Flow of River Hydro -- Using
Only Stream Velocity to Drive a Turbine
If you have a river or stream flowing through your property, you have
probably thought about harvesting some power from it. If the
stream has some elevation drop that you can use, then you probably want
to look at a conventional small
hydro system like the ones here...
But, if your stream has little to no elevation drop, its clear that there is
still a lot of energy there just in the velocity of the flowing water. So, is
there some way to just use the streams flow velocity to make some
electricity?
The answer is maybe, but its a real challenge -- see the material
below.
Note that these comments also apply to systems that utilize tidal
flow.
Power Available from Flowing Water
Click on plot for full size
For fast flowing streams
Click on plot for full size
For slower flowing streams
Update: Aug 25, 2012 -- corrected error in the two charts
above -- thanks to Richard for pointing this out.
The graphs above show the power that might be harvestable from a stream with
a turbine or underflow water wheel that intercepts 1 sqft of flow areas.
So, if you have a submerged turbine whose blades sweep 1 sqft of stream area (a
13.5 inch diameter turbine) in a 5 mph stream, and it has an efficiency of 10%,
then it can generate about 50 watts, which would be 1.2 KWH per day.
For larger turbines, just ratio up by the area that the turbine blades sweep --
for example, a 2 ft diameter turbine would have a blade area of 3.14 sqft, and
for the same conditions would generate (3.14/1.0)(50 watts) = 157 watts.
To estimate the daily energy production, just multiply the average power by 24
hours -- the 157 watt example would produce (157 watts)(24 hrs) = 3768 watt-hrs
or 3.8 KWH per day.
This is the kinetic energy of the flowing water per unit
time.
So, what efficiency might a real turbine achieve?
There are theoretical factors that limit the maximum possible efficiency for
wind or water turbines that are driven by the stream velocity to
around 60% -- this has to do with the idea that the turbine cannot slow the water
down to zero velocity -- there has to be some velocity to carry the water away
from the turbine. But, real world efficiencies for the machines I was able
to find are much lower.
The bottom line answer appears to be around 35% for a very good turbine, but
substantially less for most turbines and waterwheels. The efficiencies often quoted
for real world undershot water wheel designs are in the 15 to 20% area for
carefully designed ones. The
example underwater turbines listed below have efficiencies that range from about
10% up to about 35%.
You can use the graph above with one of the efficiency lines to get an idea
of what the potential energy harvest might be from your stream. I think
that you would have to be a very good turbine blade designer/builder to (or very
optimistic) to assume more than 10 or 15% efficiency.
So, its pretty clear that for slower flowing streams with small turbine
areas, the amount of harvestable power is quite small.
On the other hand if you have a fairly fast moving stream that has enough
width and depth to support a descent size turbine or waterwheel,
then the potential power can be significant. Since the power goes up with
the cube of velocity, the velocity you have available is very important -- be
sure you measure it carefully.
To be successful, flow of river operations need to handle a large amount of
flow and that requires a relatively large piece of equipment for the power
generated compared to conventional hydro installation that work on water
dropping through elevation. But, as the examples below show, this it can
be done.
Update 2015: I just ran across an article that goes over methods for sizing the building low to no head undershot water wheels -- its the first entry in the References section. Design Calculations for no-head, low-head waterwheels.
Some Examples
I was able to find a small number of water wheels and turbines that make use
only of the stream flow velocity to generate electricity. If you know of others,
please let me know.
Ampair UW100
This is
a commercially available submerged water turbine. It is 12.2 inches in
diameter, and generates about 50 watts in a 5 mph flow. The generator
output is 12, 24 or 48 volts DC. Price is around $1000.
For a design that has been around for a very long time, I found surprisingly
little actual design and construction information. What I did find
is listed just below...
Grid-tied Large Undershot in Utah
This is a pretty amazing
homemade undershoot waterwheel in Utah.
This project makes it pretty clear that it is possible to harvest very useful amounts
of energy IF you have a lot of fast flowing water AND you are willing to build a pretty
large contraption. Something that generates 1/5th of this amount of power
would satisfy the needs of many households.
Thanks to Dan Fink at Otherpower.com for the posting and video.
Very Nice Large Undershot on Floats
This is a beautifully built true undershot water wheel.
The size of the wheel is 10 ft diameter by 6 ft wide, 2200 lbs.
Its not really clear from the writeup what the stream velocity is, or what the
power generation is, but it might be around 200 watts at the time of the
posting. The builder was working on ways to get more power.
Alan's Undershot Wheel Project
This is an (sort of)
undershot water wheel project turned in by Alan. I say sort of because
it makes use of a small amount of head that the side creek that feeds the
waterwheel provides. But, lots of helpful construction information.
HydroCat A Commercially Available Floating Undershot Waterwheel
The video below claims that the HydroCat is available in a range of sizes.
After looking at their website, I'm not so sure, but you can contact them and
find out what they have.
It seems that even most of the undershoot and Poncelet wheel projects make use
of a small amount of head -- even if its less than a foot. Maybe there is
a message in this fact?
This is a vertical axis turbine that is said to have high efficiency (35%) and
to provide a smooth delivery of torque through its full rotation.
Prototypes of this turbine up to at least 1 meter in diameter by 2.5 meters long
have been made and tested. It is a patented design, but I
suppose that exceptions might be granted for one off, non-commercial
applications (or not).
I found mentions of the Tyson turbine in several places, but was unable to
determine if it is still available or not. It is a submerged turbine often
suspended below a floating platform.
The first reference paper has some details on comparing the Tyson turbine to
another similar turbine.
Hydrovolts
Hydrovolts.com makes a series of flow of river turbines that range from 100
watts up to 16kw.
There are a number of large underwater turbine offerings that are aimed at
large (multi mega watt) tidal flow installations. The 2nd
reference has
detailed information on a number of these.
Spiral Pump
The spiral pump is a water pump (not a power generator) that can be driven by
an undershot waterwheel.
I want to emphasize that I don't even remotely consider myself an
expert in this area. I have just gotten enough questions on the
practicality of flow of river projects to feel it was worth spending a day
uncovering what I could and passing it on. If you have more information,
or see errors on this page, please let me know either by email, or via the
Comments section below. If you have done a flow or river hydro
installation, or make equipment for flow of river installations, I'd love to
hear about it and get the details. If you know this area, and are willing
to do an article or contribute some material or design information, I'd love to
provide a home for it here.
I just ran across this article in 2015. It covers sizing and building undershot water wheels for no head and low head situations and has some hands on hints for constructing them as well. Definitely worth a careful read.
The Hydro forum at Otherpower.com
There are some folks here who are very knowledgeable
about small hydro, and a number of back posts that
should be helpful: http://www.fieldlines.com
THE EVALUATION OF AN AXIAL
FLOW, LIFT TYPE TURBINE FOR HARNESSING THE KINETIC ENERGY IN A TIDAL FLOW,
W. J. Swenson, Northern Territory Centre for Energy Research, Northern Territory
University
This is a very interesting paper providing the results of
a couple study of a Tyson turbine compared to a turbine with an optimized
turbine blade. Efficiencies just over 30% including drive train
losses were achieved.
The study involved using the turbines under real
conditions, and some of the practical challenges of running these turbines
suspended from floating platforms in debris laden water are covered.
This is a beautifully built true undershot water wheel. 
A
very low head water wheel project with some good construction information
