Wind energy storage technologies - Wind Power Generation and Wind Turbine Design - page 665

system fl exibility is maximised. An ideal option to achieve this is by integrating
the three primary sectors within any energy system: the electricity, heat and
transport sectors. HESS, TESS and EVs provide unique opportunities to inte-
grate these three sectors and hence increase the renewable energy penetrations
feasible. As a result it is diffi cult to compare HESS, TESS and EVs to the other
energy storage technologies directly as energy storage is only part of the purpose
of those systems.
4.1 Pumped-hydroelectric energy storage
PHES is the most mature and largest capacity storage technique available. A pump
and turbine have been combined in a single device optimised for this purpose.
PHES consists of two large reservoirs located at different elevations and a number
of pump turbine units (see Fig. 1). During off-peak electrical demand, water is
pumped from the lower reservoir to the higher reservoir where it is stored until it
is needed. When required (i.e. during peak electrical production) the water in the
upper reservoir is released through the turbines, which are connected to generators
that produce electricity. Therefore, during production a PHES facility operates
similarly to a conventional hydroelectric system.
The effi ciency of operational pumped storage facilities is in the region of 50-85%
with more modern units at the upper end. However, variable speed machines are
now being used to improve this. The effi ciency is limited by the effi ciency of the
pump/turbine unit used in the facilities [2]. Until recently, PHES units have always
used fresh water as the storage medium. However, in 1999 a PHES facility using
seawater as the storage medium was constructed [6] (see Fig. 2); corrosion was
Figure 1: Pumped-hydroelectric energy storage layout [ 5 ].