Technology

Lower Lea Streamwheel 2017
The Three Mills tidal basin streamwheel takes energy from the fast moving water passing through the mill race. This energy drives a small compressor that feeds air to the stagnant areas of the tidal basin. As with a fish tank, air bubbling through the water oxygenates the water, allowing it to support more fish. The streamwheel is made from laser-cut aluminium, alloy scaffolding tube and off-the-shelf hardware.


If you would like to construct one of these,  download the vector files, which can then be sent for lasercutting. The files can be viewed within Adobe Illustrator or other vector graphics applications. The image below gives an indication of what the files contain.

Thames Turbine 2013
The Active Energy Hydrokinetic Turbine optimises existing hydrokinetic river turbine design. Wide, thin, curved blades are efficient at slow river flow speeds and a spiral blade leading edge reduces weed and debris entanglement. A duct or shroud around the blades protect the blades and also draw more water through the turbine. The turbine has been engineered to use readily available components, making it a practical design for remote or non-industrialised communities. The Thames installation affords an opportunity to test and refine these new developments.

Turbine development from LoraineLeeson
Slides by Toby Borland for turbine development with the Geezers

The next phase of ActiveEnergy centres around the design, construction and installation of a hydrokinetic turbine to be tested in the Thames. A hydrokinetic turbine is equivalent to a wind turbine, capturing the energy of moving water. Like a wind turbine, a hydrokinetic turbine has a rotor that is turned by flowing water in a river, or a sea tide. As water is much heavier than air, there is far more energy to be captured in flowing water than there is in the equivalent volume of air. Most hydropower projects such as hydroelectric dams, water wheels and impulse turbines use the energy of falling water however a hydrokinetic turbine relies on the energy in passing water.

Hydrokinetic turbines are an active research field at the moment, Seagen [1] is an example of a 1.2 MW test turbine placed in Strangford Lough for trials. Numerous smaller hydrokinetic turbines have been tested and developed as a source of power in remote communities, examples include the Garman turbine in Sudan, a Brazilian 2kW turbine developed by the University of Brasil, the Tyson turbine developed in Australia and the EnCurrent Canadian Darrius turbine [2]. The design of hydrofoils for low speed, low Reynolds number flow as found in small hydrokinetic turbine generators installations is in it's infancy; experimental tests of turbine blades at this scale do not fully agree with the current design theories. In particular, the effect of surrounding the turbine rotor with a shroud can multiply the power available to the rotor.

This project is using low Reynolds number airfoils that have been optimised for use on a turbine rotor [3], it is hoped that the operation and measured efficiency of the turbine will contribute to the design of small scale hydrokinetic turbines. Its second important goal is the documentation and design of a turbine from readily available components, facilitating the development of low capital turbine installations suited to energy generation at remote sites or in developing economies.

Turbines in dams and propellors on boats both use carefully designed blades that maximise the power extracted from or given to the surrounding water. The blades used in small hydrokinetic turbines appear to have been repurposed from more generic windturbine airfoils, or are designed for relatively fast moving water. The hydrofoil design used here has been adapted for low speed efficiency using an airfoil that has been demonstrated to generate high lift and low drag at low flow speeds. A second issue is sensitivity to fouling. Weeds and marine organisms will foul submerged surfaces over time. This build up reduces the efficiency of hydrofoils sensitive to inperfections in their shape. Wind tunnel tests indicate that debris and fouling on the leading edge at the front of the blade has little effect on the lift/drag performance. To further reduce the incidence of clogging, the blades have a gentle scimitar shaped curve (a logarithmic spiral) introduced along their length. Debris caught on the blade will be pushed to the edge of the blade, where it will hopefully drift free. This same shape is successfully used in sludge mixing fans that have to churn dense and often fibrous material.

This turbine is mounted in a river, along with weeds and small debris carried in the river, there is a hazard of the rotating blades striking a boat or a floating object. A shroud, or ring is mounted around the rotating blades, preventing damage from strikes. Shrouds provide the possibility of amplifying the volume of water passing through the blades, increasing the power from the turbine. Although there is great potential claimed for shrouded hydrokinetic turbine installations, there have been remarkably few studies made. Diffuser shrouds appear to operate with the same principles and issues associated with high lift aircraft wings. The air or water moving along the surface of the wing becomes "unstuck" when the wing is tilted at high angles. Once the fluid seperates from the surface of the wing, the wing stops providing lift. In the case of a shroud, it stops diverting so water through the turbine blades. The shroud is made from High Density PolyEtheylene, cut from standard plastic 205 litre drums. The profile of the shroud is formed from two concentric rings. Both rings are formed into cones, water flowing between the two rings maintains the boundary layer. This arrangement allows a shorter cone to provide the effect of a longer conical diffuser shroud.

Most engineering research is devoted to large hydrokinetic turbines, situated in fast flowing tidal streams. These machines produce several hundred times more power than the Active Energy turbine design. The generators used in Megawatt installations are a significant fraction of the expense of these turbines. Small river-based installations require efficient and relatively cheap generators. For a generator to be cheap at this scale, an off-the-shelf unit has to be used. Most generators are designed to be efficient at several thousand revolutions per minute; if the blades of a hydrokinetic turbine were designed to rotate at that speed, imploding bubbles (cavitation) would destroy the blades. First generation wind turbines used gearboxes to match the speed of the slow turning rotor to the high rotational speed of a standard generator. Unfortunately, gearboxes reduce the amount of power going to the generator. Most modern renewable energy turbines use generators that are redesigned to work efficiently at slow rotational speeds using a large number of permanent magnets. This design has adopted the motor from a hub mounted electric bicycle. The motor has no gears and uses multiple powerful permanent magnets to achieve high torque or twisting force at low speeds. This motor is readily reconfigured as a generator. As the motor is designed to be mounted in a bicycle frame, it spins while the axle remains stationary.

Providing sealed shaft drives that prevent the generator from getting wet are expensive and cause friction. The solution adopted is to use a long turbine drive shaft angled into the water like a sampan or peque-peque motor.

The shaft is threaded steel 3/4" tubing rotating in a 2" outer tube. An opposing pair of taper roller bearings are installed in the dry end. These bearings support the drive shaft, allowing it to rotate while preventing it from sliding in or out of the outer tube, in a similar fashion as bicycle steering (headset) bearings. At the bottom end is mounted a solid bearing made from graphite filled nylon. This bearing was machined on a lathe, the only machining required for the design. It may also be possible to mix powdered graphite (powdered lock lubricant) with polycapalactone, or PLA (sold as "Polymorph") and mould the material directly around the shaft without need of machining.

The rest of the design is a floating support raft, made of drums and scaffolding. This hydrokinetic turbine has been designed to use readily available and economical components with a minimum of fabrication work.

Links

[1]  http://www.marineturbines.com/Projects

[2]  http://mhk.pnnl.gov/wiki/images/e/ef/Evaluation_of_small_axial_flow_ hydrokinetic_turbines_for_remote_communities.pdf

[3]  http://wind.nrel.gov/designcodes/simulators/HARP_Opt/


The turbine blades are skewed in two planes, the angle of the hydrofoil section relative to the axis of rotation varies with the diameter of the blade. The slower moving profile near the hub has a correspondingly steeper angle. The leading edge of the blade is curved to encourage entangled debris to slide off, this curve is a logarithmic spiral. It remains to be seen whether this arrangement will necessitate a greater clearance between blade tip and shroud.

Where the blade meets the hub (the blade root), the main consideration is an even transfer of load from the blade to the hub. Early configurations used an extrapolated hydrofoil profile projected onto the curved hub surface. The smoothest transition was found to be a loose ellipse composed of two intersecting radii. It has proved more reliable and expedient to rely on a mathematical description of the curves that define the blade surface. A significant proportion are exported from a numerical visualisation program (Scilab) for defining the surfaces in the solid modelling program (Solidworks).