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Hydropower 271

different countries. Classification according to size is administratively simple, but
is—to some degree—arbitrary: concepts like “small” or “large hydro” are not techni-
cally or scientifically rigorous indicators of impacts, economics, or other characteris-
tics [1]. Hydropower projects cover a continuum in scale from units <1kW up to
megaprojects like China’s Three Gorges with 22.5-GW installed capacity
(22,500,000kW) and an annual generation of 93.5 TWh [7].

8.3.2 Run-of-river hydropower plants
An RoR hydropower plant is a plant where little or no water storage is provided; it
generates electricity from the available flow of the river. Such plants may sometimes
include a short-term storage or “pondage,” with some storage capacity ranging from a
few hours up to daily flexibility in adapting generation to the demand profile. The
generation profile will mostly be determined by the natural river flow conditions or
by the release profile from upstream storage if it is part of a cascade. In the absence
of any pondage or upstream reservoirs, generation depends entirely on flow and typ-
ically has substantial daily, monthly, seasonal, and annual variations.

8.3.3 Storage hydropower plants
Storage hydropower plants include a dam and a reservoir to impound water, which is
stored and released later when needed. Water stored in reservoirs provides flexibility
to generate electricity on demand and reduces dependence on the variability of inflow.
Very large reservoirs can store inflow for months or even years, but they are usually
designed for seasonal storage, to supply water during dry seasons. Storage hydro-
power plants are more flexible than RoR plants, and can be operated to provide
baseload power, as well as peakload through its ability to be shut down and start
up again at short notice, depending on the demand in the power system. Given their
ability to control water flows, storage reservoirs are often built as multipurpose sys-
tems, providing additional benefits such as flood control, water supply, irrigation, nav-
igation, and recreation. The primary advantage of hydropower plants with storage is
their ability to store large volumes of energy and respond to variable load require-
ments, from short term (daily peaking) to weekly and seasonal variability. Such res-
ervoirs are becoming increasingly important and valuable also for storing energy from
other renewables, such as wind and solar power.

8.3.4 Pumped-storage hydropower plants
In a pumped-storage hydropower (PSH), water is pumped from a lower reservoir into
an upper reservoir when electricity generation exceeds demand and released back
from the upper reservoir through turbines to generate electricity when demand is
higher than the supply. Both reservoir HPPs and PSPs store potential energy as ele-
vated water for generating on demand. The difference is that PSPs can take energy
from the grid to pump the water up to a higher level, then return the energy back
to the grid later when needed. This cycle can happen several times a day. The