A Reader Asks: Isn’t there a Hydro Turbine with Variable Speed?

Jul 06

The Original 2 Questions –

Subject: Hydro turbine questions

From: Peter

Date: Wed, July 01, 2009 4:58 am

To: smallhydroblog@smallhydro.com

I have a few questions that I need answers to. I have a seasonal water fall with 38 feet of head. The flow rate varies with the weather.

Q: My first question is why isn’t there a turbine that works on variable speed? As flow increases, the output should increase.

Q: My second question is in your opinion would it be worth investing a small turbine knowing that I may only get 180 days of power.

I hope to build on this site and will be off the grid. I know I will need a hybrid system with solar/possible wind or water. I want the best, but yet the cheapest way to go.
Thanks for your time!

My Answers –

That’s a great pair of questions about Variable Flow & Speed.  There are hydro turbines to solve the variable flow problem, but that is not the same situation as variable speed. The two parameters are related, since an increase in water turbine shaft speed due to pressure changes will cause a proportinate increase in turbine water flow rate for a fixed load. The problem is that variations in turbine RPM will cause generator output frequency to vary too.
For AC generators, transformers and AC motors variable frequency is a very bad thing, it will cause heating and damage insulation, solder and machines. Frequency variation will cause transmission and synchronization problems, leading to brownouts and other problems too. 
This varible speed turbine control ability is an area of recent hydroelectric system research. DC alternator designs can tolerate this Hydro turbine generator RPM variation because the final AC power output frequency is defined by the inverter design not the alternator speed (output power will vary though.)
So, let’s look back at the possibility of site flow variation solutions and not hydro generator speed variation;

A: Several variable flow Small & Micro Hydro turbine options exist for you – 

A few variable flow Hydro Turbine configurations that come to mind are:

  • Multi jet Turgo or Pelton wheels may work but may not be as efficient as they are when that type of turbine is used for higher head >100ft 30m sites. Low head sites favor reaction turbine types.
  • A better turbine choice would be crossflow with multiple inlet gates.
    • Typical multi gate crossflow uses 2 -3 sections on turbine inlet for 1/3, 2/3, 3/3 or 1/4, 1/2/, 3/4 and 4/4 flow curves respectively.
  • You can use other low head variable flow options, Francis with wicket gate, Kaplan with wicket gate >$, etc.
  • Propeller and simple PAT (unregulated Francis type) systems operate in a narrow efficiency band or curve, yet if you can put 2 or more of these in parallel you can sequence their inlet valves much like the crossflow. Say that you have a 5 kW and a 10 kW pair of units, now you get a 5, 10, 15 kW range efficiently over a broader range of flows – if the turbine curves overlap effectively.

A: Some things to consider about Site Stream Flow Variability and Availability –

The site with flow variation and some months with zero flow may or may not be economic to set up for hydropower, depending on your Flow Duration Curve or FDC and the overall ROI for your hydropower equipment costs. An FDC is a pareto chart or ordered histogram of a stream’s frequency of flows at a given Q or water flow rate. We’ll cover FDC’s and their creation later in the 12 step Small & Micro Hydropower evaluation process.

Here’s some Additional Questions for you to Answer about your Potential Hydroelectric Site…

1. How much peak, avg vs low flow do you have? (Start with a guess, get an FDC later)
2. You said that you have 38 ft head, is there any storage or just natural falls?
3. Do you have flow data showing avg daily flow for greater than 10-12 years?
FYI – Climate change tends to follow 11 yr sunspot cycles more than anything.
4. Do you have water rights or can you get them?
180 days of 100% flow at your sweet spot may work fine energy wise,  if you can punch out enough power. Hybrid solar may help. If you can grid tie, then Hybrid with net metering may work well for you $ ROI wise. If enough power then you can become an Independent Power Producer…
DoradoVista, Inc.
PS.  Let us know how we can help with your turbine selection.  I wanted to let you know that we have some great turbine and water to wire hydropower system sources too depending on what units you are looking for.

Hydro Prospector Jess

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