Hydroelectric Power Generation - Renewable Energy: Technology, Economics and Environment - page 357

ter hammer); from there it flows to the turbine through penstocks or high pressure
tunnels. The entire plants can be built into the neighbouring rocks (cavern power
station). In the case of dam power plants, the power station is at the foot of the
dam. The large reservoirs of the European Alps are all part of diversion type hy-
dropower schemes. The power stations themselves are often far away from the
reservoir in the lower valley of the main river.
Water from the reservoirs is sent to the power station for power generation ac-
cording to demand. A distinction is made between daily, weekly, monthly and an-
nual reservoirs plus interannual reservoirs. Annual reservoirs, for example, store
the water from snowmelt in spring and summer in order to produce electricity in
the following winter to cover the peak demand. The higher the available head, the
smaller the reservoir may be, still producing at the same amount of energy.
Pumped storage power stations can have the same tasks as power stations with
a reservoir. In addition, water can be pumped into the upper reservoir and thus
store water from base-load power stations for a certain period of time; with tur-
bines this stored energy can be converted into electrical energy again at times of
peak electricity demand. Pumped storage power stations thus serve as electricity
refiners (i.e. "conversion" of base-load into peak-load electricity). In addition,
they can be used for frequency control. Pumped storage power stations with and
without natural inflow have to be differentiated here; both types can be found.
Auxiliary plants. Nowadays, auxiliary plants can be increasingly found within
drinking water supply systems. Pressure pipes take the water from high-level res-
ervoirs to the consumer, and turbines or pumps running backwards, can be im-
plemented in order to regain surplus energy. Thus only part of the energy used to
pump the water into the high-level reservoirs can be regained (i.e. no utilisation of
renewable energy). Renewable energy can only be obtained from wells or springs
at a naturally higher elevation. The advantage of such plants is that only a turbine
in the form of a reverse pump or a special axial turbine linked to a submersible
generator is added. Furthermore reverse-running spiral pumps are used in some
cases in sewage plants where the water level of the reservoirs is significantly
above the outlet level. Thus the input energy can partly be regained.
A similar situation occurs in plants where dams were built for flood protection,
to increase low flows or as drinking water or irrigation water reservoirs. The wa-
ter that needs to be distributed can also be released via a turbine. The main pur-
pose of such a reservoir is in those cases no longer energy production; thus the
plant efficiency of such power stations is relatively low compared to the size of
the dam or reservoir.
Another type of auxiliary plant is a turbine built into the dam of large diver-
sion-type power stations to release certain instream flows into the river bed as re-
quired for ecological reasons (see Fig. 8.4). Only the direct height of the dam can
be used as the head in this case and not the available overall head at the power sta-
tion itself; however, part of the energy can be regained that would normally be
lost due to the release of in-stream flows. The discharge required for a (not shown