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272 Managing Global Warming

round-trip efficiency (pumping-generation) is usually high, from 75% to 85%.
Pumped storage currently represents 99% of “on-grid” electricity storage [8]. Some
PSH projects can also have natural inflow to the upper reservoir, which will increase
the generation. Energy stored in a PSH is directly proportional to the water volume
stored in the upper reservoir and the elevation difference between reservoirs. Tradi-
tionally, PSH have been used to supply peaking power during the day and allow large
fossil and nuclear plants to run at near-constant load.
Another major advantage of PSHs is their ability to interact with other variable
renewables such as wind and solar power. PSH installations can store excess energy
during periods of high wind or high insolation, and provide back-up reserve, which is
immediately dispatchable during periods when the other variable power sources are
unavailable.

8.3.5 In-stream (hydrokinetic) hydropower plants

In-stream energy can be derived from the movement (kinetic energy) of water in riv-
ers, streams, and canals, and also from tidal flow and ocean currents. This technology
differs from traditional hydropower plants, which rely on the elevation difference
(head) between the intake and outlet. Hydrokinetic devices are placed directly in
the stream of flowing water and energy is extracted with turbines similar to those used
in tidal plants or in the ocean. The major difference to ocean currents is that the river
current is unidirectional. The kinetic energy in the flowing water is converted to
mechanical energy that drives a generator to produce electricity. Because it is powered
by kinetic energy instead of potential energy, it is also known as a “zero-head” turbine.
As such, no dams and/or head differential is necessary for the operation of this device;
the course of a river remains in its natural state. So far, very few of these power plants
have been put in operational use in rivers, but they represent an interesting alternative
for harnessing energy from the large number of streams and canals where traditional
low-head HPPs cannot be used.

8.4 Classification according to size—Small and large
hydro

It has become popular to classify hydropower plants either as “small” or “large,”
depending on installed capacity. There seem to be a belief that “small is beautiful”
and that small hydro is more environmental friendly. In some countries, small hydro
schemes are accepted for development and receive subsidies, while larger hydro
schemes are not accepted. Sometimes one can even hear the argument that “Large
hydro” is not renewable, and should not be included in the group of renewable tech-
nologies. This is of course completely wrong and it is difficult to see the logic behind
such arguments. All hydropower (except PSH) is powered by the hydrological cycle,
which again is powered by solar energy, and completely renewable. What can be dis-
cussed is whether all types of hydropower is sustainable or not. In order to answer this