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Optimisation of the clam wave energy converter
Renewable Energy, Vol.5, Part IL pp. 1464-1466, 1994
Elsevier Science Ltd erinted in Cn'emBritain 0960-1481/94 $7.004-0.00
Pergamon
OFI'IMISATION OF THE C L A M W A V E ENERGY CONVERTER.
L.J.Duckers, F.P.Lockett, B.W.Loughridge, A.M.Peatfield, M.J.West and P.R.S.White Coventry University, Coventry, UK
ABSTRACT The most promising UK offshore device is the Clam, a floating rigid toroid about 80m in diameter with access to a huge wave energy resource. The rated output would be 3MW, at a cost of 8p/kWh. A lack of Government support has almost stopped R&D on the Clam. This paper describes ongoing mathematical optimisation which would provide a foundation for a future prototype programme intended to reduce costs to 4p/Kwh. INTRODUCTION Since the Wave energy review (1) produced by the Chief Scientists Group in December 1992 gave the Clam a healthy technical appraisal but relatively poor economic prospects the team have considered optimisation of the design and of the components in order to reduce the cost of delivered electricity from the 8p/Kwh quoted in the review towards a more acceptable 3-4p/Kwh. In order to capture energy from sea waves it is necessary to intercept the waves with a structure which can respond in an appropriate manner to the forces applied to it by the waves. If the structure is fixed to the seabed or seashore then it is easy to see that some part of the structure may be allowed to move with respect to the fixed slructure and hence convert the wave energy into some mechanical energy (which is probably subsequently converted into electricity). Floating structures can be employed, but then a stable frame of reference must be established so that the 'active' part of the device moves relative to the main structure. This can be achieved by the application of inertia or by making the structure so large that it spans several wave crests and hence is reasonably stable in most sea states. The Clam is a circular floating wave energy converter which carries 12 air chambers around its circumference. As air passes between these chambers its energy is extracted by air turbines (2). On its passage from and to the chamber the air passes through an air turbine generator and so produces eleclricity. A novel air turbine, the Wells, which is self rectifying and has aerodynamic characteristics of low drag, linear pressure/flow relationship and high rotational speed (and so produce electricity on a generator fixed directly on its shaft) making it particularly suitable for wave application, is proposed for many wave energy designs. A full scale Clam would be 60 to 80m in diameter and rated at one to three MW. Because it is operated offshore in water over 40m deep it is possible to envisage arrays of many Clams generating electricity and transmitting it to shore along submarine cables. Extensive tests of the Clam at scales of 1:15 have been conducted in Loch Ness and have been used to validate a comprehensive mathematical modal which permits optimisation of the design for a specified
1464
1465
wave climate. Turbine damping, bag spring rate and device diameter are among the parameters which can be investigated. Full scale component testing of the turbine and membrane has been carried out and designs of the structure have been produced in steel and in concrete. A full scale sea going prototype costing £10M is needed to prove the concept and refine the design. With little funding the optimisation exercise is being focused on key areas which represent the major cost centres or the most economically sensitive parameters. A mathematical model of the overall system has been developed and has been validated against a considerable body of experimental evidence. This model can now be used with confidence to predict device performance and therefore to evaluate design changes. Work on the air turbine technology is progressing on both the performance assurance and on economic consWaction techniques. MATHEMATICAL
MODELLING
Linearized wave hydrodynamic studies have been carded out for a wide range of wave energy schemes. The method is robust and successful in predicting responses in all but extreme wave conditions. With its simple wetted profile, linear damping and fimited membrane motions, the Clam is a very suitable case for such investigation. Over the last few years, a comprehensive linear model has been developed to determine a complete description of the response of the system to monochromatic wave excitation. Response in spread, mixed spectrum seas can then be deduced by superposition. The device is viewed as a moored floating body, free to move in heave, surge, pitch sway and roll as well as in a further 12 modes corresponding to the surging motions of the membrane panels forming the outer wetted surfaces of the CLAM air cells. For the analysis, wave diffraction and radiation forces are calculated numerically using the well-known source distribution technique. The wetted surface of the device is represented in this case by 1792 plane facets in 128 radial strips of 14 each. Hydrodynamic coefficients for this geometry show singularities due to resonances of the water pool interior to the torus, as shown in Fig. 1 for example. Air flow within the device is modelled using a linearized gas law applied to the separate air cells connected by linear dampers to represent the resistance of the Wells turbines running at near constant speed. In this way, compressibility effects are incorporated, thereby illuminating an uncertainty in the extrapolation of small scale model tests to full scale. To validate the model, absorption efficiency was calculated from the system response to incident plane waves of periods from 3 to 18 seconds for the geometry of the model tested on Loch Ness. Mean efficiency in seas with a Peirson-Moskowitz spectral spread were then deduced. Fig. 6 compares results obtained in this way with a mean curve obtained from the Loch Ness results. Theoretical results for two choices of pneumatic tuning, that is the turbine damping rate and membrane hydrogeometric spring rate, are shown. The 'hard' tuning case is representative of the experimental settings, the 'soft' case more desirable in terms of mean seasonal capture efficiency. The results show good agreement bearing in mind that the spectral spread for the short-fetch, rapidly generated seas typical of Loch Ness is narrower than the P-M spread of North Atlantic seas. The value of this modelling is the capability to assess the sensitivity in productivity to variation of a wide variety of design parameters. For example, Figs. 2 to 4 show the relative merits of varying damping rate, Structure diameter and water depth at various wave periods.
1466
By considering these data with economic costing calculations we can identify the best combination of parameters for the most cost effective design and so are able to envisage cutting the cost of delivered energy by a factor of 2. Clearly this would have to be verified by the testing of a Clam at large scale, ideally at full scale.
The wave energy review placed the Clam at the front of the offshore devices but reported a cost of 8p/Kwh. With the optimisation technique described here the team is confident that future costs of 34p/Kwh are achievable.
Fig. 1 Normalized Heave Added Mass (+) Fig.2 Capture Efficiency for and Added Damping (o) Hard( ) to SoR(---) Turbine Damping Rate Fig.4 Capture Efficiency for
Fig.4 Capture Efficiency for
60m( ) to 80m(...)
30m( ) to 60m(---) Water Depth
CLAM Torus diameter xl0 4
3
1.5 +
2
o
1
oOOOooooo
1
o°
+++++o
om
0.5
~~++~+
0 -I 0
i
i
i
5
10
15
Wave P e r i o d
0 0
20
5
10
Wave P e r i o d
Seconds
15
~0
Seconds
o
o
0.5
0.5 c~
0
0 0
5
10
W a v e Period
15
Seconds
20
0
5
10
Wave P e r i o d
15
20
Seconds
RREF.J~I~ (1) A Review of Wave Energy. Thorpe T W (vols l&2) ETSU R72 (1992) (2) Towards a Prototype Floating Circular Clam Wave Energy Converter. L.J.Duckers, F.P.Lockett, B.W.Loughridge, A.M.Peatfield, M.J.West and P.R.S.White. Renewable Energy. Ed Sayigh A M volume 5, pp2541-2546 1992
Elsevier Science Ltd erinted in Cn'emBritain 0960-1481/94 $7.004-0.00
Pergamon
OFI'IMISATION OF THE C L A M W A V E ENERGY CONVERTER.
L.J.Duckers, F.P.Lockett, B.W.Loughridge, A.M.Peatfield, M.J.West and P.R.S.White Coventry University, Coventry, UK
ABSTRACT The most promising UK offshore device is the Clam, a floating rigid toroid about 80m in diameter with access to a huge wave energy resource. The rated output would be 3MW, at a cost of 8p/kWh. A lack of Government support has almost stopped R&D on the Clam. This paper describes ongoing mathematical optimisation which would provide a foundation for a future prototype programme intended to reduce costs to 4p/Kwh. INTRODUCTION Since the Wave energy review (1) produced by the Chief Scientists Group in December 1992 gave the Clam a healthy technical appraisal but relatively poor economic prospects the team have considered optimisation of the design and of the components in order to reduce the cost of delivered electricity from the 8p/Kwh quoted in the review towards a more acceptable 3-4p/Kwh. In order to capture energy from sea waves it is necessary to intercept the waves with a structure which can respond in an appropriate manner to the forces applied to it by the waves. If the structure is fixed to the seabed or seashore then it is easy to see that some part of the structure may be allowed to move with respect to the fixed slructure and hence convert the wave energy into some mechanical energy (which is probably subsequently converted into electricity). Floating structures can be employed, but then a stable frame of reference must be established so that the 'active' part of the device moves relative to the main structure. This can be achieved by the application of inertia or by making the structure so large that it spans several wave crests and hence is reasonably stable in most sea states. The Clam is a circular floating wave energy converter which carries 12 air chambers around its circumference. As air passes between these chambers its energy is extracted by air turbines (2). On its passage from and to the chamber the air passes through an air turbine generator and so produces eleclricity. A novel air turbine, the Wells, which is self rectifying and has aerodynamic characteristics of low drag, linear pressure/flow relationship and high rotational speed (and so produce electricity on a generator fixed directly on its shaft) making it particularly suitable for wave application, is proposed for many wave energy designs. A full scale Clam would be 60 to 80m in diameter and rated at one to three MW. Because it is operated offshore in water over 40m deep it is possible to envisage arrays of many Clams generating electricity and transmitting it to shore along submarine cables. Extensive tests of the Clam at scales of 1:15 have been conducted in Loch Ness and have been used to validate a comprehensive mathematical modal which permits optimisation of the design for a specified
1464
1465
wave climate. Turbine damping, bag spring rate and device diameter are among the parameters which can be investigated. Full scale component testing of the turbine and membrane has been carried out and designs of the structure have been produced in steel and in concrete. A full scale sea going prototype costing £10M is needed to prove the concept and refine the design. With little funding the optimisation exercise is being focused on key areas which represent the major cost centres or the most economically sensitive parameters. A mathematical model of the overall system has been developed and has been validated against a considerable body of experimental evidence. This model can now be used with confidence to predict device performance and therefore to evaluate design changes. Work on the air turbine technology is progressing on both the performance assurance and on economic consWaction techniques. MATHEMATICAL
MODELLING
Linearized wave hydrodynamic studies have been carded out for a wide range of wave energy schemes. The method is robust and successful in predicting responses in all but extreme wave conditions. With its simple wetted profile, linear damping and fimited membrane motions, the Clam is a very suitable case for such investigation. Over the last few years, a comprehensive linear model has been developed to determine a complete description of the response of the system to monochromatic wave excitation. Response in spread, mixed spectrum seas can then be deduced by superposition. The device is viewed as a moored floating body, free to move in heave, surge, pitch sway and roll as well as in a further 12 modes corresponding to the surging motions of the membrane panels forming the outer wetted surfaces of the CLAM air cells. For the analysis, wave diffraction and radiation forces are calculated numerically using the well-known source distribution technique. The wetted surface of the device is represented in this case by 1792 plane facets in 128 radial strips of 14 each. Hydrodynamic coefficients for this geometry show singularities due to resonances of the water pool interior to the torus, as shown in Fig. 1 for example. Air flow within the device is modelled using a linearized gas law applied to the separate air cells connected by linear dampers to represent the resistance of the Wells turbines running at near constant speed. In this way, compressibility effects are incorporated, thereby illuminating an uncertainty in the extrapolation of small scale model tests to full scale. To validate the model, absorption efficiency was calculated from the system response to incident plane waves of periods from 3 to 18 seconds for the geometry of the model tested on Loch Ness. Mean efficiency in seas with a Peirson-Moskowitz spectral spread were then deduced. Fig. 6 compares results obtained in this way with a mean curve obtained from the Loch Ness results. Theoretical results for two choices of pneumatic tuning, that is the turbine damping rate and membrane hydrogeometric spring rate, are shown. The 'hard' tuning case is representative of the experimental settings, the 'soft' case more desirable in terms of mean seasonal capture efficiency. The results show good agreement bearing in mind that the spectral spread for the short-fetch, rapidly generated seas typical of Loch Ness is narrower than the P-M spread of North Atlantic seas. The value of this modelling is the capability to assess the sensitivity in productivity to variation of a wide variety of design parameters. For example, Figs. 2 to 4 show the relative merits of varying damping rate, Structure diameter and water depth at various wave periods.
1466
By considering these data with economic costing calculations we can identify the best combination of parameters for the most cost effective design and so are able to envisage cutting the cost of delivered energy by a factor of 2. Clearly this would have to be verified by the testing of a Clam at large scale, ideally at full scale.
The wave energy review placed the Clam at the front of the offshore devices but reported a cost of 8p/Kwh. With the optimisation technique described here the team is confident that future costs of 34p/Kwh are achievable.
Fig. 1 Normalized Heave Added Mass (+) Fig.2 Capture Efficiency for and Added Damping (o) Hard( ) to SoR(---) Turbine Damping Rate Fig.4 Capture Efficiency for
Fig.4 Capture Efficiency for
60m( ) to 80m(...)
30m( ) to 60m(---) Water Depth
CLAM Torus diameter xl0 4
3
1.5 +
2
o
1
oOOOooooo
1
o°
+++++o
om
0.5
~~++~+
0 -I 0
i
i
i
5
10
15
Wave P e r i o d
0 0
20
5
10
Wave P e r i o d
Seconds
15
~0
Seconds
o
o
0.5
0.5 c~
0
0 0
5
10
W a v e Period
15
Seconds
20
0
5
10
Wave P e r i o d
15
20
Seconds
RREF.J~I~ (1) A Review of Wave Energy. Thorpe T W (vols l&2) ETSU R72 (1992) (2) Towards a Prototype Floating Circular Clam Wave Energy Converter. L.J.Duckers, F.P.Lockett, B.W.Loughridge, A.M.Peatfield, M.J.West and P.R.S.White. Renewable Energy. Ed Sayigh A M volume 5, pp2541-2546 1992