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Advances in Mechanical Engineering
Volume 2013 (2013), Article ID 340501, 9 pages
http://dx.doi.org/10.1155/2013/340501
Research Article

New Concept for Assessment of Tidal Current Energy in Jiangsu Coast, China

1State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, China
2College of Harbor, Coastal and Offshore Engineering, Hohai University, Nanjing 210098, China

Received 29 June 2013; Revised 10 October 2013; Accepted 14 October 2013

Academic Editor: Luigi Cappelli

Copyright © 2013 Ji-Sheng Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Tidal current energy has attracted more and more attentions of coastal engineers in recent years, mainly due to its advantages of low environmental impact, long-term predictability, and large energy potential. In this study, a two-dimensional hydrodynamic model is applied to predict the distribution of mean density of tidal current energy and to determine a suitable site for energy exploitation in Jiangsu Coast. The simulation results including water elevation and tidal current (speed and direction) were validated with measured data, showing a reasonable agreement. Then, the model was used to evaluate the distribution of mean density of tidal current energy during springtide and neap tide in Jiangsu Coast. Considering the discontinuous performance of tidal current turbine, a new concept for assessing tidal current energy is introduced with three parameters: total operating time, dispersion of operating time, and mean operating time of tidal current turbine. The operating efficiency of tidal current turbine at three locations around radial submarine sand ridges was taken as examples for comparison, determining suitable sites for development of tidal current farm.

1. Introduction

The rapid development of economics and science technology has caused an enormous consumption of fossil fuel energy and environmental pollution. As marine renewable energies are clean and sustainable, they can offer a solution to relieve energy crisis and to reduce environmental impacts. It is well known that ocean covers nearly 71% of the earth’s surface and holds a large amount of energy more than 2 × 103 TW [1]. Ocean energies can be exploited in many different forms, including tide, wave, tidal current, thermal, salinity gradients, and biomass [2]. Tidal current energy differs from tidal energy. Application of tidal current energy is to transform kinetic energy of tidal current to electricity, while application of tidal energy is about the usage of potential energy due to sea level variation [3]. Among all forms of ocean energies, tidal current energy is preferable mainly due to its advantages of the high energy density (approximately 832 times greater than wind) [2], long-time predictability, and potentially large resource [4]. For these reasons, tidal current energy draws more and more attentions from the public in recent years.

China has an excellent resource of tidal current energy with a huge capacity of approximate 13950 MW [5]. Exploitation of tidal current energy will be an important supply as a clean and reliable power in the near future. Jiangsu is a province on the eastern coast of China with a coastline of 954 Km. A radial submarine sand ridge was formed in the northern part of Jiangsu Coast due to strong tidal current and, undoubtedly, tidal current energy mainly concentrated in this area. Maximal current speeds in Huangshayang channel and Xiyang channel are over 2.5 m/s and 2.0 m/s, respectively. Therefore, assessment of tidal current energy in Jiangsu Coast is of great practical significance.

Assessment of tidal current energy is usually applied to identify location of greatest potential and to estimate level of energy production that can be achieved [6]. In the past ten years, some studies regarding assessment of tidal current energy in China have been conducted. For example, Li et al. investigated the distribution of tidal current energy and recommended some possible location for exploitation in China [7]. Liu et al. summarized distribution of tidal current energy in some several water channels around China and calculated the theoretical value of tidal current power [8]. After reviewing previous two-dimensional model for evaluation of tidal currents energy, Chen implemented a three-dimensional, semi-implicit Euler-Lagrange finite element model (SELFE) to assess the potential tidal current energy of three locations around Kinmen Island in Taiwan and analyzed the impacts of energy extraction on hydrodynamics in Taiwan Strait [3, 6].

Previous assessment of tidal current energy mainly focused on simulation of specific coast, distribution of energy, and potential channel for future exploitation. Operating time of current turbine is acknowledged as an influencing factor on energy extraction, because frequently interrupted working condition will reduce efficiency of current turbine. Time-dependent magnitude of current velocity shows a process of periodic variation, and a current turbine has an upper limit and a lower limit to current speed within which it can transform energy properly. Therefore, operating time of current turbine is discontinuous and periodic. Period and interruption interval of current turbine during operation are mainly dependent on tidal cycle. Consequently, a water channel with more steady and consecutive tidal current is more suitable for transformation from tidal current energy to electricity.

Main objective of this study is to propose a new concept for assessment of tidal current energy in terms of total operating time, dispersion of operating time, and mean operating time. A two-dimensional hydrodynamic model is developed in Jiangsu Coast, which is based on the commercial software, MIKE 21 FM package, and the simulated results (water elevation and current velocity) are validated with the measurement data. Then, numerical results from validated model are used for assessment of tidal current energy. Density of tidal current energy, total operating time, dispersion of operating time, and mean operating time at three locations are compared to identify the most suitable site for deployment of tide current turbine.

2. Numerical Model

2.1. Governing Equations of Hydrodynamic Model

A two-dimensional model is built within the commercial MIKE 21 FM package to simulate tidal hydrodynamics in Jiangsu Coast. The two-dimensional incompressible Reynolds-Averaged Navier-Stokes (RANS) equations for describing tidal hydrodynamics can be written as where are horizontal Cartesian coordinates; is time; is surface elevation; is still water depth; is total water depth; and are velocity components in and directions; is Coriolis parameter (in which is angular rate of revolution and is geographic latitude); is gravitational acceleration; is density of water; , , , and are components of radiation stress tensor; is atmospheric pressure; is reference density of water; is magnitude of discharge due to point sources; is velocity by which water is discharged into the ambient water; are lateral stresses including viscous friction, turbulent friction, and differential advection; and overbar indicates a depth-averaged value.

2.2. Calculation Formulation of Tidal Current Energy

When tidal flow passes through a vertical cross-section of unit area perpendicular to the flow direction per unit time, the current energy extracted can be calculated by the method of kineticenergy density [9]: where is density of tidal current energy within unit area, is turbine efficiency coefficient, and is magnitude of flow velocity averaged over cross-section. Mean density of tidal current energy, , over an arbitrary period can be calculated by

2.3. Computational Domain and Boundary Conditions

The computational domain covers Jiangsu Coast and its adjacent ocean areas (Figures 1 and 2), with a distance of 246 Km in longitude from east (123°22′17′′E) to west (120°23′36′′E) and a distance of 300 Km in latitude from south (30°52′22′′N) to north (35°56′43′′N). As shown in Figure 3, this domain is divided into a series of unstructured triangular grids with 75882 nodes and 149926 elements. A small grid with a size of 200 m is used along coastal boundary, while a large grid with a size of 5000 m is applied at open-sea boundary.

340501.fig.001
Figure 1: Regional map of study area in China (downloaded from Google map).
340501.fig.002
Figure 2: Map and bathymetry of study area.
340501.fig.003
Figure 3: Unstructured triangular grid of study area.

Wind stress, surface net heat, and moisture flux can be imposed on model system via surface boundary, but they are not considered in this study. At sea bottom, bottom shear stress induced by bottom friction is specified. Time-dependent water elevation clamped open boundary condition is provided along open-sea boundary, while long-term averaged runoff from the Yangtze River is prescribed on the land side. Initial surface elevation is set as 0 m with no velocity, and a spin-up period of 24 hours is adopted in the simulation to avoid the impact of initial condition.

3. Model Validation

To check/ensure accuracy of hydrodynamic model, the calculated results of water elevation and current velocity are compared with field measurement. During 22/August/2006–25/August/2006 (spring tide) and 29/August/2006–01/September/2006 (neap tide), a serial of field measurement including water elevation and current velocity was carried out in Jiangsu Coast. WFH-2 Absolute Machinery Coded water level meter is used for recording water elevation at Dafeng station (120°48′12′′E, 33°16′55′′N), while Acoustic Doppler Current Profiler (ADCP) is adopted for monitoring current velocity at stations (120°51′48′′E, 33°10′18′′N), (121°8′48′′E, 32°34′60′′N), and (121°29′42′′E, 32°39′36′′N) (see Figure 4).

340501.fig.004
Figure 4: Locations of stations Dafeng, , , and .

Figure 5 shows the comparison of calculated water elevation and measured data during spring tide and neap tide, indicating a reasonable agreement between numerical model and field measurement. The values of mean absolute error (MAE) for spring tide and neap tide are 0.17 and 0.07, the values of root mean square error (RMSE) for spring tide and neap tide are 0.21 and 0.08, and the coefficients of determination () are 0.998 and 0.999.

fig5
Figure 5: Comparison of calculated water elevation and measured data at Dafeng station.

A 24-hour spring tide from 08:00 24/August/2006 to 08:00 25/August/2006 and a 24-hour neap tide (12:00 31/August/2006 to 12:00 01/September/2006) are chosen for the comparison (Beijing time). Figure 6 demonstrates the comparison of current magnitude and current direction at stations , , and , and Table 1 gives the values of MAE and RMSE between numerical simulation and field measurement. It can be seen that agreement between simulation and measurement at is much better than the others. However, some differences between the simulation and measurement are obvious, which may be ascribed to (i) complex bathymetry within the radial submarine sand ridges, (ii) various roughness coefficients during tidal cycle, and (iii) unsteady surface wind.

tab1
Table 1: Values of MAE and RMSE between calculated and observed tidal current.
fig6
Figure 6: Comparisons of depth-averaged tidal current between numerical simulation and field measurement for ((a)–(f)) spring tide and ((g)–(l)) neap tide at stations , , and .

4. Assessment of Tidal Current Energy

Numerical results from validated model are used to evaluate distribution of tidal current energy in Jiangsu Coast. In addition to conventional method calculating mean density of tidal current energy within the domain, a new concept with three parameters, total operating time, dispersion of operating time, and mean operating time of tidal current turbine, is introduced to describe the total working condition of current turbine. With these three new indicators, three water channels are taken as examples to identify the most suitable site for exploitation of tidal current energy.

4.1. Mean Density of Tidal Current Energy

Mean density of tidal current energy is a concept indicating the ability of energy production over a given period. Density of current energy density is calculated by (2), and average of these values is taken as mean density of tidal current energy (3). It is noted that a typical value of 0.3 is adopted for the parameter (the percentage of power that can be extracted from the tidal stream, taking into account the losses due to Betz’ law and those assigned to internal mechanisms within the turbine) [6, 10]. Figure 7 shows the distribution of mean density of tidal current energy in spring tide and neap tide. It can be seen that tidal current energy concentrates around the radial submarine sand ridges with a highest value of 0.94 Kw/m2 during spring tide and a highest value of 0.60 Kw/m2 during neap tide.

fig7
Figure 7: Distribution of mean density of tidal current energy during (a) spring tide and (b) neap tide (unit: 100 W/m2).

The radial submarine sand ridges consist of many water channels that are ideal locations for exploitation of tidal current energy. According to long-term tidal measurement, three water channels (Xiyang, Huangshayang, and Lanshayang) with wide width and rapid current are considered in this study. After calculation of the potential mean density of energy, one representative point in each water channel is chosen for the comparison of tidal current energy to find a suitable site for tidal turbines. As displayed in Figure 8, locations (121°00′58′′E, 33°00′06′′N) in Xiyang, (121°26′51′′E, 32°39′01′′N) in Huangshayang, and (121°39′32′′E, 32°34′04′′N) in Lanshayang are around the radial submarine sand ridges. Table 2 gives values of mean density of tidal current energy at these three positions, showing that has a maximal value of mean density of tidal current energy among these three locations.

tab2
Table 2: Mean density of tidal current energy at , , and .
340501.fig.008
Figure 8: Locations of , , and .
4.2. New Concept for Assessment of Tidal Current Energy

It is well known that tidal current turbine has a low limit of tidal current velocity for power generation. In 2010, Benelghali et al. compared Doubly-Fed Induction Generator (DFIG) and Permanent Magnet Synchronous Generator (PMSG) for marine current turbine applications, and they found that DFIG is with a lower limit of 1.3 m/s [11]. In this study, the velocity of 1.3 m/s is taken as the lower limit of tidal current turbine. Due to the existence of the lower limit, tidal current turbine is with discontinuous operating in practical application. Total operating time, dispersion of operating time, and mean operating time consequently become three new important parameters in assessment of tidal current energy and evaluation of operating efficiency of tidal current turbine.

Total operating time is defined as the sum of operating times over a given period, indicating the duration of effective operation of tidal current turbine: where is current velocity and is a given period.

According to the low limit of 1.3 m/s of tidal current turbine, all the durations with a tidal current speed over 1.3 m/s are accumulated and their sum is considered as total operating time. As listed in Table 3, during 72-hour spring tide and neap tide, total operating time at locations and is longer than that at location . For example, total operation time at location is 31.8 hours and 18.3 hours for spring tide and neap tide, respectively, indicating that tidal current turbine works in 44% of time in spring tide and 25% of time in neap tide.

tab3
Table 3: Total operating time during spring tide and neap tide at , , and .

Dispersion of operating time is defined as interruption number of turbine performance during a given period, which is calculated by counting the status shift from power-off (with a current velocity below 1.3 m/s) to power-on (with a current velocity over 1.3 m/s) of tidal current turbine:

Values of dispersion of operating time during spring tide and neap tide at , , and are given in Table 4, showing that interruption number of turbine performance is the largest. It is reasonable to have a larger interruption number when total operating time of tidal current turbine is longer. It is necessary to point out that, in these two indicators, total operating time of tidal current turbine is the main indicator for assessment of tidal current energy. In the case of similar amount of total operating time, a smaller value of dispersion of operating time is preferred.

tab4
Table 4: Dispersion of operating time and mean operating time during spring tide and neap tide at , , and .

It is also useful to define mean operation time of tidal current turbine as the ratio of total operating time to dispersion of operation time:

As listed in Table 4, mean operating time at stations and is obviously larger than that at . It can be seen that a higher efficiency of turbine operation can be achieved at and (due to a higher value of total operating time) although interruption number of turbine operation is larger. Therefore, stations (Xiyang) and (Lanshayang) are more suitable sites for deployment of tidal current turbines among these three locations. However, the final decision on site selection for tidal current farm is also dependent on the large-scale ocean space-use plan along Jiangsu Coast.

5. Conclusion

A two-dimensional hydrodynamic model was developed with Mike 21 FM package to assess tidal current energy and seek for suitable sites for deployment of tidal current turbine. For model validation, calculated water elevation and current velocity are compared with field measurement, indicating a generally reasonable agreement. Then, simulation results were used to evaluate tidal current energy resources in Jiangsu Coast. In addition to traditional method calculating mean density of tidal current energy, a new concept in terms of total operating time, dispersion of operating time, and mean operating time of tidal current turbine was proposed for assessment. These three indicators were well defined in the study, and they were found to be very useful in determining suitable sites for deployment of tidal current turbine. It is noted that total operating time of tidal current turbine is the most important indicator for assessment of tidal current energy in this new concept. In the case of similar amount of total operating time, a smaller value of dispersion of operating time or a larger value of mean operating time is preferred. Based on this new concept, Xiyang and Lanshayang are the more suitable sites for deployment of tidal current turbine in Jiangsu Coast. However, the final decision on site selection for tidal current farm is also dependent on the large-scale ocean space-use plan along Jiangsu Coast.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (51137002), Natural Science Foundation Project of Jiangsu Province (BK2011026), and the 111 project (B12032).

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