TECHNICAL FIELD

[0001] The present invention relates generally to hydro- electrical generators and, in particular, to fully submersible hydroelectric turbines for harnessing kinetic energy contained in underwater currents.

BACKGROUND

[0002] Hydroelectric generators produce electrical power from the movement of water. There is now tremendous interest in this form of renewable energy as it does not produce, during operation, any harmful greenhouse gas emissions, like hydrocarbon combustion, nor does it raise other environmental and health concerns like nuclear power.

[0003] Although there has been much interest over the past decades in hydroelectric generating stations with dams, there has also been some interest in developing underwater hydroelectric turbines to harness kinetic energy contained in river currents, tidal currents or ocean currents. Because installation underwater of these turbines is expensive and complex, these underwater turbines have very demanding operating requirements, particularly in terms of reliability (i.e. service life) and energy-conversion efficiency.

[0004] A number of different technologies have been developed in recent years. Some examples include US 6,409,466 (Lamont) ; US 4,274,009 (Parker); US 5,100,290 (Berger) ; US 5,798,572 (Lehoczky) ; US 4,613,279 (Corren et al.); US 5,440,176 (Haining), US 4,335,319 (Mettersheimer) ; US 4,219,303 (Mouton) ; US 4,306,157 (Wracsaricht) ; US 6,648,589 (Williams); US 2002/0088222 (Vauthier) ; US 7,471,009 (Davis et al.); US 7,378,750 (Williams); US 4,421,990 (Heuss et al.); US 2,634,375 (Guimbal); US 7,279,803 (Bosley) ; US 4,026,587

(Hultman et al.) . Despite these various technologies, there remains a need for a more reliable and efficient underwater turbine. Such a turbine is disclosed herein.

SUMMARY

[0005] In broad terms, the present invention is a novel submerged, or submersible, hydroelectric turbine that is capable of generating electrical power from underwater currents such as, for example, from river currents or tidal currents. The turbine has novel self-contained front end and rear end bearing units, or bearing assemblies, that include their own self-powered lubricant circulation, cooling and filtering systems and their own automatically adaptive pressure-compensation system. The innovative design of the bearing units substantially prolongs the underwater service life of the turbine, thus minimizing manufacturing, operating and maintenance costs.

[0006] The self-powered, self-priming lubricant circulation system uses only a fraction of the mechanical power of the rotating central shaft of the turbine to circulate lubricant through an internal circuit to lubricate, clean and cool the bearings .

[0007] The automatically adaptive pressure-compensation system uses one or more equilibrium chambers to balance the lubricant pressure inside the lubricant circulation system with the ambient water pressure outside the bearing chambers. By maintaining almost zero pressure differential, even in the face of changing ambient water temperature, there is virtually no leakage (or, at most, only a minuscule amount of leakage) of oil (lubricant) out of the lubricant circulation system of the turbine or, conversely, of water into the lubricant circulation system. Precluding the seepage of water into the lubricant circulation system is crucial to ensure ongoing proper lubrication of moving parts and thus to ensure prolonged service life. Precluding the leakage of lubricant into the ambient water is also important, both to ensure sufficient quantity and pressure of lubricant within the circulation system but also for environmental reasons, a biodegradable lubricant has been used.

[0008] Accordingly, one main aspect of the present invention is a underwater hydroelectric turbine having a turbine housing having an inlet and an outlet, a central shaft extending axially inside the turbine housing, a self-contained, sealed front bearing unit for rotationally supporting a front end of the central shaft, and a self-contained, sealed rear bearing unit for rotationally supporting a rear end of the central shaft. The front bearing unit and the rear bearing unit each includes a self-powered lubricant circulation system that draws a fraction of the mechanical power from rotation of the central shaft to circulate lubricant through each respective bearing unit .

[0009] In one set of embodiments of this invention, the circulation system includes a central axial channel through the central shaft extending from a front end of the central shaft to a pair of radial channels which are disposed closer to a geometrical center of the central shaft than is a bearing within each bearing unit, the radial channels delivering lubricant to an annular passageway that enables lubricant to flow through the bearing to continuously lubricate, to clean the lubricant (by filtering out metallic particles) and to cool the bearing. [0010] In another set of embodiments of this invention, the lubricant circulation system further includes a filter for filtering the lubricant before the lubricant enters the central axial channel.

[0011] In yet another set of embodiments of this invention, the front bearing unit is axially movable with respect to the rear bearing unit, allowing mechanical and/or thermal stress relief .

[0012] Another aspect of the present invention an underwater hydroelectric turbine having a turbine housing having an inlet and an outlet, a central shaft extending axially inside the turbine housing, a self-contained, sealed front bearing unit for rotationally supporting a front end of the central shaft; a self-contained, sealed rear bearing unit for rotationally supporting a rear end of the central shaft; and an equilibrium chamber for automatically equilibrating lubricant pressure inside a lubricant-containing chamber of the turbine with ambient water pressure outside the bearing chambers.

[0013] In one set of embodiments of this invention, the equilibrium chamber includes a tank and a bladder having an elastic membrane within said tank, the bladder being pre- charged with a pressurized inert gas to expand and contract to balance the pressure change inside the lubricant-containing chamber of the turbine with the ambient water pressure outside the bearing chamber.

[0014] In another set of embodiments of this invention, there are successive annular chambers around the central shaft. These three chambers comprise a first chamber having a first mechanical high-performance seal to isolate the first chamber from ambient water, a second chamber adjacent to the first chamber and having a second mechanical seal to isolate the second chamber from the first chamber, and a third chamber adjacent to the second chamber and having a third seal to isolate the third chamber from the second chamber and vice- versa .

[0015] In yet another set of embodiments of this invention, the first chamber comprises a first equilibrium chamber, the second chamber comprises a second equilibrium chamber and the third chamber comprises a third equilibrium chamber.

[0016] Other aspects, features and advantages of this novel technology will become apparent with reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

[0018] FIG. 1 is a front view of a front hub assembly of an underwater hydroelectric turbine in accordance with an embodiment of the present invention;

[0019] FIG. 2 is a cross-sectional view of the front hub assembly taken through section A-A in FIG. 1;

[0020] FIG. 3 is a cross-sectional view of a front bearing unit of the front hub assembly, which is identified as Detail B in FIG. 2 ;

[0021] FIG. 4 is an enlarged rear view of the front hub assembly depicted in FIG. 1;

[0022] FIG. 5 is an isometric view of a rear bearing unit in accordance with an embodiment of the present invention; [0023] FIG. 6 is a front view of the rear bearing unit depicted in FIG. 5;

[0024] FIG. 7 is a cross-sectional view of the rear bearing unit taken through section A-A in FIG. 6;

[0025] FIG. 8 is a schematic depiction of a three-chamber auto-adaptive pressure-compensation system for an underwater hydroelectric turbine in accordance with one embodiment of the present invention;

[0026] FIG. 9 is a cross-sectional view of a common central shaft rotationally supported by a front bearing unit and a rear bearing unit similar to the ones presented above; and

[0027] FIG. 10 is an enlarged cross-sectional view of the front bearing unit depicted in FIG. 9.

[0028] It will be noted that throughout the appended drawings, like features are identified by like reference numerals. It should furthermore be noted that the drawings are not necessarily to scale.

DETAILED DESCRIPTION

[0029] In general, and by way of overview, the present invention provides an innovative underwater hydroelectric turbine. This turbine has innovative self-contained front and rear bearing units which rotationally support the central shaft of the turbine. These bearing units play a crucial role in prolonging the service life of the turbine, enhancing the efficiency of energy recovery and minimizing environment impact. There are two main novel aspects of the bearing units .

[0030] First, the bearing units include a self-powered, self- priming circulation system that harnesses the power of the rotating central shaft in order to circulate the lubricant. The self-powered lubricant circulation system lubricates, cleans and cools the bearings. Furthermore, by circulating this bearing lubrication fluid into the heat-exchanging zone in the nose portion and through the filter, the lubricating fluid is both cooled and filtered (cleaned) of metallic particles .

[0031] Second, the bearing units include an automatically adaptive pressure-compensation system. Equilibrium chambers, that may be pre-charged with inert pressurized gas, are used to balance the lubricant pressure with the ambient water pressure to prevent, or at least greatly inhibit, lubricant from leaking out of the lubricant circulation system and also to prevent, or at least greatly inhibit, water from seeping into the lubricant circulation system.

[0032] The first and second novel aspects of the bearing units are preferably used together in the same turbine design to provide optimal performance, although they may also be used independently. It should also be understood that the same technologies are used in both the front and rear bearing units of the turbine, but may exceptionally be used in a variant in only one of the two bearing units.

[0033] Main illustrative embodiments of this invention are now described below having regard to the appended figures.

[0034] FIG. 1 depicts the front hub assembly that contains the front bearing unit of the turbine. As shown in this introductory figure, the turbine has a plurality of profiled guide vanes extending radially from the turbine housing to the axially disposed central shaft. In this particular embodiment, there are five profiled guide vanes, although it will be appreciated that the number of vanes may be varied. These guide vanes both support the central shaft (on bearings, as will be described in detail below) and pre-swirl the water flowing into the inlet of the turbine in order to increase efficiency. The water flowing into the inlet causes a rotor (or runner) to rotate with the central shaft. The central shaft rotates on the front and rear bearing units that are supported by respective sets of front and rear vanes. Magnets in the tips of the rotor rotate with respect to fixed windings fixed in the stator, which is disposed within the housing of the turbine, to induce electrical voltage and current flow in the external electrical circuit, thereby generating electrical power. This electrical power is carried to the shore by submersible cable where the electrical power may be consumed locally and/or fed into an electrical grid.

[0035] FIG. 2 is a cross-sectional view of this front hub assembly shown in FIG. 1. FIG. 2 is a sectional view taken through section A-A of FIG. 1. Accordingly, FIG. 2 shows the guide vanes extending radially from the turbine housing to the front bearing unit which supports the front end of the rotatable central shaft. The longevity (service life) of the turbine is dependent upon the front and rear bearing units. The novel bearing design disclosed herein is expected to enable the turbine to operate maintenance-free for approximately 100,000 hours. Given the expense and difficulty of accessing underwater turbines for repair and maintenance, the longevity and reliability of the turbine are of paramount importance .

[0036] FIG. 3 is a cross-sectional view of the front bearing unit of the front hub assembly. The front bearing unit depicted in FIG. 3 is identified in FIG. 2 as Detail B. The novel design of this front bearing unit, like that of the rear bearing unit to be described below, has two main innovative aspects: (1) the bearing unit has a self-powered, self-priming lubricant circulation system that uses a fraction of the rotational power of the central shaft to drive an impeller that circulates the lubricant through the circulation system; and (2) the bearing unit has an auto-adaptive pressure- compensation system that uses one or more equilibrium chambers for balancing the lubricant pressure with the ambient water pressure. These features may be used together or independently to provide superior turbine bearing performance. In addition to these novel aspects, the front bearing unit, like the rear bearing unit, has a spherical roller bearing to properly rotationally balance and support the central shaft and also includes a plurality of seals to isolate the bearing and its lubricant (s) from the surrounding (ambient) water.

[0037] In the embodiment depicted in FIGS. 1-4, a spherical roller bearing is provided in the front bearing unit (as it is for the rear bearing unit) . It is to be appreciated that in other variants it may be possible to have more than one such bearing, to have a different type of bearing, or to have a combination of different types of bearings, journals or sleeves .

[0038] As depicted in FIG. 3, the lubricant circulation system continually circulates lubricant through the bearing

(as long as the central shaft is rotating) to thereby lubricate, cool, and clean the bearing lubricant, therefore protecting the bearing. Continual lubrication of the bearing prolongs the life of the bearing. Cooling of the bearing is also of prime importance to dissipate the heat generated by the normal operation of the bearing. The bearing produces heat and normal wear during operation. This heat is dissipated by the flow of lubricant, which carries away heat. Heat is transferred through wall of the housing with ambient

(cooler) water. Cleaning is also of importance. Over time, the bearing race ways may wear and small metallic particles or flecks of metal may break loose, which can accelerate wear of the bearing. The lubricant also cleans these or any other particles from the bearing and carries them to the filter media installed at each end of the axial central shaft.

[0039] An impeller is connected to the axial central shaft. This impeller has impeller blades that circulate the lubricant. The axial central shaft has an axial lubricant channel (or axial lubricant passageway) therein that extends from a front end of the shaft toward the interior of the shaft (i.e. toward the geometrical center of the shaft) . Radial lubricant channels (radial lubricant passageways) extend from the interior of the axial channel to the annular passageway. The spherical roller bearing is positioned within this annular passageway. In operation, the rotation of the central shaft causes the impeller to rotate in unison with the shaft. The impeller blades of the impeller impel the lubricant so that it circulates through the circuit of the circulation system. Lubricant enters the axial channel, travels inwardly to the radial channels and then travels radially outwardly until it reaches the annular passageway. The lubricant then flows back through the spherical roller bearing and past the impeller blades to the nose portion of the rear bearing unit housing where it exchanges heat through the thin steel wall of the bearing unit housing with the cooler ambient water that is flowing past the outside of this housing. The cooled lubricant then re-enters the axial channel for further circulation through the radial channels, annular passageway, etc.

[0040] This self-contained, self-powered lubrication system has the inherent advantage of providing proportional lubrication. In other words, the faster the central shaft turns, the more heat is generated and the more the bearing wears. However, the faster the central shaft turns, the faster the lubricant is circulated (as this is directly proportional to the angular velocity of the impeller) . Therefore, the faster the shaft rotates, the greater the cooling and lubrication. This design therefore provides an innovative way to provide proportional, self-powered cooling, lubrication and cleaning functions. Furthermore, the lubrication system is self-priming. No external pump and filtering system is required to circulate and clean the lubricant in the lubrication system, nor to prime the pump. This reduces cost, complexity, size and improves service life. It is expected that the novel design will enable the turbine bearing system to operate maintenance-free for 100,000 hours (i.e. over 10 years) . Because of the reduced maintenance cost, the energy extraction cost becomes very competitive as compared to other renewable energy production options.

[0041] As further depicted in the particular embodiment illustrated in FIG. 3, a high-performance filter medium is provided at the inlet to the axial lubricant channel to filter the lubricant and catch particles, metallic or otherwise, that are generated in, or which ingress into, the bearing chamber. As further depicted in the embodiment of FIG. 3, a magnetic candle is provided to magnetically attract and trap metallic particles. The magnetic candle and filter medium remove metallic and other unwanted particles from the lubricant to ensure that only clean lubricant flows through the bearing. This magnetic cleaning and filtering further prolongs bearing life.

[0042] As further depicted in FIG. 3, the auto-adaptive pressure-compensation system has one or more equilibrium chambers to automatically equilibrate the lubricant pressure with the ambient water pressure. The ambient water pressure is a function of depth (which remains constant once the turbine is installed) , flow rate (which can vary as currents go faster or slower) and temperature (which also can vary with weather and the seasons) . Water temperature also has the effect of changing the temperature of the turbine components and of the lubricant (s) inside the turbine. Because of these temperature effects, there is potential for a pressure differential to develop between the lubricant inside the turbine and the ambient water outside the turbine. This potential pressure differential is problematic since it may induce leakage of lubricant out of the turbine into the surrounding water or, alternatively, seepage of water into the sealed-off chambers of the turbine. In the former case, leakage of lubricating oil into the water is potentially bad for the environment, although for this particular turbine, care has been taken to utilize an oil that is biodegradable and environmentally-friendly. Another problem with leakage of oil is that, over time, the turbine will lose lubricant quantity and pressure, thus eventually degrading performance. Seepage of water into the sealed-off chambers of the turbine is also highly problematic as this dilutes the lubricant, changing its composition and efficacy. Therefore, it is highly desirable to minimize, if not outright eliminate, any pressure differentials between the inside chambers of the turbine and the ambient water. The present invention provides an automatically adaptive pressure-compensation system that reacts to changes in the ambient pressure of the exterior water to adapt (adjust) the pressure inside the chambers of the turbine to match the ambient water pressure, so as to quickly eliminate any developing pressure differential. By restoring the pressure equilibrium, the potential for leakage or seepage into or out of the oil-containing chambers of the turbine is eliminated (or at least greatly minimized) . The auto-adaptive pressure-compensation system equilibrates pressure inside and outside the oil-containing chambers using one or more equilibrium chambers. Each chamber is provided with its own equilibrium chamber that automatically compensates for pressure changes.

[0043] Each equilibrium chamber comprises a tank and a bladder having an elastic membrane within said tank. The bladder is pre-charged with a pressurized inert gas (such as for example a gas like N ₂ or CO ₂ ) . This gas expands and contracts to balance the pressure inside the lubricant- containing chamber of the turbine with the ambient water pressure outside the turbine. The gas is pre-charged to a pressure based on the operating depth (i.e. based on the water column at that depth) so as to provide a range of pressure compensation for likely operating conditions of the turbine.

[0044] In the specific embodiment depicted in FIG. 3, one such equilibrium chamber is positioned inside the nose portion of the housing of the front bearing unit. In the embodiments presented below, three equilibrium chambers are provided for each bearing unit (one for each of three distinct chambers) . This will be described in greater detail below.

[0045] Another feature of this turbine is that the front bearing unit is axially movable with respect to the rear bearing unit. This potential for axial movement accommodates thermal expansion or other strain in the turbine, therefore relieving any stress beyond design stress. This axial movement of the front bearing unit relative to the rear bearing unit is made possible by virtue of a front bearing unit that comprises a bearing that slides within a bearing housing to enable axial displacement of the front bearing unit relative to the rear bearing unit. In other words, the inner sleeve of the bearing is tightly fitted to the outside of the central shaft whereas the external sleeve slides inside its respective housing. This enables some degree of axial displacement to accommodate thermal expansion or other mechanically-induced strain.

[0046] FIG. 4 is an enlarged rear view of the front hub assembly depicted in FIG. 1. FIG. 4 shows the location, in this particular embodiment, of a biaxial accelerometer 161 designed to measure axial and radial vibrations. A monitoring system may be provided to receive signals from the accelerometer and to provide warnings when vibrations exceed a predetermined threshold. A control system may also be provided to control the angular velocity of the central shaft if the vibrations exceed a predetermined threshold to avoid damage to the turbine.

[0047] In the specific embodiment depicted by way of example in FIGS. 1-4, the front hub assembly has a front hub 101, as shown in FIG. 3, with a Varilip® seal support 102 and threaded fasteners 103. A spherical roller bearing 104 is fitted between the hub and the central shaft (or main shaft) 162. Front bearing sleeve 105 and nut 106 hold the internal sleeve of the bearing in place. Also visible are the pumping cover 107, filter spacer 108, filter sheet 109, washer 110 and fasteners 111 and O-ring 112. On the top side are the guide vane inlet 113 and guide vane key 114. Visible at the rear of the front bearing unit is seal cartridge 115. A seal spacer

116 is provided along the shaft, as shown. A magnetic candle

117 is disposed in the nose portion, as described above.

[0048] For the first equilibrium chamber connected to the first chamber, the equilibrium chamber includes a bladder top cover 118, a 0.6 litre bladder 119, a bladder tube 120, and a bladder bottom cover 121. The 1.15 litre bladder of the third equilibrium chamber is designed by numeral 122. The second equilibrium chamber comprises a bladder top cover 137, a 0.16 litre bladder 138, a bladder tube 139 and a bladder bottom cover 140.

[0049] Also visible in FIG. 3 are the following parts: stay ring 123, guide vane adjuster sleeve nut 124, washer 125, front protection support 126, threaded fastener 127, 0-rings 128 and 129, fasteners 130, 131 and 132, sealing rim 133 and 134, and fasteners 135 and 136, and 0-ring boss plug 142.

[0050] As shown in FIG. 3, the front bearing unit includes a front cover 143. Also at the front end of the front bearing unit is a bearing chamber oil manifold 150, an oil manifold rod 151, Swagelok® fittings 152, 153.

[0051] The front bearing unit includes three submersible pressure transducers 157 mounted via respective National Pipe Thread (NPT) adapters 156, as shown in FIG. 3. Three moisture (humidity) sensors 158 are also visible in this cross- sectional view.

[0052] FIGS. 5-8 depict the rear bearing unit. Many of the components and features of the rear bearing unit are identical or analogous to those of the front bearing unit and, accordingly, will not be redundantly described herein. The rear bearing unit, like the front bearing unit, has a self- powered lubricant circulation system and/or an auto-adaptive pressure-compensation system in order to prolong service life, to minimize environmental impact, to improve energy extraction efficiency and to minimize manufacturing cost and complexity.

[0053] FIG. 5 is an isometric view of the rear bearing unit in accordance with one embodiment of the present invention. FIG. 5 shows the substantially cylindrical bearing unit housing that encloses the bearing unit. Two equilibrium chambers (tanks) are visible aft of the rear bearing unit housing. These two equilibrium chambers are disposed downstream of the rear bearing unit to minimize hydrodynamic drag and exposure to foreign objects (e.g. logs) that may traverse the turbine.

[0054] FIG. 6 is a front view of the rear bearing unit depicted in FIG. 5. The two external equilibrium chamber are visible. Inside each equilibrium chamber is a respective bladder, representing by a jagged line showing that the bladder is partially deflated (as it adapts the pressure inside its respective oil chamber to the ambient water pressure) . The first and second external equilibrium chambers are used for first and second oil-containing chambers, respectively.

[0055] FIG. 7 is a cross-sectional view of the rear bearing unit taken through section A-A in FIG. 6. One of the two side-by-side external equilibrium chamber is visible in this figure as well as the internal equilibrium chamber used for a third lubricant-containing chamber. The third chamber is the largest chamber. This is the chamber that includes the lubricant-circulation system (axial channel, radial channels, and annular passageway through the spherical roller bearing) . This third chamber also includes, as shown in FIG. 7, a nose portion. The nose portion defines a generally frusta-conical volume. As noted above, lubricant in this nose portion exchanges heat with the cooler ambient water through the steel walls of the housing. Also the nose portion contains the magnetic candle which, along with the filter medium at the mouth of the axial channel, filters and cleanses the lubricant. It is inside this nose portion that the third equilibrium chamber is positioned, as shown in this same figure. The third equilibrium chamber thus adapts the pressure in the third chamber to match the ambient water pressure outside the housing of the bearing unit. [0056] With reference still to FIG. 7, the rear bearing unit comprises a bearing unit housing 201, a spherical roller bearing 202, a rear bearing sleeve 203, a nut 204 and a rear bearing bushing 205. A Varilip® seal support 206 restrains the seal cartridge 213 on the main shaft (central shaft) 240. O-ring 214 and seal spacer 216 are provided as shown. Note that central shaft 240 is the rear portion of the same central shaft designated in FIG. 3 by reference numeral 162.

[0057] As illustrated in FIG. 7, the rear bearing unit, like the front bearing unit, has a self-powered lubrication system having an impeller driven by the central shaft to cause lubricant to flow through a closed-loop circuit formed by axial and radial channels and an annular return passageway through the bearing, as described in detail above. This lubricant-circulation system also has, in this specific implementation, a filter composed of a filter spacer 210, filter sheet 211 and washer 212. In this particular embodiment, the circulation system also comprises a magnetic candle 215 for filtering out metallic particles from the lubricant .

[0058] As shown in FIG. 7, the rear bearing unit, like the front bearing unit, has an auto-adaptive pressure-compensation system composed of three chambers and three equilibrium chambers, one per chamber. As shown, the first equilibrium chamber includes a bladder top cover 217, a 0.6-litre bladder 218, a bladder bottom cover 219, a bladder tube 220 and a sealing rim 221. The second equilibrium chamber includes a bladder top cover 225, a 0.16-litre bladder 226, a bladder tube 227 and a bladder bottom cover 228. The third equilibrium chamber includes a 1.15-litre bladder 222. As depicted in this figure, the third equilibrium chamber in this embodiment is secured in a nose portion of the housing of the rear bearing unit, i.e. in a bulbous extension of the housing. The rear bearing unit may also include a cover 229, as shown.

[0059] In terms of sensors, the rear bearing unit includes three submersible pressure transducers 236 mounted via respective NPT adapters 237, three moisture sensors 238, and one biaxial accelerometer 239. Note that the pressure transducer 238 visible in FIG. 7 is mounted to a bearing chamber oil manifold 230 to which a downstream oil pipe 233 is connected via a Swagelok® fitting 232. The pipe 233 is connected at its upstream with another Swagelok® fitting 235. Oil manifold rods 231 extend rearwardly from the bearing unit housing to the oil manifold 230.

[0060] FIG. 8 schematically depicts the three-chamber sealing system used in one embodiment of the present invention. Three successive sealed oil-containing chambers are employed to provide a substantially leakage-free bearing unit. Each of the three lubricant-containing chambers has its own respective equilibrium chamber for independently and automatically adapting the pressure inside the lubricant-containing chamber to match the ambient water pressure. This tiered (layered) approach to sealing the bearing unit attenuates the effects of water seepage into the bearing unit by successively diluting the water-oil mixture. For example, consider a scenario where a small amount of water seeps into the first lubricant- containing chamber, e.g. due to a transient pressure imbalance across the first seal. This small amount of water mixes with the oil in this lubricant-containing chamber to form a mixture of mainly oil with a small amount of water. Even if this mixture in the first lubricant-containing chamber further seeps into the second lubricant-containing chamber, the resulting mixture in the second lubricant-containing chamber is predominantly oil, since the liquid that seeps into the lubricant-containing chamber is mainly oil with only a small amount of water. Likewise, even if the mixture in the second lubricant-containing chamber seeps into the third lubricant- containing chamber, the resulting mixture contains only a very small quantity of water. In other words, the tiered configuration of lubricant-containing chambers has the effect of successively diluting the amount of water in each successive lubricant-containing chamber (so that even if water were to seep into the third lubricant-containing chamber, the amount would be rather negligible as it would represent a dilution of a dilution of a dilution) .

[0061] As depicted by way of example in FIG. 8, the third lubricant-containing chamber has the largest volume, the first lubricant-containing chamber has the second largest volume and the second lubricant-containing chamber has the smallest volume. Connected to the first lubricant-containing chamber is a first equilibrium chamber. Connected to the second lubricant-containing chamber is a second equilibrium chamber. Connected to the third lubricant-containing chamber is a third equilibrium chamber.

[0062] Solely by way of example, the first lubricant- containing chamber may have a volume (capacity) of approximately 3 litres, the second lubricant-containing chamber a volume of 1 litre, and the third lubricant- containing chamber a volume of 10 litres. It will be understood that the volumes (capacities) of these lubricant- containing chambers may be varied and are presented herein strictly as an example. Again solely by way of example, the first equilibrium chamber may have a volume of 0.6 litres, the second equilibrium chamber a volume of 0.16 litres, and the third equilibrium chamber a volume of 1.15 litres. In this specific configuration, the third equilibrium chamber has a larger volume than the first equilibrium chamber which has a larger volume than the second equilibrium chamber. It bears emphasis that these equilibrium chamber volumes may be varied and are presented herein strictly as an example for a turbine that is expected to be mounted in a river at a depth of approximately 10 metres where water temperatures are expected to vary between 4 and 18 degrees Celsius. Note how the ratio of the lubricant-containing chamber volume to the equilibrium chamber volume increases from the second chamber to first chamber to the third chamber, reflecting the design pressure compensation that is required (relative to each chamber capacity) . In other words, the first lubricant-containing chamber is most exposed and thus requires the greatest leakage-free capability relative to its size. The second lubricant-containing chamber is somewhat protected by the first lubricant-containing chamber and thus the pressure- compensation capability of its equilibrium chamber is not as great as it is for the first lubricant-containing chamber. The third lubricant-containing chamber is protected by both the first and second lubricant-containing chambers. Its pressure-compensation equilibrium chamber can thus be different as the variability in pressure differential between the third lubricant-containing chamber and ambient water is moderated by the first two lubricant-containing chambers.

[0063] As further depicted schematically in FIG. 8, there is a static seal (e.g. 0-ring or other sealing element) at the water-side interface of the first chamber. Another seal is present between the first and second chambers. A seal and grease barrier is provided between the second and third chambers. One or more of these seals may be part of a sealing cartridge .

[0064] As further depicted schematically in FIG. 8, the first lubricant-containing chamber and the second lubricant- containing chamber may contain the same bio-degradable fluid (same lubricant) whereas a different type of oil (lubricant) may be used for the third lubricant-containing chamber which actually contains the spherical roller bearing.

[0065] Each of the three lubricant-containing chambers may include its own humidity sensor and/or its own pressure sensor and/or its own temperature sensor. These sensors send signals to a control system that is typically to be located on shore that monitors the performance of the turbine over time. These sensors play an important role in delivering performance data and other metrics to the operator to enable the operator to know when the turbine is due for maintenance.

[0066] The turbine may include a forwardly extending submersible pressure transducer connected to a front lubricant manifold of the front bearing unit for sensing ambient water pressure. Similarly, a rearwardly extending submersible pressure transducer connected to a rear lubricant manifold of the rear bearing unit for sensing ambient water pressure.

[0067] FIG. 9 illustrates a central shaft 405 supported by both front and rear bearing units. The bearing units depicted in this particular figure are from an earlier iteration of the technology, and thus slightly different in design. FIG. 10 is a cross-sectional view of the front bearing unit of FIG. 9. The embodiment shown in FIG. 10 also includes a spherical roller bearing 401, seal cartridge 402, impeller 403, filter 404, and axial channel 406 leading to the same radial channels and annular passageway. A bearing unit housing 411 encloses the bearing unit. This earlier iteration of the turbine shows that the components may be configured differently without departing from the inventive concepts presented herein. For example, the equilibrium chambers may be located beneath the bearing units connected to a port 408, as shown in FIG. 10. As will be appreciated, piping arrangements may be devised to locate the equilibrium chambers virtually anywhere, although the best mode known to Applicant at the time of filing is to locate the third equilibrium chamber inside the nose portion as depicted in FIG. 3 and FIG. 7 and to position the first and second equilibrium chamber outside and behind the bearing units where they are protected from damage.

[0068] FIG. 10 also shows the cover 409 that is secured to the bearing unit to ensure that seals remain as much as possible under static pressure as opposed to under dynamic pressure. The cover attenuates the rate of change in pressure at the seal, thereby giving the equilibrium chamber (s) time to adapt to the change in pressure. The cover therefore cooperates with the equilibrium chamber to maintain a close- to-zero pressure differential across the seals of each of the bearing units.

[0069] FIG. 10 also clearly shows the dynamic rotating friction-type floating seal 407 that is used to isolate the lubricant from the ambient water. This type of seal allows the shaft to turn while maintaining a watertight seal.

[0070] Most of the structural components of the front and rear bearing units are preferably made of stainless steel, although it should be appreciated that other alloys or materials may be substituted as would be understood by a person of ordinary skill in the art.

[0071] The present invention has been described in terms of specific embodiments, examples, implementations and configurations which are intended to be exemplary or illustrative only. Other variants, modifications, refinements and applications of this innovative technology will become readily apparent to those of ordinary skill in the art who have had the benefit of reading this disclosure. Such variants, modifications, refinements and applications fall within the ambit and scope of the present invention. Accordingly, the scope of the exclusive right sought by the Applicant for the present invention is intended to be limited solely by the appended claims and their legal equivalents.