|
AC/DC:
Micro-Hydro-Electric Options
Paul Cunningham
& ROBERT G. FIFE
When considering a micro-hydroelectric installation,
one of the primary issues is whether it win be all AC
direct system or battery based system.
AC direct systems
consist of a turbine-generator unit producing AC power
which is used as needed. That is, it is fed directly
to the appliances. Governing of such a system is
usually done electronically, with reliable,
off-the-shelf equipment that is readily available. In
order to maintain the correct voltage and frequency
within the parameters required, the power is monitored
and that which is not used by the appliances is
directed to an alternate load, such as heating. This
also means that the appliance load can not exceed the
power generated, as this win result m system collapse.
The generated power is monitored cycle by cycle and is
diverted as required.
In a battery based
system, the generated power is used to charge a
battery bank, then the power is sent to DC loads, or
to an inverter to power AC loads, or both. Regulation
consists of diverting excess power to an alternate
load to prevent battery overcharge. The
battery/inverter combination can provide large surges
of power to handle loads such as pumps, lights, tools
etc. As well, with battery based systems, other
sources of power call be easily integrated (i.e. PV
cells, or wind turbines) and fed to the batteries.
An AC direct
micro-hydroelectric scheme is simpler in its overall
design than battery based systems, and for this reason
they are sought by many people. However, the output of
AC direct systems must be capable of handling all of
the power requirements at any instant, which can be
substantial when startup surges are considered. For
instance, incandescent lights typically require ten
times their running current at turn-on; induction
motors, such as those typically found in refrigeration
and water pumps, may require five to seven times their
operating current for starting. This power must be
available when needed for the system to continue
functioning, as exceeding its capacity win cause an
electrical collapse. Since AC power cannot be stored,
and kinetic energy can, the addition of a flywheel to
the turbine can help carry the system through such
power overdraughts. A battery based system stores the
generated electricity chemically, and so only the
average usage needs to be generated. The batteries
handle the peaks and valleys of the electrical loads.
The generation components of the system can even be
taken out of service for repairs or maintenance
without immediately affecting the power delivered to
the loads.
Both AC and battery
based systems can supply AC power to appliances that
is indistinguishable from commercial power. The AC
direct system usually requires far more power to be
generated than in a battery scheme. This may be the
most important factor in determining the system type
at any given site. As an example, when a refrigerator
drawing running power of 200 W starts, there is a
surge of less than a second during which it may
require up to 1500 W. If this power is not available,
beyond the other loads operating coincidentally, the
system voltage will drop to the point of failure. AC
direct systems, for these reasons, seldom have a
capacity of less than 2 kW. This contrasts with
battery systems which typically require generator
outputs of around 300 W in order to meet the needs of
standard household electrical loads (excluding heat).
Exceptions to this are some residents who use AC
direct induction systems to produce only a few hundred
watts to meet their needs for lighting and small
electrical appliances (mention was made of a system of
this sort in the September 1998 issue of Renewable
Energy World, 'Long Distance Power Transmission for
Renewable Energy Systems', p. 72 ). Note that if power
on this scale was used to charge batteries, then far
more substantial loads could be sustained. The big
advantage to the large output AC direct systems is
that they meet the need for appliances and lighting
while the excess power is usually sufficient to meet
all the hot water needs and most, if not all of the
space heating requirements.
If there are sufficient
resources to implement an AC direct micro-hydro
system, there arises one significant consideration:
the infrastructure required to complete such a system
is much more weighty, both physically, and in the
finances necessary to procure it at the outset, than
that of a battery charging system. Firstly, the
pipeline used to feed a battery scheme is seldom
larger than 6 inches (about 15 cm), and is typically 4
inches (10 cm) or less. Compare this with the much
larger piping necessary to carry the flows required
for AC production, and the price difference can be
prohibitive, not to mention the considerable toil and
expense required to move and bury large-scale pipe.
Secondly, consider the power generating components,
and the equipment necessary to support them. Beginning
at the pipeline, the differences between the AC direct
and the battery based systems can easily be seen,
primarily, in the actual size of the generators.
Usually, a generator that produces a few hundred watts
can be on the scale of the typical automotive
alternator, while a generator in the multi-kW range is
certainly much larger, and depending on whether it is
a synchronous or induction generator, the price can be
even more disproportionate. Add to this the turbine
runners that would necessarily be much larger in the
AC system, and one can easily see how the initial
costs involved in the generation components would
outweigh a battery based micro-hydro generator. From
the generator, leading to the point of usage, run the
conductors, in the form of copper or aluminum wire.
The size of the wire is dependent upon the voltage and
current of the transmission and the distance over
which it is to travel. In long distance situations, AC
travels well as it is usually high voltage, thereby
minimizing line loss on a given wire gauge; in battery
based schemes, the voltage is determined by the
battery bank, so in the cases of low voltage battery
banks, transformers may be necessary in order to step,
then step down voltage, so as to minimize line loss.
Large gauge wire can be used to the same effect. On
the side of battery charging systems, if the
transmission distance is modest, the wire gauge
necessary to conduct hundreds of watts is
significantly smaller than that required to carry
thousands.
While there are many
factors to consider when choosing a
micro-hydroelectric scheme, if the pertinent details
involved are given adequate attention, an optimal
solution can be found for the generation potential of
any given site. It is hoped that the information
necessary to begin this process has been summarized in
this article so as to be a starting point for the
system designer where ever the site may be.
Access
PAUL CUNNINGHAM & ROBERT G. FIFE
Energy Systems and Design
P.O. Box 4557, Sussex, N.B.
Canada E4E 5L7
Phone: +1 506 433 3151
Fax: +1 506 433 6151
e-mail: hydropow@nbnet.nb.ca
|
