For over 25 years (since 1982), the Scottish island of Fair Isle (population 70,
area 5.6 km2) has had two electricity networks that distribute power from
two wind turbines and, if necessary, a diesel-powered electricity generator.
Standard electricity service is provided on one network, and electric heating
is delivered by a second set of cables. The electric heating is mainly
served by excess electricity from the wind-turbines that would otherwise
have had to be dumped. Remote frequency-sensitive programmable relays
control individual water heaters and storage heaters in the individual
buildings of the community. The mains frequency is used to inform heaters
when they may switch on. In fact there are up to six frequency channels
per household, so the system emulates seven grids. Fair Isle also successfully
trialled a kinetic-energy storage system (a flywheel) to store energy
during fluctuations of wind strength on a time-scale of 20 seconds.
If 30 million electric vehicles were willing, in times of national electricity
shortage, to run their chargers in reverse and put power back into the grid,
then, at 2 kW per vehicle, we’d have a potential power source of 60 GW –
similar to the capacity of all the power stations in the country. Even if only
one third of the vehicles were connected and available at one time, they’d
still amount to a potential source of 20 GW of power. If each of those
vehicles made an emergency donation of 2 kWh of energy – corresponding
to perhaps 20% of its battery’s energy-storage capacity – then the total
energy provided by the fleet would be 20 GWh – twice as much as the
energy in the Dinorwig pumped storage facility.
There are lots of ways to store energy, and lots of criteria by which storage
solutions are judged. Figure 26.13 shows three of the most important
criteria: energy density (how much energy is stored per kilogram of storage
system); efficiency (how much energy you get back per unit energy
put in); and lifetime (how many cycles of energy storage can be delivered
before the system needs refurbishing). Other important criteria are: the
maximum rate at which energy can be pumped into or out of the storage
system, often expressed as a power per kg; the duration for which energy
stays stored in the system; and of course the cost and safety of the system.
Figure 26.15 shows a monster flywheel used to supply brief bursts of
power of up to 0.4 GW to power an experimental facility. It weighs 800 t.
Spinning at 225 revolutions per minute, it can store 1000 kWh, and its en-
ergy density is about 1 Wh per kg.