Hydrokinetic energy conversion systems: A technology status review

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/268519416 Hydrokinetic energy conversion systems: A technology status review Article in Renewable and Sustainable Energy Reviews · March 2015 DOI: 10.1016/j.rser.2014.10.037 CITATIONS READS 5 322 2 authors: Mehmet Ishak Yuce Abdullah Muratoglu 34 PUBLICATIONS 229 CITATIONS 14 PUBLICATIONS 7 CITATIONS Gaziantep University SEE PROFILE Batman Univesity SEE PROFILE Some of the authors of this publication are also working on these related projects: Experimental analysis of TIGRIS-27H hydrokinetic turbine View project All content following this page was uploaded by Mehmet Ishak Yuce on 28 April 2015. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately. Renewable and Sustainable Energy Reviews 43 (2015) 72–82 Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser Hydrokinetic energy conversion systems: A technology status review M. Ishak Yuce a,n, Abdullah Muratoglu a,b a b University of Gaziantep, Civil Engineering Department, 27310 Gaziantep, Turkey Batman University, Civil Engineering Department, 72060 Batman, Turkey art ic l e i nf o a b s t r a c t Article history: Received 7 November 2013 Received in revised form 29 September 2014 Accepted 18 October 2014 Available online 21 November 2014 Hydrokinetic energy conversion systems are the electromechanical devices that convert kinetic energy of river streams, tidal currents, man-made water channels or waves into electricity without using a special head and impoundment. This new technology became popular especially in the last two decades and needs to be well investigated. In this study, the hydrokinetic energy conversion systems were reviewed broadly. They have been categorized into two main groups as current and wave energy conversion devices. Their technology, working principles, environmental impacts, source potential, advantages, drawbacks and related issues were detailed. & 2014 Elsevier Ltd. All rights reserved. Keywords: Hydrokinetic energy Current energy devices River energy conversion Wave energy Tidal power Contents 1. 2. 3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Source potential. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Theory and design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Current energy conversion (CEC) systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Wave energy conversion systems (WEC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Existing technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Realistic applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Technology survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Economic aspects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Environmental impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Operating conditions and mooring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Discussion and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction Increasing energy demand, harmful environmental effects of conventional energy production technologies, increasing cost and running out reserves of fossil fuels, climate change, spreading health problems and social pressure have led scientists and engineers to find alternative non-consuming, harmless, cheaper and sustainable energy production methods. Renewable energy n Corresponding author. Tel.: þ 90 507 702 0560. E-mail address: [email protected] (M.I. Yuce). http://dx.doi.org/10.1016/j.rser.2014.10.037 1364-0321/& 2014 Elsevier Ltd. All rights reserved. 72 74 75 75 76 76 77 78 78 78 79 80 80 80 technologies offer many environmental benefits over conventional energy sources [1]. The hydropower is the world’s largest and cheapest [2] source of renewable energy. It is also the most efficient way to produce electricity [3]. Approximately 18% of world’s electricity is supplied from hydropower [4]. Predictability, regularity and having worldwide spreading sources make hydropower one of the most attractive choices of energy production. There are mainly two approaches to harness energy from water, namely, hydrostatic and hydrokinetic methods. Hydrostatic approach is the conventional way of producing electricity by M.I. Yuce, A. Muratoglu / Renewable and Sustainable Energy Reviews 43 (2015) 72–82 storing water in reservoirs to create a pressure head and extracting the potential energy of water through suitable turbo-machinery [5]. In hydrokinetic approach, the kinetic energy inside the flowing water is directly converted into electricity by relatively small scale turbines without impoundment and with almost no head [2]. Hydrokinetic turbines are also called free flow turbines, ultra-low or zero head hydro turbines [5]. Hydrokinetic technologies are designed to be installed in natural streams like rivers, tidal estuaries, ocean currents, waves, man-made waterways [6] and other flowing water facilities with an optimum velocity. Hydrokinetic energy technologies have some advantages over the conventional hydropower production methods. Hydrokinetic systems require minimum amount of civil work [5]. There is no extra cost to construct a dam or a reservoir to accumulate the water. The kinetic energy is harnessed based on water motion in the form of current and waves. Although the hydrokinetic turbines have relatively small scale power production, they can be installed as multi-unit arrays like wind farms to increase energy extraction [6]. The hydrokinetic systems provide more valuable and predictable energy than wind and solar devices [7]. Especially river streams and tidal currents are highly predictable. In developed countries the suitable sites for large scale hydropower plants have been mostly exploited. Furthermore, across some river valleys dam construction may be either technically or economical infeasible, due to the topography, geology of the site, non-availability of construction materials, seismic hazards, right of way cost, etc. Hydrokinetic energy conversion systems provide a good choice for electrification of such sites. On the other hand, employing hydrokinetic turbines can be the most suitable and cheap way of supplying electricity to remote and off-grid areas where transmission lines do not exist [8]. According to United Nations Development Programme (UNDP), in 2008, there were globally about 1.5 billion people who lived without electricity especially in least developed countries and in sub-Saharan Africa [9]. Reliable energy can be supplied with hydrokinetic systems to remote areas having rich water resources such as South Africa [10]. Additionally, hydrokinetic systems have minimal environmental impacts compare to dams [5]. Large scale hydroelectric power plants have some unfavorable effects on the environment such as; people relocation, inundation of agricultural, historical and habitat areas, sedimentation of fertile lands, methane (CH4) gas emission, altering the river regime, etc. Contrarily, the natural tissue of the energy production site is not seriously affected by hydrokinetic systems. Hydrokinetic technology has several drawbacks compare to the other energy production methods. These systems have relatively small scale power production with lower power coefficients. The maximum efficiency that an in stream hydrokinetic turbine can reach is 59.3% which is also known as Betz limit. Only high quality professional systems can reach 50% efficiency. On the other hand, Power density (W/m2) 1000 73 cavitation is one of the biggest constraints of hydrokinetic turbines. It is defined as the formation of water bubbles or voids when the local pressure falls below the vapor pressure. Cavitation can significantly damage the turbine. Especially high speed moving parts can be subjected to cavitation [11,12]. Harsh marine environment is another disadvantage of hydrokinetic systems. Especially wave energy conversion devices should be strongly designed to withstand high and irregular water loads. On the other hand hydrokinetic systems can have small scale environmental risks. Installation of hydrokinetic systems can block the navigation and fishing. The turbine parts, chemical agents, noise and vibration can badly affect the water habitat. Bad environmental influences of hydrokinetic systems are still investigated by scientist. There have been limited studies on hydrodynamic characteristics of hydrokinetic turbines. These systems are still in their infancy and need to be well investigated. The scientific background behind in-stream energy conversion systems is very similar to that of wind energy conversion technologies. The main principles such as utilization of blade sections, BEM theory, Betz limit, etc. are learned from aerodynamic and hydrodynamic applications, wind turbine and ship propeller methodologies apart from a number of fundamental differences [13]. The design of hydrokinetic systems requires interdisciplinary study of environmental, hydraulic, hydrologic, electric and mechanical branches. Considerable amount of power can be obtained from an instream hydrokinetic turbine comparing with the equally sized wind turbine [5]. A hydrokinetic turbine operating with a rated speed of 2–3 m/s can produce four times energy of similarly rated wind turbine [14]. The approximate fluid densities are 1000 kg/m3 and 1.223 kg/m3 for water and wind, respectively. Wind turbines are usually designed to operate at rated wind speed of 11–13 m/s [15]. In contrast, the rated velocity for hydrokinetic turbines is between 1.5 and 3 m/s. The comparison of power densities for water and wind turbines are given Fig. 1. The power density of a hydrokinetic turbine operating with 2 m/s free stream velocity is same as that of wind turbine running with approximately 16 m/s flow speed. Several hydrodynamic models have been developed in order to model tidal, river and wind driven circulations (Mecca, MIKE, etc.) [16]. Many analytical and numerical modeling efforts have been made to calculate the amount of extractable power from river, marine and tidal resources [17]. One dimensional (1-D) analytical models are used for the effects on water level and velocity, whereas, advanced 2-D and 3-D are implemented to calculate the source potential [18]. Yang et al. [17] gives an updated list of models that employed to determine the tidal stream resources. Majority of developed models are based on tidal power. A riverine kinetic energy model was discussed recently by Khan et al. [5,18]. y = 150V3 750 y = 0.183V3 500 Water turbine Wind turbine 250 0 0 5 10 Velocity (m/s) 15 Fig. 1. Comparison of power density for in-stream water and wind turbines. 20 74 M.I. Yuce, A. Muratoglu / Renewable and Sustainable Energy Reviews 43 (2015) 72–82 2. Source potential The resource potential for hydrokinetic energy is investigated for three different cases as river, tidal and wave energy. Various independent studies have been established to reveal hydrokinetic energy potential at the regional and global scale. While tidal movements and river flows are highly predictable, it is relatively difficult to properly estimate the wave resources due to its irregular characteristics. On the other hand, the technical potential is a specific part of the theoretical potential and changes according to technology used. Therefore, technical potential estimations vary based on the proposed technology. Hydrokinetic source potential and technology analyses are mostly practiced by USA, UK and Canada. The technically recoverable hydrokinetic energy capacities of these countries and global predictions are given in Table 1. In order to predict the power in a river, temporal and spatial flow analyses must be conducted, the cross-section area, depth, velocity and other related characteristics should be known. The river energy is generally studied catchment based especially for the dam feasibility analyses. Accordingly, there is limited number of studies on the regional river energy potentials. On the other hand, global river databases are not ready to present a worldwide river energy resource assessment for hydrokinetic applications [5]. To date, the largest study within this scope has been concluded by EPRI [19] in order to assess the theoretical and technically recoverable river energy in the United States. According to this study, the theoretical riverine hydrokinetic energy resource assessment of the continental US is estimated to be 1381 TW h/yr. 120 TW h/yr of this amount is predicted to be technically recoverable [19]. The theoretical global tidal resource is estimated to be 8800 TW h/yr [20]. Technically recoverable tidal energy potential is predicted as 800 TW h/yr [7,23]. US, Canada and UK are the Table 1 Technically recoverable hydrokinetic energy resources [7,19–24]. Country USA UK Canada Global Energy (TW h/yr) Wave Tidal River 210 105 N/A 750 250 94 110 800 120 N/A N/A N/A leading countries having high potential of tidal energy. Total theoretical power available along the U.S. coasts is estimated to be 50 GW. The technical resource is estimated to be 250 TW h/yr [21]. Alaska shares the vast majority of this amount. UK is estimated to have a theoretical tidal stream potential between 50 and 94 TW h/yr with about half of the European resource. Finally, Canada has 110 TW h/yr technically feasible tidal energy potential [22,23]. According to the recent analyses, the gross theoretical worldwide wave energy resources are estimated to be up to 80,000 TW h/yr [25–28]. It is predicted that up to 750 TW h/yr of this amount is technically recoverable [23]. World Energy Council proposes that the technical recoverable wave energy can be increased to 2000 TW/yr if the wave energy systems are sufficiently improved [29]. The theoretical and technically recoverable wave energy potential of the US is estimated as 2100 and 210 TW h/yr, respectively. The highest wave activity is found between the latitudes of 301 and 601 on the north and the south hemispheres [20]. UK, Canada, Norway and Ireland, Denmark and France have relatively higher resources of wave energy. The total amount of European technically recoverable wave energy resource Table 2 Annual average wave power for some countries [31]. Location Estimated wave power (kW/m) Belgium Canada Denmark France, Atlantic Ocean France, Mediterranean Greece India Ireland Italy Japan Norway Portugal United Kingdom USA, California USA, Hawaii USA, Maine USA, New England USA, North Carolina USA, Massachusetts USA, Oregon 10 33 7–24 40 4–5 2–4 10–32 57–77 10–5 6–7 20–40 30–40 45–75 10–32 15 14 4–22 5–15 5 21 Fig. 2. Approximate global distributions of wave power (kW/m of wave front) [30] and global tidal ranges (m) [20]. M.I. Yuce, A. Muratoglu / Renewable and Sustainable Energy Reviews 43 (2015) 72–82 is estimated to be up to 236 TW h/yr [23]. UK itself has 105 TW h/ yr recoverable wave energy capacity [24]. The global tidal ranges and approximate wave power distributions are given in Fig. 2. Annual average wave power distributions for various countries are given in Table 2. 3. Theory and design Existing hydrokinetic energy technologies can be classified according to the working principles, mainly in two categories as current energy conversion (CEC) and wave energy conversion (WEC) systems. CEC devices which are also called in-stream or rotating energy conversion systems have been designed to harness energy from a flowing stream through a rotating turbine. Tidal instream energy converters (TISEC) [32], marine current turbines (MCT) [33] and river energy conversion systems (RCECS) are among the CEC systems. WEC devices extract the energy of the irregular waves basically by creating a system of reacting forces in two or more bodies relative to each other [34]. CEC and WEC systems have different working principles and design techniques. The fundamental theories for designing CEC and WEC devices are separately explained in the following sections. 3.1. Current energy conversion (CEC) systems Current energy conversion (CEC) systems mainly have a propeller with two or more blades rotating around a horizontal or vertical shaft by the effects of the hydrodynamic forces generated by the free stream. Each blade is basically designed from one or more hydrofoils. The blades rotate with the torque that is produced by the lift force. Selecting a high performance hydrofoil having large lift/drag ratio is important in the design process. The main principles that are used to model rotating hydrokinetic turbines are one dimensional momentum (actuator disk), rotor disk and blade element momentum (BEM) theories. The BEM theory is a universally accepted method which is the combination of both blade element and momentum theories. It is more complicated than the actuator and rotor disk theories. Actuator disk theory presents a very rough approach and it is incapable of determining the detailed design specifications of a turbine rotor. The BEM theory provides a detailed turbine design procedure and it is used to determine the lift and drag forces, thrust and power coefficients, also rotational speed, twist and pitch angle distributions. The theoretical performance curves of horizontal axis turbines can be extracted from BEM theory [35–37]. In BEM theory, the blade is divided into a number of imaginary small segments. The rotor is assumed to have infinite number of blades and radial flow effects are neglected. The lift and drag forces of each blade segment are taken from the blade sections’ two dimensional characteristics [35,38,39]. The aerodynamic loads and performance of each blade element is evaluated iteratively. The resultant loads on a blade section (hydrofoil) are seen in Fig. 3. Each blade section has an optimum angle of attack which is the angle between the relative velocity and the blade section’s chord line. The extracted power is proportional with the relative velocity which is the vector sum of the axial and tangential velocities. The angle of attack varies (α) from hub to tip with the effect of the tangential velocity. Therefore, the turbine blades should be twisted around the twist axis to keep angle of attack constant. On the other hand, hydrokinetic turbines are subjected to high thrust and torsional loads due to density of water causing high bending moment at the blade root [40]. So, thicker blade sections are preferred near the hub. The maximum efficiency that an ideal turbine can reach is known as Betz Limit. Betz law proposes that the theoretical 75 maximum power coefficient for a rotating turbine in a fluid stream is 0.593. This criterion can be applied to all hydrokinetic turbines working in a free stream such as tidal and river currents [15]. The typical efficiency for a hydrokinetic turbine with low mechanical losses is approximately 30% [14]. For a well-designed system, the overall power coefficient is between 0.4 and 0.45 [41]. Generally hydrokinetic turbines are modeled to have a fixed speed rotor in which propeller turns with a constant rotational speed (rpm). More professional systems use variable speed mechanisms for better efficiency. Similarly, the performance of hydrokinetic turbines can also be increased by assigning variable pitch mechanism to the propeller [42]. The power output of rotating current energy conversion systems are evaluated as follows; 1 P ¼ ρAV 3 C P 2 ð1Þ where; P is the total power output from the turbine in Watts, ⍴ is the density of the fluid, A is the swept area of the rotor blades (m2), V is the flow velocity (m/s) and CP is the power coefficient of the turbine which also represents the overall efficiency. Fig. 4 illustrates the relationship between power, rotor diameter and free stream velocity in CEC systems. As it is seen from Eq. (1), the total power is directly proportional to the cubic power of the flow velocity. Therefore, in hydrokinetic turbines, the flow velocity has a particular importance. Fig. 3. The resultant loads on a blade section for a typical blade section (Vrel is the relative velocity, FL is the lift force, FD is the drag force and α is the angle of attack). Fig. 4. The relation of diameter and velocity for in-stream hydrokinetic turbines with power. 76 M.I. Yuce, A. Muratoglu / Renewable and Sustainable Energy Reviews 43 (2015) 72–82 P crest length ¼ 1 ρg 2 H 2 T 32π ð4Þ Approximate power per unit of wave front (kW/m) for irregular waves is [50]: P wavefront ffi0:42H s T P ð5Þ where; A is wave amplitude (m), T is period (s), ⍴ is the water density (kg/m3), g is the ground acceleration coefficient (m/s2), H is the wave height (m), Hs is significant height (m) and Tp is peak wave period (s). Detailed theory of wave energy is given in [48–51]. 4. Existing technology Fig. 5. A typical power curve for in-stream CEC devices. A typical power curve of hydrokinetic turbines is given in Fig. 5. Each rotating turbine has a characteristic cut-in and rated velocity. The maximum power that can be generated by a particular turbine is called as rated power. The speed of the free stream at the rated power is known as rated velocity. Designing a turbine blade is a complex process having many parameters. In order to make an ideal design, an optimization algorithm should be employed to test all probable configurations for an optimum solution. Several design and optimization algorithms have been developed in order to maximize the efficiency of the turbines (Harp_Opt [43], JavaProp [44]). Traditional gradient based optimization methods (e.g. Newton’s method) provide fast convergence time but they can fail in the existence of multiple optimum solutions. Contrarily, the genetic algorithms can effectively search the global optimum solutions and select the best local optimum result although taking more time [45]. 3.2. Wave energy conversion systems (WEC) Wave energy conversion (WEC) is a hugely varying stochastic process due to diffraction and radiation [46]. Therefore, the theory is mainly device based. Various hydraulic and pneumatic power conversion systems have been developed to convert the dispersed movements of waves into the mechanical power. WEC devices have reciprocating and rotating parts to use hydrodynamic lift force created by the flow over a hydrofoil or lifting structure producing high torque and low speed output [33]. They vary in size, orientation and distance from the shore [47]. These systems can be bottom and shore mounted or floating. Each wave energy conversion device, extracts certain amount of power from the wave resource in accordance with its efficiency. The power calculation from waves is still not explained sufficiently due to the complexity and the stochastic progression of waves. Various types of wave turbines have been designed based on different working principles. Therefore, in this study, the power in a typical wave source is presented rather than the extraction principles of energy from each wave converter. The wave power density (W/m2) which is the energy per wave period is [48]; P density ¼ pgA2 2T The hydrokinetic energy technology which was evolved in last two decades is one of the newest and fastest growing sector of renewable energies [52]. The technology has gained a significant attention especially for current and wave energy conversion devices. Majority of the systems are at the research and development stages and very few devices are at the pre-commercial deployment stage [53]. The industry is growing rapidly with more than 100 conceptual design of wave, tidal and current energy turbines [54,55]. More recent comprehensive status of the technology and the industrial trends have been given in a number of studies [52,53]. The abundant tidal resources, non-carbon based renewable energy need and economic effects have led the UK to provide a roadmap and assess the environmental effects of tidal energy conversion technologies [52]. With the establishment of the European Marine Energy Centre (EMEC) in UK, Fundy Ocean Research Center (FORCE) in Canada and Ocean Renewable Energy Coalition (OREC) in the U.S. the hydrokinetic device industry has gained momentum. More than $50 million has been invested for the development of the technology by the USDOE (United States Department of Energy), Wind and Water Power Program. The RITE (Roosevelt Island Tidal Energy) project of Verdant Power is one of the very first projects of in-stream energy conversion technologies [56]. ð2Þ Powers per meter of wave front and crest length are [49]; P wavefront ¼ 1 2 2 ρg A T 8π ð3Þ Fig. 6. Classification of hydrokinetic systems. M.I. Yuce, A. Muratoglu / Renewable and Sustainable Energy Reviews 43 (2015) 72–82 4.1. Classification The classification of the hydrokinetic device technology is given in Fig. 6. CEC systems can be sorted as horizontal axis, vertical axis, helical and ducted turbines. Majority of hydrokinetic turbines have a horizontal axis of rotation parallel to the flow direction. The first vertical axis hydrokinetic turbine was designed by Darrieus in the 1920s [33]. In this system, there are a number of hydrofoil shaped blades vertically placed between a top and bottom support, rotating around perpendicular axis relative to the flow direction [57]. Both horizontal and vertical axis converters can be designed to have 2, 3 or multi-bladed propellers. Multi-bladed turbines generate greater starting torques than the two and three-bladed turbines without any balancing problem, however, they cause greater hydrodynamic loses than the other two types [33]. The helical turbines are a different form of Darrieus turbines designed by wrapping blades in a helical shape [58]. The first helical turbine was designed by Gorlov to solve the vibration problems that Darrieus turbines suffered from. In the helical 77 turbines, the axis of rotation is perpendicular to the water flow [47] and they can capture the water motion from every direction even in very low speeds [59]. They can be installed horizontally or vertically with respect to ground [46]. This particular property increases the efficiency and practicability of the helical turbines. Augmentation decreases the pressure within the confined area thus increases the flow velocity [60]. Concentrated fluid flow around the ducted turbines provides high level of energy to be extracted [5]. Ducted turbines are not subjected to Betz limit; therefore, further investigations will help increase the efficiency of these turbines to a desirable level [61]. WEC systems can be categorized according to the energy extraction method as oscillating water columns (OWC), overtopping devices (OTD) and wave activated bodies (WAB). Oscillating water columns (OWC) are partially submerged structures with a collector below the sea level, containing a column of water. When wave enters the collector, the water column moves up and down depressurizes an air column by the compression force of rising and falling of water level. The energy of oscillating air flow is then Fig. 7. (a) SeaGen [66], (b) Verdant Power [61], (c) Pelamis [65] and (d) PowerBuoy [67]. Table 3 Technical specifications of some current energy conversion turbines (Source: [54,60,68–70]). Turbine type Horizontal axis turbines SeaGen Verdant Power Tidal Stream TidEl System(twin propeller) Hammerfest Strøm Tidal Stream Turbine (Tidal Generation ltd.) Open Hydro(twin propeller) Amazon AquaCharger Vertical axis turbines EnCurrent Hydro Turbine (Non-ducted) Davis Hydro Turbine Exim Tidal Turbine Ponte Di Archimede Helical turbines GCK Gorlov Helical Turbine Lucid Energy Technologies GHK Ducted turbines Underwater Electric Kite (twin propeller) Rotech Tidal Turbine Clean Current Turbine EnCurrent Hydro Turbine (ducted) Clean Current Power System Hydroreactor Stream Accelerator Dimensions (m)a Rated Power (kW) Rated Velocity (m/s) Cut-in Speed (m/s) No. of blades 18 5 20 18.5 20 18 15 1.8 1200 35 1000–2000 2  500 – 1000 1520 0.5 2.4 2.2 – 2.3 2.5 3.5 2.57 1.5 0.7 0.7 1 0.7 – 2.5 0.7 0.45 2 3 2 2 3 3 Multi 3 1.6  0.8 6.1 13 65 12.5 250 44 25 4 3 3 2 2 1.5 0.7 Multi 4 2 3 1  2.5 1,2,3 180 40–150–360 7.72 4.5 0.5 0.5 Multi Multi 4 25 18 31 1.7, 2.9, 4 1, 1.5, 2 400 2000 1700–5000 18 16, 44, 84 16, 37, 67 3 3.1 3.5 2.8 3 2.5 1.54 1 1 1.5 1.5 0 Multi Multi Multi Multi 3 Multi a First and second character is the diameter and the length of turbine. If characters are divided by comma (,) that shows there are different dimensions of that type of turbine. 78 M.I. Yuce, A. Muratoglu / Renewable and Sustainable Energy Reviews 43 (2015) 72–82 extracted by a turbine. Overtopping devices (OTD) have a partially submerged floating reservoir which creates a water head. The energy is extracted by using the water level difference between the reservoir and the sea. Wave activated bodies (WAB) have an oscillating mechanism which reciprocate by the effect of the waves and produce energy [34,62]. 4.2. Realistic applications There are various CEC device projects that are working with different principles. Most of the technologies are still at the testing process. Some of the most popular hydrokinetic energy projects are explained below: SeaGen turbine (Fig. 7a) is the first largest attempt to harness in-stream hydrokinetic energy and developed by Marine Current Turbines Company. The device has twin horizontal axis bidirectional two bladed propellers each 18 m in diameter. The system was installed in Strangford, Ireland, in 2008. The turbines were mounted on a steel pipe of 21 m diameter which is anchored to the seabed and they can be raised over the water surface for better adjustment and maintenance. The system is grid connected and capable of delivering up to 6000 MW h electricity. The design has been made to withstand very severe conditions of the ocean. The cut in and rated velocities of the rotor are 0.7 and 2.4 m/s, respectively. Both propellers produce 1.2 MW power at the rated velocity [59,63,64]. Verdant Power turbine (Fig. 7b) is a fixed speed stall regulated device which is designed by Verdant Power Company. It has a three-bladed rotor in 5 m diameter. The propeller has a constant rotational speed of 40 rpm. The cut in and rated velocities are 0.7 and 2.2 m/s, respectively. The turbine delivers 35 kW power with the efficiency between 0.38 and 0.44. The company has installed 6 full scale grid connected hydrokinetic turbines at the East River, New York in 2006 (RITE project) and delivered 70 MW h of energy [52,65,66]. Lucid Energy Company developed a new technology by placing the vertical axis helical turbines inside the pipes having different diameters. The technology uses the advantages of helical and ducted turbines and enables to utilize multiple devices in a single pipe. Up to 100 kW power can be produced in a 1.5 m diameter pipe with 2.1 m/s water velocity. The technology extracts a certain amount of water head from the system [67]. Pelamis (Fig. 7c) is the one of the most popular wave energy conversion devices. In this technology, four cylindrical sections are connected and aligned with the direction of wave. The connection points contain hydraulic structures which pumps oil to the motor and driving the electric generators [68]. Pelamis Wave Power Company is working on a bunch of different projects to set up wave farms at the coasts of Scotland and Portugal. Two WEC devices each has 0.75 MW capacity have been deployed near Orkney, Scotland. The technology is still being tested within the scope of different projects [69]. PowerBuoy (Fig. 7d) has been designed by the American Company of Ocean Power Technologies. It has axisymmetric two body heaving mechanism. The power is produced by the relative motion of both bodies. A 40 kW prototype has been constructed at Spain, in 2008. It is planned to increase net power with multiple applications [68]. The technical specifications of some prominent current and wave energy conversion systems are given in Tables 3 and 4, respectively. 4.3. Technology survey US Department of Energy’s water power program provides the largest database [76] on hydrokinetic energy devices. In this database wide variety of hydrokinetic turbines have been assessed and listed according to their technology, company and project status. The systems were categorized in five phases (undeveloped, siting/planning, site development, device testing and deployed). Globally, since the beginning of 2013, about 280 different companies have been worked on current and wave energy converters. Totally there are more than 300 projects from undeveloped to deployed phases. The number of hydrokinetic devices at the stage of testing and deployment are 81 and 55 for current and wave energy conversion systems, respectively. Fig. 8 shows the rates of hydrokinetic turbine projects at different phases. Fig. 9 shows the country based distributions for all stages except undeveloped phase. Fig. 10 illustrates the quantities of different categories of hydrokinetic turbines. 4.4. Economic aspects The energy cost is one of the most important properties for the majority of renewable technologies. The net energy cost is depend on the capital cost, mooring, maintenance and operation cost, simplicity of the design, diversity of the applications, scalability, labor engagement, system reliability, performance and social acceptance [5]. There is no definite universally accepted energy cost for hydrokinetic applications because it is a new emerging area and the technologies are still at the development and testing stage. In order to supply suitable and reliable amount energy to the grid, these technologies should be better investigated in forms of array installations. Table 4 Technical specifications of some wave energy conversion systems [67,71–74]. System name Oscillating water columns Limpet Energetech OreCon Sperboy Overtopping devices Wave Dragon Wave activated bodies Pelamis Wavebob PowerBuoy 500 Wave Roller Archimedes Wave Swing Aqua buoy a Dimensions (m) Weight (t) Water depth (m) Rated powera (kW) – 35 m (Parabolic width) 32 m (Diameter) – – 450 1,250 – – 50 m 4 50 – 500 500–2000 100 1000 4 25 4000 4 50 4 50 – – 43 m 4 50 750 1000 500 100 4000 250 260  300 150  4.63 (Diameter) 15 Diameter – – 7  18 (Diameter) 6 Diameter The rated power is for one unit, if there is multiple application of the device.  30,000 380 440 – – – – M.I. Yuce, A. Muratoglu / Renewable and Sustainable Energy Reviews 43 (2015) 72–82 79 Deployed Wave Current Device Testing Site development Siting/planning Undeveloped 0 20 40 60 80 100 Fig. 8. Hydrokinetic turbine projects at different phases (data is based on [75]). Belgium Japan Scotland Finland Denmark Portugal Germany Sweden Singapore Netherlands Italy Ireland Norway France Australia Canada United Kingdom United States WEC CEC 179 0 5 10 15 20 25 30 Fig. 9. Country based distributions of hydrokinetic turbine projects (data is based on [75]). Axial flow turbine Cross flow turbine Oscillating water column Overtopping device Wave activated body 0 50 100 150 200 Fig. 10. Categorization according to working principle (data is based on [75]). The electric energy generation cost per kW by means of hydrokinetic technology is still much more expensive than the conventional hydropower. However, it is believed that, the hydrokinetic energy sector will become a major source of electricity and supply more cost-effective and reliable energy by the year of 2050 with the further developments [77]. Today, the capital cost required to install offshore hydrokinetic projects are 1.5–2 times greater than on land applications Additional costs are required in order to marinize a device. The first investment cost is highly depend on the project location and technology. On the other hand, the operation and maintenance costs are three times of land devices [47]. Some of the considerable economic analyses on hydrokinetic power have been summarized by Bahaj [46]. An extensive and recent economic analysis on the tidal and wave energy is given in [77]. It is estimated that, levelised cost of energy for early array applications varies between 24–47 c€/kW h for tidal and 34–63 c€/kW h for wave energy. The overall deployment cost of wave applications is far greater than that of tidal devices [77]. 5. Environmental impacts In contrast to conventional hydrostatic systems, the hydrokinetic devices are working without significantly altering the natural pathway of the stream [55] and water habitat. The impact of rotors or other parts to the marine life and underwater noise is relatively low, compared with wind turbines or ship propellers, due to the low speed running characteristics and less surface area of the propellers [7]. Since the hydrokinetic turbine technologies are still at the development stage, they may have some concerns associated with the environment. Uncertainties regarding these new turbines should be well studied. Some of the bad environmental impacts of these systems have been noted by scientists. Interaction of hydrokinetic devices with marine ecosystem is still under investigation. The turbine blades or other moving parts can strike the aquatic organisms such as fishes, diving birds, etc. Mobile animals can be entangled in submerged cables. The electromagnetic impacts of 80 M.I. Yuce, A. Muratoglu / Renewable and Sustainable Energy Reviews 43 (2015) 72–82 transmission lines may affect the underwater animals. The noise, vibration and turbulence generated by the moving or rotating parts can damage the aquatic wildlife environment. The chemical contaminants leached from underwater machine parts can pollute the water. Due to the energy extraction from the system, the volumetric flows, current patterns and tidal ranges can be affected. The hydrologic regime such as, natural flow depth, velocity distributions and sedimentation of the site can vary, especially in shallow waters. The river navigation and fish migrations may be blocked by the devices [78–81]. The most sensible way to prevent the environmental impacts of hydrokinetic devices is to avoid the areas having sensitive habitats such as the regions with high biodiversity and fragile sites. On the other hand some of the seasons should play a critical role for the biodiversity such as migration and reproduction periods. An extensive study has been reported by USDOE (United States Department of Energy) on the potential environmental effects of hydrokinetic energy technologies [82]. Similarly the type of the device and technology plays an important role for the environmental impacts of the system. These effects should be studied at the designing process. Laboratory experiments or CFD simulations should be conducted before deployment. Fast moving and rotating parts should be avoided in critical environments. 6. Operating conditions and mooring Hydrokinetic turbine parts are exposed to harsh and salty marine environment. The propeller and other metallic parts should be carefully constructed to prevent corrosion. Increasing the thickness of the still and coating are the solutions to avoid corrosion. The turbine nacelle should be water resistant and wellsealed. On the other hand, seaweed and other filamentous plants can foul the blades and reduce the performance [14]. Wave energy converters are more difficult to design relative to the tidal and river applications due to complexity of platform and system motion [83]. The turbine itself, mooring and other structures, especially in wave devices are subjected to severe conditions [46]. The turbine system should be sufficiently strong in order to overcome the drift force of wave and currents. The requirements of mooring technology could be transferred from offshore oil and gas systems. Electric transmission lines on marine environment should be well protected and tension loads must not be permitted [62]. River current turbines are functioning in relatively calm environment. Scouring at the bottom of rivers can change the flow regime thus, decrease the efficiency of turbines. Sedimentation in rivers may block the turbine parts. Cavitation is one of the major threats for the turbine rotors. The design of the blades should carefully be conducted in order to prevent cavitation. The amount of cavitation and its region could be determined with the CFD (computational fluid dynamics) simulations of rotating or moving parts. exceed 80 to 90% levels in dams, hydrokinetic turbines can barely achieve 35%. In current energy conversion (CEC) systems, the power output is directly proportional to the cubic power of the flow velocity. Therefore, in hydrokinetic systems, higher flow velocity provides higher power. So called Betz limit is one of the major obstructions on the efficiency of in-stream hydrokinetic turbines. Betz law proposes that a free flow turbine cannot exceed 59.3% theoretical efficiency. However, the Betz limit can be achieved with the proper augmentation of the turbine propeller. Augmented turbines can provide higher level of energy extraction with increasing flow velocity and inducing sub-atmospheric pressure within a constrained area. Malipeddi and Chatterjee stated that, the power coefficient of a straight bladed-Darrieus turbine can be increased up to 0.72 with augmentation [84]. Similarly, helical devices present a promising technology. Further scientific investigations on the helical and ducted turbines would increase the overall efficiency of hydrokinetic technology. On the other hand, in order to increase the performance, variable speed and variable pitch hydrokinetic turbines should be further investigated [85]. Hydrokinetic technology has less environmental impacts than that of conventional hydropower. The technology work without significantly altering the natural pathway of stream; however, it still poses some environmental concerns such as; striking the marine animals, noise, vibration, electromagnetic impacts and other regional effects. The environmental impacts and suitable precautions are still being investigated by the scientists. Harsh and sediment-rich water environment is one of the biggest drawbacks of hydrokinetic energy conversion systems. Especially wave energy devices should be projected and assembled to withstand severe environment effects such as, the drift force of wave and currents. On the other hand, effect of sedimentation on riverine hydrokinetic devices should be better studied. Most of the sites that are suitable for hydroelectric energy production have been occupied by dams in the developed counties. The hydrokinetic technology presents an alternative way to produce electricity in such sites. On the other hand, these devices would provide the solution of the electrification problem for remote areas and less developed countries which are grid independent. The governments also play a significant role for the development of the technology. The leading activities such as research and development studies, encouraging scientists to build prototypes, developing, design and testing standards, licensing, leasing, permits, establishing required organizations and other formal works should be concluded by the government activities [86]. Hydrokinetic energy conversion devices need further and more rigorous studies. Feasibility, efficiency, impact and reliability analyses [87] need to be conducted. Hydrodynamic characteristics of the hydrokinetic devices should be better investigated. The ways of increasing the flow velocity through augmented channels, improving overall efficiency of the turbines, cavitation and other structural problems should be adequately studied. This clean and great source of energy embedded in water flow should not be overlooked. 7. Discussion and conclusion Hydrokinetic energy is a newly emerging area of renewable energy technologies. 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