station power
(million m3)
energy stored
Ffestiniog 0.36 320–295 1.7 1.3
Cruachan 0.40 365–334 11.3 10
Foyers 0.30 178–172 13.6 6.3
Dinorwig 1.80 542–494 6.7 9.1

using turbines just like the ones in hydroelectric power stations.

Britain has four pumped storage facilities, which can store 30 GWh be-
tween them (table 26.4, figure 26.6). They are typically used to store excess
electricity at night, then return it during the day, especially at moments of
peak demand – a profitable business, as figure 26.5 shows. The Dinorwig
power station – an astonishing cathedral inside a mountain in Snowdonia
– also plays an insurance role: it has enough oomph to restart the national
grid in the event of a major failure. Dinorwig can switch on, from 0 to
1.3 GW power, in 12 seconds.

Dinorwig is the Queen of the four facilities. Let’s review her vital statis-
tics. The total energy that can be stored in Dinorwig is about 9 GWh. Its
upper lake is about 500 m above the lower, and the working volume of 7
million m3 flows at a maximum rate of 390 m3/s, allowing power delivery
at 1.7 GW for 5 hours. The efficiency of this storage system is 75%.

If all four pumped storage stations are switched on simultaneously,
they can produce a power of 2.8 GW. They can switch on extremely fast,
coping with any slew rate that demand-fluctuations or wind-fluctuations
could come up with. However the capacity of 2.8 GW is not enough to
replace 10 GW or 33 GW of wind power if it suddenly went missing. Nor
is the total energy stored (30 GWh) anywhere near the 1200 GWh we are
interested in storing in order to make it through a big lull. Could pumped

Table 26.4. Pumped storage facilities in Britain. The maximum energy storable in today’s pumped storage systems is about 30 GWh.
Figure 26.5. How pumped storage pays for itself. Electricity prices, in £ per MWh, on three days in 2006 and 2007.
Figure 26.6. Llyn Stwlan, the upper reservoir of the Ffestiniog pumped storage scheme in north Wales. Energy stored: 1.3 GWh. Photo by Adrian Pingstone.