Tidal Current Energy

Hydroelectric Power
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tremendously. These include technical variables, specific investment costs, total
production costs, peak load production possibilities and external effects. For
example, there is a very big difference between small-scale hydro plants and the
Three Gorges mega-dam in China, between run-of-river schemes and pumped
storage schemes, between schemes in the rainforest in South America and arid
areas in the Near East or schemes in the Swiss Alps. Sometimes hydropower
schemes are categorized by the size of installed capacity and divided into micro
( � 100 kW), mini ( � 1 MW), small ( � 10 MW) and large ( � 10 MW) hydropower
plants [7] . However, these definitions are relative and vary from country to country.
This strong heterogeneity of hydropower schemes complicates the definition
and characterization of hydropower [8] . In contrast to other electricity-producing
technologies, where the plants are much more homogeneous, varying mostly in
block size, hydropower technology is very much more complicated [9] .
2 .
Technology
The global water cycle, driven by the sun, is the renewable resource for hydropower.
Basically, water’s potential (or kinetic) energy is converted into electricity
using water turbines and electric generators. Details can be found in Ref. [10] .
As a very mature technology, hydropower has not undergone any fundamental
changes over the past 100 years. However, technical details are continually being
improved, turbines are becoming more efficient, and the setting of water inflow,
tunnels and reservoirs is reaching perfection. The overall efficiency of hydropower
ranges from 75% to 90%, which is the highest efficiency reached by any kind of
electricity generation technology. For example, modern gas turbines achieve 57%,
coal plants 37–42%, nuclear plants 40%, photovoltaics 5–15% and wind turbines
19–33% [10] . It is not only the efficiency of hydropower that often makes it the
best option of all the electricity-producing technologies. A comparison of payback
ratios also points to the superiority of hydropower. The energy payback ratio is
defined as the ratio of the sum of energy generated over the life span and the
sum of energy used for construction, operation and decommissioning of a power
plant [10] . While all the other technologies reach only factors of between 9 and 39,
the energy payback ratio for hydropower is around a factor of 200, making it the
most effective technology for electricity production [11] . Figure 11.3 depicts the
energy payback ratios of various energy technologies.
In the last century, hydropower technology has not undergone dramatic changes
and basically only two types of scheme exist. These two, run-of-river and storage
schemes, differ in the ability to store a substantial part of the annually available
water in a reservoir. The continuous discharge of the incoming water is a characteristic
of run-of-river schemes. They have no storage capacity and the head pond
usually has the same height above sea level all the year round. On the other hand,
storage schemes are characterized by reservoirs enabling them to shift the use of
the water inflow from times of low electricity demand to times of high electricity
demand. These plants are able to shut down their engines on an hourly or
daily profile, with electricity generation timed for peak demand only. All types of