Tidal Current Energy

Hydroelectric Power
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in mountainous regions and reach head heights of several hundred meters. In
contrast to run-of-river schemes, they use relatively low flows over high heads.
Storage schemes provide peak-load electricity, and as a result electricity generation
is usually spread over about 2000–3000 full-load hours a year.
Pumped storage schemes use pumps between an upper and a lower reservoir
to pump up water after turbination. Because the water is turbinated and
pumped through the same penstocks and therefore over the same head, these
plants use approximately 1.3–1.4 times more energy to pump than they actually
produce. Depending on the size of the reservoirs and the installed capacity, the
circulation of the water takes place on a diurnal, weekly or seasonal basis. As
long as the electricity prices between times of turbination (peak load) and times
of pumping (off-peak) show a ratio larger than 1.3–1.4, these schemes are economically
viable [13] .
The reservoirs of storage and pumped storage schemes, mostly in mountainous
regions, are created through the optimal damming of valleys. Geological, topological
and hydrological factors determine the quality of a site and the dam, which in
turn defines the size of the reservoir and the electricity generation patterns of future
hydropower schemes. Dams are usually classified into two main groups: concrete
and embankment dams. The choice depends on factors such as the geology and
topology of the site, the local climatic and hydrological conditions, and the availability
of construction resources (labor and material), as well as the appearance in
the landscape. Of course, the most important factor is the construction costs, which
reflect the sum of many local factors. Concrete dams can be categorized into arch
dams, buttress dams and gravity dams, while embankment dams can be rock-fill or
earth-fill dams. Dams have life spans of more than 100 years, reflecting the long-term
character of hydropower technology. The dimensions of dams can be enormous,
with crests of up to several hundred meters and dam heights up to 285 meters.
From the reservoirs or head ponds, the water is often diverted through tunnels
and penstocks in order to gain additional hydraulic head. The type of turbine used
depends on the type of plant, the height of the gross head and the site characteristics.
There are basically two types of hydraulic turbine: impulse and reaction turbines.
The former uses only the velocity of the water and is used for high-head,
low-flow applications, while the latter uses the combined effect of pressure and
moving water, and is suited for lower heads and higher flows [14] . Kaplan, Francis
(both reaction turbines) and Pelton (impulse turbine) are the most frequently used
types of turbine. While Kaplan turbines are mainly used in run-of-river plants
with low heads ( � 40 m) but up to 100 MW installed capacity, Francis and Pelton
turbines are used for high-head run-of-river, storage and pumped storage schemes
with high heads and up to hundreds of megawatts of installed capacity. For heads
between 750 and 2000 m, only Pelton turbines can be used which allow for very
fast power regulation [14] . The use of Francis and Pelton turbines overlaps in the
range of 200–750 m of head, while the choice for installation depends on site characteristics
and finally on the energy economic aspects [10] .
Hydropower schemes are often linked to purposes other than electricity production.
This is especially true in arid areas, where hydropower dams are used