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.
|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.
The equation plotted in the graphs is:
Power = (1/2)(Turbine Efficiency) (Water Density) (Turbine Flow Area) (Water Velocity)^3
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.
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.
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.
The UW100 Manual: http://www.ampair.com/downloads/UW_100_manual_rev_Feb_2007.pdf
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...
This is a pretty amazing homemade undershoot waterwheel in Utah.
It generates 3KW and is actually grid-tied.
This Otherpower.com forum posting provides some detail on it. Be sure to watch the YouTube video...
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.
This is a beautifully built true undershot water wheel.
Details in this post on the Otherpower forum...
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.
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.
The curved paddles improve efficiency.
A bit of design information on the Poncelet...
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.
Youtube Video of HydroCat in Operation...
A very low head and very small water wheel with lots of construction detail from Otherpower...
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).
A commercial version of the turbine may be available here: http://www.gcktechnology.com/GCK/pg2.html
Cost at the current very low production volume is reported to be $6000 per unit for the 1 by 2.5 meter turbine.
It looks like a good metal worker might be able to make one of these turbines in a not to fancy shop.
A homemade(?) Gorlov turbine that has been operating in Brazil since 2001.
Wikipedia article on Gorlov Turbine...
This is a good paper by Dr. Gorlov (the inventor), with quite a bit of design information and a number of application ideas: Development of the Helical Reaction Hydraulic Turbine, Alexander Gorlov, 1998
The Gorlov turbine needs to be installed such that the the top of the turbine is will below the surface of the water to prevent the turbine from forming a vortex that sucks air into the turbine and reduces its efficiency: http://oceanenergy.epri.com/attachments/streamenergy/reports/004TISECDeviceReportFinal111005.pdf
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.com makes a series of flow of river turbines that range from 100 watts up to 16kw.
They have a good reference page on flow of river turbines...
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.
The spiral pump is a water pump (not a power generator) that can be driven by an undershot waterwheel.
Some history, design information, construction details and project pictures here: http://lurkertech.com/water/pump/tailer/
Several spiral pump projects: http://wildwaterpower.com/
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.
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.
EPRI Ocean Energy Web Page
Practical Action has a page on river turbines
Gary November 15, 2011