GB2490729A

GB2490729A - Hydro kinetic water turbine duct

Info

Publication number: GB2490729A
Application number: GB1108027.2A
Authority: GB
Inventors: Alan Saunders
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-05-13
Filing date: 2011-05-13
Publication date: 2012-11-14
Also published as: GB201117705D0; GB2487448A; GB201108027D0; GB2487448B

Abstract

A hydro-kinetic water turbine duct especially for the generation of electricity in zero head, run of the river applications, comprising an inlet duct, a turbine housing 1 and an outlet duct. The trailing edge of the outlet duct and the leading edge of the inlet duct are radiused Re to increase the flow rate through the turbine housing by causing parallel flow streamlines. Preferably there is a fairing 2 around the outside of the duct with a constant external cross sectional area. Power limiting devices with no moving parts may located in the inlet duct so that the average speed of water through the duct is constant once the open water speed is above a minimum level. The power limiting device may be in the form of a radius intersection between the inlet and turbine sections Rt, a disrupter in the form of a slot, a gap or a step in the ducting or a projection either partly or entirely across the duct.

Description

Hydro-kinetic Water Turbine Duct

Field of the invention

This invention relates to a hydro-kinetic water turbine duct adapted to house a turbine intended for the generation of electricity.

Background

The use of turbines to extract energy from flowing water is well known and various mechanisms have been employed for many centuries. In the current climate of attempting to implement renewable energy technologies of a high efficiency and that are cost effective it is seen that hydro-turbines are behind the complementary technology of wind turbines in terms of the number of installations. The reasons for this are many but include the fact that the environment in which they are installed is more harsh than the "dry" environment of a wind turbine which is in itself a rugged environment. The cost of installation per unit of electricity generated has been higher. The proposals here will increase the efficiency of the hydro-kinetic generator which means that the machine is smaller and more cost effective than formerly.

Any turbine used where the working fluid flow rate varies, requires a control system in order to maintain the correct rotational speed for the generator taking into account its capacity and the available energy in the fluid. Control systems in use are either mechanical or electrical/electronic. Any mechanical system is difficult to design to be reliable without it needing a high degree of maintenance. Moving parts immersed in seawater are notorious for failing and requiring frequent maintenance. Designs with blades that furl or move to create a stalled condition have been proposed by others. Electrical and electronic control systems tend to be complex adding both cost and a higher likelihood of failure to the system.

Others have been working in the field but there has been no evidence that the concepts utilised in the presently proposed duct have been put to use. In Patent GB259558A of 1926 Darrieus covers a cross flow turbine in a flume (duct) to accelerate the fluid flow rate. However, there is no mention of control and this has been a problem for turbines working in this way ever since. The presently proposed duct here gets around those problems in a way that has not been seen by

others working in the field.

Another Patent, U52006008351, has a two bladed Darrieus turbine in a duct. Here the generator is connected to the turbine via a complex arrangement using a ninety degree gearbox to allow all machinery to be above the surface of the water. The speed control is by using the generator as a brake, during which period no electricity is generated. In the presently proposed duct here electricity is generated whenever the turbine is rotating.

Patent EP1430220 of Clean Current Power Systems describes a cylindrical duct used to accelerate the water flow around an axial flow turbine. Drawings show both a faired and un-faired design, but the significance of this is not discussed in the patent.

According to a first aspect of the invention, there is provided a hydro-kinetic water turbine duct adapted to house a turbine intended for the generation of electricity when the duct is in flowing water, the duct comprising an outlet duct leading from a turbine housing, the outlet duct comprising a mouth defined by a trailing edge spaced from the turbine housing, the trailing edge being radiused such that, in use in flowing water, the radiused trailing edge is operative to change the direction of the flow streamlines in the water flow to be parallel to increase the flow rate of water through the turbine housing and thus the power generated when the duct is in use with a turbine.

According to a second aspect of the invention there is provided, a plurality of ducts of the first aspect of the invention fixed laterally together to form a fence.

Other aspects of the present invention may include any combination of the features or limitations referred to herein.

The present invention may be carried into practice in various ways, but embodiments will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a sectional side view of a duct in accordance with the present invention; Figure 2 is a sectional side view of another duct in accordance with the present invention; Figure 3 is an end view of a further duct in accordance with the present invention; Figure 4 is a sectional side view of a further duct in accordance with the present invention; Figure 5 are end views of a duct in accordance with the present invention on various different mounting structures; and Figure 6 is a graph showing a relationship between the radius of part of a duct and the performance of a turbine mounted in the duct.

Description

With reference to the figures, a kinetic water turbine duct is provided adapted to house a turbine intended for the generation of electricity.

The duct is appropriate for both rivers where the water is running in one direction only and for tidal, i.e. reversing flows. Generally the former will generate about three times the amount of electricity compared to the latter. Thus there is a greater need for a tidal turbine to be efficient in order to make it cost effective.

Any tidal turbine has to cope with a varying velocity of water from zero at the turn of the tide to a maximum midway between the extremes of the tidal levels. In addition tidal heights vary from a minimum at neap tides to a maximum at spring tides. The turbine and generator could be sized to cope with the maximum velocity and hence maximum power. However, this is only required for about an hour once every two weeks, whilst for the rest of the time the velocity and power is substantially less. As is well known, for a fluid turbine, the power is related to the fluid velocity by the cubic power. Thus for a typical tidal flow where the spring tidal flow rate may be twice that of the neap tidal flow the power could be eight times larger. In order to capture the energy at a reasonable cost the turbine and generator have to be sized to be smaller than the peak flow and power, but large enough to give enough power to pay back the investment in the device.

In addition if the installation is in a river which may flood, or run faster for other reasons, at certain times of year the duct described here will protect it from damage and ensure the best compromise between cost and the amount of electricity to be generated.

The duct keeps the average velocity in the turbine housing virtually constant above a critical fluid velocity so that there is no need for a separate speed control of the turbine or generator. In addition it allows a simpler and hence lower cost electrical output control system, often based on inverter technology, to manipulate the output electrical voltage and wave form. The flow control covered by this duct is achieved by one or any combination of the following methods.

The first method is by designing the duct so that it causes the flow to alter its characteristics above a certain inlet fluid flow rate. The key part of the fluid flow is where it enters the narrowest part which is where the turbine is situated, Figure 1, item 1. The ideal flow would be in streamlines parallel to each other without eddies or reverse flows or other disruptions of any kind. This is achieved by first increasing the velocity at which the flow changes characteristic by incorporating a radius, Figure 1 R, where a classically designed venturi would have an angle. This is at the joint between the converging part and the parallel turbine housing.

In a classical venturi a "vena contracta" is incorporated in the flow so that the performance of the venturi is predictable and this also allows it to be used as a measuring instrument. However, to ensure this, the included angle of the contraction of the duct is limited, typically to about 22 degrees or so, and the discharging duct has to have a very shallow angle indeed, typically 7 degrees. As a result a classical venturi is very long.

Incorporating the radius increases the velocity at which the flow comes away from the duct walls and becomes disrupted. The included angle of the duct walls can then be increased so that at the chosen fluid flow rate the fluid does become disrupted with the result that the average flow rate through the turbine remains the same.

It has been found that for the rates of flow likely to be encountered in average tidal rivers that this effect takes place where the ratio of the Hydraulic Diameter of the inlet area, Figure 1 item 2, to the Hydraulic Diameter of the turbine area, Figure 1 item 1, is greater than 1.6 and the ratio of the orthogonal distance, Figure 1 item 3, between these areas to the Hydraulic Diameter of the turbine area is less than 1.8.

This can give an acceleration of the stream fluid flow to the turbine housing fluid flow of about two.

The second method takes advantage of an inherent feature in the construction of the duct proposed here. The turbine, generator and associated housing are mounted in the central, narrow, portion of the ductwork so that it may be removed easily for maintenance. By the nature of the duct design in conjunction with a removable turbine there will be disrupter in the form of a slot, gap or step, Figure 1 item 4, around the joint between the turbine housing and duct. This will have little effect when the fluid flow rate is low, but be more disruptive as the flow rate increases. Calculation and test work allows this effect to be optimised.

The third method is to use disrupters comprising hydrodynamic "spoilers", figure 3 item 1, that are designed to have little effect when the flow rate is low but to act in a similar way to the gap described earlier and to become more disruptive as the flow rate increases. These spoilers may be finger-like protuberances into the fluid flow from the duct wall. The number and style of them can, once again, be determined by calculation and test work. These protuberances may cross the duct entirely, rather like a large mesh. They may have an aerofoil section, or deliberately have sharp edges in order to disrupt the flow. The exact number and design of the protuberances depends on the fluid flow velocities that are being targeted in the particular design and should be selected after using classical fluid dynamic analysis or a technique such as Computational Fluid Dynamics as well as practical testing to confirm the outcome.

Figures 1 and 2 show a symmetrical ductwork design which is intended for use when the machine is fixed in orientation and it situated in a reversing tidal flow.

This compromises the maximum amount of energy that can be extracted from the fluid stream. If the machine is to be installed where the flow is only in one direction then it may not need to be symmetrical and it is likely that greater efficiencies can be achieved if the discharge duct is of a design in order to optimise the pressure drop across the machine and enhance the fluid flow, Figure 4. The discharge duct can be of a geometry that maximises the pressure drop through the machine thus increasing the fluid flow rate. This is achieved by changing the angle and form of the duct side walls.

The radii at the leading and trailing edges, see Figure 2 feature Re, affect the performance of the duct and it has been seen both by the use of CFD calculations and experimentation that there is an optimum radius for a given duct. Figure 6 shows the effect. The use of the correct radius improves the overall efficiency of the unit, increasing the acceleration of the water.

In addition it has been found, again by the use of CFD calculations and by observation during experiments, that for a square or rectangular duct the use of a fairing around the outside of the duct improves the fluid flow and helps prevent backflows and eddying. Backflows and eddying reduce the acceleration of the water that is achieved by the duct. See figure 2, feature 2. The fairing allows the angle of the converging and diverging portions to be maximised which allows the maximum acceleration to be achieved for any duct configuration.

The duct can be installed using one of many different techniques which will depend on the details of the particular site. Figure 5 item 1 shows brackets fixed to a steel or concrete river wall, bridge stanchion or similar upon which the machine is fixed.

The height of the brackets would be such that the machine is underwater at all states of all tides when a significant flow is running to ensure that the maximum amount of energy is captured.

Also shown are methods of constraining the machine, Figure 5 items 2 and 3, to allow vertical movement up and down with the tide without any lateral movement which could both disrupt the flow of water into the machine and cause damage to it and/or its surroundings if it was impacting on, as an example, a pontoon. Figure 5 item 2 shows vertical rods, cables, shafts or similar which are fixed at both ends or at both sides, or both. These would run through appropriate rings or similar mounted on the machine. Figure 5 item 3 shows constraints external to the machine, perhaps right-angled structures at each corner which will allow the machine to rise and fall but prevent lateral movement.

If the duct is constrained to move vertically with the tide then it would be fitted with a float or floats which would ensure that it does indeed rise and fall with the tide but keeps the inlet of the duct under water at all times. Typically the top of the inlet of the duct would be 0.5 m below the surface of the water. In a similar way to the use of floats it could be mounted beneath a pontoon or similar structure which itself floats up and down with the tide.

The duct could be mounted on legs or a stand, Figure 5 item 4, which themselves are fixed to the river or sea bed. Other installation methods include the use of mooring lines to a pontoon, wharf or dock wall or to pier or bridge legs.

In addition the duct could be fixed to a mooring weight, or equivalent, Figure 5 item 5, and allowed to turn with the tide, or between mooring weights or buoys or equivalent to prevent turning with the tide.

The nature of the design also allows the ducts to be fixed together laterally in a "fence" and then installed across waterways, or partially across waterways, for example under bridges or from a river wall out to a pontoon. A duct of this type could also be fitted to, or under, a raft or other boat or floating apparatus which could then be moored using conventional methods.

In order to simplify the design, reduce costs and improve reliability the turbine bearings, when situated under water, are to be lubricated by that water which flows through or around the machine. Materials are available that will give a good life time if the components are designed and manufactured to the state-of-the-art by those skilled in the practice.

For the same set of reasons the turbine and generator can be fixed on the same shaft. Other designs offer complex mechanisms that are required to be kept free from water and the designs have to go to great lengths to do so thus adding complexity and cost to the machine.

The duct and all other features apply equally to an axial flow or cross flow turbine.

A cross flow turbine is preferably used as this offers a greater area to the fluid flow and it square or rectangular elevation allows the use of square or rectangular ducting which is simpler to fabricate at a reasonable cost when compared to a circular duct.

A problem for most applications of this type will be fouling by marine organisms. If internal to the duct it will reduce the fluid flow velocity thereby reducing the power extracted from the fluid. If the fouling is on the turbine itself it will reduce the efficiency by a relatively large amount. Conventional methods would include painting the duct with a poisonous anti-foul paint such as that used on most marine craft. This would require annual maintenance and does put some poison into the local marine environment, which is undesirable. Another option is to use a copper based paint which appears to have a longer life, probably of the order of 10 years.

This too, works by poisoning the organisms, but is kinder to the environment.

Other options are becoming available, but to date have not been used in devices such as this but only on marine craft for which they have been invented and are being commercialised. These include the use of ultrasonic frequencies, seawater electrolysis and the use of a naturally occurring microscopic fungus called streptomyces avermitilis' that exists in some large fish and prevents small organisms such as barnacles from adhering to the surface. These three latter options are preferred for use in this duct. The first two of these require a small electrical current in order to operate. This can be derived from the generator output.

The duct could be damaged if large objects such as tree branches, weed, bundles of fishing line and other river debris entered the duct and went into the turbine. Thus it is proposed to use a mesh basket that projects from the inlet area having sides as well as a frontal area so that it is unlikely to get blocked up, and in any case when the tidal flow reverses the revised flow direction will clear the mesh basket of any debris. The mesh will also have the effect of preventing any swimmers or other river users from getting injured due to the rotating parts of the turbine.

It is essential that fish are protected and a combination of an inlet mesh and a gap between the turbine and the housing slightly larger than the mesh size to allow free passage of those fish that are smaller than the mesh size will ensure the minimum amount of damage. Others have reported that fish are not drawn towards a rotating Darrieus mechanism but turn away from it.

It is important that such a duct is of as low a cost as is practical given the required robustness and reliability that is required. Designing the duct to be low cost will ensure that the user or owner gets a good "pay back" period making it attractive to purchase. Methods of manufacture which may be economical, depending on the volumes required and physical size include rotational moulding of a polymer and the use of fibre reinforced plastic or fibre reinforced concrete. These processes and materials allow a float to be built in, rather than having a separate item which is fastened to the duct.

Such ducts may be subject to the ingress of small amounts of weed, fishing line and light rope despite the mesh baskets fitted. Thus it may incorporate rope cutters between the end plates of the turbine and the turbine housing to the turbine from rotating. These are typically stainless steel protuberances or fixed stainless steel blocks or folded steel sheet fixed to both the end plates of the turbine and the turbine housing such that as the turbine rotates these cutters pass each other at an angle neither orthogonal or parallel and the clearance between them is of the order of 0.5 mm or so. Any weed or similar caught in this area will be chopped and then pass harmlessly through the machine.

The preferred embodiment for use in a reversing tidal flow area such as a river has a symmetrical converging-diverging duct of a square or rectangular section which has a radius as it enters the parallel walled turbine housing which contains a Darrieus type rotor. The slot and step at the interface between the inlet duct and the turbine housing, along with the angle of the converging duct in combination with the appropriate radius are sufficient to ensure that the average fluid flow becomes constant above a particular free stream flow rate. The ratio of areas and length of the machine are as described previously.

The edges of the inlet and discharge ducts have a radius and the outside surface has a fairing.

This duct may be mounted in one of several ways, which may include the use of floats, according to the site conditions. The turbine and generator, i.e. all of the moving parts requiring maintenance, are fixed together as a sub-assembly and arranged to be easy to remove from the duct. Mesh baskets protect the inlets from large debris entering the turbine and causing damage or preventing it from rotating. Smaller debris is catered for by 1rope cutters" that will slice the debris up into pieces that will safely pass through the machine. There is no danger to fish or similar from the rope cutter device. An ultrasonic device, or other method, is used to keep the machine clear of marine growth without harming the local marine environment. The design allows for a simple method of assembly from easily produced parts where the design allows for multiple identical parts to be produced thus being cost effective to manufacture.

The duct may therefore comprise part of a ducted turbine system such that the velocity of the water flowing through the duct is proportional to the free stream velocity up to a velocity at which fixed features of the duct, with no moving parts, may cause the water velocity to become near constant. Thus, as the rotational speed of the turbine is proportional to the power, the power generated can be constant allowing the generator to run at or near to its capacity. Thus the generator is neither overloaded, which causes damage to it or requires complex controls, nor is it over-sized to cope with every rotational speed due to the particular fluid flow that it is in. In the latter case the generator would cost many times what the generator proposed here would cost.

This arrangement is particularly suited to a turbine for use in a zero-head, or run-of-river, application. The duct which could be parallel sided if control without acceleration is required but is preferably of a converging-diverging nature with the turbine fitted at the narrowest part where the fluid flow is fastest. Such a duct allows more power to be extracted from a given fluid flow when compared to a turbine of the same projected area of the duct opening. This is because the power extracted from the fluid is related to the third power of the fluid velocity as is well documented.

Thus another benefit the above described duct brings is to greatly increase the fluid flow rate when the free stream velocity is low, and the turbine will then start at much lower velocities than other designs, particularly those of a turbine without a duct. This will increase the overall amount of electricity generated at any particular site or installation.

Any duct system inserted into a fluid flow alters the flow of that fluid. The above described duct may maximise the flow rate through the duct. In a free stream the duct is a resistance to flow and much of the fluid will prefer to flow around it, depending on the nature of any adjacent structures. Certain features of the duct can improve or degrade the likelihood of fluid flowing through the duct as described above.

Claims

CLAIMS1. A hydro-kinetic water turbine duct adapted to house a turbine intended for the generation of electricity when the duct is in flowing water, the duct comprising an outlet duct leading from a turbine housing, the outlet duct comprising an outlet aperture defined by a trailing edge spaced from the turbine housing, the trailing edge being radiused such that, in use in flowing water, the radiused trailing edge is operative to change the direction of the flow streamlines in the water flow to be parallel to increase the flow rate of water through the turbine housing and thus the power generated when the duct is in use with a turbine.
2. The duct of claim 1 comprising an inlet duct having a mouth defined by a leading edge, the leading edge being radiused such that, in use in flowing water, the radiused leading edge is operative to accelerate the water flow to increase the flow rate of water through the turbine housing.
3. The duct of claim 1 or 2 wherein the exterior of the duct comprises a fairing around the outside of the duct, the fairing being of substantially constant external cross sectional area.
4. The duct of any one of the preceding claims comprising power limiting means having no moving parts and being located at the inlet duct, and operative, in use in flowing water, to cause the average water speed to remain substantially constant once the water speed is above a minimum level.
5. The duct of claim 4 wherein, the power limiting means comprises at least one of: a. the intersection of walls of the inlet duct with walls of the turbine housing walls, the intersection being radiused; b. a disrupter in the ducting at or adjacent the turbine housing; and; c. a projection attached to the duct so as to project into the interior of the duct
6. The duct of claim S wherein the disrupter comprises at least one of a slot, a gap or a step in the ducting.
7. The duct of claim S or 6 wherein the disrupter is located at the intersection of the inlet duct walls to the turbine housing walls.
8. The duct of any one of claims 5 to 7 wherein the disrupter is located at a position at the inlet or outlet to the turbine housing.
9. The duct of any one of the preceding claims 5 to 8 wherein the projection projects entirely across the duct.
10.The duct of any one of claims 5 to 8 wherein the projection projects only partially across the duct.
11.The duct of any one of the preceding claims wherein the ratio of the Hydraulic Diameter of the mouth of the inlet duct to the Hydraulic Diameter of the turbine housing is greater than 1.6, and the ratio of the orthogonal distance therebetween to the Hydraulic Diameter of the turbine housing is less than 1.8.
12.The duct of any one of the preceding claims wherein the inlet duct is of reducing transverse cross sectional area so as to taper radially inwardly towards the turbine housing, the turbine housing being of substantially constant cross sectional area..
13.The duct of any one of the preceding claims wherein the outlet duct is of increasing transverse cross sectional area, the inlet portion tapering radially outwardly from the turbine portion,
14.The duct of any one of the preceding claims wherein the inlet and outlet ducts are symmetrical to allow the duct to be fixed in orientation whilst in a reversing tidal flow.
15.The duct of any one of claims 1 to 13 wherein the inlet and outlet ducts are not symmetrical, but have been optimised to maximise the fluid flow rate through the turbine housing when the fluid flow is unidirectional.
16.The duct of any one of the preceding claims further comprising a mounting bracket operative to mount the duct to a river or dock wall or pier legs or equivalent such that at least the turbine housing is underwater at all states of the tide.
17. The duct of claim 16 wherein the duct and bracket are arranged to constrain the duct to move in a vertical direction only, the duct being provided with at least one float to ensure that the duct operates, with the turbine housing below the water surface by a fixed amount.
18. The duct of any one of claims 15 to 17 wherein the bracket comprises at least one leg operative to support the duct on the river or sea bed.
19. The duct of any one of claims 15 to 18 being further provided with mooring lines adapted to connect the duct to a river or dock wall or pier legs or the like.
20.The duct of any one of the preceding claims provided with a mooring weight.
21.The duct of any one of the preceding claims further comprising a combined turbine and generator module that is removably mounted on the turbine housing, the module being secured thereto by a fixing, a part of the fixing that is arranged to be manipulated in order to release the module being arranged to be above water level.
22.The duct of claim 21 wherein the turbine and generator module comprises water lubricated bearings.
23.The duct of claim 21 or claim 22 having the turbine and generator mounted on the same shaft.
24.The duct of any one of claims 21 to 23 wherein the turbine is at least one of axial flow or cross flow.
25.The duct of any one of the preceding claims wherein an ultrasonic device is provided to prevent fouling by marine growth, the device taking its operating electrical current as a by-product of the generator.
26.The duct of any one of the preceding claims wherein a seawater electrolysis device is provided to prevent fouling by marine growth, the electrolysis device taking its operating electrical current as a by-product of the generator.
27.The duct of any one of the preceding claims comprising a paint finish comprising a microscopic fungus called streptomyces avermitilis' within a paint finish to prevent fouling by marine growth.
28.The duct of any one of the preceding claims further comprising at least one float arranged such that, in use, the water inlet duct is always beneath the water level.
29.The duct of any one of the preceding claims manufactured from at least one of a rotationally moulded polymer, fibre reinforced concrete (GRC or FRC), and fibre reinforced polymer (GRP or FRP), integrally incorporating a float without the float being a separate part.
30.The duct of any one of the preceding claims wherein the inlet duct is provided with a mesh inlet basket that projects from the inlet duct into the flowing fluid to restrict marine debris, fish and people from entering the duct.
31.The duct of any one of claims 21 to 30further comprising rope cutters between end plates of the turbine and the turbine housing to cut debris such as weed, fishing line and light rope that may have entered the duct and which would otherwise prevent the turbine from rotating.
32.A plurality of ducts of any one of claims 1 to 31 fixed laterally together to form a fence.
33.A duct substantially as described herein and as shown in Figures 1 to 4.