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Novelapproachofbidirectionaldiffuser-augmentedchannelssystemforenhancinghydrokineticpowergenerationinchannels

ARTICLEinRENEWABLEENERGY·NOVEMBER2015

ImpactFactor:3.48·DOI:10.1016/j.renene.2015.05.038

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AlyHassanElbatran(A.H.Elbatran)

UniversitiTeknologiMalaysia

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OmarYaakob

UniversitiTeknologiMalaysia

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YasserAhmed

UniversitiTeknologiMalaysia

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M.RajaliJalal

UniversitiTeknologiMalaysia

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Renewable Energy 83 (2015) 809e819

Contents lists avai

Renewable Energy

journal homepage: www.elsevier .com/locate/renene

Novel approach of bidirectional diffuser-augmented channels systemfor enhancing hydrokinetic power generation in channels

A.H. Elbatran a, c, O.B. Yaakob a, b, Yasser M. Ahmed a, b, d, *, M. Rajali Jalal b

a Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysiab Marine Technology Centre, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysiac Faculty of Engineering, Arab Academy for Science and Technology and Maritime Transport, 1029 Alexandria, Egyptd Faculty of Engineering, Alexandria University, Alexandria, Egypt

a r t i c l e i n f o

Article history:Received 15 May 2014Received in revised form26 April 2015Accepted 21 May 2015Available online

Keywords:Hydrokinetic energyMicro scale channelAugmented channelNozzleCFDEfficiency

* Corresponding author. Faculty of Mechanical EngMalaysia, 81310 Skudai, Johor, Malaysia.

E-mail addresses: yasser@mail.fkm.utm.my, haaaly

http://dx.doi.org/10.1016/j.renene.2015.05.0380960-1481/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

Hydrokinetic is a recently introduced type of hydropower energy, having been proven as the mosteffective and predictable renewable energy source available around the world, especially in the rural andelectrification areas. Most of these sites are dependent on small and micro scale stations to producecheap but abundantly available and effective electrical energy. Hydrokinetic energy that can be har-nessed from the flow of water in the irrigation and rainy channels is a promising technology in countrieswith vast current energy. Micro hydrokinetic energy scheme presents an attractive, environmentallyfriendly and efficient electric generation in rural, remote and hilly areas, as effort to reduce the ever-increasing greenhouse gas emissions and fuel prices in these sites. Though potential, this scheme isyet to be fully discovered to the considerable extent, as researchers are still searching for solution for themain problem of low velocity of current in the open flow channels. Deploying acceleration nozzle in thechannel is a unique solution for increasing the channels current flow systems' efficiency. Accelerationnozzle channel method has numerous advantages especially on the environmental impact, yet has notbeen given much attention in the renewable energy field. This paper proposes a novel system config-uration to capture as much as kinetic energy from in stream current water. This system, known asbidirectional diffuser-augmented channel functions by utilizing dual directed nozzles in the flow, sur-rounded by dual cross flow turbines. This type of turbine is commonly used for hydropower applications;and this study proposes the employment of this turbine for hydrokinetic power generation. Numericalinvestigations had been performed using finite volume Reynolds-Averaged NaviereStokes Equations(RANSE) code Ansys CFX to investigate the flow field characteristics of the new system approach withand without the turbines. The performance of the twin (lower and upper) cross flow turbines had alsobeen studied. It was found that the highest efficiency of 0.52 was recorded for lower turbine at tip speedratio (TSR) of 0.5.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

From the world population of 7 billion, 1.3 billion people stillremain without access to electricity, especially those in the ruraland poor areas in developing countries. From this number, 22%are those living in developing countries, mostly living in the sub-Saharan Africa and developing countries in Asia. Nevertheless, itis expected that in the beginning of year 2040, almost one billionpeople will gain access to electricity while nearly 500 million stillremain without access. Renewable energy is also expected to

ineering, Universiti Teknologi

2@live.utm.my (Y.M. Ahmed).

represent 50% of the total power generation in these areas [1,2].This is because renewable resources promise efficient solution toachieve a perfect connection between renewable energy andsustainable development in the future [3]. Tidal current power isone of the most prominent, clean and predictable power resourcefrom water in the world, especially regarding micro applicationswhich can be utilized in remote and hilly areas for electrificationprocess [4e6]. Micro hydropower scheme is the most suitable andefficient option of renewable energy, since it has low environ-mental effects and lower operation and maintenance costs [7].Most rural and hilly areas use small and micro hydropower plantsto produce cheap, available and effective electricity supply [8,9].Hydrokinetic or in-stream technology is a novel type of small andmicro hydro-power, which functions by deploying hydrokinetic

A.H. Elbatran et al. / Renewable Energy 83 (2015) 809e819810

turbines in flow of rivers or canal to generate energy [9e11].Harnessing kinetic energy from the flow of water in open chan-nels is closely similar to current power generation, thus canoptimize existing facilities like weirs, barrages and falls [12]. Inprevious years, researches had been solely focusing on studyingthe water stream technology from both flow and turbines systemspoints of view, proposing improvements on the open channelflow and the most suitable turbine systems for use in thesechannels. Literature review lists various existing current energytechnologies which can be employed to extract power from oceantidal current, horizontal axial flow and horizontal and verticalcross flow turbines such as in-plan, Darrieus, H- Darrieus, Savo-nious and helical turbines. The free stream flow of these systemsrequires high amount of mass flow with low velocities andpressures to be able to extract energy; nevertheless, their effec-tiveness in all aspects is debatable as previous conventional tur-bines are more suitable for application in areas with high pressureand flow rate [13]. Due to this shortcoming, the flow in the newsystems needs to be accelerated to increase the harness of flowingpower. Using nozzle is the most efficient choice to accelerate theflow, as nozzle can be utilized in stream of the channel flow orducted around turbines. Khan et al. [14] and Elbatran et al. [15]carried out analytical and numerical studies to accelerate theflow by deploying nozzles for run-of-river turbines in openchannels. Kirke [16] studied the application of a ducted turbine, inwhich he found more power generation than using the openturbine. Furukawa et al. [17] studied the design parameters ofducted Darrieus-type water turbine principle to achieve the mostperformance at low head hydropower. Shimokawa et al. [18]proposed more simplified runner casing of Darrieus, and exam-ined the inlet nozzle and the small upper-casing through exper-iments to improve turbine performance.

One of the most attractive turbines is cross flow turbine(CFT), known as Banki and Ossberger turbines. This type ofturbine is more practical than other types of existing microhydropower turbines, easy to construct and cost effective [19].These turbines are also familiar with high and medium flow rateand low head [20,21], capable of generating average efficiency of80% for small and micro power outputs [21]. These values are,however, lower than those of other most popular hydro turbineslike Pelton, Turgo, Francis and Kaplan [22]. In comparison, themain advantage of this turbine is its ability to keep maximumefficiency with higher range of flow [23]. Geometrical parame-ters like number of blades, runner diameter ratio, nozzle entryarc, guide vanes and angle of attack are the cornerstone of theCFT efficiency. Refs. [19,24e29] studied CFT by utilizing differentgeometric parameters under flow/head variations. The crossflow turbine is more proper for run-of-river applications becauseits requirement for large head is lower, depending on the flowrate than other types of turbines [19]. Kim et al. [13] presented anew configuration of cross flow turbine for harnessing tidalenergy by utilizing a larger area of the channel. Choi et al. [30],Choi et al. [31] and Prasad et al. [32] also implemented andconducted experimental and numerical studies on the cross flowturbine as Direct Drive Turbine (DDT) for wave energy produc-tion field.

As referred from the literature, many researches had con-ducted studies to investigate the flow characteristics in nozzlesand ducts, as well as studying the performance of CFT; in whichall concluded that CFT is indeed suitable for current flow. In thisresearch work, we present a new configuration system whichutilizes bidirectional nozzles in two directions of channels,including twin cross flow turbines, to increase the hydrokineticpower efficiency. This idea is fairly a new approach in the hy-drokinetic energy generation field. This research also presents the

current arrangement for use at rivers, irrigation/rainy channelsand shallow waters.

2. Diffuser-augmented channel and cross flow turbinesmodels

Model diffuser e augmented channel as shown in Fig. 1 iscomposed of an inlet nozzle in Y-direction, followed by NACA0025as a nozzle positioned in the plan direction, a diffuser at the exitside of channels in Y and Z directions and dual cross flow turbinerunners. In this configuration, the nozzle incoming water has a freesurface of 0.4m. The upper intake occupies 0.15m of the inlet waterlevel, while water enters the lower part of the inlet nozzle at 0.25 mwater because the lower turbine is in a normal mode situation, thusis expected to achieve higher performance. The free surface at thespecial system inlet enables the deployment of current arrange-ment in shallow water or at the surface of river applications inaddition to channels.

As shown in Fig. 1a, the inlet nozzle is not necessarily sym-metrical. The use of CFT runners in channels and the currentdiffuser e augmented channel system is new design concept pro-posed by the authors. Fig. 2 shows the runner, which has length, L,of 0.38 m, outer diameter of Do ¼ 0.2 m and inner diameter ofDi ¼ 0.1368 m; having diameter ratio of around 0.67, as the runnerwould be more efficient at this ratio [17]. There are a total of 26blades [12] formedwith an arc camber having 2.7 mm thickness (t).The blade inlet and outlet angles are positioned at 90� and 30�

respectively, as these angles configuration has been mentioned inthe literature as able to produce the best results [12,17,21,28].Meanwhile, the clearance between the blade tip and the casing isset 5 mm.

Nowadays, many countries have irrigations or rainy microchannels, which is already a provision for further developing thistechnology. Development of open flow micro channels in openflow that suit these countries has a main problem of low velocityof current, hence the deployment of nozzle in stream of openchannels flow is the most possible method for increasing thechannels current flow systems efficiency. Thus, in this study pa-per, we propose the utilization of bidirectional diffuser e

augmented channels for micro scale channels. The geometry ofthe micro scale channel model is shown in Fig. 3, where Lc ¼ 20 mindicates the overall length of channel, W ¼ 0.6 m is the width ofthe channel, T ¼ 0.4 m is water level at the channel inlet, andh ¼ 1.2 m is the channel depth. The channel has a rectangularcross section, and the diffuser e augmented system is impededand positioned in the middle of the channel plan. This system isset with 10 m long in up and down streams to diminish the effectsof the disturbance of the flow, and 1.2 m height to help the solverfor predicting the free-surface flow pattern accurately at thedifferent simulations.

3. Governing equations

Ansys CFX was used to construct the mathematical descriptionof the free surface flow, based on the homogenous multiphaseEulerian fluid approach. In this approach, both fluids (air andwater)share the same velocity fields and other relevant fields, such astemperature and turbulence, and they are separated by a distinctresolvable interface. The governing equations for the unsteady,viscous, turbulent flow are the NaviereStokes equations, which canbe written in the following form:

v

vtðrÞ þ v

vxiðruiÞ ¼ 0:0 (1)

Fig. 1. Schematic diagrams of diffuser e augmentation channel surrounding the runners.

Fig. 2. The cross flow turbine runner.

A.H. Elbatran et al. / Renewable Energy 83 (2015) 809e819 811

ðruiÞ þv

vxi

�ruiuj

� ¼ �vpvxi

þ v

vxi

�� ru0

iu0j

�

þ v

vxj

"m

vuivxj

þ vujvxi

� 23dijvulvxl

!# (2)

where

�ru0iu

0j ¼ mt

vuivxj

þ vujvxi

!� 23

�pkþ vui

vxi

�dij (3)

and

r ¼X2a¼1

rara; m ¼X2a¼1

rama;X2a¼1

ra ¼ 1 (4)

The shear stress transport (SST) turbulent model can beexpressed in the following mathematical form:

vðpkÞvt

þ v

vxiðpkuiÞ ¼

v

vxj

Gk

vkvxj

!þ Gk � Yk (5)

Fig. 3. Schematic of the test model showing different parts.

A.H. Elbatran et al. / Renewable Energy 83 (2015) 809e819812

vðpuÞvt

þ v

vxiðpuuiÞ ¼

v

vxj

Gu

vu

vxj

!þ Gu � Yu þ Du (6)

In the previous equations, r is water density (kg/m3), ui¼ (u, v,w) represents velocity components in the directions of xi¼ (x y z), kisturbulence kinetic energy, u is specific turbulence dissipation, m iswater dynamic viscosity (N s/m2). Gk and Gu represent the effectivediffusivity for k and u. Gk represents the generation of turbulencekinetic energy due to mean velocity gradients, and Gu representsthe generation of u. Yk and Yu represent the dissipation of k and u

due to turbulence, while Du represents the cross-diffusion term.

4. Grid generation

A grid generator of the Reynolds-Averaged NaviereStokesEquations (RANSE) code (ICEM) computational fluid dynamic (CFD)was used for meshing the system with hybrid (structured hexa-hedron and unstructured tetrahedral) grids. Several computationalgrids had been tested in this study to check the solution sensitivity,and ultimately, mesh with structured and unstructured elementshad been chosen for the model channel and diffuser-augmentedsystems in the current study. The computational grid was dividedinto four domains; model channel, bidirectional diffuser-augmented channel, and lower and upper similar turbines, asshown in Fig. 4. The total number of elements in the augmenteddiffuser channel configuration system was estimated about2,311,268.

5. Numerical method

In this research, finite volume method was used for discretizingthe governing equations in Ansys CFX. The convective terms werediscretized using Second Order Upwind scheme, and the pressurewas interpolated using linear interpolation scheme, while thecentral difference scheme was utilized for diffusion terms. For thepressureevelocity coupling, SIMPLE (Semi-Implicit Methods forPressure-Linked Equation) was utilized. Unsteady simulation(transient simulation) was performed based on Reynolds averagedNaviereStokes (RANS) equations with the keε shear stress trans-port (SST) used for turbulence modeling. Convergence was moni-tored through making dimensionless residual sum for all variablesacross the computational points. The convergence of the solutionwas set to 5 revolutions. The inlet velocity and water depth (T) atthe inlet boundary of the model channel in this researchwere set at1 m/s and 0.4 m, respectively. The velocity also had normal direc-tion to the water model channel inlet. The outlet boundary condi-tion was specified according to the outflow, and the reference

pressure was set to atmospheric 1 bar, in which the boundaryconditions were constructed as a wall (no slip condition). 6e8parallel computers were used to complete the current research. Inorder to get various values of tip speed ratio (TSR), the angularvelocity (u) was varied, where the tip speed ratio was defined asthe ratio of the circumferential velocity of the turbine to the meanturbine runner inlet velocity. TSR was calculated using the equationbelow:

TSR ¼ urV

(7)

where r is the turbine outer radius and V is the mean turbinerunner inlet velocity.

The following results had been obtained using an inlet modelvelocity channel “U” of 1 m/s, which was generic for use in all cases.In addition, the runner blade tip speed ratio (TSR) as defined inEquation (7) had been varied from 0.08 to 1.2 at a constant radius of0.1 m for upper and lower rotors.

6. Results and discussions

6.1. Flow field characteristics through system

The flow field characteristics were studied through special andunique system to evaluate the system efficiency. Increasing flowcharacteristic could be achieved using ducted diffuser whileconcentrating the flow by applying two ducted rectangulars sur-rounding the turbine runners. From Figs. 5 and 6, it can be seenclearly that the presented application of bidirectional diffuser-augmented channels is really significant to increase the flowingspeed of the fluid. Increasing the speed is a crucial factor indesigning oceans low speed current appearance. The main advan-tage of the current system is that the flow through the channel isenhanced twice by the bidirectional nozzles before entering therotor, as shown in Figs. 5 and 6, which illustrate the flow charac-teristics through the present system in two planes in the absence ofthe turbines. In the simulation, it also noticed that the inlet velocityof the diffuser-augmented channel was diminished compared tothe model channel free stream velocity, which reached 0.6 m/s. Thefirst nozzle stage accelerated the flow to regain the velocity value tothe origin of almost 1 m/s, as shown in Fig. 5, and then the seconddirected nozzle increased the mean runner inlet velocity values,which had values slightly more than twice than that of the modelchannel free stream velocity, in which the velocity recorded 2 m/sand 1.6 m/s for the lower and upper turbine locations, respectively.The second directed nozzle also developed a flow pattern especiallyat the walls, as can be seen in Fig. 6, which was due to theenhancement in the kinetic energy absorbed by the turbines, thus

Fig. 4. Meshing for (a) model channel, (b) bidirectional diffuser augmented channel and (c) the turbine runner.

Fig. 5. Velocity vectors showing the flow of water along the proposed channel.

A.H. Elbatran et al. / Renewable Energy 83 (2015) 809e819 813

increasing the system efficiency.It was also important to measure the flow properties of the

system with and without turbines to get the amount of kineticenergy utilized by the turbines. Fig. 7 displays the flow character-istics through the current diffuser-augmented channel with tur-bines in two axis plan. The inlet runner velocity reduced whenreaching the turbine, in which the mean inlet velocity recordedalmost 1.7m/s and 1.45m/s for lower and upper runners' entrances.After that, the velocity started to decrease behind the turbines, andin contrast, the flow was more accelerated in the absence of tur-bines. The generated energy by the new system is gained mainly

from the kinetic energy, pressure drop, and potential difference(small) due to the special design of the system. Thus, the differencesbetween the values of velocities behind the turbine in two casesplay a significant role in the amount of energy harnessed by tur-bines. Moreover, diffuser-augmented could redirect the flowperpendicular to the rotor blade and increase the flowat the runnerintake.

The velocity at the turbine inlet was varied using TSR for thelower and upper turbines cases, as shown in Fig. 8. It was observedthat the mean inlet of runner velocity for two cases considerablydecreased with the increment of TSR. On the other hand, the mean

Fig. 6. Velocity vectors showing the flow of water through the proposed channel in Z-plan.

A.H. Elbatran et al. / Renewable Energy 83 (2015) 809e819814

entrance velocity for the lower case gained higher values than theupper turbine, because the augmentation channel at the upper partintake exploited 0.15 m of the inlet water level, whilst the waterentered the lower part of the augmented channel with 0.25 mwater level. This resulted in that the kinetic energy available to thelower turbine was more than the upper turbine; consequently, thepower output of upper turbine was expected to decrease asdescribed in the next section regarding turbine powercharacteristics.

Fig. 9 shows the velocity vectors for the flow through the lowerand upper rotors blades. The water flow was directed towards therunners through the bidirectional nozzles. When the water flowedthrough the second directed NACA nozzle, the first stage power(A) was generated. The flow then crossed the vaneless inside ofthe runner and departed through, producing the second stagepower (B). Some higher velocity water flow was entrained be-tween the turbine blade tips and casing, producing jet flow (C)which contributed to added energy generation; that in fact, thisstage power was created by the NACA nozzle. Hence, the flow ofwater through the turbine passed over the blades twice beforeexiting the runner, which transmitted the kinetic energy of flow tothe turbine blades. Jet stage was added to increase the energygenerated by the turbines. The kinetic energy of the flow wasabsorbed by the blades, causing them to move and rotate in thedirection of counter clockwise due to decelerating and the accel-erating regions, as shown in Fig. 10. It was noticed that the bladesangle was not ideal for use in this configuration. Optimum anglewas needed to make the flow more perpendicular on the blades tocapture as much torque as possible. Furthermore, since the cross-flow turbine was originally classified as impulse turbine, weexploited this nature to generate jet flow to present the third jetregion at the blade tips to assist the blade for rotating andgenerating as much power as possible, as can be referred fromFig. 10. On the other hand, vortex would be formed in the runner,but the vortex formed in the upper rotor occupied more area thanthe lower rotor, but the vortex strength was rather higher in theupper turbine runner. The vortex had a bad effect on the flow fieldcharacteristics, which decreased the generated energy and theefficiency of the upper turbine. Moreover, the vortex caused thewater entered through the upper turbine against the gravitationaleffect.

In this study, we also considered the pressure variation in thediffuser-augmented channel with the upstream and downstream ofthe lower and upper turbines in XZ and XY planes (see Fig. 11). Thepressure recorded higher values in front of the runners, and laterbegan to reduce through and behind the turbines, which causedpressure difference. Furthermore, the pressure difference appearedon the blades surfaces in two stages (dark blue circles), caused byhigher pressure in the decelerating region (lower surface “A”) andthe lower pressure created in the accelerating region (upper surface“B”). At this moment, energy was produced due to pressure dif-ference. It was also observed that low pressure difference wasproduced through the upper rotor, which had a bad effect to theenergy transfer process and the efficiency of upper turbine.

6.2. Performance characteristics of CF turbines

The main interest of the current work is investigating the per-formance characteristics of the proposed bidirectional diffuser-augmented channels system to increase the effectiveness of thehydrokinetic flow stream energy in micro channels. This was donethrough measuring the performance properties of the lower andupper cross flow turbines performance. Efficiency (h) of turbineswas used to reflect the turbine performance, as presented inEquation (8):

h ¼ T*uPc

(8)

where T is the turbine torque, u is the angular velocity of the tur-bine; Pc is the maximum extractable power that can be harvestedfrom the channel model, as described in Equation (9):

Pc ¼ 0:5rAU3 (9)

where r ¼ 1000 kg/m3 is the density of water, (A) is the sectionalarea of the flow at the inlet of diffuser-augmented channel, andU ¼ 1 m/s is the flow stream velocity.

The lower and upper cross flow turbines were tested at differenttip speed ratio. Fig. 12 shows the output power variations atdifferent tip speed ratio (TSR) for lower and upper turbines. It wasnoticed that the maximum power output recorded for the lower

Fig. 7. Flow characteristics in two plan of the channel with turbines at TSR ¼ 0.42.

A.H. Elbatran et al. / Renewable Energy 83 (2015) 809e819 815

turbine case was around 39Wat a tip speed ratio around 0.5, whilethe peak output power was about 18.36 Wat 0.64 tip speed ratio inthe case of upper turbine. Consequently, the efficiency, h, wasconsiderably higher for lower turbine for most of the tip speed ratioranges studied in this work. 0.52 was considered as the most sig-nificant value for lower CFT, while 0.4 was considered as the highestefficiency in case of the upper turbine, as shown in Fig. 13. Theresults showed that maximum efficiency was obtained at adequatevalues of tip speed ratio (0.4 and 0.64), in which these values werequite acceptable and in accordance to the theoretical study by Balji[33]. Furthermore, the highest output power that could be har-nessed was 57.36 W for overall current system. According to thediscussion in previous section, it was expected that the flow fieldperformance characteristics of the water flowing through the lowerturbine would be more impacted than that through the upper

turbine. Besides, the pressure difference through the lower runnerwasmore than the upper rotor, as can be referred from Figs. 8, 9 and11. Hence, higher power generation could be obtained in the case oflower turbine. This proved the importance of enhancing the flow ofwater to get significant power extraction.

6.3. Cost issues

The main problem in supplying electrical energy to rural andremote sites is that, the electrical energy generation by conven-tional ways is very costly, in addition to financial and economy is-sues already faced by these sites. Micro hydropower scheme ispotential as the cheapest but most efficient solution for electricsupply generation, especially in rural and poor areas. The microhydropower schemes present an effective solution which has been

Fig. 8. Runner inlet velocity variations at different tip speed ratios for lower and uppercases.

Fig. 9. Velocity vectors showing the flow of water through the two runners atTSR ¼ 0.478 for lower turbine and TSR ¼ 0.534 for upper.

A.H. Elbatran et al. / Renewable Energy 83 (2015) 809e819816

recommended by many international organizations such as UnitedNations Industrial Development and World Bank [8]. The WorldBank states that the initial cost of micro hydropower scale is only 6cents/hour, compared to those of wind and solar which cost 7cents/hour and 10 cents/hour, respectively [34]. The proposed hy-drokinetic system is a novel category of micro hydropower systems,which is significantly cost effective for socio-economic develop-ment of poor areas.

The cross flow turbines runner proposed here has simple designand fabrication because it is only composed of two circular discs

joined by curved horizontal blades. The runner blades are, there-fore, possible to fabricate from cheap material and then weldedonto the two discs. Hence, it can be manufactured by local work-shops without requiring special tools or specific knowledge. Forthis reason, the cross flow turbine can be applied in micro hydro-power stations for developing countries. The low cost schemes,including, CFT have been installed in Nepal, Sri Lanka, Peru andIndonesia by Swiss Centre for Appropriate Technology (SKAT),German Technical Operation Agency (GTZ), and entec Consulting &Engineering [7,35,36]. The total cost of these projects of micro hy-dropower schemes varies from $1500 to $3000 according to kilo-watt of produced power [37,38]. The cost of turbines plays animportant role in determining the total budget of micro powerplants, as turbine and generator set constitutes nearly 30% of thetotal cost of the scheme [8]. It is estimated that the present CFTwithgenerator costs around 450 up to 900 $US/kW. Furthermore, theCFT runners here are implemented without the need for specialcasing, inlet nozzles, drums and guide vanes; hence the cost ofremoving parts can compensate the cost of two directed nozzlesand diffuser of the proposed configuration. According to ourcurrently prototype experience and low velocity regions inMalaysia, the cost of bi-directed duct and two cross flow turbinesrunners including labor, material and manufacturing is nearly 1000$US/kW. This amount is expected to decrease in the sites with highvelocity ranges because the tidal current power available in thiscase for the same configuration dimension is comparably high.

On the other hand, based on the prototypes cost analysis of thetidal current plants which can be utilized in the current configu-ration as a suitable alternative scheme, the cost of prototype pro-posed by Carbon Trust (CT) was 12,500 $US/kW, and some devicesalso were installed for under 7500 $US/kW. However, other com-mercial projects indicated that average total cost of turbine andgenerator set for shallow and deep tidal current stream is around5000 $US/kW, and around 5156 $US/kW for deep tidal stream [39].Thus, the mean cost of turbine generator set which may bedeployed in the present system is from 1500 $US/kW to 2250 $US/kW. According to the cost analysis, the initial cost of CFT withgenerator set for the proposed scheme is around one third of thecost of tidal current turbine with generator. Thus, CFT is the mostsuitable option which can be considered for the proposed schemecompared to alternative configuration of tidal current turbine.Another advantage of the current configuration is that it can beutilized in rainy and irrigation channels, provided that these re-gions have been proven technically and economically viable formicro power production scale [40].

6.4. Main features, advantages and limitations of the system

The current system is divided into twomain parts, which are theaugmentation channel and cross flow turbines. The current channelis uniquely designed for micro channels applications. This system isthe first ever in using dual nozzle and diffuser at different di-rections, which enhances the flow velocity more than once beforeentering the turbines, especially for low velocity districts. In addi-tion, the use of NACA shape for developing the convergence-divergence nozzle is fairly new concept. Deployment of the crossflow turbines in the present augmentation channel is also a newand most effective solution to overcome the low efficiency ofconventional tidal current turbines, because of their advantage ofbeing able for use as hydrokinetic turbines.

From the simulation conducted in this study, it was noticed thatthe current configuration was the most effective in the presence ofthe free surface at the new proposed channel system inlet. Thismeans that the current configuration is most potential for appli-cation on the surface of river, shallow water and channels. This

Fig. 10. Velocity vectors showing the flow of water through the lower runner blades in the first stage at TSR ¼ 0.478.

Fig. 11. Pressure contours of the flow of water through the runners at TSR ¼ 0.478 for lower turbine and 0.534 for upper turbine.

A.H. Elbatran et al. / Renewable Energy 83 (2015) 809e819 817

configuration can also be developed to be utilized in the waveenergy extraction field. This system promises sufficient perfor-mance and higher efficiency, in comparison with conventionalhydrokinetic turbines. Consequently, it can be presented as themost suitable solution for rural and remote areas micro stations.

The capability of being generated by the current system arrange-ment is also very close to some similar configurations. The presentnumerical results had been compared with the experimental andnumerical analysis works by Choi et al. [30], Choi et al. [31], Parasadet al. [32] and Kim et al. [13]. A good agreement between numerical

Fig. 12. Power output variations of the lower and upper turbines at various TSR.

Fig. 13. Efficiency variations of the lower and upper turbines at different TSR.

A.H. Elbatran et al. / Renewable Energy 83 (2015) 809e819818

and experimental results was observed for turbine performancewhich the maximum efficiency obtained in this research wasaround 0.52, close to 0.51 found in similar study [13]; even withother comparable configurations [30e32] which obtained 0.516,0.517 and 0.55, respectively. There is more; the efficiency of thecurrent augmentation channel could still be increased by con-ducting modifications on the turbine runner.

The application of cross flow turbines as tested in this study stillposes a question: How can the cross flow runner design be furthermodified to optimize the turbine efficiency and effectiveness for ifgiven special configuration of new channel design? The currentstudy does not accurately signify optimum angle of the runnerblades, but proposes more effective angle in order to make the inletflow in perpendicular with the blade. This proposal is able to in-crease the area of the blades exposed to the incoming water,consequently increasing the torque, power and efficiency. Apartfrom achieving the most power generation by adjusting the angles,other solutions include addition of the number of blades, andchanging the cross section of the blades into aerofoil section.

The main advantage of the present system is that it is able towork efficiently with dual turbine runners. Moreover, it canwork atnumerous runners if the system is repeated. Nevertheless, it needsa generator for every turbine, by which the turbines should be inparallel axes and spinning in opposite rotational axis orientationdirections. This, however, can cause increment in the cost of initialbudget of the system. The generators also are mounted in the

turbine shaft and positioned outside at side of the channel. Ac-cording to the current micro channels which have small width anddepth, turbine generator is easy to install because it is not sub-merged. In addition, it does not need special arrangement formooring, which contributes to decrease in the cost. In contrast, thefabrication of turbine generator in large channels or in rivers israther complicated, by which the system in this case becomesfloated and the generator is submerged. Subsequently, it is ex-pected to increase the initial and running cost of the system. Incomparison, the cost of the present system is lower because it isinstalled at micro channels schemes, and it is expected to increasein case of large channels and rivers, in which the cost is acceptableto the ranges of the similar stations, and its efficiency is verysatisfactory.

7. Conclusion

The use of two directed nozzles can accelerate the water flow inmicro channels, especially in low velocity regions. This study hasshown that bidirectional nozzle is capable to concentrate the flowdirection twice whilst increasing the flow velocity. The use ofconventional current turbine is no longer a feasible solution toemploy in the current configuration for generating electricitybecause its efficiency is too low. The proposed cross flow turbine ispotential as the most suitable option for micro hydrokinetic ap-plications. Numerical analysis on the performance of two cross flowturbines inside bidirectional diffuser-augmented channels has beencarried out to investigate the flow properties through the channel,output power and efficiency of the turbines. The results of thisstudy showed that the water flow can be accelerated in the chan-nels when the bidirectional diffuser-augmented channels are uti-lized. The accelerated water flow increases the energy absorbed bythe turbines. In this configuration, the kinetic energy provided tothe lower turbine is more than the upper turbine. By this, the lowerturbine is capable to produce maximum power output nearly 39W,compared to only 18.36 W produced by the upper turbine. Theefficiency of the lower turbine is also increased by 1.3 times of thatthe upper case. In this study, the maximum efficiency obtained wasaround 0.52. Besides hydrokinetic energy harnessing, the currentconfiguration can be applied in other types of energy production,such as wave energy production. Moreover, it is more economical toimplement compared to other similar methods.

Acknowledgment

The authors would like to express sincere gratitude to UniversitiTeknologi Malaysia (UTM) for providing assistance and access todata compilation for this publication.

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