Civil Engineering Reference
In-Depth Information
The average annual wave power ranges from 5 to 20 kW/m on the basis
of 8,760 h/year with a wave depth from 30 to 60 m depending on the
distance from the shore and on the position in the ocean. The power of a
single device is generally less than a few hundreds of kW.
The devices that convert ocean thermal energy into electricity can best work
when the temperature difference between the warmer top layer and the colder deep
ocean water (mainly in tropical oceans) is about 20
C (36
F). They must be
installed close to the shore and require a large-diameter intake pipe to bring the cold
water to the surface. Warm water evaporates at low pressure (1) in specially
designed evaporators and then expands in a vapor turbine (2) in a vacuum chamber
where the flash-evaporated water vaporizes a low-boiling-point fluid in a closed
loop that drives a turbine producing electricity (see also ORC cycle, Sect.
9.2
).
Deep cold water is used to condensate the vapor in both devices.
Potential heat recovery and production of freshwater are additional
advantages of the ocean thermal energy conversion. Typical power
ranges from 20 to 30 kW to few MW.
4.7
Renewable Sources: Hydraulic Energy
Hydroelectric power stations are designed to convert the gravitational energy of
water into mechanical and then into electric energy. Both large and small power
stations can be classified as follows: (1) run-of-river power station when no
significant regulating reservoir exists; (2) pondage power station which is run-of-
river with small reservoirs able to delay the production of electricity for a short
period (hours, days); (3) seasonal power station with a reservoir to regulate the
water supply to the turbines; and (4) pumped-storage power station when a reser-
voir is filled exclusively or partially by pumps.
Large power plants, greater than a fewMW, belong exclusively to utilities; small
and mini-power stations down to few hundreds of kW usually belong to single
factories for their own use.
Three main types of turbine are employed in hydroelectric power
stations.
Propeller turbines, such as the Kaplan turbine, are employed for low
heads of not more than 40-50 m (130-160 ft) and high flow rates ranging
between 2 and 40 m
3
/s (70-1,400 ft
3
/s). They provide high rotor velocity
for relatively low water through-flow velocities and may attain a good
(continued)
