Measuring Streamflow Q. Using a Rectangular Slotted Weir

Dec 21
2009
Leave a
Comment

One method to accurately measure flow Q for small and medium-size streams is through the use of a slotted Weir.

Methods for Measuring Your Stream Flow…

Stream Flow x Fall = Hydro Power

Stream Flow x Head Pressure = Power

Stream levels will change through the seasons, so it is important to measure FLOW at various times of the year. We will need these varied flow measures to create an FDC or flow Duration curve, more on the FDC in a later post. If this seasonal variable flow measure is not possible, attempt to determine various annual flows by discussing the stream with a neighbor, or finding US geological survey flow data for your stream or a nearby larger stream. Also keep in mind that fish, birds, plants and other living things rely on your stream for survival. Especially during low water seasons, avoid using all the water for your hydro system. FLOW is typically expressed as volume per second or minute. This is also called a “FLOW rate” since it is a dynamic volume per time interval. Common examples of volume units are gallons or liters per second (or minute), and cubic feet or cubic meters per second (or minute):

A rectangular slotted Weir consists of a temporary dam structure with a rectangular slot are opening gate.

This slotted Weir gate has the following characteristics;

  1. All stream flow to be measured, Q. is constrained to go through the slotted gate.
  2. The bottom of the rectangular slotted Weir gate is leveled horizontally.
  3. A reference stake or pole is driven into the stream bed below the water line. So that it is exactly level with bottom of the Weir gate.
  4. The stake must be placed upstream at least four times the distance of the maximum Weir gate water depth.
  5. Water must be allowed to exit the Weir gate freely, such that there is an air gap beneath it as it flows over the Weir. A “sharp” 90 degree edge lip helps here.
  6. Water upstream of the Weir must move freely and not have major disturbances.
  7. Water will contract or shrink in width x depth, as it increases speed, when it approaches and flows through the opening.

     

Given both the width and depth of the water flowing over the Weir; it is a simple procedure to look up the value for the water flow using a Weir table.


Measure Stream Flow Q using a Rectangular Weir (contracted) Measure Stream Flow Q using a Rectangular Weir (contracted)

The following table is based on a reference Weir gate 1 inch wide.

An example of use is as follows:

Assume your Weir gate is 1 foot wide or 12 inches, you measure the water passing over it at 6 1/4 inches.

Using the table, you look up 6+1/4 and read 6.2 5 CFM per inch of width.

Multiply 6.25 CFM/in x 12 in = 75 CFM. That’s a pretty decent flow, if you have enough head you may be in business.

FYI – Metric Formula for a rectangular notched Weir is: Q = 2/3 x Cd x , 2g^1/2 x (L – 0.2h) x h^3/2, Where Cd is the coefficient of discharge.

Take Cd = 0.6 (normal case) then Q = 1.8 x (L – 0.2h) x h^3/2 in liters/sec

—

Inches
  +0/8 +1/8 +1/4 +3/8 +1/2 +5/8 +3/4 +7/8
0 0.00 0.01 0.05 0.09 0.14 0.19 0.26 0.32
1 0.40 0.47 0.55 0.64 0.73 0.82 0.92 1.02
2 1.13 1.23 1.35 1.46 1.58 1.70 1.82 1.95
3 2.07 2.21 2.34 2.48 2.61 2.76 2.90 3.05
4 3.20 3.35 3.50 3.66 3.81 3.97 4.14 4.30
5 4.47 4.64 4.81 4.98 5.15 5.33 5.51 5.69
6 5.87 6.06 6.25 6.44 6.62 6.82 7.01 7.21
7 7.40 7.60 7.80 8.01 8.21 8.42 8.63 8.83
8 9.05 9.26 9.47 9.69 9.91 10.13 10.35 10.57
9 10.80 11.02 11.25 11.48 11.71 11.94 12.17 12.41
10 12.64 12.88 13.12 13.36 13.6 13.85 14.09 14.34
11 14.59 14.84 15.09 15.34 15.59 15.85 16.11 16.36
12 16.62 16.88 17.15 17.41 17.67 17.94 18.21 18.47
13 18.74 19.01 19.29 19.56 19.84 20.11 20.39 20.67
14 20.95 21.23 21.51 21.80 22.08 22.37 22.65 22.94
15 23.23 23.52 23.82 24.11 24.40 24.70 25.00 25.30
16 25.60 25.90 26.20 26.50 26.80 27.11 27.42 27.72
17 28.03 28.34 28.65 28.97 29.28 29.59 29.91 30.22
18 30.54 30.86 31.18 31.50 31.82 32.15 32.47 32.80
19 33.12 33.45 33.78 34.11 34.44 34.77 35.10 35.44
20 35.77 36.11 36.45 36.78 37.12 37.46 37.80 38.15

A Weir is especially effective for measuring FLOW during different times of the year. Once the Weir is in place, it is easy to quickly measure the depth of the water and chart FLOW at various points in time. Design Flow Even though your Flow may be very high after exceptionally rainy periods, it probably won’t be cost effective to design your turbine system to handle all that water for just a few days of the year. Instead, it makes sense to build a system that uses Flow you can count on for much of the year. This is called Design Flow, and it is the maximum Flow your hydro system is designed to accommodate. Design Flow, along with Net Head, determines everything about your hydro system, from pipeline size to power output.  For more on measuring stream flow you may want to visit Canyon Hydro – Measuring Flow, or British Hydro – Flow Measuring.

Written by Jess in: Small & Micro Hydro | Tags: , , , ,

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

Jul 06
2009
Leave a
Comment

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!
Pete

My Answers –
 

Peter,
 
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…
 
Sincerely,
Jess
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.
 
 
Written by Jess in: Hydropower system, Small & Micro Hydro, Water Generators, Water Turbines | Tags: , , , , ,

Hydropower Calculation Example in Metric (SI) & British (US) units

May 14
2009
Leave a
Comment

Last week I received a request for a working example of  hydropower calculation procedures.  Along with that request was another question asking what the 1/11.8 factor meant when working with feet vs. the 9.800 factor using meters and kW .  The latter 9.800 factor was what that reader was familiar with.  Please bear with me, there is a lot of interesting math ahead!

I will deal with both of these requests and a little bit more about hydropower efficiency computations  in today’s SmallHydro.com blog post.

Part of the mystery hydro conversion factor issue stems from the different measurement units used (SI) or metric vs. (US) or British imperial units.  These units of length, volume and time all play a role in getting the “right” answer when computing Small & Micro hydro power.  Above all, memorize this relationship:

HydroPower = Efficiency x Pressure drop x Flow rate = η x (ρ · g · h) x Q

Another interesting note shown at the bottom of the chart is that when computing the sites turbine inlet hydrostatic pressure = (rho x g x h) the pressure coefficient value for (rho x g) is quite different for each measurement unit type. This coefficient varies with the inverse of the 4′th power of the length for a given unit, hence all the strange conversion factors, like 1/11.82 for (US- kW) and 9.807 for (SI-kW), etc.

Generally it is best to stick with a single unit standard when working these problems to avoid mistakes! Metric really makes the conversion factors simpler, but the US is still using the British imperial system long after others have moved on, so I include it for comparison. Of interest too is that this site is right on the edge of Small or Micro Hydropower qualification.  It would make a nice renewable hydro power addition to a ranch, small village or community.

This handy chart shows an example of a 150 meter hydropower site with 0.095 cubic meters per second flow rate. The British (US) unit equivalents of 492.126 ft and 3.355 CFS are shown computed as well as incorporating both turbine and generator efficiency for accuracy.

Here is a PDF file of the same chart for printout: Hydropower-calculations-metric-si-british-us

Sincerely,
Jess

Hydropower Calculation Example SI & US units

Hydropower Calculation Example SI & US units

Written by Jess in: Small & Micro Hydro, Small Hydro News | Tags: , , , , ,

What’s on Your Mind Regarding Small & Micro Hydropower?

May 07
2009
Leave a
Comment

What are the essential one or two most important things that are blocking you from moving forward with your Hydropower site ideas or plans?

Now is your chance to pipe up about what is the biggest thing you think is preventing you from starting or completing your Small or Micro Hydroelectric project?

For example:  I’m blocked by; ‘Regulations…’, ‘Cash or capital issues…’, ‘Understanding the site…’, ‘Not sure how to proceed with…’, etc. You fill in the missing words in your comment on this post,  Try to keep discussion focused on the  issue that most impacts your progress.

Use the comments on this post to gain or give insight and provide feedback between all the readers.

This is also where a few of our more experienced Renewable Hydropower readers can really help by adding your comments to any threads as you have insight..

Sincerely,
Jess

PS.  This is a great time to think hard about what is really blocking your way forward with hydropower.

Written by Jess in: Small & Micro Hydro | Tags: , , , , ,
« Older Entries

Hydro Prospector Jess

Got SmallHydro?sidebar-pic
How may we help you Prospect for Small & Micro Hydropower Resources?