A Kaplan turbine is basically a propeller with adjustable blades inside a tube. It is an axial-flow turbine, which means that the flow direction does not change as it crosses the rotor. Figure 1 shows a simplified Kaplan turbine.
The inlet guide-vanes can be opened and closed to regulate the amount of flow that can pass through the turbine. When fully closed they will stop the water completely and bring the turbine to rest. Depending on the position of the inlet guide-vanes they introduce differing amounts of ‘swirl’ to the flow, and ensure that the water hits the rotor at the most efficient angle for the highest efficiency. The rotor blade pitch is also adjustable, from a flat profile for very low flows to a heavily-pitched profile for high flows (see Figure 2). This adjustability of both inlet guide-vanes and rotor blades means that the flow operating range is very wide (a characteristic from the inlet guide-vanes) and the turbine efficiency is high and the efficiency curve very flat (a characteristic from the adjustable rotor blades allowing optimum alignment of the blade to the oncoming flow).
The nose cone on a Kaplan turbine is important hydro-dynamically to reduce losses and prevent the formation of a core ‘rope vortex’, and also provides the space for the complex blade pitching mechanism inside. The draft tube is also a critically important part. Although a static fabricated part, the geometry of the draft tube is carefully designed to extract any remaining kinetic energy from the flow by reducing the water pressure at the exit of the rotor.
There are variants of Kaplan turbines that only have adjustable inlet guide-vanes or adjustable rotor blades, which are known as semi-Kaplan’s. Although the performance of semi-Kaplan’s is compromised when operating across a wide flow range, for applications where the flow does not vary much they can be a more cost-effective choice. Figure 3 below shows how the efficiency varies across the operating flow range for a full-Kaplan (curve A), a semi-Kaplan with adjustable blades (curve B) and a semi-Kaplan with adjustable inlet guide-vanes (curve D). It also shows the efficiency curve for a propeller turbine (a Kaplan with fixed blades and fixed inlet guide-vanes (curve C).
Kaplan turbines could technically work across a wide range of heads and flow rates, but because of other turbine types being more effective on higher heads, and because Kaplan’s are relative expensive, they are the turbine of choice for lower head sites with high flow rates. Typically they are used on sites with net heads from 1.5 to 20 metres and peak flow rates from 3 m3/s to 30 m3/s. In the UK this tends to be on lowland rivers with low heads (1.5 to 5 metres) and relatively high flow rates (up to 30 m3/s). Such systems would have power outputs ranging from 75 kW up to 1 MW.
The smallest good quality Kaplan turbines available have rotor diameters of 600 mm, though these tend to be prohibitively expensive, at least a very low heads, so generally speaking the smallest rotors are 800 mm. The largest rotors available have 3 to 5 metre diameters. For even larger sites multiple-turbines tend to be used rather than increasing the diameter further. Kaplan turbines are available in three basic configurations; vertical axis, horizontal axis (also called S-turbines) and bulb turbines.
The commonest orientation currently being installed in the UK (at least by Renewables First) is vertical axis. Vertical-axis Kaplan’s have the advantage of requiring the smallest footprint or land-take. A typical layout is shown in Figure 4. The Kaplan turbine is built into the concrete structure, with the inlet volute (basically a snail shell shaped pipe that wraps around the inlet guide-vanes and distributes the water equally around the whole circumference) and draft tube cast into the concrete at the construction phase. So critical is the perfect geometry of the intake volute and draft tube that it is normal practice for the turbine manufacturer to supply the wooden formwork for these parts to be used by the civil engineering contractor.
Horizontal axis or ‘S-turbines’ and bulb turbines are technically slightly more efficient than vertical axis Kaplans because the inlet flow does not have to change direction so should have lower hydraulic losses. In reality there is no discernible difference, so the decision on the orientation is normally made through choice of supplier and price. S-turbines do require a larger system footprint which can be a disadvantage in space-constrained sites. A typical S-turbine layout is shown in Figure 5.
Bulb Kaplan turbines have all of the drive system and generator accommodated inside a streamlined ‘bulb’ that sits within the main flow. They are only practical on large hydro projects where it is physically possible for a person to climb down into the bulb for maintenance and are normally used on large systems only. A typical cross-section is shown in Figure 6.
Figure 7 below shows an HSI horizontal-axis Kaplan turbine with a 1 metre diameter rotor being lifted into position. Figure 8 shows an S-turbine in operation. If you would like to work with Renewables First on a Kaplan turbine hydro project, please get in touch.