- Email: [email protected]
An experimental study of compressed air generation using a pendulum wave energy converter
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
290
2010, 22(5), supplement :290-295 DOI: 10.1016/S1001-6058(09)60209-2
An experimental study of compressed air generation using a pendulum wave energy converter Shuji Ogai1, Shinya Umeda2*, Hajime Ishida2 1
2
LWJ Co., Ltd., Nishiyodogawa-ku, Osaka, Japan School of Environmental Design, Kanazawa University, Kanazawa, Ishikawa, Japan * Email: [email protected]
ABSTRACT: This study describes a novel system of compressed air generation using a pendulum wave energy converter installed in a coastal defense structure. The objective of this study is to understand how much energy from the incoming waves can be captured for use by the system. Laboratory experiments were carried out to determine the effects of wave and system load conditions on energy conversion efficiency and wave reflection. The test results show that the system can operate with a high degree of efficiency under standing wave and high-load conditions. A reduction of wave reflection can be achieved by the system under a wide range of wave conditions. KEY WORDS: Wave energy; wave reflection; pendulum motion; compressed air; renewable energy.
1 INTRODUCTION Ocean waves represent one of the most significant potential sources of renewable energy. Isaacs and Seymour [1] estimated that the wave energy available from the world’s coastlines is about 2.5 × 1012 W. Takahashi and Adachi [2] calculated that the energy available at the Japanese coastline averages at 3.6 × 1010 W. This is roughly equivalent to 1/3 of the electric power generated in Japan. The use of wave energy may provide a low environmental impact way to meet the important challenge of increasing global demand for energy. A number of wave energy converters have been designed and tested over a period of decades[3-9]. However, some problems remain in terms of safety, reliability, maintenance and economics. Several kinds of devices have been invented, of which the surging type wave energy converter is one of the more promising ones[5, 9]. Surging devices exploit the horizontal velocity component in wave motion to drive a pendulum flap. The pendulum flap usually
drives an electric generator, and is one of the simplest methods for wave energy extraction. The method is potentially superior in durability and maintenance demand. In addition, the wave energy converter can be combined with a coastal defensive structure. This feature is economically promising because the total combined cost of a multi-functional system is less than that of separate installations. To make progress toward the establishment of a wave energy system, we have invented an effective system of compressed air generation using a pendulum wave energy converter [10] (see Fig. 1). The system produces compressed air instead of electricity. This is because it is not desirable to convert wave energy directly into electric power in large amounts due to both low energy conversion efficiency and the consequent irregularity of the electric output. Compressed air is easy to store and safe to use in coastal areas. In addition to a number of industrial applications, compressed air is commonly used for water aeration in port and fishery installations. The system described in this paper is designed to increase the efficiency of compressed air production by the introduction of a novel air compressor and gear system. The objective of this study is to understand how much energy from incoming waves can be captured for use by the system. Detailed experiments were carried out to evaluate the effects of wave and system load conditions on both the energy efficiency and the hydrodynamic characteristics of the system. 2 COMPRESSED AIR GENERATING SYSTEM Figure 1 shows a typical system of compressed air production with a pendulum-type wave energy
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China converter. The system principally consists of a hinged pendulum flap, a gear transmission system and in this case, a novel air compressor. Wave-induced motion of the flap is transmitted via the gear transmission system into the air compressor. The gear system is devised so that, independent of the direction of wave motion, the pivot can rotate in only one direction. This is achieved by using a pair of one-way ratchet gears. The air compressor adopts a new piston configuration shown in Fig. 2. The new piston design has a hypocycloid mechanism with a linear moving piston rod and a planetary gear. This has the effect of increasing the generator efficiency by reducing the friction between piston and cylinder. Tank
the other is for full standing waves. For the experiment using full standing waves, the wave absorber was removed and the model was placed near the first node of the waves (D/L=0.22). The conditions of each wave are shown in Table 1. The wave period, T was varied between 1.0 to 5.0 s. Incident wave heights, H0 up to 25 cm were generated without wave breaking. The wave reflection coefficients shown in Table 1 are estimated by measuring the waves before installation of the model. Measurements were carried out, examining the waves, the motion of the pendulum flap, and the volume and pressure of the generated air under forced loading and unloading conditions. The load intensity for the system was controlled by means of a valve in the air tank.
Valve Air
Piston-type air-compressor Gear system
291
Wave generator
Pendulum flap
Wave gages
Hinge
Incident wave
Hinge
WG1
Wave absorber
WG2
140
h＝60
Wave length, L
178
5
Incident wave
220， 300
130
680
210
Pendulum flap 230
1640
D＝330
Unit: cm
Fig. 1 Compressed air generation system using a pendulumtype wave energy converter
Table 1 Experimental conditions
Cylinder
Cylinder
Wave Wave Load Test period, condition condition number T (s)
Piston Piston Piston rod Gear wheel
Fig. 3 Wave flume setup showing the pendulum flap installation
Piston rod
Planet gear Gear wheel
(a) Linear motion type (b) Normal type Fig. 2 Comparison of the new piston-type air compressor and a normal one
3 EXPERIMENTAL SETUP Experiments were conducted in a wave flume 22 m in length, 1 m wide and 1.8 m deep, as shown in Fig. 3. At one end of the flume an actuator equipped with a wave absorbing control system generates the test waves and at the other end a wave absorber is installed. The model compressed air system was installed at a distance of about 16 m from the wave generator. Regular waves were generated with 0.6 m in water depth. This paper presents two series of experiments. One is for partial standing waves, and
Incident wave height H0 (cm)
Wave Wave energy reflection E /B coefficient 0 (W/m)
Parcial No-loaded standing waves Loaded
68 1.0 - 5.0 2.4 - 25
Full No-loaded standing Loaded waves
14
3.0
17.3
0.47
78.1
10
3.0
14.4
1.0
54.3
8
0.04～0.5 0.7～118
3.0 4.4 - 14.5 0.95～1.0 4.5～49
4 EFFICIENCY COMPUTATION The energy conversion efficiency of the compressed air generation system is defined as the average air power to the average incident wave energy for a given flap width. The efficiency η is calculated from the following equation. 1 P air T η = = E0
∫
T 0
p(t )Q (t ) dt E0
(1)
where, Pair is the average air power, p(t) the instantaneous air pressure, Q(t) the rate of the air flow, and E0 is the average incident wave energy over a wave period, given by the following equation.
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
292
⎡
E0 =
⎤ 4π h ⎥ L ⎢1 + ⎥B ⎢ sinh ⎛ 4 π h ⎞ ⎥ ⎟ ⎜ ⎢⎣ ⎝ L ⎠ ⎥⎦
ρ g H02L ⎢ 16 T
(2)
where, ρ is the water density, g the acceleration due to gravity, h the water depth, L the wave length, and B the width of the pendulum flap. 5 RESULTS AND DISCUSSIONS 5.1 Flap motion and wave reflection Figure 4 shows swing angles of the pendulum flap under no-load conditions for partial standing waves. The angle Ө is measured from a vertical line passing through the hinge. Ө > 0 corresponds to the flap inclined onshore. In general, it is found that the swing amplitude increases with increasing incident wave height H0 and wave period T. The mid-point of the swing tends to move onshore (i.e. away from the wave generator) as the waves become higher and steeper. This movement is related to the crest-trough asymmetry of waves. While the flap is significantly pushed toward shore during the crest phase, the flap is not pulled offshore to the same extent during the trough phase. Fig. 5(a) and (b) show variations in the flap swing amplitude Өa as functions of H0 and ξh50, respectively. ξh50 is the horizontal displacement component through which a fluid particle at the barycenter of the immersed part of the flap moves during one wave period. ξh50 was calculated from linear wave theory. Fig. 5(a) shows that the influence of the wave period on the swing amplitudes is significant. Fig. 5(b) shows that the swing amplitude of the flap for relatively mild wave is related to the horizontal particle displacement. The particle displacements are equivalent to the horizontal distance covered by the barycenter. However, the distance for steep waves becomes larger than the theoretical displacement.
Fig. 5 Variation in pendulum flap swing amplitude
Fig. 6 Effect of the presence of the system on wave reflection
Fig. 4 Swing angle of the pendulum flap under no-load conditions for partial standing waves
The effect of the system on reduction in wave reflection is illustrated in Fig. 6. The wave reflection coefficients estimated by measuring the waves before and after installation of the model are shown in Fig.
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China 6(a) and (b). The reflection coefficients tend to increase with wave height, unless the waves are very small. Under the same conditions of wave height and period, the reflection coefficient is significantly reduced by the presence of the model. The reduction in wave reflection depends on the wave condition. For relatively high waves of T = 3s, the wave reflection reduction effect is most enhanced. The model can reduce the reflection coefficient by almost one-half. However, for very low waves at T = 3s, the reflection coefficient increases due to small movements of the pendulum flap. 5.2 Volume, pressure and power of generated air Figure 7 shows a typical time series of water waves and generated air under no-load conditions for
Fig. 7 Time series of water wave and generated air (for Incident wave energy, E0 /B = 48-50W/m.)
293
partial standing waves. Though the incident wave energy E0 for T = 2, 3 and 4s is much the same, the wave shapes are different from each other. The time series of the water waves shows that high frequency components are enhanced for long period waves. The volumes of generated air for T ≤ 3s show greatest fluctuation at double the frequency of the incident waves, while the fundamental frequency switches to triple between T = 3s and T = 4s, and is still triple for T = 5s. The air volume for T = 4s achieves a momentary maximum value, but its time average is less than that of T = 3s. The average air power generated by the system is shown in Fig. 8. Fig. 8(a) shows that the influence of wave period on the generated air power is significant. For T = 2.5s and 3s, the system can produce compressed air with a high degree of efficiency. The frequency response of the system is closely related to the specific period of the pendulum flap. The specific period measured after draining the wave flume is about 2.5s. The ideal specific period of a simple pendulum of the same length as the flap is calculated at 2.7s. The real specific period of the system is considered to be between the two periods, accounting for the hydrodynamic mass associated with the flap. It follows that the period can be comparable to the optimum period of the system. Figure 8(b) shows the variation of air power production with respect to the average angular velocity of the flap. The angular velocity ωa is defined
Fig. 8 Average air power for partial standing waves
as the swing amplitude Өa to the wave period. The air
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9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
power increases with increasing ωa. It is found that the average air power for T = 2.5 – 5.0s can be reasonably evaluated as a function of ωa. The angular velocity ωa can be predicted with the horizontal particle displacement ξh50 shown in Fig. 5(b). 5.3 Effect of the system load intensity The effect of system load intensity on compressed air generation is shown in Fig. 9. The wave condition was set at a partial standing wave of T = 3s, H0 = 17.3cm, produced relatively high power. The volume of the generated air gradually decreases with increasing system load intensity, and then decreases abruptly when the system load intensity ε attains a
the volume of the air produced can be reduced at high system load conditions. The relative variation of the pressure is less than that of the volume. Fig. 10 shows the average air power with respect to the system load intensity. It can be seen that air power increases up to ε = 94%. Any further increase in ε results in a reduction in air power. A maximum air power of about 8.4W is achieved when ε = 94%. Its energy conversion efficiency η is calculated to be about 12%. 5.4 Compressed air generation for full standing waves Figure 11 shows the reducing effect of the model on wave reflection for full and partial standing waves with T = 3s. The system works at ε = 0%. It can be seen that a 0.2 to 0.3 decrement of the reflection coefficient occurs for the full standing wave. The decrement due to the system gradually decreases with increasing wave height. The decrement is comparable to that of the partial standing wave. The variations of average air power with incident wave energy E0 for full and partial standing waves are shown in Fig. 12. The air power generated by the system for the full standing wave is around 1.5–2 times greater than that for the partial standing wave. Since the horizontal
Fig. 11 Reduction of wave reflection due to the system for full and partial standing waves of T = 3s (ε = 0%) Fig. 9 Compressed air generation at differing system load intensities (T = 3s, H0 = 17.3cm)
Fig. 12 Air power for full and partial standing waves (ε = 0%) Fig. 10 Average air power for variation of the system load intensity (T = 3s, H0 = 17.3cm)
90% degree of shutoff. In contrast, the pressure of the generated air increases with ε. The time variation in
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
295
triple the frequency between T = 3s and T = 4s. (4) The average air power can be reasonably evaluated as a function of the average angular velocity of the pendulum flap ωa. (5) The system produces compressed air with a high degree of efficiency when the wave period is comparable to the specific period of the flap. (6) The system load control can stabilize the volume of the generated air and increases its pressure. An energy conversion efficiency of about 34% is attained for the full standing wave of T = 3s with ε = 93%. Fig. 13 Energy conversion efficiency for variation of the system load intensity
displacement of a fluid particle is amplified around nodes of the full standing wave, the air power increases with the moment of force to the pendulum flap and the swing amplitude. It is seen that the energy efficiency for the full standing wave tends to increase with increasing E0. The variation of the energy conversion efficiency with system load intensity for the full standing wave is shown in Fig. 13. The efficiency starts to rise sharply when the system load is beyond ε = 90%. A maximum efficiency of about 34% is achieved when ε = 93% with the present model configuration with a standing wave. Further study is required to improve the efficiency of the system with a different configuration of the mechanical components and for different wave conditions. 6 CONCLUSIONS An experimental study was conducted to understand the energy conversion efficiency and the hydrodynamic characteristics on the new system of compressed air generation from wave energy. The main results obtained in this experiment can be summarized as follows: (1) Although the horizontal distance covered by the barycenter of the underwater flap is equivalent to the theoretical displacement of a fluid particle, the distances produced by steep waves tend to be larger than the theoretical displacement. (2) The system can reduce the wave reflection for a wide range of wave conditions. The maximum reduction ratio due to the presence of the system is found to be about 50%. (3) The volume of generated air shows maximum fluctuation at double the frequency of the incident wave, while the fundamental frequency switches to
ACKNOWLEDGMENTS The present work is supported by a Grant-in-Aid for Scientific Research (No. 20560474) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, for which we are grateful. REFERENCES [1] Isaacs J D, Seymour R J. The ocean as a power resource. Int J Environ Studies.1973, 4: 201-205. [2] Takahashi S, Adachi T. Wave power around Japan from a viewpoint of its utilization. Technical Note of the Port and Harbor Research Institute, Japan, 1989, 654: 3-18. [3] Isaacs J D, Castel D, Wick G L. Utilization of the energy in ocean waves. Ocean Engineering, 1976, 3: 175-187. [4] Evans D V. Power from water waves Annual Review of Fluid Mechanics, 1982, 13: 157-187. [5] Kondo H, Watabe T, Yano K. Wave power extraction at coastal structure by means of moving body in the chamber. Proc. of Int. Conf. on Coastal Engineering 1984, 3: 28752891. [6] Watanabe K, Nakagawa H, Sawamoto M, et al. Characteristics of pneumatic wave power conversion system with water valve rectifier. Coastal Engineering in Japan, 1992, 35(2): 245-261. [7] Thiruvenkatasamy K, Neelamani S. On the efficiency of wave energy caissons in array. Applied Ocean Research, 1997, 19: 61-72. [8] Orer G, Ozdamar, A. An experimental study on the efficiency of the submerged plate wave energy converter. Renewable Energy, 2007, 32: 1317-1327. [9] Clement A. et al. Wave energy in Europe: current status and perspectives. Renewable & Sustainable Energy Reviews, 2002, 6: 405-431. [10] Ogai S, Umeda S, Ishida H. Experimental study on automatic production of compressed air with wave force turbine for exploiting natural energy. Annual Journal of Civil Engineering in the Ocean, JSCE, 2009, 25: 383-388 (in Japanese).
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2010, 22(5), supplement :290-295 DOI: 10.1016/S1001-6058(09)60209-2
An experimental study of compressed air generation using a pendulum wave energy converter Shuji Ogai1, Shinya Umeda2*, Hajime Ishida2 1
2
LWJ Co., Ltd., Nishiyodogawa-ku, Osaka, Japan School of Environmental Design, Kanazawa University, Kanazawa, Ishikawa, Japan * Email: [email protected]
ABSTRACT: This study describes a novel system of compressed air generation using a pendulum wave energy converter installed in a coastal defense structure. The objective of this study is to understand how much energy from the incoming waves can be captured for use by the system. Laboratory experiments were carried out to determine the effects of wave and system load conditions on energy conversion efficiency and wave reflection. The test results show that the system can operate with a high degree of efficiency under standing wave and high-load conditions. A reduction of wave reflection can be achieved by the system under a wide range of wave conditions. KEY WORDS: Wave energy; wave reflection; pendulum motion; compressed air; renewable energy.
1 INTRODUCTION Ocean waves represent one of the most significant potential sources of renewable energy. Isaacs and Seymour [1] estimated that the wave energy available from the world’s coastlines is about 2.5 × 1012 W. Takahashi and Adachi [2] calculated that the energy available at the Japanese coastline averages at 3.6 × 1010 W. This is roughly equivalent to 1/3 of the electric power generated in Japan. The use of wave energy may provide a low environmental impact way to meet the important challenge of increasing global demand for energy. A number of wave energy converters have been designed and tested over a period of decades[3-9]. However, some problems remain in terms of safety, reliability, maintenance and economics. Several kinds of devices have been invented, of which the surging type wave energy converter is one of the more promising ones[5, 9]. Surging devices exploit the horizontal velocity component in wave motion to drive a pendulum flap. The pendulum flap usually
drives an electric generator, and is one of the simplest methods for wave energy extraction. The method is potentially superior in durability and maintenance demand. In addition, the wave energy converter can be combined with a coastal defensive structure. This feature is economically promising because the total combined cost of a multi-functional system is less than that of separate installations. To make progress toward the establishment of a wave energy system, we have invented an effective system of compressed air generation using a pendulum wave energy converter [10] (see Fig. 1). The system produces compressed air instead of electricity. This is because it is not desirable to convert wave energy directly into electric power in large amounts due to both low energy conversion efficiency and the consequent irregularity of the electric output. Compressed air is easy to store and safe to use in coastal areas. In addition to a number of industrial applications, compressed air is commonly used for water aeration in port and fishery installations. The system described in this paper is designed to increase the efficiency of compressed air production by the introduction of a novel air compressor and gear system. The objective of this study is to understand how much energy from incoming waves can be captured for use by the system. Detailed experiments were carried out to evaluate the effects of wave and system load conditions on both the energy efficiency and the hydrodynamic characteristics of the system. 2 COMPRESSED AIR GENERATING SYSTEM Figure 1 shows a typical system of compressed air production with a pendulum-type wave energy
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China converter. The system principally consists of a hinged pendulum flap, a gear transmission system and in this case, a novel air compressor. Wave-induced motion of the flap is transmitted via the gear transmission system into the air compressor. The gear system is devised so that, independent of the direction of wave motion, the pivot can rotate in only one direction. This is achieved by using a pair of one-way ratchet gears. The air compressor adopts a new piston configuration shown in Fig. 2. The new piston design has a hypocycloid mechanism with a linear moving piston rod and a planetary gear. This has the effect of increasing the generator efficiency by reducing the friction between piston and cylinder. Tank
the other is for full standing waves. For the experiment using full standing waves, the wave absorber was removed and the model was placed near the first node of the waves (D/L=0.22). The conditions of each wave are shown in Table 1. The wave period, T was varied between 1.0 to 5.0 s. Incident wave heights, H0 up to 25 cm were generated without wave breaking. The wave reflection coefficients shown in Table 1 are estimated by measuring the waves before installation of the model. Measurements were carried out, examining the waves, the motion of the pendulum flap, and the volume and pressure of the generated air under forced loading and unloading conditions. The load intensity for the system was controlled by means of a valve in the air tank.
Valve Air
Piston-type air-compressor Gear system
291
Wave generator
Pendulum flap
Wave gages
Hinge
Incident wave
Hinge
WG1
Wave absorber
WG2
140
h＝60
Wave length, L
178
5
Incident wave
220， 300
130
680
210
Pendulum flap 230
1640
D＝330
Unit: cm
Fig. 1 Compressed air generation system using a pendulumtype wave energy converter
Table 1 Experimental conditions
Cylinder
Cylinder
Wave Wave Load Test period, condition condition number T (s)
Piston Piston Piston rod Gear wheel
Fig. 3 Wave flume setup showing the pendulum flap installation
Piston rod
Planet gear Gear wheel
(a) Linear motion type (b) Normal type Fig. 2 Comparison of the new piston-type air compressor and a normal one
3 EXPERIMENTAL SETUP Experiments were conducted in a wave flume 22 m in length, 1 m wide and 1.8 m deep, as shown in Fig. 3. At one end of the flume an actuator equipped with a wave absorbing control system generates the test waves and at the other end a wave absorber is installed. The model compressed air system was installed at a distance of about 16 m from the wave generator. Regular waves were generated with 0.6 m in water depth. This paper presents two series of experiments. One is for partial standing waves, and
Incident wave height H0 (cm)
Wave Wave energy reflection E /B coefficient 0 (W/m)
Parcial No-loaded standing waves Loaded
68 1.0 - 5.0 2.4 - 25
Full No-loaded standing Loaded waves
14
3.0
17.3
0.47
78.1
10
3.0
14.4
1.0
54.3
8
0.04～0.5 0.7～118
3.0 4.4 - 14.5 0.95～1.0 4.5～49
4 EFFICIENCY COMPUTATION The energy conversion efficiency of the compressed air generation system is defined as the average air power to the average incident wave energy for a given flap width. The efficiency η is calculated from the following equation. 1 P air T η = = E0
∫
T 0
p(t )Q (t ) dt E0
(1)
where, Pair is the average air power, p(t) the instantaneous air pressure, Q(t) the rate of the air flow, and E0 is the average incident wave energy over a wave period, given by the following equation.
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
292
⎡
E0 =
⎤ 4π h ⎥ L ⎢1 + ⎥B ⎢ sinh ⎛ 4 π h ⎞ ⎥ ⎟ ⎜ ⎢⎣ ⎝ L ⎠ ⎥⎦
ρ g H02L ⎢ 16 T
(2)
where, ρ is the water density, g the acceleration due to gravity, h the water depth, L the wave length, and B the width of the pendulum flap. 5 RESULTS AND DISCUSSIONS 5.1 Flap motion and wave reflection Figure 4 shows swing angles of the pendulum flap under no-load conditions for partial standing waves. The angle Ө is measured from a vertical line passing through the hinge. Ө > 0 corresponds to the flap inclined onshore. In general, it is found that the swing amplitude increases with increasing incident wave height H0 and wave period T. The mid-point of the swing tends to move onshore (i.e. away from the wave generator) as the waves become higher and steeper. This movement is related to the crest-trough asymmetry of waves. While the flap is significantly pushed toward shore during the crest phase, the flap is not pulled offshore to the same extent during the trough phase. Fig. 5(a) and (b) show variations in the flap swing amplitude Өa as functions of H0 and ξh50, respectively. ξh50 is the horizontal displacement component through which a fluid particle at the barycenter of the immersed part of the flap moves during one wave period. ξh50 was calculated from linear wave theory. Fig. 5(a) shows that the influence of the wave period on the swing amplitudes is significant. Fig. 5(b) shows that the swing amplitude of the flap for relatively mild wave is related to the horizontal particle displacement. The particle displacements are equivalent to the horizontal distance covered by the barycenter. However, the distance for steep waves becomes larger than the theoretical displacement.
Fig. 5 Variation in pendulum flap swing amplitude
Fig. 6 Effect of the presence of the system on wave reflection
Fig. 4 Swing angle of the pendulum flap under no-load conditions for partial standing waves
The effect of the system on reduction in wave reflection is illustrated in Fig. 6. The wave reflection coefficients estimated by measuring the waves before and after installation of the model are shown in Fig.
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China 6(a) and (b). The reflection coefficients tend to increase with wave height, unless the waves are very small. Under the same conditions of wave height and period, the reflection coefficient is significantly reduced by the presence of the model. The reduction in wave reflection depends on the wave condition. For relatively high waves of T = 3s, the wave reflection reduction effect is most enhanced. The model can reduce the reflection coefficient by almost one-half. However, for very low waves at T = 3s, the reflection coefficient increases due to small movements of the pendulum flap. 5.2 Volume, pressure and power of generated air Figure 7 shows a typical time series of water waves and generated air under no-load conditions for
Fig. 7 Time series of water wave and generated air (for Incident wave energy, E0 /B = 48-50W/m.)
293
partial standing waves. Though the incident wave energy E0 for T = 2, 3 and 4s is much the same, the wave shapes are different from each other. The time series of the water waves shows that high frequency components are enhanced for long period waves. The volumes of generated air for T ≤ 3s show greatest fluctuation at double the frequency of the incident waves, while the fundamental frequency switches to triple between T = 3s and T = 4s, and is still triple for T = 5s. The air volume for T = 4s achieves a momentary maximum value, but its time average is less than that of T = 3s. The average air power generated by the system is shown in Fig. 8. Fig. 8(a) shows that the influence of wave period on the generated air power is significant. For T = 2.5s and 3s, the system can produce compressed air with a high degree of efficiency. The frequency response of the system is closely related to the specific period of the pendulum flap. The specific period measured after draining the wave flume is about 2.5s. The ideal specific period of a simple pendulum of the same length as the flap is calculated at 2.7s. The real specific period of the system is considered to be between the two periods, accounting for the hydrodynamic mass associated with the flap. It follows that the period can be comparable to the optimum period of the system. Figure 8(b) shows the variation of air power production with respect to the average angular velocity of the flap. The angular velocity ωa is defined
Fig. 8 Average air power for partial standing waves
as the swing amplitude Өa to the wave period. The air
294
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
power increases with increasing ωa. It is found that the average air power for T = 2.5 – 5.0s can be reasonably evaluated as a function of ωa. The angular velocity ωa can be predicted with the horizontal particle displacement ξh50 shown in Fig. 5(b). 5.3 Effect of the system load intensity The effect of system load intensity on compressed air generation is shown in Fig. 9. The wave condition was set at a partial standing wave of T = 3s, H0 = 17.3cm, produced relatively high power. The volume of the generated air gradually decreases with increasing system load intensity, and then decreases abruptly when the system load intensity ε attains a
the volume of the air produced can be reduced at high system load conditions. The relative variation of the pressure is less than that of the volume. Fig. 10 shows the average air power with respect to the system load intensity. It can be seen that air power increases up to ε = 94%. Any further increase in ε results in a reduction in air power. A maximum air power of about 8.4W is achieved when ε = 94%. Its energy conversion efficiency η is calculated to be about 12%. 5.4 Compressed air generation for full standing waves Figure 11 shows the reducing effect of the model on wave reflection for full and partial standing waves with T = 3s. The system works at ε = 0%. It can be seen that a 0.2 to 0.3 decrement of the reflection coefficient occurs for the full standing wave. The decrement due to the system gradually decreases with increasing wave height. The decrement is comparable to that of the partial standing wave. The variations of average air power with incident wave energy E0 for full and partial standing waves are shown in Fig. 12. The air power generated by the system for the full standing wave is around 1.5–2 times greater than that for the partial standing wave. Since the horizontal
Fig. 11 Reduction of wave reflection due to the system for full and partial standing waves of T = 3s (ε = 0%) Fig. 9 Compressed air generation at differing system load intensities (T = 3s, H0 = 17.3cm)
Fig. 12 Air power for full and partial standing waves (ε = 0%) Fig. 10 Average air power for variation of the system load intensity (T = 3s, H0 = 17.3cm)
90% degree of shutoff. In contrast, the pressure of the generated air increases with ε. The time variation in
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
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triple the frequency between T = 3s and T = 4s. (4) The average air power can be reasonably evaluated as a function of the average angular velocity of the pendulum flap ωa. (5) The system produces compressed air with a high degree of efficiency when the wave period is comparable to the specific period of the flap. (6) The system load control can stabilize the volume of the generated air and increases its pressure. An energy conversion efficiency of about 34% is attained for the full standing wave of T = 3s with ε = 93%. Fig. 13 Energy conversion efficiency for variation of the system load intensity
displacement of a fluid particle is amplified around nodes of the full standing wave, the air power increases with the moment of force to the pendulum flap and the swing amplitude. It is seen that the energy efficiency for the full standing wave tends to increase with increasing E0. The variation of the energy conversion efficiency with system load intensity for the full standing wave is shown in Fig. 13. The efficiency starts to rise sharply when the system load is beyond ε = 90%. A maximum efficiency of about 34% is achieved when ε = 93% with the present model configuration with a standing wave. Further study is required to improve the efficiency of the system with a different configuration of the mechanical components and for different wave conditions. 6 CONCLUSIONS An experimental study was conducted to understand the energy conversion efficiency and the hydrodynamic characteristics on the new system of compressed air generation from wave energy. The main results obtained in this experiment can be summarized as follows: (1) Although the horizontal distance covered by the barycenter of the underwater flap is equivalent to the theoretical displacement of a fluid particle, the distances produced by steep waves tend to be larger than the theoretical displacement. (2) The system can reduce the wave reflection for a wide range of wave conditions. The maximum reduction ratio due to the presence of the system is found to be about 50%. (3) The volume of generated air shows maximum fluctuation at double the frequency of the incident wave, while the fundamental frequency switches to
ACKNOWLEDGMENTS The present work is supported by a Grant-in-Aid for Scientific Research (No. 20560474) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, for which we are grateful. REFERENCES [1] Isaacs J D, Seymour R J. The ocean as a power resource. Int J Environ Studies.1973, 4: 201-205. [2] Takahashi S, Adachi T. Wave power around Japan from a viewpoint of its utilization. Technical Note of the Port and Harbor Research Institute, Japan, 1989, 654: 3-18. [3] Isaacs J D, Castel D, Wick G L. Utilization of the energy in ocean waves. Ocean Engineering, 1976, 3: 175-187. [4] Evans D V. Power from water waves Annual Review of Fluid Mechanics, 1982, 13: 157-187. [5] Kondo H, Watabe T, Yano K. Wave power extraction at coastal structure by means of moving body in the chamber. Proc. of Int. Conf. on Coastal Engineering 1984, 3: 28752891. [6] Watanabe K, Nakagawa H, Sawamoto M, et al. Characteristics of pneumatic wave power conversion system with water valve rectifier. Coastal Engineering in Japan, 1992, 35(2): 245-261. [7] Thiruvenkatasamy K, Neelamani S. On the efficiency of wave energy caissons in array. Applied Ocean Research, 1997, 19: 61-72. [8] Orer G, Ozdamar, A. An experimental study on the efficiency of the submerged plate wave energy converter. Renewable Energy, 2007, 32: 1317-1327. [9] Clement A. et al. Wave energy in Europe: current status and perspectives. Renewable & Sustainable Energy Reviews, 2002, 6: 405-431. [10] Ogai S, Umeda S, Ishida H. Experimental study on automatic production of compressed air with wave force turbine for exploiting natural energy. Annual Journal of Civil Engineering in the Ocean, JSCE, 2009, 25: 383-388 (in Japanese).