Tidal Current Energy for Indian Coastal Lines – A State Art of Review

India has about 7500 km of coastal lines with a significant potential to harvest marine energy. Tidal energy, one of the types of marine energy that emerges has a practical option to supply reliable and renewable energy to the Indian coastal areas. This paper focuses on tidal current turbines that can be adapted to various potential sites available along Indian coasts. Tidal energy projects all over the world and the type of turbines used by their applications are presented. This is followed by preliminary analysis of the Indian sites that includes current speeds and depth, possible adoption of current tidal current technology to those sites and the likely power output. Finally, challenges facing those who adopt the current technology are briefly discussed.


Introduction
India with a fast growing population requires a diverse power generation to comply with the demand for energy in the country. The growth in the power sector during 2009-10 to 2016-17 is shown in figure 1 [1]. The present installed capacity of various renewable energy sources in India are shown in figure 2 [1]. Present clean energy sources installed in the country are not sufficient to replace the whole or partial conventional energy plants polluting air. So adopting more renewable energy sources is of great importance for overcoming emerging issues of environmental concerns in India [1]. Marine energy is a valuable source of energy to the Indian economy due to the long Indian coastal line, but it requires more attention and development to make it valuable for the energy sector [2]. Generally the generation of power from marine energy can be classified as ocean thermal energy, current, wave, tidal energy and from salinity gradients, all are completely renewable and reliable source for efficient energy generation.
There are several review articles on tidal turbine technologies and their suitability for various places [3][4] [5] but in this article the discussions are focused on suitable Indian sites and the kind tidal current turbines that may be appropriate for those sites. This paper will give some trace for researchers to decide the type of turbines to be adopted while implementing tidal energy technologies in India.
A Tidal current turbine can be described as an underwater wind mill with blades rotating at slow speed. It can be operated at water depths between 35 and 100 m .This technology neither creates visual or audible impact near the shore nor affects the ship route if it is well placed underwater [6]. The tidal turbines are of four types. They are vertical axis, horizontal axis, oscillating hydrofoil and IOP Publishing doi: 10.1088/1742-6596/1716/1/012008 2 venturi effect. The first two are current turbines and the others can exploit the rise and fall in the water level . figure 3 gives a brief understanding of the different types of turbines existing in the world to extract energy. This paper is organized as follows: Section 2 generally describes tidal energy aspects and Section 3 describes several types of commercially available turbines of various worldwide companies and operation envelopes in terms of suitable current speeds and water depth. Section 4 gives a detailed study of potential sites in Indian coastal lines to show the feasibility of installing tidal turbines in the near future, where site"s tidal current speed and water depth are leading parameters in choosing the appropriate type of a tidal current turbine. Conclusions are given in Section 5.

Tidal Energy
Tides occur due to the existence of gravitational force between the Sun, the Earth and the Moon. The distance between the Moon and the Earth is shorter, and so the force between them is higher than that of between Sun and the Earth. So the tide action will be higher during the night than in the day time [7]. Tidal currents are formed when the water move horizontally due to the tidal action in the shore. The current which moves towards the land are known as flood tides and which moves away from the sea are known as ebb tides [8]. Due to the change in gravitational force between the Earth and the celestial bodies the high and low tides occur. Tidal range is defined as the difference between high tides and low tides. Figure 4 represents the tidal range at the time of high and low tides near the seashore [9]. Generally the energy generated out of the tidal action in water is known to be as tidal energy.       Harnessing of tidal energy can be implemented either by using the potential energy or kinetic energy of the tides. The tidal barrage technology make use of the potential energy of water by building a barrage in place experiencing tidal range of 5 m or higher whereas the tidal current turbine technology make use of the kinetic energy of the flowing water [10].
At present various countries are going ahead in implementing tidal turbine for energy production. The technically harvestable tidal energy near the coast all around the world is 1TW [11] but due to the high capital cost and maintenance, this technology is preferred only in sites with a potential to produce significant energy. Although tidal energy plant is expensive to carry over, its performance and benefits are proven to be worth in countries like France, UK, China etc [12]. Some of the few tidal energy projects and their power generation capacity around the world are given in table 1.
Approximately there are 20 locations worldwide with significant potential tidal energy and India is one of them. The highest tidal range can be found in Canada at the Bay of Fundy, with a maximum tidal range of up to 16.3 meters [13]. The Gulf of Cambay and the Gulf of Kutch in Gujarat are the places with the highest tidal range in India i.e.,11 m and 8 m respectively. They possess average tidal ranges of about 6.8 m and 5.2 m respectively [14]. These tides brings an average tidal speed ranging between 2.5 m/s to 3 m/s, making tidal turbines a viable option for those sites without the need to build barrages.

Tidal Current Turbines
The tidal plants listed in table 1 are only few and many more plants are being developed around the world. For assessing the tidal energy potential in an area, its affecting factors need to be checked. The sea bed should be uniform to minimise turbulence and loss of velocity. The current speed at the site is a major factor responsible for efficient extraction of tidal energy and this can be seen through equation (1) [15].
Where P is the available power from the water stream, "ρ" denotes the density of water, "v" denotes velocity of the stream, "A" denotes cross section area of the rotor,"Cp" denotes the coefficient power of turbine and it depends on the design of the turbine. The actuator disk theory show that Cp is limited to below 0.6 for a single stage hydro-kinetic un-shrouded turbine. This the so-called Betz limit. Thus the site resource depends on the current speed, depth and turbine efficiency [16].
This section describes some of the commercially available tidal current technologies that are used for various sites worldwide. However, first we revisit the tidal current turbine classification.
Tidal current devices are typically designed as horizontal or vertical axis turbines for energy extraction. When the tidal currents flow in the direction of axis of rotation of the rotor it is horizontal axis turbine. For structural stability three blades are preferred in most places. The blades are connected with the hub. The hub is connected with shaft. The shaft is the one which is connected with gearbox and a generator. At the rear end of the hub, the generator is placed [17].
The entire setup can be deployed by depending on the seabed arrangement and depth of water. For shallow water region the gravity base, monopile, and pile jacket are considered to be a base structure. The floating type of structures can be used for supporting the assembly in case of larger depth. The design, manufacturing and maintenance of this turbine is simpler than vertical axis turbine. The Horizontal axis tidal current turbine which is commonly called as HATCT, moves slowly and so it should not affect the marine animals [17]. The various HATCT companies around the world and their stage of turbine development are given in detail in the table 2.
In the vertical axis turbine, the flow of tidal currents is in the direction perpendicular to the axis of rotation of the rotor. As the tidal current crosses through the turbine, it is also known as a cross flow turbine. Here it is possible to place the gear box and generator on the floating vessel and the shaft output can be transferred vertically above the sea surface, whereas in horizontal axis turbine it is not possible [17].
However it is complex task to design the vertical axis tidal current turbine (VATCT) and also weighs heavier than the HATCT. Due to the cross flow, the turbine is much prone to cavitation which needs to be driven up. It is not self-starting, generally costlier and having a lower Cp than the HATCT [17]. Still it has been adopted in various sites due to the advantage that it can operate even at low current speed and shallow depth. Another advantage is that VATCT has a simple structure and have straight blades, hence there is no issue regarding direction during the tidal flow changes its direction diurnally [18]. Some of the VATCT projects with their power production capacity at each stage are shown in the table 3 [11].
3.1 Commercially available Tidal turbines that may be suitable for the Indian sector As discussed earlier, the important factors that affect the tidal energy potential are the current speed and the depth of the site. Minimum of 1 -2m/s of current speed is required for the turbine to rotate for which the optimum energy can be extracted when the current speed ranges between 1.5 m/s and 3 m/s [18]. A site with 2.5 m/s is considered to have exceptionally high energy resource [19]. However there are also some turbines designed to run at 1m/s and below [19]. The cross-section area of the turbine over which efficient energy can be extracted depends on the water level above the surface of the turbine [16]. The section below describes some of the tidal technologies of different famous companies adopted in various sites all over the world.

SeaGen.
The SeaGen technology has been implemented in 2008 by Marine current Turbine (MCT) which is now a part of the Atlantis resources in UK. The SeaGen technology was developed especially to establish Strangford Lough project. The 1.2MW project of SeaGen technology is the world"s first grid connected commercial scale tidal device. So far this technology has generated more than 8GWh of electricity. This MCT uses the experience gained in the Strangford Lough project for further development of the SeaGen technology [20].
The SeaGen marine current turbine technology consists of axial flow rotors which drives a generator through a gearbox. This looks like a wind turbine or a hydroelectric turbine which is as in figure 5.a. The devices are connected to a monopole foundation and are secured to the seabed by a pinpiled quadrapod [20]. The pitch controlled type of rotors is used to minimise the forces on the structure which also optimises the energy extraction. The SeaGen system placed in Strangford lough is shown in figure 5.b in which it has a 20 m rotor diameter is adoptable in marine environment up to 38 m water depth. The rated power of the turbine was found to be achieved when the tidal currents were 2.4 m/s or higher than . Generally maintenance is always an important tedious task for water turbines.
Here with the support of the structure, that task is simplified by enabling the turbine to be taken out from the water during maintenance work [21].  After the successful installation of the 300 kW tidal energy project in Norway, a 1 MW device named HS1000 was developed, tested and installed in December 2011 in cooperation with the Scottish power renewables [6]. HS1000 is the first ever pre-commercial tidal turbine to validate the technology for the world"s first tidal power array system at the Europen Marine Energy Center (EMEC), Orkney, Scotland [24]. The device was connected to the grid by February 2012 and has the production capacity of about 3.1GWh per year and satisfies the need of over 500 houses [6].

MeyGen.
Meygen is the world"s largest tidal stream project currently under construction [26]. Meygen is 85% owned by the Atlantis Resources solely for the development of the inner sound tidal site. [26]. For the first phase of the project they have two turbine suppliers, Atlantis Resources Ltd and Andritz Hydro Hammerfest [26].
They use the turbine AR1500 device from Atlantis Resources Ltd. Its rated capacity is 1.5 MW at 3 m/s. The turbine has 360 degree yawing capacity and active pitch blades that yields maximum energy. The deployment and connection work is made easy and can be done in 90 minutes [25].
HS1500 of Andritz Hydro Hammerfest has the same features as of HS1000 but with rated capacity of 1.5 MW. The rated capacity is achieved at a current speed of 3 m/s. A sensor driven monitoring system governed by an automatic control software adjust the leading edge to extract optimum output from the tidal currents [25]. For initial development of the project, 1 X AR1500 and 3 X HS1500 devices have been selected that will provide overall capacity of 6 MW which will grow upto 400 MW in the later phase. The turbines will be given a gravity based mooring system [27].  [28]. They use the cross flow turbines named as Enermor Kobold turbine for harnessing energy from the currents. It is a three bladded turbine with the length of 5 to 6 m and rotor diameter of about 6 m. The Enermor Kobold turbine is a uni-directional vertical axis turbine with a high starting torque and provides spontaneous start, so it avoids the purpose of ignition system to start even in worse condition [29]. The picture of this turbine is shown in the figure 8.
The Enermor project located in Strait of Messina along the Sicilian coast was installed in a depth of 18-25 m in 2001. The designed power capacity is 80 kW and produces 25 kW in the place with 2 m/s current speed [30]. Table 4 provides the detail on the tidal turbines, type of the turbines with its rated capacity at different current speed.

Tidal Turbine Technologies for Indian sites
India is one of the few tidal energy potential sites in the world which is covered by ocean on three sides. This enables the ocean to attain high tidal range at some places. The potential sites in India are shown in the figure 9 [22].
In India places like Gujarat and Sundarbans were already found to have a significant tidal potential sometime ago. A study made by Indian Institute of Technology-Madras reveals the distribution of current speed along the Indian coast lines, which are listed in table 5 [11]. Based on the available data about the current speed the probable type of turbine that can be adopted for Indian sites are suggested further.
The Gulf of Khambat is one of the richest locations of tidal energy potential in India. It experiences a tidal range of 5 to 11 m. The site has a typical current speed of 2.5m/s and an average potential energy of 10.9MW/km 2 and kinetic power of the tidal current of about 4000W/m 2 . per crosssection area of the turbine. Among the above discussed turbines in Section 3; SeaGen, Andritz Hydro Hammerfest, and Enermor Kobold turbines can be suitable in places with a current speed below 2.5m/s. The Gulf of Kutch having 4 to 9m tidal range is the location with the highest current speed of 3m/s. The average potential energy available is 7.2MW/km 2 and average kinetic power of about 4500W/m 2 per cross-section area. Any of the turbines discussed in the above section can be used in places where the current speed is less than 3m/s. Sundarbans with a tidal range of 4 to 7m is another important tidal energy potential site in India that has current speeds of 2 to 3m/s. The average potential energy available in this location is 7.2MW/km 2 with an averaged kinetic power of about 4000W/m 2 per cross-section area. Since the current speed varies between 2 to 3m/s, it is always safer to prefer the turbine for lower condition which can produce electricity even below 2m/s. In this approach the Andritz Hydro Hammerfest and Enermor Kobold turbines may be suitable for this location. South of Gujarat has a tidal range of 2 -4m and current speed of 2m/s. The average available potential energy is 1.5 MW/km 2 and an averaged kinetic power of about 4000W/m 2 per cross-section area. For this again, the Andritz Hydro Hammerfest and Enermor Kobold turbines may be suitable.
All other places like Maharastra, Karnataka, Kerala, and Orissa have a low current speed of 1.5m/s while Andhra has a minimum of 1m/s for which the turbines from Andritz Hydro Hammerfest can be suitable for those locations. Table 6 gives an overview on the preferable tidal turbine technologies for various Indian sites with respect to the available current speed and water depth Figure 5.a) The SeaGen system full view [31]. b) The SeaGen system adopted in Strangford lough, UK [31].   [30] . Figure 8. Atlantis AR1000 Tidal turbine [32].