Learning From Field Testing Experiences for Developing Small Scale Tidal Current Energy Conversion System

Tidal current energy conversion system (TCECS) has been tested in potential straits such as the Larantuka Strait and Madura Strait. In its development, the technology will be improved by increasing its capacity to 100 kW. The new capacity is planned to use a farming system with vertical axis turbines. The experience of installing and operating a TCECS prototype with a capacity of 10-20 kW in Indonesia could be a valuable experience for the implementation phase of a TCECS with a higher capacity. This paper aims to report the experience of TCECS installation in Indonesia. According to previous installation experience, TCECS was made floating using a catamaran type as a supporting structure. This was done to facilitate the transportation process from the production site to the installation site. TCECS production was carried out in an area that is quite far from the installation area because there is not enough land around it to assemble or produce the technology. This condition is one of the factors that need to be considered in the TCECS installation step in Indonesia. Another factor to consider is the size of the TCECS technology to be utilized. The Installation facilities in Indonesia, especially in the eastern region, have been yet inadequate in using large equipment, e.g., a heavy crane. Therefore, the dimensions of the TCECS to be installed should not have large dimensions. To take the experience of the installation, other factors are described in several chapters in this paper.


Introduction
Along with the global target for reducing fossil fuels related to global warming, Indonesia has begun to commit to achieving the net zero emission target from 2017 [1], with an initial target of achieving 31% energy mixing by 2050.Then the road-map is accelerated with a 100% achievement target in 2060 or sooner [2].This target has its own challenges because geographically Indonesia is an archipelagic country.The distribution of the islands causes the fulfillment of electricity to be off-grid.Furthermore, many islands are remote areas.Electricity currently uses diesel, which is it is the target to be replaced with renewable energy, and at the same time as the fulfillment of electrification ratio which currently is 99,76% [3].Ocean currents are an interesting option to look at because of their stable and predictable nature.For this reason, the contribution of ocean currents in fulfilling renewable energy gets a fairly large portion, which is 13.4 GW [4].However, what needs to be considered in its development must be 1265 (2023) 012016 IOP Publishing doi:10.1088/1755-1315/1265/1/012016 2 able to adapt to the characteristics of the islands and the support system for those area.Research on the energy potential of ocean currents and their generation technology has developed in Indonesia.BPPT engineers since 2005 [5] have been mapping the energy potential of ocean currents numerically, reviewing its application in Eastern Indonesia using a Darrieus type turbine, conducting hydrodynamic tests, and designing prototypes of ocean currents power plants.In 2009 the prototype was installed for the first time in the Larantuka Strait (https://www.youtube.com/user/erw4ndi).
In 2011 together with marine energy lovers, the Indonesian Ocean Energy Association (ASELI) was established with the aim of promoting the potential of marine energy [6].Since then, research on energy sourced from the sea has been mostly carried out by academics from ITS since 2014 together with other research teams.These studies with the topic of renewable ocean current energy have been carried out numerically as published by Rahmawati et al [7], Satrio et al [8] and Junianto et al [9].After that, these studies were physically tested in the laboratory as published by Satrio et al [10] and Musabikha et al [11], [12].Technology testing is continued in a controlled field on an open channel as has been done by Madi et al [13].
This technology research journey is expected to enter the field-testing phase in 2022, namely in potential locations in the archipelago, both in terms of resources and electricity needs.Therefore, the selection of potential locations becomes very important for the application of the technology to be right on target.Several potential areas have been presented through a map of the potential for renewable marine energy published by the Ministry of Energy and Mineral Resources together with the Indonesian Marine Energy Association (ASELI) (2014).However, the map cannot be used directly because it is theoretical.For this reason, an assessment of the potential for ocean currents is needed which can later be converted into renewable energy with a more accurate estimate.The target of this work is to identify ocean current turbine technology that is suitable for the speed of ocean currents in Indonesia.

Power Generation of Tidal Current Energy
The vertical axis tidal current turbine has three blades that use an airfoil.This type of blade has a thin profile.Utilization of this airfoil causes this type of turbine to use the lift to rotate the shaft of the tidal current turbine as illustrated in Figure 1.The lift force on the turbine is different from the lift force created on the airplane wing.The lift force on the turbine occurs because of the difference in the direction of flow around the turbine with that across the turbine.Due to this difference, the fluid flow forms a resultant with a new flow direction which eventually forms an angle of attack.
The available power in a seawater body is defined as the density of kinetic energy available at a certain flow velocity against the turbine sweep area that is traversed and the density of seawater [14].Velocity is assumed to be uniform in space and constant in time.This available power calculation can be calculated by (1).
Where: P = available power (Watt) ρ = density of sea water (kg/m 3 ) A = turbine sweep area (m 2 ) v = tidal current velocity (m/s) Figure 1.Fluid velocity around turbine airfoil [14] The rotation of the vertical axis turbine due to the lift force produces turbine power (Pt) or actual power.The power is generated from the available power in the sea water body.Thus, the ratio of actual power to available power is the power coefficient achieved by the turbine.This power coefficient or power coefficient (CP) is calculated using (2).
The value of CP on a turbine shows the ability of the turbine to convert the kinetic energy available in the seawater body as wide as the turbine sweep into mechanical energy.Albert Betz stated that a rotor cannot convert kinetic energy into mechanical energy of more than 59% in other words it is maximum CP is 0.59 [14], [15].This condition is known as the Betz Limit.

Important Aspects Of TCECS Technology Identification
In the studies that have been carried out, several important aspects that need to be considered in identifying TCECS technology are economic aspects, field conditions aspects, and regulatory aspects.These three aspects become the basis of thought in determining TCECS technology in Indonesia.These three aspects are further elaborated as follows: then reduced to the level of performance of turbine blades.6. Turbine hydrodynamic test and turbine blade performance.7. Design of turbine prototypes, platforms, Cremona system connecting floaters, floater connection systems for easy assembly, turbine raising and lowering systems transportation and operation data, mooring system calculations, and others.8. Mechanical transmission system design.9. Electrical system design.10.Manufacturing of turbines, platforms, mooring systems, mechanical transmissions, electrical and others.11.Planning for mobilization to the site.

TCECS Implementation And Utilization Strategy
Considering this, another strategy is needed for the implementation and utilization of TCECS.By considering the three aspects mentioned above and considering it of the available facilities and the addition of the population distribution in the Eastern Indonesia region, the approach to making mediumscale TCECS cannot be implemented.For this reason, it is necessary to approach the manufacture of small-scale TCECS but in large quantities [16].This strategic approach has the following advantages: 1. TCECS is a completely new item and has not been used en masse.It is still in the research stage so that with the approach of making small-scale TCECS it will be able to reduce costs and losses that may occur to a minimum.2. The distribution of the population in Eastern Indonesia is not as dense and as much as Java Island, so there is no need for TCECS on a large scale.3. The manufacture of small-scale TCECS will be able to reduce installation costs compared to largescale TCECS which demands the support of heavy equipment in the installation process.4. Small-scale TCECS is easy to manufacture, easy to operate, and easy to maintain.5. Components of small-scale TCECS will be easily made domestically so that if the TCECS will be developed in large quantities then domestic industry support will be easy to implement and this means also supporting government programs in the development of technology-based industries.6. Small-scale TCECS will be easily operated by local human resources who can be drawn from vocational schools that are already evenly distributed in Eastern Indonesia.

Conclusion
Based on a study of the experience of implementing national tidal currents turbines, the use of vertical axis tidal currents turbines has its own charm, especially in the relatively easy production process and uncomplicated maintenance system.For the support system, this turbine can be placed on a floating structure, jetty structure, or existing jetty in a potential location.Later, it is necessary to calculate the reinforcement when the turbine is added to the structure.However, if the condition of the existing structure is not possible, it is necessary to design a new support structure that can support the performance of the tidal current turbine in a potential location.
Experiences that can be used as a reference in implementing TCECS in Indonesia are as follows:1.Identification of infrastructure, supporting facilities, logistics, and speed of ocean currents.2. Prediction of potential ocean currents in several straits in Bali, NTB, NTT, West Papua, Riau Islands, Sunda Strait, and others.3. Measurement of ocean current velocity at candidate locations.4. Selection of turbine technology for ocean current energy conversion. 5. Design of turbines and turbine blades based on strait-level mesoscale numerical simulations, Furthermore, and this is very important, ocean currents in Indonesia are typical ocean currents of the Equatorial Region (equator).The equatorial region is an area where the influence of the Coriolis force is small.The Coriolis force is a force caused by the rotation of the earth.The magnitude of this force is strongly influenced by the angle of position on the earth.The Coriolis force function is a sine function of the angle of the earth's position.The following equation is the Coriolis force of Equation 3:Where:CF stands for Coriolis Force (N).Ω is the angular velocity of the earth = 360 deg/day = 7.292e-5 rad/s.v is the speed of ocean currents / wind (m/s).Sin (latitude) is the angle of the earth's position.