WO2009130730A1

WO2009130730A1 - Variable geometry diffuser augmentation device for wind or marine current turbines

Info

Publication number: WO2009130730A1
Application number: PCT/IT2009/000168
Authority: WO
Inventors: Francis Allen Farrelly
Original assignee: Francis Allen Farrelly
Priority date: 2008-04-16
Filing date: 2009-04-16
Publication date: 2009-10-29
Also published as: ITRM20080204A1

Abstract

The object of description is an improved diffuser for augmenting a turbine characterized by a variable geometry diffuser exit section. This invention has application in the field of renewable energy from marine currents or wind power. The innovation disclosed describes a device acting as a difϊuser whose geometry may be controlled so that the flow of fluid through the turbine or turbines may be varied. The invention presented here furnishes an effective solution directly aimed at solving the issue of excessive structural loading at higher than rated fluid speeds. Furthermore, this invention provides an operative means by which the boundary layer in the difϊuser augmentation section may be reattached to the diffiiser wall should this occur, say under the influence of turbulent fluid flows entering the diffiiser.

Description

Variable Geometry Diffuser Augmentation Device for Wind or Marine Current Turbines

DESCRIPTIOIV

Technical field of the invention

This invention relates generally to the field of renewable energy from marine currents or wind power and, more particularly, to an improved variable geometry diffuser for augmenting a turbine. The innovation disclosed by this invention is specifically directed at turbines fitted with diffuser-type augmentation devices and describes a device acting as a diffuser whose geometry may be controlled so that the flow of fluid through the turbine or turbines may be varied- Background Art A diffuser is a passage appropriately shaped so as to enhance the flow of a fluid, be it water or air, through the same passage which also contains within it at least one turbine. Generally, appropriate streamlining of the passage is necessary in order to achieve the required flow enhancement. Diffusers may also be fitted with slots through which fluid may pass from the exterior of the diffuser to its interior. The aim of these slots is to energize the boundary layer of the fluid flow on the inner walls of the diffuser so that the walls may diverge at a greater angle then would otherwise be possible. Turbines fitted with these types of diffosers are know as Diffuser Augmented Turbines (DAT).

DAT systems hold a great potential in the cost-effective production of renewable energy, although an optimal application of these augmentation systems is still an unmet goat. Numerous theoretical and experimental studies have been conducted on fitting turbines with diffusers in the effort to reduce the cost of producing renewable energy from these energy conversions systems. With respect to simple turbines, DAT systems seek both to increase the mass-flow as well as the conversion efficiency so that more power per unit of mass-flow is extracted from the fluid (water or air) passing through the turbine impellers.

Experiments on Diffuser Augmented Wind Turbines (DAWT) have been conducted seriously from the 1950's on. Recent experiments have shown very promising results (e.g. US4482290), though cost-effectiveness is still to be proved. In the field of marine current turbines several systems have been tried and tested (e.g. WOOl 06122 or EP1430220B1) and appear to be very promising.

Drag loading and its dependence with augmentation has been identified as a major drawback in the DAWT concept as loads acting on the diffuser can be considerable. Supporting structures can also be subject to excessive loads when the fluid's velocity increases noticeably above the systems rated values. Object

The invention presented here furnishes an effective solution directly aimed at solving the issue of excessive loading incurred by DAT at higher than rated fluid velocities. Furthermore, this invention provides an operative means by which the boundary layer in the diffuser augmentation section may be reattached to the diffuser wall should this occur, say under the influence of turbulent fluid flows entering the diffuser. Studies have shown that the structural costs represent a substantial portion of the total construction costs in DAT devices. This is because the structure must withstand not just rated conditions in the fluid velocity, but also those present in worst case scenarios. By introducing a means by which it is possible to vary augmentation by controlling the diffuser geometry, it is possible to increase augmentation for fluid flows slower than rated speed and then decrease augmentation for faster flows. The second important action made possible by controlling the geometry is related to the boundary layer control mechanism; introducing slots in the difruser, as described by US4482290, the boundary fayer on the inner walls of the difruser can be energized, improving the augmentation action. However, if a large portion of the diffiiser's inner wall is affected by the separation of this boundary layer, these slots become ineffective. This invention provides a means by which it is possible to recover from this loss of effective augmentation by controlling the geometry of the diffuser walls. Should a large portion of the boundary layer become detached, the walls of the difruser are repositioned so as to have a considerably smaller angle of divergence greatly increasing the chance of a reattachment of the boundary layer. Once this has occurred, the walls of the difϊuser are repositioned increasing the angle of divergence of the difruser so as to achieve the augmentation sought for. Statement of Invention

It is an object of the present invention to implement an apparatus capable of varying its geometry with the aim of providing a controllable amount of augmentation to one or more turbines, irrespective of whether these be of the axial or cross-axial type. In general a diffuser augmented nozzle is composed of a concentrator, a throat and a difruser section. Typically the exit area of the difruser section is 120 to 300 % of the sectional area of the throat. Most often when a single axial turbine is placed in the nozzle's throat the difruser's outlet has a circular cross-section, whereas when one or more cross-axial turbines are placed in the throat, the diffuser's exit cross-section is typically rectangular, thus forming a linear nozzle. Best mode for carrying out the invention In one embodiment of the present invention a variable geometry difruser augmented turbine (see Fig. 1), composed of a recfangular-like linear nozzle with a concentrator section (1), a throat (2) suitable for housing at least one turbine, and a variable geometry difϊuser section (4). The cross-axial turbines are placed in the throat section but for simplicity are not show in the figure. In this figure the fluid flows from the left hand side of the figure with part of the flow entering the inlet (1) flowing through the throat (2) and out of the difϊuser exit (3). In this linear nozzle configuration, die diffiiser's trailing edge forms a rectangular section whose sides are the difruser's 4 rims. In order to vary the geometry of the difruser section the two longest rims are positionable. As might the flaps of an airplane wing, these rims are capable of rotating in an arc about a pivotal point offset outside the diffuser, thus being angled outwardly and at a distance from the fixed portion of the difruser. The gap thus formed has a function similar to the slots used for boundary layer control in other DAT systems. In this deployed configuration, the fluid flowing outside the duct reaches the rim and is split into two components: one diverted outside around the rim; the second flowing through the gap between the difϊuser wall and the rim. This latter component of the fluid flow has higher specific kinetic energy than the fluid flow forming the turbine wake in the difruser and is thus capable of re-energizing it. Furthermore, it tends to form a steadier boundary layer around the positionable rim. One possible mechanism which can be used to position these rims, derived from systems in use in airplane wings, is shown in Fig.2. The difruser wall (5) is fitted with a spar (7) which has a pivotal point offset on the outside of the difϊuser. About this pivotal point the positionable rim's (8) spar can be made to rotate. This simple mechanism, which enables the rim to be positioned at an angle and displaced from the fixed part of the difruser (5) is, in many ways, similar to a full-span slotted flap on an aircraft wing. In practice the designer may choose whether to make all or some of the rims positionable or even just a single one. In the case in which adjacent rims are positionable, a space between adjacent rims may form when they are deployed. This space may cause a noticeable disruption to the fluid flow; the designer may choose to fit a semi-rigid slip-plate overlapping adjacent rims thus ensuring that the space between these adjacent rims remains impermeable to the fluid flow. Should it be necessary the positionable rims may be brought to a rest position, show as a dotted profile labeled 6 in Fig.2, which is more generally aligned to the fixed differ walls (5) which decreases the augmentation of the fluid flow through the turbine. In this position the forces on the structure are reduced and any detachments of the boundary layer on the inner wall of the difruser will tend to re-adhere. From this rest position the rim can be redeployed controlling an actuator which causes the rim to pivot to one of its deployed positions. A further embodiment, suitable for elliptic-like shaped diffusers, extends this technique of subdividing the rim in adjacent movable segments fitted with overlapping semi-rigid slip-plates so as to achieve a controlled variable exit area difruser with an expandable fluid-proof surface.

In a further embodiment suitable for diffusers with circular or elliptical cross section, the positionable rim is segmented in multiple sections, which when deployed form an annular ring flap. Again, if need be, the space which forms between adjacent rim sections as they separate can be rendered impenetrable to the fluid flow by fitting adjacent rims with overlapping semi-rigid slip-plates.

In all embodiments the effectiveness of the augmentation difruser can be improved by increasing the effective inlet to exit area ratio in a multiple slotted configuration, so as to reduce boundary layer break-down problems. This is best achieved by cascading sets of movable rims. Fig. 3 shows an example of this multi-slotted rim configuration in which the an inner section (10) and an outer section (11) of the rim can be positioned in deployed positions (12 and 13 respectively) bringing these sections awav from the fixed difruser section (9\ and into the exterior fluid flow.

In practice details in the execution may vary while still in keeping with the invention and thus in the mtent scope.

Claims

CLAIMSWhat is claimed is:

1. A difϊuser augmented turbine, wherein the improvement comprises in the diffiiser having a controllable exit geometry; the said difϊuser to be placed in the general direction of the fluid flow and having 2 extremities; the first being the leading extremity to be positioned generally upstream and the second being the trailing extremity to be placed generally downstream; the said difϊuser' s trailing extremity incorporating at least one positionable rim; the said positionable rim having a rest position so as to act generally as with the trailing section of the diffiiser; the said positionable rim capable of being deployed in at least one position so as to form a gap between the rim itself and the fixed section of the difϊuser; furthermore the said positionable rim, in this aforementioned deployed position, may be angled outward, so as to alter the geometry of the said difϊuser generally increasing its exit area, with the aim of controlling the fluid flow through the turbines contained within the said difϊuser.

2. A difϊuser augmented turbine, according to claim 1, wherein the said positionable rim is fitted with at least one semi-rigid slip-plate overlapping an adjacent rim, whether this be positionable or not.

3. A difϊuser augmented turbine, according to claim 1, wherein the fluid flowing through the said difϊuser is water

4. A difϊuser augmented turbine, according to claim 1, wherein the fluid flowing through the said diffiiser is air.

5. A diffuser augmented turbine, according to claim 1 , wherein the cross-section of the said diffiiser is of rectangular shape and the said positionable rim takes the form of a controllable flap.