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W2P: A high-power integrated generation unit for offshore wind power and ocean wave energy
Ocean Engineering 128 (2016) 41–47
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W2P: A high-power integrated generation unit for offshore wind power and ocean wave energy ⁎
Weixing Chena, , Feng Gaoa, Xiangdun Menga, Bin Chena, Anye Renb a b
State Key Laboratory of Mechanical System and Vibration, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China Institute of Aerospace System Engineering Shanghai, Shanghai, China
A R T I C L E I N F O
A BS T RAC T
Keywords: W2P High power Integrated generation Offshore wind power Ocean wave energy
Energy resources of offshore wind and ocean wave are abundant, clean and renewable. Various technologies have been developed to utilize the two kinds of energy separately. We present a high-power integrated generation unit for offshore wind power and ocean wave energy (W2P). The unit includes that: (1) The wind wheel with retractable blades and the 3-DOF (degrees of freedom) mechanism with the hemispherical oscillating body are used to collect the irregular wind and wave power, respectively; (2) The energy conversion devices (ECDs) are utilized to convert mechanical energy from both the wind wheel and the 3-DOF mechanism into hydraulic energy; (3) The hydraulic energy is used to drive the hydraulic motors and electrical generators to produce electricity. Some analyses and experiments have been conducted to obtain the performance of the key components of the unit. Based on the layout method, the single row wind-wave power plant is established.
1. Introduction Offshore wind power and ocean wave energy originate from the solar energy. Both of the two kinds of energy are abundant, widely distributed and renewable energy resources awaiting exploration (Zheng et al., 2012, 2014; Cornett, 2008; Wu et al., 2015). There are mainly three types of offshore floating wind turbine generator (WTG) classified by the floating platform including Spar floater (Skaare et al., 2007; Karimirad et al., 2009), Tension leg platform (Bir and Jonkman, 2007; Sclavounos et al., 2010) and Semi-submersible platform (Henderson et al., 2003; Carballo and Iglesias, 2013; Abanades et al., 2014). The conventional WTGs (the gear-driven train and the direct drive train) are used for offshore wind power utilization (Bilgili et al., 2011; Sun et al., 2012). However, the gear-driven train has the disadvantages of the failure of the gearbox and the high maintenance cost of the generator and the gearbox in the nacelle. The direct drive train has the expensive, high-torque and precisely controlled generators which are larger than ordinary ones, increasing the cost. So far, there have emerged some design concept of offshore WTG (Jones et al., 2012; Jones and Chao, 2011). The hydraulic WTG can remove the gearbox and install the generator on the ground, which can reduce the mass on the top of the tower and the cost of installations and maintenance. The ocean wave energy converters (WECs) are still in the research stage (Falnes, 2002; Scruggs and Jacob, 2009; Cruz, 2007). There are
⁎
Corresponding author. E-mail address: [email protected] (W. Chen).
http://dx.doi.org/10.1016/j.oceaneng.2016.10.017 Received 7 May 2016; Received in revised form 6 September 2016; Accepted 7 October 2016 0029-8018/ © 2016 Elsevier Ltd. All rights reserved.
three principles of WECs (Falcão, 2010), including the oscillating water column (Masuda, 1986; Brito-Melo et al., 2008; Setoguchi and Takao, 2006), the oscillating body system (Salter, 1974; Budar and Falnes, 1975; Weinstein et al., 2004; Ruellan et al., 2010; Henderson, 2006; AlHabaibeh et al., 2010) and the overtopping converter (Kofoed et al., 2006; Vicinanza and Frigaard, 2008). Among various technologies, the oscillating body systems have been investigated widely in the recent years, which almost have one DOF. The performance of various WECs have been numerically studied (Babarit et al., 2012), and the results indicate that the efficiency of numerous oscillating systems are not very high. The combined exploitation of offshore wind power and ocean wave energy is a very recent research topic (Pérez-Collazo et al., 2015). Currently, there are two types of combined wind-wave systems: colocated and hybrid, some of which emerge on the website. Co-located systems combine the wind farm and the wave array with independent foundation, such as Wave Star (2012), Wave Treader (Power-technology.com, 2010) and WEGA (Renewable Energy Focus, 2010). Hybrid systems are that offshore WTGs and WECs are installed on the same platform working as an unit, such as W2Power (Pelagic PowerAS, 2010) and Poseidon (Floating Power Plant AS, 2013). The modeling and testing of some combined systems are also performed (Peiffer et al., 2011; Peiffer and Roddier 2012; Muliawan et al., 2013). Besides these designs, some experts believe that the combined systems have great perspectives (Astariz et al., 2015a, 2015b; Astariz and Iglesias,
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W. Chen et al.
Nomenclature
Hs Te v D1 D2 θ ψ Φ β Pwave η
S (ω ) q N Pwind P0 ηt ηg
significant wave height wave energy period wind speed diameter of the wind wheel diameter of the hemisphere output angle of the heave motion of WEC output angle of the pitch motion of WEC output angle of the roll motion of WEC wave direction mechanical power absorbed of the WEC from waves efficiency of the ECD
q M T S P
irregular incident wave spectrum factor of WEC array number of WECs power absorbed by the wind wheel power output of one unit hydraulic transmission efficiency the efficiency of the hydraulic motor and the electrical generator average of q factors number of the units horizontal distance among units vertical distance among units power output of the plant
including several groups of accumulators, hydraulic motors and electrical generators. The wind wheel with retractable blades (Fig. 2b) collect the irregular wind power, which can adapt to a wide range of wind speeds and withstand extreme wind conditions because of using the retractable blades. The 3-DOF mechanism with the hemispherical oscillating body (Fig. 2d) is used to collect the irregular wave power efficiently, which can decouple the three motions (heave, roll and pitch) of the oscillating body. The ECDs (Fig. 2c and e) are utilized to convert mechanical energy from both the wind wheel and the mechanisms into the same hydraulic energy. Finally, the stable hydraulic energy stored in accumulators is used to drive the hydraulic motors and electrical generators to produce electricity (Fig. 2f). The W2P has some characteristics (2E3S):
2016). In this paper, the concrete design of a hybrid system W2P is presented, which is able to convert the two kinds of energy into electricity through the hydraulic energy transmission and the same ECDs. When the significant wave height Hs , the wave energy period Te and the wind speed v are equal to 4 m, 10 s and 13.5 m/s respectively, the unit and wind-wave power plant are presented. Based on the numerical model and experiment, the power output of the unit and wind-wave power plant is calculated to be 16.7 MW and 100 MW, respectively.
2. Principle and performance of the W2P The principle of the integrated generation is shown in Fig. 1. The wind wheels, the oscillating bodies and the absorption mechanisms collect and deliver wind power and wave energy, and convert them into mechanical energy. The energy converters transform mechanical energy into hydraulic energy which is stored in accumulators. The hydraulic energy is transformed into electrical energy through the hydraulic motors and electrical generators. Then the wind-wave power plant established by the principle can provide power for the residential electricity, the mining equipment, the hydrogen production, the sea water desalinization and so on. Based on the principle of the integrated generation, the high-power W2P is proposed (Fig. 2a). The W2P consists of one large floating platform, one WTG, three WECs and a set of generating equipment
(1) Enhanced power export. The unit can increase the energy yield per unit area of seas. The W2P can be grouped to realize great power output. (2) Efficient power yield. The wind wheel can adjust the diameter and the velocity according to the wind speeds to achieve the maximum power coefficient. The 3-DOF mechanism can extract mechanical energy from three motions of oscillating bodies. (3) Smoothed power output. The wave resource is more predictable and less variable than the wind resource. Some generators can be shut off to achieve the optimal rotational speed when the incoming wind power and wave energy decrease. The accumulators are used
Fig. 1. Schematic of the integrated generation.
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Fig. 2. Schematic of the high-power W2P. (a) The unit. (b) The variable radius wheel. (c) ECDs of the WTG. (d) 3-DOF mechanism with oscillating body of the WEC. (e) ECDs of the WEC. (f) Generating equipment.
to keep the oil pressure stable and store hydraulic energy. (4) Simplified installations and maintenances. The generators of the WTG are placed on the floating platform. The same ECDs are adopted in the energy conversion, which simplifies the system composition. The shared use of these installations and technicians will reduce the cost. (5) Strengthened reliability. In terrible weather conditions, the blades of the wind wheel can shrink to the minimum length to protect the WTG. Only the oscillating body of the WEC contacts with sea water, which can reduce the corrosion.
rotations are delivered to ψ and Φ relative to the platform. The mechanism can make the three motions of the oscillating body decoupled in the delivery process. β is the wave direction. The diameter of the hemisphere D2 is 60 m. Based on Cummins equation and Lagrange’s equation the dynamic model is established.
To achieve optimum tip-speed ratio, the diameter of the wind wheel shown in Fig. 2b can be adjusted from D1/2 to D1 according to the wind speeds by the retractable blades driven by the rhombus mechanisms and the velocity of the wind wheel is also changed. Each rhombus mechanism is driven by one hydraulic cylinder (Chen et al., 2016a). For the hemispherical body, the reciprocating motions in heave, roll and pitch caused by the excitation wave forces and the restoring forces facilitate the power absorption of WECs. Therefore, it is significant to design the 3-DOF WEC to improve the power absorption efficiency. The mechanical energy extracted by the oscillating body from the ocean wave energy is delivered by the 3-DOF mechanism shown in Fig. 3a. The mechanism mainly consists of the four-bar linkage and the 2-DOF spherical joint. The heave motion of the oscillating body is transmitted to the rotation feature θ relative to the floating platform by the four-bar linkage which has a similar straight line guide way in the point O. The pitch and roll motions of the oscillating body are captured by the 2-DOF spherical joint. The spatial RSSR linkage changes the rotational direction of the roll motion vertically. Then, the two parallel
The mean time-series power output of the WEC in the all directions can be written as:
(1)
M (χ ) χ ̈ +V (χ , χ ̇ ) +G (χ )=F (χ , χ ̇ )
where M (χ ) is the mass matrix, V (χ , χ ̇ ) is the damping matrix, G (χ ) is the stiffness matrix and F (χ , χ ̇ ) is the wave force (moments) matrix.
χ =[θψΦ ]T
(2)
Pwave=Pθ +Pϕ +Pψ t ∫0 Tθ θ ̇ dt
(3) t ∫0 Tϕ ϕ̇ dt
t ∫0 Tϕ ϕ̇ dt
, Pϕ = and Pψ = . Pθ , Pϕ and Pψ are the where Pθ = t t t mean power outputs of the three reciprocating motions, respectively. Tθ , Tϕ and Tψ are the constant torques acting on the WEC by ECDs. Then the numerical model is established in Simulink. Through numerous simulation, the power absorption atlas of the WEC in different sea states ( β=180°) is shown in Fig. 3b. The power extracted by the WEC Pwave is up to 4.2 MW in the sea state: Hs=4m, Te=10s which is used in previous works (Vicente et al., 2013), and the efficiency is over 80%. 3. Experiments of the ECD The mechanical energy of both the wind wheel and the 3-DOF
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Fig. 3. Schematic and performance of the 3-DOF WEC. (a) Schematic of the 3-DOF mechanism of the WEC. (b) The power absorption atlas of the WEC in different sea states ( β=180°).
4. Wind-wave power plant
mechanism can be converted into hydraulic energy by the ECD invented by us. The ECD is shown in Fig. 4a. There are 8 pinions assembled around the main gear. In the two sides of each pinion are assembled two sets of crank-rockers. The motion of the shaft is transmitted from the main gear to the pinions which drive the crankrockers to generate reciprocating motions of the rods. Whatever the rotation direction, the hydraulic cylinders conduct oil suction and discharge alternately to export high pressure oil, converting mechanical energy into hydraulic energy. The ECDs can be superposed to realize high-power conversion. The ECD is utilized for the conversion of both wind power and wave energy. Consequently, the efficiency of the ECD is significant for the W2P. The experimental scheme by using the power recovery method is designed to test the efficiency of the ECD. And the test platform is built, which is shown in Fig. 4b. Through the reduction gear box the output of the hydraulic motor is transmitted to the ECD, which produce high pressure oil cycling to the experimental hydraulic motor. The power loss of the circulation is supplemented by the high pressure oil from the main pump outside. The power loss of the reduction gear box can be calculated using the data of the torque and speed sensors. The flywheel increases the rotation inertia to weaken the fluctuation of speed. In the stable configuration, the power loss of the ECD can be calculated by the difference between the circulation power loss which equals to the power supplement of the main pump and the sum of the power loss of the reduction gear box and the hydraulic motor. The method has the advantage in avoiding the energy consumption of the simulated load. The efficiency of the ECD under various pressures and speeds shown in Fig. 4c demonstrates that the efficiency η is over 80%.
When the power plant is established, the interactions among WECs will emerge. Due to the interactions among the WECs, each WEC has 3DOF even under the one-directional wave. The frequency-domain dynamic model of the jth direction of the mth WEC is expressed as (Chen et al., 2016b): N ⎞ ⎛ N ⎞ ⎛ 3 3 −ω 2 ⎜⎜Mmj + ∑ ∑s =1 μmns (ω) ⎟⎟ ηmj (ω) + iω ⎜⎜ ∑ ∑s =1 Bmns (ω)+cmj ⎟⎟ ηmj (ω) ⎠ ⎝ n =1 ⎠ ⎝ n =1
+ k mj ηmj (ω) = Fmj (ω)
(4)
where N is the total number of WECs. ω is the wave frequency. Mmj is the mass or moment of inertia of the mth WEC in the jth direction. μmns (ω) is the hydrodynamic added mass of the mth WEC in the jth direction induced by the sth motion of the nth WEC. Bmns (ω) is the hydrodynamic damping of the mth WEC in the jth direction induced by the sth motion of the nth WEC. The power take-off system of the mth WEC in the jth direction can be simplified as the linear damping term cmj . ηmj (ω) is the motion amplitude of the mth WEC in the jth direction. k mj is the hydrostatic stiffness coefficient of the mth WEC in the jth direction. Fmj (ω) is the amplitude of the wave excitation force acting on the mth WEC in the jth direction. Then the total power absorption of the OWEC array in regular incident waves can be expressed as: N
p (ω)=
3
∑∑ m =1 j =1
44
1 2 cmj ω 2ηmj (ω ) 2
(5)
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Fig. 4. Schematic, efficiency test platform and the efficiency results of the ECD. (a) Schematic of the ECD. (b) The efficiency test platform of the ECD. (c) The efficiency of the ECD under various pressures and speeds.
respectively. When the large-scale wind-wave power plant needs to be established, the layout of the W2Ps should be investigated. Because the destructive effect seems to be increasingly significant with increasing number of rows (the lines of WECs perpendicular to the incident wave direction) (Babarit, 2013). The layout method of the single row plant is researched. Provided that the distance among units is equal and that one unit is influenced only by the neighbor ones. The units are endowed with three topological positions A, B and C (Fig. 5a). Type A has only one neighbor, type B has two central symmetric neighbors, and type C has two axisymmetric neighbors. The q factor is related to the topological type and the distance among units. The dynamic response of the WEC array can also be calculated by AQWA. If the neighbor unit in the left is set as the coordinate origin, the q of each topological type at different positions can be obtained (Fig. 5b). Higher value of q means better performance of the unit. When the horizontal distance and the vertical distance among the units are 8D2 and 2D2 respectively, the q factors of the three topological types are high. The average value q of the factors for all the units describes the performance of various layouts of single row power plant. For the power plant with the five units, the q factors of the five layouts are analyzed (Fig. 5c) based on the data from Fig. 5b. The type fifth layout is with the best performance (q =1.477). Then the wind-wave power plant is shown in Fig. 5d. The diameters of the oscillating bodies and the wind wheels are 60 m and 126 m respectively. The dimension of the triangle platform side is 200 m. The horizontal distance T and the vertical distance S among the units are 480 m and 120 m respectively. The generated power of the W2P layout can be:
The total power absorption in the irregular incident wave spec⎛ −1054 ⎞ 263H2 trum:S (ω)= 4 5s exp ⎜ 4 4 ⎟of the WEC array can be written as: ⎝ Te ω ⎠ Te ω
PN =
∫0
∞
2S (ω) p (ω) dω
(6)
Theq factor can be written as:
q=
PN NP0
(7)
where P0 is the power output of a single isolated WEC. If q > 1, the wave interactions have constructive effects on the power outputs of the array. Reversely, if q < 1, the effects are destructive. The W2P is composed of one WTG and three WECs. The three WECs are placed in a triangular array to diminish the negative influence of WECs (Borgarino et al., 2012) and to facilitate the absorption of ocean wave from multiple directions (de Andrés et al., 2014). The frequency domain dynamic of the WEC in array can be obtained by the software AQWA. Based on Eq. (5)–(7), the distances among WECs are determined to be 5D2 which means the five times of the diameter of the oscillating body, and the q factor is equal to 1.336 (Hs=4m, Te=10s , β=180°). When the rated wind and sea conditions are determined (Hs=4m, Te=10s, β =180°, v = 13.5m / s ) and the optimum wind power coefficient is about 0.436 (Zhang et al., 2004), the Pwind for the wind power absorption by the wheel reaches 8.6 MW (D1=126m ). Therefore, the generated power of the independent W2P can be obtained:
P0=(Pwind +3Pwave q ) ηηt ηg=16. 7MW
(8)
where ηt =0.95 and ηg=0.96 are the hydraulic transmission efficiency and the efficiency of the hydraulic motor and the electrical generator,
P=(Pwind M +3Pwave qM ) ηηt ηg=100MW 45
(9)
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Fig. 5. The layout design of units. (a) Three topological types of the unit. (b) The q factors of the three topological types. (c) Five layouts of a single row power plant consisting of five units. (d) 100 MW wind-wave power plant.
the cost of energy. However, the cost is related to the prospect of the unit. Future research about the cost of the integrated generation unit and the wind-wave farm will be conducted.
where M is the number of the units. By using the layout method, the high-power wind-wave power plant based on the W2P can be designed, for example, the 1000 MW power plant with the 50 units.
Acknowledgement 5. Conclusions This research was supported by National Natural Science Foundation of China (Grant no. 51335007).
In this paper, the principle of the integrated generation for offshore wind power and ocean wave energy is proposed, which are converted both through hydraulic energy. Based on the principle, the integrated unit consisting of one WTG and three WECs is invented. The wind wheel with retractable blades can obtain optimum wind power coefficient through adjusting the diameter and velocity of the wheel. The 3-DOF mechanism with the hemispherical oscillating body can collect the wave power efficiently. The mechanical energy collected from the wind power and the wave energy can be converted by the same ECDs into hydraulic energy which drives the hydraulic motors and electrical generators to produce electricity. The performance of the WEC is obtained and the efficiency of the ECD is tested. The array of the unit is determined. The power output of the unit is calculated to be 16.7 MW. Based on the layout method, the single row wind-wave power plant is established. The rated power of the plant is up to be 100 MW. Here, the integrated generation unit is designed, not considering
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W2P: A high-power integrated generation unit for offshore wind power and ocean wave energy ⁎
Weixing Chena, , Feng Gaoa, Xiangdun Menga, Bin Chena, Anye Renb a b
State Key Laboratory of Mechanical System and Vibration, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China Institute of Aerospace System Engineering Shanghai, Shanghai, China
A R T I C L E I N F O
A BS T RAC T
Keywords: W2P High power Integrated generation Offshore wind power Ocean wave energy
Energy resources of offshore wind and ocean wave are abundant, clean and renewable. Various technologies have been developed to utilize the two kinds of energy separately. We present a high-power integrated generation unit for offshore wind power and ocean wave energy (W2P). The unit includes that: (1) The wind wheel with retractable blades and the 3-DOF (degrees of freedom) mechanism with the hemispherical oscillating body are used to collect the irregular wind and wave power, respectively; (2) The energy conversion devices (ECDs) are utilized to convert mechanical energy from both the wind wheel and the 3-DOF mechanism into hydraulic energy; (3) The hydraulic energy is used to drive the hydraulic motors and electrical generators to produce electricity. Some analyses and experiments have been conducted to obtain the performance of the key components of the unit. Based on the layout method, the single row wind-wave power plant is established.
1. Introduction Offshore wind power and ocean wave energy originate from the solar energy. Both of the two kinds of energy are abundant, widely distributed and renewable energy resources awaiting exploration (Zheng et al., 2012, 2014; Cornett, 2008; Wu et al., 2015). There are mainly three types of offshore floating wind turbine generator (WTG) classified by the floating platform including Spar floater (Skaare et al., 2007; Karimirad et al., 2009), Tension leg platform (Bir and Jonkman, 2007; Sclavounos et al., 2010) and Semi-submersible platform (Henderson et al., 2003; Carballo and Iglesias, 2013; Abanades et al., 2014). The conventional WTGs (the gear-driven train and the direct drive train) are used for offshore wind power utilization (Bilgili et al., 2011; Sun et al., 2012). However, the gear-driven train has the disadvantages of the failure of the gearbox and the high maintenance cost of the generator and the gearbox in the nacelle. The direct drive train has the expensive, high-torque and precisely controlled generators which are larger than ordinary ones, increasing the cost. So far, there have emerged some design concept of offshore WTG (Jones et al., 2012; Jones and Chao, 2011). The hydraulic WTG can remove the gearbox and install the generator on the ground, which can reduce the mass on the top of the tower and the cost of installations and maintenance. The ocean wave energy converters (WECs) are still in the research stage (Falnes, 2002; Scruggs and Jacob, 2009; Cruz, 2007). There are
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Corresponding author. E-mail address: [email protected] (W. Chen).
http://dx.doi.org/10.1016/j.oceaneng.2016.10.017 Received 7 May 2016; Received in revised form 6 September 2016; Accepted 7 October 2016 0029-8018/ © 2016 Elsevier Ltd. All rights reserved.
three principles of WECs (Falcão, 2010), including the oscillating water column (Masuda, 1986; Brito-Melo et al., 2008; Setoguchi and Takao, 2006), the oscillating body system (Salter, 1974; Budar and Falnes, 1975; Weinstein et al., 2004; Ruellan et al., 2010; Henderson, 2006; AlHabaibeh et al., 2010) and the overtopping converter (Kofoed et al., 2006; Vicinanza and Frigaard, 2008). Among various technologies, the oscillating body systems have been investigated widely in the recent years, which almost have one DOF. The performance of various WECs have been numerically studied (Babarit et al., 2012), and the results indicate that the efficiency of numerous oscillating systems are not very high. The combined exploitation of offshore wind power and ocean wave energy is a very recent research topic (Pérez-Collazo et al., 2015). Currently, there are two types of combined wind-wave systems: colocated and hybrid, some of which emerge on the website. Co-located systems combine the wind farm and the wave array with independent foundation, such as Wave Star (2012), Wave Treader (Power-technology.com, 2010) and WEGA (Renewable Energy Focus, 2010). Hybrid systems are that offshore WTGs and WECs are installed on the same platform working as an unit, such as W2Power (Pelagic PowerAS, 2010) and Poseidon (Floating Power Plant AS, 2013). The modeling and testing of some combined systems are also performed (Peiffer et al., 2011; Peiffer and Roddier 2012; Muliawan et al., 2013). Besides these designs, some experts believe that the combined systems have great perspectives (Astariz et al., 2015a, 2015b; Astariz and Iglesias,
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Nomenclature
Hs Te v D1 D2 θ ψ Φ β Pwave η
S (ω ) q N Pwind P0 ηt ηg
significant wave height wave energy period wind speed diameter of the wind wheel diameter of the hemisphere output angle of the heave motion of WEC output angle of the pitch motion of WEC output angle of the roll motion of WEC wave direction mechanical power absorbed of the WEC from waves efficiency of the ECD
q M T S P
irregular incident wave spectrum factor of WEC array number of WECs power absorbed by the wind wheel power output of one unit hydraulic transmission efficiency the efficiency of the hydraulic motor and the electrical generator average of q factors number of the units horizontal distance among units vertical distance among units power output of the plant
including several groups of accumulators, hydraulic motors and electrical generators. The wind wheel with retractable blades (Fig. 2b) collect the irregular wind power, which can adapt to a wide range of wind speeds and withstand extreme wind conditions because of using the retractable blades. The 3-DOF mechanism with the hemispherical oscillating body (Fig. 2d) is used to collect the irregular wave power efficiently, which can decouple the three motions (heave, roll and pitch) of the oscillating body. The ECDs (Fig. 2c and e) are utilized to convert mechanical energy from both the wind wheel and the mechanisms into the same hydraulic energy. Finally, the stable hydraulic energy stored in accumulators is used to drive the hydraulic motors and electrical generators to produce electricity (Fig. 2f). The W2P has some characteristics (2E3S):
2016). In this paper, the concrete design of a hybrid system W2P is presented, which is able to convert the two kinds of energy into electricity through the hydraulic energy transmission and the same ECDs. When the significant wave height Hs , the wave energy period Te and the wind speed v are equal to 4 m, 10 s and 13.5 m/s respectively, the unit and wind-wave power plant are presented. Based on the numerical model and experiment, the power output of the unit and wind-wave power plant is calculated to be 16.7 MW and 100 MW, respectively.
2. Principle and performance of the W2P The principle of the integrated generation is shown in Fig. 1. The wind wheels, the oscillating bodies and the absorption mechanisms collect and deliver wind power and wave energy, and convert them into mechanical energy. The energy converters transform mechanical energy into hydraulic energy which is stored in accumulators. The hydraulic energy is transformed into electrical energy through the hydraulic motors and electrical generators. Then the wind-wave power plant established by the principle can provide power for the residential electricity, the mining equipment, the hydrogen production, the sea water desalinization and so on. Based on the principle of the integrated generation, the high-power W2P is proposed (Fig. 2a). The W2P consists of one large floating platform, one WTG, three WECs and a set of generating equipment
(1) Enhanced power export. The unit can increase the energy yield per unit area of seas. The W2P can be grouped to realize great power output. (2) Efficient power yield. The wind wheel can adjust the diameter and the velocity according to the wind speeds to achieve the maximum power coefficient. The 3-DOF mechanism can extract mechanical energy from three motions of oscillating bodies. (3) Smoothed power output. The wave resource is more predictable and less variable than the wind resource. Some generators can be shut off to achieve the optimal rotational speed when the incoming wind power and wave energy decrease. The accumulators are used
Fig. 1. Schematic of the integrated generation.
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Fig. 2. Schematic of the high-power W2P. (a) The unit. (b) The variable radius wheel. (c) ECDs of the WTG. (d) 3-DOF mechanism with oscillating body of the WEC. (e) ECDs of the WEC. (f) Generating equipment.
to keep the oil pressure stable and store hydraulic energy. (4) Simplified installations and maintenances. The generators of the WTG are placed on the floating platform. The same ECDs are adopted in the energy conversion, which simplifies the system composition. The shared use of these installations and technicians will reduce the cost. (5) Strengthened reliability. In terrible weather conditions, the blades of the wind wheel can shrink to the minimum length to protect the WTG. Only the oscillating body of the WEC contacts with sea water, which can reduce the corrosion.
rotations are delivered to ψ and Φ relative to the platform. The mechanism can make the three motions of the oscillating body decoupled in the delivery process. β is the wave direction. The diameter of the hemisphere D2 is 60 m. Based on Cummins equation and Lagrange’s equation the dynamic model is established.
To achieve optimum tip-speed ratio, the diameter of the wind wheel shown in Fig. 2b can be adjusted from D1/2 to D1 according to the wind speeds by the retractable blades driven by the rhombus mechanisms and the velocity of the wind wheel is also changed. Each rhombus mechanism is driven by one hydraulic cylinder (Chen et al., 2016a). For the hemispherical body, the reciprocating motions in heave, roll and pitch caused by the excitation wave forces and the restoring forces facilitate the power absorption of WECs. Therefore, it is significant to design the 3-DOF WEC to improve the power absorption efficiency. The mechanical energy extracted by the oscillating body from the ocean wave energy is delivered by the 3-DOF mechanism shown in Fig. 3a. The mechanism mainly consists of the four-bar linkage and the 2-DOF spherical joint. The heave motion of the oscillating body is transmitted to the rotation feature θ relative to the floating platform by the four-bar linkage which has a similar straight line guide way in the point O. The pitch and roll motions of the oscillating body are captured by the 2-DOF spherical joint. The spatial RSSR linkage changes the rotational direction of the roll motion vertically. Then, the two parallel
The mean time-series power output of the WEC in the all directions can be written as:
(1)
M (χ ) χ ̈ +V (χ , χ ̇ ) +G (χ )=F (χ , χ ̇ )
where M (χ ) is the mass matrix, V (χ , χ ̇ ) is the damping matrix, G (χ ) is the stiffness matrix and F (χ , χ ̇ ) is the wave force (moments) matrix.
χ =[θψΦ ]T
(2)
Pwave=Pθ +Pϕ +Pψ t ∫0 Tθ θ ̇ dt
(3) t ∫0 Tϕ ϕ̇ dt
t ∫0 Tϕ ϕ̇ dt
, Pϕ = and Pψ = . Pθ , Pϕ and Pψ are the where Pθ = t t t mean power outputs of the three reciprocating motions, respectively. Tθ , Tϕ and Tψ are the constant torques acting on the WEC by ECDs. Then the numerical model is established in Simulink. Through numerous simulation, the power absorption atlas of the WEC in different sea states ( β=180°) is shown in Fig. 3b. The power extracted by the WEC Pwave is up to 4.2 MW in the sea state: Hs=4m, Te=10s which is used in previous works (Vicente et al., 2013), and the efficiency is over 80%. 3. Experiments of the ECD The mechanical energy of both the wind wheel and the 3-DOF
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Fig. 3. Schematic and performance of the 3-DOF WEC. (a) Schematic of the 3-DOF mechanism of the WEC. (b) The power absorption atlas of the WEC in different sea states ( β=180°).
4. Wind-wave power plant
mechanism can be converted into hydraulic energy by the ECD invented by us. The ECD is shown in Fig. 4a. There are 8 pinions assembled around the main gear. In the two sides of each pinion are assembled two sets of crank-rockers. The motion of the shaft is transmitted from the main gear to the pinions which drive the crankrockers to generate reciprocating motions of the rods. Whatever the rotation direction, the hydraulic cylinders conduct oil suction and discharge alternately to export high pressure oil, converting mechanical energy into hydraulic energy. The ECDs can be superposed to realize high-power conversion. The ECD is utilized for the conversion of both wind power and wave energy. Consequently, the efficiency of the ECD is significant for the W2P. The experimental scheme by using the power recovery method is designed to test the efficiency of the ECD. And the test platform is built, which is shown in Fig. 4b. Through the reduction gear box the output of the hydraulic motor is transmitted to the ECD, which produce high pressure oil cycling to the experimental hydraulic motor. The power loss of the circulation is supplemented by the high pressure oil from the main pump outside. The power loss of the reduction gear box can be calculated using the data of the torque and speed sensors. The flywheel increases the rotation inertia to weaken the fluctuation of speed. In the stable configuration, the power loss of the ECD can be calculated by the difference between the circulation power loss which equals to the power supplement of the main pump and the sum of the power loss of the reduction gear box and the hydraulic motor. The method has the advantage in avoiding the energy consumption of the simulated load. The efficiency of the ECD under various pressures and speeds shown in Fig. 4c demonstrates that the efficiency η is over 80%.
When the power plant is established, the interactions among WECs will emerge. Due to the interactions among the WECs, each WEC has 3DOF even under the one-directional wave. The frequency-domain dynamic model of the jth direction of the mth WEC is expressed as (Chen et al., 2016b): N ⎞ ⎛ N ⎞ ⎛ 3 3 −ω 2 ⎜⎜Mmj + ∑ ∑s =1 μmns (ω) ⎟⎟ ηmj (ω) + iω ⎜⎜ ∑ ∑s =1 Bmns (ω)+cmj ⎟⎟ ηmj (ω) ⎠ ⎝ n =1 ⎠ ⎝ n =1
+ k mj ηmj (ω) = Fmj (ω)
(4)
where N is the total number of WECs. ω is the wave frequency. Mmj is the mass or moment of inertia of the mth WEC in the jth direction. μmns (ω) is the hydrodynamic added mass of the mth WEC in the jth direction induced by the sth motion of the nth WEC. Bmns (ω) is the hydrodynamic damping of the mth WEC in the jth direction induced by the sth motion of the nth WEC. The power take-off system of the mth WEC in the jth direction can be simplified as the linear damping term cmj . ηmj (ω) is the motion amplitude of the mth WEC in the jth direction. k mj is the hydrostatic stiffness coefficient of the mth WEC in the jth direction. Fmj (ω) is the amplitude of the wave excitation force acting on the mth WEC in the jth direction. Then the total power absorption of the OWEC array in regular incident waves can be expressed as: N
p (ω)=
3
∑∑ m =1 j =1
44
1 2 cmj ω 2ηmj (ω ) 2
(5)
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Fig. 4. Schematic, efficiency test platform and the efficiency results of the ECD. (a) Schematic of the ECD. (b) The efficiency test platform of the ECD. (c) The efficiency of the ECD under various pressures and speeds.
respectively. When the large-scale wind-wave power plant needs to be established, the layout of the W2Ps should be investigated. Because the destructive effect seems to be increasingly significant with increasing number of rows (the lines of WECs perpendicular to the incident wave direction) (Babarit, 2013). The layout method of the single row plant is researched. Provided that the distance among units is equal and that one unit is influenced only by the neighbor ones. The units are endowed with three topological positions A, B and C (Fig. 5a). Type A has only one neighbor, type B has two central symmetric neighbors, and type C has two axisymmetric neighbors. The q factor is related to the topological type and the distance among units. The dynamic response of the WEC array can also be calculated by AQWA. If the neighbor unit in the left is set as the coordinate origin, the q of each topological type at different positions can be obtained (Fig. 5b). Higher value of q means better performance of the unit. When the horizontal distance and the vertical distance among the units are 8D2 and 2D2 respectively, the q factors of the three topological types are high. The average value q of the factors for all the units describes the performance of various layouts of single row power plant. For the power plant with the five units, the q factors of the five layouts are analyzed (Fig. 5c) based on the data from Fig. 5b. The type fifth layout is with the best performance (q =1.477). Then the wind-wave power plant is shown in Fig. 5d. The diameters of the oscillating bodies and the wind wheels are 60 m and 126 m respectively. The dimension of the triangle platform side is 200 m. The horizontal distance T and the vertical distance S among the units are 480 m and 120 m respectively. The generated power of the W2P layout can be:
The total power absorption in the irregular incident wave spec⎛ −1054 ⎞ 263H2 trum:S (ω)= 4 5s exp ⎜ 4 4 ⎟of the WEC array can be written as: ⎝ Te ω ⎠ Te ω
PN =
∫0
∞
2S (ω) p (ω) dω
(6)
Theq factor can be written as:
q=
PN NP0
(7)
where P0 is the power output of a single isolated WEC. If q > 1, the wave interactions have constructive effects on the power outputs of the array. Reversely, if q < 1, the effects are destructive. The W2P is composed of one WTG and three WECs. The three WECs are placed in a triangular array to diminish the negative influence of WECs (Borgarino et al., 2012) and to facilitate the absorption of ocean wave from multiple directions (de Andrés et al., 2014). The frequency domain dynamic of the WEC in array can be obtained by the software AQWA. Based on Eq. (5)–(7), the distances among WECs are determined to be 5D2 which means the five times of the diameter of the oscillating body, and the q factor is equal to 1.336 (Hs=4m, Te=10s , β=180°). When the rated wind and sea conditions are determined (Hs=4m, Te=10s, β =180°, v = 13.5m / s ) and the optimum wind power coefficient is about 0.436 (Zhang et al., 2004), the Pwind for the wind power absorption by the wheel reaches 8.6 MW (D1=126m ). Therefore, the generated power of the independent W2P can be obtained:
P0=(Pwind +3Pwave q ) ηηt ηg=16. 7MW
(8)
where ηt =0.95 and ηg=0.96 are the hydraulic transmission efficiency and the efficiency of the hydraulic motor and the electrical generator,
P=(Pwind M +3Pwave qM ) ηηt ηg=100MW 45
(9)
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Fig. 5. The layout design of units. (a) Three topological types of the unit. (b) The q factors of the three topological types. (c) Five layouts of a single row power plant consisting of five units. (d) 100 MW wind-wave power plant.
the cost of energy. However, the cost is related to the prospect of the unit. Future research about the cost of the integrated generation unit and the wind-wave farm will be conducted.
where M is the number of the units. By using the layout method, the high-power wind-wave power plant based on the W2P can be designed, for example, the 1000 MW power plant with the 50 units.
Acknowledgement 5. Conclusions This research was supported by National Natural Science Foundation of China (Grant no. 51335007).
In this paper, the principle of the integrated generation for offshore wind power and ocean wave energy is proposed, which are converted both through hydraulic energy. Based on the principle, the integrated unit consisting of one WTG and three WECs is invented. The wind wheel with retractable blades can obtain optimum wind power coefficient through adjusting the diameter and velocity of the wheel. The 3-DOF mechanism with the hemispherical oscillating body can collect the wave power efficiently. The mechanical energy collected from the wind power and the wave energy can be converted by the same ECDs into hydraulic energy which drives the hydraulic motors and electrical generators to produce electricity. The performance of the WEC is obtained and the efficiency of the ECD is tested. The array of the unit is determined. The power output of the unit is calculated to be 16.7 MW. Based on the layout method, the single row wind-wave power plant is established. The rated power of the plant is up to be 100 MW. Here, the integrated generation unit is designed, not considering
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