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Source:
World Renewable Energy Congress VI (WREC2000)
(c)2000 Elsevier Science Ltd. All rights
reserved.
Editor: A.A.M. Sayigh
Abstract
This paper
covers the development of a water wheel driven electricity
generator by a small UK charity. Engineering problems
and solutions are exemplified by reference to two working
prototypes, one in the UK and the second in the Third
World, and the beneficial characteristics of a water
wheel as a prime mover are established. Site requirements
and potential power output are identified, again with
practical references. The efficiency of the components
and of the whole system is briefly analysed. A description
of construction and installation, with reference to
both civil works and engineering components, stresses
simplicity and appropriateness for "local"
involvement. Comments on capital costs and maintenance
allow comparison with other forms of hydropower. It
is suggested that this simple form of technology is
particularly appropriate for non-technical individuals
and communities to relate to. Thus construction, power
management and maintenance requirements are made easier.
As a tool to help combat environmental pollution and
to improve living standards in the Third World, this
system seeks recognition.
Background
Pedley Water
Wheel Ltd is an extension of the Pedley Wood Conservation
Trust, a small UK charity aimed at increasing environmental
awareness, particularly of children. One way we did
this was by constructing a wooden water wheel, fed by
our small woodland stream in Cheshire in 1991. The water
wheel ran a DC truck dynamo, driven through an old tractor
back axle and gearbox. We produced electricity, the
children learnt, and so did we!
Nine years
later, the same basic concept has lead to the main working
prototype, which runs continuously and provides up to
2 kW of AC electricity, supporting most of our domestic
and commercial electrical equipment.
This prototype
continues to provide the basis for designs and installations
currently ongoing in the UK and in the Third World.
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Engineering
Problems & Solutions
We have learnt
to recognise that water wheels are gravity driven machines.
They turn slowly - traditionally at, say, 4-10 rpm,
depending upon diameter. To generate electricity we
now use standard induction motors running as generators
at 1600 rpm. The resultant gearing problem we now resolve
in various ways, dependent upon end-user requirements.
For example, the wheel in Pedley Wood uses a geared
induction motor with a ratio of 133:1. Gearbox losses
are less than 5% - a great improvement from the tractor
days. We also run our wheels as fast as gravity, centrifugal
force and speed of water entry will allow, in order
to minimise torque and associated component size and
gearing problems.
For our installation
in Sri Lanka (1998) we adopted a different approach.
Although the prime requirement was to provide electricity
to twenty four households, we recognised the benefits
which mechanical power could offer (rice hulling, milling,
woodworking machinery, etc.) and thus installed an intermediate
standalone gearbox offering mechanical power take-off
facilities. This also avoids the 20% losses in the generator
- an important advantage when dealing with a relatively
small power output.
The water
wheel is such a forgiving and flexible "prime mover",
that it encourages lateral thinking .... we are currently
experimenting with a belt-driven system, using the wheel
as a large driving pulley, and thus reducing the torque
on the shaft and at the gearbox by approaching 90%,
with corresponding financial savings. We even toyed
with the idea of using the wheel as the rotor of a giant
generator .... !
DC electricity
is of limited use in most parts of the world. Modern
equipment usually requires a 240volt AC supply. To achieve
this, we have adopted the same electronic control system
used in many small water turbine installations. By controlling
the voltage, the wheel is held at a constant speed,
and allows total flexibility in power use.
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Site Requirements
and Power Output
To maintain
simplicity of design and construction, we select sites
offering a head of 2.5 - 6m, with a flow of 40 -200
litres per second.
Because the
wheel has a broad operating band, it can function with
wide fluctuations in flow. Thus by careful operation,
maximum advantage can be taken of available water power.
In the UK, for example, maximum rainfall/flow occurs
during winter months, when electrical power requirements
are greatest (unlike solar power!). Nevertheless, the
wheel will continue to produce meaningful electrical
power during the drier summer months. Our small stream
drives a 3m diameter wheel and provides up to 2 kW in
winter, reducing to 0.5 kW in summer, when flow drops
to around 30 l/s
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Efficiencies
A well designed
overshot water wheel can produce an efficiency at the
shaft exceeding 80% of the theoretical maximum power
available in the water course. This compares very favourably
even with costly, sophisticated low-head turbines, operating
under similar site conditions. Gear losses, as mentioned,
are in the order of 5%, and high efficiency induction
motor/generator losses are about 20%. Thus the output
in electrical
power should exceed 60% of the theoretical power available
at the site. At the extremes of its operating band,
efficiencies naturally fall.
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Construction
and Installation
Civil Works:
These are dictated by individual site conditions and
location. Generally, however, the lowhead is a simplifying
factor. The requirements are: a regulated channel from
the water source to the head of the wheel, a basic sluice
control, a re-inforced concrete foundation supporting
the wheel shaft, and a channel returning the spent water
to the water course. The geared generator and electronic
control unit requires simple protection against the
elements and human interference, or it can be housed
in a structure to take advantage of mechanical power,
if required.
In Sri Lanka,
the 3.5m diameter wheel conveniently links two irrigation
channels spaced 5m vertically apart - thus minimising
costs, and with the spent water continuing to fulfil
irrigation requirements.
Any small
hydro installation requires the appropriate input of
practical and professional skills. Although simple,
the Pedley Wheel is no exception. The wheel size, complexity
of bucket design to achieve maximum efficiency, and
the end-use distribution of electrical power will dictate
the levels of skills required.
The Wheel
& Generator: The Pedley Wheel is designed to encourage
local fabrication - particularly with Third World locations
in mind, where employment is an important factor. Basic
engineering facilities are required to cut, bend, drill
and weld metal. Fabrication takes place in a local workshop
and components are bolted or welded together on site.
Conventional machining is required for the shaft. The
two bearings and geared motor are standard industrial
components, generally available worldwide. The electronic
control system is of the same basic design as that used
for the regulation of small turbines.
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Costs
& Maintenance
The cost
of the civil works is largely a function of site conditions
and labour costs. In Third world installations, our
experience is that these works are within the abilities
of a local community, and thus costs are minimal.
From installations
to date, a figure of around GBP1,750 per kilowatt installed
is indicated for the wheel construction and associated
engineering components. As usual, economies of scale
apply and this figure may reduce significantly if the
wheel is locally manufactured in numbers.
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Potential
Users
In the developed
world, tens of thousands of derelict water mill sites
exist, where a Pedley Wheel could be installed relatively
cheaply and simply. In addition, there are also new
low-head opportunities, including, for example, water
treatment plant outflows, reservoir overflows, flood
protection weirs, etc.
In the Third
World, the majority of the rural population has no electricity,
but desperately needs it to provide improved facilities
for health care, education, communications, etc.
For all,
global pollution problems urgently demand an initiative.
We appreciate that changes have to be made on a large
scale, but whilst politicians waste precious time debating
and arguing about solutions at government level, the
Pedley Wheel, in its very small way, offers an immediate
way forward for simple and affordable renewable energy
production.
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Conclusion
The Pedley
Wheel has additional special charms. Its gentle pace
and sound offer an accepted therapy in this hectic and
stressful world, whilst its simplicity enables the non-technical
user to understand its basic engineering aspects, and
thus to respect and care for it through a long and useful
life.
We have recently
designed and supplied the components to facilitate electricity
generation from a 650 year old (14th Century) water
mill. It is no exaggeration to say that most folk, out
on a Sunday stroll, would not fail to stop and take
a long look at a working water wheel. We have found
that this is the first step to further enquiry, leading
often to keen interest in the whole subject now under
discussion. What better way to stimulate public awareness,
which surely is the first important step, if the pollution
problems of the planet are to be addressed.
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