GB2478218A - Integrated offshore wind and tidal power system - Google Patents

Abstract

An integrated offshore wind and tidal power system where the tidal power system has a plurality of submerged impeller driven pumps 4 connected in closed fluid loop to drive a turbine such as a reaction turbine which powers a generator 3 which is mounted above the water surface on an offshore wind turbine support structure 2 or on a separate structure next to the wind turbine support structure. Preferably each impeller drives a centrifugal pump via a gear box and each of the pump units are connected in series. The impeller and pump units may be mounted on submerged structures secured by piling or by a gravity base on the seabed.

Description

Integrated Wind and Tidal Energy System Conventional near shore wind power developments either in the planning or operations phase can offer a significant additional synergy with tidal energy in an integrated package where there is tidal energy availability coupled to a windfarm location. No truly viable combination of wind and tidal on a large industrial application scale has been deployed. Conventional near shore wind farm developments employing horizontal axis wind turbines mounted on monopile structures could be adapted either in operational phase or preferably in design phase to incorporate a tidal energy system as described in this invention as an integrated package. Currently tidal and wind energy systems have been conceived as standalone concepts and no truly integrated viable schemes have been deployed or put forward to address this issue. For the majority of cases where near shore wind turbines are deployed it is usually offshore on a tidal flat area where there are strong winds and the turbine can be mounted relatively far from land fall but at a shallow water depth. These types of environment can also lend themselves well to tidal energy concepts if one was available that could operate effectively in a shallow near shore marine environment. However in a number of cases the tidal energy available is not sufficient on its own to sustain a tidal energy infrastructure using existing concepts thus creating a missed opportunity for capture of energy from these tidal areas.

The invention to be described here fulfils all of the criteria required to truly integrate existing wind turbine technology with tidal energy in a single combined and viable concept for widespread application to existing and new wind farms located in near shore shallow marine and estuarine environments.

In the invention described here, a very simple modular system that can be applied to any location involving a fast flowing tidal or localised current effect at sea bed is located at sufficient depth to allow the submerged modular impeller driven pump units (described in following paragraphs) 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 can be installed at a site location for relatively low capital expenditure and minimal environmental impact during the installation phase compared to tidal systems driving a dedicated generator. A large hydroelectric generator can be located within a dedicated pump/impeller network and for an expansive sea bed location a number of these networks can be used to occupy the seabed acreage. The system integrates with wind turbine technology either by sharing the monopile for the wind turbine with the hydroelectric generator or in existing wind farms mounting another monopile adjacent to the existing wind turbine for the tidal power generator. This configuration can also allow sharing of a number of services with the existing wind turbines in the wind farm principally the grid connection, transformers and switchgear. This is achieved by having all major high voltage connections and generation equipment associated with the tidal energy component of the system mounted above sea level and splash zone at the wind turbine structure.

A key feature of this invention is that the hydroelectric power fluid is circulated within a self contained closed loop connecting the submerged impeller driven pump units similar to the invention described in GB Patent application GB 2470447 A. The impeller on the impeller driven pump units are driven by high velocity subsea currents and hence provide the driving power to rotate the centrifugal pumps and indirectly the electrical power generator via the fluid circulating in the loop.

Centrifugal pumps were chosen instead of positive displacement type pumps to ensure continuous steady state flow through pumps to the reaction turbine section and avoid the need for pressure relief systems to be incorporated for system overpressure protection. 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, thereby making the system more reliable. Avoidance of intake of seawater from the marine environment also eliminates 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 self contained loop as will be demonstrated in the main description using fluid energy conservation equations allows the hydroelectric generator to be mounted above the sea/water's surface directly on an existing or new wind turbine monopile or an adjacent located monopile to a wind turbine. It is this concept that makes the invention unique and a creative solution to unlocking the combined benefits of wind and tidal power generation in a single package.

According to the present invention a tidal current hydroelectric power generation system where a plurality of series connected submerged impeller driven pump units within a closed loop generate fluid movement collectively through pipes connected between pump suction and discharges to elevate fluid pressure to drive a hydroelectric power generation turbine within the same closed loop, the aforementioned hydroelectric turbine mounted above water surface on an offshore wind turbine support or adjacent structure allowing both to function as a combined power generation installation.

Preferably an integrated offshore wind and tidal power system where a tidal energy gathering system utilising series or parallel connected submerged impeller driven pumps connected in a closed fluid loop drives a hydroelectric generator within the same closed loop mounted above water surface on an offshore wind turbine support or adjacent structure.

Preferably a plurality of submerged impeller driven pump units where each pump is connected to a dedicated impeller via a gear box mechanism driving a centrifugal pump to circulate fluid within closed ioop network to elevate pressure sufficiently to drive a hydroelectric turbine mounted above the water surface located on the support structure of an offshore wind turbine.

Preferably a plurality of submerged impeller driven pumps where the high pressure output of the closed ioop system drives a hydroelectric power generation turbine mounted above water surface on a structure adjacent to a wind turbine support structure where both units function as a combined power generation installation.

Preferably a plurality of submerged impeller driven pump units connected via closed loop piping generate sufficiently elevated pressure to mount a reaction turbine above water surface that is housed on a wind turbine structure or on an adjacent structure that allows the wind and tidal power units to act as an integrated power generation installation.

Preferably a tidal power generation system utilising a closed loop of fluid circulation drives a hydroelectric turbine with fluid that is non corrosive and excludes surrounding marine environment while allowing the hydroelectric turbine to be mounted above water surface on a wind turbine structure or adjacent structure that allows both to function as a combined power generation installation.

Preferably a tidal power generation system where the impeller and pump units are mounted on submerged structures secured by micro piling, a monopile or gravity base on to the seabed.

An embodiment of present invention will now be described by the following figures: Figure 1.Overview of a typical combined wind and tidal energy system.

Figure 2.lllustration of main power generator mounted on wind turbine monopile structure and showing submerged impeller driven pump system.

Figure 3.Side elevation section through an alternating pitch impeller driven pump system.

Figure 4.Schematic of multiple submerged impellers driven pump hydroelectric power generation system.

Figure 5.Side elevation section of monopile structure incorporating hydroelectric reaction type turbine Figure 6.Side elevation of combined tidal energy and wind turbine with reaction turbine incorporated within monopile structure By reference to figures ito 6 a hydroelectric turbine 3 preferably of the reaction type is co-mounted on a wind turbine 1 monopile structure 2. The use of a reaction turbine 23 is referred to here for description of the invention, though a Pelton wheel could also be employed. A reaction turbine is best employed in a fully closed fluid loop of interconnecting lines 5, 6, 7 where the fluid is contained completely and thereby virtually eliminates the effects of cavitation in the centrifugal pump 14 sections. The centrifugal pumps 14 are driven directly via a gear box 18 coupled to a tidal turbine 12 employing variable pitch impellers 9 to account for the ebb and flow of the tidal system in which the combined wind and tidal energy system is situated.

The preferred material for the interconnecting piping system is flexible armoured plastic coated pipe though other materials in rigid pipe construction can be used. For a given number of submerged impeller driven pump units 4 the first stage takes its suction 15 from the discharge 26, 6 of the hydroelectric turbine 23 section via the main suction flow line 6 and discharges to the suction of the next unit and so on to the final impeller driven pump unit. The discharge 16 of the final impeller driven submerged pump unit feeds into the intake 25 of the power generation turbine 23 via a main discharge flow line 5. The system therefore operates in a closed loop with energy input to the pumps 14 via the interaction of the impellers 9 and the water flow that rotates them to drive the pumps 14 contained within the nacelle 11 via an optional gear box 18. The preferred embodiment of this invention utilises an alternating pitch impeller 9 to maintain unidirectional rotation when the tide is in the required direction and ensure unidirectional fluid flow from the pump 14. Figure 4 illustrates the concept in terms of a schematic illustrating five submerged impeller driven pump units 14 in a closed loop in series. Each flow line 5, 6, 7 has a frictional pressure loss generated by fluid flow through the line 5, 6, 7. This pressure loss is illustrated as /iPf in the following equations with n being the number of submerged impeller driven pump units. Each individual submerged impeller driven pump unit 4 generates a pressure increase LiPuntn. The difference in the summation of pump pressure increases across the series connected impeller and pump units 1 and pressure losses across lines 5, 6, 7 is equal to the available pressure to the hydroelectric power generation turbine 23.

Pressure Across Turbine = Sum of submerged impeller driven pump lift pressures -Sum of head and frictional losses in connecting piping system Sum of head losses in piping system = friction head losses in piping + sum of head losses due to momentum change at bends + elevation head loss from sea bed to turbine intake.

P1-P0= =1APunitn-=11iPfn+1 Power = (P1 P) . flelec.

flelec = Fluid to electrical power conversion efficiency (%) Q = Fluid volumetric flowrate within pump/generator loop (m3/s) P = Power turbine intake pressure (N/m2)= Closed loop output (high pressure side) PD = Power turbine discharge pressure (N/m2)= Closed loop input (low pressure side) = Pressure increase across submerged impeller driven pump unit (N/rn2) LPmn Pressure losses due to pipe internal friction and elevation head (N/rn2) n = Number of submerged impeller driven pump units Note that pump units when installed in series elevate the pressure across the whole system cumulatively. The result is a much higher pressure available for a given flow to the hydroelectric turbine unit 23 than if one submerged impeller driven pump unit 4 had been used. The advantage of a series connected installation for this system in a closed loop is that it also allows the fluid to be lifted to the intake of the hydroelectric power reaction turbine 23 at a high pressure required to allow this type of fluid device to operate efficiently. Additionally the use of the series connected pumps allows a smaller diameter flow line 5, 6, 7 to be utilised. Note that elevation head loss from sea bed to turbine intake is offset by increased suction head available at first pump section, due to the flow loop 5, 6, 7 being closed and hence the height from turbine discharge riser 26 to suction 15 provides elevated suction head and pressure to the first stage pump 14. Hence the turbine can be raised above sea level with minimal efficiency losses in comparison to a system operating in a completely submerged environment.

The use of pumps connected in series and in a closed loop allow for the hydroelectric power generation unit to be located above water surface and operate efficiently as a power generation concept.

It is also possible using the system as illustrated in figure 4 to alternatively connect the submerged impeller driven pump units 4 in parallel, in this case the pump head generated would be low and equal to that from a single centrifugal pump 14 contained within the submerged impeller driven pump unit 4 and the flow would be maximised instead of pressure as is the case in a series connected system. Although a parallel connected system generates a low head, it will allow a larger flow of fluid in the pump 14 and piping network 5, 6, 7 and utilisation of a different type of turbine 23 within hydroelectric turbine unit 3. However the low head would preclude mounting the hydroelectric turbine above water surface and the piping would also be larger and require a more complex manifold arrangement. It is therefore impractical to utilise a tidal energy system where the pump units are connected in parallel if the turbine section was to be mounted above sea surface.

However for the purposes of describing the invention the connected flow circuit referred to will be the series connected configuration as the preferred embodiment of this invention. Other alternative configurations can involve dispensing with the pipe 6 between the discharge of the hydroelectric power turbine 26 and the suction of the first submerged impeller driven pump 14 in the network.

This may offer marginally improved power output in open loop configuration. However the discharge of the hydroelectric power turbine tail race 26 may cause significant scouring on the seabed or monopile structure adjacent to its discharge point. An additional issue with the open loop configuration is that the intake 15 of the first stage impeller driven pump unit 14 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 15 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 with the isolation of 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 would be entirely feasible and minimise any potential environmental impact. Therefore the use of a closed flow loop via piping 5, 6, 7, totally excludes seawater from the surrounding seabed environment.

Referring to figure 1 to 6 a number of submerged modular impeller driven pump units 4 in tide ebb and flow configuration are submerged on a seabed location where the impeller units 12 are driven by the flow of water due to prevailing tidal currents in the area. With reference to figure 3 the submerged impeller driven pump unit Consists of a number of main features: the impeller 12, 9 and its ducting 8, the nacelle 11 housing the centrifugal pump 14 and drive components; disc brake 19 and gear box 18 itself connecting to the pump via a main shaft 17. In figure 6 the base is illustrated by a traditional monopile seabed structure, however other configurations of seabed structure could be used, for example micropiles or gravity bases. The gear box 18 increases the speed of the pump 14 shaft by the impeller 12 speed multiplied by the gear ratio to drive the pump 17 shaft and intake and discharge fluid via the pump 14. The nacelle housing 11 is of streamlined shape to avoid propagation of wake from the impeller 12 and limit the efficiency of downstream units. In figure 3 the blades 9 are assumed to be alternating pitch similar to those installed in devices presently in use and will allow pump 14 direction of rotation to maintain constant in one direction. The pump 14 intake 15 is at the side of the nacelle 11 via an opening. The centrifugal pump 14 discharges via 16 through another opening on the opposite side of the nacelle 11. The pump 14 intake 15 is connected to a suction line 7 either from the discharge of a previous submerged impeller driven pump unit or from the discharge line 6 from the power generation hydroelectric turbine 23. The nacelle 11 can either be sealed or allowed to operate in flooded mode. It is essential that the sealed components are: the gear box 18 and associated bearings, centrifugal pump 14 internal bearings via pump 14 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 entirely suitable. The nacelle 11 and main rotational components contained within, sit on a sub assembly 13; the sub assembly 13 supporting the nacelle 11 is made up of a circular section and angled struts to enclose the pump 14 suction and discharge line 15, 16. The sub assembly 13 can be constructed in a number of ways and do not need description here for the purpose of describing the invention. However the main feature of the sub assembly 13 is to provide a support for the nacelle 11, duct 8, pump 14 and the direction of the suction and discharge 15, 16 to reduce energy losses in the fluid loop 5, 6, 7.

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 utilising either diver support or ROV technology to support all phases of construction and maintenance.

As described the series connection of the submerged impeller driven pump paired units 4 allows a substantial increase in pressure across the network and use of high pressure drop reaction type turbines 23 as illustrated in figures 4 to 6. To allow a network of submerged impeller driven pump units 4 to operate a number of ballasting weights or trench and burial would need to be used on the interconnecting flow lines 5, 6, 7 and the main hydroelectric turbine discharge flow line26 and hydroelectric turbine suction flow lines 25 would have to be supported either inside the monopile 21 or using riser supports 20 outside the monopile. Ballasting weights or conventional burial and possibly rock dumping would be used to absorb hydraulic loads induced by flow transients within the piping 5, 6, 7. 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 14 damage, is avoidance of cavitation and ensure the closed loop system has flow lines 5, 6, 7 and risers 25, 26 completely filled with fluid with no air pockets or voids. To achieve a complete fill of the closed loop system 5, 6, 7 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. The mounting of the reaction turbine at the highest point in the hydraulic system makes this an achievable process to accommodate a high point vent and method that can allow a complete system fill. The fill and vent through isolation valves would be achieved by connecting fill and vent hoses to the valves to and from a surface construction vessel and probably dispense with the need for diver operations during this phase of the commissioning. The complete fill of the system is a key component in successful operation of this invention and an additional key feature that makes the closed loop configuration attractive and feasible for this application.

Momentum transfer across the reaction turbine 23 section requires that the risers either internal 26 or external 25 as illustrated in figure 5 are secured by supports 20, 21 to absorb reaction forces in this part of the system. Additionally the reaction turbine 23 needs restraint within the monopile 2 structure possibly by a recessed part of the structure 22 that is sufficiently robust.

Claims (6)

1. Claims 1. An integrated offshore wind and tidal power system where a tidal energy gathering system utilising a plurality of submerged impeller driven pumps connected within a closed fluid loop drives a hydroelectric generator within the same closed loop mounted above water surface on an offshore wind turbine support structure.
2. 2. As defined in claim 1 a plurality of submerged impeller driven pump units where each pump is connected to a dedicated impeller via a gear box mechanism driving a centrifugal pump to circulate fluid within closed loop network to elevate pressure sufficiently to drive a hydroelectric turbine mounted above the water surface located on the support structure of an offshore wind turbine.
3. 3. As defined in claim 1 a plurality of submerged impeller driven pumps where the high pressure output of the closed loop system drives a hydroelectric power generation turbine mounted above water surface on a monopile structure adjacent to a wind turbine monopile so that both units function as a combined power generation installation.
4. 4. As defined in claim 1 a plurality of submerged impeller driven pump units connected in series via closed loop piping that generates sufficiently elevated pressure to mount a reaction turbine above water surface that is housed on a wind turbine structure or an adjacent structure that allows the wind and tidal power units to act as a combined power generation installation.
5. 5. As defined in claim 1 a tidal power generation system where the impeller and pump units are mounted on submerged structures secured by piling or gravity base on to the seabed.
6. 6. A tidal power generation system as defined in claim 1 that utilises a closed loop of fluid circulation to drive a hydroelectric turbine with fluid that is non corrosive and excludes surrounding marine environment.