The actual power that any turbine can produce is dependent on your water resource.
Designing A Hydro System - The Basics
Solar PV is easy. Just install it, connect it up and it does the rest. It starts to generate electricity when enough light falls onto the solar modules in the morning and how much it will generate through the day depends on the amount of light available. Then at night it simply shuts itself down until the next morning.
Wind Turbines are quite easy. They typically start generating electricity when the wind speed reaches 3 metres per second and will continue to generate over a wide and fluctuating wind speed up to around 15 metres per second when they automatically shut themselves down for safety reasons.
Hydro is different. The energy source is a volume of water falling through a vertical distance. (Flow in litres per second and head in metres) Just like solar PV and wind, the amount of water available can vary widely over winter and summer, or following heavy and prolonged rainfall.
Unlike solar PV and wind turbines which can cope with wide variations in light and wind speed respectively, the hydro electric turbine requires a constant and steady flow of water to generate electricity efficiently 24 hours a day, 365 days a year.
Every Hydro Electric Turbine is designed specifically for the site where it is going to be installed.
There is no "Off The Shelf” or "One Size Fits All” turbine available.
A typical installation will have a small impoundment dam in a stream well up a hill from where a pipe (penstock) takes off some of the water available in the stream and conveys it down the hill to the turbine site where the water passes through nozzles at high velocity to strike an impellor causing it to rotate and drive the alternator to generate electricity. The water then falls out of the bottom of the turbine housing and is diverted back into the stream again.
The amount of energy (electricity) we can get out of the turbine depends on the volume of water delivered to the turbine and the vertical distance it is falling. (Flow and head)
The head is the vertical distance measured between the turbine inlet and the point in the stream at which you will position the penstock inlet and is generally measured in metres. The flow is the volume of water you will take out of the stream and down the penstock to the turbine. This is measured in litres per second. The formula for calculating the potential power output in Watts of a specific site is quite simple:-
Head (metres) x Flow (l/s) x Gravity (9.81) x Turbine efficiency (50%). A stream with a head of 16 metres and an abstraction rate of 10 litres per second will look like this:-
16 x 10 x 9.81 x 0.50 = 785 Watts (0.785 kW)
There are many variables which can affect the output:-
- The volume of water available in the stream during the period when you want to generate electricity. Ideally this would be all year around, but on some sites it is only over a 4 month winter period.
- The volume of water you can abstract from the stream without adversely affecting the ecology of the stream between the point of abstraction and return which is called the depleted reach. This might be only 50% of the flow available.
- The length and diameter of the penstock pipe, the material it is made of and the number and severity of the bends in the pipe, all of which will restrict the efficiency of the pipe.
- The diameter of the nozzle jets in the turbine itself.
- The size of the electricity cable and its length.
When all of these measurements have been taken and the calculations made, your turbine can be manufactured to suit your site specifically. If any of the parameters change then the turbine may not operate efficiently, or at all without modifications to the nozzle sizes and perhaps the generator as well.
Five Steps To A Successful Hydro Installation (and becoming an alcoholic)
- Location. Identify the mostly likely site for the point of abstraction and the location of the turbine.
- Consider the distance from the turbine to the point of connection to your house or buildings. Long cable runs can be expensive.
- How will you route the penstock from the abstraction point to the turbine. Steep runs will require a lot of support for the penstock pipe whilst longer shallow runs will have greater frictional losses in the pipe and will be more expensive.
- The abstraction point will require regular maintenance to clean the intake screens and ensure the water flow is adequate.
- Measurements. Record the water flow and the head.
- The specification for your turbine is based on the design flow you have available in your water course. The design flow (measured in litres per second) will be the amount of water which can flow down a pipe of fixed size and length from the point of abstraction to the turbine location. The design flow must always be less than the minimum amount of water in the watercourse.
- The volume of water in your stream which is over and above the design flow will pass over a spill way or fish ladder and continue down the stream through the depleted reach.
- The Environment Agency (NR Wales) may specify that the design flow is not greater than 50% of the minimum flow so that the ecology in the depleted reach is not damaged.
- Permissions and Registration
- Submitting an overview of your project to the Environment Agency (NR Wales) as a preliminary application will identify if will be acceptable or will be rejected.
- Local Authority Planning permission may be required for the modification or construction of any permanent features such as dams or turbine houses.
- With the EA and planning "in the bag” you can start the pre accreditation process to qualify for the feed in tariff through OFGEM
- Your turbine will be ordered from PowerSpout by Westflight and manufactured in accordance with the design specification from the PowerSpout calculator. Delivery is generally 2 – 3 weeks from order.
- All site work and infrastructure including the impoundment (if you have one), intake screen (essential) penstock and turbine base should be constructed.
- On the electrical side the cabling from the inverter location to your point of grid connection has to be specified, installed and tested by a qualified electrician.
- The turbine can then be installed and connected to the penstock.
- A number of tests must be completed on the installation to ensure that it is operating correctly before it can be turned on. The turbine rotor speed and open circuit voltage must be within the design limits or the inverter may be seriously damaged.
- With everything in place and signed off you can turn the switch and sip the champagne as you contemplate the sheer pleasure and economic benefits that hydro power provides.
- Maintenance and administration
- On a regular basis you must check the intake screen for both accumulated debris and its effectiveness.
- On a daily basis you should monitor the output in kWh for the previous day. Any change in output may mean that a turbine jet is blocked with leaves or similar.