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A conceptual study of floating axis water current turbine for low-cost energy capturing from river, tide and ocean currents

Hiromichi Akimoto1, Kenji Tanaka2 and Kiyoshi Uzawa2 1 Division of Ocean Systems Engineering, Korea Advanced Institute of Science and Technology 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea

2 Department of Systems Innovation, Graduate School of Engineering, The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

akimoto@kaist.ac.kr

Abstract. The cost of utilizing kinetic energy of river stream, tidal and ocean current is considered to be higher than that of wind power generation because of difficulties in construction and maintenance of devices installed in seawater. As a solution to the problem, the authors propose a new concept of water stream turbine. The main idea is in the manner of supporting turbine. Although it is similar to a vertical axis turbine, the direction of turbine axis is not firmly fixed and its tilt angle is passively adjustable to the stream velocity. Since it does not have to keep the turbine axis in upright position, required structural strength and weight of the device will be reduced significantly. This paper describes the application ranging from the small hydro power in river streams to large application of tidal and ocean current turbine. In the large capacity plant for tidal stream and ocean current, the main mechanism of turbine axis support is the same as that of wind turbine authors proposed in the previous paper. It leads to the further opportunity of cost reduction. The sample design of a multi-megawatt ocean current turbine shows the possibility of high economic performance of the concept. The results show that the cost of energy in the concept can be comparable to land based wind turbine.

Keywords Ocean current turbine; tidal stream turbine; floating axis turbine; vertical axis turbine; conceptual study

1. Introduction The needs of renewable energy have been increasing after decisions of countries on the reduction of nuclear power following the nuclear power plant accident in Japan, 2011. Although, at present, utilization of hydrokinetic, i.e., tidal stream turbine and ocean current turbine, is more expensive than that of wind power, their stable and predictable power favorably replace some part of existing thermal and nuclear power. The stable characteristics lead to higher capacity utilization ratio of turbine plant and the predictability is suitable for the connection to onshore grids where fluctuation of power leads to difficulties in the planned operation of existing power resources.

Although these water currents have suitable characteristics, their costs of energy are considered to be higher than that of wind power. In ocean environment, installation and maintenance costs increase significantly due to the limited accessibility of the site and required underwater works. Also, the treatments of corrosion and biofouling in seawater are big issue of R&D. To alleviate these problems,

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researchers have proposed varieties of concept. Gorlov [1, 2] designed a helical stream turbine for utilizing the low head stream of tidal energy without dam construction. The concept avoids the unrecoverable damage to the environment caused by dam construction and reduces the cost of power plant. University Naples and Ponte di Archimede S.p.A tested their tidal stream turbine in Messina Strait, Italy [3-6]. It is a vertical axis turbine (Kobold turbine) mounted under a float. Its stream direction independency and easy installation will reduce the total cost of the plant. Marine Current Turbine Ltd. developed commercial tidal current turbines, SeaGen [7]. Their concept is twin horizontal axis turbine mounted on a tubular monopile. It has an elevation mechanism for above water maintenance of rotor units. OpenHydro Group Ltd. [8] developed Open-Center turbines which are to be deployed on sea bed for capturing tidal stream energy. Their direct drive generator and lubricant-free design reduce maintenance cost. Also, there are many other ongoing researches on the utilization of hydrokinetic energies and their reviews such as Khan et al. [9] and Electric Power Research Institute [10].

The above mentioned researches show that reducing the cost of maintenance is the key for practical use of turbines. In addition, they show that the sweep area of turbine is relatively small in the total size of device. Although reducing the supporting structure of turbine is favorable for high economic performance, the heavy hydrodynamic load of water stream increases the required size and weight of the plant. To solve the problem, the authors propose a new configuration of water stream turbine. Although it resembles a vertical axis turbine, we do not try to keep the upright position of turbine axis and its tilt angle is passively adjustable to water current speed. This paper shows that the concept reduces the size and cost of the power plant. It also provides sample designs for a tidal stream turbine and a multi-megawatt ocean current turbine system. The main mechanism of converting turbine torque to electricity is common to that of floating wind turbine concept which has been proposed by the authors [11].

2. Floating axis water stream turbine

2.1. Inclined axis stream turbine Figure 1 shows major configurations of stream turbine. Turbines are placed on the sea bottom as

shown in Fig. 1(a) or supported by a float in Fig. 1(b). There are both horizontal and vertical axis layouts. Their turbine units need to be raised above the water surface for regular maintenance and cleaning. It requires a specially designed service vessel or an elevation mechanism as in SeaGen [7].

(a) (b) Figure 1. Water stream turbine configurations: horizontal axis and vertical axis turbines placed on

sea bed (a) and supported by a float (b)

In the above mentioned configurations, the sweep area of turbine tends to be relatively small in the total dimensions of device. Although we can use buoyancy for partially supporting submerged structures, utilized range in depth direction has a significant limitation because of high hydrodynamic load on the turbine. If we expand the size of the turbine, it leads to considerable increase of the supporting structures and the structural strength at the root of the vertical axis turbine to keep the upright position of the turbine system.

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To solve these problems, the authors abandon the concept of keeping upright position of the turbine as shown in Fig. 2. It is similar to a vertical axis turbine. However, the turbine axis swings around the pivot on a supporting structure or on a float. The inclination angle is passively adjustable to stream speed and is determined in the balance of hydrodynamic force, buoyancy and weight of the turbine. The torque converting unit that contains a generator and gearbox swings with the turbine axis so that the unit does not experience high bending moment. For maintenance access, the turbine axis swings to near the water surface by being pulled up at the lower end of the turbine using a cable outside of the system. Since bending moment on the root of turbine axis is no longer a significant problem, we can employ long blades and obtain larger swept area with high aspect ratio. It utilizes wider depth range of water stream without scaling up the support structure. The device can be installed on onshore structure or on a float as shown in Figs. 2(a) and (b). The array of moored turbines captures water stream energy efficiently as shown in Fig. 2(c).

(a) (b) (c)

Flow direction

Figure 2. Inclined axis water turbine mounted on a supporting structure (a) and on a float (b);

small farm of moored inclined axis turbines (c)

Although the proposed idea is very simple and does not require any high technology, the features mentioned above are favorable for low cost small hydro power. It is applicable not only to river stream but also to tidal stream of alternative flow direction because of the swing mechanism of turbine axis. Pillars on water bottom or other onshore structures can be used as a basis for the supporting arm or mooring point for the float. In river environment, there may be the limitation of water depth and seasonal change of water level. However, the present concept is easily adaptable to them and can provide low-tech and low-cost electricity in developing regions where they have rivers or tidal stream of sufficient velocity.

2.2. Floating axis tidal turbine The inclined axis concept is only applicable to small hydro power because the design of pivot

mechanism is difficult in heavy turbine load. For large capacity turbine, the authors use the same approach of Floating Axis Wind Turbine (FAWT) concept proposed by the authors [11]. Figure 3 shows the artist’s illustration of FAWT. The wind turbine is on a rotating spar buoy which does not have any internal rotary machinery. The torque of wind turbine is derived by contacting rollers installed off-turbine-axis on the secondary float. The position of secondary float is kept by catenary cables connected to on-deck mooring points. The same torque converting unit for offshore wind application can be used in the water stream turbine.

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(a) (b) Figure 3. Images of Floating Axis Wind Turbine based on [11];

(a) front and side views; (b) perspective view

Figure 4 shows a sample design of tidal stream turbine. The assumed site is Kurushima strait in Seto Inland Sea, Japan. The speed of tidal stream at the site is over 2m/sec in 60% of a year. Its maximum velocity is 4.5m/sec. Main particulars of the turbine are in Table 1. In the plan, we adjust the ballast water in the turbine column so that a specified length of its head is above the water surface. The rotary energy of the turbine can be derived by rollers contacting on the cylindrical surface of the column. Since the weight of turbine is supported by buoyancy, the contacting rollers sustain horizontal component of the thrust force on the turbine. The tilt moment on the turbine balances with the weight of central column and the pendulum weight at the lower end of column.

(a)

Front view Side view

pendulum (b)

Figure 4. Tidal stream turbine for Seto Inland Sea, Japan; (a) front and side views; (b) perspective view

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Table 1. Parameters of 139kW tidal stream turbine Rated power [kW] 139 Assumed flow speed [m/sec] 2.0 Turbine

Diameter [m] 6.0 Height [m] 25.0 Tilt angle at rated power [degree] 30.0 Effective sweep area [m2] 135.9 Blade chord length [m] 0.40

Revolution speed [rpm] 14.0 Thrust force [kN] 198

Center column Extern diameter at mid point [m] 0.650 Thickness [mm] 19.5 Float part diameter [m] 2.0

Pendulum tension [ton] 33.4 Power coefficient 30% Total efficiency 25% Total displacement [ton] 40.0

The assumed power coefficient of the turbine is 30% and the total efficiency of electricity generation is 25% as a moderate performance of vertical axis turbine. Even when the direction of tidal stream is reversed, flexible support of the turbine provides swing of rotating axis and the direction of rotation is unchanged.

The center column is a steel (SS400) cylinder which has inner ballast tanks for the adjustment of buoyancy. The dimension of the pipe is determined based on the maximum bending moment that occurs at the middle point of the turbine with safety factor 2.0. Turbine blades and their supporting arms are made of composite materials. The composition of fiber reinforced plastic (FRP) material is designed to have a density near the salt water so that the weight of blades is neutralized by buoyancy. Light-density core materials, hollow fiberglass and hollow carbon fiber can be used for the adjustment of blade density. The assumed tilt angle at the rated power is 30 degree in 2.0m/sec water stream. In the condition, about 50% of righting moment is provided by the distributed weight of central column and remaining 50% is by the pendulum. The drag (thrust) coefficient of vertical axis turbine is higher than that of horizontal axis turbine (assumed CD is 1.0 in the above estimation). However, different from horizontal axis turbine case, we do not need to keep the upright position of turbine.

Tilt restoring moment is proportional to the tilt angle. However, too large tilt may decrease the performance of turbine. The condition of tilted turbine is similar to the small vertical axis wind turbine mounted on a rooftop of building. Balduzzi et al. [12] examined the influence of skewed flow caused by rooftop position to turbine performance. Their experimental and numerical results in skew angle up to 40 degree show the increase of energy harvesting (up to 12%). The present tilt angle is set to be in the range of their analysis. Since the analysis of Balduzzi et al. [12] is for a low aspect ratio turbine, the power increase by tilt is not counted in the present estimation of high aspect ratio turbine.

When maintenance of turbine is required, it can be laid afloat near the sea surface by discharging internal ballast water and loosing the cable to the pendulum. In the condition, maintenance of blades is possible over the water surface.

The blades are in helical shape as in the Gorlov Helical Turbine [2] for smoothing and leveling the turbine torque. Since the curvature of helical blade is moderate due to the high aspect ratio of the turbine, they can be made as straight blades and attached to support arms with the specified curvature.

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Figure 5 depicts the merit of floating axis configuration in the comparison to vertical axis turbine supported by a floater. The comparative concept of Kobold turbine [3] and the present tidal stream turbine has the same diameter of 6.0m and rated stream velocity of 2.0m/sec. With the tilt angle of 30 degrees, the floating axis turbine has 4.3 times larger sweep area (front projection area) with high aspect ratio while the increase of displacement is only 14%. The power weight ratio (=output/displacement weight) of the plant is about four times higher than that of the comparative Kobold turbine. However, it should be noted that the output of Kobold turbine in the comparison is from an experimental power plant [6] while that of floating axis turbine is only by a conceptual study.

Floating Axis TurbineOutput:139kW at 2.0m/sDisplacement: 40ton

m0.6=φ

m0.5=H

Floating Vertical Axis TurbineOutput: 30kW at 2.0m/sDisplacement: 35ton

m0.6=φ

m0.25=H

(a) (b)

Figure 5. Comparison of device size; vertical axis turbine supported by a floater (a); floating axis turbine (b)

2.3. Multi-megawatt ocean current turbine Kuroshio Current is a strong ocean current in the western North Pacific Ocean. Since the current

passes near the dense populated Pacific Ocean side of Japanese main islands, utilization of its stable water stream energy is very attractive to the country. It is about 100km away from the electricity transmission network of Japan as shown in Fig. 6. The width of the current is about 100km and the maximum surface speed is 4 knot (2.06 m/sec). The peak of flow speed occurs in the depth less than 200m and reaches to 2.5m/sec [13].

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Figure 6. Ocean currents which surround Japan islands [13] (1. Kuroshio 2.Kuroshio-Zokuryu 3.Kuroshio-Hanryu 4.Tsushima-Danryu 5.Tsugaru-Danryu 6.Sohya-Danryu 7.Oyashio 8.Liman-Kanryu)

Figure 7 shows the image of ocean current turbine. For cancelling the reaction torque of electric

generators, supporting floats of counter rotating turbines are rigidly connected. It reduces the complexity and load of mooring system. The rated total output of twin turbine is 2MW (1MW each at 2.5m/sec current). The basic design of each turbine is almost the same as that of the tidal stream generator in the previous section. The assumed tilt angle is 30 degrees at the rated power. Table 2 shows the main particulars of twin turbine.

(a) (b) Figure 7. Ocean current turbine of 2MW rated power for Kuroshio Current; (a) front and side views; (c) perspective view

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Table 2. Particular items of 2MW ocean current turbine Rated power 2MW(1MW×2) Rated current speed [m/sec] 2.5 Turbine Number of turbines 2 Diameter [m] 12.0

Height [m] 40.1 Tilt angle at rated power [degree] 30.0 Effective sweep area [m2] 417 Blade chord length [m] 0.80

Revolution speed [rpm] 8.8 Thrust force [kN] 1500

Center column Extern diameter at mid point [m] 1.70

Thickness [mm] 45.0 Float part diameter [m] 4.00 Pendulum tension [ton] 245 Power coefficient 35% Total efficiency 30% Total displacement [ton] 2×310

Although there are many uncertainties in this start-up research, it is worthwhile to show a preliminary estimation of economic performance of the concept. We refer to the 5MW offshore floating horizontal axis wind turbine in the feasibility study of Fulton et al. [14] since we could not find reliable cost information of ocean current turbines. The HAWT is on a tension leg platform (TLP) proposed by National Renewable Energy Laboratory (Musial et al. [15]). Table 3 shows the comparison of component costs of three turbines. The first column is the 5MW HAWT [14] and the second is the hypothetical model of 2MW HAWT estimated from the 5MW model. The third column is the present ocean current turbine. In the reduction from 5MW to 2MW wind turbine, it is assumed that component costs are proportional to the rated power. Assumed distance of the site from onshore power grids is 100 km and the cost of electrical infrastructure is per turbine cost of a 100-unit wind farm.

The assumed cost of drive train in the ocean current turbine is the same as that of 2MW HAWT. Since the drive train is not subject to the limitation of weight and nacelle size, it does not require cutting edge technology. Also, it does not require high R&D costs of large capacity drive train because the load of thrust and electric generation can be distributed to multiple small rotor-generator units. For the consideration of uncertainty in the dynamics of tilted turbine, anchoring cost is assumed to be the same as that of 5MW floating wind turbine studied by Bulder et al. [16].

Although ocean current is more stable than wind, we do not have reliable performance records of their utilization. In the present evaluation, the authors assume that the capacity factor is 50%. It should be noted that the present estimation is arranged to show higher cost of energy of the plant and that the accuracy is less than first order.

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Table 3. Cost of 5MW wind turbine and 2MW ocean current turbine (cost is in 1000USD unit) Type Floating 5MW

Wind turbine* Floating 2MW Wind turbine

2MW Ocean current turbine

Rated power [kW] 5000 2000 2000 Sweep area dimension φ125m φ79m (12m×40m)×2 Distance from shore [km] 100 100 100 Rotor/Turbine cost (subtotal) 1070 428 952 Blades 691 276 276 Hub 235 94 - Pitch mechanism and bearings 144 58 -

Central column - - 626 Pendulum - - 49

Drive train, nacelle 2,111 844 844 Control, safety system 10 4 4 Tower 796 318 - Balance of station 8,933 3,573 1,524 Mobilization, plant and equipment 232 93 93 Permits, engineering 57 23 57 Gravity anchor structure 1,602 641 - Semi-submersible platform 1,783 713 - Tension legs, winches, porches 1,823 729 - Deployment 161 64 64 Electrical infrastructure** 3,275 1,310 1,310 Anchoring*** - - 840 Initial capital cost 12,920 5,168 4,164 Installed cost per power [USD/kW] 2.58 2.58 2.08 Levelized replacement cost 54 22 22 Operation & Maintenance at $20/kW/year 100 40 40 Net annual energy production [MWh] 18,965 7,586 8,760 Fixed charge rate 11.85% 11.85% 11.85% Cost of energy [USD/kWh] 0.087 0.087 0.063

* Data source [14], **assumed cost in a 100-units wind farm, ***[16]

The bottom line of Table 3 shows that the cost of energy in the 2MW twin turbine is 0.063USD/kWh. The performance is attributable to its reduced device size and the stable feature of ocean current (high utilization ratio of device). The power weight ratio of twin turbine is 3.2kW/ton while that of SeaGen is 2.58kW/ton (1.2MW rated power by 465ton weight [7]). However, it is not a direct comparison because SeaGen is a bottom fixed tidal current turbine and 79% mass of the present twin turbine is for pendulum weight which will be made of low-cost materials. There are many factors those are still under investigation and the authors have to admit that the level of research is before providing uncertainty analysis of the cost estimation. However, the preliminary estimation shows that the proposed concept has a potential in the utilization of ocean renewable energy.

3. Conclusion The authors proposed a concept of floating axis water stream turbine. In the concept, inclination of the turbine axis is passively determined in the balance of hydrodynamic loads, buoyancy and weight. The configuration is suitable for small low-cost hydro power. The concept is extendable to large applications of tidal stream turbine and ocean current turbine. The sample design of 2MW twin ocean current turbine showed that the concept has a good potential in economic performance. The main

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mechanism of converting turbine torque to electricity is the same as that of Floating Axis Wind Turbine proposed by the authors. The R&D of the device can be shared with our offshore wind applications.

There are many engineering problems those cannot be discussed in the limited pages. They include dynamics of turbine, fatigue strength, cost of biofouling protection and precise estimation of efficiency. They are under investigation and will be shown in the future work.

Acknowledgement This research was supported by WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R31-2008-000-10045-0).

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