Hydro-power
Introduction
• Hydro-power term is used for the generation of shaft
power from falling water. The power is then used for
direct mechanical purposes or for generating electricity.
• Hydro-power accounts for about 20% of world’s electric
generation.
• In India, hydro power plants with capacity of 25 MW or
below are classified as small hydro.
• Hydro turbines have a rapid response for power
generation and so the power may be used to supply
both base load and peak demand requirements on a
grid supply.
• Power generation efficiencies may be as high as 90%.
https://mnre.gov.in/small-hydro/current-status
Hydro-power principles
• A volume per second, Q, of water falls down a slope. The rate of potential
energy lost by the falling fluid is
P0 is the energy change per second (Watts)
• A hydro-power system convert this power to shaft power with some frictional
losses (minor), hence the actual output is close to the design output
• The site must have sufficiently high Q and H. this requires a rainfall
>∼40cmy−1 dispersed through the year, a suitable catchment and, if possible,
a water storage site.
• Costs of system are associated with civil work, turbine, pipe work
Available head
• Hf allows for friction losses in the pipe and channels leading from the
source to the turbine
• It is possible to keep Hf <∼Ht/3
• Hf increases in proportion to the total length of pipe, so that the best
sites for hydro-power have steep slopes
Measurement of flow rate Q
• The flow through the turbine produces the power, and this flow will usually be less
than the flow in the stream
• The flow in the stream varies with time, the minimum (dry season) flow should be
known for power generation and for aquatic life
• The maximum flow and flood levels should be known to avoid damage to
installations
• The flow rate can be calculated by
Cont.
1 Basic method 2 Refined method I
accurate and is ideal for small flows
Cont.
3 Refined method II 4 Sophisticated method
most accurate method for large streams
forward speed u is measured with a small flow
Measuring the horizontal distance required for the float to rise
meter at the points of a two-dimensional grid
gives the speed. Gives mean speed averaged over depth
extending across the stream
Cont.
5 Using a weir
If Q is to be measured throughout the year
for the same stream, measurement can be
made by building a dam with a specially
shaped calibration notch. Such a dam is
called a weir.
The height of flow through the notch gives
a measure of the flow.
The system is calibrated against a
laboratory model having the same form of
notch.
Turbines
• Reaction turbines: the turbine is totally embedded in the fluid and powered
from the pressure drop across the device.
• Impulse turbines: the flow hits the turbine as a jet in an open environment,
with the power deriving from the kinetic energy of the flow.
For small values of H
Impulse turbine
Reaction turbine
An impulse turbine: Pelton wheel
• The potential energy of the water in the reservoir is changed into kinetic energy of one
or more jets.
• Each jet then hits a series of buckets or ‘cups’ placed on the perimeter of a vertical
wheel
• The tangential force applied to the wheel causes it to rotate
Impulse turbine: Power transfer
• The cup moves to the right with steady speed uc and the input jet speed is uj
• Relative jet speed (uj− uc)
• friction is negligible for polished cup is smooth, so the jet is deflected smoothly
through almost 180° with no loss in speed; u2= uj
The force F experienced by the cup
in the direction of the jet
The power P1 transferred to the single cup is
By differentiation with respect to uc this is a maximum for constant uj
when
this ideal turbine has 100% efficiency
in practice efficiency range from 50% for small units to 90% for accurately
machined large commercial systems
Impulse turbine: Jet velocity and
nozzle size
Applying Bernoulli’s theorem at two points, considering pipe
friction (available head Ha)
If there are n nozzles, each of area a, then the total flow from
all jets is
If the efficiency of transforming the water jet power into mechanical
rotational power is , then the mechanical power output Pm from the
turbine with n jets is The output power is proportional to the total
𝑃 𝑚 =𝜂𝑚 𝑛 𝑃 𝑗 jet cross-sectional area A = na
• a is limited by the size of cup:
large turbine size
• increase the number of nozzles n:
but complexity increases for n ≥
total flow Q through the turbine cannot
be more than the flow in the stream 4, n = 2 is the most common
Impulse turbine: Angular velocity
and turbine size
• Selection of nozzle size fixes cup size but not overall size of the wheel that
determined by geometric constraints, and by the required rotational speed for
electric generators (have greatest efficiency at large rotational frequency, commonly
∼1500rpm)
• If the wheel has radius R and turns at angular velocity ω
Considering circular cross section of
nozzle
shape number of the turbine
Cont.
• instead of the dimensionless shape number, in engineering usually use a dimensioned
characteristic called ‘specific speed’, Ns
Example
Determine the dimensions of a single jet Pelton wheel to develop 160kW under a head
of (1) 81m and (2) 5.0 m. What is the angular speed at which these wheels will perform
best ()? Consider water is the working fluid. Consider r=R/12
Shape number 0.129,
for 81m, ω is 43 rad/s, R=0.55 m
for 5 m, ω 1.33 rad/s, R=3.72 m
• For low head, Pelton wheels would be unwieldy and costly, especially because the size of framework
and housing increases with the size of turbine.
• In practice therefore, Pelton wheels are used predominantly for high-head/small-flow installations.
Reaction turbines
• In order to maintain the same ω and P with a lower H, we require a turbine with
larger shape number.
• One way to increase the number of nozzles on a Pelton wheel: pipework becomes unduly
complicated n>4 figure (a)
The entire periphery of the wheel is made
into one large ‘slot’ jet which flows into
the rotating wheel, as in Figure (b). Such
turbines are called reaction machines’
because the fluid pushes (or ‘reacts’)
continuously against the blades.
Energy transfer
Electricity generation
using Hydroelectric
systems
Hydro-power
Water pumping using
hydraulic ram pump
