Sea Trial of an Impulse Turbine for Wave Energy Conversion

Transcription

1 Sea Trial of an Impulse Turbine for Wave Energy Conversion Manabu Takao 1, Eiji Sato 2, Takayuki Takeuchi 3, Shuichi Nagata 4, Kazutaka Toyota 4 and Toshiaki Setoguchi 4 1. Department of Mechanical Engineering, Matsue National College of Technology, Matsue, Japan 2. Hokuriku Regional Development Bureau, Ministry of Land, Infrastructure and Transport, Niigata, Japan 3. Corporate Technology Development Office, Fuji Electric Systems Co., Ltd., Tokyo, Japan 4. Institute of Ocean Energy, Saga University, Saga, Japan Abstract: A sea trial of wave energy plant with an impulse turbine for bi-directional airflow has been carried out at Niigata-nishi Port in order to demonstrate usefulness of the impulse turbine. Oscillating water column (OWC) based wave energy plant has been installed at the side of a breakwater. The wave energy plant consists of an impulse turbine with a tip diameter of 458 mm, a generator with a rated power of 500W and an air chamber with a sectional area of 4 m 2. The impulse turbine has fixed guide vanes both upstream and downstream, and these geometries are symmetrical with respect to the rotor centerline in order to rotate a single direction in bi-directional airflow generated by OWC. The experimental data obtained by using the impulse turbine was compared to these of Wells turbine which is the mainstream of air turbine for wave energy conversion. As a result, total efficiency of the plant using the impulse turbine was considerably higher than that of Wells turbine, especially when higher wave power exists on the sea. Therefore, it was concluded in the study that the impulse turbine is superior to Wells turbine as air turbine for wave energy conversion. Keywords: Natural Energy, Wave Energy Conversion, Impulse Turbine, Wells Turbine, Sea Trial 1. INTRODUCTION Several of the wave energy devices being studied under many wave energy programs in the United Kingdom, Japan, Portugal, India and other countries make use of the principle of an oscillating water column (OWC) [1]. Figures 1 and 2 show the principle of OWC based wave energy conversion and its energy conversion chain respectively. In such wave energy devices, a water column which oscillates due to wave motion is used to drive an oscillating air column which is converted into mechanical energy. The energy conversion from the oscillating air column can be achieved by using a self-rectifying air turbine such as Wells turbine which was introduced by Dr. A. A. Wells in 1976 [2, 3]. This turbine rotates in a single direction in oscillating airflow and therefore does not require a system of non-return valves. Furthermore, this turbine is one of the simplest and probably the most economical turbines for wave energy conversion. However, according to previous studies, the Wells turbine has inherent disadvantages: lower efficiency and poorer starting characteristics in comparison with conventional turbines [2, 3]. These drawbacks are due to severe stalling. Therefore, although a number of OWC based wave energy plants using Wells turbine have been constructed and tested to date, the total conversion efficiencies of the plants were approximately 15 % at the best [4-6]. Moreover, Wells turbine has a higher noise level and a maintenance problem because of high rotational speed. Recently, in order to develop a high performance self-rectifying air turbine for wave energy conversion, an impulse turbine for wave energy conversion has been proposed by the authors [7, 8]. There are many reports which describe the performance of the turbine both at Figure 1 Outline of OWC based wave energy plant Potential hydro energy of ocean waves Air chamber Potential and kinetic energy of air Air turbine Mechanical energy of rotation Generator Electricity Figure 2 Energy conversion chain in OWC based wave energy plant 211

2 2m Wave energy plant Breakwater 2m Cross section of air chamber Turbine Generator Air chamber OWC Ocean wave Figure 3 Wave energy plant and breakwater (No. 1) at Niigata-nishi Port Figure 5 Outline of wave energy plant Figure 4 Wave energy plant installed at breakwater Figure 6 Turbine and generator Figure 7 Outline of impulse turbine and generator starting and running conditions. The experimental results of the model testing show that the efficiency of the impulse turbine is high in a wide range of flow coefficient, though its peak efficiency is almost the same to that of Wells turbine [7, 8]. However, a sea trial of wave energy plant with impulse turbine has not been carried out so far. The objective of this study is to clarify the performance of wave energy plant with the impulse turbine and to demonstrate the usefulness of the impulse turbine for wave energy conversion. This study was carried out as a cooperative research by Saga University, Matsue National College of Technology and Ministry of Land, Infrastructure and Transport. 2. EXPERIMENTAL APPARATUS AND PROCE- DURE 2.1. OWC based wave energy plant The sea trial was performed by the use of an oscillating water column (OWC) based wave energy plant which 212

3 was installed at a breakwater (No. 1) of Niigata-nishi Port The rotor blade profile consists of a circular arc on the as shown in Fig. 3. Figures 4 and 5 show the wave energy plant used in the study. An air chamber which is A radius of the circular is 46.4 mm and the ellipse has pressure side and part of an ellipse on the suction side. primary conversion device has a cross sectional area of 4 semi-major axis of 193 mm and semi-minor axis of 63.5 m 2 (= 2 m 2 m). Figures 6 and 7 show the impulse mm. The chord length is 83.2 mm; solidity of 2.02 at turbine and the generator (Ryokuseisha WAG-500A). mean radius; blade inlet (or outlet) angle of 60 ; thickness ratio of 0.3; tip diameter of 458 mm; tip clearance of These are mounted on the air chamber as shown in Fig. 5. The generator starts to generate electricity at 1450 rpm 1 mm; hub-to-tip ratio of 0.7. Figure 9 shows the impulse type rotor installed in the turbine. Note that the and a rated power is 500W at 2600 rpm. adopted turbine rotor is the most promising one [3, 4] Impulse turbine for wave energy conversion The camber line of guide vane consists of a straight As shown in Fig. 8, the turbine configuration employed in the study is the impulse type one having fixed guide vanes both upstream and downstream, and these geometries are symmetrical with respect to the rotor centerline. The specifications of the impulse turbine rotor adopted in the sea trial are as follows. line with a length of 53.7 mm and a circular arc with a radius of 49 mm. The guide vane with chord length of mm are symmetrically installed at the distance of 30.7 mm downstream and upstream of the rotor as shown in Fig. 8. Detailed information about the guide vane is as follows: solidity of 2.27 at mean radius; thickness ratio of ; camber angle of 60. Regarding a setting angle of guide vane, according to previous studies, the optimum value is 30º [7, 8]. However, the guide vane was set at 15 in order to enhance the whirl velocity at inlet of the rotor and to increase the rotational speed. NACA Rotor Hub Casing Figure 8 Configuration of impulse turbine Figure 10 Configuration of Wells turbine Unit: mm Figure 9 Turbine rotor Figure 11 Wells turbine installed in casing 213

4 According to previous studies, the impulse turbine rotates at lower speed in comparison with conventional turbines [3, 4]. In the sea trial, the impulse turbine is coupled to a drive pulley through a drive shaft as shown in Fig. 7, in order to increase the rotational speed of generator to 4 times because the generator is operated at higher speed Wells turbine A sea trial of wave energy plant using Wells turbine has been already performed by Ministry of Land, Infrastructure and Transport, Fuji Electric Systems Co., Ltd. and Mitsui Engineering and Shipbuilding Co., Ltd., from October to November in Figure 10 shows the configuration of rotor adopted in the sea trial. The blade profile is NACA0020 with chord length of 131 mm; solidity at mean radius: 0.829; aspect ratio: 0.5; tip diameter: 458 mm; tip clearance: 1 mm; hub-to-tip ratio: The rotational speed of Wells turbine is much higher than that of the impulse turbine. Therefore, the turbine is directly coupled to the generator through the drive shaft, that is, V-pulleys and V-belt are not necessary in the plant. The air chamber and the generator are the same to the sea trial for the impulse turbine. Figure 11 shows Wells turbine installed in the turbine casing. 3. RESULTS AND DISCUSSIONS 3.1. Output Figure 12 indicates the results of the sea trials, where the ordinate and the abscissa are the average output P o and the wave power P w respectively. It is obviously understand from the figure that P o in the case of impulse turbine is quite high than that of Wells turbine in whole region of P w. Especially, its value at higher wave power is more than twice the output power of Wells turbine. This result is caused by the following reasons [7, 8]. (1) The impulse turbine does not stall, though Wells turbine experiences a severe stall at higher flow coefficient. Accordingly, the impulse turbine can convert pneumatic energy into mechanical energy in comprehensive range of P w. (2) When the impulse turbine is operated in the plant, the pressure in the air chamber is lower in comparison with Wells turbine. Therefore, by using the impulse turbine, the OWC freely oscillate comparatively. Then, the primary conversion efficiency may be improved. It is concluded from the above reasons that the impulse turbine is suitable for wave energy conversion than Wells turbine Procedure of sea trial Impulse turbine Wells turbine In the sea trial, the output and the rotational speed of the generator were observed for 5 minutes every 2 hours and these were stored in PC. This measurement is timed to the observation of wave climate by an offshore buoy which is placed at 8 km away from Niigata-nishi Port by Ministry of Land, Infrastructure and Transport [9]. The period of sea trial for the impulse turbine was from March to April in Evaluation of turbine performance The performance of the turbine is evaluated by a relationship between the average output of generator P o and the incident wave power to the plant P w in the study. P w is defined as follows [1]: P w = 0.5 H 1/3 2 T 1/3 B (1) where H 1/3 and T 1/3 are the significant wave height and period at the sea trial site, respectively, and B is the width of air chamber. H 1/3 and T 1/3 were estimated from the wave climate which is observed by the offshore buoy. The equations for estimation, which were formulated by Ministry of Land, Infrastructure and Transport, are as follows: H 1/3 = H b1/ (2) T 1/3 = T b1/ (3) Figure 12 Comparison of average output of generator Impulse turbine Wells turbine where H b1/3 and T b1/3 are the significant wave height and period at the offshore buoy, respectively. Figure 13 Comparison of time variations of rotational speed of generator 214

5 Here note that the total conversion efficiency (= P o / P w ) is from 2 % to 4 % as shown in the figure. The reasons of this deterioration are as follows: (1) The relation between the sectional area of plant and the turbine size was optimized for the use of Wells turbine. Accordingly, the size of impulse turbine does not match the air chamber used in the sea trial. (2) Since the generator operates at higher rotational speed, the generator used in the sea trial is suitable for Wells turbine. In the case of the impulse turbine, V-belt and V-pulleys must be adopted in order to increase the rotational speed of the shaft, and then, friction loss occurs at these parts. (3) The significant wave height and period used for the evaluation of the turbine was estimated from wave climates observed by the offshore buoy. The estimation might not be accurate Rotational speed Figure 13 shows time variations of the rotational speed n. The wave conditions are H 1/3 = 0.67 m and T 1/3 = 8.2s for the impulse turbine (2:00 April 1st, 2006) and H 1/3 = 0.67 m,t 1/3 = 7.7 s for Wells turbine (16:00 October 23rd, 2004). These two wave power are almost the same. As shown in the figure, the change of rotational speed of the impulse turbine is much larger than that of Wells turbine. It is because torque of the impulse turbine is higher than that of Wells turbine and rotor stall does not occur for the impulse turbine [7, 8]. Then, the turbine rotor accelerates very well. Conversely, rapid decrease of the rotational speed is due to friction between V-belt and V-pulleys, and friction in bearings which is made of ABS resin. Here note that, in the case of impulse turbine, the rotational speed of generator is 4 times of the turbine by using V-belt and V-pulleys. 4. CONCLUSIONS In the study, the sea trial of oscillating water column (OWC) based wave energy using the impulse turbine was carried out in order to investigate the performance of the impulse turbine. The obtained results were compared to that of Wells turbine. As a result, it is concluded that the impulse turbine which was proposed by the authors is superior to Wells turbine as air turbine for wave energy conversion. This study was performed under the Cooperative Research Program of IOES, Institute of Ocean Energy, Saga University (Accept#06004D). REFERENCES 1. R. Bhattacharyya and M. E. McCormick, Wave Energy Conversion, Elsevier, (2003). 2. T. Setoguchi and M. Takao, Energy Conversion and Management, 47, (2006), pp S. Raghunathan, The Wells Air Turbine for Wave Energy Conversion, Prog. Aerospace Sci., 31-4 (1995), pp H. Osawa, Y. Washio, T. Ogata, Y. Tsuritani and Y. Nagata, Proc. 12th Int. Offshore and Polar Eng. Conf., (2002), pp Wavegen, Islay LIMPET Project Monitoring Final Report, (2002). 6. A. Sarmento, A. Brito-Melo and F. Neumann, ed28ec aa45f7.doc, Proc. WREC 2006, (2006). 7. T. Setoguchi, M. Takao, Y. Kinoue, K. Kaneko, S. Santhakumar and M. Inoue, Int. J. Offshore and Polar Eng., 10-2 (2000), pp T. Setoguchi, S. Santhakumar, H. Maeda, M. Takao and K. Kaneko, Renewable Energy, 23 (2001), pp NOWPHAS, index.html 215