The head (measured
in metres(m)) is the vertical drop from the top of system (where the
water enters your pipe/penstock) to the bottom (where it is released).
Head should not be confused with the distance between the top and bottom
of the system (which should be kept to a minimum). It is the difference
in height, the drop, that counts. The greater the head, the higher the
potential power. There are several ways you might measure the head.
You could check an ordinance survey map for contour lines. You could
measure the straight line distance between top and bottom and the angle
between the two and use trigonometry. You could even borrow an altimeter
(some gimmicky watches have them).
The
flow (measured in litres per second (l/s)), is the volume of water
which flows past any given point in the system within the period of
one second. It needs to be measured before the turbine is in place in
order to find the potential power because the flow will be reduced when
it is fitted. The greater the flow, the higher the potential power.
There are several ways to measure flow but the simplest is to use a
bucket of a known capacity and time how long it takes to fill. Eg. If
it take 5 seconds to fill a 10 litre bucket you have 2 litres per second.
Calculating the power
(measured in watts (W)), is done by multiplying the head and flow by
the force of gravity (around 9.8 m/s/s which you can't easily change
significantly without leaving the planet). For ease of calculation you
could use 10 m/s/s as the force of gravity - no one will notice (unless
they are very unfit).
To calculate the potential
power..
head (m) x flow (l/s)
x gravity (10) = power available (w)
For example, if you
have just 0.25 l/s and a head of 20 metres, the power available
would be 20 x 0.25 x 10 = 50 watts
Note: What
you consider practical or a useful amounts of power, may well
differ from other peoples ideas so bear that in mind when you
see figures sometimes quoted for minimum usable head and flow.
If all you want is to provide is a trickle charge to keep a battery
toped up when not in use for long periods of time, you might be
happy with a fraction of a watt from a home made system rather
than buy expensive solar panels or have to keep buying new batteries.
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The main parts of a hydro system are the penstock and the turbine. The
penstock is any artificial method by which you get water to your turbine
from the highest point in your system with the minimum of obstruction
and resistance. Penstock may be in the form of pipes or an artificial
channel but sometimes the landscape may have provided a natural solution.
Other factors that effect the amount of power available include the
resistance experienced by the water while on its way to your turbine
which is directly proportional to the length of your penstock. If you
are using pipe it is important to use a large enough diameter and keep
it as short as possible while still providing the amount of head you
want. Avoid any sharp bends or joins etc.
The type of turbine you use will dictate how much of the available
power you can actually provide to the generator and the efficiency of
the generator will dictate the amount of electrical power provided.
You will probably get less than half the available power. Your losses
don't end there. If your turbine is located a great distance from the
site you plan to use you power you may also have transmission losses
in the cable. But what the hell, even with the losses, something is
better than nothing.
This page
will be updated to include;
- Information on
the use of pumps as turbines,
Our first system used a rotary pump as a turbine. These are fairly
easy and cheap to obtain and can provide quite high efficiencies.
Information about suitable types of pump can be obtain from a book
produced by Intermediate Technology (see below).
- Possible designs
for building turbines from scratch
We have build a pelton wheel from flat plate aluminium. Other ideas
include using teaspoons or casting individual pelton cups and fitting
them onto a hub. If you want to chat about these ideas, please do
get in touch.
- The
reuse of impellers and small propellers from model boats etc.
Just an ideal but it should work. There is a name for this but I can't
recall it. Coaxial / In flow ?
Basically you would place a propeller like turbine inside a curved
section of pipe so that the shaft could exit the pipe and be attached
to a generator.
- How
to build a filter
Our first system failed fairly often due to a poorly designed filter.
A system using a pump as an turbine will need a good filter. Our latest
system doesn't suffer from the same problem since small object can
not jam the turbine.
- The
use of DC motors as generator
Permanent DC motors are a cheap and easy source of generators. They
work in the same way as a dynamo. There are more efficient ways of
doing this but this is certainly the easiest. DC motors can be found
in scraped cars (fan motor etc.). Turn them fast enough and you've
got power.
- How
to generate AC and why you might want to
There are reasons why you might want to generate AC even if you only
use 12v DC systems. High voltage AC can be transferred long distances
in thin cable while much thicker cable would be required for low voltage
DC. AC can be stepped up or down easily and efficiently using transformers.
The same is not true of DC. For these reasons it often makes sense
to start with AC. Induction motors are a very common form of AC motor
that can be obtained very cheaply. They come in a variety of sizes
and can be made to act as generators. See the book from Intermediate
Technology to find out how to do this (and then let us know).
Keep It Simple Stupid: Don't forget that you don't have to produce
electricity when you need power. When it is possible to do so, you will
find it more efficient to use the available mechanical power directly.
For example; powering machinery such as a saw mill, grinder, stone polisher
etc.
For more detailed information on the subject of small scale hydro
power you might like to consult these outside links:
The
Micro Hydro Centre - Research, information and development projects.
Pico
hydro website - Articles, newsletter, useful guides.
Hydropower
literature - A comprehensive list of books on the subject. We can't
recommend any specifically because we haven't got read any yet, however
we are sure that the books published by Intermediate
Technology on the use of
pump as turbines and the use of induction
motors as generators both sound very useful and would be a great
addition to our library (hint hint).
Note:
If you are planning on setting up a hydro power system in the United
Kingdom you may require a water extraction license under the Water
Resources Act 1991. There are exceptions from the annual fee based
on the power generating capacity of the system and also on the level
of water use and type of installation. To find out more, contact
the Environment Agency. Similar legal considerations may apply in
other countries. |
See also: General information
about hydro electric power
Our own micro hydro system at Steward
Wood
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