GB2470447A - Paired tidal turbines drive pumps connected in series - Google Patents

Abstract

A system of sea bed mounted tidal / marine current turbine driven pumps 1 is connected by submerged pipes 2, 5, 9 to drive a power generation turbine 8. The turbines 1 may be ducted and may be mounted in opposite facing pairs. The turbines 1 may drive the associated pump via gearing and a one-way clutch, so that only the turbine facing the flow direction will generate power. The pumps may be connected hydraulically in series. Installation, maintenance and construction is achieved by use of subsea support vessel 32 and remote operated vehicle 33 technology.

Description

Submerged Impeller Driven Pump Tidal Power Generation System Subsea tidal power generation offers an immense opportunity as a viable alternative to a number of low carbon emission electrical power generation concepts. However current tidal power schemes often involve significant engineering challenges associated with reliability of subsea systems and equipment thus making them extremely high Cost as an alternative to other renewable energy sources such as onshore and offshore wind power. A major disadvantage of the main forms of subsea tidal systems currently in use is that multiple generators have to be coupled to the driving device, usually an impeller of sufficiently large size to capture energy from tidal currents. Even the smallest of subsea tidal power schemes involve multiple power generators and cables between the generating units. The invention described here is mainly applicable to fast flowing tidal and subsea current systems and utilises a number of impeller and pump coupled units to drive water flow at higher pressures via a subsea piping network to a single large power generator. In large tidal power generation schemes employing this invention multiple networks could also be employed. The other advantage that this invention presents is that it is constructed in modular form and can be installed with lower capital expenditure compared to similar sized installations of similar power generating capacity. Additional advantage offered by this system is that it is low maintenance and of extremely simple construction and therefore the failure modes effects and criticality analysis for such a system are not complex thereby making subsea repair and retrieval operations relatively simple as there are relatively few failure modes. A fundamental concept in this invention has been application of high reliability, simple technology that will make it economic through reduced through life costs compared to other subsea tidal power generation technology. This will be described in some detail

in the main description.

The invention described here presents a very simple modular system that can be applied to any location involving a fast flowing tidal or localised current effect at sea bed of sufficient depth to allow the submerged modular impeller driven pump units to operate. The submerged impeller driven modular pump units offer a significant advantage in that they can be manufactured relatively cheaply on a mass scale and be installed at a site location for relatively low capital expenditure and minimal environmental impact during the installation phase compared to similar systems. A large hydroelectric generator has to be located within a dedicated pump/impeller network and for an expansive sea bed location a number of these networks may occupy the seabed acreage. Where a system location is near shore, e.g. bays with high speed tidal currents, the generator could be located above surface or even onshore. However for the purposes of describing this invention generically all electrical power transmission and generator cabling connection is located on the seabed. As described previously a key feature of this invention is that the hydroelectric power fluid is contained within a self contained closed loop connecting the submerged impeller driven pump units.

The impeller on the pump/impeller Units are driven by high speed subsea currents and hence the driving power to power the pumps and indirectly the electrical power generator via the fluid circulating in the loop. The only interaction with the flow of water in the environment is between the impellers and water flow around the submerged nacelles, structures and piping. The design of this system also lends itself very well to the use of non corrodible materials in the main impeller, pump and interconnecting pipework. The use of a closed loop allows the system to avoid having seawater directly circulated through the pumps and generator's hydroelectric turbine. Avoidance of intake of seawater from the marine environment also avoids interaction with marine life being ingested in to the piping system and pumps. A uniform quality of circulated water is maintained and this can also be treated with appropriate chemicals e.g. biocides, friction reducers and antifreeze agents without any environmental impact as the system is self contained.

The invention will now be described by the following figures: Figure 1.Overview of a typical installation at a seabed location.

Figure 2.Simple line drawing schematic of multiple submerged tidal current paired fixed pitch impeller driven pump system.

Figure 3.Line diagram for a pair of impeller driven pump systems with fixed pitch impellers Figure 4.Elevation through the drive train of a typical paired fixed pitch impeller and pump system.

Figure 5.Front and rear elevation of a paired submerged fixed pitch impeller driven pump system.

Figure 6.Side elevation of a paired submerged fixed pitch impeller driven pump system.

Figure 7.lllustrative diagram of installation of a impeller driven pump system and nacelle mating with subsea structure and interconnecting piping system.

Figure 8.Typical schematic of a reaction type turbine for use on multiple submerged impeller driven pump hydroelectric power generation system.

Figure 9.lllustration of main power generator showing interaction with piping system and diver and ROV operations.

Figure 1O.Side elevation section through an alternating pitch impeller driven pump system within a nacelle.

Figure 11.Simple line drawing schematic of multiple alternating pitch impeller driven pump hydroelectric power generation network.

By reference to figure 1 each submerged impeller driven pump unit 1 is connected in series to another unit via an interconnecting pipe 2. The pipe 2 can be constructed from a number of materials, the preferred material type being a plastic made with an armour coating similar to those on offer by a number of manufacturers who specialise in subsea flow line installations particularly for water injection applications in subsea oil and gas developments. However rigid metallic pipe could also be used but is not the favoured approach for a number of reasons, mainly corrosion, difficulty in installation and cost, for the purposes of description of this invention materials will not be referenced directly. However the preferred material for the interconnecting piping system is flexible armoured plastic coated pipe. For a given number of submerged impeller driven pump units 1 the first stage takes its suction 3 from the discharge of the hydroelectric turbine section 4 via the main suction flow line 5 and discharges to the suction of the next unit and so on to the final impeller driven pump unit. The discharge 6 of the final impeller driven submerged pump unit feeds into the intake 7 of the power generation turbine 8 via a main discharge flow line 9. Therefore the system operates in a closed loop with energy input to the pumps 12 via the interaction of the impellers 10 and the water flow that rotates them to drive the pump 12 contained within the nacelle 11 via an optional gear box 14 and a clutch 13 to engage the impeller 10 that is driven by current flow in correct direction to rotate pump 12. The clutch mechanism 13 is a suggested method of operation of a fixed pitch impeller 10 unit, alternatively an alternating pitch impeller 42 may be used in a single impeller pump unit with no need for a clutch 13 to engage rotation when the tide is in the required direction and ensure unidirectional fluid flow from the pump 12. In reference to figure 3 When the tidal direction changes the clutch 13b disengages on the offline fixed pitch impeller 10 b and pump 12b section and the opposing mounted impeller 10 a and pump 12 a system engages the clutch 13a and the pump 12a is rotated by the fixed pitch impeller lOa ensuring water flow is maintained within the closed loop. The configuration illustrated in figure 3 extracts maximum power from tidal flow in both directions, a single alternating pitch impeller 42 system while dispensing with the need for a clutch 13 or other rotational motion engagement device and additional paired impeller and pump unit 1 does have limitations. The main limitation of a single impeller and pump unit with alternating pitch impeller 42 is that the shadow of the nacelle created when the tidal current is flowing in the reverse direction i.e. to the rear of the impeller ducting 17 limits the available energy to the alternating pitch impeller 42 in the reverse direction. However some installations selection based on size and cost may favour that configuration to allow this invention to operate and hence for completeness it is described. Figure 2 illustrates the concept in terms of a schematic illustrating four submerged impeller driven pump units 1 pairs in a closed loop in series. Each flow line 2, 5, 9 has a frictional pressure loss generated by fluid flow through the line 2, 5, 9. This pressure loss is illustrated as ttPf in the following equations with n being the number of submerged impeller driven pump units. Each individual submerged impeller driven pump unit 1 generates a pressure increase �Punit n. The difference in the summation of pump pressure increases across the series connected impeller and pump units 1 and pressure losses across lines 2,5,9 is equal to the available pressure to the hydroelectric power generation turbine 35.

Pressure Across Turbine = Sum of submerged impeller driven pump lift pressures -Sum of head and frictional losses in connecting piping system P1 -P0 tPunit n - LPf n + 1 Power = (P1 -P0) . 1leiec. 0 tlelec = Fluid to electrical power conversion efficiency (%) O = Fluid volumetric flowrate within pump/generator loop (m3/s) P = Power turbine intake pressure (N/rn2) PD= Power turbine discharge pressure (N/rn2) Pressure increase across submerged impeller driven pump unit (N/rn2) P1 Pressure losses due to pipe internal friction and elevation head (N/rn2) n = Number of submerged impeller driven pump units* is applicable for number of paired impeller and pump units for fixed pitch impeller 10 systems and for the number of single impeller driven pump units for alternating pitch impeller 42 systems Note that pump units when installed in series elevate the pressure across the whole system additively. The result is a much higher pressure available for a given flow to the hydroelectric turbine unit 8 than if one submerged impeller driven pump unit 1 had been used.

It is also possible using the system as illustrated in figure 2 to alternatively connect the submerged impeller driven purnp units 1 in parallel, in this case the pump head generated would be low and equal to that from a single centrifugal pump 12 contained within the submerged impeller driven pump unit 1 and the flow would be maximised instead of pressure for series connected system. A parallel connected system although it generates a low head will allow a larger flow of fluid in the pump 12 and piping network 2, 5, 9 and utilisation of a different type of turbine 35 within hydroelectric turbine unit 8. However for the purposes of describing the invention the connected flow circuit referred to will be the series connected configuration. Other alternative configurations can involve dispensing with the pipeS between the discharge of the hydroelectric power turbine 8 and the suction of the first submerged impeller driven pump 1 in the network. This may offer marginally improved power output in open loop configuration. However the discharge of the hydroelectric power turbine tail race 4 may cause significant scouring on the seabed adjacent to its discharge point. An additional issue with the open ioop configuration is that the intake 3 of the first stage impeller driven pump unit 1 would need to have some form of solids removal as suspended solids would be likely to be present and ingress to the system via the pump 12 suction in the form of organic and inorganic material would be highly likely. Hence a closed system offers a number of reliability and environmental advantages mainly associated by isolating the hydroelectric power fluid from the environment. Additionally the use of a closed loop also allows a self contained fluid system using any other desired fluid than seawater. A de-ionised water or potable water system containing chemical additives such as: oxygen scavengers, friction reducers, antifreeze or biocides. Therefore the use of a closed flow loop via piping 2, 5, 9 totally excludes seawater from the surrounding seabed environment.

Referring to figure 3 it is evident that in a tidal system ebb and flow of the tide would dictate the rotation of the impeller. One approach to overcome this and maximise energy extraction efficiency from the current uses impeller and pump units 1 in a paired configuration. When the tidal direction is in the flow direction as illustrated in figure 3 clutch 13 a engages and the impeller 10 a drives the pump 12a through the gear box 14 a. As the impeller 10 a drives the pump 12 a in the flow direction of the tide non return valve 39 a opens and forward flow of the fluid within the network continues.

On the adjacent paired impeller pump unit 1 b the clutch 13 b disengages as the flow of the tide forces the impeller 10 b away from the clutch 13 b engagement direction. The common shaft bearing 19 is of a type that combines rotational motion confinement while allowing the shaft 24 to move back and forward along its centre line. This backward and forwards motion allowed in the main shaft 24 by the bearing 19 allows the clutch 13 to engage and disengage. When pump 12 b is not engaged by the impeller 10 b as the flow direction is causing the clutch 13b to disengage the non return valve 39 b is forced to the closed position by the flow of fluid from the discharge of pump 12a.

Referring to figure 1 a number of submerged modular impeller driven pump units 1 in tide ebb and flow configuration are submerged on a seabed location where the impeller units 10 are driven by the flow of water due to prevailing tidal currents in the area. With reference to figure 4 the submerged impeller driven pump unit consists of a number of main features: the impeller 10 and its ducting 17, the nacelle 11 housing the centrifugal pump 12 and drive components; bearings 19, 20, 21 and gear box bearings 22 and the gear box 14 itself. In figure 5 the base 18 is a traditional piled seabed structure with piles 23 located at points on the structure to secure it to the sea bed. The impeller 10 is connected to a common shaft 24 that is supported by a sealed bearing 19 leading to a clutch 13 and then via a confining bearing 21 to the main gearbox 14. The impeller duct is connected to the nacelle for support via the sub structure 29 and also support from the base structure 18. The gear box 14 increases the speed of the pump 12 shaft by the impeller 10 speed multiptied by the gear ratio to drive the pump 12 shaft and intake and discharge fluid via the pump 12. The nacelle housing 11 is of streamlined shape to avoid propagation of wake from the impeller 10 and limit the efficiency of downstream units. In figure 10 illustrating a section through a single impeller driven pump unit 1 with alternating pitch impeller 42 it is clear that the hydrodynamic efficiency of the nacelle 11 is critical to energy extraction in the reverse flow direction. The nacelle 11 is an optional feature of this invention and where a sufficiently large pump and impeller unit 1 is located on the seabed and is extremely heavy then the nacelle 11 may be dispensed with to allow subsea access to individual components for removal to surface. The pump 12 intake 25 is at the rear of the nacelle 11 via an opening 26 at that point. The centrifugal pump 12 discharges via 27 through another opening 28 in the nacelle 11. The pump 12 intake 25 is connected to a suction line 2 either from the discharge of a previous submerged impeller driven pump unit or from the discharge line 5 from the power generation hydroelectric turbine 8. The nacelle 11 can either be sealed or allowed to operate in flooded mode. It is essential that the sealed components are: the main common shaft bearing 19, clutch 13, main bearing 21, gear box 14, centrifugal pump 12 internal bearings via pump 12 internal seals. Proprietary equipment for marine environments can be used for all of these applications and no particular manufacturer is required to be recommended and a number of component designs offered in the market place are suitable. The nacelle 11 and main rotational components contained within sit on a sub assembly 29; the sub assembly 29 supporting the nacelle 11 is made up of two separate sub assemblies. Sub assembly 29 connects to another sub assembly 18 that is the base structure piled in to the seabed. The sub assemblies 29, 18 can be mated in a number of ways, either by a mechanical locking device using male and female components or by bolting 30 in a vessel assisted operation as illustrated in figure 7. In figure 7 the pre-installed and piled 23 base structure 18 is already installed on the seabed and the impeller and pump unit 1 is lowered on a lifting frame 31 with the sub assembly 29 already connected to the impeller and pump unit 1. The sub assembly 29 and impeller and pump unit 1 are lowered in to position and mated with the base structure sub assembly 18. The two assemblies can then be secured by using divers or remote operated vehicles (ROVs) to complete bolting 30 the two structures 18, 29 together. As referred to previously in this description, bolting is only one of a number of possible methods of mating sub-structures 18, 29.

To minimise environmental impact during installation it is essential that the procedures reflect the sensitivity of the installed location and the hazards associated with installing submerged equipment in a fast flowing seabed tidal environment. The installation procedures would be specific to the conditions where the unit was installed and would in most foreseeable cases involve the use of a subsea construction vessel 32 utilising either diver support or ROV 33 technology to support all phases of construction and maintenance.

As described the series connection of the submerged impeller driven pump paired units 1 allows a substantial increase in pressure across the network and use of high pressure drop reaction type turbines 35 as illustrated in figure 8. To allow the installation and operation of a reaction type turbine 35 to drive a generator 34 a series connection of the submerged impeller driven pump units 1 as illustrated in figure 1, 2, 3 and 11 would be required. To allow a network of submerged impeller driven pump units 1 to operate a number of ballasting weights or trench and burial would need to be used on the interconnecting flow lines 2 and the main hydroelectric turbine 8 discharge flow line and hydroelectric turbine 8 suction flow lines 9. Ballasting weights or conventional burial and possibly rock dumping would be used to absorb hydraulic loads induced by flow transients within the piping. As mentioned previously flow induced transients are an undesirable effect in using this method of hydroelectric power generation in particular in the closed loop configuration described. A key part of avoiding flow induced transients and centrifugal pump 12 damage is to avoid cavitation and ensure the closed loop system has flow lines 2, 5, 9 completely filled with fluid with no air pockets or voids. To achieve a complete fill of the closed loop system 2, 5, 9 it is essential that the fill of the system is completed via a valve line up that allows all air to be vented off and displaced through a high point in the system. To achieve complete system fill water is introduced with the following valve line up by reference to figure 9, the main isolation valve 36 is in the closed position prior to initiating the fill up of the system and fluid is introduced via the inlet fill valve 37 and air is vented of through the vent valve 38 until fresh water is seen to flow continuously from this point indicating that the whole system is completely filled and that no air pockets that could propagate cavitation are trapped within the system. The fill and vent through valves 37 and 38 would be achieved by connecting fill and vent hoses to the valves to and from a subsea support and construction vessel 32 and valve operations would either be achieved by ROV 33 or diver operations.

A pumping unit located on the subsea support and construction vessel 32 would be required to ensure adequate line 2, 5, 9 flushing and preparation. Once the system fill is confirmed the main isolation valve 36 is opened and the vent valve 38 and fill valve 37 are then closed and the system is available to operate. The complete fill of the system is a key component in successful in operation of this invention.

As previously referred to this description of the invention refers to a series configuration to describe how the power to a hydroelectric turbine 8 and generator 34 can be increased by increasing pressure across a number of submerged impeller driven pump units 1 connected in series configuration. However as cited but not required for this description of the invention the impeller and pump unit 1 pairs can be connected in parallel to allow a low pressure head and high flow driven power output to be achieved.

Although it is not specific to this inventive concept, for illustrative purposes in figure 8 a typical reaction turbine 35 schematic is shown. However it is key that the forces involved in this unit must be managed and such a structure 48, 43 housing the main turbine 35 would need to be piled 46 to the seabed, additionally a concrete fill 47 may be required within the containment structure 48, 43.

In figure 9 a simple illustration of side elevations of the main turbine housing structure 48,43 are illustrated with the generator 34 mated to it. The generator 34 would ideally be able to be uncoupled mechanically at the turbine and generator coupling 45 and disconnected from power cable(s) 40 subsea to allow retrieval to surface and access to the main turbine 35 internals. However dependent on size of unit and subsea support and construction vessels used both the turbine unit 8 and generator 34 may be able to retrieve to surface in a single lift and possibly using diverless ROV 33 intervention alone.

Claims (6)

1. Claims 1. A tidal current hydroelectric power generation system where submerged impeller driven pump units generate fluid movement collectively in a system using interconnecting pipes between the pumps to drive a hydroelectric power generation turbine unit.
2. 2. As defined in claim 1 a submerged impeller driven pump unit with a ducted impeller system connected via a gear box mechanism driving a pump to circulate fluid in the network to drive the hydroelectric turbine.
3. 3. As defined in claim 1 multiple submerged fixed pitch impeller driven pump units driven by tidal flow with direction of pumping activated by a clutch mechanism when tidal flow is in the required direction while causing pump to stop by disengaging clutch on paired unit facing opposite direction of tidal flow.
4. 4. As defined in claim 1 multiple submerged impeller driven pump units connected via piping that is primed via a system of isolation, fill and vent valves to expel air pockets within the interconnecting piping system ensuring that piping is completely liquid filled to eliminate pump cavitation.
5. 5. A tidal power generation system as defined in claim 1 that allows a closed loop of fluid circulation to drive a hydroelectric turbine with fluid that is non corrosive and excludes surrounding marine environment.
6. 6. A submerged impeller driven pump unit as defined in claim 1 employing an alternating pitch impeller to allow constant rotational direction to maintain unidirectional fluid pumping motion.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWSClaims 9 1. A tidal current hydroelectric power generation system where a plurality of paired submerged impeller driven pump units connected in series with each other within a closed fluid ioop generate fluid movement continuously in the same direction within the closed loop to elevate fluid pressure sufficiently to drive a reaction type power generation turbine within the same closed loop piping system regardless of tidal ebb and flow direction.2. As defined in claim 1 a tidal current hydroelectric power generation system where a plurality of paired submerged fixed pitch impeller driven pumps facing opposite to each other are driven by tidal flow with direction of pumping activated by engagement of a clutch mechanism that engages when tidal flow is in the required direction while causing pump to stop by disengaging clutch on the paired unit facing opposite direction of tidal flow with fluid flow direction within the closed loop directed by non return valves on the pump discharges.3. A method of priming a tidal current hydroelectric power generation system according to claim 1 by connecting fill and vent hoses to fill and vent valves located on the closed loop piping with fill and discharge from a subsea support and construction vessel with valve movement achieved by remote operated vehicle (ROV) or diver operations utilising a fluid pumping unit located on the subsea support and construction vessel for flushing, preparation and priming of the subsea closed piping loop.4. A tidal current hydroelectric power generation system as defined in claim 1 where the impeller and pump units are mounted on submerged structures secured by piling on to the *si seabed.SI* 5. A tidal current hydroelectric power generation system as defined in claim 1 where a ** hydroelectric reaction type turbine mounted on a seabed location is enclosed in a subsea *ISa containment structure piled on to the seabed where the structure enclosing hydroelectric turbine is able to contain fluid forces exerted by the hydroelectric reaction turbine. * .*, ** * S **S * S