Diffusor For The Air Turbine Of A Wave Power Plant The invention concerns a diffusor for the air turbine of a wave power plant, in particular for a turbine, through which flows a liquid axially, with the air flowing 5 from both sides and rotating in a single direction, as an axially arranged Wells turbine. Wave chambers fitted with an access opening situated below the water surface can be used for using the motive energy of waves. At high tide, the water surface rises in the 10 wave chamber and compresses an air volume situated thereon, at low tide an eddy current is generated in the wave chamber. If a pneumatic connection is created by a flow channel between the air volume enclosed in the wave chamber and the surrounding atmosphere, an oscillating flow results in the flow channel, which is transformed by means of a bidirectional air turbine facing the incoming flow into a 15 rotational movement for driving an electrical generator. Wave power plants which operate according to the principle explained above are also designated as OWC power plants (for oscillating wave column). Such a plant is described by way of example by document US 5,191,225. 20 Air turbines with the air flowing from both sides, which keep the same rotation direction when changing the inflow direction, are used preferably for operating OWC power plants. This property is satisfied by a Wells turbine, as described by way of example in document US 4,313,711. Consequently, a Wells turbine typically presents a plurality of turbine blades, which are provided with drop-shaped, 25 symmetrical profile and whose profile chords lie on the rotation plane of the turbine. The turbine blades designed as such are carried by a hub-shaped element. Additionally, a pulse turbine can be used as an alternative embodiment of a bidirectional air turbine facing the incoming flow, as disclosed for instance in document WO 06137696 Al. 30 A way of increasing the degree of efficiency of a generic wave power plant includes using a diffusor downstream of the air turbine. Said diffusor is typically provided in the mouth region of the flow channel to the surrounding atmosphere. Indeed, the 01/0211 I,dh-18983 - speci - cdm.doc, I -2 diffusors used previously, fitted with a widening external pipe and a cylindrical core tube in the flow shadows of the fixed parts of the air turbine, in particular of the generator, have not proven sufficiently efficient. 5 The object of the invention is then to install a diffusor subsequent to a air turbine at the mouth of a flow channel outgoing from a wave chamber of an OWC-power plant, which is characterised by a high increase in the degree of efficiency of the air turbine and by as short as possible an axial construction length. Moreover, the diffusor should be straightforward from construction and manufacturing technical viewpoints. 10 According to the present invention there is provided a wave power plant, including a wave chamber a flow channel which creates a pneumatic connection between the wave chamber and the surrounding atmosphere, an air turbine with the air flowing from both sides and through which flows a liquid axially, with the same rotation 15 direction, which is arranged in the flow channel, a diffusor, which forms the termination of the flow channel to the surrounding atmosphere, wherein the diffusor follows an axial direction, and the diffusor presents a core tube, whose maximum radial extension continuously decreases perpendicular to the axial direction in a first axial section and continuously increases in a second axial section, wherein the 20 second axial section follows the first axial section in the exhaust direction and reaches up to the surrounding atmosphere. The invention recognises that a diffusor must be adapted to the strong turbulent component in the after-running region relative to the air turbine. The result is an 25 unusually large opening angle for the outer shell of the diffusor, to guarantee that the product substantially remains constant thanks to the peripheral speed of the flow and radius. In a next step a core tube should be provided which again presents an opening angle which is sufficiently high, that is to say a major radial widening as seen in the exhaust direction. Consequently it has been considered, that for pipe 30 flows subjected to turbulence a dead zone is set aside in the centre for the flow, which dissipates energy. This highly turbulent area should therefore be blocked with said widening core tube. 0110211 Idh-18983 - spcci - cdm.doc.2 -3 The result is an axial-axial-diffusor with large opening angle respectively an axial radial-diffusor, whereas both variations are provided with a highly widening core tube. Along those lines, the invention goes one step further inasmuch as the inventors have acknowledged that a rigid geometry of the core tube and of the outer 5 shell of the diffusor is not equally suitable for all operating ranges for recycling a large stationary differential pressure. It is thus of paramount importance that the diffusor's flow-through of a generic plant changes permanently. In addition to an oscillating movement in the flow channel, a large fluctuation portion appears which results from a permanently varying wave characteristic, which causes stochastic 10 pressure fluctuations in the wave chamber. It is moreover of paramount importance that the variation of the rotation speed of the air turbines used typically presents a longer time constant with respect to the permanent change in the inflow characteristic. The result is that the ratio between the flow velocities caused by the rotational movement of the air turbine along the periphery and the axial flow 15 components changes permanently. To solve this problem, the diffusor according to the invention presents a core tube, whose contour is suitable to fix locally a quasi stationary vortex which influences the effective contour of the core tube. A core tube is used to that end which is subdivided 20 in a first axial section and a second axial section. Consequently, the first axial section begins on the inlet region of the diffusor facing the air turbine and ends on a radial bottleneck of the core tube. The add-on second axial section extends from the radial bottleneck to the diffusor's outlet side, i.e. to the surrounding atmosphere. Accordingly, the maximum radial extension of the core tube decreases continuously 25 in the first axial section in the exhaust direction to the radial bottleneck and increases continuously in the second axial section. As a result, a quasi stationary flow cylinder forms when running through between both axial sections at the radial bottleneck. As the contour of the core tube is receding radially and inwardly, said typically toroidal shaped whirl is fixed locally and hence taps from the flow only a minimal amount of 30 energy which is significantly smaller compared with a permanent new formation of vortex, which occurs with vortices carried away with the flow. 01/02/11 ,dh-18983 - spcci - cdm.doc,3 -4 The outer shell of the diffusor is designed in such a way , that a free flow cross section of the diffusor is present in the exhaust direction which monotonously increases. Accordingly, in particular in the area of the first axial section up to the radial bottleneck of the core tube, the outer shell of the diffusor may have a section 5 with an inner diameter tapering in the exhaust direction, to avoid any flow separation on the inside wall of the outer shell. Due to the localised, quasi stationary formation of vortex, the actual contour of the core tube is determined by an encasing contour, which encloses the quasi stationary 10 vortex. This again offers the advantage that the otherwise highly turbulent dead zone of the flow is efficiently blocked centrally in the diffusor and that at the same time permanent adaptation of this effective contour is permitted, inasmuch as according to the actual diffusor's flow-through the quasi stationary vortex is formed with various vorticity. 15 In a first extreme case for a first operating mode during the exhaust phase there is no or only a vanishing small stationary vortex and the first axial section presents a large effective increase in flow cross-section. In the second conversely extreme operating condition and also during the exhaust phase, the quasi stationary vortex is strongly 20 marked and for blocking the central cross-section to the main flow, the effective contour leads to a large effective opening angle, which is efficient, in particular for a high volume stream with large turbulence. Additionally, the diffusor according to the invention has proven advantageous for the aspiration cycle. Due to the opposite through-flow, the diffusor here acts as a confusor, wherein the bottleneck of the core 25 tube shifts the flow downstream thereof and hence in the subsequent air turbine markedly radially and outwardly while increasing its degree of efficiency. Moreover, any vortex is again avoided on the core tube through the fixing effect at the radial embayment during freighting by using the flow, so as to prevent any permanent new formation of vortex. 30 Besides, the diffusor according to the invention involves a short construction length in axial direction and due to the efficient flow deceleration during the exhaust phase and to the guiding of the flow radially and outwardly at the outlet of the diffusor to 01/02/11 ,dh-18983 - speci - cdm.doc,4 -5 the surrounding atmosphere, it also enables to provide additional components. A shutoff device and/or a guiding apparatus is particularly preferably arranged in this area so that these components again need not be accommodated in the flow channel area, in which the air turbine is situated, and that the total construction length of the 5 plant is further reduced in axial direction. Consequently, any additional shutoff valve in the flow channel for more reliable shutdown of the plant can be dispensed with. The invention is described more in detail below using exemplary embodiments and in connection with figure illustrations, which represent the following details: 10 Figure 1 shows an axial-axial-diffuser according to the invention connected to a bidirectional air turbine in longitudinal section. Figure 2 shows a further embodiment of a diffusor according to the invention in longitudinal section, which is arranged as an axial-radial-diffuser. 15 Figure 3 shows a generic water power plant with a diffusor according to the state of the art. Figure 3 represents schematically and in a simplified fashion the basic components of a generic OWC water power plant. The wave chamber 1 is clearly visible in which 20 the fluctuating water surface 24 causes a pressure variation in the airspace 23 situated thereon. A flow channel 3 extends therefrom, which transmits kinetic energy by exhaust and aspiration to an air turbine 4 with air flowing from both sides and the same rotational direction, predominantly a Wells turbine. The known diffusor 5 illustrated on Figure 3 on the mouth region of the flow channel 2 presents a core tube 25 8 with a constant diameter. Such a geometry has shown that the differential pressure coefficient Cp-DIFF as ratio between the recovered static pressure and the dynamic pressure presents a too small value on the inlet side of the diffusor, which can be ascribed in particular to the high turbulent component in the wake flow relative to the air turbine 4. Additionally, major flow separations occur in the area of the diffusor 30 which is close to the wall. Figure 1 shows a first exemplary embodiment of a diffusor according to the invention in the form of an axial-axial-diffuser in longitudinal section, which forms 01/02/11 ,dh-18983 - speci - cdm.doc,5 -6 the mouth region of the flow channel 2. In the operating area of the exhaust cycle, the diffusor 5 is downstream of the air turbine 4. For these parts, the rotating portion is represented schematically and in a simplified fashion with the air turbine 20 as well as the fixed portion, which includes the electrical generator 21, that is arranged 5 centrally in the flow channel 2 by means of the struts 22.1, 22.2, 22.3 and 22.4 and carries the air turbine rotor 20. The diffusor according to the invention presents an outer shell 7 with a large opening angle 2a. The semi-opening angle a characterising the wall slope is preferably 10 selected greater than 4'. The result is an opening angle of at least 8' for the outer shell 7. Still larger opening angles in the region above 15* are however particularly preferable. There is also a core tube 8, which is subdivided in a first axial section 9 and a second 15 axial section 10 in axial direction 6. Consequently, the axial direction 6 is determined for the present embodiment by the rotation axis 25 of the air turbine 4. Moreover, the axial direction 6 shown on figure 1 points out to the surrounding atmosphere 3. Embodiments with an elbow in the flow channel 2 downstream of the air turbine 4 can also be envisioned. In such a case, the axial direction 6 is defined by the exhaust 20 direction as an averaging of all velocity components in the flow channel on a cross section matching the diffusor's inlet side. According to the invention, the first axial section 9 constricts itself in the exhaust direction, which translates in a negative opening angle in this area for the present 25 rotation symmetric embodiment. Accordingly, the maximum radial extension ri is greater than the further downstream maximum radial extension r2, which marks at the same time the position of the strongest bottlenecks. Here is connected the second axial section 10 of the core tube 8 which presents an increasing radial extension in the exhaust direction. Accordingly, the third radial extension r3 selected by way of 30 example is smaller than the fourth radial extension r4 situated downstream during the exhaust phase. Consequently, the cross-section decreases in the first axial section 9 as continuously as the cross-section increases in the second axial section 10 to provide an advantageous embodiment. Embodiments can further be envisioned, 01/02)11 .dh-18983 - speci - cdm.doc,6 -7 wherein the radial tapering respectively the radial widening extends step by step. Such a configuration is not represented in detail on the figures. The effect of the radial narrowing on the core tube 8 is as follows: A quasi stationary 5 vortex 11 is fixed at least for certain operating situations in the transition between the radially tapering first axial section 9 and the radially broadening second axial section 10. This is represented sketchily on figure 1 for a flow-through of the diffusor in the exhaust direction. The flow displacement effect is clearly visible through the vortex 11, which modifies the actual cross-section of the core tube 8. The flow is hence 10 displaced radially and outwardly relative to the outer shell 7 of the diffusor 5, which increases the degree of efficiency of the diffusor for the present flow that is strongly subjected to turbulence. The vortex described above is not available in the pronounced form in the case of a tapering volume stream during the exhaust phase and/or during the aspiration cycle. Accordingly, the core tube 8 is adapted to the 15 flow. The bottleneck in the transition between the first axial section 9 and the second axial section 10 enables to fix the nascent vortices so as to avoid any permanent vortex separation. The result is that energy is not removed permanently from the flow for 20 obtaining a separation vortex, wherein the increased efficiency provided by the diffusor according to the invention is explained further. Moreover, the configuration according to figure 1 shows an opening angle 2b for the core tube 8 in the second axial section 10 thereof, which is larger than the opening 25 angle 2a of the outer shell 7. The consequence is relatively small surface area ratio between the outlet and the inlet of the diffusor 5 and there is a strong radial widening for efficient use of the flow portions subjected to turbulence. Besides, the second axial section (10) of the core tube (8) comprises an external half (15) reaching into the surrounding atmosphere (3), which fills up the central dead zone (12) of the flow 30 in the diffusor (5). Figure 2 shows an alternative embodiment, for which compared with the configuration according to figure 1 the outlet side of the diffusor 5 according to the 01/0211 Idh-18983 - speci - cdm.doc,7 -8 invention is changed. The bottleneck according to the invention between the first axial section 9 and the second axial section 10 of the core tube 8 is also available. The sole deviation is the configuration as an axial-radial-diffusor, which involves a short axial construction length particularly efficiently. A further embodiment is 5 besides represented for which additional components are arranged on the outlet side on the diffusor, always related to the exhaust direction, more precisely we are here dealing with a shutoff device 13, which makes any additional shutoff valve in the flow channel 2 superfluous. Additionally, the shutoff device 13 is formed in such a way that it serves as a guiding apparatus 14 at the same time. A separate staggered 10 succession of both these components can also be envisioned, but not shown in detail in the graphical representation. Baffle plates serving for soundproofing can be provided on the outlet side of the diffusor as further components not represented in detail, since the flow is decelerated efficiently in this area and the influence of such components for blocking the flow accordingly reduced. 15 Further embodiments of the invention can be contemplated. It is hence possible in particular, to adapt the diffusor more or less to a space structure encompassing the outlet by departing from the rotation symmetry. Moreover, the concept according to the invention can also be used for a curved exhaust section in a flow channel, 20 providing that a core tube 8 presents a radial indentation perpendicular to the exhaust direction. Additional variations of the invention can be found in the claims below. Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and 25 "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. The reference to any prior art in this specification is not, and should not, be taken as 30 an acknowledgment or any form or suggestion that the prior art forms part of the common general knowledge in Australia. 01/02/1 .dh-18983 - speci - cdm.doc,8 -9 List of Reference Numerals I Wave chamber 2 Flow channel 5 3 Surrounding atmosphere 4 Air turbine 5 Diffusor 6 Axial direction 7 Outer shell 10 8 Core tube 9 First axial section 10 Second axial section 11 Vortex 12 Dead zone 15 13 Shutoff device 14 Guiding apparatus 15 External half 20 Air turbine rotor 21 Electrical generator 20 22.1,22.2 22.3, 22.4 Struts 23 Airspace 24 Water surface 25 Rotation axis 25 ri, rI, r2, r3, r4 Radial extension a, b Semi opening angle 01/02/11 ,dh-18983 - speci - cdm.doc,9