Experimental Research on Performances of Air Turbines for a Fixed Oscillating Water Column-Type Wave Energy Converter

A fixed oscillating water column (OWC)-type wave energy converter is composed of an air chamber for primary conversion and an air turbine for secondary conversion. In the optimal design method of a fixed OWC-type wave energy converter, it is necessary to develop a design method which can consider the characteristics of incident wave motion, the motion of the internal free surface affected in the structure such as a partly submerged wall, the fluctuation of air pressure in an air chamber, the rotation of the air turbine. In this paper, the 2-dimensional wave tank tests in regular waves for the performance evaluation of the air turbines in a fixed OWC-type wave energy converter were conducted to obtain the data needed to make this design method. As the results, the effects of the impulse turbine specification such as the rotor inlet/outlet angle, the guide vane's number and the vane's setting angle on the primary and secondary conversion efficiencies are clarified experimentally. Furthermore, the performances of the Wells turbines with different number of blade are presented for comparison of the operating condition.


I. INTRODUCTION
As for renewable energy resources in the world, we can newly exploit the mini-/micro-hydro, the wind, the solar and the ocean energy, etc. Wave energy which is one of the renewable energies attracts attention as a promising resource that can reduce CO2 emissions. Wave energy converter (WEC) which converts wave power into electric power has been developed all over the world and many types of WECs [1] such as the movable body type [2,3] and the overtopping wave type [4] were proposed. There is an oscillating water column (OWC)-type wave energy converter as one of the WECs. This device is composed of an air chamber, an air turbine and a generator and is expected to be safe even under storm conditions. Many studies on this device have been performed experimentally and theoretically since the early 1970s. In the performance evaluation of the OWC-type WEC, it is necessary to consider the characteristics of the incident wave motion, the motion of the internal free surface affected in the structure such as a partly submerged wall, the fluctuation of air pressure in an air chamber, and the rotation of the air turbine. However, most of the past studies were carried out by dividing into two steps of the primary conversion and the secondary conversion. To estimate the primary conversion performance, the many researches [5,6] on the air chamber with the nozzle as the substitute of the turbine were conducted. Wilbert et al. [7] carried out the wave flume tests to evaluate the hydrodynamic performance of Double Chamber Oscillating Water Column (DCOWC). Ning et al. [8] investigated the hydrodynamic performance of a fixed OWC wave energy device under various wave conditions and geometric parameters in a wave flume. In this study, the power take-off was implemented through a nozzle situated on the roof of the chamber. Though, the effects of the turbine are unclear in the above studies. As concerns the secondary conversion, by means of the test rig having a piston-cylinder to generate the oscillating air flow, the performance of the air turbine was evaluated in a series of studies. Setoguchi et al. [9,10] reviewed a variety of experimental results concerning the performances of the Wells turbine and the impulse turbine. Besides, Takao et al. [11] showed that the impulse turbine was superior to the Wells turbine in a wide flow rate range. On the other hand, there are the studies about the performance of the OWC with the turbine [12], but the most studies make mention of the performance of the OWC with the Wells turbine. Besides, the influences of the turbine geometry on the primary conversion and secondary conversion performances were not clarified. This paper discusses the geometry effects of the impulse turbine on the primary and secondary conversion efficiencies in the fixed OWC-type WEC based on the experimental data needed to make this design method. Additionally, the performance of the Wells turbine is compared with the one of the impulse turbine. Fig. 1 shows the arrangement of the experimental devices in the 2-dimensional wave tank. This tank is 18.5 m long, 0.8 m wide and contains 0.8 m water depth. An absorbing wave generator was installed at the end of the tank and the model turbine was located at the other end of the tank. Four wave height gauges (TS-DWG) produced by TECHNO SERVICE Co., Ltd. were arranged to measure the amplitudes of the incident wave accurately [13]. The wave data is fed into the computer through the analog-todigital converter (PCI-3165) from the Interface Corporation. The incident wave height was configured as high as possible in this wave tank, and the value was 0.1 m. Fig. 2 shows the model of OWC-type wave energy converter with the impulse turbine. In the experiments, the turbine is rotated by the alternating-current synchronous motor (HG-JR73) manufactured by the Mitsubishi Electric Corporation. The torque transducer (SS-005) and the electromagnetic rotation detector (MP-981) produced by ONO SOKKI Co., Ltd. were located at the end of the turbine shaft. The air chamber length is 0.7 m, the curtain wall depth is 0.1 m and the thickness of the curtain wall is 0.005 m. The schematic design of the air chamber was conducted in a series of numerical studies [14]. Fig. 3 indicates the location of the pressure gauge (AP-10S) and the wave height gauges (FW-H07 and UD320) made by KEYENCE CORPORATION at the top of the air chamber. The rectangular orifice of the air chamber is located at the center. The data of waves, the turbine rotational speed, the torque, the pressure and the water surface elevation in the air chamber were measured simultaneously. The sampling frequency is 50 Hz and the data collection was started after the lapse of 30 seconds from the start of the wave generator. Fig. 4 shows the basic configuration of the rotor and the fixed guide vanes. This turbine configuration was adopted on the basis of the air turbine test results [9]. The numbers of the rotor blades and the single-stage guide vanes are 30 and 26, respectively. The inlet/outlet angle  of rotor is 60 degrees and the setting angle  of guide vane is 30 degrees. The inner diameter D of turbine casing is 170 mm, the hub ratio  is 0.7 and the clearance between the rotor blade tip and the casing is 0.3 mm. The inner diameter D of the turbine casing was determined based on the condition that the total pressure drop at the turbine is equal to the total pressure difference at the nozzle which the ratio of cross-section between the nozzle and the air chamber is 1/100.

II. EXPERIMENTAL APPARATUS
Besides, we conducted the steady flow turbine test. In this test without waves, the bottom of the air chamber was closed by an acrylic board as shown in fig. 5 and the steady flow was generated by a centrifugal fan. Fig. 6 shows a comparison of the efficiencies between the three cases with different number Zg of guide vanes. The abscissa is the ratio between the wave length  and the air chamber length L. In this experiment, the wave length  was changed while keeping the time-averaged rotational speed of turbine N = 700 rpm. The wave periods are T = 1.00 sec., 1.15 sec., 1.30 sec., 1.41 sec., 1.50 sec., 1.56 sec., 1.65 sec., 1.73 sec., 1.87 sec., 2.03 sec., 2.30 sec. and 2.63 sec. The primary conversion efficiency 1, the secondary conversion efficiency 2 and the generating efficiency  are defined as:

Impulse Turbine 3.1.1 Effect of number of guide vanes
where Pair is the time-averaged power of the air, Pwave is the time-averaged power of the incident wave and Ptorque is the turbine output. The definitions of these parameters are as follows: where S, p, , w, g, i, Cg, W, T0 and  denote the crosssection area of air chamber, the pressure, the six averaged water level in the air chamber, the water density, the gravitational acceleration, the incident wave amplitude, the group velocity, the chamber width, the turbine output torque and the angular velocity of turbine, respectively.   fig. 8, the efficiencies are compared between the above three cases. The number of guide vanes is Zg = 26 giving the high efficiency. The ratio /L was kept constant at 6.3. As shown in fig. 8, the effect of the  on the 1 is small at about N = 700 rpm giving the maximum . Besides, the maximum value of the  is highest at  = 30.0 degrees.
Figs. 9 and 10 show the amplitudes of the pressure and the water surface elevation in the air chamber. It is found that the pressure increases at smaller . Meanwhile, the amplitude of the water surface elevation decreases inversely with the pressure rise. Therefore, the difference of the 1 due to the  is small in fig. 8.

International Journal of Advanced Engineering Research and Science (IJAERS)
[ where , va, U, ST and p are the air density, the axial velocity, the circumferential velocity at mean radius r [= D(1+)/4], the flow passage area of turbine and the total pressure drop at the turbine. In the steady flow test, the inlet/outlet angle  of rotor is 60 degrees, the number Zg of guide vanes is 26 and the setting angle  of guide vane is 30 degrees.
In fig. 11(a) fig. 11(c). This fact corresponds to the pressure rise in fig. 9. In the case of  = 22.5 degrees, along with the pressure rise, the turbine output torque becomes higher at small flow rate due to the increase of the whirl velocity of flow from the upstream guide vane as shown in fig. 11(b). This is the reason why the peak of 2 appeared in the small flow rate.

Effect of inlet/outlet angle of rotor
The inlet/outlet angle of rotor was changed from  = 60 degrees to 50 degrees. Fig. 12 shows the variations of the efficiencies due to the rotational speed.
In addition, fig. 13 shows the secondary conversion efficiency as a function of the flow coefficient. The trend shifting the peak of 2 to the small flow rate region at  = 22.5 degrees in fig. 13 is similar to the one of the case of   = 60 degrees in fig. 11(a), although the maximum value of 2 decreased. This reduction of 2 causes the deterioration of the  as shown in fig. 12.

Wells Turbine
The energy conversion efficiency of the OWC model with the Wells turbine was measured for comparison with the one of the above impulse turbine. In fig. 15, the blades numbers of the Wells turbine without the guide vane are Zr = 7, 6 and 5. In all three cases, the chord length is 51 mm, blade profile is NACA0020, the aspect ratio is 0.5 and the hub ratio is 0.7. In the case Zr = 6, the solidity at mean radius is 0.67, and this value was adopted in the previous research on a performance in the steady flow condition [15].   becomes lower in the case Zr = 7, because the increase of CT corresponding to the increase of CA is not sufficient. This is caused by the fact that the flow passage area at hub side is extremely narrow as shown in fig. 15.

IV. CONCLUSIONS
This paper discussed the effects of the impulse turbine specification such as the rotor inlet/outlet angle , the guide vane's setting angle  and the guide vane's number Zg on the primary conversion efficiency 1, the secondary conversion efficiency 2 and the generating efficiency  (= 12) in the fixed OWC-type WEC based on the results of the 2-dimensional wave tank tests.
In the experiments, the rotor inlet/outlet angle was changed between three cases  = 70 degrees, 60 degrees and 50 degrees, and the guide vane's setting angle was varied from  = 30.0 degrees to 37.5 degrees and 22.5 degrees. Besides, the single-stage guide vane's number was changed from Zg = 26 to 32 and 20. Furthermore, the performances of the Wells turbines with different number Zr of blades were investigated. The following concluding remarks are obtained. 1. The maximum generating efficiency of about 28% is