Small Hydro Power Plants.
By Emil Bedi, CANCEE and Hakan Falk, "Energy Saving Now".
SMALL HYDRO POWER PLANTS
 Small and micro or nano hydropower schemes combine the advantages of large hydro on the one hand an
decentralized power supply, on the other. They do not have many of the
disadvantages, such as costly transmissions and environmental issues in
the case of large hydro, and dependence on imported fuel and the need for
highly skilled maintenance in the case of fossil fuelled plants. Moreover, the
harnessing of small hydro-resources, being of a decentralised nature, lends
itself to decentralised utilization, local implementation and management,
making rural development possible mainly based on self-reliance and the
use of natural, local resources.
There are in fact many thousands of small hydro plants in operation today
all over the world. Modern hydraulic turbine technology is very highly developed
with the a history of more than 150 years. Sophisticated design and
manufacturing technology have evolved in industrialised countries over
conventional technology the last 40 years. The aim is to achieve higher
and higher conversion efficiencies, which makes sense in large schemes where 1
percent more or less may mean several MW of capacity. As far as costs are
concerned, such sophisticated technology tends to be very expensive. Again, it
is in the big schemes where economic viability is possible. Small installations
for which the sophisticated technology of large hydro is often scaled down
indiscriminately, have higher capital cost per unit of installed capacity.
On the other hand environmental impacts due to small hydro stations are
generally negligible or are controllable because of their size. Often they are
non-existent.
Small hydro power plants are in large majority connected to the
electricity grids. Most of them are of the “run-of-river” type, meaning simply
that they do not have any sizeable reservoir (i.e. water not stored behind
the dam) and produce electricity when the water provided by the river flow is
available but generation ceases when the river dries-up and the flow falls below
a predetermined amount. Power can be supplied by a small (or micro) hydro power
plant in two ways. In a battery-based system, power is generated at a level
equal to the average demand and stored in batteries. Batteries can supply power
as needed at levels much higher than that generated and during times of low
demand the excess can be stored. If enough energy is available from the water,
an alternating current (AC) direct system can generate power. This system
typically requires much higher power level than the battery-based system. Small
hydropower in developing countries, on the other hand, implies decentralisation.
Energy produced is usually supplied to relatively few consumers nearby, mostly
with a low-tension distribution network only.
Small hydro schemes have different configurations according to the head.
High head schemes are typical of mountain areas, and due to the fact that for
the same power they need a lower flow, they are usually cheaper. Low heads
schemes are typical of the valleys and do not need feeder canal. Of the numerous
factors which affect the capital cost, site selection and basic lay-out are
among the first to be considered. Adequate head and flow are necessary
requirements for hydro generation.
Most hydro power systems require a
pipeline to feed water to the turbine. The exception is a propeller machine with
an open intake. The water must pass first through a simple filter to block
debris that may clog or damage the turbine. The intake is usually placed off to
the side of the main water flow to protect it from the direct force of the water
and debris during high flow.
High safety standards in construction works are often not necessary, even
the rupture of a small dam would not usually threaten human life, and the risks
are smaller anyway if initial costs are kept down. This makes it possible
to use mainly local materials and local construction techniques,
with a high degree of local labour participation.
Small hydro
systems can require more maintenance than comparable wind or photovoltaic
systems. It is important to keep debris out of the turbine. This is done by
reliable screening and construction of a settling basin. In the turbine
itself, only the bearings and brushes will require regular maintenance and
replacement.
COST OF SHPP
Hydropower plants are characterised by high initial
capital-investment (according to World Bank total costs are between USD
1800 and USD 8800 per kW for heads from 2,3 to 13,5 m and USD
1000 to USD 3000 for heads between 27 and 350 meters.) and low
operation and maintenance cost. The investment costs include:
 Construction
(dam, channel, machine house),
Parts for electricity generation
(turbine, generator, transformer, power lines),
Other (engineering,
ground property, commissioning).
Usually equipment for low head and low output becomes very costly and
equipment cost ranges from 40 to 50 % of total cost in conventional hydro
installations. As far as costs of civil construction-components are concerned,
no standard cost unit can be given. Dams, canals and intakes will
obviously cost a very different share of the total for different sites. Much
depends on the topography and the geology, and also on the construction
method applied and the materials used. Just to mention some examples the
total cost of new small hydro power plants in Germany was 10-16 DM/W (5-9 ECU/W)
and are divided in most cases 35% (construction) - 50% (electricity parts) - 15%
(other). There are of course some differences between countries e.g. costs of 8
kW turbine (Banki type with regulation) in Czech republic is 4000 USD ,
equivalent to 3500 ECU or 0,45 ECU/W.
The high investment costs is
the largest barrier in development of small hydro power schemes. Despite this
obstacle and long pay-back times (7-10 years in some countries e.g. Slovakia)
small hydro power plants are often cost-effective because of their long
life-time (often more than 70 years) and low maintenance costs. As a general
rule, total costs of operation and maintenance without major replacements
account for approximately 3 to 4% of capital costs for small and
micro-hydropower installations.
THE CONTEXT FOR SMALL HYDROPOWER STATIONS
Decentralised, small power
demand; small industry, individual farms and enterprises, rural
communities.
Low tension distribution networks and eventually sub-regional
micro-grid systems.
Individual, co-operative or communal
ownership with semi-skilled labour requirements and co-operative
administration.
Short gestation period with local materials and skills applicable
depending on potential, it can make a considerable impact on the quality
of rural life.
Its flexibility regarding adaptation to quick load variations makes
it a favoured component in any integrated power system.
Plants can last for very
long time. Some are more than 70 years old and still in operation. Plants
commissioned recently may show even longer life span and thus can serve
consumers over several generations without polluting the atmosphere.
Investment in small hydro power have proved to be safe and secure over several
decades.
SMALL HYDRO POWER PLANTS FOR DEVELOPING COUNTRIES
 In developing countries the
domain where small hydropower can potentially have an important impact on
development is in domestic lighting and in providing stationary motive
power for such diverse productive uses as water-pumping, wood and metal
working, grain milling, textile fibre spinning and weaving. While much of the
discussion is concerned with the generation of electricity, it must be
recognised that the same source of power can perform mechanical tasks
directly via gears and belt drives, very often more economically.
Emphasis is on the use of currently available know-how, using
simple equipment that can be made locally, and the use of local
construction materials and techniques. The aim is to reduce capital costs
as far as possible. Rather than scaling down large-scale technology, this
may lead to a more appropriate upgrading of local technology for larger
schemes at a later stage.
CHINA
The
construction of small hydropower stations has been a very meaningful in
the past 25 years. Besides the development of large resources, much
emphasis was given to small-scale developments resulting in an estimated 100.000
stations around the vast countryside with installed capacity approaching 10.000
MW.
The first large-scale campaign to establish many small waterworks
started in 1956. An ambitious plan called for the construction of 1000 small
stations of a multi-purpose character, combining irrigation, flood control
and power generation, in one year, reaching a total capacity of 30 MW. Although
industrial capability permitted construction of large turbines, and the
range under which small hydropower falls in China was extended to 12 MW, this
indicates that construction of very small units continued. In fact, a range of
miniature turbine-generators with outputs from 0,6 to 12 kW was developed,
suitable for scattered mountain villages with small hydropower
resources.
The development activities in this field were entirely
relying on local resources -materials, skill and labour - and the results
achieved are from this perspective even more impressive. Hydropower development
in China faces some major natural obstacles. The regional distribution of
resources is very uneven and concentrated in regions that are thinly populated.
Flow variations in many rivers are considerable. The maximum recorded
flood flow in the Huang Ho river was 88 times larger than the minimum discharge
and in smaller rivers this ratio is likely to be much higher.
MICRO HYDRO SYSTEMS
 Microhydro systems are defined as
hydroelectric systems that produce less than 1000 Watts. At the high end,
microhydro systems produce enough power to run three electrically efficient
households. No other form of renewable energy is so reliable or powerful for
what it costs. Micro hydro system means that the site has either very little
fall or very small flow of water, but probably not both. At sites with lower
flow rates, systems are usually tied to a battery bank and configured to produce
direct current. With larger hydro resources, systems may be configured to
produce alternating current without the use of a battery bank. These systems
must be able to directly power peak loads. In some case excess power produced is
transferred to an alternate load such as a hot water heater.
 A
hydropower turbine appropriate for household use can be bought for about USD
1000. These simple units are about the size of a breadbox and use a rewired
automobile alternator to produce direct current. The direct current is used to
charge batteries, then converted to AC power with an inverter.
A typical micro hydro installation diverts a small portion of stream flow
across a screen into a water storage e.g. 200 litre drum. The drum acts as a
settling basin and the screen collects debris from the water which may clog the
intake to the turbine. The water flows from the drum to the turbine through PVC
piping (usually 5 to 10 centimetres in diameter), and then returns to the
stream. Additional costs for piping, controls, batteries, and wiring vary
depending on the particular application, but range from USD 1000 to USD
5000.
 Micro
hydro turbines come in two basic forms. One uses an alternator, just like an
automobile. The other (nano hydro systems) uses a permanent magnet (permag)
generator/motor. The alternator based machines are for larger systems producing
from 100 to 1000 watts, while the permag units are best suited to systems
producing under 80 Watts.
Larger systems use shunt diversion for
regulation. This prevents overspeeding of the turbine and premature wear of
parts. Smaller systems use regulation schemes that unload the alternator when
power is not needed. In all cases, these controls need to be user adjustable.
Micro hydro systems are easy to fit with batteries. The turbine produces
constant power all the time. The battery acts as a “flywheel” to smooth out the
inevitable peaks of consumption. Micro hydros refill the batteries almost
immediately after even a little power is consumed from the battery. These
systems are “shallow-cycling” and ordinary batteries will last a long time.
Usually spending money on good pipe and an efficient turbine is more effective
than spending it on batteries. In a microhydro system the length and diameter of
the pipe must be specified to suit the situation and the turbine. Using long
runs of small diameter pipe will make even the finest turbine ineffective.
 
NANOHYDRO - PERMAG
 |
What sets nano hydro systems apart from other hydro generators is
the use of permanent magnet generators for the power source. The advantage
to this is that no power is fed back into the machine to electrically
generate a magnetic field, as is the case with most alternators, so all of
what is produced will feed the batteries. The disadvantage of a permag
set-up is that the maximum output is limited by the inherent strength of
the magnets. Normally that’s not a problem in a nano hydro situation
because usually flow and head of water are too small for a larger, more
powerful system anyway. |
BATTERY-BASED SYSTEMS
Most micro and nano hydro systems are battery-based. They require far
less water than AC systems and are usually less expensive. Because the energy is
stored in batteries, the generator can be shut down without interrupting the
power delivered to the loads. Since only the average load needs to be generated
in this system, the pipeline, turbine, generator and other components can be
much smaller than those in AC system. For conversion of DC battery power to AC
output (type of power needed by most of home appliances) inverters are used. The
input voltage to the batteries in battery-based system usually ranges from 12 to
48 Volts DC. If the transmission distance is not long then 12 V system is used.
For longer transmission distances higher voltage is used.
AC SYSTEMS
Alternating current (AC) hydro power
systems are those used by utilities, but it can also be used on a home power
scale under the appropriate conditions. In home power scale system power is not
sent to the utility grid, but is directly used by a homeowners appliances
(load). AC system does not need batteries. This means that the generator must be
capable of supplying the continuous demand, including the peak load. The most
difficult load is the short-lasting power surge drawn by motors in
refrigerators, washing machines and some other appliances. Usually in typical AC
system, an electronic controller is keeping voltage and frequency within
prescribed limits. The output from hydro power plan can not be stored and any
unused power is sent to a “shunt” load, which can be e.g. a hot water heater.
There is almost always enough excess power from this type of system to heat
domestic hot water and provide space heating as well.
PUMP AS TURBINE
High costs of equipment and civil works, or more generally, the
capital-intensive nature of small hydropower plants, has long been a major
constraint. However, in many situations it is necessary not only to achieve a
better relation of costs compared to other energies, but to reduce them in
absolute terms. This is possible to some degree by standardising equipment, but
the scope for using such standardised equipment remains limited since no two
sites are exactly the same. Efforts at cost reduction through indigenous
manufacture are more promising, largely due to much lower labour costs. To make
this possible, standards of design, performance and sometimes reliability must
be lowered and all unnecessary sophistication avoided. The same is true in civil
construction work, where local materials and techniques should be used to
the largest possible extent.
In developing countries and especially in rural areas, it is generally
recognized that small hydropower may play a significant role. However, high
initial investment costs of small hydropower plants have restricted rapid
development of this energy potential in many countries. The use of standard
pumps as turbines (PAT) may often be an alternative with a considerable economic
advantage and might therefore contribute to a broader application of
micro-hydropower. Direct drive of machinery, electricity generation (in parallel
to a large grid or isolated) or combinations of these are possible just as with
a conventional turbine. The only difference is that a PAT cannot make use of the
available water as efficiently as a turbine due to its lack of hydraulic
controls.
FIELDS OF APPLICATION OF PUMPS USED AS TURBINES
Pumps (rotational fluid machines) are
completely reversible and can run effectively as a turbine. Standard pumps not
intentionally designed to operate as turbines are now more and more used in
small and micro-hydropower schemes due to their advantages mentioned above.
However, performance in both modes are not identical although the theory of
ideal fluids would predict the same. Without exception, the optimum flow and
head in the turbine mode is greater than in pumping mode. The main reason for
this difference is related to the hydraulic losses of the machine.
Applications of PAT range from direct drive of machinery in
agro-processing factories and small industries (flour mills, oil expellers, rice
hullers, saw mills, wood and metal workshops) to electricity generation both in
stand-alone and grid-linked stations.
In most instances, no design
changes or modifications need to be made for a pump operating as a turbine
provided that selection has taken into account the higher operating head and
power output of the machine in turbine mode and consequently, nominal turbine
speed has been taken well below maximum permissible pump speed. However, a
design review is also required to check any adverse effects occurring from the
reverse rotation in turbine mode.
Advantages of PAT
the investment costs of PATs
may be less than 50% of those of a comparable turbine (especially for small
units below 50 kW). This might be an important issue for projects with limited
budgets and loan possibilities
construction: the absence of a flow
control device, usually felt as a drawback, is at the same time an advantage
since the pump construction is usually simple and sturdy
availability: due to their
widespread application (irrigation, industry, water supply), standard pumps are
readily available (short delivery times) and manufacturers and their
representatives are world-wide present
spare parts: spare parts are readily
available since major pump manufacturers offer after- sales services almost
throughout the world
maintenance: no special equipment and
skills are required.
Disadvantages
No hydraulic control device:
therefore, a control valve must be incorporated in the penstock line (additional
costs) to start and stop the PAT. If the valve is used to accommodate to
seasonal variations of flow, the hydraulic losses of the installation will
increase sharply.
Lower efficiency at part load: a conventional turbine has an
effective hydraulic control (adjustable guide vanes, nozzles or runner blades)
to adjust the machine to the available flow or the required output. If PATs are
operated at other than the design flow, i.e. below their best efficiency
point a relatively rapid drop of efficiency will occur.
The disadvantages of PATs can be reduced to a minimum if the PAT is very
carefully selected and only applied where justified. Poor performance due to an
inappropriately selected machine or application will lead to a reduction of
gains. Summed up over the entire lifetime of the machine, this reduced output
might by far offset the cost advantage of the PAT (lower investment costs) in
comparison to a conventional turbine.
DIFFERENCES BETWEEN PUMPS AND TURBINES
Pumps are usually operated with constant speed, head and flow. A pump is
therefore designed for one particular of operation (duty point) and does not
require a regulating device (guide vane). Ideally, the duty point coincides with
the maximum efficiency of the pump.
Turbines operate under
variable head and flow conditions. In an small hydro power plant, flow must be
adjustable to either accommodate to seasonal variations of the available water
or to adjust power output according to the demand of the consumers. Adjustable
guide vanes and/or runner blades (or nozzles controlled by a streamlined valve)
regulate the flow.
TYPE OF PUMP TO BE USED AND EFFICIENCY IN TURBINE MODE
Virtually any type of pump may be used
as turbine. However, the main advantage of a PAT, i.e. lower costs than a
conventional turbine, is very pronounced for standard centrifugal and mixed flow
pumps whereas axial flow pumps are less advantageous in that respect. The vast
field of different pump designs and power ranges provides a suitable PAT for
almost any application with heads from about 10 m up to several hundred meters.
Large flows may be accommodated with double-flow pumps. Even submersible pumps
may be used as PATs which, when integrated in the water course or pipe system,
are completely hidden away underground, an important factor for the conservation
of the environment. Efficiencies of pumps used as turbines may be the same as in
pump mode but are more often several percent (3 - 5%) lower.
Direct drive of machinery, electricity generation (in parallel to a large
grid or isolated system) or combinations of these are possible just as with a
conventional turbine. Although the PATs cover a wide range of the small
hydropower domain, they cannot replace conventional turbines everywhere. Since
PATs have no hydraulic control device such as guide vanes, they are usually
unsuitable to accommodate variable flow conditions. Throttling flow by means of
a control valve in the penstock is inefficient and only applicable over a small
range.
The lack of a hydraulic control device of a PAT has long been seen as a
disadvantage also in terms of constancy of PAT speed under variable load.
Grid-linked electricity generation or direct drive of machinery are either
constant load applications or do not require precise speed control. These
applications are therefore very suitable for PATs. Stand alone electricity
generation on the other hand requires some form of governing to keep voltage and
frequency within acceptable limits under changing load. The use of PATs in
free-standing electricity generation is, however, not excluded due to the recent
development of electronic load controllers which provide effective governing in
conjunction with both induction and synchronous generators. Electronic load
controllers keep the load on the PAT constant by switching in ballast loads
whenever the electricity demand of the consumers drops.
HYDRO RAM PUMP
Hydro ram is not an animal but a self-driven pump first installed at the
turn of the century when they were popular with farmers who had natural water
courses on their land. With the coming of grid electricity and mains water, many
rams were left to rot and rust in the post-war period. Nevertheless this device
is a useful source of cheap energy even today. Ram pumps do not produce
electricity but the mechanical work for pumping water to higher elevations. They
use a downhill water pressure to pump a portion of that water higher
uphill to a holding tank. No other source of power is needed. The hydro rams are
complete in themselves and designed to work with the minimum of attention, and
to suit all the ordinary conditions.
The hydro ram has proved to be
one of the most reliable devices used for water pumping. Many over 100 years old
are still in use, and it remains one of the few really practical and efficient
uses of renewable energy today. Hydro rams are relatively cheap, will last
almost indefinitely and with no moving metal parts and its simplicity require
only minimum of maintenance. If the two essentials are provided – a supply of
water (spring or stream, as little as 4 litres per minute will suffice) and the
ability to provide a “fall” for that water – the hydro ram can reduce or even
eliminate costly water bills. Typical uses of hydro rams include :
Village water
supplies
Irrigation
Water pumping and circulation in
industry
Water circulation for heat pumps
Water circulation for solar panels
How a Ram Pump Works
All ram pumps work on the principle of momentum which is controlled by a
cycle set up by the interaction of two valves in the pump. The water, being
admitted into the drive pipe, flows through it by gravitation until it reaches
the ram, passes through the ram and through the pulse valve into the waste
drain. As the water flows, its velocity increases until the pulse valve is no
longer able to pass the volume of water flowing, and on this point being reached
the pulse valve is suddenly closed. The outlet thus being closed, the flow of
water suddenly stops. This produces a concussion of more or less severity in the
body of the ram, according to the height and distance from which the water is
flowing, and a result of this concussion is that a portion of the water in the
body of the ram is forced upwards through the delivery valve into the air
cylinder. At the same time the recoil allows the pulse valve to return to its
original position. The outlet being thus reopened, the water which was brought
to rest by the closing of the pulse valve recommences to flow through the ram
till it acquires the necessary velocity to raise the pulse valve a second time ,
closing the outlet, producing a concussion, and forcing more water into the air
chamber through the delivery valve. This series of events occurs from 40 to 90
times per minute, according to the size of the hydro ram, fall of water driving
ram, etc. The ram will continue working automatically for months, the pulse
valve rubber and the delivery valve rubber being the only moving parts.
The water, which is forced into the air chamber, finds its way from it
through a pipe, known as the rising main or delivery pipe, to the place where it
is required for use, a continuous flow being maintained so long as the ram
remains working.
The fall of water necessary to work a ram may be as
low as 0,5 meter and with such a fall, water may be raised to 10 to 15 meters.
With higher falls, such as from 2 to 10 meters and over, water can be raised to
upwards of 100 meters in height and more than 1 kilometre in distance.
The installation is extremely simple. All that is required – water at the
point of by constructing a pool. From this running downwards on an even gradient
to the point of location of the ram itself, runs the drain pipe which has to be
heavy gauge galvanised steel or cast iron pipe and of appropriate length which
is dependent upon the height to which the water is to be pumped. Although it is
not essential that this pipe should be buried, it is preferable in order to
avoid interference from wild life and unauthorised persons. The ram chamber
itself can vary considerably but all that is required is a concrete base which
securely hold the ram in place. Hydro rams are working unaffected by the
temperature changes (especially low temperatures which might cause a
conventional system to freeze unless some form of heat is provided.)
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