There is a large range of different hydropower technologies, from Archimedes screw to Zuppinger wheel. For smaller sites there are four in common use.
Archimedes Screw
The Archimedes Screw, as its name implies, has been around a long while and is still used for lifting or moving water in sewage plants or moving grain in grain stores. It is very simple with only one moving part and two bearings.
This lifting device can be used in reverse, where water travels down the screw, causing it to turn.
The screw turns much more slowly (normally between 20-30 rpm) than modern turbines, which in itself has advantages in that it causes less damage to fish. In reality the water is not turning at all, it is the screw that turns and the water (and any fish) get a free ride down. The screw being “open” to the air means that the device operates at atmospheric pressure and so does not adversely affect the swim bladders of fish. Waterwheels are the only other devices that share this advantage.
- Fish friendly
- Low pressure, so swim bladders of fish not affected
- No draft tube so reduced civil costs.
- Can be as simple as a foundation of two concrete piers
- High efficiency over wide range of flow – 100l/s-15m³/s
- No fine screening, so no trash cleaner and debris passes through
- High visual impact, mechanism visible
- Normal rotation speeds 25-30rpm
- Only suitable for low head schemes
Kaplan turbine
At first glance a Kaplan turbine consists of a propeller, similar to a ship’s propeller. In low head situations, the common six bladed design is reduced to three and the water flow is regulated by the movement of wicket gates. The blade pitch is simultaneously varied with the wicket gates to maintain high efficiency under partial flow conditions. The wicket gates are carefully profiled to induce tangential velocity (whirl) into the water flow. This complex engineering and the necessary control systems make Kaplan turbines expensive for both small scale and micro hydro installations.
Considerations
- At designed flow rate most efficient turbine.
- All the mechanism can be architecturally hidden
- Relatively small mesh fish screens required.
- Automatic trash screen clearance needed in most installations.
- Efficiency between 20% & 30% of flow rate is less than other turbine.
- Complex control systems required.
- Penstock and tailrace design crucial to efficiency.
- Civil works are extensive and need to be of high standard to warrant costs of turbine
Crossflow turbine
The crossflow turbine in its simplest form is a drum with slots cut into the sides and the remaining metal angled to catch the water. It is in effect similar to a small waterwheel but the design of the blades allows energy to be extracted both as the flow first strikes the drum and as it exits.
Theoretically, the runner length can be increased to any length without affecting the hydraulic characteristics so doubling the runner length results in a doubling of the power at the same speed. The lower the head the longer the runner becomes. Controlling this length means the output can be variable and so can work “off grid”.
The design will “work” on heads as low as 3m but more commonly operates above 4m. The simplicity means turbines can be fabricated in small workshops and many have been built.
Considerations
- Small machinery size
- Improved part load efficiency can be achieved by varying the width of the turbine blade over which the water is directed
- Power output is a function of runner length
- Fine screening required
- Can operate off grid without battery back up
- Can be “home” constructed (but watch out for FiTs!)
Pelton wheel
(High head micro-schemes)
Once the available head starts to increase then the Pelton wheel becomes the technology to use, being both efficient and having a long life. The runner is basically a wheel with cups mounted around its rim. The devices can work with single or multiple jets.
A common variant on the Pelton wheel is the Turgo wheel where the jets of water strike the wheel at an angle instead of directly.
Considerations
- normally 20m minimum head
- cost of penstock
- installation can involve multiple landowners
- civil works can be very basic
Water Wheels
There were some 10,000 mill sites in the UK of which it is estimated that 2,000, at least, could produce electricity and be commercially viable. Re-installing a water wheel is often a strong consideration in generating electricity at these sites. The majority would have been operating with around 6 – 8 feet (2 – 2.5m) of head and so many of them can be considered for refurbishment.
Overshoot wheels are a viable solution for heads of 2-7m. Undershoot wheels are designed for heads of under 2m and as low as 0.3m.
The electrical output is often not large and the original use of the mill may have only needed and intermittent flow. However many sites will already have their civil structures predominantly intact, substantially saving costs.
