Pumped storage is not really a renewable energy source, but a way of
storing energy. This system uses two reservoirs, one above the turbine
and one below. At times of low electricity demand (for example,
at night when most people are asleep), electricity powers the turbine
to pump water from the lower reservoir up to the higher one. This water
can then be released when electricity is most needed (for example,
when most people come home from work and make tea). Finding ways
to store energy is becoming more and more important as we get more of
our energy from renewable sources, which cannot be switched on and
off when needed.