|
Microhydro & Small Hydro
BCSEA
position on run-of-river hydro power ( PDF,
221kb)
Most of the large hydro-electric power generation projects
we see and hear about involve dams and storage reservoirs.
They have the advantage of storing energy for release when
there is peak demand or during periods of the year when water
flows are lower. Some of the disadvantages are that the storage
reservoirs inundate and alienate large areas of land, affect
local climate, and release toxins as organics break down.
Micro and small hydro projects seldom use storage reservoirs
and are often referred to as run-of-river. They have some
significant advantages over other renewables. The amount of
power that can be generated is highly predictable, is available
24 hours a day and the installed cost per kilowatt is lower
than either wind or photovoltaic. The disadvantage is that
the power potential is very site specific.
Micro and small hydro developments are similar in terms of
impacts and operation with their main difference being size.
While definitions of the terms "micro" and "small
hydro" vary significantly, BC Hydro considers microhydro
developments to have an installed capacity of less than 2
MW (2,000 kW) and small hydro developments have installed
capacities between 2 and 50 MW. Some use the terminology "pico"
and "nano-hydro" to described very small systems
of up to several kW that are common in rural BC.
The site is the determining factor. Power potential is a
function of head (the change in elevation of the water) and
the available flow. An approximate potential can be calculated
using the formula P (in kw) = QH x 7.83 . Q is the flow in
cubic meters/sec and H is the head in meters. In evaluating
a site some of the things to consider are the flow duration
curve, and the potential head vs. the infrastructure required.
If you can achieve a 20 meter head with 100 meters of pipe
but have to go another 400 meters to get an additional 5 meters
of head, the extra cost isn't likely worthwhile.
Most small streams have a much higher flow in the spring
freshet than for much of the year so systems should be sized
to operate using the flows that are most consistently available.
What is available 80% of the time is the rule of thumb although
some equipment can operate through a range of flows. There
is a lot of stream flow data available in BC for many streams.
Extrapolations can be made for other streams in the area
same area. If you measure the flow on the stream you are
interested in periodically, and compare this flow to the
flow of a local stream that you have historical data on it
is easier to establish a flow duration curve. The other important
bit of information needed is a quite accurate head measurement.
A very good estimate can be obtained by hiking the site with
an accurate altimeter.
Another important consideration is having land access available
for the works to be built. If it all falls within your own
property it is no problem but if a right of way is needed
across Crown or neighbour's land it is important to approach
them early on in the process.
Once you are satisfied that there is sufficient head and
flow available for your purposes there is a range of equipment
choices available. On lower head systems (under 30m) reaction
turbines (Francis, Kaplan, recycled pumps) are commonly used
because the exhaust, or tailrace, water increases the turbine's
efficiency. This type of turbine is like a propeller mounted
n a pipe and must have a consistent flow. The efficiency
of these turbines drops dramatically if the turbine blades
are not fully immersed. To accommodate a range of flows it
is necessary to have two or more smaller units so one or
more can be shut down and the water diverted to the other.
Higher head systems usually use impulse turbines (Pelton,
Turgo, Crossflow) in which jets of water are directed at
cups of various shapes. These turbines spin in the air and
the tailrace water is simply directed away from the turbine
so it doesn't interfere with it. The useful head does not
include the tailrace in this type of turbine. They have the
advantage that a variable number and size of nozzles can
be used to accommodate variable flows with little loss of
efficiency.
For any turbine the most important factor in its longevity
is clean water. Careful design of intake structures to eliminate
trash and abrasives is very important. In fish bearing streams
it is also important to minimize any impact. Most intakes
involve a grate then a finer screen. Larger systems may have
a settling basin as part of the intake.
For all the turbines there are huge size ranges available,
from units five cm in diameter to the units many meters across
used by BC Hydro. In all cases however they must be coupled
to some type of generator to produce electricity.
For small home systems a stock or modified automotive alternator
may be selected with a battery bank to even out the variable
demands on the system. Larger systems that are isolated from
other power sources generally use synchronous generators
to get the 60 cycle power standard in Canada. If a system
is connected to the grid it is easier and cheaper to use
an induction generator. It takes a 'signal' from the grid
and naturally cycles in time with it. If the grid power is
interrupted for any reason, such as a tree on the line, the
induction generator loses it's signal and generation ceases.
This is a necessary feature in systems that are grid connected
for the safety of repair crews.
This interconnection increases the cost of a project but
provides some significant advantages. The system can either
be sized to provide part of the required household demand,
the balance being drawn from the grid, or can be sized to
meet maximum demand with any surplus being sold into the
grid on what is referred to as 'net metering'. Still larger
systems may be planned to use part or none of the output.
An essential part in any system is a control system that
creates an equilibrium between the energy of the water, which
is trying to push the turbine faster, and the energy output
of the generator, which acts like a brake to slow the turbine
down. This matching of generator size and rotational speed
with the available energy potential is a very important calculation.
In a simple system using an alternator, speed is not as critical
because the alternator output is direct current (no cycles).
Alternators do have optimum speed ranges and will have limited
life expectancy if run too fast because they are trying to
'push out' too many amps. The batteries serve much of the
control function but if the batteries become fully charged
energy must be "dumped" or redirected, usually into some
heating load. On 60 cycle systems generator speed is more
critical because it determines the cycles/sec.
To maintain the correct load on larger stand alone systems
fairly large amounts of power sometimes need to be "dumped".
Grid connected systems are able to feed this surplus power
into the grid which is a significant economic benefit both
to the producer and to the system at large, allowing Hydro
to retain water to meet peak demands. Their reservoirs are
used as batteries for the grid.
There are many good sites on the net discussing equipment
options and site evaluation but one of the best starting points
is www.retscreen.gc.ca.
It has downloadable programs for site evaluation.
A net search of microhydro provides a wealth of sources of
equipment and information on their intended uses.
Credits
Written by Don Cavers.
|
