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Crossflow turbines

Crossflow turbines are widely used on UK small hydro sites with typical power outputs from 5 kW up to 100 kW, though they can actually be up to 3 MW in size on the very largest systems. Generally speaking there are better turbine choices for these higher power outputs.

They will work on net heads from just 1.75 metres all of the way to 200 metres, though there are more appropriate turbine choices for sites with heads above 40 metres. They will work on average annual flows as low as 40 litres/second up to 5 m3/s, though on the higher flow rates there may be better other turbine types to consider.

Figure 1 – Crossflow turbine in cross-section

Crossflow turbines gets their name from the way the water flows through, or more correctly ‘across’ the rotor as shown in Figure 1 below (hence across flow or crossflow). The water flows over and under the inlet guide-vane which directs flow to ensure that the water hits the rotor at the correct angle for maximum efficiency. The water then flows over the upper rotor blades, producing a torque on the rotor, then through the centre of the rotor and back across the low rotor blades producing more torque on the rotor. Most of the power is extracted by the upper blades (roughly 75%) and the remaining 25% by the lower blades. Obviously the rotor is rotating, so what are the upper blades one moment will be the lower blades the next.

One of the advantages of crossflow turbines are that they are self-cleaning to a degree in that leaves etc. that could get pushed into and stuck on the upper blades are washed off by the exiting water on the lower blades. Also the centrifugal force tends to throw trapped debris outwards, further increasing the self-cleaning capabilities. This isn’t such a great benefit nowadays because fine fish screens are required at the intake, which tends to exclude debris anyway.

The mains parts are shown in Figures 1 and 2. The water enters through the inlet adaptor, also called the ‘round to square adaptor’ because it fits between the round water supply pipe and the rectangular inlet to the turbine. The inlet adaptor can be in a horizontal orientation (shown in Figure 1) or vertical (shown in Figure 2) to suit the site conditions.

Exploded view of crossflow turbines

Figure 2 – Main parts of a crossflow turbine

Next the water comes to the inlet guide-vanes, which both regulate the flow rate through the turbine and direct the flow onto the rotor at the optimum angle for maximum efficiency. The rotor spins on a horizontal axis and looks similar to a cylinder lawn mower rotor, though is of a much heavier construction.

Crossflow turbines are impulse turbines, which means (amongst other things) that the rotor is spinning air and is not fully-flooded like in a reaction (e.g. Kaplan) turbine. The water exits the rotor and falls into the draft tube, which can be many metres long (though not normally more than 1/3 of the total net head across the system). The exiting water fills the draft tube from just below the rotor all of the way to the discharge water level. This column of water waters wants to ‘fall out’ of the draft tube, but cannot because the air that would need to fill void is prevented from entering quickly by the air inlet valve on the front turbine casing. This creates a negative pressure inside the turbine cavity and is called the ‘suction head’ across the turbine. The positive water pressure from the upstream side of the rotor is called the ‘pressure head’ and the sum of the pressure and suction head equals the net head across the hydro system.

It is important when setting up crossflows to adjust the air inlet valve correctly so that it allows just the right amount of air into the turbine chamber; too much and the column of water in the draft tube will fall, reducing the suction head and the efficiency of the system. Too little and the column of water in the draft tube will rise and ‘drown’ the rotor, which causes excessive drag and also reduces efficiency.

The advantage of using a draft tube to maximise the suction head is that the long draft tube moves the main body of the crossflow turbine upwards, away from the discharge water level and (hopefully) above the floodwater level during high flow events. If the main body of the turbine was still likely to get flooded during high flow events it would have to be built into a ‘tanked’ enclosure.

Another major advantage of a crossflow turbine is their wide operating flow range with a high efficiency across the whole range. This is possible because (most) crossflows have two inlet guide-vanes, one 1/3 of the intake width and the second 2/3 of the width (a so-called 1/3 : 2/3 split). This means that during lower flow periods the 2/3 inlet guide-vane can be completely closed allowing no water through, and the turbine will operate on just the 1/3 guide-vane which effectively means that only 1/3 of the rotor is in use. If ‘average’ flow rates are available the 1/3 guide-vane can close and the turbine operates on just the 2/3 side, then when high flow rates are available both guide-vanes can work together. This is shown diagrammatically in Figure 3 along with the efficiency curve. Note that the peak efficiency shown of 86% is a little high: 82% is more realistic for a high quality crossflow turbine. The horizontal axis (labelled ‘Q (%)’) is the percentage of the maximum flow rate, so 100% is the maximum flow rate, 50% half of maximum etc. The turbine efficiency is shown on the vertical axis.

Crossflow turbine efficiency curve

Figure 3 – Efficiency curve for 1/3, 2/3 and 3/3 inlet guide-vanes in operation

In good quality crossflows the guide-vanes fit the turbine casing with such precision that they can stop the water flow 100%. This is a useful feature because it means the rotor can be stopped fully when the turbine is shut down, though in some situations it is better to leave the rotor turning very slowly even when shut down to prevent various creepy crawlies taking up residence inside the generator if it shut down for a period during summer low flows.

A typical low-head crossflow turbine installation is shown in section in Figure 4. What is notable is the depth of the discharge sump underneath the draft tube exit. This is required to make sure that the discharge water can exit efficiently, but does mean that a very deep excavation is required below the site downstream water level during the construction phase. This can be problematic if 100% effective coffer dams cannot be built to dry-out the area due to the site layout, and is quite an expensive feature to construct.

Figure 4 – Cross-section of a typical low head crossflow turbine installation

Figure 4 – Cross-section of a typical low head crossflow turbine installation

Crossflows are available in a number of rotor diameters, normally in 100 mm steps from 100 mm to 500 mm. The smaller diameters are for higher-head sites. For low head sites from 2.5 to 5 metres 300 mm diameter rotors are normally used. The 400 and 500 mm rotors are used on very low head sites. Renewables First have successfully installed 500 mm rotor crossflows on net heads as low as 1.75 metres.

Many of the parts of crossflow turbines are standardised and only the width of the rotor is bespoke designed to match the expected range of flows at the hydro site.

Crossflow turbines require relatively little maintenance. The main rotor bearings are grease lubricated via two grease nipples, one on each side of the turbine and require greasing from a grease gun one per month. Annually the air inlet valve position should be checked. Hydro systems with crossflow turbines normally have 12 mm intake screens which prevent almost all but the tiniest debris entering the turbine, and this debris can pass through without any problem. Good quality crossflow turbines should operate efficiently for at least 40 years, and the oldest turbines are 60 years old and still operating reliably and efficiently and in most cases have only required new main bearings and guide vane bushes.

On the outside of the turbine the two inlet guide-vanes are moved via ‘actuator arms’ (see Figures 5 and 6) which are moved either by electric or hydraulic actuators. The amount and direction of actuator movement is governed by the system controller.

The turbine rotor normally rotates more slowly that the generator, so either a belt-drive or a gearbox is used to increase the speed. On smaller systems the drive pulley can be attached directly to the turbine shaft, and on larger systems an intermediate layshaft is used to avoid applying excessive radial loads onto the turbine drive shaft. The best (and most expensive) option is to use a gearbox which connects to the turbine via a flexible coupling, as shown on Figure 7.

The following pictures show a range of different crossflow turbines installed by Renewables First over the years. If you are interested in a crossflow turbine-based hydropower system for your site please get in touch to discuss the options further.

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