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Hydropower 279

8.5.5 Surge tanks in hydropower plants

Surge tanks are applied in hydropower plants with long water conduits to reduce pres-
sure forces during the acceleration of the large water masses. They are constructed as
intermittent water reservoirs close to the turbines, either with open access to atmo-
spheric air or as a closed volume filled with pressurized air. The surge tanks reduce
the length of the water column to be accelerated, and thus the resulting pressure forces.
This reduction is necessary to limit the design pressure for structural components and
to enable “speed governing” of the turbines.
Closed surge tanks for hydropower plants may be constructed as rock caverns or
steel tanks filled with pressurized air (“Air-cushion chambers”), and are applied where
the topography or other factors render them more feasible than open surge tanks con-
nected to atmospheric air. The three upper examples in Fig. 8.5 show traditional
design with open surge tanks, the lower (From 1975 on) shows the typical layout
for a hydropower plant with a closed surge tank. There are two main advantages in
using a closed surge tank compared to the open type:
– More flexibility in locating the powerhouse and shorter tunnel system
– Shorter distance from surge tank to turbine—more efficient regulation
A few such air-cushion chambers have been built in Norway, the first ones in Driva
HPP (1973), Jukla HPP (1974), and Brattset HPP (1981). Later seven more have been
3
built, with volumes ranging from 2000 to 110,000m .
The transition from fossil to renewable power increases the need for flexible hydro-
power plants in order to balance the highly variable production from v-RES such as
wind and solar. When the size of the power plants increases, the surge tank design
becomes more important owing to larger water masses and pressure forces. More fre-
quent start-stop operations further increase the importance of the surge tank, and more
surge tank research is considered vital to prepare for future challenges.
Recent research at NTNU has provided better understanding and improved math-
ematical models of the complex hydrodynamic and thermodynamic processes in
closed surge tanks. Improved computer models combined with physical-scale models
that can be used to design and operate these optimally have been developed and will be
further improved [15].

8.6 Hydropower resources—Potential

8.6.1 Definition of potential
The potential assessment process for hydropower is different from those used for other
renewables (wind, solar, bio, …) since hydropower is always site specific, and deter-
mined by the combination of available water flow and usable head at each location.
All estimates for hydropower potential are therefore based on data from known sites
where flow and head is measured and technical design optimized to fit these.
In order to compute the potential for hydropower within a wider area (catchment,
region, country, …), it is necessary to identify all feasible sites where a suitable