Review on Tidal Energy Technologies and Research Subjects

Tidal current power is one of the promising and reliable renewable energies with the advantage of continuous and predictable resource. It can make stable electricity regardless of weather conditions or seasons all year around. The required technologies for tidal current power in the ocean have been developed for years and now recognized that it could be commercialized after intensive field tests and successful demonstrations. There are several tidal farm development projects in the world, such as the MeyGen project in UK with its commercialization at hand. However, various research subjects in the tidal current energy field are seeking improvements and industrialization of tidal current power in terms of economy and technical reliability. This paper introduces the resource assessment procedure of tidal energy that has been applied in Korea coastal regions. The key research subjects for tidal current power together with the interaction effect of multi-arrangement is described. Also, this paper is to introduce the research output of each subject such as turbine design, experimental validation, turbine interaction and wake, multi-array module, FSI (fluid-structure interaction), and duct application.


Introduction
There are four major ocean energy sources: tidal current, tidal barrage, wave power, and OTEC (ocean thermal energy conversion). Tidal current power generates electricity by converting the current kinetic energy to the rotational energy using turbines. It is known to be a reliable, continuous, and predictable energy source regardless of weather conditions or seasons. The power extracted from tidal current can be predicted accurately in advance by harmonic analysis. This advantage is highly attractive to grid management (Funke et al., 2014). Fig. 1 shows typical tidal devices in the stage of commercialization.
Tidal barrage power exploits the potential energy in the height difference between high and low tides using a dam containing seawater. Tidal barrage power has the same advantages as tidal current power in continuous and predictable energy. However, it needs to construct a dam structure which not only requires a large area for a basin with enormous construction cost but also imposes a great deal of environmental impact on the surrounding area. Fig. 2 shows Sihwa tidal power plant with a total power output capacity of 254 MW. This is the world's largest tidal barrage power in commercial operation (Bae et al., 2010).
Wave power utilizes the kinetic and potential energies of  ocean waves by operating a turbine or by using a mechanical or hydraulic device. Since a standard power generation mechanism for wave power is not established, various wave power devices are being developed and tested. One of them is the point absorber floats on the water surface being supported by cables up to the seabed. This type can absorb wave energy by radiating a wave with destructive interference to the incoming waves. The surface attenuator acts a flexing motion created by swells that drive hydraulic pumps to generate electricity. Other type is oscillating water column which has an air chamber that compresses air inside forcing through the inlet to rotate the turbine to create electricity. Other type is overtopping devices which can fill the reservoir with waves. The potential energy in the reservoir height is captured with a low-head turbine. Fig. 3 shows the wave device named Wave Star in point absorber type and Pelamis in surface attenuator.
OTEC uses the temperature difference between cooler deep and warmer surface seawater to run a heat engine that produces electricity. OTEC is one of the renewable energy sources with continuous availability. Traditionally, there are two types of OTEC: closed-cycle and open-cycle types. The working fluids for a closed-cycle OTEC are refrigerants that have low boiling points. The most common heat cycle for a closed-cycle OTEC is the Rankine cycle, using a low-pressure turbine. Open-cycle OTEC uses vapor from seawater as the working fluid producing pure freshwater. Fig. 4 shows a closed-cycle OTEC power plant and basic concept.
In this paper, the tidal energy application technologies and turbine performance in multi-arrangement are described. The procedure and methodology of resource assessment are suggested. The tidal application technologies such as turbine design, experimental validation, turbine interaction and wake, multi-arrangement effects, FSI (fluid-struc-

Resource assessment
2.1 Energy potential definition Resource assessment is to be preceded before any detailed planning and development for renewable energy. The results of resource assessment are used to estimate development scale and economic feasibility. Many organizations have introduced methodologies for renewable energy resource assessment dividing energy potential definitions into 3−5 steps as Table 1. These can be categorized as theoretical, technical, and economic potentials.
KIER (Korea Institute of Energy Research) has announced the renewable energy potential definition which categorizes renewable energy potential into four steps: theoretical, geographical, technical, and market potentials, as shown in Fig. 5. The theoretical potential is the total natural energy in a region; the geographical potential the total natural energy disregarding areas with restrictions; the technical potential the extracted energy using present technology considering energy efficiency, utilization factor, etc; and the market potential the economically feasible energy with energy demand, standard market price, government support, etc. This definition has been applied in renewable energies such as solar power generation, biomass generation, wind power generation, geothermal power generation, ocean energy generation and etc.

Procedure and methodology
The first step of resource assessment is to define theoretical energy density in accordance with the definition of energy potential. Variables in the equation should be properly adopted not to overestimate or underestimate energy density considering the characteristics and mechanism of the energy sources. Then a literature survey is conducted along the coastline. This includes investigation into restricted areas for geographical potential. The technical potential can be calculated considering tidal device type and technical capabilities. The ocean survey is to be carried out on the basis of the literature data. The ocean survey equipment includes echo sound, side scan sonar, multi-beam sonar, bottom profiler, ADCP (acoustic Doppler current profiler), etc. The entire area of interest can be numerically modelled and simulated based on ocean survey data. From this procedure, the energy potentials can be estimated according to CFD simulation with the surveyed data.

Tidal current energy potential calculation
The west coast of Korea is well known for abundant tidal energy resources up to 10 m tidal range. In addition, with 3600 islands or so along the coastline, the tidal current is accelerated between islands. Therefore, there are many attractive and exploitable areas for tidal current power in the west and south coasts of Korea. When the resource comes from the kinetic energy of a fluid stream, the power density (PD) is widely used to describe the energy density (Kabir et al., 2014). The power density of tidal current is the amount of power per unit turbine disk area which is given by ρ where is sea water density and U tidal current speed.
The power density is the kinetic energy flux density as a function of tidal current speed but cannot be used to describe the quantity of energy in the region. Therefore, API (average power intercepted) is introduced to calculate tidal current energy density. API is theoretically extractable energy potential per unit area, and the power density is converted to API as: When a turbine diameter is D, the swept area is given by The occupied area per tidal device is determined as 15D 2 (10D×1.5D), assuming that lateral distance is 2D and longitudinal distance 10D based on the recent studies, as shown in Fig. 6 (Myers et al., 2011;Mycek et al., 2014).
Energy potential can be evaluated using IGC (installed generating capacity) and AEP (annual energy production). IGC is the utilizable tidal current potential in a region and can be calculated based on averaged API and sea area of a region as: AEP is the annual power generation of electricity from installed generating capacity that can be calculated based on IGC as: The limit of applied area is by EEZ (exclusive economic zone), where a state has special rights regarding the ex-ploration and use of marine resources, including energy production from water and wind. The study area was divided into 10 regions according to the administrative district, as shown in Fig. 7. However, the east coastal area having low tidal current speed was excluded in this resource assessment.
Surface average velocities from 1902 measuring points were categorized according to regions. These observational data were unable to describe the detailed distribution of tidal current speed. Therefore, numerical simulation of flow circulation was conducted. Table 2 summarizes the tidal current energy potential in Korea. The theoretical potential was estimated as IGC of 439 GW and AEP of 3844 TWh. Considering geographically restricted areas (such as environmental preservation, military zone, sea route, artificial reef and fishing zone, water depth limitation, and subsea cable and pipeline), the geographical potential was estimated as about 63.8% in theory. The technical potential was calculated as IGC of 44.7 GW and AEP of 392 TWh/y, considering the existing technology development.

Tidal current projects in Korea
Korea has a strong current on the west coast with up to 10 m tidal range and up to 6 m/s current speed between island on the south coast, and thus there are many suitable locations for TCP (Tidal Current Power) application. The first tidal current power facility was installed in 2003 in Uldolmok by KORDI (Korea Ocean Research & Development Institute) It was the first VAT device of a 100 kW helical turbine in Korea. In March 2009, a 1 MW test TCP facility was constructed in the narrow channel in Uldolmok (Fig. 8). The maximum speed in the region was 6.5 m/s with rock seabed condition.
In 2008, Ocean Space Ltd. and Inha University developed and tested a 25 kW tidal device which was the first HAT (horizontal axis turbine) in Korea. This device was installed in the cooling water channel in Samcheonpo power plant. The 100 kW floating HAT tidal device with 8 m 3bladed turbine was developed based on the experiment from the 25 kW tidal device test. This was exploited in the Yeosu area in 2010 and tested successfully as shown in Fig. 9.
An active-controlled 200 kW tidal device has been developed since 2011, as shown in Fig. 10 (Ko et al., 2019). The device is supported by a caisson-type gravity-based support structure. In April 2009, Incheon Metropolitan City, Ongjin County, Inha University, POSCO E&C, and Korea South-East Power Co. signed an MOU to develop a 200 kW tidal current farm in Incheon area. If the project goes well, this site will be the largest TCP farm in the world. It can be secured a production of 613 GWh per year supplying electricity of 160000 households. Table 3 shows the TCP projects in Korea.
In 2017, the tidal current testbed construction project of 4.5 MW was launched (Ko et al., 2019). This project consists of five berths with grid connection and the facility can test various tidal devices to prove the function and reliability of system. Fig. 11 shows the conceptual drawing of the tidal current testbed.

Turbine design
The turbine is the first element that converts kinetic energy of tidal current to rotational energy. The performance of tidal turbines is a key factor that greatly affects the tidal system efficiency. The tidal turbine is designed using the blade element momentum theory (BEMT) which consists of blade element theory and momentum theory (Masters et al., 2011). BEMT describes the forces acting on a blade element as shown in Fig. 12. The momentum theory describes the interaction of a turbine and a flow through a turbine based on the law of momentum conservation. BEMT introduces axial and tangential induction factors that are used as   Chul Hee JO, Su Jin HWANG China Ocean Eng., 2020, Vol. 34, No. 1, P. 137-150 141 a design parameter of a turbine. Fig. 13 shows the turbine design algorithm. The initial condition is set as Betz limit where turbine power is the maximum according to the momentum theory. The inflow angle is determined by axial and tangential induction factors. The normal and tangential force coefficient is calculated from the inflow angle. For the spanwise flow that a vortex from the tip is excluded in BEMT, the loss correction factor is used to correct BEMT. Chord length and axial and tangential induction factors are calculated with modified variables. This process is repeated until the values of induction factors have converged to their final value.
A tidal turbine was designed using the turbine design algorithm with the diameter of 10 m, rated velocity 2.5 m/s and design TSR (tip speed ratio) 5. S814 foil, developed by NREL (national renewable energy laboratory) for a wind turbine. The turbine was modeled by CATIA, as shown in Fig. 14.
The performance of the designed turbine is evaluated by 3D CFD (computational fluid dynamics) using ANSYS CFX program. Grid generation was carefully conducted for smooth convergence and reliable results. The thickness of the near-wall grid layers was considered according to the application of the turbulence model.
Since the flow around the blade of a horizontal axis turbine can be regarded stationary, the analysis field was assumed to be incompressible, three dimensional, and steadystate. The analysis field of two domains consists of one pas-    sage with one blade; an internal rotating domain encompasses the blades, and a stationary domain covers the remaining area of the flow field, as shown in Fig. 15. The computational domains were calculated by assuming that the flow between adjacent passages is periodic in the rotating direction.
Solid boundaries and blade surfaces were defined as non-slip walls. The hub and top boundary were treated as a free slip wall to eliminate their effect. In the rotating domain, angular velocity was prescribed for each case to give a TSR using the moving reference frame (MRF) method. The surfaces between the two domains were interfaced using the general grid interface (GGI) method, and the frozen rotor method was applied for the MRF interface. A rotational periodic model was used for the interface between the side-wall boundaries. A normal velocity was applied to the inlet, and the static pressure was set to 1.0 atm at the outlet. The shear stress transport (SST) turbulence model was used as a turbulence closure.
Since the turbine converts the available power of the flow to rotational energy, its performance can be evaluated by calculating the energy conversion efficiency. The turbine power characteristic is affected by factors including the current speed, turbine size, and rotational speed, but can be normalized and compared with others effectively as plotting the power curves for TSR. Thus, performance analysis is widely applied using TSR. The power coefficient is defined using Eq. (6) and TSR is a dimensionless value representing the rotating speed based on the inflow velocity, as shown in Eq. (7) (Jo et al., 2013). Hence, ω where C p is the power coefficient, T torque, the rotational speed, A turbine swept area, and R the radius of the turbine.
The results show that the maximum efficiency of the designed turbine is about 47.6% at the rated TSR of 5. As shown in Fig. 16, turbine performance is steady and stable for various current speeds.

Experimental validation
The numerical simulation is widely used to evaluate turbine performance significantly reducing the time and cost associated with experimental testing. On the other hand, because the results of numerical simulation are affected by input data such as flow field modelling, grid generation, boundary condition, etc., the methodology and results of numerical simulation should be validated by comparing with the experiment.
Circulating water channels (CWC) can be used to conduct the tidal turbine performance test. Fig. 17 shows the CWC with a test section of 2.3 m×1.0 m×0.9 m and a maximum velocity of 1.2 m/s at Inha University.
The performance of the designed turbine was estimated using CFD and verified by comparison with experiments, as shown in Fig. 18. To measure the output power of the turbine, a torque meter with an RPM sensor was equipped in the nacelle, and control and monitoring devices installed with power cables connected to a generator. It can display the measured data and can control the loads at the generator. The nacelle was made a streamlined shape with a long shaft to reduce the interference effect between the blades and the device.
The results show the rotating speed was controlled successfully from 122 RPM to 275 RPM by applying electrical loads. The maximum power of 22 W was generated at 150 RPM corresponding to rated TSR 5. The C p curve from the experiment shows good agreement with CFD results consid-

Turbine interaction and wake
For the commercialization of tidal current power, the tidal farm composed of many tidal current devices is required to meet the economic feasibility. As there are many tidal current devices installed in a tidal farm, there will be the efficiency drop due to the interactions between turbines. The narrower the distance between the tidal turbines, the less the output of the downstream turbines by the influence of wake that occurs at the upstream turbines as shown in Fig. 20. On the contrary, the wider the distance between the turbines, the less the total electricity generation of the tidal farm as the number of turbines is decreased in the allowed area. Hence, the economic feasibility can be improved by maximizing the power production optimizing the spacing of turbines.
The turbine interaction and wake is the important subject for the tidal farm arrangement. Fig. 21 depicts the streamwise velocity in the domain for the rotating turbine, highlighting the measuring sections. Obtained were velocity data from 17 points across the flow field and 5 distances downstream as shown in Fig. 21. Velocity distribution changed for every single step and the average data were reflected to calculating the velocity deficit.
It shows the characteristics of the wakes of the turbine more intuitively by analyzing the velocity deficit (Jo et al., 2014). Velocity deficit V def can be defined as Eq. (8). The distance apart from the turbine has each velocity distribution and can analyze the wake characteristics.
In the previous study, the velocity measurement was estimated to be a total of 85 points. The velocity deficit for distance from the turbine (4D, 8D, 12D, and 18D) was calculated. Fig. 22 shows the wake pattern and recovery along the downstream. A heavy flow disturbance was observed at 2D and 4D distances, with the maximum velocity deficits of 54.5% and 37.2% respectively. The large and rapid recovery was seen from 2D to 8D, followed by a much slower recovery beyond. It should be noted that, for this configuration, the wake has not fully recovered by 20D downstream.     From the 2D and 4D distances, the velocity deficit was calculated as the maximum of 54.52% and 37.20%. As the distance gets shorter, the area of the turbine effect gets narrower. The velocity recovered rapidly up to 4D yet more slowly after 4D than the wake region nearby.
The turbine power characteristic is affected by design variables such as current speed, turbine size, rotational speed, etc. Power generated by the turbine can be non-dimensionalized by available power flowing into the turbine swept area. Flows that pass the upstream turbine lose their energy, and streamwise velocities significantly decrease behind the turbine, as observed in Fig. 23 (Jo et al., 2012). Since it is difficult to specify the accurate U ∞ to calculate C p , available power in the wake area is not clear and precise. Thus, the output power generated by the downstream turbine can be directly compared with the upstream turbine, and then power decrements be calculated. Fig. 24 shows the power curves of the two turbines. The maximum power of the upstream turbine was 61.0 W at 72 RPM, and 79% decreased for a downstream turbine located at 7D away.
The effects of axial and diagonal arrangements have been investigated through the measurement of RPM decrement of downstream rotors. All rotors have the same configuration and positioned at various gaps as shown in Fig. 25. The rotors are located at the interval of 1D up to 3D. The RPM of each rotor is measured and compared. The interaction between rotors located in a diagonal direction is also studied with three rotors as shown in Fig. 26. The distance between A1 and A2 is 0.5D, and A12 is 1D from the front rotors.
As shown in Fig. 27, RPM increases as the increase in the distance between rotors. It is confirmed that the interference effect decreases due to the upstream rotor as the distance increases. When rotors are arranged in multi-arrays, a diagonal arrangement is more advantageous for interference effects than a straight-line arrangement.
The interference effect can be reduced when the rotors are arranged diagonally, as shown in Fig. 28. Even though the downstream rotor is close to the upstream rotor (1D), the RPM decrement is relatively low with an average of 11.2% and RPM is enhanced by up to 34.9%.

Multi-arrayed module
The multi-arrayed module is developed for easy installation and maintenance by ballast control as shown in Fig. 29 (Jo et al., 2015a). This system has been successfully designed and tested by Inha University. Depending on the number of the device and its size, the module can accommodate more than one device. The module can be lifted to the surface for maintenance as well as repair and lowered down by filling water in the ballast tanks. This concept can be a high-cost savings in the operation of tidal turbines.
The static and dynamic simulations have been conducted for multi-arranged tidal devices, located at specific distances. A static condition means that the rotor is fixed at an incoming current, and the dynamic is used for the rotating rotors with an incoming flow. The boundary conditions, rotor specifications, and supporting base structures of the experiment are applied in the numerical modeling, as shown in Fig. 30.   Fig. 31 shows the streamlines in the field. The streamlines around the devices are very complicated behind the front unit. It also indicates there is a complex interaction between devices.
Each interference effect, according to the variation of rotor placement is observed in Figs. 32 and 33 (Jo et al., 2010). There is little or no interaction between rotors in a transverse location, such as in Case 1. The upstream rotor affects the downstream rotor in the axial direction. In the case of a diagonal arrangement, which shows a minimum decrement from the experiment, the influence of the rotor on the rotary motion is weak.

FSI (fluid structure interaction)
CFD analysis is conducted to evaluate turbine performance, assuming a rigid body for the turbine. However, the turbine blades are to be deformed by hydro-forces in reality, and the resulting deformation can affect the performance of the turbine. This effect should be considered to estimate the output of tidal current power.
The turbine deformation can be simulated by FSI meth-   od. In FSI analysis, the fluid pressure on the solid structure is calculated using CFD and mapped onto a finite element method (FEM) solid model to evaluate the structural deformation. Fig. 34 shows the flowchart for one-way FSI analysis.
A static FEM analysis was conducted to calculate the turbine deformation. Hydro-forces acting on the blade were imported from the CFD results and applied to the blade surface as a loading condition. A composite material, made of E-glass fiber and epoxy, was used for the blade modelling. The shapes of the deformed blades were obtained for various velocities, as shown in Fig. 35. No significant changes were observed below 1.5 m/s, but as the velocity increases, the margin of the increase grows bigger. The streamwise deformations along the blade section were plotted in Fig. 36. The blade was deformed 411 mm in streamwise direction at the upstream velocity of 3.5 m/s. It corresponded to 8.2% of the blade length. The streamwise deformation was the most significant, but the chordwise deformation was also observed up to 122 mm. This deformed shape was transferred to the CFD model.
The performance of the deformed turbine was estimated using CFD. The configurations of the computational domain, boundary conditions, and grid system were the same as those used in the rigid turbine analysis. The power coefficient (C p ) of the deformed turbine was compared with that of the rigid turbine at a rated TSR of 5. At the upstream velocity of 3.5 m/s, the efficiency was calculated as 44.9%, and that of the undeformed turbine as 47.4%, as shown in Fig. 37. The C p decreased by approximately 2.5%, which could cause a notable loss in the power production of tidal arrays. Since the quantity of power production is a crucial parameter, especially concerning economic feasibility, a small power deficit would induce considerable changes.    Chul Hee JO, Su Jin HWANG China Ocean Eng., 2020, Vol. 34, No. 1, P. 137-150 4.6 Duct application The flow speed is the most significant and critical factor for power generation from tidal current because the power extracted by a tidal device is proportional to the cube of the flow speed (Blunden and Bahaj, 2007). The duct system can be used for tidal current power to increase the flow speed passing through the tidal turbine (Jo et al., 2016). It can also potentially expand the application areas of tidal devices to relatively low current speed areas.
Three streamlined duct shapes with the same overall length are introduced, as shown in Fig. 38. The first is a nozzle type that has an open inlet and a cylindrical outlet; the second a diffuser type, which is the reverse shape of the nozzle type, having a cylindrical inlet and an open outlet; and the last a nozzle combined with a diffuser type with an open inlet and outlet (Jo et al., 2015b).
CFD was conducted to simulate flow patterns around three duct types. The computational domain was designed, considering the specification of a circulating water channel (CWC) facility (Fig. 39) in which the experimental study was conducted. All analysis cases have the same external boundary dimensions, as shown in Fig. 40, with different duct shapes in the inner domain. The flow around the duct was assumed to be incompressible, three-dimensional, and steady-state. The time scale for the steady-state analysis was controlled using the auto time scale function. A normal velocity was defined as 0.2 to 1.2 m/s at the inlet. The opening condition was set at the outlet. Fig. 41 shows the flow speed at each point in the longitudinal direction relative to the center of the duct, which is a point (0, 0, 0). The flow speeds of the nozzle type decreased by about 30% at the inlet area but recovered at the center of the duct. The flow speed amplification by the nozzle type duct is imperceptible (Fig. 41a). In contrast, Fig. 41b illustrated the amplified flow speeds from the inlet area of the diffuser type duct. For the nozzle combined with a diffuser type duct (Fig. 41c), the increase was the highest in the rate of the flow speeds. For this type, depending on the upstream speed, the flow speed was increased up to about 46% relative to an upstream flow of 1.2 m/s. The basis for these flow patterns is in Fig. 42. In the nozzle type duct, there is no sucking shape, and the opening shape of the inlet produces a strong and steady pressure. However, in the diffuser type and the nozzle combined with a diffuser type, there is a pressure drop in the duct due to the opening shape of the outlet. The pressure drop in the duct causes an increase of the flow speed in it. The difference between the nozzle type and the nozzle-diffuser type is the shape of the outlet. For the nozzle-diffuser type, as previously mentioned, the opening shape of the outlet can create a suction effect so that the static pressure near the inlet decreases. For the nozzle type, the cylindrical shape of the outlet cannot produce a suction effect. For this reason, the flow in the nozzle type duct is slower than that in other types.
The flow speed amplification by the nozzle-diffuser type duct was verified by experimental fluid dynamics (EFD). The experimental model duct is an identical copy of the

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Chul Hee JO, Su Jin HWANG China Ocean Eng., 2020, Vol. 34, No. 1, P. 137-150 CFD model (Fig. 43). The flow speed in the duct was measured using an acoustic Doppler velocimeter according to the change in upstream velocity. The upstream velocity was controlled from 0.2 to 1.2 m/s, and the measuring points were from (0, +0.06, 0) to (0, −0.24, 0) relative to the center of the duct. Fig. 44 shows the flow speed in the duct according to upstream velocity. The rate of flow speeds was the highest at the center of the duct in all cases. The results of EFD were close to CFD results at the low upstream velocity. It was shown that a higher upstream velocity produced a higher output gap. It seemed that the flow speed increased up to about 64% relative to an upstream flow of 1.2 m/s. A 64% increase in flow velocity multiplied the generated power about four times.
A small-scale ducted tidal device has been developed and installed near Hong Kong's west coast in Tuen Mun (Jo et al., 2018). The project location is in the Hong Kong Gold Coast, where the water is part of the South China Sea. The speed of the tidal current at the project site is relatively low from almost 0 to 0.8 m/s. To accelerate the current speed for possible generation, a duct upstream of the turbine was applied. This was the method to harness energy from the low current speed region.
In this project, the nozzle-diffuser duct was attached to increase the low current speed. According to the CFD results, the duct increased the current speed by more than 50% in an open boundary condition. Specifically for the project site conditions, Inha University designed and manufactured the tidal current turbine, which consisted of two parts: the generating and electronic parts. The generating part included a rotor, a nozzle-diffuser duct, supporting structures, and a magnetic coupling. The electrical part included a generator, control box, battery, circuit protection, LED panels, and cables.
The experiments were conducted in a circulation water channel (CWC) to validate the results of CFD. An ADV was used to measure the velocity inside the duct. The experimental flow rate similar to the simulated flow rate was 0.4 m/s to 0.8 m/s. After the laboratory tests had confirmed the functionality and workability of the turbine, it was installed at the test site as shown in Hong Kong (Fig. 45). The prototype turbine with a diameter of 35 cm and the designed power generation up to 34 W was successfully demonstrated at flow speed of as low as 0.4 m/s.

Conclusion
This paper describes the various tidal energy application technologies. There are important research subjects in the tidal current energy filed to confirm and improve the device efficiency. The reliable and valid academic contributions are required to industrialize tidal energy technologies.  Chul Hee JO, Su Jin HWANG China Ocean Eng., 2020, Vol. 34, No. 1, P. 137-150 149 The collaboration among researchers is crucial, and sharing the knowledge and experiences is required to solve the technical problems and improve device reliability. There are still areas to study in the development of tidal current farm, especially the interaction between turbines and optimization of multi arrangement are to be further investigated. Though there are a good number of sites with high tidal energy potential in Asian regions, most of tidal current farm projects have been postponed due to governments' low compensation rates. Therefore, it is a critical issue to hold the realistic and economically feasible value for the commercialization of tidal energy. The researchers are to try to make this resource widely known to the public as well as government officers, invite their attention and support for the implication of the tidal energy technologies, therefore benefitting the public with clean power.