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.
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.
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
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.Figure 5 – Small Ecowave crossflow turbineFigure 6 – Ossberger crossflow turbineFigure 7 – Crossflow turbine with a gearbox drive system
Our Crossflow turbines are designed correspondent to the Ossberger type. They are made of standardized components configured according to customer requirements – i.e. the quantity of water and the head of the particular location. Such a modular system enables short delivery times and good prices at the same time.
Crossflow turbines have a long service life and they are almost maintenancefree. During operation, they do not require any costly or complex spare parts; repairing them is feasible on site. A specific advantage of Crossflow turbines is the possibility of using them in drinking water systems, even in very long conduits, not causing undesirable water hammer effects and thus not affecting the quality of drinking water during operation. This has been successfully tested by our company several times in numerous countries around the world.
Operating turbine range
- Head height: H = 3… 200 m
- Flow: Q = 0,03… 16 m³/s
- Capacity: N = 10… 7 000 kW
Principle of a 2-cell Crossflow Turbine
Crossflow turbines are radial, slightly overpressure turbines with tangential injection of the runner blades and with a horizontal shaft. They rank among low-speed turbines. The water flow comes through an inlet pipe, then, it is regulated by guide vanes and finally enters the runner of the turbine. After passing through, the water leaves on the opposite side of the runner, providing so additional efficiency. Finally, it flows from the casing either freely or through a draft tube to a stilling basin under the turbine.
If the water flow is variable, then the Crossflow turbine is designed with two cells. The standard division of the inlet cells is 1:2. The narrower cell processes small water flow and the wider cell processes medium flow. Both cells together process full flow. As a result, the water flow is used from 100 to 12 % with maximum efficiency, and the turbine is able to start operation even with only 6% of the design flow.
In practice, the water flow in the runner provides a self-cleaning effect. Any impurities that are pushed between the blades when water enters the runner are also pulled out by centrifugal force.After a half revolution of the runner, the water takes the impurities out of the runner and flushes them away to the stilling basin.
Level of turbine efficiency
The total efficiency of small Crossflow turbines with a small head is between 80-84% throughout the flow. The maximum efficiency of medium and big turbines with a higher head is 87%.
The advantages of a partially loaded Crossflow turbine are illustrated by the efficiency curve shown in fig.3. The actual river flows are very small for several months of the year, if compared to the design flow of the turbine. During those months, the energy production depends solely on the ability of the turbine to utilize these partial flows efficiently. As a result, our two-cell Crossflow turbines with their flat curve of efficiency achieve higher annual energy output, than turbines reaching high efficiency with full flow, but rather low efficiency with partial flow.
The casing of a Crossflow turbine is made of structural steel; it is robust, resistant to impacts and frost. If there is high contents of abrasive material in the water (e.g. sand, silt) or if the the actual composition of the water is considered aggressive (e.g. sea water, acidy water), all parts of the turbine in contact with water are made of stainless steel.
In a split Crossflow turbine, the working water is directed by two force-balanced profile guide vanes. The vanes split the water beam, balance it and let it enter the runner smoothly – independently of the width of the cells. Both rotary guide
vanes are set precisely in the turbine casing and they can serve as the closing device of the turbine if there is a lower head. Then it is not necessary to use a shutoff valve between the pressure piping and the turbine. Both guide vanes are fitted independently with extended arms, to which automatic or manual control is connected. Guide vanes are placed in highly resistant slide bearings, which do not require any maintenance. The turbine will be able to close by gravitation in the event of its shutdown, by added weights to the arms ends.
The runner is the most important part of the turbine. It is equipped with blades which are made of polished drawn profile steel by a well-proven method. Depending on the actual hydraulic data, either structural steel or stainless steel is used for their construction. Both ends of the blades are fitted in runner discs and welded with intermediate discs of the runner following a specific procedure. Depending on the size, the runner has up to 37 blades. The linearly slanted blades create only slight axial force and therefore reinforced axial bearings with complex fitting and lubrication are not required. Blades of wider runners are supported by multiple discs. Runners are carefully balanced before final installation of the turbine and are subject to crack detection control.
Crossflow turbines are equipped with self-aligning roller bearings having several advantages, such as low rolling resistance and simple maintenance. The design of the bearing housing prevents water leakage into bearings and contact of lubricants with working water. This is an important quality of the patent design of our Crossflow turbine bearing housing. Furthermore, the runner is centred in the turbine casing by means of the bearings. Such an inventive technical solution is completed with maintenance-free sealing elements. Apart from grease change every year, the bearings do not require any maintenance. Moreover, the used technical solution enables simple replacement of the runner without taking the entire turbine out of its position.
The Crossflow turbine is a free stream turbine, just like a Pelton turbine. However, in case of medium or low head, it is possible to apply a draft tube in order to utilize the entire head. The water column in the draft tube must be controllable, though. This is ensured by a regulation air valve, affecting the suction pressure in the turbine casing. In such a manner, turbines with a suction head from 1 to 3 m can be optimally used without any danger of cavitation.
- 10 kW-7.000 kW/unit
- Head range: 5-200 m
- Run of river SHPP
- Dam SHPP
- SHPP in drinking water systems
- SHPP in wastewater treatment plants
- SHPP for electrification of remote areas
- Increased annual production because of high efficiency
- from 12% to 100% of flow
- Starts operation with only 6% of flow
- Tailor-made for every installation
- Non-clogging runner
- Highly tolerant against varying head
- Simple installation and minimal civil works
- Almost maintainance-free
- Highly tolerant against foreign substances
- Proven quality from almost 10.000 installations