Page 25 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

16 Fundamentals of Ocean Renewable Energy

Dinorwig pumped power station in North Wales (UK) can attain full power
output, from standby, in under 20 s. Although this power station can only operate
for 6 h before exhausting its ‘fuel’ supply, this is an excellent renewable example
of dispatchable power. Further, although wind energy is nondispatchable, it does
provide a useful degree of flexibility when the ‘fuel’ is abundant—for example,
under windy conditions, wind turbines can easily be controlled to provide less
than maximum output [9].

1.3.1 Supply vs. Demand
At any point in time, the electricity that is supplied to a grid must equal demand.
Demand for electricity follows a reasonably predictable pattern. For example,
demand is generally greater in winter months when increased electricity is
required for heating, and during office hours when workplaces are lit and heated,
and workers use electrical equipment such as computers. Those tasked with
controlling electricity grids, for example, the National Grid in the UK, forecast
demand using a number of factors such as:

● weather patterns
● time of year
● day of week
● time of day
Typical electricity demand time series for the United Kingdom are shown in
Fig. 1.12 for a year, a month, and a week. There is a clear seasonal trend, with
more demand for electricity during winter months, but considerable variability
within each month, week, and day. For example, demand over the week October
19–25 shown in Fig. 1.12C varies from around 25 GW at around 04:00, and
peaks at over 45 GW at around 18:00 on most week days.
At the scale of a national grid system, demand throughout the day will
fluctuate gradually, because increased demand in one part of the grid (e.g. an
electrical heater switched on in one household) is offset by reduced demand
in another part of the grid (e.g. an electrical oven switched off in another
household). This gradual variation in demand for electricity is known as the
‘diversity of demand’.

1.3.2 Grid Inertia

System inertia is considered to be one of the most important system parameters
upon which the synchronized operation of present day power systems is based
[11]. Synchronization is the process of matching the speed and frequency of
a power source to a running network. An AC generator cannot deliver power
to an electrical grid unless it is running at the same frequency as the network
(i.e. 50 Hz for most grid systems). Presently, the majority of electricity is