Inlet geometry effect of wave energy conversion system

Transcription

1 Journal of Mechanical Science and Technology 6 (9) (01) 793~798 DOI /s Inlet geometry effect of wave energy conversion system Jin-Seok Oh * and Sung-Hun Han Division of Mechatronics Engineering, Korea Maritime University, Busan, , Korea (Manuscript Received May 16, 011; Revised March 7, 01; Accepted April 3, 01) Abstract A floating marine facility, such as a buoy, plays an important role as an aid to navigation for maritime safety. Due to development of the wave energy conversion system, installing an oscillating water column (OWC) with rechargeable batteries on a buoy has become the trend. In order to extend the battery replacement cycle, the battery voltage must be kept within a certain range by improving the energy conversion efficiency. This paper suggests using inlet geometry modification of the OWC to increase vertical displacement. Two types of inlet geometry, the trumpet type and the cylindrical type, are designed and compared with each other through a scale model experiment in a -D wave tank, as well as through a full-scale experiment at sea. The results show that the trumpet-shaped inlet of the OWC generates more electric power than does the cylindrical-shaped inlet. Keywords: Buoy; Efficiency; Inlet geometry; Modification; OWC; Wave energy Introduction Due to concerns regarding exhaustion of petroleum resources and global warming, the demand for renewable energy has been increasing. There are many renewable energy sources that can replace fossil fuels such as solar, wind, geothermal heat, tidal, and ocean wave energy. Among these renewable energy sources, ocean wave energy has attracted the world s attention because of its magnitude. Muetze and Vining [1] reported that the ocean s wave energy is able to provide 15-0 times more available energy per square meter than either wind or solar energy. Scientists have been developing various types of wave energy converters for many years to extract more power. The most well-known form of wave energy converter is the oscillating water column (OWC). OWC converts hydraulic energy into pneumatic energy (bidirectional airflow). This pneumatic energy is converted by an air-driven turbine installed on the top of the OWC device. As the demand for electric power of stand-alone power systems increase, OWC has been applied to aids to navigation such as buoys. However, its low energy conversion efficiency, which is vulnerable to weather conditions, often causes the storage battery to over discharge. As a result, the replacement cycle of the battery is shortened, and maintenance costs are raised. Therefore, the battery s voltage should be kept within a * Corresponding author. Tel.: , Fax.: address: ojs@hhu.ac.kr Recommended by Associate Editor Moon Ki Kim KSME & Springer 01 certain rage by improving energy conversion efficiency. In order to raise the energy conversion efficiency, the vertical displacement of the free water surface in the OWC chamber has to be increased. Numerous factors affect the vertical displacement, such as wave height, wave length, wave period, OWC chamber dimensions, total mean water depth, phase control, and so on [-5]. McCormick [6] discovered that the length of the internal water column with additional fluid mass can be excited by increasing the center-pipe length within a certain range. In this paper, we deal with the seawater inlet geometry of the OWC to increase output power of the wave energy conversion system through experiments that use two types of OWC: cylindrical and trumpet. The aim of these experiments is to raise pneumatic energy for the air-driven turbine installed on the top of the OWC by reducing entrance loss. The experiments are divided into two parts: a scale model test in the -D wave tank and a full-scale buoy test at sea. In both experiments, the internal wave heights of the OWC and the output power of the wells turbine are measured, respectively.. Summary of the theory.1 Internal wave height and seawater inflow To simplify the theoretical analysis, several things are assumed: (1) Air is incompressible. () Wave forces are far greater than the air force. (3) Pressure at the exhaust of the OWC is atmospheric. (4) External wave is linear (sinusoidal).

2 794 J.-S. Oh and S.-H. Han / Journal of Mechanical Science and Technology 6 (9) (01) 793~798 Fig. 1. Diagram of the inlet geometry of the OWC. (5) Viscous effects are ignored. (6) Seawater flow is irrotational. (7) Heaving motion of the buoy is ignored (stationary buoy). (8) Depth of water is greater than half of the wavelength. (9) Internal wave height of the trumpet type is dependent on the seawater inflow, which is proportional to the opening surface area at the bottom of the OWC. McCormick [7] demonstrated that the floating generation equipment in a buoy consists of a circular floating body containing a vertical water column that has free communication with the sea. Thus, the water surface in the center of the OWC rises and falls with the same period as that of the external wave. The inlet geometry of the OWC is shown in Fig. 1. According to McCormick [8], air velocity, excited by both the buoy s heaving and wave motion, is proportional to the wave height. Assuming the internal wave height is proportional to the external wave height, the average internal wave height, H 1, is given by H = ς H 1 (1) where ς is not specifically identified as the function or the proportional factor; thus, we assume that it is the proportional factor. The average internal wave height of the cylindrical type is the same as H 1 ; however, the height of the trumpet type depends on the assumed seawater inflow. The seawater inflow of both types is, respectively, Q = π r H () C( Cylindrical ) 1 QT ( Trumpet ) K R HC C = π (3) where K is the flow coefficient and depends on the inlet geometry. The inlet geometry depends on the ratio of E/D and F/D in Fig., where D is the diameter of the OWC chamber, and E and F are the horizontal length and vertical length, respectively. The curve, representing the entrance section of the seawater, is obtained from the equation of the ellipse: (x/e) +(y/f) =1, where x is the horizontal axis and y is the vertical axis. The curve is not identified theoretically; rather, it is based on the experimental result using the full-scale buoy at sea. Thus, the internal wave Fig.. Diagram of the OWC. heights can be inferred as H H C( Cylindrical ) 1 = T T ( Trumpet ) π r1 = H (4) Q where R is the inlet radius of the trumpet type, r 1 is the inlet radius of the cylindrical type, and Q C (cylindrical type) and Q T (trumpet type) are the seawater inflow of both types, respectively. McCormick [9] remarked that the average vertical velocity, V 1, depends on the average free surface displacement and angular frequency. Assuming the water column has an average free surface displacement, the free surface displacement and the average vertical velocity are, respectively, H1 δ1= cos( ωt) (6) dδ1 ωh1 V1 = = sin( ωt) dt (7) where δ 1 is the average free surface displacement and is the natural angular frequency [9].. Available power to the turbine According to McCormick [8], the airflow above the internal free surface of the OWC is determined by seawater inflow, whereas the displacement of the seawater in the OWC is as same as the amount of airflow. From the equation of continuity, the axial velocity, V, in the turbine passage is as follows: r 1 V = V1. r McCormick [9] added that the power, E, available to the turbine is dependent on the pressure gradient and volume rate of airflow, Q, across the turbine. The available power, E, is calculated as follows: (5) (8)

3 J.-S. Oh and S.-H. Han / Journal of Mechanical Science and Technology 6 (9) (01) 793~ E = P Q (9) where P is the pressure difference between P and P 3, and P is the upstream pressure in the OWC. Here, P 3 is atmospheric pressure, which is a zero gauge pressure, as shown in Fig.. McCormick [9] demonstrated that the pressure difference in the power expression equation is obtained from the linear momentum equation: Table 1. Specifications of the scale model. Item Value Unit L (length of the OWC) 313 mm R (radius of trumpet type) 68.8 mm r 1 (internal radius of the OWC ) 31.4 mm r (internal radius of turbine passage) 0.0 mm A 1 (area of water surface) mm A (area of turbine passage) mm r 1 φ1 Q P= ρa + ρa ( V V1 ). r t π r (10) Here, P can be derived from the energy equation due to Bernoulli: 1 φ1 1 φ P1 + ρ V1 + ρ = P + ρ V + ρ a a t a a t (11) where P 1 is the pressure in the OWC chamber and ρ a is the density of the air. The velocity potentials, 1 and, are approximated by V1H 1 φ1 V1δ 1= cos( ωt) (1) φ A 1 r1 1 1 A φ = r φ (13) Fig. 3. Diagram of the experimental configuration. where A 1 is the area of the water surface in the OWC chamber and A is the area of the air turbine passage [9]. 3. Experimental setup 3.1 Experiments in -D wave tank To verify the correlation between the inlet geometry and the water height of OWC, experiments were carried out in the -D wave tank using scale models. The wave tank used was 5.0 m in length, 1.3 m in width, and 1.0 m in height. The depth of the water was 0.8 m. In order to minimize the effects of wave reflections, a wave absorber was installed at the end of the wave tank. The experimental configuration for the test of the scale model is shown in Fig. 3. The model was designed to adjust to the center of gravity and to the buoyancy using weights. A mooring line connected to the model to prevent horizontal shifts by wave was fixed on the bottom of the -D wave tank. The model was made of acrylic and was 1014 mm long and 70 mm wide. The scale factor of 1/10 was applied to the buoy model. The specifications of the scale model are listed in Table 1. In the model tests, the wave period, T, and wave height, H, were taken as variable input parameters. The external wave height was measured using a wave height meter (servo-type). (a) Scale model test Fig. 4. Experiments in the -D wave tank. (b) Scale model The internal water height of the OWC was measured by a camcorder installed next to the wave tank. To distinguish the internal wave height, scales were marked on the surface of the OWC. The tests were conducted for four different values of T and six different values of H. The two types of inlet geometry, trumpet and cylindrical, were designed and compared with each other s performances. The cylindrical type and the trumpet type were tested in the same conditions for comparison. Fig. 4 shows the scale model test in the -D wave tank.

4 796 J.-S. Oh and S.-H. Han / Journal of Mechanical Science and Technology 6 (9) (01) 793~798 (a) T = 1.3 s Fig. 5. Experiments at sea. 3. Experiments at sea Full-scale buoys were tested at sea near the Achi Islands in Busan, Korea. Fig. 5 shows the top side of the full-scale buoy. The buoys were placed along the side of the island at intervals of less than 100 m at sea. Thus, we assume that the experimental conditions for each buoy are nearly identical. The buoys were designed with reference to the data obtained by experiments in the -D wave tank. Each buoy consisted of the OWC, wells turbine, generator, power controller, and so on. Two types of inlet geometry were manufactured, for the OWC: trumpet and cylindrical. The wells turbine was installed on the top of the OWC for each buoy. It rotated in one direction regardless of the direction of airflow. The generator was connected to the shaft of the wells turbine. The power controller converted AC into 4 DC through switching and rectification. The output power of the wave energy conversion system was collected every minute from each buoy by a wireless communication module. The data acquisition module was made up of a PIC microcontroller, a radio frequency transceiver, a current sensor, and so on. (b) T = 1.6 s 4. Result 4.1 Experiments in -D wave tank (c) T = 1.9 s The internal wave height of the trumpet type was higher than the cylindrical type in all sections, as shown in Fig. 6. The internal wave height was proportional to the external wave height. The difference of the water height between both types showed a tendency to increase in section T = s. The difference was the highest at T = 1.6 s with H = 0.1 m in Fig. 6(b). The difference was not significant despite a variation of the external wave change at T =. s. 4. Experiments in the sea The output data were collected from buoys for 4 h on August 4th, 010, as shown in Fig. 7. The output range of the (d) T =. s Fig. 6. Experimental results from the -D wave tank.

5 J.-S. Oh and S.-H. Han / Journal of Mechanical Science and Technology 6 (9) (01) 793~ (a) Cylindrical type Fig. 8. Internal wave height ratio of trumpet type to cylindrical type: H T/H C (scale model). (b) Trumpet type Fig. 9. Hourly mean output ratio of trumpet type to cylindrical type: P T/P C (full-scale buoy). Fig. 7. Experimental results at sea (August 4th, 010). cylindrical type of the buoy was within W in Fig. 7(a). However, the output of the trumpet type fluctuated between 0 and 43.1 W, as shown in Fig. 7(b). The average power from the wells turbine was 1.74 kwh and kwh for the cylindrical type and the trumpet type, respectively. Table shows the results of the experiment. 5. Discussion Eqs. (1)-(13) involve assumptions to enhance understanding of the wave energy conversion. In reality, however, calculating energy conversion efficiencies using the equations already presented can be difficult. Because the weather changes every moment at sea, we are unable to predict the variation of the parameters such as vertical displacement of the OWC, buoy motion, external and internal wave heights, and wave period. For this reason, we conducted an empirical study on the application of the OWC for a buoy. Based on experimental results from both the scale-model test and full-scale test, the effectiveness of the trumpet type OWC was confirmed. Using the scale-model tests in the wave tank, we found that the internal wave height of the trumpet type OWC was higher than the cylindrical type OWC. Fig. 8 represents the internal wave height ratio of the trumpet type to the cylindrical type: H T /H C. The ratio that depends on both external wave height and wave period was larger than 1.0 in all cases, and was the Fig. 10. Hourly mean output power of wells turbine (full-scale buoy). largest (approximately 4.0) at T = 1.3 m with H = 0.0 m. We also determined that the turbine output of the trumpet type buoy was more than that of the cylindrical type buoy through a full-scale buoy tests at sea. The turbine output and its ratio of trumpet type to cylindrical type, P T /P C, are shown in Figs. 9 and 10. The ratio, the hourly mean value for every 1 h, was the largest of approximately 51 at 10 p.m., It means that pneumatic energy, converted from wave energy, is directly influenced by the seawater inlet geometry. This paper is unable to show the conversion efficiency of wave energy to electric energy because the experiments does not include measurement data such as buoy motion, external wave height, air velocity of turbine passage, and air pressure of OWC chamber. However, it presents the effectiveness of

6 798 J.-S. Oh and S.-H. Han / Journal of Mechanical Science and Technology 6 (9) (01) 793~798 the trumpet-shaped OWC for the wave energy conversion system by the comparative method, based on experimental results. To clarify the energy conversion efficiency in more detail, additional study is needed through experiments, including the measurements mentioned above. 6. Conclusion Inlet geometry, which is directly related to the wave energy conversion efficiency, affects the internal wave height of the OWC. The performance of the trumpet type OWC was better than that of the cylindrical type at all times in the experiments. To optimize the inlet geometry of the OWC, additional study should be conducted. The modification of the inlet geometry can improve the wave energy conversion efficiency and help reduce the maintenance cost by extending the battery replacement cycle for stand-alone renewable energy systems such as the buoy. Acknowledgment This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology ( ). Nomenclature H : External wave height H 1 : Internal wave height of the OWC H C : Internal wave height of the OWC (Cylindrical type) H T : Internal wave height of the OWC (Trumpet type) ς : Proportional factor (H = ς H 1 ) r 1 : Inlet radius of the cylindrical type r : Radius of turbine passage R : Inlet radius of trumpet type Q : Seawater inflow Q C : Seawater inflow (Cylindrical type) Q T : Seawater inflow (Trumpet type) K : Flow coefficient δ 1 : Average free surface displacement V 1, V : Average vertical velocity : Angular frequency t : Time E : Available power to the turbine P : Pressure difference P 1 : Pressure in the OWC chamber P : Pressure in turbine passage a : Density of the air 1, : Velocity potential A 1 : Area of water surface A : Area of turbine passage L : Length of the OWC P C : Turbine output (Cylindrical type) : Turbine output (Trumpet type) P C References [1] A. Muetze and J. G. Vining, Ocean wave energy conversion, industry applications conference, 41st IAS Annual Meeting, Conference Record of IEEE, 3 (006) [] J. S. Oh and K. J. Jo, Design program for a wave energy converter in buoy, International Society of Offshore and Polar Engineers, Beijing, China, 3 (010) [3] H. H. Lee and M. L. Jeng, Feasible study on the wave power converter applied to offshore platform system, Ocean Engineering Conference, Taiwan (006) [4] J. Falnes, Principles for capture of energy from ocean waves. phase control and optimum oscillation, Department of Physics, Trondheim, Norway (1995). [5] G. Nunes, D. Valerio and P. Beirao, Modeling and control of a wave energy converter, Renewable Energy, 36 (011) [6] M. E. McCormick, B. H. Carson and D. H. Rau, An experimental study of a wave-energy conversion buoy, MTS Journal, 9 (1975) [7] M. E. McCormick, Analysis of a wave energy conversion buoy, Journal of Hydronautics, 8 (1974) [8] M. E. McCormick, A modified linear analysis of a wave energy conversion buoy, Ocean Engineering, 3 (1976) [9] M. E. McCormick and L. David, Ocean wave energy conversion, John Wilet & sons (1981) 61-71, Jin-Seok Oh is a professor in the College of Maritime Science, Korea Maritime University. He received his B.E. in Marine Engineering from Korea Maritime University in He worked for Zodiac Maritime (UK) as an engine officer for 4 years. He worked at the Agency for Defense Development (ADD) as a researcher from 1989 to 199, and earned his M.A. and Ph.D. in Marine Engineering from Korea Maritime University. He studied Design of Energy System in Kyushu University from 006 to 009, and received his Ph.D. in Design of Energy System from Kyushu University in 009. His research interests have led him to pursue and combine several areas: smart grid control algorithms, hybrid generation systems, wave energy conversion buoys, and energy-saving systems for ships. Sung-Hun Han received his B.E. in Marine Engineering from Korea Maritime University in 007. He worked for Hyundai Merchant Marine as an engine officer from 007 to 010. He is pursuing his Master s degree in the Department of Mechatronics Engineering, Korea Maritime University.