Because of the variable inflow and operating conditions of the plant these
levels vary. The effective or net head, H, is the head available for energy
production after the deduction of losses in the conveying system of the
plant (Figs 12.8 and 12.15).
The water falling from a high-level source drives turbines, which in turn
drive generators that produce the electricity. The hydraulic power is given by
P gQH/1000 (kW) (12.1)whereis the turbine efficiency and Qis the flow rate (in m^3 s^1 ) under a
head of H(m). The hydraulic efficiency of the plant is the ratio of net head
to gross head (i.e. H/H 0 ), and the overall efficiency is equal to the hydraulic
efficiency times the efficiency of turbine and generator. The installed
capacity of a hydroplant is the maximum power which can be developed by
the generators at normal head with full flow. The unit of electrical power is
the kilowatt, and that of the electrical energy, defined as the power
delivered per unit time, is the kilowatt-hour (kW h).
Primary, or ‘firm’, power is the power which is always available, and
which corresponds to the minimum streamflow without consideration of
storage. Secondary, or surplus, power is the remainder and is not available
all the time. Secondary power is useful only if it can be absorbed by reliev-
ing some other station, thus affecting a fuel (thermal) saving or water
saving (in the case of another hydrostation with storage).
12.5 Types of water power development
(a) Run-of-river plant (local development)
A weir or barrage is built across a river and the low head created is used to
generate power, the power station often being an integral part of the weir
structure. It has very limited storage capacity and can only use water when
available. Its firm capacity is low, because the water supply is not uniform
throughout the year, but it can serve as a baseload plant. Some plants may
have enough upstream storage to meet the peak demands of the day. Two
typical layouts of the run-of-river plant are shown in Fig. 12.2.
TYPES OF WATER POWER DEVELOPMENT 499
Fig. 12.2 Run-of-river plants