Lü Qin,LI De-tang,LI Da-te,TANG Wen-tao,CAO Wei-nan,JIN Huo-ran,HU Xing-chen

(Ship and Marine Engineering College of Zhejiang Ocean University,Zhoushan 316000,China)

0 Introduction

As the fossil energy is decreasing gradually,energy prices are rising[1-2].At the same time,environmental pollution which is caused by using fossil energy has a great influence on people’s normal life.In order to solve the bottleneck problem of energy supply in social development,finding alternative,renewable and clean energy has become the consensus of various countries around the world[3-5].Thus,the research for ocean wave energy ushered in an unprecedented climax.

However,ocean wave power generation devices are most far from land,which results in a number of high precision but bulky test equipment can not play its due role in research.In recent years,some researchers used some advanced testing techniques to improve the accuracy of the test system and simplify the system,and they have made some achievements.Meanwhile,using LabVIEW plays a crucial role[6-8].

In this paper,a new energy harvesting efficiency of buoy test system based on‘Haiyuan 1’ wave power generating platform and Virtual Instrument Software LabVIEW is developed.This test system can achieve the functions of data acquisition,data processing and display,generating report,analyzing signal,and so on,which has reference value for engineering application[9].

1 The‘Haiyuan 1’wave power generating platform and the buoys

‘Haiyuan 1’is a jack-up absorption wave power generating platform designed and developed by School of Ship and Marine Engineering College of Zhejiang Ocean University(As Fig.1 shows).The wave power generating platform mainly consists of a support platform,three groups of wave energy collection module,hydraulic systems and power output system.The main structure is shown in Fig.2,buoys are the pile-oriented,the top are equipped with two wave plates and each wave plate cooperates through a group of rolling cylinders.The wave-powered buoy we selected is a rotating body(Buoy is shown in Fig.3).Buoy can be simply divided into two parts,the upper is a cylinder with the diameter of 3 200 mm and the height of 625 mm;the lower part is a curved surface similar to hemisphere,and the height is 835 mm.

Fig.1 ‘Haiyuan 1’wave power generating platform

Fig.2 The overall structure of‘Haiyuan 1’wave power generating platform

When the‘Haiyuan 1’wave power generating platform works in real sea conditions,waves push the buoys to do the heave movement along the piles.The wave plates also do the same movement because they are connected to the buoys,which finally propels the electric generator outputting electricity.The hydraulic system uses bidirectional hydraulic transmission form,which enables the buoys to drive the hydraulic system continuously by moving up or down.Thus,electricity can be continuously taken-off.Besides,hydraulic energy accumulators are stalled in the hydraulic system,which can store hydraulic energy and stabilize the fluctuate energy.

Fig.3 The buoy of‘Haiyuan 1’wave power generating platform

2 Calculation method of energy harvesting efficiency

2.1 Calculation of the wave energy

In Real Sea Conditions,the wave energy[10-11]can be calculated as:

where ρ means seawater density,g means gravitational acceleration,H1/3means significant wave height,cgmeans wave velocity,Aωmeans the waterplane area of buoy,T means wave cycle.

Because of the randomness and irregular of waves in real sea conditions,this paper will use the method of Cross-zero to calculate T and H1/3,as is shown in Fig.4.

Fig.4 The method of Cross-zero

2.2 Calculation of the delivered power of buoy

The delivered power[12]of wave power generating platform buoy of‘Haiyuan 1’can be calculated as:

where t means the motion time of buoy,FPTOmeans hydraulic damping force,it can be calculated from Eq.(3).L can be calculated as the distance of hydraulic piston movement in the time of t,which is equal to the movement distance of the buoy.As the hydraulic system of‘Haiyuan 1’wave power generating platform is bi-directional piston system,which means that the upand-down motion of buoys is effective,and it can be calculated from the buoy motion curve below.

where k means system pressure,A refers to piston area,and the design value is 100 cm2.

2.3 Calculation of energy harvesting efficiency of buoy

The energy harvesting efficiency of buoy is the ratio of delivered power and wave energy.It is also called the first conversion efficiency and can be calculated as:

3 Design of the test system

3.1 Hardware design of the test system

The test signal was transformed to current signal by sensor,which is the function of the hardware units in the test system,and the current signal was recuperated by signal conditioning circuit,then it was converted into digital signal that could be recognizable by computer through NI data acquisition card[13].The YF-YJ50 Laser displacement sensor,WPT50-05-0C Pressure sensor,CT15-BB6 Flow sensor and Temperature sensor were used to gather data and the DAQBook-1600A-CAM multi-channel data acquisition instrument was applied to transform the current signal to digital signal in this test system.Parts of the equipments is shown in Fig.5.

Fig.5 Hardware equipments

The test wave data is obtained by measuring the movement of buoy which is placed on the sea.During the experiment,we used two YF-YJ50 laser displacement sensors to record the motion trail of wave and buoy and get the motion curve at the same time.

3.2 Software design of the test system

Fig.6 Test system

Fig.7 Test system

Fig.8 Test system

Fig.9 Calculation system

The test system is designed using Labview 2011,it can run on the mainstream operating system like Windows 7 and Windows XP,it has the characteristics that multiple processes are able to run simultaneously and multiple pages show on the same window.The test system includes wave testing system,buoy test system,hydraulic test system and the energy efficiency calculation system,the interface is shown in Fig.6.The system has a good human-machine interface which makes the operation convenient.Fig.7 shows the wave test system and the buoy test system on which the curve of the wave amplitude and vertical displacement of buoy over time is displayed real time.Relative data of energy efficiency can be acquired from the curve.Fig.8 shows the hydraulic test system which monitors the flow,pressure and the temperature of the piston in the system.Fig.9 shows the energy harvesting efficiency calculation system from which the efficiency can be acquired with the input of relative parameters.Fig.10 shows the controller of test system,with different button controls different system.

Not only the test data is monitored at any time,but also the test data and diagrams are operated conveniently such as saving and playback and so on in this test system,which is contributed to deal with the test data.The whole experimental operational process was simplified just only from the front panel of the operating procedure to the statement generated finally in this test system.

Fig.10 The controller of testing system

4 Experimental analysis of the test system

4.1 System pressure correction

The hydraulic system pressure was the important calculation basis of the delivered power of the buoy.The system pressure of the wave power generating platform of‘Haiyuan 1’was set artificially,which means that the platform can run at any system pressure of any prescribed scope.However,due to the multiform energy consumption,it leads to the decrease of the system pressure,compared to the predetermined value,at hydraulic piston position.The system pressure at piston position is supervised in time,so it made the energy harvesting efficiency more correct by the data acquisition system in this passage.The system pressure correction was shown as Tab.1.

Tab.1 System pressure correction

4.2 Analysis of energy harvesting efficiency

Fig.11 The relationship between system pressure and energy harvesting efficiency of the buoy in No.1 sea condition

Fig12 The relationship between system pressure and energy harvesting efficiency of the buoy in No.2 sea condition

In this experiment,we compare and analyze energy harvesting efficiency of the buoy in three different sea conditions.Their water depth are 2.0 m,2.5 m and 2.8 m,respectively.

Figs.11-13 show the relationship between system pressure and energy harvesting efficiency of the buoy in different real sea conditions.It is clearly shown in figures that energy harvesting efficiency increases with the system pressure rising when system pressure is lower than 2.0 MPa or so.When system pressure is more than 2.0 MPa,the energy harvesting efficiency of the buoy decreases with the system pressure rising.In three real sea conditions,the maximum energy harvesting efficiency of the buoy were 49.16%,42.5%and 45.65%,respectively.

Fig.13 The relationship between system pressure and energy harvesting efficiency of the buoy in No.3 sea condition

In the premise that the delivered power of the buoy ensured a high value,system pressure should be as small as possible.On one hand,if system pressure is too large,the buoy might hardly be driven when income wave is small.In this situation,the wave energy could not be absorbed.On the other hand,large designed system pressure would reduce the reliability of the hydraulic system and increase the cost and the building difficulty of the hydraulic system.Figures show that 2.0 MPa is considered to be a suitable system pressure because all curves reach a peak point near this value.

5 Conclusion

In this paper,a new energy harvesting efficiency of buoy test system based on‘Haiyuan 1’wave power generating platform and LabVIEW is designed.When we tested‘Haiyuan 1’wave power generating platform in Zhujiajian Dongsha area,Zhoushan in real sea conditions,we find this test system is stable and easy operation.We can obtain effective results and data from it.The transformation of the model test system and optimization of test system is achieved in this paper.A more intelligent hydraulic machine test system is of great significance to the design of buoy of wave power generating platform and the increase of the energy harvesting efficiency.A foundation is laid for the automatic test technology of the wave energy power generation;the basis in theory and in practice is provided and it has an important engineering significance.

The authors would like to thank the National Ocean Bureau of China special funds for renewable energy project under Contract No.ZJME2011BL04.This work was also supported by the Research Foundation of the State Key Ocean Engineering Laboratory of Shanghai Jiao Tong University supporting this research under Contract No.1205.

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