Archimedes Screws have been used for centuries as pumps, originally using animal power and more recently electric motors to rotate a “corkscrew” within a tube to lift water from one level to another, the most common traditional application being in irrigation systems. They are still used today as pumps, with tens of thousands being used to move solids or sludges where conventional high speed rotational pumps would either damage the material being moved (e.g. bulk grain) or the material being moved would damage or clog the pump (e.g. sewage sludges).
If the screw is not used as a pump, but is supplied with water at its top end, then the pressure of the water on the blades of the screw turns the “corkscrew” as the water flows down the tube or trough under gravity. This rotational energy can be captured through a gearbox and used to drive a generator.
The energy that can be captured in this way is predominantly the potential energy of the water as it falls from the water level upstream of the screws to the lower level downstream of the screws. Due to the slow rotational speed of the screws (about 3 seconds per complete turn) there is very little kinetic energy in the water that is released from the lower end of the screws, resulting in a high efficiency.
The slow rotational speed of the screws, coupled with the large cross section of the spaces in which the water moves down the screw, results in their being very “fish friendly” compared with conventional turbines, which can result in high fish mortality. Independent tests confirm that Archimedes screws are the most fish friendly type of turbine, and result in very little damage indeed to all types of fish.
Our optimal three screw construction
The configuration at our weir: