Measuring Water Head for Hydro Systems
What is a Water Head?
You want to install a hydro system.
Of course, there are several factors to consider.
Some of them will directly affect the operations of your hydro system.
One of them is the water head.
But, what is a water head?
Simply defined, water head is the height difference between the point at which water gets into a hydro system and where it leaves.
This height difference is measured in meters.
As you know, there are different types of hydro systems.
This means that the water head is not all the same in all the hydro systems.
In one case, the difference can be between at the turbine entrance and where it discharges.
In other cases, it can be between the intake screen and where it gets back to the watercourse.
- How a Hydro System Works
We would like to know better about the water head.
Before doing that, there is one term that keeps coming up.
That is hydro system.
What is a hydro system?
How does a hydro system work?
Basically, a hydro system is a system that comprises water that is within a specific aquatic environment.
This system should exist within a specific geographical entity.
How do hydro systems work?
This is another important thing that you need to know.
Knowing how a typical hydro system works will help you to appreciate the importance of measuring water head.
Despite that there are different types of hydro systems, all of them have the same working principle.
They all convert the pressure in the head of water into rotary mechanical power.
That is not all.
This mechanical power is then transformed into electricity.
The final consumers then use this electricity for various purposes.
Well, that sounds so general.
Let’s get into details of how a hydro system works.
First, water flows into the intake of a hydro system.
Of course, this intake is on the upstream.
Also, the intake section has a screen which performs two key functions.
These are:
-Prevent the debris from getting into the system and causing mechanical damage.
-Prevent aquatic animals such as fish from getting into the system.
Once the water has passed through the intake screen, it passes through a penstock pipe straight to the turbine.
The penstock pipe is designed to raise the pressure of the water.
The turbine is positioned right at the end of the penstock pipe.
This turbine has a rotor.
The rotor turns the high-pressure water into mechanical energy.
Water from the turbine is discharged at a very slow rate.
This is done with the intention of maximizing the head level.
The draft tube does the work of minimizing the water pressure from the turbine.
Water inside the draft tube is subjected to the force of gravity.
In doing so, it creates negative pressure under the turbine.
Do you need the mechanical energy from the turbine?
Definitely no.
This is why this mechanical rotational energy is directly converted into electricity.
The low-speed power from turbine is converted into high -speed.
This is achieved by simply adjusting the gearbox to make the turbine rotate faster.
The high-speed power is then used to drive a generator.
The main function of this generator is to convert mechanical energy to much-needed electricity.
This electricity is then transferred to private homes or commercial buildings where it is will be consumed.
Sometimes, the electrical energy is exported to the main grid where it is sold.
Well, basically, that is how a micro hydro system works.
With this information, you will have a crystal-clear understanding of how a water head is measured.
As we had defined, water head is the pressure created due to the elevation difference between the turbine and the pipeline.
The value of water head is measured in meters.
However, some people measure it in terms of feet.
Alternatively, you can measure the water head in terms of pressure.
Here, the dimensions used are newtons per square inch or square meter.
Conventionally speaking,a high water head produces more pressure.
This happens regardless of the size of your stream.
More pressure means that there will be more output from the turbine.
- Direct Water Head Measurement
There is no doubt that hydro systems can be quite massive and hectic to work on.
So how can you determine the water head of your hydro system?
One method is through direct measurement.
You can achieve this using a laser level or even a contractor ’s level that is positioned on top of a tripod.
Do you want to measure the water head on the downhill of a hydro system?
You may consider using a tall pall that is calibrated with the actual measurements.
Alternatively, you can fix a measuring tape on this pole.
Remember to adjust tape after recording each measurement.
Also, keep moving the pole downhill as you move closer towards the generator.
In doing all these, ensure that the pole remains in a vertical position..
This will help you to get more accurate readings.
Write down the measurements together with the height level that you get.
Do a summation of all the measurements.
Subtract all the level heights that you collected.
From this, you will be able to determine the total water head.
Here is the graphical representation of measuring the water head when on the downhill of a hydro system
Image source: how to measure water head
-First, subtract the height of the level from the measurement on the stick to determine the head of each leg.
-Repeat for several legs from the intake location to the turbine location.
-Add the head of each leg so that you find the total head.
Measuring the Water Head Uphill
-The height of the level is equivalent to the head of each leg.
-Repeat measuring for several legs starting from the location of the turbine to the location of the intake screen.
-Multiply the height of the level with the number of the legs.
-The value that you will get will be the total head.
- Measuring the Pressure of the Water to Determine the Water Head
Now that you have measured the distance between the inlet, you can move to the next stage
You need to find the pressure difference between these two critical points.
But, how do you do so?
You can use flexible plastic tubing or even garden hoses.
If the distance between the turbine and the inlet is short, you can use garden hoses.
The first step that you should take is to run the hose or the tubing fro0m the proposed intake location to the turbine location.
If you attach hoses to each other, ensure that they form a very tight connection.
Attach an accurate pressure gauge right at the bottom end of the hose.
Fill the upper side of the hose with water.
Ensure that the hose does not have high spots which are capable of trapping air resulting to inaccurate results.
One way of eliminating this air is to flush some water through the hose before connecting the gauge.
After that, you can start taking measurements of the water head.
Move the hose through different locations and take note of the readings.
Unless you are on a very steep hillside, the psi readings will always be 0.5 psi for every vertical foot.
It is advisable to use a long hose to minimize chances of errors.
You should also use a very accurate pressure gauge.
- Calculating the Net Water Head
Image Source:Finding the total head
At this point, you have recorded the above measurements.
You have the measurements for the difference in height and the pressure difference between the turbine and the intake.
So what next?
You can use the measurements that you have obtained to find the net water head.
But, what do we mean by the term “net water head?”
Net water head is the actual pressure at the bottom of the pipeline when water is flowing through the turbine.
This value will always be less than the gross overhead.
The lower value is as a result of the friction losses in the pipeline.
One of the key components for finding the net water head is the water flow figures.
The friction loss is likely to be high in the long pipelines that have small diameters.
Because of this, the design of the pipeline matters when trying to compute the net water head.
A good pipeline should give at least 85% to 90% of the gross water head.
All in all, you should be more careful with the net water head than the gross water head.
This is because you will use it to know the electrical output of your hydro system.
To get the best results, ensure that you talk with your turbine supplier.
The supplier will customize the specifications of your pipeline.
Water Head Loss
When measuring the water head, there is one more thing that you need to know.
That is the water head loss.
What is water head loss?
This is the loss of water power mainly because of the friction inside the pipeline.
What if the diameter of the pipeline is wide enough?
This won’t matter.
You may still have to deal with this pertinent issue of water head loss.
The joints and corners of the pipe may still provide the frictional drag.
This ends up slowing the water that is passing through the pipe.
It is almost impossible to measure or predict the water head loss.
However, you can know this value only when the water is flowing through the pipe.
Large pipes have a tendency of minimizing this water head loss.
This is because the frictional drag in these pipes is less significant.
In the end, they deliver more power to the turbine.
The result is more output in terms of electricity.
However, these advantages come at a cost.
Larger pipes are more expensive than the smaller ones.
This means that you need to make a choice between the head loss and the system cost.
So what is the best thing to do?
Choose a pipe that does not lose more than 15% percent of its total head to head loss.
However, it all depends on the size and the capability of the hydro site.
It is acceptable for high-end sites to experience more losses.
However low, end sites should work towards minimizing the water head losses.
This leads to one question.
Does the length of the pipe matter?
Yes.
The length can affect the efficiency of the hydro site.
It also has an impact on the total cost of the system.
Measuring the length of the pipeline is not a difficult task.
You can just use a tape measure.
Run your tape measure between the location of the intake and the location of the turbine.
Stick to the route where the pipeline will pass through.
Keep in mind that you should run the pipeline up to the creek bed.
This will help to prevent any possible damage especially when the water levels get high.
- What is the Minimum Head and Flow
There is something that everyone who installs a hydro system wants.
To have the maximum water head.
As we had already stated, maximum head guarantees high output.
As you keep in mind the maximum head, you need to be careful with the minimum head.
You also need to consider the minimum water flow that is produced by the micro-hydro system.
A combination of the minimum water head and water flow determines the gross power output.
To have a better understanding, the table below gives a graphic presentation of the minimums.
| Lower Head Hydro power site | High head hydro power sites | ||||||
| Gross Head 2M | Gross head 5m | Gross Head 10M | Gross Head 15m | Gross Head 50m | Gross Head 100m | ||
| 25 kW | 1.9 m3/s | 0.75 m3/s | 0.38 m3/s | 0.15 m3/s | 75 litres/s | 38 litres/s |
- Other Factors to Consider When Choosing a Hydro Site
Image Source: Hydropower site
Apart from the water head, there are other factors to consider when choosing a hydro site.
Here are some of them:
(I)Good Water flow
What is the nature of the water flow?
The stream should have sufficient flow to turn the turbine.
Even if the site has low head, it should at least have enough flow.
However, there is nothing like a perfect water flow rate.
You only need to assess your energy.
From there, you will know whether the hydroelectric generator system will be economically viable.
(II)An excellent grid connection
Your hydroelectric generator may produce more than enough energy.
So, where should you take the surplus electricity?
Most people sell the energy back to the grid.
To achieve this goal, the hydropower site needs to be closer to a good grid connection.
The grid should be strong enough to take the power that is generated from your hydropower.
(III)Highly accessible
Is your hydropower site easily accessible?
Keep in mind that there will be lots of movements to and from the site during the construction stage.
The hydro site will also require regular maintenance even after the construction.
The accessibility of the site should be highly considered.
(IV)Environmental issue
You should not ignore the environmental issue of the site.
Will the hydro generator cause any form of environmental pollution?
The good news is hydroelectric turbine rarely causes environmental degradation.
(V)Legal issues
There are existing laws regardless of where you are living.
Seek legal consent before installing a water turbine generator.
Otherwise, you may end up incurring heavy losses in case the local authorities are of a different opinion regarding the site.
So, what next?
I hope that this you have learned several things from this article.
You now know how to measure and calculate a water head of a hydro site.
Before doing that, you should consider factors for choosing a hydro site.
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