SMALL HYDRAULIC POWER PLANT DEVELOPMENT 529
while the riser itself responds quickly to maintaining the desired
level. The differential tank with an extended penstock, which acts as
a central riser, is shown in Fig. 12.21(e).- Surge tanks with venturi mounting.Considering the velocity energy
under the surge tank (V^2 /2gE 0 ), Thoma’s critical section can be
written as
AscV^20 AtLt/2g(P 0 E 0 )H 0. (12.35)More economical sections may result by providing a venturi contrac-
tion (Fig. 12.21(f )) under the surge tank (thus increasing the velocity
head,E 0 (Escande, Dat and Nalluri, 1962)). For the detailed theory
and design of surge tanks, see Jaeger (1977), Novak (1983), Wylie
and Streeter (1993) and Popescu et al. (2003).12.11 Small hydraulic power plant development
There are some inconsistencies in the use of the term ‘small’ hydraulic
power, but the following classification adopted by the United Nations
International Development Organisation is widely used: micropower
100 kW, minipower100 kW to 1 MW, small power 1 MW to 10 MW.
Another term which could be added to the above classification is ‘Picohy-
dro’ used for developments of less than 5 kW.
The potential for the development of small hydraulic power plants
worldwide is substantial and its use particularly in rural or isolated areas is
growing at such a rate that any statistics become quickly obsolete. As an
example in UK the technically feasible potential for small hydraulic power
plants (5 MW) is about 4.6 TWh/year, the economically feasible poten-
tial about 1.1 TWh/year, of which about 12% has been developed (Bartle
and Hallowes, 2005). In China small plants owned by municipalities and
local authorities account for about 16 000 MW; Indonesia’s micro-potential
(between 250 kW and 500 kW) is estimated at about 500 MW (Hayes,
2004).
Very small plants (pico – micro) require the development of special
suitable machinery (e.g. fixed blade turbines) and many can be sited at
disused mills, which used water wheels. Although the installation of new
water wheels disappeared virtually early in the 20th century, their potential
use as a cost effective low head low flow hydraulic energy converter (con-
trary to turbines operating under atmospheric conditions) is making them
attractive again to a certain degree. The overshot water wheels were
employed for head differences 2.5–10 m and flow rates of 0.1–0.2 m^3 /s, the
breastwheels for 1.5–4 m and 0.5–0.95 m^3 /s and the undershotwheels for
0.5–2.5 m and 0.5–0.95 m^3 /s. ‘Modern’ water wheels employ the potential