Archimedean screws are relative newcomers to the small-scale hydro world having only arrived on the scene over the last ten years.
However, they have been around for many decades as pumps where tens-of-thousands have been installed worldwide, particularly in sewage treatment works. The same manufacturers that dominate the pump market are now the main suppliers into the hydropower market as well.
As the name suggests, Archimedes is widely acknowledged as the inventor of the screw back in 250 BC, though the credit has been wrongly attributed because they were actually in use in Egypt many years before then. Historically they were used in irrigation to lift water to a higher level and were generally powered by oxen, or even humans on smaller versions. The basic principle of an Archimedean screw pump is shown in the diagram above. If the handle at the top was turned in an anti-clockwise direction it would draw the water up from the lower level to the top.
When used as a hydro turbine the principle is the same but acts in reverse. The water enters the screw at the top and the weight of the water pushes on the helical flights, allowing the water to fall to the lower level and causing the screw to rotate. This rotational energy can then be extracted by an electrical generator connected to the main shaft of the screw.
Archimedean screws for hydropower are used on low head / high flow sites. They can work efficiently on heads as low as 1 metre, though are not generally used on heads less than 1.5 m (more for economic reasons than technical ones). Single screws can work on heads up to 8 metres, but above this multiple screws are generally used, though in many cases for heads above 8 metres there may be more appropriate turbines available with much smaller footprints.
The maximum flow rate through an Archimedean screw is determined by the screw diameter. The smallest screws are just 1 metre diameter and can pass 250 litres / second, then they increase in 250 mm steps all of the way up to 5 metres in diameter with a maximum flow rate of around 14.5 m3/s. The 5 metre maximum is really based on practical delivery restrictions, and in many cases 3 metres is the maximum diameter that can be delivered to a site. If there is more flow available, multiple screws can be installed in parallel.
In terms of power output, the very smallest Archimedean screws can produce as little as 5 kW, and the largest 500 kW.
The main parts of an Archimedean screw used as a hydro generator are shown below. The actual screw is below the upper bearing. The helical screw or ‘flights’ are made from rolled flat steel plate that is then welded to a central steel core. Most Archimedean screws have three flights, or three separate helices winding around the central core.
Archimedean screws typically rotate at around 26 rpm, so the top of the screw connects to a gearbox to increase the rotational speed to between 750 and 1500 rpm to make it compatible with standard generators. Even though they rotate relatively slowly Archimedean screws can splash water around, though this is reduced significantly by the use of a splash guard shown running down the left-hand side of the screw as shown below.
Also quite a few Archimedean screws have been installed without any protective guarding over the screw itself, though we would recommend having the whole screw covered to prevent large debris, animals or even people falling in and becoming entrained. The guarding can be designed sympathetically so that the screw is still visible if required.
Archimedean screws are normally set at an angle of 22 degrees from horizontal, which is the optimum for the most cost-effective installations. There is scope to adjusting the angle slightly if the site requires it (to fit into a particular space for example).
The best Archimedean screws are variable-speed in operation, which means that the rotational speed of the screw can be increased or decreased depending on the flow rate available in the river. This is much better than having a fixed-speed screw and varying the flow rate through an automated sluice, which creates high head losses and impacts the overall system efficiency. Variable-speed screws are also quieter in operation and don’t suffer from ‘back slap’ at the discharge-end of the screw.
A typical efficiency curve for a good quality variable-speed Archimedean screw is shown below. This is the mechanical efficiency, so doesn’t include the gearbox, generator and inverter losses (these are approximately 15% on in total). It’s worth noting that there are some Archimedean screw suppliers that ‘over sell’ the efficiency of screws, so be careful when comparing performance. A lower claimed efficiency may not be because a particular screw is inferior; it could just be that the supplier is more honest!
A significant advantage of Archimedean screws is their debris tolerance. Due to the relatively large dimensions of the screw’s flights and slow rotational speed, relatively large debris can pass through unhindered and without damaging the screw, and certainly all small debris such as leaves can pass through without any problems at all. This means that fine screens are not required at the intake to the screw and they can manage with course screens with 100 or 150 mm bar-spacing. This leads to relatively modest amounts of debris build-up on the course screen and removes the requirement for (expensive) automatic intake screen cleaners which are normally required on larger low-head hydropower systems.
The low rotational speed and large flow-passage dimensions of Archimedean screws also allow fish to pass downstream through the screw in relative safety. Archimedean screws are often touted as ‘fish friendly’ hydro turbines, which they undoubtedly are, though we at Renewables First would say that all hydro systems should be fish friendly, regardless of turbine type. In non-screw hydro systems this just means well designed intake screens and fish passes / by passes would be required. Note that if upstream fish passage is required at an
The final advantage of the Archimedean Screw is simplified civil engineering works and foundations. Because screws don’t have draft tubes or discharge sumps, it means that the depth of any concrete works on the downstream-side of the screw is relatively shallow, which reduces construction costs. The civils works are also relatively simple, the main part being the load-bearing foundations underneath the upper and lower bearings. In softer ground conditions the load-bearing foundations can be piled. A typical cross section of an Archimedean Screw installation recently designed by Renewables is shown below.