Design of a Water Wheel For a Low Head Micro Hydropower System

Abstract

This research work performs an analysis, evaluation and systematic representation of techniques of a low head micro hydropower system using a fixed water source and designing of a water wheel for the system. This is a support system to the national grid and is designed to provide electricity to a house when the national grid fails. Water is pumped up to the upper reservoir during the off-peak hour and when needed water is released from the upper reservoir through the turbine to generate electricity. This system makes use of a fixed amount of water that stored in a lower reservoir after circulating through the turbine. A water wheel is chosen as a turbine for the system and this paper is focused on to design and construction of a water wheel suitable for the micro hydropower site. Water wheels have been known since antiquity. With the industrial revolution, hydraulic sciences were developed and new materials such as wrought iron became available allowing for a rational analysis and improved strength and geometry of water wheels. A detailed study of the available literature covering the engineering design of water wheel was conducted. The design of water wheel was dominated by the requirement for a geometry which would minimize losses, and retain the water as long as possible in the machine. Theoretical output power has been calculated for a range of water flow and a graph has been plotted using the relevant values.

Full text

Journal Basic Science And Technology, 1(3),1-6,2012
ISSN : 2089-8185
1
Design of a Water Wheel For a Low Head Micro
Hydropower System
Ayesha Zaman
Dept. of Applied physics, Electronics
& Communication Engineering
University of Dhaka
Dhaka, Bangladesh
ayeshazaman@univdhaka.edu
Taslima Khan
Dept. of Applied physics, Electronics
& Communication Engineering
University of Dhaka
Dhaka, Bangladesh
Abstract—This research work performs an analysis, evaluation
and systematic representation of techniques of a low head micro
hydropower system using a fixed water source and designing of a
water wheel f or the system. This is a support system to the
national grid and is designed to provide electricity to a house
when the national grid fails. Water is pumped up to the upper
reservoir during the off-peak hour and when needed water is
released from the upper reservoir through the turbine to
generate electricity. This system makes use of a fixed amount of
water that stored in a lower reservoir after circulating through
the turbine. A water wheel is chosen as a turbine for the system
and this paper is focused on to design and construction of a water
wheel suitable for the micro hydropower site. Water wheels have
been known since antiquity. With the industrial revolution,
hydraulic sciences were developed and new materials such as
wrought iron became available allowing for a rational analysis
and improved strength and geometry of water wheels. A detailed
study of the available literature covering the engineering design
of water wheel was conducted. The design of water wheel was
dominated by the requirement for a geometry which would
minimize losses, and retain the water as long as possible in the
machine. Theoretical output power has been calculated for a
range of water flow and a graph has been plotted using the
relevant values.
Keywords-Breastshot water wheel; flow rate; efficiency factor;
hydro power.
I.
I
NTRODUCTION
Hydropower systems are classified as large, medium, small,
mini and micro according to their installation power generation
capacity. Electrical power is measured in watts (W), kilowatts
(kW) or megawatts (MW). A micro-hydropower system is
generally classified as having a generating capacity of less than
100 kW. Close to a quarter of the energy of the sun that reaches
the earth’s surface causes water to evaporate and hence a
proportion of this energy causes vapor to rise against the
earth’s gravitational pull. This vapor then condenses into rain
and snow, which again falls back to the earth’s surface. This is
called the water cycle and is the fundamental reason why
hydropower is possible [1]. When rain and snow fall onto any
ground above sea level some of the sun’s energy is conserved
in the form of potential energy. This energy is then dissipated
in currents as water runs down in streams. By catching this
water in the controlled form of pipes, one can exploit the
kinetic energy that becomes available with the movement of
water. These pipes are then used to direct the stream of water,
under pressure, onto a turbine blade. The water then strikes the
turbine blade to create mechanical energy [1].
This mechanical energy is then transmitted to an electrical
generator through a rotating shaft [2]. This simple process is
essentially how all hydropower is generated – a process tried
and tested over hundreds of years which currently supplies
over 715,000 MW or 19% of the world’s total electricity [3].
II. M
ICRO
-H
YDRO AND
R
URAL
D
EVELOPMENT
Access to electricity is one of the fundamental need for
rural development and a necessity for the improvement of
infrastructure. An estimated 1.5 billion people in developing
countries do not have access to electricity [4], severely limiting
the possibilities of economic growth. An increased focus on
decentralized energy generation, where the state cannot viably
connect population centers to the main electricity grid,
significantly improves the development prospects of struggling
communities. Micro-hydro provides a reliable, affordable,
economically viable, socially acceptable and environmentally
sound energy alternative for rural development. It is the small
scale harnessing of energy from falling water, generating
typically less than 100 KW [5] and powering small
communities or factories.
III. T
YPES OF
H
YDROPOWER
As with any other type of renewable energy source, there
are many types of hydro power. This includes: impoundment,
diversion and pumped storage. Impoundment describes a
certain hydro facility where a dam is used to store water. The
water is used to run the turbine to create the electrical energy.
These are the most widely recognized styles although they are
actually not very common and are quite infeasible for most
residential areas due to their costs and complexities. Diversion
is almost the same except it channels a portion of the river
through a canal or penstock. Diversion is also called a “run-off-
river,” in some cases. Pumped storage, another type of micro
hydro utility, needs its own facility. In pumped storage, water
is pumped from a lower reservoir to an upper reservoir. When
water is released from the top, energy is produced [6].

Journal Basic Science And Technology, 1(3),1-6,2012
ISSN : 2089-8185
2
IV. B
ASIC
C
OMPONENTS OF
A
M
ICRO
-H
YDROPOWER
S
YSTEM
Basic components of a typical micro-hydro system are
mainly civil works components (headwork, intake, headrace
canal, fore bay, penstock/pipe and tailrace) and powerhouse
components (turbines, generators, drive systems and
controllers). Civil works structures control the water that runs
through a micro-hydropower system, and conveyances are a
large part of the project work. It is important that civil
structures are located in suitable sites and designed for
optimum performance and stability. In case of power house
components turbine or waterwheel is the most essential part. A
turbine unit consists of a runner connected to a shaft that
converts the potential energy in falling water into mechanical
or shaft power. The turbine is connected either directly to the
generator or is connected by means of gears or belts and
pulleys, depending on the speed required for the generator. The
choice of turbine depends mainly on the head and the design
flow for the proposed micro-hydropower installation. The
selection also depends on the desired running speed of the
generator. All turbines tend to run most efficiently at a
particular combination of speed, head and flow. In order to suit
a variety of head and flow conditions, turbines are broadly
divided into four groups (high, medium, low and ultralow
head) and into two categories (impulse and reaction) [7]. Fig 1
shows a schematic diagram of a typical micro-hydro power
system.
Figure 1. A Micro Hydropower system with a fixed water source
V. H
YDRO
E
NERGY
S
CENARIO IN
B
ANGLADESH
Hydropower generation is an eco-friendly clean power
generation method. It is an established source of electricity and
currently accounts for about 20% of electricity generation
worldwide [8]. Renewable energy exploitation in Bangladesh is
not new. People are using renewable energy sources like solar,
wind, hydro power for different purposes from primordial time.
The only hydro power station of the country, the Karnafuly
Hydro Power Station with a generating capacity of 230 MW
by 5 units across the river Karnafuly . This plant is the only
hydroelectric power plant operated by (2 x 50MW = 100MW)
Bangladesh Power Development Board(BPDB). BPDB is
considering extension of Karnafuli hydro station to add another
100MW capacity. The additional energy will be generated
during the rainy season when most of the year water is spilled.
Apart from Kaptai, two other prospective sites, for hydropower
generation at Sangu and Matamuhuri River, are identified by
BPDB.
VI. W
ATERWHEEL
The waterwheel is one of the oldest hydraulic machines
known to humankind and has been in use since antiquity.
Originally built of wood, the availability of new materials,
namely wrought iron, and the increasing demand for
mechanical power during the industrial revolution led, in
combination with the development of hydraulic engineering, to
the rational design of waterwheels, resulting in much increased
performance and efficiency. Three distinct types of
waterwheels evolved the overshot, the breastshot and the
undershot wheel. Among the three types the Breastshot
waterwheel is the most suitable for the application of low-head
microhydro system. It is typically used for used for head
differences of 1.5 to 4m, and flow rates of 0.35 to 0.65 m³/s per
m width [9].
VII. D
ESIGN
P
ROCEDURE
One of the first steps in planning is to measure the power
potential of the stream. The amount of power that can be
obtained from a stream depends on the amount of water flow,
the height from which the water falls (head) also the efficiency
of the plant to convert mechanical energy to electrical energy.
Hence the amount of power available from a micro-
hydropower system is directly related to the flow rate, head and
the force of gravity. This is calculated using the following
equation:
]/8.9[tan
][
][/
][
..
2
smtconsnalgravitatiog
mheadgrossH
srateflowusuableQ
KWoutputpowerltheoreticaPwhere
gHQP
th
th
=
=
=
=
=
To calculate the most realistic power output from the site,
there must be taken into account the friction losses in the
penstock pipes and the efficiency of the turbine and generator.
When determining the head, it is needed to consider gross head
and net head. Gross head is the vertical distance between the
top of the penstock that conveys the water under pressure and
the point where the water discharges from the turbine. Net head
is the available head after subtracting the head loss due to
friction in the penstock from the total (gross) head. So (net
head = gross head – losses in the penstock). Therefore, to
determine a realistic power output, the theoretical power must
be multiplied by an efficiency factor of 0.5 to 0.7, depending
on the capacity and type of system,
]7)[7.05.0(
...
−=
=
factorefficiencyewhere
egHQP

Journal Basic Science And Technology, 1(3),1-6,2012
ISSN : 2089-8185
3
VIII. D
ESIGN PARAMETERS OF
B
REASTSHOT
W
ATERWHEELS
A. Implementation of Bach Methodology
In the case of a breastshot waterwheel, the loss at exit and
entrance are minimized. The most advanced design method
was developed by the German engineer Carl von Bach (1886).
According to Bach A waterwheel has to be designed for a
given head difference
H
and a flow rate Q . The diameter
D
of a breastshot wheel can be taken as twice the head difference
H
or slightly more. Bach recommends a value of D
mH 5.3
+
=
. This however leads to quite a large wheel
diameter for smaller values of
H
.
For a low head micro-hydro power system if the required
power output is 1kw; flowrate has to be ensured theoretically
as
sm /
3
04.0
.
Here according to Bach D=5.2m + 3.5m = 8.7m, but In this
case the diameter is taken as 8m in order to design a smaller
and thus more economical wheel.
A waterwheel of 8m diameter with a head difference of
5.2m and a flow rate of
sm /
3
04.0
will be designed using the
method and recommendations given by Bach. The tangential
velocity should be within 1.5–2 m/s. Tangential velocity is the
instantaneous linear velocity of a body moving in a circular
path; its direction is tangential to the circular path at the point
in question.
smvtakeletsand
QmHmDNow
t
sm
/5.1
04.0,2.5,8
/
3
=
===
The optimal rotational speed of a breast shot wheel is
approximately:
)(5.7
21 elyapproximatrpm
D=
The depth d of the cells can be calculated as a function of
the head difference and the diameter with the following
empirical equations:
mtomd
tod
H
D
to
H
D
d
15.008.0
5.15.05.14.0
5.04.0
33
33
=
=
=
Fig 2 shows the side elevation of a breastshot water wheel
pointing out the relevant design parameters.
A depth d of 0·15 m was chosen for the model. The width B
of the wheel follows
m
dv
Q
B
t
35.0
15.05.05.1
04.0 =
××
==
ε
Figure 2. Sideways of elevation wheel

Journal Basic Science And Technology, 1(3),1-6,2012
ISSN : 2089-8185
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B. Design of downstream case
At the outflow, the water must move with the tangential
speed of the wheel of 1·8 m/s. The submerged depth at the exit
r
t
can be determined by
mttakeslet
m
Bv
Q
t
r
t
r
01.0'
08.0
5.135.0
04.0
=
=
×
=
=
Figure 3. Design consideration: outflow case
In order to avoid losses at the exit of the blade out of the
water, the blade should be curved in such a way so that the
angle of intersection of blade and water surface is always
normal to the water surface from point A to point B along the
wetted length
w
l
.
The radius
a
r
of the lower section of the blade is given by:
0
13
4
1.04 =
−
=
−
=arcs
R
tt
arcs
r
β
m
t
r
r
a
44.0
13sin
1.0
sin
0
===
β
The blade follows this radius from the outer edge to the
point of maximum submergence
).(
r
tRei
−
. It is then curved
with a smaller radius
b
r
to approach the maximum depth d of
the cell tangentially.
C. Design of inflow
The downstream condition determines the geometry of the
blade. The inflowing water enters the cell at a depth
1
x
below
the upstream water level at point C as shown in Fig 1. In order
to maximize the use of the kinetic energy of the water, the
inflow velocity
in
v
should be approximately twice the
tangential velocity
t
v
of the wheel. It is important to avoid
turbulent losses at the inflow generated by the effective inflow
vector
eff
v
, which constitutes the vector sum of the inflow
vector
in
v
and the tangential velocity vector of the wheel
blade
t
v
, is parallel to the blade. The angle α, under which the
water enters the cell is usually chosen so that tan α= 0·5 or α=
26 (sometimes up to 30). The larger α becomes, the faster
the wheel or the slower the water has to move. The entry
velocity
in
v
of the water can now be determined by
s
m
v
v
t
in
/
43
.
0
)261390sin(
13sin5.1
)90sin(
sin
=
−−
×
=
−−
=
αβ
β
With this velocity, the depth of the inflow underneath the
upstream water level can be calculated, assuming a loss factor
of 0·1
m
g
x01.0
2
43.0
10.1
2
1=×=
Simplifying, an inflow detail with one opening can now be
designed with a loss factor of 0·08 for the inflow.
With an inflow velocity of 0.43 m/s, the required
theoretical depth
op
d
for a single inflow opening as shown in
Fig. 2 can be determined by
m
Bv
Q
d
in
op
29.0
35
.
0
43
.
0
92
.
0
04.0
92.0
=
×
×
=
××
=
This is the width of the opening close to the water wheel.
IX. S
IMULATION AND
R
ESULT
A. Calculation of Theoretical Power Output
Theoretical output power is calculated using Matlab for 10
different values of flow rates. The results are given in the
table1.

Journal Basic Science And Technology, 1(3),1-6,2012
ISSN : 2089-8185
5
Figure 4.
Flowrates for different low head
TABLE I. T
HEORETICAL POWER OUTPUT FOR A RANGE OF FLOW RATE
In the same way, output power is calculated for four
different values of head like, H=5m, 5.1m, 5.3m and 5.4m.
These values of output powers are plotted against the flow rates
in Fig 4.
X. O
VERALL
D
ISCUSSION
This research work has been done considering the
establishment of a low-head microhydro power system.
Projected people considered here are the mediocre people.
Such a family requires about 1 kw of power to use their house
hold appliances in case of interruption of electricity from the
national grid line. Though the establishment cost is a bit higher
but after constructing the system once, it works up to 20 to 25
years . The breast shot water wheel that has been designed and
simulated provides approximately the desired amount of output
power and is most effective for getting higher power
requirements also.
R
EFERENCES
Head
H
Gravitational
constant, g
Efficiency
e
Flow rate
Q Theoretical power output
0.035 0.8918
0.036 0.9173
0.037 0.9428
0.038 0.9682
0.039 0.9937
0.040 1.0192
0.041 1.0447
0.042 1.0702
0.043 1.0956
5.2m 9.8 0.5
0.044 1.1211

Journal Basic Science And Technology, 1(3),1-6,2012
ISSN : 2089-8185
6
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