity
for energy storage.

Dispatchable
Electricity generating plants that can be turned on or off, or which can vary their
power output in response to changes in demand, are known as dispatchable.
Base load power plants such as coal and nuclear are not dispatchable, because
it will take many hours to adjust their electrical output, and so they cannot
closely match demand. However, open cycle gas turbine (OCGT) power plants
are a good example of dispatchable electricity, because the power output of an
OCGT plant can be adjusted within a short-time frame. For example, from
cold start, an OCGT power plant can be fully operational in around 30 min
[10]. It is difficult for renewable sources to be dispatchable, but conventional
hydroelectric schemes are one exception, because it is possible to control the
volume of water that flows through the penstock. In addition, the 1.7 GW

6. By contrast, gas power stations are dispatchable, because their electrical output can be varied at
short timescales to respond to changes in demand.