River or Tidal Current Counter-Oscillating Pump for Tidal or Hydro Power
It was naively thought that the Flo’Pump could be inverted to make a fluttering oscillating water current mill below a small floating platform with the blade and pump the only submerged moving parts. But unless the blade is prohibitively heavy and unbalanced such as cast solid iron, the ratio of blade inertia to that of circumscribing cylinder of fluid is not sufficiently maintained for flutter to occur. Whilst flutter is a ubiquitous possibility with much heavier-than-air aircraft structures, it is virtually unknown with much lower ‘mass ratio’ ship hydrodynamic surfaces.
Calculation shows that too low a pitch inertia ratio making the pitch-only motion much more overdamped than in air is the most critical. The only practical way to increase it enough (given the majority of the blade aft of the axis at the 23% chord point must be near neutrally buoyant to not be too tailheavy) is with gearing up to a flywheel.
Conversely the low natural mass ratios mean that an articulation mechanism to mechanically produce oscillation of fixed amplitude will have to balance much less dominant blade inertial reactions. The frequency of oscillation and pumping can increase as the current speed and power increases to a definite upper limit (unlike the windspeed) without the inertial loading becoming intolerable.
Whereas the highly flutter-prone Wing’d Pump is easily started by the high gustiness of the wind, the unsteadiness of a watercurrent is much lower and insufficient to start a less unstable water wing. So at least a starting mechanism is necessary anyways. The bidirectionality of the tide tends to further complicate the mechanisms, unless the platform can swing at anchor.
The inertia in water of a floating platform is just sufficient for it to not move too much in reaction to the unbalanced wing and torque of the Flo’Pump, but it would excessively with the much higher forces on a watercurrent blade. So floating requires 2 counteroscillating blades counter rolling and counterpitching which can be integrated with the pitch gearing for flutter
A simpler alternative is based on observation of the low windspeed oscillation of the Flutterwing when its coefficient of performance is highest. The wing pitch is a virtual square wave of 180 swing. So just fix the blades tangentially and make them a bidirectional circular steel section. youtube This is just like a mainsail driving a boat into roll oscillations when running. It couldn’t be used as a windmill because of the high drag in high winds even if the oscillation could be stopped, but again there is low upper limit to the tidal current. The oscillation will self-start due to Von-Karman vortex shedding from the staggered blades. The counteroscillating configuration allows a very simple pump cable geometry that is very non-linear to absorb all the power and contain the amplitude. Simple cabling also makes the blades counteroscillate and allows them to be winched to the surface for easy cleaning.
However the downwind thrust is high. Whereas the wing downwind torque opposed any torque from bow anchoring, the water blade torque augments it so exacerbating the very high platform pitch stiffness needed. And the platform must swing at anchor with a swivel needed on the output line.
Unlike rotors oscillating fins do not wrap weeds or errant ropes to jam and the above does not have any bearings at all in the silt-laden if not corrosive stream. It is better to have well marked visible audible presence and warning rather than being hidden underwater at the mercy of (dragging) boat anchors and commerical fishing gear. Units can be built at a small shipyard, pretested and towed to a site for easy installation in one tide window, and just as easily moved.
The prime niche would be pumping water, either ashore for use or to generate electricity off-grid. As in Wing’d Pump generation water would be pumped into a large high pressure tank against trapped ( isothermal) air. A used ‘propane’ tank (rated at 215 psi new) could stand vertical with a silicon oil film floating on top of the water to reduce air absorption and corrosion. This could be efficiently converted to constant voltage and frequency electricity on demand by a small impulse wheel close to the hydropneumatic tank. Testing is planned of a cheap arc valve that swings to vary the jet orifice area with little hydraulic power or flow loss or control resistance. For the time being, not quite as fine a control is to have multiple fixed nozzles on the same Pelton wheel each turned on by energizing a 150 psi (household) irrigation solenoid costing about $20 and using about 5W. By having say 4 nozzles each twice the previous in flow area, different combinations of on and off give 16 equal steps in flow rate. The control would 'count' through these to keep the generator rpm constant and output voltage and frequency constant as the load varies. Because the waterflow is from airpressure and not water height, fast control without water hammer is feasible unlike in micro-hydro where standby heating "loads" (of high waste) must be switched on and off. The system cost should be very competitive with underwater turbines charging battery banks with much smaller system losses.
Underwater tidal generators have bearings, gearboxes, and generators submerged in seawater. If the seals fail, these expensive components will be ruined by corrosion and the oil will pollute the water. So the component, installation foundation and maintenance costs are much higher than the Flutterfin. Of the rotor designs to feed the grid the best option would seem to be a Vawt cantilevered below a floating ‘endplate’ platform, synchronously started from the grid, possibly even from the tide tables. The tidal current power spectrum is usually sufficiently narrowbanded that the Vawt’s narrow operating peak won’t lose much and stall can be avoided just beyond the max tidal speed. Again underwater bearings can be entirely avoided though not underwater cleaning. Foil fouling is critical given the Vawt’s high drag losses. Most tidal windmill proposals seem totally blind to the severe fouling and corrosion problems of saltwater and are deliberately blind in submerging everything beyond visual monitoring and visual avoidance by (fishing) boats and anchoring ships. Our well experience is that working blind in a corrosive environment is a major development and eventual maintenance handicap.
There are several potential ways to prevent clogging of the pump system with marine growth. Firstly as with the fins all external surfaces at least would be coated with antifouling paint. Secondly the outside of the pump would be cleaned with long poles from the surface though not as effectively as the wings brought to the surface. Just before an extended slack tide, the pump system could be fed with a watersoluble biocide which would then get pumped into the pipeline and dwell and kill internal growth during the typical 2 or 3 days of insufficient currents, but then breakdown before output at the shore station. Chlorine generated from the seawater is used in seawater cooling systems. Less frequently in such a slack tide interval the pipeline would be rotor-rooted from shore and the entire pump could be replaced with a spare. The removed unit would be disassembled ashore for thorough internal cleaning and inspection if not replacement of the piston cup seals. A complete solution would always be a closed pump above water with dual pipelines of hydraulic oil, preferably a benign natural type for the worst case scenario of a spill.