Designing a pico hydro turbine and generator

[mech_boss]

I'm working on a project right now which requires a hydro turbine generator system requiring 120W of power output capable of working with the following specs:

Flow: 0.003m^s/s
Pressure: 6m head

By my calculations, this should require an efficiency of ~66%. All flow must be in the axial direction, as the space requirements of the application mean the turbine must be no more than 3" diameter or so. I plan to use a standardized off the shelf generator (if possible) and then design the turbine somewhat from scratch. I will conduct a CFD analysis once I have determined what exactly I should be looking to optimize and what rough blade profile I should be using. I have a few things I am uncertain about though:

1) I would like to use direct drive coupling between the turbine and generator in order to minimize cost and complexity of the system, and I know this will be based on the generator RPM, but what exactly determines the turbine RPM that will be achieved? Also what will determine the torque of the turbine/ shaft/ generator?

2) What more information will I require about the system? At this time I don't know exactly the load resistance that the generator will be connected to. Would this be useful to know?

3) What type of generator should I be looking at specifically? I have seen similar applications (mostly for wind turbines) in this power range that use permanent magnet generators. Would this be the best option?

4) Are there any formulae other than P=Tω and P=ηρghQ that I should know in order to help determine the necessary parameters of the system?

Thanks a lot for your help everyone.

 

[DEvens]

Will you really be doing CFD (computational fluid dynamics) analysis of this system? The motion of water around a turbine is pretty complicated. You would need to make a fairly sophisticated and accurate model to get useful results. On the plus side, with 120Watts of power and 6 meters of head you are not likely to get fluid flow velocities more than a few meters per second. That will reduce the complexity a little.

But you might find it easier, and quicker, to do some practical tests of different turbine designs. Producing water flow in this range isn't particularly difficult. If you can borrow a pump such as is used to empty a swimming pool you should get enough flow. If I'm doing the arithmetic correctly you are looking at around 3 kg/s flow. Looking inside some such pumps might give you hints as to the nature of turbine you want, though many of those pumps will be centrifugal impeller type pumps.

The things you will want to vary to optimize your turbine: number of blades, length of blade, width of chamber the turbine sits in, angle of the blades. You could also consider whether your turbine should be one layer or two or even three. To get really fancy, do you want a classical windmill kind of turbine? Or do you want a turbine with axis perpendicular to flow rather than parallel to it?

Or you could get really sneaky and build something like a water wheel. A flow of 3 kg/s isn't that big. A water wheel 12 meters tall might actually work.

 

[Nidum]

Your questions are far too complex for quick answers but here are a few hints in the right direction :

Any likely turbine design will have a small number of relatively wide blades and only one row .

Maximum blade velocity is determined only by the incoming flow velocity . On load blade velocity will drop a lot .Typically blade velocity will be about half incoming flow velocity on full load . Actual rotational speed is determined by blade velocity and geometry of turbine .Rotational speed is not going to be very high in any case .

To design any turbine you need to understand relative velocity and velocity triangles . Best to look these up .

Permanent magnet dynamo's are ok but there are other alternatives .

If you can ask more specific questions I will attempt to answer them . Ideally sketch out a tentative design and we'll evaluate it .

 

[mech_boss]

Will you really be doing CFD (computational fluid dynamics) analysis of this system? The motion of water around a turbine is pretty complicated. You would need to make a fairly sophisticated and accurate model to get useful results. On the plus side, with 120Watts of power and 6 meters of head you are not likely to get fluid flow velocities more than a few meters per second. That will reduce the complexity a little.

But you might find it easier, and quicker, to do some practical tests of different turbine designs. Producing water flow in this range isn't particularly difficult. If you can borrow a pump such as is used to empty a swimming pool you should get enough flow. If I'm doing the arithmetic correctly you are looking at around 3 kg/s flow. Looking inside some such pumps might give you hints as to the nature of turbine you want, though many of those pumps will be centrifugal impeller type pumps.

The things you will want to vary to optimize your turbine: number of blades, length of blade, width of chamber the turbine sits in, angle of the blades. You could also consider whether your turbine should be one layer or two or even three. To get really fancy, do you want a classical windmill kind of turbine? Or do you want a turbine with axis perpendicular to flow rather than parallel to it?

Or you could get really sneaky and build something like a water wheel. A flow of 3 kg/s isn't that big. A water wheel 12 meters tall might actually work.

- It's for an academic research project so ideally I'll get some CFD analysis done.
- Because of the nature of the enclosure the turbine must fit in, it must fit within a 3" diameter circular cross section. Any configuration is fine as long as it fits in this area.

Thanks for your reply!

 

[mech_boss]

Your questions are far too complex for quick answers but here are a few hints in the right direction :

Any likely turbine design will have a small number of relatively wide blades and only one row .

Maximum blade velocity is determined only by the incoming flow velocity . On load blade velocity will drop a lot .Typically blade velocity will be about half incoming flow velocity on full load . Actual rotational speed is determined by blade velocity and geometry of turbine .Rotational speed is not going to be very high in any case .

To design any turbine you need to understand relative velocity and velocity triangles . Best to look these up .

Permanent magnet dynamo's are ok but there are other alternatives .

If you can ask more specific questions I will attempt to answer them . Ideally sketch out a tentative design and we'll evaluate it .

Why would a small number of blades be the most suitable design for this application? Like DEvens said above, I have looked at the impellers for some pumps that operate in this range and their geometry is a little more complex:

http://ca.grundfos.com/content/gca/en_CA/products/find-product/redi_flo/_jcr_content/tabbedpanel/brochures/par2/downloads/download/file/file.res/LRFSL005_0400_RediFlo.pdf

I have seen permanent magnet dynamos as a pretty common generator type in this range but they seem a little large for this application in terms of physical size. Could I get away with using a permanent magnet alternator or a DC motor running in reverse? What are the intrinsic losses when you run a motor in reverse (in terms of percent efficiency)?

Thanks for your help!

 

[Nidum]

An axial pump can only ever generate a small pressure rise /stage efficiently whereas an axial turbine can utilise a huge pressure drop/stage efficiently .

Therefore large number of stages in axial pumps and small numbers of stages in axial turbines for same pressure rise/drop .

Design of turbine stages (at low temperatures) is relatively simple whereas designing pump stages is very complex .

So for your proposed design one simple stage will probably do almost all you want .

Choice of actual number and detail design of blades is a matter for investigation but in principle it will certainly turn out to be a relatively small number of wide blades .

Studying pumps gives little guidance to designing turbines .

 

[Nidum]

For the generator I would look at external rotor designs . Very high efficiency and high power output in small frame sizes .

 

[mech_boss]

For the generator I would look at external rotor designs . Very high efficiency and high power output in small frame sizes .

Do you have a link to examples of any models?

 

[Nidum]

Seems to be a shortage of published information on the generator variant . Plenty on motors .

I'll investigate further .

 

[mech_boss]

Do you think any of these motors in the 120W or 150W range would be a good choice if run in reverse? How often do you have to change a brushed motor if the system is running constantly?

http://www.maxonmotor.com/maxon/view/catalog

 

[mech_boss]

Also can someone explain why there is a direct relationship between torque with current and speed with voltage in a generator?

 

[Nidum]

Do you think any of these motors in the 120W or 150W range would be a good choice if run in reverse? How often do you have to change a brushed motor if the system is running constantly?

http://www.maxonmotor.com/maxon/view/catalog

Generally speaking most motors are optimised as motors and most generators are optimised as generators . There are dual function machines but these are usually specially designed .

As for suitability of a specific motor you need to get technical information from manufacturer otherwise we would just be guessing .

Something to establish early is the range of speeds turbine is likely to run at so as to look for suitable matched generators .

Brushed motors of higher quality are usually reliable for long periods .

Whilst there are many motor designs available in small sizes there are very few generators so there is a potential problem in finding one .

Quite feasible to design and manufacture a generator that does exactly what you want if nothing suitable available commercially .

 

[Nidum]

How much axial length is available for assembly of turbine and generator together with any pipework et al ??

 

[Rogue909]

By my calculations, this should require an efficiency of ~66%.

From my understanding (granted, it is rather limited) a turbine in any fluid cannot pull more than 59% of energy from a fluid. This is referred to as the Betz Limit.

https://en.wikipedia.org/wiki/Blade_element_momentum_theory